Cation Selectivity in Sodium-Calcium, Sodium-Magnesium, and Calcium-Magnesium Exchange on Wyoming Bentonite at 298 K1

Abstract

Exchange isotherms were prepared for Na** plus in Na** plus -Ca**2** plus and Na** plus -Mg**2** plus exchange reactions, and for Mg**2** plus in Ca**2** plus -Mg**2** plus exchange reactions, at 298 K on Wyoming bentonite suspended in a 0. 05M perchlorate background. These isotherms were essentially congruent with the appropriate thermodynamic nonpreference exchange isotherms. It was concluded from this fact that there is essentially no difference in the affinity of montmorillonite clay for Ca**2** plus vs. Mg**2** plus .

Full text

Cation
Selectivity
in
Sodium-Calcium,
Sodium-Magnesium,
and
Calcium-Magnesium
Exchange
on
Wyoming
Bentonite
at 298
K1
GARRISON
SPOSITO,
KENNETH
M.
HOLTZCLAW,
CLAIRE
JOUANY,
AND
LAURENT
CHARLET2
ABSTRACT
Exchange isotherms
were
prepared
for
Na+
in
Na+—»Ca2+
and
Na+^Mg2+
exchange reactions,
and for
Mg2+
in
Ca2+—»Mg2+
ex-
change reactions,
at 298 K on
Wyoming
bentonite
suspended
in a
0.05M
perchlorate
background.
These
isotherms
were
essentially
congruent
with
the
appropriate
thermodynamic
nonpreference
ex-
change isotherms. It was concluded
from
this
fact that there is es-
sentially
no
difference
in the
affinity
of
montmorillonite
clay
for
Ca2+
vs.
Mg2+.
However, exchange isotherms prepared
for
Na+
in
Na(I)
—
Ca(II)
and
Na(I)
—
Mg(II)
exchange,
as
well
as
published
ones
for
Mg(II)
in
Ca(II)
—
Mg(II) exchange
on
montmorillonite
suspended
in a
0.05M
chloride background consistently indicate
a
slight
preference
on the
clay
for
Ca(II)
over
Mg(II).
This
preference
for
Ca(II)
is
concluded
to be the
result
of the
formation
of
CaCl+
complexes, which
are
more stable
thermodynamically
and
have
a
greater
affinity
for the
clay than
MgCl+
complexes.
Additional
Index
Words:
cation exchange, chloride complexes, sa-
line
soils,
thermodynamics of cation exchange.
Sposito,
G.,
K.M.
Holtzclaw,
C.
Jouany,
and
L.
Charlet.
1983. Cat-
ion
selectivity
in
sodium-calcium,
sodium-magnesium,
and
cal-
cium-magnesium exchange
on
Wyoming
bentonite
at 298 K.
Soil
Sci.
Soc.
Am.
J.
47:917-921.
O
NE
OF THE
CENTRAL
PROBLEMS
in the
chemistry
of
arid-zone soils
is the
quantitative character-
ization of an exchange complex bearing sodium, cal-
cium,
and
magnesium ions.
Rahman
and
Rowell
(1979)
and
Bresler
et
al.
(1982,
part
1)
have reviewed
this problem recently
and
have brought into clear
fo-
cus
the two objectives of understanding the patterns
of
Na-Ca-Mg
exchange reactions
and of
predicting
the
effect
of
exchanger composition
on
soil physical
properties.
The
consensus
of the
literature
is
that
ex-
changeable sodium percentage (ESP), electrolyte con-
centration,
and
soil mineralogy
are the
three master
chemical variables on which the water-conducting
properties
of
arid-zone soils
depend.
What remains
is
to
develop
a
unified,
quantitative description
of the
interrelations among these variables that
is of
predic-
tive value.
Much
of the
research
on
Na-Ca-Mg
exchange
re-
actions
has
involved montmorillonite clay
or
mont-
morillonitic
soils. In respect to exchange selectivity,
the
trends that have emerged
from
the
results
of
pub-
lished studies are:
(i)
for a
given distribution
of
Na
and
bivalent cation
in the
aqueous solution phase,
the
resulting
ESP on the
clay
is
somewhat higher
in
Na-
Mg
exchange than
in
Na-Ca
exchange,
and
(ii)
in
Ca-
Mg
exchange,
a
small
preference
for Ca
exists
on the
exchange
complex
(Schwertmann,
1962; Clark,
1966;
Dolcater
et
al.,
1968;
Levy
and
Hillel,
1968;
Hunsaker
1
Contribution
from
the
Dep.
of Soil and Environmental Sciences,
Univ. of
California,
Riverside,
CA 92521. Received 10 Nov. 1982.
