Microplastics Promote Light-Driven Ferrihydrite Transformation by Altering the Balance between Interfacial Aggregation and Redox Processes

Ferrihydrite (Fh) phototransformation regulates iron cycling and the mobility of nutrients and contaminants in aquatic and terrestrial environments. Microplastics (MPs) increasingly coexist with Fh, yet whether intact MP particles regulate Fh phototransformation through particle-scale interfacial effects remains unclear. We investigated how MPs (polystyrene (PS), polyvinyl chloride (PVC), and polylactic acid (PLA)) affect Fh transformation under simulated solar irradiation. Without MPs, irradiation produced predominantly hematite (Hm), with goethite (Gt) as a minor product, and 9.17% of Fh was transformed after 25 d. MPs increased the transformation without changing detectable products. MPs suppressed Fh reaggregation by shifting surface charge and disrupting Fh–Fh contacts, while enhancing irradiation-induced charge separation, surface Fe(III) reduction, Fe(III)/Fe(II) cycling, and Fe mobilization. The promoting effects outweighed aggregation inhibition, causing net promotion of Fh transformation. Notably, PLA hydrolysis and photo-oxidation lowered pH and released acidic, Fe-complexing photoproducts, amplifying Fe mobilization and the aqueous Fe pathway. Overall, the PLA > PVC > PS trend reflects coupled polymer chemistry, particle properties, intact-particle interactions, and irradiation susceptibility, with PLA-derived aqueous components providing an additional solution-phase contribution. These findings show that intact MPs and polymer-derived aqueous products jointly modify Fe mineral evolution, linking plastic aging to iron biogeochemistry in illuminated surface environments.