Polymer-specific plant and microbial responses in constructed wetland microcosms under combined microplastic and heavy metal exposure

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Microplastics (MPs) serve as vectors for pollutants, particularly heavy metals (HMs), yet their combined effects in constructed wetlands (CW) remain largely unexplored. Batch-fed CW microcosms were used to investigate how aged polyethylene (PE) spheres and polyester (PES) fibers, together with Zn, Mn, Pb, Cu, and Ni, influence nutrient removal, metal retention, plant physiological responses, and microbial community dynamics. HM removal from synthetic wastewater exceeded 90% in all treatments, indicating a strong retention capacity, likely governed by sediment sorption processes. However, retention of Zn, Mn, Pb and Ni declined slightly over time, suggesting progressive saturation of available binding sites. MPs did not significantly affect metal removal. The addition of HMs and MPs led to increased removal of total nitrogen, carbon, phosphate, and sulfate. Mean time-weighted removal efficiencies across treatments were 76.7% for total nitrogen, 98.7% for nitrate, 75.7% for total carbon, 45.5% for phosphate, and 31.7% for sulfate. Plant responses differed by polymer type, with PES and HM+PES inducing higher oxidative stress in Iris pseudacorus , possibly due to differences in polymer shape, color and additive leaching. Microbial diversity increased over time, although PES and HM+PE reduced species richness. Time was the primary driver of community turnover, while MPs and HMs acted as secondary modulators promoting polymer-specific and metal-tolerant taxa. Overall, results showed that young experimental CWs maintained treatment performance under combined MP and HM exposure over the study period, with system maturation identified as an important factor influencing functional and microbial dynamics.