Use of biochar as a low-cost adsorbent for removal of heavy metals from water and wastewater: A review

In recent years, the removal of heavy metals from water and wastewater systems remains a challenging issue globally. Adsorption technology is widely used for the removal of heavy metals from aquatic systems due to several advantages including its simple operation, low cost and high efficiency. Biochar, which is an environmental benign and low-cost adsorbent, is increasingly used for remediation of heavy metal pollution in aquatic systems. This review critically analyzes recent developments on the biochar-based adsorptive removal of heavy metals from water and wastewater. Biochar is effective for the removal of several toxic metals (As, Cr and Mn) from groundwater and drinking water. Biochar also shows promising performance for the removal of various heavy metals (Cr, Pb, Cu, Cd and Zn) from wastewater. The heavy metal adsorption capacity of the biochar largely depends on the water/wastewater chemistry (pH, competing ions and initial concentrations of adsorbates), contact time between adsorbates and the adsorbent, adsorbent dose and environmental parameters (temperature). The adsorption isotherms and kinetics can be well explained by the Langmuir and pseudo-second order models, respectively. The key mechanisms which drive the heavy metal adsorption onto biochar include precipitation and ion exchange. Biochar-based adsorptive decontamination of heavy metal pollution is sustainable due to low environmental impacts, low energy consumption and low-costs. The key knowledge gaps and future research directions for improvement of biochar-based sorption capacities are highlighted. This review would help for the development of effective biochar-based adsorption technology for the removal of heavy metals from aquatic environments.