Immobilization of hexavalent chromium in soil and groundwater using synthetic pyrite particles

In this study, reactive pyrite (FeS₂) particles were prepared through a modified hydrothermal method and tested for immobilization of Cr(VI) in contaminated soil and synthetic groundwater. The addition of a NaAc buffer in the synthetic process resulted in pyrite particles of greater specific surface area, more uniform size, and more crystalline structure. The particles can effectively immobilize Cr(VI) in both water and a model Chinese loess soil. Over 99.9% of Cr(VI) was rapidly removed from water at pH 6.0 (Initial Cr(VI) = 25 mg/L, FeS₂ dosage = 0.48 g/L), and the removal remained high (>82%) even at pH 9.5. Both adsorption and reductive precipitation were found operative in the Cr(VI) immobilization, with ∼66% of Cr immobilized due to reduction. Fe(II) ions associated on the FeS₂ surface played a key role in the reduction of Cr(VI) to Cr(III), and S₂^2- also facilitated the reductive removal of Cr(VI). The presence of humic acid enhanced Cr(VI) removal at pH 4.0, but the effect was negligible at pH 6.0. Batch kinetic tests showed that treating a Cr(VI)-laden soil with 0.48 g/L (as Fe) of FeS₂ decreased the equilibrium water-leachable Cr(VI) by >99.0% at pH 6.0 and by >70.0% at pH 9.0. The distribution coefficient (K_d) value of the pyrite-amended soil was 1477.8 at pH 6.0, which is 306 times higher than that for the untreated soil. Column elution tests showed that installation of a 3-cm reactive layer of FeS₂ in a soil column was able to capture the leachable Cr(VI) from the soil, and the retardation factor (R_d) for the 3-cm FeS₂ layer sample was 381 times higher than that for the plain soil. The synthetic pyrite particles may serve as a reactive material for effective removal or immobilization of Cr(VI) in contaminated water or soil.