Biochar as a low-cost adsorbent for aqueous heavy metal removal: A review

As an efficient and low-cost adsorbent, biochar has attracted extensive attention and research in the field of heavy metal pollution water treatment due to its superiority in heavy metal adsorption. However, research progress on the physicochemical properties of biochar, adsorption mechanism, modified biochar and the factors affecting the adsorption of heavy metal, there is still a lack of relevant information in the field application of heavy metal polluted water bodies. This review firstly summarizes the effects of physicochemical properties such as pore distribution, element composition, pH value, specific surface area, cation exchange capacity and surface functional groups on the adsorption of heavy metals by biochar under different raw materials and pyrolysis temperatures. The physicochemical properties and effects of biochar improved by physical and chemical modification methods in recent years are discussed, as well as the dosage of biochar, pH value of water, temperature of water, initial concentrations and types of other cations and heavy metals in water, ageing process of biochar and other factors affecting the removal effect of heavy metals. The different mechanisms of heavy metals adsorption by different physicochemical properties are also given, which can be mainly divided into physical adsorption, ion exchange and surface complexation, electrostatic interaction and redox effect, and precipitation effect. Finally, research gaps and uncertainties of biochar in the water treatment of heavy metal polluted water bodies are pointed out, and further research needs and future research directions are proposed.