Lanthanides, distribution coefficients

Enough studies have been done to demonstrate unequivocally the value of mathematical models to estimate lanthanide behavior during rock and magmaforming processes. Most results are rather semiquantitative, subject to substantial uncertainties. The extent to which lanthanide distributions can yield more quantitatively accurate information such as lanthanide concentrations in source regions, or fractions of liquids solidified or solids melted, or to enable establishment of clear-cut genetic relationships is still being evaluated. One of the crucial steps in this evaluation is accurate determination of values for lanthanide distribution coefficients and the extent to which those values depend on pressure, temperature, and composition. Several studies have been done, but few systematic measurements to show effects of temperature, pressure, and composition have been done. 

Schnetzler and Philpotts (1968) obtained values for lanthanide distribution coefficients by the phenocryst-host matrix method. Phenocrysts are large mineral crystals found in otherwise fine-grained frozen lavas. The phenocrysts are presumed to have grown in equilibrium with the lava at depth prior to eruption of the lava. On eruption, the phenocrysts are swept along with the host lava, which quickly chills around them. By measuring the lanthanide concentrations in the phenocrysts (solid phase) and the host matrix (liquid phase) and taking the ratio, values for distribution coefficients are obtained. Accuracy depends on whether the phenocrysts truly grew in equilibrium with the liquid, whether the host matrix really represents that liquid, and whether both phases can be sampled without contamination by each other and accurately analyzed. 

Fig. 21.19. Typical values of lanthanide distribution coefficients for common, rock-forming minerals. Values for apatite inferred from analysis of Skaergaard intrusion (Paster et al., 1974) rest from phenocryst-matrix analyses (Schnetzler and Philpotts, 1970).

Drake and Weill (1975) used the same experimental technique as Grutzeck et al. to determine lanthanide distribution coefficients for feldspar crystallized from a silicate liquid. Values fell about the middle of the range observed for phenocryst-matrix pairs. They found, as expected, that D for Eu increased as the partial pressure of oxygen was decreased (fo2 ranged from lO to 10 ). 

Lanthanide distribution coefficients were determined by Shimizu and Kushiro (1975) on a synthetic garnet. The garnet was separated from the quenched parent liquid for measurement of lanthanide concentrations by isotope dilution mass spectrometry. Values for the heavier lanthanides (fig. 21.20) are not appreciably disturbed by possible contamination by quenched liquid, but those for the lightest lanthanides may be substantially too high. 

Therefore the extent of extraction or back-extraction is governed by the concentration of X ia the aqueous phase, the distribution coefficients, and selectivities depending on the anion. In nitrate solutions, the distribution coefficient decreases as the atomic number of the REE increases, whereas ia thiocyanate solutions, the distribution coefficient roughly increases as the atomic number of the REE increases. The position of yttrium in the lanthanide series is not the same in nitrate and thiocyanate solutions, and this phenomenon has been used for high purity yttrium manufacture in the past. A combination of extraction by carboxyUc acids then by ammonium salts is also utilized for production of high purity yttrium. 

Fig. 11.19 Distribution coefficients for lanthanide extraction with Aliquat 336 from nitrate (solid squares) and thiocyanate (solid circles) media. (From Ref. 1.)... 

The rate of extraction of the lanthanides by the 2-ethylhexyl ester of 2-ethylhexylphosphonic acid (e.g., PC88A, P507) is slower than for DEHPA but the higher distribution coefficients allow extraction from more dilute solutions. Also, as acidic stripping is easier than with DEHPA, the reagent has been used commercially to separate lutetium from terbium and ytterbium. 

The distribution coefficients for europium also appear to be strongly influenced by low values of the solution pH. Such behavior is similar to that of barium and silver and again suggests that the sorption of the lanthanides may be due to at least two separate mechanisms, which are also probably essentially the same as those discussed for barium. 

