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Adsorption of 226Ra by soils in the presence of Ca2+ ions. Specific adsorption (II)
Chemosphere No. 5, PP 293 - 299. ~)Pergamon Press Ltd. 1979. Printed in Great Britain.
0045-6535/79/0501-0293Z02.00/0
ADSORPTION OF 226Ra BY SOILS IN THE PRESENCE OF Ca 2+ IONS. SPECIFIC ADSORPTION (II).
Jatln S. Nathwani and Colin R. Phillips Department of Chemical Engineering and Applied Chemistry University of Toronto Toronto, Ontario M.SS IA4
INTRODUCTION Sorption phenomena play an important role in the release, mobility and biological availability of radium-226 from uranium mill railings.
In a previous study I, adsorption of
226Ra by soils over a wide range of 226Ra concentration was described in terms of standard Freundlich and Langmuir isotherms.
It was also found that organic matter and clay were the domi-
nant constituents of soils contributing to adsorption of 226Ra.
These studies, however, were
carried out under conditions where the influence of other cations in solution was not addressed. Adsorption from soils under natural conditions normally takes place in the presence of calcium and other cations in amounts large enough to prevent adsorption on normal cation exchange sites 2. It is the objective of the present study to examine the specific adsorption of 226Ra by soils in the presence of Ca 2+ ions and to establish the validity of sorption isotherms under these conditions.
MATERIALS AND METHODS The soils, the equilibration procedures and the methods of analysis used are described in Part I.
Adsorption of 226Ra by soils was determined at 25°C ± 0.5°C on soll suspensions containing
varying amounts of 226Ra spiked in 0.05 M CaCl 2 stock solutions.
The influence of the Ca 2+ ion
concentration of the solution on Kd, the distribution coefficient, was studied by initially
satu-
rating the soil with Ca 2+ ions and subsequently equilibrating with solutions containing a small amount of 226Ra, but different amounts of Ca 2+ ions. at i0 pCi/ml.
293
The amount of 226Ra was always kept constant
z94
No. 5
RESULTS AND DISCUSSION The forms of the Freundlich and Langmuir equations results
for specific adsorption
the lower and upper limits of experimental The adsorption
parameters
Table I; the soil series
(samples)
concentration
soils.
that the At
the data deviate from the two iso-
for the specific adsorption of 226Ra on soils are presented and the numbers
correspond
isotherms,
in solution),
cation for soils 3'4, and may indicate are bound by different
The
isotherms over a wide range of concentrations.
concentrations,
The initial slope of the Freundlich vs C (equilibrium
in Part I I.
of 226Ra on soils in the presence of Ca 2+ ions indicated
data fit both the Langmuir and the Freundlich
therms.
used were described
to those in Part I.
plotted as S (amount adsorbed,
pCig -I)
has been used as a measure of the affinity
the relative
in
of the
strength with which initial amounts of 226Ra
The initial slopes of Freundlich
isotherms were determined
the tangent at low values of C where the curve can be considered as a straight line.
from
The
initial slopes of the curves obtained by plotting S vs C for adsorption of 226Ra by soils in the presence of Ca 2+ ions is different
from that found for total adsorption
of 226Ra from dilute
solutions I (see Fig. i). This effect can be explained in dilute solutions, completely
it exhibited such an affinity
adsorbed by all soils.
curves obtained are similar
gressively
for the adsorption
surface
that it was almost
The initial part of the isotherm is therefore vertical
the data for adsorption
in the plot of S vs C, showing
5
that as more adsorption
In the presence
sites are filled,
sites on the soils.
there is pro-
These curves for
of 226Ra on soils are very close to the L curves of Giles et al. 5 and the
(n and K 2) describing bonding energy are different
for different
soils
(Table I).
The initial slope values for specific adsorption of 226Ra were significantly with the Freundlich for KI, r = 0.976).
and Langmuir parameters This suggests
representing
influenced more by the maximum adsorption
K2C 8 i + K2C
correlated
(for K, r s 0.983;
in the presence of a large amount
sites, the initial slope of the isotherm is
capacity of the soil than by its affinity
form of the Langmuir equation
parameter has been suggested6:
maximum adsorption
that when 226R~ is adsorbed
of Ca 2+ ions filling a great number of adsorption
A different
and the
of 226Ra on soils displayed an initial
less chance for the solute to find available
specific adsorption parameters
when 226Ra was present as the dominant cation
to those classified as H curves by Giles et al.
of large amounts of Ca 2+ ions, curvature
as follows:
for 226Ra.
to provide an estimate of the bonding energy
No. 5
295
where 0 is the fraction of the adsorption C, and K follows
226Ra concentration,
is a constant related to the bonding energy of the soll with 226Ra. that when 8 = 0.5 (one half saturation),
estimate
divided by the respective By plotting the affinity
the energy bonding parameter.
equilihri~
concentration
an exponential
This can be a
the soil affinity
S,
for 226Ra.
