1. Quantification of Rapid Environmental Redox Processes with Quick-Scanning X-ray Absorption Spectroscopy (Q-XAS) M. Ginder-Vogel, G. Landrot, J. S. Fischel, and D. L. Sparks Delaware Environmental Institute, Department of Plant and Soil Sciences, University of Delaware Quantification of the initial rates of environmental reactions at the mineral/water interface is a fundamental prerequisite to determining reaction mechanisms and contaminant transport modeling and predicting environmental risk. Until recently, experimental techniques with adequate time resolution and elemental sensitivity to measure initial rates of the wide variety of environmental reactions were quite limited. In this study, we apply quick-scanning x-ray absorption spectroscopy (Q-XAS), at sub-second time-scales, to measure the initial oxidation rate of As(III) to As(V) by hydrous manganese(IV) oxide. Using Q-XAS, As(III) and As(V) concentrations were determined every 0.98 s in batch reactions. The initial apparent As(III) depletion rate constants (t < 30 s) measured with Q-XAS are nearly twice as large as rate constants measured with traditional analytical techniques. Our results demonstrate the importance of developing analytical techniques capable of analyzing environmental reactions on the same time scale as they occur. Experimental setup used to collect Q-XAS data. M. Ginder-Vogel, G. Landrot, J. S. Fischel, and D. L. Sparks, “Quantification of rapid environmental redox processes with quick-scanning x-ray absorption spectroscopy (Q-XAS)“, PNAS, 106, 38, 16124–16128 Work performed on beamline X18B X18B

2. Kinetics of Chromium(III) Oxidation by Manganese(IV) Oxides Using Quick Scanning X-ray Absorption Fine Structure Spectroscopy (Q-XAFS) G. Landrot, M. Ginder-Vogel, and D. L. Sparks Delaware Environmental Institute, Department of Plant and Soil Sciences, University of Delaware 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. 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. G. Landrot, M. Ginder-Vogel, and D. L. Sparks, “Kinetics of Chromium(III) Oxidation by Manganese(IV) Oxides Using Quick Scanning X-ray Absorption Fine Structure Spectroscopy (Q-XAFS)“, Environ. Sci, Technol., 44, 143-149 Work performed on beamline X18B Cr(III) oxidation kinetics using a QXAFS technique, at pH 2.5, [Cr(III) ]=100mM, [HMO]=20g/L and 0-240 seconds. Each XANES spectrum shown represents 3 seconds of the reaction (average of four 0.75s spectra). X18B