Measurement of the Reactive Surface Area of Clay Minerals Using Solid-State NMR Studies of a Probe Molecule†
Rebecca L. Sanders, Nancy M. Washton and Karl T. Mueller*
J. Phys. Chem. C, 2010, 114 (12), pp 5491–5498
Abstract: Understanding the surface reactivity of clay minerals is necessary for accurate prediction of natural weathering rates due to the ubiquity of clays in the environment as weathering products of primary minerals. However, the reactivity of the heterogeneous surfaces of a clay can be difficult to characterize as clay mineral edge sites often react at different rates or via different mechanisms than sites on the basal planes. Ultimately, a method is needed to probe quantitatively the reactive surface sites in order to predict clay mineral dissolution rates. In this study, solid-state NMR spectroscopy has been utilized to investigate surface hydroxyl species and their relation to clay surface reactivity. The surfaces of two kaolinite samples (KGa-1b and KGa-2) and two montmorillonite samples (STx-1b and SWy-2) were reacted with the probe molecule (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS), which binds selectively to reactive non-hydrogen bonded Q3Si hydroxyl sites. Quantification of 19F spins in the TFS-treated samples using 19F magic angle spinning NMR peak intensities provides a sensitive measure of the number of reactive hydroxyl sites on a mass normalized (per gram) basis. The reactive surface site densities of KGa-1b and KGa-2 were found to be proportional to published atomic force microscopy edge site fractions. An example from KGa-1b dissolution after 10 days at pH 2.9 and 21 °C revealed no significant change in Brunauer−Emmett−Teller specific surface area, but a 25% decrease in reactive surface site density. We posit this site density determined by solid-state NMR is proportional to the reactive surface area of each clay mineral and its use in future dissolution studies is warranted to investigate how changes in reactive surface area can be tied to decreases in rates of silicon and aluminum release into solution.