Arsenate-reducing bacteria-mediated arsenic speciation changes and redistribution during mineral transformations in arsenate-associated goethite.

The fate of Fe(III)-(oxyhydr)oxides-bound As was generally regulated by dissimilatory As(V)-reduction. However, the impact of pH and bacterial conditions on the coupled processes of microbially-mediated As speciation changes and Fe-mineral transformation remains unclear. Our study therefore incubated As(V)-associated goethite with different As(V)-reducing bacteria at a range of pH. Results show that As reduction was most prominent at pH 7 as the bacterial growth was optimal. However, aqueous As concentration was the lowest (0.8-3.7 mg/L), due to rapid microbial Fe(II) formation at pH 7 triggered secondary mineralization and significant As-readsorption. Our study provides the first spectroscopic evidence for mineral-phase temporal evolution, and indicates in the presence of phosphate, vivianite will precipitate first and adsorb large amount of As(III) (40-44% of solid As). Thereafter, continuously increased Fe(II) may catalyze lepidocrocite and eventually magnetite formation, which further sequestrate aqueous As(III). Conversely, at pH 5 and 9, bacterial growth was inhibited, the corresponding lower microbially-derived Fe(II) concentrations caused no secondary minerals formation. Released As(III) was therefore largely remained in solution (6-9.7 mg/L). Our study demonstrates that As-bound Fe(III)-(oxyhydr)oxides could pose greater risks under acidic or alkaline conditions in biotic reactions. Additionally, bacterial species could strongly impact Fe-mineral transformation pathways and As solid-solution redistribution.