Exchange-adsorptive Cathodic Stripping Voltammetry Research Articles

Recommendations for best practice for iron speciation by competitive ligand exchange adsorptive cathodic stripping voltammetry with salicylaldoxime

The method of competitive ligand exchange followed by adsorptive cathodic stripping voltammetry (CLE-AdCSV) allows for the determination of dissolved iron (DFe) organic speciation parameters, i.e., ligand concentration (LFe) and conditional stability constant (log KFe′Lcond). Investigation of DFe organic speciation by CLE-AdCSV has been conducted in a wide range of marine systems, but aspects of its application pose challenges that have yet to be explicitly addressed. Here, we present a set of observations and recommendations to work toward establishing best practice for DFe organic speciation measurements using the added ligand salicylaldoxime (SA). We detail conditioning procedures to ensure a stable AdCSV signal and discuss the processes at play during conditioning. We also present step-by-step guidelines to simplify CLE-AdCSV data treatment and interpretation using the softwares ECDSoft and ProMCC and a custom spreadsheet. We validate our application and interpretation methodology with the model siderophore deferoxamine B (DFO-B) in a natural seawater sample. The reproducibility of our application and interpretation methodology was evaluated by running duplicate titrations on 19 samples, many of which had been refrozen prior to the duplicate analysis. Nevertheless, 50% of the duplicate analyses agreed within 10% of their relative standard deviation (RSD), and up to 80% within 25% RSD, for both LFe and log KFe′Lcond. Finally, we compared the sequential addition and equilibration of DFe and SA with overnight equilibration after simultaneous addition of DFe and SA on 24 samples. We found a rather good agreement between both procedures, with 60% of samples within 25% RSD for LFe (and 43% of samples for log KFe′Lcond), and it was not possible to predict differences in LFe or log KFe′Lcond based on the method applied, suggesting specific association/dissociation kinetics for different ligand assemblages. Further investigation of the equilibration kinetics against SA may be helpful as a potential way to distinguish natural ligand assemblages.

Identifying potential sources of iron-binding ligands in coastal Antarctic environments and the wider Southern Ocean

The availability of iron (Fe) to marine microbial communities is enhanced through complexation by ligands. In Fe limited environments, measuring the distribution and identifying the likely sources of ligands is therefore central to understanding the drivers of marine productivity. Antarctic coastal marine environments support highly productive ecosystems and are influenced by numerous sources of ligands, the magnitude of which varies both spatially and seasonally. Using competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) with 2-(2-thiazolylazo)-p-cresol (TAC) as a competing artificial ligand, this study investigates Fe-binding ligands (FeL) across the continental shelf break in the Mertz Glacier Region, East Antarctica (64 - 67°S; 138 - 154°E) during austral summer of 2019. The average FeL concentration was 0.86 ± 0.5 nM Eq Fe, with strong conditional stability constants (Log KFeL) averaging 23.1 ± 1.0. The strongest binding ligands were observed in modified circumpolar deep water (CDW), thought to be linked to bacterial Fe remineralisation and potential siderophore release. High proportions of excess unbound ligands (L’) were observed in surface waters, as a result of phytoplankton Fe uptake in the mixed layer and euphotic zone. However, FeL and L’ concentrations were greater at depth, suggesting ligands were supplied with dissolved Fe from upwelled CDW and particle remineralisation in benthic nepheloid layers over the shelf. Recent sea-ice melt appeared to support bacterial production in areas where Fe and ligands were exhausted. This study is included within our newly compiled Southern Ocean Ligand (SOLt) Collection, a database of publicly available Fe-binding ligand surveys performed south of 50°S. A review of the SOLt Collection brings attention to the paucity of ligand data collected along the East Antarctic coast and the difficulties in pinpointing sources of Fe and ligands in coastal environments. Elucidating poorly understood ligand sources is essential to predicting future Fe availability for microbial populations under rapid environmental change.

