Enhancing soil redox dynamics: Comparative effects of Fe-modified biochar (N–Fe and S–Fe) on Fe oxide transformation and Cd immobilization

Biochar and modified biochar have gained wide attention for Cd-contaminated soil remediation. This study investigates the effects of rape straw biochar (RSB), sulfur-iron modified biochar (S–FeBC), and nitrogen-iron modified biochar (N–FeBC) on soil Fe oxide transformation and Cd immobilization. The mediated electrochemical analysis results showed that Fe modification effectively enhanced the electron exchange capacity (EEC) of biochar. After 40 days of anaerobic incubation, compared to the treatment without biochar (CK), the concentrations of CaCl2-extractable Cd in N–FeBC, S–FeBC, and RSB treatments decreased by 79%, 53%, and 23%, respectively. Compared with S–FeBC, N–FeBC significantly decreased the soil Eh and increased soil pH within the first 15 days, which could be attributed to its higher EEC and alkalinity. There is a negative correlation between the concentration of CaCl2-extractable Cd and soil pH (p < 0.01). The sequential extraction results showed that both N–FeBC and S–FeBC promoted Cd transfer from acid-soluble to Fe/Mn oxides bound fraction (Fe/Mn–Cd). N–FeBC significantly increased the concentration of amorphous Fe oxides (amFeox) from 4.0 g kg−1 in day 1 to 4.6 g kg−1 in day 15 by promoting the NO3−-reducing Fe(II) oxidation process, while S–FeBC significantly increased amFeox from 4.0 g kg−1 in day 15 to 4.8 g kg−1 in day 40 by promoting the Fe(II) recrystallization. There is a positive correlation between the concentration of amFeox and Fe/Mn–Cd (p < 0.01). The scanning electron microscopy analysis showed that Cd was bound to the amFeox coating on the surface of Fe-modified biochar. By acting as an electron shuttle, the active surface of Fe-modified biochar may serve as a hotspot for Fe transformation, which promotes amFeox formation and Cd immobilization. This study highlights the potential of Fe-modified biochar for the remediation of Cd-contaminated soils and provides valuable insights into the development of effective remediation approaches for Cd-contaminated soils.