Copper(II) Binding by Dissolved Organic Matter: Importance of the Copper-to-Dissolved Organic Matter Ratio and Implications for the Biotic Ligand Model

The ratio of copper to dissolved organic matter (DOM) is known to affect the strength of copper binding by DOM, but previous methods to determine the Cu(2+)-DOM binding strength have generally not measured binding constants over the same Cu:DOM ratios. In this study, we used a competitive ligand exchange-solid-phase extraction (CLE-SPE) method to determine conditional stability constants for Cu(2+)-DOM binding at pH 6.6 and 0.01 M ionic strength over a range of Cu:DOM ratios that bridge the detection windows of copper-ion-selective electrode and voltammetry measurements. As the Cu:DOM ratio increased from 0.0005 to 0.1 mg of Cu/mg of DOM, the measured conditional binding constant ((c)K(CuDOM)) decreased from 10(11.5) to 10(5.6) M(-1). A comparison of the binding constants measured by CLE-SPE with those measured by copper-ion-selective electrode and voltammetry demonstrates that the Cu:DOM ratio is an important factor controlling Cu(2+)-DOM binding strength even for DOM isolates of different types and different sources and for whole water samples. The results were modeled with Visual MINTEQ and compared to results from the biotic ligand model (BLM). The BLM was found to over-estimate Cu(2+) at low total copper concentrations and under-estimate Cu(2+) at high total copper concentrations.     

Biotic ligand model, a flexible tool for developing site-specific water quality guidelines for metals

The biotic ligand model (BLM) is a mechanistic approach that greatly improves our ability to generate site-specific ambient water quality criteria (AWQC)for metals in the natural environment relative to conventional relationships based only on hardness. The model is flexible; all aspects of water chemistry that affect toxicity can be included, so the BLM integrates the concept of bioavailability into AWQC--in essence the computational equivalent of water effect ratio (WER) testing. The theory of the BLM evolved from the gill surface interaction model (GSIM) and the free ion activity model (FIAM). Using an equilibrium geochemical modeling framework, the BLM incorporates the competition of the free metal ion with other naturally occurring cations (e.g., Ca2+, Na+, Mg2-, H+), togetherwith complexation by abiotic ligands [e.g., DOM (dissolved organic matter), chloride, carbonates, sulfide] for binding with the biotic ligand, the site of toxic action on the organism. On the basis of fish gill research, the biotic ligands appear to be active ion uptake pathways (e.g., Na+ transporters for copper and silver, Ca2+ transporters for zinc, cadmium, lead, and cobalt), whose geochemical characteristics (affinity = log K, capacity = Bmax) can be quantified in short-term (3-24 h) in vivo gill binding tests. In general, the greater the toxicity of a particular metal, the higher the log K. The BLM quantitatively relates short-term binding to acute toxicity, with the LA50 (lethal accumulation) being predictive of the LC50 (generally 96 h for fish, 48 h for daphnids). We critically evaluate currently available BLMs for copper, silver, zinc, and nickel and gill binding approaches for cadmium, lead, and cobalt on which BLMs could be based. Most BLMs originate from tests with fish and have been recalibrated for more sensitive daphnids by adjustment of LA50 so as to fit the results of toxicity testing. Issues of concern include the arbitrary nature of LA50 adjustments; possible mechanistic differences between daphnids and fish that may alter log K values, particularly for hardness cations (Ca2+, Mg2+); assumption of fixed biotic ligand characteristics in the face of evidence that they may change in response to acclimation and diet; difficulties in dealing with DOM and incorporating its heterogeneity into the modeling framework; and the paucity of validation exercises on natural water data sets. Important needs include characterization of biotic ligand properties at the molecular level; development of in vitro BLMs, extension of the BLM approach to a wider range of organisms, to the estuarine and marine environment, and to deal with metal mixtures; and further development of BLM frameworks to predict chronic toxicity and thereby generate chronic AWQC.     

A biotic ligand model predicting acute copper toxicity for Daphnia magna: the effects of calcium, magnesium, sodium, potassium, and pH

