Sorption characteristics of⁹⁹Tc onto bentonite material with different additives under anaerobic conditions

Abstract

Summary The radionuclide 99 Tc is one of the fission products of spent nuclear fuel. The chemical forms of this element depend primarily on the pH and E h of the environment. The aim of this study was to describe the sorption of the pertechnetate anion onto bentonite under anaerobic conditions, which accurately simulate the environment of a deep repository. Several additives were tested to determine whether they increase the retention of 99 Tc on this material — Fe, FeS, Fe 3 O 4 and activated carbon. It was found that it is very difficult to create the same conditions present in the deep repository in the laboratory, which is why the reduction of the pertechnetate anion to its lower oxidation state (Tc(IV)) was not achieved. The retention of the pertechnate anion on bentonite is very low ( K D ∼1 mL/g), the uptake of the pertechnetate anion onto bentonite was achieved by the addition of suitable additives to the bentonite mixtures e.g. activated carbon, Fe and FeS ( K D ∼102–103 mL/g). Various chemical forms of technetium were obtained in the solution, dependant on the various reaction mechanisms of the pertechnetate anion. Fe and FeS enabled the reduction of the pertechnetate anion to its lower oxidation state in the form of insoluble TcO(OH) 2 or TcO 2 · n H 2 O, physical sorption is the dominant mechanism with activated carbon.