Competitive Ligand Equilibration Research Articles

Effect of salinity and temperature on the determination of dissolved iron-binding organic ligands in the polar marine environment

It is widely accepted that iron (Fe)-binding organic ligands play a crucial role in Fe distribution in the marine environment and thus in Fe biogeochemistry. Although Competitive Ligand Equilibration – Adsorptive Cathodic Stripping Voltammetry (CLE-AdCSV) is a well-established technique to investigate Fe chemical speciation in marine samples, several impediments still need to be addressed. These include the extrapolation of laboratory measurements to in-situ conditions, the harmonization of the analytical procedures used, and the applicability of the methods over salinity ranges wider than seawater (e.g., sea ice). This work focusses on the calibration of 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and 1-nitroso-2-naphthol (NN), along the salinity range 1–90, and titration of natural samples at two different temperatures (4 °C and 20 °C). The artificial ligand concentration was 10 μM for TAC and 5 μM for SA and NN. Calibrations showed that increasing salinity caused a decrease in the conditional stability constants (logK'Fe’AL) for NN and SA (although different behaviours were noted for the two species FeSA and FeSA2). Less accuracy was noted using TAC, which behaved inconsistently outside the 21 < S < 35 range, and its use is therefore discouraged in fresh and highly saline waters. Titrations of natural samples showed that only SA covered the salinity range selected, up to 78, and its use is therefore recommended in sea-ice studies. The side reaction coefficient (logα'Fe’AL) of each artificial ligand was found to be influenced by temperature differently: logα'Fe’SA was higher at lower temperature (4 °C), whereas logα'Fe’SA2 and logα'Fe’NN3 increased with increasing temperature (to 20 °C). Although titrations performed at 4 °C highlighted that the uncomplexed Fe fraction was 14% lower than at 20 °C, with potential consequences on primary productivity, the percentage of natural Fe complexed was >99%. Future investigations should consider the analysis of the samples at a temperature as close as possible to in-situ conditions to reduce the potential temperature effects.

Copper-binding ligands in deep-sea pore waters of the Pacific Ocean and potential impacts of polymetallic nodule mining on the copper cycle

The release of potentially toxic metals, such as copper (Cu), into the water column is of concern during polymetallic nodule mining. The bioavailability and thus toxicity of Cu is strongly influenced by its speciation which is dominated by organic ligand (L) complexation in seawater, with L-complexes being considered less bioavailable than free Cu2+. The presence of CuL-complexes in deep-sea sediments has, however, not been systematically studied in the context of deep-sea mining. We thus analyzed the Cu-binding L concentration ([L]) in deep-sea pore waters of two polymetallic nodule provinces in the Pacific Ocean, the Peru Basin and the Clarion-Clipperton-Zone, using competitive ligand equilibration–adsorptive stripping voltammetry. The pore-water dissolved Cu concentration ([dCu]) ranged from 3 to 96 nM, generally exceeding bottom water concentrations (4–44 nM). Based on fitting results from ProMCC and Excel, Cu was predominantly complexed by L (3–313 nM) in bottom waters and undisturbed pore waters. We conclude that processes like deep-sea mining are unlikely to cause a release of toxic Cu2+ concentrations ([Cu2+]) to the seawater as > 99% Cu was organically complexed in pore waters and the [Cu2+] was < 6 pM for 8 of 9 samples. Moreover, the excess of L found especially in shallow pore waters implied that even with a Cu release through mining activities, Cu2+ likely remains beneath toxic thresholds.

