Ligands In Seawater Research Articles

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.

The importance of kinetics and redox in the biogeochemical cycling of iron in the surface ocean.

It is now well established that Iron (Fe) is a limiting element in many regions of the open ocean. Our current understanding of the key processes which control iron distribution in the open ocean have been largely based on thermodynamic measurements performed under the assumption of equilibrium conditions. Using this equilibrium approach, researchers have been able to detect and quantify organic complexing ligands in seawater and examine their role in increasing the overall solubility of iron. Our current knowledge about iron bioavailability to phytoplankton and bacteria is also based heavily on carefully controlled laboratory studies where it is assumed the chemical species are in equilibrium in line with the free ion association model and/or its successor the biotic ligand model. Similarly most field work on iron biogeochemistry generally consists of a single profile which is in essence a “snap-shot” in time of the system under investigation. However it is well known that the surface ocean is an extremely dynamic environment and it is unlikely if thermodynamic equilibrium between all the iron species present is ever truly achieved. In sunlit waters this is mostly due to the daily passage of the sun across the sky leading to photoredox processes which alter Fe speciation by cycling between redox states and between inorganic and organic species. Episodic deposition events, dry and wet, are also important perturbations to iron cycling as they bring in new iron to the system and alter the equilibrium between iron species and phases. Here we utilize new field data collected in the open ocean on the complexation kinetics of iron in the surface ocean to identify the important role of weak iron binding ligands (i.e., those that cannot maintain iron in solution indefinitely at seawater pH: αFeL < αFe′) in allowing transient increases in iron solubility in response to iron deposition events. Experiments with the thermal source SOTS-1 also indicate the short term impact of this species on iron solubility also with relevance to the euphotic zone. This data highlights the roles of kinetics, redox, and weaker iron binding ligands in the biogeochemical cycling of iron in the ocean.

Loose ligands and available iron in the ocean

The ocean covers ≈70% of the Earth's surface and plays a major role in regulating global climate and sequestering carbon dioxide from the atmosphere. Diverse populations of phytoplankton, namely photosynthetic microalgae and cyanobacteria, abound in sunlit surface waters of the ocean and account for half of global primary production. Phytoplankton require a variety of essential nutrients, including iron, to fix carbon dioxide and fuel ocean food webs, but the oxidized form of iron, Fe(III), that prevails in the ocean is only sparingly soluble in oxygenated seawater. The solubility of iron is enhanced through chelation with organic ligands, and nearly all of the dissolved iron in seawater is bound to natural ligands (1). The molecular identity and reactivity of seawater ligands has largely eluded discovery by the iron workers, leaving a gaping hole in our understanding of the availability of iron to phytoplankton. This has ramifications for the global C cycle, because phytoplankton productivity is limited by iron availability in vast regions of the surface ocean that have an ample supply of other critical nutrients, such as N and P (2). In PNAS, Hassler et al. (3) provide experimental evidence indicating saccharide-enhanced iron utilization by phytoplankton cultures and natural planktonic communities in surface seawater. Apparently, saccharides chelate iron in a bioavailable form, an observation that is likely to transform current thinking about iron cycling in the ocean. Saccharides (also known as carbohydrates) are abundant, bioreactive components of dissolved organic matter (DOM) that are produced by phytoplankton in the surface ocean (4, 5). It seems that saccharides play a critical role in sustaining ocean productivity through a positive feedback that links the C and Fe cycles (Fig. 1A).

