Silica: Assessment methods of synthesis from rice husk, main physical-chemical characteristics and practical applications: monograph 9786010437739

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AL-FARABI KAZAKH NATIONAL UNIVERSITY

S. Azat

SILICA: ASSESSMENT METHODS OF SYNTHESIS FROM RICE HUSK, MAIN PHYSICAL-CHEMICAL CHARACTERISTICS AND PRACTICAL APPLICATIONS Monograph

Almaty «Kazakh university» 2018

UDC 661 LBC 35.50 A 99

Recommended for publication by the Academic Council (Protocol №1, 29.09.2018) and the Editorial and Publishing Council of al-Farabi Kazakh National University (Protocol №1, 11.10.2018) Reviewers: Prof. Prikhodko N.G. (Almaty University of Power Engineering and Telecommunications) Prof. Su Xin Tai (South China University of Technology, China) Prof. Ongarbaev Ye.K. (al-Farabi Kazakh National University)

A 99

Azat S. Silica: Assessment methods of synthesis from rice husk, main physical-chemical characteristics and practical applications: monograph / S. Azat. – Almaty: Kazakh University, 2018. – 151 p. ISBN 978-601-04-3773-9 This monograph contains theoretical and experimental results of the evaluation of various types of the method for the synthesis of silicon dioxide from rice husk of Kazakhstan (Almaty, Kyzylorda and Turkystan) and its application in water purification (Bylkyldak). Based on literature data, it can be noted that the disposal of solid waste such as rice husk and rice straw is a global problem, the burning of which can produce greenhouse gases that adversely affect the environment. In this regard, considerable attention in this monograph is paid to the creation of a complete technological sequence for processing rice husk and the development of economically and environmentally beneficial applications. The monograph is designed for a wide range of specialists in the field of chemistry, nanotechnology, agrochemistry and technology of production and processing of agricultural products, as well as students, undergraduates and Ph.D. doctoral students of relevant specialties.

UDC 661 LBC 35.50

ISBN 978-601-04-3773-9

© Azat S., 2018 © Al-Farabi KazNU, 2018

CONTENTS LIST OF ABBREVIATIONS..................................................................... 6 ILLUSTRATIONS......................................................................................7 PREFACE...................................................................................................14 ACKNOWLEDGMENTS..........................................................................16 INTRODUCTION......................................................................................17 1. RICE HUSK AS A SOURCE FOR THE PRODUCTION OF SILICA NANOPARTICLES: OVERVIEW.........................................19 1.1. Structure and components of rice husk (RH).......................................19 1.2. Recycling of Rice Husk.......................................................................23 1.3. What is SiO2?.......................................................................................25 1.4. Application of rice husks in the production of silica...........................28 1.5. Investigation of the composition of rice husk...................................... 31 REFERENCES........................................................................................... 34 2. TRADITIONAL METHODS FOR SYNTHESIS OF SiO2 NANOPARTICLES & SYNTHESIS OF SILICA FROM RH...................41 2.1. Methods for synthesis of SiO2 nanoparticles.......................................41 2.2. Preparation of silica from RH.............................................................. 44 REFERENCES 2........................................................................................ 51 3. METHODOLOGY OF OBTAINING SiO2 USING PROCESSING OF RICE HUSKS..............................................................56 3.1. Extraction of silica nanoparticles.........................................................57 3.2. Methodologies of obtaining SiO2 by using processing of rice husks, waste from rice production by different treatment methods........................................................................58 3.2.1. Investigation effect of different calcination temperature on the formation of white rice husk ash during direct incineration of water-washed rice husk...........58 3.3. Effect of different treatment methods on the silica yield and the study of the elemental composition of the samples........................61 3.4. Silica oxide sintering method without acidification (Thermally treatment).................................................................................62 3.4.1. Yield of WRH from RHs of different region of Kazakhstan (without HCl) during direct incineration in the muffle furnace...........64 3.5.1. Pre-treatment with Hydrochloric acid........................................ 65 3.5.2. Pre-treatment with Citric Acid...................................................70 3

4

Contents

3.5.3. Effect of Different Pre-Acid Wash Treatment on Silica Yield Pre-treatment with Hydrochloric acid..................................................72 3.6. Study of the elemental composition of the different stages of rice husks ....................................................................................76 3.7. Structural features of silica samples from rice husk............................78 3.7.1. Results of X-Ray Diffraction analysis of the samples...............78 3.7.2. Results of Raman spectroscopic analysis of the samples of white rice husk and silicon dioxide.................................... 80 3.7.3. Results of Fourier Transform Infrared Spectroscopy Analysis..81 3.8. Thermogravimetric analysis................................................................. 82 3.9. Investigation of the surface of silica samples from rice husk..............83 3.9.1. Morphology of RH before and after direct incineration (Scanning Electron Microscopy)......................................................... 83 3.9.2. Morphology of rice husk silica after pre-treatment with hydrochloric acid (Scanning Electron Microscopy)............................84 3.9.3. Analysis of the specific surface area, average pore size of the obtained samples........................................................84 3.10. Conclusion of the experimental part..................................................86 REFERENCES 3........................................................................................87 4. SILICA FROM RICE HUSK: APPLICATIONS....................................89 CONCLUSION...........................................................................................97 REFERENCES 4........................................................................................97 5. RICE HUSK BASED SILICA/Ag NANOPARTICLES COMPOSITE MATERIALS AS A NEW ADSORBENT FOR REMOVAL OF AQUEOUS MERCURY IONS FROM WATER............................................................ 102 5.1. Composite materials in water treatment..............................................102 5.2. Creation Hydride Silica Composites for Rapid and Facile Removal of Aqueous Mercury......................................................... 103 5.3. Relevance of Silica based Composites for Rapid and Facile Removal of Aqueous Mercury Bylkyldak Storage Lake.................107 5.4. Collection of “real” water samples and lake sediments from the lake-reservoir Bylkildak, Pavlodar...............................................110 5.5. Materials and methods......................................................................... 111 5.5.1. Materials and chemicals.............................................................111 5.5.2. Characterizations........................................................................111 5.5.3. Synthesis of silica oxide by thermal treatment.......................... 112 5.5.4. Modification surface of rice husk silica samples with silicon hydride groups..................................................................112 5.5.5. Formation of Ag nanoparticles on the surface of silica.............112 5.6. Results and discussion.........................................................................114

Contents

5

5.6.1. Analysis of samples by the method of low-temperature adsorption-desorption of nitrogen........................................................ 114 5.6.2. Thermal analysis of initial and modified samples of silica samples................................................................................... 115 5.6.3. FTIR analysis of initial and modified samples of silica samples................................................................................... 116 5.6.4. Effect of Silver NPs concentration.............................................118 5.6.5. Characterization of composites SiO2/AgNPs ............................ 120 5.6.6. Mercury-Adsorption Experiments.............................................121 5.6.6.1. Incineration of silver nanoparticles on silica substrate with aqueous solution of mercury (Modular solution)..............................................................................121 5.6.6.2. Incineration of silver nanoparticles on silica substrate with aqueous solution of mercury (Real solution).......123 5.7. Results of SEM and EDX & TEM аnаlysіs of the silica based composite & its interaction with mercury ions.......................124 Conclusion ................................................................................................127 REFERENCES 5........................................................................................128 CONCLUSION...........................................................................................130 APPENDIX ................................................................................................132

LIST OF ABBREVIATION

SiNp RH RHA WRH

Rice Husk Rice Husk Ash White Rice Husk

WRHA

White Rice Husk Ash

TEOS

Tetraethyl orthosilicate

TES CTAB

Triethoxysilane Cetrimonium Bromide

SEM

Scanning Electron Microscopy

TEM

Transmission electron microscopy

XRD

X-ray Diffraction

XRF

X-ray Fluorescence

FT-IR TGA IUPAC BET EC

6

Silica Nanoparticle

Fourier-Transform Infrared Spectroscopy Thermogravimetric Analysis International Union of Pure and Applied Chemistry Brunauer–Emmett–Teller (method) European Communities

ILLUSTRATIONS Figures 1

Rice monitoring of the world market

17

2

Rice producing regions of Kazakhstan

18

3

Rice grain and respective constituent structures and rice husk

19

4

Integrated strategy of agricultural wastes (e.g., RH) valorization for the production of biofuels, carbon and silica/silicon materials

20

5

Scheme of obtaining various substances from rice husks

22

6

General scheme of silica synthesis processes

24

7

Dynamics of the number of articles on the production and application of silica from rice husk ash

26

8

Flow chart of a typical sol-gel process for preparing nanosilica powder

37

9

Schematic diagram of experimental apparatus for the synthesis of silica nanoparticles by flame spray pyrolysis using two-fluid nozzle spray

38

10 Different methods for producing different structural silica from

39

11 Rice husks from different regions of Kazakhstan (Objects of

49

12 Pre-washing of RH with water & drying in air condition

49

13 Appearance of raw rice husk, white rice husk ash and extracted

50

14 Experimental sequence of the direct incineration of rice husk at

51

RH

experiments)

silica from treated rice husk in 2 M HCl acid followed by combustion at 600 ºC

600 °C for 3 hours in muffle furnace (SNOL, 2009) in order to obtain white rice husk ash

7

8

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

15 Percent yield of the white rice husk (WRH) and rice husk silica

54

16 Calcination of rice husks in the muffle furnace at the

55

17 Higher flaming times for the rice husk in the muffle furnace

55

18 White rice husk ash product from burning rice husk inside the

55

19 Filtration of the water soluble sodium silicate solution

55

20 Scheme of the silicon oxide production from the raw rice husk

56

21 Percent yield of the white rice husk (WRH) extracted from the

57

22 Percent yield of the white rice husk (WRH) extracted from the

57

23 Initial raw materials (Rice husks)

58

24 Treatment of the raw material with 2M hydrochloric acid

58

25 Calcination of rice husks in the muffle furnace at the

58

26 Combustion of carbon in the composition of RH at high

58

27 Formation of the white rice husk ash

59

28 Mass of obtained white rice husk ash

59

29 Treatment of the WRHA with 2M NaOH solution

59

30 Installation of vacuum filtration

59

31 Formation of the final product (Silicon dioxide)

59

32 Filtering process of the obtained silicon dioxide

60

33 The obtained pure silicon dioxide after washing with hot water

60

34 Scheme of the silicon oxide production from the raw rice husk

61

(RHS) extracted from the rice husk (RH) of different region of Kazakhstan by different methods temperature of 600 °C (600 ºC)

muffle furnace (600 ºC)

by thermal treatment

rice husk (RH) of different region of Kazakhstan (without HCl) rice husk (RH) of different region of Kazakhstan (without HCl)

solution

temperature of 600 °C temperature

by pre-treatment with hydrochloric acid

9

Illustrations 35 Graphical representation of the scheme of experiments

61

36 Formed white rice husk ash after pre-treatment with citric acid

62

37 Formed final product (Silicon dioxide)

63

38 Scheme of the silicon oxide production from the raw rice husk

63

39 Percent yield of the white rice husk (WRH) and rice husk silica

64

40 Percent yield of the white rice husk (WRH) and rice husk silica

65

41 Influence of Volume of HCl on the pH of RH washing

66

42 Percent yield of the white rice husk (WRH) and rice husk silica

67

43 Percent yield of the white rice husk (WRH) and rice husk silica

70

44 X-ray diffraction pattern of SiO2 obtained from rice husk

70

45 Sample 1 (White rice husk)

70

46 Sample 2 (SiO2+HCl)

71

47 Sample 3 (HCl+SiO2+CTAB)

71

48 Sample 4 (SiO2+25 ºC+H3PO4+hot water+not centrifugation)

71

49 Sample 5 (HCl+CTAB+700 ºC (6h))

71

50 Sample 6 (HCl+SiO2+CTAB+900 ºC (2h))

72

51 FT-IR spectra of rice husks (Rice husk 1 – Almaty rice husk,

72

52 FTIR spectra for silica from rice husk

73

53 Thermograph of RH from different region

74

54 SEM images of rice husks before heat treatment (a) and SEM

74

by pre-treatment with citric acid

(RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with HCl) (RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with HCl)

(RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with citric acid) (RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with citric acid)

Rice husk 2 – Kyzylorda rice husk, Rice husk 3 – Turkystan rice husk)

micrograph of the SiO2

10

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

55 SEM micrograph of the SiO2 obtained from the rice husk ash

74

56 The general scheme of experimental sequence for silica

77

57 A graphical representation of the application of silica

83

58 a) Map showing the location of the study area in Kazakhstan; b)

93

59 The Bylkyldak lake-reservoir appearance, view on the Pavlodar

94

60 Schematic illustration of the experimental procedure of

96

61 Silver nanoparticles generation on the surface of modified silica

99

62 Nitrogen adsorption-desorption isotherm of initial RHA-Si

100

63 Nitrogen adsorption-desorption isotherm of a silane-modified

100

64 Thermograph of RHA-Si sample

100

65 Thermograph of RHA-Si/HSi sample

101

66 FTIR spectra for silica from rice husk (a), after TES

102

67 TEM images of silver nanoparticles synthesized based on

103

synthesis from rice husk ash

nanoparticles (SiNP) in biomedical research. Majorly it is useful in drug delivery as drug delivery carriers and protein. The genetic materials (e.g. DNA, miRNA and siRNA) could be delivering into the cellular system through gene delivery methods using SiNP and photodynamic therapy Map of the Pavlodar Northern Industrial Zone with key study areas indicated; c) Sampling locations for surface sediment samples and sediment cores in Lake Balkyldak. The outfall pipe from the factory is indicated by a dashed and dotted line and effluents enter the lake close to point 1.

prom zone

composite materials fabrication substrate sample

RHA-Si/HSi sample

modification (b) and silver nanoparticles attachment (c) different reaction times

11

Illustrations 68 X-ray diffraction patterns for silver nanoparticles on silica

105

69 X-ray diffraction patterns for silver nanoparticles on silica

105

70 X-ray diffraction patterns for silver nanoparticles on silica

105

71 The dependence of adsorption capacities and concentration of

107

72 The dependence of adsorption capacities and concentration of

108

73 Effect of contact times on the adsorption efficiency of mercury

108

74 TEM image of silica particles acting as the support site for

109

75 SEM images of silica based composite decorated with silver

109

76 SEM images of silver containig composite after interaction with

109

77 EDX graph of Ag0 nanoparticles on the silica support

110

78 Graph of EDX analysis of silver containing sample after

111

subtrate synthesized from 0.2 mmol Ag/g SiO2 concentration of silver subtrate synthesized from 0.3 mmol Ag/g SiO2 concentration of silver subtrate synthesized from 0.4 mmol Ag/gSiO2 concentration of silver Hg in water after sorption (Modular Solution) 2+

Hg2+ in water after sorption (Real solution)

(II) ions from aqueous solution (Real solution) AgNPs

nanoparticles Hg2+

interaction with Hg

2+

12

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Tables 1

Typical composition of rice husk

19

2

Typical husk analysis from various literature sources

27

3

The composition of rice husk ashes according to geographical 28 region

4

Overview of nanosilica preparation using various approaches

5

Comparison of capital cost in production of nano silica from 41 commercial and agro-waste resources

6

Cost estimation of silica production per kg using RHA as source

7

Effect of temperature and residence time on the formation of 51 white rice husk ash during direct incineration of water-washed rice husk in the muffle furnace

8

Yield of WRH, Silica from RHs (Almaty region, Kyzylorda region, 53 Turkystan region) during different methods of treatment

9

Yield of WRH from RHs (Almaty region, Kyzylorda region, 56 Turkystan region) during direct incineration in the muffle furnace

10

Yield of WRH, Silica from RHs (Almaty region, Kyzylorda 64 region, Turkystan region) during pre-treatment with hydrochloric acid

11

Influence of Ph & Volume of HCl on the yield of obtained final 65 product

12

Yield of WRH, Silica from RHs (Almaty region, Kyzylorda region, 66 Turkystan region) during pre-treatment with citric acid

13

The elemental analysis of the amorphous silica samples (600 °C) 67

14

I The elemental composition of different stages of rice husks II The elemental composition of different stages of rice husks

15

Specific surface and average pore sizes of silica samples (SiO2) 75 treated under different conditions

16

Samples of amorphous silica from rice husks obtained with pre- 75 treatment with hydrochloric acid

17

Different applications and purity levels required of rice husk 84 silica and recommended processes

40

41

67

Illustrations

13

18

Volume of production of caustic soda, and the actual consumption 95 of mercury in the PO «Khimprom», Pavlodar

19

The amount of deposited Ag nanoparticles on the surface of silica

98

20

Texture characteristics of samples

99

21

Different silver containing silica samples for interactions with 106 mercury nitrate solution

22

Removal of Hg2+ from aqueous solution

23

Residual mercury concentration and adsorption rates (%) 107 given in respect to the silver nanoparticles contain in nature sourced materials (silica’s obtained from rice husk) and time of interaction with “real” mercury containing aqueous samples of the lake-reservoir Bylkyldak

24

Results of energy-dispersive X-ray spectral microanalysis of 110 zero-valent silver nanoparticles on the surface of silica

25

Interpretation of the EDX graph for a silver containing sample 111 after interaction with Hg2+

106

APPENDIX E 1

Results of X-ray fluorescent analysis of the white rice husk 144 specimens calcinated at different temperature

2

Results of X-ray fluorescent analysis of the white rice husk 145 specimens calcinated at different temperature

3

Results of X-ray fluorescent analysis of different rice husk and 146 rice husk silica specimens (Pre-treated & Untreated specimens)

4

Results of X-ray fluorescent analysis of different white rice 147 husk and rice husk silica specimens (Pre-treated & Untreated specimens)

PREFACE

According to the Food and Agriculture Organization of the United Nations (FAO), the global paddy rice production in 2016 is estimated to be 748.0 million tons. Based on this, the amount of rice husks (RHs) accounts for ~20% of paddy rice production by weight. Much is treated as a waste and either thrown into rivers or put to landfill, often creating pollution problems as it decays or simply returned to the fields where it can become airborne. Therefore, RHs are often considered as an agricultural waste. Applications of pristine RH have been very limited. RHs could be a suitable candidate of feedstock for silica based materials because of their high silica content (15-28 wt %) and large availability. In recent years environmental demand and sustainable development have become increasingly important. It is important to study and utilize RH biowaste, and convert RHs into valued materials. This is the focus of this research. The work is reported and summarized in six Сhapters in this monograph. Chapter 1 is an overview of the preparation of silica based materials from rice husk agricultural waste. Influencing factors such as the temperature of the combustion process, the structural composition of the ash and the geographical location of the rice husk are also described. Research has been conducted on application of RHs as a raw material to synthesize a number of silica compounds, including silica. The applications of such materials are very comprehensive. Synthesis of these silica materials from RHs and their applications has reviewed in this chapter. Chapter 2 is focused on the traditional methods for silica synthesis and synthetic methods of silica fabrication from RH. The nano silica powder is generally prepared by using vapour-phase reaction, sol-gel and thermo-decomposition methods. In most of the abovementioned methods, nano silica is synthesised using chemicals as precursors. In chemical methods, it is easy to control size, shape and purity of the material but the starting reagents are costly. In industrial applications, low costs and large quantities of initial precursor are needed. So that why to reduce using chemicals to production silica 14

Preface

15

it was using extraction of nanosilica from agro waste. It could be observed that this process was comparatively cheaper and also very energy intensive as compared to commercial and industrial methods being used for the production of silica. Also, attention was paid to the reviews the processing methods of rice husk for silica, highlighting their advantages and disadvantages. Chapter 3 represents the results of the experimental procedure of obtaining silica samples from RH grown in different regions (Almaty, Kyzylorda and Turkystan) by using thermal treatment, pre-acid wash treatment with hydrochloric or citric acids and direct combustion of RH. Morphology and intrinsic structure, including surface area and crystallinity of silica was characterized by SEM, XRD and BET measurement. More details about the physical-chemical characterisation of silica are discussed in this Chapter, including the results of Raman spectroscopy, FTIR and TGA. In addition to all Chapter 3 described effect of different treatment methods on the final silica yield. Chapter 4 is devoted to the consideration of various beneficial applications of silica synthesized from the rice husk. Reviewed studies suggests an applicability of this value-based product in diverse fields of industry and environment, such as adsorption materials, porous silica, silica-supported catalysts, silica-supported metal and oxide nanoparticles, photocatalytics, drug delivery and so on. Chapter 5 was aimed at the practical application of a hydride composite material based on silicon dioxide from RHs in the purification of the Bylkyldak storage lake from aqueous mercury. Modification of the SiO2 by triethoxysilane helped in the further work of silver nanoparticles generation on its surface. In our case, aqueous mercury solution interacts with silver metal at a stoichiometric ratio of 1:2, resulting in zero valent mercury. It was of great interest to test the capability of silica modified with hydride groups (≡SiH) for the removal of mercury from water, as these functionalized substrates potentially provide a selective, low-cost, easily manufactured adsorbent. Furthermore, creation of rice husk silica based hydride composite solves the several large man-made environmental disasters of mercury polluted Lake.

ACKNOWLEDGMENTS

Thanks to: Colleagues in the Chair of chemical physics and materials science of Al-Farabi Kazakh National University; Colleagues in the “Institute of combustion problems”; Colleagues in the Open type National nanotechnological laboratory of Al-Farabi Kazakh National University; Colleagues in the Nazarbaev University (Project «Hyperstiochiometry Activity in Metal Nanoparticle Interaction (HYPER Activ))

16

INTRODUCTION Rice, which provides a major source of food for billions of people, covers 1 % of the surface of the earth. Rice husk (RH), the outer covering of paddy rice, is an important by-product during the milling process and residue ash is generated after the burning of RH and the ash is called rice husk ash (RHA), which is a primary waste material in the agricultural industry. The main components in RH are lignin, cellulose, and hemicellulose, which are generally named lignocellulose. RH also contains approximately 15 to 28 wt% of silica. The high content of silica in RH presents opportunities for the preparation of valueadded silicon based materials. Since the 1970s, various silicon based materials, including silica, silicon carbide, silicon nitride, silicon tetrachloride, zeolite, and silicates, have been successfully synthesized using RH as the silicon source. This field of research has been significantly advanced and expanded in the past decade spurred by the global attention on sustainable and renewable resources. Extraction of silica from rice husk is an emerging trend in the current research field. Large amount of rice husk (RH) are treated as waste and disposed off at the landfill site. These waste materials can also cause fire, which may lead to severe environmental pollutions. The airborne particles produced from dust may induce respiratory disease to human beings. The burning of rice husk results in the formation of rice husk ash (RHA) with major SiO2 content with 10 to 20 % of carbon and traces of other organic components depending on the burning conditions, the furnace type, the rice variety, the climate and the geographical area. Moreover, the commonly used silica precursor like tetraethyl orthosilicate (TEOS) is more expensive, and hence rice husk ash (RHA) and other waste sources having silica are used as an alternative. Acid leaching of the rice husk ash was carried out to remove soluble elemental impurities and hence it increases the purity of the silica content. The organic compounds in rice husk and other waste materials can be decomposed under burning conditions. The metallic impurities can be transferred to soluble ions by simple acid treatment. 17

18

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

The development of technology for the production of silica from rice husks is gaining popularity due to the growing consumption of amorphous silicon dioxide in many countries, including Kazakhstan, which has the widest application in various industries. Considering the above, the relevant task is search of new economically feasible methods for development silicon-containing materials from rice husk. In this work the treatment of rice husk with different preparation methods to prepare high purity silica is investigated: acid leaching technique (hydrochloric acid and citric acid) and thermally treatment.