Spectrophotometric Determination of Fe(II) and Fe(III) Using 5-(4-Ethylphenyl azo ) 2-Hydroxybenzaldehyde Oxime Reagent

Spectrophotometric Determination of Fe(II) and Fe(III) Using 5-(4-Ethylphenyl azo ) 2-Hydroxybenzaldehyde Oxime Reagent

Organic complexation of iron by strong ligands and siderophores in the eastern tropical North Pacific oxygen deficient zone

Continental margins are an important external source of dissolved iron to the marine environment. However, the mechanisms responsible for the offshore transport of dissolved iron is impacted by the resulting iron speciation. We characterized the iron speciation in the Eastern Tropical North Pacific (ETNP) oxygen deficient zone (ODZ), including dissolved iron, organic iron-binding ligands, and reduced iron. Organic iron-binding ligands were measured using both competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) and liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS) in order to explore the impact of organic ligands on dissolved iron (dFeT) and iron(II) biogeochemistry in the region. Organic ligands were present in high concentrations (1.06–5.30 nmol L−1) and exceeded dissolved iron concentrations (0.36–4.52 nmol L−1) at all locations. Iron-binding strengths (logKFeL,Fe′cond) ranged 11.22 to 12.75 and were elevated in the ODZ layer relative to the oxygenated water column. LC-ESI-MS revealed the presence of siderophores, or bacterially-produced organic ligands with high Fe-affinity, in all samples analyzed, suggesting these compounds may be produced by microbes in the ODZ despite high ambient dFeT concentrations. This study is the first to characterize siderophores in an ODZ environment to date, and the three siderophores found (amphibactin B, synechobactin c9, synechobactin c10) could contribute to the observed elevated logKFeL,Fe′cond of ligands in the ODZ. Comparative analysis of organic ligand logKFeL,Fe′cond values in other low oxygen environments suggests that strong ligands, including siderophores, could be present in other low oxygen regions. In a simple model of the shelf-to-offshore iron transport mechanism, strong organic iron-binding ligands had a large impact on the longevity and transport of iron in the ODZ. These results suggest that organic ligand composition can have an impact on iron distributions in the ETNP ODZ and regulate the offshore transport of iron to the open ocean.

Wide-range detection of Cu-binding organic ligands in seawater using reverse titration

The toxicity and bioavailability of Cu in seawater were estimated using its chemical speciation. In this study, the concentrations and conditional stability constants (K'CuL) of Cu complexes with Cu-binding organic ligands (L) in seawater were determined using the reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry method. The concentrations of strong and weak ligands (L1 and L2, respectively) were determined using reverse titration via the addition of Cu to seawater samples to achieve ligand saturation. Our results revealed that the reverse titration method can be successfully used for the detection of high concentrations of L2. Using the reverse titration method, we detected L2 concentrations that exceeded 80 and 120 nM in seawater samples from the subtropical western North Pacific Ocean and Otsuchi Bay Japan, respectively. The K′CuL1 and K′CuL2 values obtained using the reverse titration method were comparable with those obtained using the forward titration method. Therefore, the reverse titration method can be used to determine the chemical speciation of Cu in the ocean and coastal regions as supplement to the more conventional forward titration method.

Predicting Copper Speciation in Estuarine Waters-Is Dissolved Organic Carbon a Good Proxy for the Presence of Organic Ligands?

A new generation of speciation-based aquatic environmental quality standards (EQS) for metals have been developed using models to predict the free metal ion concentration, the most ecologically relevant form, to set site-specific values. Some countries such as the U.K. have moved toward this approach by setting a new estuarine and marine water EQS for copper, based on an empirical relationship between copper toxicity to mussels (Mytilus sp.) and ambient dissolved organic carbon (DOC) concentrations. This assumes an inverse relationship between DOC and free copper ion concentration owing to complexation by predominantly organic ligands. At low DOC concentrations, the new EQS is more stringent, but above 162 μM DOC it is higher than the previous value. However, the relationship between DOC and copper speciation is poorly defined in estuarine waters. This research discusses the influence of DOC from different sources on copper speciation in estuaries and concludes that DOC is not necessarily an accurate predictor of copper speciation. Nevertheless, the determination of ligand strength and concentrations by Competitive Ligand Exchange Adsorptive Cathodic Stripping Voltammetry enabled the prediction of the free metal ion concentration within an order of magnitude for estuarine waters by using a readily available metal speciation model (Visual MINTEQ).

