Radiolytic alterations to neptunium extraction and redox in 30 % tri-n-butyl phosphate

The extraction of neptunium in the +IV, +V, and +VI oxidation states was studied in irradiated solutions of 30 % tri-n-butyl phosphate (TBP) in dodecane as well as in synthetic mixtures of TBP with its primary radiolysis product, di-n-butyl phosphoric acid (HDBP). Extraction of all three oxidation states was increased by irradiation, with the most dramatic effect for Np(IV). Without stabilization of the neptunium oxidation state, the irradiated organic phase displayed redox activity toward the Np(VI/V) couple, achieving a Np(VI):Np(V) ratio set by absorbed dose, independent of starting oxidation state. In synthetic TBP-HDBP mixtures, Np(VI) was extracted better than in the irradiated case, while Np(IV) displayed similar behavior between the two systems. The presence of HDBP resulted in greater equilibrium Np(VI):Np(V) ratios when contacting a Np(V) solution in 4 M HNO3 with the synthetic mixture. Tetravalent neptunium also formed precipitates in the presence of HDBP at low acidities or high HDBP concentrations. All evidence points toward an ion-exchange extraction mechanism, where DBP-, rather than nitrate, coordinates to Np(IV) or Np(VI). Reductive stripping of Np(VI) from degraded organic phases using nitrous acid was unsuccessful, indicating that Np(VI) reduction is inhibited by the presence of HDBP.