Clay Minerals (1994) 29, 145-151
C O M P A R I S O N OF M E A S U R E D A N D C A L C U L A T E D
DIFFUSION COEFFICIENTS FOR IODIDE IN
COMPACTED
CLAYS
D. W. OSCARSON
AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE 1LO
(Received 25 May 1993; revised 15 July 1993)
A B S T R A C T: An understanding of the behaviour of contaminants in compacted clays is important in
assessing the effectiveness of clay-based barrier materials used in many waste containment strategies. Here
the diffusion and sorption behaviour of I- with selected compacted clays is examined (129Iis a relatively longlived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients, D, and distribution
coefficients, Ka, were measured for I with four clays: bentonite, Lake Agassiz clay (a glacial lake clay
composed mainly of smectite, illite, kaolinite, and quartz), interstratified illite-smectite, and kaolinite. For
the diffusion experiments the clays were compacted to a dry bulk density, p, of -1.2 Mg m -3. The mean
measured D values, Din, were as follows: bentonite, 310 ~tm2 s-l; Lake Agassiz clay, 0.62 ~m2 s 1; illitesmectite, 190~tm2s a; and kaolinite, 74~tm2s l. The measured values were compared with those calculated,
De, from the following model: D = Do~an/(n + pKd), where D Ois the diffusion coefficient in pure bulk water,
ra the apparent tortuosity factor, and n the solution-filled porosity of the clay. Except for the Lake Agassiz
clay, where Dm was about an order of magnitude lower than De, the Dm and D~ values agreed within a factor
of about two. The Lake Agassiz clay has a markedly higher organic carbon content than the other three clays,
and this could affect ra, which may be overestimated in the model.
C o m p a c t e d clay-based materials are i m p o r t a n t
b a r r i e r s in m a n y waste disposal strategies. In
C a n a d a , for e x a m p l e , a disposal concept is b e i n g
e v a l u a t e d in which d e n s e clay-based materials
would b e used to s u r r o u n d c o n t a i n e r s holding
high-level n u c l e a r fuel waste in a vault e x c a v a t e d
500-1000 m in granitic rock in the C a n a d i a n
Shield ( H a n c o x & Nuttall, 1991). T h e s e materials
would also b e used to backfill a n d seal large
p o r t i o n s of t h e excavated vault after the waste
c o n t a i n e r s were emplaced.
Because the permeability of d e n s e clay-based
materials is very low ( O s c a r s o n et al., 1990), the
principal m e c h a n i s m of mass t r a n s p o r t t h r o u g h
these materials is m o l e c u l a r diffusion. Diffusion
coefficients are, t h e r e f o r e , critical p a r a m e t e r s for
predicting m i g r a t i o n rates t h r o u g h these barriers.
In the long-term disposal of high-level n u c l e a r
fuel waste, t h e fission p r o d u c t 129I is of c o n c e r n ,
b o t h because of its long half-life of 1.6 • 107 yr
and b e c a u s e it is poorly s o r b e d o n m o s t clay
materials ( O s c a r s o n et al., 1984). O f all the
radioisotopes p r e s e n t in n u c l e a r fuel waste, 129I
has t h e greatest p o t e n t i a l to r e a c h the b i o s p h e r e
b e f o r e decaying to insignificant levels.
This study h a d two objectives: to e x p e r i m e n tally m e a s u r e diffusion coefficients for I - in
c o m p a c t e d clays h a v i n g a wide r a n g e of properties, a n d to c o m p a r e the m e a s u r e d diffusion
coefficients with t h o s e calculated from a m o d e l
containing easily m e a s u r e d or readily available
p a r a m e t e r s . Since diffusion coefficients are relatively difficult a n d t i m e - c o n s u m i n g to m e a s u r e in
c o m p a c t e d clays, it is desirable to h a v e o t h e r
m e a n s of estimating t h e m ; h e n c e the n e e d to
c o m p a r e m e a s u r e d a n d calculated diffusion
coefficients.
MATERIALS
AND
METHODS
c~s
F o u r clays were used; t h e i r origin a n d composition are s u m m a r i z e d below a n d some selected
p r o p e r t i e s are given in T a b l e 1.
