Aqueous phase, speciation

Figure 5.1 shows the modelled sorption curve together with the experimental sorption data of phyllite. It is noteworthy that the calculated sorption curve of the phyllite is based exclusively on surface complex formation and surface acidity constants obtained from individual batch experiments with pure mineral phases. No experimental data of phyllite were used for the optimization procedure. The modelling of the associated aqueous phase speciation of uranium(VI) was based on the recommended NEA data set (Grenthe et al., 1992). 

To model the adsorption of copper cyanide complexes, the multicomponent Langmuir isotherm can be modified to correlate the aqueous phase speciation and the basicity of the ICAA. The available protonated sites on the ligands attached to the surface also can be assumed to be a function of pH. 

Due to the complexity of the precipitation process, the saturation index (SI) is calculated to estimate the calcium carbonate precipitation in water, and is used to describe the saturation state (from a thermodynamic point of view) of the aqueous phase composition versus different solids. It is widely used to estimate the potential precipitation of different solids from an equilibrated aqueous phase speciation. 

Among the above-defined thermodynamic entities, the individual ionic activity coefficients are particularly useful, because they allow practical calculation of the speciation state of an aqueous phase, linking individual ionic molalities to the energy balance. We will see in the following section how these coefficients may be derived. 

In addition, a model is needed that can describe the nonideality of a system containing molecular and ionic species. Freguia and Rochelle adopted the model developed by Chen et al. [AIChE J., 25, 820 (1979)] and later modified by Mock et al. [AIChE J., 32, 1655 (1986)] for mixed-electrolyte systems. The combination of the speciation set of reactions [Eqs. (14-74a) to (14-74e) and the nonideality model is capable of representing the solubility data, such as presented in Figs. 14-1 and 14-2, to good accuracy. In addition, the model accurately and correctly represents the actual species present in the aqueous phase, which is important for faithful description of the chemical kinetics and species mass transfer across the interface. Finally, the thermodynamic model facilitates accurate modeling of the heat effects, such as those discussed in Example 6. 

Hutton, C., Bryce, D.W., Russeau, W. et al. (2005) Aqueous and solid-phase speciation of arsenic in Cornish soils. Mineralogical Magazine, 69(5), 577-89. 

Liang, L., M. Horvat, and N.S. Bloom. 1993. An improved speciation method for mercury by GC/CVAFS after aqueous phase ethylation and room temperature precollection. Talanta 41 371-379. 

The formal similarity between adsorption and complexation reactions can be exploited to incorporate adsorbed species into the equilibrium speciation calculations described in Sections 2.4 and 3.1. To do this, a choice of adsorbent species components (SR r in Eq. 4.3) must be made and equilibrium constants for reactions with aqueous ions must be available. A model for computing adsorbed species activity coefficients must also be selected.8 Once these choices are made and the thermodynamic data are compiled, a speciation calculation proceeds by adding adsorbent species and adsorbed species (SR Mp(OH)yHxLq in Eq. 4.3) to the mole-balance equations for metals and ligands, and then following the steps described in Section 2.4 for aqueous species. For compatibility of the units of concentration, njw) in Eq. 4.2 is converted to an aqueous-phase concentration through division by the volume of aqueous solution. 

Diffuse layer metal retention and outer sphere complex formation involve electrostatic attractive forces, which are characteristically weaker than co-ordinative interactions leading to inner sphere surface complex formation. A number of factors influence metal interactions with surfaces, including the chemical composition of the surface, surface charge, and the nature and speciation of the metal ion. The importance of the pH of the aqueous phase in these interactions will be discussed further in Section 3.2.4.1. 

Phase speciation. The distribution of components among two or more phases. In a wet soil, iron can be present in at least three phases as the X-ray amorphous oxyhydroxide, ferrihydrite as goethite and as dissolved aqueous iron. 

It influences the distribution of substances between the aqueous phase and particulate matter, which, in turn, affects their transport through the various reservoirs of the earth. The affinity of the solutes to the surfaces of the conveyor belt of the settling inorganic and biotic particles in the ocean (and in lakes) regulates their (relative) residence time, their residual concentrations, and their ultimate fate. Adsorption has a pronounced effect on the speciation of aquatic constituents. 

In soils and sediments, the speciation in the dissolved aqueous phase is primarily important to assess bioavailability and toxicity. The total concentration (adsorbed and solid phase) gives an idea of the capacity of a reactive element, which could under certain circumstances (e.g., acidification, complex formation) be mobilized. 

Natural particles suspended in the air can be transported to regions far from their sources. This is important for transporting many metals and metalloids in the ecosystem. A few metals and metalloids, most notably Hg, As, and Se, can exist not only in the solid and liquid phases but also as gases in ambient environments. The loss of Hg from the aqueous phase can result from reduction of Hg " " to Hg and alkylation to form methyl- or dimethylmercury. Through microbial activity, the methylated forms can be converted to Hg, which is more volatile and less toxic. Microbial mediation can also transform several other trace elements (e.g., As, Se) to organometallic compounds (Gadd, 1993). These volatile organometallic compounds can dominate the transport of these trace elements in local environments. However, bacterial mediation of alkylation of metals such as Hg is influenced substantially by Hg speciation. Mineral colloids vary in their ability to affect the bioavailability and methylation of Hg(II) in aqueous systems... 

While Davis and his colleagues illustrated the significance of soil metal speciation in risk assessment, Morrison et al. (1989) pointed out that the toxicity of metals is related to the forms in which they exist in the aqueous phase. This is because the interaction of metals with intracellular compartments is highly dependent on chemical speciation. Some species may be able to bind chemically with extracellular proteins and other biological molecules, some may adsorb onto cell walls, and others may diffuse through cell membranes. Consequently, toxicity is more related to the concentrations of metals in a particular species, than to the total concentrations. Geochemical modeling... 

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. 

Aqueous speciation is normalized to aqueous phase only. ftNo other aqueous Be species available in the database. 

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