Thermodynamics of extraction of tetravalent plutonium, uranium, thorium and zirconium

Abstract

Thermodynamic treatment of the experimental data on the extraction of quadrivalent Pu, U, Th and Zr with tri-n-butyl phosphate (TBP) from nitric acid solutions is presented. It is shown that the extraction of all the quadrivalent metals studied is going according to the same mechanism: M(OH)⁴⁻ⁱ+(4−i)NO ⁻₃ +2TBP⇌M(OH)_i(NO₃)_4−i·2 TBP. For Zr, i=0, 1, and 2; for the remaining M(IV), i=0 and 1. The thermodynamic constants of extraction of M(IV) with the kerosene solutions of TBP according to the above mentioned equation are as follows: Zr: K ⁰₀ =0.6; K ⁰₁ =14; K ⁰₂ =5. Pu: K ⁰₀ =380; K ⁰₁ =4.8·10⁴. U: K ⁰₀ =300; K ⁰₁ =1.8·10⁴. Th: K ⁰₀ ∼150. It has been established that Zr and Pu(IV) are extracted into 2-thenoyltrifluoracetone (HA) from perchloric acid solutions under the formation of MA₄ and M(ClO₄)A₃ species. For the extraction from nitric acid solutions, the species formed are ZrA₄ and Zr(NO₃)A₃ in the case of Zr, PuA₄ and Pu(OH)A₃ in the case of Pu. The differences in the qualitative and quantitative characteristics of the extraction of M(IV) with TBP and HA from nitric and perchloric acids are explained by the effect of the character of the acid and of ionic potential upon the structure of the hydration shell of M ⁴⁺_aq .