Technetium Sorption by Cementitious Materials Under Reducing Conditions

Kaplan, Daniel I. ; Estes, Shanna L. ; Powell, Brian A.

Savannah River Site (S.C.)

关键词: K{Sub D} Value;    Grout;    Eh;    Saltstone;    12 Management Of Radioactive And Non-Radioactive Wastes From Nuclear Facilities Cement;   

DOI  :  10.2172/1056467
RP-ID  :  SRNL-STI--2012-00596
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  1056467

美国|英语

来源: UNT Digital Library

【 摘 要 】

The objective of this study was to measure technetium ({sup 99}Tc) sorption to cementitious materials under reducing conditions to simulate Saltstone Disposal Facility conditions. {sup 99}Tc(VII) batch sorption experiments were conducted for 319 days in an inert glovebag with a variety of cementitious materials (aged cement, Vault 2, TR545, and TR547) containing varying amounts of blast furnace slag. Between 154 and 319 days, the {sup 99}Tc aqueous concentrations tended to remain constant and samples amended with different initial {sup 99}Tc concentrations, tended to merge at about 10{sup -9} M for Vault 2 (17% slag) and TR545 (90% slag) and 10{sup -8} M for TR547 (45% slag). This data provided strong evidence that solubility, and not adsorption (K{sub d} values), was controlling aqueous {sup 99}Tc concentrations. Laboratory data superimposed over thermodynamic speciation diagrams further supported the conclusion that solubility, and not adsorption controlled {sup 99}Tc aqueous concentrations. The oxidation state of the aqueous {sup 99}Tc at the end of the sorption experiment was determined by solvent extraction to be almost entirely {sup 99}Tc(VII). The pH of the present system was ~11.8. Previously proposed solubility controlling phases including Tc-sulfides may be present, but do not appear to control solubility. After the 319 day sorption period, the suspensions were removed from the glovebag and a desorption step under oxic conditions was conducted for 20 days by adding oxic, pH-buffered solutions to the suspensions. {sup 99}Tc aqueous concentrations increased by more than an order of magnitude and Eh increased by several hundred millivolts within 24 hours after the introduction of the oxic solutions. These desorption results are consistent with re-oxidation and dissolution/desorption of {sup 99}Tc(IV) phases possibly present in the cementitious materials after the anoxic sorption step of the experiment. Aqueous {sup 99}Tc concentrations continued to increase slowly until the termination of the desorption experiment after 20 days. Although the cementitious materials investigated demonstrated the ability to strongly sequester aqueous {sup 99}Tc under anoxic conditions, the introduction of oxygen resulted in the rapid remobilization of {sup 99}Tc. These studies provide experimental support for the use of a solubility based model under reducing saltstone conditions where Tc(IV) is expected to be the dominant species. The existing Kd model would be appropriate for oxidized saltstone conditions (aged saltstone, not directly evaluated in this study) where Tc(VII) is expected to be the dominant species.