Model predictions of copper speciation in coastal water compared to measurements by analytical voltammetry

Trace metal toxicity to aquatic biota is highly dependent on the meta?s chemical speciation. Accordingly, metal speciation is being incorporated in to water quality criteria and toxicity regulations using the Biotic Ligand Model (BLM) but there are currently no BLM for biota in marine and estuarine waters. In this study, I compare copper speciation measurements in a typical coastal water made using Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) to model calculations using Visual MINTEQ. Both Visual MINTEQ and BLM use similar programs to model copper interactions with dissolved organic matter-DOM (i.e., the Stockholm Humic Model and WHAM-Windermere Humic Aqueous Model, respectively). The total dissolved (<0.4 μm filter) copper concentration, [CuT] in the study sites ranged from <10 nM close to the open Baltic Sea to ca. 50 nM in the vicinity of a marina in the Stockholm Archipelago. The corresponding free copper concentration [Cu2+], measured by CLE-ACSV ranged from 10–13.2 M to 10–12.0 M for the reference and marina sites, respectively, whereas the corresponding [Cu2+] modeled calculations ranged from 10–12.5 M to 10–11.6 M. The low copper to DOM ratios (similar to 0.0004 mg Cu per mg DOC) in these coastal waters ensured that ambient dissolved copper was overwhelmingly chelated to strong Cu–binding ligands (12 < log KCuL1,Cu2+Cond >14). The modeled [Cu2+] could be fitted to the experimental values better after the conditional stability constant for copper binding to fulvic acid (FA) complexes in DOM in the SHM was adjusted to account for higher concentration of strong Cu-binding sites in FA.     

Comparison of copper speciation in estuarine water measured using analytical voltammetry and supported liquid membrane techniques

The supported liquid membrane (SLM) is a promising separation and preconcentration technique that is well-suited for trace metal speciation in natural waters. The technique is based on the selective complexation of metal ions by a hydrophobic ligand (carrier) dissolved in a water-immiscible organic solvent immobilized in a porous, inert membrane. This membrane separates two aqueous solutions: the test (or donor) solution and the strip (or acceptor) solution. The metal carrier complex is transported by diffusion across the membrane from the source to the strip solution where metal ions are back-extracted. The technique offers great potential to tune the selectivity by incorporating different complexing ligands in the membrane. A SLM was used to analyze the dissolved (<0.45 microm) copper speciation from two sites in the San Francisco Bay estuary; Dumbarton Bridge, [Cu]total approximately 27 nM, and San Bruno Shoals, [Cu]total approximately 23 nM. The sites were also characterized independently by differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE). The SLM employed 10 mM lasalocid, a naturally occurring carboxylic polyether ionophore, in nitrophenyl octyl ether (NPOE) asthe membrane complexing ligand, supported by a microporous, polypropylene, hydrophobic membrane. This is the first study where SLM technique has been compared with an independent speciation technique in marine waters. Results of copper speciation measurements from Dumbarton Bridge, a site in South San Francisco Bay where copper speciation has been well-characterized in previous studies using various voltammetric techniques, indicated that only about 3% (0.9 nM) of the total dissolved copper was SLM labile. The corresponding DPASV labile copper fraction was <0.4% (<0.1 nM) of total dissolved copper. The concentration of total copper binding ligands measured by the membrane technique was 471 nM as compared to 354 nM measured by DPASV, more than 1 order of magnitude higher than the total dissolved copper concentration. The SLM measurements were consistent with earlier copper speciation measurements that were made in South San Francisco Bay using other voltammetric stripping techniques.     

