Silica Synthesis from Oil Palm Mill Boiler Ash Under Different Concentration of NaOH and Extraction Time

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IOP Conference Series: Earth and Environmental Science

PAPER • OPEN ACCESS

Silica Synthesis from Oil Palm Mill Boiler Ash Under Different Concentration of NaOH and Extraction Time

To cite this article: Firda Ferdianita Adha and Endang Warsiki 2023 IOP Conf. Ser.: Earth Environ.

Sci. 1187 012012

View the article online for updates and enhancements.

Silica Synthesis from Oil Palm Mill Boiler Ash Under Different Concentration of NaOH and Extraction Time

Firda Ferdianita Adha¹ and Endang Warsiki¹

1Department of Agroindustrial Technology, IPB University, Bogor, Indonesia

E-mail: [email protected] and [email protected]

Abstract. Boiler ash from palm oil mill contains high SiO2compound. The boiler ash used in this study was from PKS Cikasungka PTPN VIII, Bogor. The research objectives were to (i) optimize the conditions of silica synthesis process from boiler ash, (ii) analyze the silica content of the synthesis, (iii) and analyze the performance of silica as a moisture absorber. This research was designed using a central composite design (CCD) and the variables were optimized using the response surface method (RSM), silica was synthesis using sol-gel method and silica content was measured using XRF. The performance of silica as a moisture absorber was also studied to determine the appropriate adsorption kinetics at 32%, 64%, 75%, 85% and 97% RH conditions. The results showed that the optimum process conditions that produced the highest silica yield (41.17%) were solvent concentration of 7.39 M and extraction time 78.13 minutes. The silica gel produced is a white powder with SiO2content of 79.20%. The results showed that the water vapor adsorption kinetics at 64% RH was simulated with a first order model, while other treatments with 32%, 75%, 85% and 97% RH were simulated with a second order pseudo model. At 32% RH had a higher k value of 0.086 g/g/day than the 97% RH of 0.011 g/g/day.

1. Introduction

Processing of palm oil into crude palm oil (CPO) in several industry produces a lot of waste solid in the form of biomass such as empty bunches, shells and fibers [1]. Solid waste from palm oil industry when improper handling will impact on pollution environment. Various attempts have been carried out to process and increase the economic value of waste solid palm oil [2]. The process of burning the shell and fiber in boiler produces product side in the form of fly ash and bottom ash or also known as boiler slag. Boiler ash contains silica (SiO2) who potential to be used and replace other sources of silica which comes from nature. Noted that amount of SiO2 content 2 boiler ash in the form of bottom ash by 71.14% [3] ,60.75% [4] and by 65.3% [5]. Potency development of silica from boiler ash as well based on material area silica based in industry.

Source of industrial grade silica (IGS) currently used are minerals quartz and silica sand by process calcination at high temperature, silica in the two types of raw materials crystalline so that requires more energy large process. Superiority silica from palm oil solid waste amorphous form which is more reactive (easy to react), not hard (low crystallinity), no requires a large process energy, and has a specific surface area (SSA) which is quite high, cheap and easy to find [6]. The method used for extracting silica i.e. extraction alkaline or ashing method. this method easy and relatively inexpensive [7]. This method is based on solubility of amorphous silica in solution alkali. Frequent alkaline solutions used for extraction is KOH and NaOH. Extraction of silica from ash boiler has been carried out with high yields generated by 15.87% [8]. Therefore yield is still low and not much research has

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been done about optimization of silica extraction process from boiler ash, so that in research this is done by optimizing the extraction of silica from boiler ash with variable experiments in the form of solvent concentration NaOH and extraction time for get the optimum conditions and highest yield, analyze the purity of the silica compound generated and get an overview performance of synthesized silica as water vapor absorber.

2. Methodology

This research consists of several stages, such as material characterization, sample preparation, silica content purification, silica synthesis and optimization, silica content test, and water vapor adsorption test.

2.1. Material Characterization

Boiler ash characterization by test X-Ray Fluosrescence (XRF) to find out the mineral content, especially percentage of silica in boiler ash. Characterization of boiler ash in this research uses sources from research introduction with same raw materials.

