Speciation-solubility Models

Van den Berg, C. M. G., and J. R. Kramer (1979), "Conditional Stability Constants for Copper Ions with Ligands in Natural Waters", in E. Jenne, Ed., On Chemical Modeling Speciation, Sorption, Solubility and Kinetics in Aqueous Systems, ACS Symp. Series. 

Morse J.W. and Berner R.A. (1979) The chemistry of calcium carbonate in the deep oceans. In Chemical Modelling—Speciation, Sorption, Solubility, and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 499-535. Amer. Chem. Soc., Washington, D.C. 

Plummer. L.N. and Wigley, T.N.L. Critical review of the kinetics of calcite dissolution and precipitation, in Jenne, E.A., ed., "Chemical Modeling. Speciation, Sorption, Solubility and Kinetics in Aqueous Systems," Amer. Chem. Soc. Symp. Series, Washington, D.C. (this volume), de Kanel, J. and Morse, J.W. The chemistry of orthophosphate uptake from seawater onto calcite and aragonite, Geochim. Cosmochim. Acta 1335-1340 (1978). 

Most applications in the regulatory environment have used speciation-solubility models. A few used surface complexation models. Applications of surface complexa-tion models mostly used the model and data from Dzombak and Morel (1990). Reaction path calculations are mostly limited to the titration and mixing calculations of two fluids. 

In general, geochemical models can be divided according to their levels of complexity (Figure 2.3). Speciation-solubility models contain no spatial or temporal information and are sometimes called zero-dimension models. Reaction path models simulate the successive reaction steps of a system in response to the mass or energy flux. Some temporal information is included in terms of reaction progress, f, but no spatial information is contained. Coupled reactive mass transport models contain both temporal and spatial information about chemical reactions, a complexity that is desired for environmental applications, but these models are complex and expensive to use. 

Speciation-solubility geochemical models can answer three questions, given concentrations of constituents measured analytically in a system at the temperature and pressure of interests ... 

Speciation-solubility models deal with a closed, static, batch or beaker-type system. However, speciation-solubility models also serve as the basis for the reaction path and reactive transport models discussed below. Equilibrium calculations are also useful to evaluate kinetic rates as a function of the deviation from equilibrium. Direct applications of speciation-solubility models include assessment of bioavailability because the toxicity of some contaminants (e.g., chromium and arsenic) varies drastically for different species. 

Two types of algorithm are used in speciation-solubility models those that use equilibrium constants and those that use free energy minimization. Most geochemical models we described in the text belong to the first type. Interested readers are referred to Anderson and Crerar (1993) for a more detailed discussion. 

Reaction path models calculate a sequence of equilibrium states involving incremental or step-wise mass transfer between the phases within a system, or incremental addition or subtraction of a reactant from the system, possibly accompanied by an increase or decrease of temperature and pressure (Helgeson, 1968, 1969). The calculated mass transfer is based on the principles of mass balance and thermodynamic equilibrium. Unlike speciation-solubility calculations, which deal with the equilibrium state of a system, the reaction path model simulates processes, in which the masses of the phases play a role. 

In this chapter we shall limit our discussion of the codes to speciation-solubility and reaction path modeling codes. Coupled reactive mass transport codes are much more mathematically and computationally complex. The readers can find recent discussions in Lichtner (1996) and Steefel and MacQuarrie (1996). 

The mathematical formulation of equilibrium speciation-solubility models and the numerical methods to solve the algebraic equations are described elsewhere, and we shall not repeat them here. The reader can consult Anderson and Crerar (1993), Bethke (1996), and Westall et al. (1976). Mathematical formulations for the reaction path models are described in detail in Anderson and Crerar (1993) and Bethke (1996). 

For instance, V.N. Ozyabkin (1995) subdivides them depending on the complexity and dimension of the forecast object. J.Rubin (1983), W. Kinzelbach (1992), S.R. Kraynov et al. (2004) base the classification on phase uniformity of the medium and velocity of chemical processes. In this connection they distinguish thermodynamical, transport and kinetic models. In Europe and the USA are broadly used classifications based on typization of local problems (Chen Zhu, Anderson G., 2002, Bethke C. M., 2008, etc.). In connection with this all hydrogeochemical models are subdivided there into three groups speciation-solubility models or batch models, reaction path modelsor mass transfer models reactive transport models or couplet mass transport models. In the second group of this classification is non-uniquely identified the role of mass transfer kinetics. 

Figure 6.3a shows the plot of log S versus pH of an ampholyte (ciprofloxacin, pKa values 8.62 and 6.16, log So — 3.72 [pION]). In Figs. 6.1b, 6.2b, and 6.3b are the log-log speciation profiles, analogous to those shown in Figs. 4.2b, 4.3b, and 4.4b. Note the discontinuities shown for the solubility speciation curves. These are the transition points between a solution containing some precipitate and a solution where the sample is completely dissolved. These log-log solubility curves are important components of the absorption model described in Section 2.1 and illustrated in Fig. 2.2. 

Langmuir, D., Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symposium, American Chemical Society, Washington, DC, 1979, pp. 353-387. 

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. 

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. 

Atlantis II Deep (Red Sea) and the uptake of amino acids by synthetic P-FeOOH Cln. Geochim. Cosmochim. Acta 47 1465-1470 Holm,T.R., Anderson, M.A., Iverson, D.G. Stanforth, R.S. (1979) Heterogeneous interaction of arsenic in aquatic systems. ACS Symposium Ser. Chemical modelling of aqueous systems Speciation, sorption, solubility, kinetics. 711-736... 

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