Approved
19
Apr. 1983.
2
Professor
of
Soil Science, Research Associate
IV,
Visiting
Post-
doctoral
Fellow,
and
Graduate
Research
Assistant,
respectively.
and
Pratt,
1971;
Levy
and
Shainberg,
1972; Levy
et
al.,
1972;
van
Bladel
et
al.,
1973;
Jensen
and
Babcock,
1973;
Gheyi
and van
Bladel,
1975;
Macs
and
Cremers,
1977;
Rahman
and
Rowell, 1979; Rowell
and
Shain-
berg, 1979; van Bladel and Gheyi, 1980; Shainberg et
al.,
1980).
The
extent
to
which
Na-Mg
exchange
re-
sults
in
larger
ESP
values than
Na-Ca
exchange
is not
pronounced,
the
differences
being
the
order
of 3 to 5
in
ESP (Rahman and Rowell, 1979). The same is true
of the
selectivity
of
montmorillonite
for
calcium over
magnesium.
For
example,
van
Bladel
and
Gheyi
(1980)
have reported a mean value of
0.8
±
0.1
for the
overall
Vanselow
selectivity
coefficient,
KTV
=
(xTMg
CaT)/(xTCa
MgT)
[1]
which
describes
the
stoichiometric
cation exchange
re-
action:
CaX2(s)
+
MgCl2(aq)
=
MgX2(s)
+
CaCl2(aq),
[2]
where
XT
is the
mole
fraction
of all
species
of a
metal
in
the
exchanger phase,
MT
(M
=
Ca or
Mg)
is the
total
molarity
of all
species
of a
metal
in the
aqueous
solution phase,
and X
refers
to
1
mol
of
negative charge
on
Camp
Berteau
montmorillonite clay. This mean
value
of
KTV
is
only
a
little smaller than 1.0,
the
value
that
indicates
no
overall preference
for
calcium
vs.
magnesium
on the
clay.
A
common feature
in all of the
experimental studies
cited above [except one, that
of
Macs
and
Cremers
(1977)]
is the use of a chloride background ionic me-
dium
in
which
to
carry
out the
exchange reactions.
Sposito et al.
(1983)
have shown recently that, in chlo-
ride media,
Na-Ca
and
Na-Mg
exchange reactions
on
montmorillonite (Wyoming bentonite) actually
are
ternary
cation exchange reactions, involving
Na+,
M2+,
and the
complex,
MC1+
(M
=
Ca or
Mg). According
to the
data
presented
by
Sposito
et al.
(1983),
as the
charge
fraction
of
Ca(II) increases beyond
0.3 in
mixed
NaCl/CaQ2
solutions reacting with
a
Na-montmoril-
lonite,
the
species
of
Ca(II) that competes
effectively
with
Na+
on the
external
surfaces
of the
quasicrystals
that have formed is the complex,
CaCl+,
not the
free
cation,
Ca2+.
In the
case
of
Na-Mg
exchange,
the
same
process
was
shown
to
occur,
but to a
lesser extent
because montmorillonite apparently exhibits
a
lesser
affinity
for
MgCl+
than
for
CaCl+
(Sposito
et
al.,
1983,
Fig.
1).
The
significance
of the
results
of
Sposito
et al.
(1983)
for
cation selectivity
in
Na-Ca-Mg
exchanges
on
montmorillonite
is the
possibility that
the
small over-
all
preference
of
Ca(II) over Mg(II) that
has
been
ob-
served repeatedly in chloride background media may,
in
fact,
be
produced
by the
greater
affinity
of the
spe-
cies
CaCl+
for the
clay relative
to the
species
MgCl+.
It is possible that, without the presence of
monovalent
chloride complexes, montmorillonite would exhibit no
differences
in
Ca(II)
vs.
Mg(II) selectivity
in
exchange
reactions involving solely
the
free
cations,
Na+,
Ca2+,
and
Mg2+.
