Cation exchange element separation

Influence of unstirred layers near the membrane. Near the membrane there exist unstirred layers which under unfavourable conditions can exert a considerable influence on the fluxes and the membrane potential too. F. Helfferich (57) has drawn the attention to this effect. The thickness of these layers depends on the rate of stirring. Under good stirring conditions the film-thickness amounts to 20 to 1 10-3 cm. Under extreme conditions it can be reduced to 10-4 cm. It is not always possible to eliminate its influence (139). The transport in the films is diffusion-controlled. In some cases the effect of the films can be involved in the calculations. As an example the case of selfdiffusion is given here. A cation-exchange resin separates two solutions of identical chemical composition. The cations on either side are isotopes of the same element. 

Various gel- and porous-type resins have been examined for use in the cation-exchange chromatographic separation of the transplutonium elements from the fission-product lanthanides with an eluent of 11.7 M hydrochloric acid (17). In the case of gel-type resins, very fine ones such as colloidal aggregate, are needed to perform good separation. The number of theoretical plate obtained... 

Element 104. The first experimental results on the cation exchange, CIX, separations [213] have shown that Rf is a homolog of Zr and Hf The elution of those elements by 6 M HCl demonstrated that the chloride complexation of Rf is similar to that of Hf and much stronger than that of the actinides. However, disagreements in the sequences of the values for Zr, Hf and Rf complexes sorbed by cation and anion resins fi-om HF and HCl aqueous solutions have been revealed (see [12]). Also, various trends in the hydrolysis of Rf and other group-4 elements were established by various experiments [217,218]. 

Ion-exchange separations can also be made by the use of a polymer with exchangeable anions in this case, the lanthanide or actinide elements must be initially present as complex ions (11,12). The anion-exchange resins Dowex-1 (a copolymer of styrene and divinylben2ene with quaternary ammonium groups) and Amherlite IRA-400 (a quaternary ammonium polystyrene) have been used successfully. The order of elution is often the reverse of that from cationic-exchange resins. 

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

The Multi-Purpose Processing Facility was installed in F Canyon (reprocessing plant) at SRP for separation of Californium and trans-californium elements using newly developed, high-pressure, chromatographic cation exchange processes. 

The ratio of the distribution coefficients of pertechnetate and perrhenate is about 1.6 to 2, comparable to adjacent rare earth metals. Technetium and rhenium may be separated by ion-exchange chromatography. However, efficient separations require some care and tend to be slow. On the other hand, cation exchange resins adsorb technetiiun only to a negligible extent so that pertechnetate can be rapidly separated from cationic elements . 

The new chemistry is based on a Sr-90/Y-90 separation using a-hydroxyisobutyric acid (a-HIB) and cation exchange chromatography (5). Once the activities are loaded onto the column, the steps to prepare the column for the a-HIB elution remove several of the possibile contaminants including rubidium and cobalt. Finally, the a-HIB elution also removes a wide range of other elements as well, leaving strontium on the ion exchange column (6). 

Trace amounts of rare earth elements that exist as impurities in other materials have also been analyzed using cation-exchange HPLC [97]. Ion chromatography was used to separate 14 rare earth elements and to eliminate interferences from polyatomic ions upon direct introduction of the eluent into the HPLC system (Fig. 8). The detection limits of the elements were in the range 1-5 pg ml in solution and ng g 1 in the solid. The linear range extended to six orders of magnitude from 10 pg ml 1 to 10 pig ml. R.S.Ds. were also favourable ( 1% for Lu). 

Strelow, F. W. E., C. R. van Zyl, and C. R. Nolle Separation of Alkaline Earth Elements by Cation-Exchange Chromatography in Ammoniummalonate Media. Anal. Chim. Acta 40, 145 (1968). 

As described in Figure 3.7, TRU separation is performed by implementing the DIDPA process on pretreated PUREX raffinates. A front-end denitration step by formic acid is thus required to reduce the nitric acid concentration of the feed down to 0.5 M to allow the TRU elements to be extracted by the cation exchanger di-fvo-dccyl-phosphoric acid (DIDPA). This preliminary step, however, induces the precipitation of Mo and Zr (and thus the potential carrying of Pu), which requires filtration steps. The TRU and Ln(III) elements are coextracted by a solvent composed of the dimerized DIDPA and TBP, dissolved at 0.5 and 0.1 M, respectively, in n-dodecane. The An(III) + Ln(III) fraction is back-extracted into a concentrated 4 M nitric acid solution, whereas Np and Pu are selectively stripped by oxalic acid. 

There have been few elemental speciation studies in the literature involving cation-exchange chromatography (CEC) coupled to ICP-MS. A cation-exchange column was used by Larsen et al. [57,69] for arsenic speciation in several seafood sample extracts. The chromatography was optimized for the separation of arsenocholine, trimethylarsinic, trimethylarsine oxide, inorganic As, and two unknown cationic arsenic compounds. A mobile phase of 20 mM pyridinium ion, at pH 2.65, was used to perform the separation (Fig. 10.10). 

K. E. 0degard, W. Lund, Multi-element speciation of tea infusion using cation-exchange separation and size-exclusion chromatography in combination with inductively coupled plasma mass spectrometry, J. Anal. Atom Spectrom., 12 (1997), 403-408. 

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