Effect and mechanism of vernadite for red soil remediation and cadmium immobilization

Mn oxide minerals have a large specific surface area and structural vacancies, which have attracted increasing attention in the remediation of Cd-contaminated soil. Vernadite, as a kind of common Mn oxide mineral, was applied to Cd-contaminated red soil to carry out a passivation experiment, to evaluate the remediation effect on Cd. After applying 0.2-4% vernadite to Cd-contaminated red soil and passivating it for 90 d, the soil pH value increased by 0.17-1.29 units, the soil cation exchange capacity (CEC) increased by 11.86-62.25%, and the soil organic matter decreased by 3.42-31.09%. The Cd-contaminated soil with 4% vernadite had the best passivation effect on Cd, which significantly reduced the activity of Cd in the soil. The available Cd content in the soil decreased from 3.22 mg kg^-1 to 1.69 mg kg^-1. The proportion of exchangeable Cd fraction in total soil Cd decreased from 43.61% to 19.11%, while that of the Fe-Mn oxide bound Cd fraction increased from 11.42% to 25.38%. Additionally, the residual Cd fractions increased from 24.41% to 40.37%. These results suggested the application of vernadite had a positive remediation effect on soil contaminated with Cd. Vernadite can indirectly enhance Cd adsorption by altering soil physicochemical properties, whereas direct surface complexation between Mn-OH and Cd forms Mn-O-Cd. These findings offer a new approach for remediating Cd-contaminated red soil in south China, and provide theoretical support for the application of Mn oxide minerals in soil remediation.