D. Mccubbin et Ks. Leonard, LABORATORY STUDIES TO INVESTIGATE SHORT-TERM OXIDATION AND SORPTION BEHAVIOR OF NEPTUNIUM IN ARTIFICIAL AND NATURAL SEAWATER SOLUTIONS, Marine chemistry, 56(1-2), 1997, pp. 107-121
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.).