LABORATORY STUDIES TO INVESTIGATE SHORT-TERM OXIDATION AND SORPTION BEHAVIOR OF NEPTUNIUM IN ARTIFICIAL AND NATURAL SEAWATER SOLUTIONS

A number of short-term laboratory experiments have been carried out, u sing radiotracers and artificial and natural seawater solutions, to ex amine the possible variables which could influence the behaviour of ne ptunium in seawater. Parameters considered include initial oxidation s tate, sorption by suspended particulate material and chemical composit ion of the solution phase. Results indicate that the uptakes of Np(IV) and Np(V) species by suspended sediment exhibit different kinetics. F or Np(IV), rapid initial sorption was observed followed by desorption over longer time periods. In contrast, initial sorption of Np(V) was f ollowed by further uptake of the species, but at a slower rate. No evi dence was obtained to suggest that Np(V) is reduced to Np(IV). Althoug h a large proportion of Np(IV) was rapidly oxidised to Np(V) under neu tral and alkaline pH conditions, the rate of conversion slowed progres sively with time. Only a small, but nevertheless measurable, amount of dissolved Np(IV) remained after equilibrating for 24 h. In addition, the kinetics of Np(IV) oxidation and Th(IV) sorption were remarkably s imilar in seawater containing a relatively small amount of particulate material. Although it has been shown that the major constituents of s eawater can influence Np(IV) sorption and oxidation kinetics, to a fir st approximation, a consistent redox equilibrium was attained within a relatively short period (24 h). Laboratory data are discussed in the context of extrapolating trends to natural systems and, environmental observations of the behaviour of Np-237 discharged into the Irish Sea from the BNFL nuclear fuels reprocessing plant at Sellafield (U.K.).