Fast iron speciation in seawater by catalytic Competitive Ligand Equilibration-Cathodic Stripping Voltammetry with tenfold sample size reduction

Iron speciation analysis in seawater is a fundamental step to understand the cycling of this element in oceanic waters, in view of its central role in regulating primary productivity and its connection to global planetary cycles. At present, analytical procedures are the bottleneck for speciation analysis, in term of both time and sample size requirement. Here we present a novel instrumental configuration for the speciation analysis of iron by the Competitive Ligand Equilibration - Cathodic Stripping Voltammetry (CLE-CSV) procedure. The new system features a 1 mL microcell and a silver wire pseudoreference enabling a tenfold reduction of the sample volume. 2,3-dihydroxynaphthalene was used as the complexing ligand and atmospheric oxygen as the catalytic enhancer because they ensured the best analytical performances in terms of detection capabilities. The side reaction coefficient for the FeDHN complex αFe’DHN was calibrated against EDTA and an average value of 9.25 for logK’Fe’DHN was calculated. The method was successfully validated in UV digested seawater using diethylenetriaminepentaacetic acid (DTPA), which has known stability constant for iron. The method was lastly applied to six samples from the Ross Sea water column (Antarctica), demonstrating its fit for purpose for the detection of trace amounts of iron ligands in seawater. Thanks to the employed instrumental configuration and the high sensitivity, the proposed method achieved a tenfold reduction in sample size, a tenfold increase in sensitivity compared with other methods employing DHN and halved the analysis time with respect to the fastest method reported in the literature. Half an hour is enough to measure a 12 point titration, making the analysis of at least three titrations per day feasible. It is expected that the application of this procedure will foster the sample throughput, thanks to the reduced analysis time, and make possible the analysis of limitedly available and challenging samples, like porewater and vent fluids via the tenfold reduction in sample size.

Influence of organic complexation on dissolved iron distribution in East Antarctic pack ice

Abstract Since Antarctic sea ice covers an area larger than the Antarctic continent itself, the discovery that it can fertilize the Southern Ocean with iron (Fe) has fostered a new breadth of research in recent years. In order to test the hypothesis that Fe-binding organic ligands control the distribution of dissolved iron (DFe) in Antarctic pack ice, iron organic speciation was investigated in samples collected during the Sea Ice Physics and Ecosystem eXperiment-2 (SIPEX-2) voyage in Austral winter/spring 2012. Dissolved Fe was measured using sector field inductively coupled plasma mass spectrometry, and iron organic speciation parameters were determined by competitive ligand equilibration - adsorptive cathodic stripping voltammetry method, using 1-nitroso-2-naphthol (NN) as the added ligand. The concentration of Fe-binding organic ligands (Lt) ranged from 4.9 nM to 41 nM (average of 14.9 ± 8.4 nM, n = 34), and was always higher than the corresponding DFe (average of 7.5 ± 4.5 nM, n = 34). Conditional stability constants (log K′ Fe’L = 11.7–13.0) were similar to those previously observed in land-fast ice. Concentrations of DFe and Lt displayed similar depth profiles; their strong correlation (Spearman's ρ = 0.80, p 1). Estimates showed that pack ice would have released 0.45 μmol/m 2 /d of Lt during spring melt, 0.21 μmol/m 2 /d of which are free from Fe binding, and hence available for further complexation. Therefore, it is suggested that this excess of Fe-free ligands may play a key role in controlling the solubility of free or newly formed Fe in surface waters before the peak of primary production, outcompeting the Fe-binding organic ligands already present in seawater.

Determination of the Side-Reaction Coefficient of Desferrioxamine B in Trace-Metal-Free Seawater

