Effect and mechanism of magnesium-modified biochar from citrus peel on cadmium remediation of water, soil, and plant

Cadmium (Cd) pollution influences environmental quality and human health, and magnesium (Mg)-modified biochar can efficiently clean Cd. However, to date, the effects and mechanisms of Mg-modified biochar generated from agricultural waste on comprehensive Cd remediation in water, soil, and plant is limited. In this study, the Mg-modified and unmodified biochars were produced from citrus peels. It was found that the surface area and pore volume of modified biochars were higher than those of unmodified biochars. After Mg modification, increase in H/C, O/C, and (O + N)/C ratios indicated Mg-modified biochars had a greater proportion of aromatic structures. In aqueous experiment, maximum Cd adsorption capacity and removal rate were observed in MgBC600, which was 182.24 mg/g and 97.75%. In pot experiment, MgBC600 significantly reduced Cd concentration of pakchoi by 44.40–46.20% and enhanced biomass by 40.68–112.50%. The application of MgBC600 promoted Cd immobilization by converting bioavailable Cd into insoluble forms. Mineral precipitation was the main mechanism (63.82–86.04%) of biochars for Cd remediation. XPS and XRD analysis proved the form of precipitation was CdO, Cd(OH)₂, and CdCO₃. Therefore, this study provides a theoretical basis for the resource utilization of citrus peels and the future application of Mg-modified biochar for environmental purification and safe crop production. Cadmium (Cd) pollution influences environmental quality and human health. This is the first study to integrate multiple perspectives (the effects of water, soil, and plant) to reveal the Cd adsorption mechanism of Mg-modified biochar produced from the agricultural waste citrus peel. We pointed out that the precipitation formation of CdO, Cd(OH)₂, and CdCO₃ was the main mechanism of biochars for Cd remediation in both water and soil. This study provides a solid theoretical basis for the resource utilization of citrus peels and the future application of Mg-modified biochar for environmental purification and safe crop production (<i>e.g.,</i> vegetables).