Exploring the interactions between microplastics and heavy metals during the processes of biotic and abiotic aging towards increased environmental relevance

Understanding the interactions between microplastics and heavy metals is essential for assessing the environmental risks of plastic pollution, but the factors governing this interaction remain poorly understood. In this study, we investigated how biotic (aging in stream water) and abiotic aging (UV irradiation) influence the interactions between trace concentrations (5 µg/L) of metal ions (Pb, Cd) and microplastics (polyethylene (PE) and polyvinyl chloride (PVC)). A combination of bulk digestion and laterally resolved laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) provided complementary insights into the overall adsorption of Pb and Cd as well as laterally resolved elemental distributions within individual particles. Both aging conditions led to surface alterations, with PVC exhibiting pronounced degradation under abiotic aging. Biotic aging promoted the formation of biofilms, especially on PE, which correlated with increased adsorption of metal ions. Biofilms incorporated biogenic elements (Fe, Mn, Mg) during aging (from natural stream water), highlighting their key role in nutrient cycling. Despite the detection of Zn and Sr additives in pristine PVC, no significant leaching was observed during aging. Elemental mapping revealed that the investigated heavy metals were predominantly associated with the biofilm on the surface of the microplastics, with the exception of UV-aged PVC, in which Pb showed a concentration gradient from the surface into the polymer matrix, suggesting the potential migration of the metal beyond surface adsorption. Overall, this study highlights the critical role of environmentally relevant aging in influencing metal-microplastic interactions and the potential of LA-ICP-MS to advance the understanding of these complex processes.