The extent to which Ca2+, Mg2+, Na+, K+ ions and pH independently mitigate acute copper toxicity for the cladoceran Daphnia magna was examined. Higher activities of Ca2+, Mg2+, and Na+ (but not K+) linearly increased the 48-h EC50 (as Cu2+ activity), supporting the concept of competitive binding of these ions and copper ions to toxic action or transport sites at the organism-water interface (e.g. fish gill, the biotic ligand). The increase of the EC50 (as Cu2+ activity) with increasing H+, however, seemed to suggest cotoxicity of CuOH+ rather than proton competition. Based on the biotic ligand model (BLM) concept, we developed a methodology to estimate stability constants for the binding of Cu2+, CuOH+, Ca2+, Mg2+, Na+, and H+ to the biotic ligand, solely based on toxicity data. Following values were obtained: log K(CuBL) = 8.02, log K(CuOHBL)= 7.45, log K(CaBL) = 3.47, log K(MgBL) = 3.58, log K(NaBL) = 3.19, and log K(HBL) approximately 5.4. Further, we calculated that on average 39% of the biotic ligand sites need to be occupied by copper to induce a 50% acute effect for D. magna after 48 h of exposure. Using the estimated constants, a BLM was developed that can predict acute copper toxicity for D. magna as a function of water characteristics. The presented methodology can easily be applied for BLM development for other organisms and metals. After validation with laboratory and natural waters (including DOC), the developed model will support efforts to improve the ecological relevance of presently applied risk assessment procedures.     

Modeling complexometric titrations of natural water samples

Complexometric titrations are the primary source of metal speciation data for aquatic systems, yet their interpretation in waters containing humic and fulvic acids remains problematic. In particular, the accuracy of inferred ambient free metal ion concentrations and parameters quantifying metal complexation by natural ligands has been challenged because of the difficulties inherent in calibrating common analytical methods and in modeling the diverse array of ligands present. This work tests and applies a new method of modeling titration data that combines calibration of analytical sensitivity (S) and estimation of concentrations and stability constants for discrete natural ligand classes ([Li]T and Ki) into a single step using nonlinear regression and a new analytical solution to the one-metal/two-ligand equilibrium problem. When applied to jointly model data from multiple titrations conducted at different analytical windows, it yields accurate estimates of S, [Li]T, Ki, and [Cu2+] plus Monte Carlo-based estimates of the uncertainty in [Cu2+]. Jointly modeling titration data at low-and high-analytical windows leads to an efficient adaptation of the recently proposed "overload" approach to calibrating ACSV/CLE measurements. Application of the method to published data sets yields model results with greater accuracy and precision than originally obtained. The discrete ligand-class model is also re-parametrized, using humic and fulvic acids, L1 class (K1 = 10(13) M(-1)), and strong ligands (L(S)) with K(S) > K1 as "natural components". This approach suggests that Cu complexation in NW Mediterranean Sea water can be well represented as 0.8 +/- 0.3/0.2 mg humic equiv/L, 13 +/- 1 nM L1, and 2.5 +/- 0.1 nM L(S) with [CU]T = 3 nM. In coastal seawater from Narragansett Bay, RI, Cu speciation can be modeled as 0.6 +/- 0.1 mg humic equiv/L and 22 +/- 1 nM L1 or approximately 12 nM L1 and approximately 9 nM L(S), with [CU]T = 13 nM. In both waters, the large excess (approximately 10 nM) of high-affinity, Cu-binding ligands over [CU]T results in low equilibrium [Cu2+] of 10(-14.5 +/- 0.2) M and 10(-13.3 +/- 0.4) M, respectively.     

Determination of mercury complexation in coastal and estuarine waters using competitive ligand exchange method

While many studies have examined Hg(II) binding ligand in natural dissolved organic matter, determined ligand concentrations far exceed natural Hg(II) concentrations. This ligand class may not influence natural Hg(II) complexation, given the reverse relation between ligand concentration and metal-ligand binding strength. This study used a new competing ligand, thiosalicylic acid, in a competitive ligand exchange method in which water-toluene extraction was used to determine extremely strong Hg(II) binding sites in estuarine and coastal waters (dissolved [Hg] = 0.5-8 pM). Thiosalicylic acid competition lowered the detection limit of Hg(II) complexing ligand by 2 orders of magnitude from values found by previous studies; the determined Hg(II) complexing ligand ranged from 13 to 103 pM. The logarithmic conditional stability constants between Hg(II) and Hg(II) complexing ligand (Kcond' = [HgL]/([Hg2+][L']), [L'] = total [L] - [HgL]) ranged from 26.5 to 29.0. Applying the same method for chloride competition detected another class of ligand that is present from 0.5 to 9.6 nM with log conditional stability constants ranging from 23.1 to 24.4. A linear relationship was observed between the log conditional stability constant and log Hg(II) complexing ligand concentration, supporting the hypothesis that Hg(II) binding ligand should be characterized as a series or continuum of binding sites on natural dissolved organic matter. Calculating Hg(II) complexation using the conditional stability constants and ligand concentrations determined in this study indicates that >99% of the dissolved mercury is complexed by natural ligand associated with dissolved organic matter in estuarine and coastal waters of Galveston Bay, Texas.     

