Clays and Clay Minerals. Vol. 46. No. 2, 132 [38, 1998.
D E H Y D R A T I O N , D I F F U S I O N A N D E N T R A P M E N T OF ZINC IN B E N T O N I T E
Y. B. MA AND N. C. UREN
School of Agriculture, La Trobe University, Bundoora, Victoria 3083, Australia
Abstract--Interactions with bentonite are important in the chemical speciation and fate of heavy metals
in soils and other ecosystems. The interactions of Zn with bentonite were studied using X-ray diffraction
(XRD), dehydration, kinetic and sequential extraction procedures. The species and activity of Zn retained
by bentonite were affected markedly by pH. The Zn(OH) + was retained by bentonite prepared at pH -->
6.9. The extent of dehydration of Zn(OH)+-bentonite was higher than that for Zn-bentonite. At a relative
humidity of 55.5%, the basal spacing of the Zn(OH)*-bentonite was from 1.21 to 1.26 nm with 1 water
sheet and that of the Zn-bentonite was 1.51 nm with 2 water sheets. The greater affinity of Zn(OH) + for
bentonite than Zn was associated with a lower degree of hydration. When an aqueous suspension of Cabentonite was incubated with soluble Zn, the concentration of Zn retained by the Ca-bentonite was linearly
related to the square root of time. The rate of the interaction was controlled probably by the interlayer
diffusion and subsequently by the diffusion into the ditrigonal cavities in bentonite. The Zn retained by
bentonite was dehydrated in situ so as to increase the bonding of Zn with surfaces of bentonite. With
hydrothermal treatment the retained Zn could diffuse easily into the cavities and transform increasingly
to the residual forms that are associated with the entrapped form.
Key Words--Bentonite, Entrapment, Interaction, Micropore Diffusion, Speciation, Zinc.
INTRODUCTION
I n t e r a c t i o n s o f Z n w i t h smectites, such as b e n t o n i t e ,
are i m p o r t a n t in the c h e m i c a l speciation a n d fate o f
Z n in soils a n d o t h e r e c o s y s t e m s . T h e interactions inv o l v i n g surface c o m p l e x f o r m a t i o n ( i n n e r a n d outer
sphere) h a v e b e e n studied f r e q u e n t l y in the short t e r m
(days), b u t t h e y h a v e n o t b e e n s t u d i e d o v e r relatively
long t e r m s ( m o n t h s ) . T h e l o n g - t e r m p r o c e s s e s that decrease the activity a n d extractability o f Z n r e t a i n e d b y
smectites are b e l i e v e d to b e i m p o r t a n t in the availability o f Z n a d d e d to soils a n d in the r e m e d i a t i o n o f
soils c o n t a m i n a t e d w i t h Zn.
T h e species a n d activity o f Zn, w h e t h e r in solution
or r e t a i n e d b y smectites, are a f f e c t e d m a r k e d l y b y pH.
H i g h p H s g e n e r a l l y result in greater specific adsorption o f Z n (Farrah a n d P i c k e t i n g 1976). T h e m e c h a n i s m o f the specific a d s o r p t i o n at h i g h p H is b e l i e v e d
to b e associated w i t h the f o r m a t i o n o f Z n ( O H ) § (Farrah a n d P i c k e r i n g 1976; Tiller et al. 1984). A l t h o u g h
Z n h a s a h i g h d e g r e e o f h y d r a t i o n (Pass 1973), in sol u t i o n the Z n will i n c r e a s i n g l y f o r m Z n ( O H ) + w i t h inc r e a s i n g pH. F u r t h e r m o r e , p r o t o n d i s s o c i a t i o n f r o m
w a t e r m o l e c u l e s o f t e n takes place m o r e readily in the
i n t e r m i c e l l a r spaces t h a n in b u l k solutions ( F a r m e r
1978; Tan 1993). Q u i r k a n d P o s n e r (1975) p o s t u l a t e d
that at low pH, Z n c a n h y d r o l y z e to Z n ( O H ) § at the
surface o f F e oxides. A n d F r e n k e l (1974) r e p o r t e d that
the d e g r e e o f d i s s o c i a t i o n o f a w a t e r m o l e c u l e ads o r b e d o n m o n t m o r i l l o n i t e is 107 t i m e s h i g h e r t h a n in
liquid water. T h e reaction:
[Zn(H20)n] 2+ ~ [ Z n ( O H ) ( H 2 0 ) . . . . i] +
+ xH20 + H §
Copyright 9 1998, The Clay Minerals Society
[1]
is p r o m o t e d b y partial d e h y d r a t i o n , since t h e r e s i d u a l
w a t e r m o l e c u l e s a r o u n d Z n are m o r e h i g h l y p o l a r i z e d
a n d h e n c e m o r e acidic ( F a r m e r 1978). T h e greater affinity o f Z n ( O H ) § o v e r Z n is a p p a r e n t l y due to the
l o w e r d e g r e e of h y d r a t i o n o f the f o r m e r w h i c h leads
to b o t h steric a n d electrostatic f a v o r i t i s m in the sorption process.
