**8. Anion exchange**

In the separation and preparation of rare earth elements with a high degree of purity and separation of macro quantities from micro quantities practical application of anion exchangers to this end took place much later than that of cation exchangers because the mechanism of the processes involved in anion exchangers was more complex and for a long time was not fully explained.

Rare earth elements(III) show little tendency to form anionic complexes with simple inorganic ligands and are poorly sorbed on the anion exchangers from aqueous solutions of hydrochloric and nitric(V) acids. They are also weakly sorbed from sulphuric(VI), phosphoric(V) and mixture of hydrochloric and hydrofluoric acid solutions (Jegorov & Makarova, 1971). Much better results of rare earth elements(III) sorption were obtained from the solutions of such salts as chlorides, nitrates(III), nitrates(V), sulphates(IV), sulphates(VI), thiocyanates, thiosulphates and carbonates (Marcus & Nelson, 1959), which for the chloride system was interpreted by HCl2 formation which as a stronger anion than HCl acid has greater affinity for the anion exchanger than Cl ion (Minczewski et al. 1982). It was also shown that using the gradient elution of 6-3 M LiCl solutions at 351 K the separated elements are eluted in the order: Cs(I), Ba(II), Yb(III), Eu(III), Sm(III), Nd(III), Pr(III), Ce(III), La(III). For 3 M solution of Mg(NO3)2 there was obtained the analogous elution series: Gd(III), Eu(III), Sm(III), Nd(III), Pr(III), Ce(III), La(III) and the heavy lanthanides are poorly separated.

116 Ion Exchange Technologies

Hubicki, 1983b).

Y(III) from Sm(III), Eu(III) and Gd(III).

process of lanthanum(III).

**8. Anion exchange** 

time was not fully explained.

for the chloride system was interpreted by HCl2-

high concentrations in the eluate.

acids were used (Jegorov & Makarova, 1971; Hubicka & Hubicki, 1983a; Hubicka &

Of them, special attention should be paid to pyruvic acid (Hubicka & Hubicki, 1983a; Hubicka & Hubicki, 1983b). The pyruvic acid solutions at the concentration 0.15-0.4 M at pH 3.5 and 5.0 proved to be useful for separating such pairs of elements as Y(III)-Nd(III), Sm(III)-Nd(III) as well as for separation of lanthanum(III) from other light lanthanides(III), yttrium(III) from heavy lanthanides using the cation exchanger Wofatit KPS with 4 and 8% DVB. Using pyruvic acid the elution of rare earth elements proceeds in the order of

decreasing atomic numbers. Yttrium becomes similar to the medium lanthanides(III).

In the case of thiodiglycolic acid application for the separation of rare earth elements, the unusual position of Y(III) in the elution series can be seen. It can be as follows: Sm(III), Eu(III), Gd(III), Nd(III), Pr(III), Dy(III), Ho(III), Er(III), Yb(III), Lu(III), Y(III), La(III). Yttrium(III) elutes after heavy lanthanides, which enables its separation from Dy(III). The thiodiglycolic acid solution at the concentration 0.15 M and pH 5.5 can be applied for the separation of Y(III) from Nd(III); Sm(III) from light lanthanides(III) and Y(III) as well as

Using -hydroxoethylideno-1,1-diphosphonic acid proved that Y(III) behaves as a medium lanthanide(III) and can be separated from heavy lanthanides(III) (especially from Lu(III), Yb(III) and Tm(III)) as well as from Nd(III) (Hubicka & Hubicki, 1980). Availability and low price of this acid also provides an opportunity to use it as an eluent in the purification

The disadvantage of ion exchange separation of mixtures of rare earth elements on the polystyrene-sulphonic cation exchangers is the lack of universal eluent, which would allow for selective separation of light, medium and heavy lanthanides(III) as well as to achieve

In the separation and preparation of rare earth elements with a high degree of purity and separation of macro quantities from micro quantities practical application of anion exchangers to this end took place much later than that of cation exchangers because the mechanism of the processes involved in anion exchangers was more complex and for a long

Rare earth elements(III) show little tendency to form anionic complexes with simple inorganic ligands and are poorly sorbed on the anion exchangers from aqueous solutions of hydrochloric and nitric(V) acids. They are also weakly sorbed from sulphuric(VI), phosphoric(V) and mixture of hydrochloric and hydrofluoric acid solutions (Jegorov & Makarova, 1971). Much better results of rare earth elements(III) sorption were obtained from the solutions of such salts as chlorides, nitrates(III), nitrates(V), sulphates(IV), sulphates(VI), thiocyanates, thiosulphates and carbonates (Marcus & Nelson, 1959), which

formation which as a stronger anion than

In practice, the anion exchangers were therefore used only for the separation of thorium(IV) and uranium(IV, VI) from rare earth elements(III) from the solutions of mineral acids (Buddery et al. 1959; Marhol, 1982). Thorium(IV) in the nitric(V) acid and uranium(VI) in hydrochloric acid medium form stable, anionic complexes and therefore their separation from rare earth elements(III) forming less stable, cationic, neutral or anionic complexes is possible. Using 7 M HNO3 solution selective separation of thorium(IV) from rare earth elements(III) was achieved (Danon, 1960).

