Solution speciation

To determine species in solution, calculations as a function of pH were carried out in the range from 2 to 12. The calcium concentration was in this case fixed at 0.5 mM. At this calcium concentration no calcite precipitation occurs over the entire pH range. 

The speciation of sodium is not quite as spectacular, with sodium being completely dissolved as monovalent Na. However, at pH 10, sodium begins to be present in the form of the anionic ion pair 

For calcium the transition from divalent Ca-+ to neutral CaCOs (aq) occurs above pH 8, thus below the pH value of 10 used in the experiments. 

These effects affect rejection of sodium and calcium as a function of pH and need to be considered in the interpretation of results. 

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A number of zinc halide complexes, including bridging halides, have already been mentioned in the context of the other ligands systems discussed in this Chapter. Examples will be presented of catalytic systems and solution speciation, particularly in the presence of other coordinating ligands. There are now also a few zinc fluoride species that have been well characterized. 

SOLUTION SPECIATION OF TRACE ELEMENTS IN ARID ZONES... 

Since carbonate and high pH are unique characteristics of arid and semi-arid soils, we will first examine solution speciation and the equilibrium reactions of the C02-H20 system. We will then examine the solution speciation of Ca and Mg, Zn, Cu, Ni, Cd, Pb, Cr(III) and Cr(IV), Hg, and Se. 

In addition to soil solution, speciation of trace elements in water of the Nahr-Ibrahim river valley of Lebanon was studied with the AQUACHEM model. The results indicate that a high percentage of Pb and Zn is present as carbonate species, but in low percentages in free hydrated ion species. Cadmium exhibits as a high percentage of a free hydrated Cd2+. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

Figure 3.4. Effects of phosphate levels on Cd and Zn solution speciation in California soils that received sludge application (data extracted from Villarroel et al., 1993)...

Figure 3.5. Effects of Cl concentrations (NaCl or CaCl2) on Cd solution speciation in a California soil (data extracted from Bingham et al., 1983)...

Candelaria L.M., Chang A.C. Cadmium activities, solution speciation, and solid phase distribution of Cd in cadmium nitrate and sewage sludge-treated soil systems. Soil Sci 1997 162 (10) 722-732. 

Power et al. (2005) show the effeet of pH and initial As(III) coneentration on the kineties of arsenite oxidation at bimessite-water interfaees, when a competitive metal (e.g., Zn) is present in an adsorbed or nonadsorbed state (Fig. 16.5). Two well-defined trends in the As(III) oxidation reactions can be distinguished (1) the extent of As(III) oxidation decreases with increasing pH from 4.5 to 6.0 and (2) oxidation on a percent basis is suppressed with increasing initial As(III) concentration from 100 to 300 dM. The pH effects on As(III) oxidation may have been influenced by competitive adsorption reactions between As(III) and reaction products (e.g., Mn(II)) and were not influenced by arsenic solution speciation. The suppressed As(III) oxidation rate constant may be a result of differences in the amount of Mn(II) release, which compete with dissolved As(III) species for unreacted Mn(IV) surface sites, and of Mn(II) adsorption, which inhibit the reaction between As(III) and Mn(IV) surface sites. 

As already stated, speciation is the characteristic distribution of various ionic and/or neutral species in an aqueous solution. Speciation calculation, allowing practical estimation of the reactive properties of an aqueous solution, acidity, redox state, the degree of saturation of the various solids, and so on, is carried out on a thermodynamic basis starting from the chemical composition of the solution of interest and using the reaction constants of the various equilibria of the type seen in equation 8.19. 

Examples of reaction rates for different metals are given in Tables 9.5 and 9.6. Reaction rates that are extremely fast (>107s 1) or very slow (<10 8s 1) will not affect assumptions concerning solution equilibrium. However, caution is required in the application of chemical thermodynamics to reactions with intermediate rates (Sposito, 1986 1989). The importance of kinetics in solution speciation depends on the time frame of the experiment or application. Solution reactions that take days to come to equilibrium will tend to have a minor impact on conclusions or predictions concerning long-term behaviour (e.g. soil formation), but could have important implications for short-term situations, such as the growth of an annual pasture or storm water runoff. 

The seawater chemistries of Mn, Fe, Co, Ni, Cu and Zn are, in many respects, quite diverse. One characteristic that these elements have in common, however, is an accessible +11 oxidation state. Except in the case of iron, which exists dominantly as Fem in seawater, the solution speciation of these elements is dominated by the +11 oxidation state. The aspect of these elements seawater speciation which most distinguishes them from other cations in the Periodic Table is their substantial involvement in organic complexation. 

Model Calculation of Predominant Solution Speciation of Fluoride in Soil Water Extracts for 0-15 cm (A) and 15-30 cm (B) Depths from Processing Plant Transect... 

From the evolution of the two 27A1 NMR signals with time and temperature, the authors claim a solution speciation first with primary monophosphate VIA1 complexes units (Fig. 10(a)) which loose one water molecule at higher temperature, giving a fivefold coordination to Al. Then prenucleation clusters form in the solution by dimerization of these 5-coordinated complexes (Fig. 10(b)) which will be involved in the nucleation and crystallization steps. 

Use of solute speciation models to help predict metal availability to organisms in natural systems has not been attempted to date. Statistical comparisons of calculated Cu activities in various natural waters with concentrations of Cu in resident phytoplankton populations would provide a relatively simple test of the laboratory-based models of Cu availability. For organisms other than phytoplankton, however, the problem of modeling the bioavailability of metals in natural systems will be more complex. 

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