Adsorption of Cu, Cd, and Ni on goethite in the presence of natural groundwater ligands

The adsorption of copper, cadmium, and nickel on goethite was examined in natural groundwater samples from an infiltration site of the river Glatt at Glattfelden (Switzerland). Unfractionated dissolved organic matter was used at its natural concentrations. Metal concentrations were close to environmental conditions. Cu, Cd, and Ni presented the typical pH adsorption edge of cations. The major influence on metal adsorption was due to a strong organic ligand L(I), which inhibited adsorption of Cu, Cd, and Ni in the alkaline pH region. Complexation of Cu, Cd, and Ni by the natural organic ligands was described with a model defining a minimum number of discrete ligands: a strong ligand L(I) at low concentration and additional weaker ligands with higher concentrations. The adsorption of Cu, Cd, and Ni on the goethite surface in the presence of the natural organic ligands was adequately described by considering only surface complexation and complexation in solution by organic ligands. No ternary complexes had to be invoked in the model. The major effect was complexation by the strongest ligand, whereas interactions with other cations and anions had only a minor influence. Competition reactions between Cu and Ni for complexation with the same strong ligand L(I) were observed.     

Forward modeling of metal complexation by NOM: II. prediction of binding site properties

An a priori model of metal complexation by natural organic matter (NOM) has previously been shown to predict experimental data at pH 7.0 and 0.1 M ionic strength (Cabaniss, S. E. Environ. Sci. Technol. 2009). Unlike macroscopic models based only on stoichiometry and thermodynamics, this a priori model also predicts the ligand groups and properties of complexed (occupied) molecules. Ligand molecules with strong binding sites form complexes at low metal concentrations and have average properties (molecular weight, charge, aromaticity) which can differ significantly from the average properties of bulk NOM. Cu(II), Ni(II) and Pb(II) preferentially bind to strong amine-containing sites which are often located on small (MW < 1000), lower-aromaticity molecules. Cd(II) and Zn(II) show generally weaker binding, although they also prefer amine-containing sites to pure carboxylates and bind to smaller, less aromatic molecules. Ca(II) shows no real preference for amine over carboxylate ligand groups, preferentially binding to larger and more negatively charged molecules. Al(III) has a unique preference for phenol-containing sites and larger, more aromatic molecules. While some predictions of this model are consistent with a variety of experimental data from the literature, others await validation by molecular-level analysis.     

Competitive binding of CU2+ and Zn2+ to live cells of shewanella putrefaciens

Binding of Cu2+ and Zn2+ to live cells of Shewanella putrefaciens was measured at pH 4, 5.5, and 7 for dissolved metal concentrations ranging from 0.1 to 100 microM. Release of organic compounds by the cells resulted in concentrations of dissolved organic carbon (DOC) between 0.5 and 1.6 mM. A discrete site, nonelectrostatic model was used to describe Cu2+ and Zn2+ binding to the cells. Binding of Zn2+, which increased with increasing pH over the entire range of dissolved Zn2+ concentration, could be explained by invoking two types of cell wall binding sites: acidic and neutral functional groups. Binding of Cu2+ exhibited a more complex pH dependence: at dissolved metal concentrations below 1 microM, binding to the cells actually increased with decreasing pH. This behavior could be reproduced by (1) assuming the existence of a small fraction of high-affinity binding sites in the cell wall (approximately 5%) and (2) including metal complexation by dissolved organic ligands. The latter compete with the neutral cell wall groups and decrease Zn2+ and Cu2+ binding at pH 5.5 and 7. The observed isotherms implied that binding of the metals was only weakly affected by cell wall charging. Model parameters derived from the single-metal binding isotherms were able to account for the observed competition of Zn2+ and Cu2+ for cell wall sites when both metals were present.     

