Solid State 15N NMR Study of Pyridine Adsorption on Clay Minerals

A solid state nuclear magnetic resonance (NMR) method is presented for spectroscopic adsorption studies of organic pollutant compounds on surfaces of powders under environmentally relevant hydration conditions. The applicability of the solid state NMR spectroscopy was investigated using pyridine as a model adsorbate and a clay mineral, hectorite, as an adsorbent. Solid state 15 N magic angle spinning (MAS) and cross-polarization (CP)/MAS NMR methods were used to study adsorption of [ 15 N]pyridine on fully hydrated and on dehydrated homoionic K-, Ca-, Mg-, and Al-hectorites. Powder X-ray diffraction (XRD) was used to determine the expansion of basal spacings and to confirm intercalation of pyridine into clay interlayers. With the exception of hydrated Al-hectorite, more intense signals were obtained with MAS than with CP/MAS, and cross-polarization was less efficient on dehydrated clay/pyridine samples than on hydrated ones, indicative of a higher mobility of pyridine in the interlayers of dehydrated clays compared to hydrated ones. Pyridine interacted primarily through hydrogen bonding to the interlayer water on the hydrated hectorites. On the dehydrated K-hectorite ( d 001 = 10.7 Å before pyridine adsorption), pyridine solvated the interlayers. Dehydrated Ca- and Mg-hectorites ( d 001 = 12.9 and 12.8 Å, respectively, before pyridine adsorption) retained one layer of hydration water to which pyridine hydrogen bonds. Intercalation of pyridine was facilitated by the presence of water, the extreme case being Al-hectorite where almost no intercalation was observed without water. Spinning sidebands were only observed for the pyridine resonance between −90 and −94 ppm. This anisotropic species is assigned to the pyridine keying into the ditrigonal hole, hydrogen bonded to the proton of structural OH. The results clearly show that 15 N NMR of an adsorbed N-containing organic compound is a powerful tool for identifying and distinguishing various surface complexes and adducts on hydrated mineral surfaces.