Microplastics exacerbate heavy metal pollution stress in the surface water of a mining city: Occurrence, drivers, and vector effects

Coal mining activities cause surface subsidence and heavy metal contamination, threatening regional water security. Microplastics (MPs) released during activities such as farmland reclamation in mining areas can act as vectors, exacerbating heavy metal transport and pollution. However, systematic understanding of the occurrence and combined pollution of MPs and heavy metals in surface water of intensively disturbed coal mining regions remains limited. Therefore, this study focused on a typical coal-mining city. It assessed the pollution status of MPs and heavy metals in the surface water and revealed the environmental role of MPs in the aquatic ecosystem. The results showed average MP abundances of 216.7 ± 81.9 n/L in surface water and 1496.7 ± 562.5 n/kg in sediments. MPs were predominantly transparent fibers and films smaller than 0.1 mm, composed mainly of polyethylene (43.50%) and polypropylene (36.29%). Risk assessment indicated a low overall MP pollution load, but the Polymer Hazard Index suggested Grade III (high risk). Cd, As, and Ni were the key risk elements in the aquatic environment. MP abundance in surface water was significantly influenced by TN, NO 3 ⁻–N, Cr, and Pb (p < 0.05), while pH, Cr, and Pb were key regulators in sediments (p < 0.05). Furthermore, MPs acted as critical carriers for heavy metals such as Cd, Cr, Cu, and Zn, with adsorption capacity comparable to the sediment matrix and functioning as dynamic “transport‑release” vectors. These findings offer a basis for managing composite MP‑heavy metal pollution and for supporting ecological health in waters of coal‑based cities. • The main sources of MPs and HMs differ across the environmental functional zones of Huaibei City. • pH, TN, NO 3 ⁻-N, Cr, and Pb were significantly correlated with the abundance of MPs. • Cd, Cr, Cu, and Zn were the main enriched metals on the surface of MPs. • This study revealed the key drivers and mechanisms of HMs adsorption onto MPs.