Effect of Chemical Washing Pre-treatment of Empty Fruit Bunch (EFB) biochar on Characterization of Hydrogel Biochar composite as Bioadsorbent

Hydrogel biochar composite (HBC) is a recent interest among researchers because of the hydrophilic characteristic which can adsorb chemical fluid and showed a versatile potential as adsorbent in removing hazardous material in wastewater and gas stream. In this study, the effect of chemical washing pre-treatment by using two different type of chemical agent Hydrochloric Acid (HCL) and Hydrogen Peroxide (H2O2) was analysed and investigated. The raw EFB biochar was prepared using microwave assisted pyrolysis under 1000W for 30 min under N2 flow with 150 mL/min. To improve the adsoprtion ability, the EFB biochar has been chemical washed pre-treatment with Hydrochloric Acid (HCl) and Hydrogen Peroxide (H2O2) before polymerization process with acrylamide (AAm) as monomer, N,N’-methylenebisacrylamide (MBA) as crosslinker and ammonium persulfate (APS) as initiator. The characterization has studied by using Fourier transform infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). FTIR result shows that, the formation of Raw EFB to Hydrogel Biochar Composite (Raw EFB > EFB Biochar > Treated Biochars (HCl & H2O2) > Hydrogel Biochar Composite) have changed in functional group. For DSC result it shows that the thermal behaviour of all samples is endothermic process and have high thermal resistance.


Introduction
Palm oil industry is the largest agricultural industry in Malaysia aligned with wastes and residues produced and became the major disposal problems [1]. Palm fibre (PF), palm kernel (PK), Empty Fruit Bunch (EFB), and palm oil mill effluent (POME) are the biomass produced from oil palm industry [2]. It is a good oportunity to produce more beneficial product from the agriculture waste such as biochar which is high potential in adsorption sector. Some methods were used to produced biochar they are included the fixed bed pyrolyser, carbonizer and furnace [3]. But in recent years, researchers found a new alternative technique applied for pyrolysis in producing biochar with high efficiency, energy saving, selective, no pollution and also friendly used which is by using microwave assisted pyrolysis [4]. Microwave is very effective equipment in producing chars product from biomass such as Empty Fruit Bunch or materials have high moisture content because microwave mechanism in particle heating where the electromagnetic field will penetrates the solid and interacts directly with dipoles in the chemical structure to vaporize the moisture [5]. On the other hands, Hydrogel Biochar Composite (HBC) is recent findings combination of biochar and hydrogel polymer. Hydrogel is a unique material in terms of physical and chemical properties because it is hydrophilic, swellable and modifiable composite which is good in adsorption process [6]. Then the aim of this study is to enhance the performance of biochar as bioadsorbent by chemical washing pre-treatment with Hydrochloride Acid (HCl) and Hydrogen Peroxide (H2O2) in order to remove the impurities and enlarge the porosity. Uses of dilute acid such as 0.1M HCl as chemical agent because the dilute HCl will not damage the lignocellulose [7].
Meanwhile, H2O2 is an oxidizing agent which have potential to bleach and in the same time will remove impurities and toxical element [8]. Then two types of Hydrogel Biochar Composite (HBC) were produced, which are the biochar were pre-treated with HCl (EFB-HBC H100) and the other one is biochar were pre-treated with H2O2 (EFB-HBC P100). This paper are going to compare the characteristic of EFB-HBC H100 and EFB_HBC P100. As previous sturdy, experiment by Sanyang et al. [9] was successful with 99% and 35.75 mg/g sorption capacity of zinc removal in wastewater by modified RH-HBC (hydrogel biochar composite form rice husk). Earlier researcher, Karakoryun et al. [10] has conclude that, hydrogel biochar composites has great application in removing of toxic organic species such as phenols from wastewater. As expected, the HBC will apply in industrial waste treatment for example in removing hazardous gas with high temperature. This study will compare and investigate in terms of their functional group and thermal behaviour in production of HBC.

Raw Material & Chemical
Empty Fruit Bunch (EFB) has chosen as raw material and obtained from one of the palm oil industry located in Banting, Selangor. The chemical washing agent for this study are 0.1M of dilute Hydrochloric acid (HCl) and 0.1M Hydrogen Peroxide (H2O2) for biochar washing pre-treatment and distilled water is used as diluting agent. Other chemical for HBC synthesize such as Acrylamide (AAm) as monomer, N,N′-methylenebisacrylamide (MBA) as cross-linker and ammonium persulfate (APS) as initiator are supplied from R&M chemical.

Biochar Production
Microwave assisted pyrolysis technique has applied in producing EFB biochar in this study. 200g of raw Empty Fruit Bunch (EFB) were placed in quartz reactor in the microwave pyrolyzer at 1000 W of microwave power level 30 minutes under nitrogen flow at 150 mL/min. The method to prepared EFB char by using microwave assisted pyrolysis is referring to Zakiuddin Januri et al. [5] with some modification.

Acid Washing Pre-treatment
The acid washing experiments were carried out using Hydrochloric Acid (HCl) and Hydrogen Peroxide H2O2 [10] [11]. 2 L of diluted 0.1 M HCl and 0.1 M H2O2 solutions were prepared in 2 L volumetric flask for stock chemical solution. Then 10g of EFB biochar were pre-treated with 200 mL of prepared acid solutions in a closed beaker for 6 hrs. Finally, the biochar were washed with distilled water until a neutral pH is obtained and oven dried at 80 ˚C overnights. Then pre-treated biochar named as biochar H100 and biochar P100 for HCl and H2O2 solutions respectively.

