Kinetic degradation of oil palm mesocarp fibre pyrolysis and application of biochar for removal of methylene blue

Abstract

Malaysia is currently having a big problem on agricultural waste such as residues from palm oil industry and extra attention is required to handle this issue. The waste from oil palm mills is a renewable resource that potentially can be used to produce chemical feedstocks, fuels, and adsorbents through a process called pyrolysis. This study used oil palm mesocarp fibre (OPMF) from Kulai Palm Oil Mill located in Johor, Malaysia as sample. The sample was characterized (proximate, ultimate, lignin content, pore size, functional group, surface morphology) and kinetic properties was investigated using thermogravimetric analyser. Three non-isothermal methods namely Kissinger, Ozawa-Flynn-Wall (OFW) and Kissinger-Akahira-Sunose (KAS) were used to analyse the solid-state kinetics, and data obtained were compared. The sample was heated using different heating rates of 5, 10, 15, 20, and 30 Kmin-1 ranging from 298 K to 1173 K. The thermal decomposition process revealed that three main stages existed; dehydration, active, and passive pyrolysis. The activation energy (Ea) and pre-exponential factor achieved from Kissinger, KAS and OFW methods were 159.74 kJ/mol, 161.90 kJ/mol, 163.29 kJ/mol and 1.02E+10 min-1, 1.90E+13 min-1, 3.79E+17 min-1, respectively. A variance of Ea with conversion was noticed when both KAS and OFW models were employed. This revealed that the pyrolysis of OPMF progresses through more complex and multi-step kinetics. The results achieved from Kissinger method represents the actual values of kinetic parameters which were the same for the whole pyrolysis process. KAS and OFW methods represented the apparent values of kinetic parameters. All devolatilization processes were described by first order single reaction and the results showed that values of kinetic parameters using three different methods were in good agreement. However, OFW and KAS methods were more efficient in describing the degradation mechanism of the solid-state reactions. Further pyrolysis of OPMF with certain conditions was also carried out to obtain char product for adsorption study. The dye removal performance of one selected char was investigated using methylene blue (MB) with some parameters; initial concentration of MB, biochar dosage (adsorbent), temperature, and pH. Results showed that the rate of removal of methylene blue depends on concentration of solute (MB), pH value, MB concentration and adsorbent dosage. However, temperature of surrounding did not significantly affect the adsorption efficiency. Highest adsorption capacity were recorded at 400 mg/L with 42.53 mg/g removal, at pH 12 with 97.81 % removal, and at 0.5 g adsorbent dosage with 19 mg/g at shortest time of 240 minutes. The equilibrium isotherm study for adsorption of MB showed that Temkin isotherm model was best fit amongst all isotherm models. Langmuir and Freundlich models indicate that the the heat of adsorption of all molecules in the layer linearly decreases with coverage because of adsorbent-adsorbate interactions. Lastly, pseudo-second order was found as the best fit kinetics to the MB removal by OPMF-C followed by Elovich, pseudo-first order kinetic and Weber-Morris.