Metal speciation approaches

METHODOLOGICAL APPROACHES TO THE INVESTIGATION OF THE METAL SPECIATION IN THE NATURAL WATER... 

Brugmann [784] discussed different approaches to trace metal speciation (bioassays, computer modelling, analytical methods). The electrochemical techniques include conventional polarography, ASV, and potentiometry. ASV diagnosis of seawater was useful for investigating the properties of metal complexes in seawater. Differences in the lead and copper values yielded for Baltic seawater by methods based on differential pulse ASV or AAS are discussed with respect to speciation. 

After a brief introduction on terminology, this overview summarizes the experimental and theoretical modelling methods applied in trace metal speciation studies, emphasizing the dissolved fraction as defined by 0.95 pm filtration. The experimental approach comprises interactions with organic - and inorganic ligands, speciation schemes, biological experiments and interactions with particles and colloids. 

Different approaches in trace metal speciation studies (with some examples). 

Rohl, R., 1982. Trace metal speciation in seawater - a paper electrophoretic approach. Anal. Chim. Acta, 135 99-110. 

As shown in the preceding section, toxic metals may be present in a wide variety of physicochemical forms in surface waters, wastewater, landfill leachates, soils, or sediments. Early on, metal speciation in surface waters was determined, using a chemical approach (Giesy et al., 1978). We now know that metal speciation affects their bioavailability and potential toxicity to aquatic organisms (Tessier and Turner,... 

The ideal speciation procedure is one which allows positive identification and quantitative evaluation of one particular species. Some of the better known approaches are summarised in Table 2.1. Techniques (usually spectroscopic) which have been used to identify and determine directly a particular chemical species in biological samples, are summarised in Table 2.2 and the topic of Direct Methods of Metal Speciation is dealt with in Chapter 3. 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

Apte, S.C. and Batley, G.E. (1995) Trace metal speciation of labile chemical species in natural waters and sediments non-electrochemical approaches. In Metal Speciation and Bioavailability in Aquatic Systems (eds Tessier, A. and Turner, D.R.). John Wiley and Sons, Chichester, pp. 259-306. 

Rubio, R. and Ure, A.M. (1993) Approaches to sampling and pretreatment for metal speciation in soils and sediments. Int.f. Environ. Anal. Chem., 51, 205-217. 

Venditti, D., Berthelin, J., and Durecu, S. (2000b). A multidisciplinary approach to assess history, environmental risks, and remediation feasibility of soils contaminated by metallurgical activities. Part B Direct metal speciation in the solid phase. Arch. Environ. Contam. Toxicol. 38, 421-427. 

Analytical Approach to Metal Speciation and Metal Analysis... 

Tests to evaluate the accuracy of the DPASV approach to metal speciation have previously been reported using model ligands to study the complexation of Cd, Cu, Pb and Zn in sea water (32-34, 108). Reported results show that the ligand concentrations and conditional stability constants obtained are in agreement with the theoretical data, as set forth in Table 5.4. 

The results of measurements carried out on surface samples collected during two oceanographic expeditions at the Terra Nova Bay were analyzed by a multivariate statistical approach. The Principal Components Analysis was used to observe association between variables and it showed an opposition between the Cd concentration (total and labile) and the ligand that complexes it (134). These results show that the metal speciation could affect its distribution. It, in particular, could emphasize the direct involvement of complexation in the transfer of Cd from the dissolved phase to the particulate affecting the total dissolved distribution. 

The fate of toxic metals in soils is influenced by its initial chemical form and the environmental and edaphic conditions, such as pH, redox status, and soil organic matter content. The metal speciation in soils can be discussed broadly using the following approaches ... 

A variety of methods for the determination of total metals and metal species/fractions have been described. While the most common method of sample decomposition of environmental samples is probably acid digestion, other approaches, most notably the use of fusion, have some specialist application areas, e.g. geological samples. The area of metal speciation is an ever expanding one, with new approaches being developed and evolving on a regular basis. The major limitation to this area of activity is the development of robust and reliable approaches to analysis. Some selected examples have been provided to assist the reader to evaluate some of the procedures that have already been developed. However, after reading this chapter you will have quickly realized that the methods are limited to only a few metal species. 

Eeenandez a, Teeneeo M, Barracan F and Jimenez J (2000) An approach to characterization of sources of urban airborne particles through heavy metal speciation. Chemosphere - Global Change. Science 2 123-136. 

Using basic chemical theory, Sposito (1986) and Morgan (1987) have outlined the principles for elucidating metal speciation in the natmal environment. Sposito and Mattigod (1980) and Sposito and Coves (1988) have developed computer-based geochemical modeling approaches specifically for soils. 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

The calculated thermodynamic properties (Table 7) show that, in the presence of chloride ligands, bisulfide complexation of Zn, Cd and Hg is unimportant even up to 300°C. The thermodynamics of reactions between simple gas-phase molecules can be calculated easily from first-principles and may provide an approach to understanding metal speciation in supercritical fluids where solvation is minimal. 

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