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In order to understand the complex and dynamic neptunium process chemistry in the TBP-HNO3 system, the kinetics involved reversible redox reaction and extraction mass transfer was investigated. The results indicates that the mass transfer rate of Np(VI) is much faster than the redox reaction in aqueous solution. The concentrations of nitric acid and nitrous acid not only can change the Np(V) oxidation reaction and Np(VI) reduction reaction rate, but also can ultimately determine the distribution of neptunium extraction equilibrium. The variety of temperature can only influence the extraction equilibrium time, but cannot alter the equilibrium state of neptunium.

In order to understand the equilibrium state, conversion process and mechanism of Np(V)/Np(VI) in nitric acid solution, the redox kinetics of neptunium with nitrous and nitric acids was studied spectrophotometrically by an nonlinear fitting method. The rate equation of the redox reaction was acquired. Apparent activation energy of forward reaction and reverse reaction are approximately the same of 80 kJ/mol. Varying the concentrations of nitric and nitrous acids will change the reaction rate of forward and reverse reaction in different degrees. As a result, the equilibrium state of Np(V)/Np(VI) is altered, but it is not influenced by temperature.