Tuesday, 11 July 2006 - 4:40 PM

X-Ray Absorption Spectroscopy Investigation of Copper Adsorption Mechanisms on Clay Minerals.

Daniel Strawn, University of Idaho, Soil and Land Resources Division, PO Box 442339 Ag Sci. 242, Moscow, ID 83844-2339

Clay minerals are ubiquitous in the environment and play critical roles in the fate and transport of metals. Cation exchange is a dominant reaction mechanism for metals on clay. However, in recent years evidence for surface precipitation and adsorption on variable charge edge sites has been confirmed by molecular spectroscopy, suggesting that sorption phenomena on clay minerals is varied and complex. The goal of the research presented here is to investigate reaction mechanisms occurring on clay mineral surfaces as a function of mineral and solution properties. Reaction mechanisms were investigated by conducting macroscopic sorption experiments and probing the reaction products using molecular-scale techniques, such as near-edge (XANES) and extended-fine structure (EXAFS) X-ray absorption spectroscopy (XAS). Because clay minerals preferentially align themselves with respect to their crystallographic axis, and synchrotron X-rays are polarized, the potential to preferentially probe the surface complexes using polarized XAS (P-XAS) exists. P-XAS allows for added insight into metal speciation on the clay mineral surfaces. In this research XAS and P-XAS was used to investigate copper sorption on vermiculite and montmorillonite as a function of pH, ionic strength, and loading level. At high ionic strength and pH 6 sorption occurred primarily on clay mineral edges. Polarized EXAFS and XANES indicated that Cu sorbed on Llano vermiculite was aligned with its equatorial ligands in the ab plane of the clay particle. The polarized XANES results were interpreted using molecular orbital theory, which confirmed the molecular environment and orientation of the sorbed Cu. In contrast to the Llano vermiculite, sorption of Cu on Wyoming montmorillonite indicated that binuclear-Cu clusters were sorbed on the clay surface. In none of the Cu-equilibrated clay minerals were vast multinuclear complexes observed, such as have been observed for sorption of Ni, Zn, and Co in systems in which the metals are unsaturated with respect to oxide and hydroxide phases. It is well known that the atomic characteristics of Cu are distinct from the other first row transition metals in that its molecular coordination favors an axially distorted environment, known as Jahn-Teller distortion. Interpretation of the XAS data confirm this atomic property, and offers an explanation for the unique behavior of Cu compared to Ni, Zn, and Co. Thus by using advanced spectroscopic methods we have gained atomic and molecular level insight into Cu speciation on clay mineral surfaces. By understanding reaction mechanisms occurring on the clay surfaces better models can be developed that will lead to improved understanding of metal fate in the environment.

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