Classical speciation

Classical speciation of radionuclides is described in Chapter 13. Methodology for single and sequential extraction of soil to assess radionuclide availability to plants is similar to that used for heavy metals, and has recently been reviewed (Kennedy et al., 1997). Therefore, only recent applications of sequential extraction to speci-ate both natural and anthropogenic nuclides are discussed below. 

The approach usually adopted in geochemistry to describe the speciation state of aqueous solutions is that of ionic coupling-complexation, based on the classical model developed by Garrels and Thompson (1962) for seawater. This model con-... 

Regarding the liquid/liquid extraction from the metal standpoint is rather different. This is the classical approach of coordination chemistry (most of the publications in this area). Today, it is still difficult to establish a direct link between the two descriptions of the organic extractant phases. To better understand liquid/ liquid extraction, the aggregation number and coordination number must be measured separately for each system and set of initial conditions. This is the only way to determine the role of the aggregates in the extraction efficiency. This important point was emphasized by Yaita et al. (61). In this way, Gannaz et al. has used an approach combining studies on both supramolecular and molecular speciation of extractant systems of the DIAMEX-SANEX process (36). 

Because of these distinct advantages over classical extractive sample preparation techniques, it is easy to understand why SPME has become one of the most popular sample preparation techniques for water analysis. Numerous monographs and reviews document this great popularity and discuss in detail the theory and practice of SPME,102 103 as well as applications in the field of environmental analysis104-105 and speciation analytics.106-107... 

As was stressed above, in case of direct excitation of the lanthanides, the differences in the absorption spectra between the solvated ion and the complex formed are usually considered as slight but allow nevertheless the use of spectrophotometry for speciation purposes (for a recent example, see Giroux et al. (2000)). In the case of U(VI), the absorption changes observed upon complexation have often been used to determine equilibrium constants (for the classical example of U(VI) hydrolysed species, see Dai et al. (1998), Meinrath (1998)). For curium, no information of this kind is available the low molar absorption coefficient implies the use of solutions with high total curium concentrations, which are almost impossible to handle nowadays due to safety reasons. The absorption spectra of curium are known only for aqueous solutions of HCIO4 (Carnall et al., 1958), HC1, H2SO4 and HNO3 (Pascal, 1962). 

One of the classic methods for speciation of metals in soils was developed by Tessier et al. [125], and this method is still substantially in use, although several modifications of this method have also been published. Again, it is important to stress that even small modifications of the methods used can have substantial effects on the data obtained. 

Wilkins, R. G., Kinetics and Mechanism of Reactions of Transition Metal Complexes, VCH Publishers, New York, 1991. This classic textbook, by one of the key figures in the study of complexation reactions, offers a wealth of detail on the experimental aspects of aqueous speciation. 

According to International Union of Pure and Applied Chemistry (IUPAC), the terms speciation and chemical species should be reserved for the forms of an element defined as to isotopic composition, electronic or oxidation state and/or complex or molecular structure (Templeton el al, 2000). This classical definition, appropriate to speciation in solution samples, would exclude most speciation studies on solid materials, such as soils and sediments, more properly defined as fractionation studies. The terminology used in this chapter is based on the broader definition of speciation given by Ure and Davidson (2002), which encompass the IUPAC s narrow definition and includes the selective extraction and fractionation techniques of solid samples. 

Classical wet chemical extraction procedures have also been used to assess the solid-phase speciation of arsenic, but care must be taken not to oxidize As(III) during extraction (Demesmay and Olle, 1997). Extractions should be carried out in the dark to minimize photochemical oxidation. 

As mentioned earlier, complex formation reactions at hydrous metal oxide surfaces can be treated as an extension of classic coordination chemistry metal centers on mineral surfaces participate in inner-sphere and outer-sphere coordination reactions with molecules adsorbed from overlying solution, including H2O, OH , O, and solute molecules (Schindler, 1981 Schindler and Stumm, 1987). A variety of protonation/deprotonation and complex-formation reactions determine the speciation of surface sites. A few... 

Classical chemistry literature provides comprehensive lists of rate data that can also be applied for predicting abiotic reaction rates of environmental importance when the pathways are well identified. For some reactions, such as hydrolysis or OH radical reactions, enough data are available to determine siructure-reactivity correlations that enable one to interpolate within series of chemically related compounds of known reactivity [for review, see Brezonik (Chapter 4, this volume) and Lyman et al. (1982)]. The aquatic chemist is, however, more often confronted with the fact that many classical studies have been performed in organic solvents, but that the speciation of many dissolved chemicals in aquatic systems may vary with pH (caboxylic acids, phenols, etc.) and with ligand concentrations (e.g., all dissolved heavy metal species) or that the chemical species may be adsorbed on surfaces or absorbed by colloidal organic materials. In all these cases, reaction-rate constants must be determined for each individual aqueous species contributing to the overall kinetics. The equilibrium distribution of these species must also be accounted for. Reductions of the... 

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