Speciation seawater

Laughlin, R.B., Jr., H.E. Guard, and W.M. Coleman III. 1986b. Tributyltin in seawater speciation and octanol-water partition coefficient. Environ. Sci. Technol. 20 201-204. 

The aqua Zn ion is dominant in organic matter-free freshwater while the free Zrf+ ion and chloride complexes dominate in seawater (Stanley and Byrne 1990 Millero 1996). The free Cu + ion is dominant in freshwater, while the carbonate complexes CuCO, and [Cu(C03)2] are preponderant in seawater. Speciation and solubility of Zn in Cl-rich hydrothermal solutions has been investigated by Wesolowski et al. (1998). Speciation and solubility of Cu have been investigated by Mountain and Seward (1999) for hydrothermal solutions dominated by sulfides and by Xiao et al. (1998), Liu et al. (2002), and Archibald et al. (2002) for solutions dominated by chlorides. 

While seawater speciation of Mo is dominated by Mo04, trace intermediates like... 

Elements considered in seawater speciation calculations can be separated into major and minor components. Such a separation is possible because the vast majority of seawater constituents have concentrations so low that they do not significantly influence the activities of the major cations and anions in seawater. As such, the equilibrium behaviour of the major ions in seawater can be understood (calculated) independently of the numerous minor constituents and these results can then be applied to calculations involving individual minor constituents. 

In contrast to equilibria dominated by chloride complexation, equilibria involving hydrolysis or carbonate complexation are strongly influenced by pH. A cation s tendency towards hydrolysis, and its complexation behaviour in general, can be described in terms of the ratio of its ionic charge squared (z2) to its ionic radius (r) in Angstroms (Turner et al., 1981). Cations with z2/r in the intermediate range, 7 < z/r < 25, have seawater speciation schemes that are strongly influenced by solution pH. Carbonate equilibria of the form... 

The transition metals in Period 4 between Group 7 (Mn) and Group 12 (Zn) have remarkable and varied seawater speciation behaviours that are generally quite distinct from those of other elements in Groups 7-12. On this account, the speciation of these Period 4 metals (Mn, Fe, Co, Ni, Cu and Zn) will be discussed together, subsequent to the discussion of the other metals of Groups 7-18. 

The seawater chemistries of Mn, Fe, Co, Ni, Cu and Zn are, in many respects, quite diverse. One characteristic that these elements have in common, however, is an accessible +11 oxidation state. Except in the case of iron, which exists dominantly as Fem in seawater, the solution speciation of these elements is dominated by the +11 oxidation state. The aspect of these elements seawater speciation which most distinguishes them from other cations in the Periodic Table is their substantial involvement in organic complexation. 

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. 

The redox conditions, of course, have to be considered also. A number of elements with several redox states have a very different speciation under oxic and anoxic conditions. We will discuss inorganic seawater speciation in Section 6.8. 

Tableau 6.6 shows the equilibria and mole balances considered and gives the results on the inorganic seawater speciation.

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. 

Koschinsky, A. and Hein, J.R., 2003. Uptake of elements from seawater by ferromanganese crusts solid-phase associations and seawater speciation. Marine Geology, 198 331-351. 

The thermodynamic data compilations of Sillen and Martell catalyzed rapid advances in equilibrium models of seawater speciation. These works were followed by additional compilations that were critically important to modern sea-water speciation assessments. In view of these developments, and additional extensive experimental analyses appropriate to seawater. Principal Species assessments ten to fifteen years after the pioneering work of Sillen demonstrated a much improved awareness of the importance of hydrolysis in elemental speciation. 

An additional major speciation assessment provided a greatly improved, comprehensive view of inorganic complexation in seawater. Based on the analogous characteristics of metal complexation by carbonate and oxalate, Turner et al. concluded that rare earth element complexation in seawater is dominated by carbonate. Subsequently, as the result of approximately twenty years of progress in seawater speciation, the Principal Species assessment of Bruland listed seventeen elements with carbonate-dominated Principal Species. 

The lanthanide chloride stability constants which are recommended for seawater speciation calculations are shown in table 8. The logci/3 values shown in table 8 were calculated from the data of Mironov et al. (1982) and are nearly identical to the logcii i values used by Lee and Byrne (1993c) to correct the fluoride stability constant data of Bilal and co-workers (Bilal et al. 1979, Bilal and Becker 1979, Bilal and Koss 1980, Bilal 1980, Bilal and Becker 1980, Becker and Bilal 1985) for chloride ion pairing. 

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