Sorption and surface complexation modeling of Cs-137 on Fe(II)-montmorillonite clay mineral relevant to nuclear waste disposal

摘要

Bentonite, a natural clay, is a promising candidate to be used as a buffer/backfill material in the geological disposal systems for management of high level waste (HLW). Fe released due to corrosion of canister/overpack may result in alteration of Na-montmorillonite (Na-Mt), present in bentonite, to Fe-montmorillonite (Fe-Mt) in a span of several years after the emplacement of vitrified HLW in deep geological repositories. For realistic performance assessment, it is essential to understand the sorption behavior of altered Mt with regard to the radionuclides present in HLW. Cs is one of the high-yield (Cs-137 t(1/2) = 30.1 y, 6%) and long-lived (Cs-135 t(1/2 )= 2 x 10(6) y) fission products in spent fuels. The objective of present study is to understand the effect of various parameters, viz., time (0-48 h), pH (3.0-9.0), ionic strength (0.001-1 M) [Cs(I)] (10(-10)-10(-3) M) and Fe dissolution on sorption behavior of Cs(I) on Fe(II)-Mt through batch sorption experiments. Fe(II)-Mt was synthesized by reducing Fe(III)-Mt using ascorbic acid as reducing agent in N-2 atmosphere. The near-constancy in Cs(I) sorption on Fe(II)-Mt with pH (>= 4), and decrease with increasing ionic strength, illustrate the ion exchange as dominant mode of Cs(I) sorption. Further, Cs(I) sorption isotherm on Fe(II)-Mt is found to be linear. The estimation of dissolved iron in the supernatant of Fe(II)-Mt suspensions demonstrated that dissolved Fe decreased with increase in pH and increased with increase in ionic strength. Moreover, the Fe2+/Fe-total ratio determined in all experiments was close to unity, thereby depicting that Fe(II) did not oxidize to Fe(III), except when suspension pH was >= 5.5. For comparison, Cs(I) sorption was also studied on Na(I)-Mt and compared with that on Fe(III)-Mt. Surface complexation modeling of Cs(I) sorption on the three clay minerals, viz., Na -Mt, Fe(II)-Mt and Fe(III)-Mt, has been successfully carried out.