Significant improvement in the sorption and separation processes of nitrate(V) complexes of rare earth elements(III), due to higher rates of separation, may be obtained by the addition of methanol to the aqueous solution of nitric(V) acid (Stewart & Faris, 1956, Faris & Wharton, 1962). A similar role is also played by higher order alcohols, derivatives of ethylene glycol, dioxane, acetone and tetrahydrofurane.

In the case of such eluents as bufforic solutions of citric and -hydroxyisobutiric acids rare earth elements from strongly basic anion exchangers are eluted in the reverse order than in the case of their elution from the cation exchangers: La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III). Yttrium(III) occupies the position near dysprosium(III).

The studies begun by Dybczyński (Minczewski & Dybczyński, 1962a; Minczewski & Dybczyński, 1962b) on application of complexing agents from the group of aminopolycarboxylic acids (EDTA, CDTA) for the micro quantities of rare earth elements separation reveal that their separation mechanism depends on the factors affecting differentiated anion exchange affinity in the studied system.

Determined by Dybczyński for radioactive indicators the order of elution of rare earth elements(III) using EDTA from the strongly basic anion exchanger Dowex 1x4 in the EDTA form is as follows: Lu(III), Yb(III), Tm(III), Sc(III), Er(III), Y(III), Ho(III), La(III), Dy(III), Ce(III), Tb(III), Pr(III), Nd(III), Gd(III), Pm(III), Eu(III), Sm(III). While the elution order for radioactive indicators of rare earth elements(III) with CDTA from the same anion exchanger in the CDTA form is as follows: Lu(III), Yb(III), Tm(III), Er(III), Y(III), Ho(III), Dy(III), Tb(III), Sc(III), La(III), Gd(III), Eu(III), Ce(III), Pr(III), Sm(III), Nd(III), Pm(III).

Selectivity coefficients of complexes of rare earth elements(III) with EDTA and CDTA in both systems initially grow with the increasing atomic number, pass through a maximum and then decrease (Dybczyński, 1964; Wódkiewicz & Dybczyński, 1968; Wódkiewicz & Dybczyński, 1972). The change of the maximum values of selectivity coefficients from the position of Sm(III) (when EDTA was used as the eluent) to the position of Pm(III) (when

CDTA was used as the eluent) can be conditioned by the necessity of the presence of metal ion with a larger ionic radius, in order to ensure optimal packing of ligands around the central ion. Selectivity coefficients of complexes of rare earth elements(III) with CDTA, in most cases, are lower than in the case of rare earth elements(III) complexes with EDTA. The higher values of the theoretical plates designated for the system with CDTA also demonstrated a less favorable kinetic reaction of ion exchange than in the case of EDTA.

It was also shown that not only the type of eluent used affects the quality of the separation of the above mentioned complexes of rare earth elements(III). In the selected chromatographic system, the values of the separation coefficients can also be modified by changes in temperature and the degree of cross linking of the anion exchanger (Dybczyński, 1964).

Increasing the temperature generally results in improvement of the kinetics of ion exchange (an increase of diffusion coefficients in the ion exchange phase and solution), which significantly reduces the height of the theoretical plates. The temperature rise also affects the selectivity coefficients and separation factors, for example for CDTA the selectivity coefficients increase with the increasing atomic number from La(III) to Pm(III) and then decrease with further increase in the atomic number. At higher temperatures they reach the maximum value for Nd(III). The order of elution of rare earth elements(III) at 365 K is thus as follows: Lu(III), Yb(III), Tm(III), Er(III), Y(III), Ho(III), Dy(III), Tb(III), Gd(III), La(III), Eu(III), Sm(III), Ce(III), Pm(III), Pr(III), Nd(III).

