Laboratory studies to investigate short-term oxidation and sorption behaviour of neptunium in artificial and natural seawater solutions

A number of short-term laboratory experiments have been carried out, using radiotracers and artificial and natural seawater solutions, to examine the possible variables which could influence the behaviour of neptunium in seawater. Parameters considered include initial oxidation state, sorption by suspended particulate material and chemical composition of the solution phase. Results indicate that the uptakes of Np(IV) and Np(V) species by suspended sediment exhibit different kinetics. For Np(IV), rapid initial sorption was observed followed by desorption over longer time periods. In contrast, initial sorption of Np(V) was followed by further uptake of the species, but at a slower rate. No evidence was obtained to suggest that Np(V) is reduced to Np(IV). Although a large proportion of Np(IV) was rapidly oxidised to Np(V) under neutral and alkaline pH conditions, the rate of conversion slowed progressively 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 similar in seawater containing a relatively small amount of particulate material. Although it has been shown that the major constituents of seawater can influence Np(IV) sorption and oxidation kinetics, to a first 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 237Np discharged into the Irish Sea from the BNFL nuclear fuels reprocessing plant at Sellafield (U.K.).