Jurnal Kimia Sains dan Aplikasi 24 (2) (2021): 37-42 37

Jurnal Kimia Sains dan Aplikasi 24 (2) (2021): 37-42 »«14**-•»•?j-QJurnal KimiaH3Sains dan AplikasiISSN:1410-8917

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Jurnal Kimia Sains dan AplikasiJournal of Scientific and Applied ChemistryJournal homepage: http://ejournal.undip.ac.id/index.php/ksa

Rice Husk Demineralization: Effect of Washing Solution on ItsPhysicochemical Structure and Thermal DegradationHesti Wijayanti 3 *, Iryanti Fatyasari Nataa, Chairul Irawan 3, Rinny Jelitaa H)

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a Department of Chemical Engineering, Faculty of Engineering, Lambung Mangkurat University, South Kalimantan,Indonesia

Corresponding author: hesti.wijayanti@ulm.ac.id

https://doi.org/lo.i47io/ jksa.24.237-42

A r t i c l e I n f o A b s t r a c t

Article history:

Received: 29th December 2020Revised: 26th February 2021Accepted: 6th March 2021Online: 15th March 2021

Generally, biomass consists of various amounts of minerals. These mineralsinfluence the biomass characteristics and behavior during their use in athermochemical process such as pyrolysis. The conversion during pyrolysis andits final product will be affected. This research was carried out to study theimpact of washing treatment in water and acid solutions on the rice husk as theraw material for pyrolysis. Also, the effect of acid strength (citric acid as theweak acid while nitric acid as the strong acid ) and its concentration (1, 5, and 10wt.%) was investigated. The results confirmed from the thermogravimetry(TGA/DTG) analysis, surface analysis (SEM), and spectra (FTIR) analysisdescribe the treatment using water caused less change on the rice husk surfacestructure and its thermal degradation. However, it seems hard to reduce theminerals (proved from XRF analysis). Meanwhile, the treatment using acidssolution resulted in lower mineral composition than the rice husk withouttreatment. This result is more visible for demineralization using a 5 wt.% nitricacid solution. However, for a higher concentration (washing treatment using 10wt.% solutions of nitric acid), the degradation on rice husk structure was moreoccurred.

Keywords:rice husk; demineralization;acid; physicochemicalstructure; thermal degradation

change easily during storage due to inorganic minerals.Minerals accelerate the polymerization reaction becauseit increases the viscosity of the pyrolysis oil [5].

Many researches have been conducted to remove theminerals from biomass. The most used method forremoving the minerals is acid washing treatment. Thismethod is effective in ash removal. Vamvuka et al. [4]used HC1, CH3COOH, and HCl/HF solutions. Tan andWang [6] found the order of mineral removal using acidsolutions from white pine and rice husk wasHC1>H2S04>H3P04. They also stated that pyrolysis’s bio-oil yield increased due to the increase in the HClconcentration from 3 to 7%. Numerous previous researchstudies have investigated the efficiency of various acidsolutions during the washing process. Most of themstudied the demineralization of various biomass usingdifferent acids at fixed concentrations [4, 6, 7, 8].Meanwhile, Stefanidis et al [9] studied the effect ofstrong and weak acid at low concentrations (up to 1

1. IntroductionThe decline of fossil fuel deposits is turning

attention to find alternative energy for fossil fuelsubstitution. Biomass is one of the promising sources ofrenewable energy. Pyrolysis is a thermochemicaldecomposition of biomass results in fuel and/orchemicals at high temperature in the condition of oxygenabsence. The important reasons should be evaluated forthe effective design of pyrolysis reactor, not only on thesource of biomass, operating conditions, and reactorconfiguration [1] but also on the minerals contained andtheir raw biomass composition [2]. In general, even atlow content, these minerals, especially alkali andalkaline earth metals such as sodium, potassium,calcium, iron, and so forth, causes adverse effects suchas accumulation of fouling on the surface of equipment,thus causes the overall rate of combustion decline [3, 4].The physical and chemical properties of pyrolyzed oilresulting from the high mineral content of biomass can

Jurnal Kimia Sains dan Aplikasi 24 (2) (2021): 37-42 38wt.%) and the effect of heating during demineralization.However, so far, only a few have reported the effect ofdifferent concentrations of strong and weak acidsolutions during demineralization treatment on thebiomass’s physicochemistry. As reported in earlierresearch, acid treatments for biomass can significantlyreduce minerals. However, at the same time, it was alsoprobably washing away the acid-soluble componentssuch as hemicellulose and cellulose [10]. This processcan be observed through the change of physicochemicalstructure of biomass itself [5]. Therefore, this researchwas conducted to investigate the impacts ofdemineralization using water, nitric acid (strong acid ),and citric acid (weak acid) at different concentrations ofrice husk physicochemistry. Furthermore, the thermaldegradation of treated rice husk was observed usingthermogravimetric analysis (TGA). The weightdecomposition of a sample for the temperature change ismeasured using TGA [11]. From the TGA curve and itsderivative curve (DTG), the sample’s weight loss withincreasing temperature could be obtained [12].