Surface complexation of copper(II) on soil particles: EPR and XAFS studies

The interactions of transition metals with natural systems play an important role in the mobility and the bioavailability of these metals in soils. In this study, the adsorption of copper(II) onto natural soil particles was studied as a function of pH and metal concentration. The retention capacity of soil particles was determined at pH 6.2 to be equal to 6.7 mg of copper/g of solid. The Langmuir and Freundlich isotherm equations were then used to describe the partitioning behavior of the system at different pH values. A combination of EPR, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) spectroscopies was used to probe the Cu atomic environment at the soil particles/aqueous interface. The spectroscopic study revealed that copper(II) ions are held in inner-sphere surface complexes. It also revealed that Cu was in an octahedral coordination with first-shell oxygen atoms. A weak tetragonal distortion was pointed out due to the Jahn-Teller effect, with a mean Cu-Oequatorial bond distance of 1.96 A and a Cu-Oaxial bond distance of 2.06 A. A detailed analysis of the spectroscopic data suggested that Cu(II) was bonded to organic matter coated onto the mineral fraction of soil particles.     

Copper toxicity to larval stages of three marine invertebrates and copper complexation capacity in San Diego Bay, California

Temporal and spatial measurements of the toxicity (EC50), chemical speciation, and complexation capacity (Cu-CC) of copper in waters from San Diego Bay suggest control of the Cu-CC over copper bioavailability. While spatial distributions of total copper (CuT) indicate an increase in concentration from the mouth toward the head of San Diego Bay, the distribution of aqueous free copper ion (Cu(II)aq) shows the opposite trend. This suggests that the bioavailability of copper to organisms decreases toward the head of the bay, and is corroborated by the increase in the amount of copper needed to reach an EC50, observed for larval stages of three marine invertebrates (Mediterranean mussel, Mytilus galloprovincialis, sand dollar, Dendraster excentricus, and purple sea urchin, Strongylocentrotus purpuratus), and by the increase in Cu-CC heading into the head of the bay. The amount of Cu(II)aq required to produce a 50% reduction in normal larval development (referred to here as pCuTox,) of the mussel, the most sensitive of the three marine invertebrates, was generally at or above approximately 1 x 10(-11) mol L(-1) equivalents of Cu (i.e., pCuTox approximately 11 = -(log [Cu(II)aq])). These results suggest that the copper complexation capacity in San Diego Bay controls copper toxicity by keeping the concentration of Cu(II)aq at nontoxic levels.     

Modeling kinetics of Cu and Zn release from soils

Kinetics of Cu and Zn release from soil particles was studied using two surface soils with a stirred-flow method. Different solution pH, dissolved organic matter (DOM) concentrations, and flow rates were tested in this study. A model for kinetics controlled sorption/desorption reactions between soils and solutions was globally fit to all experimental data simultaneously. Results were compared to a model that assumes local instantaneous equilibrium. We obtained one unique set of model parameters applicable to different pH, dissolved organic carbon (DOC), and flow conditions. We included DOM complexation of copper ions, which decreased their sorption. The effect of pH was included by assuming proton competition with metal ions for binding sites on soil particles. These results provide the basis for developing predictive models for metal release from soil particles to surface waters and soil solution.     

Co-removal of hexavalent chromium through copper precipitation in synthetic wastewater

The mechanisms of hexavalent chromium [Cr(VI)] co-removal with copper [Cu(II)] during homogeneous precipitation were studied with batch tests using a synthetic solution containing Cr(VI) and Cu(II). Metal precipitation was induced by adding Na2CO3 stepwise to different pH, and the respective removals of Cu(II) and Cr(VI) were measured. At the same time, the relative quantities of Cu(II) and Cr(VI) in the precipitates were also analyzed to establish their stoichiometric relationship. The results indicated that, in a solution containing 150 mg/L Cu(II) and 60 mg/L Cr(VI), the initial co-removal of Cr(VII with Cu(II) began at pH 5.0 and completed at pH 6.2. At pH 5.0-5.2, coprecipitation took place through the formation of copper-chromium-bearing solids [such as CuCrO4 and/or CuCrO4 x 2Cu(OH)2]. Thereafter, the remaining soluble copper started to react with carbonate in a heterogeneous environment to form the negatively charged basic copper carbonate precipitates [CuCO3 x Cu(OH)2], which subsequently adsorbed additional Cr(VI) (or HCrO4-) at pH 5.2-6.2. The maximum Cr(VI) co-removal took place at pH 6.2. Between the two mechanisms, co-precipitation accounted for about 29% of the total chromium's co-removal while the remaining 71% was attributed to surface adsorption, mainly through electrostatic attraction and ligand exchange. When the solution pH was increased to beyond 7.5, a surface charge reversal took place on the basic copper carbonate solids, and this led to some Cr(VI) desorption. Thus, the extent of Cr(VI) adsorption is highly pH dependent.     

