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.     

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.     

Determination of the zinc complexing capacity in seawater by cathodic stripping voltammetry of zinc—APDC complex ions

A novel technique to determine complexing capacities for zinc is presented. The free zinc concentration is determined by cathodic stripping voltammetry preceded by adsorptive collection of complexes of zinc with ammonium pyrrolidine dithiocarbamate (APDC). The reduction peak of zinc is depressed as a result of ligand competition by natural organic material in the sample. Sufficient time is allowed to reach equilibrium between this material and added APDC, and equilibrium is maintained during the measurement. Both electrochemically reversible and irreversible complexes can therefore be investigated. Values for K_ZnAPDC^′ are calibrated against NTA and EDTA in seawater of several salinities; log K_ZnAPDC^′ was found to be 4.40 at 36‰, 4.36 at 24‰, 4.43 at 12‰, and 4.87 at 2.3‰. The ligand concentration and conditional stability constant, K_ZnL^′, for complexing ligands in a sample from the Irish Sea were determined in the presence of 4 × 10^?5 M APDC and with added zinc concentrations between 5 × 10^?9 and 3 × 10^?7 M. The data best fitted a complexation model containing two ligands with concentrations of 2.6 and 6.2 and 10^?8 M, and with values for log K_ZnL^′ of 8.4 and 7.5, respectively. These results are comparable to those obtained with other equilibrium techniques, but the values of the constants are greater than those from ASV measurements.     

Complexation of copper and nickel in the dissolved phase of marine sediment slurries

Speciation of copper and nickel in the water phase of incubated marine slurries under aerobic conditions was performed with MnO₂ and Sep—Pak C₁₈ cartridges. Changes in time during the incubations of concentrations of dissolved organic carbon (DOC), dissolved copper and nickel and inorganic nitrogen were followed. The influence of organic complexation on the dissolved concentrations of copper and nickel was investigated as well as competition between copper and nickel for dissolved organic ligands.Two pools of dissolved organic ligands could be distinguished. With the MnO₂ method a relatively strong ligand group was determined that was subjected to degradation. The conditional stability constant for copper with the relatively strong ligand was 10^11.1. The conditional stability constant for the relatively strong nickel ligand was difficult to determine due to saturation of the ligand sites; it was found to be around 10¹⁰. However, it could not be ascertained whether nickel was reversibly com-plexed with the organic ligands.With Sep—Pak a relatively weak Hgand group was detected that was probably more resistant to degradation. The conditional stability constant of the weaker ligand could not be estimated, an approximation revealed that it was weaker than the ligand group determined with the MnO₂ method. For copper the difference between binding strength of the ligand groups was at least 100, for nickel the difference was less.Competition between copper and nickel for the ligands could not be detected. Only during the first day of the experiment, when the system was not in equilibrium was competition suspected. However, the replacement of nickel by copper from the ligand sites was not straightforward and could not be accounted for by our model.The concentration of total dissolved copper during the first week of the experiment was found to be controlled on the one hand by release from the sediment of copper already associated with dissolved organic matter (DOM) and on the other hand by concentration of the strongest ligand. The calculated free copper concentration increased from 10⁻¹² to 10⁻⁹mol l⁻¹ due to the oxidation of the strongest ligand. After saturation of the strongest ligand the relatively weak ligand controlled the free copper concentration. A continuing release of copper from the sediment by degradation of particulate organic matter (POM) will not increase the free copper concentration until the ligand sites of the weaker ligands get saturated.The total dissolved nickel concentration seemed only to be determined by the sum of the concentrations of the organic ligands. A degradation of ligands resulted in a decrease of the total dissolved nickel concentration. The calculated free nickel concentration did not change with time.     

