GEOCHEM, computer

Kulik, D. A. 2002. Minimising uncertainty induced by temperature extrapolations of thermodynamic data A pragmatic view on the integration of thermodynamic databases into geochemical computer codes. Proceedings of the Workshop on The Use of Thermodynamic Databases in Performance Assessment , 29-30 May 2001, Barcelona, Spain. Organisation for Economic Cooperation and Development OECD, Paris, France, 125-137. 

Once formed, H2S° would partially dissociate into HS- and H+. A small amount of H2As03 and H+ would form from the dissociation of H3As03°. Some H3As03° and H2S° could also react to produce thioarsenic species, such as AsS(OH)HS (see also Section 2.7.3). Depending on the accuracy and completeness of their thermodynamic databases, geochemical computer models may be able to identify the major reactions and estimate the activities of their products. 

Additional information on adsorption mechanisms and models is in Stollenwerk (2003), 93-99 and Prasad (1994). Foster (2003) also discusses in considerable detail how As(III) and As(V) may adsorb and coordinate on the surfaces of various iron, aluminum, and manganese (oxy)(hydr)oxides. In adsorption studies, relevant laboratory parameters include arsenic and adsorbent concentrations, adsorbent chemistry and surface area, surface site densities, and the equilibrium constants of the relevant reactions (Stollenwerk, 2003), 95. Once laboratory data are available, MINTEQA2 (Allison, Brown and Novo-Gradac, 1991), PHREEQC (Parkhurst and Appelo, 1999), and other geochemical computer programs may be used to derive the adsorption models. 

Mitzman, S. (1999) The application of PHREEQCi, a geochemical computer program, to aid in the management of a wastewater treatment wetland. Master s Thesis, Texas A M University, College Station, p. 163. 

This chapter has discussed the fundamental importance of the master variables ps and pH in determining species distribution under the conditions prevailing in natural systems (pe —10 to - - 17 and pH 4 to 10). The equilibria contained in Sections 3.2.2 3.2.4 also provide the basis for many of the currently available geochemical computer models (e.g. WATEQ4F, PHREEQC, and MINTEQA2 ), which are being... 

Overall, geochemical computer models can be extremely useful in the description of chemical equilibria occurring in the aquatic environment. In some cases, predictions about reaction kinetics and transport of species can also be made. The application of geochemical models is not limited to natural aquatic systems but has been usefully extended to predict the eflfectiveness of certain remediation strategies in the treatment of waters emanating from contaminated sites." ... 

Charlton S. R., Macklin C. L., and Parkhurst D. L. (1997) PHREEQCI—A Graphical User Interface for the Geochemical Computer Program PHREEQC. US Geol. Surv. Water-Resour. Invest. Report 97-4222, 9pp. 

Bradbury, M. H., and Baeyens, B. (1994) Sorption by Cation Exchange Incorporation of a Cation Exchange Model into Geochemical Computer Codes. Bericht No. 94-07, Paul Scherrer Institute, Wiirenlingen. 

Selecting the least components (also called master species) is one of the fundamental and essential input decisions made in geochemical computer codes such as PHREEQE (Parkhurst et al. 1990), WATEQF (Ball and Nordstrom 1991), and M1NTEQA2 (Allison et al. 1991), for example. 

In studies of the state of saturation of minerals in natural waters and in most of the geochemical computer codes, the saturation index (SI) is used. The index is defined as 5/= log,o((2/ eq). so that 5/ = 0 at equilibrium (at saturation) of the mineral with the solution. The saturation index and AG, are related through SI = AG,./(2.3026 RT). If the reaction is written with the mineral as the reactant, then when SI and AG, are both negative, the mineral is undersaturated and so will tend to dissolve. When both are positive, the mineral is supersaturated and will tend to precipitate from solution. 

Calculate the volume-corrected titration curve for the titration of 0.01 N HCl with 0.01 N NaOH between pH 2 and 12 using a geochemical computer code and compare the result to the corresponding curve in Fig. 5.5. This problem can be solved using the mixing option in SOLMINEQ.88 (Kharaka et al. 1988). The titration is simulated by mixing different fractions of solution 1 (pH = 12.0, Na" = 0.01 mol/kg) with solution 2 (pH = 2.0, Cr = 0.01 mol/kg). In the output Cg = Na". Computed results are given here. 

Geochemical computer modeling and the abundance of limestone indicates saturation of the groundwater with respect to anhydrite and calcite. Calcium, sulfate alkalinity, and TDS... 

So far we have used phase diagrams to visualize clay mineral stabilities and phase relations involving the clays in natural waters. Given the complex chemistries of mixed-layer clays in particular, geochemical computer codes offer a more rigorous way to evaluate their stabilities. The thermodynamic data bases of most of these codes list stability constants for a variety of clay minerals which, except for kaolinite, are usually of nonideal composition. Most of these stability constants have been obtained from solubility measurements and are of mixed reliability. It is appropriate to... 

PHREEQE A geochemical computer code based on PC (w/PHRQ- the ion-pairing model which calculates INl T and pH, redox potential and mass transfer. 

The other approach employs a geochemical computer model, such as PHREEQC (Parkhurst 1995 also Chap. 15) with an input of a complete seawater analysis. Such a model will then calculate the activity coefficients and the species distribution of the solution according to the complete analysis and the constants of the thermodynamic database used. These constants are well known with an accuracy which is usually better than the accuracy of most of our analyses at least for the major aquatic species. Together with the real constant of the solubility product a reliable saturation index (SI = log Q) is then calculated. The constants of solubility products are not accurately known for some minerals, but for calcite, and also for most other carbonates, these constants and their dependence on temperature and pressure are very well documented. 

The geochemical computer models, EQ3/6 (Wolery 1978), SOLMINEQ.88 (Kharaka et al. 1988), PATHARC.94 (Perkins and Gtmter 1995) and TOUGHREACT (Xu et al. 2000, 2004) have been used to model water-rock reactions driven by the formation of carbonic acid when waste CO2 is injected into deep aquifers using experimentally determined rate data (e.g., Gunter and Perkins 1993). These calculations have shown that carbonate aquifers are limited in the quantity of CO2 that can be trapped, while siliciclastic aquifers have superior potential for trapping CO2 through the precipitation of carbonates, particularly,... 

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