A Valence Control Method Based on a Pt-Aided Hydrogen Peroxide Treatment for Yielding Tetravalent Neptunium and Plutonium in Nitric Acid Solutions

Abstract

There are several possible causes of the recovery decrease sometimes encountered in the chemical separation procedures for actinides. Although the interferences present in the samples could be related to the recovery decrease in the actinide separations, an incomplete valence control of the actinides in the sample solution conditioning step often causes problems of a significant loss of them in the subsequent sample loading step. Thus, control of the valence states of the actinides in the sample solutions is a basis for the separation methods such as anion exchange, extraction chromatography, solvent extraction and precipitation. In some chromatographic separation procedures for actinides, both Np and Pu are required to be adsorbed as their tetravalent state on the sorption media such as anion exchange resins or UTEVA-resin. 1,2 The most prominent factor influencing the individual separation of actinides may be an adjustment of the valences of Pu to a desired one, which is nearly the same as in the case of Np. Therefore, special attention must be given to a completion of a valence control for Pu and Np together in a sample solution prior to its loading on a separation column. Various reducing agents such as ferrous sulfate, hydroxylamine, hydrazine, SO2 and SnCl2 are used to reduce Np(VI) to Np(IV) and all the Pu to Pu(III). 3 Ferrous sulfamate reduces Np in higher valence states to Np(IV), while it reduces all the Pu to Pu(III). 4 Np and Pu can be adjusted to Np(IV) and Pu(IV) by the use of sodium nitrite following an addition of ferrous ammonium sulfate. 5 However, these reagents introduce foreign ions into sample solutions which would complicate the subsequent separation steps. Reduction of Np(VI) to Np(IV) and Pu(VI) to Pu(IV) with H2O2 has sometimes been adopted in the valence control step of the actinide separation procedures used to determine individual actinides. 1,3,6 However, poor and inconsistent recoveries for Np and/or Pu were often observed when using H2O2 as a valence control agent in HNO3 media samples. 7-11 The purpose of this study is to develop a simple and efficient method that affords a valence adjustment of both Np and Pu to a tetravalent state for an effective adsorption onto the UTEVA-resin and the anion exchange resins from HNO3 media. A H2O2-based sample solution treatment followed by a Pt-metal catalytic decomposition of the excess H2O2 was investigated to adjust both the Np and Pu to the required tetravalent state. The effectiveness of the H2O2based valence control method proposed in this study was assessed by its adaptation to an UTEVA-resin chromatography for an individual separation of U, Np, Pu, and Am by using a synthetic mixture of their isotopes. Experimental Section