Chromate Reduction by Hydroxycarbonate Green Rust

Key words: mineral/water interface, reduced Fe minerals, natural attenuation To address the potential for abiotic natural attenuation of chromate, we have studied the kinetics and mechanism of chromate removal in the presence of hydroxycarbonate green rust. The kinetics of chromate removal were studied in batch systems to evaluate the effect of green rust surface area and pH. The rate of chromate removal was dependent on the green rust surface area concentration, and a surface area normalized rate constant (k SA = 0.044 L m-2 min-1) was determined. The rate of chromate removal increased with decreased pH. The effect of pH, however, was modest, with only a five-fold increase in rate observed over five pH values (from pH 9.0 to 5.0). Of the three potential pathways hypothesized for chromate removal (reduction, adsorption, or interlayer exchange), reduction was found to be the dominant pathway controlling chromate loss from solution. Negligible adsorption or interlayer exchange of chromate ions was found to occur independently of reduction. The rapid reduction of chromate appears to result in precipitation of Cr(III) phases, such as Cr(OH) 3 or [Fe,Cr](OH) 3 .