Electrochemical techniques like adsorptive cathodic stripping voltammetry with competitive ligand equilibration (ACSV-CLE) can determine total concentrations of marine organic ligands and their conditional binding constants for specific metals, but cannot identify them. Individual organic ligands, isolated from microbial cultures or biosynthesized through genomics, can be structurally characterized via NMR and tandem MS analysis, but this is tedious and time-consuming. A complementary approach is to compare known properties of natural ligands, particularly their conditional binding constants, with those of model organic ligands, measured under suitable conditions. Such comparisons cannot be meaningfully interpreted unless the side-reaction coefficient (SRC) of the model ligand in seawater is thoroughly evaluated. We conducted series of potentiometric titrations, in non-coordinating medium at seawater ionic strength (0.7 M NaClO4) over a range of metal:ligand molar ratios, to study complexation of the siderophore desferrioxamine B (DFOB) with Mg and Ca, for which it has the highest affinity among the major seasalt cations. From similar titrations of acetohydroxamic acid in the absence and presence of methanesulfonate (mesylate), it was determined that Mg and Ca binding to this common DFOB counter-ion is not strong enough to interfere with the DFOB titrations. Stability constants were measured for all DFOB complexes with Mg and Ca including, for the first time, the bidentate complexes. No evidence was found for Mg and Ca coordination with the DFOB terminal amine. From the improved DFOB speciation, we calculated five SRCs for each of the five (de)protonated forms of DFOB in trace-metal-free seawater, yet we also present a more convenient definition of a single SRC that allows adjustment of all DFOB stability constants to seawater conditions, no matter which of these forms is selected as the 'component' (reference species). An example of Cd speciation in seawater containing DFOB illustrates the non-trivial use of different SRCs for polyprotic, polydentate organic ligands.

Dissolved iron distribution and organic complexation in the coastal waters of the East China Sea

The distribution of total dissolved iron and organic ligands in the coastal waters of the East China Sea, especially in the Yangtze River estuary, was investigated using competitive ligand equilibration–adsorptive cathodic stripping voltammetry (CLE–ACSV) with 2,3-dihydroxynaphthalene (DHN) as the competing ligand.The seasonal variation of total dissolved iron (D-Fe) and its organic ligands (Lt) were observed in spring and autumn. The average D-Fe concentration in both surface and bottom waters in spring was higher than that in autumn. With respect to the horizontal distribution of D-Fe and Lt content, the highest values were observed in the estuary stations located in the Yangtze River Estuary and the Hangzhou Bay area and the lowest values were observed in offshore stations. The average surface D-Fe concentration of estuary stations in spring and autumn was 39.4±26.6 (the standard deviation) nmol/L and 20.5±11.0nmol/L, respectively. The average bottom D-Fe concentrations of estuary stations in spring and autumn were 76.1±58.6nmol/L and 78.5±39.0nmol/L, respectively.Higher surface Lt concentrations were observed during the spring. The average surface Lt concentrations of estuary stations in spring and autumn were 40.0±26.8 and 23.2±7.8nmol/L, respectively. The concentration of D-Fe and Lt in the bottom water was higher than that observed in surface water and exhibited less seasonal variability. Strong ligands were observed in spring estuary surface water. The average surface logK'FeL values in spring and autumn were 12.9±0.8 and 11.7±0.5, respectively. The total ligand concentration in this study was generally in excess of the dissolved iron, particularly in autumn at offshore stations. Higher excess organic ligand concentrations were also generally observed in the upper water column compared to bottom waters in autumn.

Organic Cu-complexation at the shallow marine hydrothermal vent fields off the coast of Milos (Greece), Dominica (Lesser Antilles) and the Bay of Plenty (New Zealand)