I

RICE HUSK AS A SOURCE FOR THE PRODUCTION OF SILICA NANOPARTICLES: OVERVIEW

1.1. Structure and components of rice husk (RH) Critical economic and environmental situations of the current days encourage companies and researchers to develop and improve technologies intended to reduce or minimize industrial wastes. Consequently, much effort has been expended in different areas, including the agricultural production. Rice is the second largest produced cereal in the world. Its production is geographically concentrated in Asia with more than 90 percent of world output. The United States and Brazil are the most important non-Asian producers and Italy ranks first in Europe. According to information from the Food and Agriculture Organization of the United Nations, world rice production in 2016 was 748.0 million tons in the global rice market (Figure 1) [1].

Figure 1 – Rice monitoring of the world market [1]

While in Kazakhstan, according to the information of Ministry of Agriculture of the Republic of Kazakhstan, the main regions in the territory of the republic involved in rice production are: Kyzylorda Region (78,4 thousand ha), Almaty Region (11,1 thousand ha) and 19

20

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Turkystan Region (3 thousand ha) (Figure 2) [2-3]. Rice growing in Kazakhstan is developed mainly in the land areas of the lower reaches of the Syr Darya River, in the territory of the Kyzylorda Region. The location of this region below sea level, state of the soil, ameliorative and agricultural conditions have become the reason for the creation of a special irrigation engineering system for watering of rice fields. In most varieties rice is composed by approximately 20 % of rice husk, which contains a fibrous materials and silica (from 1 ton of paddy it is possible to produce 220 kg of rice husk); however the amount of each component depends on the climate and geographic location of rice crop. Therefore, due to its high percentage in the grain composition, the husk is considered a by-product in the mills and creates disposal and pollution problems [4-5].

Figure 2 – Rice producing regions of Kazakhstan [2-3]

RH is a major agro-industrial by-product of rice milling. It is the natural protective outer shell that develops during the growth of rice and its main components are palea and lemma [6]. Representative dissected structure of mature rice grain, respective constituent

I. Rice Husk as a Source for the Production of Silica Nanoparticles ...

21

structures is shown in Figure 3. These two components interlock with each other after the pollen grains are shed and provide an enclosure to rice grain. The lemma and palea remain firmly attached to rice grain providing shelter during its growth and are separated during rice milling. Structure and internal organization of lemma are different from palea; however, their surface morphology is similar [7]. In addition, morphology of the outer surface of lemma/palea is different from the inner ones, which houses the rice grain. The inner surface of this protective shell (layer) is comparatively smooth owing to the presence of wax and natural fats, while the outer surface is highly irregular [6-8]. The presence of waxy materials in the inner surface of RH, which houses the rice grain, provides great asylum to rice grain (paddy) against termites and other harmful microorganisms [9]. On the other hand, the presence of these waxy materials is considered to be responsible for the poor affinity/interactions between RH particles and the polymeric matrices owing to their ability to hide the reactive sites available for interaction [10].

Figure 3 – Rice grain and respective constituent structures and rice husk Source: adapted from Buggenhout et al. [11] and Kieling [12]

Major constituents of RH are cellulose, hemicellulose, lignin, and silica, and a typical composition of these constituents is tabulated in Table 1 [13-15].

22

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Typical composition of rice husk [13-15] Composition Cellulose Hemicellulose Lignin Silica Soluble Moisture content

Table 1

Percentage 25-35 18-21 26-31 15-17 2-5 5-10

Reports have been published on the composition, properties, and intended uses of RHs since at least as early as 1871 [16]. Silicon enters the rice plant through its root in a soluble form, probably as a silicate or monosilicic acid, and then moves to the outer surface of the plant, where it becomes concentrated by evaporation and polymerization to form a cellulose silica membrane. There is quite general agreement that the silica is predominantly in inorganic linkages, but some of the silica is also bonded covalently to the organic compounds. This portion of the silica cannot be dissolved in alkali and can withstand very high temperatures [18]. Characterizations by SEM, energy-dispersive X-ray analysis, AES, etc., suggest that the silica is mainly localized in the tough interlayer (epidermis) of the RH and that it also fills in the spaces between the epidermal cells [16-17, 19-20]. Harvesting silica or silicon from agricultural wastes including RH can not only take full advantage of their highest potential value, but also minimize the related environmental issues from the valorization of RH [25]. Thermochemical treatment such as pyrolysis, gasification is considered as the most promising approach that can converted RH to biofuels (e.g., bio-oil, syngas) and biochars simultaneously [26]. Subsequently, bio-oil can be converted to renewable biofuels (e.g., biodiesel) by using the RH-derived catalysts. Vapors from organic matters decomposition can be upgraded to value-added syngas for energy application or chemical synthesis by using the RH silica materials, which are used for gas cleaning [27-28], catalytic reforming [29-30]. In general, the applications of RH-derived biochars mainly include soil remediation, pollutants removal, silicon materials, and so on (Figure 4).

I. Rice Husk as a Source for the Production of Silica Nanoparticles ...

23

Figure 4 – Integrated strategy of agricultural wastes (e.g., RH) valorization for the production of biofuels, carbon and silica/silicon materials. Adapted from [25-30]

Conclusion. It is now worldwide accepted that lignocellulosic biomass (rice straw and sugarcane bagasse) has tremendous potential for provided economic, energy saving, ecology benefits and improvements in properties of material. Production of ethanol, charcoal, building materials (concrete, cement, and bricks), paper from rice straw, rice husk, sugarcane bagasse is the major renewable or recyclable residues. Also, production of silicon nanoparticle from rice husk has potential to contribute majorly in biomedical research especially in drug delivery system. Properties and applications of agricultural solid wastes show better options for the utilization of rice and sugarcane biomass instead of burning or leaving them on agricultural land after harvesting. Because the open burning or leaving them on field may cause sever eco-toxicity and heath problems like, cardiovascular disease, neurodegeneration and possibly cancer [24]. 1.2. Recycling of Rice Husk Much of the husk produced from the processing of rice is either burnt or dumped as waste. Even though some of this husk is converted into end products such as feedstock and adsorbent most are burnt

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

openly, causing environmental and health problems especially in poor and developing countries [31-32]. Therefore, it is very important to find pathways to fully recycling of the rice husk. Burning rice husks as a substitute for fuel for energy production is a useful solution that is used in many industries. However, it leads to new wastes, called rice husk ash (RHA). Ash of rice husks are widely used as raw materials for various applications, such as the production of building materials and concrete. It can also serve as adsorbent for the adsorption of an organic dye, inorganic metal ions, and waste gases and as a carrier catalyst [33-35]. In recent decades, the RHA has been widely used as a building material for concrete products [3638] or as an adsorbent for an organic dye such as indigocarmine dye [39] and inorganic metals such as lead, mercury [40], Cd2+ and Zn2+ [41]. Because of the high silicon content, the RHA (80-97 %) can be an economic raw material for the production of silicates and silicon materials [42-43]. Depending on the combustion process, the RHA can contain silica in an amorphous form. The amorphous form of silica is obtained from the RHA obtained by burning rice husks at controlled temperatures below 700 °C. The transformation of this amorphous state into a crystalline state occurs if the ash is exposed to high temperatures above 850 ºC [44]. While crystalline silica is used in ceramics [45] and in the cement industry [46], amorphous silica has even more applications [47]. Therefore, this residue can be considered as a new economically advantageous raw material for the production of silica [48]. Rice husk ash, obtained by burning rice husks, can contain more than 90 % of silica and an amount of metallic impurities. Metallic impurities such as iron (Fe), manganese (Mn), calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg), which affect the purity and color of silica, can be eliminated by pre-treatment with hydrochloric acid, sulfuric acid or nitric acid before burning [49]. Silica, obtained from rice husk, is used in cement and concrete constructions as good pozzolan [50-51] and as an anticorrosive agent for other applications. Various value-added products, such as airgel [52-53], silicon carbide, porous carbon [54], zeolites [55], high purity silicon [56], and others can be similarly produced by using rice husk silicium. These applications are related to the used treatment or method, which often affects the characteristics of the product. For example,

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studies by H. Hamdan, M.N.M Muhid, S. Endud, E. Listiorini, and Z. Ramli have shown that various methods for the production of rice husk silica affect different morphology, structure and reactivity. India, Pakistan, Bangladesh, Sri Lanka, Australia, Thailand, Indonesia, and USA were pioneers in the recycling of rice husk during 1970–1985, which were supported by government and other organizations. Unique characteristics of rice husk in comparison with other agricultural residues, such as high silica contents (87-97 wt% SiO2), high porosity, lightweight and very high external surface area make it a valuable material for industrial applications. Figure 5 depicts some of the applications of rice husks in different industrial fields, taking advantages of special characteristics of rice husk [57].

Figure 5 – Scheme of obtaining various substances from rice husks [57]

1.3. What is SiO2? The chemical compound silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

of diatoms. Silica is manufactured in several forms, including fused quartz, crystal, fumed silica, colloidal silica, silica gel, and aerogel. Silica is the common name for materials composed of silicon dioxide (SiO2) and occurs in crystalline and amorphous forms. Crystalline silica exists in multiple forms. Quartz and, more specifically a-quartz, is a widespread and well-known material. Upon heating, a-quartz is transformed into b-quartz, trydimite and cristobalite. Porosil is the family name for porous crystalline silica. Quartz exists in natural and synthetic forms, whereas all porosils are synthetic. Amorphous silica can be divided into natural specimens (eg. diatomaceous earth, opal and silica glass) and human-made products [58-59]. Amorphous silica particles are formed by polymerization of monomers in the aqueous solution supersaturated with silicic acid. Various silica materials are produced in liquid phase processes (Figure 6). Colloidal silica or silica sol is most often produced in a multistep process in which the alkaline silicate solution is partially neutralized with a mineral acid. Alternatively, this pH neutralization can be achieved by electrodialysis. The resulting silica suspension is stabilized by pH adjustment. Finally a solid concentration up to 50 wt% is reached by water evaporation. Silica sol nanoparticles show a perfect spherical shape and identical size as a result of extensive Ostwald ripening [60]. Stober silica sol is prepared by controlled hydrolysis and condensation of tetraethylorthosilicate (TEOS) in ethanol to which catalytic amounts of water and ammonia are added. The Stober procedure can be used to obtain monodisperse spherical amorphous silica particles with tunable size and porosity [61]. Silica gel is obtained by destabilizing silica sol. Silica gel is an open 3-D network of aggregated sol particles. The pore size is related to the size of the original silica sol particles composing the gel. Precipitated silica is formed when a sol is destabilized and precipitated. Ordered mesoporous silica is obtained by a supramolecular assembly of silica around surfactant micelles. Typical surfactant molecules are amphiphilic polymers such as tribloc copolymers or quaternary alkylammonium compounds. These organic supramolecular templates are evacuated from the mesopores, typically via a calcination step. Calcination is a controlled combustion process

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leading to oxidation and decomposition of the template molecules into small volatile products such as NOx, CO2 and H2O, which can leave the pores. The diameter of the mesopores (2-50 nm) is determined by the type of surfactant applied [62-63]. A completely different synthesis route of amorphous silica starts from SiCl4 in the vapor phase. Silicon tetrachloride is oxidized in a hydrogen flame at temperatures exceeding 1000 °C and polymerized into amorphous non-porous SNPs. This nanopowder has very low bulk density and high specific surface area, typically 200 to 300 m2/g. This material is called pyrogenic or fumed silica, referring to the special synthesis conditions [60]. The synthesis of dense crystalline silica such as quartz from aqueous solution is a slow process requiring heating the solution to accelerate the formation process in a so-called hydrothermal synthesis [60]. Alternatively, under high pressure, amorphous silica can be transformed to crystalline material by microcrystallization. The appearance of quartz ranges from macroscopic crystals to microcrystalline powders. Large crystals are grown at high temperature and pressure in industry. Smaller quartz crystals are conveniently obtained by grinding large crystals. Alpha-quartz is formed under moderate temperature and pressure conditions and is the most abundant form of quartz. At temperatures exceeding 573 °C, a-quartz can transform into b-quartz [64]. At atmospheric pressure and temperatures higher than 870 °C, quartz is transformed into tridymite and at temperatures more than 1470 °C into cristobalite [60, 65]. These high-temperature polymorphs of quartz have the same elemental composition but a different crystal structure and can persist metastably at lower temperatures. Dense and porous crystalline materials can be distinguished by framework density. The framework density is conveniently defined as the number of tetrahedrally coordinated atoms (T-atoms) per nm3. For dense structures, such as quartz, tridymite and cristobalite, values of 22 to 29 T-atoms/nm3 are common, whereas for porosils belonging to the zeolite material family, as few as 12.1 T-atoms/nm3 are present [66]. The framework structure of a porosil is denoted with a 3-letter code [67]. Porosils are crystallized in aqueous media in the presence of organic molecules that act as porogens or template molecules defining the size and shape of the pores. Their evacuation is typically achieved through calcination. Among the porosils are clathrasils and zeosils

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

[68-69]. Zeosils have cages with windows or channels of a sufficiently free dimension to allow molecules to diffuse in and out, a property known as molecular sieving [70]. Clathrasils have cages with windows that are delineated with a 6-membered ring of SiO units, thus presenting a free aperture of barely 0.28 nm. Even a molecule as small as oxygen has no access to the cavities of a clathrasil. The organic template molecules engaged in the crystallization of a clathrasil cannot be removed easily from the pores [68-69]. When heated above 1700 °C, any type of silica (amorphous or crystalline) melts. During cooling, the disordered structure is solidified, and a dense amorphous silica glass or vitreous silica is formed [60].

Figure 6 – General scheme of silica synthesis processes. Adapted from [71]

1.4. Application of rice husks in the production of silica Rice husk during combustion gives ash, which contains a very high percentage of crystalline silica. Nevertheless, if it is burned under controlled conditions is obtained amorphous silica which has a high reactivity. M.A. Hamad, I.A. Khattab studied the thermal decomposition of rice husks in the laboratory [72]. They noted that

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the ash, which formed at lower temperatures (773-873 K), consisted of amorphous silica. Crystalline forms of cristobalite and tridymite were found at temperatures >1073 K and >1423 K, respectively. Subsequent to this, it was evaluated the ash, formed during the burning of rice husks in a fixed bed at different air velocities. At lower combustion rates is formed amorphous silica, while at higher speeds the silica crystallizes. Crystallization of silica should be prevented by controlling the temperature and the burning time, in order to obtain the maximum amorphous material. The grinding of ash to produce a high surface area can increase the reactivity. The heat, generated by burning rice husk, eventually converted into mechanical energy for grinding the ash. RHA during mixing with Portland cement produces high-strength concrete. The conducted studies indicate the possibility of converting RH into amorphous silica with high reactivity, which can be applied in high-strength concrete. R. K. Chouhan, B. Kujur, S. S. Amritphale, N. Chandra reported [73] that the temperature of complete combustion of the rice husk affects the compression strength of the lime-RHA solution. And here the main role is played by the reactivity of silica. Thus, the quality of the RHA depends on various factors, such as the temperature of complete combustion, the time, the heating rate, the type of oven/kiln, etc. However, the efficiency of combustion, the optimal use of energy is extremely important for determining the economic benefits of the entire process, which requires careful study. Guanyi Chen, Guiyue Du, Wenchao Ma, Beibei Yan, Zhihua Wang, Wenxue Gao [74] used fluidized bed combustion (bed material: quartz sand) to produce amorphous silica whose purity level does not exceed 94.8 % and, therefore, this method is not suitable for the production of many value-added products. The method of using 1M NaOH was used by U. Kalapathy, A. Proctor, and J. Shultz to obtain silica from rice husk ash with a purity of more than 97 %, in order to create a silicate heat-insulating material [75]. Later, in 2011, Xiaoyu Ma, Bing Zhou, Wei Gao, Yuning Qu, Lili Wang, Zichen Wang, Yanchao Zhu was proposed a recyclable technology for the production of silica powder using rice husks and ammonium fluoride. The purity of the final dioxide reached up to 94.6 % [76]. T-H. Liou [77] obtained amorphous nanostructured silica powders with an average particle size of 60 nm and a high specific surface area

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by using nonisothermal decomposition of rice husk at temperatures between 27-727 °C by using different burning rates. N. Yalcin, V. Sevinc [78] demonstrated that the homogeneous particle size distribution of nano-sized silica can be obtained by burning pre-treated rice husks at 600-800 °C in an atmosphere of pure oxygen. However, by analyzing these methods, it can be concluded that methods that are capable to obtaining high purity nano-silica are chemical based on hazardous and environmentally hazardous products, and can be expensive, requiring additional precautions. Sodium silicate is used as a source of silicon in the industrial production of silica. However, sodium silicates obtained by melting quartz sand and sodium carbonate at 1300 °C require not only a large amount of energy but also further purification (Affandi, S., H. Setyawan, S. Winardi, A. Purwanto, R. Balgis, 2009 [79]). In addition, this can be the cause of widespread environmental pollution. Alternative solution of this problem can be the low-temperature extraction of amorphous silica from plant biomass, which makes it possible to produce a highquality, environmentally friendly and economical product (TzongHorng Liou, Chun-Chen Yang, 2011 [80]). Ghorbani, F., Y. Habibollah, Z. Mehraban, M. S. Celik, A. A. Ghoreyshi, M. Anbia, 2013 showed that 600 °C is the optimum temperature for the production of amorphous silica [81]. Nevertheless, Rozainee, M., S. P. Ngo, A. A. Salema, K. G. Tan [82] found that the RHA remains amorphous at calcination at 700 °C for 6 hours. Many researchers used heat treatment of husks and processing with various chemical reagents (HCl, H2SO4, HNO3, NaOH, NH4OH, etc.) to produce amorphous high purity silica from rice husks [83-86] before and after burning at temperatures from 773 to 1673 K and at different time intervals. Chemical processing before burning was more beneficial. Whereas the formation of black particles in the composition of silica from the untreated husk was found to be higher than the acidtreated husks. It has been proved that potassium in the husks causes this phenomenon, which is largely removed by acid treatment [84-85]. The following diagram can serve as a proof fact of increasing production of silica from the rice husk ash by visually representing the relevance of the existing research direction. The dynamics of the number of articles over the past 10 years shows that the number of studies on the production and application of the silica from rice husks

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I. Rice Husk as a Source for the Production of Silica Nanoparticles ...

increases every year (Figure 7). The present dynamics was created according to the ScienceDirect database [87].

Figure 7 – Dynamics of the number of articles on the production and application of silica from rice husk ash [87]

1.5. Investigation of the composition of rice husk Rice husks have low moisture content, generally in the range of 8 % to 10 % [88-89]. The following are typical chemical analyses of rice husks: Typical husk analysis from various literature sources Property Bulk density (kg/m3) Length of husks (mm) Hardness (Moh’s Scale) Ash Carbon (%) Hydrogen (%) Oxygen (%) Nitrogen (%) Sulphur (%) Moisture (%)

Reference

Table 2

Reference

96-160

[90]

128

[92]

2.5-5

[90]

–

–

5.5-6.5

[90]

–

–

22.24 35.77 5.06 36.59 0.32 0.082 8.05

[88] [88] [88] [88] [88] [88] [88]

13.2-29.0 36.66 4.37 31.68 0.23 0.04 8.76

[91] [92] [92] [92] [92] [92] [92]

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

The silica in the rice husks is at the molecular level, and is associated with water. It occurs in several forms (polymorphs) within the husks. In nature, the polymorphs of silica (SiO2) are: quartz, cristobalite, tridymite, coesite, stishovite, lechatelerite (silica glass), and opal; the latter two being amorphous [88]. For RHA as a potentially product we need only distinguish between amorphous silica and crystalline silica. From the technical literature, two forms appear to predominate in combustion and gasification. These are lechatelerite (silica glass), an amorphous form, and cristobalite, a crystalline form. SiO2 can also occur in a very fine, submicron form. This form is of the highest commercial value although it is the most difficult to extract. SiO2 is generally determined as ‘total’ SiO2, since the proportion of crystalline to amorphous silica requires further costly analysis, usually by X-Ray Diffraction (XRD). Determining the quantity of these polymorphs is fundamental to investigating the ash. The colour of the ash generally reflects the completeness of the combustion process as well as the structural composition of the ash. Generally, darker ashes exhibit higher carbon content (with the exception of those that may be darker due to soil chemistry/ region. Lighter ashes have achieved higher carbon burnout, whilst those showing a pinkish tinge have higher crystalline (tridymite or cristobalite) content. Influence of geographical region on ash properties. It has been reported that chemical variations in husk composition (and consequently ash composition) are influenced by such things as the soil chemistry, paddy variety and climate. However, only one report of a change in the physical and chemical properties of ash influenced by region was found [93]. A variation in colour and trace metal was found in ash from husks burnt in different regions, with ash produced from husks from Northern India resulting in a much darker ash than husks from the US [93]. The colour variation was not related to differences in the carbon remaining in the ash, although it is not known the precise regional features that affected the ash. It could be due to the agronomy of the paddy as studies have shown that differences in mineral composition of ash can be attributed to fertilizers applied during rice cultivation, with phosphate having a negative affect on the quality of the ash in terms of its ability to act as a pozzolan [94]. It has also been said that the high K2O found in

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some ashes could be a consequence of K-rich fertilizers used during the paddy cultivation [95]. Thus, different authors [57, 96-98] have published different values of the composition of rice husk ash which is largely dependent on geographical location (Table 3). Table 3 The composition of rice husk ashes according to geographical region Elemental composition of rice husk ash SiO2 K 2O P 2O 5 CaO SO3 MgO Al2O3 Fe2O3 MnO Rb2O ZnO CuO Na2O Cl PtO2 Tm2O3

XRF [57] Kazakhstan 90.194 2.949 – 2.618 2.021 0.311 0.23 0.570 0.138 – 0.037 0.071 0.365 0.278 0.047 0.172

XRF [96] Canada 91.56 4.76 – 0.78 0.29 – 2.36 0.11 0.07 – 0.01 0.01 – – – –

ICP [97] Egypt 91.5 1.23 0.30 0.57 – 0.30 0.62 0.42 0.04 – – – 0.18 – – –

XRF [98] Malaysia 91.25 3.829 2.45 0.875 0.661 0.573 0.18 0.0866 0.0726 0.0143 0.0111 – – – – –

From Table 3, it is clear that although the composition of the rice husk may be dependent on some factors, the percentage of silica in the ashes ranges between 90-92 %.

