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

Laboratory experiments were carried out to assess the influence of temperat ure and photochemical reactions upon the dissolution of Pu239 + 240 and Am- 241 from an Irish Sea sediment in seawater. Supplementary information was o btained from experiments to evaluate the rate and extent of Np-239 tracer d esorption, and Pu-238(V) tracer uptake. A steady state distribution of 241A m and Pu239 + 240(IV) species between dissolved and particulate phases was rapidly attained (<1 h), The extent of their remobilisation was largely una ffected by temperature changes within the range approximate to 4-25 degrees 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 revers ible first order reaction. Remobilisation of Pu239 + 240(V), and uptake of Pu-238(V) tracer, species increased concomitant with temperature and light irradiation due to simultaneous promotion of oxidation and reduction reacti ons upon the surface of the suspended particles. It is suggested that, in t erms of band-gap theory, the rate determining step may be migration of vale nce band holes and conduction electrons to Pu sites. These laboratory data are discussed in the context of extrapolating trends to the remobilisation behaviour of Pu239 + 240 and Am-241 from contaminated sediments in the Iris h 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 pha ses. These estimates are compared with the predicted half-time for dissolut ion of Pu239 + 240 bound to Irish Sea sediments. Crown Copyright (C) 1999 P ublished by Elsevier Science Ltd. All Fights reserved.