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 (As-V), 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 As-V 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. (C) 2007 Elsevier Inc. All rights reserved.