In Situ Chromium(VI) Reduction Using Iron(II) Solutions: Modeling Dynamic Geochemical Gradients

Previous experiments were conducted using Atlantic Coastal Plain soils and subsurface sediments to evaluate the effectiveness of Fe(II) solutions (FeCl 2 or FeSO 4 ) as an in situ means of remediating Cr(VI) contamination. Although Fe(II) was effective in reducing Cr(VI), the subsequent precipitation of Cr(III) was inhibited by the decrease in pH accompanying hydrolysis of Fe(III) and Cr(III). Including acetate buffer (pH 5.6) enhanced Cr(III) precipitation in batch equilibrium experiments, but added SO 4 2− and acetate enhanced Cr(VI) mobility. Reactive transport modeling based on available data and constants derived from the literature was used to describe the dynamic geochemical gradients associated with advective–dispersive conditions encountered during the application of such a remediation strategy. Two mechanisms of Cr partitioning were simulated: (i) Cr(VI) (CrO 4 2− ) sorption to and competition with SO 4 2− for weak binding sites associated with Fe oxides using the diffuse double layer model based on surface protonation and complexation constants derived from the literature, with reactive site densities derived from batch data; and (ii) Cr(III) precipitation in the form of a mixed Cr(III)–Fe(III) hydroxide. Adjusting for only two optimized parameters, apparent site density and solubility, the transport model predictions were qualitatively consistent with observed Cr(VI) behavior during both the contamination and remediation phases, including the enhanced migration of Cr(VI) induced by SO 4 2− competition and the continued migration of Cr(III) associated with acidification induced by oxidation of nonbuffered Fe(II) treatments. Discrepancies between experimental results and model simulations can be attributed to the kinetics of both sorption and redox processes.