Hydrothermal vents at the seafloor release large volumes of metal-rich fluids into the ocean. Some of these metals are biologically essential (such as Fe), while others may be toxic (e.g. Cu). Organisms living at these habitats produce small organic molecules, ligands that are able to form complexes with different metals, to either enhance their bioavailability or to decrease their toxicity. While some deep-sea vents have been studied with respect to metal complexation, comparable studies at shallow marine hydrothermal vent systems are rather rare. In this study, total dissolved Cu concentrations ([Cu]T) and corresponding Cu binding ligand concentrations ([L]Cu) at the shallow water hydrothermal vent fields off the coast of Milos (Greece), Dominica (Lesser Antilles) and the Bay of Plenty (New Zealand) were examined by a voltammetric ligand titration, using competitive ligand equilibration–adsorptive stripping voltammetry (CLE–AdCSV).Milos, Dominica and the Bay of Plenty are three very different environments and our data show that they are relatively depleted in total dissolved Cu, compared to deep-sea hydrothermal vents where [Cu]T and [L]Cu concentrations up to 400nM and 4000nM have been found, respectively. In this data set, [Cu]T rises up to 21.53nM, however, most samples are in the range of 0.5nM–3nM. [L]Cu varies between 2.07nM and 23.9nM, with one exception, reaching 56.8nM. Conditional stability constants (Log K) range between 11.57 and 14.01.Assuming that the three investigated sites are representative for shallow marine hydrothermal vents, the Cu-flux into the ocean due to complexation appears to be relatively low compared to deep-sea systems. Our data indicate a stable complexation of Cu in most cases, which may lower the toxic effect of Cu on these mixed photic–chemosynthetic communities. However one pore water sample from Dominica showed an excess [Cu]T over [L]Cu, which would create a highly toxic environment for most marine organisms. We further show that ligand–Cu ratios and complex stability constants are in a similar range as the ones at deep-sea vents, which may indicate similar complexation parameters and processes at shallow and deep-sea hydrothermal vents.

Miniaturization in voltammetry: Ultratrace element analysis and speciation with twenty-fold sample size reduction

Voltammetric techniques have emerged as powerful methods for the determination and speciation of trace and ultratrace elements without any preconcentration in several research fields. Nevertheless, large sample volumes are typically required (10mL), which strongly limits their application and/or the precision of the results. In this work, we report a 20-fold reduction in sample size for trace and ultratrace elemental determination and speciation by conventional voltammetric instrumentation, introducing the lowest amount of sample (0.5mL) in which ultratrace detection has been performed up to now. This goal was achieved by a careful design of a new sample holder. Reliable, validated results were obtained for the determination of trace/ultratrace elements in rainwater (Cd, Co, Cu, Ni, Pb) and seawater (Cu). Moreover, copper speciation in seawater samples was consistently determined by competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV). The proposed apparatus showed several advantages: (1) 20-fold reduction in sample volume (the sample size is lowered from 120 to 6mL for the CLE–CSV procedure); (2) decrease in analysis time due to the reduction in purging time up to 2.5 fold; (3) 20-fold drop in reagent consumption. Moreover, the analytical performances were not affected: similar detection capabilities, precision and accuracy were obtained. Application to sample of limited availability (e.g. porewaters, snow, rainwater, open ocean water, biological samples) and to the description of high resolution temporal trends may be easily foreseen.

Evaluation of copper speciation in the extract of Eichhornia crassipes using reverse and forward/CLE voltammetric titrations

Reverse and forward titrations coupled with the competitive ligand equilibration (CLE) technique were, for the first time, used to investigate the chemical organic speciation of copper ions in an aqueous extract of Eichhornia crassipes (E.C) plant growing in Egypt. Salicylaldoxime (SA) was used as the competitive ligand. Two samples were collected from huge submerged masses in the Nile water at Sohag and Mansura cities. In this study the concentration of the natural complexing ligands (CLn) and their conditional stability constants with copper (KCuLn′) in the aqueous extract were determined by linear and nonlinear modeling. Reverse titration resulted in two classes of copper complexation (CuL1 and CuL2) in both samples. The strongest class, CuL1, is predominant in Sohag-sample with average logKCuL1′ and CL1 of 15.66 and 435.5nM, respectively, and also dominated in the Mansura-sample with average 16.77 and 162nM, respectively. The second class (CuL2) is minor in both samples. For comparison, forward titration was performed for the Sohag-sample. The results indicated that reverse titration showed a breakthrough ability to detect, for the first time, different types of metal complexation present in equilibrium in the aqueous extracts of biological samples. Moreover, the presence of E.C in Nile water is so useful in transforming free Cu ions to less toxic forms by organic complexation.

