When ionized, the common charged state for the elements in the lanthanide series (or lanthanon series) is +3. Such ions can be retained by a sulfonic acid cation resin and exhibit the following order of selectivity:
Lu3+ < Yb3+ < Tm3+ < Er3+ < Ho3+ < Dy3+ < Tb3+ < Gd3+ < Eu3+ < Sm3+ < Pm3+ < Nd3+ < Pr3+ < Ce3+ < La3+
Total ionic strength of a feed stream will influence both selectivity and resin capacity. If significant amounts of alkalies (Li+, Na+, K+, Rb+, Cs+) or alkaline earths (Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ra2+) are in solution with the lanthanides, a two column cation resin system may be worth investigating. The first column is used to remove the bulk of the alkalies and alkaline earths and the second is used to concentrate and hold the lanthanides.
Often these metals are being separated from other closely associated metals so resins with the maximum number of theoretical separation plates are needed. Strong acid cation fine mesh resins like DOWEX™ 50WX8 100 200 Resin or DOWEX 50WX4 100 200 Resin are proposed for the lanthanides.