Determination of conditional stability constants of organic copper and zinc complexes dissolved in seawater using ligand exchange method with EDTA

To clarify the nature of organic metal complexes dissolved in seawater, a ligand exchange reaction between ligands of natural origin and an aminopolycarboxylic acid (EDTA) was used to determine the conditional stability constants of organic metal complexes. The results indicate that more than two organic molecules complexed with copper and zinc exist in surface seawater. It is found that the conditional stability constants of these naturally-occurring organic metal complexes are 1–3 orders of magnitude higher than those of EDTA-Cu and EDTA-Zn complexes. These estimates of the conditional stability constants for the dominant species of organic copper and zinc complexes are 10^11.8 and 10^9.3, respectively, at pH 8.1. The results indicate that these naturally-occurring organic metal complexes are stable species and not easily dissociated or displaced with others in the marine environment.     

Determination of the complexing capacity and conditional stability constants of complexes of copper(II) with natural organic ligands in seawater by cathodic stripping voltammetry of copper-catechol complex ions

A new method is proposed for the determination of complexing capacities and conditional stability constants for complexes of copper(II) with dissolved organic ligands in seawater. This method is based on ligand competition by the added ligand catechol for free metal ions. The concentration of copper-catechol complex ions is measured with great sensitivity by cathodic stripping voltammetry. The concentration of the free copper ion is calculated from the concentration of copper-catechol complex ions. Ligand concentrations and conditional stability constants are obtained from a titration of the ligands with copper. Two techniques for treatment of the data are compared. A seawater sample, originating from open oceanic conditions, is analysed and two complexing ligands were detected, having concentrations of 1.1 × 10^?8 and 3.3 × 10^?8 M, and conditional stability constants (log K′_CuL) of 12.2 and 10.2, respectively.     

A comparison of iron limitation of phytoplankton in natural oceanic waters and laboratory media conditioned with EDTA

The solubility of iron in oxic waters is so low that iron can be a limiting nutrient for phytoplankton growth in the open ocean. In order to mimic low iron concentrations in algal cultures, Ethylenediaminetetraacetate (EDTA) is commonly used. The presence of EDTA enables culture experiments to be performed at a low free metal concentration, while the total metal concentrations are high. Using EDTA provides for a more reproducible medium. In this study Fe speciation, as defined by EDTA in culture media, is compared with complexation by natural organic complexes in ocean water where Fe is thought to be limited. To grow oceanic species into iron limitation, a concentration of at least 10⁻⁴ M EDTA is necessary. Only then does the calculated [Fe³⁺] concentrations resemble those found in natural sea water, where the speciation is governed by natural dissolved organic ligands at nanomolar concentrations. Moreover, EDTA influences the redox speciation of iron, and thus frustrates research on the preferred source of Fe-uptake, Fe(III) or Fe(II), by algae. Nowadays, one can measure the extent of natural organic complexation in sea water, as well as the dissolved Fe(II) state, and can use ultra clean techniques in order to prevent contamination. Therefore, it is advisable to work with more natural conditions and not use EDTA to create iron limitation. This is especially important when the biological availability of the different chemical fractions of iron are the subject of research. Typically, many oceanic algae in the smallest size classes can still grow at very low ambient Fe and are not easily cultivated into limitation under ambient sea water conditions. However, the important class of large oceanic algae responsible for the major blooms and the large scale cycling of carbon, silicon and other elements, commonly has a high Fe requirement and can be grown into Fe limitation in ambient seawater.     

Determination of copper complexation with natural organic ligands in seawater by equilibration with MnO2 I. Theory

The theory is discussed which describes the distribution of copper ions between a weak ion exchanger, as exemplified by MnO₂, and natural organic complexing material in seawater. Application of this theory and experimental procedures are outlined in part II of this series. It is apparent from the theory that titration with Cu²⁺ of one or more organic complexing ligands can be graphically represented by straight lines; slope and y-axis intercept provide information on the conditional stability constants and the ligand concentrations. Model calculations show that measurement of metal complexation at ligand concentrations higher than normally present in seawater may produce erroneous results because of possible changes in the metal to ligand ratio in the complexes. It is therefore advisable to measure metal complexation in the original, unaltered, water sample.     

