Immobilization of Chromate from Hyperalkaline Waste Streams by Green Rusts and Zero-valent iron

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). ZVI is effective at acid to circum-neutral pH but Cr(VI) reduction rates in hyperalkaline (>pH 10) effluent from chromite ore processing residue (COPR) wastes are unknown. 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 chromate solutions at pH 7.7-12.3 and a COPR leachate (pH ≈12.2). Green rust removes chromate from high pH solutions (pH 10-12.5) very rapidly (<400 seconds). Chromate reduction rates for both green rust phases were consistently higher than for ZVI throughout the pH range studied; the surface area normalised rate constants were two orders of magnitude higher in the leachate solution at pH 12.2. The performances for both green rusts were unaffected by changes in pH, which contrasted with that of ZVI which exhibited a marked decline in reduction rate with increasing pH to become almost ineffective above pH 12.