Experimental study on injection of ferrous sulphate for remediation of a clayey soil contaminated with hexavalent chromium

In-situ remediation of the Cr(VI) contaminated clayey soil remains challenging due to the low hydraulic conductivity and high ion adsorption capacity of the clays. In this study, the injection tests of ferrous sulphate (FeSO₄) for a clayey soil contaminated with hexavalent chromium (Cr(VI)) were performed to explore the feasibility of FeSO₄ solution for remediation of Cr(VI) contaminated soil under different injection pressures. The results show that the injection of FeSO₄ solution under the pressure of 30 kPa, 70 kPa, and 100 kPa caused the hydraulic conductivities of the specimens to decrease by 36.5%, 38.7% and 32.2%, respectively. This phenomenon could be attributed to the formation of the mixed iron(III)/chromium(III) hydroxide, a process for which scanning electron microscopy with energy-dispersive X-ray microanalysis provided direct evidence. Moreover, the mobility capacity of Fe(II) was stronger under high pressures (70 kPa, 100 kPa) as indicated by Fe profiles in specimens after remediation under different injection pressures. The range of iron increment at 100 kPa was 3.2–11.9 g/kg. However, the maximum iron increment at 30 kPa was only 5 g/kg and the migration of Fe(II) was limited, which was less than 2.5 cm. In addition, the injection pressure of 70 kPa was the most efficient for remediation of Cr(VI) contaminated soil under the experimental condition in present work, which the remediation efficiency was 87.43–95.12%. As injection pressure increased, Cr(VI) leaching increased. Therefore, these experimental results can be used as a reference for remediation of hexavalent chromium contaminated soil by in-situ injection.