2.2. Sample Preparation

Boiler ash samples used in this study were taken from PTPN VIII Cikasungka Palm Oil Mill, Bogor, West Java. The boiler ash produced by Cikasungka POM is the result of burning of fruit fiber and oil palm shells. First process is boiler ash sieved using a 100 mesh sieve. After that, the boiler ash was soaked in hot water for 2 hours to separate the materials other than ash so that these materials do not become impurity in the silica extraction process. The ash is then dried using oven at 80^oC for 24 hours.

2.3. Purification of Silica Content in Boiler Ash

The ash is further purified or through a leaching process with soaking use 1.2 M HCl for 24 hours to dissolve metal oxides other than SiO2 so that the levels of impurities in the ash boiler decreased. The mixture is then filtered with paper ash-free filter, then washed until neutral using distilled water. The ash boiler is then dried in the oven at 80⁰ C for 24 hours. The results of the oven that will be used for the process of silica extraction.

2.4. Synthesis Silica and Process Optimization

Silica synthesis from boiler ash carried out using the sol-gel method. Purified sample then added the NaOH solution (4M, 6M and 8M) by comparison between sample and solvent is 1:4 and then heated at a temperature of 75^o C while stirring using a magnetic stirrer with varying times (30, 60, and 90 minutes). Next, the filtrate is a sodium silicate solution separated from the residue using vacuum filters. filtrate then added slowly with 1 M HCl up to pH 7 to bind SiO2 so the solution turns white turbidity due to the formation of a precipitate in the form of white gels. Then gel let stand for 18 hours or process aging so that the gel becomes ripe or rigid. Next obtained gel filtered using a vacuum filter and washed with distilled water repeatedly to remove NaCl. Gel then dried in the oven at temperature 105^o C for 24 hours. Furthermore, dry gel crushed with mortar to obtain silica powder. Calculation of silica gel yield obtained by the formula based on [8,9]

Yield (%) = 𝑠𝑖𝑙𝑖𝑐𝑎 𝑔𝑒𝑙 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔𝑟)

𝑠𝑎𝑚𝑝𝑙𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔𝑟) x 100%

The experimental design used to determine the optimal condition for silica synthesis using the sol-gel method, namely RSM (Response Surface method). The experimental design used is CCD (Central Composite Design). This experiment consists of 13 runs with 5 reps on center point.

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2.5. Silica Content Test

Silica is synthesized under the optimal process condition testing using X-Ray Fluorescence to find out elemental composition of Si and SiO2 compounds of the silica it contains. With analysis using XRF also show the percentage of impurities that affect the purity of silica.

2.6. Performance Test of Silica as Moisture Absorber

Water vapor adsorption process using silica gel as much as 3 grams stored in a cup that is placed in a jar containing various saline solutions to set humidity in the jar based on ASTM E104 (2012). Saline solution that used include MgCl, NaNO2, NaCl, KCl, and K2SO4 for conditioning humidity 32%, 64%, 75%, 85% and 97%. From the data that obtained, then compiled the kinetic model equation and determine suitable kinetic model for the process water vapor adsorption by looking at the rate absorption by silica in various room humidity variations. Kinetic models used in research are first order, second order, pseudo first order and pseudo second order are appropriate with [10]. The linear equation of the first order of reaction and two are expressed in the formula as following:

ln qt = ln q0 – kt 1 𝑞𝑡⁄ - 1

⁄𝑞𝑜 = kt

Where qt is the mass at time t = t, qo is the mass at time t=0, k (g/g/day) is a kinetic constant and t is the time (day). While the model pseudo first-order and pseudo . kinetics second order is expressed by the equation:

ln (qe – qt) = ln qe– k1t 𝑡⁄𝑞𝑡 = ¹⁄_{𝑘2𝑞𝑒2} + ¹⁄_𝑞𝑒 t

The notation qt (mg/g) is the amount of a substance that adsorbed at time t (days), qe (mg/g) is the amount of substance that adsorbed at equilibrium, and k1 (g/g/day) is the process rate constant adsorption.

3. Results and Discussion

3.1. Chemical Content of Palm Oil Mill Boiler Ash

Research of [11] has conducted a characterization of palm oil mill boiler ash obtained from PTPN VIII Cikasungka, Bogor using X-Ray Fluorescence (XRF). The test results on the ash content of palm oil mill boilers are shown in Table 1.