Fe-Binding Dissolved Organic Ligands in the Oxic and Suboxic Waters of the Black Sea

In the oxygen-rich layer of the Black Sea, above the permanent halocline, the Fe and nitrate concentrations are low where fluorescence is relatively high , indicating uptake by phytoplankton. In this study we used ligand exchange adsorptive cathodic stripping voltammetry (CLE-aCSV), using 2-(2-Thiazolylazo)-p-cresol (TAC) as measuring ligand, to investigate the role of Fe-binding dissolved organic ligands in keeping Fe in the dissolved phase and potentially biologically available. The conditional stability constant, logK´, was between 21 and 22 in most samples, which is on average lower than in ocean water. The Fe-binding dissolved organic ligand concentrations varied between 0.35 and 4.81 nEq of M Fe, which was higher than the dissolved concentration of Fe (DFe) as found in most samples. At two stations ligands were saturated in the surface. At one station ligands were saturated near the oxycline, where ligand concentrations seemed to increase, indicating that they play a role in keeping Fe in the dissolved phase across the redox gradient. At the fluorescence maximum (between 40 and 50 m), the dissolved organic ligand binding capacity (alphaFeL=K´*[L´]) of Fe was at its highest while the concentration DFe was at its lowest. Here, we find a statistically significant, positive relationship between fluorescence and the logarithm of alphaFeL, along with fluorescence and the ratio of the total ligand concentration over DFe. These relationships are best explained by phytoplankton utilizing Fe from Fe-binding organic ligands, resulting in an increase in free Fe-binding ligands.

Iron-binding ligands and humic substances in the San Francisco Bay estuary and estuarine-influenced shelf regions of coastal California

Dissolved iron (dFe) and organic dFe-binding ligands were determined in San Francisco Bay, California by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) along a salinity gradient from the freshwater endmember of the Sacramento River (salinity <2) to the mouth of the estuary (salinity >26). A range of dFe-binding ligand classes was simultaneously determined using multiple analytical window analysis, involving titrations with multiple concentrations of the added ligand, salicylaldoxime. The highest dFe and ligand concentrations were determined in the low salinity end of the estuary, with dFe equal to 131.5nmolL−1 and strong ligand (log KFeL,Fe′cond≥12.0) concentrations equal to 139.5nmolL−1. The weakest ligands (log KFeL,Fe′cond<10.0) were always in excess of dFe in low salinity waters, but were rapidly flocculated within the estuary and were not detected at salinities greater than 7. The strongest ligands (log KFeL,Fe′cond>11.0) were tightly coupled to dFe throughout the estuary, with average excess ligand concentrations ([L]–[dFe]) equal to 0.5nmolL−1. Humic-like substances analyzed via both CLE-ACSV and proton nuclear magnetic resonance in several samples were found to be a significant portion of the dFe-binding ligand pool in San Francisco Bay, with concentrations ranging from 559.5μgL−1 to 67.5μgL−1 in the lowest and highest salinity samples, respectively. DFe-binding ligands and humic-like substances were also found in benthic boundary layer samples taken from the shelf near the mouths of San Francisco Bay and Eel River, suggesting estuaries are an important source of dFe-binding ligands to California coastal shelf waters.

