Chemo-degradation of chelating and complexing agents in a simulated, mixed nuclear waste

Abstract

The effect(s) of the chemical composition of stored mixed wastes on the stability of their organic content was investigated. A nonradioactive simulant of a mixed waste was formulated to mimic a complex concentrate waste stored at the U.S. Department of Energy's Hanford Site. The simulant consisted of an alkaline inorganic matrix plus the chelating agents ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and N-(2-hydroxyethyl)ethylene-diaminetriacetic acid (HEDTA), and the complexing agent citric acid. The simulant's total organic content remained fairly constant (82–102% recoveries) for 171 days at ambient temperature, suggesting that little organic degradation had occurred. However, detailed analyses of the organics by gas chromatography/mass spectrometry (GC/MS) revealed that, collectively, 61.7% of the parent organics had degraded. Moreover, the individual parent compounds degraded differentially, with the HEDTA being the most labile. Twenty-one degradation products were identified, consisting mainly of complexor and chelator fragments. Such extensive changes in the organic composition of the simulant will undoubtedly have to be factored into the management of mixed wastes.