Acid Isolation Techniques for Silica Isolation from Rice Husk and Determination of Its Physicochemical Properties

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DOI: 10.21776/ub.jpacr.2022.011.01.641 J. Pure App. Chem. Res., 2022, 11 (1), 72-82 27 April 2022 X

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Acid Isolation Techniques for Silica Isolation from Rice Husk and Determination of Its Physicochemical Properties

Siti Mutrofin,^1* Leo Krisna,¹ Diah Mardiana,¹ and Rachmat Triandi Tjahjanto,¹

1Department of Chemistry, Faculty of Mathematics and Natural Sciences, Brawijaya University

*Corresponding e-mail: [email protected] Received 01 November 2021; Accepted 27 April 2022

ABSTRACT

Previous research showed that rice husk has a high silica content so it becomes an alternative for processing rice waste. The acid leaching method is a technique to isolate silica by adding an acid solution. The research aim is to obtain the optimum silica yield and determine the chemical ash content of rice husks (rice IR-64,). Rice husks were first calcined at two various temperatures and times. The average yields obtained were 19.54% (500^oC / 1 hr), 18.42%

(500ôC / 2 hrs), 25.03% (700ôC /1 hr), and 22.4% (700ôC/2 hrs). The physical appearance of ash was grains and white (700 ôC/1hr), that were rinsed with 1 M HCl solution to remove impurities. Added 1 M NaOH was to form sodium silicate. The last step was to isolate silica using different concentrations of HCl and HNO3 (3 M, 2 M, and 1 M). The highest result was 99.87% of silica under HNO3 1 M solution. An infrared study supports that the isolated product was silica, with the presence of prominent peaks at 1102 cm^-1 (stretching Si-O) and 471 cm^-1 (bending Si-O). A unique peak at 958 cm^-1 for Si-O-Ca present at the isolated silica using HCl 3 M, gives information on inosilicate type structure. X-ray diffraction analysis with QualX application showed that silica had cristobalite and wollastonite peaks, and the value of crystallinity index was about 63.91%.

Keywords: rice IR-64, rice husk, silica

INTRODUCTION

Silica is the most abundant element in the earth's crust followed by alumina. Silica is generally formed together with oxides of magnesium, aluminum, calcium, and iron. During the long process (hydrothermal), silica is rarely found pure because of the dissolving action of water so that it will appear as quartz[1]. Silica and water have something in common, namely that they have oxygen atoms that affect the density characteristics of each element. In addition, silica and gel relate to each other through hydration reactions and dehydration reactions. Based on this reaction, silica can be used to absorb a certain amount of water as used in reducing the humidity of a box and allowing it to be reused by a dehydration reaction[2].

Indonesia produces 54,649,202.24 tons of rice in 2020 and the figure is predicted to continue to increase over time (based on data from the previous 5 years). In addition, production by-products such as rice husks will certainly be present in large quantities. Currently, rice husk is still rarely used as a source of income after being made into a more useful product[3]. In Malang, rice production can be found almost all over Malang, especially in the Malang district. However, it can be seen that the knowledge of farmers to maximize income is still low, especially for the

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utilization of all crops including the by-product that will be discussed, namely rice husk[4]. Rice husk is generally used as an energy source in the form of biological coal. However, the use of rice husk as an energy source is said to be very dangerous because it can cause carcinogenic effects and respiratory problems. The solution to abundant rice husk is to isolate silica because rice husk has a high silica content. Silica obtained from rice husk ash can be used as a mixture of building cement, adsorbents, catalysts, pesticides, fertilizers, and good insulators[5–7]. The potential of silica content as a catalyst can be used to produce biodiesel. This biodiesel comes from the conversion of methanol which is almost 100% with the addition of LiSiO³from the calcined rice husk⁸. Rice husk can be processed into silica by burning and the ash will be processed. Based on the results

of previous studies, the content of SiO2 is very rich so it must be utilized. The result of the X- Ray Diffractometer or XRD shows the content of rice husk ash consisting of SiO2 (91.45%), K2O (2.27%), P2O5 (1.6%), CaO (1.24%), SO3 (1.13% %), Cl (0.99 %), Fe2O3 (0.62 %), ZnO (0.27 %), MnO (0.24 %), TiO2 (0.12% %), and Nd2O3 (0.01%). However,further investigating is needed because without the proper method burning ash can damage human health. This occurs due to the presence of inorganic impurities such as K and Na, which can be removed by using acids such as HCl or HNO3.[7,9,10].

