The role of thermal and photochemical reactions upon the remobilisation of Pu from an Irish Sea sediment

Laboratory experiments were carried out to assess the influence of temperature and photochemical reactions upon the dissolution of 239+240Pu and 241Am from an Irish Sea sediment in seawater. Supplementary information was obtained from experiments to evaluate the rate and extent of 239Np tracer desorption, and 238Pu(V) tracer uptake. A steady state distribution of 241Am and 239+240Pu(IV) species between dissolved and particulate phases was rapidly attained (<1 h). The extent of their remobilisation was largely unaffected by temperature changes within the range ≈4–25°C and light irradiation. Dissolution of Pu(V) species appeared to occur as an independent process and the data could be modelled in terms of a reversible first order reaction. Remobilisation of 239+240Pu(V), and uptake of 238Pu(V) tracer, species increased concomitant with temperature and light irradiation due to simultaneous promotion of oxidation and reduction reactions upon the surface of the suspended particles. It is suggested that, in terms of band-gap theory, the rate determining step may be migration of valence band holes and conduction electrons to Pu sites. These laboratory data are discussed in the context of extrapolating trends to the remobilisation behaviour of 239+240Pu and 241Am from contaminated sediments in the Irish Sea (UK). An assessment is made of the half-time taken for Pu(V) species to attain a steady-state distribution between dissolved and particulate phases. These estimates are compared with the predicted half-time for dissolution of 239+240Pu bound to Irish Sea sediments.