Tiller a n d H o d g s o n (1962) p o s t u l a t e d that the Z n
u n e x t r a c t a b l e with 2.5% H O A c is associated w i t h the
filling o f lattice v a c a n c i e s o f layer silicates, b u t X R D
analysis h a s failed to r e v e a l a n y c h a n g e s in the structure o f the layer silicates ( R e d d y a n d Perkins 1974).
R e d d y a n d Perkins (1974) p o s t u l a t e d that the Z n enters
the interlayer spaces o f the e x p a n d e d lattice (due to
h y d r a t i o n ) a n d is t r a p p e d as the crystal lattice c o n tracts u p o n drying. H o w e v e r , there is little direct evid e n c e for lattice cavity e n t r a p m e n t o f Z n b y s w e l l i n g
layer silicates.
F e w studies h a v e b e e n carried out to i n v e s t i g a t e the
loss o f activity a n d extractability o f Z n at h i g h p H o v e r
m o n t h s a n d years. I n t e r a c t i o n s in the interlayer spaces,
such as w i t h ditrigonal cavities, m a y b e r e s p o n s i b l e
a l o n g w i t h o t h e r p r o c e s s e s s u c h as d e h y d r a t i o n o f
Z n ( O H ) § species. T h e o b j e c t i v e o f this study w a s to
i n v e s t i g a t e the species, d e h y d r a t i o n a n d interlayer diff u s i o n a n d e n t r a p m e n t o f Z n in d i t r i g o n a l cavities o f
bentonite.
MATERIALS AND METHODS
Hydration and Dehydration---Theory and Experiment
T h e cations with low h y d r a t i o n energies, s u c h as K,
NH4 + a n d Rb, are easy to d e h y d r a t e a n d t e n d to b e
e n t r a p p e d in interlayer positions o f swelling layer silicates ( D i x o n a n d W e e d 1989). T h e free e n e r g y o f sol132
Vol. 46, No. 2, 1998
Zinc interactions with bentonite
Table 1. Salts used to prepare saturated electrolyte solutions
for constant water activity at 20 ~
Salt
Water activity (aw)
P205
LiC1.H20
CaC12.6H20
Ca(NOa)z.4H20
NaNO2
0.00
0.12
0.35
0.55
0.65
From Denis et al. (1991).
133
The chemical analysis o f Zn-saturated bentonite at
p H 5.6 was carried out by X-ray fluorescence ( X R F )
spectroscopy and the results were presented as follows: SiO2 647 g kg -1, A1203 205 g kg -1, M g O 40 g
kg -1, K20 33 g kg -1, N ~ O 11 g kg -1, Fe203 7.7 g
kg -1, C a t 7.2 g kg -1, T i t 2 1.8 g kg -1, P2Os 0.4 g kg -1,
M n O 0.1 g kg -1 and Z n O 44.3 g kg -1. The structural
formula was obtained by calculation ( N e w m a n and
B r o w n 1987; Bain and Smith 1994):
Ko.51Na0.26Ca0.09Zn 0.40(A12.85Fe(III)0.07Ti0.02Mg0.73)
( S i 7.gA10.1 )O20(OH)4
vation of an ion m a y be calculated directly from the
Born equation (Pass 1973):
8,fi.~0ri
1-
[21
where: L = the A v o g a d r o constant = 6.002 • 1023
m o l - l ; Z = the numerical value of the charge on the
ion; e = the magnitude of the electronic charge =
1.602 • 10 -19 C ; r i = the radius of the ion (nm); 60 =
the permittivity of a v a c u u m = 8.854 • 10 -12 F m - l ;
= the permittivity of the solvent (F m 1); and Er
de0 = the relative permittivity (also called the dielectric constant). Expressions for the hydration enthalpy
and hydration entropy m a y be derived from this equation, which on substitution reduce to:
=
A H ~ = - 6 9 . 9 • 103Z2
(nm J mo1-1)
[3]
(nm J K -1 tool -1)
[4]
ri
Z2
AS ~ = - 4 . 1 0 • - ri
The ionic radii are often difficult to ascertain. Generally, the crystallographic radii rc corrected by the
additive term c, with a constant c o m m o n value (0.085
n m for Pauling's ionic radii) for cations is used: ri =
rc + c (Koryta and Dvorfik 1987).
