Chemical speciation and toxicity of nickel species in natural waters from the Sudbury area (Canada)

Metal complexation properties of dissolved organic carbon (DOC) in freshwaters are recognized but poorly understood. Here, we investigated the release of free nickel from Ni-DOC complexes using nickel-polluted freshwaters from Sudbury (Canada). We used the Competing Ligand Exchange Method with Chelex-100 as the competing ligand to measure the rate of free Ni2+ ion released by the dissociation of Ni-DOC complexes. The kinetic studies showed that the fastest kinetically distinguishable component representing approximately 30-95% of the total nickel had a dissociation rate coefficient similar to that reported for [Ni(H20)6]2+. High concentrations of Ca2+ and Mg2+ caused a larger amount of the DOC-bound nickel to be released as free Ni2+ ion. Growth inhibition of the freshwater alga Pseudokirchneriella subcapitata was highly correlated with the Ni/DOC ratio, the free plus labile nickel concentration, and the dissociation rate coefficient. While the levels of metals were not sufficient to kill Daphnia magna, these test organisms were immobilized in the same samples that showed algal growth inhibition. Only one sample caused 22% death of Hydra attenuata. The algal toxicity tests were consistent with the kinetic speciation results and are consistent with the hypothesis that dissolved [Ni(H20)6]2+ plus other labile nickel species are toxic forms of Ni present.     

A kinetic study of nickel complexation in model systems by adsorptive cathodic stripping voltammetry

Adsorptive cathodic stripping voltammetry (AdCSV) in conjunction with the competing ligand-exchange method (CLEM) was investigated as a tool for measuring dissociation rate coefficients of nickel complexes in model systems. Dimethylglyoxime (DMG) was used as the competing ligand. Citric acid (CA) and a well-characterized fulvic acid (FA) were used as model ligands. The rate coefficients were calculated, and the consistency of equilibrium and kinetic data was discussed. The contributions of the disjunctive pathway (proceeding by the dissociation of the initial complex) and the adjunctive pathway (proceeding by the formation of an intermediate complex as a result of direct attack of the competing ligand on the initial complex) on the overall reactions were investigated. The reactions of Ni-CA or Ni-FA complexes with DMG were demonstrated to proceed by both disjunctive and adjunctive pathways. The predominant pathway for the overall reaction depends on the nickel-to-initial ligand and the DMG-to-initial ligand ratios. The reactions follow predominantly the disjunctive pathway for [DMG] > or = 3 mM and Ni-to-dissolved organic carbon (DOC) ratios greater than 10 nM Ni 2+/g of DOC. Since free nickel ion in freshwaters is reported to be toxic, its rate and pathway of formation are of environmental concern.     

Interpretation of in situ speciation measurements of inorganic and organically complexed trace metals in freshwater by DGT

The dynamic speciation technique, diffusive gradients in thin-films (DGT), has been used in freshwater to determine simultaneously, from a single set of in situ measurements, (1) the equilibrium distribution of metal ions between simple inorganic complexes and larger organic complexes and (2) information on the rates of dissociation of these complexes. DGT devices with different diffusion layer thicknesses (0.3, 0.54, 1.34, and 2.14 mm) were used to estimate the in situ dissociation kinetics. Information on the species distribution was obtained by using two types of gel, which allow relatively free (polyacrylamide, APA) and more retarded (restricted, RES) diffusion of the metal complexes. The full theoretical basis of the technique is developed and applied to in situ measurements of Mn, Fe, Co, Ni, Cu, Cd, and Pb in a pristine river (Wyre, U.K.), with high DOC(15mg L(-1)), assuming that organic complexes are dominated by fulvic acid. These first DGT measurements that do not rely on assumptions about complex lability or the distribution of species, are compared to total dissolved measurements, previously reported speciation calculations and measurements using alternative speciation techniques. Examination of calculation consistency suggests that the effective mean diffusion coefficients of metal complexes with organic matter under in situ conditions may be larger than those measured in the laboratory using extracted fulvic acid.     

