Graphical abstractPreparation and applications of iron/biochar composites in remediation of heavy metal contaminated soils: Current status and further perspectives

Biochar, as a cost-effective, efficient, and environmentally friendly material, has been widely applied in the environment. However, difficult solid-liquid separation, relatively underdeveloped pore structure, and poor mechanical properties limited its development. In recent years, to over those disadvantages, iron/biochar composites have been efficiently used for pollutant removal. Although many reports have reported the application of iron/biochar composites, none of them have yet focused on heavy metal contaminated soils. Given this, we summarized and compared the preparation approaches of iron/biochar composites in our work. The efficient application of iron/biochar composites on heavy metals removal in contaminated soils and their mechanisms of action were also analyzed. We found that chemical reduction, thermal conversion, hydrothermal carbonization, co-precipitation, ball milling, and green synthesis are commonly used methods for iron/biochar composite preparation. The iron/biochar composites have stronger application potentials compared with pristine biochar because the iron group can enrich the cation exchange capacity, specific surface area, elemental morphology, functional groups, aromatic structure, and degree of graphitization of biochar and enhance the removal ability of heavy metals from contaminated soils. However, the quantitative relationship between preparation technologies of iron/biochar composites and removal efficiency for heavy metals, their long-term remediation behavior, and practical validation remained unsolved. The joint application of iron/biochar composites and plants or soil animals may be a more effective approach to soil remediation.