X-Ray Absorption and Electron Paramagnetic Resonance Studies of Cu(II) Sorbed to Silica: Surface-Induced Precipitation at Low Surface Coverages

Does silica addition affect translocation and leaching of cadmium and copper in soil?

Soil and groundwater contamination with potentially toxic elements (PTEs) including cadmium (Cd) and copper (Cu) has become a serious problem for ecosystem functioning. Silicon (Si) may precipitate these metals as silicates, and may also form, at undersaturation of silicates, 'Si-contaminant compounds', i.e. particles of polymerized silica with PTEs incorporated or adsorbed by inner-sphere complexes. While the formation of these compounds in aqueous solution has been proven, their formation in soil remains unclear yet. Therefore, we conducted column experiments with a topsoil horizon artificially contaminated with Cd or Cu solutions (10 mM) in the presence (10 mM) and absence of monomeric Si, and monitored the elemental composition of the eluates during 12 irrigation steps with artificial rainwater by microwave-plasma atomic emission spectrometry, the size and charge of the particles eluted by dynamic light scattering and phase analysis light scattering, and determined the spatial distribution of total and exchangeable Cd and Cu in soil after the experiments. When Si was previously applied to soil, significantly larger particles (up to > 200 nm) in the eluates indicated Si polymerization and formation of Si-contaminant compounds. However, Cd and Cu concentrations were very low (<0.4 μM), pointing to efficient retardation in soil. In any variant, the particles formed were slightly negatively charged (-11 mV). The molar metal:Si ratios in the eluates and significant correlations between the amounts of Si and metals in soil extracted by NH₄NO₃ pointed to the formation of Si-contaminant compounds, too. More Cu than Cd was retained in soil, and significantly more in the presence of Si, but less Cu than Cd was in exchangeable form. While particularly Cu formed Si-contaminant compounds, which reduced the concentration of Cu ions, the Si-contaminant-compound particles in the eluates remained very small, thus potentially susceptible to particulate export from soil into the groundwater.

Formation of Cd precipitates on γ-Al2O3: Implications for Cd sequestration in the environment

Apart from surface complexation, precipitation of minerals also plays an important role in reducing the mobility and transport of heavy metals in the environment. In this study, Cd(II) sorption species on surfaces of γ-Al₂O₃ at pH 7.5 were characterized using multiple techniques. Results show that in addition to adsorption complexes, Cd hydroxide phases (Cd(OH)₂ precipitates and Cd_x(OH)_y polynuclear complexes) were formed at the initial stages of Cd(II) sorption and gradually transformed to CdCO₃ with time. In addition, Cd(II) formed CdAl layered double hydroxide (LDH) on γ-Al₂O₃ under various conditions, independent of temperature and Cd loadings. The formation of Cd hydroxide phases and CdAl LDH could be ascribed to surface-induced precipitation because the bulk solution was undersaturated with respect to hydroxides. CdAl LDH formation on the Al-bearing mineral here is rather surprising because typically this occurs with elements of ionic radii similar to that of Al³⁺; this formation is unknown for metals such as Cd(II) with a much larger ionic radius. The thermodynamic feasibility of CdAl LDH formation was further confirmed by laboratory synthesis of CdAl LDH and density function theory (DFT) calculations. These results suggest that Cd precipitation on Al-bearing minerals can be an important mechanism for Cd immobilization in the natural environment. Additionally, the finding of CdAl LDH formation on Al-bearing minerals and the thermodynamic stability of CdAl LDH provides new insights into the remediation of Cd-polluted soils and aquatic systems.