Strong copper-binding behavior of terrestrial humic substances in seawater

In coastal areas, strong complexation of copper generally reduces its toxicity; our ability to monitor and regulate copper as a toxin therefore depends on our understanding of the sources and sinks of the copper-binding ligands. Terrestrial humic substances (HS) are well-recognized contributors to weak ligand concentrations in aquatic systems. In this work, we show that HS are likely contributors to both stronger and weaker ligand classes controlling copper speciation in coastal areas receiving typical inputs of terrestrial organic matter. We used competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV), with the added ligands benzoylacetone and salicylaldoxime, to examine copper binding by terrestrial HS in a seawater matrix, at HS and copper concentrations typical of coastal waters. Copper titration data of 1 mg/L Suwannee River humic acid (SRHA) in seawater could be modeled using conditional stability constants of 10(12.0) and 10(10.0) and total ligand concentrations of 10.4 and 199 nM for a stronger and weaker ligand, respectively. Similar results were obtained for Suwannee River fulvic acid (SRFA). Strong copper binding by SRFA in seawater was weaker than previously reported for a freshwater at similar pH, possibly indicating effects of Ca and Mg competition or ionic strength. Nevertheless,the concentrations and binding strengths of copper ligands we observed are comparable to the range reported in previous coastal speciation studies. In addition, we show that the weaker copper ligands cause internal calibration techniques to significantly underestimate the sensitivity of ACSV in the presence of HS concentrations typical of coastal waters. To address this issue, we demonstrate the use of "overload titrations", using a high enough concentration of added ligand to outcompete all natural ligands as an alternative calibration technique for analysis of coastal samples.     

Influence of metal speciation in natural freshwater on bioaccumulation of copper and zinc in periphyton: a microcosm study

The free ion activity model (FIAM) has already been confirmed under laboratory conditions for many trace metals but has still to be validated under natural conditions where the presence of natural organic ligands influences metal speciation and bioavailability. The goal of this study was to test if the FIAM is followed under natural conditions by measuring copper and zinc speciation as well as metal accumulation in periphyton. Periphyton was exposed in microcosms to natural river water with different added concentrations of copper (25-258 nM) or zinc (18-501 nM) and additions of a synthetic ligand (NTA). Free Cu2+ was in the range of 10(-16.5)-10(-14.5) M and Zn(2+) was in the range of 0.7-8.7 nM, as measured by competitive ligand exchange coupled with cathodic/anodic stripping voltammetry. Other metal complexes were either measured or computed. Bioaccumulation of zinc in periphyton appeared to be controlled by the free zinc ion concentration, confirming the FIAM. In contrast, bioaccumulation of copper was controlled by weakly complexed copper (including Cu2+ plus inorganic and weak organic complexes), which is in disagreement with the FIAM, and appears to be caused by limitation of copper diffusion due to very low free Cu2+ occurring in natural environments.     

Numerical approach to speciation and estimation of parameters used in modeling trace metal bioavailability

Speciation affects trace metal bioavailability. One model used to describe the importance of speciation is the biotic ligand model (BLM), wherein the competition of inorganic and organic ligands with a biotic ligand for free-ion trace metal determines the ultimate metal availability to biota. This and similar models require natural ligand concentrations and conditional stability constants as input parameters. In concept, the BLM is itself an analogue of some analytical approaches to the determination of trace metal speciation. A notable example is competitive ligand equilibration/cathodic stripping voltammetry, which employs an artificial ligand for comparative assessment of natural ligand concentrations and discrete conditional stability constants (i.e., BLM parameters) in a natural sample. Here, we report a new numerical approach to voltammetric speciation and parameter estimation that employs multiple analytical windows and a two-step optimization process, simultaneously generating both parameters and a complete suite of corresponding species concentrations. This approach is more powerful, systematic, and flexible than those previously reported.     

Kinetically inert Cu in coastal waters

Many studies have shown that Cu and other metals in natural waters are mostly bound by unidentified compounds interpreted to be strong ligands reversibly complexing a given metal. However, commonly applied analytical techniques are not capable of distinguishing strongly but reversibly complexed metal from metal bound in kinetically inert compounds. In this work, we use a modified competitive ligand exchange adsorptive cathodic stripping voltammetry method combined with size fractionation to show that most if not all of the apparently very strongly (log K > or = 13) bound Cu in samples from five New England coastal waters (1-18 nM, 10-60% of total Cu) is actually present as kinetically inert compounds. In three of the five samples examined by ultrafiltration, a significant portion of the 0.2-microm-filtrable inert Cu was retained by a 0.02-microm-pore size filter, suggesting that at least some of the Cu was kinetically inert because it was physically sequestered in colloidal material. The rest of the ambient Cu, and Cu added in titrations, were reversibly bound in complexes that could be modeled as having conditional stability constants of 10(10)-10(13). The Cu-binding ability of these complexes was equivalent to that of seawater containing reasonable concentrations of humic substances from terrestrial sources, approximately 0.15-0.45 mg of C/L. Both the inert compounds and the reversible ligands were important for determining [Cu2+] at ambient Cu levels in our samples.     

