Microplastics modulate triclosan abiotic methylation: Effects of polymer type and photoaging

Microplastics (MPs) are ubiquitous, diverse and undergo aging in water environments. Although it is well-known that MPs are important carriers for co-existing contaminants, their roles in contaminant transformation remain unclear. This study investigated the impacts of 11 pristine and photoaged MPs on the abiotic methylation of co-existing triclosan (TCS). Among pristine MPs, polyamide 6 (PA6) addition uniquely enhanced TCS methylation efficiency compared to MP-free controls and other MP suspensions (p < 0.05), probably due to its amide groups catalyzing the reaction by creating a polar protic environment. Biodegradable MPs inhibited TCS methylation by limiting CH₃I accessibility to TCS, whilst other conventional MPs (except polypropylene (PP)) showed negligible effects. Furthermore, photoaging altered MP impacts on TCS methylation. Photoaged PA6 reduced methyl TCS production by 93.4% relative to pristine PA6, attributed to amide bonds cleavage. The inhibitory effects of biodegradable MPs and PP diminished after photoaging, while photoaged polystyrene showed an opposite trend. Overall, both surface functional groups and adsorption capacities of MPs are key factors governing TCS abiotic methylation. This study is the first to examine how MP type and photoaging behavior modulate TCS abiotic methylation, providing new insights into the impacts of MPs on co-existing contaminants' environmental fate in aquatic environments.