Investigations of iron coordination and redox reactions in seawater using Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using ⁵⁹Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10⁶M⁻¹s⁻¹ and 20 M⁻¹s⁻¹, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH)₃^o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater.In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.     

Measurement of manganese, zinc and cadmium complexation in seawater using Chelex ion exchange equilibria

Equilibria between Chelex 100* and manganese, zinc and cadmium ions were used to determine the complexation of these trace metals in 36‰ Gulf Stream seawater at 25°C and pH 8.2. The method utilized radiotracers (⁵⁴Mn, ⁶⁵Zn, and ¹⁰⁹Cd) to quantify trace metal adsorption from trace metal-amended seawater and from seawater containing a series of ethylenediaminetetracetate (EDTA)—metal ion buffers. Results were consistent with Chelex adsorption of both trace metal ions and trace metal—EDTA chelates. Equilibrium models fitted to the data were used to establish conditional stability constants for Chelex adsorption of manganese, zinc and cadmium ions and for adsorption of EDTA-chelates. These models also yielded ratios of free metal ions to total dissolved trace metal concentrations in seawater: 10^−0.1 for manganese, 10^−0.2 for zinc, and 10^−1.5 for cadmium. Independent measurements with a cadmium ion-selective electrode also yielded a free: total cadmium ratio of 10^−1.5.     

The speciation of Fe(II) and Fe(III) in natural waters

The interactions of Fe(II) and Fe(III) with the inorganic anions of natural waters have been examined using the specific interaction and ion pairing models. The specific interaction model as formulated by Pitzer is used to examine the interactions of the major components (Na⁺, Mg²⁺, Ca²⁺, K⁺, Sr²⁺, Cl⁻, SO₄⁻, HCO₃⁻, Br⁻, CO₃²⁻, B(OH)₄⁻, B(OH)₃ and CO₂) of seawater and the ion pairing model is used to account for the strong interaction of Fe(II) and Fe(III) with major and minor ligands (Cl⁻, SO₄²⁻, OH⁻, HCO₃⁻, CO₃²⁻ and HS⁻) in the waters. The model can be used to estimate the activity and speciation of iron in natural waters as a function of composition (major sea salts) and ionic strength (0 to 3 M). The measured stability constants (K_FeX^*) of Fe(II) and Fe(III) have been used to estimate the thermodynamic constants (K_FeX) and the activity coefficient of iron complexes (γ_FeX) with a number of inorganic ligands in NaClO₄ medium at various ionic strengths: In(K_FeX/γ_Feγ_X) = InK_FeX − In(γ_FeX) The activity coefficients for free ions (γ_Fe, γ_x) needed for this extrapolation have been estimated from the Pitzer equations. The activity coefficients of the ion pairs have been used to determine Pitzer parameters (B_FeX, B_FeX⁰, C_FeX^φ) for the iron complexes. These results make it possible to estimate the stability constants for the formation of Fe(II) and Fe(III) complexes over a wide range of ionic strengths and in different media. The model has been used to determine the solubility of Fe(III) in seawater as a function of pH. The results are in good agreement with the measurements of Byrne and Kester and Kuma et al. When the formation of Fe organic complexes is considered, the solubility of Fe(III) in seawater is increased by about 25%.     

