Cation and Water Interactions in the Interlamellae of a Smectite Clay

Advances in NMR instrumentation and availability have led to increased application to mineral systems and to environmental problems. The sensitivity of high-field NMR systems is nearly sufficient to work at real environmental concentrations. Even with limited sensitivity, the amount of chemical information obtained through NMR spectroscopy makes it a very valuable technique in many model systems. The application of NMR spectroscopy in mineral systems has been primarily limited to studies of the structural metals aluminum and silicon. However, in recent years there have been several publications on mobile cations in minerals, including work on the exchangeable cations in clays. Our interests lie in understanding the sorption of cations in clays, the structural sites available for that sorption, and the role of water in cation–clay interactions. Our goal is to eventually understand the molecular interactions that determine the adsorption and diffusion of cations in clays and, thus, the role of clays in determining cation transport through the geosphere. This fundamental understanding has applications in the fate of heavy metals, radionuclides, and even the mobility of nutrients for plants. It is well known that there are very strong interactions between metals and humic materials and these are also strong contributors to cation mobility. However, for simplicity, we have chosen to focus on the interactions of mobile metal ions with well-characterized clays. An NMR-based approach to this problem can take two complementary directions: first, studies of the structural components of clays such as 29Si and 27Al NMR as a function of cation or hydration; second, NMR studies of probe molecules—which in this case are the cations themselves. High magnetic field, multinuclear NMR spectrometers make it quite possible to study various “uncommon” nuclei with relative ease. It is our experience that using the cations as probe nuclei for studying sorption phenomena yields more information than studies of structural nuclei. This chapter is basically a report of work in progress on several systems that are starting to yield interesting results, which it is hoped will lead to a general understanding of these complex systems.