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[43] J.D. Martinez, T. Pineda, J.P. Lopez, M. Betancur. Assessment of the rice husk lean-combustion in a bubbling fluidized bed for the production of amorphous silica-rich ash // Energy. – 2011. – Vol. 36, Iss. 6. – P. 3846. [44] V.R. Shelke, S.S. Bhagade, and S.A. Mandavgane. Mesoporous Silica from Rice Husk Ash // Bulletin of Chemical Reaction Engineering & Catalysis. – 2010. – Vol. 5, Iss. 2. – P. 63. [45] C. S. Prasad, K. N. Maiti, and R. Venugopal. Effect of RHA in White Ware Compositions // Ceramics International. – 2001. – Vol. 27, Iss. 6. – P. 629635. [46] J. C. Saha, K. Diksit, and M. Bandyopadhyay. Comparative Studies for Selection of Technologies for Arsenic Removal From Drinking Water // BUET-UNU International Workshop on Technologies for Arsenic Removal from Drinking Water, Bangladesh.UNDP Sustainable Development Networking Program, Technical Session II, May 5th. – 2001. – P. 76-84. [47] S. Siriwandena, H. Ismail, and U. S. Ishakiaku. A Comparison of White Rice Husk Ash and Silica as Filler in Ethylene-propylene-diene Terpolymer Vulcanizates // Polymer International. – 2001. – Vol. 50. – P. 707-713. [48] Della, Viviana Possamai; Hotza, Dachamir; Junkes, Janaína Accordi and Oliveira, Antonio Pedro Novaes de. Comparative study of silica obtained from acid leaching of rice husk and the silica obtained by thermal treatment of rice husk ash // Quimica Nova. – 2006. – Vol. 29, Iss. 6. – P. 1175. [49] Rohani Abu Bakar, Rosiyah Yahya, Seng Neon Gan. Production of High Purity Amorphous Silica from Rice Husk // Procedia Chemistry. – 2016. – Vol. 19. – P. 189-195. [50] Shazim Ali Memon, Muhammad Ali Shaikh, Hassan Akbar. Utilization of Rice Husk Ash as viscosity modifying agent in Self Compacting Concrete // Construction and Building Materials. – 2011. – Vol. 25, Iss. 2. – P. 10441048. [51] M.F.M. Zain, M.N. Islam, F. Mahmud, M. Jamil. Production of rice husk ash for use in concrete as a supplementary cementitious material // Construction and Building Materials. – 2011. – Vol. 25, Iss. 2. – P. 798-805. [52] Qi Tang, Tao Wang. Preparation of silica aerogel from rice hull ash by supercritical carbon dioxide drying // The Journal of Supercritical Fluids. – 2005. – Vol. 35, Iss. 1. – P. 91-94. [53] Nayak J, Bera J. Preparation of silica aerogel by ambient pressure drying process using rice husk ash as raw material // Transactions of the Indian Ceramic Society. – 2009. – Vol. 68, Iss. 2. – P. 1-4. [54] Yupeng Guo, Jingzhe Zhao, Hui Zhang, Shaofeng Yang, Jurui Qi, Zichen Wang, Hongding Xu. Use of rice husk-based porous carbon for adsorption of Rhodamine B from aqueous solutions // Dyes and Pigments. – 2005. – Vol. 66, Iss. 2. – P. 123-128. [55] Halimaton Hamdan, Mohd Nazlan Mohd Muhid, Salasiah Endud, Endang Listiorini, Zainab Ramli. 29Si MAS NMR, XRD and FESEM studies of rice husk silica for the synthesis of zeolites // Journal of Non-Crystalline Solids. – 1997. – Vol. 221, Iss. 1-2. – P. 126-131. [56] Yu-Bin Im, Rizwan Wahab, Sadia Ameen, Young-Soon Kim, O-Bong Yang, and Hyung-Shik Shin. Synthesis and Characterization of High-

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ... Purity Silica Nanosphere from Rice Husk // Journal of Nanoscience and Nanotechnology. – 2011. – Vol. 11, Iss. 7. – P. 5934-5938. [57] S. Azat., A.V. Korobeinyk, N. Meirbekov., R.B. Kozakevych, R.L.D. Whitby., Z.A. Mansurov. Nano-SiO2 from rice husk ash, synthesis and characterization // International Symposium «Physics and chemistry of carbon materials / nanoengineering», International Conference «Nanoenergetic materials and nanoenergy». – 2016. – P. 28-31. [58] Robert J. P. Corriu and Dominique Leclercq. “Recent developments of molecular chemistry for sol-gel processes” // Angewandte Chemie International Edition In English. – 1996. – Vol. 35, Iss. 13-14. – P. 14201436. [59] Dorota Napierska, Leen CJ Thomassen, Dominique Lison, Johan A Martens and Peter H Hoet. The nanosilica hazard: another variable entity // Particle and Fibre Toxicology. – 2010. – Vol. 39, Iss. 7. – P. 1-2. [60] Von R. K. Iler. John Wiley and Sons. The Chemistry of Silica. Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry. // Angewandte Chemie. – 1979. – Vol. 92, Iss. 4. – P. 1-328. [61] Stober W, Fink W, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range // Journal of Colloid and Interface Science. – 1968. – Vol. 26, Iss. 1. – P. 62-69. [62] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Ordered Mesoporous Molecular-Sieves Synthesized by A Liquid-Crystal Template Mechanism // Nature. – 1992. – Vol. 359. – P. 710-712. [63] Zhao D, Feng J, Huo Q, Melosh N, Fredrickson GH, Chmelka BF, Stucky GD. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores // Science. – 1998. – Vol. 279. – P. 548-552. [64] Andre P. Legrand. The Surface Properties of Silicas // John Wiley & Sons, Ltd. (UK). – 1998. – P. 494. [65] Heaney PJ. Structure and Chemistry of the Low-Pressure Silica Polymorphs. In Silica: physical behanior, geochemistry, and materials applications. // Edited by: Heaney PJ, Prewitt CT, Gibbs GV. Washington, D.C.: Mineralogical Society of America. – 1994. – P. 1-40. [66] Liebau F. Structural Chemistry of Silicates. Structure, Bonding, and Classification // Springer-Verlag Berlin Heidelberg. – 1985. [67] Baerlocher Ch, McCusker LB, Olson DH. Atlas of Zeolite Framework Types // Amsterdam: Elsevier. – 2007. [68] Barrer RM. Hydrothermal Chemistry of Zeolites // London: Academic Press. – 1982. [69] Higgins JB. Silica zeolites and clathrasils. In Silica: physical behanior, geochemistry, and materials applications // Edited by: Heaney PJ, Prewitt CT, Gibbs GV. Washington, D.C.: Mineralogical Society of America. – 1994. – P. 507-543. [70] Tosheva L, Valtchev VP. Nanozeolites: Synthesis, Crystallization Mechanism, and Applications // Chemistry of Materials. – 2005. – Vol. 17. – P. 2494-2513. [71] Brinker CF, Schrerer GW. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing // 2 edition. London: Academic Press. – 1990.

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[72] M.A. Hamad, I.A. Khattab. Effect of the combustion process on the structure of rice hull silica // Thermochimica Acta. – 1981. – Vol. 48, Iss. 3. – P. 343-349. [73] R. K. Chouhan, B. Kujur, S. S. Amritphale, N. Chandra. Effect of temperature of washing of rice husk on the compressive strength of lime-rice husk silica mortar // Silicates Industriels. – 2000. – Vol. 65, Iss. 5/6. – P. 67-71. [74] Guanyi Chen, Guiyue Du, Wenchao Ma, Beibei Yan, Zhihua Wang, Wenxue Gao. Production of amorphous rice husk ash in a 500 kW fluidized bed combustor // Fuel. – 2015. – Vol. 144. – P. 216. [75] U. Kalapathy, A. Proctor and J. Shultz. Silicate thermal insulation material from rice hull ash // Industrial & Engineering Chemistry Research. – 2003. – Vol. 42, Iss. 1. – P. 46-49. [76] Xiaoyu Ma, Bing Zhou, Wei Gao, Yuning Qu, Lili Wang, Zichen Wang, Yanchao Zhu. A recyclable method for production of pure silica from rice hull ash // Powder Technology. – 2011. – Vol. 217. – P. 497-501. [77] Tzong-Horng Liou. Preparation and characterization of nano-structured silica from rice husk // Materials Science and Engineering: A. – 2004. – Vol. 364, Iss. 1-2. – P. 313-323. [78] N. Yalçin, V. Sevinç. Studies on silica obtained from rice husk // Ceramics International. – 2001. – Vol. 27, Iss. 2. – P. 219-224. [79] Samsudin Affandi, Heru Setyawan, Sugeng Winardi, Agus Purwanto, Ratna Balgis. A facile method for production of high-purity silica xerogels from bagasse ash // Advanced Powder Technology. – 2009. – Vol. 20, Iss. 5. – P. 468-472. [80] Tzong-Horng Liou, Chun-Chen Yang. Synthesis and surface characteristics of nanosilica produced from alkali-extracted rice husk ash // Materials Science and Engineering: B. – 2011. – Vol. 176, Iss. 7. – P. 521-529. [81] Farshid Ghorbani, Habibollah Younesi, Zahra Mehraban, Mehmet Sabri Çelik, Ali Asghar Ghoreyshi, Mansoor Anbia. Preparation and characterization of highly pure silica from sedge as agricultural waste and its utilization in the synthesis of mesoporous silica MCM-41 // Journal of the Taiwan Institute of Chemical Engineers. – 2013. – Vol. 44, Iss. 5. – P. 821-828. [82] M. Rozainee, S.P. Ngo, Arshad A. Salema, K.G. Tan. Fluidized bed combustion of rice husk to produce amorphous siliceous ash // Energy for Sustainable Development. – 2008. – Vol. 12, Iss. 1. – P. 33-42. [83] Madhumita Sarangi, S. Bhattacharyya, and R.C. Behera. Effect of temperature on morphology and phase transformations of nano-crystalline silica obtained from rice husk // Phase Transitions. – 2009. – Vol. 82, Iss. 5 – P. 377-386. [84] R.V. Krishnarao, J. Subrahmanyam, T. Jagadish Kumar. Studies on the formation of black particles in rice husk silica ash // Journal of the European Ceramic Society. – 2001. – Vol. 21, Iss. 1. – P. 99-104. [85] Ali M., Ul Haq E., Abdul Karim M., Ahmed S., Ibrahim A., Ahmad W., Baig W. Effect of leaching with 5-6N H2SO4 on thermal kinetics of rice husk during pure silica recovery // Journal of Advanced Research. – 2016. – Vol. 7, Iss. 1. – P. 47-51.

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ... [86] Rosario Madrid, C. A. Nogueira, F. Margarido. Production and characterisation of amorphous silica from rice husk waste // 4th International Conference on Engineering for Waste and Biomass Valorisation. – 2012. – Vol. 4. – P. 1-10. [87] http://www.sciencedirect.com [88] Velupillai, L., Mahin, D.B., Warshaw, J.W and Wailes, E.J. (1997). A Study of the Market for Rice Husk-to-Energy Systems and Equipment. Louisiana State University Agricultural Center, USA [89] Mahin, D. B. (1990). Energy from Rice Residues, Biomass Energy and Technology Report. Winrock International Institute for Agricultural Development. Arlington, USA. [90] Houston, D.F. (1972) . Rice Chemistry and Technology. American Association of Cereal Chemists, St Paul, MN, USA. pp689-695 [91] Dodson, Tortech. (2002). Personal Communication. www.torftech.com/ news.htm [92] Agrilectric (2001, 2002). Personal Communication. www.agrilectric.com [93] Hsu, Wen-Hwei and Luh, Bor S. Rice Hulls, Chapter 22. In: (source unkown) pp736–761. [94] Boateng, A.A. and Skeete, D.A. (1990). Incineration of rice hull for use as a cementitious material: the Guyana experience. Cement and Concrete Research Vol 20 pp795-802. [95] Mehta, P.K. (1994). Rice Husk Ash – A Unique Supplementary Cementing Material. In: Advances in Concrete Technology. MSL Report 94-1 (R) CANMET. pp419-444. [96] K. K. Larbi: Synthesis of High Purity Silicon from Rice Husks // MSc thesis, Graduate Department of Materials Science and Engineering, University of Toronto. – 2010. – P. 1-128. [97] Rasha M. Mohamed, Reda M. Radwan, Mohamed M. Abdel-Aziz, Magdy M. Khattab. Electrical and thermal properties of γ-irradiated nitrile rubber/ rice husk ash composites // Journal ofAppliedPolymer Science. – 2009. – Vol. 115. – P. 1495-1502. [98] Iyenagbe B. Ugheoke, Othman Mamat. A critical assessment and new research directions of rice husk silica processing methods and properties // Maejo International Journal of Science and Technology. – 2012. – Vol. 6, Iss. 03. – P. 430-448.

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TRADITIONAL METHODS FOR SYNTHESIS OF SiO2 NANOPARTICLES & SYNTHESIS OF SILICA FROM RH

2.1. Methods for synthesis of SiO2 nanoparticles Advancement in nanotechnology has lead to the production of nano-sized silica, SiO2, which has been widely used in both scientific research and engineering development [1]. Synthetic silica (colloidal silica, silica gels, pyrogenic silica and precipitated silica) is pure and produced mostly in amorphous powder forms compared to natural mineral silica (quartz, tridymite, cristobalite) which are in crystalline forms [2]. For the synthesis of silica nanoparticles various methods are employed such as reverse microemulsion processing, flame synthesis, sol-gel processing etc. [5-7]. Irrespective to the method of synthesis, the main focus was to control the particle size, particle surface reactivity and morphology. The most common process to synthesize silica nanoparticles is a sol-gel technique which involves the simultaneous hydrolysis and condensation reaction of the metal alkoxide. Among several methods, sol-gel method has some advantages such as low temperature synthesis and control of reaction kinetics by varying the composition of chemicals. By employing sol-gel method, SiO2 particles of 1 μm were first reported by Stober et al. [8], however, nanoparticles of few hundred nanometers to several micrometers size was reported with respected to the control hydrolysis of TEOS in ethanol [9]. The size and shape of silica nanoparticles are controllable with electrolytes, surfactants and organic acids etc. In similar manner, silica nanoparticles were reported by using electrolyte, sodium iodide, whereas control mechanism of particle size was reported by using ammonium bromide [10, 11]. Further, Wang and coworkers have reported preparation of cubic silica nanoparticles with the use of organic acid (tartaric acid) [12]. Similarly, the preparation of silica nanoparticles with hexagonally arrayed mesopores has been reported by using binary surfactants method [13]. The preparation of nanometer 41

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

sized mesoporous silica particles with the addition of triethanolamine as base and cationic surfactant as template was reported [14]. Synthesis of hexagonal shaped mesoporous silica particles have also been been reported [15]. Synthesized mesoporous silica nanospheres were reported with uniform size and morphology with the use of polyvinyl pyrrolidine and different types of cationic surfactants [16]. The solgel technique provides many advantages over other methods, mainly owing to facility, versatility, purity, homogeneity and modifications of material properties by changing synthesis parameters [17, 18]. The sol-gel process is widely applied to produce ceramic materials due to its ability to form pure and homogenous products at mild conditions. The process involves hydrolysis and condensation of metal alkoxides [19, 20] such as tetraethyl orthosilicate (TEOS) or inorganic salts [20] such as sodium silicate (Na2SiO3). Silicate particles mostly synthesized in the presence of mineral acid (e.g. HCl) or base (e.g. NH3) as catalyst. Flow chart of a typical sol-gel process which leads to the production silica nanoparticles using silicon alkoxides (Si(OR)4), is shown in Figure 8.

Figure 8 – Flow chart of a typical sol-gel process for preparing nanosilica powder [21]

II. Traditional Methods for Synthesis of SiO2 Nanoparticles & Synthesis ...

43

Condensation of hydroxide molecules by elimination of water leads to the formation of sol (colloidal silica). After a prolonged ageing process, the colloidal particles will link together to form network structure, resulting in a porous gel. Removal of solvent from the sol or gel will produce silica powder. Since the sol-gel process starts with the nanosized hydroxide units, and undergoes reaction on the nanometer scale, it results in the formation of nanometer silica particles [3]. Optimizing the reaction conditions of sol-gel process such as concentration of reactants, concentration of catalyst and reaction temperature [22, 23] and addition of electrolytes (metal salts) [24] are some of the recent attempts made by the researchers to reduce the silica size using the sol-gel platform. The smallest possible silica nanoparticles with average diameter of 14 to 20 nm, in colloidal form have been produced using these approaches [22-24]. Flame Synthesis. Silica nanoparticles also can be produced through high temperature flame decomposition of metal-organic precursors. This process also referred as chemical vapor condensation (CVC) [30]. In a typical CVC process, silica nanoparticles are produced by burning silicon tetrachloride, SiCl4 with hydrogen and oxygen [2]. Difficulty in controlling the particle size, morphology and phase composition is the main disadvantages of the flame synthesis [3]. Nevertheless, this is the prominent method that has been used to commercially produce silica nanoparticles in powder form. Example of schematic diagram of the experimental setup for the synthesis of SiO2 nanoparticles by flame synthesis is shown in Figure 9 [31].

Figure 9 – Schematic diagram of experimental apparatus for the synthesis of silica nanoparticles by flame spray pyrolysis using twofluid nozzle spray [31]

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

2.2. Preparation of silica from RH From the different synthesis methods, the chemical method consisting of simple acid leaching and post annealing is of the most simple and successful techniques to synthesize the ultrafine SiO2 nanoparticles from RHA. Leaching of RH with a solution of HCl, H2SO4, H3PO4, HNO3, NH4OH and NaOH before thermal treatment at different ranges of temperature and time can be so effective in accelerating the hydrolysis of cellulose and hemicelluloses in RH and removing most of the metallic impurities. This allows producing white-color silica completely, with high specific surface area [3237]. Figure 10 summarizes the common procedures for producing silica from RH, including impurities removal before and after thermal processes. Economic and environmental assessment of the process. The cost of nano silica is also high as compared to common silica. Therefore, extraction of nanosilica using agro waste is economic and environmentally benign [39]. An overview of nanosized silica production by different thermal treatment methods is provided in table 4. Bakar, R.A., Yahya, R., Gan, S.N., 2016 synthesized silica of 0.5-0.7 μm size from HCl leached rice husk ash by heating in range of 500 °C-900 °C. Silica preparation of 100-200 nm scale was done by using acid precipitation method and husk was washed with HCl prior to extraction using polyethylene glycol (PEG) as template by An, D., Guo, Y., Zou, B., Zhu, Y., Wang, Z., 2011 [42]. Various studies are reported for silica preparation with the use of costly templates and surfactants and use of acid washing under high temperature and atmospheric pressure. However very few studies have reported silica extraction in which use of surfactants were avoided. It can be seen in the comparative account that under thermal treatment and alkalization of the rice husk, present study has eliminated the step of acid leaching and use of templates and processing temperature is also comparatively low in order to keep the process simple and green. Also, the purity and nanoscale size of the silica were not compromised and yielded good comparative results.

II. Traditional Methods for Synthesis of SiO2 Nanoparticles & Synthesis ...

45

Figure 10 – Different methods for producing different structural silica from RH. Adapted from [38]

Comparison of cost estimate of silica production from agro waste and industrial process is presented in table 5. It could be observed that this process was comparatively cheaper and also very energy intensive as compared to commercial and industrial methods being used for the production of silica. Further, an economic analysis of cost of nanosilica production per kg was evaluated (Table 6). Considering cost price of raw materials and energy used in production of nano silica from RHA and market selling price of commercial nanosilica of 10-20 nm range and >98 % purity net profit of 65.75 $ was estimated. Owing to industrial importance of the element, this study provides a simple and economic approach of waste utilization and waste minimization for the production of raw material.

46

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Table 4 Overview of nanosilica preparation using various approaches Ash used

Rice husk ash Rice husk ash

Size of silica 100200 nm

0.50.7 μm

Rice husk ash

NA

Rice husk ash

6070 nm

Rice husk ash

4050 nm

Rice husk ash

75252 nm

Acid washing

Method of extraction

Template used

Temperature

Purity index

Reference

HCl washed prior to extraction

Acid precipitation method

PEG

Room temperature

99.6 %

[42]

HCl leached

Thermal method

NA

500 °C-900 °C

99.58 %

[43]

Acid washing with 1N HCl Boiled in HCl solution Boiled and refluxed with dilute HCl solution Treated with dilute HCl

Acid precipitation method Calcination method

NA

90 °C

NA

[44]

NA

Calcined at 700 °C

NA

[45]

Carbonization method

NA

70-95 °C

NA

[46]

Acid precipitation Method

No template

90 °C

NA

[47]

5 % by weight polyethylene glycol (PEG)

Room temperature

98.2 %

[48]

NA

500 °C 600 °C 700 °C

NA

[49]

Rice husk ash

100200 nm

Pretreated with HCl

Acid precipitation method

Rice husk ash

1030 nm

Washed with 1 N HCl

Thermal treatment method

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II. Traditional Methods for Synthesis of SiO2 Nanoparticles & Synthesis ... Vietnamese Rice -husk ash Rice husk ash Rice husk ash Rice husk ash

Acidwashed with 10 % 3 nm HCl and 30 % by weight H2SO4 NA Boiled in HCl solution Not Acid in washed at nano boiling scale temp.point 60 Acid nm washed with HCl

Rice husk ash

5-30 nm

Rice husk ash

1015 nm

Acidwashed with hot HCl No acid washing

Acid precipitation method Magnesium reduction method Acid precipitation method Nonisothermal decomposition Dissolutionprecipitation technique Acidprecipitation technique

CTAB/ water/ butanol solution

60 °C

NA

[50]

NA

620 °C

99.5 % [51]

No template

100 °C

96 %

[52]

NA

27 °C-727 °C

95 %

[53]

No template used

50 °C

99.4 % [54]

No template used

80 °C

99 %

[40]

*NA‒ not available

Table 5 Comparison of capital cost in production of nano silica from commercial and agro-waste resources [40, p. 10] Comporative Unit Raw materials Processing temperature Chemicals used

Agro-waste resources Current Unit Price method Rice husk ash

Zero cost

600 °C NaOH pellets 1N HCl

7.42 $ per kg 7.3 $ per L

Commercial resources Commercial Method

Unit Price

Sodium meta silicate, Tetra ethyl silicate

238.13 $ per kg 61.79 $ per L

1300-2000 °C CO2

114.45 $ per L

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ... Table 5 (continued)

Energy cost/ power Extraction temperature

1.5 KW/h

0.70 $ per KW

80 °C

11-13 MW/h

0.89 $ per KW

120 °C

Cost estimation of silica production per kg using RHA as source [40, p. 10] Raw materials used Rice husk NaOH pellet Hydrochloric Acid Energy cost Total cost of nano silica, per kg Market price of nano silica* Net profit

Table 6

Cost per kg Zero cost 89.04 $ 0.0073 $ 0.21 $ 89.257 $ 155 $ 65.75 $

The following discussion reviews the processing methods of rice husk for silica, highlighting their advantages and disadvantages: 1) Direct incineration without pre-treatments (Thermal treatment). The rice husk is directly incinerated to produce silica of varying purity, with or without the use of pre-treatments [57, 58]. In the overall process, the temperature of incineration, holding time and pre-treatment techniques employed affect the character, especially the surface area and brightness (whiteness), of the silica produced. As the incineration temperature increases, there appears to be some accompanying phase changes. It is noted, however, that rice husk silica produced between 500-650 °C with incineration holding (soaking) time of 2.5-6 hours is considered ideal for producing white amorphous silica while crystallinity sets in when incineration temperature increases beyond 700 °C. The quantity of the operational phase, whether cristobalite or tridymite, is dependent on the applied temperature range and the impurity level in the rice husk. Including the incineration temperature grossly affects the surface area and hence the reactivity of silica produced from direct incineration process. Direct incineration of rice husk can be accomplished in open air as reported by H. Hamdan, MNM Muhid, S. Endud, E. Listiorini,