@ (see Fig. 2), it can be seen that
to surface
coverage.
At low values of 8, when few sites are occupied,
Influence
to I/C.
The amount ol 226Ra adsorbed,
shape for both total and specific adsorption
lower in the presence
is equal
C, may represent
these values of S/C vs the corresponding of soils for 226Ra is sensitive
From this it
the Langmuir parameter K2, which provides an
of the bonding energy of the soil with the adsorbate,
useful method for evaluating
226Ra.
maximum at a given equilibrium
The curves obtained reveal
(in the presence of Ca 2+ ions) of
the affinity
for 226Ra is significantly
of Ca 2+ ions than when only 226Ra is present.
of Ca 2+ Concentration
In these experiments, Ca 2+ concentration
the amount of 226Ra added was kept constant
in solution was varied from 0.005-0.05
saturated with Ca 2+ ions.
M.
(10 pCiml -]) while
the
The soil samples were initially
The sorption by soils is characterized
by Kd, the distribution
coeffi-
cient, which is defined as Amount of
Kd(£/g)
In Fig.
=
226Ra adsorbed
Concentration
by soil (pCi/g)
of 226Ra in solution
(pCi/£)
3, K d is plotted as a fumction of the Ca 2+ ion concentration
226Ra by all soils tested was reduced considerably
in solution.
when the Ca 2+ concentration
Sorption of
of the solution
was increased.
CONCLUSIONS The specific adsorption
of 226Ra on soils in the presence
with both Langmuir and Freundlich In the presence therm is influenced
isotherms
of Ca 2+ ions is in accordance
over a wide range of concentrations.
of a large excess of Ca 2+ ions,
more by the maximum adsorption
the initial slope of the Freundlich
iso-
capacity of the soil than by its affinity
for 226Ra. At low values of surface
coverage,
the presence
of Ca 2+ ions has a significant
upon sorption as indicated by the Langmuir bonding energy parameter coefficient.
effect
and, Kd, the distribution
296
No. 5
REFERENCES i.
J.S. Nathwani and C.R. Phillips, Part I. Chemosphere.
2.
R.G. McLaren, D.V. Crawford, J. Soil Sci., 24, 443 (1973).
3.
K.G. Tiller, J.F. Hodgson, M. Peach, Soil Sci., 95, 392-399 (1963).
4.
K.G. Tiller, J.L. Honeysette, E.G. Hallsworth, Aust. J. Soil Res., !, 43 (1969).
5.
C.H. Giles, T.H. MacEvans, S.N. Nakhwa, D. Smith, J. Chem. Soc., 1960, 3973 (1960).
6.
J.H. de Boer.
In The Dynamical Character of Adsorption, p. 58, Oxford Univ. Press (1968).
Table I Summary of Freundlich and Langmuir Isotherm Parameters for the Specific Adsorption of 226Ra on Soils Langmuir Soil Series
Soil No.
KI
Freundlich K2
K
n
Initial Slope
Wendover
1
4761.9
0.127
1779.7
0.213
202.2
Haldimand
2
1596.1
0.048
470.1
0.201
22.3
Rideau
3
368.4
0.015
101.6
0.154
3.28
Vaudreuil
4
493.9
0.0048
43.1
0.283
4.26
Grimsby
5
65.1
0.011
19.5
0.135
0.62
St. Thomas
6
18.6
0.027
i0.i
0.071
0.131
No. 5
297
i0000 f
f
U
5000
~g
4000
3000 Z
2000
I000
900
-..: i0
: 20
EQUILIBRIUM
Figure
I.
Adsorption presence
Isotherms
; 30
. . . . . . 40 50
226Ra CONCENTRATION
for adsorption
of Ca 2+ ions (specific)
dilute solutions.
, 60
. 70
IN LIQUID,
, 80 C
eq.
. 90
( pCil -I)
of 226Ra by soil no.l in the
and for total adsorption
from
i00
298
No. 5
Soll no. 1 (Wendover) 8000 Soil no. 5 (Grimsby)
15
5000
S/C 10
si c
1000 0.2
0.6
1.0
e
0.2
0.6
1.0
Soil no. 6 (St. Thomas) !otal S/c
!
0
0.2
0.6
1.0
" versus surface cGverage @ Figure 2. Plots of the affinity of soils for 226Ra - (/SC)
No.
5
299
103
102
~
Kd
10
i
m
s
b
y
)
~a.
•
s)
rl
1.0
!
I
I
t
0.01
|
0.02
, |
0.03
CONCENTRATION OF Ca 2+ IN SOLUTION (
0 04 molesl-I)
Figure 3. Plot of the distribution co-efficient, Kd, for the sorption of 226Ra by soils as a function of Ca2+- concentration in solution.
