Reduction of Np(VI) and Pu(VI) by organic chelating agents

The reduction of NpO22+ and PuO22+ by oxalate, citrate, and ethylenediamine tetraacetic acid (EDTA) was investigated in low ionic strength media and br ines. This was done to help establish the stability of the An(VI) oxidation state in the presence of organic complexants. The stability of the An(VI) oxidation state depended on the pH and relative strength of the various oxi dation state-specific complexes. At low ionic strength and pH 6, NpO22+ was rapidly reduced to form NpO2+ organic complexes. At longer times, Np(IV) o rganic complexes were observed in the presence of citrate. PuO22+ was predo minantly reduced to Pu4+, resulting in the formation of organic complexes o r polymeric/hydrolytic precipitates. The relative rates of reduction to the An(V) complex were EDTA > citrate > oxalate. Subsequent reduction to An(IV ) complexes, however, occurred in the following order: citrate > EDTA > oxa late because of the stability of the An(V)-EDTA complex. The presence of or ganic complexants led to the rapid reduction of NpO22+ and PuO22+ in G-Seep brine at pHs 5 and 7. At pHs 8 and 10 in ERDA-6 brine, carbonate and hydro lytic complexes predominated and slowed down or prevented the reduction of An(VI) by the organics present.