In
this
paper,
the
first
measurements
of Ca
917
Published September, 1983

918
SOIL SCI.
SOG.
AM.
J.,
VOL.
47,
1983
—»
Mg
exchange
on
montmorillonite
in a
perchlorate
background,
where
monovalent
complexes
do not oc-
cur,
will
be
presented.
Thermodynamic
methods
will
be
applied
to
examine
cation
selectivity
in
Na
—>
Ca,
Na
—>
Mg, and Ca
—»
Mg
exchange
reactions
on Wy-
oming
bentonite
at 298
K
in
both
perchlorate
and
chloride
background
media.
On the
basis
of
this
anal-
ysis,
the
role
of the
complex,
MC1+
(M
=
Ca or
Mg),
will
be
evaluated
in
relation
to the
frequent
observa-
tion
that
montmorillonites
and
montmorillonitic
soils
adsorb
Ca(II)
preferentially
to
Mg(II).
MATERIALS
AND
METHODS
Wyoming
Bentonite
Montmorillonite
SWy-1,
obtained
from
the
Source Clays
Repository
of the
Clay Minerals Society,
was
used
in
this
study. The unit cell formula for
this
Crook County, Wyo-
ming,
bentonite is
(Sposito
et
al.,
1983):
where
M+
refers
to 1
mol
of
monovalent exchangeable cat-
ion
charge.
The
procedures employed
to
purify
the
clay
and
prepare it in the sodium-saturated
form
in either a perchlor-
ate or
a
chloride background medium have been described
in
detail
by
Sposito
et al.
(1981,
1983).
For the
exchange
experiments involving only calcium
and
magnesium,
the so-
dium-clay
in the
perchlorate medium
was
converted
to a
calcium-saturated clay
by
repeated washing (including
a
1
5-
min
shaking period)
and
centrifugation
with
2M
Ca(ClO4)2.
Exchange
Experiments
Sodium-montmorillonite
samples were reacted at 25.0 ±
0.3°C
with mixed salt solutions
of
either
NaClO4/Ca(ClO4)2,
NaClO4/Mg(ClO4)2,
NaCl/CaCl2,
or
NaCl/MgCl2,
as de-
scribed
by
Sposito
et al.
(1983).
Calcium-montmorillonite
samples
were reacted
at
25.0
±
0.3°C
with
Ca(ClO4)2/
Mg(ClO4)2
solutions.
The
reaction times varied between
24
and 60
h.
In the
Na-exchange
experiments,
the
anion
mo-
larity
was
maintained
at
0.05
1
±
0.004M
and the
pH
value
was
7.0 ±
0.5.
In the
Ca-Mg
exchange experiments,
the
perchlorate
molarity
was
0.048
±
0.003M
and the pH
value
was
7.1
±
0.3.
The
clay stock suspension concentration
was
23.2
± 0.2
g
clay/kg suspension.
The
methods
by
which
the
supernatant solutions
and
clay slurries were analyzed
for Na,
Ca, and Mg
after
the
exchange experiments were completed
have been described
by
Sposito
et al.
(1983).
Data
Analysis
The
surface excesses
of Na, Ca, and Mg
were calculated
as described by Sposito et al.
(1981):
IV1")
=
nt
-
Mwmt,
(i
=
Na+,
Ca2+,
or
Mg2+),
[3]
where
r/w>
is the
surface excess,
in
moles
per
kilogram
of
clay;
n,
is the
total number
of
moles
of
metal species
/
in
the
clay slurry
per
kilogram
of
clay;
Mw
is the
mass
of
water
in
the
clay slurry
per
kilogram
of
clay;
and
mt
is the
molality
of
metal species
/'
in the
supernatant solution.
The
quantity
lY1"*
is the
number
of
excess moles
of
metal species
/',
per
kilogram of clay, relative to the number of moles of the
metal species present
in a
bulk aqueous solution
of
molality
m,
that contains
the
same mass
of
water
as in the
clay slurry.
The
adsorbed metal species charge
was
calculated with
the
following
equation:
q{
=
Z,T,M
(i
=
Na+,
Ca2+,
etc.),
[4]
where
Z,
is the valence of metal species i. The total adsorbed
metal charge,
Q0,
was
calculated
as the sum of all
terms
<?,
for
the
specific
metal cation species adsorbed
by the
clay
in
a
given exchange experiment.
