Speciation, chemical systems

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.)...

Today it has become clear that the effect of trace elements in living systems, in food, and in the environment depends on the chemical form in which the element enters the system and the final form in which it is present. The form, or species, clearly governs its biochemical and geochemical behaviour. lUPAC (the International Union for Pure and Applied Chemistry) has recently set guidelines for terms related to chemical speciation of trace elements (Templeton et al. 2000). Speciation, or the analytical activity of measuring the chemical species, is a relatively new scientific field. The procedures usually consist of two consecutive steps (i) the separation of the species, and (2) their measurement An evident handicap in speciation analysis is that the concentration of the individual species is far lower than the total elemental concentration so that an enrichment step is indispensable in many cases. Such a proliferation of steps in analytical procedure not only increases the danger of losses due to incomplete recovery, chemical instability of the species and adsorption to laboratory ware, but may also enhance the risk of contamination from reagents and equipment. 

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. 

As discussed in Chapter 5 of this volume [104], chemical speciation can be defined as the physicochemical distribution of a chemical among all of its possible forms. In environmental systems, ligands range from simple ligands... 

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. 

The actual form in which a contaminant molecule or ion is present in natural water, as result of a change in the coordinative relationship, emphasizes a specific chemical speciation. A chemical species is defined by lUPAC as the isotopic composition, electronic or oxidation state, and/or complex or molecular stracture, and the speciation of an element as the distribution of an element amongst defined chemical species in a system (Templeton et al. 2000). 

Robinson, B., Outred, H., Brooks, R. Kirkman, J. 1995. The distribution and fate of arsenic in the Waikato river system, North Island, New Zealand. Chemical Speciation and Bioavailability, 1, 89-96. 

The techniques and methods that could be applied to chemical speciation in biological systems are surveyed and the limitations are highlighted. In addition, changes that occur in the samples that may have a detrimental effect on the results are examined. 

In the past, most analytical problems related to environmental or biological systems were addressed by measuring the total concentrations of the elements. However, at present, there is an increasing awareness of the importance of the chemical form in which an element is present (e.g. the oxidation state, the nature of the ligands or even the molecular structure) since its chemical, biological and toxicological properties critically depend on it. Hence there is a clear need for rapid and robust analytical tools to perform chemical speciation, and atomic spectroscopy is undoubtedly one of the most important tools for such studies. 

Part II considers speciation in specific compartments of the environment viz. the atmosphere, biological systems, soils, sediments and natural waters, and with particular aspects of the speciation of environmentally important radionuclides. Two new chapters have been added to make the coverage even more comprehensive. These new chapters are Chapter 10, Chemical Speciation in Soib and Related Materials by Selective Chemical Extraction by the editors, and Chapter 12, Speciation in Seawater by R.H. Byrne of the University of South Florida. 

Computer simulation is now used extensively as a tool to help to understand and predict the transport of radionuclides through environmental systems. Most models relate to waste disposal and are based on measured parameters such as water movements, salinity, suspended load and the radionuclide concentration in the solute, suspended particulate matter and bottom deposits. Comparatively few attempts appear to have been made to include chemical speciation into this type of model, presumably because of the added complexity involved. Some modellers have attempted to take into account the characteristics of the major chemical phases such as those present in different particles or coatings (e.g. Martinez-Aquirre et al., 1994). Others have noted the importance of including details of particular chemical species present in industrial waste releases when constructing models to predict dispersion (Abril and Fraga, 1996). 

The number of analytical methods developed for the study of the distribution of metal- and metalloid-containing species in the last decade has been impressive. However, a majority of these are as yet to be applied to real biological materials. With the greater appreciation of the pre- and post-sampling factors that influence chemical speciation, and the development of appropriate quality control materials the results of these studies will become more reliable. Consequently, the use of chemical speciation data will become indispensable to accurate environmental impact assessment, and to our understanding of the roles that metals and metalloids play in biological systems. 

Considerable recent research has focused on the topic of chemical speciation in the environment. It is increasingly realised that the distribution, mobility and biological availability of chemical elements depend not simply on their concentrations but, critically, on the forms in which they occur in natural systems. Continuing developments in analytical chemistry have made speciation practicable even where analytes are present at trace levels (as is often the case in natural samples). 

Fig. 7-2. Summary of environmental pathways by which terrestrial plants may become contaminated with radionuclides. In the case of an input from atmosphere, or as a result of the process of resuspension , any external radionuclide burden may be reduced by field loss mechanisms conversely, an initially external radionuclide deposit (Rat) may become internalised (i int) following foliar absorption and translocation. Radioactive contaminants of soils may be derived either from atmospheric inputs or from seepage in ground waters. Partitioning of radionuclides in soil—soil water systems controls their availability for root absorption, which normally occurs exclusively from the liquid phase. The chemical speciation of the nuclide in this phase, however, provides a further control on bioavailability which is highly radionuclide specific.

Tipping, E., Lofts, S., and Lawlor, A.J. (1998) Modeling the chemical speciation of trace metals in the surface waters of the Humber system. Sci. Total Environ. 210/211, 63-77. 

Witters H.E. (1998) Chemical speciation dynamics and toxicity assessment in aquatic systems. Ecotoxicology and Environmental Safety 41 90-95. 

The effect of pH is illustrated by the example of the chemical speciation of hydrocarbonate, which is abundant and usually the most important anion of groundwaters. The hydrocarbonate content of groundwaters is usually high. Hydrocarbonate can be present as different species, depending on pH. It is shown in Figure 1.2, where the speciation of a typical Hungarian groundwater is shown at different pH values (467 mg/dm3 = 7.5 mmol/dm3 hydrocarbonate, 48 mg/dm3 = 1.2 mmol/dm3 calcium content). The chemical equilibria in a closed system are as follows. 

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. 

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