Avonlea bentonite. This clay is from the B e a r paw F o r m a t i o n of U p p e r C r e t a c e o u s age in
s o u t h e r n S a s k a t c h e w a n , C a n a d a ( O s c a r s o n et al.,
1990). It contains - - 8 0 % smectite ( m o n t m o r i l lonite), 10% illite, 5% quartz, a n d m i n o r
9 1994 The Mineralogical Society
146
D. W. Oscarson
TABLE 1. Selected properties of the clays.
Clay
Avonlea bentonite
Lake Agassiz
Illite-smectite
Kaolinite
Cation
exchange
capacity a
(cmolc kg 1)
Specific
surface areab
(103 m2 kg 1)
Organic
carbonc
(wt%)
pH d
Kde
(m3 Mg 1)
60
50
43
2.3
480
303
292
10
0.31
1.2
0.16
0.06
7.3
7.1
7.4
3.9
0.25 _+ 0.014f
12 _+ 0.20
0.22 _+ 0.009
1.7 _+ 0.18
a Calcium and Mg2+ were the index and replacing cations, respectively (Jackson, 1975).
b Ethylene glycol monethyl ether method (Carter et al., 1986).
c Determined by igniting a sample at 900~ in an O2 stream; CO2 released was measured
with an Astro 2001 organic C IR Analyser. Clays were pretreated with FeSO4 and
HzSO4 to remove inorganic C (Nelson & Sommers, 1982).
d Synthetic groundwater solution-to-clay ratio of 2:1.
e For I using eqn. 7.
e Mean _+ 1 standard deviation (n = 4).
a m o u n t s of gypsum, feldspar a n d c a r b o n a t e
( O s c a r s o n & Dixon, 1989). T h e clay is a comp o n e n t of the reference buffer material in the
disposal c o n c e p t d e v e l o p e d within the C a n a d i a n
Nuclear Fuel W a s t e M a n a g e m e n t P r o g r a m
( C N F W M P ) . T h e buffer m a t e r i a l is a 1:1 mix by
dry mass of b e n t o n i t e a n d sand c o m p a c t e d to a
dry bulk density of 1.7 M g m -3.
Lake Agassiz clay. This clay originates from
Pleistocene f r e s h w a t e r lake s e d i m e n t s of glacial
Lake Agassiz in s o u t h e r n M a n i t o b a , C a n a d a . It
contains -- 35% smectite, 20% illite, 15% quartz,
10% kaolinite, 10% calcite a n d m i n o r a m o u n t s of
feldspar a n d d o l o m i t e ( O s c a r s o n & D i x o n , 1989).
It is a c o m p o n e n t of the r e f e r e n c e backfill
material in the C N F W M P . T h e backfill m a t e r i a l is
a 1:3 mix by dry mass of L a k e Agassiz clay a n d
crushed granite aggregate, c o m p a c t e d to a dry
bulk density of ~ 2 . 0 mg m -3.
lllite-smectite mixed-layered clay. This clay was
o b t a i n e d f r o m the Source Clays R e p o s i t o r y of the
Clay M i n e r a l s Society, C o l u m b i a , Missouri; it is
d e s i g n a t e d ISMt-1. It is f r o m t h e M a n c o s Shale,
U S A , a n d is described as a r a n d o m interstratification of 60% illite and 40% smectite layers. This
clay was included in the study b e c a u s e in a
h y d r o t h e r m a l e n v i r o n m e n t , such as t h a t in a
nuclear fuel waste disposal vault, t h e r e is a
p o t e n t i a l for the smectite c o m p o n e n t of the
b a r r i e r materials to gradually t r a n s f o r m o v e r
t h o u s a n d s of years into an illite-smectite mixed-
layered material ( O s c a r s o n & H u m e , 1993).
H e n c e , its diffusive p r o p e r t i e s are i m p o r t a n t for
assessing the l o n g - t e r m p e r f o r m a n c e of smectiteb a s e d b a r r i e r materials.
Kaolinite. This clay was also o b t a i n e d f r o m the
Source Clays R e p o s i t o r y a n d is d e s i g n a t e d K G a 1. It is a well-crystallized clay f r o m W a s h i n g t o n
County, G e o r g i a , U S A .
Solution
T h e clays were s a t u r a t e d with a synthetic
g r o u n d w a t e r solution ( S G W ) with a p H of 7.2,
effective ionic s t r e n g t h of 220 mol m -3, a n d the
following c o m p o s i t i o n (in mol m 3): Na, 83; K,
0.36; Mg, 2.5; Ca, 53; HCO3, 0.28; C1, 170; a n d
SO4, 11. It was c h o s e n to r e p r e s e n t g r o u n d w a t e r
f o u n d deep in granitic rock in the C a n a d i a n Shield
( F r a p e et al., 1984).