Chinese white bentonite was provided by C o m m e r cial Minerals Limited, Victoria. Bentonite (5 g) was
saturated with K, Ca, M g and Zn by shaking for 4 h
with 50 m L of 0.1 M KCI, CaC12, MgCI2 and ZnCI/
solutions, centrifuged and the supernatants decanted.
The residues were washed twice for 1 h each time with
25 m L o f distilled water. The pHs o f the bentonite
suspensions were adjusted with KOH/HC1 to the appropriate values (initial pH) and the suspensions were
allowed to stand overnight to achieve p H equilibrium;
then the final p H was measured. The suspensions were
centrifuged and the supernatants decanted. All the residues were air-dried at r o o m temperature except the
Mg-bentonites, which were dried at 40 ~ for easy
grinding. After grinding, s o m e o f the samples were
stored in a desiccator for at least 1 w e e k above a saturated solution of Mg(NOs)2.6H20 to obtain constant
relative humidity (55.5%) prior to X R D with a Siem e n s D1500 X-ray Diffractometer.
The chemical composition and the formula of the bentonite suggest that the bentonite is a dioctahedral illite-smectite and the predominant charges are generated in octahedral sheets ( N e w m a n and B r o w n 1987).
The h o m o i o n i c bentonite samples were incubated at
a water activity (aw) of 1.0 for 1 w e e k and then placed
in desiccators at water activities o f 0, 0.12, 0.35, 0.55
and 0.65 (Table 1) at 20 ~ for 1 week. The water
contents of the samples at the different aw were measured by weighing.
The bentonites saturated with Zn (Zn-bentonite) at
different pHs were extracted with 1 M NH4C1, KC1,
NaC1 and LiC1 solutions at p H 7.0 (twice), respectively. The solutions (200 m L ) were added to the centrifuge tubes with Zn-bentonite (1 g) at different pHs and
shaken for 24 h. After shaking and centrifuging, the
supernatants were decanted and filtered. The second
extraction was conducted by adding another 200 m L
extractant to the residue f r o m the previous extraction.
A f t e r shaking for 24 h, the filtrates were obtained as
before. Calculations were m a d e to take into account
the amount o f Zn left o v e r f r o m the previous extraction. The filtrates were diluted 100 times for Zn analysis by atomic absorption spectrophotometry (VarianTechtron A A - 1 4 7 5 series).
K i n e t i c s - - T h e o r y and E x p e r i m e n t
Bentonite saturated with Ca was prepared by shaking bentonite (20 g) with 100 m L 0.5 M CaCI2 at p H
7.0 for 3 times, each for 24 h, and then the sample
was washed twice with distilled water, air-dried and
passed through a 100-mesh sieve.
The kinetics o f the reaction of Zn with the Ca-bentonite (4 m g L -1 Zn in 0.01 M CaC12 in 1% suspension) was studied by monitoring the Zn concentration
in solution while the suspension was continuously
stirred. The concentration of Zn in solution decreases
as Zn enters the Ca-bentonite because there is a limited
v o l u m e o f solution. For the diffusion f r o m a limited
source (such as a stirred solution o f limited volume)
into a plane sheet, the process can generally be expressed by the following equation (Crank 1975):
YIYm
=
1 -
e rl~
erfcX/T/et 2
[5]
where Y is the amount o f diffused Zn, Ym is the total
134
Clays and Clay Minerals
Ma and Uren
200
-
Y/Ym = 13
~X/'~--~X/i
[91
A
g
w h e r e 13 = 1 + w (w is a n error f r o m the d e l e t i o n o f
h i g h e r - o r d e r terms). A plot o f Y/Ym versus 1/X/t s h o u l d
give a straight line w i t h a n i n t e r c e p t 13 a n d a slope cd
X/-~-D/l2. T h e v a l u e o f c~ c a n b e e s t i m a t e d b y the equation, c~ = 1/F - 1, w h e r e F is the fraction o f the total
a m o u n t o f Z n finally t a k e n up b y the C a - b e n t o n i t e . In
this e x p e r i m e n t , the v a l u e o f F is g i v e n b y the e q u a t i o n
YIYm = Ft/(t + tl/2), w h e r e ill2 is a c o n s t a n t ; w h e n t --~
150
c
0
100'
o
.
