Transport of mercury-microplastic complexes in porous media: Roles of Hg-affinitive colloidal particles and surface...

Microplastics (MPs) have emerged as important carriers for heavy metals, yet the environmental mobility of metals adsorbed on MPs surfaces remains poorly constrained, particularly in the presence of Hg-affinitive colloidal particles. Here, we systematically investigate the transport behavior of mercury-microplastic (MPs-Hg) complexes in porous media using molybdenum disulfide (MoS₂) as a representative Hg-affinitive colloid. Column transport experiments, aggregation kinetics, spectroscopic characterization, and theoretical analyses were combined to elucidate how colloidal interactions regulate Hg mobility. Results show that MoS₂ strongly scavenges Hg from MPs surfaces via Hg-S coordination, enabling Hg co-transport through porous media. Transport behavior depends critically on MPs surface functionality: for negatively charged MPs-COOH, electrostatic repulsion suppresses aggregation and MoS₂ enhances Hg mobility primarily through a colloid-mediated carrier effect; for positively charged MPs-NH₂, electrostatic attraction induces heteroaggregation, whose stability governs mobility-suppressing transport at low MoS₂/MP ratios but promoting co-migration at higher ratios. These mechanisms persist under environmentally relevant conditions, including iron oxide-coated sand and real groundwater systems, although MoS₂ aggregation under high ionic strength attenuates its carrier role. These findings reveal that Hg-affinitive colloidal particles can fundamentally alter the transport and fate of Hg bound to MPs, highlighting an overlooked pathway for Hg migration at the water-soil interface and underscoring the importance of colloid-microplastic-metal interactions in subsurface risk assessments.