Heavy metal sorption in marine microplastic over time

Microplastics are a ubiquitous pollutant in the environment that act as carriers for hazardous chemicals, including heavy metals. Understanding the rate at which metals accumulate, and how this differs among plastic types is crucial for risk assessment and quantifying the overall ecotoxicity of microplastics. This study investigated the long-term sorption and accumulation of 15 heavy metals and metalloids across five common polymers (LDPE, HDPE, PP, PET, and PVC) during a 12-month (48-week) in situ deployment in an urban, coastal marina. Multivariate analysis (PERMANOVA) revealed that both plastic type and exposure duration significantly influenced the metal composition on the plastics. Aluminium, manganese, iron, arsenic and lead consistently accumulated across most plastic types tested. In contrast, other highly toxic heavy metals, such as chromium and cadmium, showed no evidence of accumulation. The number of elements sorbed was governed by polymer properties: LDPE (the polymer with the lowest crystallinity) adsorbed the highest number of elements (9), whereas PVC was the least reactive, with only 4 elements changing significantly in concentration over time. Crucially, a saturation point was not found for any plastic type over the 48 week study period, indicating that toxic elements, such as arsenic and lead, could continue to accumulate. This accumulation is concerning because when ingested, these toxic heavy metals have the potential to leach out and bioaccumulate in individuals, subsequently moving up the food chain. Overall, results highlighted the need to account for polymer-specific and time-dependent metal sorption in assessments of potential environmental and ecotoxicological risk associated to environmental microplastics.