A catalytic oxidation method for the determination of total nitrogen dissolved in seawater

A detailed examination of a high-temperature catalytic oxidation method for liquid samples in the analysis of total nitrogen dissolved in seawater is reported. The nitrogenous compounds in liquid samples are oxidized on a platinum catalyzer at 680°C under oxygen atmosphere and the generated NO₂ is absorbed into a chromogenic reagent, followed by a spectrophotometric determination. The results of this method are much higher than those of wet oxidation methods. Molecular size dependency of the results clearly indicates that the above discrepancy is caused by the low oxidation capacity of the wet oxidation method against high-polymer organic matter dissolved in seawater. The results revealed that the concentration of total nitrogen in seawater is nearly constant from surface to bottom, ranging from 30 to 40 μM 1^?1, which organic nitrogen concentration higher in the surface layer, and a rapid decrease with depth. An examination of molecular size distribution indicates that the concentration of high-polymer organic nitrogen decreases rapidly from surface to deeper layers, with molecular sizes ranging from 5 × 10³ to 3 × 10⁴. Because of the well-defined principle of the oxidation process, its reliability, ease of sample handling and of analytical procedure on board or in the land laboratory, the present method is much more suitable for the marine analytical chemistry of total and organic nitrogen than the other previous methods.     

Evidence for organic complexation of iron in seawater

Iron occurs at very low concentrations in seawater of oceanic origin and its low abundance is thought to limit primary production in offshore waters (Martin and Fitzwater, 1988). A new electrochemical method, cathodic stripping voltammetry (CSV), is used here to determine the speciation of iron in seawater originating from the Western Mediterranean taking advantage of ligand competition of an added electroactive ligand with the natural organic complexing matter to evaluate whether iron is organically complexed. The measurements indicate that iron occurs 99% (or 99.9% depending on which value is selected for α_Fe′) complexed by organic complexing ligands throughout the water column of the Western Mediterranean and by analogy probably also in other oceanic waters. The composition of the organic complexing ligands is as yet unknown, but the data indicate a major source from microorganisms (bacteria or phytoplankton) in and immediately below the fluorescence maximum in the upper water column. The organic complexes are apparently reversible releasing iron when the competing ligand is added and binding more iron when its concentration is increased. The organic complexing ligands occur at concentrations well above those of iron ensuring full complexation of this biologically essential element, and buffer the free iron concentration at a very low level against fluctuations as a result of removal by primary producers or inputs from atmospheric sources. The new data indicate that a re-evaluation of the concept of the bioavailable fraction of iron is required.     

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.     

Dissociation of hydrogen sulphide in seawater and comparison of pH scales

The protonization constant of HS^? (K₁₂) has been determined potentiometrically (glass electrode) at atmospheric pressure in synthetic seawater in the salinity range 2.5–40‰ at 5 and 25°C and in NaCl solutions in the formal ionic strength of 0.1–0.8 M at 5 and 25°C. The difference between synthetic seawater and an NaCl solution with the same formal ionic strength can be explained in terms of the complexation of H⁺ by sulphate in seawater. These results can be used to compare the pH scales suggested by Hansson (1973c) and Bates (1975). Furthermore, comparison between the present values of K₁₂ and those of Goldhaber and Kaplan (1975) makes it possible to compare the conventional pH scale with Hansson's titration pH scale. The conditional protonization constant of HS^? in seawater of different salinities can be used to modify the Gran plots (Hansson and Jagner, 1973) for alkalinity measurements in anoxic seawater. Ion-pair formation between HS^? and Mg²⁺ or Ca²⁺ seems to be very weak.     

An experimental study on the speciation of dissolved zinc,cadmium, lead and copper in river rhine and north sea water,by differential pulsed anodic stripping voltammetry