"
it
5O
oo, Y/Ym = F.
0
0.0
i
i
i
i
i
0.2
0.4
0.6
0.8
1.0
Water
activity
Figure 1. The water content of Zn-bentonite at different pHs
as a function of water activity: (Q) pH 5.6, (11) pH 6.3, (LS)
pH 6.9 and (A) pH 8.8.
a m o u n t o f Z n a d d e d to the s u s p e n s i o n o f b e n t o n i t e , T
= Dt/P, erfc is referred to as the e r r o r - f u n c t i o n c o m p l e m e n t , D is a c o n s t a n t d i f f u s i o n coefficient, 2l is the
sheet thickness, t is t i m e a n d cx is a c o n s t a n t w h o s e
v a l u e c a n b e g i v e n b y the f r a c t i o n o f the total a m o u n t
o f Z n finally t a k e n up b y the C a - b e n t o n i t e ( F =
1/(1 + a)).
I f T/e~2 is s m a l l ( < 0 . 0 1 ) , c o r r e s p o n d i n g to small t,
s m a l l D/I 2 a n d large a , e r/~2 ~ 1, thus one obtains:
Y/Ym = 1 - e r f c ~
+ lOV'-~-~k e~ )
2
~
2
+""
{%/T) 3
[6]
T h e h i g h e r - o r d e r t e r m s in E q u a t i o n [6] c a n b e neglected at small T/a2; h e n c e E q u a t i o n [5] c a n b e simplified to:
Y/Ym = o L ~ X / 7
[7]
I f s m a l l values o f c~ are required, c o r r e s p o n d i n g to
v e r y h i g h fractional u p t a k e s o f Z n b y a p l a n e sheet, it
is c o n v e n i e n t to r e d u c e E q u a t i o n [5] to:
Y/Y
m
=
1
X/~X/-T + 2 X / ' - ~ X / - f /
y'
4N/~- \%/-~]
+...
[8]
T h i s is o b t a i n e d f r o m E q u a t i o n [5] b y substituting
the a s y m p t o t i c e x p a n s i o n for e r/~2 erfc T V ~ - ~2 w h e n T~
cr2 is large. I f {x is small, the h i g h e r - o r d e r t e r m s in
E q u a t i o n [6] c a n b e n e g l e c t e d at large values o f t i m e
a n d E q u a t i o n [5] c a n b e simplified to:
Incubation and Extraction
B e n t o n i t e saturated w i t h C a 2+ at p H 7.0 (1 g) was
m i x e d w i t h 2 0 - m L aliquots o f 4 m g L -l Z n as ZnSO4
in 0.01 M CaC12, a n d then i n c u b a t e d at 20 ~ A f t e r
1, 30, 60, 221 a n d 4 3 0 d, the s u s p e n s i o n s w e r e centrifuged a n d the c o n c e n t r a t i o n s o f Z n in the s u p e r n a tants as water-soluble Z n ( W S - Z n ) were m e a s u r e d .
T h e Z n r e t a i n e d b y the b e n t o n i t e was extracted sequentially with 2 0 m L o f 1 M MgC12 at p H 7.0 b y
s h a k i n g for 1 h (MgClz-Zn), 2 s u c c e s s i v e 1% NaH C a E D T A in 1 M a m m o n i u m acetate at p H 8.3 b y
s h a k i n g for 2 h ( E D T A - Z n ) , 0.5 M HC1 b y s h a k i n g
for 2 h (0.5 M HC1-Zn), 6 M HC1 b y s h a k i n g for 2 h
(6 M H C I - Z n ) a n d H2SO4-HC104-HF m i x t u r e (residual
Zn). T w e n t y g r a m s o f C a - b e n t o n i t e were a d d e d to 4 0 0
m L o f 4 m g L -I Z n (ZnSO4) in 0.01 M CaC12 a n d the
s u s p e n s i o n was i n c u b a t e d at 20 ~ for 120 d. S o m e o f
the s u s p e n s i o n o f b e n t o n i t e w i t h r e t a i n e d Z n was heated in a Teflon p r e s s u r e vessel a n d s o m e o f the residue
( b e n t o n i t e w i t h Z n ) was h e a t e d in air at 2 0 0 ~ for 12
h. A f t e r heating, 2 0 m L o f the s u s p e n s i o n w a s extracted sequentially u s i n g the m e t h o d d e s c r i b e d above.
