New insight into manganese-enhanced abiotic degradation of microplastics: Processes and mechanisms

Microplastics(MPs) are an emerging environmental pollutant and have penetrated the most remote and primitive areas. MPs degradation has received widespread attention. Manganese(Mn) is a highly reactive metal element in the environment, yet its contribution to MPs degradation remains unclear. Herein,we simulated the aging of polyethylene MPs with Mn(Ⅱ) under aqueous conditions at pH 5 and 8 for720 days. Mn greatly promoted the MPs degradation, and the average particle sizes of polyethylene MPs were reduced from 9.2 μm to 5.9 μm after aging at pH 5 under light irradiation for 720 days. Plenty of oxygen-containing groups were generated on the MPs surfaces, and the carbonyl index remarkably increased, reaching four times that of the control without adding Mn. Mechanistically, the adsorbed Mn(Ⅱ)on the MPs surfaces were primarily oxidized to high-valence Mn(Ⅲ/Ⅳ) profited from the photoproduced radicals, followed by the MPs oxidation via Mn(Ⅲ/Ⅳ), which were reduced to regenerate Mn(Ⅱ), initiating a new redox cycling. During the degradation, dissolved organic matter was continuously released, mainly including bisphenol A and phthalic acid esters. Mn acts as a catalyst to accelerate the MPs degradation by redox cycling. Our results provide a new insight into the mechanisms of abiotic degradation of MPs in aqueous environments.