Kinetics of Cr(VI) reduction by carbonate green rust

被引:231
作者
Williams, AGB [1 ]
Scherer, MM [1 ]
机构
[1] Univ Iowa, Dept Civil & Environm Engn, Seamans Ctr 4105, Iowa City, IA 52242 USA
关键词
D O I
10.1021/es010579g
中图分类号
X [环境科学、安全科学];
学科分类号
08 ; 0830 ;
摘要
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, [(Fe4Fe2III)-Fe-IIl(OH)(12)][4H(2)O . CO3], was synthesized by induced 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 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)]) 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 x 10(-3) to 11.2 x 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 COO concentrations (< 200 muM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appear to deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher COO concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).
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页码:3488 / 3494
页数:7
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