Chemical speciation, metals

Prediction of the chemistry of plutonium in near-neutral aqueous media is highly dependent on understanding reactions that may be occurring in such media. One of the most important parameters is the stability and nature of complexes formed by plutonium in its four common oxidation states. Because Pu(III), Pu(IV), and Pu(VI) are readily hydrolysed, complexation reactions generally are studied in mildly to strongly acidic media. Data determined in acid media (and frequently at high concentrations of plutonium) then are used to predict the chemical speciation of plutonium at near-neutral pH and low concentrations of the metal ion. 

Fig. 15-11 Effects of strong complexation on metal ion toxicity, (a) Increasing concentration of NTA, a strong multi-dentate complexing agent, decreases the toxicity of Cd to grass shrimp. All systems have equal concentrations of total Cd. (b) When the results are replotted showing survival as a function of Cd concentration, the data for all concentrations of NTA collapse to a single curve. (Reprinted with permission from W. G. Sunda et al. (1978). Effect of chemical speciation on toxicity of cadmium to grass shrimp, Palaemonetes pugio importance of free cadmium ions. Environ. Sci. Technol. 12,409-413, American Chemical Society.)...

The chemical speciation of the metal is defined as its distribution among different phases and different dissolved forms. When heavy metals enter aquatic... 

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

Twiss, M., Errecalde, O., Fortin, C., Campbell, P., Jumarie, C., Denizeau, F., Berkelaar, E., Hale, B., and van Rees, K., Coupling the use of computer chemical speciation models and culture techniques in laboratory investigations of trace metal toxicity, Chem Spec Bioavailab, 13 (1), 9-24, 2001. 

A method for estimating the TSCF for equation 14.24 is given in Table 14.10. The root concentration factor is also defined in Table 14.10 as the ratio of the contaminant in the roots to the concentration dissolved in the soil water (pg/kg root per pg/L). This is important in estimating the mass of contaminant sorbed to roots in phytoremediation systems. The values of TSCF and RCF for metals depend on the metals redox states and chemical speciation in soil and groundwater. 

Cadmium is a silver-white, blue-tinged, lustrous metal that melts at 321°C and boils at 767°C. This divalent element has an atomic weight of 112.4, an atomic number of 48, and a density of 8.642 g/cm3. It is insoluble in water, although its chloride and sulfate salts are freely soluble (Windholz et al. 1976 USPHS 1993). The availability of cadmium to living organisms from their immediate physical and chemical environs depends on numerous factors, including adsorption and desorption rates of cadmium from terrigenous materials, pH, Eh, chemical speciation, and many... 

Numerous and disparate copper criteria are proposed for protecting the health of agricultural crops, aquatic life, terrestrial invertebrates, poultry, laboratory white rats, and humans (Table 3.8) however, no copper criteria are now available for protection of avian and mammalian wildlife, and this needs to be rectified. Several of the proposed criteria do not adequately protect sensitive species of plants and animals and need to be reexamined. Other research areas that merit additional effort include biomarkers of early copper stress copper interactions with interrelated trace elements in cases of deficiency and excess copper status effects on disease resistance, cancer, mutagenicity, and birth defects mechanisms of copper tolerance or acclimatization and chemical speciation of copper, including measurement of flux rates of ionic copper from metallic copper. 

As noted above, biouptake involves a series of elementary processes that take place in the external medium, in the interphasial region, and within the cell itself. One of the most important characteristics of the medium is the chemical speciation of the bioactive element or compound under consideration. Speci-ation not only includes complexation of metal ions by various types of ligands, but also the distribution over different oxidation states, e.g. Fe(II) and Fe(III), and protonation/deprotonation of organic and inorganic acids of intermediate strength. The relationship between speciation and the direct or indirect bioavailability1 of certain species has received a lot of recent attention. 

Chemical Speciation of Organics and of Metals at Biological Interphases... 

Colloidal and particulate metal species also contribute to maintaining low free metal ion concentrations. The behaviour of colloidal species of similar size may however be very different, depending on their chemical speciation and on their structure. Metals bound to HA of large size may exchange with the... 

Bioaccumulation is a complicated process that couples numerous complex and interacting factors. In order to directly relate the chemical speciation of an element to its bioavailability in natural waters, it will be necessary to first improve our mechanistic understanding of the uptake process from mass transport reactions in solution to element transfer across the biological membrane. In addition, the role(s) of complex lability and mobility, the presence of competing metal concentrations and the role(s) of natural organic ligands will need to be examined quantitatively and mechanistically. The preceding chapter... 

Escher, B. and Sigg, L. (2004). Chemical speciation of organics and metals at biological interphases. In Physicochemical Kinetics and Transport at Biointerfaces. eds. van Leeuwen, H. P. and Koster, W., Vol. 9, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems, Series eds. Buffle, J. and van Leeuwen, H. P., John Wiley Sons Ltd, Chichester, pp. 205-269. 

An earlier study on the speciation of heavy metals and geochemical mapping of the total metal content of the surface soils of Delhi7 showed the total Al content to vary between 1.87 to 5.34% with a mean of 3.57%. In view of the above, the objective of this research was to ascertain primarily the status of Aluminum in Delhi soils by studying its chemical speciation in the soil profile in order to improve the understanding of its distribution in the solid-phase pool of the soil. More precisely, the objectives were ... 

As for all elements, the distribution of Mo in the environment depends critically on chemical speciation, including oxidation state (Bertine and Turekian 1973 Morford and Emerson 1999). However, Mo is somewhat unusual in both respects. In terms of ligand coordination. Mo is one of a small number of transition metals that commonly form oxy anions and coordinate only weakly with other environmentally common ligands such as Cl" or OH". Other such metals include Cr and W, which sit above and below Mo, respectively, in Group VI of the Periodic Table, as well as Tc, Re, Os and U. Hence, Mo chemistry has some analogies with these metals, as well as with nonmetals such as S, Se, P and As which also form oxyanions. 

The trace metals listed in Table 11.2 (with the inclusion of Sn) are of particular concern as they are toxic at low concentrations. For historical reasons, these elements are commonly referred to as the heavy metals. The degree to which the heavy metals cause toxic effects is dependent on their concentration, chemical speciation, and other environmental conditions, such as temperature. As illustrated in Table 28.6, the type and physiological state of the target organism are also important as these fectors determine the degree to which internal metabolic processes can detoxify or eliminate the pollutant. 

Table 28.7 Some Factors Affecting Chemical Speciation and Thus Toxicity of Trace Metals in Estuarine and Marine Organisms.

The solubilized metals form complexes with organic and inorganic anions. The chemical speciation of these complexes changes as the metal moves seaward through the estuary due to increasing salinity. These shifts can be predicted from equilibrium speciation calculations as described in Chapter 5.7. Two examples are shown in Figure 28.23 for... 

Mudroch A, Arafat N, Davies S. 1984. Changes in chemical speciation of soluble metals in Quebec lakes. Environ Tech Lett 5 237-244. 

Recent reviews on chemical speciation are published by e.g. Stumm and Brauner (1975), Florence and Batley (1980) and Leppard (1983) sometimes, with special reference to metal-organic interactions (Mantoura, 1982) or complexation in natural waters (Kramer and Duinker, 1984b). Bruland (1983) summarized the distribution and behaviour of trace elements in ocean waters. The occurrence of certain species is largely dependent on the environmental conditions. There exists a strong competition of trace metals with H+ or major cations like Ca2+ and Mg2+ in seawater, but also with other trace metals which might form more stable complexes with the ligand in question on the other side, many potential ligands or chelators compete for one trace element. 

---