Immobilization of Cadmium in Soil by the Addition of Humic Acid-Modified Montmorillonite, Sepiolite, and Albite

To support secure and sustainable agricultural production, immobilization agents were developed in this study. Montmorillonite (Mont), sepiolite (Sep), and albite (Alb) were modified with humic acid (HA) to remediate cadmium (Cd-contaminated soil. Characterization analysis showed HA-Mont, HA-Sep, and HA-Alb had greater pore diameter than the unmodified forms which may favor the immobilization of Cd. During the short-time incubation (15 days) experiment, the immobilization efficiencies of HA-Mont, HA-Sep, and HA-Alb at a 4% addition rate were 12.87%, 5.86%, and 6.20% higher than those of Mont, Sep, and Alb. The reduction in the soil DTPA-Cd content was greater under the 0.5% HA-Sep treatment (31.35%) than under the 4% HA-Mont (26.95%) and the 4% HA-Alb (15.44%) treatment. Successive BCR extractions confirmed that HA-Mont, HA-Sep, and HA-Alb promoted the transformation of unstable Cd fractions to stable Cd fractions. Application of 4% HA-Sep produced the highest immobilization effect, with a 19% decrease in exchangeable fraction and reducible fraction. The findings of the long-term incubation (120 days, 1% application rate) experiment showed that the immobilization efficiencies of HA-Mont, HA-Sep, and HA-Alb increased rapidly during the first 30 days and then gradually increased or decreased slowly. In the HT (pH = 7.46) soil, HA-Mont was found to have the highest immobilization efficiency with 25.87% at the 30th day of incubation. HA-Sep had promising potential for long-term Cd immobilization, with the highest immobilization efficiency (48.27% and 29.97%) in the LT (pH = 5.17) and MT (pH = 6.56) soil occurring on the 120th day of incubation. The increase in pH was one of the important mechanisms for Cd immobilization of the LT and MT soil. Overall, humic acid modification of minerals is a beneficial strategy for remediating Cd-contaminated soil while aligning with sustainable agricultural goals.