Novel insight into the aging process of microplastics: An in-situ study in coastal wetlands

Coastal wetlands, the critical interface between the terrestrial and marine environments, provide a dynamic and unique environment for the aging of microplastics (MPs). Nevertheless, both abiotic and biotic processes that contribute to the aging of MPs in coastal wetlands have been largely neglected. In this study, the aging of MPs was continuously characterized in Hangzhou Bay, a representative coastal wetland in Zhejiang, China. Three-month exposure of polymers in sediment-water interface induced the aging phenomenon with embrittlement and exfoliation, as evidenced by simultaneous observed alternations in crystallinity and functional groups. A first-order kinetic model was fitted to describe the rate and degree of aging quantitatively. As evidenced by the carbonyl index, the residence time of all the examined MPs exhibited significant variance, ranging from 335 to 661 days. These variations might be caused by the selective attachment of plastic-degrading microorganisms (such as Moraxella sp. and Rhodococcus sp.). A positive correlation between the carbonyl index, the number of OTUs in the MP-associated biofilm, and irradiation was observed (p < 0.001), suggesting that the aging process may be co-regulated by natural sunlight and wetland microbial colonization. This study sheds new light on the long-term environmental fate of MPs and their associated ecological risks.