Kinetics and Mechanism of the Oxidation of Neptunium(IV) by Nitric Acid in Tributyl Phosphate Solution

Abstract

Kinetics / Rate equation / Tri-butyl phosphate / Neptunium(IV) / Nitrous acid / SpectrophotometryAbstract Np(IV) oxidation by nitric acid in tributyl phosphate solutions is negligible at ambient temperatures whilst at an elevated temperature it proceeds according to the stoichiometric equation (without accounting for TBP complexes):The reaction is an autocatalytic reaction that is due to the formation of more reactive (compared to HNO3) oxidants, i.e. HNO2 and Np(VI). The main contribution is made by the Np(V) reproportionation reaction. Hence Np(IV) oxidation follows the scheme: 2 Np(IV) + HNO3 = 2 Np(V) + HNO2 (slow) Np(V) + HN02 = Np(VI) + N0 (quick) Np(IV) + Np(VI) = 2 Np(V) (slow) 2 NO + HNO3 + HjO = 3 HNO2 (quick) The reaction rate is described by the equation: ^/[Np(IV)] dt <:r[Np(IV)] ^ ^Np(IV)][Np(VI)][H30]' [H.O]'" [HNOj]"where /t4 = (9.5±0.8)X 10"" min"' at 60.2°C and k" is an extremal function of the HNO3 concentration in TBP Solution with the maximum at =0.4 M HNO3; at [HNO3] = 0.2 M and T = 60.2°C, k" = (0.70 + 0.43) mol'" 1-° min"'. The activation energy of the reaction represented by k^ is 90±2 kJ/mol.

Oxidation Mechanism of Tetravalent Uranium in Nitric Acid System Synthesized by Hydrogen Reduction Process

“…This observed oxidation behavior is in complete agreement with the speculated mechanism of tetravalent uranium oxidation, which is consistent with the oxidation behavior of neptunium-(IV). 21 Furthermore, the observed trend of faster oxidation rates at higher acidity levels at 60 °C also aligns with this proposed mechanism. Additionally, at 60 °C, the oxidation process of tetravalent uranium exhibits a relatively long induction period during which the concentration of nitrite ions in the solution remains low.…”

The Oxidation of U(IV) by Nitric Acid in 30% Tri-Butyl Phosphate Solutions

“…The initial organic solutions obtained by this technique may contain nitrous acid (nitrogen oxides), the concentration of which is determined by many factors (phase contact time, hydrazine concentration in the aqueous phase, temperature etc. )· Since it is known that nitrous acid (nitrogen oxides) plays a leading role in the oxidation of Pu(III) and Np(IV) with nitric acid in TBP solutions [12][13] and in the oxidation of U(IV) in an aqueous HN0 3 solution [14], this is a potentially serious problem. Indeed, it was the varying concentration of HN0 2 that was used by the authors of reference [5] to explain the non-reproducibility of the induction period that they observed in their experiments on the oxidation of U(IV) in 20% TBP.…”

“…It is well known that water can substantially affect the rates of actinide reactions in TBP ( [13] and references therein). In the oxidation of U 4+ to U0 2 2+ in TBP, the uranium atoms are oxygenated -most probably by water molecule oxygen atoms.…”

X-ray absorption spectroscopy and actinide electrochemistry: a setup dedicated to radioactive samples applied to neptunium chemistry

“…Next, the Np V reduction was investigated by a stepwise decrease of the applied potential to 0, À100, À150, À200 and À250 mV/(Ag/AgCl). In HNO 3 solution, the Np IV ion is difficult to stabilize due to its reoxidation with HNO 2 in equilibrium with nitrate ions (Taylor et al, 1998;Koltunov et al, 1997). The corresponding XANES spectra are displayed on Fig.…”