Separation of lanthanide elements by ion recognition of polymer gel copolymerized with acidic phosphorus compound

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The extraction of lanthanide ion between an aqueous solution and a thermosensitive polymer gel, n-isopropylacrylamide (NIPA) gels copolymerized with 2-metacrloxyethylacidphospate (MR) was examined. This polymer gel shows a phase transition phenomena, that is, the structure of gel changes from swelling to shrinking with increasing temperature. The extraction equilibrium of lanthanide ions, La(III) and Sm(III), in an aqueous solution was measured batchwise. The distribution ratio, Kd, increased with increasing temperature. These metal ions can be extracted in the gel at the shrinking condition and cluted by the conformation change of polymer network from shrinking to swelling of gel with decreasing temperature.The extraction mechanism of lanthanide ion, which was estimated from the pH-dependency on Kd, was described simply asLn^<3+> + 3 RH <double arrow> LnR_3 + 3 H^+at both the shrinking and swelling conditions. The number of effective phosphorus molecules for the extraction of lanthanide ion, which was derived from the extraction constants at the shrinking and swelling conditions corresponded to about 80% of copolymerized phosphorus molecules at the shrinking condition and was reduced to about 30% at the swelling condition.The thermal swing tests were carried out to examine the possibility of practical use. It is important practically to maintain the extraction capability of gel in a long-term use. The temperature-dependency of Kd was maintained for repcating the extraction and elution by the thermal swing operation. These results suggest that the phase transition phenomena of thermosensitive gel is applicable to the extraction and elution operations of metal ion.