Reduction behavior of uranium in the presence of citric acid

We examined the reduction behavior of UO22+ in citrate media at pH 2.0-7.0 by column electrodes. At pH 2.0, UO22+ 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 UO22+ shifted to lower values with an increase in pH from 2.0 to 7.0. At pH 6.0 and 7.0, UO22+ was not reduced to U(IV) completely at the electrode potential above -0.8 V vs. silver/silver chloride electrode. Ultraviolet-visible spectroscopy and speciation calculation of UO22+ in citrate media indicated that uranium existed as a mixture of UO22+, [UO2Cit] and [(UO2)2Cit2]2- at pH 2-3, and a predominant species at pH 3-5 was [(UO2)2Cit2]2- (H3Cit: citric acid). At pH 5-7, polymeric complexes, probably, [(UO2)3Cit3]3- and [(UO2)6Cit6(OH)10]16- were present. These findings suggest that the reduction of UO22+ is more difficult by polymerization of UO22+ with citric acid at higher pHs. Absorption spectra of the reduced complexes showed that U(IV) forms soluble complexes with citric acid at pH 2.0-5.0, and presence of U(V) species was not observed during the reduction of UO22+. (orig.)