A theoretical study of actinide and lanthanide extraction with carbamoylmethylphosphine oxide ligands

With the development of nuclear energy, safe disposal of the spent nuclear fuel especially high level liquid waste (HLLW) generated during the PUREX (Plutonium Uranium Extraction) process has become the key factors affecting the sustainable development of nuclear energy. The n-octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide (CMPO) used in the so-called TRUEX (Transuranium Extraction) process was found to possess excellent extracting ability for actinide and lanthanide cations in acidic media. In this work, The UO22+, NpO2+, Pu4+, Am3+ and Eu3+ extraction complexes with CMPO and diphenyl-N,N-diisobutyl carbamoyl phosphine oxide (Ph2CMPO) have been investigated by density functional theory (DFT) in conjunction with relativistic small-core pseudopotentials. For most extraction complexes, the CMPO and Ph2CMPO molecules are coordinated as bidentate chelating ligands through the carbonyl oxygen and phosphoric oxygen atoms. The metal-ligand bonding is mainly ionic for all of these complexes. The neutral UO2L2(NO3)2, NpO2L2(NO3), PuL2(NO3)4, AmL3(NO3)3 and EuL3(NO3)3 complexes are predicted to be the most thermodynamically stable molecules according to the metal-ligand complexation reactions. As reported in the literature, the extractability of these actinides decrease in the order of Pu4+ > UO22+ > Eu3+ ≈ Am3+ > NpO2+. In addition, hydration energies may play an important role in the extractability of CMPO and Ph2CMPO for these actinide ions. In most cases, the complexes with CMPO and Ph2CMPO ligands have comparable metal-ligand binding energies, i.e., the substitution of a phenyl ring for the n-octyl at the phosphoryl group of CMPO has little influence on the extraction of these actinides and lanthanides. (author)