Facile Extraction and Characterization of Silica Nanopowder from Marine National Park beach sand via Alkali Fusion Route

 

Samuel N. Ndung’u1*, Ruth N. Wanjau1, Esther W. Nthiga2

1Department of Chemistry, Kenyatta University, P.O Box 43844-00100, Nairobi, Kenya.

2Department of Chemistry, Dedan Kimathi University of Technology, P.O Box 657-10100, Nyeri, Kenya.

*Corresponding Author E-mail: samuelndungu530@gmail.com

 

ABSTRACT:

The current study showed utilization of locally available beach sand from Marine National Park in facile extraction of highly amorphous silica via a simple alkali fusion method. The purification of the silica sand was done using 6 N HCl solution to increase its purity. The method entailed formation of sodium silicate (Na2SiO3) solution from silica sand, precipitation using HCl acid to form tetraortosilicate acid, Si(OH)4 (silica gel) which is then heated to form amorphous silica (SiO2). The percentage yield of the extracted silica was 35.0139±0.11%. The extracted silica was characterized using X-Ray Fluorescence Spectrometer (XRF), X-Ray Diffractometer (XRD) and Fourier Transform Infrared Spectrophotometer (FT-IR). The XRF characterization revealed that the extracted silica content was 94.16±0.47 % which increased from 81.32±0.81% (Silica sand). The XRD results revealed a highly crystalline quartz as the main component of silica sand. The XRD spectra of extracted silica showed a broad diffraction peak at 2θ = 21.76o revealing its amorphous nature. The average particle size of the extracted silica was 45.15nm. The FT-IR characterization of the extracted silica showed hydroxyl (-OH) in silanol (Si-OH) and siloxane (Si-O-Si) as important functional groups. The results showed a low cost technique for the production of highly pure amorphous nanosized silica as a potential mineral that can be employed in vast industrial fields.

 

KEYWORDS: Marine National Park, Alkali Fusion, extracted silica, amorphous, silica sand.

 


1. INTRODUCTION:

Silica (SiO2) is obtained naturally from minerals, biogenic marine organisms and agricultural wastes1. It is found in mainly in crystalline, gel or amorphous forms2. In the industrial fields, highly amorphous silica is widely used in vast industries such as electronics, cement, textiles, cosmetics, rubber, glass, waste water treatment, toothpaste, paint, concrete, healthcare, paper, ceramics amongst others3,4. The emerging technological inventions has heightened the demand for various forms of silica such as powder silica, precipitated silica, silica gel, fumed silica and silica sol5. This has been due to their desired features such as chemically stable, friendly to the environment, easily biocompatible, easily fabricated and highly pure for use in different industries3,6. Amorphous silica can be extracted from natural minerals and agricultural wastes such as quartz sand7, rice husk2, siliceous sands8, sugarcane bagasse9, Douiret sand10, flying ash sludge11 and mud12 amongst others.

 

The natural sand is among the most abundant materials found in beaches, seas, coastal oceans and some of the fresh shallow lakes13. It is formed mainly from weathering and erosion resulting to finer and cleaner grains of the sand14. The aggregates consist of mainly silica - based quartz (SiO2) with some crystalline such as tridymite and cristobalite15, micro- and cryptocrystalline polymorphs16, strained quartz polymorphs17 with others glassy volcanic materials18,19. Apart from the non - reactive crystalline quartz, other quartz polymorphs (opal, granite, quartzite, phyllite, quartzite, gneiss, granodiorite, chert, chalsedony and siltstone amongst others) are reported to contain active silica from silica - silicate minerals which are potential for chemical dissolution via alkali - silica reactions19. The silica sand is also reported to contain amorphous structured silica materials20 from biogenic decomposition of siliceous organisms such as diatomaceous earth21 and siliceous sponges22. The silica sand is not found in pure form but as a mixture of other oxides such as Iron (III) oxide (Fe2O3), Aluminium (III) oxide (Al2O3), Titanium (IV) oxide (TiO2), Calcium (II) oxide (CaO), Magnesium (II) oxide (MgO) and Potassium oxide (K2O) among others15

 

The quartz polymorphs (partly crystalline) lose their crystal structure when heated in strongly concentrated alkali solutions (activating agents) forming soluble silicates which are then employed to synthesize obtain pure amorphous silica23,24. This is due to their change in oxygen bridge of the siloxane (Si-O-Si) bond energies, hence allowing the bond angles to rotate freely and easily to form disordered structure with different shapes25,24. Amorphous silica has hydrophilic properties because of their attached hydroxyl group on the surface26. Various methods of extraction of highly pure amorphous silica from the natural silica sand such as electrocoagulation27, sol-gel28, hydrothermal29 and alkaline fusion30 have been reported.

