Speciation of metals

Chromium is known in all oxidation state between 0 and VI, but is commonly found in oxidation states III and VI. Cr(III) species in acidic solution exist as Cr(H20)g ions and in concentrated alkali have been identified as Cr(OH)g and Cr(0H)5(H20) . These ions are regular octahedral. Cr(VI) in basic solution above pH 6 exists as tetrahedral yellow chromate, CrO ion. Between pH 2 and 6, HCrOj and the orange-red dichromate, Cr207, are in equilibrium. Under strongly acid conditions, only dichromate ion exists. Addition of alkali to dichromate gives chromate. 

There have been several reports [8-13] describing the speciation of chromium. Table 8.1 describes the conditions used in these reports. Many of these reports described using DC plasma atomic emission (DCPAE), ICP atomic emission (ICPAE) and atomic absorption (AA) 

Sample Species (ret. time, min) Eluant Column Detector Detection limits Ref 

The basis of many of the separations has been to convert Cr(III) to an anion by adding a complexing agent. Cr(VI) is already an anion (usually CrO ), hence, anion chromatography can be used to separate mixtures. An example is separations carried out in the presence of KSCN converting Cr to CrCSCN).  

Pyridine dicarboxylic acid (PDCA) was used as a complexing agent for Cr to form anionic Cr(PDCA)2 The sample pH is critical to the separation. Optimum results were reported at pH 6.8. Both Cr(III) and CrCVI) were detected using UVA S detection and diphenylcar-bohydrazide as a post-column reagent. The method was found to be applicable for plating baths and waste water analysis. 

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Examples were given earlier of a few systems for which the theoretical speciation of metals was calculated, and a few others in which it was analyzed experimentally, at least into some subgroups if not specific species. These approaches have rarely been combined to critically evaluate data or hypotheses regarding metal transport through a real system. One study, already noted... 

From the beginnings of ecology as a discipline, the mining industry has been at the center of the battle over preservation versus exploitation. As discussed in the introduction to this chapter, human activities such as mining, power production from fossil fuels and discharges of industrial and municipal wastes not only increase the rate at which metals enter the biosphere but may also drastically alter the speciation of metals from what it would be in the undisturbed geologic cycle. 

Cornelis R (1996) Involvement of analytical chemistry in chemical speciation of metals in clinical samples. Ann Clin Lab Sd 26 252-263. 

Some selected applications (chemical class, mobile phase, stationary phase, column, detector) are as follows fatty acids, C02, PEG or CN-PS, OTC, FID organotin, C02, PhMe-PS, OTC, FPD [305], ThepSFC-ICP system is an effective tool for speciation of metal-containing compounds [362], even in the presence of modifiers to control the retention of components. 

While QWASI is an easy to use multimedia fate modeling tool, it has been originally designed as a fugacity model. Even though an adaptation to ionic substances exists and it has been applied to lead before, it needs to be recognized that it does not take speciation of metals into account. This adds to the overall uncertainty of results. 

Courchesne F., Seguin V., Dufresne A. Solid phase speciation of metals in the rhizosphere of forest and industrial soils. Proceedings of the 5th International Conference on Biogeochemistry of Trace Elements, 1999, Austria. 

In recent years, several techniques based on synchrotron radiation have been applied to determine, with pm-spatial resolution, the elemental distribution and speciation of metals in mine-contaminated soils (Manceau et al. 2003 Morin et al. 1999, 2001). 

In conclusion it must be noted that a lot of developments are still needed in order to increase the possibility of on-line/on-site monitoring of mineral constituents, including the speciation of metallic compounds with regard to health risks. 

Stability constants of metal complexes with inorganic ligands are generally well known [30], Inorganic speciation of metals can therefore easily be evaluated by thermodynamic calculations if the composition of the solution is known [29,31],... 

A combination of direct observation and extraction may be carried out. The whole soil may be analyzed by various methods and then specific components sequentially extracted and measured. This approach has been used in the investigation of the speciation of metals in soil under various conditions. Using X-ray spectroscopy, metals and their various ionic forms in soil can be directly identified [1],... 

DAmore JJ, Al-Abed SR, Scheckel KG, Ryan JA. Methods for speciation of metals in soils a review. J. Environ. Qual. 2005 34 1707-1745. 

The retardation equation can also be applied to inorganic soluble substances (ions, radionuclides, metals). But here we have to consider, in addition to the sorption or ion exchange process, that the speciation of metal ions or ligands in a multi-... 

The mobility of metals in soil solutions is controlled by several processes (1) desorption or dissolution (rate depends on the solubility of metal-mineral form) (2) diffusion (depends on speciation of metal, soil oxidation/reduction potential, and pH) (3) sorption or precipitation (depends on soil solution concentration and rhi-zosphere effects) and (4) translocation in the plants (depends on plant species, soil solution concentration, and competing ions) (McBride... 

In 2006, the speciation of metals and metalloids (As, Bi, Hg, Pb, Sb, Se and Sn) associated with alkyl groups and biomacromolecules in the environment was critically reviewed by Hirner.85 More than 60 species of alkylated metals and metalloids have been found in different ecosystems and terrestrial locations all over the world.85-87 These alkylated metals or metalloids are of interest due to their toxicological properties (e.g. monomethyl mercury, MMHg, which gained worldwide attention during the Minamata tragedy, and are not only known to be produced by microbial methylation within most anaerobic compartments of the environment, but also in the course of enzymatic transformation during human metabolism.85... 

Lauder, L. (1987) Speciation of Metals in Water, Sediment and Soil Systems (Proceedings of the International Workshop). Springer-Verlag, Berlin. 

Langford, C.H. and Cook, R.L. (1995) Kinetic versus equilibrium studies for the speciation of metal complexes with ligands from soil and water. Analyst, 120, 591-596. 

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