Removal of heavy metals from soil with biochar composite: A critical review of the mechanism

Heavy metals are considered main pollutants in soil, which can be transferred to food chain through crops. So far, there have been several methods such as physical, chemical, biological and methods to remove the heavy metals from contaminated soil. These methods have their own advantages and disadvantages in varied aspects. This review briefly discussed the progress of each method for treatment of heavy metals in contaminated soil with particular focus on the use of biochar composites for removal of heavy metals. This is because biochar can be produced from renewable biomass and it contains various metals and oxygen-containing functional groups, which can remove the heavy metals from soil through physical sorption, precipitation, complexation, ion exchange and electrostatic interaction mechanisms. Depending on the type of heavy metals, different methods including doping metals or adding non-metals on the surface of biochar have been used to enhance the adsorption rate of heavy metals. The main route for removing heavy metals is via ion-exchange, while non-metals in biochar composites enhance the rate of removal through physical sorption or complexation mechanisms. The amount of metals and non-metals on the surface of biochar should be balanced for enhancing the efficiency of removing heavy metals in contaminated soil. In addition, the issues associated with the use of biochar for remedy of contaminated soil were discussed in detail and recommendations for future research are proposed.