ProMCC: An all-in-one tool for trace metal complexation studies

A new windows-based, user friendly program (ProMCC) for (i) the determination of metal complexation parameters (ligand concentration (L) and conditional stability constants (K′)) and for (ii) theoretical simulation of metal complexometric titration, assuming discrete ligand model, is developed. Although primarily intended for treatment of experimental data obtained either by anodic stripping voltammetry (ASV) or competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV), it could manage titration-type data of other techniques (e.g. ISE, sorption isotherm). Currently, the program is capable to process (fit and/or simulate) titration data up to three discrete ligand classes. Procedure for adjustment of “true” analytical sensitivity incorporated in ProMCC was found to provide reasonably good estimates of sensitivity either for one-ligand or two-ligand system. The particular feature of ProMCC is that it incorporates two complementary fitting methodologies: (1) a non-linear fitting of conventional linearized transformations (e.g. Ružić/Van Den Berg, Langmuir/Gerringa) and (2) a “complete complexation model” — a matrix based optimization of mass balance equations. Comparison test of different non-linear fitting modes and titration types revealed that a slight underestimation of ligand concentration and overestimation of conditional stability constant may occur if titration is performed in logarithmic mode, mainly due to unfavorable noise distribution. An advantage of implemented “complete complexation model” fitting mode is that it allows simultaneous analysis of titrations obtained at multiple detection windows as unified dataset (multi-detection window approach), providing complexation parameters for up to three ligand classes. A new alternative “RAL-approach” in analyzing complexometric titrations obtained at multiple detection windows for copper–salicylaldoxime (Cu–SA) system is suggested. It assumes that the analytical sensitivity is changing along the titration curve respecting the true speciation of Cu–SA in sample. An adapted empirical equation for calculation of relative intensity (RAL) is proposed. Flexibility in adjusting parameters, immediate graphical feedback and visualizations make ProMCC handy for treatment of large set of experimental data, and a tool for research in refinement of the methods of metal complexing capacity determination which is continuously improving.

The relevance of ligand exchange kinetics in the measurement of iron speciation by CLE–AdCSV in seawater

CLE–AdCSV (competitive ligand exchange–adsorptive cathodic stripping voltammetry) has proved to be a decisive technique in recognising the key role that organic matter plays in modulating the biogeochemical cycle of iron in aquatic ecosystems. As the method requires equilibrium among all the constituents of the solution before measurements are taken, accurate knowledge of the kinetics of the undergoing ligand exchange is essential. The kinetics of the process has been most frequently described in terms of a dissociative (or Eigen–Wilkins) mechanism, with some authors even claiming that, under such approach, iron complexes would dissociate during CLE so slowly that the method would be invalidated. The complexity of the processes—largely exceeding the simple mechanistic description of the Eigen–Wilkins mechanism—is discussed on the basis of a broad overview of possible reaction routes. Evidence of the role of matrix components of natural waters is presented and the impossibility of exporting kinetic constants to systems characterised by different molecular mechanics, mostly ignored in previous studies, is highlighted. Experimental evidence obtained from various methodological approaches allows inferring the range of CLE equilibration times to be applied in CLE–AdCSV measurements and further proves that the Eigen–Wilkins mechanism is just one of the many reaction routes applicable in natural waters. Studying iron CLE mechanisms for proposed competing ligands before designing any experiment using CLE–AdCSV is recommended.

Dissolved copper speciation in the Tasman Sea, SW Pacific Ocean

The speciation of copper (Cu) in seawater is dominated by organic complexation but the sources, distribution and nature of these natural organic ligands remain unclear. The spatial distribution and character of organic copper binding ligands at two sites in the Tasman Sea were investigated using competitive ligand exchange adsorptive cathodic stripping voltammetry, utilising salicylaldoxime as the competing ligand. The first station was located in the oligotrophic north Tasman Sea and was contrasted with a second, mesotrophic, station in the south of the region where sampling coincided with a coccolithophore bloom. The northern P1 site (0–3000m) was characterised by detection of a single, strong, class of ligand with a uniformly distributed conditional stability constant (log K′CuL=12.97±0.26). At the two shallowest depths of the southern P3 site (13 and 28m), two ligands were detected. A single, stronger, ligand was detected at all other depths for this site. Below 120m, conditional stability constants at the southern P3 site (0–3500m) were uniformly distributed (log K′CuL=13.32±0.21). At both stations, ligand concentrations usually exceeded the total dissolved Cu concentration and mirrored the bio-intermediate Cu profile at depth. Dissolved Cu was more than 99% complexed by organic ligands at all depths except one, when dissolved Cu concentration exceeds the dissolve ligand concentration. The variability in ligand concentrations was greatest in the top 0–50m at the northern P1 site and 0–80m at the southern site. Based on a combination of phototrophic cell counts and biomarker pigment concentrations, the source of organic Cu-binding ligands in the north Tasman Sea is thought to be from cyanobacterial production, namely Synechococcus spp which dominates in the region. The detection of two organic Cu binding ligand classes at the southern P3 station indicates a more complex Cu-binding ligand pool in the surface waters. At this site, the ligands are likely sourced from cyanobacteria and eukaryotes, namely coccolithophores. At the northern P1 station only, a relationship between the proportion of unbound ligand available and the dissolved oxygen concentration was obtained. Given the simpler ligand pool dynamics of the northern station relative to the south, this relationship may reveal that complexation of organic Cu-binding ligands is important to prevent or slow ligand degradation in the deep sea.