RESULTS AND DISCUSSION
Effects o f p H o n Species, H y d r a t i o n a n d
Extractability o f R e t a i n e d Z n
A l t h o u g h the B o r n e q u a t i o n is a r o u g h a p p r o x i m a tion, it c a n b e u s e d to c o m p a r e the h y d r a t i o n effect o f
Z n species. O n e expects the h y d r a t i o n e n t h a l p y (AH ~
o f Z n ( O H ) + to b e less t h a n that o f Z n b e c a u s e o f the
l o w e r c h a r g e a n d greater size o f the former. T h e - A H ~
o f Z n ( r c = 0.074 n m ) is e s t i m a t e d to b e 2 0 5 6 k J mo1-1
w h i l e the - A H ~ o f Z n ( O H ) + ( r c ~ 0.21 n m ) c a n b e
e s t i m a t e d to b e a b o u t 237 kJ mol-~, w h i c h is 8 t i m e s
less t h a n that o f Z n a n d is less t h a n the - A H ~ for K
(322.1 kJ m o l 1) a n d R b (292.8 kJ m o l 1) (Pass 1973).
T h u s o n e expects that Z n ( O H ) + h a s a low d e g r e e o f
h y d r a t i o n a n d d e h y d r a t e s easily.
T h e w a t e r c o n t e n t o f the Z n - b e n t o n i t e s at different
aw d e c r e a s e d w i t h i n c r e a s i n g p H (Figure 1). T h e degree o f h y d r a t i o n o f Z n - b e n t o n i t e at the " h i g h " p H
was l o w e r t h a n that at the " l o w " pH. T h e f o r m a t i o n
o f Z n ( O H ) + is p r o b a b l y r e s p o n s i b l e for the l o w e r water c o n t e n t o f Z n - b e n t o n i t e at the " h i g h " pH.
Vol. 46, No. 2, 1998
Zinc interactions with bentonite
135
1oo
20
80
,,Q
g
E
C
10
0
,"
.9
80
P
4o
"0
-r
a.
20
04
0.0
0.2
0.4
0.6
0.8
Water
activity
1.0
0
,
6
5
,
7
Figure 2. The hydration number of Zn retained by bentonite
at different pHs as a function of water activity: (0) pH 5.6,
(11) pH 6.3, (A) pH 6.9 and (A) pH 8.8.
The hydration number (n), which is the number of
H20 molecules bound to a single ion, was estimated
from the ratio of adsorbed water to Zn retained (H20
molecules/Zn cation). At the " h i g h " pH, n was found
to be --<7; thus the H20 molecules probably existed
only in the primary shell or coordination shell and
would be bonded to Zn by ion dipole forces or coordinate bonds. At the " l o w " pH, n was always greater
than that at the " h i g h " pH and was from 7 to 20 at
a~ from 0.12 to 1.00 respectively; here there were 2
shells in the hydration s p h e r e - - a primary shell and a
secondary shell (Figure 2). The basal spacing (001) of
Zn(OH)+-bentonite at the " h i g h " pH was from 1.21
to 1.26 nm with 1 water sheet; that of Zn-bentonite at
the " l o w " pH was 1.51 nm with 2 water sheets; the
basal spacings (001) of the bentonites saturated with
K, Ca and Mg at high pH and low pH were found to
be similar and were from 1.44 to 1.51 nm (Figure 3).
~
p
H
6
8
10 12
pH
Figure 4. The proportion of Zn extracted with 1 M chloride
solutions (twice) for Zn-bentonites prepared at different pHs:
(0) NH4C1, (m) KC1, (A) NaC1 and (A) LiC1.
Compared to K, Ca and Mg, the species and hydration
of Zn retained and the basal spacing were markedly
affected by pH.
The concentrations of Zn removed in 2 successive
extractions with 1 M chloride solutions of some monovalent cations for Zn-bentonites prepared at different
pHs are shown in Figure 4. The exchangeability of K,
Na and Li markedly decreased with increasing of pH,
but the percentage of Zn extracted successively by
NHaCI solution showed little change with increasing
of pH with respect to the other monovaient cations.
