Speciation models seawater

Although the details of the equilibrium model are still uncertain, the general trends are likely reliable. As shown in Figme 5.16, most of the Fe(III) in seawater is predicted to be in the form of the FeL complex. The equilibrium model also predicts that this degree of complexation should enhance iron solubility such that 10 to 50% of the iron delivered to the ocean as dust will eventually become dissolved if equilibrimn is attained. If this model is a reasonable representation for iron speciation in seawater, uptake of [Fe(III)]jQjgj by phytoplankton should induce a spontaneous dissolution of additional particulate iron so as to drive the dissolved iron concentrations back toward their equilibrium values. 

The species distributions shown in Table 12.2 depict results from one of the earliest (Garrels and Thompson, 1962) and one of the most recent (Millero and Schreiber, 1982) seawater ion pairing models. These results and others (Kester, 1975a) are consistent with the following general characteristics of major ion speciation in seawater ... 

It would be difficult to find more comprehensive or more detailed studies on the physical chemistry of seawater than those done at the University of Miami (Millero, 2001). Several programs were developed for calculation of activity coefficients and speciation of both major ions and trace elements in seawater. The activity coefficient models have been influenced strongly by the Pitzer method but are best described as hybrid because of the need to use ion-pair formation constants (Millero and Schreiber, 1982). The current model is based on Quick Basic computes activity coefficients for 12 major cations and anions, 7 neutral solutes, and more than 36 minor or trace ions. At 25 °C the ionic strength range is 0-6 m. For major components, the temperature range has been extended to 0-50 °C, and in many cases the temperature dependence is reasonably estimated to 75 °C. Details of the model and the parameters and their sources can be found in Millero and Roy (1997) and Millero and Pierrot (1998). Comparison of some individual-ion activity coefficients and some speciation for seawater computed with the Miami model is shown in Section 5.02.8.6 on model reliability. 

Turner et al. (1981) and Byrne et al. (1988). Turner et al. (1981) used a database of stability constants for more than 500 metal complexes to calculate the inorganic speciation for 58 trace elements in model seawater at pH 8.2, 25 °C, and 1 atm. Byrne et al. (1988) extended this work by considering the influence of temperature and pH on speciation. The free hydrated divalent cation dominates the dissolved inorganic speciation of Zn(II) and the first transition series metals Mn(II), Co(II), and Ni(II). Strongly hydrolyzed trace metals include Be(II), Al(III), Fe(III),... 

The concept of chemical modeling of natural hydrologic systems was introduced by Garrels and Thompson Q) in a paper that described the distribution of chemical species in seawater. Their approach was to construct a rigorous thermodynamically based model that was (1) mathematically but not conceptually decoupled from flow, and (2) could provide quantifiable information about the chemical processes active in an aqueous system, such as seawater or groundwater. Their initial model considered 17 species, was restricted to 25 °C and remarkably enough, clearly quantified the predominant ion and ion pair speciation in seawater. This work set the framework for a number of the computer codes used today. 

Ion-Association and lon-Hvdration. Aqueous solutions of electrolytes have been chemically described using a variety of theories. The original theoretical approach used by geochemists to model aqueous systems was based on the concept of ion-pairing or ion-association. The ion association approach as described by Carrels and Thompson (1) accurately depicted the speciation of seawater and later many other aqueous solutions. This approach was subsequently found to be inadequate for defining the chemistry of more complex and more concentrated aqueous solutions or those solutions near the critical point of water. This deficiency led to the use of other theoretical approaches to describe these systems, such as the ion-interaction, mean salt, and ion-hydration theories. 

Bc2 produced by flash photolysis of seawater decays by parallel first- and second-order reactions. The environmentally important exponential decay is a pseudo first-order reaction of Br2 with the carbonate/ bicarbonate system in seawater. A chemical speciation model for the free ions and ion-pairs in seawater and in solutions at seawater ionic strength allowed us to measure the dependence of the pseudo first-order rate term, a, on individual C02-containing species. A predictive equation based on reaction of Br2 with free C03 and the hgC03°, NaC03 and CaC03 ion pairs accounts for the mean seawater a at pH 8.1 within experimental uncertainty. The reaction productfs) are unknown. 

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. 

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. 

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. 

This simple two component model for the Fe isotope composition of seawater does not consider the effects of the Fe isotope composition of dissolved Fe from rivers or from rain. Although the dissolved Fe fluxes are small (Fig. 19) the dissolved fluxes may have an important control on the overall Fe isotope composition of the oceans if they represent an Fe source that is preferentially added to the hydrogenous Fe budget that is ultimately sequestered into Fe-Mn nodules. In particular riverine components may be very important in the Pacific Ocean where a significant amount of Fe to the oceans can be delivered from rivers that drain oceanic islands (Sholkovitz et al. 1999). An additional uncertainty lies in how Fe from particulate matter is utilized in seawater. For example, does the solubilization of Fe from aerosol particles result in a significant Fe isotope fractionation, and does Fe speciation lead to Fe isotope fractionation ... 

The approach usually adopted in geochemistry to describe the speciation state of aqueous solutions is that of ionic coupling-complexation, based on the classical model developed by Garrels and Thompson (1962) for seawater. This model con-... 

Plummer. L.N. and Wigley, T.N.L. Critical review of the kinetics of calcite dissolution and precipitation, in Jenne, E.A., ed., "Chemical Modeling. Speciation, Sorption, Solubility and Kinetics in Aqueous Systems," Amer. Chem. Soc. Symp. Series, Washington, D.C. (this volume), de Kanel, J. and Morse, J.W. The chemistry of orthophosphate uptake from seawater onto calcite and aragonite, Geochim. Cosmochim. Acta 1335-1340 (1978). 

The results calculated are in reasonable agreement with the seawater speciation data from Turner et al. (1981) quoted in Table 6.5 and with the qualitative features of the Garrels and Thompson model. Of course, the selection of different suites of stability constants leads to somewhat different speciation pictures. For example, the calculations made by Garrels and Thompson, Dickson et al., and in Tableau 6.6 are based on the assumption that chloride complexes with the major cations are unimportant. This assumption may be wrong and ion pairs with CP may represent nonnegligible fractions of the major cation concentration. Then, of course, a different speciation picture would result however, the extension of these results to trace metals (see the column for seawater in Table 6.5) would require a reinterpretation of the original experimental coordination data with equilibrium constants with the CP ion pairing model. 

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