Diffusion experiments
T h e half-cell m e t h o d used to m e a s u r e diffusion
coefficients in c o m p a c t e d clays is described in
detail by Sawatsky & O s c a r s o n (1991). A d i a g r a m
of the diffusion cell is s h o w n in Fig. 1. T h e
t e c h n i q u e involves the m e a s u r e m e n t of the
a m o u n t of a diffusant t h a t has diffused across t h e
interface of a r a d i o i s o t o p e - t a g g e d clay plug a n d
an u n t a g g e d plug within a given time.
Diffusion of iodide in clays
tainless steel cap
tainless steel
uter cylinder
Stainless steel
~ i n n e r cylinders
:------Clay and tracer
~-~0-
ring
~
---Teflon
~0
[
'= p l u g
I
2
3
4
l
I
l
I
5cm
FIG. 1. Diagram of the diffusion cell.
The clays were compacted to a target dry bulk
density, p, of --1.2 Mg m -3 (this is the approximate density of the clay c o m p o n e n t of both the
reference buffer and the backfill material in the
Canadian concept for the disposal of high-level
nuclear fuel waste). Synthetic groundwater or
S G W spiked with Na125I was added to the loose
clays (before compacting) in amounts such that
the clays would be as close to saturated as possible
when compacted to the target density.
Before the diffusion experiments, either two
tagged or untagged clay plugs (--4 cm long and 2
cm in diameter) were placed in the diffusion cells
for seven days to allow the moisture and 125I, in
the case of the tagged plugs, to distribute
uniformly throughout the plugs. After this equilibration period, and in preparation for the diffusion experiments, the clay plugs in the diffusion
cells were switched so that tagged plugs were in
contact with untagged plugs. A thin layer of a 1:1
mixture of the appropriate clay and S G W was
added to one end of each untagged plug. This plug
was then placed in contact with a tagged plug
outside the diffusion cell; most of the clay paste
oozed out and was immediately wiped off, leaving
only a small amount of paste at the interface. It is
felt that this procedure provided an excellent
147
contact between the two plugs, and, at the same
time, introduced only a slight error. The clay
plugs were then placed in the diffusion cells and
the cells immersed in a water bath at 23 +__0.1~
for the diffusion period, which ranged from --2-5
days. For each clay, experiments were conducted
at least in triplicate.
A f t e r the diffusion period, the clay plugs were
sectioned, starting at the end of the clay plug that
was initially inactive, into slices 2 4 m m thick.
The moisture content of a subsample of each slice
was determined gravimetricalty by heating at
105~ to constant mass; generally, the clays were
> 9 5 % saturated during the tests, based on the
calculated porosity of the compacted clay and i t s
specific gravity. The activity of 125I in a second
subsample of each slice was measured by g a m m a
spectroscopy using an Ortec, high-purity germanium detector ( E G & G Ortec, O a k Ridge,
Tennessee). The activity of 125I in each slice from
the initially untagged plug was summed to obtain
the total activity of 125I in the plug after the
diffusion period; this value was used to obtain the
diffusion coefficient as described below.
The clay plugs were sectioned after the diffusion runs to obtain concentration profiles for 125l
in the clay. The shape of these profiles can often
provide useful information on the behaviour of
diffusants in porous materials (Sawatsky & Oscarson, 1991).
D i f f u s i o n coefficients
If coupled processes are neglected and transport is assumed to occur in only one direction, say
the x-direction, the transient diffusional transport
of a solute through an homogeneous and isotropic
porous m e d i u m is given by Fick's second law:
Oc/3t = D( ~Zc/3x2),
(1)
where c is the bulk concentration (based on the
total v o l u m e - - s o l u t i o n plus c l a y - - o f the system)
of the diffusant, t is time, and D the diffusion
coefficient (this D-value is often called the
apparent diffusion coefficient). This D-value is
related to the diffusion coefficient in pure bulk
water under stationary conditions, Do, by
D = Dovan/(n + pKd),
(2)
where r, is the apparent tortuosity factor (which
includes not only the actual geometric tortuosity
D. W. Oscarson
148
b u t also all o t h e r factors t h a t m a y b e i n h e r e n t in
its m e a s u r e m e n t , such as t h e i n c r e a s e d viscosity
of w a t e r a d j a c e n t to surfaces of clay particles); n is
the total solution-filled porosity of the clay; a n d
Ka is the distribution coefficient, defined as the
c o n c e n t r a t i o n of a species s o r b e d o n t h e solid
p h a s e to the c o n c e n t r a t i o n of t h a t species in
solution at equilibrium.