Experimental study and modelling of inorganic Cd2+ speciation in natural waters

Environmental contextCadmium(II) is considered as one of the most dangerous pollutants in natural waters. We performed a complete study on the interactions of cadmium(II) with inorganic components of natural waters, by using new experiments and some literature data. The body of results can be considered an improvement in modelling the inorganic speciation of cadmium(II) in natural waters. AbstractAn analysis of literature data together with new potentiometric and voltammetric studies on the interaction between Cd2+ and the common environmental inorganic ligands, such as OH–, Cl–, F–, CO32– and SO42–, was reported at t = 25°C. New formation constant values for CdCl+, CdCl20, CdCl(OH)0, CdSO40, CdCO30 and CdF+ species at different ionic strengths (0.1 ≤ I ≤ 1 mol L–1) were determined. The cumulative seawater ligand capacity was evaluated by using a model in which synthetic seawater is expressed as a single salt BA, where the major cations (Na+, K+, Ca2+ and Mg2+) are represented by a single cation Bz+ and the anions (Cl– and SO42–) by a single anion Az– (|z| = 1.117). The formation of CdA0.883+ and CdA(OH)0.117– species was proposed and formation constants are reported at different salinities (5 ≤ S ≤ 45). The ionic strength dependence of the stability constants was studied by means of Pitzer and SIT (Specific ion Interaction Theory) equations.

Influence of sediment acidification on the bioaccumulation of metals in Ruditapes philippinarum

The influence of pH (range 6.5-8.5) on the uptake of Zn, Cd, Pb, Cu, Ni, Cr, Hg, and As by juveniles of the clam Ruditapes philippinarum was examined in order to understand whether variation in sediment pH has significant repercussions on metal bioaccumulation. Clams were exposed to sediments collected in three locations in the Gulf of Cadiz (Huelva, Guadalquivir and Bay of Cadiz) and to contaminated particles derived from an accidental mining spill in Spain. With a notable exception of metal Cd, the concentration of metals within clams significantly increased (p < 0.1) when sediment pH was lowered by one or two units. Moreover, the magnitude of this effect was dependent on the type of sediment contamination. Lower pH increases metal solubility and reduces or invert the metal sorption of metals to sediments. Increases in free metal ions in water favors metal uptake by clams, hence pH is an important factor controlling the mobility of these metals within sediments and their subsequent bioaccumulation within biota. Although sediment-water exchange of Cd can increase with acidification, this excess may be counterbalanced by the presence of ligands in seawater preventing the uptake by organism. Besides chlorines, Cd has also an affinity with carbonates and other ligands present in sea water. These Cd-carbonate complexes may reduce the bioavailable to organisms. These results highlight the potential implications of sediment acidification, either due to the storage excess of organic matter or to the forced capture of CO(2), on the increasing metal availability to benthic organisms. This kind of studies should be increased to address the influence of acidification in the behavior, bioavailability, toxicity, and risk assessment of contaminants associated with sediments either above sub-seabed geological formations in marine environments or in high enriched by organic matter in estuarine areas. Recently, the capture of CO(2) in marine environments has been approved and started; it is necessary to address the potential impacts associated with leakages or other events occurring during the procedure of injection and storage of CO2.

Chemistry of Marine Ligands and Siderophores

Marine microorganisms are presented with unique challenges to obtain essential metal ions required to survive and thrive in the ocean. The production of organic ligands to complex transition metal ions is one strategy to both facilitate uptake of specific metals, such as iron, and to mitigate the potential toxic effects of other metal ions, such as copper. A number of important trace metal ions are complexed by organic ligands in seawater, including iron, cobalt, nickel, copper, zinc, and cadmium, thus defining the speciation of these metal ions in the ocean. In the case of iron, siderophores have been identified and structurally characterized. Siderophores are low molecular weight iron-binding ligands produced by marine bacteria. Although progress has been made toward the identity of in situ iron-binding ligands, few compounds have been identified that coordinate the other trace metals. Deciphering the chemical structures and production stimuli of naturally produced organic ligands and the organisms they come from is fundamental to understanding metal speciation and bioavailability. The current evidence for marine ligands, with an emphasis on siderophores, and discussion of the importance and implications of metal-binding ligands in controlling metal speciation and cycling within the world's oceans are presented.

