In Situ Reduction of Fe(III) (Hydr)oxides-Bound As(V) via Electron Shuttling in the Presence of Organic Matter

Abstract

Although the reduction of pentavalent arsenic (As(V)) bound to Fe(III) (hydr)oxides (As(V)-containing FeOOH) coupled with organic matter (OM) degradation has received extensive attention, little is known about in situ reduction of FeOOH mineral-bound As(V) (As(V)_(s)) via electron shuttling in the presence of As(V)-reducing bacteria (AsRB). Here, anthraquinone-2,6-disulfonate (AQDS), Suwannee River humic acid (SRHA), and fulvic acid (SRFA) were used to establish an agar electron shuttle system with ferrihydrite-As(V)/goethite-As(V) and AsRB, which was isolated from high-As groundwater as the candidate strain (RA-1). We found that AQDS was more efficient in shuttling electrons to reduce As(V)_(s) and Fe(III) than SRHA and SRFA. High contents of quinone moieties in AQDS indicated that quinone moieties were conducive to As(V)_(s) and Fe(III) reduction. The reduction kinetic rates (K_red) of As(V)_(s) were higher than those of Fe(III), indicating the occurrence of in situ As(V)_(s) reduction via electron shuttling. Furthermore, K_red of As(V)_(s) for ferrihydrite-As(V) systems was greater than that of goethite-As(V) systems, demonstrating that As(V) bound to ferrihydrite via monodentate was more easily reduced than that bound to goethite via bidentate. The relative expression levels of As metabolic genes (arsC, arrA, and arsB) initially increased and then decreased in the late stage of experiments. High As concentrations in suspension inhibited the transcriptional activity of As metabolic genes in the late stage, reducing the electron production efficiency of RA-1 and subsequently slowing in situ reduction of As(V)_(s). This study highlights the importance of OM electron shuttling in the reduction of As(V)_(s), offering new perspectives in As enrichment in groundwater.