In this study, I - did not r e a c h the ends of the
clay plugs within the diffusion period, so the plugs
can b e c o n s i d e r e d to b e infinite in extent. T h e
a p p r o p r i a t e solution to eqn. 1 is t h e n ( C r a n k ,
1975),
C/Co = 1/2 erfc[x/2(Dt)l/2],
(3)
subject to the following initial a n d b o u n d a r y
conditions:
c(x,O) = 0 for x > 0
= CofOrx~<0
and
c( + ~ , t ) = o
c ( - ~ , t ) = Co,
w h e r e Co is t h e initial b u l k c o n c e n t r a t i o n of the
diffusant in the tagged clay plug.
F o r the half-cell m e t h o d , the total q u a n t i t y of
125I, Q1, t h a t has diffused f r o m the tagged plug
across the b o u n d a r y at x = 0 (the interface of the
two plugs) into the initially u n t a g g e d plug in time t
is most readily m e a s u r e d . To o b t a i n this quantity,
the integral of eqn. 3 is r e q u i r e d ; this is given by
Ol
=
of clay were s u s p e n d e d in 10 cm 3 of the S G W
spiked with 125I in 50 cm 3 p o l y c a r b o n a t e centrifuge tubes. T h e initial 125I c o n c e n t r a t i o n was 23
n m o l m - 3 - - a b o u t the same c o n c e n t r a t i o n as used
in t h e tagged plugs in the diffusion e x p e r i m e n t s
described above. T h e t u b e s were capped, sealed
in d o u b l e p o l y e t h y l e n e bags, a n d t h e n placed in a
w a t e r b a t h at 23 + 0.1~ for 10 days. T h e t u b e s
were s h a k e n periodically. A f t e r t h e r e a c t i o n
period, t h e t u b e s were centrifuged at 5500 g for 10
min. T h e p H of the s u p e r n a t a n t solution was
m e a s u r e d with a glass electrode. A 1-cm 3 aliquot
of the s u p e r n a t a n t was w i t h d r a w n a n d a d d e d to 18
cm 3 of Instagel liquid scintillation cocktail. T h e
activity of 125I was m e a s u r e d b y liquid scintillation
c o u n t i n g ( B e c k m a n LS 5801, B e c k m a n Instrum e n t s , Inc., Irvine, California). C o n t r o l experim e n t s , c o n d u c t e d identically b u t w i t h o u t the clay,
s h o w e d that n o d e t e c t a b l e I was s o r b e d o n the
centrifuge tubes. D i s t r i b u t i o n coefficients were
calculated from
Kd = [(Ai/Ae) -- 1](S - Sa)/Pw,
(7)
w h e r e A i is the n e t c o u n t rate of the solution
initially a d d e d to the clay, A e t h e n e t c o u n t rate of
the solution after the r e a c t i o n period, S the
solution to clay ratio ( o n a mass basis) a n d Sa t h e
solution to clay ratio (or the gravimetric m o i s t u r e
c o n t e n t ) of the air-dried clay, a n d Pw the density
of the S G W .
- D A fro[C3c(O,t)/Ox]dt = A c o ( D t / ~ ) 1/2, (4)
w h e r e A is the cross-sectional area of the clay
plug. T h e total q u a n t i t y of 1251, Qt, in t h e tagged
clay plug at t = 0 is
Qt = A h c o ,
(5)
w h e r e h is the length of the clay plug (half-cell).
T a k i n g t h e ratio of Q 1 / Q t = f, one o b t a i n s
f = (Ol/~h2) 1/2.
(6)
E q u a t i o n 6 is a simplified form of a m a n i p u lated solution to eqn. 1 ( C r a n k , 1975). T h e
simplified relation is close to the exact relation
w h e n f < 0.3 ( K e m p e r , 1986), which is the case in
this study. A n explicit expression for D is t h e n
o b t a i n e d by r e a r r a n g i n g eqn. 6.
