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

Abstract

Abstract--X-ray absorption spectroscopy (XAS) was used to determine the local molecular environment of Co(II) surface complexes sorbed on three different kaolinites at ambient temperature and pressure in contact with an aqueous solution. Interatomic distances and types and numbers of backscattering atoms have been derived from analysis of the extended X-ray absorption fine structure (EXAFS). These data show that, at the lowest amounts of Co uptake on kaolinite (0.20--0.32 ~mol m-2), Co is surrounded by ~60 atoms at 2.04-2.08 A and a small number orAl or Si atoms (N = 0.6-1.5) at two distinct distances, 2.67-2.72 A and 3.38-3.43/~. These results indicate that Co bonds to the kaolinite surface as octahedrally coordinated, bidentate inner-sphere mononuclear complexes at low surface coverages, confirming indirect evidence from solution studies that a fraction of sorbed Co forms strongly bound complexes on kaolinite. In addition to inner-sphere complexes identified by EXAFS spectroscopy, solution studies provide evidence for the presence of weakly bound, outer-sphere Co complexes that cannot be detected directly by EXAFS. One orientation for inner-sphere complexes indicated by XAS is bidentate bonding of Co to oxygen atoms at two A1-O-Si edge sites or an A1-O-Si and AI-OH (inner hydroxyl) edge site, i.e., comersharing between Co octahedra and AI and Si potyhedra. At slightly higher surface sorption densities (0.51-0.57/~mol m-2), the presence of a small number of second-neighbor Co atoms (average Nco < 1) at 3.10-3.13/~ indicates the formation of oxy-or hydroxy-bridged, multinuclear surface complexes in addition to mononuclear complexes. At these surface coverages, Co-Co and Co-AI/Si distances derived from EXAFS are consistent with edge-sharing between Co and AI octahedra on either edges or (001) faces of the aluminol sheet in kaolinite. Multinuclear complexes form on kaolinite at low surface sorption densities equivalent to < 5% coverage by a monolayer of oxygen-ligated Co octahedra over the N2-BET surface area. These spectroscopic results have several implications for macroscopic modeling of metal ion uptake on kaolinite: 1) Primary binding sites on the kaolinite surface at low uptake are edge, non-bridging AI-OH inner hydroxyl sites and edge AI-O-Si bridging oxygen sites, not Si-OH sites typically assumed in sorption models; 2) specific adsorption of Co is via bidentate, inner-sphere complexation; and 3) at slightly higher uptake but still a small fraction of monolayer coverage, formation of Co multinuclear complexes, primarily edge-sharing with A1-OH octahedra, begins to dominate sorption.

Adsorption of Zinc from Aqueous Solutions Using Modified Brazilian Gray Clay

“…It is generally agreed that clay is an efficient adsorbent for some heavy metals, especially for lead [12], copper [13], cadmium [14], zinc [1,5,15,16] and nickel [17]. Although kaolinite exhibits the least exchangeability, some studies have confirmed the potential of natural and modified kaolinites for the adsorption of metal ions from solutions [16,[18][19][20][21][22][23][24].…”

XPS Study of Pb(II) Adsorption on .GAMMA.-Al2O3 Surface at High pH Conditions

“…20) On the other hand, extended X-ray absorption fine structure (EXAFS) revealed that Co on kaolinite formed inner-sphere surface complex at neutral pH. 21) So, it needs various experimental methods to understand adsorp-tion mechanisms. In this study, effect of ionic strength, XPS measurement, and MO calculation were applied to elucidate adsorption mechanism between Pb(II) and substrate γ -Al 2 O 3 at high pH condition.…”

EXAFS Studies about the Sorption of Cadmium Ions on Montmorillonite

“…Recent studies using EXAFS (extend X-ray absorption fine structure) have shown that heavy metals are sorbed on clay and oxide surfaces accompanied by the formation of multinuclear or polynuclear surface complexes (i.e., surface precipitate) much more frequently than previously expected based on the thermodynamical models. [5][6][7][8][9] These microscopic studies have promoted our understanding of the real sorption mechanism. EXAFS analyses of Cd species on the montmorillonite, however, have never been reported in spite of its importance for soil pollution mainly because the K-edge EXAFS requires X-rays with higher energy than 23 keV.…”