Based on the description above, the extraction of silica from rice husks needs to be studied further, such as the effect of temperature, type of solvent, and solvent concentration. It is hoped that this research can solve the problems mentioned above and increase the utilization of rice waste in Indonesia

EXPERIMENT

Chemicals and instrumentation

The chemicals used in this study were 37% hydrochloric acid (Merck) and 65% nitric acid (Merck). Rice husks come from the "sahabat” rice mill which is in the Karangploso district, Malang, East Java. The instruments used are FTIR-8400 Shidmadzu and XRD PANalytical.

Rice husk washing

The rice husks were firstly washed with distilled water 3 times and dried in the oven at 60^oC for 24 hours. After drying, the rice husks were grounded and sieved to the size of 200 mesh.

Then, the rice husks were soaked in a different solution, with 1 M HCl solution for 24 hours at room temperature. Furthermore, the rice husks were dried in the oven for 24 hours at a temperature of 60^oC[11].

Calcination of rice husks with temperature variations

It was about five (5.0) g of the washing product was calcined into a furnace with a temperature variation of 500^oC and 700^oC. The ash extraction was repeated 2 times for each variation of the calcination temperature. Visually analysis (visuals can be seen from the photo of the rice husk that has been taken) and infrared analysis of the Fourier transform is carried out as a first step to decide on candidate materials to be examined further[11,12].

Washing with dilute acid (HCl)

The selected husk ash was immersed in 3 M HCl(l) with a ratio (1:10) of 5.0 g of ash to 50 mL of solution. This solution was stirred for 2 hours at room temperature. The precipitate was then filtered off using filter paper. The residue (solid phase) was dried for 12 hours at a

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temperature of 120^oC. The purpose of this step is to remove impurities, especially lignin, hemicellulose, cellulose, etc[10,11].

Silica extraction by adding the sodium hydroxide

Rice husk ash that passed the selection from the previous research stage was put into a beaker of as much as 1.0 g and added with 10 mL of 1 M NaOH(l) (1:10). Then, the solution was stirred continuously at 80^oC for 1 hour. The precipitate formed was filtered and dried in the oven at 110^oC for 2 hours[11,12]

Precipitation of pure silica with various solutions

Rice husk ash that passed the selection of the previous stage was put into a beaker as much as 0.5 g, added with different treatments by adding 5 mL of HCl(l) (1:10) and HNO3(l) solution with a concentration of 1 M; 2 M; and 3 M in different be. The solution was stirred continuously for 2 hours at 80 ^oC. After that, the precipitate formed was filtered and put in the oven, until the weight obtained was constant. This step is repeated 2 times for each variation of solvent concentration[10–12]

Silica characterization

Characterization was conducted by FTIR and XRD analysis. The product of silica was characterized using Shidmadzu's FTIR-8400 that was set at a wavenumber of 400-4000 cm-1. The data obtained is the relationship between the percentage of transmittance (%T) and wavenumber.

Silica with the same amount was characterized using the PANalytical XRD at the 2θ onset was between 10-90o. The data obtained was the relationship of intensity with degree 2θ, and for more detailed analysis, the software QualX was applied.