An Electroanalytical Approach to the Understanding of Copper Exportation in Glaciated Catchments

AbstractThe use of electroanalytical techniques for the speciation analysis of copper in four glaciated catchments is presented. The method is based on a competitive ligand equilibration scheme coupled with cathodic stripping detection (CLE‐CSV procedure). Issues related to the particular nature of the samples, i.e. their very low ionic strength, and their effects on the speciation procedure are presented and discussed in detail. The presence of strong complexing ligands was evidenced in all of the investigated catchments (30 samples, ligand concentrations 0.2–8.9 nM; K′=1011.51−1013.60). Copper species distribution was for the first time determined in ice samples.

The flux of soluble organic‐iron(III) complexes from sediments represents a source of stable iron(III) to estuarine waters and to the continental shelf

Iron speciation in the Satilla River Estuary was investigated using competitive ligand equilibration–adsorptive cathodic stripping voltammetry (CLE‐ACSV) with the ligand 1‐nitroso‐2‐naphthol (1N2N). The blackwater Satilla River contains high concentrations of dissolved organic matter and iron, suggesting that it could provide a source of dissolved iron to the continental shelf. Total dissolved iron in the water column decreases along the estuary, likely due to flocculation and precipitation processes associated with the increase in salinity. Simultaneously, the percentage of dissolved organic‐Fe(III) complexes measured by CLE‐ACSV in overlying waters of sediment cores increases with salinity. The speciation of dissolved iron in the pore waters indicates that these complexes originate in the underlying sediments. Diffusive fluxes calculated from depth profiles in the sediments indicate that only 8% of the sedimentary flux of Fe(III) is delivered to the continental shelf during low riverine discharge. During normal flow conditions, however, the sediment flux of Fe(III) represents 63% of the total riverine flux of Fe(III). These findings suggest that the flux of dissolved iron to the continental shelf is controlled by the sedimentation of iron in the estuary and the remobilization of soluble organic‐Fe(III) complexes produced either during iron reduction deep in the sediment or oxidation of Fe(II) close to the sediment–water interface. The Satilla River Estuary generates five to eight times higher concentrations of dissolved Fe(III) than the average major world rivers, suggesting that it is, along with other small blackwater rivers, currently underrepresented in world average river flux calculations.

Assessment of accuracy and precision in speciation analysis by competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV) and application to Antarctic samples

The analytical performances of Competitive Ligand Equilibration with Cathodic Stripping Voltammetric detection of the labile fraction (CLE–CSV) were assessed. This speciation method enables the concentration of natural ligand(s) and their conditional stability constants for the complexation of the investigated metal to be determined through thermodynamic considerations. Literature data were discussed and general trends in the precision of the determined parameters identified: ligand concentrations were affected, on average, by a 10% relative percentage standard deviation (RSD%), whereas conditional stability constants showed much lower precision, with an average RSD% of 50%. New experimental data were collected to obtain a complete assessment of accuracy and precision attainable for the determination of strong ligands at the ultra trace level, enabling the whole protocol to be evaluated. Firstly, the side reaction coefficient alpha for the formation of the complex between the added ligand and the investigated metal ( α CuL) was determined. The method was subsequently applied to the analysis of solution containing ligand at trace levels (5–50 nM) with known complexing characteristics. Copper was used as the model metal ion and ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) as the model ligands. Results evidenced that the CLE–CSV protocol is not affected by systematic errors in the determination of both ligand concentration and the conditional stability constants. Good precision is obtained for ligand concentrations, with an average relative standard deviation (RSD%) of 5%; an average RSD% of 23% was calculated for the conditional stability constants. Including the contribution of the uncertainty in the value of α CuL in the evaluation of the uncertainty in the latter parameter increased the RSD% up to 40%. The CLE–CSV protocol was subsequently applied to the detection of strong ligands in water samples collected in Antarctica: precision was shown to be comparable with literature data.

