Reduction and immobilization of Cr(VI)-contaminated soil using FeSO4 combined with (NH4)2HPO4: The remediation efficiency, mechanisms, and long-term stability

Abstract

The chemical reduction technique is widely used to remediate Cr(VI)-contaminated soil due to its cost-effective and convenient operation. However, it also suffers from the "yellowing" issue and re-migration of Cr(VI) after on-site remediation. This study hence investigated a new combined treatment material, (NH4)2HPO4 and FeSO4, to enhance the immobilization of Cr(VI) in soil, ultimately inhibiting the "yellowing" phenomenon. The Cr(VI) remediation efficiency, mechanisms, and the long-term stability using FeSO4 with (NH4)2HPO4 were investigated. The Cr(VI) immobilization mechanisms were elucidated using the European Community Bureau of Reference (BCR) sequential extraction procedure, X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). The results suggested that, following the reduction of Cr(VI) to Cr(III) by FeSO4, the presence of (NH4)2HPO4 promoted the formation of residual Cr (CrPO4). At the optimal dose of FeSO4:(NH4)2HPO4:Cr(VI), 5:10:1, 88 % of the 450 mg/kg Cr(VI) in soil was reduced, all the HOAC-extractable Cr (58.67 %) disappeared, while the residual Cr was increased from 6.74 % to 50.61 %. The H2O2 oxidation and simulated rainfall experiments suggested that the extra addition of (NH4)2HPO4 significantly inhibited Cr reoxidation and leachability, possibly due to the formation of residue CrPO4 and FexCr1-x(OH)3, especially when the rainfall`s SO42-:NO3- ratio was 8:1. It is believed that the combination of FeSO4 and common synthetic fertilizer (NH4)2HPO4 is a promising method for in-situ remediation of Cr(VI)-contaminated soil due to its strong resistance to oxidation and acid rain erosion.