Effects of doped Mn on ferrihydrite transformation and the associated arsenic behavior during sulfidation-reoxidation process.

In paddy soils, the fate of As is tightly linked to sulfidogenic transformation of ferrihydrite and fluctuation of redox conditions. Manganese often coexists with ferrihydrite and inevitably influences ferrihydrite geochemistry and As fate. However, how doped Mn affects sulfidogenesis in ferrihydrite and the associated As behavior under redox-dynamic conditions remains unclear. In this study, batch experiments coupled with geochemical and mineralogical analyses were conducted to elucidate these processes. Results showed that under low SO42- loadings, ferrihydrite transformed into magnetite, lepidocrocite, and minor amounts of mackinawite under anoxic conditions, resulting in minimal release of pre-sorbed As into solution. Under high SO42- loadings, however, the transformation of ferrihydrite to iron sulfides resulted in 19.37-33.14% of the pre-sorbed As being released into solution. Concurrently, 50.74-57.43% of the total As was converted to residual-phase As. The doped Mn inhibited ferrihydrite reduction by suppressing electron transfer from S2- to Fe(III), thereby reducing As release during the anoxic stage. Additionally, the doped Mn promoted mackinawite formation and increased the proportion of residual-phase As. Specifically, at the end of the anoxic stage, the aqueous As concentration in the High-Fh-Mn10% treatment (25.18 μmol/L) was significantly lower than that in the High-Fh treatment (43.08 μmol/L), while the proportion of residual-phase As in the former treatment (57.43%) was higher than that in the latter (50.74%). Upon transition to oxic conditions, the previously formed Fe(II) and As(III) were re-oxidized to Fe(III) and As(V), and the desorbed As was re-immobilized by the newly formed Fe(III) oxides. Meanwhile, aqueous Mn(II) was partly converted to Mn(III) species, which are powerful oxidants that can facilitate the above re-oxidation and re-immobilization processes. These findings provide critical insights into the geochemistry of Fe-S-Mn-As in paddy soils and offer valuable implications for As remediation strategies.