Clays and Clay Minerals,
Wol.45, No. 5, 733-744, 1997.
E F F E C T OF I N T E R A C T I O N B E T W E E N C L A Y P A R T I C L E S A N D
Fe 3§ I O N S O N C O L L O I D A L P R O P E R T I E S O F
KAOLINITE SUSPENSIONS
KUNSONG M A AND ALAIN C. PIERRE
University of Alberta, Edmonton, Canada
Abstract--Fine kaolinite suspensions were mixed with unaged or aged FeC13 in this experiment. The
interaction between clay particles and Fe 3+ hydrolysis products was studied by transmission electron
microscopy (TEM) and scanning electron microscopy (SEM). The proportion of Fe adsorbed was measured and the electrical charge on the clay particles was determined by electrophoresis. The effect of this
interaction on flocculation of clay suspensions was investigated in a series of sedimentation tests. The
Fe 3+ ions acted as counterions when their concentration was low and when unaged FeC13 solution was
used. Otherwise, their hydrolysis complexes acted as a bonding agent between kaolinite particles. The
dispersion-flocculation behavior of kaolinite suspensions was found to be in agreement with the theory
of Derjaguin, Landau, Verwey and Overbeek (DLVO), as the sedimentation behavior could be predicted
from the data of zeta potentials (4).
Key Words--Colloid, Electron Microscopy, Flocculation, Iron, Kaolinite, Sedimentation, Zeta Potential.
INTRODUCTION
(Blackmore 1973; E1-Swaify and E m e r s o n 1975; Reng a s a m y and Oades 1977; Young and Ohtsubo 1987)
In a previous study (Ma and Pierre 1992), the sedimentation b e h a v i o r o f a fine kaolinite suspension in
the presence of Fe 3+ electrolytes was systematically
investigated in aqueous media. The sedimentation behavior was explained qualitatively in terms of D L V O
theory (Verwey and O v e r b e e k 1948; H i e m e n z 1977).
It was shown that w h e n the repulsion o f 2 electrical
double layers around kaolinite particles was strong, the
colloidal system r e m a i n e d dispersed. As the Fe 3+ concentration increased, the electrical double layer was
compressed. Consequently, the dispersed kaolinite suspension was flocculated.
In the present study, the interaction b e t w e e n clay
particles and Fe 3+ ions was e x a m i n e d in more detail.
The variation of the electrical charge o f kaolinite particles as a function of p H and Fe 3+ concentration was
measured. The supercritical drying technique was applied to m a k e clay sediment samples with original
structure. The linkage b e t w e e n clay particles was observed by electron microscopy. The results are reported in the f o l l o w i n g paragraphs.
Understanding the interaction of metal ions with
clays in aqueous m e d i a is o f fundamental importance
to controlling the ceramic process as well as the mining process. Since Fe 3+ ions m a y be present naturally
in clay minerals or can be added artificially into a pure
clay, the interaction b e t w e e n Fe 3§ ions and clays in
aqueous media and the effect of the interaction on colloidal properties of a clay suspension have been studied extensively (Greenland 1975; R e n g a s a m y and Oades 1977; Young and Ohtsubo 1987; M a and Pierre
1992; Z o u and Pierre 1992).
A c l a y - - s u c h as kaolinite, montmorillonite or ill i t e - - i s an aluminosilicate mineral with platelike particles (Brindley 1958). The platelike particles have different crystal structures on their edges and faces. Thus,
the electrical charges on edges are often different f r o m
that on faces. Clay particles interact in edge-to-edge
(EE), edge-to-face (EF) and face-to-face (FF) m o d e s
and f o r m a " c a r d - h o u s e " structure in aqueous m e d i a
(van Olphen 1977). However, the structure characterization or the colloidal behavior depends largely on
the nature of the clay and other aqueous medium.