Table 1. Chemical content of palm oil mill boiler ash

Component Total (%)

SiO2 52.41

K2O 14.47

CaO 7.64

Al2O3 5.10

MgO 4.54

Fe2O3 4.18

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3.2. Silica Synthesis

In this study, boiler ash was carried out by a washing or leaching process before the synthesis process was carried out. Boiler ash washing uses HCl solution which aims to dissolve other oxides besides SiO2 in the form of metal oxides such as MgO, K2O, and Ca2O so that the level of impurities decreases and the silica content increases. The choice of NaOH to extract silica in this study is because silica compounds are easily soluble in alkaline conditions, and will precipitate in acidic conditions [12]. This process aims to simplify the process dissolving SiO2 and converting silica compounds in boiler ash into a solution of sodium silicate (Na2SiO3) with the following reaction [13]. (Handayani et al. 2015):

SiO2(s) + 2NaOH (aq) Na2SiO3(aq) + H2O (l)

The reaction results obtained form sodium silicate (Na2SiO3) in liquid form. Sodium silicate solution is the basic ingredient or precursor in the manufacture of silica gel. The sodium silicate solution formed later added with 1 M HCl solution until it reaches pH 7 to bind SiO2 so that SiO2 is produced in the form of a white gel. White gel is formed because there are no more organic impurities that interfere with the isolation process [14]. The appearance of the sodium silicate solution before adding HCl solution and after adding HCl is shown in following figure

The reaction that occurs after the addition of HCl is the exchange of Na+ ions in Na2SiO3 with H+

ions in HCl [15]. The formation of silicic acid that occurs according to [12].

Na2SiO3(aq) + 2HCl(aq) H2SiO3(s) + 2NaCl(s)

H2SiO3 SiO2 + H2O(l)

The solid particles obtained are white silicic acid (H2SiO3). The addition of hydrochloric acid little by little until the pH of the solution reaches 7, causes the silicic acid to polymerize by forming siloxane bonds (Si-O-Si). The polymerization will continue until the solution forms a soft gel called a hydrogel. The formed gel was allowed to stand (aging) for 18 hours. The gel maturation process aims to make the gel stiffer, strong and also shrinks in solution. This stage is called syneresis. Synersis is a gel shrinkage process that occurs due to the evaporation of liquid from the pores. The gel formed is then washed using distilled water with the aim of removing impurities and dissolving salt ions which are by-products of the silicic acid polymerization reaction [14]. The moisture content that affects the silica is removed by a drying process using an oven at a temperature of 105^oC for 24 hours to obtain a white powdered silica called xerogel. The appearance of silica gel before drying and after drying is shown in the following figure

Figure 1. Sodium silicate solution before added HCl

Figure 2. Sodium silicate solution after added HCl

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3.3. Optimization of the Synthesis Process

The results of the silica synthesis can be seen in Table 2. The highest silica yield produced in this study was in accordance with the experimental design using Design Expert of 43.7%. Table 2 shows the highest yield at 8 M NaOH solvent concentration with an extraction time of 90 minutes. The lowest yield of several combinations of the above treatments was the treatment with 3.17 M NaOH solvent concentration and extraction time of 60 minutes.

Table 2.Yield of silica from boiler ash

Actual Coded Yield (%)

Run X1 X2

Solute Concentratio n (M)

Extraction Time

(menit) Y

1 -1 -1 4 30 10.77

2 1 -1 8 30 20.22

3 -1 1 4 90 18.37

4 1 1 8 90 43.7

5 -1.414 0 3.17 60 6.99

6 1.414 0 8.83 60 21.3

7 0 -1.414 6 17.57 13.31

8 0 1.414 6 102.43 27.96

9 0 0 6 60 38.97

10 0 0 6 60 38.55

11 0 0 6 60 38.98

12 0 0 6 60 39.01

13 0 0 6 60 38.73

The optimum process conditions were determined based on the best model, analysis of variance (ANOVA), and determination of the stationary point. The best model is determined based on the value of the SMSS (sequential model sum of squares) which is significant or the lowest (P<0.05), the value of lack of fit which is not significant (P>0.05), the highest R² and adjusted-R² values and or the difference between the two values. the smallest, and the lowest PRESS (prediction residual error of sum square) [16].