Determination of the mercury complexation characteristics of dissolved organic matter in natural waters with "reducible Hg" titrations

A new method for the determination of the concentration and conditional stability constant of dissolved organic matter that binds mercury (Hg) has been developed using an in vitro assay of reducible Hg. The technique is a wet chemical analogue to electrochemical approaches now in use for ligand studies of many other trace transition metals in natural waters. Ligand characteristics are obtained from additions of ionic Hg to buffered lake, river water, and seawater and determination of the wet chemically reducible fraction following equilibration of the spike. This approach is robust, as demonstrated by (i) analysis using three reducing agents of varying strengths, (ii) replicate analyses, (iii) comparison to well-characterized complexing species (chloride and EDTA) using a competitive ion-exchange resin, and (iv) kinetic studies. Results indicate that Hg-complexing equivalents are present in the dissolved phase (<0.2 microm) ranging from <1 to >60 nN concentrations and with log conditional stability constants (log K') in the range of 21-24. Only one ligand class was found in the natural waters analyzed. There was indirect evidence for a class of organic ligands that formed reducible complexes with Hg in freshwater. Such ligand characteristics indicate that the vast majority of ionic inorganic Hg dissolved in freshwater and coastal saltwaters is associated with organic complexes. Concentrations, affinities, and kinetics implicate multidentate chelation sites as the principal complexing moieties for Hg and discourage the use of humic carboxylic acids as a proxy for the ligands/functional groups.     

Speciation of Cu and Zn in drainage water from agricultural soils

Inputs of copper and zinc from agricultural soils into the aquatic system were investigated in this study, because of their heavy agricultural usage as feed additives and components of fertilizers and fungicides. As the mobility and bioavailability of these metals are affected by their speciation, the lipophilic, colloidal and organic fractions were determined in drainage water from a loamy and a humic soil treated with fungicides or manure. This study therefore investigates the impact of agricultural activity on a natural environment and furthers our understanding of the mobility of metals in agricultural soils and aquatic pollution in rural areas. Marked increases in the total dissolved metal concentrations were observed in the drainage water during rain events with up to 0.3 microM Cu and 0.26 microM Zn depending on the intensity of the rainfall and soil type. The mobile metal fractions were of a small molecular size (<10 kD) and mainly hydrophilic. Lipophilic complexes originating from a dithiocarbamate (DTC) fungicide could not be observed in the drainage water; however, small amounts of lipophilic metal complexes may be of natural origin. Cu was organically complexed to > 99.9% by abundant organic ligands (log K 10.5-11.0). About 50% of dissolved Zn were electrochemically labile, and the other 50% were complexed by strong organic ligands (log K 8.2-8.6). Therefore very little free metal species were found suggesting a low bioavailability of these metals in the drainage water even at elevated metal concentrations.     

Determination of strong ligand sites in sewage effluent-impacted waters by competitive ligand titration with silver

A competitive ligand titration, employing Ag+, is used to determine the binding capacity of the small amounts of strong ligands (SL) in natural water samples. Strong ligands are defined here as high-affinity binding sites for group 11 and 12 metals such as Cu(I), Hg(II), and Ag(I). In addition, the conditional binding strength (log K') is determined for Ag- and SL. Diethyldithiocarbamate (DEDC) is the competitive ligand employed. The system is set at constant pH (8.1), ionic strength (0.1 M), and excess-fixed DEDC (10 microM) to determine SLs with log K' for Ag+ of >10. Silver was chosen as the titrant metal because it binds predominantly with S(-II) versus other ligands and reduced sulfur is thought to comprise the majority of SLs in natural waters. A two-phase system, water and 1,2-dichloroethane (DCE), is required due to the insolubility of Ag-DEDC in water. Added silver partitions into Ag+ and Ag-SL in the aqueous phase and into Ag-DEDC in the DCE phase. An automated system is used to add aliquots of silver and measure Ag-DEDC by UV absorbance in the DCE phase and [Ag+] by specific ion electrode in the aqueous phase. Excess addition of silver and a "Gran's" analysis gives the binding capacity of SL. The stability constant can also be determined for each addition of silver for an overall one-site SL assumption. Cysteine was used to test the method, and urban waters revealed SL capacities from about 50 to 150 nM and log K'(Ag) of 11-12. An independent analysis of chromium-reducible sulfide correlates well with the SL capacity.     

