Kinetics of Cr(VI) Reduction by Carbonate Green Rust

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII 4FeIII 2(OH)12][4H2O·CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(II)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 ± 7 m2 g-1 was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higher than the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (k obs) ranging from 1.2 × 10-3 to 11.2 × 10-3 s-1 were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (