Modeling geochemical speciation

Modeling geochemical speciation and dissolution/precipitation pH-stat leaching experiments MINTEQA2 Identification of controlling processes for pH-stat leaching behavior of contaminants [6]... 

Parkhurst, D. L., 1995, User s guide to PHREEQC, a computer model for speciation, reaction-path, advective-transport and inverse geochemical calculations. US Geological Survey Water-Resources Investigations Report 95-4227. 

Geochemical speciation modelling indicated saturation with respect to gypsum and several carbonates, slight under-saturation with respect to calcium arsenate (Ca3[As04]2) and ferrihydrite. 

Application of geochemical speciation mass-transfer models... 

Allison, J. D., and Brown, D. S. (1995). M1NTEQA2/PRODEFA2 a geochemical speciation model and interactive preprocessor. In Chemical Equilibrium and Reaction Models, ed. Loeppert, R. H., Schwab, A. P., and Goldberg, S., Soil Science Society of America, Madison, WI, 241-252. 

Examples showing that metal speciation is important to metal toxicity include arsenic, copper, selenium, and chromium. While ionic copper (Cu2+) and CuClj are highly toxic, Q1CO3 and Cu-EDTA have low toxicity (Morrison et al, 1989). Toxicity tests show that As(III) is about 50 times more toxic than As(VI). Trivalent chromium is much less toxic than hexavalent chromium, probably because Cr(VI) is much smaller and the chemical structure of chromate is similar to sulfate. A special channel already exists in biomembranes for sulfate transport. While modeling metal speciation is not always possible, and redox equilibrium is not achieved in all natural waters, geochemical modeling of equilibrium species distribution remains one of the methods of discerning metal speciation. 

In the second step of the modeling exercise, speciation and surface adsorption in the neutralized water were modeled using the geochemical code minteqa2. Analytical concentrations of Cd, Ni, Be, and U (5.5, 65,4.1, and 22 x 10-6 molL-1, respectively) were used as input concentrations. The surface properties and surface complexation constants were taken from Dzombak and Morel (1990). Due to the lack of experimental data for A1 hydroxides, we assume that all A1 hydroxides have the same sorptive properties as Fe hydroxide and that the mass of A1 hydroxide was added to the Fe hydroxide concentrations. The total sorbent concentration is 5.3 g/L as Fe(OH)3, which is calculated from the first step. minteqa2 was used to calculate the partitioning of these ions between the aqueous phase and ferric iron hydrous oxide (HFO) surfaces. 

Allison, J.D. Brown, D.S. (1995) MINTEQA2/PROGEGA - A geochemical speciation model and interactive processor. In R.H. Loeppert etal. (ed.), Chemical equilibria and reaction models. Soil Set Soc. Am. Spec. Pub. Madison, WI. 241-252. 

MinteqA2 is a geochemical speciation code which was released by the U.S. EPA. Version 3.11 was used in this study. The software is available on the internet from the U.S. EPA Center for Exposure Assessment and Modeling (CEAM) as are several manuals (Allison etal. (1991)). 

Geochemical speciation SEM/EDX and XRD mineralogical composition of the unleached material SOLTEQ-B (a modified version of MINTEQA2 that includes Berner s model of CSH dissolution) Prediction of leaching from cement-stabilized waste [3]... 

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. 

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