Degradation of citric acid in a simulated, mixed nuclear waste: Radiolytic versus chemical forces

Our laboratory investigated the effect(s) of radiolysis and waste chemistry on the stability of citrate in a nonradioactive simulant of a mixed waste, formulated to mimic a complex concentrate waste stored at the U.S. Department of Energy's Hanford Site. The parent compound citrate and any degradation products formed were analyzed as methyl esters by gas chromatography (GC) and GC/mass spectrometry (GC/MS). In general, citrate proved to be sensitive to both waste radiolysis and chemistry, but in different ways. Gamma irradiation of the simulant in a 60Co source produced extensive citrate degradation (58.6% after only 100 h), but yielded only one degradation product, the complexor fragment propanedioic acid. In contrast, chemo-degradation of citrate in the simulant was slower (64% after 180 days), but was more diverse, yielding several complexor fragments (ethanedioic, propanedioic, and butanoic acids), along with traces of other mono- and dicarboxylic acids. Ethanedioic acid dominated in the chemo-degradation study; propanedioic acid was much less abundant, unlike the radiolysis study. Radiolysis resulted in dispersive organic degradation; 41.4% of the waste's total organic content (TOC) disappeared after 100 h of irradiation. In contrast, chemo-degradation of citrate appears to be fairly constant over most of the 180-day study, with the TOC loss averaging 17.6%. Citrate degradation will undoubtedly have an impact on the management of mixed wastes.