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49

Z. Ramli [67] or in a muffle furnace (at temperatures 500, 600 and 700 °C) [59]. Another method used by some researchers [56-58] is the fluidized bed combustion technique to produce rice husk silica, even though the purity was not more than 95 %. So whether in static or flowing air, complete incineration can be achieved with some varied effects on the properties of the silica produced. 2) Pre-treatment effects on silica production from rice husk. One of the reasons why it is difficult to obtain silica with purity in excess of 97 % from rice husk by the direct incineration process is a consequence of the effects of the metallic impurities the husk contains. For instance, S. Chandrasekhar, P. N. Pramada, L. Praveen [60] reported that oxides, especially K2O, impart black color on the particles. Some explanations to support this phenomenon is that there exists a strong interaction between oxides, especially those of potassium and sodium, contained in rice husk and the silica therein, such that it can result in the surface melting of SiO2 particles and accelerate an early crystallization of amorphous SiO2 into cristobalite, as implied by research results [61]. This is one of the reasons why U. Kalapathy, A. Proctor, J. Shultz [62] couldn’t achieve a purity of up to 98 % even after 14 hour of their solgel treatment of rice husk ash with bases and acids. The surface melting of these oxides on the silica grossly reduces the surface area, thereby reducing the reactivity of the particles. Thus, it is often necessary to use some pre-treatment methods, which can either be done through acidic or basic medium, to reduce or remove metallic impurities in order to increase the chances of obtaining silica of higher purity and surface area than is achievable in the direct incineration method. Three main pre-treatment methods have generally been used in the production of high purity silica from rice husk. These are acid leaching, basic pre-treatment and microbiological pre-treatment, usually in combination with some acids. Several kinds of acids, both mineral and organic, have been used to pre-treat rice husk before processes such as incineration begin [55, 60, 61, 63-66]. Hydrochloric acid (HCl) has proved to be most effective in removing metallic impurities from the husk and so it is by far the most widely used. S. Chandrasekhar, P. N. Pramada and L. Praveen [60], found that leaching of rice husk in 1N HCl is effective in removing most of the metallic impurities. Other researchers used organic acids and compared results with those obtained by using other different

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

mineral acids in pre-treating rice husk and concluded that HCl is better. However, Junko Umeda, Katsuyoshi Kondoh [67] reported a very high purity (>99.5 %) silica from rice husk on pre-leaching it with citric acid. While acid leaching affects the chemical composition of the husk, it does not affect the structure, whether crystalline or amorphous, of the silica. Thus, the change of phase from amorphous to crystalline is not affected by the pre-treatment method employed. An insight into this dynamics was presented in the research report of Concha Real, Maria D. Alcala, and Jose M. Criado [61]. They found that preliminary leaching of rice husk with a solution of HCl before incineration at 600 °C, if properly done, can result in a high purity silica (approximately 99,5 %) with high specific surface area (approximately 260 m2/g). They indicated that the high-surface-area silica produced was unaffected even after being heated at 800 °C. They also performed the HCl leaching on the white ashes obtained from incineration of untreated rice husk at 600 °C and obtained an amorphous silica with the same purity, although its specific surface area decreased to as low as 1 m2/g. They explained the kinetics of this drastic change in surface area, attributing it to the interaction between alkali oxides, specifically K2O and SiO2. Other acids such as H2SO4, HNO3 and their mixture, have also been used in the acid pre-treatment [68]. The general leaching effects of H2SO4, HNO3 and HCl are similar, but HCl is superior to H2SO4 and HNO3 in removing the metallic ingredients. U. Kalapathy, A. Proctor, J. Shultz also attempted chemical pre-treatment of incinerated rice husk using HCl but the results were inferior to those of the pretreatment [62]. Some alkalis such as NaOH and NH4OH have been used to pretreat rice husk [55, 59]. However, the effects of alkali pre-treatment were not as obvious or satisfactory as those of acid pre-treatment. 3) Hydrothermal method. Hydrothermal synthesis has been defined as a process that utilizes single‒ or heterogeneous phase reactions in aqueous media at elevated temperature (T>25 ºC) and pressure (P>100 kPa) to crystallize ceramic materials directly from solution [69]. Rice husk contains organic compounds and oxides of metals. Under high temperature, high pressure and acidic or basic medium with strong oxidizing activity, the organic compounds are decomposed and the trace metals turned into soluble ions; then, silica is obtained. This

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processing method can achieve the purification of silica from the husk with only the use of water. However, achieving complete dissolution of the organic matter in the rice husk is a task that is near impossible. So, practically this process still requires an incineration step, though the soaking time may be less compared to incinerating the untreated or pre-treated rice husk. The method does not affect the amorphicity of the silica in rice husk. 4) Other Methods. X. W. Yu, G. H. Xu, Y. Y. Zhou, G. P. Zhao, S. N. Shang reacted carbonized rice husk with Na2CO3 solution in a proper ratio for 3 hour, followed by incineration step at 600-650 ºC for varied soaking time between 3-7 hours to obtain silica. The silica made from this method has good reinforcing properties in rubber [70]. 5) Conclusion of the processing methods. Most methods [39-70] of the production of rice husk silica were mostly done on a laboratory scale. If they were to be scaled up to commercial level, the cost and personnel risks involved would be quite high, since acids and other corrosive media would be worked with at high temperatures. The quality of the silica obtained from fluidized bed combustion incinerator is not more than 95 %, which restricts its application to chemically insensitive areas like cement and concrete admixtures where highpurity silica is not essential. For this reason, therefore, it is necessary to evolve a system that can produce high-purity silica at volumes of production capable of supporting industrial needs. REFERENCES 2 [1] Sun, Y., Zhang, Z. and Wong, C. P. (2005). Study on mono-dispersed nanosize silica by surface modification for underfill applications. J. Colloid Inter. Sci. 292: 436. [2] Vansant, E. F., Voort, P. V. D. and Vrancken, K. C. (1995). Characterization and chemical modification of the silica surface, Elsevier Science, New York. [3] Klabunde, K. J. (2001). Nanoscale materials in chemistry. WileyInterscience, New York. [4] Reverchon, E. and Adami, R. (2006). Nanomaterials and supercritical fluids. J. Supercritical Fluids 37: 1. [5] D. Nagoo, H. Osuzu, A. Yamada, E. Mine, Y. Kobayashi, M. Konno, J. Colloid Interface Sci. 274, (2004) 143-149. [6] S. Vemury, S.E. Pratsinis, L. Kibbey, J. Mater. Res. 12, (1997)1031-1042. [7] T. Tani, N. Watanabe, K. Takatori, J. Nanoparticle Res. 5, (2003)39–46.

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[33] Chakraverty A, Mishra P, Banerjee H. Investigation of combustion of raw and acid-leached rice husk for production of pure amorphous white silica. J Mater Sci 1988; 23:21–4. [34] Ghosh A, Ahmed S, Mollah M. Synthesis and characterization of zeolite NaY using local rice husk as a source of silica and removal of Cr (VI) from wastewater by zeolite. Bangladesh J Sci Ind Res 2013; 48:81–8. [35] Carmona V, Oliveira R, Silva W, Mattoso L, Marconcini J. Nanosilica from rice husk: extraction and characterization. Ind Crops Prod 2013; 43:291–6. [36] Kumproa K, Singhykaew S, Nuntiya A. Effects of reaction time and hydrochloric acid concentration on acid hydrolysis of rice Husk by reflux method. Adv Mater Res 2012; 550:592–7. [37] Ang TN, Ngoh GC, Chua ASM. Comparative study of various pre-treatment reagents on rice husk and structural changes assessment of the optimized pre-treated rice husk. Bioresour Technol 2013; 135:116–9. [38] Yafei Shen. Rice husk silica derived nanomaterials for sustainable applications // Renewable and Sustainable Energy Reviews. – 2017. – Vol. 80. – P. 454. [39] Liu, D., Zhang, W., Lin, H., Li, Y., Lu, H., Wang, Y., 2016. A green technology for the preparation of high capacitance rice husk-based activated carbon. J. Clean. Prod. 112, 1190–1198. doi:10.1016/j.jclepro.2015.07.005 [40] Suman Mor, Chhavi K. Manchanda, Sushil K. Kansal, Khaiwal Ravindra, 2016. Nanosilica extraction from processed agricultural residue using green technology. Journal of Cleaner Production. 143, 1284-1290. doi: 10.1016/j. jclepro.2016.11.142 [41] Pipatti, R., Sharma, C., Yamada, M., Svardal, P., Guendehou, G.H.S., Koch, M., Hockstad, L., Pipatti, R., Yamada, M., Doorn, M.R.J., Towprayoon, S., Manso Vieira, S.M., Irving, W., Palmer, C., Pipatti, R., Wang, C., IPCC, 2006. Incineration and open burning of waste. 2006 IPCC Guidel. Natl. Greenh. Gas Invent. 5, 1–26. doi:WAS-01 [42] An, D., Guo, Y., Zou, B., Zhu, Y., Wang, Z., 2011. A study on the consecutive preparation of silica powders and active carbon from rice husk ash. Biomass and Bioenergy 35, 1227–1234. doi:10.1016/j.biombioe.2010.12.014 [43] Bakar, R.A., Yahya, R., Gan, S.N., 2016. Production of High Purity Amorphous Silica from Rice Husk. Procedia Chem. 19, 189–195. doi:10.1016/j.proche.2016.03.092 [44] Kumar, A., Singha, S., Dasgupta, D., Datta, S., Mandal, T., 2015. Simultaneous recovery of silica and treatment of rice mill wastewater using rice husk ash: An economic approach. Ecol. Eng. 84, 29–37. doi:10.1016/j. ecoleng.2015.07.010 [45] Li, Y., Lan, J.Y., Liu, J., Yu, J., Luo, Z., Wang, W., Sun, L., 2015. Synthesis of gold nanoparticles on rice husk silica for catalysis applications. Ind. Eng. Chem. Res. 54, 5656–5663. doi:10.1021/acs.iecr.5b00216 [46] Liu, Y., Guo, Y., Zhu, Y., An, D., Gao, W., Wang, Z., Ma, Y., Wang, Z., 2011. A sustainable route for the preparation of activated carbon and silica from rice husk ash. J. Hazard. Mater. 186, 1314–1319. doi:10.1016/j. jhazmat.2010.12.007 [47] Zulkifli, N.S.C., Ab Rahman, I., Mohamad, D., Husein, A., 2013b. A green sol–gel route for the synthesis of structurally controlled silica particles

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[62] U. Kalapathy, A. Proctor, J. Shultz. A simple method for production of pure silica from rice hull ash // Bioresource Technology. – 2000. – Vol. 73, Iss. 3. – P. 257-262. [63] L. A. Zemnukhova, A. E. Panasenko, E. A. Tsoi, G. A. Fedorishcheva, N. P. Shapkin, A. P. Artem’yanov and V. Yu. Maiorov. Composition and structure of amorphous silica produced from rice husk and straw // Inorganic Materials. – 2014. – Vol. 50, Iss. 1. – P. 75-81. [64] Da Silva J, Da Cunha C, De Carvalho F, Rodrigues Filho U, Oliveira P, Segatto Silva M. Obtaining high purity silica from rice hulls // Quimica Nova. – 2010. – Vol. 33, Iss. 4. – P. 794-797. [65] Dongmin An, Yupeng Guo, Bo Zou, Yanchao Zhu, Zichen Wang. A study on the consecutive preparation of silica powders and active carbon from rice husk ash // Biomass and Bioenergy. – 2011. – Vol. 35, Iss. 3. – P. 12271234. [66] Ping Lu, You-Lo Hsieh. Highly pure amorphous silica nano-disks from rice straw // Powder Technology. – 2012. – Vol. 225. – P. 149-155. [67] Junko Umeda, Katsuyoshi Kondoh. High-purification of amorphous silica originated from rice husks by combination of polysaccharide hydrolysis and metallic impurities removal // Industrial Crops and Products. – 2010. – Vol. 32, Iss. 3. – P. 539-544. [68] P. Sidheswaran, A. N. Bhat. Recovery of Amorphous Silica in Pure Form from Rice Husk // Transactions of the Indian Ceramic Society. – 1996. – Vol. 55, Iss. 4. – P. 93-96. [69] W. L. Suchanek, R. E. Riman. Hydrothermal synthesis of advanced ceramic powders // Advances in Science and Technology. – 2006. – Vol. 45. – P. 184-193. [70] X. W. Yu, G. H. Xu, Y. Y. Zhou, G. P. Zhao, S. N. Shang. Study of factors influencing extraction rate of white carbon black produced from rice husk // Chinese Journal of Chemical Engineering. – 1998. – Vol. 26. – P. 51-56.

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METHODOLOGY OF OBTAINING SiO2 BY USING PROCESSING OF RICE HUSKS

As research objects served the shells of rice (Rice Husks), selected from different regions of the Republic of Kazakhstan (Figure 11). Samples of rice husk with a length of about 5-10 mm were pre-washed with water and dried in air (Figure 12). Prepared samples were treated and analyzed using a number of physicochemical methods.

Figure 11 – Rice husks from different regions of Kazakhstan (Objects of experiments)

Figure 12 – Pre-washing of RH with water & drying in air condition

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3.1. Extraction of silica nanoparticles The versatile material RHA has generated great amount of interest in researchers world-wide. Researchers have worked on various techniques for the synthesis of silica nanoparticles using different templates to control the shape and size of the particles. However in this present study, a simple, template free different methods such as pre-acid wash treatment (hydrochloric acid, citric acid) and thermal treatment was used to prepare silica nano particles (Figure 13, Appendix A). The aim of acid pre-treatment is to improve the purity of silica product. It proves to be an effective way in substantially removing most of the metallic impurities and producing ash silica completely white in colour. The acid treatment also gives a high surface area for the silica when it is precipitated. In addition the quality of producing nano-silica powder depends on the calcinations or combustions temperature and duration of calcinations or combustions. WRHA which predominantly contains silica dissolves in sodium hydroxide solution after pre-treatment. Sodium silicate formed is used as precursor for silica synthesis. The principal reaction can be represented as NaOH + SiO2 → Na2SiO3 +H2O Na2SiO3 + 2HCl →H2SiO3↓+2NaCl H2SiO3 → SiO2 + H2O Reaction of hydrochloric acid with sodium silicate promotes a silanol (R3Si–OH) groups formation and condensation of which leads to the formation of extended three-dimensional Si–O–Si network.

Figure 13 – Appearance of raw rice husk, white rice husk ash and extracted silica from treated rice husk in 2 M HCl acid followed by combustion at 600 ºC

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3.2. Methodologies of obtaining SiO2 by using processing of rice husks, waste from rice production by different treatment methods 3.2.1. Investigation effect of different calcination temperature on the formation of white rice husk ash during direct incineration of water-washed rice husk RHA in essence is the product of the high temperature burning process of RH. In other words, the structure and properties of RHA must to some extent be dependent on the combustion temperature. Furthermore, a good understanding of the combustion temperature dependence of RHAs is essential for the structure, property optimization of RHA and to choose optimal conbustion temperature for different methods of production rice husk silica. Combustion conditions influence the chemistry (composition) and amount of ash produced (yield). This is because combustion causes an expulsion of water, the conversion of combustible carbon into CO2 and steam, the loss of carbon dioxide from carbonates, the conversion of metal sulphides into metal oxides, metal sulphates and sulphur oxides and other chemical reactions. The final ash product from combustion water-washed rice husk was completely white. This enabled the distinct determination of the rice husk combustion times, whereby the first formation of completely white ash signified the complete combustion of rice husk (Figure 14).

Figure 14 – Experimental sequence of the direct incineration of rice husk at 600 °C for 3 hours in muffle furnace (SNOL, 2009) in order to obtain white rice husk ash

The white ash samples obtained from the direct incineration of water-washed rice husk from different region of Kazakhstan were compared in Table 7.

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Table 7 Effect of temperature and residence time on the formation of white rice husk ash during direct incineration of water-washed rice husk in the muffle furnace Calcination temperature (ºC) & Residence time

Water-washed Rice Husk

500 ºC, 4 hours without HCl

550 ºC, 4 hours without HCl

600 ºC, 4 hours without HCl

650 ºC, 4 hours without HCl

The appearance of the white rice husk ash subjected to calcination at different temperatures

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700 ºC, 4 hours without HCl

750 ºC, 4 hours without HCl

800 ºC, 4 hours without HCl

The colour of the ash was affected by the temperature and duration of the combustion process as «black ash» samples were obtained at 400-500 °C while «white ash» was obtained from 550 °C to 700 °C (for 4 hours). The RHA colour at high temperatures (750 °C and 800 °C) was predominantly grey ash due to high combustion inefficiency resulting in a high residual carbon. The silica (SiO2) content of the ash samples, which is of significant importance in colour of the ash, was found to be in the range from 73 % (sample WRH+H3PO4) to 97 % (sample WRH+H2SO4) (Table 14, results of PANalytical Axios mAX XRF analysis) during the combustion at 600 °C for 4 hours. According to Table 7, it was suggested that 600 °C is the optimal temperature for obtaining WRHA from RH because of colour of the ash, economical energy consumption in the combustion of raw materials and the high silica content of the sample.

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3.3. Effect of different treatment methods on the silica yield and the study of the elemental composition of the samples Different grades of silica with different yields can be obtained based on the calcinations or combustions temperature and duration of calcinations or combustions in different treatment methods (Table 8, Figure 15). The percent yield of the extracted silica from the RHA was estimated according to the following formula (1): (1) where, W1 = Weight (g) of the extracted silica W2 = Weight (g) of the rice husk ash Table 8 Yield of WRH, Silica from RHs (Almaty region, Kyzylorda region, Turkystan region) during different methods of treatment Methods Pre-treatment with HCl Pre-treatment with citric acid Thermal treatment

RH-1 WRH SiO2 19 17.4 19 13 20 15

Yield, wt % RH-2 WRH SiO2 16 11.5 15.4 10.7 14.9 11.6

RH-3 WRH SiO2 17 11 17.7 12 18 12.7

Figure 15 – Percent yield of the white rice husk (WRH) and rice husk silica (RHS) extracted from the rice husk (RH) of different region of Kazakhstan by different methods

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It was noted from the experiments that the yield of the precipitated silica was dependent upon the applied processing parameters, such as concentration of sodium hydroxide, extraction time, solid / solution ratio, etc. As such, these parameters were varied in order to explore the optimum conditions for maximum recovery of SiO2 from the RHA, employed in this study. 3.4. Silica oxide sintering method without acidification (Thermally treatment) Rice husk (Almaty region, Kyzylorda region, Turkystan region) which is an agricultural residue was used as the main raw material for synthesizing silicon oxide by thermal treatment (Figure 16). The samples of rice husks were previously washed with water for the purification of the composition from foreign substances. Then the initial raw materials were dried in the laboratory drying oven at the temperature of 90 °C for 2 hours (for complete evaporation of the water in the composition). All prepared samples (50 g) were calcinated at 600 °C for 4 hours in a muffle furnace (AAF series, Carbolite (UK)) to produce white rice husk ash (WRHA). After the end of the process, all organic compounds in the rice husk are burned completely (at the temperature of 600 °C for 4 hours) and eventually the ash of white rice husks is formed (Figures 17, 18).

Figure 16 – Calcination Figure 17 – Higher Figure 18 – White rice of rice husks in the muffle flaming times for the rice husk ash product from furnace at the temperature husk in the muffle furnace burning rice husk inside of 600 °C (600 ºC) the muffle furnace (600 ºC)

Subsequently the WRHA was mixed with 100 ml of 2M NaOH at 90 °C at continuous vigorous stirring for 2 hours in order to extract the solid silica into water-soluble sodium silicate.

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The water soluble sodium silicate solution was filtered via the vacuum pump to remove insoluble residues (Figure 19).

Figure 19 – Filtration of the water soluble sodium silicate solution

After filtration, the water-soluble sodium silicate solution (the filtrate) converted into insoluble silicic acid by reaction with concentrated HCl (30 minutes, under continuous stirring). Scheme of rice husk conversion into the silica by thermal treatment is presented on a Figures 20.

Figure 20 – Scheme of the silicon oxide production from the raw rice husk by thermal treatment

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3.4.1. Yield of WRH from RHs of different region of Kazakhstan (without HCl) during direct incineration in the muffle furnace The WRH yields obtained at the specified incineration conditions (500 °C-800 °C, 4 hours) were found to be in the range of 14.9 % (600 °C, RH 2) to 28.2 % (650 °C, RH 1) (Table 9). Diagrams of ash yields (Figures 21, 22) as a function of temperature indicated that lower calcinations temperatures (500 to 650 ºC) gave higher ash yields than higher temperatures (700 ºC to 800 ºC). The ash yields however did not vary significantly with prolonged calcinations time (5-6 hrs), showing a greater dependence of higher ash yields on the temperature of calcinations than on the duration. Table 9 Yield of WRH from RHs (Almaty region, Kyzylorda region, Turkystan region) during direct incineration in the muffle furnace Temperature of calcinations (τ=4 hours) 500 °C 550 °C 600 °C 650 °C 700 °C 750 °C 800 °C

RH 1 21.4 19.7 20 28.2 19.6 19 19.6

Yield, wt % RH 2 RH 3 17.9 18.7 15.4 17.6 14.9 18 16 26 15.5 17.6 15.5 17.4 15 17

Raw RH 100 100 100 100 100 100 100

Figure 21 – Percent yield of the white rice husk (WRH) extracted from the rice husk (RH) of different region of Kazakhstan (without HCl)

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Figure 22 – Percent yield of the white rice husk (WRH) extracted from the rice husk (RH) of different region of Kazakhstan (without HCl)

3.5.1. Pre-treatment with hydrochloric acid Rice husks (Almaty region, Kyzylorda region, Turkystan region) which is an agricultural residue was used as the main raw material for synthesizing silicon dioxide (Figure 23). The samples of rice husks, which selected in the Almaty region in the village of Bakanas, were previously washed with water for the purification of the composition from foreign substances. Then the initial raw materials were dried in the laboratory drying oven at the temperature of 90 °C for 2 hours (for complete evaporation of the water in the composition). The sample (50 g) of the raw material was treated with the 2M hydrochloric acid solution (500 ml) and dried at 90 °C for 2 hours (Figure 24).

Figure 23 – Initial raw materials (Rice husks)

Figure 24 – Treatment of the raw material with 2M hydrochloric acid solution

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The all prepared samples were continuously washed with distilled water until pH 7 and separated from solution via filtration. Thereupon, RH was dried at 105 °C for overnight and then calcinated at 600 °C for 4 hours in a muffle furnace (AAF series, Carbolite (UK)) to produce white rice husk ash (WRHA) (Figure 25). The main purpose of heat treatment (calcination) is production pure silicon dioxide from rice husks by burning of organic compounds and carbon in the composition of raw materials at high temperature (Figure 26).

Figure 25 – Calcination of rice husks in the muffle furnace at the temperature of 600 °C

Figure 26 – Combustion of carbon in the composition of RH at high temperature

After the end of the process, all organic compounds in the rice husk are burned completely (at the temperature of 600 °C for 4 hours) and eventually the ash of white rice husks is formed (Figure 27). As can be seen from Figure 28, the mass of the white rice husk ash obtained after the combustion process was 8.707 grams.

Figure 27 – Formation of the white rice Figure 28 – Mass of obtained white rice husk ash husk ash

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Subsequently the WRHA was mixed with 100 ml of 2M NaOH at 90 °C at continuous vigorous stirring for 2 hours in order to extract the solid silica into water-soluble sodium silicate (Figure 29). The water soluble sodium silicate solution was filtered via the vacuum pump to remove insoluble residues (Figure 30).

Figure 29 – Treatment of the WRHA with 2M NaOH solution

Figure 30 – Installation of vacuum filtration

After filtration, the water-soluble sodium silicate solution (the filtrate) converted into insoluble silicic acid by reaction with concentrated HCl (30 minutes, under continuous stirring) (Figure 31).

