Enhanced extraction and separation of zirconium(IV) and hafnium(IV) with 3-phenyl-4-benzoyl-5-isoxazolone in presence of various neutral organophosphorus extractants

Various 3-phenyl-4-aroyl-5-isoxazolones, namely, 3-phenyl-4-benzoyl-5-isoxazolone (HPBI), 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI) and 3-phenyl-4-(4-toluoyl)-5-isoxazolone (HTPI) were synthesized and examined with regard to the solvent extraction behavior of Zr(IV) and Hf(IV) from hydrochloric acid solutions. The results demonstrated that Zr(IV) and Hf(IV) extracted into chloroform with 3-phenyl-4-aroyl-5-isoxazolones (HA), as ZrOA₂ and HfOA₂, respectively. The equilibrium constants of the extracted complexes follow an order HFBPI > HPBI > HTPI, which is in accordance with their pK_a values. The synergistic extraction of Zr(IV) and Hf(IV) was also investigated with mixtures of HPBI and tri-n-octylphosphine oxide (TOPO), trialkylphosphine oxide (TRPO), and tri-n-butylphosphate (TBP), where the complexation strength follows the order TOPO > TRPO > TBP. The equilibrium constants of the synergistically extracted complexes have been correlated with the donor ability of phosphoryl oxygen of the neutral organophosphorus extractants in terms of their ³¹P NMR chemical shifts and their basicity values. The addition of neutral organophosphorus extractant to the metal chelate system not only enhances the extraction efficiency but also improves the selectivity between these metal ions.