Organic and inorganic speciation of copper in the Irish Sea

The metal complexing ability of surface water of the Irish Sea has been measured by the MnO₂ adsorption method. In all samples strong copper-chelating compounds are present at concentrations of 60–150 nM, with conditional stability constants (log values) of 10.0–10.4. The concentrations of Cu, Pb and Cd in the samples are 16–39 nM, 1–7 nM and 0.1–2 nM, respectively; much less than the ligand concentrations. The organic compounds form complexes with 94–98% of dissolved copper, and therefore constitute the major form of copper in surface water of the Irish Sea. Recalculation of speciation of the inorganic fraction of copper in seawater reveals that the major complex ion is that of CuCO₃⁰ (60%), followed by CuOH⁺ (16%) and Cu(OH)₂⁰ (16%). Complexes with borate ions form a small and rather insignificant fraction of 1%.     

Thermodynamic characterization of the partitioning of iron between soluble and colloidal species in the Atlantic Ocean

Recent electrochemical measurements have shown that iron (Fe) speciation in seawater is dominated by complexation with strong organic ligands throughout the water column and have provided important thermodynamic information about these compounds. Independent work has shown that iron exists in both soluble and colloidal fractions in the Atlantic Ocean. Here we have combined these approaches in samples collected from a variety of regimes within the Atlantic Ocean. We measured the partitioning of Fe between soluble (< 0.02 μm) and colloidal (0.02 to 0.4 μm) size classes and characterized the concentrations and conditional stability constants of Fe ligands within these size classes. Results suggest that equilibrium partitioning of Fe between soluble and colloidal ligands is partially responsible for the distribution of Fe between soluble and colloidal size classes. However, a significant fraction of the colloidal Fe was inert to ligand exchange as soluble Fe concentrations were generally lower than values predicted by a simple equilibrium partitioning model.In surface waters, strong ligands with conditional stability constants of 10¹³ relative to total inorganic Fe appeared to dominate speciation in both the soluble and colloidal fractions. In deep waters these ligands were absent, and instead we found ligands with stability constants 12–15 fold smaller that were predominantly in the soluble pool. Nevertheless, significant levels of colloidal Fe were found in these samples, which we inferred must be inert to coordination exchange.     

Chemical speciation of trace metals in seawater: Implication of particulate trace metals

Chemical speciation of particulate metals in seawater was examined theoretically. Mass balance considerations showed that the apparent conditional stability constant, defined for organically binding metals in suspended particles, coincides with the conditional stability constant determined for the corresponding metal-organic complexes dissolved in seawater. This hypothesis suggests that some metals, which are present as organic complexes (e.g. copper), are directly associated with particulate organic matter. Metals, whose free ion is buffered by organic and/or inorganic ligands, may be used as indicators of the presence of particulate organic matter in the marine environment.     

Uptake of Zn and Mn into body tissues and renal concretions by the Southern Quahog, Mercenaria campechiensis (Gmelin): Effects of elevated phosphate and metal concentrations

The seawater chemistry of potentially toxic metals can affect their availability to marine organisms. Investigation of the relationship between metal chemistry and metal bioavailability has progressed slowly due to difficulties in controlling and measuring metal speciation in uptake media. Recent work with strong metal chelators such as NTA and EDTA has allowed a closer examination of how metal chemistry relates to biological accumulation and toxicity.^1–3 However, the presence of a strong chelator at membrane transport sites and the possible alteration of microenvironments by strong chelators could create unnatural uptake behavior. This study presents another method for stabilizing metal chemistry in accumulation experiments. A cation exchange resin was used to study Mn⁵⁴ accumulation by a small bivalve Donax variabilus. The resin proved an effective method for buffering manganese chemistry in seawater and could provide a useful tool to look for subtle effects present in other metal buffered seawater systems.     

