Deposition of Uranium Oxide Following the Reduction in Weak Acid Solution Using Electrochemical Quartz Crystal Microbalance (EQCM)
Authors: Ouchi Kazuki , Otobe Haruyoshi , Kitatsuji Yoshihiro , Yamamoto Masahiro 
Published: February 2020

To evaluate the safety of the geological disposal of high-level radioactive waste, it is important to understand the behavior of actinide (An) in nature environment. An ions having large electric charge easily form complexes and colloids. The migration behaviors of An species differ from one another, hence formation reaction and chemical state of aggregates such as colloids should be clarified. While it was reported that the reduction of U(VI) to U(IV) leads to the formation of UO2 nanoparticles, the scheme of nanoparticulation following the reduction has not been understood. In previous study (Y. Kitastuji et al., Electrochim. Acta, 2014, 141, 6-12.), the authors investigated the reduction of U(VI) in a weak acid solution. It was found that aggregates of U(IV) formed in reduction process of U(V) enhanced the rate of the disproportionation and the electrolytic reduction of U(V). In this study, we investigated the aggregation of U(IV) following the valence change of U as electrodeposition using electrochemical quartz crystal microbalance (EQCM).
EQCM (ALS420C, BAS) were measured by using the 8 MHz AT-cut quartz crystal (diameter 13.7 mm) with Au electrodes (diameter 5.1 mm) on both sides in 1 M NaClO4 solutions containing 1 × 10-3 M UO2
2+. The pHs of the solution were adjusted to be 2.0 and 4.0 by adding NaOH. A platinum wire and a silver-silver chloride electrode (SSE) with 1 M LiCl were employed as the counter electrode and the reference electrode, respectively. The applied potential was -0.35 V at which the current for the reduction of U(VI) to U(V) was observed.
When EQCM measurements of the reduction of U(VI) in a weak acid solution were performed, the frequency was changed negatively. This indicates that deposits of U(IV) were formed on the electrode surface. We consider that the deposition is divided into the three steps according to deposition rates; I) the induction step which is until starting deposition, II) metastable aggregation step of which reaction rate is temporary fast, and III) stable aggregation step of which rate is constant. The oxidative potential of deposits at each aggregation step was investigated by stripping voltammetry measurement. The oxidative potential of deposit at stable aggregation step was higher than that at metastable aggregation step. This indicates that deposits are stabilized with growth. The pH dependence of the deposition rate was investigated. The deposits of U(IV) formed at more than pH 3.1. When pH was higher, the induction period was shorter, and deposition rates at metastable and stable aggregation steps were high. This suggests that a U(IV) hydroxide relates to the formation of deposits. In addition, the mass change of deposits was measured to investigate the chemical reaction after stopping the electrolysis. Mass increase was observed in the a little time from the end of electrolysis. This increase proposed that the slow chemical reaction, i.e. disproportionation, makes progress and U(IV) aggregates grow up. It presumes that this reaction leads to the deposition of U(IV) aggregates. The oxidative potential of deposits after stopped electrolysis was shifted to positive potential and also increased in the residual amount of deposited species on the electrode surface. It is considered that formed deposits change more stable one with time.
From these results, we proposed the following aggregation mechanism (see Figure). In the induction step, U(IV) is formed by the disproportionation following the reduction of U(VI) to U(V). In metastable aggregation step, U(IV) produced a U(IV) hydroxide. The U(IV) hydroxide aggregates as a core and form into the metastable aggregate of various stable states. Also, the increase of reduction current at metastable aggregation step was observed due to reduction of U(V) by deposits on the electrode surface. We consider that decrease of deposition rate at stable aggregation step depends on dehydration of metastable aggregates. In stable aggregation step, those aggregates are dehydrated, and more stable U oxide is finally formed.
This work was supported by JSPS KAKENHI Grant Number 15H04247.

Figure 1