Distinct pools of dissolved iron‐binding ligands in the surface and benthic boundary layer of the California Current

Organic dissolved iron (dFe)—binding ligands were measured by competitive ligand exchange—adsorptive cathodic stripping voltammetry (CLE‐ACSV) at multiple analytical windows (side reaction coefficient of salicylaldoxime, αFe(SA)2 = 30, 60 and 100) in surface and benthic boundary layer (BBL) samples along the central California coast during spring and summer. The weakest ligands were detected in the BBL at the lowest analytical window with average log KcondFeL,Fe' = 10.2 ± 0.4 in the summer and 10.8 ± 0.2 in the spring. Between 3% and 18% of the dFe complexation in the BBL was accounted for by HS, which were measured separately in samples by ACSV and may indicate a source of dFe‐binding ligands from San Francisco Bay. The strongest ligands were found in nearshore spring surface waters at the highest analytical window with average log KcondFeL,Fe' = 11.9 ± 0.3, and the concentrations of these ligands declined rapidly offshore. The ligand pools in the surface and BBL waters were distinct from each other based on principal components analysis, with variances in the BBL ligand pool explained by sample location, and variance in surface waters explained by water mass. The use of multiple analytical window analysis elucidated several distinct iron‐binding ligand pools, each with unique distributions in the central California Current system.

New approach to analysis of voltammetric ligand titration data improves understanding of metal speciation in natural waters

In this study, previously published multiple analytical window datasets of competitive ligand exchange—adsorptive cathodic stripping voltammetry titrations for dissolved copper with salicylaldoxime as the competing ligand were reanalyzed. A new numerical method was applied, the Sander‐Wells (S‐W) method, which is able to simultaneously analyze multiple analytical window titrations as a unified dataset and calculate parameters for one to three ligand classes in the samples. The two datasets used were from separate sampling events at Dumbarton Bridge, San Francisco Bay. The aim of this study was to see if discrete ligand classes can be resolved from these data, and how these compare with results obtained by applying commonly used manual calculations of individual analytical window titrations separately: Scatchard and van den Berg/Ružic linearizations, and the Gerringa non‐linear regression. The united multiple analytical window dataset could only be analyzed simultaneously by the S‐W method, and two or three discrete ligand classes were resolved for each sample. Applying the four different data analysis methods independently to each of the five analytical windows resulted in solutions spanning a continuum of conditional stability constants and ligand concentrations. It was shown that the sensitivity S, as well as data structure and iterative optimization, are the major reasons for different results. This study demonstrates the utility of simultaneous analysis of multiple analytical window titration curves to better determine the distribution of trace metal species in aqueous systems and the advantages of adopting such a ‘unified data set’ approach over fitting individual titration curves.

The stabilisation and transportation of dissolved iron from high temperature hydrothermal vent systems

Iron (Fe) binding phases in two hydrothermal plumes in the Southern Ocean were studied using a novel voltammetric technique. This approach, reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry, showed that on average 30±21% of dissolved Fe in the hydrothermal plumes was stabilised by chemically labile binding to ligands. The conditional stability constant (logK′FeL) of the observed complexes was 20.61±0.54 (mean±1 SD) for the two vent sites, intermediate between previous measurements of deep ocean ligands (21.4–23; Kondo et al., 2012) and dissolved weak estuarine ligands (<20; Gerringa et al., 2007).Our results indicate that approximately 7.5% of all hydrothermal Fe was stabilised by complexation with ligands. Furthermore, 47±26% of the dissolved Fe in the plume existed in the colloidal size range (0.02–0.2µm). Our data suggests that a portion (∼7.5%) of hydrothermal Fe is sufficiently stabilised in the dissolved size fraction (<0.2µm) to make an important impact on deep ocean Fe distributions. Lateral deep ocean currents transport this hydrothermal Fe as lenses of enhanced Fe concentrations away from mid ocean ridge spreading centres and back arc basins.

