Chemical speciation changes

Nuclear magnetic resonance (NMR) has been used to characterize chemical speciation changes occurring in both soluble and insoluble fractions of silicone polymers for decades (d-9). One of the most common strategies for... 

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... 

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). 

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

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. 

The changes in the three-dimensional structures of the biological molecules with storage, and the length of time that a sample can be stor i without adverse effects are points that have to be addressed in any futme study on the effect of storage on the chemical speciation of elements. 

Crecelius, E.A. (1977) Changes in the chemical speciation of arsenic following ingestion by man. Environmental Health Perspectives, 19, 147-50. 

A component can undergo considerable physico-chemical speciation alterations in an estuary. With respect to dissolved constituents, the composition and concentration of available ligands changes. Depending upon the initial pH of the riverine water, OH may become markedly more important down the estuary. Similarly, chlorocomplexes for metals such as Cd, Hg and Zn become more prevalent as the salinity increases. Conversely, the competitive influence of seawater derived Ca and Mg for organic material decreases the relative importance of humic complexation for Mn and Zn. 

Finally, models were calculated which predict the variations in chemical speciation of copper resulting from changes in the chemical parameters pH, carbonate alkalinity, concentration of dissolved organic matter, and concentration of total dissolved copper. 

Another approach to the assessment of ion speciation in the resin and solution phases with equal ility for the determination of formation constants of complexes in these two environments is provided by NMR spectroscopy. The method can provide sharp, well-separated signals for each successive complex species formed, the peak area being proportional to the atomic concentration of the element being measured, irrespective of its complex form. For a completely labile system, a well-defined chemical shift change is often observed with successive complex formation. These features make this analytical procedure well suited for study of metal ion complexation by the ion-exchanger phase. 

Finally, we must somehow resolve the vexing problem of the chemical speciation of trace elements in seawater what is the chemical nature of the various metal chelators whose existence has been demonstrated by electrochemistry Is the chemistry of several metals in surface seawater really controlled by metallophores released by prokaryotes Or are dissolved metals chiefly present as parts of metalloproteins in the process of remineralization How do metal chelators affect the residence times of metals (in particular, scavenged elements such as iron and cobalt) and in turn how do those chelators influence the global carbon cycle via changes in marine primary productivity ... 

Changes in the oxidation state of trace metals can occur depending on the redox condition of the environment. Redox reactions are thus important in influencing the chemical speciation of a number of metals and metalloids, notably Hg, As, Se, Cr, Pu, Co, Pb, Ni, and Cu (Oscarson et al., 1981 Bartlett and James, 1993 Alloway, 1995 Myneni et al., 1997 Huang, 2000 James and Bartlett, 2000 Adriano, 2001 Sparks, 2003). Redox reactions also exert a great influence in the transformation and reactivity of Fe and Mn oxides in soils tliat have an enormous capacity to adsorb metals and metalloids (Huang and Germida, 2002). Furthermore, reduction of sulfate to sulfide in an anerobic environment also affects... 

The environmental impact of metallic contaminants in soils is dependent both on the chemical speciation of the metal and the response of the matrix to biological and physicochemical conditions. These factors are responsible for the mobilisation of the metal from the solid into the aqueous phase hence transport within the immediate vicinity, has an impact on the rate of dispersal, dilution, uptake and transfer into living systems. The impact of changing environmental conditions on the contaminant inventory can be to enhance or moderate these phenomena, with subsequent consequences for the broader risk assessment of the contaminants. During the last ten years, extensive research in analytical chemistry was initiated to develop highly specific and sensitive methods for measuring potentially harmful substances in various... 

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