Equilibrium surface speciation

An overall reaction describing the adsorption or desorption of soil solution species by a soil adsorbent can be written 

Some special cases of the reaction in equation (9.2) are listed in Table 9.7. In each case, S represents the adsorbent structure not involved directly in the adsorption-desorption reaction. Equation (9.2) can be generalised (Sposito, 1983) to permit more than lmol of the species SRZsR(s) to react, to replace Mm+ by a metal-hydroxy polymer (e.g. Al OH) ), or to replace Ll by a polyanion (e.g. fulvic acid). Note that the adsorbent can be either inorganic or organic (see the first two reactions in Table 9.7). 

An equilibrium constant for the heterogeneous reaction in equation (9.2) can be defined in terms of activities (denoted by bold parentheses)  

The relationship between mole fraction and activity, and, therefore, between fQds and -fQdsc, is made through the rational activity coefficients  

If the adsorbent-adsorbate solid phase at equilibrium comprises only the two species, SR C(s) and SR(s), the straightforward methods of chemical thermodynamics can be applied to derive an experimentally accessible relationship between either /sr c or /SR and the conditional equilibrium constant, KMlsc (Sposito, 1983). 

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An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

Figure 9.24. (a) Calculated surface speciation as a function of pH at ionic strength 0.1 (1 1 electrolyte) for a 10 M hydrous ferric oxide suspension, (b) Calculated equilibrium speciation as a function of pH for zinc in a 10 M suspension of hydrous ferric oxide TOTZn = 10 M, / = 0.1 M. (Adapted from Dzombak and Morel, 1990.)... 

Correcting for Coulombic Interaction. The surface speciation as a function of solution variables can be computed if we can correct our equibrium constants for electrostatic attraction or repulsion. Westall (21, 22, 23) has developed a computer program that permits one to compute iteratively the composition of the surface and its charge from a set of equilibrium constants. Figures 12 and 13 illustrate the application of this computation to the interaction of o-phosphate with goethite ( -FeOOH). This interaction is rather involved because various mono-dentate and bidentate surface species have to be assumed to account for the experimental observations (18, 24) ... 

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. 

The equilibrium speciation of a metal ion influenced by cation exchange is dependent on the relative concentrations of the cations competing for the negatively charged sites on the particle s surface and their relative affinities for adsorption. Since one cation displaces another from the negatively charged sites, this process is termed cation exchange. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

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