Lanthanide fractionation

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

Raw material for the 4 Gd target was produced by spallation reactions in a tantalum metal target by 750-MeV protons at the Isotopes Production Facility at LAMPF. The details of the irradiation and the chemistries associated with separating the hafnium and lanthanide fractions have been reported previously.9 ... 

Large-scale purification of americium, curium, and californium with pressurized cation exchange has been planned at SRP for many years (1). Initial work involved SRP batch extractions to isolate a crude actinide-lanthanide fraction followed by solvent extraction and ion exchange in the SRL high level caves (1J. For large-scale purification, a single step was substituted for batch extraction and solvent extraction. Plant Purex solvent (30 vol % tri-n-butyl phosphate in n-paraffin) was used to minimize flush time and cross-contamination of solvent. 

D ecent development of our knowledge of lanthanide distributions in nature encouraged us to believe that variations in ratios of these elements might well characterize individual water masses as summarized by Haskin et al. (5), considerable lanthanide fractionation has occurred in the formation of the earth s crust it might be expected that these fractionations would be reflected in the lanthanide patterns of material eroded from diflFerent regions and supplied to the oceans. Since, on the other hand, the lanthanide patterns of marine shales and sediments (5, 6, 11) do not reflect these regional diflFerences but are essentially uniform on a world-wide basis, sea water should express the diflFerential residues on a... 

Of typical separations methods, those based on volatility (like distillation) are of little importance in lanthanide/actinide separations. Precipitation methods and other biphasic separations processes are by far the most useful. For the separation of macroscopic amounts of the individual lanthanides, fractional crystallization was the principal technique in the early days of the investigation of the lanthanides. The small solubility differences required hundreds or even thousands of repetitions to achieve useful separation of the elements (Moeller 1963). 

Lanthanide distribution models (Elderfield 1988, Byrne and Kim 1993, Erel and Morgan 1991, Erel and Stolper 1993) provide a basis for the comparative shale-normalized lanthanide concentrations (lanthanide fractionations) which are observed in the oceans. Lanthanide fractionation models are formulated in terms of competitive complexation equilibria involving, on one hand, solution complexation, and on the other, surface complexation on marine particles. Following the developments of Elderfield (1988) and Byrne and Kim (1993), shale-normalized lanthanide concentrations (Mj)sn in seawater can be expressed as... 

Heavy-lanthanide enrichments in seawater are reasonably explained by eqs. (1) and (2) wherein the extent of lanthanide solution complexation increases between La and Lu to a much greater degree than is the case for surface complexation. It is probable that competitive solution/surface complexation exerts the dominant role in heavy-lanthanide enrichments in seawater. However, it should be noted, as well, that lanthanide phosphate co-precipitation also appears to promote heavy-lanthanide enrichments in solution. Although the existence of lanthanide phosphate precipitation in seawater has not been directly demonstrated, such controls are consistent with observed total lanthanide concentrations in some environments (Byrne and Kim 1993, Johannesson et al. 1995) and are consistent, as well, with the general features of lanthanide fractionation in the oceans. 

Coagulation is caused by salting out of river-borne colloid-containing lanthanides. This removal process leads to lanthanide fractionation. 

In summary, the lanthanides undergo an array of reactions in estuaries which affect both their absolute and relative lanthanide abundances. Hence, estuaries are excellent natural laboratories to test and study the influence of aquatic geochemical processes on lanthanide fractionation. To what extent does fractionation in estuaries control the composition of seawater Is the release of dissolved lanthanides from estuarine shelf sediments a quantitatively important process with respect to the ocean budget and composition Answers to these and other related questions will require more detailed study and modeling. 

Fig. 15. Schematic model of lanthanide fractionation between particles and seawater reproduced from Sholkovitz et al. (1994). Main features include (1) the systematic variation in the relative affinity of lanthanide(lll) for complexation to solution carbonates and binding to particles,...

Interelement lanthanide fractionation in the oceans allow insights into basic processes controlling the concentrations and relative abundance of trace elements. The best synthesis of lanthanide fractionation in the oceans appears in Bertram and Elderfield (1993). This subsection relies heavily on this paper and focuses on strictly trivalent elements. 

Fig. 41. Concentration of dissolved Ce, Nd, Eu, and Yb in the upper porewater (0-1 cm) as a function of time for the CHEER time-series study of Chesapeake Bay. Dashed line for Yb indicates that no heavy-lanthanide fraction was measured for samples on days 138 and 166. From Sholkovitz et al. (1992).

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