Phosphate- and Bicarbonate-Enhanced Arsenic Mobilization and Redistribution in Indigenous Bacillus Strain–Sediment Microcosms

Groundwater contaminated with arsenic (As) represents a significant health risk on a global scale. The migration and transformation of As in high-As sediments are influenced by microbial activity and water chemistry, yet the role of competing anions such as phosphate and bicarbonate is not fully understood. In this study, the indigenous Bacillus strain D2201 from the Datong Basin promoted arsenic and iron (Fe) mobilization in sediment microcosms under stimulated laboratory conditions. Phosphate treatment produced lower dissolved Fe(II) but higher HCl-extractable Fe(II) than the basal-medium control (CK), whereas bicarbonate treatment maintained higher dissolved Fe(II). Aqueous As increased and subsequently declined. By day 9, total dissolved As [As(T)] had decreased from the respective treatment-specific maximum concentrations by 77.6%, 71.0%, and 77.8% in the phosphate treatments at pH 6.0, 7.0, and 8.0, respectively, while the corresponding within-treatment decreases in the bicarbonate treatments were 47.2%, 32.5%, and 34.9%, respectively. Phosphate also altered the distribution of operationally defined solid-phase As fractions, characterized by a decrease in the fraction associated with well-crystalline Fe/Al (hydr)oxides and concomitant increases in the non-specifically and specifically sorbed fractions. Excitation–emission matrix (EEM) spectra showed qualitative differences in fluorescence patterns among the treatments at pH 7.0. The As/Fe responses observed under phosphate and bicarbonate amendments at different pH values provided insights into arsenic mobilization under simulated laboratory conditions. These results provide a scientific theoretical basis for the biogeochemical research of arsenic.