An ATR-FTIR spectroscopic approach for measuring rapid kinetics at the mineral/water interface

This study presents a methodology for studying rapid kinetic reactions for IR active compounds. In soils, sediments, and groundwater systems a rapid initial chemical reaction can comprise a substantial portion of the total reaction process at the mineral/water interface. Rapid-scan attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy is presented here as a new method for collecting rapid in situ kinetic data. As an example of its application, the initial oxidation of arsenite (As III) via Mn-oxides is examined. Using a rapid-scan technique, IR spectra were collected with a time resolution of up to 2.55 s (24 scans, 8 cm(-1) resolution). Through observation and analysis of IR bands corresponding to arsenate (AsV), rapid chemically-controlled As III oxidation is observed (initial pH 6-9) with 50% of the reaction occurring within the first one min. The oxidation of As III is followed by rapid binding of AsV to HMO, at least in part, through surface bound Mn II. The experimental data indicate that rapid-scan FTIR is an effective technique for acquisition of kinetic data, providing molecular scale information for rapid reactions at the solid/liquid interface.