X-ray spectroscopic studies of the high temperature reduction of Cu(II)O by 2-chlorophenol on a simulated fly ash surface

The reaction of 2-chlorophenol on Cu(II)O at 375 degrees C is studied using X-ray absorption near edge structure (XANES) spectroscopy. A mixture of copper(II) oxide and silica is prepared to serve as a surrogate for fly ash in combustion systems. 2-Chlorophenol is utilized as a model precursor for formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). The Cu K-edge spectra shiftto lower binding energy, reflecting the reduction of the copper. The substrate is found to form a mixture of Cu(II), Cu(I), and Cu(O), with the dominant species being Cu(I). The data are fitted well with a first-order reaction scheme, with a time constant at 375 degrees C of 76 s. This is the first application of XANES spectroscopy for studying the kinetics and mechanism of heterogeneous reactions relevant to combustion processes, and the results demonstrate the utility and desirability of such X-ray spectroscopic studies.     

Cu(II) complexation of high molecular weight (HMW) fluorescent substances in root exudates from a wetland halophyte (Salicornia europaea L.)

High molecular weight (HMW) fractions are important components in root exudates. However, there is little available information concerning complexation of Cu(II) to the HMW fractions in root exudates. In the present study, complexation of root exudates from Salicornia europaea L. with Cu(II) was investigated using excitation emission matrix (EEM) fluorescence spectroscopy. Two protein-like fluorescence peaks were identified in the EEM spectra of root exudates. Fluorescence of both peaks was clearly quenched by Cu(II). The increase of conditional stability constant with increasing temperature indicates that the fluorescence quenching of the protein-like fluorescence by Cu(II) may be controlled by a dynamic process. The values of conditional stability constants (logK(a)) were in the range of 4.32-4.69, which were close to those of complexation of fulvic acid with Cu(II). This shows that the HMW fluorescent substances in root exudates from S. europaea L. were strong organic ligands for Cu(II). Our study suggests that the HMW fluorescent substances may affect chemical forms, mobility, and thus the fate of copper in wetland.     

Impact of pH on Cu accumulation kinetics in earthworm cytosol

We studied the interaction between toxic stress and accumulation in the earthworm Aporrectodea caliginosa, as induced by different soil copper pools and soil constituents (especially pH). Earthworms were exposed in quartz sand, spiked soils, and field soils with different Cu concentrations and varying soil composition. The copper content in the earthworms was determined in the following: the cytosolic fraction, a granular fraction and a fraction consisting of tissue fragments, cell membranes and intact cells. The highest amount of Cu was found in the cytosolic fraction. The other fractions varied only slightly in response to changes in any of the copper pools in soil. Cytosolic copper was the best predictor of Cu availability to earthworms collected from soils at constant pH, as statistically significant correlations were obtained with pore water pCu at constant pH in earthworms exposed in quartz sand. This correlation was lost for cytosolic Cu concentrations in earthworms exposed to spiked soils and field soils at differing pHs. Instead, cytosolic copper correlated well to Cu in either pore water or solid phase. Soil pH not only plays an important role in the availability of metals and therefore on their uptake fluxes, but internal competition of Cu2+ and H+ at physiologically active binding sites also explained these apparent contradictions and increased the predictability of body burdens significantly.     

