Complexation of zinc by exudates from Skeletonema costatum grown in culture

Measurements have been made, by amperometric titration using anodic stripping voltammetry, of the zinc binding properties of organic material released by Skeletonema costatum in culture, after the stationary growth phase had been reached. Titration curves showed two inflection points, interpreted as indicating the presence of two analytically distinguishable ligand assemblages (L₁ and L₂). The complexing capacities of these assemblages increased with time and the associated conditional stability constants K₁^′ and K₂^′ decreased with time. This apparent reduction in stability constants could be accounted for by complex dissociation. Several methods for the calculation of complexing capacity and stability constant were employed and although the calculated concentrations of C_L varied, each method showed similar trends.Size fractionation of the organic moieties revealed that C_L, was composed of organic assemblages with apparent molecular weight greater than 3 × 10⁴. The ligands L₂ were of mixed size composition.     

Complexation of zinc by exudates of Thalassiosira fluviatilis grown in culture

Measurements have been made, by amperometric titration using anodic stripping voltammetry, of the zinc-binding properties of organic material released by Thalassiosira fluviatilis in culture over a period from the beginning of growth to senescence. After the onset of the stationary phase, titration curves showed two inflection points, interpreted as indicating the presence of two analytically distinguishable ligand assemblages (L₁ and L₂). The complexing capacities (C_L) of these assemblages changed with time, C_L1 increasing linearly from <0.5 × 10^?8 M to 2.0 × 10^?6 M into the stationary phase and then more rapidly during senescence, and C_L2 increasing after its initial appearance to a final value of 4.4 × 10^?6 M. These changes were accompanied by systematic decreases in the corresponding conditional stability constants (K′) calculated for 1 : 1 associations of the ligands with zinc. Values of log K′₁ decreased from 6.6 to 5.8, and those of log K′₂ from 7.2 to 6.2. The presence of the second ligand assemblage may be related to the release of intracellular material during senescence.     

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%.     

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.     

Determination of the apparent copper complexing capacity of seawater by anodic stripping voltammetry

A new experimental technique using differential pulse anodic stripping voltammetry with a thin-film mercury electrode and efficient mixing of the electrolyte for the apparent copper complexing capacity of seawater is proposed. The effects of some factors such as type of electrode and cell, pH of the solution, potential of deposition, time of equilibration, and influence of a non-ionic surface active agent on the measurements are examined. Experimental results for the apparent copper complexing capacity of seawater are calculated presuming 1:1 inert complex formation. By using EDTA as a model ligand, it is shown that the proposed procedure with efficient mixing of the solution excludes any appreciable kinetic influence upon the electroanalytical results.     

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 two voltammetric techniques for determining zinc speciation in Northeast Pacific Ocean waters

Two independent voltammetric techniques, differential pulse cathodic stripping voltammetry (DPCSV) and differential pulse anodic stripping voltammetry (DPASV), determined that 95% of the dissolved zinc is organically complexed at two depths (60 and 150 m) within the surface euphotic zone at an open ocean station in the Northeast Pacific. Average values for the concentrations of the natural zinc-complexing organic ligands (C_L) obtained from duplicate determinations at these two depths by DPCSV versus DPASV are in excellent agreement: 1.60 ± 0.01 versus 1.76 ± 0.03 nM at 60 m, and 2.14 (n=1) versus 2.22 ± 0.06 nM at 150 m. Average values for the conditional stability constants (with respect to free Zn²⁺) of the natural zinc-organic complexes (log K^′_ZnL) from duplicate determinations at both depths by DPCSV versus DPASV are 10.3 ± 0.2 versus 11.2 ± 0.2. Additional research is required to assess the significance of the difference in the conditional stability constants determined by these two techniques. These results confirm recent observations that strong zinc complexes formed with an organic ligand class existing at nanomolar concentrations dominates zinc speciation in the North Pacific.     

Organic complexation and its control of the dissolved concentrations of copper and zinc in the Scheldt estuary

Cathodic stripping voltammetry (CSV) is used to determine total (after UV-irradiation) and labile dissolved metal concentrations as well as complexing ligand concentrations in samples from the river Scheldt estuary. It was found that even at high added concentrations of catechol (1 m for copper and 0·4 m for iron) and of APDC (1 m for zinc) only part of the dissolved metal was labile (5–58% for copper, 34–69% for zinc, 10–38% for iron); this discrepancy could be explained by the low solubility of iron which is largely present as colloidal material, and by competition for dissolved copper and zinc by organic complexing ligands. Ligand concentrations varied between 28 and 206 n for copper and between 22 and 220 n for zinc; part of the copper complexing ligands could be sub-divided into strong complexing sites with concentrations between 23 and 121 n and weaker sites with concentrations between 44 and 131 n . Values for conditional stability constants varied between (logK′ values) 13·0 and 14·8 for strong and between 11·5 and 12·1 for weaker copper complexing ligands, whereas for zinc the values were between 8·6 and 10·6. The average products of ligand concentrations and conditional stability constants (a-coefficients) were 6 × 10² for zinc and 6 × 10⁶ for copper.The dissolved zinc concentration was found to co-vary with the zinc complexing ligand concentration throughout the estuary. It is argued that the zinc concentration is regulated, in this estuary at least, by interactions with dissolved organic complexing ligands. A similar relationship was apparent between the dissolved copper and the strong copper complexing ligand concentration. The total copper complexing ligand concentrations were much greater than the dissolved copper concentrations, suggesting that only strongly complexed copper is kept in solution.These results provide evidence for the first time that interactions of copper and zinc with dissolved organic complexing ligands determine the geochemical pathway of these metals.     