Kinetic limitations in measuring stabilities of metal complexes by competitive ligand exchange-adsorptive stripping voltammetry (CLE-AdSV)

The kinetic limitations of Competitive Ligand Exchange-Adsorptive Stripping Voltammetry, CLE-AdSV, for the determination of very stable metal complexes are explained in detail. For a given type of metal, from a certain lower limit of the complex stability constant, K, the usual simple equilibrium interpretation of CLE-AdSV measurements is not generally valid. By critical assessment of data for natural waters we show that in many cases the reported stability constants appearto derive from nonequilibrium conditions in the bulk sample and hence overestimate the real values. Fe(II) is a special case due to the particular kinetic features of hydroxide as a ligand. Our results call for validation of such data by analysis on the basis of the kinetics involved and/or by independent kinetic-free experimental approaches. Earlier speculations from CLE-AdSV results on very strong ligands and derived features such as the potential bioavailability of trace metals in natural waters require reconsideration.     

Physical and kinetic speciation of copper and zinc in three geochemically contrasting marine estuaries

The physical and kinetic speciation of Cu and Zn in three impacted marine estuaries was examined. Contrasts in sources of metal-binding ligands, solution chemistry, and hydrologic forcing between and withinthethree study systems (Cape Fear River Estuary, North Carolina; Norfolk-Hampton Roads-Elizabeth River, Virginia; San Diego Bay, California) were exploited to enhance our understanding of Cu and Zn speciation. Trace metal-optimized tangential-flow ultrafiltration at 1 kDa nominal molecular weight limit (NMWL) was used to fractionate <0.4 microm species into colloidal and "dissolved" pools. Colloidal species of dissolved organic matter (DOM) and copper were significant and often the dominant pools in each of the three study systems. Characteristic colloidal fractions of both DOM and Cu ranged from near 70% of <0.4 microm concentrations in Cape Fear to 50% in San Diego Bay. Colloidal Cu and DOM were strongly coupled, and variability in observed <0.4 microm Cu concentrations was closely related to the concentrations of colloidal-associated metal. Colloidal fractions were much smaller for Zn than that of Cu; ranging from 10-30% in Cape Fear to less than 5% in San Diego Bay, and no relationship to DOM was observed. Kinetic separations on Chelex resin revealed the presence of large nonlabile pools of Cu in each of the study systems, with the highest fractions (70-100%) in Cape Fear and Norfolk and lowest (30-50%) in San Diego Bay. A close relationship was observed between colloidal and nonlabile Cu species, implying slow reactivity of colloidal-bound Cu. The fraction of filterable Zn labile to Chelex averaged 97%, 85%, and 60% in San Diego, Norfolk, and Cape Fear, respectively. Anthropogenic Zn appeared almost exclusively in the <1 kDa fraction, while anthropogenic Cu was distributed between dissolved and colloidal pools. Copper particle-partition coefficients (Kd) followed the trend: San Diego > Norfolk > Cape Fear and were inversely correlated with DOC concentrations. Colloid-based partition coefficients were significantly greater, in many cases an order of magnitude greater, than particle-based partition coefficients. The partitioning data suggest the presence of metal-enriched bacterial-derived exudates and/or discrete metal phases in colloidal-sized particles in impacted regions of these estuaries. The strong relationships observed between Cu and DOC indicate that Cu partitioning behavior over a range of estuarine environments may be modeled effectively with a limited set of coefficients. Our measurements of metal lability and size distribution imply that the fraction of <0.4 microm Zn that is likely to be bioavailable is greater than that for Cu, especially in impacted regions of the study systems.     