Response of the Iron Chalcogenide Glass Membrane Ion‐Selective Electrode in a Seawater Ligand Mimetic Calibration Buffer

AbstractAs a continuation of recent mechanistic studies into the influence of seawater ligands on the surface chemistry of the iron chalcogenide glass membrane ion‐selective electrode (ISE), the present study has investigated the response of the iron(III) ISE in a seawater ligand mimetic system to examine its suitability as a calibration medium for the electroanalysis of raw or natural seawater. Significantly, dip method calibrations of the ISE in a mixture of salicylate, ethylene diamine tetraacetic acid (EDTA), ethylene diamine and minor amounts of dissolved iron(III) and copper(II) yielded the expected Nernstian response of 30 mV/decade according to the known ion‐exchange/electron transfer response mechanism of this ISE. Furthermore, ideal Nernstian response of the electrode is also obtained in a continuous flow analysis (CFA) mode, noting that this provides scope for using a hydrodynamic flow regime to minimize the electrode release of iron and the concomitant detection limit of the ISE. Ultimately, repetitive CFA analyses of free iron(III) in raw or natural seawater yielded a free iron(III) level commensurate with the expected inorganic and organic speciation of iron(III) in seawater.

Photochemistry of Organic Iron(III) Complexing Ligands in Oceanic Systems

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (>99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photo-chemically produced radical species, such as superoxide (O2-). The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimental/observational studies of the natural seawater ligand pool.

Chemical Speciation of Trace Metals in Seawater: a Review

The recent development of the chemical speciation of trace metals in seawater is described. The speciation studies reveal that metal ion complexation is one of the most important processes in seawater; especially, most bioactive trace metals, such as Fe(III) and Cu, exist as complexes with ligands in dissolved organic matter. The organic ligands in seawater are characterized with metal ions selected by the HSAB concept. A strong organic ligand, which originates from marine microorganisms, is classified as a hard base including carboxylates. The free organic ligand concentrations in seawater are buffered by complexation with excess amounts of Ca and Mg in seawater. The chemical equilibrium model suggested that the concentrations of bioactive free metal ions are at an optimal level to activities of marine microorganisms. For chemical speciation, it is important to have a better understanding of the ecological roles of trace metals in seawater.

In situ electrochemical impedance spectroscopy/synchrotron radiation grazing incidence X-ray diffraction—A powerful new technique for the characterization of electrochemical surfaces and interfaces

A marriage of electrochemical impedance spectroscopy (EIS) and in situ synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) has provided a powerful new technique for the elucidation of the mechanistic chemistry of electrochemical systems. In this study, EIS/SR-GIXRD has been used to investigate the influence of metal ion buffer calibration ligands, along with natural organic ligands in seawater, on the behaviour of the iron chalcogenide glass ion-selective electrode (ISE). The SR-GIXRD data demonstrated that citrate – a previously reported poor iron calibration ligand for the analysis of seawater – induced an instantaneous and total dissolution of crystalline GeSe and Sb 2Se 3 in the modified surface layer (MSL) of the ISE, while natural organic ligands in seawater and a mixture of ligands in a mimetic seawater ligand system protected the MSL's crystalline inclusions of GeSe and Sb 2Se 3 from oxidative attack. Expectedly, the EIS data showed that citrate induced a loss in the medium frequency time constant for the MSL of the ISE, while seawater's natural organic ligands and the mimetic ligand system preserved the medium frequency EIS response characteristics of the ISE's MSL. The new EIS/SR-GIXRD technique has provided insights into the suitability of iron calibration ligands for the analysis of iron in seawater.

Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: Influence of pH and ionic strength

The sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide at low ionic strength (0.01 M ≤ I ≤ 0.09 M) was investigated over a wide range of pH (3.9 ≤ pH ≤ 7.1). YREE distribution coefficients, defined as i K Fe = [MS i] T / (M T[Fe 3+] S), where [MS i] T is the concentration of YREE sorbed by the precipitate, M T is the total YREE concentration in solution, and [Fe 3+] S is the concentration of precipitated iron, are weakly dependent on ionic strength but strongly dependent on pH. For each YREE, the pH dependence of log i K Fe is highly linear over the investigated pH range. The slopes of log i K Fe versus pH regressions range between 1.43 ± 0.04 for La and 1.55 ± 0.03 for Lu. Distribution coefficients are well described by an equation of the form i K Fe = ( S β 1[H +] − 1 + S β 2[H +] − 2 ) / ( S K 1[H +] + 1), where S β n are stability constants for YREE sorption by surface hydroxyl groups and S K 1 is a ferric hydroxide surface protonation constant. Best-fit estimates of S β n for each YREE were obtained with log S K 1 = 4.76. Distribution coefficient predictions, using this two-site surface complexation model, accurately describe the log i K Fe patterns obtained in the present study, as well as distribution coefficient patterns obtained in previous studies at near-neutral pH. Modeled log i K Fe results were used to predict YREE sorption patterns appropriate to the open ocean by accounting for YREE solution complexation with the major inorganic YREE ligands in seawater. The predicted log i K Fe′ pattern for seawater, while distinctly different from log i K Fe observations in synthetic solutions at low ionic strength, is in good agreement with results for natural seawater obtained by others.