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.

Chemical and physical speciation of mercury in Offatts Bayou: A seasonally anoxic bayou in Galveston Bay

A chemical equilibrium model was used to predict the solution speciation of dissolved mercury (Hg) in the stratified water column of Offatts Bayou, a subestuary in Galveston Bay, Texas, which undergoes seasonal anoxia in bottom waters. Chemical equilibrium modeling was conducted using conditional stability constants and concentrations of Hg‐complexing organic ligands experimentally determined by competitive ligand equilibration methods. Dissolved Hg complexation was dominated by interactions with sulfide and dissolved organic matter (DOM) (HOHgHS0, HOHgHS(DOM), HgSHS−, and HgS22‐ at all depths. Sulfide and glutathione competed for methylmercury (MeHg) complexation in oxic layers; in anoxic waters, sulfide complexation dominated MeHg speciation. The particle‐water distribution coefficient (Kd) of Hg decreased in the anoxic layer of the water column, where the dissolved sulfide concentration increased, providing evidence that sulfide complexation influences the solubility of Hg. The solubility of MeHg was elevated in the anoxic as compared to the oxic layers, and this distributional feature was coincident with a change in the solution speciation of dissolved MeHg from glutathione/sulfide complexation in the oxic layers to a predominantly sulfide complexation in the anoxic layers. Maximum enrichment of Hg, MeHg, and iron (Fe) in suspended particulate matter was observed in the lower layer of the pycnocline, most likely resulting from formation of insoluble Fe oxide, which scavenged dissolved Hg‐sulfide and MeHg‐sulfide species. The concomitant decrease in dissolved inorganic Hg, Fe, and sulfide in the anoxic layers is suggested to result from scavenging of inorganic Hg by FeS, which is in accordance with the Hg speciation model. Overall, Hg cycling in the water column of Offatts Bayou was associated with sulfide and DOM complexation, Fe dissolution/precipitation, water column production of MeHg, and/or efflux of MeHg from anoxic sediment.

Comparative study of organic Cd and Zn complexation in lake waters - seasonality, depth and pH dependence

Environmental context. The bioavailability of trace metals such as zinc and cadmium strongly depends on what chemical form they are in, and not simply on the total metal concentration. Zinc is an essential micronutrient, whereas cadmium is extremely toxic, but when they occur in the same environment there is potential for the two metals to compete for the same biological binding sites. In this study we have studied the trends in Cd and Zn complexation in three alpine lakes in New Zealand. We conclude that, although the total concentration of cadmium is much lower than that of zinc and copper, it bares the highest risk of toxicity for organisms. Abstract. The variation with depth, pH dependence, seasonal variability and selectivity of strong, natural organic cadmium and zinc-binding ligands from three New Zealand alpine lakes (Hayes, Manapouri and Hauroko) have been investigated. Competitive ligand equilibration–cathodic stripping voltammetry (CLE-CSV) with APDC (ammonium pyrrolidine dithiocarbamate) was used to measure the ligand complexation by Zn2+ and anodic stripping voltammetry (ASV) for complexation of Cd2+. In all lakes, the total dissolved cadmium concentration [CdT] averaged 0.040 nM (standard deviation σ = 0.114), while the average concentration of Cd-binding ligands [LCd] was 5.17 ± 1.79 nM (1σ), with conditional stability constants relative to free Cd2+, log K′Cd2+L′, that ranged from 7.92 at depth to 10.58 at the surface. Calculated concentrations of the free aquo ion [Cd2+] averaged 0.0147 ± 0.0616 nM (1σ), and showed a higher percentage of Cd complexed by strong ligands in the summer and in surface waters. The selectivity of cadmium-binding organic ligands was low, and ligand-bound Cd2+ was easily displaced by Zn2+ and Cu2+. Total dissolved zinc concentrations [ZnT] were highly variable, and ranged from 1.04 to 10.94 nM. The corresponding ligand concentrations of strong zinc-binding ligands [LZn] were between 2.14 and 15.52 nM, with conditional stability constants log K′Zn2+L′ as low as 8.78 in deep water collected in summer from Lake Hauroko, up to a maximum of 12.41 at a depth of 5 m in Lake Hayes. The calculated concentrations of the free aquo complex [Zn2+] ranged widely between 0.001 and 1.620 nM.