Silica yield

The silica yield was calculated respectively from different calcination temperatures and different solvents by dividing the silica yield obtained by the rice husk ash used. The formula used is as below:

amount of silica obtained

% 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 = a𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 of 𝑟𝑟𝑦𝑦𝑟𝑟𝑦𝑦 ℎ𝑚𝑚𝑢𝑢𝑢𝑢 𝑎𝑎𝑢𝑢ℎ 𝑚𝑚𝑢𝑢𝑦𝑦𝑦𝑦

Calculation of crystallinity index and crystal size

Crystallinity index and crystal size were measured by using Origin software. The crystallinity index was calculated by dividing the peak area by the total area. The formula is as follow:

% crystallinity index

=

^{𝑝𝑝𝑦𝑦𝑎𝑎𝑢𝑢}^{𝑎𝑎𝑟𝑟𝑦𝑦𝑎𝑎} × 100%

𝑚𝑚𝑚𝑚𝑚𝑚𝑎𝑎𝑦𝑦 𝑎𝑎𝑟𝑟𝑦𝑦𝑎𝑎

The crystal size can be found using the Scherrer equation provided that the calculation results do not exceed 200 nm. The equation as follows:

𝐷𝐷 = 𝐾𝐾 λ β cos θ D = Crystal size (nm)

K = Scherrer constant (0,94 for cubic symmetry)

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λ = Lambda (1,5406)

β = FWHM (Full width at half maximum) RESULT AND DISCUSSION

Variation of calcination temperature and time

Calcination of rice husks at high temperatures aimed to remove organic impurities such as cellulose, hemicellulose, and protein. In this research, time variations of 1 hour and 2 hours were carried out with temperature variations of 500^oC and 700^oC. The silica amount was 5.0 g and the treatment is repeated 2 times. The results of the variations are shown in Table 1 below:

Table 1. Rice husk ash yield

Temperature 500^oC 700^oC

Time 1 h 2 h 1 h 2 h

Yield 19.54 % 18.42 % 25.03 % 22.4 %

Based on the results, the longer the calcination time was the less rice husk ash yielded. But, the longer time is better for removing more impurities. [5,12]. The physical form of the silica obtained from calcination variation is shown in Figure 1 below:

500^oC; 1 h Brownish

500^oC; 2 h Grey

700^oC; 1 h

700^oC; 2 h

White

White Figure 1. Pictures of calcined results with variations in temperature and time

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Silica treated at 500ôC-1 hours was brown and gray at 500ôC-2 hours. The longer the calcination time, the fewer impurities were obtained. Impurities have been reduced significantly at temperatures of 700ôC at 1 hour and 2 hours, white rice husk ash products were gained. During the stirring process of rice husk ash under treatment of 700ôC-2 hours, black grains were found attached to the magnetic stirrer (Figure 2) but these black grains were not found in rice husk ash yielded from the 700ôC 1 hour calcination. It was estimated that these grains have magnetic properties which in the previous studies, rice husks may rice husks contain iron in the form of hematite (Fe2O3) [13]. It confirmed no carbon content because all carbon atoms decomposed to CO2 at 400ôC–500ôC. The product at 700 ôC-1 hours was selected to be the best result.

Figure 2. Black particles are attached to a magnetic stirrer (left) and a clean stirrer example (right) The yield of silica with various acid solutions and concentrations

Selected rice husk ash (700^oC-1 hours) is rinsed. In this study, HCl and HNO3 solutions of 1 M, 2 M, and 3 M in order to precipitate pure silica. The silica yield obtained is shown in Table 2 below:

Table 2. Silica yield under different acid treatments

Solution HCl HNO3

Concentration (M) 1 2 3 1 2 3

Yield (%) 98.95 98.75 97.41 99.87 99.82 98.79

The yield percentage of silica using HCl solution was less than the HNO3 ones, it might be due to the NO^3- steric effect from the HNO3 solution. It contributed to increasing the number of products. The T-test shows there was no significant difference.

