Single extraction radionuclides

There are special extractants to extract each class of radionuclides crown ethers for cesium and strontium and phosphine oxides, carbamoylmethylphosphine oxides, and diamides for actinides, etc. It is unrealistic to have a single extractant that can extract all target nuclides with nearly the same effectiveness. So, a promising technical decision is to mix extractants for different radionuclides and extract them simultaneously. 

Equilibration usually is reached in a few minutes, but should be checked for each method. Typically, the radionuclide in the aqueous phase is extracted into the organic phase under one set of conditions, and is then back-extracted under a second set of conditions. Washing steps commonly are inserted after extraction to improve the specificity of the radionuclide transfer. A single extraction and back-extraction cycle may suffice for purification or several cycles may be necessary. In analogy to ion-exchange systems, column separations have been developed with countercurrent flow of the two liquids. 

Some extractants have been used in the development of selective sensors for radionuclides. These have been noted in Tables 9.1 through 9.3 and briefly mentioned above. They were recently reviewed in detail.99 In addition to column sensor formats, extractants have been coated onto passivated ion-implanted planar silicon (PIPS) diodes to create selective radionuclide sensors.152153 Though not a focus of this review, it is worth noting the such sensors, combining separation and detection in a single functional unit, have potential for use in process-monitoring applications. 

Classical speciation of radionuclides is described in Chapter 13. Methodology for single and sequential extraction of soil to assess radionuclide availability to plants is similar to that used for heavy metals, and has recently been reviewed (Kennedy et al., 1997). Therefore, only recent applications of sequential extraction to speci-ate both natural and anthropogenic nuclides are discussed below. 

Kennedy, VH., Sanchez, A.L., Oughton, D.H. and Rowland, A.R (1997) Use of single and sequential extractants to assess radionuclide and heavy metal availability from soil to roots. Analyst, 122, 89R-100R. 

Extractants from liquid-liquid systems such as HDEHP [di(2-ethylhexyl) or-thophosphoric acid] and CMPO/TBP (carbamoylmethylphosphine oxide derivative and tri-n-butyl phosphate) are supported on the solid material, as are newer ion-selective crown ethers (such as 4,4 (50-di-f-butylcyclohexano 18-crown-6 for Sr). Various SPE columns available commercially from Eichrom Industries have proven useful to separate radionuclides such as Sr, Tc, Ra, Ni, Pb, Am, Pu, Th, U, Np, Cm, and lanthanides. These columns usually are small (approximately 2 ml resin bed). Their effectiveness depends on their specificity for the ion that includes the radionuclide of interest, but the small volume limits the amount (i.e., less than 10 mg) of carrier that can be retained. The specificity of each product shows promise for development of procedures for sequential radionuclide analyses from a single sample aliquot. (Burnett et al. 1997, Horowitz et al. 1998)... 

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