The b e h a v i o r o f Fe 3§ ions is v e r y complicated in
aqueous media, and has been r e v i e w e d by H e n r y et
al. (1990) and L i v a g e et al. (1988). Fe 3-- ions undertake a series of c h e m i c a l reactions through c o m p l e x ation, hydrolysis and condensation in aqueous m e d i a
to f o r m different Fe hydroxylated species, precipitates
and gels o v e r specific p H ranges. These c h e m i c a l
products interact with clay particle surfaces and
change the electrical properties of clay particles in
aqueous m e d i a significantly. Therefore, the dispersion-flocculation b e h a v i o r of clay suspensions is exceedingly c o m p l e x in the presence of Fe 3+ ions
Copyright 9 1997, The Clay Minerals Society
EXPERIMENTAL
Hydrite U F kaolinite f r o m Georgia Kaolin C o m p a ny was used in this study. The m e d i a n particle size
was 0.20 pom (Georgia Kaolin C o m p a n y 1990). This
kaolinite was processed by acidified salt washes (Rand
and Melton 1977), f o l l o w i n g the Schofield and Samson (1954) method, to r e m o v e adsorbed impurities and
convert the kaolinite to sodium form. B e f o r e any sedimentation experiment, a 0.125 M Na4P207 solution
was added to the kaolinite suspension so as to disperse
it, in a ratio of 1 m L solution to 0.5 g kaolinite. This
733
734
Ma and Pierre
kaolinite was charged negatively on both edges and
faces (Ma 1995).
Two types of FeC13 solutions were used in this
study. In the first type, FeC13 was dissolved in distilled
water at room temperature, and added to the kaolinite
suspensions just before a sedimentation experiment.
This type of aqueous FeC13 solution is called "unaged" throughout this paper. The other type of solution is called "aged", and it was prepared by the following procedure. The FeC13 was dissolved in distilled
water at room temperature at a concentration of 1.0
M. This solution was allowed to age for 1 mo before
it was mixed with a kaolinite suspension. Precipitation
occurred during aging. A part of the precipitate was
collected and dried in a desiccator for X-ray diffraction (XRD) analysis.
Kaolinite suspensions were prepared in graduated
glass cylinders with an inside diameter of 28 mm. The
Na-kaolinite content was 0.5 g in each cylinder. The
sedimentation experiments were carried out at pH 2,
4, 6, 8, 9.5, 10 and 11 with unaged FeC13 and at pH
2, 4, 6, 9.5 and 11 with aged FeC13. The pH values of
the suspensions were adjusted with diluted HC1 or
NaOH. Then FeC13 solution was added to the cylinders. At each pH level, 6 concentrations of unaged
FeC13 were examined: 0.0, 0.17, 0.33, 0.67, 1.67 and
3.33 raM, respectively, and 6 concentrations of aged
FeC13: 0.33, 0.67, 1.67, 3.33, 5,00 and 10.00 mM, respectively. The initial volume of the suspension in a
cylinder was 100 mL after it was adjusted to a required
pH value and the concentration of FeCI3. Each cylinder was covered with a piece of parafilm, then shaken
vigorously to homogenize the clay suspension. All
sedimentation tests were carried out at room temperature. When a clear interface was formed in a suspension, the position of this interface was recorded. The
final sediment thickness was defined as the thickness
after 360-h sedimentation. A height of 1 cm corresponded to a volume of 6.16 mL in a graduated cylinder.
In order to observe the structure of the kaolinite
sediments under SEM, the samples were dried by the
supercritical method. This technique is efficient to
eliminate capillary force on samples during drying and
to preserve the original structure of the sediments that
were formed. To do this, some suspension samples
were transferred from a glass cylinder to dialysis tubes
from the Spectrum Company and sedimentation was
carried out in these tubes. After sedimentation was
complete, the dialysis tubes were allowed to make a
very slow liquid exchange for ethanol. T h e n the dialysis tubes were placed in a supercritical point dryer
(Bio-Rad E3000 Series). In this dryer, ethanol was replaced by liquid COz. The temperature and pressure of
CO2 were raised above critical point values (Tr = 31
~ and Pc = 74.433 kPa (Smart and Tovey 1982)),
Clays and Clay Minerals
where it was evacuated. Finally, the supercritical-dried
samples were examined under a Hitachi S-2700 SEM.