Based on the results of the analysis using Design Expert 11.1.2 Trial (Table 3), the recommended model is quadratic. The determination of the model is based on the SMSS (sequential model sum of squares) parameter value of 0.0002 which indicates the model is significant (p <0.05) which indicates the model is appropriate. The SMSS value of the quadratic model is the lowest among other models,

Figure 3. Silica gel before drying Figure 4. Silica gel after drying

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this indicates that the error rate or the probability of error from a quadratic model is less than 5% . The value of R² shows how much influence the independent variables used in the experiment are able to influence the results of the experiment. The higher the R² value, the better the model [16]. The R² value of the experimental results was obtained at 0.9450 which indicated that the solvent concentration of NaOH and extraction time had an effect on 94.50% of the response, while the remaining 5.50%

were other variables that were not applied in this study. Another variable that can be a factor in influencing the silica yield is the extraction temperature. According to [17], the silica yield will increase with increasing temperature in the extraction process. The adjusted R² value of the quadratic model has a value that is closest to its R², which is 0.9057, it shows that the quadratic model is the model that best fits the variables used. The greater the adjusted R², the better the R² value [18]. The PRESS value of the quadratic model is the lowest at 774.57, meaning that the quadratic model has the ability to predict the best response with the lowest error rate. The lack of fit value for the quadratic model is significant, which is <0.0001 (P>0.05) indicating that there is a discrepancy between the silica yield response data and the model so that this model still has drawbacks when used as a prediction. This is not a problem because the model obtained is still good in describing the yield.

Table 3. Parameter values for determining the best model for silica yield response Model

SMSS (p-

value) R2 Adjusted

R2 PRESS Lack of fit

(p-value) Description Linear 0.1075 0.3599 0.2318 2130.94 < 0.0001 - 2FI 0.5101 0.3917 0.1889 2295.97 < 0.0001 - Quadratic 0.0002 0.9450 0.9057 774.57 < 0.0001 Suggested Cubic 0.3207 0.9651 0.9163 4418.25 < 0.0001 Aliased

The quadratic model as the recommended model is then analyzed using the Analysis of Variance (ANOVA) to determine the relationship between several variables, namely the concentration of NaOH solvent and extraction time. The results of ANOVA in this study are presented in the following table.

Table 4. ANOVA Results Sumber Sum of

squares df Mean of squares

F Value P Value

Prob>F Description

Model 1874.32 5 374.86 24.06 0.0003 Significant

A-Solvent

Concentration 378.36 1 378.36 24.29 0.0017 B-

Extraction Time

335.38 1 335.38 21.53 0.0024

AB 63.04 1 63.04 4.05 0.0842

A² 823.89 1 823.89 52.89 0.0002

B² 405.81 1 405.81 26.05 0.0014

Residual 109.05 7 15.58

Lack of Fit 108.89 3 36.30 900.20 < 0.0001 Significant

Pure Error 0.1613 4 0.0403

Cor Total 1983.37 12

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ANOVA results show the effect of each factor to the response silica yield. F value calculated on the model equation is 24.06 shows significant value. Parameter significant influence of a factor is the magnitude of the p-value "Prob>F" must be less than 5% (0.0003) which means that the resulting model significant. In analysis of variance (ANOVA), the most important factor affect the yield response silica is a quadratic interaction solvent concentration with value Fcount of 52.89. The greater the value of F-count, the greater the effect real. These factors have an effect significant to the yield response silica marked with p-value “Prob>F” smaller than 0.05 (0.0002). Solvent concentration (A), time extraction and quadratic extraction time (B2) also has a significant effect on the response of silica yield. Score interaction of solvent concentration and time extraction (AB) is not significant because is at a value of 8% (0.0842).

3.4. Performance of Silica as a Moisture Absorber

The silica gel produced in this study was tested for performance to determine the adsorption power of H2O by silica gel so that it is known how far or how effective the ability of silica gel in absorbing silica gel is.water vapor when applied in packages with certain RH conditions. The amount of water vapor adsorbed for a certain time can be seen in Figure 5. Figure 5 shows that the adsorbed water vapor increases with time at 64%, 75%, 85% and 97% RH conditions until it reaches the equilibrium point. However, at 32% RH, silica gel run into weight loss. According to [19], the decrease in weight is caused by the value of the water content of the materialwhich is higher than the environmental RH.