Dissolved electroactive concentrations of zinc, cadmium, lead and copper in river Rhine and North Sea samples have been measured at natural and lower pH values by differential pulsed anodic stripping voltammetry using a Kemula-type hanging mercury drop electrode. Average concentrations detected in North Sea samples at salinities $\text{? 32‰ S}$ and their range are (in μgl^?1): 3.9 (2.0–7.5) for zinc, 0.23 (0.13–0.31) for cadmium, 0.3 (0.1–0.6) for lead and 0.3 (0.25–0.60) for copper (pH 8.1). The APDC-MIBK extracting/concentrating method, followed by AAS measurement applied to the same samples, resulted in 3.9 (2.0–7.5) for zinc, 0.11 (0.01–0.27) for cadmium, 0.5 (0.2–0.9) for lead and 1.6 (0.7–3.2) for copper. A fraction of the electroactive concentrations at pH 2.7 (6.1 for Zn) is electroactive at pH 8.1. The fractions are 100% for Cd, 20% for Cu, 13% for Pb and 40% for Zn. The remaining fractions are partly composed of organically bound species in solution. The low value for lead may be caused by the presence of particulate lead that is dissolved at low pH.Ionic copper and lead species, added separately to seawater at pH 8.1 are removed from the electroactive form, and taken up in (organic) complexes in the same ratio (at least for copper) as the species already present. Added ionic zinc is not removed within the time scale of the measurements (30 min). North Sea water at the natural pH has a complexing capacity, probably due to the presence of dissolved organic compounds, in a concentration equivalent to 3.10^?7M copper. The complexing capacity is zero at pH 2.7. The usual method of standard addition for the determination of electroactive copper and lead concentrations may lead to erroneous results in samples where complexation of this type occurs.     

Kinetic and equilibrium studies of copper-dissolved organic matter complexation in water column of the stratified Krka River estuary (Croatia)

An interaction of dissolved natural organic matter (DNOM) with copper ions in the water column of the stratified Krka River estuary (Croatia) was studied. The experimental methodology was based on the differential pulse anodic stripping voltammetric (DPASV) determination of labile copper species by titrating the sample using increments of copper additions uniformly distributed on the logarithmic scale. A classical at-equilibrium approach (determination of copper complexing capacity, CuCC) and a kinetic approach (tracing of equilibrium reconstitution) of copper complexation were considered and compared. A model of discrete distribution of organic ligands forming inert copper complexes was applied. For both approaches, a home-written fitting program was used for the determination of apparent stability constants (K_i^equ), total ligands concentration (L_iT) and association/dissociation rate constants (k_i¹,k_i^- 1).A non-conservative behaviour of dissolved organic matter (DOC) and total copper concentration in a water column was registered. An enhanced biological activity at the freshwater–seawater interface (FSI) triggered an increase of total copper concentration and total ligand concentration in this water layer. The copper complexation in fresh water of Krka River was characterised by one type of binding ligands, while in most of the estuarine and marine samples two classes of ligands were identified. The distribution of apparent stability constants (log K₁^equ: 11.2–13.0, log K₂^equ:8.8–10.0) showed increasing trend towards higher salinities, indicating stronger copper complexation by autochthonous seawater organic matter.Copper complexation parameters (ligand concentrations and apparent stability constants) obtained by at-equilibrium model are in very good accordance with those of kinetic model. Calculated association rate constants (k₁¹:6.1–20 × 10³ (M s)^− 1, k₂¹: 1.3–6.3 × 10³ (M s)^− 1) indicate that copper complexation by DNOM takes place relatively slowly. The time needed to achieve a new pseudo-equilibrium induced by an increase of copper concentration (which is common for Krka River estuary during summer period due to the nautical traffic), is estimated to be from 2 to 4 h.It is found that in such oligotrophic environment (dissolved organic carbon content under 83 µM_C, i.e. 1 mg_CL^− 1) an increase of the total copper concentration above 12 nM could enhance a free copper concentration exceeding the level considered as potentially toxic for microorganisms (10 pM).     

Voltammetric studies of the organic association of copper and lead in two canadian inlets

Anodic stripping voltammetry and gel filtration chromatography were used to examine the speciation and organic complexation of copper and lead in seawater. The extent of metal complexation in biologically different water types, the molecular weight ranges of the dissolved organic matter involved in metal-organic associations, and the metal uptake kinetics of these naturally occurring organic species were examined. Analyses of samples from Saanich and Narrows inlets, British Columbia, suggest organic complexation throughout the water column, including anoxic waters of Saanich Inlet. Several fractions of organic material from sedimentary interstitial water in the molecular weight range 500–10,000 were found to complex copper and lead, the concentration of complexing ligands decreasing with depth in the core. Rates of uptake of metals by organic material are slow, of the order of tens of minutes or longer.     

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%.     

