Molecular transformation of natural organic matter associated with arsenic mobility via microbial iron reduction in aquifer settings.

Natural organic matter (NOM) critically influences the biotransformation of arsenic (As) in groundwater systems. However, the molecular-level transformation of NOM during arsenic mobilization remains poorly understood. In this study, Geobacter metallireducens strain GS-15 was cultured to reductively dissolve of As-loaded iron (hydro)oxides. The results demonstrated that water-soluble organic matter (WSOM), extracted from sediments in Jianghan Plain, effectively stimulated microbial iron (hydro)oxide reduction and As release. The initial increase followed by a decline in aqueous As concentrations is ascribed to sequestration via adsorption and co-precipitation with secondary minerals, including siderite and magnetite. WSOM composed of highly unsaturated compounds with low H/C ratios, elevated O/C ratios, high double bond equivalents (DBE), and elevated nominal oxidation state of carbon (NOSC) facilitated the As mobilization. The generation of low-oxygen, highly unsaturated compounds and aliphatic compounds (lower NOSC) indicated a distinct NOM transformation pathway. The degradation of CHON2, CHON3, and CHOS molecules in particular 20-m sediments promoted further Fe and As enrichment. These observations indicate that the aquifers at depths of 20 m contained organic matter with higher oxidation states, forming a "hot spot" for microbially mediated As enrichment. These findings advance the current understanding of NOM mineralization and its coupling with Fe and As biogeochemical cycling.