Nickel speciation and complexation kinetics in freshwater by ligand exchange and DPCSV

A technique of ligand exchange with DMG (dimethylglyoxime) and DPCSV was applied to determine Ni speciation in lake, river, and groundwater samples. The working conditions related to ligand-exchange equilibrium were optimized, and the ligand-exchange kinetics were examined. The observed pseudo-first-order rate, kobsd, was about 3 x 10(-5) (s-1) for Ni(DMG)2 complex formation with an excess of DMG (microM) over Ni (nM) at pH 7.1-7.7. The second-order exchange kinetic constants, kexch, were between 1.2 x 10(2) and 5.7 x 10(3) s-1 M-1 for ligand exchange of NiEDTA with DMG and between 5 x 10(2) and 7 x 10(3) s-1 M-1 for exchange of natural ligands with DMG in the freshwater samples under similar conditions. Ni ligand exchange between natural ligands and DMG occurred over days with half-lifes of 5-95 h. Total dissolved Ni concentrations in samples from various freshwater systems in Switzerland ranged from 4 nM in an oligotrophic lake to 30 nM in a small river affected by inputs from sewage effluents and agriculture. Free ionic Ni2+ concentrations were determined in the range of 10(-13)-10(-15) M (pNi = 12.2-14.7), indicating that more than 99.9% of dissolved Ni was bound by organic ligands with strong affinity (log K 12.1-14.9) and low concentrations (13-100 nM) at pH 7.2-8.2. Because of slow ligand-exchange kinetics, Ni speciation in natural waters may in many cases not reach equilibrium.     

The evaluation of liposome-water partitioning of 8-hydroxyquinolines and their copper complexes

Bioavailability and toxicity of mixtures are urgent research issues, but usually mixtures of exclusively organic chemicals or exclusively metals are investigated. In our study, we explored the role of combinations of hydrophobic ionogenic organic compounds (HIOCs) with copper (Cu2+)for uptake and bioavailability of metals and hydrophobic metal complexes in an in vitro membrane system. We investigated the influence of the interactions of copper and 8-hydroxyquinolines, both components used in formulations of pesticides, on their partitioning into liposomes, which are model systems for biological membranes and are composed of lipid bilayers made of phosphatidylcholine. The test set of compounds comprised the parent compound 8-hydroxyquinoline and 8-hydroxyquinolines with hydrophobic (e.g., 5-chloro-8-hydroxyquinoline, 5,7-dichloro-8-hydroxyquinoline, 5,7-dibromo-8-hydroxyquinoline) and with hydrophilic (e.g., 8-hydroxyquinoline-5-sulfonic acid) substituents. Hydrophobic 8-hydroxyquinolines facilitate the passive uptake of copper into phospholipid bilayers by complex formation. Not only the neutral species of the ligands and their neutral copper ligand complexes are significantly taken up into the membrane, but also the cationic and anionic species of the ligands and the cationic complexes. The neutral, anionic, and cationic species of 8-hydroxyquinoline and the hydrophobic substituted 8-hydroxyquinolines exhibit linear correlations between their logarithmic liposome-water partitioning coefficients (log Klipw) and the logarithmic octanol-water partitioning coefficients of their neutral species (log Kow, neutral). The neutral species show the strongest partitioning followed by the anionic and cationic species. The associated quantitative structure activity relationships describing the dependency of log Klipw of the various species from log Kow, neutral of the neutral ligand species have slopes between 0.9 and 1. In contrast, the partitioning of the neutral and cationic copper-8-hydroxyquinoline complexes is dependent on several factors including the hydrophobicity of the ligand, the effective molecular size, and the polarization of the complex itself. In consequence, there is no linear relationship between log Klipw of these complexes and log Kow of the neutral species of their ligands. The complexes with very bulky substituents showed a reduced uptake. The Klipw of the nominally neutral complexes, where Cu2+ is coordinated with two ligands, were a factor three to four higher than the Klipw of the positively charged complexes with only one ligand. Although liposome-water partitioning merely describes one element of the uptake process into biological membranes, it is a key process for bioavailability of hydrophobic compounds and, presumably, also plays a crucial role for biological uptake of the described metal organic complexes.     

Determination of silver speciation in natural waters. 2. Binding strength of silver ligands in surface freshwaters

