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Mechanistic insights into the molecular selectivity and cotransport dynamics of biodegradable microplastic-derived DOM with cadmium in saturated porous media
Summary
"Biodegradable" plastics, often marketed as eco-friendly, break down and release dissolved organic compounds that make it easier for toxic cadmium to travel through soil and potentially into groundwater. In lab experiments, this effect increased contamination movement by up to nearly 27%, with the exact chemistry of the plastic (and how weathered it was) determining how much cadmium hitched a ride. This suggests that switching to biodegradable plastics may not eliminate environmental risks—it could shift them, potentially affecting drinking water sources contaminated with heavy metals.
The widespread use of biodegradable plastics (BDPs) results in the release of biodegradable microplastics-derived dissolved organic matter (BMP-DOM) during environmental weathering. The role of its molecular characteristics in regulating the subsurface transport of coexisting heavy metals, however, remains unclear. Here we investigated the cotransport of cadmium (Cd) with DOM derived from three representative BDPs (polybutylene adipate terephthalate, PBAT; polybutylene succinate, PBS; and polylactide, PLA) at two aging stages in saturated porous media. Molecular characterization revealed clear polymer-dependent signatures. PBAT-DOM was distinctively enriched in aromatic structures and lignin-like compounds, while PBS-DOM featured a prominent aliphatic/peptide-like signature, and PLA-DOM was characterized by a unique carboxylic-rich nature and high O/C ratio. FT-ICR MS analysis of the acid-stable fraction revealed a strong affinity of Cd for condensed and unsaturated molecules; however, these stable complexes showed no clear correlation with macroscopic transport behavior. Column experiments demonstrated that all BMP-DOM types enhanced Cd mobility and increased recovery by 4.5-26.6%. This enhancement was concentration dependent. Molecular composition dominated transport efficiency at low concentrations (1.5 mg C/L), whereas DOM abundance and surface site saturation dominated at higher concentrations (15 mg C/L). Furthermore, Cd transport was mainly facilitated by acid-labile DOM fractions that are often overlooked by conventional solid-phase extraction. These results demonstrate that Cd transport potential is jointly controlled by polymer type and aging stage through their influence on reactive ligand generation. This study highlights the importance of BMP-DOM chemistry in regulating metal mobility and provides a molecular basis for environmental risk assessment.