The concentrations of Zn extracted with KC1, NaC1
and LiC1 solutions were high at pH 5.6 and 6.3 (from
70 to 88%) and low at pH 6.9 and 8.8 (from 6 to 15%).
These results suggest that the Zn(OH) + retained by
bentonites was difficult to exchange with K, Na and
pH 8.3
pH 8.7
j
J
Ak~H 6.3
pH5 . . ~ k ~
4
I
9
b
8.8
].k x,.~H6-9
2
,
8
2
j
4
6
8
10 12
2
4
6
8 10 12
i
2O
Figure 3. XRD patterns of the bentonites saturated with Zn (a), Ca (b), Mg (c) and K (d) at different pHs. At a relative
humidity of 55.5%, the basal spacing (001) of the bentonite saturated with Zn at "high pH" (pHs 6.9 and 8.8) was from 1.21
to 1.26 nm; that of Zn-bentonite at "low" pH (pHs 5.6 and 6.3) was 1.51 nm; and those of bentonites saturated with Ca, Mg
and K at different pHs were from 1.44 to 1.51 nm,
136
Ma and Uren
1.0
Table 2. Characteristics of Zn-bentonite at different pH.
"High .
Species
Ionic radius (nm)
Zn retained (g kg -~)
Water in interlayer
Basal spacing (nm)
Extractabilityt
.
.
Zn(OH) +
~0.21
67.3
1 sheet
1.21-1.26
low
.
Clays and Clay Minerals
Low"
0.8 1
Zn
0.074
37.7
2 sheets
1.51
high
0.60.4-
t In 1 M KCI, NaC1 and LiC1.
0.2-
Li, and that the affinity o f Z n ( O H ) + for b e n t o n i t e s was
h i g h e r than for Zn.
B e c a u s e o f the f o r m a t i o n o f Z n a m m i n e c o m p l e x e s
at h i g h p H (Zn(NH3) 2+ > 90% at p H --> 7.0), the exc h a n g e a b i l i t y with NH4 + was h i g h e r than for K, N a
and Li. It is c o n s i d e r e d that NH4 + can e x c h a n g e with
Z n and Z n ( O H ) + s p e c i e s a d s o r b e d with b e n t o n i t e s and
the e x c h a n g e reaction b e t w e e n Z n ( O H ) + and NH4 §
m a y b e r e p r e s e n t e d as follows:
B e n t o n i t e - Z n ( O H ) + 2NH4 § ~ B e n t o n i t e - N H 4
+ Zn(NH3) 2+ + H20
[10]
There was t w i c e as m u c h Z n retained at the " h i g h "
p H than that at the " l o w " p H (Table 2) w h i c h supports
the idea that the species o f Z n retained w e r e Z n ( O H ) §
at the " h i g h " pH, and Z n at the " l o w " pH.
The greater affinity o f Z n ( O H ) + for b e n t o n i t e w a s
a s s o c i a t e d with the l o w e r d e g r e e o f h y d r a t i o n with respect to Z n (Table 2). T h e Z n ( O H ) + retained by b e n tonite w a s d e h y d r a t e d partially at the " h i g h " p H and
s o m e b e h a v e d as specifically sorbed Zn. O n e a s s u m e s
that Z n ( O H ) § in the interlayer space was c o o r d i n a t e d
directly w i t h o x y g e n ions o f the siloxane surface o n 1
layer and f o r m e d h y d r o g e n b o n d i n g b e t w e e n h y d r o x y l
and o x y g e n on the siloxane surface o f the o p p o s i t e
layer. In contrast, the Z n retained at the " l o w " p H
b e h a v e d as readily e x c h a n g e a b l e cations and e x i s t e d
in the interlayer with 2 w a t e r sheets.