Photochemistry of Organic Iron(III) Complexing Ligands in Oceanic Systems

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (>99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photochemically produced radical species, such as superoxide (O2-). The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimentallobservational studies of the natural seawater ligand pool.

Domoic acid: The synergy of iron, copper, and the toxicity of diatoms

Diatom blooms generated by the alleviation of iron limitation in high nitrate‐low chlorophyll (HNLC) regions of the oceans often are composed of pennate diatoms of the genus Pseudo-nitzschia, many of which periodically produce the potent neurotoxin domoic acid. We show that toxigenic diatoms have an inducible high-affinity iron uptake capability that enables them to grow efficiently on iron complexed by strong organic ligands in seawater. This low-iron adaptive strategy requires copper and domoic acid, a copper chelator whose production increases sharply when both iron and copper are limiting. Addition of either domoic acid or copper to seawater improves the growth of Pseudo-nitzschia spp. on strongly complexed iron during deck incubation experiments with natural phytoplankton. Our findings indicate that domoic acid is a functional component of the unusual high-affinity iron acquisition system of these organisms. This system may help explain why Pseudo-nitzschia spp. are persistent seed populations in oceanic HNLC regions, as well as in some neritic regions. Our findings also indicate that in the absence of an adequate copper supply, iron-limited natural populations of Pseudo-nitzschia will become increasingly toxic.

Phytoplankton community structure changes following simulated upwelled iron inputs in the Peru upwelling region

The effects of iron on phytoplankton community structure in 'High Nutrient Low Chlorophyll' regions of the ocean have been examined using both shipboard batch cultures (growouts) and open ocean mesoscale fertilization experiments. The addition of iron in these areas frequently results in a shift from communities dominated by small non-siliceous species towards ones dominated by larger diatoms. We used a new shipboard continuous culture experimental design in iron-limited Peru upwelling waters to examine shifts in phytoplankton structure and their bio- geochemical consequences following simulated upwelled iron inputs. By allowing the added iron to pre-equilibrate with natural seawater ligands, we were able to supply iron in realistic chemical spe- cies at rates and concentrations similar to those found in upwelled waters off Peru. The community shifted strongly from cyanobacteria towards diatoms, and the extent of this shift was proportional to the increase in iron supply. Eukaryotic nanophytoplankton were the first to respond to the iron addi- tion, followed by a community dominated by small pennate diatoms by Day 5. These community changes led to increased biogenic silica:particulate organic nitrogen (BSi:PON) and biogenic silica:particulate organic carbon (BSi:POC) production ratios, driven mainly by increases in diatom numbers with increasing iron. Our experiment demonstrated both similarities to and differences with parallel growout experiments and previous mesoscale fertilization experiments, and suggest that the shipboard continuous culture method can be applied to questions that cannot be easily addressed by either of these previous iron addition techniques.

Photochemical degradation of organic iron complexing ligands in seawater

It has been shown by a number of investigators that greater than 99% of the Fe in the open ocean is complexed with organic ligands having conditional stability constants on the order of log K = 10–12. Some of these studies have shown evidence for the presence of 2 distinct ligand classes. It is hypothesized that the strong ligand class is produced by plankton while the weaker ligand is a result of decomposition of the strong ligand. Photochemical degradation in the surface ocean (upper 1–5 m) is one possible mechanism for this decomposition. Data from 2 cruises to the Gulf of Mexico demonstrate the importance of this photochemical reaction. The photodegradation of ligands appears to occur in a ligand that is less than 1 kDa in size and is ubiquitous throughout the Gulf of Mexico. This reaction may be responsible for controlling surface water Fe concentrations throughout the ocean and will help to further refine biogeochemical models of primary production.

