Adsorption Behaviour of Se(-II) and Tc(IV) onto Granite, Shale, Limestone, Illite, and MX-80 Bentonite in Ca-Na-Cl and Na-Ca-Cl Solutions

Abstract

Canada is in the process of implementing a Deep Geologic Repository (DGR) to dispose of used nuclear waste. Adsorption behaviour of both Se(-II) and Tc(IV) onto granite, shale, limestone, illite, and MX-80 bentonite has been elucidated. Se(-II) adsorption onto granite and MX-80 bentonite displays a decrease in Rd with an increase in solution pH. Se(-II) adsorption onto granite decreases with an increase in solution ionic strength. Se(-II) adsorption onto MX-80 bentonite does not return evidence which supports an apparent effect due to the ionic strength. Tc(IV) adsorption onto shale, limestone, illite, and MX-80 bentonite remains constant as the solution pH increases. Ionic strength does not affect the magnitude of Tc(IV) adsorption across the adsorbents, however an increase in ionic strength accelerates Tc(IV) adsorption. Se(-II) surface complexation models are best simulated with the following surface complexes: ≡Feldspar_sSe-, ≡Biotite_sOH2HSe, ≡Albite_sSe-, ≡Montmorillonite_sSe-, and ≡Montmorillonite_sOH2HSe. Tc(IV) adsorption is best simulated with: ≡Biotite_sOTcO(OH), ≡Quartz_sOTcO(OH), (≡Feldspar_sOH)2TcO(OH)-, ≡Montmorillonite_sOTcO(OH), (≡Albite_sOH)2TcO(OH)-, ≡Illite_sOTcO(OH), and ≡Chlorite_sOTcO(OH). Se(-II) adsorption onto granite and MX-80 bentonite in CR-10 solution returns Rd values of (1.80 ± 0.10) m3∙kg-1 and (0.47 ± 0.38) m3∙kg-1, respectively. Tc(IV) adsorption onto granite and MX-80 bentonite in CR-10 solution returned Rd values of (1.47 ± 0.25) m3∙kg-1 and (2.19 ± 0.33) m3∙kg-1, respectively. Tc(IV) adsorption onto shale, limestone, illite, and MX-80 bentonite in SR-270-PW solution returned Rd values of (0.16 ± 0.10) m3∙kg-1, (0.44 ± 0.21) m3∙kg-1, (1.86 ± 0.44) m3∙kg-1, and (0.23 ± 0.10) m3∙kg-1, respectively. This thesis will further deepen the understanding of Se(-II) and Tc(IV) adsorption.