A review on magnetic biochar for the removal of heavy metals from contaminated soils: Preparation, application, and microbial response

Abstract

Magnetic biochar has recently received much attention due to its magnetic separation capabilities. Additionally, magnetic biochar makes an excellent soil amendment, making it an increasingly appealing choice for the remediation of heavy metals in contaminated soils. However, the explanation of the raw ingredients and procedures for synthesizing magnetic biochar lacks a complete analysis and review of magnetic biochar's influence factors, mechanism, and microbial response to remove heavy metals from the soil. Here, we provide a detailed summary of the raw ingredients and processing techniques needed to make magnetic biochar. Generally, magnetic biochar can be made from the same raw materials as biochar with magnetic precursors. Magnetic biochars can be produced in several techniques, including ball milling, co-precipitation, hydrothermal carbonization, microwave pyrolysis, and impregnation-pyrolysis. Then, the factors influencing the removal of heavy metal contamination by magnetic biochar in soil and the adsorption mechanism are described. The intrinsic properties of heavy metals, soil properties, magnetic biochar feedstock, pyrolysis temperature, and dosing amount significantly affected the adsorption and immobilization of heavy metals in soil by magnetic biochar. The removal mechanisms include ion exchange, surface/co-precipitation, physical adsorption, complexation with functional groups, π-π interactions, electrostatic attraction, and oxidation–reduction. The reaction of soil physicochemical properties (pH, TOC/DOC/Biomass C, etc.) and microorganisms addition were also highlighted. Finally, several helpful recommendations for the future paths of magnetic biochar research were offered, which had a small positive impact on the growth of magnetic biochar.