Selective chemical extraction techniques

This chapter considers methods of trace element speciation, and their application to soils, that involve selective chemical extraction techniques. It will be concerned firstly with extraction by single selective reagents and secondly with the development and application of sequential extraction procedures for soils and related materials. Sequential extraction procedures for sediments are discussed in depth in Chapter 11. Speciation in the soil solution and modelling aspects of its interaction with soil solid phases are comprehensively covered in Chapter 9 and will not be considered here. 

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). 

Spectroscopic techniques have received increased attention for the study of natural organic matter (NOM) over the past decades (Hatcher et al., 2001 Abbt-Braun et al., 2004). Such techniques allow the determination of molecular speciation in many cases without the need for extractions, derivatization, or hydrolysis. Spectroscopy is generally less selective in nature than for example chemical extraction techniques, even of chemically or thermally recalcitrant compounds (Frimmel et al., 2002 Haberstroh et al., 2006), though important restrictions for specific bonds apply for some spectroscopic techniques. Equally important are the potentials to investigate the spatial relationships between NOM and mineral phases, surface properties and alteration, and micro-scale heterogeneity within NOM. With improved capabilities and access to synchrotron facilities, worldwide efforts in applying an entire range of powerful spectroscopic tools have proliferated in all areas of science. 

Various chemical extraction techniques have been introduced in order to selectively remove metals from the different adsorption or complexation sites of natural sediments (e.g., Tessier et al, 1979 Erel et al, 1990 Leleyter et al., 1999). It is, for example, shown by Leleyter et al. (1999) that between 20% and 60% of REE in various suspended river sediments are removed by successive extractions by water, by Mg(N03)2 (exchangeable fraction), sodium actetate (acid-soluble fraction), NH2OH - - HCl (manganese oxide dissolution) ammonium oxalate (iron oxide dissolution) and a mixture of H2O2 + HNO3 (oxidizable fraction). The complexity of... 

Typical nonsieve, polar adsorbents are siUca gel and activated alumina. Kquilihrium data have been pubUshed on many systems (11—16,46,47). The order of affinity for various chemical species is saturated hydrocarbons < aromatic hydrocarbons = halogenated hydrocarbons < ethers = esters = ketones < amines = alcohols < carboxylic acids. In general, the selectivities are parallel to those obtained by the use of selective polar solvents in hydrocarbon systems, even the magnitudes are similar. Consequendy, the commercial use of these adsorbents must compete with solvent-extraction techniques. 

Oil Fields. Oil field waters in the United States containing lithium have been identified in 10 states. The greatest concentrations are in waters from the Smackover formation of southern Arkansas and eastern Texas. Concentrations from this formation have been measured from 300—600 ppm in waters originating at a 2500—3300 m depth. Recovery of lithium from this resource would only be commercially feasible if a selective extraction technique could be developed. Lithium as a by-product of the recovery of petroleum (qv), bromine (qv), or other chemicals remains to be exploited (12). 

MAP makes use of physical phenomena that are fundamentally different compared to those applied in current sample preparation techniques. Previously, application of microwave energy as a heat source, as opposed to a resistive source of heating, was based upon the ability to heat selectively an extractant over a matrix. The fundamental principle behind MAP is just the opposite. It is based upon the fact that different chemical substances absorb microwave energy... 

Normally an extraction technique is selected to give the highest recovery for a wide range of pollutants. Therefore, the extract will most likely contain a high proportion of co-extracted material. Many of the clean-up techniques have been tailored into a series of multi-residue schemes in order to maximize the use of each sample [189,402,453,454,478-481]. This is of particular value when the maximum amount of chemical information is required for each sample. 

Solutions must be concentrated or the constituents must be isolated before trace amounts of the various organics present as complex mixtures in environmental water samples can be chemically analyzed or tested for toxicity. A major objective is to concentrate or isolate the constituents with minimum chemical alteration to optimize the generation of useful information. Factors to be considered in selecting a concentration technique include the nature of the constituents (e.g., volatile, nonvolatile), volume of the sample, and analytical or test system to be used. The principal methods currently in use involve (1) concentration processes to remove water from the samples (e.g., lyophilization, vacuum distillation, and passage through a membrane) and (2) isolation processes to separate the chemicals from the water (e.g., solvent extraction and resin adsorption). Selected methods are reviewed and evaluated. 

