Particle-solution behaviour of plutonium in an estuarine environment,Esk Estuary,UK

The particle-solution (K_d) relationships of Pu(III,IV) in the Esk Estuary are investigated, using new experimental data together with a synthesis of our earlier work. Adsorption of ²³⁶Pu(IV) by a suspension of intertidal mud (40 mg L⁻¹) from the Esk Estuary was determined after 1 h, as a function of salinity and pH, in a series of controlled laboratory experiments. Desorption of ^{239, 240}Pu(III,IV) from this environmentally contaminated sediment was determined concurrently. The short term (i.e., within a tidal cycle) non-conservative behaviour of Pu(III,IV) in both the laboratory experiments and the estuary appears to be dominated by a rapid, reversible surface complexation reaction, although only a small fraction (<5%) of the ^239,240Pu(III,IV) activity of the environmentally contaminated sediment is able to participate in the reaction. The existence of this exchangeable or labile fraction can explain the apparent dependence of Pu(III,IV) K_d on sediment concentration. Thus, a single K_d value is able to describe both the observed adsorption and desorption behaviour and the effect of sediment concentration, when desorption is expressed with respect to the labile ^239,240Pu(III,IV) fraction. For example, in R. Esk water, the adsorption of ²³⁶Pu(III,IV) and the desorption of ^236,240Pu(III,IV), over a two order of magnitude concentration range of unwashed Esk Estuary sediment, can be accounted for by a single K_d of ∼3 × 10³ L kg⁻¹. The K_d for the labile Pu(III,IV) fraction varies mainly as a function of salinity. pH is not an important factor over the pH range 4–9, although H⁺ ion exchange does occur, probably through a similar surface complexation reaction. The kinetic and equilibrium characteristics of the Pu (III,IV) sorption behaviour conform to the pattern observed by Jannasch et al. (1988) and other workers for a range of trace elements.