Figure 31 – Formation of the final product (Silicon dioxide)

The final product is passed through the filter and washed with hot water to remove by-products (the NaCl residue in its composition) (Figure 32).

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a) Fltration via the vacuum pump

b) Fltration via filter paper

Figure 32 – Filtering process of the obtained silicon dioxide

The following figure 33 shows the final finished product after washing with hot water:

Figure 33 – The obtained pure silicon dioxide after washing with hot water

Finally, the obtained pure silicon dioxide was placed in the laboratory drying oven at 105 °C for the duration of a night. After completion of the final product drying, were carried out various physicochemical investigation of the obtained silicon dioxide, the details of the characterizations are given in chapter 3 followed by results and discussion. Scheme of rice husk conversion into the silica by pre-treatment with hydrochloric acid is presented on a Figures 34 and 35.

III. Methodology of Obtaining SiO2 by using Processing of Rice Husks

Figure 34 – Scheme of the silicon oxide production from the raw rice husk by pre-treatment with hydrochloric acid

I Scheme of obtaining white rice husk ash from rice husk

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

II Scheme of production amorphous silica from white rice husk ash Figure 35 – Graphical representation of the scheme of experiments

As a whole, as shown by the experiment, from 50 g of rice husks it is possible to obtain 8.71 g of white rice ash, 6 g of pure silicon oxide, which means that from 1 ton of the rice husks it is possible to obtain 120 kg of silicon oxide and from the remains of rice husks in Kazakhstan, approximately in a year 8000 tons of silicon dioxide (Appendix B). 3.5.2. Pre-treatment with Citric Acid Rice husk (Almaty region, Kyzylorda region, Turkystan region) which is an agricultural residue was used as the main raw material for synthesizing silicon oxide by pre-treatment with citric acid. The samples of rice husks were previously washed with water for the purification of the composition from foreign substances. Then the initial raw materials were dried in the laboratory drying oven at the temperature of 90 °C for 2 hours (for complete evaporation of the water in the composition). The sample (50 g) of the raw material was mixed with the 20 % citric acid solution at 90 °C at continuous vigorous stirring for 2 hours. Then all prepared samples were calcinated at 600 °C for 4 hours in a muffle furnace (AAF series, Carbolite (UK)) to produce white rice husk ash (WRHA) (Figure 36). After the end of the process, all organic compounds in the rice husk are burned completely (at the temperature of 600 °C for 4 hours) and eventually the ash of white rice husks is formed.

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Figure 36 – Formed white rice husk ash after pre-treatment with citric acid

Subsequently the WRHA was mixed with 100 ml of 2M NaOH at 90 °C at continuous vigorous stirring for 2 hours in order to extract the solid silica into water-soluble sodium silicate. The water soluble sodium silicate solution was filtered via the vacuum pump to remove insoluble residues. After filtration, the water-soluble sodium silicate solution (the filtrate) converted into insoluble silicic acid by reaction with concentrated HCl (30 minutes, under continuous stirring) (Figure 37).

Figure 37 – Formed final product (Silicon dioxide)

Scheme of rice husk conversion into the silica by pre-treatment with citric acid is presented on a Figures 38.

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Figure 38 – Scheme of the silicon oxide production from the raw rice husk by pre-treatment with citric acid

3.5.3. Effect of Different Pre-Acid Wash Treatment on Silica Yield Pre-treatment with Hydrochloric acid In the pre-treatment process, we used two different reagents (hydrochloric acid and citric acid) for washing RH. Silica is extracted from this pre-treated RH using 100 ml of 2M NaOH. It is found that the yield of silica is strongly dependent on the type of acid used for washing and concentration of NaOH. The results are shown in the Table 10 and Figures 39, 40. Table 10 Yield of WRH, Silica from RHs (Almaty region, Kyzylorda region, Turkystan region) during pre-treatment with hydrochloric acid Types of Rice Husks WRH SiO2 Raw RH

RH 1 19 17.4 100

Yield, wt % RH 2 16 11.5 100

RH 3 17 11 100

The yield of silica from the rice husks and its purity also depend on the production scheme. The highest yield of final pure product (17.4 %)

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is formed by preliminary acid treatment of sample «RH 1+HCl». The silica content varies in the range of ~90.1-99.5 % depending on the purity of used reagent for washing (acid and its composition).

Figure 39 – Percent yield of the white rice husk (WRH) and rice husk silica (RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with HCl)

Figure 40 – Percent yield of the white rice husk (WRH) and rice husk silica (RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with HCl)

The dependence of the volume of hydrochloric acid on the pH of solution used for the preliminary washing of RH demonstrates that in the neutral medium (pH = 7) yield of the final product is significantly larger (Figure 41, Table 11).

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Influence of PH & Volume of HCl on the yield of obtained final product PH of solution 1 2 3 4 5 6 7

RH 1 11.4 11.37 10.67 9.29 8.87 6.4 11.47

RH 2 11 11.11 10.02 9.24 8 – 11.75

Table 11

RH 3 11.28 11.04 9.12 8.78 6.68 – 11.47

Figure 41 – Influence of Volume of HCl on the pH of RH washing

Pre-treatment with Citric acid Silica is extracted from pre-treated with citric acid RH using 100 ml of 2M NaOH. It is found that the yield of silica is strongly dependent on the type of acid used for washing and concentration of NaOH. The results are shown in the Table 12 and Figures 42, 43. As can be seen from Table 12, the yield of final products of preliminary washing with citric acid is lower compared to pre-treatment of RH with hydrochloric acid (Table 10). For RH treated with citric acid, the percentage of SiO2 is 98.673 %, which is higher than thermally treated husk (95.598 %) but lower than HCl-treated husk (99.666 %) (Appendix E).

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Table 12 Yield of WRH, Silica from RHs (Almaty region, Kyzylorda region, Turkystan region) during pre-treatment with citric acid Types of Rice Husks Raw RH WRH SiO2

RH-1 100 19 13

Yield, wt % RH-2 100 15.4 10.7

RH-3 100 17.7 12

Figure 42 – Percent yield of the white rice husk (WRH) and rice husk silica (RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with citric acid)

Figure 43 – Percent yield of the white rice husk (WRH) and rice husk silica (RHS) extracted from the rice husk (RH) of different region of Kazakhstan (with citric acid)

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3.6. Study of the elemental composition of the different stages of rice husks According to [1], ash content varies in different parts of rice in the range (%): in grain 1.3-2.7, in straw 6-12, in husks 15-20. In this section, the elemental composition (Table 13, 14) of rice husk, white rice husk and treated with various reagents WRH is presented. The analysis was performed by X-ray fluorescence analysis (XRF). The obtained data indicate a rich and diverse elemental composition of the investigated rice husk. Table 13 The elemental analysis of the amorphous silica samples (600 °C)

Sample name

The yield of final product (η), %

WRH+ HCl

17.4

Content, % SiO2

Na2O

Al2O3

95.871

3.578

0.032 0.018 0.279 0.021 0.014 0.006

SO3

Cl

K 2O

Fe2O3

ZnO

Table 14 I The elemental composition of different stages of rice husks Samples name Elemental Rice husk White rice composition WRH+H3PO4 WRH+HCl WRH+H2SO4 (Raw) % husk Na2O

0.365

0.079

7.037

3.578

0.545

MgO

0.311

0.03

n/d

n/d

n/d

Al2O3

0.23

0.134

0.035

0.032

n/d

 SiO2

90.194

99.506

73.439

95.871

97.425

SO3

2.021

0.021

n/d

0.018

2.001

Cl

0.278

0.041

n/d

0.279

n/d

K2O

2.949

0.074

0.033

0.021

n/d

CaO

2.618

0.036

n/d

n/d

0.016

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Table 14 (continued) MnO

0.138

n/d

n/d

n/d

n/d

Fe2O3

0.570

0.071

n/d

0.014

n/d

CuO

0.071

n/d

n/d

n/d

n/d

ZnO

0.037

0.007

0.005

0.006

n/d

Tm2O3

0.172

n/d

n/d

n/d

n/d

PtO2

0.047

n/d

0.008

n/d

n/d

P2O5

n/d

n/d

19.443

n/d

0.013

NiO

n/d

n/d

0.182

n/d

n/d

*n/d – not detected

Table 14

II The elemental composition of different stages of rice husks № 1 2 3 4 5

Samples of rice husk Filtrated White rice husk (WRH) WRH+NaOH H3PO4+ Ethanol Hot water+ H3PO4+ Ethanol

Si 56.898

P

Zn

Fe

Ni

3.717 23.43 7.39 0.18

K

Ca

Ti

4.619 2.133 2.614

99.94

n/d

n/d

0.06

n/d

n/d

n/d

n/d

97,467

n/d

2,533

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

n/d

82,888 17,112 100

n/d

*n/d – not detected

In total, the content of 16 compounds is established. It should be noted that analysis of the obtained data shows that the content of oxides of sulfur, potassium, calcium, iron, sodium and magnesium in the composition of rice husks varies in the range 0.31-2.02 %. The content of oxides varies depending on the nature of the reagent used to treatment of rice husk. These indicators correlate with the solubility of inorganic compounds, which can be formed by reacting the reagent with the raw material. Therefore, the largest content of the main

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substance, SiO2, in the final product is achieved when the initial raw material is treated with hydrochloric, phosphoric and sulfuric acids, the inorganic compounds of which are characterized by high solubility. The content and properties of silicon dioxide, SiO2, the basic mineral substance in the rice husk are different depending on many factors: different types of rice, geographic location, type of fertilizer, processing method, etc. As can be seen from Table 14, the content of silicon dioxide in the composition of rice husks is 90.194 %. These data show that rice husk is the most promising raw material for obtaining high purity silica. 3.7. Structural features of silica samples from rice husk 3.7.1. Results of X-Ray Diffraction analysis of the samples XRD patterns of ash combusted at a range of temperatures from 500-1000 °C have shown a change from amorphous to crystalline silica at 800 °C, and the peak increased abruptly at 900 °C [2]. The change from amorphous to crystalline silica at 800 °C was also found in other studies [3]. In Vietnam, a series of experiments using a laboratory oven under conditions designed to simulate the conditions of combustion from a rural facility were carried out [4]. SEM analysis of the ash found that the ‘globular’ amorphous silica increased in size from 5-10 μm to 10-50 μm with rising combustion temperatures from 500-600 °C. The transition to completely crystalline silica was complete by 900 °C. However it appears that even at temperatures of over 1000 °C the amorphous structure will be retained provided the ash is quickly cooled. With increasing temperature the silica structure progressively changes into cristobelite, tridymite and quartz crystalline forms. Transition from amorphous to crystalline silica can be described by Equation (2) below: (2)

Note:

quartz converts to quartz at 573 °C quartz converts to tridymite at 870 °C tridymite converts to crystobalite at 1470 °C

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These changes affect the structure of the ash. As such, the ‘grindability’ and therefore reactivity of the ash is affected since, after grinding, a greater surface area is available for chemical reactions if the ash is to be used as a pozzolan. X-ray diffraction patterns for silica samples were recorded using the Rigaku powder diffractometer at a scanning speed of 0.02 2 θ/min, using nickel-filtered Cu-Kα radiation in the angular range from 10 to 90 of 2-theta. The obtained silica powder is in the X-ray amorphous state, which is confirmed by X-ray phase analysis. On X-ray diffraction patterns is observed one diffuse peak in the 2θ=24° region typical for the amorphous structure of the rice husk silica, while for amorphous silicon dioxide the maximum of the diffuse peak is at 2θ=30°.

Figure 44 – X-ray diffraction pattern of SiO2 obtained from rice husk

The observed broad halo (Figure 44) with maximum of intensity at 24.0 2θ/°, corresponds to the d-spacing value of 0.36 nm and confirms an amorphous structure the obtained silica. Other studies on XRD also indicated a broad peak at around 22°– 23° which confirms the amorphous structure of silica and absence of any orderly crystalline structure [5]. The amorphous structure is very porous and has high surface area so this also gives a high activity for chemical treatment and in absorption. In general silica in this condition has far more potential for commercial utilization than mineral sources of silica which are characterized by higher temperature crystalline forms.

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3.7.2. Results of Raman spectroscopic analysis of the samples of white rice husk and silicon dioxide Figures 45-50 show the typical Raman spectra of the samples of white rice husk (Sample 1) and silica obtained by treating the rice husk (Sample 2-6).

Figure 45 – Sample 1 (White rice husk)

Figure 46 – Sample 2 (SiO2+HCl)

Figure 47 – Sample 3 (HCl+SiO2+CTAB)

Figure 48 – Sample 4 (SiO2+ 25 ºC+H3PO4+hot water+not centrifugation)

Figure 49 – Sample 5 (HCl+CTAB+700 ºC (6h))

Figure 50 – Sample 6 (HCl+SiO2+CTAB+900 ºC (2h))

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81

Raman spectra mainly show two bands for Sample 2 (SiO2+HCl) ~490, 985 cm-1. On the other hand, Sample 3 (HCl+SiO2+CTAB) shows very weak eight bands ~760, 1059, 1291, 1447, 2720, 2848, 2884, 3030 cm-1 (~2800-3000 cm-1 – this is mainly C-H, -OH groups). Sample 4 (SiO2+25 ºC+H3PO4+hot water+not centrifugation) shows four bands ~430, 497, 794, 1022 cm-1 (the sample consists of large transparent particles and small needle crystals. Needles give two sharp peaks at 1026 and 1059 cm-1.). Sample 5 (HCl+CTAB+700 ºC (6h)) – almost completely cristobalite, with some impurities. Sample 6 (HCl+SiO2+CTAB+900 ºC (2h)) shows five bands ~115, 232, 419, 784, 1077 cm-1 (almost completely cristobalite, with some impurities). The bands ~115, 232 and 419 cm-1 are attributed due to symmetric stretching of Si-O-Si bond in cristobalite (SiO2) phase. The bands in the range of 430-490 cm-1 are designates to O-Si-O bending vibrations. 3.7.3. Results of Fourier Transform Infrared Spectroscopy Analysis The functional groups in the sample were determined using a Thermo Scientific FT-IR Nicolet 6700 equipped with attenuated total reflectance (ATR) accessory. The spectra were recorded with 32 scans at a resolution of 4 cm-1 in the range of 4000-400 cm-1. The FT-IR spectra of rice husks shown in Figure 51. The notable absorption peaks at 1058 cm-1, 1058 cm-1. 1058 cm-1 – was considered to result from superposition of vibrations of the C-OH bond and Si-O bond in the siloxane (Si-O-Si) groups.

Figure 51 – FT-IR spectra of rice husks (Rice husk 1 – Almaty rice husk, Rice husk 2 – Kyzylorda rice husk, Rice husk 3 – Turkystan rice husk)

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2921cm-1 – can be attributed to the symmetric and asymmetric stretching vibrations of the aliphatic C-H bonds in -CH3 and CH2 groups in the structures cellulose, hemicellulose and lignine, respectively.

Figure 52 – FTIR spectra for silica from rice husk

In the IR spectra of Silica, stretching vibrational bands of Si-O can be observed at 1860 cm-1 (Figure 52). 3.8. Thermogravimetric analysis TG analysis was used to determine the existence of organic components in the rice husk (Figure 53). It can be seen that initial weight loss occurs within the range of 50-250 °C. The second stage reveals a rapid and large weight loss at temperature between 300-450 °C. This is due to the thermal decomposition of hemicellulose and cellulose as a major organic component in the rice husk. The third stage shows a weight loss of about 25-30 % that could be due to lignin, a thermally more stable aromatic polymer which undergoes gradual decomposition between 300 and 600 °C. The residual of ash is mainly the noncombustible silica (~15 %, >600 °C).

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Figure 53 – Thermograph of RH from different region

3.9. Investigation of the surface of silica samples from rice husk 3.9.1. Morphology of RH before and after direct incineration (Scanning Electron Microscopy) Comparisons of the surface structures of rice husk and the resulting silicon oxide were represented by SEM Quanta 3D (FEI company, USA). Figure 54 shows SEM images of the original rice husks before direct incineration. As can be seen from the figures, the samples in the initial form are very dense surfaces.

Figure 54 – SEM images of rice husks before heat treatment (a) and SEM micrograph of the SiO2

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Figure 54 (a) illustrates the morphology of outer surfaces of rice husk, the outer surface of rice husk is uneven and highly roughened. Figure 54 (b) is a low-magnification SEM image of the silica sample. The nano-scale roughness comes from the morphology of silica nanoparticles, dispersed within the bulk. 3.9.2. Morphology of rice husk silica after pre-treatment with hydrochloric acid (Scanning Electron Microscopy) The morphology of the particles of the obtained silica powder was studied using the scanning electron microscope of the brand Quanta 200i 3D (FEI Company, USA) (National Nanotechnology Laboratory of Al-Farabi Kazakh National University, Almaty, Kazakhstan) (Figure 55).

Figure 55 – SEM micrograph of the SiO2 obtained from the rice husk ash

The low-magnification SEM image of the silica sample (Figure 55) represents a nano-scale roughness, which comes from the morphology of silica nanoparticles, dispersed within the bulk. 3.9.3. Analysis of the specific surface area, average pore size of the obtained samples The specific surface area and the average pore diameter characterizing the porosity of the studied silica samples are shown in Table 15. The values of the specific surface area calculated by the BET method (Autosorb-iQ) show that the samples obtained from the RH have the highest values of S

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III. Methodology of Obtaining SiO2 by using Processing of Rice Husks

(465 m 2/g), which at the same time depend on the method of production. The specific surface area of the SiO 2 samples varies in the range, depending on the nature of the reagent used to pretreat of the rice husk. It is known that samples prepared from rice husks by heat treatment have the low specific surface area. The analysis of the obtained data confirms that the pre-treatment with the acid of the raw material significantly increases the specific surface area of amorphous silica, which decreases sharply upon crystallization of the substance. Samples of silica of mineral origin are characterized by specific surface values in the range of ~75-101 m2/g, depending on their composition [6]. On the specific surface area and pore diameter of amorphous silica are affected, as shown by the results of the study, not only the nature of the acid used for the pre-treatment of the raw material, but also the region where the rice grows (accordingly, the plant variety) as follows from Table 16. Table 15 Specific surface and average pore sizes of silica samples (SiO2) treated under different conditions

Sample name

Reagent

Specific surface area, m2/g

Specific pore volume, sm3/g

The average pore diameter, nm

White rice husk

–

197.632

0.084

1.706

SiO2

HCl

465

0.127

1.715

SiO2+CTAB

HCl

14.842

0.006

1.714

SiO2+ 25 °C+hot water+not centrifugation

H3PO4

1.527

0.001

0

SiO2+CTAB +900 °C (2h)

HCl

0.520

0

0

*CTAB – cetyltrimethylammonium bromide (template agent)

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Samples of amorphous silica from rice husks obtained with pre-treatment with hydrochloric acid Place of harvest  Almaty region Krasnodar region Saipan Island

Table 16

Concentration of HCl

Burning temperature, °С

S, m2/g

daverage, nm

References

2M

600

465

~1.7

Present work

0.1N

300; 600

230.8

4.4

[6]

n/d* n/d*

600 800

260 211

~1.5 ~1.5

[7]

*n/d – no data about the concentration of acid

The comparison of the characteristics of amorphous high-purity silicon dioxide with the results of [6-7] shows that samples obtained from pre-treated of rice husks have different values of specific surface area and pore diameter. These differences are due to a number of factors: the place of the selection of raw materials, the concentration of the reagent used to pre-treat the husks and the temperature of burning. In structure of silicon dioxide from RH predominate the pores with a diameter of 1.7 nm, therefore, these samples have a mostly microporous structure. 3.10. Conclusion of the experimental part The experimental data indicate the dependence of the characteristics of the silicon-containing samples extracted from the rice husks on the processing conditions of the raw materials: the size of the formed particles, their shape, the specific surface area and the average pore diameter. Analysis and advanced characterization of the initial and modified silica by means of low-temperature nitrogen adsorption-desorption, analysis of particle size distribution and thermal analysis was also provided. The main adsorption data of silica’s were evaluated from the isotherms of low-temperature physical adsorption-desorption of nitrogen, measured by the volumetric method on the Autosorb-iQ

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87

surface and porosity analyzer. The specific surface area was determined by the Brunauer-Emmett-Teiler method (BET). The specific surface area, porosity and adsorption properties of silica were studied by the method of low-temperature adsorptiondesorption of nitrogen. Amorphous silica of high purity obtained from the rice husk, pre-treated with acid has a specific surface area of – 465 m2/g. The particle size distribution of silica was investigated by the Zetasizer (Malvern) instrument. The method of thermal analysis on the TG/DSA 6000 instrument (Perkin Elmer) was used for study the dependence of the mass loss of a sample as a function of temperature. Figure 56 shows the general scheme of the method for processing rice husk, which makes it possible to obtain silicon dioxide, SiO2∙nH2O.

Figure 56 – The general scheme of experimental sequence for silica synthesis from rice husk ash

The proposal of the scheme for the processing of rice husks, of course, should be determined by the expediency of producing the product (silicon dioxide) in the region where the rice husk is formed and in the needs of the market. REFERENCES 3 [1] Сергиенко, В. И. Возобновляемые источники химического сырья: комплексная переработка отходов производства риса и гречихи / В. И. Сергиенко, Л. А. Земнухова, А. Г. Егоров, Е. Д Шкорина, Н. С. Василюк // Российский химический журнал (Журнал Российского химического

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Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ... общества им. Д. И. Менделеева). ‒ 2004. ‒ Т. 48. ‒ № 3. ‒ С. 116 ‒ 124. [2] National Research Development Corporation (A Government of India Enterprise) www.nrdcindia.com/pages/ricehu.htm [3] Kashikar, S.R.R. (2000). Preparation and characterization of rice husk silica compacts. M.Tech Thesis. http://www.iitk.ac.in/mme/ mtTheses/2000/9810619/html [4] Bui, D.D. and Stroeven, P. Rice Husk Ash-based Binders and their Use in High Strength Concrete in Vietnam. (no further source details available). [5] An, D., Guo, Y., Zou, B., Zhu, Y., Wang, Z., 2011. A study on the consecutive preparation of silica powders and active carbon from rice husk ash. Biomass and Bioenergy 35, 1227–1234. doi:10.1016/j.biombioe.2010.12.014 [6] Холомейдик Анна Николаевна. Получение, состав и свойства кремний‒ и углеродсодержащих продуктов переработки плодовых оболочек риса: Диссертация на соискание ученой степени кандидата химических наук. Институт химии. Владивосток. 2016. [7] Real C., Alcala M.D., Griado J.M. Preparation of silica from rice husks // Journal of the American Chemical Society. 1996. V. 79. № 8. P. 2012 – 2016.