The temperature rise does not always lead to improvement of the separation process. Distribution coefficients and the values of the theoretical plates can either increase, decrease, or remain constant with the increasing temperature, depending on the system. For elution of rare earth elements(III) with EDTA on the anion exchanger Dowex 1x4 in the EDTA form, for some pairs of elements as, for example, Pm(III)-Eu(III) and La(III)-Tb(III) reverse of selectivity takes place (Minczewski et al. 1982).

Dybczyński (1970) examined the impact of the degree of cross linking of the anion exchanger Dowex 1 on the effectiveness of separation of rare earth elements(III). He stated that the separation coefficients generally increase on a regular basis with the increasing degree of cross linking. However, for the complexes [Ln(edta)] the change in the degree of cross linking from 4 to 16% of DVB causes increase of the value of the theoretical plate up to two orders of magnitude. This has an important impact on the separation. Resolving power, which is good for anion exchange resins with the optimal degree of cross linking (4% DVB for Dowex 1 - which corresponds to the smallest value of the theoretical plate) is less than unity for the anion exchange resins with a high degree of cross linking. This is related to the exclusion of large ions from the anion exchange phase by the 'sieve effect'.

On the basis of the thermodynamic studies of the anion exchange process with application of EDTA and CDTA it was stated that the unusual and non-monotonic affinity of the rare earth elements(III) complexes with the above mentioned complexing agents is associated with differences in the structure of these complexes and therefore with their different hydration (Surls & Choppin, 1957).

It is not bound, as in the case of cation exchange, with different values of their stability constants. Sizes of complex ions such as [Ln(edta)] and [Ln(cdta)] do not change on a regular basis with the change of the central ion radius. On a regular basis, the degree of hydration of these ions does also not change as evidenced by the designated value of the standard thermodynamic potential (Go). The hydration degree decreases (from La(III) to Nd(III)) and then increases (from Pm(III) to Ho(III)), then it increases but much more slowly, from Ho(III) to Lu(III) .

The papers by Dybczyński (Minczewski & Dybczyński, 1962a) confirmed the hypothesis made by Speeding and Mackey according to which, together with the decreasing ionic radius of lanthanide(III) in the yttrium group a decrease of dentate of EDTA is observed. Edta4- and cdta4- anions act as pentadentate ligands for light and heavy(III) lanthanide(III) while their hexadentate character is enhanced in the group of medium lanthanides (Fig.8).

**Figure 8.** The structure of [La(edta)(H2O)3]- complex.

118 Ion Exchange Technologies

1964).

Eu(III), Sm(III), Ce(III), Pm(III), Pr(III), Nd(III).

selectivity takes place (Minczewski et al. 1982).

hydration (Surls & Choppin, 1957).

degree of cross linking. However, for the complexes [Ln(edta)]-

exclusion of large ions from the anion exchange phase by the 'sieve effect'.

CDTA was used as the eluent) can be conditioned by the necessity of the presence of metal ion with a larger ionic radius, in order to ensure optimal packing of ligands around the central ion. Selectivity coefficients of complexes of rare earth elements(III) with CDTA, in most cases, are lower than in the case of rare earth elements(III) complexes with EDTA. The higher values of the theoretical plates designated for the system with CDTA also demonstrated a less favorable kinetic reaction of ion exchange than in the case of EDTA.

It was also shown that not only the type of eluent used affects the quality of the separation of the above mentioned complexes of rare earth elements(III). In the selected chromatographic system, the values of the separation coefficients can also be modified by changes in temperature and the degree of cross linking of the anion exchanger (Dybczyński,

Increasing the temperature generally results in improvement of the kinetics of ion exchange (an increase of diffusion coefficients in the ion exchange phase and solution), which significantly reduces the height of the theoretical plates. The temperature rise also affects the selectivity coefficients and separation factors, for example for CDTA the selectivity coefficients increase with the increasing atomic number from La(III) to Pm(III) and then decrease with further increase in the atomic number. At higher temperatures they reach the maximum value for Nd(III). The order of elution of rare earth elements(III) at 365 K is thus as follows: Lu(III), Yb(III), Tm(III), Er(III), Y(III), Ho(III), Dy(III), Tb(III), Gd(III), La(III),

The temperature rise does not always lead to improvement of the separation process. Distribution coefficients and the values of the theoretical plates can either increase, decrease, or remain constant with the increasing temperature, depending on the system. For elution of rare earth elements(III) with EDTA on the anion exchanger Dowex 1x4 in the EDTA form, for some pairs of elements as, for example, Pm(III)-Eu(III) and La(III)-Tb(III) reverse of

