Extraction of lanthanide elements by thermosensitive polymer gel copolymerized with acidic phosphorus compound

The extraction of lanthanide ion between an aqueous solution and a thermosensitive polymer gel, N-isopropylacrylamide (NIPA) gels copolymerized with 2-methacryloyl-oxyethylacidphosphate (MR)! was studied. This polymer gel shows a phase transition phenomena. The phase transition involves the structural change of the gel 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, decreased with decreasing temperature. Lanthanide ions can be extracted at the shrinking state and then released by a conformational change of the polymer network during swelling. The temperature-dependence of Kd was more clear and sharper as the content of NIPA in the gel was increased. The difference of Kd between swelling and shrinking states tended to be larger as the content of crosslinker was decreased. The extraction mechanism of lanthanide ion, which was evaluated from the pi-I-dependence of Kd, was described simply as Ln(3+) + 3 RH reversible arrow LnR(3) + 3 H+ (Ln=lanthanide ion) at both the shrinking (38 degrees C) and swelling (8 degrees C) states. The number of MR molecules which are available for the extraction of lanthanide ion corresponds to about 80% of copolymerized MR molecules at the shrinking state (38 degrees C) and is reduced to about 30% at the swelling state (8 degrees C). These results suggest that the extraction behavior of metal ion into the gel is affected strongly by the phase transition phenomena of NIPA-MR gel.