IV

SILICA FROM RICE HUSK: APPLICATIONS

Silica nanoparticles have gained attention in recent years for numerous applications owing to their many attractive properties in the field of catalysis, drug application, gene therapy, as additives in plastics, food additives, also as pigments, pesticides, thin-film substrates and thermal insulators [1,2]. Silica is precursor of various chemicals used in different industries such as polymer-based industries, electronics, rubber, concrete and ceramic industries [3]. The silica obtained from RHA has found its use in production of solar photovoltaic cells, mesoporous compounds, ceramic industries, and photo catalytic catalyst [4]. Li, Y., Lan, J.Y., Liu, J., Yu, J., Luo, Z., Wang, W., Sun, L., 2015 synthesized gold nano particles on silica derived from RHA for catalytic applications [5]. Silica catalyst synthesized via sol-gel method from RHA was evaluated for its catalytic performance in CO2 hydrogenation process for production of methanol [6]. Silica extracted from rice husk ash was incorporated with chromium metal and benzoic acid. The heterogeneous catalyst made was successfully employed for the conversion of cyclohexane to cyclohexanone by Adam, F., Retnam, P., Iqbal, A., 2009 [7. Synthesis of nano-sized zeolites was achieved by extracting silica from rice husk ash by Sari, Z.G.L.V., Younesi, H., Kazemian, H., 2015 [8].The suitability of silica extracted from RHA was also investigated as dental filler and reported by Tolba, G.M.K., Barakat, N.A.M., Bastaweesy, A.M., Ashour, E.A., Abdelmoez, W., El-Newehy, M.H., Al-Deyab, S.S., Kim, H.Y., 2015 assessed the silica derived from RHA as an adsorbent for the removal of methylene blue [9]. The following studies from reviews suggest applicability of this value-based product in diverse fields of industry and environment: 1) Adsorption material. Due to its residual metal content and porous structure, RH silica has been used to adsorb free fatty acid in soy and palm oil. Some or all of these metal centers participate in the adsorption of the fatty acid [10,11]. Based on this, RH silica modified 89

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by Al3+ ion was synthesized from RHA using the sol-gel technique [12]. Water-treated RH were calcined at 800 °C for 5 hours to produce RH silica which was leached by HNO3 solution for 24 hours to removes the metal impurities. The high-purity RH silica was dissolved in 6.0 M NaOH solution. After filtering, the filtrate was titrated with 3.0 M HNO3 containing 10 % (w/w) Al3+ until the pH value was 5 and aged for 6 days to form a gel. RHA-Al was produced after filtering and drying the solid residue at 110 °C for 24 hours. The RHA-Al was a very good adsorbent for palmytic acid because of electrostatic interaction between the Al3+ in the silica matrix with the oxygen atoms of carbonyl group in the fatty acid [12]. RH silica has been chosen as an adsorbent in water treatment processes because of its granular structure, water insolubility, chemical stability, high mechanical strength, and its large availability. In order to enhance the adsorption capacities for heavy metals, RHA with 87.5 % silica content prepared by directly calcination of RH at 650 °C for 2 hours was used to synthesize poly inorganic silica with Fe and Al ions, which are more favorable in removal of heavy metals from waste water [13]. In this process, sodium silicate produced by refluxing RHA in NaOH solution for 1 hour was diluted with distilled water to form 1.25 M silicate solution followed by the addition of 0.25 M HCl solution to pH≈2. The above solution was mixed with a blend of AlCl3 and FeCl3 and neutralized by NaCO3. The maximum adsorption capacity of the yield copolymer for Pb, Fe, Mn and As ions are found to be 416, 222, 158, 146 mg/g respectively. Surface modification is another effective method to improve the adsorption performance of silica. Mesoporous silica produced from RHA by alkaline extraction was modified with silane using 1.2-dichloroethane as a spacer [14]. The modified silica contained the cylindrical pores that open at both ends, which allow free diffusion of Cd2+ ions to the adsorption sites on the silica surface. The silanemodified silica showed specific adsorption for Cd2+ of 44.52 mg/g at a Cd2+ concentration of 100 mg/L. Cd2+ adsorption was increased 100fold compared to the non-surface modified silica [14]. Cu2+ selective ligand, pyridine-2-carboxaldehyde was used to modify the amorphous silica gel obtained from RHA by NaOH extraction. The modified silica selectively absorbs Cu2+. Its mechanical stability was not as good as that of the commercial product. Its selectivity for Cu over Fe was not

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as high as the commercial product. But this work defined the problems associated with using RHA as a starting point for composite materials in general. 2) Silica-supported catalysts. Many heterogeneous catalyst systems are supported by mesoporous materials such as silica [15,16]. Silica-supported catalysts are being developed at a tremendous pace because of their high catalytic behavior derived from the high surface area. Many examples have been reported on the preparation and synergies of catalysts supported by silica obtained from RH. 3) Silica-supported metal catalysts. Chang, F. W., Tsay, M. T., Kuo, M. S. use RHA as a support for nickel catalysts by an incipientwet method [17-19]. RHA was impregnated in Al(NO3)3 solution for 24 hours followed by drying at 110 °C in an oven. The dried samples were calcined in air at 350 °C for 5 hours or at 850 °C for 2 hours to produce the substrate material (RHA-Al2O3), respectively. The RHAAl2O3 obtained at 350 °C was impregnated in Ni(NO3)2 solution for 24 hours and dried at 110 °C in an oven [18,19]. Following calcination at 500 °C for 4 hours, the samples were activated by using a reducing atmosphere of H2/Ar (5/95) stream at 800 °C for 3 hours. On the other hand, RHA-Al2O3 synthesized at 850 °C was ion exchanged to produce the catalyst [17]. In this process, concentrated ammonia was mixed with the Ni(NO3)2 solution prior to the addition of RHA-Al2O3 support. The mixture was kept at 25 °C for 24 hours with stirring. The exchange of Ni2+ with hydroxyls on the surface of the support is enhanced at pH=8.5 with dilute ammonia. The RHA-Al2O3 supported catalyst was produced by washing, drying, and calcining of the above ion exchanged product [17]. It was found that RHA-supported Ni catalyst exhibited high Ni surface area, easy dispersion and high selectivity when used for the hydrogenation of CO2 [17-19]. The hydrogenation reactivity was almost independent of calcination and reduction temperatures for the catalysts produced using RHA-Al2O3 with calcination at 350 °C [18,19]. The conversion of CO2 and the yield of CH4 were strongly dependent on the calcination and reduction temperatures for the catalysts obtained by ion exchange method. Adam, F., Andas, J., Ab Rahman, I., Ahmed, A. E., Iqbal, A., Thankappan, R., Chew, T. S., Kandasamy, K., Balakrishnan, S. prepared metal loaded catalysts from RHA [20-26]. Silica supported iron catalyst was synthesized from RHA via the sol–gel technique

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[25]. The RHA produced by pyrolysis of the water rinsed RH at 700 °C for 5 hours was treated with 1.0 M HNO3 for 24 hours. The obtained RHA was dissolved in 6.0 M NaOH to form a sodium silicate solution. 3.0 M HNO3 containing 10 wt% Fe3+ was added drop wise to the solution until the pH reached 5. The gel was aged for 4 days followed by filtering, washing, and drying to produce the RHA-Fe catalyst. The RHA-Fe catalyst showed high activity in the reaction between toluene and benzyl chloride. It was reusable without loss of activity and with no leaching of the metal [25]. Similarly, In, Al, Ga, Cr, and V were successfully incorporated with RHA as the substrate [20, 22, 24, 26]. The catalytic activity of Al, Ga and Fe catalysts supported by silica were tested in the benzylation reaction [26]. It was found that RHA-Fe demonstrated the highest catalytic activity, while RHAGa gave the highest selectivity [26]. However, RHA-Al was found to be inactive under the reaction conditions studied. These catalysts could be recycled and reused several times without significant loss in activity and selectivity [26]. Bimetallic modified RH silica catalysts have attracted much attention since two or two more active metals can improve performance in a reaction system [23]. High surface area Cu and Ce incorporated RH silica catalysts were prepared by a template assisted sol–gel precipitation method [23]. RH silica was produced by calcination of HNO3 pre-treated RH at 600 °C for 6 hours. The obtained silica was dissolve in NaOH solution to form sodium silicate solution followed by addition of cetrimonium bromide (CTAB) as the structure directing agent. 3 M HNO3 containing Cu2+ and Ce3+ salts was used to titrate the above solution until the pH value reached 3. The gel was aged for 24 hours. After filtering, washing, and drying, the sample was calcined at 500 °C in air to produce the xerogel which was ground to a powder to get the silica supported catalyst. Study showed the catalysts have good catalytic performance with high selectivity in the reaction of converting benzene to phenol due to the synergy between the Cu2+ and Ce3+ [23]. 4) Silica-supported organic molecule catalysts. Modification of silica with organic molecules can generate silica supported catalyst. For example, 7-amino-1-naphthalene sulfonic acid was immobilized onto RH silica and followed by HNO3 treatment for 24 hours to form strong Bronsted acid sites [27]. The catalyst showed good catalytic

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activity towards esterification of n-butyl alcohol with different mono‒ and di-acids with 88 % conversion and 100 % selectivity towards the ester [27]. 5) Silica-supported TiO2 and V2O5 photocatalysts. TiO2 is a wellknown photocatalyst for photocatalytic degradation. The absorption of UV-Vis energy by TiO2 excites electrons to a conduction band with positive holes in valence band that generates hydroxyl radicals, which oxidize organic compounds. The photoactivity of TiO2 is decreased by electron-hole recombination and particle aggregation. This can be overcome by dispersing TiO2 on materials with high surface area. Artkla, S., Wantala, K., Srinameb, B. O., Grisdanurak, N., Klysubun, W., Wittayakun, J., Kim, W., Choi, W. synthesized a hybrid photocatalyst that consisted of TiO2 nanoparticles dispersed on mesoporous RH silica [28,29]. The results showed that the morphology and band gap of TiO2 were not affected by the support. The support prevented the TiO2 nanoparticles from agglomerating. The activity of the hybrid photocatalyst for the photocatalytic degradation of tetramethylammonium in aqueous slurry was significantly higher than that of the unsupported TiO2 [29]. V2O5 is an important industrial catalyst for partial oxidation of organic compounds. Selective oxidation with rupture of C-C bonds is normally supported on different carriers depending on the type of the reaction to be catalyzed. Bulk V2O5 cannot be directly used as a catalyst. Sugunan and coworkers prepared RHA-CeO2 supported V2O5 catalysts. The catalytic activity was evaluated in liquid-phase oxidation of benzene. Selective formation of phenol was observed. This is attributed to the presence of highly dispersed active sites of V over the support [30]. 6) Preparing porous silica. For the last decades, many efforts have been made to prepare porous silica with high surface area starting from RH. Jang research group and Chareonpanich research group synthesized SBA-15 mesoporous silica using RHA derived from non-treated RH [31,32]. RHA was first converted to sodium silicate solution and then mixed with a template agent (usually a surfactant). After pH adjustment, aging, filtering, washing, and drying, the solid was calcined at 500 °C for 6 hours in air to produce SBA-15. It was found that the ultrasonic mixing gives highly ordered hexagonal porearrangement and a narrow pore size distribution with a much shorter

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hydrolysis–condensation period. The highest SBA-15 surface area was 860 m2/g when the ultrasonication time was 6 hours [32]. MCM-41 was prepared and studied by researchers using RH silica [33]. Grisdanurak, N., Chiarakorn, S., Wittayakun, J. used HBr solution to post treat RHA to get high-purity silica. The posted treated RHA was converted to sodium silicate solution with base. By using cetrimonium bromide (CTAB) as template agent, the authors produced MCM-41 with a surface area approximately 750-1100 m2/g. The pore distribution had an average diameter of about 2.95 nm. The authors also tried to surface modify the MCM-41 with trimethylchlorosilane (TMCS) and phenyldimethylchlorosilane (PDMS). To reduce the MCM-41 surface polarity. The results showed that the surface hydrophobicity of MCM-41 obtained from RH could be enhanced by the silylation with silane [34]. Zhu and coworkers successfully shortened the porous silica preparation time (within 10 hours) without compromising the specific surface area [35,36]. In their synthesis, the aging step was skipped and the microwave oven was used for drying to save time. During synthesis, by using a much lower pH, the amount of PEG incorporated into the silica-PEG composites increased. This resulted in higher surface area porous silica (792 m2/g at pH 5.7 and 1018 m2/g at pH 3.2) [36]. 7) Rice husk ash based silica nanoparticles (SiNP) in biomedical research. The use of nanotechnology for medical applications has undergone rapid development in various disciplines like, nanocarrier for delivering nanomedicine and nanoparticles for therapeutic applications. Silica nanoparticles (SNPs) have drawn widespread attention due to their applications in many emerging areas. I.I. Slowing, B.G. Trewyn, S. Giri, V.S.Y. Lin reported that mesoporous silica nanoparticles (MSNs) with a pore size ranging from 2 to 50 nm are excellent candidates for drug delivery and biomedical applications by comparing with other porous silica nanocarriers [37]. Silica nanoparticles are developed for a host of biomedical and biotechnological applications such as cancer therapy, DNA transfection, drug delivery and enzyme immobilization. The application of nanoparticles sounds more interesting, after synthesizing from agriculture waste materials like rice husk. The synthesis of amorphous silica nanoparticle is one of the best examples of solid waste management. Figure 57 represents the application of silica nanoparticle in biomedical sciences [38].

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Figure 57 – A graphical representation of the application of silica nanoparticles (SiNP) in biomedical research. Majorly it is useful in drug delivery as drug delivery carriers and protein. The genetic materials (e.g. DNA, miRNA and siRNA) could be delivering into the cellular system through gene delivery methods using SiNP and photodynamic therapy. Adapted from [38]

8) Selection of rice husk silica production process for specific applications. The impurities contained in rice husk silica can have implication in some of the applications. It is therefore important to know what characteristics are needed in an intended application, which will therefore affect the choice of the purification degree that is necessary for such an application. Table 17 gives a summary of applications of rice husk silica. The levels of purity required and the recommended processes are also indicated. Due to containing a large quantity of amorphous silicon dioxide and being prepared by a simple synthetic method, RHA is widely considered as an organic, environmentally friendly, and recyclable material. For example, aluminum and iron hydroxides were modified by RHA to remove fluoride and arsenic from drinking water [55].

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Different applications and purity levels required of rice husk silica and recommended processes Application

Desirable qualities

Purity level

Aerogels

High purity

> 99.5 %

Filler in polymers

Ability to retard thermooxidative and photo degradations. Possession of some silanol group to enhance coupling and good level of residual carbon to inhibit photo degradation

95-98 %

Cement and Concrete

High reactivity, high surface area, absence of crystallinity

88-98 %

Zeolites

High purity and high porosity

> 99.5 %

Cordierite

High purity/reactivity

> 99.5 %

Recommended process

Table 17

Reference

Acid pre-treatment before incineration at temperature less than [39-40] 700ºC in regulated environment

Hydrothermal process

[41-44]

Acid pre-treatment before incineration [45-52] at temperature less than 700ºC Acid pre-treatment before incineration at temperature less than [53-55] 700ºC in regulated environment Pre-treatment before incineration at [56-57] temperature more than 700ºC

In a future study, RHA with nanoscale microstructure, fine and homogeneous pore size can be produced to effective application of nanomaterials in adsorbent and catalyst. On the other hand, it is a prospective evolution research orientation that RHA is synthesized to thermal insulation and building material, because of possessing the performance of small thermal conductivity.

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Conclusion In recent years, recycled industrial and agricultural by-products containing silica are considered to be rich sources of silica. In addition to transforming waste into materials with high silica content, the recycling processes allow solving the problems of waste disposal and guarantee the long-term availability of natural silicon resources. Compared with synthetic sources of silica, the use of natural sources of silica allows the use of pre-made silicon precursors from raw materials, which significantly reduces the number of chemical modifications and energy consumption. As a rule, the choice of natural raw materials and the methods used to extract the silicate material determines the quality of the synthesized product. Natural sources of silica usually allow the synthesis of well-ordered mesoporous materials using gas adsorption, molecular separation or catalysis, which compete with the application of similar materials synthesized from conventional sources of silicon dioxide. Moreover, it is assumed that the silica precursors obtained from the secondary waste will be widely used in the near future, because the silica includes a large number of applications in industrial materials (cement, glassbased household products, electronic materials, catalysts, drugs etc.). REFERENCES 4 [1] Alshatwi, A.A., Athinarayanan, J., Periasamy, V.S., 2015. Biocompatibility assessment of rice husk-derived biogenic silica nanoparticles for biomedical applications. Mater. Sci. Eng. C 47, 8–16. doi:10.1016/j.msec.2014.11.005 [2] Klabunde, K. J. (2001). Nanoscale materials in chemistry. WileyInterscience, New York. [3] Gu, S., Zhou, J., Yu, C., Luo, Z., Wang, Q., Shi, Z., 2015. A novel twostaged thermal synthesis method of generating nanosilica from rice husk via pre-pyrolysis combined with calcination. Ind. Crops Prod. 65, 1–6. doi:10.1016/j.indcrop.2014.11.045 [4] Liou, T.-H., Yang, C.-C., 2011. Synthesis and surface characteristics of nanosilica produced from alkali-extracted rice husk ash. Mater. Sci. Eng. B 176, 521–529. doi:10.1016/j.mseb.2011.01.007 [5] Li, Y., Lan, J.Y., Liu, J., Yu, J., Luo, Z., Wang, W., Sun, L., 2015. Synthesis of gold nanoparticles on rice husk silica for catalysis applications. Ind. Eng. Chem. Res. 54, 5656–5663. doi:10.1021/acs.iecr.5b00216 [6] Siriworarat, K., Deerattrakul, V., Dittanet, P., Kongkachuichay, P., 2016. Production of methanol from carbon dioxide using palladium-copper-zinc

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[19] Chang, F. W.; Kuo, M. S.; Tsay, M. T.; Hsieh, M. C., Effect of calcination temperature on catalyst reducibility and hydrogenation reactivity in rice husk ash-alumina supported nickel systems. Journal of Chemical Technology and Biotechnology 2004, 79 (7), 691-699. [20] Ahmed, A. E.; Adam, F., Indium incorporated silica from rice husk and its catalytic activity. Microporous and Mesoporous Materials 2007, 103 (1-3), 284-295. [21] Adam, F.; Andas, J.; Ab Rahman, I., A study on the oxidation of phenol by heterogeneous iron silica catalyst. Chemical Engineering Journal 2010, 165 (2), 658-667. [22] Adam, F.; Iqbal, A., The oxidation of styrene by chromium-silica heterogeneous catalyst prepared from rice husk. Chemical Engineering Journal 2010, 160 (2), 742-750. [23] Adam, F.; Thankappan, R., Oxidation of benzene over bimetallic Cu-Ce incorporated rice husk silica catalysts. Chemical Engineering Journal 2010, 160 (1), 249-258. [24] Adam, F.; Chew, T. S.; Andas, J., Liquid Phase Oxidation of Acetophenone over Rice Husk Silica Vanadium Catalyst. Chinese Journal of Catalysis 2012, 33 (3), 518-522. [25] Adam, F.; Kandasamy, K.; Balakrishnan, S., Iron incorporated heterogeneous catalyst from rice husk ash. Journal of Colloid and Interface Science 2006, 304 (1), 137-143. [26] Ahmed, A. E.; Adam, F., The benzylation of benzene using aluminium, gallium and iron incorporated silica from rice husk ash. Microporous and Mesoporous Materials 2009, 118 (1-3), 35-43. [27] Adam, F.; Hello, K. M.; Ben Aisha, M. R., The synthesis of heterogeneous 7-amino-1-naphthalene sulfonic acid immobilized silica nano particles and its catalytic activity. Journal of the Taiwan Institute of Chemical Engineers 2011, 42 (5), 843-851. [28] Artkla, S.; Wantala, K.; Srinameb, B. O.; Grisdanurak, N.; Klysubun, W.; Wittayakun, J., Characteristics and photocatalytic degradation of methyl orange on Ti-RH-MCM-41 and TiO2/RH-MCM-41. Korean Journal of Chemical Engineering 2009, 26 (6), 1556-1562. [29] Artkla, S.; Kim, W.; Choi, W.; Wittayakun, J., Highly enhanced photocatalytic degradation of tetramethylammonium on the hybrid catalyst of titania and MCM-41 obtained from rice husk silica. Applied Catalysis B-Environmental 2009, 91 (1-2), 157-164. [30] Radhika, T.; Sugunan, S., Influence of surface and acid properties of vanadia supported on ceria promoted with rice husk silica on cyclohexanol decomposition. Catalysis Communications 2006, 7 (8), 528-533. [31] Bhagiyalakshmi, M.; Yun, L. J.; Anuradha, R.; Jang, H. T., Utilization of rice husk ash as silica source for the synthesis of mesoporous silicas and their application to CO2 adsorption through TREN/TEPA grafting. Journal of Hazardous Materials 2010, 175 (1-3), 928-938. [32] Chareonpanich, M.; Nanta-Ngern, A.; Limtrakul, J., Short-period synthesis of ordered mesoporous silica SBA-15 using ultrasonic technique. Materials Letters 2007, 61 (29), 5153-5156.

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V

RICE HUSK BASED SILICA/Ag NANOPARTICLES COMPOSITE MATERIALS AS A NEW ADSORBENT FOR REMOVAL OF AQUEOUS MERCURY IONS FROM WATER

5.1. Composite materials in water treatment In recent decades, many researchers have studied the design and controlled fabrication of nanostructured materials prepared from multifunctional composites. The interest in these materials has resulted from the increasing understanding that their unusual chemical, mechanical, optical, electrical, and optoelectrical properties are affected by their size, composition, and structural order. The incorporation of silver NPs into silica microsphere to form a composite structure is one of the most exciting challenges for colloid chemistry. Silver NPs are attracting the most attention for their particle sizes, which dramatically influence their special optical and electronic properties. They have great promise for use as materials exhibiting antibacterial (through the release of Ag+ ions), bio-sensing (through surface plasmon resonance) and electromagnetic wave shielding (through free electrons) properties. As such composites have been contemporarily applied in a nanoformulation process for adsorption of mercury from waste water using zero-valent nanoparticles of noble metals. The interaction of metal nanoparticles with other species has shown great promise for a number of applications and in particular silver nanoparticles (AgNPs) have been used as an anti-microbial agent in textiles and composites and for the destruction of pesticides or the removal of mercury from industrial effluents and other waters. With the formation of silver nanoparticles through a silicon hydride reduction, we expect that as the size of the particle is reduced to the nanoscale and in turn it will increase the interaction with aqueous mercury. In this regard, nowadays it is popular development of new approach of using silicon hydride which is useful for generating 102

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high purity selected metal nanoparticles with superior functional performance, which can be applied into a variety of applications, in particular, decontamination of toxic heavy metals from water, improved toxic metal vapor sensing, detoxification of biological systems, (semi)precious catalytic metal recovery from waste streams, and heterogeneous phase catalysis. Hence an attempt has been made for the synthesis of rice husk silica based composite adsorbents decorated with silver nanoparticles for decontamination of toxic heavy metals from water. 5.2. Creation Hydride Silica Composites for Rapid and Facile Removal of Aqueous Mercury Currently water contaminations with heavy metals have great interest. Even at low concentrations these metals have a toxic affects to environment and microorganisms. Mercury is one of the most harmful pollutants among heavy metals. It is widespread in natural waters, groundwater, draining water in urban areas, and industrial waste [1]. Consequently, development of efficient clean-up technologies for removal of mercury from aqueous media has been required testing various methods such as ion exchange, precipitation, reduction, solvent extraction, reverse osmosis and so on. However, compared to adsorbents all these methods have deficiencies such as high cost, low efficiency, formation of by-products, and unsuccessful at low toxic metal concentrations (1–20 mg∙L-1) etc. Hence, many researches have been considered adsorption is the most advantageous technique for elimination Hg2+ ions from wastewater [2]. According to United States Environmental Protection Agency (USEPA) the minimum allowable limit of mercury concentration for drinking water is 0.002 mg/l, whereas for World Health Organization this was set at 0.006 mg/l [3]. Besides, mercury discharge into water sources has been increased in Asia, South America, and Africa due to elevated mercury pollution from industrial plants [4]. Kazakhstan is also vulnerable in terms of ecological problems concerning mercury pollution of water resources. The brightest example is the technical reservoir Balkyldak, which is located near the industrial district of Pavlodar city in the north of Kazakhstan. This waterbody was intended to store and evaporate industrial

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wastes of several large-scale plants in Pavlodar, among them are the Pavlodar Oil Chemical Refinery (POCR) LLP, the Pavlodar Chemical Plant “Caustic” JSC and the heat electric generation plant [5]. The industrial effluent entering the reservoir contains various pollutants, such as petrochemicals, heavy metal salts (Zn, Fe, Cr, Hg etc.), chlorine, sulphates etc. Recent research conducted by Karaganda State Technical University on monitoring of toxic metal contamination in the northern district of Pavlodar showed that the mercury content in soil and groundwater exceeds allowable limit, which verifies that this region still remains the main focus of mercury pollution. In addition, mercury discharges into Balkyldak reservoir was greatly enhanced by operation of an industrial object so called ex-“Chimprom”, which produced chlorine and sodium through electrolysis with mercury cathode between 1973 and 1992. During 14 years the 1089.36 tons of metallic mercury was consumed. In addition to small discharge of mercury into waterbody during regular plant operations, the significant leakages occurred during the shutdown of the plant. Mercury discharge into aquatic systems of the lake-accumulator Balkyldak has an adverse impact on flora and fauna in this region. The analysis of tench fish, which is inhabitant of the lake, showed that mercury concentration in fish exceeds the allowable minimum limit in 0.53 and 3.73 times. The maximum amount of mercury content was found in perch, which is equal to 5.6 times maximum permissible concentration (MPC) [5]. Furthermore, comparison of water content in the Balkyldak Lake and in groundwater showed that mercury concentration has increased in groundwater, whereas the concentration of zinc and chromium remains unchanged. The major concern relates to the spread of mercury pollution into the Irtysh River, which is one of the largest waterbodies in Kazakhstan. In 1950s mercury pollution from a chemical plant in Minamata Bay caused contamination of fish, which was the main food supply for inhabitants of modest village. As a result of this human tragedy 2252 people were affected and 1043 people died. Therefore, efficient mercury remediation technologies evolvement is extremely urgent [6, 7]. Activated carbon, carbon nanotube, zeolites, clays and mesoporous silicas are have been widely used for removal metal ions from aqueous system. However because of their relatively high cost today especial attention has been devoted to finding inexpensive adsorbents [8].

