Solubility control of technetium release from Saltstone by TcO2·xH2O

Abstract

Saltstone leaching experiments were conducted using a modified single-pass flow-through method under anoxic conditions. The analytical results of leachates collected from these experiments were evaluated using thermodynamic modeling to determine if the data were consistent with potential solubility controlling phases. The results demonstrate that technetium release from Saltstone under anoxic conditions is controlled by the solubility of TcO2·xH2O (likely TcO2·1.6H2O). In our system the solubility of TcO2·1.6H2O appears to have been reached equilibrium within 2weeks, with a Tc concentration of approximately 1.5×10−6M. Because Saltstone is a cementitious material, it is a continuously reacting solid with component phases that continue to change over very long time periods (up to hundreds of years). As a result of this process, the concentrations of technetium in equilibrium with TcO2·1.6H2O are likely to vary as the composition of Saltstone pore fluid evolves over time. In a disposal scenario where the initially high pH values (∼12.5–13) decrease (due to carbonation over very long time periods), the solubility of TcO2·xH2O would decrease significantly. The thermodynamic data used to determine the solubility of TcO2·1.6H2O were taken from the tabulation of critically selected thermodynamic data determined by the Nuclear Energy Agency.Solid phase characterization to demonstrate the presence of TcO2·xH2O was not possible due to the low concentrations of technetium in our samples. Previous solid phase characterization studies with cementitious waste forms that were very similar to our Saltstone samples as well as reaction products derived from reductive immobilization of TcO4- by amorphous FeS clearly indicate the presence of TcO2 with varying degrees of hydration. Although, the presence of TcSx or other reduced technetium sulfide phases are likely present in our samples, release of technetium from Saltstone will be controlled by TcO2·1.6H2O because of its higher solubility.Our results clearly demonstrate that the release mechanism of technetium from Saltstone under reducing conditions controlled by the solubility of TcO2·xH2O (likely TcO2·1.6H2O); however, empirical distribution coefficients (Kds), that describe sorption and not solubility, were calculated for comparison with past literature values. After 84days of reaction under anoxic conditions, the average Kd value for technetium was determined to be 610mL/g. This value is similar to a value determined previously for a similar Saltstone sample under reducing conditions at 56days (712±81mL/g).