Laboratory studies of the chemical behaviour of plutonium associated with contaminated estuarine sediments

Laboratory experiments, with contaminated Esk Estuary sediment and uncontaminated River Esk and North Sea waters, have provided detailed information on Pu remobilisation behaviour. Of the 977 mBq g⁻¹ Pu (III, IV) associated with the sediment, 2·6% is subject to a rapid reversible exchange reaction. Well-defined distribution coefficients (K_d) have been determined with respect to this labile fraction (e.g. 5 × 10³ litres kg⁻¹ for remobilisation into river water). A low level of Pu (III, IV) remobilisation can be maintained following complete removal of the labile fraction, probably through a rate-limited reaction involving a more strongly bound form of Pu. Plutonium (V, VI) remobilisation occurs both through rapid exchange of a comparable labile component and through a mechanism involving photo-oxidation of Pu (III, IV).A dominant proportion of the remobilised Pu (III, IV) in solution is anionic with a molecular weight1000 daltons, probably in the form of an organic complex. Remobilised Pu (V, VI) is mainly in true solution in variously charged forms. Addition of natural colloidal organic carbon (COC) to river water increased the level of labile Pu (III, IV) remobilisation. The effect can be described by two competing equilibria in which labile Pu (III, IV) (a) is complexed by COC, and (b) is adsorbed by the sediment. The same additions of COC reduced the level of Pu (V, VI) remobilisation, either through its action as a reducing agent or through the indirect effect of associated pH changes. Increasing the carbonate alkalinity (NaHCO₃) in river water decreases the remobilisation of both Pu (III, IV) and Pu (V, VI).