For TEM observation, a diluted clay suspension was
dropped on a carbon-coated copper grid supplied by
the SPI Company. A JEOL-2010 analytical transmission electron microscope (ATEM) with an X-ray detector was used in this study.
The electrophoretic measurements were performed
with a particle micro-electrophoresis M A R K II apparatus from the Rank Brother Company. This piece of
equipment had a flat and rectangular cell. The electrophoretic mobility of a m i n i m u m of 5 particles was
successively measured, first when applying the electric
field in 1 direction, then after reversing the direction
of the applied electric field. For the equipment being
used, the zeta potential could be calculated from the
following formula (Plitt 1993):
4~rqu
= - x (300) 2
eE
[11]
where ~ is the zeta potential in V, ~q is the liquid viscosity in poise, e is the relative dielectric constant of
the liquid (as an approximation, the viscosity and dielectric constant of water were used), u is the average
velocity of clay particles in cm/s and E is the electric
field in V/cm. For the cell being used, E = V/3.27
where V is the applied voltage.
Because the original concentration (0.5% by mass)
of kaolinite suspensions was too dense to directly measure their zeta potential, the suspensions were diluted
to a concentration of 0.05% (by mass) with distilled
water. In each case, just before measuring the zeta potential, the pH was adjusted to the value that the suspension had before dilution, with a HC1 or NaOH solution. Depending on the sample, unaged or aged
FeC13 solution was added to the clay suspension so as
to maintain the initial electrolyte concentration existing before dilution.
The Fe concentration present in the supernatant liquid of clay suspensions was measured by atomic absorption (AA) with a Perkin-Elmer 4000AAS A A
spectrophotometer. The samples analyzed were prepared in the same way as those for sedimentation, and
we waited 12 h before performing an analysis, In clay
suspensions where flocculation occurred, the Fe content in the clear supernatant liquid was analyzed directly. In clay suspensions where flocculation did not
occur, the suspension was centrifuged until a clear supernatant liquid separated from the suspension.
RESULTS
The sedimentation behavior of kaolinite suspensions
could be classified into 3 types: 1) accumulation sedimentation; 2) ttocculation sedimentation; and 3)
mixed accumulation-flocculation sedimentation. As an
example, the dynamics of kaolinite sedimentation in
the presence of unaged and aged FeCI3 at pH 4 are
Vol. 45, No. 5, 1997
Colloidal properties of a fine kaolinite with Fe3+ ions
735
16
Unaged FeC13 eoneenu'ation
14
O o.0o mM
9
0.67 mM
[] 1.67mM
9 3.33mM
[]
tn
tA
,-,
l
,
0
100
200
300
400
(a)
0
i
I
i
500
600
T i m e (rain.)
16
Aged FeC.l 3 concentration
14
12
0
o.oo mM
9
1.67mM
[] 5.00 mM
10
9
10.00ndd
4
~..~,~
""
0
_
100
E
m
i
ass
,
200
- -
-
0
,
!
300
400
0
(b)
500
601
Time (min.)
Figure 1. Sediment kinetics of 0.5% Na-kaolinite suspensions at pH 4; a) displacement of sharp interfaces with unaged
[FeCI3] = 0, 0.67, 1.67 and 3.33 mM; b) displacement of sharp interfaces with aged [FeC13] = 0, 1.67, 5.00 and 10.00 mM.
reported in Figures l a and lb, respectively. Typical
accumulation sedimentation occurred when a relatively stable suspension was achieved, such as that without
any FeC13 and with unaged FeC13 concentration of 3.33
mM or with aged FeCI 3 concentration of 5.00 raM.
Heaviest particles settled first under gravity and the
sediment was established at the bottom of a cylinder.
