Performance evaluation of granular iron for removing hexavalent chromium under different geochemical conditions

Long-term column experiments were conducted under different geochemical conditions to estimate the longevity of Fe0 permeable reactive barriers (PRBs) treating hexavalent chromium (Cr(VI)). Secondary carbonate minerals were precipitated, and their effects on the performance, such as differences in the mechanism for Cr removal and the changes in system hydraulics, were assessed. Sequestration of Cr(VI) occurred primarily by precipitation of Fe(III)–Cr(III) (oxy)hydroxides. Trace amounts of Cr were observed in iron hydroxy carbonate presumably due to substitution of Cr3+ for Fe3+. The formation of Fe(III)–Cr(III) (oxy)hydroxide greatly decreased the reactivity of the Fe0 and thus resulted in migration of the Cr removal front. Carbonate minerals did not appear to contribute to further passivation with regard to reactivity toward Cr removal; rather, the column receiving high contents of dissolved calcium carbonate showed slightly enhanced Cr removal by means of a higher corrosion rate of Fe0 and because of sequestration by an iron hydroxy carbonate. Precipitation of carbonates, however, governed other geochemical parameters. The porosity and hydraulic conductivity in the column receiving high contents of dissolved calcium carbonate did not indicate a great loss in system permeability because the accumulation of carbonates declined as the Fe0 was passivated over time. However, the accumulated carbonates and associated Fe(III)–Cr(III) (oxy)hydroxide could cause problems because the presence of these solids resulted in a decline in flow rate after about 1400 pore volumes of operation.