The speciation of copper across active gradients in nitrogen‐cycle processes in the eastern tropical South Pacific

Copper (Cu) complexation and distribution were characterized using competitive ligand exchange adsorptive cathodic stripping voltammetry and isotope dilution inductively coupled plasma mass spectrometry along two transects (20°S and 10°S) in the eastern tropical South Pacific. In the southern and westernmost stations, Cu showed upper water column depletion to values as low as ∼ 0.26 nmol L−1, the lowest concentrations ever reported. However, Cu levels were much higher within the secondary nitrite maxima of the oxygen minimum zone (OMZ) in the northern (10°S) transect. The enrichment of Cu in the reducing conditions of the OMZ has not been reported before and probably reflects remineralization and offshore transport from the shelf. Free [Cu2+] was typically low throughout the water column, ranging from about 3.15 × 10−15 mol L−1 to 1.34 × 10−13 mol L−1, and depth profiles exhibited similar features to those for dissolved Cu, though they showed more variability near the surface. Offshore and beyond the influence of the OMZ, the lowest dissolved and free [Cu2+] was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM—iron limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation—have high Cu requirements. The low concentrations of free Cu2+ measured here could impose significant constraints on the rates of these processes.

Measurement of iron chemical speciation in seawater at 4 °C: The use of competitive ligand exchange–adsorptive cathodic stripping voltammetry

Iron is mostly bound to poorly characterised organic ligands; thus, organic ligands are paramount in defining Fe biogeochemical cycling and its control on oceanic primary productivity. Since 1994, Fe chemical speciation has been determined by Competitive Ligand Exchange–Adsorptive Cathodic Stripping Voltammetry (CLE–AdCSV) at room temperature. However, chemical speciation is strongly dependent on temperature and some organic ligands can be temperature sensitive. Here, we compare the use of the CLE–AdCSV at room temperature and at 4°C—a temperature closer to that found in the Southern Ocean, one of the largest iron-limited regions. For both temperatures, similar detection limits and total Fe concentrations were found. However, at 4°C the analytical detection window (αFe(TAC)2) was shifted by 1.4-fold towards the detection of weaker ligands, resulting in up to 2-fold lower ligand concentrations as well as a 2- to 5-fold and 10- to70-fold lower conditional stability constants with inorganic Fe (Fe′) and Fe(III), respectively. As a result, the Fe′ concentration at 4°C was 2-fold greater, resulting in direct implication for Fe bioavailability. Results show that difference in Fe chemical speciation at 4°C was not solely explained by temperature effect on thermodynamics with the exchange ligands or the diffusion of the electroactive complex towards the Hg drop. Lowering analytical window during analysis at room temperature is proposed as a first estimate of temperature effect on iron chemical speciation.

Characterisation of iron binding ligands in seawater by reverse titration

Here we demonstrate the use of reverse titration - competitive ligand exchange–adsorptive cathodic stripping voltammetry (RT-CLE–ACSV) for the analysis of iron (Fe) binding ligands in seawater. In contrast to the forward titration, which examines excess ligands in solution, RT-CLE–ACSV examines the existing Fe-ligand complexes by increasing the concentration of added (electroactive) ligand (1-nitroso-2-naphthol) and analysis of the proportion of Fe bound to the added ligand. The data manipulation allows the accurate characterisation of ligands at equal or lower concentrations than Fe in seawater, and disregards electrochemically inert dissolved Fe such as some colloidal phases. The method is thus superior to the forward titration in environments with high Fe and low ligand concentrations or high concentrations of inert Fe.We validated the technique using the siderophore ligand ferrioxamine B, and observed a stability constant K′Fe3+FoB of 0.74–4.37×1021mol−1, in agreement with previous results. We also successfully analysed samples from coastal waters and a deep ocean hydrothermal plume. Samples from these environments could not be analysed with confidence using the forward titration, highlighting the effectiveness of the RT-CLE–ACSV technique in waters with high concentrations of inert Fe.