 

The principle of alkali fusion extraction route is to break the chemical bonds in the silica sand using an alkaline solution such as KOH, NaOH and Na2CO3 at a certain temperature followed by the binding of silicon with oxygen to form nanosized amorphous silica30. In this research, an alkaline fusion chemical extraction method was used to obtain a nanosized amorphous silica.

 

2. MATERIALS AND METHODS:

2.1 Materials:

Hydrochloric acid (HCl) and sodium hydroxide (NaOH) analytical grade chemicals (Purity > 99.5 %), all sourced from Kobian limited, Nairobi (outlet of Sigma Aldrich), were used in the study. The solutions were prepared using distilled water at a room temperature (298 K). The equipment and instruments used in this research were Fourier Transform Infrared Spectrophotometer (IR Tracer - 100, Japan), Automated X-Ray Fluorescence Spectrometer (Bruker S1 Titan 600, Tracer 5/CTX), Distiller (WSB 14), X-Ray Diffractometer (XRD, Rigaku MiniFlex II; Tokyo, Japan), Grinding mill (Retsch SR 200), Drying oven (WTC binder FD53), Pulveriser rock grinding machine (Retsch RS 200), Magnetic stirrer with hot plate (WH240-HT) and Analytical weighing balance (ATX224 Shimadzu).

 

2.2 Sample collection and pre-treatment:

The procedure of sample collection was as described by31 with slight modifications. The sand samples were collected from the Malindi Marine National Park in Kilifi county, Kenya. The samples were obtained from a composite by mixing sub-samples (3) from a site of 0 - 15cm depth at a 5 × 5km grid sampling.  A 500.000g of this mixed composite was stored in cleaned polythene bags and tightly closed and transported to Kenyatta University laboratories. The sand was allowed to dry and visible physical impurities removed, sieved and oven - dried at 80 oC for 12hours.  The samples were then grinded into a very fine powder and kept in an airtight - labelled containers.

 

2.3 Silica extraction:

The procedure of silica extraction was performed as adopted from 32 with slight modifications. A 100.000 g of the silica sand powder was soaked in 6 N HCl solution in a glass beaker for 24 hours. The mixture was the filtrated, the residue washed with distilled water until there is no yellowish color and then dried at 105 °C to a constant weight. A 400 mL of 10 N NaOH solution was then added to the sand residue and heated at 150 oC while stirring for 4 hours. The filtrate (sodium silicate) was separated from the unreacted sand residues using Whatmann filter paper No. 42 and 6N HCl solution slowly added to form a white gel (pH 7). The reaction mixture was left overnight at room temperature, filtered and rinsed with distilled water. The gel was then dried at 105 °C to a constant weight and stored awaiting subsequent experiments. The percentage yield of silica extracted was calculated using equation 1.

 

The extracted silica was then characterized using XRF, XRD and FT-IR.

 

3. RESULTS AND DISCUSSION:

3.1 Silica extraction mechanisms:

The alkali fusion silica extraction involved preparation of sodium silicate (Na2SiO3) from silica sand using NaOH solution. This is as summarized by the equation 2.

 

The reaction mechanism of formation of sodium silicate is shown in Figure 1 below.

 

Figure 1: Reaction mechanism of sodium silicate formation

 

Sodium silicate is then reacted with HCl solution (precipitation agent) to form a precipitated tetraortosilicate acid, Si(OH)4 (silica gel) and NaCl residue which is then separated out. This is summarized by equation 3 and mechanism in Figure 2.

 

Figure 2: Reaction mechanism of tetraortosilicate acid (silica gel) formation

 

The silica gel is then heated to form amorphous silica as shown by the equation 4.

 

The reaction mechanism for the formation of amorphous silica is shown below (Figure 3).

 

Figure 3: Mechanism of silica gel heating

 

3.2 Percentage yield:

The percentage yield of the extracted silica (ES) is shown in table 1.

 

 

Table 1: Percentage yield of extracted silica

% Yield

(Mean ± S.D)

35.0139 ± 0.11

 

3.3 XRF analysis:

The XRF spectrometer (SI Titan) was used to determine the chemical composition of natural silica sand (MNPS) and extracted silica (ES) powder and results are presented in Table 2.