Exchange
isotherms
for
Na+
in
either
the
perchlorate
or
the
chloride background ionic medium were prepared
after
calculating
the two
charge fractions:
ENa
=
q^/Qo
£Na
=
[Na+]/0.05
,
[5]
where
the square brackets
refer
to a molar concentration.
The
mean
values
of
Q0
were 1.03
and 0.9
molc
kg"1,
for the
Na-Ca
and
Na-Mg
exchanges
in a
0.05M
perchlorate back-
ground,
and
0.96
and
1.04
molc
kg"1
for
the
same
two ex-
changes
in a
0.05M
chloride background (Sposito
et
al.,
1
983).
The
standard error
for all
four
sets
of
measurements
of
Q0
was
0.08
mole
kg"1.
Exchange isotherms
for
Mg2+
in
the
Ca-
Mg-ClO4
system were prepared with charge fractions cal-
culated similarly
to
those
in
Eq.
[5]:
=
2[Mg2+]/0.05
.
[6]
Vanselow
selectivity
coefficients
for
Na+
—
>
Ca2+,
CaCl+
—
Ca2+,
Na+
->
CaCl+,
Na+
—
Mg2+,
MgCl+
—
Mg2+,
and
Na+
—
»
MgCl+
exchanges
in the
0.05M
chloride background
were
calculated
from
the data in Tables 4 to 7 of Sposito et
al.
(1983)
with
the
following
equation (Sposito,
1981,
Chap.
5;
Chu
and
Sposito, 1981):
-
(xf'(0z')/(*?'(7)z9
,
=
1,2,3)
,
[7]
where
x
is a
mole fraction
of a
metal
species
in the
exchanger
phase,
()
refers
to an
activity
of a
metal
species
in the
aqueous
solution phase,
and
/
or
j
refer
to
metal
species
with valence
Z,
or
Zj.
The
superscript
T
emphasizes
the
fact
that
Na-Ca
and
Na-Mg
exchanges
in a
chloride
background
medium
involve
the
ternary cation system,
Na+-M2+-MCl+
(Sposito
et
al.,
1983). The activities in Eq. [7] were calculated as
products of single-ion molar concentrations and single-ion
activity
coefficients
estimated with
the
Davies
equation
(Sposito, 1981, Chap.
2).
When
no
confusion results,
CAJ
will
be
denoted
generically
by the
symbol
K$(Vanselow
coef-
ficient
in a
ternary exchange system). This selectivity
coef-
ficient
differs
from
that
in Eq.
[
1
]
because
it
contains species
concentrations and mole fractions instead of total concen-
trations
and
mole
fractions.
RESULTS
AND
DISCUSSION
The
primary
data
for
Ca2+
—>
Mg2+
exchange
in
0.05M
perchlorate
are
given
in
Table
1 in
terms
of
c,
the
equilibrium
molinity
(Whitfield,
1979,
p.
161)
in
the
aqueous
solution
phase,
and g, the
adsorbed
cation
charge.
The
mean
value
of
Q0
for the
Ca-Mg
exchange
experiments
was
0.96
±
0.07
molc
kg"1,
in
agreement
with
the
mean
value
of
0.97
±
0.06
molc
kg"1
re-
ported
by
Sposito
et al.
(1983)
for
Na-Ca
and
Na-Mg
exchanges
in
0.05M
perchlorate.
Exchange
isotherms
for
Na+
on Ca- and
Mg-mont-
morillonite
(Wyoming
bentonite)
are
shown
in
Fig.
1
Table
1—Experimental
data
on
Ca-Mg
exchange
on
Wyoming
bentonite
in a
0.05M
perchlorate
background.
