Hardness and carbonate effects on the reactivity of zero-valent iron for Cr(VI) removal

Abstract

Zero-valent iron (Fe 0) was used to remove hexavalent chromium, Cr(VI), in groundwater via a coupled reduction-oxidation reaction. Nine columns were set up under various groundwater geochemistry to investigate the effects of hardness and carbonate on Cr(VI) removal. The Cr(VI) removal capacity of Fe 0 was found to be about 4 mg Cr/g Fe 0 in the control column (i.e., column 1). A slight decrease in the Cr(VI) removal capacity was found in the presence of calcium hardness. However, there was a 17% drop in the Cr(VI) removal capacity when magnesium hardness was present at low to moderately hard level. Results also revealed that carbonate changed the morphology of the Fe 0 by formation of pale green precipitates on the iron filings. Furthermore, there was a 33% decrease in the Cr(VI) removal capacity of Fe 0 when both carbonate and hardness ions were present. In general, the presence of hardness ions and carbonate in groundwater have great impact on the Fe 0 by formation of passivated precipitates, such as CaCO 3, on the Fe 0 surface resulting in a diminished lifespan of the Fe 0 by blocking electron transfer.