Operationally defined speciation

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. 

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. 

The specificity of many of these reagents can be improved by combining a series of them in a sequential extraction scheme in which the residue from a first extraction is used as the material for a second extraction and so on through a number of stages. The soil phase attacked by each extracting reagent is thus restricted by the preceding extraction(s) in the series and is thereby made more specific. 

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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. 

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. 

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). 

Operationally defined speciation. In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction or chemical pools isolated for measurement. 

Delhi soils by studying its speciation in the soil profile and to assess if there was any spatial variability. Soils representing the Aravali Ridge and the alluvial floodplains of river Yamuna were collected as a single, undisturbed core up to a depth of lm and the profile differentiated into four layers- 0-17 cm, 17-37 cm, 37-57 cm, and 57-86 cm. Pseudo total Aluminum and Iron in the soils were speciated into the operationally defined species (weakly exchangeable, organic matter complexes, amorphous oxides and hydroxides, and crystalline or free oxides) by widely recommended selective extraction procedures. Both A1 and Fe in these soils are bound predominantly as Fe oxides and silicates and have only very low percentages in the easily mobilizable pools. 

Iron in the soil samples was also speciated to yield the following operationally defined species of Fe ... 

The concentrations of the four A1 species occurring in each layer of the soil profile are expressed as a percentage of the A1 content of the specific soil layer. While Alpstot increased with depth of the profile, the contribution of the operationally defined A1 species towards speciation however decreased with depth (Figure 3a-d). The maximum differentiation of Alpstotas A1, A1, A1, and A1 was 8.51% which was observed in the surface layer while the mean contribution was 5.43 1.60%. On the contrary, in 37-57 cm depth which had the most abundant Alpstot, the mean share of the A1 species (2.72 0.75%) was the lowest contribution to A1 in all the layers of the soil profile. This implied that pseudo total A1 in these depths are predominantly bound as silicates and hence are not available for speciation under the experimental conditions. 

Techniques can be classified into two main categories those that detect total metal concentrations and those that detect some operationally defined fraction of the total. Methods which detect total concentrations such as inductively coupled plasma spectrometry, neutron activation analysis, atomic absorption spectrometry and atomic emission spectrometry have no inherent speciation capabilities and must be combined with some other separation technique(s) to allow different species to be detected (approach A in Fig. 8.2). Such separation methods normally fractionate a sample on the basis of size, e.g. filtration/ultrafiltration, gel filtration, or a combination of size and charge, e.g. dialysis, ion exchange and solvent extraction (De Vitre et al., 1987 Badey, 1989b Berggren, 1989 1990 Buffle et al., 1992). In all instances the complexes studied must be relatively inert so that their concentrations are not appreciably modified during the fractionation procedure. 

McDuffie, 1979 Laxen and Harrison, 1981 Hoffmann et al., 1981 Florence, 1982 Buffle et al., 1987 Batley, 1989b Lund, 1990 van Loon and Barefoot, 1992 Pettersson et al., 1993 Baxter and Freeh, 1995 Das and Chakraborty, 1997). Results are operationally defined and care must be taken in comparing data from different workers (Quevauviller, 1995 Mach et al., 1996 Nordstrom, 1996 Donard and Astruc, 1997) (see Chapter 1 for speciation definition). 

Standardisation of speciation schemes - Despite the significant advances that have been made in metal speciation measurement techniques over the past 30 years, much remains to be done. The methods that have been developed do not provide an absolute breakdown of metal species, but rather operationally defined classifications. Because of this operational nature, the standardisation of the procedures becomes essential if different results are to be compared. So far, this has rarely been achieved. 

In this chapter, we shall introduce soil speciation concepts by consideration of inorganic trace elements in dissolved and adsorbed forms, with reference to both their molecular speciation and their operationally defined soil component speciation. We shall then consider the implications of chemical speciation in soils for agriculture and soil pollution. 