cCa
°Mg
9Ca
22.90
±
0.05
21.7
± 0.1
19.33
±
0.03
21.0
± 0.1
15.8
± 0.1
13.55
±
0.04
11.50
±
0.03
8.80
±
0.05
3.90
±
0.04
2.10
±
0.02
<io-s
2.45
±
0.05
5.04
±
0.04
6.60
±
0.07
8.03
±
0.03
9.87
±
0.05
9.15
±
0.07
14.86
±
0.02
19.5
±
0.10
22.3
± 0.1
0.98
±
0.02
0.88
±
0.09
0.86
±
0.13
0.67
±
0.12
0.63
±
0.11
0.52
±
0.07
0.53
±
0.05
0.38
±
0.04
0.18
±
0.01
0.06
±
0.02
9Mg
molc
kg-'
<io-4
0.106
±
0.027
0.20
±
0.07
0.19
±
0.05
0.32
±
0.05
0.36
±
0.04
0.45
±
0.07
0.67
±
0.11
0.72
±
0.04
0.89
±
0.16
Qo
0.98
0.99
1.06
0.86
0.95
0.88
0.98
1.05
0.90
0.95

SPOSITO
ET
AL.:
CATION SELECTIVITY
IN
NA-CA,
NA-MG,
&
CA-Mo
EXCHANGE
ON
WYOMING BENTONITE
919
l.0r
0.8
0.6
-No
0.4
0.2
WYOMING
BENTONITE
TN
=
50molc
tn-3
•
Na+-Ca2+
O
Na+-Ca2+-CaCI
+
0.2
0.4
0.6
0.8
1.0
-Na
Fig.
1—Exchange
isotherms
for
Na+
in Na
—,
Ca
exchange
on Wy-
oming
bentonite
in
perchlorate
(•)
and
chloride
(O)
background
media.
The
solid
curve
is the
thermodynamic
nonpreference
is-
otherm
for
monovalent-bivalent
cation exchange.
i.0r
0.8
-
0.6
-Na
0.4
0.2
WYOMING
BENTONITE
TN =
50molc
m-3
•
Na+-Mg2+
O
Na+-Mg2+-MgCI+
0.2
0.4
0.6
0.8
1.0
-Na
Fig.
2—Exchange
isotherms
for
Na+
in Na
—,
Mg
exchange
on
Wyoming
bentonite
in
perchlorate
(•)
and
chloride
(O)
back-
ground
media.
The
solid
curve
is the
nonpreference isotherm
for
monovalent-bivalent cation exchange.
and 2,
respectively.
The
solid curve
in
each
figure
is
the
thermodynamic nonpreference isotherm
for a bi-
nary
monovalent-bivalent cation exchange, calcu-
lated with
the
equation
(Sposito,
1981, Chap.
5)
ITN
/j___L
\£?,a
£N
[8]
where
T
=
7Na/7ca>
7 is a single-ion activity
coeffi-
cient,
and
TN
is the
total cation normality (equal
to
the
anion
molarity).
The
thermodynamic nonprefer-
ence
exchange isotherm
is
defined
by
these conditions:
(i)
that
the
standard
Gibbs
energy change
for the ex-
change
reaction is zero
(AGex
=
0), and
(ii)
that the
exchanger
phase activity
coefficients
have unit value
(ideal mixture). Thus,
in
chemical thermodynamics,
cation exchange
is
said
to
exhibit
no
selectivity
if the
exchange
equilibrium constant is equal to 1.0 and the
exchanger
phase has the properties of an ideal solid
solution.
The
vertical
lines
through
the
nonpreference
isotherms
in
Fig.
1 and 2
indicate
the
standard
errors
in the
measured values
of
ENa>
which were
±0.02
for
Na-Ca
exchanges
and ±
0.03
for
Na-Mg
exchanges.
The
data
in
Fig.
1 and 2
indicate that,
in the
per-
chlorate background, Wyoming bentonite shows
ap-
proximately
no
preference
for
Na+
in
either
Na-Ca
or
Na-Mg
exchange, according
to the
thermodynamic
criterion
of
nonpreference
described above.
On a
finer
scale
of
examination,
the
data
for the
perchlorate
me-
dium
in
Fig.
1 and 2
suggest that
Na+
may be
pre-
ferred slightly
over
Ca2+,
and
Mg2+
may be
preferred
slightly
over
Na+,
when
0.1
<
£Na
<
0.4.
A
Na/Ca-
or
Na/Mg-montmorillonite
suspension
with
<
30% of the
total adsorbed metal charge
ac-
counted
for by
Na"1'
comprises
fully
developed
quasi-
crystals,
with bivalent exchangeable cations princi-
pally
in the
interlayer
regions
and
Na+
relegated
to
the
external surfaces
(Shainberg
and
Otoh,
1968; Bar-
On
et
al.,
1970). Exchange reactions involving
Na+
for
ENa
<
0.4
can, therefore,
be
expected
to
occur
principally
on
external surfaces.
