Identification of Cr(VI) speciation in ferrous sulfate-reduced chromite ore processing residue (rCOPR) and impacts of environmental factors erosion on Cr(VI) leaching

Highlights: • Cs-STEM and μ-XRF were adopted to identify Cr(VI) speciation in the rCOPR. • Ettringite was demonstrated to be the host phase of Cr(VI). • Environmental factors (CO₃²⁻, SO₄²⁻ and H⁺) were found to promote Cr(VI) leaching. • The mechanism of Cr(VI) leaching from rCOPR was further revealed. -- Abstract: The issue that the residual Cr(VI) in the reduced chromite ore processing residue (rCOPR) is slowly released during deposition has attracted increasing attention. However, the speciation and leaching behaviors of Cr(VI) in rCOPR are still not clear, which is essential for revealing the release mechanism of Cr(VI). In this study, ettringite was determined to be the host phase of Cr(VI) in ferrous sulfate-reduced COPR by scanning electron microscopy (SEM), microfocus X-ray fluorescence spectroscopy (μ-XRF) and aberration-corrected scanning transmission electron microscopy (Cs-STEM). This is because the channel structure of ettringite makes it relatively easy for sulfate to be replaced by chromate with similar structure and thermochemical radius. Furthermore, the investigation on the leaching behavior and mechanism of Cr(VI) in rCOPR eroded by environmental factors showed that carbonates, sulfates and acid can promote the release of Cr(VI). Among them, the erosion effect of HCl on rCOPR is weaker than that of Na₂CO₃ and Na₂SO₄ because rCOPR possesses a high buffering reserve of alkalinity. In addition, the erosion of rCOPR by Na₂CO₃ and Na₂SO₄ can change Cr(VI) speciation in rCOPR. The results implied that the environmental risk of Cr(VI) release during the deposition of rCOPR should deserve careful assessment.