Speciation calculation

Recent advances in solution chemistry, thermodynamics and computer technology make it possible to describe quantitatively equilibrium and mass transfer among minerals and aqueous electrolytes in geochemical processes involving large numbers of components, phases, and chemical species at both high and low temperatures and pressures. 

This chapter focuses on one of the most common questions asked about natural chemical systems what are the concentrations or activities of the different species present in a system at complete chemical equilibrium We might be concerned, for example, with oxygen or sulfur fugacities, with the activities of complex ions, or activity ratios of reduced and oxidized species of the same component. 

In practice, these calculations range from trivially simple to enormously complex, depending on the number of species (and components) in the system. We will follow roughly this order—from trivial to complex—and outline some of the most common approaches used in performing speciation calculations. 

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A speciation calculation including one of the sorption models described above, or a combination of two or more sorption models, can be evaluated numerically following a procedure that parallels the technique described in Chapter 4. We begin as before by identifying the nonlinear portion of the problem to form the reduced basis,... 

Supersaturation of up to nearly 4 orders of magnitude is indicated relative to a log K= 4.9 which reflects freshly precipitated HFO. When elimination of all data points which are below the detection limits for Fe(lll) and for electrode measurements, values of Eh measured agree with Eh calculated from Fe(ll/lll) determinations and speciation calculations and the revised ferrihydrite saturation index diagram looks like fig. 3. 

Geochemical speciation calculations suggest that Sb occurs predominantly as Sb(lll) throughout the tailings pore-waters and that Sb concentrations are limited by the precipitation of common Sb(lll) minerals, such as sernamontite (Sb203, reaction 1). 

In freshwater, Mn(II) oxidation is slightly slower than in 0.1M NaClO. The difference between the Mn(II) oxidation rate in freshwater and 0.1M NaCIO, is greatest at pH 8.5, at this pH the rate of Mn(II) oxidation is only 40% lower in the freshwater than in 0.1M NaClO. In the estuarine-water at pH 8.5 the rate of Mn(II) oxidation is 20 times slower than in 0.1M NaCIO,. The speciation calculations indicate why the model predicts the oxidation is slower than in natural waters (see, for example Table VII). 

To perform speciation calculations, chemical distributions must be in the form of molar... 

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... 

More commonly, s are computed on the fly by computer programs as part of an equilibrium speciation calculation. These programs employ various numerical approaches for estimating K, such as the one shown in Eq. 5.11. In this case, the... 

Although computer programs are now used to perform speciation calculations, examining how these calculations are performed provides important insights into the limitations of the model predictions. Thus, we will step through a small part of the calculation used to generate the results presented in Figure 5.4, which represents the iron... 

Including multiple anions into speciation calculations greatly increases their complexity because the anions also imdergo multiple ion pairing reactions. The most important involve the formation of ion pairs with the major ions, Na, Ca, Mg, ... 

Various munerical techniques are used to indirectly obtain solutions to large systems of equations with too many imknowns to solve explicitly. One approach is to solve the equations iteratively. This is done by first assuming that all of the anions are unbound and, hence, their free ion concentrations are equal to their total (stoichiometric) concentrations. By substituting these assumed anion concentrations into the cation mass balance equations, an initial estimate is obtained for the free cation concentrations. These cation concentrations are substituted into the anion mass balance equations to obtain a first estimate of the free anion concentrations. These free anion concentrations are then used to recompute the free cation concentrations. The recalculations are continued imtil the resulting free ion concentrations exhibit little change with further iterations. The computer programs used to perform speciation calculations perform these iterations in a matter of seconds. 

Since the metals can be considered as the limiting reactants, the effects of metal-metal competition for the ligands can be neglected. Thus, speciation calculations for each trace metal can be performed independently. 

As with all speciation calculations, the results for the trace metals depend on the values used for the thermodynamic equilibriiun constants and activity coefficients. Determination of these constants is very difficult because direct detection of trace metal species is complicated by their low concentrations and analytical interferences... 

Speciation calculations can be performed for the weak acids and bases in a feshion similar to that presented earlier for Fe(III). The results of these calculations as a function of pH are shown in Figure 5.19. At the pH of seawater, the dominant species are carbonate, bicarbonate, ammonium, hydrogen phosphate, dihydrogen phosphate, and boric and silicic acid. In waters with low O2 concentrations, significant concentrations of HS can be present. 

The solubilized metals form complexes with organic and inorganic anions. The chemical speciation of these complexes changes as the metal moves seaward through the estuary due to increasing salinity. These shifts can be predicted from equilibrium speciation calculations as described in Chapter 5.7. Two examples are shown in Figure 28.23 for... 

As already stated, speciation is the characteristic distribution of various ionic and/or neutral species in an aqueous solution. Speciation calculation, allowing practical estimation of the reactive properties of an aqueous solution, acidity, redox state, the degree of saturation of the various solids, and so on, is carried out on a thermodynamic basis starting from the chemical composition of the solution of interest and using the reaction constants of the various equilibria of the type seen in equation 8.19. 

Table 8.9 shows the results of detailed speciation calculations carried out with the automated routine EQ3NR/EQ6 (Wolery, 1983). The molalities of the ions in solution resulting from this calculation are tabulated in order of decreasing abundance. 

The precise speciation calculation indicates that four minerals are at almost perfect saturation ... 

Of the various factors that cause redox disequilibria, the most effective are biologic activity (photosynthesis) and the metastable persistence of covalent complexes of light elements (C, H, O, N, S), whose bonds are particularly stable and difficult to break (Wolery, 1983). For the sake of completeness, we can also note that the apparent redox disequilibrium is sometimes actually attributable to analytical error or uncertainty (i.e., difficult determination of partial molalities of species, often extremely diluted) or even to error in speciation calculations (when using, for instance, the redox couple Fe /Fe, one must account for the fact that both Fe and Fe are partly bonded to anionic ligands so that their free ion partial molalities do not coincide with the bulk molality of the species). 

Detailed speciation calculations for the fluid coupled with mineralogical investigation of the solid paragenesis generally allow a sufficiently precise estimate of the T of equilibrium. As figure 8.30A shows, the chemistry of the fluid is buffered by wall-rock minerals, so that the saturation curves of other phases not pertaining to the system of interest are scattered (figure 8.28B). 

This approach will be rigorous if a complete speciation calculation is done for P and E (as described in standard textbooks of aquatic chemistry) and all the necessary values of ki are available. Fortunately, the analysis can usually be simplified to reaction between one or two dominant species, and most of the available rate constants are for these same species. 

The use of the thermodynamic equilibrium constants is featured prominently in any speciation calculation therefore, it is necessary to be aware of some of the conventions used and how to manipulate the equations. 

Several different types of equilibrium constant are described in the literature and it is well to be aware of the type of constant needed for speciation calculation. 

Table 5.12 MINTEQA2 model inputs for a chemical speciation calculation of soluble complexes in two acidic stream waters...

Once a choice of molecular model for adsorbed species activity coefficients has been made and its parameters are measured, an equilibrium speciation calculation can be performed by exact analogy with that for aqueous species (see Table 9.3). Computationally, adsorbent species and adsorbed species (SR M OH H in... 

Table 9.10 Results of a chemical speciation calculation involving inner-sphere surface ...

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