In situ measurements of metal complex exchange kinetics in freshwater

Trace metals were measured in situ in a freshwater river draining a peat catchment (DOC = 15 mg L(-1)) using diffusive gradients in thin-films (DGT) devices with a range of gel layer thicknesses (0.16-2.0 mm). The reciprocal of the accumulated mass of each metal varied linearly with the thickness of the diffusive layer. These plots allowed calculation of the thickness of an apparent diffusive boundary layer (ADBL). A constant value was obtained from the plots of Cd, Pb, and Zn. The observed increase in the ADBL for the other metals (Mn相似文献   

Adsorption of Me(II), HEDP, and Me(II)-HEDP onto boehmite at nonstoichiometric Me(II)-HEDP concentrations

In aqueous environments, certain heavy metals are toxic even at very low concentrations. The main pathway of metal removal in the aquatic systems is via adsorption onto surfaces. These are desired processes that help decrease the dissolved fraction of metals in natural water. The presence of organic ligands as mono- and polyphosphonates may produce drastic changes in the mobility of the heavy metals. 1-Hydroxyethane-(1,1-diphosphonic acid) (HEDP) is a very strong chelating agent widely used in industrial applications. This study examines the effect of HEDP on the adsorption of Cu(II), Zn(II), and Cd(II) onto boehmite in nonstoichiometric conditions, with the HEDP concentration higher than the corresponding Me(II) cations. At high surface loading and low pH, HEDP removes Zn(II) and Cd(II) from solution to an appreciable extent. The data are modeled assuming an anionic-ternary complex formation. In the same conditions, Cu(II) adsorption is significantly suppressed at intermediate values of pH, and this behavior is linked to Cu-HEDP complex formation in solution. At low surface covering, the effects of HEDP on metal adsorption are either negligible or slight. This behavior suggests that both ligand and metal are mainly adsorbed in separate form. All experimental data indicate that no changes are observed in the pH edges for phosphonate adsorption. The surface constants to fit the experimental data were calculated by applying the 2-K model constant capacitance (CCM).     

Interactions of trace metals with hydrogels and filter membranes used in DET and DGT techniques

Equilibrium partitioning of trace metals between bulk solution and hydrogels/filter was studied. Under some conditions, trace metal concentrations were higher in the hydrogels or filter membranes compared to bulk solution (enrichment). In synthetic soft water, enrichment of cationic trace metals in polyacrylamide hydrogels decreased with increasing trace metal concentration. Enrichment was little affected by Ca and Mg in the concentration range typically encountered in natural freshwaters, indicating high affinity but low capacity binding of trace metals to solid structure in polyacrylamide gels. The apparent binding strength decreased in the sequence: Cu > Pb > Ni approximately to Cd approximately to Co and a low concentration of cationic Cu eliminated enrichment of weakly binding trace metal cations. The polyacrylamide gels also had an affinity for fulvic acid and/or its trace metal complexes. Enrichment of cationic Cd in agarose gel and hydrophilic polyethersulfone filter was independent of concentration (10 nM to 5 microM) but decreased with increasing Ca/ Mg concentration and ionic strength, suggesting that it is mainly due to electrostatic interactions. However, Cu and Pb were enriched even after equilibration in seawater, indicating that these metals additionally bind to sites within the agarose gel and filter. Compared to the polyacrylamide gels, agarose gel had a lower affinity for metal-fulvic complexes. Potential biases in measurements made with the diffusive equilibration in thin-films (DET) technique, identified by this work, are discussed.     

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.     

In-situ speciation of Ni and Zn in freshwaters: comparison between DGT measurements and speciation models