Water vapor can move from the surroundings to the product until an equilibrium condition is reached. The attainment of the equilibrium condition is indicated by a constant weight of the product [20]. Figure 5 also shows the greater the value of RH, the amount of steam. There will also be more water in the environment where at 97% RH it can absorb up to 6.09 g water vapor/g adsorbent when it reaches equilibrium. Meanwhile, RH 85%, RH 75% and RH 64% only reached 3.07 g/g, 1.25 g/g and 0.30 g/g at equilibrium. The weight for silica at 97% RH reached equilibrium after being stored for 48 days, while silica at 32% RH reached equilibrium after being stored for 14 days. The higher the RH or aw storage value, the higher the equilibrium moisture content, the longer the time to reach equilibrium [20].

The amount of water vapor that is desorbed at RH is 32% and that is absorbed at humidity conditions of 64%, 75%, 85% and 97% by silica gel were analyzed using first-order, second-order,

7 6 5

4 ^32%

64%

3 _75%

2 ^85%

97%

1

0

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

-1 t (day)

Figure 5. Performance graph of the experimental silica absorber at various RH (g water vapor/g

adsorbent)

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pseudo-first-order and pseudo-second-order kinetic models. Data on the amount of water vapor absorbed in silica gel can be used to determine the mechanism of the absorption process and predict the absorption rate of the process [21]. Determination of the appropriate modeling for experimental data based on the relation coefficient (R²). In the application of pseudo first-order and second-order kinetic models the correlation coefficient must be high enough and qe,exp must be close enough to qe,calc [22]. The value of the adsorption rate constant (k) is a parameter adsorption kinetics which shows sooner or later the adsorption process takes place. The higher the value of k the faster the adsorption takes place [23].

Table 5. Value of the reaction rate constant (k) and the relation coefficient of the order 1

RH (%) Equation R2 K

32 ln qt = -0.003t + 1.943 0.8963 0.003

64 ln qt = 0.0059t + 1.1085 0.9823 0.059

75 ln qt = 0.0082t + 1.1863 0.9163 0.0082

85 ln qt = 0.022t + 1.3013 0.8688 0.022

97 ln qt = 0.0167t + 1.5575 0.814 0.0167

Table 6. Value of the reaction rate constant (k) and the relation coefficient of the order 2

RH (%) Equation R2 k

32 1/qt = 0.0004t + 0.1433 0.8996 0.0004

64 1/qt = -0.0019t + 0.3299 0.9791 0.0019

75 1/qt = -0.0022t + 0.3045 0.8905 0.0022

85 1/qt = -0.0048t + 0.2711 0.805 0.0048

97 1/qt = -0.0027t + 0.2119 0.6767 0.0027

Table 7. Result of the calculation of pseudo first RH

(%) Equation R2 k Qe,calc

(g/g)

Qe,exp (g/g) 32 ln(qe-qt) = -0.3086t –

1.0065

0.8979 0.3086 2.73 0.28

64 ln(qe-qt) = -0.246t – 0.6537

0.787 0.246 1.92 0.30

75 ln(qe-qt) = -0.1062t + 0.6065

0.7424 0.1062 1.83 1.25

85 ln(qe-qt) = -0.2008t + 1.9068

0.8398 0.2008 0.15 3.07

97 ln(qe-qt) = -0.1102t + 2.469

0.7222 0.1102 11.81 6.09

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Table 8. Result of the calculation of pseudo second order RH

(%) Equation R2 k Qe,calc

(g/g)

Qe,exp (g/g) 32 t/qt = -2,9598t - 9,972 0.9147 0.086 0.34 0.28

64 t/qt = 2.067t + 23.797 0.6451 0.18 0.48 0.30

75 t/qt = 0.5984t + 8.8064 0.9307 0.041 1.66 1.25

85 t/qt = 0.2208t + 2.814 0.9107 0.018 4.4 3.07

97 t/qt = 0.1298t + 1.8386 0.9696 0.011 7.07 6.09

The results of the calculations in Table 8 show that the value of qe, calc or qe calculated with the pseudo second order kinetic model is closer to the value of qe, exp than the pseudo first order kinetic model. In addition, the value of the relation coefficient (R²) in the pseudo-second order kinetic model with RH 32%, 75%, 85% and 97% higher than other kinetic models. In contrast to the case at 64%

