Discussion
The recovery of two lanthanoids easier than the others due to their redox chemistry. These lanthanoids are :
Answer: B
The two lanthanoids are cerium and europium, recovered as Ce4+ and Eu2+.
Almost all lanthanoids are stable only as Ln3+, with very similar radii, which is why they are so hard to separate. Cerium can be oxidised to Ce4+ (an empty 4f0 shell), whose chemistry differs sharply from Ln3+: it hydrolyses and precipitates or is extracted selectively. Europium can be reduced to Eu2+ (a half-filled 4f7 shell), which behaves like an alkaline-earth ion and can be removed, for example as insoluble EuSO4.
Pr4+, Tb4+, Nd4+ and Dy4+ exist only in a few solid fluorides and oxides and are not usable in solution, and 'Ln' is the general symbol for any lanthanoid, so options A, C and D do not describe a practical separation.
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