The technique of DGT (diffusive gradients in thin films) was used for the first time to measure in situ the distribution of Zn and Ni between inorganic species and complexes with fulvic and humic acids in natural waters. With DGT, metals are bound to a resin embedded in a layer of hydrogel after diffusive transport through an adjacent layer of hydrogel. The metal concentrations in the waters can be quantified using simple diffusion equations. By using devices with hydrogels of different pore size, large and small complex species were discriminated. Inorganic species diffuse freely through all gels, but larger organic complexes with fulvic and humic acids diffuse less freely through more restricted gels (gels with smaller pore size). Systematic differences between DGT devices containing gels of different pore size were obtained. Their calibration for the diffusion of fulvic and humic complexes allowed calculation of the concentrations of labile inorganic (Zn, 34.6 +/- 2.5 nM; Ni, 23.5 +/- 0.9 nM) and labile organic (Zn, 43.1 +/- 2.9 nM; Ni, 11.2 +/- 0.7 nM) complexes. The concentration of Zn measured by anodic stripping voltammetry in samples returned to the laboratory lay between the DGT-measured inorganic concentration and the total dissolved concentration, consistent with partial measurement of organic complexes of Zn. The speciation model WHAM successfully predicted the species distribution of Ni, Zn, and Cu, provided that competitive binding by Fe(III) was considered. Using the speciation models WHAM and ECOSAT, free ion activities of Zn and Ni were calculated from (1) the total inorganic species measured by DGT and (2) the total dissolved species and dissolved organic carbon. The calculations confirmed the good model predictions of metal-humic binding but highlighted problems with default databases used for the speciation of inorganic components.     

Comparison of Cd(II), Cu(II), and Pb(II) biouptake by green algae in the presence of humic acid

The present study examines the role of humic acid, as a representative of dissolved organic matter, in Cd(II), Cu(II), and Pb(II) speciation and biouptake by green microalgae. Cellular and intracellular metal fractions were compared in the presence of citric and humic acids. The results demonstrated that Cd and Cu uptake in the presence of 10 mg L(-1) humic acid was consistent with that predicted from measured free metal concentrations, while Pb biouptake was higher. By comparing Cd, Cu, and Pb cellular concentrations in the absence and presence of humic acid, it was found that the influence of the increased negative algal surface charge, resulting from humic acid adsorption, on cellular metal was negligible. Moreover, the experimental results for all three metals were in good agreement with the ternary complex hypothesis. Given that metal has much higher affinity with algal sites than humic acid adsorbed to algae, the contribution of the ternary complex to metal bioavailability was negligible in the case of Cd (II) and Cu (II). In contrast, the ternary complex contributed to over 90% of total cellular metal for Pb(II), due to the comparable affinity of Pb to algal sites in comparison with humic acid adsorbed to algae. Therefore, the extension of the biotic ligand model by including the formation of the ternary complex between the metal, humic acid, and algal surface would help to avoid underestimation of Pb biouptake in the presence of humic substances by green algae Chlorella kesslerii.     

Metal flux and dynamic speciation at (bio)interfaces. Part IV: MHEDYN, a general code for metal flux computation; application to particulate complexants and their mixtures with the other natural ligands

Metal flux at consuming interfaces (e.g., sensors or microorganisms) is simulated in environmental multiligand systems using a new numerical code, MHEDYN (Multispecies HEterogeneous DYNamics), based on the lattice Boltzmann method. The attention is focused on the computation of the maximum flux (i.e.,the flux controlled by diffusion-reaction in solution) of Cu(II). Part III described flux computation in the presence of simple ligands and fulvic/humic substances. This paper (Part IV) discusses the case of metal complexes formed with aggregates including a broad range of sizes and diffusion coefficients and their mixture with simple and fulvic ligands under typical natural water conditions. This paper describes the dynamic contribution of the various size classes of aggregate Cu(II) complexes for the first time. In two typical waters containing mixtures of ligands, the contribution of aggregates is found to be small, whereas that of fulvics may play a major role, even under pH conditions where the lability of their Cu(II) complexes is low. These results point out the great usefulness of MHEDYN for dynamic speciation in very complex mixtures. In all cases, MHEDYN enables us to compute the concentration profile of each complex and itstime evolution, as well as the steady-state flux and the corresponding contribution of each complex to the flux. Thus, MHEDYN should be very useful for comparing theoretical predictions with experimental measurements of metal bioavailability or of dynamic sensor response in a complete aquatic medium.     

