Zero-Valent Iron as a Sustainable Regulator for Anaerobic Digestion of MPs-Contaminated Organic Substrates: Performance Enhancement and Environmental Significance.

Under the "double carbon" goal, anaerobic digestion (AD) is irreplaceable for organic solid waste resource utilization, but ubiquitous microplastics (MPs) in substrates severely impair AD performance. Studies on regulating PE-MP-containing AD systems via zero valent iron (ZVI) remain scarce, so this study incorporated ZVI into such systems to elucidate its regulatory mechanisms through batch experiments. PE-MPs reduced substrate-microbe contact, inhibited key acid-methane enzymes, and disrupted microbial networks, lowering the acetate/propionate ratio to 0.269 (stable threshold: 1.50), decreasing CH4 production by 34.23%, and enhancing oxidative stress and virulence factors. ZVI-released bioavailable Fe irons promoted hydrolysis, with protease and cellulase activities increasing by 112.87% and 97.24%, respectively, and COD removal reaching 61.27%; optimized the acid-methane balance (acetate kinase increased by 100%, and the ratio was restored to 2.66); increased Methanosarcina abundance to 20.64% and CH4 production by 74.47%; and restored microbial communities by enriching functional bacteria and network modularity to alleviate oxidative stress, as indicated by a 63.85% decline in lactate dehydrogenase activity. CB-SEM further clarified that ZVI systematically improved the performance of AD by enhancing the stability, metabolic activity, and antioxidant capacity of microbial communities and by promoting key processes such as acetic acid methanation. This study clarified the core regulatory role of ZVI in enhancing the AD performance of PE-MP-containing organic substrates, thereby providing a feasible technical strategy for improving the stability and efficiency of AD systems in MP-contaminated environments.