99technetium speciation and pathways under boom clay reducing conditions when starting form different initial conditions

Full text of publication follows: 99Technetium (Tc) is a redox-sensitive fission product whose geochemical behaviour in Boom Clay was intensively studied during the last years [1, 2]. However, several questions remain concerning the in situ migration behaviour of Tc in reducing sediments that are rich in humic substances (HS). When adding technetium as TcVIIO4- in concentrations in the range of 10-7 - 10-4 mol/L to Boom Clay suspension batches, it was observed that the added TcVII was reduced to TcIV according to XANES data [2]. However, the thermodynamically predicted TcO2 precipitate was not formed: (1) the concentration of Tc remaining in solution after reaction with the solid phase increases with the total amount of Tc added and (2) dissolved Tc concentrations were orders of magnitude above the predicted Tc solubility. This observation could be explained and described by a hydrophobic sorption mechanism of neutral Tc species on dissolved Boom Clay HS with a constant K-value [1]. The neutral Tc species were identified as TcIV Eigen-colloids - polymeric chains of TcO(OH)2 - by EXAFS measurements [2]. Moreover, batch (ir)reversibility experiments and column migration experiments starting out from the Tc-humic substance mixed colloids showed that the interaction is to a large extent irreversible, or at least kinetically favoured, thus precluding the adsorption of TcIV on the solid phase or the precipitation of a TcO2 phase. When adding technetium as a precipitated TcO2 phase, i.e. starting from conditions of under-saturation, to systems containing Boom Clay HS, the observed geochemical behaviour is dramatically different than the pathway outlined above. If TcIV precipitates are made starting from TcVII and using H4N2 as a reductant, an amorphous solid phase which is loosely bound and consists mainly of colloidal aggregates, is observed to be formed. Its solubility in synthetic (inorganic) groundwater is generally higher than the expected theoretical solubility of Tc(IV) oxide and ranged over 2 orders of magnitude from 1.8 x 10-8 M to 3.7 x 10-6 M. Upon contacting this phase with dissolved Boom Clay HS, Tc concentrations in solution were seen to increase with increasing dissolved HS concentration, validating the interaction of Tc(IV) Eigen-colloids with HS observed in experiments starting from. However, if electro-reduction is used to prepare the TcIV precipitate, and appropriate pre- and after-treatment steps are taken, a more crystalline solid phase is formed. TcIV precipitates showed measured solubilities in synthetic (inorganic) groundwater ranging from 3.4 x 10-9 M to 2.2 x 10-8 M, which is lower compared to the solubility of Tc(IV) precipitates from chemical reduction. Upon equilibration of these precipitates with dissolved Boom Clay HS, Tc concentrations in solution were not markedly increased with respect to their solubility in inorganic groundwater. In the presence of a Boom Clay suspension (containing immobile and dissolved HS), the Tc solution concentration did not exceed 4.1 x 10-8 M. All of the above observations indicate that interactions between Tc(IV) species and dissolved HS do exist, and may play a substantial role in migration experiments, but are very much dependent on the initial conditions in the studied systems. [1] A.Maes, C.Bruggeman, K.Geraedts, J.Vancluysen (2003) Environ. Sci. Technol. 37, 747-753 [2] A.Maes, K.Geraedts, C.Bruggeman, J.Vancluysen, A.Rossberg, C.Hennig (2004) Environ. Sci. Technol. 38, 2044-2051. We kindly acknowledge NIRAS/ONDRAF for financial support from Contract Nr. CCHO 2001- 845/00/00. (authors)