Copper seawater speciation

If copper interactions were minimized in real seawater, abundant metals of lesser sulfide affinity would take up some of the slack. ITiis is partially evident from analyses of the type in Table III. For example, nickel has mixed layer concentrations on the order of nanomolar (22), and its sulfide equilibria and inorganic seawater speciations may resemble those of zinc (lv-19.31.32). Titration, however, should only lower free sulfide to a Table m SH equivalence point, or, to roughly picomolar. In a follow up to 1Z, Dyrssen and coworkers treat Cu(II) as a variable parameter, and find that in its absence, nickel, zinc and lead can all become sulfides while the bisulfide ion still hovers well above pM (18). Again, it must be emphasized that error margins in the various equilibria remain to be investigated. 

Moffett J. W. and Zika R. G. (1987) Solvent extraction of copper acetylacetonate in studies of copper (II) speciation in seawater. Mar. Chem. 21, 301-313. 

Zirino, A. and Yamamoto, S., A pH-dependent model for the chemical speciation of copper, zinc, cadmium, and lead in seawater, Limnol Oceanogr, 17 (5), 661-671, 1972. 

Sunda and Hanson [247] have used ligand competition techniques for the analysis of free copper (II) in seawater. This work demonstrated that only 0.02 -2% of dissolved copper (II) is accounted for by inorganic species. (i.e., Cu2+, CuC03, Cu(OH)+, CuCl+, etc.) the remainder is associated with organic complexes. Clearly, the speciation of copper (II) in seawater is markedly different from that in fresh water. 

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

The speciation of copper is different at high and low pH. At pH 1.0 most of the copper will be labile and a total copper concentration will be measured. At pH 7.7 there should be a smaller proportion of labile copper, as much will be complexed in various forms, depending on the constituents of the seawater. 

Brugmann [784] discussed different approaches to trace metal speciation (bioassays, computer modelling, analytical methods). The electrochemical techniques include conventional polarography, ASV, and potentiometry. ASV diagnosis of seawater was useful for investigating the properties of metal complexes in seawater. Differences in the lead and copper values yielded for Baltic seawater by methods based on differential pulse ASV or AAS are discussed with respect to speciation. 

Krznaric [799] studied the influence of surfactants (EDTA, NTA) on measurements of copper and cadmium in seawater by differential pulse ASV. Adsorption of surfactants onto the electrode surface were shown to change the kinetics of the overall electrode charge and mass transfer, resulting in altered detection limits. Possible implications for studies on metal speciation in polluted seawater with high surfactant contents are outlined. 

Donat and Bruland [804] studied the speciation of copper and nickel in seawater by competitive ligand equilibration-cathodic stripping voltammetry, differential pulse ASV, and graphite furnace AAS. 

Calculated equilibrium speciation of (a) mercury and (b) copper during estuarine mixing of hypothetical river water with seawater. Hum, humic substance. Note logarithmic scale on y-axis. Source. From Mantoura, R. F. C., et al. (1978). Estuarine and Coastal Marine Science 6, 387 08. 

Bruland, K.W., Rue, E.L., Donat, J.R., Skrabal, S.A., and Moffett, J.W. (2000) Intercomparison of voltammetric techniques to determine the chemical speciation of dissolved copper in a coastal seawater sample. Anal. Chim. Acta 405, 99-113. 

Sunda, W. G., and Huntsman, S. A. (1991) The Use of Chemiluminescence and Ligand Competition with EDTA to Measure Copper Concentration and Speciation in Seawater, Mar. Chem. 36, 137-163. 

FIGURE 4. Calculated equilibrium speciation of mercury and copper during estuarine mixing f hypothetical river water with seawater. Hum = humic substance. Adapted from Mantoura et (1978). 

Like iron complexes, copper complexes have been shown to be an important sink for photochemically-generated superoxide in seawater and, based on the high reactivity of Cu(ii) complexed by cyanobacterial-derived ligands, it is likely that redox reactions with superoxide significantly influence Cu redox speciation in the ocean [221,222]. These reactions also have important effects on the steady state concentrations of superoxide in seawater, reducing the concentration by at least an order of magnitude compared to previous estimates that ignored the reactions with copper complexes [222]. 

Bioavailability and toxicity of copper to aquatic orgaiusms depend on the total concentration of copper and its speciation. Both availability and toxicity are significantly reduced by increased loadings of suspended solids and natural orgaiuc chelators and increased water hardness. Toxicity to aquatic life is primarily related to the dissolved cupric ion (Cu+ ) and possibly to some hydroxyl complexes. Cupric copper (Cu+ ) is the most readily available and toxic inorgaiuc species of copper in freshwater, seawater, and sediment interstitial waters. Cupric ion accounts for about 1 % of the total dissolved copper in seawater and less than 1% in freshwater. In freshwater, cupric copper... 

Several complexing ligands can be employed to determine copper in seawater. However, the best results are obtained when copper is determined after adsorptive pre-concentration as a complex with salicylaldoxime (SA) Campos and van den Berg, 1994). Analysis is carried out at neutral pH values (pH 6.5-8.5 is suitable) which means that the original sample pH can be maintained which is useful for speciation studies. 

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