Effect of soil fulvic acid on nickel(II) sorption and bonding at the aqueous-boehmite (gamma-AIOOH) interface

The influence of soil-derived fulvic acid (SFA) on Ni(II) sorption and speciation in aqueous boehmite (gamma-AIOOH) suspensions was evaluated using a combination of sorption experiments and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy measurements. Co-sorption of SFA at the aqueous-boehmite interface modifies both the extent of Ni(II) sorption as well as the local structure of the sorbing Ni(II) ions. In SFA-free suspensions, Ni(II) sorbs by forming inner-sphere bidentate mononuclear complexes with surface aluminol groups. Addition of SFA increases Ni(II) sorption at pH conditions below the sorption edge observed in SFA-free suspensions and diminishes Ni(II) sorption at pH above the SFA-free sorption edge. When SFA is co-sorbed to boehmite, Ni(II) sorbs by forming both ligand-bridging ternary surface complexes (Ni(II)-SFA-boehmite) as well as surface complexes in which Ni(II) remains directly bonded to aluminol groups, that is, binary Ni(II)-boehmite or metal-bridging ternary surface complexes (SFA-Ni(II)-boehmite). The relative contribution of the individual sorption complexes depends heavily on geochemical conditions; the concentration of ligand-bridging complexes increases with increasing SFA sorption and decreasing pH. The local structure of sorbed Ni(II) does not change with increasing reaction time even though the extent of sorption continues to increase. This supports a slow uptake mechanism where surface or intraparticle diffusion processes are rate-limiting. This work demonstrates that the association of humic constituents with soil minerals can significantly modify the mechanisms controlling trace metal sorption and transport in heterogeneous aquatic environments.     

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.     

Determination of Trace Amounts of Cd(II), Cu(II), and Ni(II) in Food Samples Using a Novel Functionalized Magnetic Nanosorbent

This work describes a novel sorbent based on functionalization of magnetic nanoparticles by 2-aminobenzothiazole and its application in the extraction and preconcentration of trace amount of Cd(II), Cu(II), and Ni(II) ions. This nanosorbent was characterized by Fourier transfer infrared spectroscopy, thermal analysis, X-ray powder diffraction, elemental analysis, and scanning electron microscopy. The effects of various factors such as pH value, sorption time, sorbent dosage, type, volume, and concentration of the eluent as well as the elution time were investigated. Following the sorption and the elution of target analytes, the Cd(II), Cu(II), and Ni(II) ions were determined by flame atomic absorption spectrometry. Under the optimal conditions, the limits of detection (LODs) were 0.03, 0.009, and 0.1 μg L^?1 for Cd(II), Cu(II), and Ni(II), respectively. Linearity was within the range of 0.1–75 ng mL^?1 for Cd(II), 0.03–50 ng mL^?1 for Cu(II), and 0.5–100 ng mL^?1 for Ni(II) in the initial solution with r ² values greater than 0.9978. The relative standard deviations of the method were less than 8.4 %. The preconcentration factor of the method was 277. The sorption capacity of this new sorbent was 65, 78, and 49 mg g^?1 for Cd(II), Cu(II), and Ni(II), respectively. The proposed method was validated using two certified reference materials (LGC 6010 hard drinking water and NIST SRM 1515 apple leaves) in order to exhibit its applicability. Ultimately, this method was applied to the rapid extraction of the trace quantities of Cd(II), Cu(II), and Ni(II) ions in different food samples, and satisfactory results were obtained.     

Porewater evidence of metal (Cu, Ni, Co, Zn, Cd) mobilization in an acidic, ombrotrophic bog impacted by a smelter, Harjavalta, Finland and comparison with reference sites

