Aqueous media lanthanide ions

The potential of diluent modification in enhancing lanthanide/actinide separation factors has barely been tapped. The absence of a reliable predictive theory of solvation of both the extractant molecules and the extracted complexes hampers the development of this area. In view of the small energies required to reverse extraction order (a few hundred joules), subtle alteration of the organic diluent (or the aqueous medium in ion exchange procedures) also has the potential for significantly improving (at least) group separations. 

The cellulose phosphate was widely applied in the removal process of divalent and trivalent (Cu ", Zn ", Co, Ni, Pb ", Mn ", Fe, Fe , Cr ) ions because of the rapid adsorption of the metals ions in comparison with the synthetic polymers. Also it was observed that the cellulose phosphate functions as an ion exchanger for lanthanide ion removal from aqueous solution. In order to evaluate the adsorbent characteristics of each support in the removal process of metal ions from aqueous solution, the most widely used method by researchers has been the batchwise one. In any adsorption process the most important parameter is the pH of the medium, which depends on the nature of the support used and also on which metal ions are to be removed. In most cases, researchers worked with solutions that have an acid pH value in order to avoid the possible precipitation of the studied metal ions. A complex study of the pH influence upon the efficiency of Fe ", Cu ", Mn, Zn, Co ", and lanthanide(III) ions removal from aqueous solutions with PBC was made by Oshima et al. Their conclusion was that the lanthanide ions are adsorbed when the pH value of the aqueous solution is less than 3, and in the case of transition metal ions the adsorption percentage increases with increasing aqueous pH and reaches over 90% at a pH value of around 4.5. 

Adsorption and partition chromatography relies on the uptake of lanthanide ions by silica or alumina as a solid-phase transfer medium with aqueous chelating agents used for partitioning. 

In an aqueous medium, cations are solvated by some number of water molecules, the number of water molecules being determined primarily by the charge and size of the cation. The size of the solvent sheath carried by the cation and its complexes is clearly of significance in predicting the relative mobility of lanthanide ions as they traverse an analytical column, as transport properties are proportional to the fit of the analyte into the normal solvent structure. In the following paragraphs, we will explore a few of the more interesting aspects of these phenomena. 

Furthermore, lanthanides form stable complexes with polydentate chelators like DTPA, which exhibit a noncyclic structure. Two structures are depicted in Scheme 2. The following examples are only representatives for the variety of analyte molecules that can be determined by these kind of lanthanide complexes. Structure 9 employs a quinolyl ligand both as chromophore and acceptor for Zn ". The emission of the europium ion is strongly enhanced upon binding of Zn " and showing distinct selectivity over other biologically relevant metal cations in aqueous solution at neutral pH [29]. The luminescence of the chelate 10 is efficiently quenched by Cu " ions in aqueous medium [30]. The presence of Fe ", Co ", Ni ", Cr ", and Mn " interferes with the determination of Cu, although to a relatively small extent, whereas the ions Zn ", Cd ", Hg, and Pb do not interact with probe 10. 

Another example of an aqueous medium effect is the use of alcohol/water mixtures in several successful ion-exchange-based separation processes for the lanthanides and actinides. A third example, relevant to solvent extraction alone, is the effect of the diluent on the extraction process. The effect of these phenomena, and the reasons for their success (or failure) is the subject of the next section. 

The interaction between bromide and lanthanide ions was also studied, but experimental data are available for aqueous methanol solutions only (Kozachenko et al. 1973). Using a spectrophotometric method, the formation of rather weak outer-sphere bromo complexes was evidenced, and their stability constants for Pr, Nd, Sm, and Ho were determined in water and in 50% and 90% methanol (table 3). For solutions in 50% methanol, the stability of the outer-sphere bromo complexes is larger for Pr, Nd, and Sm Ki = 1.3—1.9) compared to Ho (0.97) and Er (0.70). Kozachenko et al. (1973) explained this behaviour as reflecting a higher stability for the solvates of the heavier lanthanide ions. A similar trend was observed in the stability constants of the chloro complexes in absolute methanol vide supra). Finally, the stability of the bromo complexes of the lanthanides increases as the dielectric constant of the medium is reduced. 

An additional phenomenon related to the anion in the aqueous phase is a so-called perchlorate effect (Gmelin 1983). It has been often observed that extraction of metal ions from perchlorate media is greater than that from equivalent nitrate or chloride solutions. Marcus reports (Gmelin 1983) the effect in two different systems involving acidic extractant molecules. The enhanced extraction of metal ions is also observed in systems based on neutral molecules. In the latter case, formation of aqueous complexes of stoichiometry 1 3 (R L) is required for neutrality in the extracted complex. As lanthanide perchlorate ion pairs normally do not exist in aqueous solution, it is something of a dichotomy that extraction from perchlorate medium should be more readily achieved than from more strongly complexing nitrate (or even chloride) solutions. An explanation for the phenomenon, and for the relative ease of extraction of metal ions from salt solutions (relative to that from equivalent acids), may lie in the effect of the solutes on water structure. 

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