TECHNETIUM HYDROLYSIS AND EIGENCOLLOID FORMATION
Breynaert, E. and Maes, A.
Laboratory for Colloid Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg, 23, B3001 Leuven, BELGIUM, (Andre.Maes@agr.kuleuven.ac.be)
The chemistry of reduced technetium species has been subject to investigations initiated from both
the medical community and the nuclear industry. Studies concerned with medical nuclear imaging
applications (planar scintigraphy, …) extensively focus on the complexation chemistry of Tc [1].
Research initiated by the nuclear industry on the other hand is mainly interested in: a) the separation of
99
Tc (t½ = 2.14 x 105 years, - emitter) from high level nuclear waste streams [2], a potential
application for the recycling of nuclear waste, and b) the geochemical behaviour of 99Tc, an important
issue with regard to the safety of the geological disposal facilities for long-term storage of nuclear
waste [3, 4]. 99Tc has been identified as one of the critical radionuclides that could impair the longterm safety of these facilities. Both medical and environmental studies concerned with the solubility
and the complexation chemistry of technetium have encountered colloidal Tc(IV)-forms. Although the
existence of the Tc colloids has been proven by various techniques [5-10], their quantitative
determination still remains problematic.
Tc(IV)-oxide colloids, radiolytically formed by γ-irradiation of aqueous TcO4- solutions, were
visualised for the first time by transmission electron spectroscopy [7]. This experiment confirmed the
existence of nano-sized (2 – 130 nm ) Tc(IV) colloids in reducing aqueous technetium solutions.
Recently X-ray Absorption Fine Structure (EXAFS) spectroscopy also provided evidence for the
existence of colloidal and/or polymeric Tc(IV) species in mixed chloride/sulphate media [10, 11] and
lab-scale natural systems containing humic substances [12, 13].
Henry et al. [14] developed a partial charge model to capable of predicting the reactivity of cations in
aqueous solutions towards condensation and complexation via the their hydrolysis behaviour. This
PCM model is based on the electronegativity equalization theorem, initially formulated by Sanderson
[15] and afterwards theoretically supported by the work of Parr et al. [16] . The PCM framework was
successfully applied to technetium to explain the pH dependent hydrolysis behaviour and reactivity of
Tc(IV) that was experimentally determined throughout the previous decades and resulted in the
definition of a pH region where Tc(IV) eigencolloid formation is possible.
The combination of the theoretical hydrolysis behaviour with recent experimental results obtained
from Colloid Precipitation Chromatography (CPC) has enabled the formulation of a more detailed
hypothesis on the formation of eigencolloids from condensation of mono-nuclear hydrolysed Tc(IV)
species into binuclear and polynuclear Tc(IV) species as shown in Equation 1.
Tcx  OH 4 x  z ( H 2O) 2 x  z   Tc y  OH 4 y  w ( H 2O ) 2 y  w 


z
with x=1→n, y=1→n, z = -1→ 1, w = -1→ 1
w
Tcx  y  OH 
( H 2O ) 2( x  y  w z ) 
4( x  y )  ( w  z )


w z
Eq. 1
The authors acknowledge a grant from KULeuven University and financial support from the
KULeuven Geconcerteerde Onderzoeksacties (GOA2000/007). We also kindly acknowledge
NIRAS/ONDRAF for financial support from Contract CCHO 20004004862
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