New insights on the role of organic speciation in the biogeochemical cycle of dissolved cobalt in the southeastern Atlantic and the Southern Ocean

Abstract. The organic speciation of dissolved cobalt (DCo) was investigated in the subtropical region of the southeastern Atlantic, and in the Southern Ocean in the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre, between 34°25´ S and 57°33´ S along the Greenwich Meridian during the austral summer of 2008. The organic speciation of dissolved cobalt was determined by competing ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using nioxime as a competing ligand. The concentrations of the organic ligands (L) ranged between 26 and 73 pM, and the conditional stability constants (log K'CoL) of the organic complexes of Co between 17.9 and 20.1. Most dissolved cobalt was organically complexed in the water-column (60 to &gt;99.9%). There were clear vertical and meridional patterns in the distribution of L and the organic speciation of DCo along the section. These patterns suggest a biological source of the organic ligands in the surface waters of the subtropical domain and northern subantarctic region, potentially driven by the cyanobacteria, and a removal of the organic Co by direct or indirect biological uptake. The highest L:DCo ratio (5.81 ± 1.07 pM pM−1) observed in these surface waters reflected the combined effects of ligand production and DCo consumption. As a result of these combined effects, the calculated concentrations of inorganic Co ([Co']) were very low in the subtropical and subantarctic surface waters, generally between 10−19 and 10−17 M. In intermediate and deep waters, the South African margins can be a source of organic ligands, as it was suggested to be for DCo (Bown et al., 2011), although a significant portion of DCo (up to 15%) can be stabilized and transported as inorganic species in those DCo-enriched water-masses. Contrastingly, the distribution of L does not suggest an intense biological production of L around the Antarctic Polar Front where a diatom bloom had recently occurred. Here [Co'] can be several orders of magnitude higher than those reported in the subtropical domain, suggesting that cobalt limitation was unlikely in the ACC domain. The almost invariant L:DCo ratio of ~1 recorded in these surface waters also reflected the conservative behaviours of both L and DCo. In deeper waters higher ligand concentrations were observed in waters previously identified as DCo sources (Bown et al., 2011). At those depths the eastward increase of DCo from the Drake Passage to the Greenwich Meridian could be associated with a large scale transport and remineralisation of DCo as organic complexes; here, the fraction stabilized as inorganic Co was also significant (up to 25%) in the low oxygenated Upper Circumpolar Deep Waters. Organic speciation may thus be a central factor in the biogeochemical cycle of DCo in those areas, playing a major role in the bioavailability and the geochemistry of Co.

Trace-Metal Biogeochemistry in the Mediterranean Thau Lagoon, a Shellfish Farming Area

The present study contributes to the knowledge of the biogeochemistry of Pb, Cd, Cu, and Ni in the Mediterranean Thau Lagoon, southern France, which is an important shellfish farming system. The concentrations of the metals were determined in sediment cores and the overlying waters using inductively coupled plasma mass spectrometry. Particular attention was given to the determination of dissolved Cu species because of their dual role as essential nutrient and toxicant to planktonic organisms. Dissolved Cu speciation was determined using the diffusive gradient in thin-film technique (DGT) and competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-ACSV). Our data indicated a significant historical contamination of the sediments, which commenced in the second half of the 19th century, with trace metal inputs persisting until the end of the 20th century. In recent years a decrease in metal contamination has become apparent. The maxima observed for Pb, Cd, and Cu profiles probably indicate the occurence of anoxia crises. A strong complexation of the dissolved Cu species was observed in the waters of the Thau Lagoon, which reduced the bioavailability of Cu. The dissolved Cu2 concentrations were probably too low to cause direct toxic effects on shellfish, but the highest concentration (5.29 pM) observed in this study can potentially influence phytoplankton communities. A comparison between the Cu speciation data indicates that up to 50% of the complexed Cu determined using CLE-ACSV was DGT labile.

Kinetically inert Cu in coastal waters.

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