Electrochemistry and Molecular Compositions Reflect Electron Shuttling of Dissolved Organic Matter in High Arsenic Groundwater

Little is known about the electron shuttle ability of dissolved organic matter (DOM) and its effects on arsenic (As) mobilization, which makes the underlying mechanism of groundwater As enrichment elusive. In this study, both the electrochemical properties and molecular compositions of DOM in high As groundwater were quantified in the Hetao Basin, China. We found that, along the flow path, the average electron-transferring capacity (ETC) of DOM, including the capacities of electron-accepting and electron-donating, continuously increased from 2.85 to 3.59 mmol_e-/gC along with As concentrations. The increasing ETC reflected an increase in electron shuttle ability of DOM. Furthermore, the increasing electron shuttle ability was mainly attributed to the recalcitrant compounds in DOM, especially CHOS and CHONS formulas in highly unsaturated structures with high oxygen (HUSHO) and CHO and CHON formulas in aromatic structures (AS). The significantly positive correlation between As concentration and ETC indicated that recalcitrant DOM promoted groundwater As enrichment through electron shuttling for inducing the reductive dissolution of As-containing Fe(III) oxide minerals, which was further supported by our culture experiments showing that goethite was more reduced [133 μM Fe(II)] in the presence of DOM with a higher ETC (3.35 mmol_e-/gC) as electron shuttling than that [65.2 μM Fe(II)] with a relatively lower ETC (2.41 mmol_e-/gC). Our study highlights that recalcitrant DOM compounds with unsaturated and AS have high electron shuttle ability, promoting As enrichment in groundwater.