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 ⁹⁹Tc 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 ⁹⁹Tc kinetics in aerobic soils and (ii) over what experimental timescales must ⁹⁹Tc kinetics be measured to make reliable long-term predictions of impact in the terrestrial environment? Soil microcosms spiked with ⁹⁹TcO₄^- 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 ⁹⁹Tc fraction. Complexation with soil humus was key to the progressive immobilisation of ⁹⁹Tc. 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 ⁹⁹Tc solubility in soils with low to moderate organic matter contents.