Chloride-induced electron enrichment strategy: Stabilization mechanism and efficacy of calcium/magnesium modified biochar against chromium contamination in soil.

The chromium (Cr) pollution in the soil poses serious ecological risks because hexavalent chromium (Cr) has high mobility and carcinogenicity, and its main source is the oxidation of the unstable form of trivalent chromium (Cr) in the soil. Traditional metal oxide-modified biochar has limited ability to passivate trivalent chromium (Cr) due to its low surface electron density. To address this issue, we propose a chlorine ion-induced electron enrichment strategy, taking advantage of the strong electrophilicity of chlorine ions (Cl). By incorporating chlorine ions into the biochar during the calcium/magnesium modification process, chlorine ions simultaneously enhance the π electron delocalization of the biochar and reduce the resistance of electron transmission, and induce the transfer of deep-layer electrons to the surface. XPS and DFT calculations show that chlorine ion doping on calcium oxide (CaO) forms charged groups, promoting the migration of electrons from deeper layers to surface oxygen atoms. In contrast, magnesium oxide (MgO) only shows a redistribution of surface charges. Therefore, the chlorine-doped calcium-modified biochar (CaBC) achieves a passivation efficiency of 80.16 % for Cr in the soil. After 42 days, the bioavailable Cr content can be reduced from 403.56 mg kg to 80.07 mg kg. This study provides a synergistic electron enrichment mechanism for stabilizing chromium at the source, offering an economically effective strategy for sustainable soil remediation.