Mitigation of polyethylene microplastic-induced estrogen degradation inhibition in anaerobic digestion by zero-valent iron: Mechanisms linking microbial community succession to metabolic pathway regulation

Zero-valent iron (ZVI) has shown promise in enhancing pollutant removal, yet its multi-scale mechanism for alleviating polyethylene microplastics (PE-MPs)-induced inhibition of estrogen degradation during anaerobic digestion (AD) remains unclear. Therefore, three treatment groups (T1, T2 and T3) were established in this study, including ZVI addition (1.0 g/L ZVI, T1), PE-MPs addition (150 particles/g TS, T2) and PE-MPs + ZVI addition (150 particles/g TS and 1.0 g/L ZVI, T3). After 60 days of AD, PE-MPs + ZVI addition (T3) reduced estrone (E1) and estradiol (E2) residues to 21.64 ± 0.23 μg/kg and 15.77 ± 0.05 μg/kg, increasing their removal efficiencies by 12.02% and 9.04% compared to T2. Specifically, ZVI reduced the relative abundance of Firmicutes and increased the relative abundance of the electroactive bacterium Desulfovibrio (21.74-fold vs T2), as well as methanogens. Metagenomic analysis revealed ZVI upregulated quorum sensing (QS) genes ( LuxQ , LuxS 1.14-fold and 1.13-fold vs T2) and direct interspecies electron transfer (DIET)-related genes ( ubiE , pilW , 6.98-fold and 7.66-fold vs T2), enhancing the connectivity between key estrogen-degrading genes and microorganisms (154 edges in T3 vs 120 edges in T2), particularly genes annotated to 17β-estradiol dehydrogenase and the EmtABCD system. Additionally, ZVI mitigated SOD activity and restored acetate kinase activity. PLS-PM modeled estrogen degradation genes as the primary drivers of estrogen degradation. This study provided multi-scale evidences for ZVI’s application in remediating combined PE-MPs and estrogen pollution in AD systems.