D & t r i b u t i o n coefficients
D i s t r i b u t i o n coefficients, Kd, for I - with the
four clays were d e t e r m i n e d as follows. Five grams
RESULTS
In this study, I - was t h e initial f o r m of I. I o d i d e is
stable across a wide r a n g e of p H a n d r e d o x
p o t e n t i a l conditions, a n d f r o m t h e data t h e r e is n o
indication t h a t m o r e t h a n o n e form of I is p r e s e n t .
T h e r e f o r e , in this study I - is a s s u m e d to b e the
only form of I.
Typical c o n c e n t r a t i o n profiles for I with each
of the four clays are s h o w n in Fig. 2, the s h a p e of
these profiles is consistent with m o l e c u l a r diffusion as given by Fick's s e c o n d law ( O s c a r s o n
et al., 1992; R o b i n et al., 1987). All m e a s u r e d D
values, Din, are given in T a b l e 2. A v o n l e a
b e n t o n i t e and the illite-smectite clay h a v e the
highest Dm values, a n d the L a k e Agassiz clay the
lowest. Consistent with eqn. 2, the D m values are
inversely r e l a t e d to Ka (Tables 1 a n d 2). Diffusion
coefficients calculated f r o m eqn. 2, De, are
c o m p a r e d with Dm values in T a b l e 2. (In eqn. 2,
Diffusion of iodide in clays
824
1.0
Avonlea bentonite
Diffusion t i m e = 7 2 h
el 9
o
o Q o
TABLE 2. Measured, Din, and calculated, De, diffusion
coefficients for I- in the clays.
DQQ
Dm
e9
P
(Mg m 3)
0.5
"o 9
9 o 9 1 4 99
ee
)
9
9
er
ee 9
mean
8
e-
Lake Agassiz clay
Diffusion time = 123 h
9
§
0.5
~O
t,-
mean
0
Dea
DmlDc
(pro2 s-l)
1.13
1.12
1.10
1,12
Avonlea bentonite
300
320
310
310
1.28
1.26
1.27
1.27
Lake Agassiz clay
0.68
0.62
0.57
0.62
1.18
1.12
1.12
1.12
1.14
Illite-smectite
250
200
150
160
190
140
1.4
1.16
1.13
1.16
1.17
1.16
Kaolinite
59
55
41
140
74
110
0.67
9 9
0
1.0
149
140
6.8
2.2
0.09
O
tO
o
1.0
e 9
Illite- sin 9
Diffusion time = 73 h
9 9
9
.>
t~~
+
0.5
mean
9
o9
0
1.0
9
9
9
.
.
Kaolinite
9
9
Diffusion
time = 50 h
mean
0.s
0"4
@
-2
()
a Calculated from eqn. 2; the Ka values are given in Table 1
and the values of the other parameters are given in the
text.
" " . .2. . .
4
Distance f r o m interface (cm)
Fro. 2. Concentration profiles for I with the four clays.
r~ was t a k e n to b e 0.25 for kaolinite (Shackelford
& Daniel, 1991a) a n d 0.1 for the o t h e r t h r e e clays
(Sawatsky & O s c a r s 9 1991; C h o et al., 1993a),
a n d Do for I - at 23~ is 1950 ~tm2 s 1 (Shackelford
& Daniel, 1991b)).
DISCUSSION
W i t h the e x c e p t i o n of the L a k e Agassiz clay, the
a g r e e m e n t b e t w e e n Dm and D~ is generally g o o d within a factor of --2. G o o d a g r e e m e n t b e t w e e n
Dm a n d D~ values was also f o u n d by R o b i n et al.
(1987) for Sr 2+ a n d by S h a r m a & O s c a r s o n (1989)
for P u ( I V ) diffusion in c o m p a c t e d clay-based
materials. T h u s , the model as given in eqn. 2
is j u d g e d to b e a d e q u a t e for predicting D for
m a n y diffusants in c o m p a c t e d clays having a
b r o a d r a n g e of p r o p e r t i e s .