On the
hypothesis
of
Sposito et al.
(1983),
the
monovalent
complexes,
CaCl+
and
MgCl+,
can
compete with
Na+
on the
external
surfaces
more
effectively
than
Ca2+
or
Mg2+.
This
competition, however,
is
less intense
for
MgCl+
than
for
CaCl+
because
the
former
complex
is
less stable
thermodynamically
and has a
lesser
affinity
for the
clay
than
the
latter complex (Sposito
et
al.,
1983,
Ta-
ble 1 and
Fig.
1).
Perhaps these
two
characteristics
are
the
cause
of the
more apparent downward
shift
of
Na+
selectivity
in
Na-Ca
exchange than
in
Na-Mg
ex-
change,
noted
in
Fig.
1 and 2,
when
the
background
anion
is
changed
from
perchlorate
to
chloride.
Table
2—Vanselow
selectivity
coefficients
and
exchanger-phase
composition
date (mole fractions)
for
Na*-M2*-MCl*
exchange
(M
=
Ca or Mg) on
Wyoming
bentonite
in
a
0.05M
chloride background.
0.989
1.10
1.33
2.05
2.08
1.23
0.971
0.788
0.596
0.473
0.292
0.739
0.925
1.36
2.18
2.72
2.03
1.41
0.972
0.563
0.339
0.131
1
=
Na* 2
=
Ca!*
5.68
x 10-
1.68
X 10-
1.53
X 10-
9.65
X 10-
5.96
X 10-
4.04
X 10-
3.02
X 10-
2.36
x 10-
1.82
X
10'
1.46
X 10-
1.14
X 10-
•j
_
3.99
X 10-
3.24
x 10-
2.52
x 10-
2.29
x 10-
1.61
x 10-
1.02
X 10-
7.67
x 10-
6.83
x 10-
6.07
X 10-
5.50
X 10-
13.2
25.6
29.5
46.0
59.0
55.1
56.7
57.8
57.3
56.9
50.6
Na'
2
=
Mg"
13.6
16.9
23.3
30.9
41.0
44.6
42.9
37.7
30.5
24.8
4.62
X 10-
16.8
3
=
CaCl'
0.806
0.684
0.542
0.410
0.293
0.263
0.222
0.183
0.147
0.108
0.064
3
=
MgCl*
0.832
0.671
0.496
0.371
0.254
0.217
0.195
0.170
0.147
0.126
0.103
0.182
0.276
0.400
0.496
0.561
0.530
0.511
0.504
0.482
0.465
0.452
0.157
0.304
0.462
0.569
0.651
0.656
0.647
0.640
0.635
0.622
0.617
0.012
0.040
0.058
0.094
0.146
0.207
0.267
0.313
0.371
0.427
0.484
0.011
0.025
0.042
0.060
0.095
0.127
0.158
0.189
0.220
0.252
0.280

920
SOIL
SCI.
SOC.
AM.
J.,
VOL.
47,
1983
60
50
40
30
20
10
•
'•
•
Na+-»CaCI+
EXCHANGE
O
Na+-»MgCI+
EXCHANGE
O
o
O
o
o
•
O
.2
.8
-Na
Fig.
3—The
Vanselow
selectivity
coefficient
for
Na+
—»
MC1+
ex-
change
(M
=
Ca
or
Mg)
on
Wyoming
bentonite
as a
function
of
the
charge
fraction
of
Na+
on the
clay.
The
differences
between Fig.
1 and 2 can be
eluci-
dated
further
through
an
examination
of
Table
2.
This
table lists values
for
three ternary Vanselow selectivity
coefficients
along with
the
mole fractions
of
Na+,
M2+,
and
MC1+
(M
=
Ca or Mg) on
montmorillonite
sus-
pended
in the
chloride background medium.
The se-
lectivity
coefficients
in the
first
three columns
of the
table were calculated with
Eq.
[7] and
refer
to
Na+
—»
M2+,
MC1+
-»
M2+,
and
Na+
-,
MC1+
exchange,
re-
spectively, where
M
=
Ca or Mg. The
exchanger com-
position
data
at a
mole fraction
of
Na+
of
0.1
or
less
indicate that approximately equal numbers of the spe-
cies,
Ca2+
and
CaCl+,
are
adsorbed. However, only
about one-half
as
many
MgCl+
are
adsorbed
as
there
are
Mg2+
on the
clay surface.
