Behavior of Np(V) in sodium diethylenetriaminepentaacetate (DTPA) solutions

Abstract

The reduction of Np(V) to Np(IV) with diethylenetriaminepentaacetate (DTPA) ions in solutions with pH 1.5–5.5 was studied by spectrophotometry. The constant pH was adjusted using potassium hydrogen phthalate with the addition of hydrochloric acid or alkali. The reaction rate equation is V = -d[Np(V)]/dt = k[Np(V)]. The rate constant increases with increasing [DTPA] in the range 5–50 mM; with increasing pH from 2 to 5, the rate constant passes through a maximum at pH 3.0–3.5. The maximum is due to the fact that an increase in pH leads, on the one hand, to an increase in the yield of the Np(V) complex with the DTPA ion and, on the other hand, to a decrease in the concentration of Н+ ions required for the transformation of NpO 2 + into Np4+. In argon-saturated solutions, one Np(V) ion oxidizes one DTPA ion. The radical formed from DTPA transforms into products inactive toward Np(V). In the presence of О2, the radical generates НО2, which launches the reaction Np(IV) + НО2 + Н+ → Np(V) + Н2О2. Thus, Np(V) plays the role of a homogeneous catalyst of DTPA oxidation with molecular oxygen.