PNNL

Abstract

In previous work we demonstrated that radiolysis of uranyl tris carbonate in near neutral pH to alkaline carbonate solutions, could be followed by 13C NMR. Radiolysis of the complex produced novel uranyl peroxo carbonate solution state species, whose structures depended on the pH and radiolytic dose rate. In this work, we investigate speciation of the uranyl carbonate trimer which is predominant in bicarbonate solution near pH 5.9. We observe radiolytically derived speciation to different mixed peroxy carbonate species than seen in the higher pH solutions. Auto radiolysis of uranium (VI) carbonate solutions between pH 5.9 and 7.2 is shown to alter the uranium speciation over relatively short periods of time and was followed by 13C NMR and visible spectrophotometry, using dissolved 233(UO2)3(CO3)6 6- both as the radiolysis source (D= 14.9 Gy/hr) and as a trap for the newly formed hydrogen peroxide. Direct addition of hydrogen peroxide to solutions of the uranyl-carbonate trimer is shown to reproduce the 13 C NMR signatures of the complexe(s) formed by radiolysis, but additionally a variety of new complexes are revealed. Ratios of H2O2/trimer 1 resulted in formation of stable higher order peroxo carbonate complexes. The 13C NMR signatures and visible spectra of these complexes are described here. Rigorous characterization of the species is an ongoing effort.