Simulated scenario models to assess the long-term effects of Cr(VI)-contaminated soils remediated with typical iron-bearing reductants

The chemical reduction of Cr(VI)-contaminated soil can be subject to re-oxidation when the external environment changes, and it is therefore crucial to assess the long-term stability of the remediated soil.

In this work, we systematically investigated the remediation performance of nano-zerovalent iron (nZVI), ferrous sulfate (FeSO₄), and zerovalent iron/pyrite (ZVI/FeS₂) in Cr(VI)-contaminated soil, with particular emphasis on assessing the long-term stability of Cr(VI)-contaminated soil after their remediation by adopting various simulated scenarios models, including freeze–thaw cycles (FTC) aging, acid rain (AR) leaching, alternating wet-dry (AWD), and extreme oxidation.

The results showed that compared to nZVI and FeSO₄, ZVI/FeS₂ exhibited superior Cr(VI) reduction performance with a reduction efficiency of 99.8% and improved the richness and diversity of soil microbial community. The stability of Cr in soils remediated with ZVI/FeS₂ and FeSO₄ was not significantly affected by FTC, AR leaching, and AWD, with almost no changes in Cr(VI) content in the two soils under any circumstance, except for a slight increase in Cr(VI) concentration in FeSO₄ treated one under AWD condition. By contrast, the addition of exogenous Mn oxides led to an increase in total Cr(VI) concentration of 167.98, 136.35, and 50.35 mg kg⁻¹ in the control, FeSO₄, and ZVI/FeS₂-treated soil, respectively. The ZVI/FeS₂ treated one exhibited preferable resistance to Cr(VI) re-oxidation.

Our findings illustrated that soil oxidation condition is an important factor affecting the long-term stability of remediated Cr(VI)-contaminated soil and provided critical information on guidance of selecting reducing materials for Cr(VI)-contaminated soil remediation.