Reduction behavior of uranium in the presence of citric acid

Suzuki, Yoshinori; Nankawa, Takuya ; Yoshida, Takahiro*; Ozaki, Takuo; Onuki, Toshihiko; Francis, A. J.; Tsushima, Satoru*; Enokida, Yoichi*; Yamamoto, Ichiro*

We examined the reduction behavior of UO in citrate media at pH 2.0-7.0 by column electrodes. At pH 2.0, UO was reduced to U(IV) through a one-step reduction process, while it was reduced to U(IV) through a two-step reduction process at pH 3.0-5.0. The reduction potential of UO shifted to lower values with an increase in pH from 2.0 to 7.0. At pH 6.0 and 7.0, UO was not reduced to U(IV) completely at the electrode potential above -0.8 V. Ultraviolet-visible spectroscopy and speciation calculation of UO in citrate media indicated that uranium existed as a mainly UO at pH 2-3, and a predominant species at pH 3-5 was [(UO)Cit]. At pH 5-7, polymeric complexes were present. These findings suggest that the reduction of UO is more difficult by polymerization of UO with citric acid at higher pHs.