2. MethodologyThis study was conducted in three stages, including

sample preparation, demineralization treatment, andsample analysis and characterization.2.1. Sample Preparation

Rice husk was obtained from Gambut, district ofBanjar, South of Kalimantan Province, Indonesia. It wasground and sieved to homogenize the particle powder to18-60 mesh. Then the powder was put in an oven at 8o°Cfor 3 hours to homogenize the sample’s initial conditionsbefore being used in the demineralization treatment.2.2. Demineralization Treatments

In the washing process, the rice husk sample (20 g)was immersed in the washing solution (200 mL). Thisstudy’s washing solutions were distilled water, citricacid solution, and nitric acid solution. The citric acidsolution concentration was 1 and 5 wt.%, while a nitricacid solution was 1, 5, and 10 wt.% of concentrations.This solution was 300 rpm magnetically stirred at roomtemperature for 2 hours. Then it was filtered in vacuumcondition. The acid solution samples were washed withdistilled water to neutralize the pH. Finally, they wereput in the oven at 8o°C for 22 h and then placed in adesiccator for further analysis.

2.3. Sample Analysis and Characterization

The proximate analysis of the treated and untreatedrice husk consists of moisture content, ash content, andthe volatile matter was conducted according to ASTMD3173, ASTM E1755, and ASTM D3175. Then, the fixedcarbon was computed by the difference. The morphologyand surface structure of samples were studied byscanning electron microscopy (SEM) Inspect S50 at 12.5kV and 1000 x magnification scale. The functionalgroup’s spectra were detected by Fourier TransformInfra-Red (FTIR) spectroscopy method using ThermoScientific Nicolet iSio, observed in the range of 500-4000 cm^1. The X-Ray Fluorescence (XRF)

PANalytical/Minipal 4 was used to determine thesamples’ composition. Thermogravimetric analysis(TGA) was observed using a Thermogravimetry analyzerfrom Linseis STA PT 1600, with approximately 15 mg ofa heated sample from room temperature to 6oo°C at aheating rate of io°C/min under 50 mL/min of nitrogenflow.

3. Results and DiscussionThe washing treatment was conducted to

demineralize the rice husk sample as the pretreatmentbefore being used as biofuel raw material via pyrolysis.This study found that the treatment changed thephysical and chemical structure of the rice husk. Besides,it affected the thermal behavior of rice husk.

3.1. Physical and Chemical Characterization of RiceHusk

Proximate analysis for treated and untreated ricehusk under free moisture conditions is presented inFigure 1.

100

Volatile matterFixed carbonAsh80 -

1 5£ FII£ ££ £1. £ £7 : £7— 60 - 7 Vil: £ £££ £ 5 £7£ ££ ££ £ £O)

7. 9. £H 40 - £ £ :£ £7. > 7. ££ £: 2 £:> V,7.£ ££ 7 7£ £7. £720 - : :£ £ £a7 £ £ ££ 7 ££a £££ £i > £ ££: ££ 27 70

/-> °'Figure 1. The proximate analysis results for untreated

and treated rice husk (dry basis).Figure 1 shows that washing treatment resulted in

lower ash content and volatile matter while increasingthe fixed carbon for all samples. After washingtreatment, the minerals content declined proportionallywith the acid’s strength and its concentration (XRFresults in Figure 4). The presence of minerals such aspotassium (K) and calcium (Ca) promotes the gas andchar formation but lowering the tar yield [13]. Also, Jianget al.[14] concluded that the alkaline earth metal such ascalcium (Ca) participated during secondary cracking oftar molecule and formation of gas-volatile. Thus,removing these minerals from the rice husk sampledecreased volatile and ash content while the fixed carbonincreased. However, the treatment in distilled waterhardly increased the content of ash compared tountreated one, possibly due to the water-insolubleorganics covering the surface [15] that is shown by whiteflake distribution that exhibited in Figure 2b SEM result.The ash removal after demineralization is calculatedfrom the percentage of ash reduction compared to theuntreated one. It was between 0.33 to 2.54%, and thehighest removal was due to the washing in 5% of nitricacid solution. After demineralization, the low value ofash removal indicated that the ash in rice husk is not

Jurnal Kimia Sains dan Aplikasi 24 (2) (2021): 37-42 39

easy to remove. Raveendran eta l.[16] also observed thatthe ash content in rice husk was hardly removed duringdemineralization. It could be because of the combinationof high lignin composition with high potassium content(can be seen in Figure 4). The potassium content alsoexplained why the volatile matter was increased due todemineralization, as this mineral influences the volatileformation [16]. Figure1also showed the highest removalwas due to the washing in 5% nitric acid solution. Itimplies the minerals that significantly affect the ashcontent much easier to remove in the strong acidcondition [6]. This result is consistent with the XRFresults explained later.