T h e r e a s o n for t h e m a r k e d l y lower Dm value
for I - c o m p a r e d with Dc in the Lake Agassiz clay
is n o t clear. This clay has by far the greatest
organic c a r b o n ( O C ) c o n t e n t ( T a b l e 1), a n d it is
possible t h a t I - diffuses into t h e O C matrix
during the equilibration period. W h e n the two
plugs are j o i n e d to b e g i n t h e diffusion process, I would first h a v e to diffuse t h r o u g h or out of this
matrix a n d into the p o r e solution b e f o r e it could
m o v e from the tagged to the u n t a g g e d plug. If so,
the Ta value used in eqn. 2 is likely to b e m u c h
lower t h a n 0.1. This a r g u m e n t is s u p p o r t e d by t h e
fact t h a t Dm for I in the L a k e Agassiz clay as
m e a s u r e d by the time-lag t e c h n i q u e in a t h r o u g h diffusion e x p e r i m e n t is 28 btm2 s -~ at p --~ 1.2
M g m 3 ( u n p u b l i s h e d d a t a ) ; this value is close to
150
D. W. Oscarson
the Dc value for this clay ( T a b l e 2). (See C h o et al.
(1993b) for a discussion of t h e time-lag t e c h n i q u e
in through-diffusion e x p e r i m e n t s . ) In t h e time-lag
t e c h n i q u e , I - is not in contact with the clay b e f o r e
the diffusion e x p e r i m e n t s as it is d u r i n g the
equilibration p e r i o d in the half-cell m e t h o d .
H e n c e , I - would not diffuse into the organic
matrix of the L a k e Agassiz clay to the e x t e n t it
does in the half-cell m e t h o d .
F o r o t h e r clays that h a v e b e e n e x a m i n e d , t h e r e
is generally good a g r e e m e n t b e t w e e n D m values
o b t a i n e d f r o m the half-cell a n d time-lag techniques. F o r example, the D m value d e t e r m i n e d
from the time-lag t e c h n i q u e for I - is 1 3 0 btm2 s 1
in the A v o n l e a b e n t o n i t e ( C h o et al., 1993b) a n d
140 btm 2 s -1 in the illite-smectite ( u n p u b l i s h e d
data) at p = 1.2 Mg m -3. T h e s e values are close to
the Dm values o b t a i n e d from the half-cell m e t h o d
(Table 2).
It appears, t h e n , t h a t the half-cell m e t h o d as
used h e r e is not reliable for o b t a i n i n g Dm values
for I in the L a k e Agassiz clay. A s suggested, this
m a y b e due to the comparatively high O C c o n t e n t
of this clay. F u r t h e r diffusion e x p e r i m e n t s with
o t h e r clays h a v i n g high O C c o n t e n t s are r e q u i r e d
to confirm this hypothesis.
In eqn. 2, the largest source of e r r o r is likely to
b e associated with Kd a n d n. T h e m e a s u r e m e n t of
Kd values in clay suspensions a n d t h e i r use in mass
t r a n s p o r t e q u a t i o n s have b e e n discussed by m a n y
a u t h o r s , including R o b i n et al. (1987) a n d Gillh a m & C h e r r y (1982).
In this study, n, the total p o r o s i t y - - c a l c u l a t e d
f r o m the dry b u l k density a n d specific gravity of
the c l a y - - w a s used in eqn. 2. ( W h e n the clay is
s a t u r a t e d or nearly so, which is the case in this
study, n is equal to the v o l u m e t r i c m o i s t u r e
c o n t e n t . ) R a t h e r t h a n n, an effective porosity, ne,
or the porosity available to the diffusant, should
b e used in eqn. 2. W i t h t h e exception of the
A v o n l e a b e n t o n i t e ( O s c a r s o n et al., 1992), however, ne for these clays for I diffusion is not
known. A s ne < n, the use of n r a t h e r t h a n n e in
eqn. 2 m e a n s D c is s o m e w h a t o v e r e s t i m a t e d , b u t
at most by less t h a n a factor of 5 ( O s c a r s o n et al.,
1992). B e c a u s e of the p r o p e r t i e s of the clays, such
as the specific surface area a n d particle size, the
o v e r e s t i m a t i o n would p r o b a b l y be greatest for
b e n t o n i t e a n d lowest for kaolinite.
