Processing pathways of organic matter under methanogenic conditions and its influence on arsenic mobilization in aquifers

Organic matter (OM) plays a crucial role in triggering the reductive dissolution of iron (oxyhydr)oxides and the release of arsenic (As), while also serving as a significant substrate for methanogenesis. Although methanogenesis has been observed to occur ubiquitously in As-affected groundwater, the selectivity of the initially degraded OM, its processing pathway, and the impact on As mobilization remain uncertain. Herein, the optical and molecular characteristics of dissolved organic matter (DOM) in the alluvial-lacustrine aquifer within the central Yangtze River basin were examined. Additionally, the hydrogeochemistry and inorganic stable carbon isotopes were analyzed to elucidate the processing pathways of OM under methanogenic conditions and its impact on As mobilization. Characterization of DOM compositions through fluorescence spectroscopy and ultrahigh-resolution mass spectrometry reveals processing pathways from oxygen-rich, highly aromatic organic compounds to highly unsaturated oxygen-poor and aliphatic compounds. Under methanogenic conditions, the thermodynamically more favorable degradation of recalcitrant OM provided a large number of electrons to the iron (oxyhydr)oxides, leading to the release of As. This research provides new insights into As accumulation mechanism in Quaternary alluvial-lacustrine aquifers from the perspective of unveiling the processing pathways of recalcitrant OM on methanogenesis and arsenic release.