An attempt to differentiate the affinity of individual lanthanides to the resin using temperature driven swelling of the thermosensitive copolymer of N-isopropylacrylamide and 2-(methacryloyloxy)ethyl phosphate - Thermodynamic studies

摘要

A thermosensitive polymeric gel: N-isopropylacrylamide (NIPAM) copolymerized with 2-(methacryloyloxy)ethyl phosphate (MR) and crosslinked with ethylene glycol dimethacrylate (EGDMA) was used in preliminary experiments aimed at determination of thermodynamic functions associated with the exchange and separation of metal ions using temperature swing adsorption/desorption. This polymer shows a phase transition, which involves the structural change of the gel from swollen to shrunken form at ca. 37 ℃. The batch experiments of ion-exchange of lanthanide ions (Ce, Eu, Tb, Tm), Sc as well as Co, Ba, Cs and Pa were carried out at 20, 30, 40, 50 and 60 ℃, that is both below and above volume phase transition temperature (VPTT) of the gel. The temperature was changed upwards and downwards and a distinct hysteresis of thermodynamic functions is observed i.e. the equilibrium depends on the thermal history of the thermosensitive ion-exchange gel. It was observed that the free energy change of ion-exchange (especially for lanthanides) shows extremum in the range 40-50 ℃, what confirms that structural changes and exclusion of water from resin phase has an effect on the equilibrium position of individual ion-exchange reaction. It was found that in the case of lanthanides the enthalpy change well exceeds 3 kcal/equiv., what suggests that not pure ion-exchange is responsible for the uptake of metal ions by the resin.