Speciation calculation redox

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. 

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). 

It is very important to understand that this kind of speciation calculation indicates that certain redox reactions can occur in soils, but not that they will occur a chemical reaction that is favored by a large value of log K is not necessarily favored kinetically. This fact is especially applicable to redox reactions because they are often extremely slow, and because reduction and oxidation half-reactions often do not couple well to each other. For example, the coupling of the half-reaction for 02(g) reduction with that for N2(g) oxidation leads to log K = -0.3 for the overall redox reaction ... 

Equilibrium predictions of Ce concentrations in seawater combine Ce(III) speciation calculations with the following redox equation (Elderfield 1988, De Baar et al. 1991) ... 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

Ball, J. W. and D. K. Nordstrom, 1991, User s manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters. US Geological Survey Open File Report 91-183. 

Aquatic chemists have defined their own electrochemical standard state to fecilitate calculation of redox speciation in aqueous solutions. In this standard state, all reactions are conducted at pH 7.0, 25°C, and 1 atm. The concentrations of all other solutes are 1 molal (unless otherwise specifically noted). Values so obtained are designated with the subscript w. The pe s for the most important redox couples in seawater are given in Table 7.4. 

One other serious criticism regarding the data on Cu speciation is the neglect of the cysteine present in blood plasma. Cu11 and cysteine undergo a facile redox reaction (Chapter 20.2). Since the reaction is irreversible, no quantitative thermodynamic quotient is available for use in the computer calculations. Another assumption often made is that the overwhelming concentration of other amino acids may prevent cysteine coordination and, as a result, stabilize the Cu11 state. Recent studies show that this assumption is totally unjustified48 and so the dilemma still has to be resolved. 

The calculated distribution of the main Cu(I) and Cu(II) species versus redox potential (EJ during the equilibrium electrolysis for solutions (a) and (b) is shown in Figure 3. The main Cu(I) species for the 0.2099 mol kg1 GuGl(aq) solution in 2.099 mol kg-1 HC1 are HCuClf and CUCI2 while the main Cu(II) species are Cu2+ and CuCK In the more concentrated anolyte solution (b) (1.172 mol kg1 CuCl in 7.031 mol kg1 HC1), there are the same Cu(I) species, but the speciation of Cu(II) becomes more versatile with the input of CuCl , CuClg and CuCl -... 

Of course, redox reactions do not occur in isolation but are coupled through complexation reactions to other species. For example, Eq. 2.30 could include terms for nitrate and amine complexes, in addition to those for free nitrate, nitrite, and ammonium ions, if a typical soil solution were under consideration. The calculation of nitrogen speciation then would proceed just as described above. Indeed, redox reactions introduce no new mathematical elements into a speciation computation, any more than would the consideration of, for example, C02 reactions. The only new item brought in is an additional variable, the pE value, which, like the partial pressure of C02(g), must be specified in order to solve mole balance equations for distribution coefficients. 

J.W. Ball and D.K. Nordstrom, Users Manual for WATEQ 4F, with Revised Thermodynamic Data Base and Test Cases for Calculating Speciation of Major, Trace and Redox Elements in Natural Waters, US Geological Survey Open File Report, Menlo Park, CA, 1991, 91. 

Redox Couples. The model calculates the redox potential of the couples H2O2/O2, H2O/O2, Fe +/Fe , NO2/NO3, S /SO , and As " /As, given the requisite concentrations of the couple members. Dissolved oxygen is all that is required for calculation of both the H2O/O2 and H2O2/O2 couples. The H2O/O2 couple is kinetically inhibited and is grossly out of equilibrium except at elevated temperatures (80). Therefore, the option of using pE from dissolved oxygen for redox speciation has been dropped from the model. 

As alluded to above, input data for total iron, Fe(II) and/or Fe(III) are accepted by the model, with solute modeling calculations done using whatever data are input. If either Fe(II) or Fe(III) are present, Fe(total) is ignored if Fe(II) only is present, speciation is done among Fe(II) complexes only, and likewise for Fe(III). To accomplish this, the reactions of the iron section have been extensively rewritten (10) and a procedure, named SPLIT IRON, has been added, which performs the mass balance calculations separately for Fe(II) and Fe(III) when they are input separately. An E value is calculated from the computed activities of Fe " and Fe " and may, by user option, be used to distribute other redox species in lieu of an input E value. If only Fe(total) is input, the input E value is used to distribute all redox species including Fe " " and Fe " if there is only Fe(total) input, and no input E value, all Fe calculations are bypassed. 

A set of five programs known as The Geochemist s Workbench or GWB was developed by Bethke (1994) with a wide range of capabilities similar to EQ3/6 and PHREEQC v. 2. GWB performs speciation, mass transfer, reaction-path calculations, isotopic calculations, temperamre dependence for 0-300 °C, independent redox calculations, and sorption calculations. Several electrolyte databases are available including ion association with Debye-Huckel activity coefficients, the Pitzer formulation, the Harvie-M0ller-Weare formulation, and a... 

The strong affinity of iron oxides for Se(IV) has been well documented (Dzombak and Morel, 1990) and calculations based on the Dzombak and Morel (1990) diffuse double-layer model and default HFO database show the principal response to pH and redox speciation changes (Figure 11). The selenate species is less strongly adsorbed by iron oxides at near-neutral pH than the selenite species (Figure 11). Clay minerals (Bar-Yosef and Meek, 1987) also adsorb Se(IV). 

To summarize, an evaluation of the oxidation state of metals in an environment is central to determining their probaple fate and biological significance. Redox reactions can lead to orders of magnitude changes in the concentration of metals in various phases, and hence in their mode and rate of transport. While equilibrium calculations are a valuable tool for understanding the direction in which changes are likely to occur, field measurements of the concentrations of metals in their various oxidation states are always needed to evaluate metal speciation, since chemical equilibrium is often not attained in natural systems. 

Part of almost all homogeneous kinetic calculations will be some method to decouple the reactive species, which are often redox species. In kinetic calculations, the species are obviously not at equilibrium with each other, at least at the start of the calculation they approach equilibrium during the calculation. But speciation programs such as phreeqc and react assume all species to be at equilibrium unless told otherwise. In this case we want CH4 to not react with other species, partly to see what it is doing during the reaction, and partly because in nature it is extremely unreactive. We also want acetate to be decoupled because it is metastable and will not even exist in the solution at equilibrium. At the time of writing, this is not necessary in phreeqc, because the... 

---