Development of a biotic ligand model and a regression model predicting acute copper toxicity to the earthworm Aporrectodea caliginosa

The purpose of this study was to develop a terrestrial biotic ligand model (BLM) for predicting acute copper toxicity to the earthworm Aporrectodea caliginosa. To overcome the basic problems hampering development of BLMs for terrestrial organisms, an artificial flow-through exposure system was developed consisting of an inert quartz sand matrix and a nutrient solution, of which the composition was univariately modified. A. caliginosa was exposed for 7 days under varying concentrations of copper and the major cations modifying toxicity: H+, Ca2+, Mg2+, and Na+. In addition copper speciation was modulated by means of EDTA or dissolved organic carbon (DOC). An increase in pH or pNa resulted in a linear decrease of 7-days median lethal concentrations. Increasing Ca2+ and Mg2+ activities had inconsistent effects. EDTA addition decreased toxicity when the total copper concentration in the pore water was kept the same. This is attributed to the strong complexation capacity of EDTA and shows that total copper is not the toxic species. DOC was more protective than could be explained by its metal complexing properties. The BLM developed incorporates the effects of H+ and Na+. This BLM was validated with the results of a set of bioassays with artificial pore water in quartz sand and by a set of bioassays in spiked field soils. Prediction error was within a factor of 2, but some predictions were not within the 95% confidence interval. Therefore a more widely applicable regression type model was developed that was able to explain >95% of the (lack of) toxicity observed. To our knowledge this is the first report of the successful development of a terrestrial BLM.     

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.     

Ni uptake by a green alga. 2. Validation of equilibrium models for competition effects

It is generally admitted that the presence of major cations and H+ can attenuate trace metal uptake. Recent models such as the biotic ligand model (BLM) aim to quantify and predict this effect by determining stability constants for each of the major competitors for any given interaction of a trace metal with a biological organism. In this study, short-term Ni internalization fluxes (J(int)) were used to quantitatively assess the binding of H+, Mg2+, Ca2+ (K(H-Rs), K(Mg-Rs), K(Ca-Rs)), and trace metals to transport sites (R(s)) leading to Ni biouptake by Chlamydomonas reinhardtii. H+ and Mg2+ are shown to compete directly for the entry of Ni with affinity constants that are of the same order of magnitude (K(Mg-Rs) = 10(5.1) M(-1); K(H-Rs) = 10(5.3) M(-1)) as that measured for Ni (K(Ni-Rs) = 10(5.1) M(-1)). The Ni internalization fluxes were also strongly linked to the Mg cell status. In contrast, the role of Ca2+ could not be explained by a simple competitive equilibrium with the Ni transport sites. Aluminum (K(Al-Rs) = 10(8) M(-1)), Zn (K(Zn-Rs) = 10(6.5) M(-1)), and Cu (K(Cu-Rs) = 10(6.6) M(-1)) were all shown to compete strongly with Ni for uptake. In addition to the determination of uptake constants, these studies provide insight into the transport mechanisms of Ni by the green alga, C. reinhardtii.     

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.     

Determination of stability constants of U(VI)-Fe(III)-citrate complexes

Ion-exchange experiments were performed to evaluate the formation of the uranium-citrate and uranium-iron-citrate complexes over a wide concentration range; i.e., environmentally relevant concentrations (e.g., 10(-6) M in metal and ligand) and concentrations useful for spectroscopic investigations (e.g., 10(-4) M in metal and ligand). The stability of the well-known uranium-citrate complex was determined to validate the computational and experimental methods applied to the more complex system. Values of the conditional stability constants for these species were obtained using a chemical equilibrium model in FITEQL. At a pH of 4.0, the stability constant for uranium-citrate complex (log beta1,1) was determined to be 8.71+/-0.6 at I = 0. Analysis of the results of ion-exchange experiments for the U-Fe-citric acid system indicates the formation of the 1:1:1 and 1:1:2 ternary species with stability constants (log beta) of 17.10+/-0.41 and 20.47+/-0.31, respectively, at I= 0.     