Adsorption of lead and cadmium ions on calcite in the Krka estuary

The adsorption of Pb²⁺ and of Cd²⁺ ions from calcareous Krka river water of various salinities (3, 14, 20 and 38 psu) on calcite (CaCO₃, Merck p.a.) was investigated. Simultaneous adsorption of Pb²⁺ and Cd²⁺ ions was studied as well. The results suggest that the two ions are adsorbed at different calcite surface sites; Pb²⁺ remained firmly bound to calcite at all salinities, whereas Cd²⁺ was firmly bound at low salinities and released at high salinities. Dissolved natural organic ligands at the freshwater-seawater interface (FSI; at 14 psu) promoted and below the FSI (at 20 psu) decreased the adsorption of Cd²⁺. The experiments were performed at metal concentrations of 8 × 10⁻⁸ mol l⁻¹ and at natural pH values around pH 8. Adsorbed amounts of trace metal ions were determined in filtered samples by differential pulse anodic stripping voltammetry (DPASV) with a three-electrode system, after 24 h of equilibration with calcite. Several adsorption models were tested, such as Freundlich's, Langmuir's and Schindler-Stumm's surface coordination model. Adsorption isotherms belong to S-1 class of empirical adsorption isotherms. None gave reasonable values of adsorption constants. The fractional partitioning of adsorbent to the solid phase when normalized to the quantity of adsorbent present (K_d) declined as solid concentration increased. It was found that the surface charge is not responsible for the observed effect. Instead, aggregate size increased, which effected a loss in surface area. This is a reasonable explanation for the observed S-shaped adsorption isotherms curves.     

The determination of sulfide in seawater by flow-analysis with voltammetric detection

Preliminary measurements of sulfide in seawater using cathodic stripping voltammetry and a hanging mercury drop electrode (HMDE) in batch-mode showed that the sulfide peak decreased rapidly with time. This decrease was not caused by O₂, H₂O₂ or IO₃⁻, and the sulfide peak was not stabilised by trace metal additions. A home-made flow-cell was constructed to enable the determination of sulfide in seawater using voltammetry with an HMDE. A stable sulfide peak was obtained by flow-analysis with voltammetric detection, with a precision of 2.8% and detection limit of 0.5 nM at a 60 s adsorption time. Several thiol compounds were found to produce a peak at, or very close to, the peak potential for sulfide. Their interference was evaluated by allowing the sulfide peak in conventional (batch) voltammetry to decay. Comparative experiments showed that waste metallic mercury is responsible for removal of sulfide in batch-mode analysis due to formation of insoluble mercuric sulfide salts causing the rapid decay of the sulfide peak. The problem is circumvented by using flow-analysis to determine sulfide.     

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.     

Study on copper complexing ligand concentrations in several China''s coastal waters

Copper complexing ligand concentrations in the Daya Bay, Qingdao coast, Jiaozhou Bay, South China Sea and Huanghe Estuary waters were determined by the anodic stripping voltammetry technique. The distribution regularity and the relationship with other parameters were discussed. The results were as follows： Copper complexing ligand concentrations of the South China Sea were a little higher than those of other sea areas, and they were apparently higher than those of the ocean. Compared with the subsurface layer （SSL） in the sea surface microlayer copper complexing ligand concentrations showed an enrichment phenomenon, of which the mechanism is similar to dissolved organic matter. The metal complexing ligand concentration profiles of the South China Sea showed that the value in the sea surface was the highest, then it decreased with depth accruing, and a higher value appeared at the bottom. Copper complex- ing ligand concentrations were higher than those of cadmium and lead. Ligands in each sea area exhibited a complicated property. In short, the distribution regularity of copper complexing ligand concentrations in China＇s coastal waters was consistent with that of other regions in the world. Meanwhile, the positive relationship between the copper complexing ligand concentrations and biological oxygen demand, chemical oxygen demand, dissolved organic carbon, and viscosity were found clearly.     