Effect of UVB Irradiation on Cu2+-Binding Organic Ligands and Cu2+ Speciation in Alpine Lake Waters of New Zealand

Environmental Context. The bioavailability of dissolved metals in natural waters is directly affected by metal-sequestering agents. These agents include soil-derived matter and compounds released by microorganisms, since copper can support or inhibit aquatic microorganisms depending on concentration. During summer the levels of copper increase in surface waters, an effect intuitively attributable to increased ultraviolet light degrading the sequestering agents more effectively, leading to a concurrent release of the metal. This paper shows that the amount of degradation attributable to light is too low to explain the metal release and that a biological influence may instead be responsible. Abstract. The influence of UVB irradiation on the Cu2+ binding by natural organic ligands in six alpine lakes on the South Island, New Zealand, has been investigated using competitive ligand equilibration with salicylaldoxime and detection by cathodic stripping voltammetry (CLE-CSV). During austral summer 2002–2003 the total dissolved Cu ([Cu]T), the concentration of strong Cu2+-binding ligands ([L]T), and their conditional stability constant K´´ were determined in surface samples of all six lakes. All lakes exhibited appreciable concentrations of a strong Cu2+ binding ligand with similar K´´ values and concentrations always exceeding [CuT], thus dominating Cu2+ speciation. Four lakes (Hayes, Manapouri, Wanaka, Te Anau) showed no appreciable trend in [LT] throughout the summer, whereas in Lakes Wakatipu and Hawea [LT] increased steadily throughout this period. Laboratory UVB irradiation of lake water samples using a 400 W mercury lamp with a Pyrex glass filter (λ &gt; 280 nm) showed that Cu2+-binding ligands are destroyed by UVB radiation, causing [L]T to decrease with a rate of –0.588 nmol L–1 h–1 (r2 0.88). From this we calculate that the in situ ligand destruction rate by UVB in summer for surface waters of these lakes is too small to significantly affect [LT], and conclude that variations in ligand concentrations must result from seasonally variable biological factors.

Strong copper complexation in an organic-rich estuary: the importance of allochthonous dissolved organic matter

River input of allochthonous organic matter dominates the strong complexation capacity of dissolved copper in the organic-rich Cape Fear River (CFR) estuary, North Carolina. This slightly stratified estuary is characterized by conservatively mixed dissolved organic carbon (DOC=200–1200 μM C), high river flow, and low biological productivity. Copper speciation data measured using competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV with 8-hydroxyquinoline (8-HQ)) for seven Cape Fear estuarine transects revealed that strong (mean detection window αCu8HQ=105.2) Cu-complexing ligands range in concentration from 7 to >200 nM (at fixed KCuL′=1013.5), are conservatively mixed below the turbidity maximum zone (S∼5), and exist in substantial excess of dissolved Cu levels (3–25 nM). Strong ligand and DOC concentrations exhibited strong linear correlations among transect samples at DOC concentrations <1000 μM C (r2=0.93, p≪0.01) and among all size fractions of ultrafiltered estuarine samples (r2=0.94, p≪0.01). A 300 μM C solution of Cape Fear River humic substances isolated using C18 extraction exhibited a strong Cu ligand concentration of 143 nM (at fixed KCuL′= 1013.5). Comparison with speciation data from estuarine transects indicates C18 isolated humics could account for 100% of the typical strong Cu-complexation capacity in the Cape Fear estuary.

