Immobilization of chromate in hyperalkaline waste streams by green rusts and zero-valent iron

Abstract

Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble, highly toxic Cr(VI) to insoluble Cr(III). This study compared the reduction rates of Cr(VI) by ZVI and two carbonate green rust phases in alkaline/hyperalkaline solutions. Batch experiments were carried out with synthetic chromate solutions at pH 7.7–12.3 and a chromite ore processing residue (COPR) leachate (pH ≈ 12.2). Green rust removes chromate from high pH solutions (pH 10–12.5) very rapidly (<400 s). Chromate reduction rates for both green rust phases were consistently higher than for ZVI throughout the pH range studied; the surface area normalized rate constants were two orders of magnitude higher in the COPR leachate solution at pH 12.2. The performances of both green rusts were unaffected by changes in pH. In contrast, ZVI exhibited a marked decline in reduction rate with increasing pH to become almost ineffective above pH12.

Keywords:

• green rust
• zero-valent iron
• chromite ore processing residue
• chromate reduction
• hyperalkaline waste

Acknowledgements

Thanks are due to Dr D.I. Stewart for his helpful comments on the manuscript, Ramboll UK Limited for access to the COPR site and to Dr R. Whittleston for collecting the leachate.

Funding

This work was supported in part by NERC [grant number NE/D014026/1]; Dr C.M. Rogers is a Daphne Jackson Fellow with funding provided by the Natural Environment Research Council and with additional support from an Alan Tetlow Memorial Bursary (The Royal Society of Chemistry, Water Science Forum).