Comparative yttrium and rare earth element chemistries in seawater

Comparisons of yttrium and rare earth stability constants for organic ligands indicate that Y(III)-organic complexation behavior most closely resembles, on average, the complexation characteristics of Sm(III). Yttrium organic complexation behavior distinctly differs from the behavior of Ho(III), whose ionic radius is most similar to that of yttrium. However, our stability constant comparisons demonstrate that the most reliable basis for predicting Y(III) organic stability constants are predictions based on the complexation behavior of Ho(III). Linear free energy relationships between Ho(III) and Y(III) exhibit better fits than those obtained for other rare earth elements, and much better fits than those obtained between Sm(III) and Y(III).The comparative abundances and distributions of yttrium and the rare earths in seawater are controlled by competitive equilibria between inorganic solution ligands and organic surface ligands. Due to yttrium solution complexation with hard inorganic ligands resembling that of Ho(III) and, yttrium surface complexation with soft organic ligands resembling that of light rare earth elements, the input normalized abundances of yttrium and the rare earths should be decoupled in seawater. In its seawater scavenging behavior, yttrium should act as a pseudolanthanide even heavier than the heaviest rare earth element, lutetium.     

Calibration of a chalcogenide glass membrane ion-selective electrode for the determination of free Fe in seawater: I. Measurements in UV photooxidised seawater

Calibration of a chalcogenide glass membrane, Fe(III)ISE [Fe_2.5(Ge₂₈Sb₁₂Se₆₀)_97.5], in buffered saline media has been undertaken in order to assess the suitability of this ISE for seawater analyses. The electrode slopes in saline citrate and salicylate buffers were 26.3 and 28.2 mV/decade, respectively, for Fe³⁺ concentrations ranging from 10⁻¹⁰ M to less than 10⁻²⁵ M Fe³⁺. The calibration lines in the citrate and salicylate buffers were essentially collinear with the response in unbuffered chloride-free standards containing >10⁻⁵ M Fe³⁺, demonstrating that the response of the FeISE is unaffected by chloride ions. A mechanism involving a combination of charge transfer and ion-exchange of Fe(III), at the electrode diffusion layer, can be used to explain the ≈30 mV/decade slope of the FeISE. The response of the FeISE in UV photooxidised seawater containing 8 nM total Fe was measured as the pH was changed from 8.27 to 3.51. The slope of the response was 24.2 mV/decade [Fe³⁺] calculated as a function of pH using Fe(III) hydrolysis constants for seawater. Moreover, the response was essentially collinear with that in citrate buffers and in unbuffered solutions containing >10⁻⁵ M Fe³⁺ and the slope for the combined data was 26.2 mV/decade. This study was restricted to organic-free seawater because the certainty in Fe(III)–ligand stability constants is insufficient to warrant the selection of an ideal calibration buffer system, and there is evidence that powerful chelating ligands (e.g., EDTA along with humic and fulvic acids) may alter the response of the Fe(III)ISE. The Fe dissolution rate of the FeISE in UV photooxidised seawater was found to be 1.6×10⁻² nmol Fe/min, as measured by cathodic stripping voltammetry (CSV). This would contaminate a 100-ml sample by 0.8–1.6 nM Fe over a typical measurement period of 5–10 min obtained using a stability criterion of 0.5 mV/min. Various methods are proposed for reducing the level of contamination in open ocean samples that contain sub-nanomolar concentrations of iron. The FeISE has the potential to detect free Fe³⁺ at concentrations typically found in natural seawater.     

Evaluation of the chemical forms of plutonium in seawater

Thermodynamic considerations based on existing data from various laboratory studies of plutonium species in aqueous solution are used to predict the speciation of this radioactive pollutant in seawater. Oxidation-reduction data for plutonium suggest that Pu(VI) should be very dominant in seawater solution compared to Pu(IV), and that Pu(III) and Pu(V) should be absent. The disproportionation reactions and the alpha reduction mechanism are probably of no consequence to the oxidation state in seawater. However, the irreversible hydrolysis of Pu⁴⁺ and the associated formation of polymeric Pu(OH)₄ colloids are important mechanisms of speciation control and plutonium removal to sediments, by adsorption onto suspended matter. Stability constants for plutonium complexation with inorganic ligands in seawater suggest that Pu(VI) dissolved in seawater will be dominantly PuO₂CO₃OH⁻.The theoretical predictions of plutonium speciation and behaviour in seawater are compared to the only available data on plutonium speciation in seawater (Nelson and Lovett, 1978). Good agreement between the predictions and field observations was obtained, within the limitations imposed by the scarcity of data on this subject.     

