Kinetics of Cr(VI) reduction by carbonate green rust

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were st udied for a range of reactant concentrations and pH values. Carbonate green rust, [(Fe4Fe2III)-Fe-IIl(OH)(12)][4H(2)O . CO3], was synthesized by induc ed hydrolysis (i.e., co precipitation) of an Fe(III)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m(2) g( -1) was measured for five separate batches of freeze-dried green rust preci pitate. Heterogeneous reduction by Fe(II) associated with the carbonate gre en rust appears to be the dominant pathway controlling Cr(VI) loss from sol ution. The apparent stoichiometry of the reaction between ferrous iron asso ciated with green rust ([Fe(II)(GR)]) Cr(VI) was slightly higher than the e xpected 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 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decr ease in rate with increasing pH (from 5.0 to 9.0). At low COO concentration s (< 200 muM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appear to devi ate from first-order kinetics and approach a constant value. Estimated amou nts of surface Fe(II) and total Fe(II) suggest that the deviation from firs t-order kinetics observed at higher COO concentrations and the 50-fold decr ease in rate observed upon three sequential exposures to Cr(VI) is due to e xhaustion of available Fe(II).