Thermodynamics of extraction of Am(III) and Eu(III) from different anionic media with Tri-n-octyl phosphine oxide

Extraction of Am(III) and Eu(III) from NO3-, ClO4-, SCN- and NO3- +ClO4- media with tri-n-octyl phosphine oxide (TOPO) in xylene has been carried out at 15, 25, 30 and 35 degreesC. Under the extraction conditions the species M(NO3)(3)(.)3TOPO, M(SCN)(3)(.)4TOPO, M(ClO4)(3)(.)4TOPO and MNO3(ClO4)(2)(.)4TOPO are predominantly extracted at all the temperatures (M = Am or Eu). The two phase equilibrium constant (log K-ex) follows an order SCN- > NO3- + ClO4- > ClO4- > NO3-. Extraction enthalpies have been calculated in each system from the temperature dependence of the K-ex The thermodynamic parameters DeltaG, DeltaH and DeltaS have been discussed and compared with other mono- and bidentate organophosphorus extractants for Am(III) and Eu(III) from same aqueous medium. The -DeltaH values for the two-phase reaction follow an order NO3- + ClO4- > ClO4- > SCN- > NO3- for both metal ions. This has been explained on the basis of heat energies involved with the formation of ML3 complex (L being the anion used), the transfer of ML3 to the organic phase and the attachment of TOPO molecules, and also on the limiting partial molal volume of the anions.