Past activities related to the chemical separations for plutonium extraction at the Hanford Site (WA) resulted in the production of hazardous liquid and radioactive wastes stored in underground storage tanks. Leaks of waste liquids containing a variety of contaminants from some of these tanks have caused subsurface contamination. Tc, a major contaminant of concern at the Hanford Site, is present as highly soluble TcO4- under the predominant oxidizing conditions at the Hanford subsurface. TcO4- is mobile in the vadose zone and in situ vadose zone remediation approaches are being considered to immobilize Tc and decrease its mobility to the groundwater. A combined NH3-H2S treatment has been proposed for the immobilization of U and Tc in the vadose zone: H2S reduces mobile Tc(VII) to immobile Tc(IV) sulfide phases; the NH3 treatment increases the aqueous phase pH promoting mineral dissolution and subsequent neo-phase formation as pH decreases back to natural values after NH3 gas treatment. The newly formed secondary minerals have the potential of covering Tc(IV) sulfide phases, physically protecting Tc(IV) from re-oxidation and mobilization. The objective of the current work is to investigate the stability of Tc sulfide phases under conditions similar to those at the Hanford subsurface. Batch Tc2S7 dissolution experiments under oxidizing conditions revealed an increase in 99Tc release rate in the aqueous phase with pH increase, whereas under reducing conditions, 99Tc release was negligible as expected. Furthermore, an increase in electrolyte concentration caused an increase in dissolution, but at the same time the nature of ligand (perchlorate or bicarbonate) did not seem to affect Tc-99 release in the aqueous phase. Sulfates were detected in the aqueous phase in the presence of perchlorates, whereas in the presence of bicarbonates sulfates and thiosulfates were determined.Current, experiments involve Tc2S7 dissolution under oxidizing conditions using synthetic groundwater saturated in bicarbonates. Preliminary solid phase post-dissolution measurements performed with X-Ray Fluorescence revealed the presence of 99Tc only in the solid phase, whereas no sulfur was detected suggesting possible conversion of the solid from a Tc-S phase to a Tc-O phase upon exposure to oxidizing conditions. Further solid phase analyses using SEM/FIB, TEM/SAED and EXAFS are undergoing.

Dissolution studies of technetium sulfide under oxidizing and reducing conditions: Effect of pH and ionic strength

2019 Spring National Meeting

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