Copper toxicity to bioluminescent Nitrosomonas europaea in soil is explained by the free metal ion activity in pore water

The terrestrial biotic ligand model (BLM) for metal toxicity in soil postulates that metal toxicity depends on the free metal ion activity in solution and on ions competing for metal sorption to the biotic ligand. Unequivocal evidence for the BLM assumptions is most difficult to obtain for native soil microorganisms because the abiotic and biotic compartments cannot be experimentally separated. Here, we report copper (Cu) toxicity to a bioluminescent Nitrosomonas europaea reporter strain that was used in a solid phase-contact assay and in corresponding soil extracts and artificial soil solutions. The Cu(2+) ion activities that halve bioluminescence (EC50) in artificial solutions ranged 10(-5) to 10(-7) M and increased with increasing activities of H(+), Ca(2+) and Mg(2+) according to the BLM concept. The solution based Cu(2+) EC50 values of N. europaea in six contaminated soils ranged 2 × 10(-6) to 2 × 10(-9) M and these thresholds for both solid phase or soil extract based assays were well predicted by the ion competition model fitted to artificial solution data. In addition, solution based Cu(2+) EC50 of the solid phase-contact assay were never smaller than corresponding values in soil extracts suggesting no additional solid phase toxic route. By restricting the analysis to the same added species, we show that the Cu(2+) in solution represents the toxic species to this bacterium.     

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.     

Characterizing dissolved Cu and Cd uptake in terms of the biotic ligand and biodynamics using enriched stable isotopes

The biotic ligand model considers the biological and geochemical complexities that affect metal exposure. It relates toxicity to the fraction of physiological active sites impacted by reactive metal species. The biodynamic model is a complementary construct that predicts bioaccumulation and assumes that toxicity occurs when influx rates exceed rates of loss and detoxification. In this paper we presume that metal influx rates are mechanistically the resulting processes that characterize transmembrane transport. We use enriched stable isotopes to characterize, both in terms of the biotic ligand and biodynamics, dissolved metal uptake by a freshwater snail at water hardness varying up to 180-fold. Upon 24 h exposure, metal uptake was linear over a range encompassing most environmental concentrations; although saturation kinetics were observed at higher concentrations. Cadmium influx rates correlate with changes in the affinity of the biotic ligand, whereas those of Cu correlate with changes in both site affinity and capacity. A relationship between metal influx rate and ligand character asks whether toxicity is the result of accumulation at the biotic ligand or the rate at which metal is transported by that ligand.     

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.     

Bioavailability and chronic toxicity of zinc to juvenile rainbow trout (Oncorhynchus mykiss): comparison with other fish species and development of a biotic ligand model

In this study, the effects of modifying Ca (0.2-4 mM), Mg (0.05-3 mM), Na (0.75-5 mM), and pH (5.5-7.5) on the chronic toxicity of zinc to juvenile rainbow trout (Oncorhynchus mykiss) were investigated using standard 30-d assays in which survival and growth were monitored. Survival was observed to be a more sensitive end point than growth, and mortality mainly occurred during the initial stages of the exposure. This suggested that the mode of action of zinc toxicity was mainly of an acute nature. A review and analysis of existing literature demonstrated similar results for most other fish species investigated. Overall, up to a 30-fold variation of zinc toxicity was observed, as indicated by no observed effect concentrations varying between 32.7 and 974 microg of Zn L(-1). Increased concentrations of Ca2+, Mg2+, Na+, and H+ (within the tested ranges) resulted in a reduction of chronic zinc toxicity by a factor of 12, 3, >2, and 2, respectively. This suggests the major importance of Ca competing with zinc and protecting against zinc toxicity, which seems to be a ubiquitous concept in fish species (and probably also invertebrate). On the basis of the toxicity data obtained, a chronic biotic ligand model (BLM) was developed that takes into account both chemical speciation of zinc and competition between zinc and the above-mentioned cations. The developed model was able to predict chronic effect concentrations with an error of less than a factor of 2 in most cases. Hence, it was concluded that the chronic Zn BLM can reduce toxicity variability due to bioavailability to a considerable extent and that the BLM can become an important tool in criteria setting and risk assessment practice of zinc and zinc substances.     

Determination of silver speciation in natural waters. 1. Laboratory tests of Chelex-100 chelating resin as a competing ligand

Batch and column experiments were performed to investigate the suitability and chemical characteristics of Chelex-100 for use as a competing ligand in ionic silver (Ag(I)) speciation determinations in natural waters. A conditional stability constant (Kcond) for Ag+ chelation by iminodiacetate groups on the surface of Chelex resin was determined by fitting results of batch and column experiments with an equilibrium speciation model. Results of experiments in which Chelex competed with cyanide ion and thiosulfate ion for aqueous Ag+ were fitted well by a model in which log Kcond(Ag-Chelex) was set to 7.2. This value is similar to literature equilibrium constants for a 1:1 Ag(+)-EDTA chelate. In batch experiments with Chelex, equilibration times of 24 h were found to be sufficient to bring samples close to equilibrium. Effects of resin counterion and total Ag(I) concentration on extent of Ag(I) chelation were found to be minor. Effect of pH on Ag(I) chelation was minor over a range of 6-10. Column experiments (detention time = 6 s, empty-column basis) in which thiosulfate competed with Chelex for Ag(I) gave similar results to batch experiments with thiosulfate. This implies that batch and column experiments could be compared to explore ligands in natural water systems with different rates of dissociation.     