Formation of Cu(I) in estuarine and marine waters: application of a new solid-phase extraction method to measure Cu(I)

Cu(II) is a key species with respect to the bioavailability and hence toxicity of copper. Therefore, it is important to elucidate the factors that control Cu(I) steady-state concentrations in natural waters. In this study, a solid-phase-extraction-based method was developed that allows Cu(I) measurements at ambient concentrations. Cu(I) is selectively enriched as a bathocuproine complex on a hydrophobic polymer column, whereas Cu(II), bound to ethylenediamine, is not retained on the column. After elution with acidic methanol, Cu is analyzed with graphite-furnace atomic absorption spectroscopy. The detection limit of the whole analytical procedure is below 1 x 10(-9) M, and the mean recovery of Cu(I) is approximately 70%. We then applied this method to determine Cu(I) in water samples collected from the River Scheldt estuary and the North Sea. Upon irradiation of these filtered water samples in the laboratory (with approximately 5 kW m(-2)), Cu(I) steady-state concentrations ([Cu(I)]ss) were established within a few minutes, and [Cu(I)]ss ranged from 5% to 80% of total dissolved copper, depending on the origin of the water samples. Measured [Cu(I)]ss can be interpreted by considering light-induced reduction of Cu(II) and stabilization of Cu(I) by chloride at high salinity, thermal reduction of Cu(II) by sulfide-containing compounds at low salinity, and fast reoxidation of Cu(I) due to stabilization of Cu(II) by strong organic ligands present at intermediate salinity.     

Effects of pH, chloride, and bicarbonate on Cu(I) oxidation kinetics at circumneutral pH

The oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions have been investigated over the pH range 6.5-8.0. The overall apparent oxidation rate constant was strongly affected by chloride, moderately by bicarbonate, and to a lesser extent by pH. In the absence of bicarbonate, an equilibrium-based speciation model indicated that Cu(+) and CuClOH(-) were the most kinetically reactive species, while the contribution of other Cu(I) species to the overall oxidation rate was minor. A kinetic model based on recognized key redox reactions for these two species further indicated that oxidation of Cu(I) by oxygen and superoxide were important reactions at all pH values and chloride concentrations considered, but back reduction of Cu(II) by superoxide only became important at relatively low chloride concentrations. Bicarbonate concentrations from 2 to 5 mM substantially accelerated Cu(I) oxidation. Kinetic analysis over a range of bicarbonate concentrations revealed that this was due to formation of CuCO(3)(-), which reacts relatively rapidly with oxygen, and not due to inhibition of the back reduction of Cu(II) by formation of Cu(II)-carbonate complexes. We conclude that the simultaneous oxygenation of Cu(+), CuClOH(-), and CuCO(3)(-) is the rate-limiting step in the overall oxidation of Cu(I) under these conditions.     

Voltammetric analysis of heterogeneity in metal ion binding by humics

The complexation of Cd, Pb, and Cu by fulvic acids at a fixed pH and ionic strength is studied by means of different voltammetric techniques at any metal-to-ligand ratio. When using Reverse Pulse Polarography (RPP) the complex species are electrochemically labile and not subject to significant electrodic adsorption. RPP titrations of fulvic acid with metal ions are interpreted on the basis of a recently proposed analytical expression for limiting currents valid for fully labile heterogeneous complexation. The voltammetric data are transformed into the corresponding binding curve, i.e., the fraction of occupied sites vs free metal concentration. Finally, the competition between metal ions and protons in their interaction with the fulvic binding sites as well as the concomitant polyelectrolytic effects are analyzed in terms of the NICCA-Donnan model. The results show that voltammetric techniques can be applied to the studies of heterogeneous complex systems in a broad range of metal-to-ligand ratios.     

Characterization of sorption sites on Pilayella littoralis and metal binding assessment using 113Cd and 27Al nuclear magnetic resonance

Metal interactions with the cellular structures of the marine alga Pilayella littoralis have been investigated to better understand how biomaterials sorb dissolved metals. Algae metal binding capacity at pH 5.0 was 2000, 850, 430, and 560 micromol g(-1) of dried material for Al(III), Cu(II), Cd(II), and Co(II), respectively. Binding site characterization was assessed by 1H and 13C nuclear magnetic resonance spectroscopy. Also, Fourier Transform Infrared spectroscopy (FTIR) provided some information about the types of functional groups that appear to be present in the algal material. The results suggested the presence of carboxylate, ether, amino, and hydroxyl groups. Investigation of metal competition for the alga binding sites was performed using 27Al and 113Cd NMR spectroscopy, which proved to be a valuable technique for Al and Cd sorption assessment. Aluminum and Cu were efficiently sorbed by the alga sites, and the binding affinity order of these metals was Al(III) > Cu(II) > Cd(II) > Co(II).     

