Solvent extraction studies of uranium(VI) from phosphoric acid: Role of synergistic reagents in mixture with bis(2-ethylhexyl) phosphoric acid

The extraction of uranium(VI) from 5.3 mol·L− 1 H3PO4 (a typical concentration of wet phosphoric acid) with a series of neutral organosphosphorus synergistic reagents (0.125–0.250 mol·L− 1) used in mixture with 0.5 mol·L− 1 bis(2-ethylhexyl) phosphoric acid (D2EHPA) in Isane IP 185 (a 100% isoparaffinic aliphatic diluent) has been investigated. The series of synergistic reagents includes tri-n-butyl phosphate (TBP), di-n-butyl n-butyl phosphonate (DBBP), n-butyl di-n-butyl phosphinate (BDBP), tri-n-butyl phosphine oxide (TBPO), tri-n-hexyl phosphine oxide (THPO), tri-n-octyl phosphine oxide (TOPO), tri-n-decyl phosphine oxide (TDPO), di-n-hexyl n-decyl phosphine oxide (DHDPO), n-decyl di-n-hexyl phosphinate (DDHP) and di-n-hexyl octyl methoxy phosphine oxide (di-n-HMOPO). It appears that the presence of oxygen atoms at the vicinity of the phosphorus atom in the hydrophobic chains of the synergistic reagents, as in BDBP, DBBP and TBP, lowers significantly the extraction properties of uranium(VI) from phosphoric acid compared for instance to TBPO. Conversely, the increase of the hydrophobicity of the synergistic reagents by increasing the number of carbon atoms in their hydrophobic chains is responsible for an improvement of the extraction efficiency of uranium(VI). Among all the studied systems, the mixture of D2EHPA and di-n-HMOPO in Isane IP 185 leads to the best recovery of uranium(VI) from 5.3 mol·L− 1 H3PO4. A physicochemical model has been developed to simulate the extraction data and identify the equilibria involved in the recovery of uranium(VI) from 5.3 mol·L− 1 phosphoric acid by D2EHPA/di-n-HMOPO. A good agreement between experimental and calculated distribution coefficients of uranium(VI) is obtained by taking into account the following species in organic phase: H3PO4-S, (HL)2-S, (HL)5-S, (HL)2-(S)2 and UO2(HL2)2, UO2(HL2)2S, UO2L2S, where L denotes the monomeric deprotonated form of D2EHPA (HL) and S refers to di-n-HMOPO.