Contaminant adsorption on nanoscale particles: structural and theoretical characterization of Cu2+ bonding on the surface of Keggin-type polyaluminum (Al30) molecular species

The adsorption of contaminants onto metal oxide surfaces with nanoscale Keggin-type structural topologies has been well established, but identification of the reactive sites and the exact binding mechanism are lacking. Polyaluminum species can be utilized as geochemical model compounds to provide molecular level details of the adsorption process. An Al30 Keggin-type species with two surface-bound Cu(2+) cations (Cu2Al30-S) has been crystallized in the presence of disulfonate anions and structurally characterized by single-crystal X-ray diffraction. Density functional theory (DFT) calculations of aqueous molecular analogues for Cu2Al30-S suggest that the reactivity of Al30 toward Cu(2+) and SO4(2-) shows opposite trends in preferred adsorption site as a function of particle topology, with anions preferring the beltway and cations preferring the caps. The bonding competition was modeled using two stepwise reaction schemes that consider Cu2Al30-S formation through initial Cu(2+) or SO4(2-) adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO4(2-) and the beltway of Al30 play a vital role in governing where Cu(2+) binds. The calculated electrostatic potential of Al30 provides a theoretical interpretation of the topology-dependent reactivity that is consistent with the present study as well as other results in the literature.

Probing the surface structure of divalent transition metals using surface specific solid-state NMR spectroscopy

Environmental and geochemical systems containing paramagnetic species could benefit by using nuclear magnetic resonance (NMR) spectroscopy due to the sensitivity of the spectral response to small amounts paramagnetic interactions. In this study, we apply commonly used solid-state NMR spectroscopic methods combined with chemometrics analysis to probe sorption behavior of the paramagnetic cations Cu(2+) and Ni(2+)at the amorphous silica surface. We exploit the unique properties of paramagnets to derive meaningful structural information in these systems at low, environmentally relevant cation surface loadings by comparing the NMR response of sorption samples to paramagnetic free samples. These data suggest that a simple sorption model where the cation sorbs as inner sphere complexes at negatively charged, deprotonated silanol sites is appropriate. These results help constrain sorption models that are used to describe metal fate and transport.

Copper--alumina--organic matter mixed systems: alumina transformation and copper speciation as revealed by EPR spectroscopy

The chemical forms and solubility of Cu in alumina-organic matter systems were studied separately (Cu/Al and Cu/OM) and in mixtures (Cu/Al/OM) during long-term (up to 8 years) equilibrations at pH 6 and 7.5. The transformation of alumina was monitored by XRD, while the chemical forms of Cu were probed by EPR spectroscopy. Total dissolved Cu was determined by voltammetry. Alumina transformation to gibbsite was more rapid and complete in the Cu/Al system equilibrated at pH 7.5 than at pH 6. The presence of colloidal organic matter (Cu/Al/OM) retarded the transformation of alumina. This effect was more pronounced in the system aged at pH 7.5, likely due to the higher pH that promotes formation of Al3+--organic matter coordination complexes. As expected, the systems at pH 7.5 resulted in lower dissolved Cu concentrations than corresponding systems at pH 6. After long-term equilibrations (8 and 5 years) at pH 6 and 7.5, however, the alumina-containing coprecipitates resulted in the lowest concentrations of Cu in solution (Cu/Al < Cu/Al/OM < Cu/OM). Analyses by EPR spectroscopy indicated that Cu forms inner-sphere complexes in all systems at both pH values. Changes in the chemical forms of coprecipitated Cu (Cu/Al and Cu/Al/OM systems) occurred with time and included Cu occupying discrete sites where Cu-O-Al bond formation was dominant followed by formation of clusters (Cu-O-Cu associations) and in some cases precipitates. The anisotropic EPR parameters of the Cu/OM systems suggested that stronger interactions exist between Cu and organic matter functional groups as compared to Cu interactions with alumina-containing coprecipitates; yet, Cu solubility was highest in the Cu/OM systems. The geochemical processes described in this investigation may be effective in forest soils and wastewater treatment plants where Al and Fe salts are used as flocculation agents and to remove metal contaminants from solution.

Chemistry of alumina, reactions in aqueous solution and its application in water treatment

Due to the presence and significance of alumina in the natural aquatic environment and its increasing application in drinking and wastewater purification, the knowledge of the structure of alumina and its possible interactions with organic and inorganic compounds in water are of great importance. This is of particular importance in both the understanding of natural aquatic environment processes and efficient industrial applications. The chemistry of alumina reactions in water is complex. The adsorption ability of alumina towards organic and inorganic compounds might be influenced by several factors such as: surface characteristics of the adsorbent (surface area, density, pore volume, porosity, pore size distribution, pH(PZC) as well as mechanical strength and purity), pH of the solution, ionic strength, composition of water and the physicochemical properties of adsorbates. The aim of this paper is to give a brief review of the properties of alumina and its reactivity with organic and inorganic compounds present in aqueous solutions. It also summarises the usage of alumina and alumina supported phases in water treatment technology.

An electron paramagnetic resonance study of Cu(II) sorbed on quartz

The nature of the interaction among Cu(II), adsorbed water, and quartz surface was studied using electron paramagnetic resonance (EPR) spectroscopy. The EPR lineshape gave information concerning the motional status of sorbed Cu(II) that revealed its binding strength at the surface. Two distinct absorption lines of sorbed Cu(II), namely, the liquid-type and the solid-type signal, were simultaneously observed at the fully hydrated surface at room temperature. The absorption lines and the variation of their intensity with experimental and measurement conditions such as degree of hydration, pH, ionic strength, and surface coverage indicated that there exist three kinds of Cu(II) entities, the inner-sphere surface complex, the outer-sphere surface complex, and the surface precipitate on the quartz surface, and that their concentrations change with experimental conditions. The reversible conversion of the liquid-type signal to the solid-type one during the drying-wetting or freezing-melting of the surface suggested the development of multiple layers of adsorbed water molecules on the quartz surface. It is assumed that the innermost layer of the water layers contains the inner-sphere Cu(II) surface complexes, while the outer layers contain the outer-sphere complexes whose binding strength decreases outward with increasing distance from the surface. The result of this work suggests that the sorption mechanism of a metal cation on a given mineral surface; hence its mobility in the environment may change significantly with the solution pH, the ionic strength, and the surface coverage.

Cu(II) Sorption Mechanism on Montmorillonite: An Electron Paramagnetic Resonance Study

The mechanism of Cu(II) sorption on montmorillonite was studied with electron paramagnetic resonance (EPR) spectroscopy. The major sorbed species were successfully identified by EPR spectroscopy as they showed distinct signals due to their strength of binding and local structure. The EPR results together with macroscopic sorption data show that the sorption involves at least three different mechanisms. The dominant sorption mechanism changed with pH and Na(+) content in solution. In the acidic pH range, the sorption is independent of pH but is dependent on Na(+) concentration. Like free copper ions, the sorbed Cu(II) shows an isotropic absorption line. It is thought that the Cu(II) is ion-exchanged in the interlayer site and is capable of free tumbling motion despite electrostatic binding force. In the near neutral pH range, the sorption is strongly pH dependent and the sorbed Cu(II) shows no EPR signal. It is interpreted that the Cu(II) is sorbed by a surface complexation mechanism to form an inner-sphere surface complex. The EPR spectrum with a dipolar splitting pattern means that the dimeric Cu(II) surface species is the dominant sorbed species in the basic pH region where the Cu-Cu internuclear distance of the dimer is estimated to be approximately 3.3 Å. Copyright 2000 Academic Press.