Three competing ligand methods were compared to determine characteristics of Ag(I) complexation by dissolved and colloidal ligands present in three rivers and one sewage treatment plant effluent. Iminodiacetate groups on Chelex-100 resin (used in batch and column experiments) and diethyldithiocarbamate (DDC) were used as competing ligands. Results of batch Chelex and DDC competition experiments show good agreement with regard to relative extent of Ag binding by natural ligands among the three river systems. Results of both methods also show a possible correlation between extent of Ag(I) complexation and organic matter concentration and/or Fe concentration. Fraction of Ag(I) associated with Chelex in both batch and column Chelex experiments was similar in each of the four systems tested, indicating that lability of Ag complexes does not change significantly on time scales ranging from seconds to 24 h. Results of Chelex and DDC competition were compared using a model based on a hypothetical single natural ligand. Under the experimental conditions used, this model quantified Ag(I) complexes with log Kcond values from approximately 12 to 14. For the three rivers studied, ligands with silver-association characteristics similar to those of reduced sulfur groups (log K = 14-16) present at subnanomolar concentrations likely dominate Ag(I) speciation in these systems. A weaker ligand (e.g., log Kcond < 12) at concentrations > 0.7 nM dominated Ag(I) speciation in the treatment plant effluent. This may result from elevated concentration of metals that compete for reduced sulfur groups rather than from a lower total concentration of these groups.     

Precipitation and growth of zinc sulfide nanoparticles in the presence of thiol-containing natural organic ligands

In sulfidic aquatic systems, metal sulfides can control the mobility and bioavailability of trace metal pollutants such as zinc, mercury, and silver. Nanoparticles of ZnS and other metal sulfides are known to exist in oxic and anoxic waters. However, the processes that lead to their persistence in the aquatic environment are relatively unknown. The objective of this study was to evaluate the importance of dissolved natural organics in stabilizing nanoparticulate ZnS that precipitates under environmentally relevant conditions. Precipitation and growth of ZnS particles were investigated in the presence of dissolved humic acid and low-molecular weight organic acids that are prevalent in sediment porewater. Dynamic light scattering was used to monitor the hydrodynamic diameter of particles precipitating in laboratory solutions. Zn speciation was also measured by filtering the ZnS solutions (< 0.2 microm) and using anodic stripping voltammetry to confirm that Zn was coordinated to sulfide during the precipitation experiments and not to the dissolved organic ligands. X-ray photoelectron spectroscopy and electron microscopy were used to confirm that amorphous particles containing Zn and S were precipitating in the suspensions. Observed growth rates of ZnS particles varied by orders of magnitude, depending on the type and concentration of organic ligand in solution. In the presence of humic acid and thiol-containing ligands (cysteine, glutathione, and thioglycolate), observed growth rates decreased by 1-3 orders of magnitude relative to controls without the ligands. In contrast, growth rates of the particles were consistently within 1 order of magnitude of the ligand-free control when oxygen- and amine-containing ligands (oxalate, serine, and glycolate) were present Furthermore, particle growth rates decreased with an increase in thiol concentration and increased with NaNO3 electrolyte concentration. These studies suggest that specific surface interactions with thiol-containing organics may be one factor that contributes to the persistence of naturally occurring and anthropogenic nanoparticles of ZnS and other metal sulfides in the aquatic environment.     

Effect of organic complexation on copper accumulation and toxicity to the estuarine red macroalga Ceramium tenuicorne: a test of the free ion activity model

Current water quality criteria (WQC) regulations on copper toxicity to biota are still based on total dissolved (<0.4 μm membrane filter) copper concentrations with a hardness modification for freshwaters. There are however ongoing efforts to incorporate metal speciation in WQC and toxicity regulations (such as the biotic ligand model-BLM) for copper and other metals. Here, we show that copper accumulation and growth inhibition of the Baltic macroalga Ceramium tenuicorne exposed to copper in artificial seawater at typical coastal and estuarine DOC concentrations (similar to 2-4 mg/L-C as fulvic acid) are better correlated to weakly complexed and total dissolved copper concentrations rather than the free copper concentration [Cu2+]. Our results using a combination of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) measurements and model calculations (using visual MINTEQ incorporating the Stockholm Humic Model) show that copper accumulation in C. tenuicorne only correlates linearly well to [Cu2+] at relatively high [Cu2+] and in the absence of fulvic acid. Thus the FIAM fails to describe copper accumulation in C. tenuicorne at copper and DOC concentrations typical of most marine waters. These results seem to indicate that at ambient total dissolved copper concentration in coastal and estuarine waters, C. tenuicorne might be able to access a sizable fraction of organically complexed copper when free copper concentration to the cell membrane is diffusion limited.     

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.     

