Indices of the dual roles of OM as electron donor and complexing compound involved in As and Fe mobilization in aquifer systems of the Datong Basin

Abstract Organic matter (OM) acts as a source of carbon and is strongly implicated in biogeochemical processes, such as metal complexation and redox reactions. To illustrate the effects of OM on As mobilization in aquifers, this study characterized fluorescence features and hydrochemical properties of OM in sediments and groundwater from an As-affected field site located in the Datong Basin. Fluorescence analysis showed sediment and groundwater OM are dominated by oxidized and reduced quinone-like compounds; shorter emission wavelengths observed in groundwater indicated more labile and protein-like organic substances than in sediments. Dissolved As concentrations were positively correlated with dissolved Fe and HCO3− concentrations in middle and deep groundwater, suggesting labile OM degradation promotes the release of As and Fe from sediments into the groundwater. This result also demonstrated more bioavailable OM occurs in groundwater and that labile OM degradation promotes As release. Grain size distribution results indicated sedimentary As, Fe and OM are associated with fine-grained fractions. Sedimentary As content was significantly and positively correlated with Fe2O3 and OM content, suggesting the potential existence of As–Fe-OM ternary complexation; this was further supported by the results of Fourier-transform infrared (FTIR) spectra and extraction experiments. In addition, the ratio of dissolved reduced quinone-like compounds to oxidized quinone-like compounds was positively correlated with both dissolved As and HCO3− concentrations, implying quinone-like compounds participate in the complexation and influence As mobilization. In the reducing environment, labile OM served as the electron donor to maintain microbial respiration and mediated reductive dissolution of Fe minerals. As–Fe-OM ternary complexation in sediments and microbial reduction have a potentially strong impact on As enrichment in groundwater, and therefore are important considerations for regulating As contamination.