Binding of transition metals to monosilicic acid in aqueous and xylem (Cucumis sativus L.) solutions: a low-T electron paramagnetic resonance study

The supplementation of monosilicic acid [Si(OH)4] to the root growing medium is known to protect plants from toxic levels of iron (Fe), copper (Cu) and manganese (Mn), but also to mitigate deficiency of Fe and Mn. However, the physicochemical bases of these alleviating mechanisms are not fully understood. Here we applied low-T electron paramagnetic resonance (EPR) spectroscopy to examine the formation of complexes of Si(OH)4 with Mn(2+), Fe(3+), and Cu(2+) in water and in xylem sap of cucumber (Cucumis sativus L.) grown without or with supply of Si(OH)4. EPR, which is also useful in establishing the redox state of these metals, was combined with measurements of total concentrations of metals in xylem sap by inductive coupled plasma. Our results show that Si(OH)4 forms coordination bonds with all three metals. The strongest interactions of Si(OH)4 appear to be with Cu(2+) (1/1 stoichiometry) which might lead to Cu precipitation. In line with this in vitro findings, Si(OH)4 supply to cucumber resulted in dramatically lower concentration of this metal in the xylem sap. Further, it was demonstrated that Si(OH)4 supplementation causes pro-reductive changes that contribute to the maintenance of Fe and, in particular, Mn in the xylem sap in bioavailable 2+ form. Our results shed more light on the intertwined reactions between Si(OH)4 and transition metals in plant fluids (e.g. xylem sap).

New insights into the effects of the herbicide imazethapyr on Cu(II) ecotoxicity to the aquatic unicellular alga Scenedesmus obliquus

Interactions between heavy metals and organic contaminants can result in speciation changes of heavy metals. Therefore, it is unknown whether organic contaminants in aquatic environments act as selective agents to mediate the toxicity of heavy metals. To elucidate these interactions, the effects of the herbicide imazethapyr (IM) on Cu(II) ecotoxicity to the aquatic unicellular alga Scenedesmus obliquus were investigated. It was found that the toxicity of Cu could be mediated by IM. To explore the mechanisms involved, complex formation, the catalytic activity of the complex, the Cu species and the distribution of Cu and Fe in the algal cell were characterized. The results showed that Cu(II) and IM formed an octahedral complex with an IM:Cu molar ratio of 2:1. These complexes also catalyzed the disproportionation of hydrogen peroxide. Analysis of the K-edge of Cu using XAFS spectroscopy indicated that when treated with Cu, the Cu was bound to polygalacturonic acid (on the cell wall), and once inside the cell, Cu may complex with the reduced glutathione (GSH) (in the cell). When the cell is treated with IM and Cu simultaneously, IM-Cu may be the primary complex formed. Once Cu combines with IM, it is difficult for it to interact with the cell wall. In addition, using scanning transmission soft X-ray microscopy, it was found that Cu could induce changes in the distribution of the essential trace element Fe, whereas IM-Cu cannot. This finding demonstrates the importance of interactions between heavy metals and organic contaminants, which are able to mediate the toxicity of heavy metals and should be considered in future risk assessments.

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.

Effect of complexing anions on europium sorption on suspended silica: a TRLFS study for ternary complex formation

The sorption of europium, Eu(III), on suspended silica was studied in perchlorate media under varying concentrations of Eu(III) and silica, at pH 4 to 8, and ionic strength from 0.20 to 1.40 M NaClO4at 298 K. The complexing anions, carbonate, acetate, citrate, oxalate, phosphate and EDTA, affected the sorption of Eu(III). A synergistic enhancement in the sorption of Eu(III) was observed for oxalate and phosphate ligands. Other anions suppressed the sorption in the order: acetate < carbonate < citrate < EDTA. The results of TRLFS studies were consistent with ternary Eu/silica/ligand complexation for oxalate and phosphate anions.

Sorption of Cu(II) onto vineyard soils: Macroscopic and spectroscopic investigations

The sorption of Cu on five vineyard soils was examined via macroscopic and spectroscopic investigations. The composition of the soils was previously determined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). X-ray absorption spectroscopy (XAS) was employed to determine the metal environment with regard to the identity and interaction of the nearest atomic neighbors, the bond distances, and the coordination numbers. The five soils present similar sorption properties and there is no XAS evidence that the nature of the soil samples affects the local chemical environment of Cu(II). The kinetics of the Cu sorption reactions is rapid, with the equilibrium loading of Cu on the surface achieving approximately 200 mumol g(-1), i.e., 12.7 mg g(-1). The XAS data indicate that Cu is adsorbed in the form of inner-sphere complexes with first shell CuO parameters of four equatorial CuO bonds equal to 1.93 A and two axial CuO bonds at 2.43 A. This is in accordance with a Jahn-Teller distorted octahedron environment around copper. Our results provide evidence of the complexation of Cu(II) onto soil organic matter coated with an inorganic surface (quartz, clay, and goethite).

