This project involves basic research directed toward determining the molecular structure of metals on hydrated clay mineral surfaces. Clay minerals are among the most important constituents of soils, particularly soils located in temperate regions. In addition clay minerals have been used as catalyst and in clay liners designed to attenuate and contain contaminants. Clay minerals are responsible for a significant portion of a soil’s ability to retain cations that are plant nutrients (e.g., manganese, copper, potassium, zinc) and contaminants (e.g., lead, nickel, cadmium, cesium). The retention of cations by soils controls their availability to microorganisms, plants and animals. Thus, it is critical that the reaction mechanisms occurring between metals and clay minerals are understood.

Clay minerals are unique from oxide minerals because the majority of their surfaces (interlayers) have large electrostatic potentials and no hydroxide ligands. This distinct structure has direct impacts on the solution chemistry near the surface. Traditionally the reaction mechanism that occurs between cations and clays was considered to be an electrostatic interaction (outer-sphere). Recent research has provided evidence that other sorption reaction mechanisms are also occurring, including sorption complexes that involve covalent bonds (inner-sphere) and clusters of cations on the clay surfaces (multinuclear complexes). The formation of multinuclear complexes has been shown to occur in clay suspensions where aqueous speciation models predict they should not be occurring. This suggests that the clay minerals are enhancing the formation of these complexes. Evidence for aqueous multinuclear and inner-sphere complexation for Pb sorbed on montmorillonite has been published. However, the mechanisms for enhanced multinuclear complexation, and the location for these complexes have not been determined. There are several hypotheses that explain why clay minerals enhance surface precipitation and aqueous complexation near their surfaces. This research will provide information that will support or refute these observations and hypotheses.

The goal of this project is to use molecular spectroscopy and clay minerals with unique sorption site characteristics to investigate the location and molecular structure of lead (Pb) and copper (Cu) sorbed onto clay minerals under various equilibrium conditions and sorption strategies (including site-blocking strategies). This research is unique in that it takes advantage of the physical properties of clay minerals to isolate sorption sites and mechanisms. The results from this research will provide new information on the ability of mineral surfaces to hydrolyze metals. Such fundamental information will be useful for soil scientists, geologists, environmental scientists, and engineers, both in practice and teaching.