PARTICLE-SOLUTION BEHAVIOR 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 Pu-236(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 Pu-239, Pu-240(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 Pu-239, Pu-240(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 Pu-239, Pu-240(III,IV) fraction. For example, in R. Esk water, the adsorption of Pu-236(III,IV) and the desorption of Pu-239, Pu-240(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 approximately 3 x 10(3) 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.