 

Table 2: Mean percentage of chemical composition of natural silica sand and extracted silica

Sample

Oxide (%)

SiO2

Fe2O3

ZrO2

TiO2

Al2O3

MnO2

ZnO

MNPS

81.32 ± 0.81

5.13 ± 0.25

0.89 ± 0.16

0.33 ± 0.45

2.06 ± 0.01

0.17 ± 0.06

0.08 ± 0.01

ES

94.16 ± 0.47

0.21 ± 0.09

0.34 ± 0.14

nd ± 0.34

0.05 ± 0.68

nd ± 0.04

nd ± 0.01

*nd – not detected

 

From the results in Table 2, the percentage of SiO2 in the extracted silica was 94.16 ± 0.47 %, which increased from 81.32 ± 0.81 % after alkali fusion. All the other oxides were below 1 %. This showed that alkali fusion increased not only the content of amorphous silica but also eliminated the impurities present in the silica sand7. These results are in in tandem with data reported by 30 on the extraction of silica from Bengkulu beach sand using alkali fusion method.

 

3.4 XRD analysis:

The XRD spectra for silica sand (before and after acid treatment) and extracted silica is shown by Figure 4.

 

Figure 4: The XRD pattern of MNPS before (A) and after (B) acid treatment and ES (C)

 

The X-ray diffractogram (A) in Figure 5 exhibit narrow peaks, indicating a high degree of crystallinity of the respective samples. The presence of the quartz phase in the highest relative intensity, Al2O3, Fe2O3 and ZrO2 impurities as minor phases was suggested by Match! software version 3.14 Build 238. The presence of amorphous silica materials from siliceous organisms in the silica sand is not evident in the XRD spectra when other crystalline, microcrystalline and cryptocrystalline phases are present33. All the diffraction peaks for Al2O3, Fe2O3 and ZrO2 have disappeared after acid treatment (diffractogram B) showing that acid leaching eliminated all the impurities that were present in the natural silica sand34.

 

The X-ray diffraction pattern of (diffractogram C) exhibits a broad characteristic diffraction peak at 2θ = 21.76o with low intensity which indicated amorphous nature of the extracted silica35,26. No crystalline phases or other peaks for oxides were observed in all the XRD spectra, confirming a high purity silica structure36. This proved the effectiveness of the alkali fusion extraction process. The average particle size was 45.15 nm showing that the amorphous silica was powder was in nanometer scale37.  These results are coherent to findings reported by 7 during their research study on the synthesis of silica nanopowder from Indonesian natural sand via alkali fusion route.

 

3.5 FT-IR analysis:

The extracted silica was characterized by FT-IR and results are shown by the Figure 5.

 

Figure 5: FT-IR spectra of extracted silica (ES)

 

The results in Figure 5 showed broad spectrum band at 3406.35 cm-1 characterized by (-OH) stretching vibrations of the silanol (Si-OH) or adsorbed water molecules on the surface of the extracted silica which could not be completely removed by heating34. This is confirmed by peak at 1609.78 cm-1 which are ascribed their bending vibrations30. The predominant bands 1044.47 cm-1 and 760.93 cm-1 corresponds to the asymmetric and symmetric stretching vibrations of the siloxane (Si-O-Si) bond38,39. The band observed at 457.01 cm-1 is due to bending vibrations of Si-O-Si bond in amorphous phase37.

 

CONCLUSIONS:

Natural silica sand from marine national park sand contain silica (SO2) as the main component with main impurities of Al2O3, Fe2O3 and ZrO2 as shown by the XRF and XRD results. Acid treatment of the silica sand eliminated the oxide impurities to < 1 %. The percentage yield of extracted silica (ES) via alkali fusion route was 35.0139 ± 0.11 %. The percentage of SiO2 in the extracted silica increased to 94.16 ± 0.47 % from 81.32 ± 0.81 %. The XRD spectra showed that the extracted silica was highly amorphous. The FT-IR results showed hydroxyl (-OH) in silanol and siloxane (Si-O-Si) as important functional groups in extracted silica.

 

ACKNOWLEDGEMENT:

The authors express their gratitude to Kenyatta University for laboratory space, chemicals, instruments, equipments and technical support.

 

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Received on 11.05.2023            Accepted on 24.06.2023     

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Int. J. Tech. 2023; 13(1):1-6.

DOI: 10.52711/2231-3915.2023.00001