Immobilization of uranium and nickel in sludges treated by solidification and stabilization

Research missions in nuclear energy have generated large volumes of mixed wastes. This research focuses on the development of an effective solidification and stabilization waste form for the immobilization of mixed waste sludges at the Oak Ridge National Laboratory K-25 Site. Nickel and uranium are the components of primary concern. Understanding the chemistry of contaminants is vital in determining the extent and possible mechanisms of immobilization, as well as predicting long-term performance.;The treatability investigation was based on a complex surrogate (or simulant) of the actual sludge. Portland cement-based systems including additives such as flyash, silica fume and sodium sulfide were studied. The binder-to-waste ratios and the replacement of cement with additives were determined based on the analysis of several tests. Four binder-to-waste ratios were studied over curing periods extending from 28 to 224 days. MacMullin number, pH, uranium and nickel porewater concentrations were analyzed to determine treatment effectiveness. The Toxicity Characteristic Leaching Procedure (TCLP) and unconfined compressive strength tests were conducted to evaluate physical integrity, leachability, and regulatory compliance from the use of S/S technology.;The surrogate formulation was spiked with uranium (900 ppm) and nickel (3000 ppm). These levels of uranium and nickel are reflective of the average waste concentrations reported in the K-25 Site sludges. TCLP analysis of treated surrogate generally show solution phase concentrations ;Modeling efforts indicate that uranium will be retained primarily by adsorption, but there are some precipitates that are favorable depending upon formation constants. Nickel immobilization predictions indicate that this metal will be retained through precipitation.;Since there was no opportunity to treat the authentic sludges, the effectiveness of cement-based treatment can only be predicted from the comparison of work performed and reported here to that performed and previously reported. However, the experimental approach and the information gained are potentially valuable not only to the K-25 Site, but also to the remediation of sludges with similar combinations of radioactive and hazardous components.