Speciation models 5.7, Table

Chemical Speciation Models. Using the stability constants derived by us for copper complexes with hydroxo and carbonate ligands (Table I) and for natural organic ligands (Table II), the Newport and Neuse Rivers were modeled for copper speciation as a function of pH, total copper, carbonate alkalinity and total dissolved organic matter. Speciation models were calculated from the equation ... 

Figure 10.3. The predicted distribution of Al-containing species at 25 °C according to the Al-citrate speciation model of Ohman (1988) (Table 10.4) as a function of pH and reaction time in 0.6 mol L NaCl with Airs = citys = 0.01 mol L (modified from Ohman, 1988). The species are identified by stoichiometric coefficients according to Eq. (10.7).

The findings in this chapter bring into question the ability to predict the influence of citrate, if not other rhizosphere products, on the soil chemistry of metals. Clearly, additional studies are needed to validate an existing model or to establish an Fe(III)-or Al-citrate aqueous speciation model that may be applied to the soil environment. Equally problematic are thermodynamic data compilations relative to the Fe(III)-or Al-citrate species that are included. Chemical models, as defined by various studies in Tables 10.4 and 10.5, cannot be mixed. They are constructed specifically to describe a given series of chemical systems. Presumably, the number of species, their stoichiometric characteristics, and their... 

The prediction by our speciation model that in multiple oxides (as in quartz) Si sites are not susceptible to adsorption by H+ is confirmed by the results of surface titrations of albite, labradorite, and anorthite (Fig. 6). One can see that the net adsorption of H + at the feldspar solution interface increases markedly with the relative number of Al sites to Si sites, which is reflected by the stoichiometry of Al and Si in the different feldspars. These results help clarify the typical features of the dependence on pH of silicate dissolution (Table 2) ... 

The potentiometric data were interpreted with the formation of only two dinu-clear species, (2,2) = Th2(OH)j and (3,2) = Th2(OH)3. Other models were not tested however, the speciation model used is consistent with that of Hietanen and Sillen [1968HIE/SIL] in 3 M NaCl, which has been confirmed by test calculations in the review of Baes and Mesmer [1976BAE/MES] and selected in the present review. The complex (14,6) = Th (OH)[ which is also included in the selected model for chloride media is not relevant under the experimental conditions in [1981MIL2], [1981SUR/MIL], [1982SUR/MIL] (pH < 3.2), cf. speciation calculations in Appendix A review of [1968HIE/SIL]. The reported conditional equilibrium constants, with the statistical uncertainties given as 3ct are summarised in Table A-52. 

The apparent dissociation constants Kj(/9tO 7) which were compiled from the literature are given in Table 3.4 and they are presented together with the corresponding apparent Gibbs free energies AG =-R7 ln[Kj(/ 0 7)]. Similarly as with dissociation constants of citric acid in pnre water, differences in reported values of constants can be attributed not only to precision of experiments, but also to differences in applied speciation models, differences in mathematical form of equations used for activity coefficients and the choice of concentration scales (molar or molal). One aspect which was found to be extremely important was that earlier assumption that alkali metal cations do not form complexes with citrate anions is only partially correct. It was established that these cations form weak complexes with citrate anions and sometimes this interfering effect was even taken into account in calculation of Ki(/ 0 7) values [16, 21, 22]. 

Table 6. Mineral saturation constraints and organic ligand concentrations for ore-fluid speciation models...

Table 15-6 Model results for metal speciation in natural waters ...

If required by the model(s) to be used, back-up data for each entry in the matrix or table may be supplied to resolve the total mass flow into spatial cells (UTM coordinates, depth or height), temporal cells (hourly frequency distributions, diurnal cycles, seasonal subdivisions or secular trends on annual intervals) or speciation cells (by valency state of anions or by hydrocarbon structure, for example). The level of difficulty encountered by the user in supplying these data may influence the choice of model(s). 

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). 

In 2000 two major petrochemical companies installed process NMR systems on the feed streams to steam crackers in their production complexes where they provided feed forward stream characterization to the Spyro reactor models used to optimize the production processes. The analysis was comprised of PLS prediction of n-paraffins, /xo-paraffins, naphthenes, and aromatics calibrated to GC analysis (PINA) with speciation of C4-C10 for each of the hydrocarbon groups. Figure 10.22 shows typical NMR spectral variability for naphtha streams. Table 10.2 shows the PLS calibration performance obtained with cross validation for... 

Some results of application of theoretical models to trace metal speciation were presented in the section on metal-inorganic interactions (tables 2 and 3) a collection of papers dealing with chemical modelling in aqueous systems, including speciation, sorption, solubility and kinetics was edited by 3enne (1979). Recently Nordstrom and Ball (198 0 summarized 58 aqueous chemical equilibrium computer programs of which 19 were dealing with trace metals. 

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

A deeper perception of the mechanistic implications of equation (9.2) can be had if the rational activity coefficients are described on the molecular level using the methods of statistical mechanics. This approach is the analogue of the statistical mechanical theory of activity coefficients for species in aqueous solution (Sposito, 1983). Fundamental to it is the prescription of surface speciation and the dependence of the rational activity coefficient on surface characteristics. Three representative molecular models of adsorption following this paradigm are summarised in Table 9.8. Each has been applied with success to describe the surface reactions of soil colloids (Goldberg, 1992). 

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... 

The species distributions shown in Table 12.2 depict results from one of the earliest (Garrels and Thompson, 1962) and one of the most recent (Millero and Schreiber, 1982) seawater ion pairing models. These results and others (Kester, 1975a) are consistent with the following general characteristics of major ion speciation in seawater ... 

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