Influence of thermal and photochemical reactions upon the redox cycling of Pu between solid and solution phases in seawater

Our objective here is to collate both new and previously reported data, from our laboratory experiments, to more fully assess the influence of thermal and photochemical reactions upon the dissolution of ambient 239+240Plutonium (239+240Pu) from, and sorption of 238Pu(V) tracer by, marine sediment in seawater. Our data indicate that equilibrium of 239+240Pu(IV) species was rapidly attained (<1 h) between dissolved and particulate phases. Variations in temperature, within the range 4–25 °C, did not appear to significantly effect the dissolution of 239+240Pu(IV) species, although it is possible that light irradiation resulted in reduction of 239+240Pu(V) to 239+240Pu(IV) species in the dissolved phase. The impact of irradiation appeared to increase concomitant with decrease in light wavelength. Under dark conditions, and using an artificial source simulating natural sunlight, the dissolution of 239+240Pu(V) species from, and sorption of 238Pu(V) species by, particulate material increased over the duration of these experiments. Reactions involving Pu(V) species are the rate-limiting step for partitioning of Pu between suspended particles and the dissolved phase in seawater. Both 239+240Pu(V) dissolution and 238Pu(V) sorption increased concomitant with temperature and light irradiation due to simultaneous promotion of oxidation and reduction reactions upon the surface of suspended particles. Irradiating suspensions using light sources with shorter wavelength output (<400 nm) resulted in an initial enhancement of both reaction rates. The rate of both processes subsequently decreased with time and approached slow rates as observed under dark conditions. Extrapolation of these laboratory data to interpret the remobilisation behaviour of 239+240Pu from contaminated sediments in the Irish Sea (UK) indicates that the half-time taken for Pu(V) species to attain a steady-state distribution between dissolved and particulate phases is likely to be <15 months. Comparison of this estimate with the predicted half-time for dissolution of 239+240Pu bound to surficial seabed sediment (in the order of 102 years) indicates that processes other than surface chemical reactions are more important in governing the rate and extent of 239+240Pu remobilisation.