Bioavailability of iron to Trichodesmium colonies in the western subtropical Atlantic Ocean

Trichodesmium provides new nitrogen (N) to marine surface waters via N2 fixation, a process that requires a substantial amount of iron (Fe). Organic ligands in seawater that bind Fe could either increase or reduce the bioavailability of Fe. Electrochemical techniques indicate that these naturally occurring ligands have Fe‐binding constants similar to those of siderophores and porphyrins, suggesting that these chelators play an important role in determining the bioavailability of Fe to cyanobacteria. We conducted Fe uptake experiments using model ligands labeled with 55Fe to compare the bioavailability of inorganic Fe(III), porphyrin‐bound Fe(III), and siderophorebound Fe(III) to field‐collected Trichodesmium colonies. Inorganic Fe(III) and siderophore‐bound Fe(III) were more bioavailable to Trichodesmium colonies than was porphyrin‐bound Fe(III). Furthermore, the bioavailability of the siderophore‐bound Fe(III) can be characterized by the functional groups of the siderophore. The dihydroxamate siderophore and an uncharacterized ligand from a cultured Synechococcus sp. increased the bioavailability of Fe compared to the trihydroxamate siderophores. Except for experiments with desferrioxamine B, dark incubations resulted in lower Fe uptake rates for all treatments, relative to parallel lighted incubations. This suggests that light enhances the photochemical dissociation of most of the ligand complexes or that light energy is required for the active transport of Fe complexed to the model ligands. The Fe uptake rate of Trichodesmium colonies also differed slightly on the basis of colony morphology, with higher uptake rates with “puffs” than “tufts.” These experiments show that Trichodesmium colonies are capable of discriminating between Fe bound to different organic complexes.

Characterization of copper-complexing ligands in seawater using immobilized copper(II)-ion affinity chromatography and electrospray ionization mass spectrometry

Electrochemical studies show that copper and other bioactive trace metals are associated almost exclusively with dissolved organic ligands in surface seawater. Some of these ligands may be released by photosynthetic marine organisms to control metal uptake. However, new analytical approaches are needed to determine the chemical composition and ecological significance of these compounds. We have combined immobilized metal-ion affinity chromatography (IMAC) with electrospray ionization mass spectrometry (ESI-MS) to selectively extract and characterize copper-complexing ligands in seawater. Samples taken from British Columbia coastal waters were filtered through mixed cellulose membranes (0.45 μm) using nitrogen overpressure. Compounds with an affinity for copper were then extracted using Cu(II)-IMAC and eluted in acidified seawater. The eluant was monitored using UV absorption (255 nm), which showed that IMAC extracts from river and surface waters contained the highest concentrations of UV-absorbing organic matter. Fractions were collected and analyzed by flow-injection (FI) negative ion ESI-MS using a custom made XAD-16 column, which gave 82% recovery and a detection limit of 1.6 pmol for salicylic acid in acidified seawater. Low-molecular-weight compounds containing nitrogen were detected in the IMAC extracts of highest UV absorbance. These compounds followed an elution profile similar to the corresponding UV chromatogram. Measured masses of 265 and 259 Da are consistent with peptides containing, respectively, one thiol and two primary amino groups, characteristics associated with copper-complexing ligands detected in marine algal cultures and surface seawater using other methods. However, the absence of positive ions under experimental conditions suggests that the extracted compounds are predominantly acidic, and may therefore be hydroxamic acids or other nitrogen-containing compounds. The use of alternative resins and FI solvents to improve recovery, increase ionization efficiency, and permit structural (MS/MS) analysis of compounds extracted by IMAC is now being investigated.

