Kinetics of chromium(III) oxidation by manganese(IV) oxides using quick scanning X-ray absorption fine structure spectroscopy (Q-XAFS).
The initial kinetics of Cr(III) oxidation on mineral surfaces is poorly understood, yet a significant portion of the oxidation process occurs during the first seconds of reaction. In this study, the initial rates of Cr(III) oxidation on hydrous manganese oxide (HMO) were measured at three different pH values (pH 2.5, 3, and 3.5), using a quick X-ray absorption fine structure spectroscopy (Q-XAFS) batch method. The calculated rate constants were 0.201, 0.242, and 0.322 s(-1) at pH 2.5, 3, and 3.5, respectively. These values were independent of both [Cr(III)] and [Mn(II)] and mixing speed, suggesting that the reaction was "chemically" controlled and not dependent upon diffusion at the time period the rate parameters were measured. A second-order overall rate was found at three pH values. This represents the first study to determine the chemical kinetics of Cr(III) oxidation on Mn-oxides. The results have important implications for the determination of rapid, environmentally important reactions that cannot be measured with traditional batch and flow techniques. An understanding of these reactions is critical to predicting the fate of contaminants in aquatic and terrestrial environments.
Landrot
Gautier
G
Department of Plant and Soil Science and Delaware Environmental Institute, University of Delaware, 152 Townsend Hall, Newark, Delaware 19716. , USA.
Ginder-Vogel
Matthew
M
Sparks
Donald L
DL
D002857:Chromium / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D017895:Manganese Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods*; D014965:X-Rays
D017895:Manganese Compounds; D010087:Oxides; D002857:Chromium; C027424:manganese oxide