In Situ Chromium(VI) ReducƟ on Using Iron(II) SoluƟ ons: Modeling Dynamic Geochemical Gradients

This study investigates the effectiveness of iron(II) solutions (FeCl2 or FeSO4) for in situ remediation of hexavalent chromium (Cr(VI)) contamination in Atlantic Coastal Plain soils and subsurface sediments. Previous experiments indicated that while Fe(II) was effective at reducing Cr(VI), the precipitation of trivalent chromium (Cr(III)) was hindered by acidic conditions created during the hydrolysis of Fe(III) and Cr(III). The research objective was to enhance Cr(III) precipitation through the inclusion of acetate buffer at pH 5.6, while assessing the impact of added sulfate (SO4^2−) on Cr(VI) mobility. A reactive transport model was developed to simulate dynamic geochemical gradients under advective-dispersive conditions during remediation application. The model proposed two principal mechanisms of chromium partitioning: Cr(VI) sorption competes with SO4^2− for weak binding sites on iron oxides, and Cr(III) precipitation as a mixed Cr(III)–Fe(III) hydroxide. The results indicate that model predictions align qualitatively with observed Cr(VI) behaviors during contamination and remediation phases, highlighting the complexities of chromium migration influenced by sulfate competition and acidity from non-buffered Fe(II) treatments. Discrepancies noted were mainly attributed to sorption and redox kinetics.