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

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 ( ars C, arr A, and ars B) 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. Plain Language Summary OM can promote microbial As(V) reduction in high As groundwater systems. As a result, we explored how FeOOH mineral-bound As(V) could be in situ reduced via electron shuttling between AsRB and different OM. Our findings showed that AQDS was more effective than other two natural organic matter (NOM, i.e., SRHA and SRFA) in reducing both As(V) (s) and Fe(III), due to its high content of quinone moieties. The results of kinetic analysis demonstrated in situ reduction of As(V) (s) , and As(V) bound to ferrihydrite was more readily reduced than As(V) bound to goethite. In addition, the transcriptional activities of three As metabolic genes initially increased but later declined due to high As concentrations, which further slowed the As(V) (s) reduction process. Overall, these findings provide new insights into organic-mediated As cycling in contaminated groundwater. Key Points High quinone contents in AQDS led to a higher reduction extent of As(V) (s) than in SRHA and SRFA The values of K red, As were greater than those of K red, Fe , demonstrating in situ reduction of As(V) (s) Changes in As metabolic genes affected the electron production efficiency of RA-1 and As(V) (s) reduction