Hydrogel Biochar Production
EFB-HBC were synthesized using 1.0 g of AAm was desolved in 1.0 mL of distilled water. Then, 0.6 g of Empty Fruit Bunch (EFB) biochar and 0.001 g of MBA were added to the AAm solution. After thorough mixing, 0.2 mL of 0.1 g aqueous solution of APS was added to initiate the polymerization. The hydrogel biochar precursor solution was immediately placed into plastic mould and placed in an oven at 40 ºC for 30 min. The plastic mould containing hydrogel biochar composites were removed from the oven after 30 min and left for 24 hrs at room temperature to ensure complete polymerization and crosslinking. EFB-HB was taken from the plastic mould, cut to desired sizes and washed several times with distilled water to remove all unreacted monomers and low molecular weight polymeric matter from the hydrogel. The washed EFB-HB was first dried in air before drying in a vacuum oven at 40 ˚C for 24 hrs. Then, hydrogel biochar named as EFB-HB H100 and EFB-HB P100. This hydrogel formation method are following to procedure used by [10].

Characterization
Fourier transform infrared spectroscopy (FTIR) will be measuring and analysing the functional group of samples. This instrument also can be applied to investigate the structural changes of biochars as a function of different sample preparation (pre-treatment process). In this research, FTIR will be performed by using Perkin -Elmer Spectrum 2000 FTIR with the wavelength 4000 -600 cm -1 [12]. DSC is a thermal analysis technique that shows the relation between material's heat capacity changes by temperature or in other word is to determine the heat required to increase temperature of sample. This instrument operated with sample weight ranging 5 to 10 mg in powder form heating from 30˚C to 300˚C at a heating rate of 10˚C/min [13]. Figure 1 shows the DSC curves for all samples (Raw EFB, EFB biochar, Biochar H100, Biochar P100, EFB-HBC H100 and EFB-HBC P100). As shown in Figure 1, DSC curve for raw EFB showed a high endothermic peak that represented as the glass transition which exhibits enthalpy relaxation [14]. Because of the raw EFB only undergo a glass transition, the raw EFB were classified as amorphous substances [15]. The same result has shown for biochars (Biochar, Biochar H100 and Biochar P100) which also undergo the glass transition. Besides, comparing in glass transition temperature, from raw EFB pyrolysed became biochars, it shows that the peaks are shifted to the right from Tg = 68 ºC to Tg = 89 ºC which mean it experienced some changes after pyrolysis (EFB biochar) and increasing the thermal transition. As seen from DSC curves for treated biochars (biochar H100 and biochar P100) are maintained at the same peak as biochar. This can be concluded that, in thermal reaction, the pretreated biochars (biochar H100 and biochar P100) has nothing much different from its biochar. This is because of, high amount of volatile matter was released during pyrolysis which make the material less reactive. Then the washing pre-treatment step is just to eliminate the remaining of volatile matter inside the biochar. Moreover, three consecutive endothermic curves which means substances absorbed heat are observed for hydrogel biochars. Both of the hydrogel biochar composites (EHB-HBC H100 and EFB-HBC P100) have a quite similar thermal behaviour, which means both of HBCs have approximately same glass transition temperature, Tg around 86ºC to 89ºC and they start to degrade around 200ºC. Another two curves are represented as the decomposition of substances. Michalak et al. [16] stated on their study if there is two peak existing in DSC curves after transition phase means, they found that it lose crystallization water in two steps. Then, in this study, DSC curves shows that both of HBCs lose crystallization water in two steps at Tm1, 298ºC and Tm2, 400ºC. In addition, the present of double endothermic peak conclude that HBCs is enantiotropic and polymorphic compound [17]. Polymorphism is a single substances that exhibits multiple different crystal structures or in other words they have various melting phase transitions [18]. : Raw EFB, EFB-HBC H100, Biochar H100, EFB biochar) FTIR were used to identify the changes of functional groups before and after chemical treatment and also in Hydrogel Biochar Composites (HBCs) form [19]. Figure 2 shows FTIR spectrum for the first part of the research. 4000 Figure 3 shows the FTIR spectrum for Raw EFB, EFB-HBC P100, Biochar P100 and EFB biochar. Based on FTIR spectrum in Figure 3, it shows that, the peaks of EFB-HBC P100 show that there is low transmittance, and hence large absorption ability compared to EFB-HBC H100. 4000 Table 1.

Conclusion
Hydrogel Biochar Composites (HBCs) is a new finding in adsorption technology. By treating the EFB biochar using HCl and H2O2 shows the changes in functional group. However, nothing to compare between biochar H100 and biochar P100 because the role of chemical agents is same which is to removed toxical material. The functional group was changed because of the diminished of CO2 from its biochar in both washing pre-treatment step HCl. The Differential Scanning Calorimetry (DSC) shows that Hydrogel biochar are enantiotropic and polymorphic compound with various melting transitions. In addition, the HBCs can be exposed to high temperature to the high degradation temperature. However, the DSC results for treated biochar (biochar H100 and biochar P100) has not much different compared to biochar. This is because the biochar itself has loss reactivity from pyrolysis process. Finally, from the FTIR spectra, the peaks of EFB-HBC P100 shows there is low transmittance, and hence large absorption ability compared to EFB-HBC H100.