Study on copper complexing ligand concentrations in several China’s coastal waters

Copper complexing ligand concentrations in the Daya Bay, Qingdao coast, Jiaozhou Bay, South China Sea and Huanghe Estuary waters were determined by the anodic stripping voltammetry technique. The distribution regularity and the relationship with other parameters were discussed. The results were as follows: Copper complexing ligand concentrations of the South China Sea were a little higher than those of other sea areas, and they were apparently higher than those of the ocean. Compared with the subsurface layer (SSL) in the sea surface microlayer copper complexing ligand concentrations showed an enrichment phenomenon, of which the mechanism is similar to dissolved organic matter. The metal complexing ligand concentration profiles of the South China Sea showed that the value in the sea surface was the highest, then it decreased with depth accruing, and a higher value appeared at the bottom. Copper complexing ligand concentrations were higher than those of cadmium and lead. Ligands in each sea area exhibited a complicated property. In short, the distribution regularity of copper complexing ligand concentrations in China' s coastal waters was consistent with that of other regions in the world. Meanwhile, the positive relationship between the copper complexing ligand concentrations and biological oxygen demand, chemical oxygen demand, dissolved organic carbon, and viscosity were found clearly.     

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.     

Cu complexation by organic ligands in the sub-arctic NW Pacific and Bering Sea

Cu speciation was characterized at three stations in the sub arctic NW Pacific and Bering Sea using cathodic stripping voltammetry with the competing ligands benzoylacetone and salicylaldoxime. A single ligand model was fit to the titration data, yielding concentrations throughout the water column of ∼3–4 nM, and conditional stability constants ranging from 10^12.7 to 10^14.1, this range being partly due to the choice of competing ligand. Free Cu²⁺ in surface waters was 2–4×10⁻¹⁴ M, in close agreement with values reported by previous workers in the NE Pacific using anodic stripping voltammetry (ASV). However, those results showed that complexation by strong organic ligands becomes unimportant below 200–300 m, while our data indicated Cu is strongly complexed to depths as great as 3000 m. Free Cu²⁺ concentrations in surface waters reported here and in previous work are close to the threshold value where Cu can limit the acquisition of Fe by phytoplankton.     

Studies in seawater and lake water on interactions of trace metals with humic substances isolated from marine and estuarine sediments: II. Voltammetric investigations on trace metal complex formation in the dissolved phase

In seawater and lake water the complexation of several heavy metals, present in the dissolved state at relevant trace levels, with humic substances isolated from two marine and an estuarine sediment has been studied by differential pulse anodic stripping voltammetry (DPASV) at a hanging mercury drop electrode (HMDE). Taking Zn(II) as an example, the aim of this study was to gain direct information on the general level of importance of humic substances for the speciation of certain heavy metals in fresh and saline waters.In seawater humic acids originating from different sediments have very similar complexing properties for Zn(II), decreasing slightly in the order HAN > HAL > HAM. In lake water HAL is most efficient for complexing Zn(II), followed by HAN and HAM.In both types of natural waters, the sedimentary fulvic acid is less efficient in complexing Zn(II) than the respective humic acid from the same site. In general, the complexing efficiency decreases in the order EDTA > NTA > humic acids > fulvic acid.The problem of humic acid adsorption at the electrode, which somewhat limits the investigation of Pb(II) and Cd(II), and the impact of pH on the amount of complexed Zn(II) are also discussed.The findings provide further direct evidence for the conclusion previously drawn from existing complexation data that because of the rather low levels of dissolved humics in large parts of the oceans, the complexation efficiency of humics for Cd, Pb and Zn is too low to affect the speciation pattern of these three heavy metals.     

Zinc speciation in the Northeastern Atlantic Ocean

Measurements of zinc and zinc complexation by natural organic ligands in the northeastern part of the Atlantic Ocean were made using cathodic stripping voltammetry with ligand competition. Total zinc concentrations ranged from 0.3 nM in surface waters to 2 nM at 2000 m for open-ocean waters, whilst nearer the English coast, zinc concentrations reached 1.5 nM in the upper water column. In open-ocean waters zinc speciation was dominated by complexation to a natural organic ligand with conditional stability constant (log K_ZnL′) ranging between 10.0 and 10.5 and with ligand concentrations ranging between 0.4 and 2.5 nM. The ligand was found to be uniformly distributed throughout the water column even though zinc concentrations increased with depth. Organic ligand concentrations measured in this study are similar to those published for the North Pacific. However the log K_ZnL′ values for the North Atlantic are almost and order of magnitude lower than those reported by Bruland [Bruland, K.W., 1989. Complexation of zinc by natural organic-ligands in the central North Pacific. Limnol. Oceanogr., 34, 269–285.] using anodic stripping voltammetry for the North Pacific. Free zinc ion concentrations were low in open-ocean waters (6–20 pM) but are not low enough to limit growth of a typical oceanic species of phytoplankton.     

Study of apparent complexing capacities of trace heavy-metals in the Huanghe River Estuary

-By the electroanalysis method combining the complexation titrating technique with the investigation of ip-Ea* characteristic curves, this paper measures apparent complexing capacities of trace heavy-metals in water samples from the Huanghe River Estuary. The results show that the order of apparent complexing capacities of trace heavy-metals in the samples isCu>Cd>Pb,and that apparent complexing capacities of near shore sample are higher than those far from shore. The effect of ultraviolet irradiation on the dissociation of organic ligands and the adsorption effect of cell walls (cells being treated with acid and seawater respectively) are investigated. The reduction (on electrodes) mechanism of species of trace heavy-metals in seawater is approached by ip-Ea* characteristic curves.     

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).