Yttrium and the lanthanide FPs, mainly cerium, praseodymium and promethium, will be present in the dissolver solution as trications, which are poorly extracted by TBP/OK. Distribution coefficient measurements show293 that the trisolvates are extracted according to equation (157). 

As reviewed by Fidelis and Mioduski57) the formation constant K of a complex with a given" ligand (or the distribution coefficient for extraction in another solvent, or an ion-exchange resin) shows a ratio (in the case of two consecutive lanthanides) which provides perceptible variations (from a constant) not only at the half-filled shell (q = 7) Gd(III) but also at the plateaux q = 3 and 4, as well as 10 and 11. These quarter-shell effects can be rationalized 217,218) by the refined spin-pairing energy theory. If D of Eq. (3) is decreased 1 % (65 cm 1) by the nephelauxetic effect in a... 

It was found that when extracting lanthanide elements with tributyl phosphate at low pH, IgD-Z showed an odd-even effect, which is observed when plotting the logarithm of distribution coefficient D versus the atomic number Z. Straight lines are plotted when Z is odd or even but the odd line is above the even one. Since this report, a lot of data have been reported and presented differently. Figure 1.14 shows typical curves for the change in lanthanide gradation. The lanthanide tetrad effect will also be very clear if the y-axis is not log D but Ig ex... 

Zone chromatography is a variant of the zone melting method, in which the mixture being separated is introduced into a column with a solid solvent and a molten zone is passed repeatedly along the length of the column to separate mixtures into separate bands of their components. Zone chromatography has been used for the separation of mixtures of lanthanides for preparative and analytical purposes The chelates used were mixtures of hydrated / -diketonates and their adducts with 2,2 -bipyridyl (bipy) and acety-lacetonimines. The distribution coefficients of different chelates and binary mixtures have been determined . 

A series of batch equilibrations was carried out to determine the relative affinities of selected lanthanides and actinides for niobate, zirconate, and titanate ion exchange materials as a function of pH. These affinities are expressed as distribution coefficients (l9 ), where ... 

We next turn to the ion-exchange separation and the liquid-liquid (solvent) extraction of the lanthanides and the actinides. Except in a few cases, the actual values of the distribution coefficient 0) defined as... 

Trivalent americium forms relatively unstable complexes with Cl and NOs and more stable complexes with the thiocyanate ion CNS. These americium complexes are more stable than those of the corresponding lanthanide compounds, so that americium can be separated from trivalent lanthanides by anion exchange with concentrated solutions of liQ, liNOs, or NH4CNS. Trivalent americium can be extracted with TBP from a concentrated nitrate solution. It can also be extracted with TBP from a molten LINO3 -KNOs eutectic at 150°C, with much higher distribution coefficients than in extraction from aqueous solutions. Americium is more readily extracted by this process than is trivalent curium [K2]. 

The development of ion-exchange methods for separating lanthanides and various fission products was instrumental in the development of atomic reactors. Tables of distribution coefficients as a function of pH for almost every cation in the... 

Following the development of more precise analytical methods for the lanthanides about 20 years ago, a quantitative approach to modelling trace element distribution during crystal-liquid equilibria was developed (see Haskin 1984 and DePaolo 1981b for relevant equations). The distribution coefficient, K, is a measure of the partitioning behaviour of an element between a crystal and a co-existing melt ... 

Quantitative modelling has been less successfully applied to rocks cff more felsic composition, such as granodiorites, dacites, granites and rhyolites. This is principally due to the ubiquitous presence in these evolved rocks of minor mineral phases, such as sphene, allanite, apatite and zircon, whose lanthanide contents may account for a substantial fraction of the total rock budget. Thus Gromet and Silver (1983) found that sphene and allanite, in a granodiorite from the Peninsular Ranges, California, contained 80-95% of the lanthanide content of the total rock. Distribution coefficients are not well known for these phases and the abundances of these trace minerals are difficult to determine accurately. 

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