Kinetics of 99Tc speciation in aerobic soils

Technetium-99 is a significant and long-lived component of spent nuclear fuel relevant to long-term assessments of radioactive waste disposal. Whilst Tc-99 behaviour in poorly aerated environments is well known, the long-term bioavailability in aerobic soils following direct deposition or transport to the surface is less well understood. This work addresses two questions: (i) to what extent do soil properties control Tc-99 kinetics in aerobic soils and (ii) over what experimental timescales must Tc-99 kinetics be measured to make reliable long-term predictions of impact in the terrestrial environment? Soil microcosms spiked with (TcO4-)-Tc-99 were incubated for 2.5 years and 99Tc transformations were periodically monitored by a sequential extraction, which enabled quantification of the reaction kinetics. Reduction in soluble 99Tc was slow and followed a double exponential kinetic model including a fast component enhanced by low pH, a slow component controlled by pH and organic matter, and a persistently soluble Tc-99 fraction. Complexation with soil humus was key to the progressive immobilisation of Tc-99. Evidence for slow transfer to an unidentified 'sink' was found, with estimated decadal timeframes. Our data suggest that short-term experiments may not reliably predict long-term Tc-99 solubility in soils with low to moderate organic matter contents.