Minor element speciation

Although the speciation of some minor elements has been determined directly by experimental means (e.g., ion selective electrodes, polarography, electron spin resonance) most of our thinking about speciation is based on equilibrium calculations. Garrels and Thompson... 

Dyrssen D, Wedborg M (1980) Major and minor elements, chemical speciation in estuarine waters. In Olausson E, Cato I (eds) Chemistry and biogeochemistry of estuaries. Wiley, New York, pp 71-119... 

The distribution of an element in biological samples collected from an ecosystem can be equally complex. In plants, for example, the distribution of minor elements or particular species is not uniform throughout the whole organism, i.e. leaf material values differ from those derived from ribs, stems, roots and fruit. For speciation, sub-sectioning of the semi-rigid matrix may thus have to precede isolation of the component of interest. 

The chemical reactivity of minor elements in seawater is strongly influenced by their speciation (see Stumm and Brauner, 1975). For example, Cu ion is toxic to phytoplankton (Sunda and Guillard, 1976), Uranium(VI) forms the soluble carbonate complex, 1102(003)3 , and as a result uranium behaves like an unreactive conservative element in seawater (Ku et al., 1977). 

Garrels and Thompson (1962) conducted speciation calculations for the major elements of seawater. They showed that the major cations (Na, K, Ca, Mg) and Cl are mostly (>90%) uncomplexed in seawater. The anions S04, COj, and HCO7 are tied up as complexes to a significant extent. When similar calculations are done for the minor elements in seawater, we find a different story. Most of the minor elements exist as complex ions or ion pairs. In particular, the metals form complexes with anions (ligands) such as CO , Cl , and especially OH . The best estimates of the speciation of the elements in seawater are given in Table 9-10. 

Since iodine in these processes is present in various chemical forms, the chemical speciation of iodine is important for understanding its geochemistry in oceans. Further, iodine is redox sensitive and the most abundant biophilic minor element in the oceans. Here, the behaviors of various iodine species in seawater will be described in regard to biological and abiological processes. 

As discussed in the previous section, trace elements are essentially retained in the solid combustion products and, because many are present on the surfaces of the particles, they are potentially leachable. Our data show the elements Mo, As, Cu, Zn, Pb, U, Tl, and Se will be readily accessible for leaching. A significant fraction of the V, Cr, and Ni, and a minor proportion of the Ba and Sr will also be potentially leachable because of the surface association, but most of these elements appear to be located in particles and will be released more slowly as the dissolution of the glass and other phases takes place. Rubidium, Y, Zr, Mn, and Nb are contained almost entirely within the particles and dissolution is potentially slower. The extent to which elements are leached also depends on their speciation and solubility in the porewaters, and the pH exerts a major control. In oxidizing solutions, elements such as, Cd, Cu, Mn, Ni, Pb, and Zn form hydrated cations that adsorb onto mineral surfaces at higher pH values and desorb at lower pH values. In contrast, the elements As, U, Mo, Se, and V, under similar Eh conditions, form oxyanions that adsorb onto mineral surfaces at low pH values and desorb at higher values (Jones 1995). 

Elements considered in seawater speciation calculations can be separated into major and minor components. Such a separation is possible because the vast majority of seawater constituents have concentrations so low that they do not significantly influence the activities of the major cations and anions in seawater. As such, the equilibrium behaviour of the major ions in seawater can be understood (calculated) independently of the numerous minor constituents and these results can then be applied to calculations involving individual minor constituents. 

The Elemental Content of Human Diets and Excreta The Elemental Constituents of Soils Mycotoxins Occurrence, Distribution, and Chemical Speciation of some Minor Dissolved Constituents in Ocean Waters. 

The widely different extraction efficiencies observed for different analytes in relation to the sample matrix and the solvent/extraction procedure utilized call for caution in the interpretation of the results of speciation studies. A compound may be found to largely predominate in an extract containing only a small fraction of the total element concentration and one should be aware that this compound could actually represent a minor species. Moreover, the concentration of the investigated element in the other extracts (if any) and in the residue should be determined to check the accuracy of results. Establishing a mass balance of the analytes in all fractions of the analytical procedure is recommended as it accounts for analyte extraction efficiency and helps evaluating the overall accuracy of speciation results (Fig. 8.4). 

The concentration and speciation of minor and trace elements vary considerably, depending on the particular zone where the ocean is located. Their presence or absence is due to atmospheric input,... 

It would be difficult to find more comprehensive or more detailed studies on the physical chemistry of seawater than those done at the University of Miami (Millero, 2001). Several programs were developed for calculation of activity coefficients and speciation of both major ions and trace elements in seawater. The activity coefficient models have been influenced strongly by the Pitzer method but are best described as hybrid because of the need to use ion-pair formation constants (Millero and Schreiber, 1982). The current model is based on Quick Basic computes activity coefficients for 12 major cations and anions, 7 neutral solutes, and more than 36 minor or trace ions. At 25 °C the ionic strength range is 0-6 m. For major components, the temperature range has been extended to 0-50 °C, and in many cases the temperature dependence is reasonably estimated to 75 °C. Details of the model and the parameters and their sources can be found in Millero and Roy (1997) and Millero and Pierrot (1998). Comparison of some individual-ion activity coefficients and some speciation for seawater computed with the Miami model is shown in Section 5.02.8.6 on model reliability. 

Another example of aqueous speciation that includes redox can be shown with the arsenic pe-pH diagram shown in Figure 1. Arsenic can exist in several oxidation states including As(-lll) as in arsine gas (ASH3), As(0) as in elemental arsenic, As(ll) as in realgar (AsS), As(lll) as in orpiment (AS2S3) and dissolved arsenite, and As(V) as in dissolved arsenate. Figure 1 shows the dominant dissolved species, arsenate and arsenite, and their hydrolysis products as a function of redox potential and pH based on the thermodynamic evaluation of Nordstrom and Archer (2003). These results show the dominance of hydrolysis for arsenate species, but it is of minor consequence for the arsenite species. 

Frarrz G, Andrehs G, Rhede D (1996) Crystal chemistry of morrazite and xenotime from Saxothrrringian-Moldanubian metapelites, NE Bavaria, Germany. Eur J Mineral 8 1097-1108 Gan H, Hess PC (1992) Phosphate speciation in potassium alrrminosilicate glass. Am Mineral 77 495-506 Gorz H, White EH (1970) Minor and trace elements in HF-soluble zircons. Contrib Mineral Petrol 29 180-182... 

If the speciation of trace elements is to be studied, measurements must be performed immediately after sampling. If for some elements minor changes in original speciation are acceptable, then the samples should be stored unacidified but frozen (see also the storage of As, Sb and Ge outlined in Chapter 12) to preserve as much as possible the original distribution of species. A few studies indicate, however, that some metal associations with humic substances might be stable even down to pH 2.3 (e.g., Helmers, 1994). 

Although CE is not directly an LC technique, it is nowadays a powerful separation device used in speciation analysis, providing efficient separation and supplementing LC methods. Analysis time is comparatively short, and separation efficiency is high. Method development and improvements in separation are quickly achieved by appropriate buffer systems. Different separation modes allow separations for nearly all element species. CE shows unique promise for speciation purposes by exerting only minor disturbance on organometallic complexes and species integrity. 

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