Investigation of the speciation of uranium(VI) in concentrated phosphoric acid and in synergistic extraction systems by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

The speciation of uranium(VI) present in aqueous solutions of 0.73-7.08 mol.L− 1 phosphoric acid or extracted from these solutions in various organic phases was investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS). The organic phases consisted of bis-(2-ethyl-hexyl) phosphoric acid (D2EHPA), bis(1,3-dibutyloxypropan-2-yl) phosphoric acid (BiDiBOPP) and synergistic mixtures of tri-n-octylphosphine oxide (TOPO) or di-n-hexyl octyl methoxy phosphine oxide (di-n-HMOPO) with D2EHPA or BiDiBOPP in Isane IP 185 (an aliphatic diluent). The present paper indicates that uranium(VI) exists as a unique species in aqueous solutions of 0.78-7.08 mol.L− 1 H3PO4 likely as UO2(H3PO4)n(H2PO4)m2-m where n + m = 3, and that a predominant uranium(VI) species (more than 90%) is extracted by the D2EHPA/TOPO synergistic mixture. This species, whose formula is likely UO2(HL)2L2TOPO with HL = monomeric D2EHPA and L = deprotonated form of D2EHPA, is characterized by the fact that the TOPO molecule is located in the first solvation shell of uranium(VI) as also confirmed by DFT calculations. Furthermore, TRLF spectra of uranium(VI) in D2EHPA/di-n-HMOPO and BiDiBOPP/di-n-HMOPO are quite different from those obtained with the D2EHPA/TOPO mixture. Indeed, the vibronic peaks of uranium(VI) TRLF spectra have partially (D2EHPA/di-n-HMOPO) or completely (BiDiBOPP/di-n-HMOPO) disappeared, which could be due to the presence of oxygen atoms in the hydrophobic chains of BiDiBOPP and di-n-HMOPO. As in the case of the D2EHPA/TOPO mixture, the fluorescence lifetime of uranium(VI) in the BiDiBOPP/di-n-HMOPO mixture indicates that uranium(VI) is extracted by this mixture from 5.3 mol.L− 1 H3PO4 as a unique or at least a predominant species.