Microplastics impair cadmium adsorption in soils via changes in dissolved organic matter and Fe/Mn oxides

The coexistence of microplastics (MPs) and cadmium (Cd) in agricultural soils has raised environmental concerns. However, the mechanisms by which MPs alter soil geochemical properties-particularly dissolved organic matter (DOM) and iron/manganese oxides-and thereby influence Cd adsorption remain unclear. We combined soil incubation, batch adsorption experiments, multi-spectroscopic characterization, and multivariate statistical analyses to investigate how MPs with varying properties affect Cd adsorption. MPs, especially biodegradable (PLA) and aged MPs, significantly reduced Cd adsorption capacity and promoted desorption. Adsorption kinetics and isotherms indicated that Cd adsorption was primarily governed by chemisorption, whereas MPs decreased adsorption rates and capacities. XPS and 2D-FTIR-COS revealed that oxygen-containing functional groups (C-O, -OH, OC-O) served as major Cd binding sites; MPs competed for these sites, disrupting normal coordination. Biodegradable MPs lowered soil pH, increased dissolved organic carbon, reduced DOM aromaticity and molecular weight, favoring soluble DOM-Cd complex formation and inhibiting Cd immobilization. MPs also decreased free and amorphous Fe/Mn oxides while increasing chelated forms, reducing reactive mineral surfaces for Cd sorption. Random forest, Mantel test, and PLS-PM identified SUVA₂₅₄, SUVA₂₈₀, DOC, and Fe/Mn oxides as key predictors of Cd adsorption. Collectively, MPs impair Cd immobilization through three pathways: competition for functional groups, DOM structural alteration, and Fe/Mn oxide transformation, thereby enhancing Cd mobility and environmental risk, with degradable and aged MPs showing the strongest effects. These findings provide mechanistic insights into MP-mediated Cd behavior and highlight DOM-mineral interactions in risk assessment and remediation.