The surface morphology of the rice husk can be seenin Figure 2. Untreated rice husk performs the regularstomata and epidermis clear surface (Figure 2a). Thewater washing treatment (Figure 2b) shows a similarsurface as the untreated one, but with white flakedistribution on the surface (supporting the ash contentresult). It was caused by the water-insoluble organicscovering the surface [15]. Figure 2c to 2g present thesurface of rice husk via acid washing treatment. Washingin acid solution in Figure 2c to 2g broke the rice husk’sstomata, thus revealing a smooth epidermis [17]. Thisindicated acid washing treatment resulted indegradation on the rice husk surface. Furthermore, thedegree of damage due to the washing was significantwith the acid strength and concentration. As the acidstrength and concentration rose, the surface degradationoccurred (Figure 2g).

The transmission spectra of untreated and rice huskunder washing treatment are shown in Figure 3. Thepeak of C-H at 2750-3000 cm-1 related to cellulose-richfunctional groups. Whereas the peak of C=0 ashemicellulose contribution presents at 1730 cm 1 [18].Lignin functional groups were observed at 1600 cm-1[19].The peak of C-H of C-0 at 1000-1100 cm 1 alsocontributed to cellulose [18]. The FTIR curves in Figure 3show that cellulose and lignin functional groups for

treated and untreated rice husk were not much different.However, it was a little change in hemicellulose andcellulose functional groups. The peak associated withcellulose at 1000-1100 cm 1 for untreated rice husk isquite clear. However, it seems only slightly decline uponthe treatment with water and citric acid solution at lowconcentration (1wt.%). However, this peak is almost notclear due to the treatment using a higher concentrationof citric acid (5 wt.%) and nitric acid solution at anyconcentration. Moreover, the hemicellulose peak ataround 1730 cm 1 for water and low concentration ofcitric acid (1wt.%) is almost similar. However, there wasa noticeable change in the shape of the peaks around theband, especially after treatment with higherconcentrations of citric acid (5 wt.%) and nitric acidsolution at any concentration.These results indicate thatC=0 bonds in hemicellulose and C-H of C-0 bondsassociated with cellulose are easier to break in an acidcondition, especially at higher concentrations. Thisresult supported the SEM and the TGA/DTG results later.

Untreated RHDistilled waterCitric acid, 1%Citric acid, 5%Nitric acid, 1%Nitric acid, 5%Nitric acid, 10%

£

17301 11600

4000 3500 3000 2500 2000 1500 1000 500

Wavelength (cm 1)

Figure 3. Rice husk FTIR spectra for various washingsolutions

0

b d

f 9eFigure 2. SEM images of rice husk (1000 x magnification) for (a) untreated, and treated by: (b) distilled water, (c) 1%

citric acid, (d ) 5% citric acid, (e) 1% nitric acid, (f ) 5% nitric acid, (g) 10% nitric acid.

Jurnal Kimia Sains dan Aplikasi 24 (2) (2021): 37-42 401 0 —1 solution. This result implies the strong acid such as

nitric acid at higher concentration resulted in the mostdissolution effect to cellulose and hemicellulosecompared to water and weak acid. This trend was alsoreported in [5]. Eom et al. [10] mentioned that thecellulose and hemicellulose of poplar wood weredecreased upon the acid treatment. The reactivity ofhemicellulose and cellulose was increased due to theremoval of minerals [22]. It can be concluded thatminerals in alkali metals contributed to the dominantrole during decomposition.This result indicates that acidwashing was lowering the thermal stability of rice husk[23]. It means rice husk’s physical structure is mucheasier to decompose in acid conditions [24], especially athigher concentrations. Therefore, the total solid residuefor each washing treatment at the end of TGA analysistemperature (6oo°C) is different.

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Figure 4. Mineral contents of untreated and treated ricehusk.