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Silica can be chosen as an efficiently adsorbent in water treatment processes because of its granular structure, water insolubility, chemical stability, high mechanical strength and its low costs. Alternative sources of silica such as rice husk and sugarcane bagasse have been used to obtain amorphous silica by costly templates, surfactants and use of acid washing under high temperature and atmospheric and thermal treatment methods [9]. For instance, RHA with 87.5 % silica content prepared by directly calcinations of RH at 650 °C for 2 hours was used to synthesize poly inorganic silica with Fe and Al ions which are more favorable in removal of heavy metals from waste water [10, 11]. Based on mercury properties, the physical adsorption between mercury and active adsorbents may not be effective. In majority of cases, the adsorbent’s surface must be modified for chemical adsorption. It has been reported that several metals, such as palladium, platinum, rhodium, gold, zinc, aluminium, copper and silver are ready to form amalgam with mercury. Moreover, these metal amalgams formed with mercury have relatively low solubility, which implies negligible release of mercury after adsorption. Among those metals, it has been noted that silver has the lowest solubility, therefore, it was selected to modify adsorbent support and create more active sites [12, 13]. Katok et al. [14] have reported synthesis of composite materials by immobilization of silver nanoparticles on the silica surface functionalized with hydride groups. They were examined potential application of hydride silica composites as adsorbents for mercury from aqueous systems. These novel adsorbents demonstrate high reactivity, pH sensitivity, capacity and can be effective candidate materials for removal mercury ions. The method of silver nanoparticle immobilization on the surface of modified silica which used in present work has a list of significant advantages over other techniques. At first, it is economically feasible since it requires the minimum expense of the silver nitrate solution as a starting material. Secondly, silica can be synthesized using rice husk as a raw material [15]. Besides, it is anticipated that the synthesis of ‘chemically pure’ NPs results in hyperstoichiometry phenomena by which more efficient toxic metal remediation from water can be performed.

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It was noted [15], aqueous mercury solution interacts with silver metal (Ag0) at a stoichiometric ratio of 1:2 (Equation 1), leading to the formation zero valent mercury. An oxidation-reduction (redox) reaction can be explained by the next type of redox reaction (reduction – gain of electrons, oxidation – loss of electrons): (1) Furthermore, the reaction is characterized by rapid formation of Ag-Hg solid amalgams and no release of silver into solution takes place. As it can be seen the mercury-silver pair system possesses the hyperstoichiometric effect, which can be a major advancement in nanoscience [15].

Figure 58 – Schematic illustration of the experimental procedure of composite materials fabrication

Thus, present research work explored the preparation and characterization of new inexpensive adsorbent prepared using agricultural waste materials, silver nanoparticles and triethoxysilane to remove aqueous mercury ions from water specimens. The use of silica as an adsorbent not only solve the problem of potential human

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health risk and ecological disturbances related with toxic heavy metals but also will expand the feasibility of turning agricultural byproduct into a valuable resource. The suggested mechanism for adsorption of Hg2+ ions on the rice husk based SiO2/AgNPs composite materials is demonstrated on figure 58. 5.3. Relevance of Silica based Composites for Rapid and Facile Removal of Aqueous Mercury Bylkyldak Storage Lake Pavlodar is a major industrial center in north-east Kazakhstan with heavy engineering machinery, aluminiumand chemical plants, an oil refinery and several power stations. The city has a population of approx. 300,000 and is one of the main ports on the 4248 km long river Irtysh connecting China, Kazakhstan and Siberian Russia (Figure 59 a). Balkyldak received industrial wastewater from various enterprises (mainly from PCP, but also from an oil-refinery, a tractor plant and power station ash dumps) from about 1970, and Hg-containing wastewater from 1975 when the chlor-alkali shop started production. Apart from Hg, wastewaters contained other metals (e.g. Zn, Cr, and Fe), sulphate, chloride and organic pollutants. According to unofficial estimates, Balkyldak currently contains 55 million m3 of settled waste, out of a total capacity of 74 million m3 (FASEP-Kazakhstan, 2000). In the second half of 1999 alone it is thought to have received more than 790,000 m3 of wastewater from various industries, containing ~100 t of chloride, ~60 t of sulphate, and 3.4 kg of Hg (FASEP-Kazakhstan, 2000). At present wastewater discharges from PCP are thought to be minor, since due to the economic climate most of the industrial processes are now out of operation. The European Union (EU) maximum permissible concentrations for mercury in potable and wastewater are 0.001 and 0.005 mg/L respectively. The maximum concentration of mercury recommended by the World Health Organization (WHO) is 1 ppb (1 ppb=0,001 mg/L). According to International medical and environmental organizations Mercury has been recognized as one of the most dangerous chemical pollutants. Previously, mercury is widely used in industry, agriculture,

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scientific experiments and daily life. Awareness of «the danger of mercury» was due to several large man-made environmental disasters experienced by mankind in the twentieth century.

Figure 59 – a) Map showing the location of the study area in Kazakhstan; b) Map of the Pavlodar Northern Industrial Zone with key study areas indicated; c) Sampling locations for surface sediment samples and sediment cores in Lake Balkyldak. The outfall pipe from the factory is indicated by a dashed and dotted line and effluents enter the lake close to point 1.

Mercury contamination Minamata Bay in Japan is the most famous of them. This disaster has caused an epidemic of new toxicological serious illness called «Minamata disease».

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Japan is the unique country in the world, which have now been completed large-scale operation to eliminate mercury pollution and its consequences. This process continued for nearly 50 years and was about $ 2 billion. Kazakhstan is one of the few countries performing demercurization large projects (in the cities of Pavlodar and Temirtau). Table 18 Volume of production of caustic soda, and the actual consumption of mercury in the PO «Khimprom», Pavlodar Year

Release 100% NaOH, ton / year

Specific consumption mercury kg / ton

1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 Total, tn

17775 9575 28285 36600 43082 42363 59338 48935 55510 66600 57464 38234 57954 61060 62750 685525

1,29 2,09 5,05 1,4 2,6 1,86 1,504 1,296 1,485 1,3 1,743 2,6 0,838 0,74 0,74 Average 1,589

Mercury consumption for one year, ton. 22,930 20,011 142,839 51,240 112,013 78,795 89,334 63,420 82,432 86,580 100,160 99,408 48,565 45,184 46,435 1089,356

The actual proportion of mercury consumption in the PO «Khimprom», Pavlodar was 1.589 kg/t, while the «scientificbased standard» regulated the mercury consumption of 0.3 kg/t, «technically reasonable rate» ‒ 0.5 kg/t, and «routine» ‒ 0.76 kg/m (Table 18).

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5.4. Collection of “real” water samples and lake sediments from the lake-reservoir Bylkildak, Pavlodar According to the project «Hyperstiochiometry Activity in Metal Nanoparticle Interaction (HYPER Activ), SOE 2015 009, Nazarbaev University from 20 to 26 August 2017 complex scientific research expedition of the research project “Hyperstoichiometry Activity in Metal Nanoparticle Interaction” along the route Astana-Lake-reservoir Balkyldak (Pavlodar-city) was fulfilled. During the field trip to the Pavlodar environmental samples of water and lake-bed sediments of the Balkyldak lake-reservoir were collected in order to evaluate environmental situation in region and for the future research with the “real” mercuric water samples. The lake-reservoir Bulkuldak was built in 1973 (simultaneously with the site of the former Pavlodar Chemical Plant, succeed to the JSC “Kaustik”) with the sole purpose of collecting industrial, household, drainage and storm sewage from the entire territory of the Northern industrial zone of the city of Pavlodar (Figure 60). The reservoir is under the responsibility of the Pavlodar city Akimat, and JSC “Kaustik” has no obligations related to its management.It is known that it is contaminated not only with mercury, but also with other heavy metals, sulfates, chlorides and organic substances. It was an excellent opportunity to collect aqueous, sediment, soil and microbiological samples of the heavily polluted area.

Figure 60 – The Bulkuldak lake-reservoir appearance, view on the Pavlodar prom zone

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5.5. Materials and methods 5.5.1. Materials and chemicals Triethoxysilane, glacial acetic acid, silver nitrate, mercury chloride were purchased from Sigma Aldrich and used without further purification. Rice husk (Almaty region, Kyzylorda region, Turkystan region) was used as the main raw material for synthesizing silicon oxide by thermal treatment methods. The mercury nitrate solution and microbiological samples collected on the Balkyldak Lake-reservoir were used in adsorption experiments. 5.5.2. Characterizations Fourier Transform Infrared Spectroscopy (FTIR) was performed using Agilent technologies, Cary 600 series FTIR spectrometer in transmission (T) mode at mid IR, wavenumbers range 500-4000 cm-1 to conduct IR measurements. Band intensities can be also expressed in absorbance mode, in which same results are expected. The powder was then dispersed in a matrix of potassium bromide (KBr) in the ratio of ~1:10. The absorption coefficient of KBr is far less than 400 cm-1, therefore no absorption peak of KBr appeared in the range of measurement. The size and purity of obtained nanoparticles were characterized by X-Ray phase analysis (XRD) using the “Dron-6” (Burevesnik). This method allows to determine structural and dimensional features, and to correlate data of this features according to synthesis conditions. Physical parameters of nitrogen adsorption/desorption for the Barret-Joyner-Halenda average pore diameter (DBJH), the BrunauerEmmett-Teller surface area (SBET) and the total pore volumes (Vtotal) was obtained by Autosorb-iQ Automated Gas Sorption Analyzer. Thermal characteristics of initial and modified samples of silica were measured by thermogravimetric analysis using TG/DSA 6000 instument (Perkin Elmer). Removal of mercury ions from aqueous solution analyzed by RA915M Mercury Analyzer with pyrolysis attachment (PYRO-915+). Transmission Electron Microscope (Libra 120 equipped with digital camera) was employed to investigate the morphology and size of the synthesized silver nanoparticles.

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5.5.3. Synthesis of silica oxide by thermal treatment The samples of rice husks were previously washed with water for the purification of the composition from foreign substances. Then the initial raw materials were dried in the laboratory drying oven at the temperature of 90 °C for 2 hours (for complete evaporation of the water in the composition). All prepared samples (50 g) were calcinated at 600 °C for 4 hours in a muffle furnace (AAF series, Carbolite (UK)) to produce white rice husk ash (WRHA). After the end of the process, all organic compounds in the rice husk are burned completely (at the temperature of 600 °C for 4 hours) and eventually the ash of white rice husks is formed. Subsequently the WRHA was mixed with 100 ml of 2M NaOH at 90 °C at continuous vigorous stirring for 2 hours in order to extract the solid silica into water soluble sodium silicate. The water soluble sodium silicate solution was filtered via the vacuum pump to remove insoluble residues. After filtration, the water soluble sodium silicate solution (the filtrate) converted into insoluble silicic acid by reaction with concentrated HCl (30 minutes, under continuous stirring). 5.5.4. Modification surface of rice husk silica samples with silicon hydride groups Batch (3 g) of the silica oxide was added into a round bottom flask equipped with a reflux condenser. The flask was placed in a water bath with constant temperature (90 °C) and solution of modifier (0.4 ml triethoxysilane (TES, Sigma Aldrich, 390143, 95%) in 60 ml of the glacial acetic acid) was added under continuous stirring. After 2 h of reaction, the mixture was cooled to room temperature and filtered. Obtained solid was dried at 90 °C. Resulting modified silica samples were used in reaction of the silver nanoparticles formation. 5.5.5. Formation of Ag nanoparticles on the surface of silica Typical procedure for silver nanoparticles formation on silica surface was as follows: 3 samples of modified silica (1.1 g. each) were immersed into silver nitrate (10 mmol∙L-1) aqueous solution at ambient temperature with different volume (22, 33, 44 ml) of silver nitrate; all experiments were carried out in the light shielded conditions to prevent the light degradation of the silver nitrate. Silver nanoparticles are formed on the surface of silica through the chemical reduction of

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silver ions into zero-valence state as result of reaction with siliconhydride groups on the silica surface (Figure 61). The obtained samples were filtered and dried for 12 h at 105 °C in the bench oven. The amount of was estimated according to the following equation (2) and obtained data was given in Table 19: Formula where, m is the mass of deposited Ag nanoparticles on the silica surface (g). Table 19 The amount of deposited Ag nanoparticles on the surface of silica № №1 №2 №3

Description

Ag NPs mass. On silica surface, g

Ag NPs with content of 0.2 mmol Ag/g SiO2 Ag NPs with content of 0.3 mmol Ag/g SiO2 Ag NPs with content of 0.4 mmol Ag/g SiO2

Figure 61 – Silver nanoparticles generation on the surface of modified silica substrate

0.034 0.051 0.067

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5.6. Results and discussion 5.6.1. Analysis of samples by the method of low-temperature adsorption-desorption of nitrogen N2 adsorption-desorption measurement were performed to characterize the textural properties of initial and TES-modified silica. Silica nanoparticles were obtained by thermal treatment of rice husk (RH) followed by separation of silicon dioxide from the ashes of rice husks (RHA) code-named RHA-Si and subsequent modification by silane code-named RHA-Si/HSi. Table 20 contains the data of low-temperature nitrogen adsorption in accordance with the sample coding. Adsorption data for the silica samples were used to calculate the specific surface area, pore diameters and total pore volume by the BET method. Texture characteristics of samples Sample RHA-Si RHA-Si/HSi

SA., m2/g 393 230

Vpore, cm3/g 0.347 0.246

Table 20 dpore, nm 3.1 3.3

It follows from Table 20 that the RHA-Si/HSi sample has a smaller specific surface area (230 m2/g) than the initial RHA-Si sample (393 m2/g), which is due to the modification of the sample by the silane, which partially clogs the silica surface.

Figure 62 – Nitrogen adsorptiondesorption isotherm of initial RHA-Si sample

Figure 63 – Nitrogen adsorptiondesorption isotherm of a silane-modified RHA-Si/HSi sample

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This is confirmed by a decrease in the pore volume of the modified sample (0.246 cm3/g) compared to the initial silica (0.347 cm3/g). Figures 62 and 63 show that the adsorption-desorption isotherms of the samples of RHA–Si and RHA-Si/HSi refer to the Type I according to the IUPAC (Appendix D). According to the low-temperature nitrogen adsorption-desorption data of the obtained samples, it can be asserted that these samples are macroporous. 5.6.2. Thermal analysis of initial and modified samples of silica samples The thermograph of the initial silica sample from the rice husks code-named RHA-Si is shown in Figure 64.

Figure 64 – Thermograph of RHA-Si sample

From the thermograph it follows that when the sample was heated to 950 °C, a monotonic mass loss occurred throughout the entire time period. In the first heating section up to 100 °C the sample loses 2 % of the mass due to evaporation of water from the structure of the sample, which is evident from the energy consumption curve (red line). Further, there is a gradual loss of mass, but starting from 500 °C, a sharp consumption of energy begins and ends at 800 °C, which indicates that the unburned sodium salt is melted in the structure of the silica. At the end of the thermal analysis for a sample of silica from

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rice husk, it was found that the sample is heat-resistant and the weight loss is 9.1 %.

Figure 65 – Thermograph of RHA-Si/HSi sample

The data of the thermograph of the RHA-Si/HSi sample (Figure 65) indicate a low mass loss (no more than 2.5 %) due to the evaporation of water at a temperature of 66 °C. Then there was a linear loss of mass without consumption or release of energy, but from 500 °C to 900 °C an endothermic reaction occurred without a sudden jump in mass loss, which indicates an intrastructural change in the sample of the material. The total weight loss did not exceed 14 %. 5.6.3. FTIR analysis of initial and modified samples of silica samples The FT-IR spectra of (a) the unmodified silica, (b) the TES-modified silica and silver NPs decorated silica were shown in figure 66. Silicon hydride groups anchored to the surface of silica particles possess weak reducing properties, which are sufficient for generating “chemically pure” zero-valent silver by the reduction of silver cation according to Equation (3). ≡ SiH+Ag + +2H 2 O →≡ SiOH +Ag 0 +H 3O + +1/2 H 2

(3)

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Figure 66 – FTIR spectra for silica from rice husk (a), after TES modification (b) and silver nanoparticles attachment (c)

In the IR spectra of Silica, stretching vibrational bands of Si-O can be observed at 1860 cm -1 (Figure 66a). After the modification of silica with triethoxysilane, the IR spectra contained an intense band with an absorption maximum at 2260 cm-1. This band corresponds to Si–H bond stretching vibrations in surface chemical compounds (Figure 66b). A broad absorption band at 3300–3800 cm–1 is evidence of the presence of adsorbed water and perturbed ≡ SiOH groups in the surface layer. After modified silica was brought in contact with a solution of silver nitrate, and silver nanoparticles were reduced in the surface layer of silica matrices (Figure 66c).

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Silver Nanoparticles generation 5.6.4. Effect of Silver NPs concentration Transmission Electron Microscope (TEM) was employed to provide further insights into the influence of the concentration on the size and morphology of silver nanoparticles. Figure 67 shows that all the generated nanoparticles have near-spherical shape with varying sizes. It is also seen from the images that the particles are not aggregated with each other agglomerated but spread over the surface of the silica. The reason is that Ag NPs appears only at those sites where the silicon hydride groups are present due to highly reactive properties of the latter component. Moreover, the surface density of the SiH groups are small which also prevents the agglomeration and the stability of the generated nanoparticles results in the higher surface area of the obtained material compared with bulk scale silver. a) Ag NPs with content of 0.2 mmol Ag/g SiO2

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b) Ag NPs with content of 0.3 mmol Ag/g SiO2

c) Ag NPs with content of 0.4 mmol Ag/g SiO2

Figure 67 – TEM images of silver nanoparticles synthesized based on different reaction times

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5.6.5. Characterization of composites SiO2/AgNPs Analysis of nanocomposite speciation AgNPs/SiO2 with silver nanoparticles (AgNPs) content equal to 0.2, 0.3 and 0.4 mmol/g. Figures 68, 69 and 70 showed, that XRD patterns of all samples contain five major peaks at 38°, 44.2°, 64.3°, 77.3° and 81.4° 2θ, respectively. Presence of given peaks confirms successful formation of silver nanoparticles, that have face-centered cubic crystalline structure. Average diameters for silver nanoparticles were calculated using XRD-graphs and their sizes in respect to the NPs loading are: 0.2 mmol/g – 50 nm, 0.3 mmol/g – 80 nm, and 0.4 mmol/g ‒ 140 nm, respectively.

Figure 68 – X-ray diffraction patterns for silver nanoparticles on silica subtrate synthesized from 0.2 mmol Ag/g SiO2 concentration of silver

Figure 69 – X-ray diffraction patterns for silver nanoparticles on silica subtrate synthesized from 0.3 mmol Ag/g SiO2 concentration of silver

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Figure 70 – X-ray diffraction patterns for silver nanoparticles on silica subtrate synthesized from 0.4 mmol Ag/gSiO2 concentration of silver

Further chemical analysis of composites obtained from modified natural materials took place in Almaty with the use of scanning electronic microscopy and energy-dispersive X-ray spectroscopy (Figures 75-78, Tables 24 and 25). The results of analysis confirm the presence of silver NPs in the composite structure and gives the content of silver in obtained composites (for sample with content of 0.4 mmol/g – 1.36 %wt; which is equal to the theoretical calculations). 5.6.6. Mercury-Adsorption Experiments 5.6.6.1 Incineration of silver nanoparticles on silica substrate with aqueous solution of mercury (Modular solution) Silver nanoparticles deposited on the silica surface were tested in reactions with mercury chloride (HgCl2), for each experiment 0.1 g of silver contаіnіng silica was placed in а conical flаsk аnd 10 ml of HgCl2 (Sigma Aldrich, M6529, ≥ 99.5%) solution (100 mg/l) was added. The mixture was continuously stirred at ambient tеmpеrаturе for 1.5 hours. After reaction, the mixture centrifuged and solution analysed for mercury content. Different concentrations of silver nanoparticles on silica substrate were synthesized and calculated stoichiometric ratio of HgII to Ag0: 0.2 mmol Ag/g SiO2, 0.3 mmol Ag/g SiO2 and 0.4 mmol Ag/g SiO2. All the obtained samples were assigned with the terms given in Table 21.

