Distribution ratios of lanthanides

FIGURE 4.11 Distribution ratios of lanthanides (10 5 M), Am, and Cm (trace level) from 3 M HN03 solutions into NPHE with CPnl, CPn2, CPn3, and CPn4. 

Under the same conditions, in contrast to what is observed for calix[4]arene-bearing CMPO moieties, with CPil2, distribution ratios of lanthanides increase from the lightest lanthanide, lanthanum, to europium. Americium can be easily separated from the lightest lanthanides (separation factor DAm/La > 20, DAm/Ce =15, /lAlll,Nd = 10, UAi /si = 7.5, DAm/Eu = 6), which are the most abundant lanthanides in fission-product solution. Cavitands bearing picolinamide (Cv5) or thiopicolin-amide (Cv6) residues seems much less selective than their calixarene counterparts, giving SAm/Eu < 2.18... 

Distribution Ratios for Lanthanides and Actinides from Aqueous Solutions 1.5 M HN03 into a NPHE Solution of OOCMPO (0.25 M), 054, and CPw3 (10 3 M)... 

The radioly tic behavior of a substituted picolinamide calix[6]arene, studied for the separation of actinides from lanthanides, was recently investigated by Mariani (263). For doses ranging from 0.014 to 0.055 MGy, the distribution ratios of both Am(III) and Ln(III) strongly increased, whereas after absorbed doses higher than 0.10 MGy, they decreased to values lower than those measured for nonirradiated samples. The selectivity for Am/Eu remained constant. Comparable experiments under a nitrogen atmosphere indicated the role of oxygen in the radiolysis, because the distribution ratios decreased by factors of 10 and 1.5-5 for Am-Eu and other lanthanides, respectively. The increase for lower doses was then explained by the formation of oxidized radiolytic products. No evidence of new products was obtained with the ESI-MS technique. 

The separation of americium from the lanthanides is feasible by either solvent extraction or ion-exchange process. For the solvent extraction, 0.25 M DIDPA diluted with diisopropylbenzene (DIPB) provides appropriate distribution ratios of americium between the organic phase and the aqueous phase composed of 0.05 M NasDTPA and 1 M lactic acid. 

The partitioning behavior of some metal ions depends greatly on the pH in the ABS and their distribution ratios may change markedly in acidic or basic media. The distribution ratios of actinides and lanthanides in the PEG-2000/(NH/i)2SO/i ABS are very low at low pH compared to those at high pH. Changing the pH can thus be used to strip metal ions back into a salt-rich phase [21. In Ref. 66, 70% Fe " was extracted into the PEG-rich phase at pH 4.5 and separated from Sc " and Cu " in a PEG-2000/ (NH4)2SO-i ABS with Chrome Azurol S. The extraction of Fe " at pH < 2 was much lower ( I0%) thus Fe " could be stripped by changing the pH. 

Yoshida (1964) investigated the solvent extraction behavior of lanthanides in the system M -HC104-TBP and reported the distribution ratio of the lanthanides to be third-power dependent upon the concentration of perchlorate ions in the aqueous phase and sixth-power dependent upon the concentration of TBP in the organic phase. The lanthanide extraction reaction can be represented by... 

What became known as the tetrad effect was first observed in the late 1960s during lanthanide separation experiments [25]. Fig. 1.3 shows a plot of log K, where is the distribution ratio between the aqueous and organic phases in a liquid-liquid extraction system. There are four humps separated by three minima, first at the f /f pair, secondly at the f point, and thirdly at the pair. 

The maximum distribution ratio for the Pm-HA system (D about 0.1) is reached in the pH range 6-7. It is well known that the lanthanides hydrolyze in this pH region, but it can be shown that the concentration of hydrolyzed species is <1 % of the concentration of PmAn species for the conditions of Fig. 

Fig. 4.15 The system La(III) acetylacetone (HA) - IM NaC104/benzene at 25°C as a function of lanthanide atomic number Z. (a) The distribution ratio Hl (stars, right axis) at [A ] = 10 and [HA] rg = 0.1 M, and extraction constants (crosses, left axis) for the reaction Ln + 4HA(org) LnA3HA(org) + 3FE. (b) The formation constants, K , for formation of LnA " lanthanide acetylacetonate complexes (a break at 64Gd is indicated) circles n = 1 crosses n = 2 triangles w = 3 squares w = 4. (c) The self-adduct formation constants, for the reaction of LnA3(org) + HA(org) LnA3HA(org) for org = benzene. (A second adduct, LnA3(HA)2, also seems to form for the lightest Ln ions.) (d) The distribution constant Ajc for hydrated lanthanum triacetylacetonates, LnAs (H20)2 3, between benzene and IM NaC104. (From Ref. 28.)...

Distribution Ratios for the Extraction of Several Lanthanides and Americium from Aqueous Solutions (3-4 M HN03) into NPHE by CPw3 and CPw17 Calixarenes... 

The grafting of CMPO moieties on the narrow rim affords a strong decrease of extracting ability toward lanthanides, trivalent actinides, and tetravalent plutonium from acidic solutions. The distribution ratios for the different calixarenes in NPHE are low, except for CPn3 for which the number of carbon atoms in the spacer is four, but even for this compound, the distribution ratios are lower than those obtained with their wide-rim counterparts (Figure 4.11). 

In order to find compounds able to perform an efficient lanthanide/actinide separation, the Twente group prepared two cavitands (Cvl and Cv2) functionalized with CMPO groups and two cavitands (Cv3 and Cv4) with carbamoylmethylphosphonate (CMP) groups192193 (see Section 4.7). Distribution ratios displayed by CMPO cavitands are much lower than those found for the calixarene counterpart. This important decrease of extracting ability of cavitand is probably due to the presence of a carbon atom between the benzene unit and the nitrogen atom, causing N-protonation below pH 2. Furthermore, the Am/Eu selectivity is less than that of CMPO-calix[4]... 

Although americium (Am) exists in seawater exclusively in the trivalent oxidation state, its profiles in Fig. 12.4 contrast sharply with those of the trivalent lanthanides. Assessments of Nd isotopic ratios in seawater (e.g. Bertram and Elderfield, 1993) indicate that more than 1000 years are required for attainment of steady-state distributions of lanthanides and chemically similar elements in seawater. On such a basis it is expected that, in spite of substantial chemical similarities to the lanthanides, 241Am, a relatively short-lived isotope (half-life 470 years) with variable and recent anthropogenic inputs, will not exhibit profiles similar to those of the lanthanides. 

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