Catalytic effect of metal colloid on uranium(V) reduction

Kitatsuji, Yoshihiro  ; Ouchi, Kazuki  ; Otobe, Haruyoshi  

Redox behaviors of actinide ions in a low-acid solution are complicated and have been unclear, because they tend to form hydroxide complexes, colloids and precipitates. Recently, autocatalysis of the reduction of U(V) to U(IV) in the solution where U(IV) forms hydroxide colloid was reported; U(IV) colloid formed as reduction product catalyzes the electrolytic reduction of U(V). In this study, possibility of catalysis of Zr(IV) colloid which was analogous of U(IV) colloid on the reduction of U(V) was investigated. When U(VI) was electrolyzed at a constant potential in order to reduce to U(V) in the solution of pH 3.0 containing 10 mM Zr(IV), the increase of the reduction current was observed in initial stage of electrolysis. This results indicates that U(VI) was reduced further to U(IV) without catalysis of U(IV) colloid, and that Zr(IV) colloid also acts as catalyst. Intensity of catalysis depends on concentration of Zr(IV), condition of preparing colloid and acidity of the solution.