Americium sorption on smectite-rich natural clay from granitic ground water

Abstract

Because of their significant retention capability, clay minerals have been proposed as a potential engineered barrier in high level nuclear radioactive waste disposal repositories. Smectite-rich natural clay is being considered as a backfill and buffer material for the Indian repository program. In the present study the sorption of Am by the clay, from granitic ground water, has been investigated. To identify the minerals in the clay controlling the sorption process, the adsorption isotherm of Eu(III), a chemical analogue of Am(III), was determined on montmorillonite–kaolinite clay mixtures having 0–20wt% kaolinite. The effect of experimental parameters, such as, pH, ionic strength, and the presence of cation and anions on Am(III) sorption was further investigated to develop a sorption model for the natural clay. Overlapping adsorption isotherms of Eu(III) obtained for different montmorillonite–kaolinite clay suspensions established montmorillonite as the main sorbent for Eu/Am(III) in the natural clay. Americium(III) sorption increases with pH in three distinct stages: at lower pH values (<4) the sorption is virtually insensitive to pH, then rises sharply (4–7) and subsequently attains a constant value at higher pH values (>8). Decreasing ionic strength increases the sorption at pH<6 indicating the dominant role of ion exchange reactions at lower pH. A surface complexation model, developed for natural clay by including ion exchange site and amphoteric sites present at edges, simulates the sorption profiles at varying pH and ionic strength well and confirms the montmorillonite fraction as the sorbent controlling Am(III) sorption. The presence of Ca(II) as well as anions (Cl−, NO3-) does not affect Am(III) sorption on clay under granitic ground water pH and ionic strength conditions. However, the profile of Am(III) sorption to Ca(II)-equilibrated clay differs from that for Na-equilibrated clay corroborating weaker exchange of Ca(II)–Am(III) in comparison to Na(I)–Am(III). The presence of SO42- in the sorption system lowers Am(III) sorption at lower pH values. Modeling the sorption data indicated the participation of SO42- containing Am surface species. The thermodynamic model developed for sorption onto natural clay was checked for Eu(III) sorption from granitic ground water at pH 6.1. The model simulates the sorption at lower metal ion concentration while there is deviation at higher metal ion concentration. Inclusion of more types of surface sites and the effect of organic material need to be tested to correct the model for this deviation.