An improvement in the small-volume technique for determining thorium-234 in seawater

The recently developed 2–5 L small-volume MnO₂ coprecipitation method for determining ²³⁴Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of ²³⁴Th and ²³⁴Th-based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO₂ precipitate onto a 25-mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 min for unfiltered waters from the South China Sea. In addition, we have modified the MnO₂ purification procedure to allow for alpha spectrometric measurements of ²³⁴Th recoveries. Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO₄ and 0.1 mg MnCl₂·4H₂O per liter of sample, while still maintaining high ²³⁴Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of the small-volume method.     

浙江西沪港重金属铜的配位容量和形态分析

根据2000年6月10～14日在浙江省西沪港采集的海水样品,利用AA-800石墨炉原子吸收分光光度技术和阳极溶出伏安法测定样品中重金属铜的含量,获得铜在海水中受不同的有机配体控制.不同粒级的铜表观配位容量表明西沪港海水过孔径1.00μm微孔膜的(ACuCC)较高,为144.4nmol/dm3;过0.40和0.20μm滤膜的(ACuCC)分别为103.0和102nmol/dm3;铜的有机配体条件稳定常数的对数值在7.25～9.14之间.铜的总量为21.72nmol/dm3.铜全部为稳定溶解态,其中pH2酸溶态占95.0%,强有机结合态占5.0%.溶解态铜中有机结合态占过滤海水中总铜的61.6%.     

The interaction of Np(V)O2+ with common mineral surfaces in dilute aqueous solutions and seawater

The extent and kinetics of Np(V)O₂⁺ adsorption from dilute aqueous solutions and seawater onto a variety of synthetic and natural solids were determined at 25°C and 1 atm total pressure. Extensive and complex adsorption reactions were found, contrary to speculations in the literature that NpO₂⁺ should behave as a simple monovalent ion with a low affinity for surfaces. When normalized to adsorption per unit solid surface area, the ranking for the synthetic solids was aragonite ? calcite > goethite ? MnO₂ ≈ clays. Natural materials generally followed the same behavior patterns as their synthetic counterparts. The dissolved/adsorbed ratio was found to be constant over a wide range (10^?13–10^?7M) of NpO₂⁺ concentrations. At higher concentrations the extent of adsorption decreased until a solubility limit was reached at approximately 10^?5 M.Solution composition had the most significant influence for NpO₂⁺ adsorption on goethite, where much more extensive adsorption occurs in dilute solutions than in seawater. When seawater is added to a dilute solution, extensive desorption of NpO₂⁺ from goethite occurs. Tests conducted on NpO₂⁺ adsorbed on carbonates indicated that it remained in the V oxidation state.There is a growing consensus that Pu dissolved in natural waters also occurs dominantly in the V oxidation state as PuO₂⁺ ion. Consequently, these results for NpO₂⁺ may serve as a guide for Pu behavior when also in the V oxidation state. The fact that most adsorbed Pu is found in the III or IV oxidation states indicates that reduction of Pu may occur subsequent to adsorption in the V oxidation state.     

Collection and detection of natural iron-binding ligands from seawater

Iron (Fe) is an essential element for the biochemical and physiological functioning of terrestrial and oceanic organisms, including phytoplankton, which are responsible for the primary productivity in the world's oceans. However, due to the low solubility of Fe in seawater, phytoplankton are often limited by their inability to incorporate enough Fe to allow for optimal growth rates in regions with dissolved Fe concentrations below 1 nM. It has been postulated that certain phytoplankton may produce compounds to facilitate the uptake of Fe from seawater to overcome this limitation. Dissolved Fe in the oceans is overwhelmingly complexed (>99%) by strong organic ligands that may control the uptake of Fe by microbiota; however, the identity, origin, and chemical characteristics of these organic chelates are largely unknown. Although it has been implied that some components of natural Fe-binding ligands are siderophores, no direct analyses of such compounds from natural seawater have been conducted. Here, we present a simple solid-phase extraction technique employing Biobeads SM-2 and Amberlite XAD-16 resins for concentrating naturally occurring dissolved iron-binding compounds from large volumes (>200 l) of seawater. Additionally, we report on the first successful determination of molecular weight size classes and preliminary iron-binding functional group characterization within those size classes for isolates collected from the surface and below the photic zone (150 m) in the central California coastal upwelling system. Electrochemical analyses using competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) showed that isolated compounds had conditional Fe-binding affinities (with respect to inorganic iron—Fe′) of K_FeL,Fe′^cond=10^11.5–10^11.9 M⁻¹, similar to purified marine siderophores produced in laboratory cultures and to the ambient Fe-binding ligands observed in seawater. In addition, 63% of the extracted compounds from surface-collected samples fall within the defined size range of siderophores (300–1000 Da). Hydroxamate or catecholate Fe-binding functional groups were present in each compound for which Fe binding was detected. These results illustrate that the functional groups previously shown to be present in marine and terrestrial siderophores extracted and purified from laboratory cultures are also present in the natural marine environment. These data provide evidence that a significant fraction of the organic Fe-binding compounds we collected contain Fe-binding functional groups consistent with biologically produced siderophores. These results provide further insight into characteristics of the Fe-binding ligands that are thought to be important in controlling the biological availability of Fe in the oceans.     