Rare earth element concentrations and speciation in organic-rich blackwaters of the Great Dismal Swamp, Virginia, USA

Concentrations of rare earth elements (REE), major inorganic solutes, and dissolved organic carbon (DOC) were measured in waters from the Great Dismal Swamp in southeastern Virginia, including Lake Drummond (pH=4.3), and its principal surface inflows and outflow. Concentrations of REE are high in the lake (average Nd=8 nmol/kg), and the acidic DOC-rich inflow and out flow waters (i.e., Nd=12.5 nmol/kg in Jericho–Washington Ditch and ∼8 nmol/kg in Feeder Ditch, respectively). The slightly acidic (pH 6–6.4) inflow waters from west of the Suffolk Scarp that comprise the chief source of surface water flow into Lake Drummond exhibit lower REE concentrations (Nd=3.4–7 nmol/kg). Great Dismal Swamp waters are all enriched in the middle REEs compared to the light and heavy REEs (LREE and HREE) when normalized to upper continental crustal (UCC) values. Rare earths are positively correlated to DOC concentrations in Great Dismal Swamp waters and inversely correlated with pH. Speciation calculations using a modified version of the well established Humic Ion-Binding Model V predict that organic matter complexes are the predominant form of dissolved REEs in Great Dismal Swamp waters despite significant competition from aqueous Fe and Al for organic matter binding sites. Adsorptive cathodic stripping voltammetry (ACSV) was employed, using a previously developed competitive ligand equilibration approach that involved adding the o-cresolphthalexon (OCP) ligand, to directly investigate REE complexation with natural organic ligands in Great Dismal Swamp waters. Although preliminary, the results of the ACS voltammetric titrations are consistent with dominance of organic matter complexes of REEs in Great Dismal Swamp waters. The results of the speciation modeling and the voltammetric titrations all point towards control of dissolved REEs in Great Dismal Swamp waters by complexation of these heavy metals with natural organic matter.

Strong copper complexation in an organic-rich estuary: the importance of allochthonous dissolved organic matter

River input of allochthonous organic matter dominates the strong complexation capacity of dissolved copper in the organic-rich Cape Fear River (CFR) estuary, North Carolina. This slightly stratified estuary is characterized by conservatively mixed dissolved organic carbon (DOC=200–1200 μM C), high river flow, and low biological productivity. Copper speciation data measured using competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV with 8-hydroxyquinoline (8-HQ)) for seven Cape Fear estuarine transects revealed that strong (mean detection window α Cu8HQ=10 5.2) Cu-complexing ligands range in concentration from 7 to >200 nM (at fixed K CuL′=10 13.5), are conservatively mixed below the turbidity maximum zone ( S∼5), and exist in substantial excess of dissolved Cu levels (3–25 nM). Strong ligand and DOC concentrations exhibited strong linear correlations among transect samples at DOC concentrations <1000 μM C ( r 2=0.93, p≪0.01) and among all size fractions of ultrafiltered estuarine samples ( r 2=0.94, p≪0.01). A 300 μM C solution of Cape Fear River humic substances isolated using C 18 extraction exhibited a strong Cu ligand concentration of 143 nM (at fixed K CuL′= 10 13.5). Comparison with speciation data from estuarine transects indicates C 18 isolated humics could account for 100% of the typical strong Cu-complexation capacity in the Cape Fear estuary.

Detection of iron(III)-binding ligands originating from marine phytoplankton using cathodic stripping voltammetry.

The sample preparation and analytical methodology are described for detecting biologically produced iron(III)-binding ligands in laboratory cultures of coastal marine phytoplankton. The iron(III)-binding ligands from the culture media were purified by passage through a column packing with a hydrophobic absorbent. The concentrations and stability constants of the ligands were determined by adsorptive cathodic stripping voltammetry with competitive ligand equilibration. The analytical results of the cultivated cultures suggest that eukaryotic phytoplankton would produce iron(III)-binding ligands in analogy with other microorganisms.