In soil research, the term speciation is often applied to operationally defined fractionation of heavy metals into five or more components.25 Typically, water soluble, exchangeable, organically bound (which includes what is in biomass), amorphous oxide bound, crystalline oxide bound, and residual fractions are measured.26 Sometimes residual fractions are further subdivided according to particle size distributions to give amounts in sand, silt, and clay fractions. Similar fractionation procedures are often applied to aquatic sediments.27 In arid regions, often the calcium carbonate bound fractions of heavy metals are also measured.28 Because of the constraints of detection limits, generally only cadmium, copper, iron, manganese, and zinc are usually monitored by flame spectrometry in such heavy metal speciation studies.28... 

Operationally defined Not chemical speciation analysis. No species... 

Physico-chemical speciation refers to the various physical and chemical forms in which an element may exist in the system. In oceanic waters, it is difficult to determine chemical species directly. Whereas some individual species can be analysed, others can only be inferred from thermodynamic equilibrium models as exemplified by the speciation of carbonic acid in Figure 9. Often an element is fractionated into various forms that behave similarly under a given physical (e.g., filtration) or chemical (e.g., ion exchange) operation. The resulting partition of the element is highly dependent upon the procedure utilised, and so known as operationally defined. In the following discussion, speciation will be exemplified with respect to size distribution, complexation characteristics, redox behaviour and methylation reactions. 

Methods of Speciation and Fractionation. It is apparent that in order to understand the mobility of arsenic and its availability for reactions, methods of speciation and fractionation must be applied to sediment samples in field and laboratory studies. In this paper speciation refers to the separation and quantitative determination of inorganic arsenic, methanearsonic acid, and cacodylic acid. Compartmentalization involves identifying the major compartments for arsenic in a heterogeneous system (e.g. aqueous, adsorbed, occluded,...) and determining the amounts of arsenic in each compartment. Fractionation involves the extraction of arsenic from operationally defined fractions of the solid phase of an aquatic system (e.g. sediment). 

It is now well known that trace-element concentrations in continental waters depend on the size of the pore filters used to separate the particulate from the dissolved fraction. This is apparent in Table 1, where results from the Amazon and Orinoco are reported using two filtration sizes the conventional 0.2 p,m filtration and filtration with membranes of smaller cutoff size (ultrafiltration). These results suggest the presence in solution of very small (submicro-metric) particles that pass through filters during filtration. The view that trace elements can be separated into particulate and dissolved fractions can thus no longer be held this has led authors to operationally define a colloidal fraction (0.20 p.m or 0.45 p.m to 1 nm) and a truly dissolved fraction (<1 nm) (e.g., Buffle and Van Leeuwen, 1992 Stumm, 1993). The existence of a colloidal phase has a major influence on the speciation calculation schemes presented above (based only on aqueous complexation), as the apparent solubihty of trace elements will be enhanced by the presence of colloids. The dynamics of colloids also completely change... 

The aim of sample treatment is frequently the differentiation of water components on the basis of their physical-chemical properties. More separation techniques can be applied to divide trace elements in sea water in fraction (55). The major assumption made by applying separation techniques is that removal of one or more components from a sample does not disturb the solution equilibria, but frequently there is evidence that this is not true e.g., it was observed that after removal of particles from water there was regrowth of filterable particles. However, operationally defined separation techniques are a useful means of comparing the characteristics of different samples. The problems and the advantages associated with specific separation methods for trace elements speciation has heen reviewed (55). 

Speciation is defined as the process of identifying and quantifying the different defined species, forms or phases present in a material or the description of the amounts and types of these species, forms or phases present . In some cases, it is possible to identify, by using single or sequential extractions, operationally defined determinations which identify groups of metals without clear identification. In this situation, it is possible to refer to, for example, ethylenediaminete-traacetic acid (EDTA)-extractable trace metals. The reasons why speciation is important is that metals and metalloids can be present in many forms, some of which are toxic. 

Because ground waters, like other natural waters, are dilute solutions of many compounds, metal speciation measurements are difficult. Therefore, the metal-complexing properties of natural waters are operationally defined by many factors, including the analytic method used for speciation, the conceptual and mathematical models used to analyze the data, the range of titrant metal concentrations used, and conditions such as pH, ionic strength, and temperature. The analytical methods used to determine metal speciation all have inherent assumptions and limitations. Most published studies of metal complexation in natural waters have used one analytical method. However, confirmation of results (e.g. stability constants, ligand concentrations) by independent methods would add confidence to such results. In the present work, three independent methods were used. 

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