Adsorption of several metal ions onto a model soil sample: equilibrium and EPR studies

Soils play an important role in the control of metallic cations in the environment. Therefore, knowledge of the adsorption properties of soil is crucial in understanding and solving pollution problems. Adsorption isotherms provide a macroscopic view of the retention phenomena. The aim of this paper is to study iron, manganese, and chromium adsorption onto a soil sample as a function of the reaction time, pH, and metal concentration. The adsorption isotherms allow the determination of the affinity order of metals for the surface of the soil sample as such: Fe(3+)>Cr(3+)>Mn(2+). The equilibrium data fit well with the Langmuir and Freundlich models and confirm the affinity order of the soil sample for these metals. These adsorption data are combined with EPR spectroscopy to obtain structural information about the surface complexes formed. Iron is held in inner-sphere complexes. Manganese is simultaneously held in outer- and inner-sphere complexes. Due to poor resolution, chromium was not detected by EPR and thus it is impossible to infer coordination sphere and coordination number. Iron and manganese are in an octahedral environment.

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.

Adsorption of Sulfate and Copper(II) on Goethite in Relation to the Changes of Zeta Potentials

The amount of adsorption of sulfate and Cu(II) from single- and binary-adsorbate systems on goethite were measured. All experiments were carried out with and without supporting electrolyte (0.01 M NaNO(3)) as a function of equilibrium pH (2 - 7), adsorbate concentration (0.21 - 1.57 mM), and temperature (15 - 35 degrees C). At a given equilibrium pH, it was shown that the adsorption isotherms of single sulfate, rather than Cu(II), could be well described by the Langmuir equation. The isosteric enthalpy of sulfate adsorption was also evaluated. In contrast to a single-adsorbate system, the adsorption of Cu(II) was enhanced and that of sulfate was inhibited in binary adsorbate systems under the conditions studied. Finally, the changes in zeta potentials of the goethite suspensions with solution pH before and after adsorption were measured, which could be macroscopically related to the amount of sulfate adsorption even in binary-adsorbate systems.

Formation of Ternary Surface Complexes by Adsorption of 2,2'-Bipyridine onto Silica Modified with Copper Ions

The adsorption of 2,2'-bipyridine on a copper-containing silica surface has been studied. The equilibrium of bipyridine binding to surface copper ions are well reproduced by the constants for the formation of ternary surface complexes from an aqueous solution containing copper ions and bipyridine. This indicates clearly that under experimental conditions all reactions are reversible and speciation is controlled by thermodynamics. Copyright 2001 Academic Press.

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.

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.

Specific Adsorption of Nickel and zeta Potential of Silica at Various Solid-to-Liquid Ratios

Theelectrokinetic curves of silica obtained at given total concentrations of heavy metal cations depend on the solid-to-liquid ratio; namely, at low solid-to-liquid ratios the sign of the zeta potential of silica is reversed to positive at relatively low concentrations of heavy metal cations, while at higher solid-to-liquid ratios the electrokinetic curves are not substantially influenced at much higher concentrations. The surface complexation model fails to properly reflect the Ni effect on the course of electrokinetic curves of silica. The course of electrokinetic curves [positions of the isoelectric point(s) and of the maxima in mobility] is correlated with a linear combination of the surface charge density in the absence of heavy metal cations at given ionic strength and the adsorption density of Ni. Copyright 1999 Academic Press.

The Structure of Amorphous Bulk and Silica-Supported Copper(II) Hydroxides

The data obtained show that at pH 7 copper(II) ions are adsorbed on a SiO2 surface as polymeric species of hydroxide nature. The structure of these species is similar to that of the bulk amorphous copper hydroxide. The amorphous state of supported Cu(OH)2 is caused by a small (ca. 11 Å) size of the surface particles. In contrast, the overstoicheometric water molecules seem to act as "amorphizers" of the bulk copper hydroxide. The structures of the bulk and dispersed amorphous copper(II) hydroxide were determined. The amorphous Cu(OH)2 has a layered structure close to the structure of the crystalline hydroxide, but the layers in the amorphous hydroxide are shifted toward one another approximately for 14 of the "c" period of the lattice. Copyright 1999 Academic Press.

