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

Abstract

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.

Molecular Structure and Binding Sites of Cobalt(II) Surface Complexes on Kaolinite from X-Ray Absorption Spectroscopy

“…Studies of Co(II) complexes on oxides (3,-A1203, TiO2, and SIO2) indicate inner-sphere, multinuclear complex formation that may be influenced by the structure of the substrate (Chisholm-Brause et al, 1990b;ChisholmBrause, 1991;O'Day et al, 1991). Inner-sphere, multinuclear complex formation is proposed for Pb(II) complexes on 3'-A1203 (Chisholm-Brause et aL, 1990a) and goethite (Roe et al, 1991), Pb(II) on hydrous Mnoxide , and Cr(III) on Mn-and Fe-oxide (Charier and Manceau, 1992;.…”

“…In all sorption samples, the shape of the Co K-edge is similar to that of reference spectra in which Co is six-coordinated (Co(NO3)2(aq) and Co(OH)2(s)), but differs markedly from the spectrum of CoAl204(s) in which Co is four-coordinated. The latter spectrum contains a prominent pre-edge feature at ~7709 eV that corresponds to a bound state ls to 3d orbital electronic transition (Brown et al, 1988;Chisholm-Brause et al, 1990b). This transition is characteristic of tetrahedrally coordinated metal cations, but it is not allowed when the bonding environment around the metal cation has a center of inversion as in octahedral coordination (Calas and Petiau, 1983;Waychunas et al, 1983).…”

Wettability Contrasts in Kaolinite and Illite Clays: Characterization by Infrared and X-Ray Absorption Spectroscopies

“…Henderson (1995) showed that numerous earth scientists have taken full advantage of its unique properties. Chisholm-Brause, Hayes et al (1990) and Chisholm-Brause, O'Day et al (1990) performed XAS studies of wet samples to determine the structure of metal complexes adsorbed onto mineral surfaces. Since the A1 and Si sites in clays are able to contract chemical interactions with the asphaltenes of the oil, the local environments around A1 and Si atoms in both illite and kaolinite have been investigated by EXAFS and X-ray absorption near edge structure (XANES).…”

Mineral surfaces and bioavailability of heavy metals: A molecular-scale perspective

“…For example, bidentate or tridentate surface complexes, attached by two and three bonds, respectively, will typically be more difficult to desorb than monodentate complexes (attached by a single bond), all other factors being equal. Another important factor in the effectiveness of sorption reactions in removing contaminant metal ions from solution is the possibility of multinuclear complex formation (49). In this case, more than one metal ion is involved in the surface complex.…”