Polylactic acid microplastics and polyhydroxyalkanoate microplastics affect cadmium mobility by regulating sediment geochemical properties

Microplastics may affect the bioavailability of heavy metals in sediments primarily through direct adsorption. However, the potential of microplastics to alter the distribution of heavy metal chemical forms by interfering with the geochemical properties of sediments which may indirectly affect the bioavailability of heavy metals, has been overlooked. This study investigated the impact of two widely used biodegradable microplastics, polylactic acid (PLA) and polyhydroxyalkanoate (PHA), on the distribution of cadmium (Cd²⁺) in cadmium-contaminated sediments.

Varying concentrations and particle sizes of PLA and PHA were added to Cd-pre-contaminated sediment and incubated under simulated submersion conditions for 60 days. The distribution of Cd²⁺ fraction in the sediment was determined, and the influence of biodegradable microplastics on cadmium mobility, through the regulation of sediment geochemical properties, was assessed.

Compared to the control group, the pH value and organic matter content of sediments treated with biodegradable microplastics significantly decreased, which in turn caused a transformation in the chemical form of cadmium. Higher concentrations and smaller particle sizes significantly promote a shift in cadmium components toward acid-soluble forms, thereby enhancing the mobility of cadmium in sediments, thereby amplifying its environmental toxicity.

These results emphasize the potential of biodegradable microplastics to indirectly affect the mobility of cadmium by regulating sediment geochemical properties.