Properties and Mobilities of Actinide Colloids in Geologic Systems
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Published 1982
SCIENTIFIC BASIS FOR RADIOACTIVEVA Werner. Wtze, editor
MTE•
- V
PROPERTIES AND MOBILITIES OF ACTINIDE COLLOIDS IN GEOLOGIC SYSTEMS
U. OLOFSSON, B. ALLARD, B. TORSTENFELT AND K. ANDERSSON Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 G~teborg, Sweden
INTRODUCTION The interaction between media,
e.g.
in
radionuclides in
solution and exposed
geologic
connection with underground storage of radioactive waste,
is
largely determined by the chemical properties of the system. The mobility of radionuclides in contact with low-capacity minerals would depend on the chemical state of the element inorganic complexes, oxidation states be feasible in
(e.g. degree of hydrolysis, formation of organic and
etc.).
(III, IV),
Particularly
for the actinides
however,
the groundwater environment.
true radiocolloids,
i.e.
in
their lower
would a formation of colloid particles These colloids could be either
aggregates of the radionuclide
itself,
or pseudo-
i.e. colloid material already present in the groundwater, onto which 1,2 . These colloidal particles may be very the radionuclide has sorbed colloids,
poorly sorbed on water exposed nuclides in
true solution.
actinide colloids
(Am,
Np,
geologic media in
Studies
comparison with radio-
on the formation and properties of some
Pu) are discussed in
this paper.
EXPERIMENTAL Chemicals All salt solutions were prepared from p.a. NaClO4 , pore filters
filtered through Nucle-
with a pore size of 400 nm and centrifuged at about 27000 g for
one hour in order to remove all impurities prior to the experiments.
Radionuclides The radionuclides used were 241Am, with 237Np as carrier. filters
237Pu with 239Pu as carrier and 235Np
All stock solutions were filtered
through Nuclepore
with a pore size of 400 nm.
Centrifugation experiments Solutions of NaClO4 at various ionic strengths (20 ml) were poured into 50 ml centrifuge tubes and the radionuclide was added to desired concentration. (See below). The pH-value was adjusted with HC1O 4 or NaOH.
756 The solutions were centrifuged at about 27000 g for half an hour, 0.5-1.0 ml samples were withdrawn and measured in counter.
Samples were also taken from agitated solutions.
sampling procedure,
and
a well-type scintillation This separating and
as well as measurements of pH, but without any pH-adjust-
ments, were repeated after various storage times. The following parameters were varied: -
pH-value of the solution (2-12)
-
initial nuclide concentration (10-7 M, 10-9 M)
-
ionic strength of the aqueous phase (NaCIO 4 ; 0.01 M, 1.0 M) storage time (6 h,
-
27 h,
1 w,
4 w,
6 w).
All experiments were carried out in
polypropylene tubes in
any particle release from the tube walls,
order to avoid
which could serve as sources of
pseudocolloids. The experiments with plutonium and neptunium were performed in with N2-atmosphere in
a glove-box
order to avoid any formation of carbonate complexes at
high pH due to the contact with the air containing CO2 . Diffus
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