Mineral-contaminant interactions in the presence of polystyrene microplastics: Implications for contaminant adsorption in aqueous environments.

Microplastics (MPs), particularly polystyrene microplastics (PSMPs), are emerging contaminants of global concern that can alter contaminant dynamics in aquatic systems. This study investigates how Polystyrene MPs influence the adsorption behaviour of three representative environmental contaminants-Cd(II), Cr(VI), and tetracycline (TC) onto natural clay and mineral substrates (goethite, kaolin, and bentonite). Batch adsorption experiments showed that all three minerals effectively adsorb these contaminants. However, the presence of PSMPs reduced their overall adsorption capacities by ∼13-45% for Cd(II), ∼7-11% for Cr(VI), and ∼9-12% for TC, depending on the mineral system. Kinetic modelings revealed distinct behaviours between pollutant classes. The adsorption of Cd(II) and Cr(VI) followed a mixed kinetic control governed by both surface reaction and intraparticle diffusion. In contrast, tetracycline uptake was best described by a revised pseudo-second-order mechanism sensitive to both solute and surface site availability. Markedly, the addition of PSMPs altered apparent adsorption rates in a highly system-specific manner, inducing, either suppression or minimal variation through a complex interplay between steric hindrance arising from site masking and a concurrent transition toward rapidly forming outer-sphere complexes. The XPS analyses suggested that, while contaminant binding in MP-free systems involved a greater contribution from inner-sphere complexation, PSMP-amended systems exhibited an increased contribution from weaker outer-sphere interactions. Although PSMP-only controls showed negligible direct contaminant uptake under the tested conditions, the presence of PSMPs altered mineral surface reactivity and aggregate structure, thereby reducing mineral-mediated contaminant retention. These findings demonstrate that PSMPs can modify contaminant partitioning and mobility in aquatic environments and highlight the importance of incorporating MP-mineral interactions into contaminant transport and environmental risk assessment frameworks.