Measuring free metal ion concentrations in situ in natural waters using the Donnan Membrane Technique

Metal toxicity is not related to the total but rather to the free or labile metal ion concentration. One of the techniques that can be used to measure several free metal ion concentrations simultaneously is the Donnan Membrane Technique (DMT) in combination with the inductively coupled plasma-mass spectrometer (ICP-MS). However, free metal ion concentrations in natural waters are commonly below the detection limit of ICP-MS. We decreased the detection limit by making use of a ligand, and we developed a field DMT cell that can be applied in situ in natural waters. A kinetic approach can be used to calculate free metal ion concentrations when the equilibrium time becomes too large. The field DMT measured in situ in natural waters a free metal ion concentration ranging from 0.015% (Cu) to 13% (Zn) of a total metal concentration ranging from 0.06 nM (Cd) to 237 nM (Zn). The free metal ion concentrations were difficult to predict using an equilibrium speciation model, probably due to the uncertainty in the nature of the dissolved organic matter or the presence of other reactive colloids. It is shown that DMT can follow changes in the free metal ion concentration on times scales less than a day under certain conditions.     

Strong colloidal and dissolved organic ligands binding copper and zinc in rivers

The speciation or physicochemical form of copper and zinc in freshwater plays an important role in reactivity, bioavailability, and toxicity. Strong metal-binding ligands, which determine speciation, were detected by voltammetric methods, both anodic stripping voltammetry (ASV) and competitive ligand equilibration adsorptive stripping voltammetry (CLE-AdSV); the latter technique can detect nanomolar levels of extremely strong (log K' > 13) ligands. Through careful field site selection and the investigation of ultrafiltration permeate samples, natural organic ligands were measured with limited interferences of colloidal inorganic iron- and aluminum-based trace metal-binding phases. Furthermore, ultrafiltration allowed measurement of colloidal and dissolved ligands independently, and differences of ligand abundance and strength in different size classes are reported. For copper, ultrafilterable (<3 kDa) organic ligand site concentrations (expressed normalized to dissolved organic carbon) were on average 33% of the colloidal level, but ultrafilterable ligand log K' values were 0.5 log units stronger than those of the 0.4 microm filterable concentration. The ultrafilterable copper-binding ligand concentration showed a smaller variation across the rivers (25% rsd) than zinc-binding ligands (90% rsd). For all field sites and size fractions, strong ligand sites greatly exceeded metal concentrations; subsequently, equilibrium speciation modeling predict picomolar levels of free metal. Modeling also indicated that the very strong ligands (detected by CLE-AdSV) predominate, so modeling based solely on ASV data in freshwater may be inadequate. Competition experiments indicated that the very strong ligand sites are metal specific for copper and zinc.     

Modeling metal removal onto natural particles formed during mixing of acid rock drainage with ambient surface water

Studies have examined partitioning of trace metals onto natural particles to better understand the fate and transport of trace metals in the environment, but few studies have compared model predictions with field results. We evaluate the application of an empirical modeling approach, using surface complexation parameters available in the literature, to complex natural systems. In this work, the equilibrium speciation computer program PHREEQC was used along with the diffuse double-layer surface complexation model to simulate metal removal onto natural oxide particles formed during the mixing of acid rock drainage with ambient surface water. End-member solutions sampled in the Coeur d'Alene (CdA) Mining District in September 1999 from the Bunker Hill Mine and the South Fork Coeur d'Alene (SFCdA) River were filtered and mixed in several ratios. Solution chemistry was determined for end-members and mixed solutions, and X-ray diffraction (XRD) was used to determine the mineralogy of precipitate phases. Predicted amounts of Fe precipitates were in good agreement with measured values for particulate Fe. Surface area and reactive site characteristics were used along with surface complexation constants for ferrihydrite (Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; John Wiley & Sons: New York, 1990) to predict ion sorption as a function of mixing fraction. Comparisons of model predictions with field results indicate that Pb and Cu sorption are predicted well by the model, while As, Mo, and Sb sorption are less well-predicted. Additional comparisons with particulate metal and Fe data collected from the CdA Mining District in 1996 and 1997 suggest that sorption on particulate Fe, including amorphous iron oxides and schwertmannite, may be described using universal model parameters.