Functionally defined speciation

Functionally defined speciation. Functionally defined species are exemplified by the plant-available species or chemical pools in which the function is plant availability. Available forms of trace metal cations are not necessarily associated with one particular chemical species or a specific soil component. Hence, to predict the availability of trace metals, we either have to establish the species involved and develop methods that specifically determine those forms only, or we have to establish an empirical relationship between an accepted diagnostic measure of the metal and plant growth. Both speciation in solution and fractionation of the solid phase to identify the chemical pools can affect plant uptake (phytoavailability) of trace metals and water pollution. 

Development of chemical speciation schemes which can be directly related to measures of bioavailability - This would allow the determination of which active trace element species merit the most intensive research from the standpoint of environmental perturbation. Some studies have attempted to correlate metal fractions determined by a particular technique (operationally defined speciation) with those that are bioavailable (functionally defined speciation) (Larsen and Svensmark, 1991 Buckley, 1994 Deaver and Rodgers, 1996). However, any correlation is only empirical and more research is required to achieve an understanding of the mechanisms involved in bioavailability and to develop rational predictive models. 

In terms of the definition of speciation given in Chapter 1, the types of speciation considered in this chapter are (1) functionally defined speciation and (2) operationally defined speciation. 

Such methods are used in more fundamental studies to elucidate the soil chemistry, to determine the structure and composition of soil components and to improve understanding of the processes in the soil that control the mobilisation and retention of nutrient and toxicant elements in soil as well as to illuminate their transport mechanisms. They are, therefore, more important for the soil physical chemist than the functionally defined procedures that are the main concern of the agronomist. Both methods are of major interest to the environmental scientist particularly in the study of the fate of environmental pollutants. Many of the extractants intended to target particular species are also used in a functional speciation role. 

More widely applied to determine the potential, plant and human bioavailability are the methods of PTMs speciation which involve selective chemical extraction techniques. Estimation of the plant- or human-available element content of soil using single chemical extractants is an example of functionally defined speciation, in which the function is plant or human availability. In operationally defined speciation, single extractants are classified according to their ability to release elements from specific soil phases. Selective sequential extraction procedures are examples of operational speciation (Ure and Davidson, 2002). 

Functionally defined No chemical speciation analysis. No species... 

At equilibrium, the reactant concentrations and products can be used to define a mass ratio called an equilibrium constant (A). This constant can then be used to predict the equilibrium concentrations of the reactants and products from the total amount of C or from either the equilibrium concentration of the products or the reactants. Although K is referred to as an equilibrium constant, it is a function of salinity, temperature, and pressure. With the appropriate value of K, calculations can be made to predict the equilibrium speciation of elements in seawater. The procedure for doing this is provided in the next section along with an expansion of K to multicomponent chemical systems. 

In both cases the species, forms or phases are defined (a) functionally, (b) operationally, or (c) as specific chemical compounds or oxidation states. This usage is employed in this book but IUPAC has proposed a useful clarification in that definition (1) above is abandoned in favour of speciation analysis and the term spe-ciation is reserved for the concept of a description of the distribution of species. 

The terminology used here includes, therefore, three types of speciation based on species defined functionally, operationally or as specific chemical compounds or oxidation states. 

In addition to their use in the functional speciation role, selective extraction methods have been used to target element species in soil, or elements bound to, or associated with, particular soil phases or compounds. Examples include the use of extractants to release, for determination, metals on exchange sites, or metals bound or associated with soil iron or manganese oxyhydroxides or with soil organic matter. Most of these extractants are, however, less specific than intended and may extract species from other phases. Such extractants, however, are commonly, and conveniently, designated by their target species, e.g. extractable metal species or carbonate-bound species, but should more strictly be regarded as examples of speciation in which the species are operationally defined, i.e. by the method used to isolate them. 

Speciation can be defined functionally, operationally, or chemically. A functional definition is one which specifies the type of role that the element may play in the system from which the sample was taken. For instance, a functional definition might be that mercury which can be taken up by plants or iron that can be absorbed from a pharmaceutical. This definition is probably closest to what the end user of the information really wants to know but is the most difficult for the analytical chemist to determine. Other than growing the plant in the contaminated water or soil sample and analyzing the plant tissue, or doing feeding studies on the pharmaceutical, it is nearly impossible to obtain this information experimentally. 

Because of these reasons, the determination of the total concentration of a metal in a given matrix is often inadequate as a useful characterization, and speciation has gained considerable ground. Speciation is the process yielding evidence of the atomic or molecular form of an analyte. It can be defined either functionally (e.g., the determination of species that have certain specific functions such as their availability to plants), or operationally (e.g., the determination of the extractable forms of an element). 

The dielectric continuum models may allow us to predict speciation in aqueous solutions as a function of temperature simply by changing dielectric constant of the polarizing medium. At first glance, this may simply appear to be a return to the Bom-model formalism. However, the inner sphere solvation would be included explicitly. To include temperature effects on the inner solvation shells, we would have to calculate the partition functions of the cluster defining the metal atom and its first and, possibly, second coordination environment. 

To understand how metals are transported in the Earth s crust, we need to predict the speciation of metal complexes in aqueous solutions as a function of pressure, temperature and composition. To do this using atomistic calculations, we must define an adequately large system (as a function of composition) and obtain a thermodynamic average of all the possible states of the system (as a function of pressure and temperature). 

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