Fate of arsenic during microbial reduction of biogenic versus Abiogenic As-Fe(III)-mineral coprecipitates.

Fe(III) (oxyhydr)oxide minerals exhibit a high sorption affinity for arsenic (As) and the reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides is considered to be the primary mechanism for As release into groundwater. To date, research has focused on the reactivity of abiogenic Fe(III) (oxyhydr)oxides, yet in nature biogenic Fe(III) (oxyhydr)oxides, precipitated by Fe(II)-oxidizing bacteria are also present. These biominerals contain cell-derived organic matter (CDOM), leading to different properties than their abiogenic counterparts. Here, we follow Fe mineralogy and As mobility during the reduction of As-loaded biogenic and abiogenic Fe(III) minerals by Shewanella oneidensis MR-1. We found that microbial reduction of As(III)-bearing biogenic Fe(III) (oxyhydr)oxides released more As than reduction of abiogenic Fe(III) (oxyhydr)oxides. In contrast, As was immobilized more effectively during reduction of As(V)-loaded biogenic than abiogenic Fe(III) (oxyhydr)oxides during secondary Fe mineral formation. During sterile incubation of minerals and after microbial Fe(III) reduction stopped, As(V) was mobilized from biogenic Fe(III) (oxyhydr)oxides probably by sorption competition with phosphate and CDOM. Our data show that the presence of CDOM significantly influences As mobility during reduction of Fe(III) minerals and we suggest that it is essential to consider both biogenic and abiogenic Fe(III) (oxyhydr)oxides to further understand the environmental fate of As.