Modeling of lithium interference in cadmium biosorption

Biosorption of Cd by the brown seaweed Sargassum polycystum biomass was experimentally investigated and mathematically modeled at different pH and ionic strength levels. From the potentiometric titration of the biomass, three types of functional groups were identified, and the dissociation constant and the numbers of these groups were determined. The carboxyl group (pK(H) 3.70 +/- 0.09) was found to play a major role in binding protons and Cd. The background ion, Li+, could interfere with the uptake of protons and Cd by competition forthe carboxyl sites. Whereas the binding mechanism on the carboxyl group was established as an ion exchange process, the second functional group, phosphonate (pK(H) 5.41 +/- 0.31), most likely bound metals by a complexation reaction. A biosorption model was developed based upon the binding mechanisms and was successfully used for predicting the isotherm and pH edge experiments. In addition, the speciation of the binding sites as a function of the pH was simulated using the developed model in order to visualize the distribution of Cd on the binding sites.     

Analysis of the effect of pH on Cu2+-fulvic acid complexation using a simple electostatic model

A simple electrostatic model was used to study the effect of pH on the binding of Cu2+ to fulvic acid in solutions containing similar amounts of dissolved organic carbon (DOC) as natural media, such as aquatic environments and soil solutions. Complexation behavior was affected by increased pH because of changes in the electrostatic interaction resulting from an increase in the negative charge on the fulvic acid molecule. Solutions of soil-extracted fulvic acid (FA), at concentrations of 25 and 35 mg L(-1), ionic strength 0.005 M, and pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, conditions that simulate those of natural freshwaters, were titrated with copper ion using differential pulse anodic stripping voltammetry. Assuming the formation of 1:1 complexes, the conditional binding parameters (stability constant and binding capacity) were calculated for each pH value. Use of a 1:1 electrostatic model allowed estimation of the contribution of the electrostatic effect to the ion binding reaction, at each pH value, as well as calculation of a binding constant that was not dependent on pH and which thus represented the contribution of the chemical heterogeneity. Furthermore, it was found that only a small proportion of the FA acid functional binding sites are involved in the formation of copper complexes.     

Stability of metal-glutathione complexes during oxidation by hydrogen peroxide and Cu(II)-catalysis

Thiol-containing ligands such as glutathione (GSH) are expected to degrade in the presence of oxygen; however, complexation by Hg, Ag, and other trace metals may protect free thiol functional groups (R-S-) from oxidation, leading to persistence in surface water environments. In this study, the stability of GSH complexes with Hg2+, Ag+, and other metals including Cd2+, Zn2+, and Pb2+ was assessed during exposure to two potential environmental oxidants: H202 and Cu2+. The results indicated that Hg-(GSH)2 and Ag(GSH) complexes were completely stable for at least 2 days in the presence of either H202 or Cu2+. In contrast, free GSH oxidized within minutes to hours. Complexation by Cd, Zn, and Pb slightly decreased or did not significantly affect the oxidation rate of GSH, depending upon the pH (tested between pH 6 and 9). Thermodynamic modeling of GSH speciation demonstrated that the observed oxidation rates were not consistent with predicted free GSH3- concentration. These results indicated that Cd-, Zn-, and Pb-GSH complexes were susceptible to oxidation by a mechanism that differs from GSH3- oxidation. In contrast, Hg- and Ag-GSH complexes were inert for days, suggesting that they are stable for relatively long periods in the oxic water column. These results demonstrate that coordination of Hg(II) and Ag(I) to thiol-containing ligands can potentially increase persistence and transport in surface waters.