The
calcium distribu-
tion, which
is
equivalent
to
two-thirds
of
Q0
satisfied
by
Ca2+
and
one-third
by
CaCl+,
is
consistent with
the
concept developed
by
Sposito
et
al.
(1983),
in
which
a
Ca-montmorillonite
suspended
in a
chloride ionic
medium consists
of
quasicrystals having
Ca2+
prin-
cipally
in the
interlayer
regions
and
CaCl+
principally
on the
external surfaces. However,
in the
case
of
Mg-
montmorillonite,
only about one-half
of the
charge
on
external
surfaces
evidently
is
neutralized
by
adsorbed
MgCl+.
This smaller adsorption
of
MgCl+
reflects
a
smaller
effect
of the
complex
on
Na+
selectivity,
in
agreement with
the
negligible
difference
between
the
isotherms
in
Fig.
2.
The
third column
of
Table
2
lists values
of
f^y
for
the
exchanges:
Na+
-»
CaCl+
and
Na+
—>
MgCl+.
These selectivity
coefficients
are
plotted against
ENa
in
Fig.
3.
Since
Af£
»
1.0 for
both exchanges,
the
clay
surface
is
selective
for
CaCl+
and
MgCl+
over
Na+.
As
E^a.
decreases below 0.4,
K$-
increases sharply
and
reaches
a
maximum value
at
£Na
«
0-15,
thereafter
appearing
to
drop precipitously.
The
most rapid
in-
crease
of
f^y
with
decreasing
£Na
occurs
after
quasi-
crystal formation, when
the
remaining exchangeable
Na+
reside
on
external surfaces
(£Na
<
0.4).
The
rea-
son for
this
behavior cannot
be
determined
on the
i.o
0.8
0.6
0.4
0.2
WYOMING
BENTONITE
TN =
50molc
m~3
Ca2+-Mg2+
EXCHANGE
0.2
0.4
0.6
0.8
1.0
-Mg
Fig.
4—Exchange
isotherm
for
MgJ+
in Ca
—.
Mg
exchange
on
Wyoming
bentonite
in
0.05M
perchlorate.
The
solid line
is the
thermodynamic
nonpreference
isotherm.
basis
of
thermodynamic
data
alone.
One
possibility
is
related
to the
gradual breakdown
of
quasicrystals that
has
been observed
to
begin when
£Na
>:
0.15
on Wy-
oming
bentonite
(Shainberg
and
Otoh,
1968;
Banin,
1968;
Bar-On
et
al.,
1970;
Dufey
and
Banin, 1979).
When
£Na
is
very near zero,
the
quasicrystals remain
intact
and
MC1+
is,
evidently, increasingly preferred
as
£Na
grows larger. When
£Na
>
0.15
and the
quasi-
crystals
start
to
disintegrate,
the
high selectivity
for
MC1+
drops
off
because
Na+
now is
exchanging
on
both external and internal surfaces. The combination
of
these two trends could produce the behavior in Fig.
3.
Figure
4
shows
an
exchange isotherm,
for
Mg2+
in
Ca2+
_»
Mg2+
exchange
on
Wyoming bentonite, which
was
constructed with
Eq. [6]
from
the
data
in
Table
1.
The
thermodynamic nonpreference isotherm
in
this
case
is the
straight line
in the
figure
that makes
a 45-
degree
angle with both coordinate axes (Sposito,
1981,
Chap. 5). The vertical bar indicates the standard error
in
the
measurements
of
EMs,
which
was ±
0.01.
It is
apparent that, within the experimental precision, there
is
no
preference
for
Mg2+
over
Ca2+
on the
clay min-
eral.
A
direct computation
of the
binary Vanselow
se-
lectivity
coefficient,
[9]
for
each
set of
exchange data
in
Table
1
resulted
in
Ky
=
1.0 ±
0.2, thereby
confirming
the
implication
of
Fig.
4.
Within
its
experimental precision, this result
is not in
contradiction with
the
slight preference
of
Na+
over
Ca2+
in a
perchlorate
background, indicated
in
Fig.
1, or the
slight preference
of
Mg2+
over
Na+
in
a
perchlorate background, indicated
in
Fig.