(Received ia USA
18
July 1978)
0.05
0045-6535/79/0501-0293Z02.00/0
ADSORPTION OF 226Ra BY SOILS IN THE PRESENCE OF Ca 2+ IONS. SPECIFIC ADSORPTION (II).
Jatln S. Nathwani and Colin R. Phillips Department of Chemical Engineering and Applied Chemistry University of Toronto Toronto, Ontario M.SS IA4
INTRODUCTION Sorption phenomena play an important role in the release, mobility and biological availability of radium-226 from uranium mill railings.
In a previous study I, adsorption of
226Ra by soils over a wide range of 226Ra concentration was described in terms of standard Freundlich and Langmuir isotherms.
It was also found that organic matter and clay were the domi-
nant constituents of soils contributing to adsorption of 226Ra.
These studies, however, were
carried out under conditions where the influence of other cations in solution was not addressed. Adsorption from soils under natural conditions normally takes place in the presence of calcium and other cations in amounts large enough to prevent adsorption on normal cation exchange sites 2. It is the objective of the present study to examine the specific adsorption of 226Ra by soils in the presence of Ca 2+ ions and to establish the validity of sorption isotherms under these conditions.
MATERIALS AND METHODS The soils, the equilibration procedures and the methods of analysis used are described in Part I.
Adsorption of 226Ra by soils was determined at 25°C ± 0.5°C on soll suspensions containing
varying amounts of 226Ra spiked in 0.05 M CaCl 2 stock solutions.
The influence of the Ca 2+ ion
concentration of the solution on Kd, the distribution coefficient, was studied by initially
satu-
rating the soil with Ca 2+ ions and subsequently equilibrating with solutions containing a small amount of 226Ra, but different amounts of Ca 2+ ions. at i0 pCi/ml.
293
The amount of 226Ra was always kept constant
z94
No. 5
RESULTS AND DISCUSSION The forms of the Freundlich and Langmuir equations results
for specific adsorption
the lower and upper limits of experimental The adsorption
parameters
Table I; the soil series
(samples)
concentration
soils.
that the At
the data deviate from the two iso-
for the specific adsorption of 226Ra on soils are presented and the numbers
correspond
isotherms,
in solution),
cation for soils 3'4, and may indicate are bound by different
The
isotherms over a wide range of concentrations.
concentrations,
The initial slope of the Freundlich vs C (equilibrium
in Part I I.
of 226Ra on soils in the presence of Ca 2+ ions indicated
data fit both the Langmuir and the Freundlich
therms.
used were described
to those in Part I.
plotted as S (amount adsorbed,
pCig -I)
has been used as a measure of the affinity
the relative
in
of the
strength with which initial amounts of 226Ra
The initial slopes of Freundlich
isotherms were determined
the tangent at low values of C where the curve can be considered as a straight line.
from
The
initial slopes of the curves obtained by plotting S vs C for adsorption of 226Ra by soils in the presence of Ca 2+ ions is different
from that found for total adsorption
of 226Ra from dilute
solutions I (see Fig. i). This effect can be explained in dilute solutions, completely
it exhibited such an affinity
adsorbed by all soils.
curves obtained are similar
gressively
for the adsorption
surface
that it was almost
The initial part of the isotherm is therefore vertical
the data for adsorption
in the plot of S vs C, showing
5
that as more adsorption
In the presence
sites are filled,
sites on the soils.
there is pro-
These curves for
of 226Ra on soils are very close to the L curves of Giles et al. 5 and the
(n and K 2) describing bonding energy are different
for different
soils
(Table I).
The initial slope values for specific adsorption of 226Ra were significantly with the Freundlich for KI, r = 0.976).
and Langmuir parameters This suggests
representing
influenced more by the maximum adsorption
K2C 8 i + K2C
correlated
(for K, r s 0.983;
in the presence of a large amount
sites, the initial slope of the isotherm is
capacity of the soil than by its affinity
form of the Langmuir equation
parameter has been suggested6:
maximum adsorption
that when 226R~ is adsorbed
of Ca 2+ ions filling a great number of adsorption
A different
and the
of 226Ra on soils displayed an initial
less chance for the solute to find available
specific adsorption parameters
when 226Ra was present as the dominant cation
to those classified as H curves by Giles et al.
of large amounts of Ca 2+ ions, curvature
as follows:
for 226Ra.
to provide an estimate of the bonding energy
No. 5
295
where 0 is the fraction of the adsorption C, and K follows
226Ra concentration,
is a constant related to the bonding energy of the soll with 226Ra. that when 8 = 0.5 (one half saturation),
estimate
divided by the respective By plotting the affinity
the energy bonding parameter.
equilihri~
concentration
an exponential
This can be a
the soil affinity
S,
for 226Ra.