Study of the Interaction Products of Some N- and O-Containing Compounds with Highly Dispersed Copper(II) Hydroxide

Composition and structure of the surface compounds formed after interaction between Cu(OH)2 supported on aerosil and bpy, py, PhOH, and H2O2 are studied. The EXAFS and electron spectroscopy methods show the main part of copper ions to remain in a hydroxide surrounding, whose structure does not change practically and does not depend on the nature of the extra ligands used. The colloidal Cu(OH)2 stabilized by starch is found to reveal the same properties as the supported one, but differs by greater sizes of hydroxide particles. Copyright 1999 Academic Press.

Role of Surface Precipitation in Copper Sorption by the Hydrous Oxides of Iron and Aluminum

Isotherms were developed at pH 6.9 for adsorption (ADS) and coprecipitation (CPT) of Cu by hydrous oxides of Fe (HFO) and Al (HAO) to study the role of sorbate/sorbent ratio in metal cation removal. For low sorbate/sorbent conditions, HFO had a higher Cu retention capacity than HAO regardless of contact methodology. For either oxide, CPT was consistently more effective than ADS in removing Cu from solution. At high sorbate/sorbent ratios, surface precipitation dominates and the oxide's net cation retention capacity depends on the nature and solubility of the precipitate formed at the oxide-water interface. X-ray diffraction patterns and isotherms of HAO for both ADS and CPT suggest formation of a solid solution [e.g., CuAl2O4(s)] with dramatically lower solubility than Cu(OH)2(s) precipitated in bulk solution. In contrast, Cu precipitated on the HFO surface exhibited a solubility comparable to the bulk precipitated Cu(OH)2(s). Therefore, at high sorbate/sorbent ratios, HAO has a higher Cu "apparent" sorption capacity than HFO. The relative utility of these oxides as metal scavengers thus depends markedly on sorbate/sorbent conditions. Copyright 1999 Academic Press.

Coagulation of Quartz Particles in Aqueous Solutions of Copper(II)

The colloidal stability of quartz suspension was determined over a wide range of pH in aqueous copper nitrate where the state of Cu(II) is changed from mainly aqua ions and monohydroxyl complexes in the acid and neutral pH to polynuclear hydroxo complexes and colloidal precipitated copper hydroxide at higher pH. Two regions of instability were observed and in both cases the particles were shown to have low electrophoretic mobility. In the neutral pH region, the uptake of Cu(II) was sufficient to reduce the mobility of the particles to zero, while in the high-pH region evidence suggested coagulation between precipitated Cu(OH)2 and the quartz particles. It was shown that in all cases the coagulation was reversible and that the uptake of Cu(II) was dependent on the uncharged surface hydroxyl density. Studies of the coagulation kinetics showed that extended time scales were involved (several minutes in the neutral pH region to tens of minutes at high pH). Copyright 1998 Academic Press.

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

The chemical environment of Cu(II) adsorbed to the boehmite (AlOOH) surface at pH 6.5 is examined by electron paramagnetic resonance and X-ray absorption spectroscopy. Adsorbed Cu(II) is always coordinated by four oxygens in an axially symmetric ligand field when adsorbed under our experimental conditions. The Cu-O bond distance is approximately 1.94 A. An oriented, inner-sphere Cu(II) surface complex is observed at low surface loading (<0.2 μmol/M2). A second population of Cu(OH)N (H2O)X(2-N)+ outer-sphere complexes is proposed at higher surface loadings to explain X-ray absorption fine structure results. Cu(II) strongly resists any tendency to form a surface precipitate on boehmite at pH 6.5. The presence of specifically adsorbing anions had little effect on the local chemical environment of adsorbed Cu(II). Copyright 1997 Academic Press. Copyright 1997Academic Press