Determination of trace metal complexes by natural organic and inorganic ligands in coastal seawater.

It is well-documented that organic compounds form strong complexes with most metals in aquatic systems, and that seawater is a complex medium which contains a large variety of organic and inorganic ligands, including colloidal matter. We suggest that most trace metals are complexed in seawater and that some inorganic metals complexes are either labile or not stable. In contrast, metal-organic complexes are often stable and need various and specific treatments to be dissociated. In this paper we try to illustrate a good tendency of some trace metals to be complexed by organic ligands in seawater. A solid-phase extraction method was applied using a C18 column as a resin that is able to separate metals complexed by neutral organic ligands, and the chelamine resin to separate metal species that are present as labile inorganic complexes. The determination of total dissolved metal concentrations was achieved by formatting a metal-8-hydroxyquinoline complex, followed by adsorption on C18 columns and ICP-AES analysis.

A multi-technique surface study of the mercury(II) chalcogenide ion-selective electrode in saline media.

X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), rotating disc electrode-electrochemical impedance spectroscopy (RDE-EIS) and synchrotron radiation-grazing incidence X-ray diffraction (SR-GIXRD) have been used to study the response mechanism of the mercury(II) chalcogenide ion-selective electrode (ISE) in saline media. XPS and SIMS have shown that the chalcogenide surface is poisoned by silver chloride, or a mixture of silver halides, on continuous exposure to synthetic and real seawater. Significantly, the in-situ SR-GIXRD study demonstrated that electrode fouling in synthetic seawater is linked to the formation of poorly crystalline or amorphous silver chloride, and that the low level of free mercury(II) in a calibration buffer (i.e., 10(-14) M) is able to undergo metathesis with silver(II) sulfide in the membrane generating mercury(II) sulfide. Significantly, the results of this detailed surface study have shown that silver chloride fouling of the electrode is ameliorated in real seawater comprising natural organic ligands, and this has been attributed to the peptization of silver chloride by the surfactant-like nature of seawater ligands at pH 8. RDE-EIS aging studies have revealed that the chalcogenide membrane experiences a sluggish charge transfer reaction in seawater, and contrary to a previous report for a static electrode, the seawater matrix does not passivate the RDE. The results of this XPS, SIMS, RDE-EIS and SR-GIXRD study have elucidated the response mechanism of the mercury(II) ISE in saline media.

Determination of organic complexation of cobalt in seawater by cathodic stripping voltammetry

A method was developed to determine the extent of cobalt complexation with organic ligands in seawater, and applied to samples from the North Eastern Atlantic Ocean. The cobalt speciation was determined using catalytic cathodic stripping voltammetry with ligand competition against the adsorptive ligand (nioxime). Optimised conditions include a nioxime concentration of 200 nM, 0.07 M ammonia/ammonium chloride pH buffer (pH 9.1), and 0.5 M nitrite. The stability of the mixed complex of cobalt, nioxime and ammonia was calibrated against EDTA. The cobalt speciation in the open-ocean surface waters was found to be dominated by complexation to natural organic ligands with conditional stability constants (log KCoL′) ranging between 15.6 and 16.1 and with ligand concentrations between 22 and 60 pM. The cobalt concentrations varied between 25 pM in the open-ocean waters and 103 pM in the English Channel, and were less than the ligand concentrations in many of the surface oceanic waters. The ligand concentration at depths between 30 and 115 m (average 22 pM) was lower than that of cobalt, whereas at other depths (shallower as well as deeper), the ligand concentrations were between 26 and 31 pM, sometimes greater than the cobalt concentrations. The cobalt and ligand concentrations in these waters are finely balanced, strongly binding all cobalt when the ligands are in excess. Free Co2+ concentrations calculated for the open-ocean surface waters are extremely low (<5 fM) which could cause cobalt limitation to certain phytoplankton such as coccolithophores and cyanobacteria if the organic complexes are unavailable. If this organic complexation of cobalt occurs also in other regions characterised by low cobalt concentrations, iron-limited waters in high-nitrate low-chlorophyll regions may be cobalt-limited too.