Comparison of copper speciation in coastal marine waters measured using analytical voltammetry and diffusion gradient in thin-film techniques

The diffusion gradient in thin-film hydrogel (DGT) probe is a promising tool for metal speciation work. Based on a passive sampling principle, it provides the potential for large data sets in complex regimes. DGT probes were deployed in waters characterized independently using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). The CLE-ACSV used benzoyl acetone as the competitive ligand in discrete water samples collected during the deployment of the DGT probes. The DGT probes used a 15% polyacrylamide/0.4% bis-acrylamide cross-linker hydrogel and a Na-form of Chelex-100 to complex metal that fluxed into the probe through the hydrogel. Probes were deployed in locations characterized by the degree of pollution impact: the relatively pristine Vineyard Sound, MA, [Cu]total approximately 6 nM, small seasonally active harbors on Cape Cod, MA, [Cu]total = 12-64 nM, as well as a large polluted estuary, the Elizabeth River, VA, [Cu]total = 44-58 nM, and a large polluted port, San Diego Harbor, CA, [Cu]total = 23-103 nM. This is the first study where DGT probes have been compared with an independent speciation technique in marine systems and used to establish the diffusion coefficient of Cu-complexing ligands in situ. Results showed that the probes produced highly precise data sets, with substantial differences in copper accumulation between contaminated and pristine waters. Comparison of DGT results with CLE-CSV indicate that at least 10-35% of the organically complexed copper derived by CLE-ACSV measurements was DGT-labile. Diffusion coefficients (corrected to 25 degrees C) of organically complexed DGT-labile Cu through the hydrogel ranged from 0.77 x 10(-6) cm2 s(-1) in Vineyard Sound to 2.16 x 10(-6) cm2 s(-1) in the Elizabeth River estuary. Accumulation rates of copper were substantially higher in contaminated waters than in pristine waters, suggesting that the probes in their current form may be useful as tracking tools to detect episodic sources of contamination.     

Chemical speciation of dissolved Cu, Ni, and Co in a contaminated estuary in southwest Spain and its influence on plankton communities

Four surveys of the Huelva Estuary in southwest Spain and its sources, the Tinto and the Odiel Rivers, were carried out between 1996 and 1998. The surveys investigated the impact of metalliferous mining of sulfide-rich ores in the catchment area on metal speciation, metal concentrations in a macrophyte, and phytoplankton diversity and abundance. Chemical speciation measurements in the lower Tinto Estuary showed that metals were predominantly electrochemically labile (> 99% of total dissolved Cu, Co, and Ni at 10 microM Cu, 424 nM Co, and 500 nM Ni, S = 28). Concentrations of Cu complexing ligands and free cupric ions [Cu2+] in the Gulf of Cádiz ranged between 5.3 and 38 nM and 0.2-7.9 pM, respectively, with conditional stability constants of the ligands of log K'(CuL) = 11.7-12.6. At enhanced dissolved Cu concentrations in the lower Huelva Estuary, Cu complexing ligands were saturated with Cu, resulting in nanomolar [Cu2+], which increased upstream. Metal tissue concentrations of the macrophyte Blindingia marginata were high, and a clear relationship between dissolved labile Cu and macrophyte tissue Cu concentrations was observed. A low biodiversity was observed in the Huelva system (Shannon-Wiener indices (H) typically < 0.2). Nevertheless, the maximum biomass was observed in the lower Tinto Estuary, which showed high labile metal and nutrient concentrations and a low biodiversity (H < 0.02), thereby suggesting adaptation through evolutionary processes of the phytoplankton community to the harsh conditions.     

Speciation of Co(II) and Ni(II) in anaerobic bioreactors measured by competitive ligand exchange-adsorptive stripping voltammetry

Competitive ligand exchange-adsorptive stripping voltammetry is applied to speciation analysis of dissolved Ni(II) and Co(II) in an anaerobic bioreactor and similar batch media. Co and Ni speciation in these media can be measured down to concentration levels of ca. 1 nM. Sulfide interference is avoided via removal as H2S. In methanogenic bioreactors, up to 95% of the dissolved Co and Ni is present in strongly bound forms, with complex stabilities > or =10(8)-10(9) and 10(7)-10(8) mol(-1) L, respectively. In effluent from sulfate-reducing bioreactors, Co is also found to be present in a strongly bound form, and up to micromolar levels of strongly complexing excess ligand was detected. The predominant presence of Co and Ni in strong complexes, with concomitant low free dissolved concentrations, is significant for limitation by these elements in anaerobic bioreactors.