The recovery of two lanthanoids easier than the others due to their redox chemistry. These lanthanoids are :
Ce4+, Eu2+
The lanthanoids are normally separated with difficulty because all of them are stable as Ln3+ ions of almost identical size and chemistry. An element becomes easy to recover only if it can be moved, in aqueous solution, to a different oxidation state with different solubility or complexing behaviour.
Cerium can be oxidised to Ce4+ (configuration 4f0, the stable xenon core). Ce(IV) is much more acidic and less basic than Ln(III), so it is separated by precipitation of the hydrous oxide at low pH or by solvent extraction.
Europium can be reduced (for example by zinc amalgam) to Eu2+ (configuration 4f7, a half-filled shell). Eu(II) behaves like Sr2+/Ba2+ and is removed as insoluble EuSO4, leaving the other Ln3+ ions in solution.
Pr4+, Tb4+ and Dy4+ occur only in certain solid oxides or fluorides and are not stable in aqueous solution, and Nd4+ is not a usable solution species, while a generic Ln2+ or Ln4+ does not name any particular element, so none of these redox states helps in a solution separation.
So the two lanthanoids whose redox chemistry makes their recovery easier are cerium (as Ce4+) and europium (as Eu2+).
Hence, Ce4+, Eu2+.
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.................... is a client server application that identifies each host on the internet with a unique user-friendly name.
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