Dybczyński (1970) examined the impact of the degree of cross linking of the anion exchanger Dowex 1 on the effectiveness of separation of rare earth elements(III). He stated that the separation coefficients generally increase on a regular basis with the increasing

cross linking from 4 to 16% of DVB causes increase of the value of the theoretical plate up to two orders of magnitude. This has an important impact on the separation. Resolving power, which is good for anion exchange resins with the optimal degree of cross linking (4% DVB for Dowex 1 - which corresponds to the smallest value of the theoretical plate) is less than unity for the anion exchange resins with a high degree of cross linking. This is related to the

On the basis of the thermodynamic studies of the anion exchange process with application of EDTA and CDTA it was stated that the unusual and non-monotonic affinity of the rare earth elements(III) complexes with the above mentioned complexing agents is associated with differences in the structure of these complexes and therefore with their different

the change in the degree of

The proposed model is also suggested by comparing the solubility of the complex salts of [MLn(edta)] and [MLn(cdta)] types (where M = Na+ or K+) to the ion exchange affinity of the complexes [Ln(edta)] and [Ln(cdta)]- (Minczewski & Dybczyński, 1962a). Ion affinities of these complexes are arranged in the opposite order to the solubility of the corresponding salts. The selectivity coefficients increase from lanthanum(III) to europium(III), and then decrease to lutetium(III), while the solubility of salt decreases from lanthanum(III) to samarium(III), europium(III) and then rises to lutetium(III).

Non-monotonic and different affinity of the complexes of rare earth elements(III) with EDTA and CDTA for strongly basic anion exchangers was used by Hubicka (Hubicka & Hubicki, 1986; Hubicka, 1989a) to separate the pairs of complexes of rare earth elements(III) in the macro-micro component system using the frontal analysis technique. It was shown that the efficiency of separation of rare earth elements(III) complexes with EDTA of [Ln(edta)] type is affected by not only the type of anion functional groups, but also their form and degree of cross linking and porosity of the used anion exchanger. Good results were obtained by purification of yttrium(III) from neodymium(III), samarium(III) and terbium(III) as well as lanthanum(III) from neodymium(III) and erbium(III) from dysprosium(III). For the pair Y(III)-Nd(III), the influence of the type of functional groups, degree of cross linking and anion exchanger form as well as structure and porosity of the

skeleton was determined. Among the tested anion exchangers Dowex 2x8 and Dowex 1x8 in the form of EDTA proved to be most advantageous. Good results were also obtained for the macroporous, weakly basic anion exchanger Lewatit MP-7080 in the EDTA form.

Studies on the separation of anionic complexes of rare earth elements(III) with CDTA [Ln(cdta)]- type, which is an analogue of EDTA showed that in the macro-micro system Y(III) from Sm(III), Eu(III) and Nd(III) can be separated (Hubicka, 1989a). It was found that the process of separation of these complexes affects the degree of cross linking and the anion exchanger form. As follows the best results of separation of Y(III) from Nd(III) and Sm(III) were obtained on Dowex 1x4 in the acetate form.

Application of anion exchangers for separation of rare earth elements(III) complexes with EDTA and CDTA by the frontal analysis technique allows, in comparison with cation exchangers and elution process, to reduce the consumption of these chelating agents, obtaining higher concentrations of rare earth elements(III) in the eluate and shortening the process time which is important from the economical point of view. Additionally, alkali and alkaline earth metal ions forming unstable complexes with EDTA, and sometimes accompanying rare earth elements(III) have no effect on the separation result.

Kutun and Akseli (1999, 2000) for the separation of milligram quantities (5 mg) of rare earth elements(III) in the anion exchange process used the solution of sodium trimethaphosphate as the eluent. The elution was carried out with a gradient of 0.007-0.01 M concentration on the strongly basic polystyrene anion exchangers of types 1 and 2.

The advantage of anion exchangers over cation exchangers in the separation of rare earth elements(III) using aminopolycarboxylic acids as complexing agents is associated with their lower consumption in comparison to other eluents used, much faster process time, achieving higher concentrations in the eluate and the lack of negative impact of alkali(I), Ca(II) and Mg(II) ions on separation.

A particular attention has been paid to separation and removal of rare earth(III) elements nitrate complexes by means of frontal analysis from the polar organic solvent-H2O-HNO3 on anion exchangers of various types. The addition of organic solvent to an aqueous solution of rare earth complexes generally improves their ability of separation. Selection of an organic component and its concentration in the mixture is to a large extent arbitrary. The separation process is frequently carried out with the HNO3, H2SO4, NH4SCN and CH3COOH solutions. The examples of such systems are presented in (Marcus, 1983).