Determination of cadmium(II), copper(II), manganese(II) and nickel(II) species in Antarctic seawater with complexing resins

The strong species of cadmium(II), copper(II), manganese(II) and nickel(II) in an Antarctic seawater sample are investigated by a method based on the sorption of metal ions on complexing resins. The resins compete with the ligands present in the sample to combine with the metal ions. Two resins with different adsorbing strengths were used. Very stable metal complexes were investigated with the strong sorbent Chelex 100 and weaker species with the less strong resin, Amberlite CG-50. Strong species were detected for three of the considered metal ions, but not for Mn(II). Cu(II) is completely linked to species with a side reaction coefficient as high as log α_M(I) = 11.6 at pH = 7.3. The ligand concentration was found to be similar to that of the metal ion, and the conditional stability constant was around 10²⁰ M^− 1. In the considered sample, only a fraction of the metal ions Cd(II) and Ni(II) is bound to the strong ligands, with side reaction coefficients equal to log α_M(I) = 5.5 and 6.5 at pH = 7.3 for Cd(II) and Ni(II), respectively. These findings were confirmed by the test with the weaker sorbent Amberlite CG-50. It can be calculated from the sorption equilibria that neither Mn(II) nor Ni(II) is adsorbed on Amberlite CG-50 under the considered conditions and, in fact, only a negligible fraction of Mn(II) and Ni(II) was adsorbed. A noticeable fraction of Cd(II) was adsorbed on Amberlite CG-50, meaning that cadmium(II) is partially linked to weak ligands, possibly chloride, while no copper(II) was adsorbed on this resin, confirming that copper(II) is only combined in strong species. These results are similar, but not identical, to those obtained for other seawater samples examined in previous investigations.     

Cadmium speciation and transport in the blood of the bivalve Mytilus edulis.

Cadmium (Cd) speciation in the blood plasma of Mytilus edulis was investigated using the metal speciation model MINTEQA2. In the presence of inorganic ions alone, Cd-chloro complexes dominated the speciation (97% of total Cd), with 3% as Cd2+. Inclusion of a novel Cd-binding histidine-rich glycoprotein (HRG) purified from mussel blood plasma decreased the contribution of chloro-complexes to 11.9%, with 86.8% of the Cd bound to the HRG and 1.3% present as Cd2+. Cd transfer from the blood plasma to the kidneys in vivo was studied by injecting 109Cd (both with and without additional chelation) into mussels. Oxine and EDTA complexed a significant amount of blood-borne Cd (23.7% Cd Oxine; 57.1% CdEDTA). In the presence of each chelator, plasma retained significantly more Cd, although there was no significant difference in Cd uptake by tissues (kidney, gill-mantle, and remaining viscera).     

An investigation of the suitability of amberlite XAD-1 resin for studying trace metal speciation in seawater

Amberlite XAD-1 resin was examined to test its suitability for extracting organic complexes of copper, zinc and iron from seawater. At low flow rates and at loading capacities far below theoretical values, the adsorption of these metals is not reproducible and the results are reminiscent of the behaviour observed when the adsorption capacity is being exceeded or flow rates are too high. It is suggested that the resin also adsorbs small but significant amounts of inorganic ions from seawater and that this effect makes the resin unsuitable for quantitative measurements of trace metal speciation.     

Interaction of metal complexes with coulombic ion-pairs in aqueous media of high salinity

The competition for anions between the cations of the alkali and alkaline earth metals (to form ion pairs) and the cations of heavy metals (to form complexes) is investigated. The interaction is shown to affect the stability constants of the heavy metal complexes and the nature of the ionic species present in aqueous media of high salinity. The theory is discussed with special reference to NaCl-NaClO₄ solutions, seawater and the labile complexes of lead and cadmium.     