D i f f u s i o n and E n t r a p m e n t o f Z n
T h e c o n c e n t r a t i o n o f Z n taken up and retained b y
C a - b e n t o n i t e i n c r e a s e d w i t h time (Figure 5). A plot o f
the p r o p o r t i o n o f Z n retained b y C a - b e n t o n i t e (Y/Ym)
versus the square root o f time s h o w s c o n f o r m i t y with
0.0
0
,'0
2'0
3o
~t (t in d)
Figure 5. A plot of measured proportion (Y/Ym) of Zn retained (Y) by bentonite from a well-stirred solution of limited
volume to the added Zn (Ym) against the square root of time
X/t (points) and theoretical diffusion curve (solid line) of
Equation [5] with D/F = 2.03 • 10 -5 d -~ and a = 4.52 •
10 3. The value of D/[2 was derived from the experimental
data YIY,n versus 1/N/t (t > 2 d) using Equation [9]. The
correlation coefficient (R) was 0.978, which was significant
at the level of probability P > 0.001. The experimental data
show approximate conformity with the diffusion model,
Equation [5].
the diffusion m o d e l f r o m E q u a t i o n [5] w i t h D / F =
2.03 • 10 5 d-i (or 2,35 • 10 -j~ sec -1) and o~ = 4.52
x l 0 3 (Figure 5). This result suggests that the interaction o f Z n with b e n t o n i t e was a d i f f u s i o n rate-limiting process. In the well-stirred system, film d i f f u s i o n
is m o s t unlikely to be rate-limiting; the rate o f the
p r o c e s s was p r o b a b l y controlled b y interlayer diffusion and s u b s e q u e n t l y by the diffusion into the ditrigonal cavities in bentonite.
The c o n c e n t r a t i o n s o f Z n in water-soluble, exc h a n g e a b l e (MgC12), and E D T A - e x t r a c t a b l e fractions
d e c r e a s e d w h i l e the c o n c e n t r a t i o n s o f Z n extracted b y
0.5 M HC1, 6 M HC1 and the residual Z n i n c r e a s e d
with incubation time (Table 3).
The Z n in residual f o r m s plus the Z n extracted with
6 M HC1, w h i c h w a s c o n s i d e r e d to b e associated with
the e n t r a p p e d form, i n c r e a s e d with incubation time.
A s s u m i n g the Z n in residual f o r m s plus the Z n extracted with 6 M HC1 in the C a - b e n t o n i t e treated at
200 ~ for 12 h is equal to the m a x i m u m o f Z n in
Table 3. The transformation of Zn added (80 mg kg -~) to Ca-bentonite at pH 7.0 and effects of heating (mg kg-t).
Incubation time (d)
Heating (200 ~
12 h)
Fraction
1
30
60
221
430
Control
20 ~
In air
In water
Water-soluble
MgC12-Zn
EDTA -Zn
0.5 M HCI-Zn
6 M HCI-Zn
Residual Zn
3.6
28
35
12
3.1
n.d.5
1-3
10
29
31
10
n.d.
1.2
8.1
25
34
12
n.d.
1.0
5.0
15
33
14
12
0.5
5.9
17
34
14
8.7
0.5
9.1
20
36
9.4
3.9
0.5
4.9
28
33
12
5.5
0.2
0.7
3.0
18
17
45
t n.d. = not detected (<0.1 mg kg l).
Voi. 46, No. 2, 1998
Zinc interactions with bentonite
0.6
~
Zn(OH)+
( ~
I--('~~~
0.4
E
137
(~
"A'~"~IO~:rtetrahedral
~"~")~("~(~ "4"" H20ininterlayer
~~,~-~41.---
t::raahed
ral
0.2
Hexagonalcavities
0.0
Figure 7. Cross-sectional view of the possible locations of
Zn(OH) § retained by bentonite at "high" pH.
1 ~0
20
310
40
~Jt (t in d)
Figure 6. A plot of measured proportion (Y/Ym) of Zn in
residual forms and extracted with 6 M HC1 (Y) to the added
Zn (Ym) against the square root of time ~ (points) and theoretical diffusion curve (solid line) of Equation [5] with DIE
= 3.12 • 10 -5 d -1 and a = 0.29. The value of D/12 was
derived from the experimental data Y/Ym versus x/~ (t < 30
d) using Equation [7]. The experimental data show approximate conformity with the diffusion model, Equation [5].
residual forms plus the Zn extracted with 6 M HCI in
the Ca-bentonite incubated with Zn at 20 ~ then the
change in the proportion o f Zn in residual forms and
extracted with 6 M HC1 to the added Zn with time can
be described by the diffusion m o d e l (Equation [5])
with DII 2 = 3.12 X 10 -5 d -1 (Figure 6). These results
suggest that diffusion in the interlayer space and into
cavities, and then further to the bottom o f cavities, led
to decreases in the activity and extractability o f the Zn
retained by the bentonite o v e r a relatively long time.
