Colloidal Processes in SIXEP
Streams
Z. Maher,1 N.D. Bryan,1 L. O’Brien,1 H. Sims,1 R.
J. Taylor,1 D. Goddard,1 P. Ivanov,2 S.L. Heath,2
F.R. Livens,2 S. Kellet,3 P. Rand3
1
2
3
National Nuclear Laboratory.
The University of Manchester.
Sellafield Ltd.
Introduction (1)
At Sellafield, alkaline
magnesium saturated
effluents arise from ponds and
silos.
Treated in the Site Ion
Exchange Plant (SIXEP) operational since 1985:
• Sand bed filter – to remove
particulates;
• Clinoptilolite ion exchange
columns – to remove
Cs+/Sr2+.
SIXEP is very effective and
has significantly reduced
discharges to sea.
Introduction (2)
SIXEP effluent streams contain
amounts of suspended particles and
colloids that could:
• Interfere with the ion exchange
process in SIXEP;
Artinite particles
• Act as a vector for the transport
of radionuclides.
Hence, it is important to understand
colloid behaviour in the SIXEP
process stream.
Brucite particles
This study
There are two parts:
1. SIXEP Bulk Storage Tanks
 Actinide (Pu and Am)
partition;
 characterisation of the
particles, samples
collected by ultrafiltration
and imaged by SEM/EDX.
2. Brucite model system
 Am(III) sorption to
colloids;
 Size distributions;
 Effect of carbonate
Actinide Chemistry in BST Liquor
Am: only Am(III) expected
Am(III) will sorb strongly to colloids and particles.
Pu: Pu(IV) or Pu(V)
Pu(V)  PuO2+: higher solubility;
weak sorption.
Pu(IV)  ‘Pu4+’: lower solubility, strongly hydrolysed;
strong sorption to colloids and
particles;
Intrinsic (Pu(OH)4)n polymeric colloid
formation (predictable).
Pu/Am Distribution in BST Liquor
Samples
Samples from BST ultrafiltered (1 and 200 nm):
Pu (IV) intrinsic
colloid/ colloid
sorbed Pu(IV - ?)
Colloid sorbed
Pu(IV - ?)
Colloid sorbed
Am(III)
Pu (%)
Am (%)
large colloid (> 200 nm)
36 ± 20
67 ± 14
small colloid (1 - 200 nm)
50 ± 6
20 ± 13
true solution (< 1 nm)
14 ± 8
13 ± 10
Soluble Pu(V)
Soluble Am(III) as Am(CO3)m(3-2m)+(aq)
 Most Pu and Am attached to colloids and particles.
BST Particles (1)
An interlocking platelet morphology is
common, associated with a distinct
Mg:Al ratio.
Hydrotalcite-like material
Hydrotalcite
theoretical
spectrum
BST Particles (2)
There are also more heterogeneous particles
material.
BST Particles (3)
Some elements are localised: Fe, Mn, La, Si, Al.
Others are distributed uniformly, particularly Mg. Also
evidence for many small Mg containing colloids.
Model Brucite System
BST colloids and particles are very complex
and heterogeneous.
A simpler brucite (Mg(OH)2) system was
studied in the laboratory.
Larger brucite particles:
hexagonal blocks
Smaller brucite colloids:
spherical
Brucite Colloid Population
100
∝ 90
80
70
60
50
∝ 40
30
20
number (%)
10
available area %
0
1 - 2.5 nm
2.5 - 6.5
nm
mass %
6.5 - 12.6
nm
> 12.6 nm
Brucite colloid population dominated by small
particles: mass, area and number.
∝ Am Size Distributions
Brucite colloids are
mostly small.
100
90
80
70
1 day
60
50
10 days
40
70 days
30
Am preferentially
associated with
larger
colloids/particles.
160
days
20
10
70 days
0
true
solution
1 - 2.5
nm
2.5 - 6.5
nm
1 day
6.5 - 12.6
nm
> 12.6
nm
Increase in larger
fractions with time
(6 months)
Note: on plant, actinides often found in large fractions:
e.g., BST Am; 67% > 200 nm.
Colloid Behaviour in Labile
Systems
A labile system such as brucite is dynamic, with constant
turnover of the colloidal surfaces.
Aggregation
/diffusional
growth
Small
colloid
Nucleation
E
Large
colloid
F
Complete
Dissolution
Disaggregation/
dissolution
Colloid Bed
Small particles
susceptible to
erosion and
dissolution
Incorporation Mechanism
In a system without incorporation of radionuclide, the sink
with the largest surface area should control sorption.
Smaller colloids:
larger specific
surface area.
Large colloids: smaller
specific surface area.
Although the small particles are
prone to dissolution, they can
compete with the larger particles
Incorporation Mechanism (2)
In a system with incorporation the larger particles gain an
advantage.
Smaller colloids:
cannot ‘hide’ Am
within structure
By incorporation following
sorption, the larger species have
an advantage  shift of Am to
larger species
Actinides can be
incorporated to a depth
of 100 nm in brucite
Am Size Distributions
(with 10 mM carbonate)
70
60
50
1 day
For [carbonate] =
1 mM, no
significant effect.
40
10 days
30
70 days
20
160 days
70 days
10
0
10 days
true
solution
1 - 2.5
2.5 - 6.5
nm
nm
1 day
6.5 12.6 nm
> 12.6
nm
At [carbonate] =
10 mM, Am
sorption to colloids
suppressed.
Due to competition from carbonate complexation:
Am(OH)n(3-n)+(aq)  Am(CO3)m(3-2m)+(aq)
Carbonate in BST samples in range: 7 x 10-4 – 10-2 M
 Helps explain Am in true solution.
Summary
Behaviour of actinides on plant is dominated by
association with colloids.
Laboratory experiments have helped to explain actinide
behaviour on plant – balance of incorporation in large
particles and carbonate complexation.
Although the particles and colloids on plant are complex
and heterogeneous, we can understand actinide
behaviour.
We can use this understanding to inform future operations
and decommissioning.
Acknowledgements
Thank you
NNL corporate funding (strategic research project)
Nuclear Decommissioning Authority
Sellafield Ltd
University of Manchester