Trace element fractions exchangeable

Table 4.3. Dissolution of some trace elements extracted by NaOAc-HOAc at various pHs after removing the exchangeable fraction (mg/kg)a... 

The bioavailability of trace elements in soil-water-plant ecosystems of arid zones is largely determined by their partitioning between solution and solid-phase components. The redistribution and transformation of trace elements among solid-phase components under various biogeochemical conditions strongly adjust their lability and bioavailability. Soluble, exchangeable and organically bound forms are bioavailable to plants. The carbonate bound fraction represents a reservoir of potentially bio-available trace elements to plants. In paddy soils, the amorphous Fe/Al oxide fraction contributes to trace element availability as well. 

Toxic trace elements were isolated from water samples by extraction with di-ethyldithiocarbamate (Table 2.1.2). Following this pre-concentration step the metal ions were adsorbed on a cation-exchange resin using a mixture of tetrahydro-furan-methylglycol-6 M HCl as sorption solution. The succesive elution was treated with 6 M HCl, 1 M HCl and 2 M HNO3 for fractional separation. In another application hexane-isopropanol-HCl mixture was used as the adsorption medium An analytical scheme which provides quantitative results, is described for ion-exchange separation of fifteen major, minor and trace elements in silicates For concentration and separation of copper, chromium, lead and iron an ion-exchanger in phosphate or OH -form was used in various combinations ... 

For equilibrium conditions, Donnan ion exchange theory (6) may be used to describe quantitatively equilibria for trace quantities of chemical elements in the presence of high homologous salt concentrations. The radionuclides 90Sr and 137Cs constitute a very small fraction of the mass of the chemical constituents in rainfall thus the equilibrium distribution of these nuclides between solid and liquid phases may be estimated as... 

When a given nucKde is used to trace the yield of its element in a chemical reaction, the fundamental assumption is that the chemical state of all isotopes of the same element is identical this is ensured through mixing and performing isotopic-exchange reactions. In tracer chemistry, it is important that there be no isotopic fractionation. If a signiftcant period of time has elapsed between the characterization of the concentration of the tracer nuclide and the measurement of its concentration in the final sample, then an appropriate radioactive decay correction must be appKed. 

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