Synthesis and characterization of oil palm frond based cryogel for heavy metals removal

Abstract

Heavy metal pollution has become one of the most serious environmental problems today and treatment for this problem, which is environmentally friendly, is vigorously sought. Realising that a large portion of the biomass from the oil palm plantation is just left to rot and returned to the field as fertiliser, this research aims to investigate and explore the preparation of cryogel from the lignin of oil palm frond (OPF). The potential of ultrasound-assisted extraction to extract lignin from OPF as compared to conventional heating was studied. The extraction was carried out using sodium hydroxide solution as a solvent and factors such as solvent concentration and extraction temperature effects were also examined. The presence of lignin in the corresponding extracts was confirmed by Fourier transform infrared spectroscopy (FTIR) analysis and thermogravimetric analysis (TGA). The extracted lignin was then reacted with furfural to synthesize resin before being freeze-dried to obtain cryogel. The cryogel was characterized for their morphology and physical properties using FTIR, field emission scanning electron microscopy, x-ray diffraction, Breuner, Emmer and Teller (BET) and TGA. The performance of cryogel towards the removal of heavy metals ions (zinc (Zn(II)), copper (Cu(II)) and chromium (Cr(VI))) via adsorption process was performed. The adsorption was also described through adsorption kinetics, isotherm and thermodynamic properties. The result obtained showed that the extraction of OPF assisted with ultrasound was more efficient in giving higher lignin yield compared to conventional heating and the optimum lignin yield was 26%. Other than that, the reaction temperature was lowered and the reaction time was also shortened when the extraction was assisted by ultrasound. From FTIR analysis, OPF lignin showed similar spectra in terms of functional groups but slightly different in intensity when compared to commercial lignin. Overall, under optimum conditions of adsorption, Zn(II) gave the highest adsorption capacity followed by Cu(II) and Cr(VI) (i.e. Zn(II)>Cu(II)>Cr(VI)). The adsorption capacity for these heavy metals were 0.995, 0.935 and 0.4895 mg/g for Zn(II), Cu(II) and Cr(VI), respectively. The experimental data were appropriately described by the Langmuir model. This was validated by the R2 values of the Langmuir model (0.999 to 0.9689) that were higher than the Freundlich model (0.8343 to 0.9539) and the Temkin model (0.8409 to 0.9502) for all the heavy metals tested. The adsorption kinetics followed the pseudo-second order. The negative values of the Gibbs free energy change indicated the adsorption was spontaneous. The negative value of enthalpy change and entropy change confirmed the exothermic nature and showed the decrement of structural changes at solid-solution interface during adsorption process, respectively. It can be concluded that the synthesized cryogel derived from OPF lignin provides a potential performance for heavy metals removal