RH, the value of the relation coefficient (R²) in pseudo-second order is the lowest and the highest is in the first-order kinetic model with a value of 0.9823 which is shown in Table 5. Therefore, the process of absorption of water vapor by silica at 32% RH, 75%, 85% and 97% correspond to the pseudo second order model, while at 64% RH it corresponds to the first order kinetic model.

From the overall treatment of humidity conditions, the higher the storage RH, the smaller the value of the water vapor absorption rate constant (k). The value of the adsorption rate constant (k) describes how fast or slow the adsorption process takes place. It can be seen in Table 10 that at 32% RH, the value of the constant (k) is higher at 0.086 g/g/day than 97% RH of 0.011 g/g/day.

3.5. Silica Content (SiO2)

Testing with X-Ray Fluoresence (XRF) on silica synthesized using NaOH and the optimum process conditions of this study aims to determine the composition of the mixture of material elements obtained. According to [24], Silica purification can be carried out by dissolving the content of elements such as Fe, Ca and Al using an acid solvent it will be advantageous to obtain silica with a pure grade because the content of elements considered as impurities can be soluble in acidic solvents while silica is insoluble. The ability of silica gel to prevent moisture by absorbing moisture/metal ions in water and air depending on the size, composition and number of impurities in the silica gel. The less impurities, silica able to bind more –OH and O from the absorbed water vapor, this causes greater absorption silica gel against water vapor [25]. The results of the XRF analysis after the synthesis can be seen in Table 9

Table 9. Results of silica gel XRF Characterization

Element Percentage (%) Compound Percentage (%)

Si 37,00% SiO2 79,20%

Na 4,60% Na2O 6,20%

Al 4,30% Al2O3 8,20%

Cl 3,40% Cl 3,40%

K 1,20% K2O 1,50%

P 0,30% P2O5 0,60%

Ca 0,10% CaO 0,20%

Fe 0,10% Fe2O3 0,20%

S 0,10% SO3 0,40%

TiO2 0,10%

ZnO 0,10%

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Table 9 shows that the SiO2 content contained in silica gel is 79.20% after purification using HCl.

Silica content in boiler ash raw material ranges from 34.988% [5], 58.02% [26] and 60.75% [4]. This indicates that the silica content increased after purification with HCl because the metal oxide content was reduced and dissolved in HCl. The results of the silica content obtained from this study are lower than some previous studies, namely the concentration of SiO2 obtained in the study of [5] by 96,129%, [27] of 81.3%, [28] of 95.33%. This is due to the presence of impurities, including Na2O, Al2O3, Cl, K2O, P2O5, CaO, Fe2O3, SO3, TiO2 and ZnO.

4. Conclusion

Silica gel has been successfully synthesized from palm oil mill boiler ash using the sol gel method and optimized using the Response Surface Method and CCD (Composite Central Design) design. The optimum process conditions were obtained using a solvent concentration of 7.39 M and an extraction time of 78.13 minutes to produce the best yield. by 41.17%. The results of XRF characterization showed that the Si and SiO2 silica gel compounds were synthesized, namely 37% and 79.20%, respectively. Performance of silica as a moisture absorbershowed that the silica gel produced was able to absorb water vapor at RH conditions of 64%, 75%, 85% and 97% which is indicated by the increase in weight with increasing storage time. Meanwhile, at 32% RH, silica gel decreased in weight due to lower RH of the material than the environmental RH so that the material would be dehydrated. Silica gel adsorption kinetics at 64% RH according to the model first order kinetics with adsorption rate of 0.059 g/g/day, while other treatments with RH 32%, 75%, 85% and 97% according to the pseudo- second-order kinetics model with adsorption rates of 0.086 g/g/day, 0.041 g/g/day, 0.018 g/g/day and 0.011 g/g/day, respectively.

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