Excessive consumption mechanism of hydrazine in the reaction with ReO4−: Re species evolution and ReO2·nH2O-catalyzed decomposition

Abstract

The reaction between pertechnetate (⁹⁹TcO₄⁻) and hydrazine is considered to be that of a complex inorganic chain in the PUREX process, in which hydrazine is excessively consumed with a high stoichiometric ratio of [N₂H₄]/[Tc]. Due to the ambiguity of the low-valent Tc species in the catalytic process, the consumption mechanism remains to be further optimized. In this study, perrhenate (ReO₄⁻) was used to simulate the redox behavior of TcO₄⁻ with hydrazine, and the consumption mechanism of hydrazine was examined. In batch experiments where different initial concentrations of ReO₄⁻ and hydrazine were used, the kinetic profile of hydrazine consisted of an induction period, a rapid reaction stage, and a termination stage. There was only a slight decrease in the hydrazine concentration during the induction period, and the length of the induction period was strongly dependent on the initial concentrations of hydrazine and ReO₄⁻. Then, the hydrazine concentration significantly decreased during the rapid reaction stage, and was completely consumed after the termination stage. Because excess hydrazine was over-consumed, the Re species in the reaction were investigated to study the hydrazine consumption mechanism. ReO₄⁻ was slowly reduced to ReO₄²⁻ and Re(IV) species during an induction period, with an absorption peak of 269 nm and 301 nm, respectively, and X-ray absorption fine structures (XAFs) also supported evidence of ReO₄²⁻ and soluble Re(IV) species. Most Re(IV) species were hydrolyzed to ReO₂·nH₂O during the rapid reaction stage. Further experiments suggest that ReO₂/ReO₂·nH₂O would catalyze the decomposition of hydrazine at room temperature, and lead to the excessive consumption of hydrazine in the reaction. Gas chromatography confirmed that hydrazine mainly decomposed into N₂ and NH₃.