Remediation of heavy-metal-contaminated soils by biochar: a review

Biochar has seized interest because of its significant effect on the remediation of heavy-metal-contaminated soil. Based on research achievements of biochar remediation of heavy-metal-contaminated soils in recent years, research status and progress in this field are reviewed from several important aspects, including production and properties of biochar, remediation mechanisms, remediation effect and affecting factors. The following findings were obtained: (a) biochar has the advantages of low price, high efficiency, greenness and soil improvement. (b) The physico-chemical properties of biochar are affected by biomass materials and pyrolysis temperatures. Activation, magnetisation, oxidation and digestion can improve the adsorption of heavy metals on biochar. (c) The effect of biochar on heavy metal mobility and bioavailability includes two conflicting aspects: immobilising heavy metals to reduce bioavailability or mobilising heavy metal to increase bioavailability. (d) Biochar can be applied with other solidifying materials to enhance the strength of treated soil. (e) The adsorption mechanisms of heavy metals on biochar include physical adsorption, ion exchange, electrostatic interaction, complexation and precipitation. Finally, future research is suggested, including establishing a uniform classification criterion of biochar; exploring the efficiency of biochar on the remediation of sites containing multicontaminants; illustrating the interactions between biochar and heavy metals in the complex soil environment; expanding the research scale from laboratory- and small-scale in situ tests to large-scale and long-term studies; and carrying out solidification experiments considering mechanical properties for biochar application in soil remediation.