Chromate reduction in highly alkaline groundwater by zerovalent iron: Implications for its use in a permeable reactive barrier

It is not currently known if the widely used reaction of zerovalent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilize Cr leaching from hyperalkaline chromite ore processing residue (COPR). This study compares Cr(VI) removal from COPR leachate and chromate solution by ZVI at high pH. Cr(VI) removal occurs more rapidly from the chromate solution than from COPR leachate. The reaction is first order with respect to both [Cr(VI)] and the iron surface area, but iron surface reactivity is lost to the reaction. Buffering pH downward produces little change in the removal rate or the specific capacity of iron until acidic conditions are reached. SEM and XPS analyses confirm that reaction products accumulate on the iron surface in both liquors, but that other surface precipitates also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si, and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that, although Cr(VI) reduction by ZVI will occur at hyperalkaline pH, other solutes present in COPR leachate will limit the design life of a PRB.