@ (see Fig. 2), it can be seen that
to surface
coverage.
At low values of 8, when few sites are occupied,
Influence
to I/C.
The amount ol 226Ra adsorbed,
shape for both total and specific adsorption
lower in the presence
is equal
C, may represent
these values of S/C vs the corresponding of soils for 226Ra is sensitive
From this it
the Langmuir parameter K2, which provides an
of the bonding energy of the soil with the adsorbate,
useful method for evaluating
226Ra.
maximum at a given equilibrium
The curves obtained reveal
(in the presence of Ca 2+ ions) of
the affinity
for 226Ra is significantly
of Ca 2+ ions than when only 226Ra is present.
of Ca 2+ Concentration
In these experiments, Ca 2+ concentration
the amount of 226Ra added was kept constant
in solution was varied from 0.005-0.05
saturated with Ca 2+ ions.
M.
(10 pCiml -]) while
the
The soil samples were initially
The sorption by soils is characterized
by Kd, the distribution
coeffi-
cient, which is defined as Amount of
Kd(£/g)
In Fig.
=
226Ra adsorbed
Concentration
by soil (pCi/g)
of 226Ra in solution
(pCi/£)
3, K d is plotted as a fumction of the Ca 2+ ion concentration
226Ra by all soils tested was reduced considerably
in solution.
when the Ca 2+ concentration
Sorption of
of the solution
was increased.
CONCLUSIONS The specific adsorption
of 226Ra on soils in the presence
with both Langmuir and Freundlich In the presence therm is influenced
isotherms
of Ca 2+ ions is in accordance
over a wide range of concentrations.
of a large excess of Ca 2+ ions,
more by the maximum adsorption
the initial slope of the Freundlich
iso-
capacity of the soil than by its affinity
for 226Ra. At low values of surface
coverage,
the presence
of Ca 2+ ions has a significant
upon sorption as indicated by the Langmuir bonding energy parameter coefficient.
effect
and, Kd, the distribution
296
No. 5
REFERENCES i.
J.S. Nathwani and C.R. Phillips, Part I. Chemosphere.
2.
R.G. McLaren, D.V. Crawford, J. Soil Sci., 24, 443 (1973).
3.
K.G. Tiller, J.F. Hodgson, M. Peach, Soil Sci., 95, 392-399 (1963).
4.
K.G. Tiller, J.L. Honeysette, E.G. Hallsworth, Aust. J. Soil Res., !, 43 (1969).
5.
C.H. Giles, T.H. MacEvans, S.N. Nakhwa, D. Smith, J. Chem. Soc., 1960, 3973 (1960).
6.
J.H. de Boer.
In The Dynamical Character of Adsorption, p. 58, Oxford Univ. Press (1968).
Table I Summary of Freundlich and Langmuir Isotherm Parameters for the Specific Adsorption of 226Ra on Soils Langmuir Soil Series
Soil No.
KI
Freundlich K2
K
n
Initial Slope
Wendover
1
4761.9
0.127
1779.7
0.213
202.2
Haldimand
2
1596.1
0.048
470.1
0.201
22.3
Rideau
3
368.4
0.015
101.6
0.154
3.28
Vaudreuil
4
493.9
0.0048
43.1
0.283
4.26
Grimsby
5
65.1
0.011
19.5
0.135
0.62
St. Thomas
6
18.6
0.027
i0.i
0.071
0.131
No. 5
297
i0000 f
f
U
5000
~g
4000
3000 Z
2000
I000
900
-..: i0
: 20
EQUILIBRIUM
Figure
I.
Adsorption presence
Isotherms
; 30
. . . . . . 40 50
226Ra CONCENTRATION
for adsorption
of Ca 2+ ions (specific)
dilute solutions.
, 60
. 70
IN LIQUID,
, 80 C
eq.
. 90
( pCil -I)
of 226Ra by soil no.l in the
and for total adsorption
from
i00
298
No. 5
Soll no. 1 (Wendover) 8000 Soil no. 5 (Grimsby)
15
5000
S/C 10
si c
1000 0.2
0.6
1.0
e
0.2
0.6
1.0
Soil no. 6 (St. Thomas) !otal S/c
!
0
0.2
0.6
1.0
" versus surface cGverage @ Figure 2. Plots of the affinity of soils for 226Ra - (/SC)
No.
5
299
103
102
~
Kd
10
i
m
s
b
y
)
~a.
•
s)
rl
1.0
!
I
I
t
0.01
|
0.02
, |
0.03
CONCENTRATION OF Ca 2+ IN SOLUTION (
0 04 molesl-I)
Figure 3. Plot of the distribution co-efficient, Kd, for the sorption of 226Ra by soils as a function of Ca2+- concentration in solution.
(Received ia USA
18
July 1978)
0.05