Porewaters were collected from three Finnish peat bogs subjected to varying inputs of atmospheric trace metals: Hietaj?rvi (HIJ), a low-background site, Outokumpu (OUT), near a Cu-Ni mine, and Harjavalta (HAR), near a Cu-Ni smelter. Samples for metal analyses were collected at depths ranging from 10 to 70 cm using a purpose-built syringe-type sampler. Metal concentrations were determined using inductively coupled plasma-sector field-mass spectrometry (ICP-SF-MS). Porewater concentrations at HIJ and OUT (Cd <0.3 nM, Co <1.4 nM, Cu, Ni <8 nM, Zn <250 nM) are independent of metal concentrations in the solid phase (peat). At OUT there is a limited release of Ni to the porewaters, but concentrations in the aqueous phase are generally below 0.3% of the total concentration in any given peat sample. These data are consistent with the immobility of these metals after deposition from the air. In contrast, porewaters at HAR are enriched in trace metals compared to the other sites by a factor of 2 (Zn), 10 (Cd), 20 (Co), and 100 (Cu and Ni) with dissolved fractions of Cu and Ni accounting for ca. 20% of the metal inventories in the cores. The elevated release of metals from solid phases at HAR is consistent with the postdepositional migration of metals at this site and reflects the predominance of oxide phases supplied to the bog surface and the much lower pH values (<3.4). The elevated proton concentrations not only promote mineral dissolution but also compete with cation exchange processes and hinder the formation of metal complexes with organic ligands.     

Graphene-Based Preconcentration System Prior to Energy Dispersive X-Ray Fluorescence Spectrometric Determination of Co,Ni, and Cu Ions in Wine Samples

A combination of dispersive micro solid-phase extraction (DMSPE), based on graphene as a solid sorbent, with energy dispersive X-ray fluorescence spectrometry (EDXRF) is proposed for preconcentration and determination of Co(II), Ni(II), and Cu(II) ions in wine samples. In the developed procedure, cupferron complexes of metal ions are adsorbed on graphene dispersed in aqueous samples. After the adsorption process, aqueous samples are passed through a membrane filter with the use of filtration assembly, and then loaded filters are directly measured using EDXRF. In order to obtain high recovery of the metal ions, various analytical parameters influencing sorption were optimized, such as pH, amount of graphene, Triton X-100 and cupferron, sample volume, and sorption time. Under optimal conditions, the calibration plots cover the 2 to 100 ng mL^?1 range for Co(II) and Ni(II), and 2 to 150 ng mL^?1 for Cu(II). The detection limits of 0.08, 0.08, and 0.07 ng mL^?1 for Co(II), Ni(II), and Cu(II) were obtained using 50 mL sample volume and 200 μg of graphene. The precision (at a 20 ng mL^?1 level for n?=?10) is lower than 3.5 %. The proposed method was successfully applied to determination of Co, Ni, and Cu in wine samples.     

Furcellaran metal salts – structure,thermal properties and their ligation with coumarin

Furcellaran formed complexes with transition metal ions. The amount of bound metal ions decreased in the following order: Fe(III) > Mn(II) > Cu(II) > Cr(III) > Zn > Ni(II), thus the hardness of these Lewis acids seemed to be the most essential factor controlling their ligation with the polysaccharide. Resulting complexes decomposed thermally at lower temperature than plain furcellaran. Neither furcellaran bound coumarin selected as the model molecule for mycotoxins nor this oxacompound coordinated to furcellaran metal salts.     

Metal flux and dynamic speciation at (bio)interfaces. Part III: MHEDYN, a general code for metal flux computation; application to simple and fulvic complexants

Metal flux at consuming interfaces (e.g., sensors or microorganisms) is simulated in environmental multiligand systems using a new numerical code, MHEDYN (Multispecies HEterogeneous DYNamics), based on the lattice Boltzmann method. The attention is focused on the computation of the maximum flux of Cu(II), that is, the flux controlled by diffusion-reaction in solution, irrespective of processes occurring at the interface. In parts III and IV of this series, three types of typical environmental complexants are studied: (a) simple ligands (OH- and C03(2-)), (b) fulvic or humic substances including many sites with broadly varying rate constants, and (c) aggregates including a broad range of sizes and diffusion coefficients. Part III focuses on computations in the presence of simple ligands and fulvic/humic substances separately, and part IV discusses the case of aggregate complexes alone and the mixtures of all ligands in typical natural waters. These papers describe the dynamic contribution of the various types of sites for fulvic and aggregate Cu(II) complexes for the first time. Whenever possible, the metal fluxes computed by MHEDYN are compared with those given by another code, FLUXY, based on a fully different mathematical approach, and very good agreement between these codes is obtained. In all cases, MHEDYN computes the concentration profile of each complex and its time evolution, as well as the steady-state flux and the corresponding contribution of each complex to the flux. The metal fluxes can be computed at a planar consuming surface such as an organism or a sensor surface, in presence of an unlimited number of complexation reactions of the metal M, and for any metal/ligand concentration ratio, with values of the physicochemical parameters ranging over many orders of magnitude.     

