Xin Jiat, Lee
(2018)
Evaluation of cost effective adsorbent and biochar from Malaysia oil palm wastes: synthesis, characterisation and optimisation studies.
PhD thesis, University of Nottingham.
Abstract
The rapid development of palm oil industry in Malaysia has generated significant amount of solid and liquid wastes, contributing to major environmental issues in the past five decades. Palm oil residues such as palm kernel shell (PKS), empty fruit bunch (EFB) and palm oil sludge (POS) are difficult to be disposed of. Thus, the potential application of the oil palm wastes for synthesis of value added products such as adsorbents for heavy metals removal and solid biochars for fuel generation, are presented in this thesis.
In the past three decades, industrialisation and urbanisation in Malaysia have led to an increase of heavy metals, such as copper, cadmium, lead, zinc, chromium and nickel, in the rivers and lakes. The presence of the heavy metals is causing harmful effects on the aquatic environment and human health, hence it is necessary to control the discharge of industrial effluent into the environment. Among various heavy metals abatement technologies, adsorption is by far the most promising technique due to its relatively easy operation and high efficiency. However, adsorption is associated with costly adsorbent, such as activated carbon which is usually made from non-renewable resource. This has motivated many researchers to investigate and develop cost effective adsorbents for the removal of heavy metals.
In this research, biosorbent was prepared from palm oil sludge. The preparation steps were relatively simple and low cost, involving mechanical treatments such as drying, milling and sieving. The POS biosorbent was tested on removal of copper (Cu2+) and cadmium (Cd2+), followed by process optimisation using response surface methodology (RSM), based on central composite design (CCD). Comparing between one-factor-at-a-time (OFAT) and RSM-CCD methods, both studies produced results which were in good agreement. The investigation was carried out to evaluate the effects of adsorbent dosage (W), initial pH, initial concentration (C0) and contact time (t), on the heavy metals removal. From optimisation study using RSM-CCD, the optimum adsorption parameters for Cu2+ removal were as follows: W = 0.3 g; pH 4.56; C0 = 200 mg L-1; t = 60 min, with maximum adsorption capacity (q) of 15.84 mg g-1, and for Cd2+ removal were as follows: W = 0.3 g; pH 5.8; C0 = 200 mg L-1; t = 60 min, with maximum q of 18.49 mg g-1. The adsorption equilibrium of Cu2+ and Cd2+ were best described by Langmuir and Freundlich models, respectively, based on the lowest sum of normalised error (SNE). The adsorption kinetic of Cu2+ and Cd2+ were best fitted with pseudo-second-order kinetic model. Thermodynamically, the adsorption processes were spontaneous, exothermic and feasible. Regeneration of POS biosorbent was carried out using hydrochloric acid (HCl) as the eluent, and the results indicated the high desorption efficiency for Cu2+ (up to 0.98) and Cd2+ (0.95) from the biosorbent, respectively.
The POS biomass was also converted to POS-char by slow pyrolysis which was subsequently used in lead (Pb2+) adsorption study. The synthesis of POS-char was optimised by RSM-CCD based on simultaneous maximisation of biochar yield and q of Pb2+. The interactive effects of nitrogen flowrate (FN2), heating rate (HR), pyrolysis temperature (Tpyro) and pyrolysis time (tpyro) on the responses were investigated. It was determined that the maximum biochar yield was 80.35 % and q was 4.11 mg g-1, formed at the following slow pyrolysis conditions: FN2 = 30 mL min-1; HR = 10 °C min-1; Tpyro = 500 °C; tpyro = 30 min. In Pb2+ adsorption study, the optimum parameters determined by RSM-CCD optimisation were as follows: W = 0.3 g, pH 3.2, C0 = 200 mg L-1 and t = 60 min, with a maximum q of 21.76 mg g-1. The adsorption equilibrium of Pb2+ was best represented by Freundlich model. This finding indicated that the sorption sites in POS-char were heterogeneous. The kinetic study revealed that at low concentrations, the kinetic of adsorption complied with pseudo-first-order model, while at high concentrations, it obeyed pseudo-second-order model. Regeneration of POS-char was successfully conducted using HCl and the adsorbent exhibited reusability up to 5 adsorption-desorption cycles, with the desorption efficiencies between 0.58 and 0.99. Beyond 3 cycles, the adsorbent showed noticeable structural damage. Overall, the adsorption of Pb2+ onto POS-char was spontaneous, exothermic and feasible.
The slow pyrolysis of PKS and EFB to biochars was investigated by simultaneously varying factors such as FN2, HR, Tpyro and tpyro. The synthesis parameters were optimised by RSM-CCD with respect to multiple responses, including biochar yield, higher heating value (HHV) and energy yield. The interactive effects of FN2, HR, Tpyro and tpyro on the three responses were in good agreement with literature data. The determined optimum conditions for PKS-char and EFB-char production by slow pyrolysis were as follows: FN2 = 30 mL min-1, HR = 18.9 – 20.0 °C min-1, Tpyro = 500.0 – 504.3 °C and tpyro = 30 min. The combustion kinetic on the optimised PKS-char and EFB-char were found to possess favourable combustion characteristics such as low activation energy (Ea), high energy yield and HHV. Overall, the combustion of PKS-char and EFB-char occurred in multi-step kinetics behaviour until burnout.
The cost analysis on synthesis of PKS-char, EFB-char, POS-char and POS biosorbent was performed based on independent case studies which considered the capital and operating costs. The results revealed that addition of the thermochemical conversion plant to existing oil palm mill was highly feasible. The unit cost for production of PKS-char, EFB-char, POS-char and POS were USD$ 3.94 kg-1, USD$ 1.21 kg-1, USD$ 2.17 kg-1 and USD$ 0.19 kg-1¸ respectively.
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