The mineral contents resulted from thedemineralization process are compared to the untreatedone in Figure 4. Potassium (K) significantly decreased byabout 90% upon the treatment with distilled water,while it was removed by acid treatment. However, theCalcium (Ca) reduction in distilled water was little, asobserved by Shi et al [20]. Calcium (Ca) was slightlydecreased after washing by water. However, in acids, itfurther declined to 45-57% (from the lowest to thehighest) in these orders, 1% of citric acid, 5% of citricacid, 1% of nitric acid, and 5% nitric acid, and 10% ofnitric acid. Manganese (Mn) also performs similarbehavior, but it is totally removed using 10% of nitricacid solution. Iron (Fe) did not appear to be affected bywater leaching. However, it decreased slightly due to theacid washing, even though the reduction was constant atabout 30% across all acid types and concentrations. Thelower Fe content towards a constant value at acidleaching at any concentration indicates that the Fe in therice husk sample is in the form of hematite which hashigh stability and is partly anhydrous [4]. Water and acidsolutions had no significant effect on the Copper (Cu)content. Its content was almost the same for both treatedand untreated ones. In other words, the ability to removethe minerals using water is lower than that of acids.Furthermore, the acid strength and composition willinfluence the decrease of mineral content as the lowestmineral content was obtained by washing with the nitricacid (strong acid ) solution at a high concentration.

3.2. Thermal Degradation of Treated and UntreatedRice Husk

The TGA results in Figure 5 presents that thedecomposition of the sample can be divided into fourranges: less than ioo°C for water removal, followed by200-350°C for hemicellulose decomposition. Then,cellulose decomposed at 350-400°C and greater than400°C was lignin decomposition [21]. The range ofwater-washed and acid-washed TGA curves is not muchdifferent compared to the untreated one. However, thetotal solid residue at 6oo°C is different, proportional tothe acid strength and its contents, which shows thelowest value for the treatment with 10% of nitric acid

100

— •— UntreatedDistilled waterCitric acid, 1%Citric acid, 5%Nitric acid, 1%

• Nitric acid, 5%Nitric acid, 10%

80 -

g \A\o> 60 - \CD

$03=3

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CH 40 -

20 -

200 300 5000 100 400 600

Temperature (°C)

Figure 5. TGA plots of rice husk.Figure 6 shows the DTG plots for the untreated and

treated rice husk. The 1st and second peaks of the DTGcurve for water and acid-washed rice husk were shiftedto higher temperatures than the untreated one. The 1stpeak, nearly 28o°C, associates with the decomposition ofhemicellulose, and the 2nd peak around 350°C indicatedthe cellulose decomposition was occurred [22].

0,0

1a

-0,2 - — Untreated RHDistilled waterCitric acid, 1%

— Citric acid, 5%Nitric acid, 1%Nitric acid, 5%Nitric acid, 10%

f:\G -0,4 -

£ -0,6 - J.i ;Q

-0,8 -

-1.00 100 200 300 400 500 600

Temperature (°C)

Figure 6. DTG plots of rice husk.The maximum weight loss rate for the1st peak varied

at 0.314-0.359%/°C whereas the maximum degradationrate at the 2nd one raised to 0.748%/°C and 0.753%/°C foruntreated and water-washed, respectively. In contrast,it increased to 0.787-0.870%/°C for acid-washed thosein line with the acid’s strength and concentration. The

Jurnal Kimia Sains dan Aplikasi 24 (2) (2021): 37-42 41

peak of the DTG curve indicates that the washingtreatment makes the rice husks more thermochemicallydegraded. Also, the reactivity of hemicellulose andcellulose was increased due to the treatment [22]. Fromthe rate of degradation, it can be concluded that cellulosewas more active than hemicellulose upon the washingtreatment. Moreover, the acid strength and itsconcentration are proportional to cellulose andhemicellulose reactivity, as observed by Ansharullah etal. [25]. They found the decrease of cellulose contentsignificant to the HC1 concentration increase used duringthe delignification process. However, it decreased at 10%of nitric acid to 0.782%/°C. Possibly the highconcentration of nitric acid caused the structural change.This result implies that water and acids washingtreatment reduced hemicellulose and cellulose left in therice husk as dilute acid or base clearly hydrolyzeshemicellulose efficiently [10] (in agreement with FTIRand SEM results). Thus, washing treatment not onlyreduces the inorganic contents but also changes thebiomass composition.

4. ConclusionAcid solutions’ strength and concentration

influenced the degree of ash removal in rice husk uponthe treatment. The minerals’ content also experiencedthe same trends. They would decline as the acid becamestronger as well as the acid concentration increased. Thiswashing treatment also changed the biomasscomposition. The best result was achieved by using a 5%nitric acid solution. However, the treatment using 10%nitric acid solution results in the treatment effectdeclined, moreover the surface degradation was moreoccurred. This result implies that too high acidconcentration will be an obstacle duringdemineralization.

AcknowledgmentThe authors acknowledge financial support from the

Directorate of Research and Community Service, TheMinistry of Research, Technology and Higher Educationof Indonesia with the fund for Research University GrantYear 2018 (Number: 040/UN8.2/PL/2018).

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