Characterization using FTIR

Spectra of rice husk ash were obtained at 700^oC (1 and 2 hours), RHAH 700-1, products of silica after dissolution at 1 M and 3 M HCl, and products of silica after dissolution of 1 M and 3 M HNO3. The characterization results are shown in Figures 3-6 below:

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Figure 3. FTIR of rice husk at 700^OC with different temperature. 700-1 (left) and 700-2 (right).

Figure 4. FTIR of acid leaching effect from rice husk ash from calcination at different temperatures. RHAH 700-1 (left) and RHAH 700-2 (right).

Figure 5. FTIR of HCl solution effect with different concentrations to rice husk ash.

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

466 802572

1101 1633 2360

3471

Transmittance (%T)

Wavenumber (cm^-1) Calcination at 700^oC 1 h

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

471 804

1101 1642

3483

2381

Transmitance (%T)

Wavenumber (cm^-1) Calcination at 700^oC 2 h

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

478 803566

1097 1640 2375

3463 Transmitance (%T)

Wavenumber (cm^-1)

Acid leaching for calcination at 700^oC 1 h

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

474 804

1112 1636 2309

3461

Transmitance (%T)

Wavenumber (cm^-1)

Acid leaching for calcination at 700^OC 2 h

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

469 804

1102 1644 2360

3479 Transmitance (%T)

Wavenumber (cm^-1) Effect of HCl 1 M solution

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

474 802 958

1095 3453

1646 2318

Transmitance (%T)

Wavenumber (cm^-1) Effect of HCl 3 M solution

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Figure 6. FTIR of HNO3 solution effect with different concentrations to rice husk ash.

Based on the results of FTIR characterization, functional groups can be analyzed on each rice husk ash. The functional groups are presented in Table 3.

Table 3. Interpretation of FTIR spectra from the experiment 700-1 (cm^-1) 700-2 (cm^-1) RHAH

700-1 (cm^-1)

HCl 1 M (cm^-1)

HCl 3 M (cm^-1)

HNO3

(cm1 M ^-1)

HNO3

(cm3 M ^-1) Interpretation

3471 3483 3463 3478 3453 3494 3484 C–H,

O–H (water) (45.08 %T) (28,39 %T) (36.93 %T) (34.78 %T) (37.03 %T) (30.52 %T) (34.07 %T)

2360 2381 2375 2360 2318 2325 2311 M – O /

NM – O (74.59 %T) (85,96 %T) (81.90 %T) (81.59 %T) (84.97 %T) (85.73 %T) (82.45 %T)

1633 1642 1640 1644 1646 1645 1646 C = C, C = O

(55.55 %T) (51,68 %T) (51.84 %T) (62.55 %T) (68.74 %T) (58.03 %T) (57.38 %T)

1101 1101 1097 1102 1095 1102 1104 Si – O

stretching (10.00 %T) (10 %T) (10.0 %T) (10.00 %T) (10.06 %T) (10.00 %T) (10.00 %T)

802 804 803 804 802 804 804 O – H bend

(52.31 %T) (55,34 %T) (45.34 %T) (66.20 %T) (69.53 %T) (61.48 %T) (58.78 %T)

466 471 478 469 473 471 476

Si – O bend (20.26 %T) (64,55 %T) (17.26 %T) (56.69 %T) (51.88 %T) (59.14 %T) (54.10 %T)

Based on the comparison of wavenumber and transmittance %, it can be seen that there was a change in the variation of the solution and concentration being studied. In compound codes 700-1 and 700-2, there was a decrease in transmittance on the interpretation of the

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

804 471

1102 1646

3494

2325

Transmitance (%T)

Wavenumber (cm^-1) Effect of HNO3 1 M solution

500 1000 1500 2000 2500 3000 3500

4000 0

25 50 75 100

804 476

1104 1646 3484

2311

Transmitance (%T)

Wavenumber (cm^-1) Effect of HNO3 3 M Solution

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functional groups C – H and O – H (water) at 700-2. It was concluded that the calcination time reduced lignin compounds in rice husks. At the peak of the M – O or NM (nonmetal) – O interpretation, there was an increase in transmittance at 700-2. This happened because of the influence of contamination during a long time of the calcination process. This was supported by the decrease in Si-O buckling transmittance[14].

