Technetium-99 is a significant and long-lived component of spent nuclear fuel relevant to long-term assessments of radioactive waste disposal. Whilst 99 Tc behaviour in poorly aerated environments is well known, the longterm 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 99 Tc kinetics in aerobic soils and (ii) over what experimental timescales must

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... 99 Tc kinetics be measured to make reliable long-term predictions of impact in the terrestrial environment? Soil microcosms spiked with 99 TcO 4 − were incubated for 2.5 years and 99 Tc transformations were periodically monitored by a sequential extraction, which enabled quantification of the reaction kinetics. Reduction in soluble 99 Tc 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 99 Tc fraction. Complexation with soil humus was key to the progressive immobilisation of 99 Tc. 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 99 Tc solubility in soils with low to moderate organic matter contents. relatively high fission yield (6.1 %) and a long radioactive half-life (213,000 years) and is, therefore, a significant and long-lived component of spent nuclear fuel and other radioactive wastes (Ashworth and