Solvent extraction of copper acetylacetonate in studies of copper(II) speciation in seawater

A liquid-liquid partition, ligand exchange procedure involving the formation of copper(II) complexes with acetylacetone is presented for the determination of stability constants and concentrations of copper chelators in seawater. Acetylacetone competes with natural ligands for copper, and the equilibrium concentration of the copper acetylacetonate complex is used in speciation calculations. The concentration of the complex is calculated by partitioning a fraction of it into an organic phase and determining the total Cu concentration in that phase by back extracting with acid, and analyzing by flameless atomic absorption spectroscopy. The concentration of Cu acetylacetonate in seawater in equilibrium with the organic phase is calculated from the partition coefficient. The simple, thermodynamically well characterized procedure offers several advantages over previous techniques. Studies using organic free seawater and model ligands show good agreement between experimental and calculated conditional stability constants. Studies from seawater in Biscayne Bay, Florida, indicate two ligand types are present; type 1, K₁ = 1.2 × 10¹², C_L1 = 5.1 × 10⁻⁹ M; type 2, K₂ = 2.8 × 10¹⁰, C_L2 = 1.1 × 10⁻⁷ M. Speciation is dominated by ligand type 1. Depth profiles of [Cu(II)]_free/[Cu(II)]_total measured with the procedure at ambient copper concentrations show an increase from < 5 × 10⁻⁵ at 50–60 m to > 1 × 10⁻³ at the surface at two stations off the Florida coast.     

Nitrite photolysis in seawater by sunlight

Nitrite is chemically stable but photochemically unstable in seawater. The net disappearance rate in abiotic low-nitrate seawater exposed to sunlight is ～ 10% per day. The primary products are the free radicals NO and OH. Quantitative aspects of the kinetics and secondary product formation are discussed in terms of a fourteen-step reaction scheme. Possible pathways explaining the results are suggested but not unequivocally identified.The rate of reaction in various marine environments is estimated from cruise data and extrapolations to vary between 0.2–60·10^?3 moles m^?2yr^?1, with a suggested global average for comparison purposes of 1–10·10³ moles m^?2yr^?1.These results confirm and quantify our previous suggestion that nitrite photolysis represents a source of OH radical in seawater. The reaction rate is large enough that significant impacts on the geochemical cycles of dissolved organic carbon and nitrogen and heavy metals may plausibly result. Effects on marine biota and atmospheric trace gas composition are also possible. However, specific reactions coupling the nitrite system to other processes have not yet been identified or demonstrated empirically.     

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 thorium isotopes in seawater by moored MnO2-fiber method

A method is described for the determination of Th isotopes (²³²Th,²³⁰Th,²²⁸Th and²²⁷Th) in seawater through analysis of Th adsorbed on MnO₂-impregnated fiber that has been moored in the deep sea for up to 10 months. Since the MnO₂-fiber adsorbs Th from seawater at a constant rate, natural²³⁴Th can be used as a yield monitor by making a correction for its decay during the period of deployment. The results obtained by the method showed good reproducibility and accuracy. The method has the advantage over the chemical coprecipitation method that the time and labor for sampling and processing a large-volume of seawater is reduced.