Technetium Sorption By Cementitious Materials Under Reducing Conditions

The objective of this study was to measure Tc sorption to cementitious materials under reducing conditions to simulate Saltstone Disposal Facility conditions. Earlier studies were conducted and the experimental conditions were found not to simulate those of the facility. Through a five month subcontract with Clemson University, sorption of 99Tc to four cementitious materials was examined within an anaerobic glovebag targeting a 0.1% H2(g)/ 99.9% N2(g) atmosphere. Early experiments based on Tc sorption and Eh indicated that 0.1% H2(g) (a reductant) was necessary to preclude experimental impacts from O2(g) diffusion into the glovebag. Preliminary data to date (up to 56 days) indicates that sorption of 99Tc to cementitious materials increased with increasing slag content for simulated saltstone samples. This is consistent with the conceptual model that redox active sulfide groups within the reducing slag facilitate reduction of Tc(VII) to Tc(IV). These experiments differ from previous experiments where a 2% H2(g) atmosphere was maintained (Kaplan et al., 2011 (SRNL-STI-2010-00668)). The impact of the 2% H2(g) reducing atmosphere on this data was examined and determined to cause the reduction of Tc in experimental samples without slag. In the present ongoing study, after 56 days, Tc sorption by the 50-year old cement samples (no slag) was undetectable, whereas Tc sorption in the cementitious materials containing slag continues to increase with contact time (measured after 1, 4, 8, 19 and 56 days). Sorption was not consistent with spike concentrations and steady state has not been demonstrated after 56 days. The average conditional Kd value for the Vault 2 cementitious material was 873 mL/g (17% slag), for the TR547 Saltstone (45% slag) the conditional Kd was 168 mL/g, and for TR545 (90% slag) the conditional Kd was 1,619 mL/g. It is anticipated that additional samples will be collected until steady state conditions are established to permit measuring more representative Kd and solubility values under these experimental conditions. The purpose of this revision is to correct Kd values reported in the original document. This document reports results for the first 56 days of a 319 day experiment. After the experiment had finished and all the data were compiled for QA/QC analysis, a mistake in the Kd calculations was found. The Kd values reported in revision 0 of this report were essentially one order of magnitude higher than the actual values. In this revision, the text references to Kd values have been updated along with the data in Figure 2 and Appendix B. Between the time the original document was issued, Janurary 2012, and this document was issued, a final report (Estes et al. 2012; SRNL-STI-2012-00596, Rev. 0) describing all the corrected data for the 319 day experiment was issued. This document does not contain any data that is not already reported in the final report (SRNL-STI-2012-00596, Rev. 0)