W h e n the bentonite with retained Zn was dehydrated by heating in air at 200 ~ the exchangeable Zn
transformed to E D T A - Z n , which is considered to be
associated with the specifically adsorbed Zn. The results suggest that the Zn retained by bentonite was
dehydrated in situ which increased the bonding of Zn
to the interlayer surfaces of the bentonite. With hydrothermal treatment, the adsorbed Zn m a y continue to
diffuse into the cavities and increasingly transform
into the residual forms that are analogous with the entrapped f o r m (Table 3).
The migration o f relatively small cations, such as
Li, Mg, Zn, Ni, Cu(II) and Fe(III), f r o m the interlayer
space into the crystalline matrix o f dioctahedral smecrites occurs on heating at temperatures around 200 to
300 ~ but the final location taken by migrating cations in heated smectite is uncertain. The migration o f
Li in montmorillonite was studied by IR spectroscopy
and it was suggested that the nonexchangeable Li is
likely to be found in the bottom of the ditrigonal cavities (Calvet and Prost 1971). Ben H a d j - A m a r a et al.
(1987) e x a m i n e d cation migration in N i - e x c h a n g e d
beidellite using X-ray m o d e l i n g techniques; their results suggested that Ni migrated into the vacant octa-
hedral sites. Luca and Cardile (1989) studied the
Fe(III) migration in smectites using electron spin resonance, M6ssbauer spectroscopy, and magnetic susceptibility measurements; no e v i d e n c e for the penetration o f Fe(III) into the vacant octahedral sites of montmorillonite was found. However, the penetration o f Zn
into the vacant octahedral sites of smectites is unlikely
under field conditions where the temperatures and degree of desiccation are relatively low w h e n c o m p a r e d
with the conditions referred to above.
A l t h o u g h saponite (trioctahedral) is different in
structure f r o m bentonite (dioctahedral), the Zn in water-soluble, exchangeable (MgC12) and E D T A - e x t r a c t able fractions was transformed or transferred into Zn
extracted with 0.5 M HC1, 6 M HCI and residual Zn
with heating. The similar heating effects on the retention o f Zn by both bentonite and saponite suggest that
the bottom of the ditrigonal cavities in bentonite is the
likely final site for Zn migration.
At high pH, Zn retained by bentonite existed as
Z n ( O H ) § The shape of Z n ( O H ) § is probably eggshaped and although the size o f Z n ( O H ) § seems to be
bigger than the cavities in the tetrahedral layer o f bentonite, the egg-shaped Zn(OH) § m a y fit neatly into the
cavities. The possible forms o f Zn retained by bentonite at " h i g h p H " (Zn sorbed on the external surface,
in the interlayer space and in ditrigonal cavities) are
shown in Figure 7.
CONCLUSIONS
The species, activity and extractability o f Zn retained by bentonite are affected markedly by pH. Hydroxy Zn (Zn(OH) § was retained by bentonite prepared at p H --> 6.9. The l o w e r degree of hydration of
Z n ( O H ) + relative to Zn gives rise to greater affinity o f
Z n ( O H ) + for bentonite than Zn. The basal spacing o f
Zn(OH)+-bentonite was from 1.21 to 1.26 n m with 1
water sheet; that o f Zn-bentonite was 1.51 n m with 2
water sheets at a relative humidity of 55.5%. The Zn
in Zn(OH)+-bentonite was less readily exchangeable
with 1 M solutions o f KCI, NaC1 and LiC1 and MgC12
than with 1 M NH4C1, which extracted about 80% o f
the Z n ( O H ) § retained by bentonite. C o m p l e x a t i o n o f
Zn with a m m o n i a is probably responsible for the su-
138
Ma and Uren
periority o f NHaCI. T h e activity and extractability o f
Z n r e t a i n e d b y b e n t o n i t e d e c r e a s e d o v e r a relatively
long time. The d e c r e a s e s w e r e p r o b a b l y due to interlayer diffusion and e n t r a p m e n t o f Z n in ditrigonal cavities; h y d r o t h e r m a l treatment p r o b a b l y increases the
extent and rates o f t h e s e p r o c e s s e s . The b e h a v i o r o f
Z n o b s e r v e d here can b e related to that in soils containing smectites and it m a y b e u s e d to advantage
w h e r e Z n pollution has o c c u r r e d and it is n e c e s s a r y to
d e c r e a s e the activity and m o b i l i t y o f the Z n in soils.
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(Received 24 October 1996; accepted 11 July 1997; Ms.
2821)