X-Ray Absorption and EPR Spectroscopic Characterization of Adsorbed Copper(II) Complexes at the Boehmite (AlOOH) Surface

Effects of Cu(II) and Zn(II) on PbO2 Reductive Dissolution under Drinking Water Conditions: Short-term Inhibition and Long-term Enhancement

Lead oxide (PbO₂) has the lowest solubility with free chlorine among Pb corrosion products, but depletion of free chlorine or a switch from free chlorine to monochloramine can cause its reductive dissolution. We previously reported that Cu(II) and Zn(II) inhibited PbO₂ reductive dissolution within 12 h. Here, we expanded on this work by performing longer duration experiments and further exploring the underlying mechanisms. Between 12 and 48 h, Cu(II) and Zn(II) had no discernible effect on PbO₂ reductive dissolution. From 48 to 192 h, Cu(II) and Zn(II) enhanced PbO₂ reductive dissolution. Dissolved oxygen (DO) concentrations followed the same trends as PbO₂ reductive dissolution, indicating that the DO was produced by PbO₂ reductive dissolution. On the basis of extended X-ray absorption fine structure spectra, we hypothesize that the inhibitory effect of Cu(II) and Zn(II) on PbO₂ reductive dissolution (<12 h) is caused by decreasing abundance of protonated sites on the PbO₂ surface. The enhanced dissolution (>48 h) may be caused by competitive adsorption of Cu(II) and Zn(II) with Pb(II), which could limit the adsorption of Pb(II) onto PbO₂ that could otherwise inhibit reductive dissolution. This study indicates that stagnation time plays a vital role in determining cations' effects on the stability of Pb corrosion products.

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.

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.

Adsorption of heavy metal ions on soils and soils constituents

The article focuses on adsorption of heavy metal ions on soils and soils constituents such as clay minerals, metal (hydr)oxides, and soil organic matter. Empirical and mechanistic model approaches for heavy metal adsorption and parameter determination in such models have been reviewed. Sorption mechanisms in soils, the influence of surface functional groups and surface complexation as well as parameters influencing adsorption are discussed. The individual adsorption behavior of Cd, Cr, Pb, Cu, Mn, Zn and Co on soils and soil constituents is reviewed.

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.

Metal Ion Coordination at the Water-Manganite (gamma-MnOOH) Interface

The local structure of Cd(II) surface complexes adsorbed onto manganite (gamma-MnOOH) has been investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. Adsorption experiments were carried out within the pH range 7.4-9.8 and with surface coverage from 2.2 to 8.9 µmol/m(2). Quantitative analysis of the EXAFS spectra shows little difference in the local coordination environment of Cd regardless of the coverage. Analysis of the first shell required a third cumulant in an asymmetric distribution model (i.e., non-Gaussian distribution), indicating distorted CdO(6) octahedra with an average Cd-O distance of 2.31 Å. A single second shell of Mn neighbors at a constant distance of 3.33 Å was detected. This comparatively short distance shows that inner-sphere complexes are formed. No Cd neighbor was found which means that cadmium hydroxide precipitation or cluster formation does not occur at the surface at the experimental conditions probed in this study. We assign the Cd-Mn distance to edge sharing between CdO(6) and MnO(6) octahedra, with a geometry similar to that in the mixed solid Cd(2)Mn(3)O(8). This adsorption mode is possible on the {110} and {010} cleavage planes of manganite. Copyright 2000 Academic Press.