2.
Since
Ky
in
this
case
is
equal
to the
ratio
of the
equilibrium
constant
for
Na+
—>
Mg2+
exchange
to
that
for
Na+
—>
Ca2+
exchange,
the
former constant could
be as
much
as 20%
larger than
the
latter constant
and
still

SPOSITO
ET
AL.:
CATION
SELECTIVITY
IN
NA-CA,
NA-MG,
&
CA-Mc
EXCHANGE
ON
WYOMING
BENTONITE
921
be
consistent with
the
reported standard deviation
of
±0.2
for
KBV.
The
isotherm
in
Fig.
4
differs
from
the
results
of
the
many studies that have been reported previously
for
Ca(II)-Mg(II)
exchange
in
chloride background
media (Clark,
1966;
Hunsaker
and
Pratt,
1971;
Levy
et
al.,
1972;
Levy
and
Shainberg,
1972;
Gheyi
and van
Bladel,
1975;
van
Bladel
and
Gheyi, 1980), which
im-
ply
a
slight preference
for
Ca(II)
over Mg(II).
The
mean
values
of the
overall
Vanselow
selectivity
coefficient
for
Mg(II)
—»
Ca(II) exchange (the inverse
of
KTV
in
Eq.
[1])
that
can be
calculated
from
the
data reported
by
Levy
and
Shainberg
(1972)
and by van
Bladel
and
Gheyi
(1980)
are
1.47
±
0.19
and
1.26
±
0.20,
re-
spectively.
The
differences
between these
two
results
and the
mean
Ky
determined
in
this study
are
con-
cluded
to be the
result
of the
greater
affinity
of the
clay
for
CaCl+
vs.
MgCl+
in a
chloride background
me-
dium.CONCLUSIONS
In
a
Q.Q5M
perchlorate
background
at
25°C,
the ex-
change
isotherm
for
Na+
competing either with
Ca2+
or
Mg2+
on
Wyoming
bentonite
is
very nearly con-
gruent
with
the
thermodynamic
nonpreference
iso-
therm. When
the
charge fraction
of
Na+
on the
clay
lies
between
0.1 and
0.4, there
is, at
most,
a
slight
preference
for
Na+
over
Ca2+
and for
Mg2+
over
Na+.
The
small enhancement
in
Mg2+
selectivity
may be a
result
of the
fact
that about one-third
of the
permanent
charge
on the
clay originates
in the
tetrahedral
sheet,
thus
endowing
the
clay with some
"vermiculite-like"
character
and a
slight preference
for
Mg2+.
In
a
0.05M
chloride background,
the
exchange
is-
otherm
for
Na+
competing with species
of
Mg(II)
does
not
appear
to be
detectably
different
from
the
corre-
sponding
isotherm in a perchlorate background. The
exchange
isotherm
for
Na+
competing with species
of
Ca(II)
in a
0.05M
chloride background does
shift
downward,
however,
to
indicate
a
slight increase
in
preference
for
calcium species over
Na+
when
£Na
drops below 0.4. This
shift
is
caused
by the
formation
of
CaCl+
complexes which compete strongly with
Na+
on the
external surfaces
of
quasicrystals.
In a
O.OSAf
perchlorate background
at
25°C,
the ex-
change
isotherm
for
Mg2+
competing with
Ca2+
on
Wyoming
bentonite
is
indistinguishable, within
ex-
perimental precision,
from
a
thermodynamic nonpre-
ference
isotherm. Reports
of
a
small preference
for
Ca(II) over Mg(II)
on
Wyoming bentonite
in
chloride
background media
reflect
the
greater
affinity
of the
clay
for
CaCl+
vs.
MgCl+.
ACKNOWLEDGMENTS
The
research reported
in
this
paper
was
supported
in
part
by
a
grant
from
the
Kearney
Foundation
of
Soil
Science
and
in
part
by a
grant-in-aid
to
L.
Charlet
from
the
Graduate
Division
of the
University
of
California,
Riverside.
Grati-
tude
also
is
expressed
for a
research fellowship
to Dr.
C.
Jouany
from
the
French
Ministry
of
Foreign
Affairs.
Dr.
Philip
Fletcher
kindly
read
this
paper
in
early draft form
and
made several helpful comments towards
its
improve-
ment.