Burton, Edward D., Bush, Richard T., Sullivan, Leigh A., Johnston, Scott G., and Hocking, Rosalie K. (2008) Mobility of arsenic and selected metals during re-flooding of iron- and organic-rich acid-sulfate soil. Chemical Geology, 253 (1-2). pp. 64-73.

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Abstract

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 reflooding 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(Ill)-reduction released high concentrations of Fell and was associated with significant mobilization of As. During the early stages of re-flooding, Fell mobility was controlled by dissolution of schwertmannite (Fe₈O₈(OH)₆SO₄) with an ion activity product (IAP) of 10^(19±2). The mobility of Fell was subsequently controlled by the precipitation of siderite (FeCO₃) with an LAP 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 Fell. 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 reflooding should be considered when planning remediation approaches for acid-sulfate soils.

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