Seawater equilibrium speciation

Sibley TH, Morgan JJ (1975) Equilibrium speciation of trace metals in freshwater-seawater mixtures. In Hutchinson HC (ed) Proceedings of international conference on heavy metals in the environment, University of Toronto, Toronto, Ontario pp 310-338... 

At equilibrium, the reactant concentrations and products can be used to define a mass ratio called an equilibrium constant (A). This constant can then be used to predict the equilibrium concentrations of the reactants and products from the total amount of C or from either the equilibrium concentration of the products or the reactants. Although K is referred to as an equilibrium constant, it is a function of salinity, temperature, and pressure. With the appropriate value of K, calculations can be made to predict the equilibrium speciation of elements in seawater. The procedure for doing this is provided in the next section along with an expansion of K to multicomponent chemical systems. 

Equilibrium speciation calculations have been performed for all of the varied types of chemical transformations that occur in seawater as listed in Table 5.2. Note that some involve only a phase change and, thus, are not chemical reactions. Equilibrium... 

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. 

Turner D. R., Whitfield M., and Dickson A. G. (1981). The equilibrium speciation of dissolved components in fresh water and seawater at 25°C and 1 atm pressure. Geochim. Cos-mochim. Acta, 45 855-881. 

Sibley, T.H. and Morgan, 3.3., 1975. Equilibrium speciation of trace metals in freshwater -seawater mixtures. In T. Hutchinson (ed.), Proc. Int. Conf. Heavy Metals in the Environment. Univ. of Toronto, Ontario, p. 319. 

Table 12.1 lists elements in seawater that have concentrations greater than 0.1/tmol kg-1. The first seven elements in Table 12.1 (Cl, Na, Mg, S, Ca, K and C) have concentrations greater than or equal to 0.002 mol kg 1 and are generally treated as the major ionic constituents of seawater. The ion pairing constants of these relatively weakly interacting elements have been studied as a function of temperature, pressure and ionic strength. An equilibrium speciation...

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). 

The general algorithm used by this program was designed by Carrels and Thompson (1962) for one of the first calculations of the equilibrium speciation of seawater. We will summarize the original description by Truesdell and Jones (1974) of the numerical method. 

Based on equilibrium data compilations including Smith and Martell, Martell and Smith, Baes and Mesmer, Turner et al., Byrne et al., and Liu and Byrne, Table 1 provides a compilation of pKn, pKn and pQn data, and equilibrium speciation schemes appropriate to seawater (S = 35) at 25°C. The first two columns ofTable 1 provide each elements atomic number and identity. The third column provides... 

Table 1. A compilation of pK, pK, pQ data and equilibrium speciation schemes appropriate to seawater (S 35) at 25°C. Equilibrium constants are expressed on the free hydrogen ion concentration scale.

Equilibrium constants are also dependent on temperature and pressure. The temperature functionality can be predicted from a reaction s enthalpy and entropy changes. The effect of pressure can be significant when comparing speciation at the sea surface to that in the deep sea. Empirical equations are used to adapt equilibrium constants measured at 1 atm for high-pressure conditions. Equilibrium constants can be formulated from solute concentrations in units of molarity, molality, or even moles per kilogram of seawater. 

Since [Fe(lll)]jojaj [Fe " ], the formation of ion pairs and complexes is greatly enhancing the equilibrium solubility of ferrihydrite. This is called the salting-in effect and illustrates why mineral solubility calculations in seawater must take ion speciation into consideration. 

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. 

Demianov, P D. DeStefano, A. Gianguzza, and S. Sammartano, Equilibrium studies in natural waters Speciation of phenolic compounds in synthetic seawater at different salinities , Environ. Toxicol. Chem, 14,767-773 (1995). 

In many experiments chemical equilibrium is assumed, however dynamic nonequilibrium processes in seawater may result in products different from those expected under equilibrium conditions. Equilibria can be complex and may involve several types of reactions simultaneously. Kinetics can therefore affect speciation of trace elements and should be taken into account. 

---