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Table 21 Different silver containing silica samples for interactions with mercury nitrate solution № 1 2 3

Sample name of silver containing silica Ag NPs with content of 0.2 mmol Ag/g SiO2 Ag NPs with content of 0.3 mmol Ag/g SiO2 Ag NPs with content of 0.4 mmol Ag/g SiO2

Interactions with mercury nitrate solution 0.1 g of 0.2 mmol of Ag/g SiO2 was mixed with 10 mL of 100mg/l of HgCl2 0.1 g of 0.3 mmol of Ag/g SiO2 was mixed with 10 mL of 100mg/l of HgCl2 0.1 g of 0.4 mmol of Ag/g SiO2 was mixed with 10 mL of 100mg/l of HgCl2

The produced silver nanoparticles immobilized on silica support were tested on removal the mercury (II) ions from aqueous solution and calculated stoichiometric ratio of HgII to Ag0 (Table 22). Removal of Hg2+ from aqueous solution №

Sample description

1 2 3

0.2 mmol Ag/g SiO2 0.3 mmol Ag/g SiO2 0.4 mmol Ag/g SiO2

Stoichiometric ratio of HgII to Ag0 0.185:1 0.12:1 0.09:1

Table 22

Concentration of Hg2+ Adsorption in water after sorption capacity (%) ng/g 17156.3 83 15557.3 84 14650.9 85

Figure 71 – The dependence of adsorption capacities and concentration of Hg2+ in water after sorption (Modular Solution)

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Removal of Hg2+ from aqueous solution analyzed by RA-915M Mercury Analyzer with pyrolyzer PYRO-915+. The concentration of Hg2+ in water before sorption was 98243.1 ng/g (Modular solution). Mercury adsorption onto hydride silica composites is a fast and efficient process, allowing the loading of up to 0.2, 0.3, 0.4 mmol of Ag/g SiO2 adsorbent with a stoichiometric molar ratio 0.185:1, 0.12:1, 0.09:1 between HgII and Ag0 on the silica surface. 5.6.6.2 Incineration of silver nanoparticles on silica substrate with aqueous solution of mercury (Real solution) The interaction of the noble nanoparticles containing nature sourced materials with “real” mercury-contain aqueous samples of the lake-reservoir Balkyldak was analyzed using Lumex RA-915+ mercury analyzer (Table 23). The initial concentration of the mercury in aqueous sample was 14836.6 ng/g. Table 23 Residual mercury concentration and adsorption rates (%) given in respect to the silver nanoparticles contain in nature sourced materials (silica’s obtained from rice husk) and time of interaction with “real” mercury containing aqueous samples of the lake-reservoir Bylkyldak Concentration Adsorption rates of mercury in water (%) after sorption, ng/l 3059.3 79.4 8354.2 43.7 5553.6 62.6 1037.6 93 766.0 94.8 2525.5 83 3292.1 77.8 2167.2 85.4 1921.3 87 1760.7 88.1 5775.1 61

№

Samples and time contacts

1 2 3 4 5 6 7 8 9 10 11

0.2 mmol Ag/g SiO2 , 15 min 0.2 mmol Ag/g SiO2 , 30 min 0.2 mmol Ag/g SiO2 , 60 min 0.2 mmol Ag/g SiO2 , 90 min 0.2 mmol Ag/g SiO2 , 120 min 0.3 mmol Ag/g SiO2 , 15 min 0.3 mmol Ag/g SiO2 , 30 min 0.3 mmol Ag/g SiO2 , 60 min 0.3 mmol Ag/g SiO2 , 90 min 0.3 mmol Ag/g SiO2 , 120 min 0.4 mmol Ag/g SiO2 , 15 min

12

0.4 mmol Ag/g SiO2 , 30 min

3074.5

79.3

13 14

0.4 mmol Ag/g SiO2 , 60 min 0.4 mmol Ag/g SiO2 , 90 min

5704.6 2441.1

61.5 83.5

15

0.4 mmol Ag/g SiO2 , 120 min

4935.9

66.7

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Figure 72 – The dependence of adsorption capacities and concentration of Hg2+ in water after sorption (Real solution)

Figure 73 – Effect of contact times on the adsorption efficiency of mercury (II) ions from aqueous solution (Real solution)

5.7. Results of SEM and EDX & TEM аnаlysіs of the silica based composite & its interaction with mercury ions After modification of the silica surface with zero-valence silver, the resulting composite structure was studied using SEM microscopy in Almaty (SEM Quanta 3D, FEI company, USA) and TEM microscopy (Libra 120 equipped with digital camera) in Astana (Figures 74, 75). As seen from the SEM images (Figures 75 and 76) after silver nanoparticles deposition on the silica surface the morphology of samples are changing as on the surface appears the zero-valence silver agglomerates. Figure 76 indicates that silver containing silica based composite interacts with mercury ions (Hg2 +).

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Figure 74 – TEM image of silica particles acting as the support site for AgNPs

Figure 75 – SEM images of silica based composite decorated with silver nanoparticles

Figure 76 – SEM images of silver containig composite after interaction with Hg2+

By comparison of the SEM images it is clearly seen that after reaction of composite sample with mercury ions the sample morphology changes dramatically; i.e. after interaction with mercury the surface becomes heterogeneous and agglomerated on the surface enlarges which can prove the formation of structures of Ag2Hg (amalgam). The graph of Energy-dispersive X-ray spectroscopy (EDX), Almaty (Figure 77) for a sample after reaction of modified silica with silver solution confirms the presence of zero valence silver on the silica surface and interpretation of the EDX graph is presented as a table (Table 24).

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Figure 77 – EDX graph of Ag0 nanoparticles on the silica support

Table 24 Results of energy-dispersive X-ray spectral microanalysis of zero-valent silver nanoparticles on the surface of silica Element OK Si K Ag L Matrix

Wt% 50.84 43.73 5.44 Correction

Atom% 66.41 32.54 1.05 ZAF

Table 24 confirms that sample contain of about 5.44 atom % of zero valence silver, 50.84 atom % of oxygen and 43.73 atom % silica. To verify a reaction of mercury ions with silica surface decorated with silver nanopatricles an EDX analysis was performed after mercury-silver interaction (Figure78) and EDX spectra was interpreted as a Table 25.

Figure 78 – Graph of EDX analysis of silver containing sample after interaction with Hg2+

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Interpretation of the EDX graph for a silver containing sample after interaction with Hg2+ Element CK OK Si K Hg M Ag L Matrix

Wt% 1.29 53.07 43.18 1.40 1.07 Correction

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Atom% 2.15 66.63 30.88 0.14 0.20 ZAF

Table 25 gives a sample composition as 1.07 atom % of zero valence silver, 53.07 atom % of oxygen, 43.18 atom % of silica, and 1.40 atom % of mercury. By the use of interpretation of the Tables 24 and 25, we can confirm the bulk scale interaction of aqueous mercury with zero-valence silver; the silver amount in samples decreases from 5.44 atom % to 1.07 atom% proves reaction of mercury ions with zero-valence silver. Conclusion 1. The adsorption of ionic mercury (II) from aqueous solution on functionalized hydride silicon materials was investigated. 2. The FTIR spectrum of a triethoxysilane surface-modified silica sample exhibits an intense absorption band at 2250 cm-1 that is typical of silicon hydride groups, in addition to the vibration bands observed for the native silica obtained from rice husk. 3. Experiments were carried out to investigate the effect of silver nitrate concentration, initial mercury concentration of the aqueous solution on mercury loading. The effectiveness of ionic mercury removal from an aqueous solution by adsorption onto silica with grafted silicon hydride groups occurs mainly by a redox mechanism: oxidation of ≡SiH groups and reduction of HgII ions. 4. Mercury adsorption onto hydride silica composites is a fast and efficient process, allowing the loading of up to 0.2, 0.3, 0.4 mmol of Ag/g SiO2 adsorbent with a stoichiometric molar ratio 0.185:1, 0.12:1, 0.09:1 between HgII and Ag0 on the silica surface.

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5. The bulk scale interaction of aqueous mercury with zero-valence silver on the silica surface was confirmed using EDX analysis. The EDX analysis detects mercury content on samples as 1.40 at. %. REFERENCES 5 [1] Mohammad Ali Azizi Ganzagh, Mardali Yousefpour, Zahra Taherian. The removal of mercury (II) from water by Ag supported on nanomesoporous silica, J Chem Biol, 2016, 9, 127-142. [2] M. Arshadi, A. Khalafi-Nezhad, H. Firouzabadi, A. Abbaspourrad. Adsorption of mercury ions from wastewater by a hyperbranched and multifunctionalized dendrimer modified mixed-oxides nanoparticles, Journal of Colloid and Interface Science, 2017. DOI: http://dx.doi. org/10.1016/j.jcis.2017.05.052 [3] Atwood, D. A., and M. K. Zaman. “Mercury removal from water.” Recent Developments in Mercury Science, Springer Berlin Heidelberg, pp. 163182 (2006). [4] Girginova, Penka I., et al. ”Silica coated magnetite particles for magnetic removal of Hg2+ from water.” Journal of colloid and interface science, 345(2), pp. 234-240 (2010). [5] Susanne M. Ullrich, Mikhail A. Ilyushchenko, Irken M. Kamberov, Trevor W. Tanton. Mercury contamination in the vicinity of a derelict chlor-alkali plant. Part I: Sediment and water contamination of Lake Balkyldak and the River Irtysh, Science of the Total Environment, 2007, 381, 1-16. [6] Schroeder, W. H., J. Munthe, and O. Lindqvist. “Cycling of mercury between water, air, and soil compartments of the environment.” Water, Air, and Soil Pollution, 48(3-4), pp. 337-347 (1989). [7] Wang, Qianrui, et al. “Sources and remediation for mercury contamination in aquatic systems – a literature review.” Environmental pollution, 131(2), pp. 323-336 (2004). [8] Yang Yu, Jonas Addai-Mensah and Dusan Losic. “Synthesis and Application of Hydride Silica Composites for Rapid and Facile Removal of Aqueous Mercury.” Sci. Technol. Adv. Mater., 13(015008), pp. 1-12 (2012). [9] Kumar S., Sangwan P., Dhankhar R. Mor V., and Bidra S. (2013) Utilization of Rice Husk and Their Ash: A Review, Research Journal of Chemical and Environmental Sciences, 5: 126-129. [10] Abo-El-Enein S.A., Eissa M.A., Diafullah A.A., Rizk M.A., Mohamed F.M. Removal of some heavy metals ions from wastewater by copolymer of iron and aluminum impregnated with active silica derived from rice husk ash // Journal of Hazardous Materials. – 2009. – Vol. 172, № 2-3. – P. 574579. [11] L. Sun and K. Gong, Silicon-based materials from rice husks and their applications, Ind. Eng. Chem. Res. 40 (2001), pp. 5861–5871. [12] Bootharaju, M. S., and T. Pradeep. “Uptake of toxic metal ions from water by naked and monolayer protected silver nanoparticles: An X-ray

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photoelectron spectroscopic investigation.” The Journal of Physical Chemistry C, 114(18), pp. 8328-8336 (2010). [13] Khunphonoi, Rattabal, et al. “Enhancement of elemental mercury adsorption by silver supported material.” Journal of Environmental Sciences, 32, pp. 207-216 (2015). [14] K.V. Katok, R.L.D. Whitby, F. Fayon, S. Bonnamy, S.V. Mikhalovsky, and A.B. Cundy. “Synthesis and Application of Hydride Silica Composites for Rapid and Facile Removal of Aqueous Mercury.” ChemPhysChem, 14(18), pp. 4126-4133 (2013). [15] Katok, Kseniia V., et al. “Hyperstoichiometric interaction between silver and mercury at the nanoscale.” Angewandte Chemie International Edition, 51(11), pp. 2632-2635 (2012).

CONCLUSION

1. The method of conversion of the rice husk into nano-SiO2 was developed and confirmed. Pre-treatment methods of rice husk are studied and the composition and content of rice husks components are determined. The main substance in the ash is amorphous silicon dioxide, the content of which differs in the studied varieties from 90.199.5 %. The obtained data indicate the existence of the dependence of the studied parameters on the plant variety and the reagent used for the treatment. 2. Different samples of amorphous silica were obtained from the rice husks by chemical reagent treatment and thermal treatment. Their elemental composition, structure and physicochemical properties are established and the dependence of the parameters on the method of production and on the plant variety is shown. 3. RHA-derived silica’s modified with silane’s were studied using method of low-temperature adsorption-desorption of nitrogen, analysis of the particle size distribution and thermal analysis. 4. It was found that when the initial samples are modified by silane’s, the specific surface area and pore volume decrease, which indicates the fixing of silane’s on the surface of the samples. 5. According to the results of thermal analysis, the initial sample of silica from rice husk loses 9.1 %, while after the modification the weight loss is 14 %. 6. Analysis of the particle size distribution for the initial and silane-modified silica’s from rice husks shows that when the sample is modified, the particle size distribution range decreases from 60-1000 μm for RHA-Si to 30-300 μm for the RHA-Si/HSi sample. Reduction of silica particles from RHA occurs in the synthesis of modified silica in high-temperature conditions and the use of glacial acetic acid, which lead to the disaggregation and stabilization of silica particles. 7. The surface characteristics of silicon dioxide samples obtained from the rice husk of different region of Kazakhstan are studied. The specific surface area is determined by the BET method (Almaty region – 465 m2/g, Kyzylorda region – 627 m2/g, Turkystan region – 150 130

Conclusion

131

m2/g). Data about the pore sizes (there are mainly micropores) are investigated. Structure of the obtained nano-SiO2 was confirmed using an XRD and SEM analyses. The broad diffused peak with maximum of intensity at 24-theta/deg was observed in XRD spectra, indicating amorphous structure and nanoscale of the obtained silica. 8. The dependence of morphology and surface characteristics of silicon dioxide samples obtained from rice husks on the method of production and on the conditions of processing raw materials is established. 9. The use of silica based-RHA as partial replacement of geopolymer concrete has been investigated in this thesis. 10. In summary, the amorphous silica-rich RH could become a potential resource of low cost precursors for the production of high value-added silica/silicon based materials for practical applications.

APPENDIX A Flowchart of the experimental procedure for the preparation of the Silica NPs from Rice Husk by different methods

132

133

Appendix

APPENDIX B Cost analysis calculations for SiO2 production by pre-treatment with hydrochloric acid are done based on the prevailing laboratory conditions as follows: The husk was purchased from rice mill located at the 3 regions of Kazakhstan (Almaty, Kyzylorda, Turkystan regions). The husk was incinerated in a muffle furnace (Laboratory type). Samples of rice husk with a length of about 5-10 mm were pre-washed with water and dried in air. The cost was arrived for the production of 1 kilogram of SiO2. a) Cost of raw material (Rice Husk): We need 715 g RH to produce 100 g of SiO2: 1 000 g – 50 KZT 715 g – х KZT / х = 35.75 KZT b) Transit cost for the husk: 0 KZT c) Cost of incineration of pre-treated rice husk: Electricity 1100 ºС – Watt (2 kW) 2h4h

0 ºС -----------/----------------/---------‒ 1100 ºС 90 600

90 ºС, 2 h (Electricity) = 2 ∙ 0.1728 kW = 0.3456 kW 600 ºС, 4 h (Electricity) = 6 ∙ 1.152 kW = 4.608 kW Total = 4.608 kW + 0.3456 kW = 4.9536 kW Total electricity: 4.9636 kW ∙ 10.87 KZT = 53.95 KZT Electrical energy required to produce 1 kg of SiO2 in muffle furnace: 809.25 KZT d) Cost of chemical reagents (NaOH, HCl) and water: We need 114.2 g NaOH to prepare 100 g of SiO2: 1 000 000 g – 105 000 KZT 114.2 g – х KZT / х = 119.91 KZT

134

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

We need 1.3 liters of HCl to prepare 100 g of SiO2. The cost of HCl is 162.7 KZT. 671 liter – 84 000 KZT 1.3 liter – х KZT / х = 162.7 KZT We need 37.5 liter H2O to prepare 100 g of SiO2. The cost of water is: 1000 liter – 140 KZT 35.7 liter – х KZT / х = 5 KZT e) Total cost of production for 1 kg of SiO2: Rice husk: 350.75 KZT Electricity: 809.25 KZT Sodium hydroxide: 1190 KZT Hydrochloric Acid: 813.5 KZT Water: 1785 KZT Total: 4139.25 KZT f) Net profit calculations: Market price of nano silica*: 155 $ = 49 839.23 KZT * http://www.us-nano.com/inc/sdetail/411 Therefore net profit of production for one kg of SiO2: 49 839.23 KZT – 4139.25 KZT = 45699.98 KZT = 142.13 $ Net profit: 142.13 $

135

Appendix

APPENDIX C The elemental composition of different stages of rice husks

1-sample. The filtered residue

2-sample. White rice husk (WRH)

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

3-sample. WRH+NaOH

136

4-sample. H3PO4+Ethanol

Appendix

137

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

5-sample. Hot water+H3PO4+Ethanol

138

Appendix

139

APPENDIX D Adsorption isotherms (Autosorb-iQ) Silica samples 01

140

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Appendix

141

142

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Silica samples 02

Appendix

143

144

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Appendix

Silica samples 03

145

146

Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

Appendix

147

85,746

0,127 2,064 6,489 5,077 0,155 0,342 -

Cl Zn Fe K Ca Mn Cu Cr Ti

WRH1, 500 ºC

Si

Elemental composition

0,017 2,059 10,648 4,099 0,133 0,154 -

82,890

WRH2, 500 ºC 1,003 0,086 2,118 4,768 4,764 0,232 0,205 -

86,825

WRH3, 500 ºC 0,203 0,949 4,252 4,792 0,234 0,131 -

89,439

WRH1, 550 ºC 0,136 1,436 7,419 3,191 0,111 0,059 -

87,647

WRH2, 550 ºC 0,361 0,141 3,518 6,581 5,495 0,286 0,178 -

83,439 0,044 0,585 7,665 5,942 0,265 0,071 0,013 0,093

85,321

Samples name WRH- WRH3, 1, 550 ºC 600 ºC 0,679 0,084 0,855 12,253 3,466 0,328 0,141 0,072

82,121

WRH2, 600 ºC 0,083 1,374 8,584 6,849 0,309 0,053 0,040 0,068

82,640

WRH3, 600 ºC 0,104 0,900 6,922 7,016 0,296 0,005 0,030

84,728

WRH1, 650 ºC

0,681 0,078 0,888 11,666 3,350 0,360 0,061 0,082 0,080

82,754

0,103 0,965 8,891 4,824 0,261 0,116

84,839

WRHWRH-3, 2, 650 ºC 650 ºC

Table 1 Results of X-ray fluorescent analysis of the white rice husk specimens calcinated at different temperature

APPENDIX E

148 Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

WRH-1, 700 ºC 86,527 0,625 0,118 0,593 6,484 5,379

0,213 0,060

Elemental composition Si Cl Zn Fe K Ca

Mn Cu Cr Ti

0,404 0,042 0,033 0,030

WRH-2, 700 ºC 84,267 0,019 1,012 10,607 3,585 0,264 0,062 0,062 0,047

WRH-3, 700 ºC 82,508 0,076 1,897 9,251 5,833 0,339 0,006 0,049 -

WRH-1, 750 ºC 87,663 0,031 1,431 5,645 4,835 0,613 0,243 -

0,216 0,095 0,047 -

Samples name WRH-2, WRH-3, 750 ºC 750 ºC 84,194 87,564 0,201 0,033 2,266 1,636 8,836 5,426 3,646 4,984

-

0,233 0,162

WRH-1, 800 ºC 88,974 0,120 1,069 3,635 5,806 0,452 0,149 0,053 -

WRH-2, 800 ºC 82,312 0,097 3,189 9,531 4,216

0,553 0,146 0,048 -

WRH-3, 800 ºC 80,584 0,066 2,474 8,564 7,565

Table 2 Results of X-ray fluorescent analysis of the white rice husk specimens calcinated at different temperature

Appendix

149

0,597

8,599

6,710 0,178

-

-

-

-

Fe

K

Ca Mn

Cu

Cr

Ti

S Ni

-

-

-

-

2,752 0,640

18,464

0,581

0,485

3,105

-

0,155

Cl

Zn

73,974

83,762

Si

Rice Husk-2

Rice Husk-1

Elemental composition

-

-

-

-

8,289 -

4,844

2,502

-

-

84,365

-

0,053

0,024

0,052

1,141 0,018

0,698

0,390

0,046

1,979

95,598

1,277 0,043

0,004

0,012

-

1,242 0,009

1,768

0,445

0,066

10,327

84,808

1,772 0,073

-

-

0,059

0,134 -

1,044

0,069

-

7,941

88,908

-

-

-

0,344

-

0,349

1,181

-

0,959

97,168

Samples name SiO2-1, SiO2-2, SiO2-3, RH-1, Rice without without without HCl Husk-3 HCl HCl HCl

-

-

0,011

0,315

0,210

-

3,727

-

6,095

89,642

RH-2, HCl

-

-

-

0,124

1,234 -

0,492

2,238

0,072

2,792

93,049

RH-3, HCl

-

-

-

-

-

-

-

-

1,792

98,208

-

-

-

-

-

-

0,020

0,122

0,791

-

-

-

-

0,200 -

0,087

0,001

0,047

-

99,067 99,666

SiO2-1, SiO2-2, SiO2-3, HCl HCl HCl

Table 3 Results of X-ray fluorescent analysis of different rice husk and rice husk silica specimens (Pre-treated & Untreated specimens)

150 Silica: Assessment Methods of Synthesis from Rice Husk, Main Physical ...

1,020

0,843

0,561

0,147

0,457

Fe

K

Ca Mn Cu Cr

-

0,077 -

-

-

Ti

Sr S Ni

-

0,229 0,082 0,096 -

-

-

Zn

0,296

-

98,758 97,433

Cl

Si

0,020

0,121

6,599 0,328 0,017

8,634

1,503

-

-

82,778

0,061

0,011

4,800 0,628 0,197 -

11,268

1,670

0,082

-

81,283

0,013

0,224

7,118 0,309 0,077 -

7,009

0,732

0,027

0,163

84,328

SiO22, citric acid

SiO23, citric acid

0,477 0,124

-

0,629 0,063 -

1,525

0,265

-

2,336

0,010

-

-

0,107 0,100 -

0,133

0,169

0,026

0,059

0,430

2,007 0,111 -

1,904

0,104

1,448

-

-

0,912 0,056 -

0,282

0,404

0,001

0,783 20,495 0,344

-

-

0,199 -

0,076

0,127

0,021

-

-

-

0,135 0,047 0,007 -

-

0,240

0,143

0,453

1,249 2,942 0,292

9,971

29,593 0,958 0,533 0,011

9,468

32,937

0,295

-

2,441 3,576 0,291

2,700

13,861 0,805 0,605 0,099

4,282

58,786

0,771

-

11,783

LightDarkcolored colored SiO2/1 SiO2/2 SiO2/3 Hard Hard Si-H/ Si-H/ Si-H/ Coal Coal TES TES TES Fly Fly Ash Ash

94,581 98,673 73,443 98,002 99,578 98,976 11,751

Elemental White White WRH-1, WRH-2, WRH-3, SiO2citric citric citric compoRice Rice 1, acid, acid, acid, sition Husk- Huskcitric 600 ºC, 600 ºC, 600 ºC, 2 3 acid 4h 4h 4h

Samples name

Table 4 Results of X-ray fluorescent analysis of different white rice husk and rice husk silica specimens (Pre-treated & Untreated specimens)

Appendix

151

Scientific publication

Seitkhan Azat SILICA: ASSESSMENT METHODS OF SYNTHESIS FROM RICE HUSK, MAIN PHYSICAL-CHEMICAL CHARACTERISTICS AND PRACTICAL APPLICATIONS Monograph Typesetting and cover design A. Кaliyeva IB №12447 Signed for publishing 28.12.2018. Format 60x84 1/16. Offset paper. Digital printing. Volume 9.4 printer’s sheet. 500 copies. Order №8252. Publishing house «Qazaq university» Al-Farabi Kazakh National University KazNU, 71 Al-Farabi, 050040, Almaty Printed in the printing office of the «Kazakh University» publishing house.