Abstract
The influence of clay minerals on the reduction of Cr 6+ by citric acid was investigated at pH values 4.0, 4.5 and 5.0 at 25 °C. The results indicate that montmorillonite and illite greatly accelerate the reduction reactions at pH 4.0 and 4.5, but their effects are dramatically reduced at pH 5.0. The role of clay minerals in accelerating the reactions is in the order: illite > montmorillonite ≫ kaolinite, which has a positive correlation with the amount of Mn 2+ adsorbed on the surfaces of these minerals. With light, Fe 3+ also significantly increases reaction rates. Ethylenediaminetetraacetic acid (EDTA) greatly suppresses the acceleration of the reduction reactions by these minerals, indicating that EDTA competes with citric acid for Mn 2+. Thus, the formation of complexes between Mn 2+ and citric acid could be a prerequisite for the acceleration of the reductions of Cr 6+ by clay minerals. In addition, there is no relationship between the specific surface area of clay minerals and the reduction rate of Cr 6+ by citric acid.
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