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Influence of Polyethylene Microplastics on Heavy Metal Enrichment and Ecological Risks in Recessed Green Spaces
Summary
This study found that tiny plastic bits (polyethylene microplastics) from stormwater runoff can actually grab onto toxic heavy metals like lead, copper, and cadmium, causing them to build up in green infrastructure like rain gardens—mostly concentrating in the top layer of soil. This matters because these "green" stormwater systems, designed to filter pollution before it reaches waterways, may become hotspots where plastics and heavy metals team up to create a more concentrated pollution risk than either would alone, potentially affecting soil and water quality in urban areas where people live and garden.
Abstract Stormwater runoff typically carries substantial pollutant loads, with reported microplastic (MP) concentrations ranging from 0.02 to 15,499 items/L and heavy metal (HM) concentrations commonly in the μg / L range. Although previous studies have separately examined the toxicity of MPs and HMs, their combined effects and ecological risks in low-impact development (LID) facilities remain insufficiently understood. Using recessed green spaces as a representative LID facility, this study investigated the influence of polyethylene (PE) MPs on the enrichment of Cu 2 + , Cd 2 + , and Pb 2 + in stormwater runoff. The results showed that the enrichment efficiency of PE for HMs followed the order Pb 2 + > Cu 2 + > Cd 2 + . Enrichment efficiency was positively correlated with HM concentration, temperature, and pH, but negatively correlated with PE particle size and salinity. Electrostatic interaction was identified as the dominant mechanism governing HM adsorption by PE. In systems where MPs and HMs coexisted, the removal efficiency of HMs was enhanced, following the order Cu 2 + > Pb 2 + > Cd 2 + . The concentrations of Cu 2 + , Cd 2 + , and Pb 2 + in the planting medium decreased with increasing soil depth, with most HMs retained and immobilized in the shallow layer (0–2 cm). Ecological risk assessment showed that, under a 24 h leaching period, combined MP and HM pollution increased the pollution load index ( PLI ) of the planting medium from 1.12 to 1.23, while the surface layer approached moderate pollution ( PLI = 1.92 ). The potential ecological risk index ( <m