Speciation models

Hamilton critically reviewed the literature on aqueous polysulfide solutions and proposed a speciation model of his own [13]. 

Therefore, the pH values of these solutions are between 11 and 12. The speciation model used by 8chwarzenbach and Fischer is certainly too simple but these authors have been the first to demonstrate the strong dependence of the polysulfide anion distribution on the alkalinity. According to Eqs. (26)-(28) higher pH values in dilute solutions will favor smaller anion sizes. 

Twiss, M., Errecalde, O., Fortin, C., Campbell, P., Jumarie, C., Denizeau, F., Berkelaar, E., Hale, B., and van Rees, K., Coupling the use of computer chemical speciation models and culture techniques in laboratory investigations of trace metal toxicity, Chem Spec Bioavailab, 13 (1), 9-24, 2001. 

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. 

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. 

Turner, D. R. (1995). Problems in trace metal speciation modelling. In Metal Speciation and Bioavailability in Aquatic Systems, eds. Tessier, A. and Turner, D. R., Vol. 3, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems, Series eds. Buffle, J. and van Leeuwen, H. P., John Wiley Sons Ltd, Chichester. 

MINTEQA2 http //www.epa.gov/ceampubl/mmedia/minteq/index.htm MINTEQA2 is an equilibrium speciation model that can be used to calculate the equilibrium composition of dilute aqueous solutions in the laboratory or in natural aqueous systems. The model is useful for calculating the equilibrium mass distribution among dissolved species, adsorbed species, and multiple solid phases under a variety of conditions including a gas phase with constant partial pressures. 

We present here the initial modeling efforts and experimental determinations of boron speciation in solution with a reactive polymer. Also, the speciation model is applied to the ultrafiltration process and compared with data obtained from PAUF of a synthetic boron-contaminated feed. 

Note that the assumption of Henry s law will lead to incorrect results for solubility of chemical systems such as C02-MEA (Figs. 14-1 and 14-2) and HCl-H20. Solubility modeling for chemical systems requires the use oi speciation model, as described later in this section. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Zelano, V., Daniele, P. G, Berto, S., Ginepro, M., Laurenti, E., and Prenesti, E. (2006). Metal ion distribution between water and river sediment Speciation model and spectroscopic validation. Anal. Chim. 96(1-2), 1-11. 

Brown, D.S. and Allison, J.D. (1987) MINTEQA 1, an Equilibrium Metal Speciation Model User s Manual., EPA/600/3-87/012.U.S. Environmental Protection Agency, Athens, Georgia. 

Most speciation modelling is based on the assumption of thermodynamic equilibrium between phases, so it is necessary to describe the various equations that are used to quantify these chemical reactions. 

The elegance of the surface complexation approch lies in the fact that it can be incorporated into the thermodynamic speciation models used for soluble complexes. Consequently many of the computer models, e.g. SOILCHEM, HYDRAQL, MINTEQA2 and ECOSAT, include several different SCMs. Some commonly used SCMs are the diffuse-double-layer model, DDLM (Huang and Stumm, 1973 Dzombak and Morel, 1990), the constant capacitance model, CCM (Stumm et al., 1970 1976 1980 Schindler et al., 1976), the triple-layer model, TLM (Davis etal., 1978 Davis and Leckie, 1978,1980 Hayes and Leckie, 1987 Hayes et al., 1988) and the 1 pK basic Stern model (Bolt and Van Riemsdijk, 1982 Van Riemsdijk et al., 1986 1987). 

Castilho, P. Del and Rix, I. (1993) Ammonium acetate extraction for soil heavy metal speciation, model aided soil test interpretation. Int.f. Environ. Anal. Chem., 51, 59-61. 

Zhang, H. 2004. In-situ speciation of Ni andZn in freshwaters Comparison between DGT measurements and speciation models. Environ. Sci. Technol. 38 1421-1427. 

Toxic Metal Speciation Models for Soil and Sediment.51... 

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