Evidence for multinuclear metal-ion complexes at solid/water interfaces from X-ray absorption spectroscopy

Metals dissolved in natural waters often become sorbed onto oxide or clay minerals, so that prediction of their chemical behaviour and transport properties requires knowledge of the structure and bonding of metal species at the solid/water interface. For many sorption systems, X-ray absorption spectroscopy (XAS) can be used to determine the identity and number of nearest-neighbour atoms and interatomic distances in aqueous complexes on solid surfaces, and thus to identify the dominant type of surface complex and the partitioning mechanism. Here we describe an XAS study of divalent cobalt (Co(II)) complexes sorbed on three different solids, gamma-Al2O3, rutile (TiO2) and kaolinite (Al2Si2O5(OH)4). We find direct evidence for the presence of multinuclear sorption complexes at surface coverages below one monolayer of Co(II) atoms. Our spectroscopic data reveal distinct differences in the number of coordinating atoms and interatomic distances in the surface complexes formed on each of the solids at the same sorption density. These results suggest that different oxide and clay surfaces influence the structure and properties of aqueous surface complexes, and therefore must be accounted for in models of metal-ion sorption.