Biosorption of heavy metals from aqueous solution by various chemically modified agricultural wastes: A review

In recent years, with the global increase in industrialization, there has been a significant increase in the amount of toxic pollutants such as heavy metals being released into water bodies. Adsorption is considered as one of the most attractive methods due to its high efficiency and ability to remove heavy metals even at low concentration. Agricultural wastes-based biosorbents have attracted great interest due to their capability to effectively remove heavy metals from wastewater. Chemical modifications on biosorbents can significantly improve the biosorption capacities and the longevity of the biosorbents. Most of the adsorption studies followed the Langmuir and Freundlich adsorption isotherm models. The pseudo-second-order kinetic model best fitted most of the adsorption kinetic studies. Moreover, ion exchange, electrostatic attraction and chelation were the governing adsorption mechanisms in most of the studies. The adsorption process was usually spontaneous and endothermic in nature. The regeneration of biosorbents was most effective when acids were used as eluents and this had allowed some biosorbents to be reused efficiently up to 10 times. To elucidate a practical perspective, analysis on column studies, adsorption performance using industrial wastewater, and cost analysis had been conducted. Cost analysis proved that agricultural wastes-based biosorbents are cheaper than traditional adsorbents such as activated carbon.