The influence of HCl and HNO3 solutions gave the same peaks with the exception of the wavenumbers at ~ 2360 cm^-1 and ~ 1640 cm^-1. The concentration of acids affected the peaks of C = C and C = O, which showed a widening of the peaks in the HNO3 solution and HCl solution, this was because the higher the concentration, the dissociation process (separation of metals) from the metal occurs, giving rise to new M = O or C = C compounds[14].

At wave numbers, ~1100 cm^-1 and ~470 cm^-1, were attributed to Si-O functional groups both stretched and bent for all rice husk ash. In the HCl solution with a concentration of 3 M, a unique peak appeared at a wavenumber of 958 cm^-1. This peak indicates Si-O stretching and bending originated from Si-O-Ca. Based on the XRF results of previous studies, calcium and iron are contained in rice husks. Then it probably comes from wollastonite from the pyroxene group of inosilicate class[15].

Characterization using XRD

The products of 3M HCl dissolution were chosen for XRD powder analysis because there was a unique peek at 958 cm^-1 based on infrared spectra. The peak was not present in other products. That peak to was supposed to be the bending vibration of Si-O-Ca, a part of wollastonite from the pyroxene group of inosilicate class[15]. The origin and analyzed (QualX application) diffractograms were displayed in Figures 7 and 8, respectively.

140 120 100 80 60 40 20

0

20 40 60 80

2Theta

Figure 7. Raw XRD data

Intensit

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Figure 8. Combined cristobalite and Wollastonite spectra (top), crystallite spectra (middle), and Wollastonite (bottom).

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The analysis was done by examining the profile of the origin XRD spectra. Preliminary analysis results were this silica amorphous because it has a lot of noise but has a crystalline peak for further analysis[16]. The analysis was carried out using the reference peaks of crystalline SiO2 and CaSiO2. Based on the infrared analysis, it was likely to have silica and wollastonite. It was estimated that the amounts are 62.8% (SiO²) and 37.2% (CaSiO2)[17,18].

Using Origin software, the peak area could be calculated, which is then used to calculate the crystallinity index of one of the peaks (2θ = 21.97). The crystallinity index can be found by sorted peak area and then dividing by the total area. The crystallinity index was 63.91%. By using the Scherrer equation, the crystal size was obtained at 5.3571 nm. The calculations are summarized below:

% crystallinity index = ^529,,9129_829,2103× 100% = 63,91%

𝐷𝐷 = 0,94 × 1,5406

𝑟𝑟𝑎𝑎𝑦𝑦𝑦𝑦𝑎𝑎𝑚𝑚 (15,7539) × 𝑟𝑟𝑚𝑚𝑢𝑢 (21,1331/2) 𝐷𝐷 = 0,94 × 1,5406

0,275 × 0,983 𝐷𝐷 = 𝟓𝟓,𝟑𝟑𝟓𝟓𝟑𝟑𝟑𝟑 𝒏𝒏𝒏𝒏 CONCLUSION

Isolated silica from Malang’s rice husk (IR64 variety) was carried out successfully using acid methods, 1 M HNO3, which yielded about 99,87 % of silica. An infrared study supports that the isolated product was silica, with the presence of prominent peaks at 1102 cm^-1 (stretching Si-O) and 471 cm^-1 (bending Si-O). However, for XRD analysis, a product-related from 3 M HCl was selected due to the presence of a unique peak at 958 cm for Si-O-Ca, which gives information on inosilicate type structure. It was supported by the value of the crystallinity index was about 63.91%. It was estimated that the product is mainly composed of 62.8% (SiO²) and 37.2% (CaSiO2).

ACKNOWLEDGMENT

The author's gratitude to Brawijaya University especially the Department of Chemistry, Faculty of Mathematics and Natural Sciences for the support of this research project.

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