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Why are EDTA and HEDTA used in rare-earth ion-exchange separation?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

In the ion-exchange process, standard resins are not selective enough to separate rare-earth elements because the tenacity with which they hold cations does not vary sufficiently between neighboring lanthanides.1

To achieve separation, complexing agents such as ethylene diamine tetraacetate (EDTA) and hydroxyethylene diamine triacetate (HEDTA) are introduced. These agents are used because they form complexes with rare-earth ions ( $R^{3 +}$ ) where the stability of the complex varies significantly from one rare earth to another. By creating these differences in complex stability, the rare earths can be eluted through the ion-exchange column in distinct, rectangular bands with minimal overlap, allowing for their effective separation.1