Production and speciation of hydrogen sulfide in surface waters of the high latitude North Atlantic Ocean

During the August 1993 Intergovernmental Oceanographic Commission's Contaminant Baseline Survey cruise to the high latitude North Atlantic, determinations of total dissolved sulfide (TDS=free sulfide, H₂S_(g)+HS⁻+S²⁻, plus dissolved metal–sulfide complexes), free sulfide, and carbonyl sulfide (OCS) were made along a horizontal transect and at six vertical profile stations. Unlike data from lower latitudes, the distributions of OCS and TDS were remarkably uniform, with surface water OCS averaging 108 pmol/l and TDS averaging 58 pmol/l; free sulfide was below the detection limits of 5 pmol/l at all stations. The vertical profiles of both OCS and TDS show surface maxima and rapid decreases into the major thermocline. For OCS this is indicative of production via photolysis of dissolved organic sulfur compounds, while TDS may be produced from the hydrolysis of OCS. The concentrations of OCS are similar to those found in coastal waters, and suggests that these sub-polar regions may be large OCS sources to the troposphere during summer. However, it is unclear whether higher concentrations of OCS precursors, a long photo period during summer, or slow rates of removal by hydrolysis due to low temperatures are responsible for the elevated OCS levels. TDS concentrations are primarily controlled by the rate of OCS hydrolysis, production by phytoplankton, and oxidative loss by oxygen and iodate. Both of the losses are affected by trace metal complexation, and to examine this, freshly collected seawater was amended by hydrogen sulfide gas and trace metal additions, and the concentration of free sulfide monitored as a function of metal concentration. This allowed the determinations of conditional stability constants for metal sulfides, with the log K_cond of Cd(HS)⁺ being 8.0±0.5, 7.0±0.6 for Ni(HS)⁺, and 7.4±0.7 for Zn(HS)⁺; attempts at measuring the K_cond of Cu(HS)⁺ were thwarted by the apparent reduction of Cu(II) to Cu(I) by sulfide. Using these constants in an equilibrium speciation model indicates that on average about 75% of the measured TDS was free, with the remaining fraction complexed with Ni, Cd, and Zn (in order of decreasing percentages). While closer to the field observations than would be found with stability constants reported by other workers, these values are still at variance with the actual speciation (i.e., <30% free). This suggests that the stability constants for Cd, Ni, and Zn are somewhat higher than found, thus reducing the concentration of free sulfide. Nevertheless, these speciation data are important for balancing the TDS budget since the loss by iodate oxidation of free sulfide exceeds all production estimates.     

Determination of copper complexation with natural organic ligands in seawater by equilibration with MnO2 II. Experimental procedures and application to surface seawater

The MnO₂ adsorption method combined with voltammetry is proposed for the direct determination of metal complexation in seawater of various salinities as a more satisfactory alternative to direct voltammetric measurements and bioassay methods. A small quantity of MnO₂ is equilibrated with copper ions in filtered seawater. Natural organic ligands in the seawater compete for copper with the MnO₂. Total dissolved copper is measured by differential pulse anodic stripping voltammetry after filtration and acidification of the sample. Preconcentration of natural water samples is unnecessary and measurement is performed at the natural equilibrium pH of the aerated sample. The analytical limit of detection of the method depends on contamination from the filtration step, and for copper complexation a ligand concentration of 5 × 10^?8 M was obtained. The sensitivity can be increased by use of radioisotopes as tracers. The method is very versatile in that complexation of various metals may be determined by any analytical method that measures total dissolved metal concentrations. Neither organic ligands nor their complexes with copper adsorb on the MnO₂ at pH8, but at pH 1.8 MnO₂ is an efficient scavenger for electroactive organic material.Samples of surface water from the Irish Sea and the Atlantic Ocean were found to contain ligand concentrations of 1.7 × 10^?7 and 1.1 × 10^?7 M, with conditional stability constants (log values) of 9.84 ± 0.13 and 9.86 ± 0.23, respectively, at pH 8.0.