Speciation and Solubility Modeling

The application of speciation-solubility in geochemistry goes back to Garrels and Thompson (1962), who calculated the aqueous speciation in seawater and saturation states with respect to mineral solubilities. Since then, this subject has been treated extensively, and hundreds of codes are available for this kind of calculation. In principle, the concentration and activity coefficients of all aqueous species can be calculated if there are the same number of equations as unknowns. Given an equilibrium constant for each complex, plus an analysis giving the total quantity of each basis species, this is usually possible. 

The interpretation of calculated mineral Saturation Indices is not simple, and is discussed in 6.2.7. 

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The predicted waste inventory for the repository indicates that potentially significant quantities of the organic ligands—acetate, citrate, oxalate, and EDTA—will be present (US DOE, 1996). Actinide interactions with these compounds were not considered in the speciation and solubility modeling, as calculations suggested that they would be mostly complexed by transition metal ions (Fe, Ni " ", Cr, and Mn " ") released by corrosion of the steel waste containers and waste components. A thermodynamic model of actinide-ligand interactions appropriate to brines will be included in solubility calculations for WIPP recertification. 

Besides the accuracy and completeness of the thermodynamic data, a model is also produced with certain input constraints, e.g., analytical concentrations of groundwater samples. This means that the accuracy and completeness of analytical data also influence a model. For example, if the water is predominantly sulfate-rich and the analysis failed to analyze sulfate, then speciation and solubility models cannot be correct because a major component was missed in analysis. This shortcoming has nothing to do with the code or program itself. 

Programs typically give the modeler various choices in the input file, e.g., choices relating to redox states. Different choices may result in different speciation and solubility models. 

The speciation and solubility modeling of six water analyses gives us ... 

Kirpichtchikova, T. A., Manceau, A., Spadini, L., Panfili, F., Marcus, M. A., andjacquet, T. (2006). Speciation and solubility of heavy metals in contaminated soil using X-ray microfluorescence, EXAFS spectroscopy, chemical extraction, and thermodynamic modeling. Geochim. Cosmochim. Acta 70, 2163-2190. 

Performance assessment calculations of actinide speciation and solubility, and of the potential releases that could result if the repository is breached, were carried out as part of the CCA) for the waste isolation pilot plant (WIPP) (US DOE, 1996 US EPA, 1998a,b,c,d). The calculations modeled actinide behavior in a reference Salado brine and a less magnesium-rich brine from the Castile Formation as described previously (see Tables 6 and 8). The performance assessment calculations will be periodically repeated with updated parameter sets as part of site recertification. 

An analysis of the waste pond water was used for calculating speciation and solubility of the injectate (Table 8.4). The modeling results show supersaturation of calcite and dolomite (and other carbonates), as well as iron oxides, quartz, and a number of unlikely silicates. 

Wanner, H. 1985. Modelling radionuclide speciation and solubility limits in the near-field of a deep repository. Scientific Basis for Nuclear Waste Management, IX, 509-516. 

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. 

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. 

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