Separating lanthanides using ion-exchange method

Question

Following statements are on Separation of rare earth metals (lanthanides) via in-exchange method

The tripositive ions are strongly sorbed by cation exchange resin: $\ce{La^{3+}}$, the largest is most strongly held; $\ce{Lu^{3+}}$, the smallest is least strongly held. Elution of $\ce{HCl}$, therefore removes $\ce{Lu^{3+}}$ first and $\ce{La^{3+}}$ last.

Generally, the smallest cation forms the strongest bond, so shouldn't $\ce{Lu^{3+}}$ be most strongly held? The most common cation exchangers, such as $\ce{RSO3^-Na+}$, where $\ce{R}$ is a cross-linked matrix are hard bases, so shouldn't according to HSAB principle also, $\ce{Lu^{3+}}$ be most strongly held?

Follow-on excerpt,

The separation is however not good, particularly between the heavier lanthanides, but is markedly improved by the addition of complexing agents such as $\ce{EDTA}$, or ammonium citrate/citric acid buffer to the eluting solution. The citrate ion complexes most strongly with the smaller elements, i.e. with the heavier elements.

Now, the two compounds (cation-exchanger and complex) helds strongly different elements, one smaller in size and other larger in size. How will presence of two enhance separating property? Also here ammonium phosphate is mono-, di- or tribasic?

Following graphs are also given

Please explain the meaning and significance of the graphs.

Answer 1

You must be reading this from really old ion exchange references. It is always good to mention the resources and references for other readers.

Ion-exchange retention can be very roughly approximated with Coulomb's law, which says that the force of interaction between two "ions" is directly proportional. The force of interaction is inversely proportional to the square of the hydrated radius of the ions. Here the word interaction means attraction or repulsion.

So you have to look up the hydrated radii of the lanthanides and try to see a trend on a simple sulfonated cation exchanger.

You second point is a misconception. The "trick" to add complexing agents works when you wish to alter the sign of the species you wish to separate. For example Cu(II) has a +2 charge, if I complex it with EDTA, the [Cu(EDTA)] has a charge of -2 Think about the Coulomb's law now. Can we use a cation exchanger now? This time we must try an anion exchanger.

One can still use a cation exchangers, in that case the complexed metallic anions will be repelled out and those ions which did not complex well will stay on the resin.