Migration '09 edition:12 location:Kennewick, Washington, USA date:September 20 - 25
The redox-sensitive Tc99 has a half-life of 2.14x105 years and is identified, among other nuclides, as an important element with regard to the storage of high level radioactive waste . Under suitable reducing conditions, e.g. in the presence of an iron(II) containing solid phase which can act as an electron donor, technetium is considered immobile due to the formation of a sparsely soluble hydrous Tc(IV) precipitate.1 Due to the previously demonstrated association of Tc(IV) eigencolloids with mobile humic substances (HS) or the formation of Tc(IV)-HS complexes, the mobility of reduced Tc species may be drastically enhanced.2 The recent discovery of very stable Tc(IV) complexes with pyrogallol, a humic substance model compound, is a strong indication for the existence of similar complexes in natural systems containing dissolved organic matter. These pyrogallol complexes, readily formed at pH 11 and stable towards technetium colloid or precipitate formation necessitate a renewed research effort into the potential existence of Tc(IV)-HS complexes.3 Since the formation of such complexes indicates an extra potential for migration of Tc(IV) , their existence would impose a strong incentive for the re-evaluation of today’s safety case associated with high-level radioactive waste disposal.
The formation of stable Tc(IV)-NOM complexes was investigated as function of pH using dissolved Boom Clay organic matter. Pertechnetate was pre-reduced in supernatant solutions containing natural BC organic matter at different pH. After the pre-reduction and re-setting of the supernatant pH to pH 8.2, the predominant pH in the Boom Clay system, the supernatant solutions were contacted with the Boom Clay solid phases. Comparison of Tc distribution in these systems with reference Boom Clay batch systems, in addition to SEC on the supernatant solutions allowed to evaluate the competition between the sorption of technetium on the BC solid phase and the formation of stable Tc(IV)-BC organic matter complexes and Tc(IV)-BC organic matter colloid/colloid associations.