Although supercritical extraction (SFE) has been known for some time, it is still a relatively new technique to the analytical chemist. Before developing an SFE method, the chemist must understand the composition of the matrix and the analyte properties. The key instrumental parameters affecting the extraction of analytes from the matrix include fluid density, temperature, and fluid composition. Both the make-up of the matrix and the analytes must be considered when selecting the extraction conditions. Consideration of the extraction parameters must be given with respect to their affect on the analytes of interest and on the compounds present in the matrix that may either coextract with the analytes or inhibit their extraction by physical or chemical means. 

Another popular and selective extraction technique widely used in bioanalysis is solid phase extraction (SPE). SPE is a separation process utilizing the affinity of the analytes to a solid stationary phase. By manipulating the polarity and pH of the mobile phase, the analytes of interest or undesired impurities pass through stationary phase sequentially according to their physical and chemical properties. For a SPE procedure, a wash step refers to the elution of the unwanted impurities which are discarded and the elution step refers to the elution of the analytes of interest which are collected. While the fundamental remains the same in decades, the continuing invention and introduction of new commercial stationary phases and accessory devices have boosted the application of SPE in bioanalysis and many other fields. 

Total concentrations in the sediment do not necessarily reflect either the biologically or chemically reactive fraction of metals or substrates. Thus, -we have used partial extraction techniques to characterize different phases of both metal and substrate. Under different estuarine conditions, hydrous oxides may vary in crystallinity and in their associations -with other substrates (e.g., organics). The nature of the organic materials may also vary greatly. Different extractants remove different quantities of these substrates, in response to differences in substrate form ( ). We may not assume that extractants selectively remove trace metals from any single sorption substrate (2 +), but differences in trace metal solubility among extractants may be useful to empirically separate metal forms susceptible to different treatments. Statistical association... 

Solid-phase speciation has been measured both by wet chemical extraction and, for arsenic, by instrumental methods principally X-ray absorption near edge structure spectroscopy (XANES) (Brown et al., 1999). La Force et al. (2000) used XANES and selective extractions to determine the likely speciation of arsenic in a wetland affected by mine wastes. They identified seasonal effects with As(El) and As(V) thought to be associated with carbonates in the summer, iron oxides in the autumn and winter, and silicates in the spring. Extended X-ray absorption fine stmcture spectroscopy (EXAES) has been used to determine the oxidation state of arsenic in arsenic-rich Californian mine wastes (Eoster et al., 1998b). Typical concentrations of arsenic in sods and sediments (arsenic <20 mg kg ) are often too low for EXAFS measurements, but as more powerful photon beams become available, the use of such techniques should increase. 

Extraction techniques that involve a chemical reaction can be classified as nonselective extraction or concentration, when more than one analyte is extracted from the solution by using the organic collectors (e.g., 8-hydroxyquinoline and dithizone derivatives) and selective extraction or separation. The first step in such an extraction technique is the formation of the complex by adding the reagent(s) to the solution of analyte, and after the extraction of the complex in an organic solvent. The problem that can arise in a SIA system with these types of extraction is the precipitate that is formed, and this can contaminate the other sample and also can block the tubing. To avoid these problems, it is necessary either to dilute the sample in such a way that the precipitation equihbria will not be reached and that all the complex will remain dissolved in the solution, or by derivatization of the hgands to... 

Thus, the mechanisms of interaction include hydrogen bonding and dipole-dipole forces (polar interactions), van der Waal s forces (nonpolar or hydrophobic interactions), size exclusion, and cation and anion exchange. Some sorbents combine several interactions for greater selectivity. The extensive line of sorbent chemical structures facilitates one of the most powerful aspects of SPE, that is, selectivity. Selectivity is the degree to which an extraction technique can separate the analyte from interferences in the original sample. The number of possible interactions between the analyte and the solid phase facilitates this selectivity. 

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