An emerging sink for phosphorus in lake ecosystems: Microplastic-enabled iron and phosphorus costabilization in the overlying water

While microplastics (MPs) are known to influence the biogeochemical cycling of phosphorus (P) in lake ecosystems, a critical gap remains in understanding their specific role as environmental vectors in the overlying water. This study investigated the mechanisms and aging effects (induced by UV irradiation) of MPs acting as novel interfaces mediating iron-phosphorus immobilization under simulated lake overlying water conditions (neutral pH and low dissolved oxygen, < 0.2 mg/L). The results indicated that unaged and aged MPs exhibited no adsorption capacity for P in only PO condition. In contrast, within the Fe(II) and PO co-existing condition, MPs mediated the surface oxidation of Fe(II) to Fe(III); the resulting Fe(III) then enabled the efficient co-immobilization with PO through distinct microscopic mechanisms specific to each polymer type. Specifically, Chlorinated Polyethylene (CPE) and Polylactic Acid (PLA) achieved this via chemical bridging (Fe-O-P bonds), whereas Polypropylene (PP) and Polyethylene (PE) relied on physically induced heterogeneous nucleation. The increased capacity of PP, PE, and CPE (12%-17.2%) correlated with the rise in surface oxygen-containing functional groups after aging. Conversely, the capacity of PLA decreased because crystallization encapsulated the active sites. This study demonstrates the effective and polymer-specific immobilization of P onto MPs in Fe(II)-rich overlying water. This process enables MPs to function as both temporary sinks and potential mobile carriers with re-release risks, highlighting the necessity of incorporating such mechanisms into eutrophication risk assessments.