Partitioning of metal species during an enriched fuel combustion experiment. speciation in the gaseous and particulate phases

Combustion processes are the most important source of metal in the atmosphere and need to be better understood to improve flue gas treatment and health impact studies. This combustion experiment was designed to study metal partitioning and metal speciation in the gaseous and particulate phases. A light fuel oil was enriched with 15 organometallic compounds of the following elements: Pb, Hg, As, Cu, Zn, Cd, Se, Sn, Mn, V, Tl, Ni, Co, Cr, and Sb. The resulting mixture was burnt in a pilot-scale fuel combustion boiler under controlled conditions. After filtration of the particles, the gaseous species were sampled in the stack through a heated sampling tube simultaneously by standardized washing bottles-based sampling techniques and cryogenically. The cryogenic samples were collected at -80 degrees C for further speciation analysis by LT/GC-ICPMS. Three species of selenium and two of mercury were evidenced as volatile species in the flue gas. Thermodynamic predictions and experiments suggest the following volatile metal species to be present in the flue gas: H2Se, CSSe, CSe2, SeCl2, Hg(0), and HgCl2. Quantification of volatile metal species in comparison between cryogenic techniques and the washing bottles-based sampling method is also discussed. Concerning metal partitioning, the results indicated that under these conditions, at least 60% (by weight) of the elements Pb, Sn, Cu, Co, Tl, Mn, V, Cr, Ni, Zn, Cd, and Sb mixed to the fuel were found in the particulate matter. For As and Se, 37 and 17%, respectively, were detected in the particles, and no particulate mercury was found. Direct metal speciation in particles was performed by XPS allowing the determination of the oxidation state of the following elements: Sb(V), Tl(III), Mn(IV), Cd(II), Zn(II), Cr(III), Ni(II), Co(II), V(V), and Cu(II). Water soluble species of inorganic Cr, As, and Se in particulate matter were determined by HPLC/ICP-MS and identified in the oxidation state Cr(III), As(V), and Se(IV).     

Influence of chloride and metals on silver bioavailability to Atlantic salmon (Salmo salar) and Rainbow trout (Oncorhynchus mykiss) yolk-sac fry

The effects of differing water chloride concentrations (0-10 mM) or competing metals [Cu(II), Cd(II), Zn(II), Pb(II), Co(II) (1-10,000 nM)] on Ag(I) uptake in yolk-sac fry of two salmonid species, the Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss), were studied. None of the metals tested were strong competitors of Atlantic salmon yolk-sac fry whole body Ag(I) influx. Inhibition of Ag(I) influx was only seen with a 100-fold excess of Cu(II) or Cd(II) or a 1000-fold excess of Pb(II) or Co(II). At these concentrations, the degree of competition appears to be directly proportional to the conditional stability constant of the competing metal to the gill (metal-gill log K). The range of [Cl-] allowed an assessment of Ag+, AgCl(aq), and AgCl2- bioavailability. The pattern of Ag(I) uptake was similar for each fish species. At <1 mM Cl-, where the [Ag+] dominates, the Ag(I) accumulation rate was constant. Above 1 mM Cl-, where the [AgCl(aq)] is dominant and the [AgCl2-] increases, there was a decline in Ag(I) uptake rate. However, even when very little Ag+ was present (i.e., at 10 mM Cl-) Ag(I) accumulated, albeit at a lower rate. This was suggestive of passive influx by AgCl(aq) and indicated little or no entry of negatively charged silver chloride complexes. The decline in Ag(I) uptake above 1 mM Cl- demonstrated that, if Ag(I) was present as both Ag+ and AgCl(aq), salmonid Ag(I) accumulation was dominated by Ag+ uptake. Therefore, the order of bioavailability of the Ag(I) species was determined as Ag+ > AgCl(aq) > AgCl2-.