Mobility of arsenic and selected metals during re-flooding of iron- and organic-rich acid-sulfate soil

The drainage-induced oxidation of iron-sulfide minerals in acid-sulfate soils has adversely affected large areas of coastal floodplains. Re-flooding of these soils, via the re-establishment of more natural drainage regimes, is a potential remediation approach. Here we describe the mobility of Al, As, Fe, Mn, Ni and Zn during controlled re-flooding of an Fe- and organic-rich acid-sulfate soil material. Soil re-flooding caused the onset of microbially-mediated Fe(III)-reduction, which raised the pH of the initially acidic (pH 3.4) soil to pH 6.0 to 6.5, thereby immobilizing Al. The process of Fe(III)-reduction released high concentrations of Fe^II and was associated with significant mobilization of As. During the early stages of re-flooding, Fe^II mobility was controlled by dissolution of schwertmannite (Fe₈O₈(OH)₆SO₄) with an ion activity product (IAP) of 10^{19 ± 2}. The mobility of Fe^II was subsequently controlled by the precipitation of siderite (FeCO₃) with an IAP spanning 10^{- 10} to 10^{- 7.5}. The formation of acid-volatile sulfide (AVS), as a product of SO₄-reduction, further retarded the mobility of Fe^II. Interactions with AVS also strongly immobilized Mn, Ni and Zn, yet had little effect on As which remained relatively mobile in the re-flooded soil. This study shows that the mobilization of As and Fe during soil re-flooding should be considered when planning remediation approaches for acid-sulfate soils.