Extraction of UO22+ by two highly sterically hindered (X1) (X2) PO(OH) extractants from an aqueous chloride phase

The comparative extraction behavior of tracer-level UO₂²⁺ into benzene solutiosn of two highly sterically hindered extractants, di(2,6-di-iso-propyl phenyl) phosphoric acid, HD(2,6-i-PΦ)P and di-tertiary-butyl phosphinic acid, H[Dt-BP], vs an aqueous 1.0 F (NaCl + HCl) phase was studied. The extraction of UO₂²⁺ in both systems is directly second-power dependent upon extractant concentration and inversely second-power dependent upon hydrogen ion concentration, the stoichiometry of extraction being UO_2A²⁺+2(HY)₂₀ ⇆ UO₂(HY₂)₂₀ + 2H_A⁺ The expression for the distribution ratio, K, is K=K₅F²/[H⁺]² the general expression for the extraction of any metallic species being K=K₅F^a/[H⁺]^b where K_s is a constant characteristic of the system, F the concentration in formality units of extractant in the organic phase, [H⁺] the concentration of hydrogen ion in the aqueous phase, and a and b the respective extractant and hydrogen-ion dependencies.Both extractants have a high degree of steric hindrance. The HD(2,6-i-PΦ)P is the more highly acidic, the pK_A value, in 75% ethanol, being 3.2. The pK_A, previously reported, for H[Dt-BP] is 6.26. The K_s for UO₂²⁺ in the system HY in benzene diluent vs an aqueous 1.0 F (NaCl + HCl) phase is 2 × 10⁴ for H[Dt-BP] and 3 × 10⁻¹ for HD(2,6-i-PΦ)P; the ratio of the K_s values, nearly 7 × 10³, favors the less acidic extractant. For comparative purposes, the K_s values for UO₂²⁺ and for Eu³⁺ in other (X₁)(X₂)PO(OH), in benzene diluent vs 1.0 F (NaCl + HCl) systems are presented. The variations are discussed in terms of the pK_A of the extractant and the steric hindrance within the extractant.