Reduction of As(V)-bearing goethite by dissolved organic matter from arsenic-prone groundwater during the repeated electron shuttling

The molecular structure of dissolved organic matter (DOM) is crucial for electron shuttling. However, the molecular transformation of DOM during the repeated electron shuttling and its impact on reduction of Fe oxides-adsorbed arsenic (As) remain poorly understood. Here, four groundwater DOM samples obtained from As-prone groundwater from the Hetao basin, China were concentrated to investigate their molecular transformations and electron shuttle ability during the repeated electron shuttling for reduction of As(V)-bearing goethite. We found that higher electron-transferring capacity (ETC) of high-As groundwater DOM resulted from the higher aromaticity and more functional moieties (i.e., total phenols and quinones) than that in low-As groundwater DOM, and irreversible loss of aromaticity and the functional moieties during the repeated electron shuttling lowered the ETC. This decreasing ETC of DOM reflected the decrease in its electron shuttle ability to reduce As(V)-bearing goethite. Concretely, the maximum concentration of Fe(II) produced from goethite reduction via electron shuttling decreased from 145 μM with the reduced DOM of the first round of electron shuttling to 111 μM with the reduced DOM of the third round of electron shuttling. Additionally, the positive correlation between total dissolved As (mainly as As(V)) and total Fe(II) concentration illustrated that electrons shuttled by the DOM preferentially reduced goethite, thus releasing the adsorbed As(V) into water. This study highlighted that the molecular transformation of DOM during the repeated electron shuttling lowered the electron shuttle ability, thereby impairing the reduction of Fe(III) oxide minerals for As mobility.