A n o t h e r u n c e r t a i n t y associated with the nvalue is t h a t in the n u m e r a t o r of eqn. 2, n is the
t r a n s p o r t porosity, w h e r e a s in the d e n o m i n a t o r it
is the storage porosity; t h e s e porosities are not
necessarily the same. T h e t r a n s p o r t porosity is the
porosity available for solute t r a n s p o r t a n d occurs
only in a fraction of the total p o r e volume. F o r
example, O s c a r s o n et al. (1992) f o u n d t h a t for
b e n t o n i t e at p --~ 1.3 M g m 3, ne for the diffusion
of I was a b o u t 20% of n. T h e storage porosity,
o n the o t h e r h a n d , is a f o r m of r e t a r d a t i o n or
s o r p t i o n and may, for e x a m p l e , involve d e a d - e n d
p o r e s a n d 'cul-de-sacs.' M o s t investigators
assume t h a t the t r a n s p o r t a n d storage porosities
are the same and, h e n c e , eqn. 2 is o f t e n given as
D = Dorff[1 + (pln)Kd].
(8)
T h e t e r m in t h e d e n o m i n a t o r is usually called
the r e t a r d a t i o n factor. Obviously, m u c h m o r e
work is r e q u i r e d o n the porosity of clays a n d soils
a n d its relation to c o n t a m i n a n t transport.
O r g a n i c m a t t e r is t h o u g h t to b e i m p o r t a n t in
t h e s o r p t i o n of I by e a r t h e n materials ( S h e p p a r d
& T h i b a u l t , 1991 a n d references t h e r e i n ) . E x c e p t
for kaolinite, t h e r e is a t r e n d to increasing Kd
values with increasing O C c o n t e n t (Table 1).
T h e Kd value for I with kaolinite is relatively
high a n d yet this clay has the lowest O C c o n t e n t of
the clays e x a m i n e d (Table 1). D e (1961) r e p o r t e d
t h a t kaolinite has a g r e a t e r s o r p t i o n capacity for
I
t h a n b e n t o n i t e a n d vermiculite, which is
consistent with the data r e p o r t e d here. D e
suggested t h a t t h e structural hydroxyl groups on
the surface of kaolinite particles may play a n
i m p o r t a n t role in sorbing I - . A l l a r d et al. (1980)
f o u n d t h a t the s o r p t i o n of I - on clay m i n e r a l s
increases with decreasing p H , p r o b a b l y b e c a u s e
of a g r e a t e r positive surface-charge density o n the
clay at lower p H values. T h e lower p H of the
kaolinite system could t h e n , at least partially,
explain the relatively high K d values. C o u t u r e &
Seitz (1983), h o w e v e r , f o u n d very little s o r p t i o n
of I - o n kaolinite in the p H r a n g e from 4-6.
W i t h r e g a r d to c o n t a i n m e n t b a r r i e r s for wastes
t h a t contain I - , the longest t r a n s p o r t time would
b e p r o v i d e d by the L a k e Agassiz clay: regardless
of the m e t h o d of m e a s u r e m e n t , D is lower in this
clay t h a n in the o t h e r clays e x a m i n e d . This is
e n c o u r a g i n g as this clay is a c o m p o n e n t of the
r e f e r e n c e backfill m a t e r i a l in t h e C a n a d i a n disposal concept.
T h e Dr, values for I - in the A v o n l e a b e n t o n i t e
a n d illite-smectite clay are virtually the s a m e
(Table 2). T h e D m values for Sr 2+, C a 2 + a n d N a +
are also similar in the two clays ( u n p u b l i s h e d
Diffusion o f iodide in clays
data). This suggests t h a t if a s m e c t i t e - b a s e d
b a r r i e r material is partially t r a n s f o r m e d to an
illite-smectite mixed-layered m a t e r i a l in a hyd r o t h e r m a l e n v i r o n m e n t , such as t h a t of a n u c l e a r
fuel waste disposal vault, t h e r e would b e n o
adverse effect with respect to mass t r a n s p o r t . It is
recognized, h o w e v e r , t h a t the fabric of a n illitesmectite clay f o r m e d from the in situ t r a n s f o r m a tion of smectite m a y b e quite different f r o m t h e
fabric of the c o m p a c t e d clay p o w d e r used in this
study, a n d this could affect D. O n the o t h e r h a n d ,
the results of C h o et al. (1993b) suggest t h a t at a
given density, clay fabric is p r o b a b l y n o t an
i m p o r t a n t factor in t h e diffusion of I - a n d Cs + in
compacted bentonite.
ACKNOWLEDGMENTS
Mr H.B. Hume is thanked for his technical assistance. The
Canadian Nuclear Fuel Waste Management Program is
jointly funded by AECL Research and Ontario Hydro
under the auspices of the CANDU Owners Group.
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