Definition of speciation

In this context the term speciation may be defined as either 

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. 

Functionally defined species are exemplified by the plant-available species mentioned in Section 1.1 in which the function is plant availability. 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

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The existence of an element in different chemical forms in the gaseous, solid or aqueous solution phase provides the conceptual basis for speciation in soils. More particularly, a chemical species in soil refers either to a specific molecular arrangement of the atoms of an element or, quite often, to the result of an operational process of detection and quantitation aimed at elucidating chemical forms (Bernhard et at., 1986, pp. 7-14). In principle, the former definition should be the outcome of the latter, methodological definition. In practice, this connection is difficult to achieve in natural systems (Bernhard et al., 1986) (see Chapter 1 for a definition of speciation). Understanding speciation is important in assessing the availability of plant nutrients, plant uptake of potentially toxic elements (e.g. Al, Cd), and the movement of both nutrient and toxic substances into waterways or other parts of an ecosystem (Da Silva et al., 1991). 

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. 

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. 

Speciation encompasses both the chemical and physical form an element takes in a geochemical setting. A detailed definition of speciation includes the following components (1) the identity of the contaminant of concern or interest (2) the oxidation state of the contaminant (3) associations and complexes to solids and dissolved species (surface complexes, metal-ligand bonds, surface precipitates) and (4) the molecular geometry and coordination environment of the metal.5 The more of these parameters that can be identified the better one can predict the potential risk of toxicity to organisms by heavy metal contaminants. Prior to the application... 

Speciation analysis of an element is usually defined as the determination of the concentrations of the individual physico-chemical forms of the element in a sample that together constitute its total concentration. The International Union for Pure and Applied Chemistry (lUPAC) has recently recommended the definition of speciation as the distribution of an element amongst defined chemical species in a system. lUPAC defines chemical species as the specific forms of an element defined as to isotopic composition, electronic or oxidative state, and/or complex or molecular stmcture. 

The term bioavailability has different meanings in different contexts and disciplines. Numerous definitions of bioavailability exist. Research on the relationship between bioavailability and chemical speciation (forms) originated in the field of soil fertility in the search for a good predictor for the bioavailability of essential plant nutrients (Traina and Laperche 1999). It is well accepted that dissolved nutrients are more labile and bioavailable to plants than solid-phase forms (including sorbed species). The same has been considered to be true for organic contaminants and their availability for microbial degradation. 

The guiding principles for the selection or development of speciation procedures are similar to those recommended for other forms of chemical analysis. For example, the initial step should be careful definition of the problem, including listing of the analytical specifications (e.g. type of analysis, concentration range, potential sources of error). This step can be followed by selection of a suitable measurement procedure, nomination of a selective separation procedure (if required) and organisation of the total protocol. 

Electrochemical methods have been used for determinations of species of elements in natural waters. Of the many electrochemical techniques, only a few have proved to be useful for studies of speciation in complex samples, and to possess the sensitivity required for environmental applications. The greatest concern is the measurement of the toxic fraction of a metal in an aqueous sample. The definition of a toxic fraction of a metal is that fraction of the total dissolved metal concentration that is recognised as toxic by an aquatic organism. Toxicity is measured by means of bioassays. Elowever, a universally applicable bioassay procedure cannot be adopted because the responses of different aquatic species to metal species vary. Nevertheless, bioassays should be used as means of evaluation and validation of speciation methods. A condition is that the test species (of the bioassay) should be very sensitive to the metals being studied so as to simulate a worst case situation (Florence, 1992). 

An operational definition is considerably more practical. Operationally determined species are defined by the methods used to separate them from other forms of the same element that may be present. The physical or chemical procedure that isolates the particular set of metal species is used to define the set. Metals extracted from soil with an acetate buffer is an operational definition of a certain class. Lead present in airborne particles of less than 10 pm is another. In water analyses, simply filtering the sample before acidification can speciate the analytes into dissolved and insoluble fractions. These procedures are sometimes referred to as fractionation, which is probably a more properly descriptive term than speciation, as speciation might imply that a particular chemical species or compound is being determined. When such operational speciation is done, careful documentation of the protocol is required, since small changes in procedure can lead to substantial changes in the results. Standardized methods are recommended, as results cannot be compared from one laboratory to another unless a standard protocol is followed [124], Improvements in methodology must be documented and compared with the currently used standard methods to produce useful, readily interpretable information. 

Computational speciation can be compared to analytical speciation for some species. There is always the problem that analytical methods also suffer from operational definitions, interferences, limits of detection, and associated assumptions. Nevertheless, there is no better method of determining accuracy of speciation than by comparing analytical results with computational results (Nordstrom, 1996). In the few instances where this has been done, the comparison ranges from excellent to poor. Examples of studies of this type can be found in Leppard (1983), Batley (1989), and Nordstrom (1996, 2001). Sometimes comparison of two analytical methods for the same speciation can give spurious results. In Table 3, measured and calculated ionic activity coefficients for seawater at 25 °C and 35%o salinity are compared, after adjusting to a reference value of yci = 0.666 (Millero, 2001). These values would indicate that for a complex saline solution such as seawater, the activity coefficients can be... 

Chapter 2 discusses several aspects of speciation analysis, e.g. definitions, existing regulations and sources of errors likely to occur at various steps of the analytical procedures. It is followed by Chapter 3 describing general principles of improvement schemes (organizational aspects) and basic requirements to be followed for the certification of reference materials. 

Both filterable and particulate forms of organic phosphorus are transferred from soils, but the lack of suitable techniques for determining specific phosphorus compounds in aqueous samples means that there is little information available on the chemical composition of either fraction. Conceptual understanding of organic phosphorus forms and their behaviour in water is also limited by the operational definitions that arise from conventional analysis procedures (see below). For detailed reviews of speciation techniques for organic phosphorus in soil waters see McKelvie (Chapter 1, this volume) and Cooper et al. (Chapter 3, this volume). 

It may not be trivial for an analyst to say that the method actually does measure what it purports to. When there are possibilities of speciation, or different isomers, then the analyst must be aware of the client s requirements and ensure that the method is specific for them. For example, analysis for total copper is different from bioavailable copper , and with the latter the definition of bioavailable must be carefully considered. 

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