Kinetics and reversibility of Tc immobilisation in soils under aerated or flooded conditions

Titre du congrès :18th World Congress of Soil Science Ville du congrès :Philadelphie Date du congrès :09/07/2006

The fate of technetium in the environment is studied because one of its isotopes, 99Tc, is an important component of nuclear waste and because it is present in discharges from some nuclear processing plants. The dynamics of this transition metal in soil depend largely on redox conditions and its interaction with organic matter. The pertechnetate anion, TcO4-, is not strongly adsorbed by soil components and so is very mobile. However under reducing conditions Tc(IV) may be immobilised by adsorption, precipitation or redox reactions with soil organic matter. Rather little is known about the soil properties that determine the extent and the reversibility of immobilisation. The aim of this study was to follow the changes in fractionation of technetium, added as 99TcO4-, in soil over a 2-month period and to identify the chemical form of the immobilised element using chemical extractions. Five contrasting soils were studied and the effect of organic matter amendment was also studied for one of them. Soils were incubated either under moist, aerated conditions, or flooded to rapidly induce reducing conditions. The extent of remobilisation of Tc after one month of flooding followed by one month of aerated conditions was also measured. Changes in solution pH, major anion and cation concentrations in solution and solubility of both organic carbon and Fe were also monitored. There was no significant immobilisation of pertechnetate in aerated soils over a one-month period. However, the concentration of Tc in the waterhead of flooded soil decreased fairly continuously over a two-month period in all soils. The rate of decrease differed between soils and there was no simple relationship with soil properties, such as pH, texture and organic matter content. Water-extractable and exchangeable Tc followed similar trends for each of the soils. Immobilised Tc could be extracted using sodium hydroxide or hydrogen peroxide, indicating that it was associated with organic matter. There was little difference in the efficiency of these extractants, suggesting that immobilised Tc was not simply adsorbed or precipitated Tc(IV). Immobilisation of Tc after flooding was only partially reversible, as demonstrated by the increase in water-extractability of Tc after a further incubation under aerated conditions, but water solubility did not return to the initial high level. There was no simple relationship between reversibility and either the extent of initial immobilisation or soil properties. Further work is required in order to integrate the complexity of Tc dynamics in soils under fluctuating redox conditions into risk assessment models.