New insight into the overlooked aging behavior of microplastics mediated by soluble Mn(Ⅲ) in subsurface environments

Microplastics (MPs) undergo pervasive environmental aging, but non-photochemical transformations in subsurface environments remain poorly understood. Here, we investigated the aging of polyethylene (PE), polystyrene (PS), and polylactic acid (PLA) by pyrophosphate-stabilized Mn(Ⅲ) under dark, circumneutral, Mn-rich accelerated conditions. The tested materials showed distinct changes in morphology, crystallinity, surface composition, and net dissolved organic carbon (DOC) accumulation. GPC showed substantial decreases in the apparent molecular weight of the recovered PE fraction, no appreciable decrease in recovered PS, and redistribution rather than a uniform molecular-weight decrease in recovered PLA. After correction for the plastic-only and MP-free controls, net DOC accumulation ranged from 5.05 to 9.81 mg C L^-1 after 28 d. Mn speciation, electrochemical, EPR, quenching, oxic-anoxic comparison, and density functional theory analyses indicated coupled interfacial coordination, Mn redox cycling, and radical reactions. Oxygen availability particularly amplified the aging responses of PE and PS. Responses were weaker at 10 μM than at 100 μM Mn(Ⅲ), supporting concentration dependence. These findings identify ligand-stabilized Mn(Ⅲ) as a previously overlooked potential contributor to non-photochemical MP aging under Mn-rich subsurface conditions, with implications for plastic-associated DOC accumulation and MP behavior in groundwater systems.