Impact of Organic Matter on Microbially-Mediated Reduction and Mobilization of Arsenic and Iron in Arsenic(V)-Bearing Ferrihydrite

Under anoxic conditions, the interactions between As-bearing ferrihydrite (Fh) and As(V)-reducing bacteria are known to cause Fh transformations and As mobilization. However, the impact of different types of organic matter (OM) on microbial As/Fe transformation in As-bearing Fh-organic associations remains unclear. In our study, we therefore exposed arsenate-adsorbed ferrihydrite, ferrihydrite-PGA (polygalacturonic acid), and ferrihydrite-HA (humic acid) complexes to two typical Fe(III)- and As(V)-reducing bacteria, and followed the fate of Fe and As in the solid and aqueous phases. Results show that PGA and HA promoted the reductive dissolution of Fh, resulting in 0.7–1.6 and 0.8–1.9 times more As release than in the OM-free Fh, respectively. This was achieved by higher cell numbers in the presence of PGA, and through Fe-reduction via electron-shuttling facilitated by HA. Arsenic-XAS results showed that the solid-phase arsenite fraction in Fh-PGA and Fh-HA was 15–19% and 27–28% higher than in pure Fh, respectively. The solid-associated arsenite fraction likely increased because PGA promoted cell growth and As(V) reduction, while HA provided electron shuttling compounds for direct microbial As(V)-reduction. Collectively, our findings demonstrate that As speciation and partitioning during microbial reduction of Fh-organic associations are strongly influenced by PGA and HA, as well as the strains’ abilities to utilize electron-shuttling compounds.