The thickness of the sediment increased as sedimentation time lapsed. Typical flocculation sedimentation
occurred with unaged FeC13 concentration of 0.67 m M
or with aged FeC13 concentration of 1.67 mM. A sharp
interface between the sediment and the clear supernatant liquid was observed and this interface kept
moving downward until equilibrium was established.
Mixed flocculation-accumulation sedimentation was
observed in the suspensions with unaged FeC13 concentration of 1.67 m M or with aged FeC13 concentration of 10.00 raM. In this case, sedimentation followed
the dynamic of an accumulation sedimentation at the
beginning. Then it acted like that of a flocculation sedimentation. These different kinds of sedimentation behaviors were characterized in Ma and Pierre (1992).
Figure 2 shows the final sediment thickness of suspensions with different concentrations of unaged and
aged FeC13 at pH 2 and 9.5. At pH 2, both with unaged
or aged FeC13, flocculation sedimentation occurred,
while accumulation sedimentation occurred at pH 9.5.
In both cases, the final sediment thickness increased
736
Ma and Pierre
4.0
3.5
~
with the concentration of unaged or aged FeC13. Unaged FeC13 increased the thickness m o r e effectively
than aged FeCI3.
W h e n unaged Fe 3+ electrolytes were m i x e d with kaolinite suspensions, the Fe 3+ ions undertook hydrolysis
in the aqueous media. Ferric hydrolysis c o m p o u n d s
did precipitate on the clay particle surfaces, and Figures 3a and 3b show the T E M observations of interaction o f such hydrolysis products with kaolinite particles. The energy dispersive X - r a y ( E D X ) analyses o f
selected areas, represented by small circles in Figures
3a and 3b, are also reported in Figures 3c and 3d. T h e
Fe precipitates had the appearance of rodlike or platelike particles (Figure 3a), eventually joining to each
other to f o r m a thin film (Figure 3b). The corresponding E D X analysis indicated that these particles and
films were Fe compounds.
We m e n t i o n e d previously that precipitation o c c u r r e d
in FeC13 solutions during aging. The X R D analysis
indicated that these precipitates included ferric hydroxides ( F e O O H ) and ferric oxides (hematite Fe203
and FezO3-H20) ( M a 1995). By A A analysis, w e found
Unaged FeCI3
3.0
~o 2.5
Feel3
0.
.Ig
"w, 2.0
..
1.5
1.0 (
i.C3/[$3""
0.5 ,,,
Unaged FeCl~
Aged FeCls
0.0
~l-t -I- ~'I-.-- -,- - . - - -,- D
0.0
0.5
1.0
1.5
I , ,
2.0
2.5
,
I
3.0
Clays and Clay Minerals
,
3.5
Fe Concentration (mM)
Figure 2. Sedimentation behavior of 0.5% Na-kaolinite suspensions as a function of the concentration of unaged or aged
FeC13. The solid lines represent the final sediment thickness
by flocculation sedimentation at pH 2; the dashed lines represent the final sediment thickness by accumulation sedimentation at pH 9.5.
100
K~
(c)
75
Si K,~
AI I,
Z
Fe K=~
50
Z
Cu K=j
25
0
2
0
4
6
8
I0
KeV
200
(d)
Fe Kc~l
150
[-,
Z 100 O Ka
[..,
Z
I Cu IGI
50
[ ,hi
[Ill
CI.Ka
:1
o.
0
Cu KaI
Fe Ka2
Cu Kco
SiK~
.
2
.
" J
.
4
6
8
"~'~
10
KeV
Figure 3. TEM micrographs of the Fe materials deposited on the kaolinite particles in sediments made with 10 mM FeC13
at pH 9.5: a) rodlike and platelike particles; b) thin film; c) EDX spectrum of the surface of the kaolinite particles corresponds
to circle in (a); and d) EDX spectrum on the surface of the kaolinite particles corresponds to circle in (b).
Vol. 45, No. 5, 1997
Colloidal properties of a fine kaolinite with Fe3§ ions
737
4O
Inidalunag~FcCl3c~176
0 0.33
"Z
35
om
9 1.67
.~ 30
E
r~
25
20
t_
.<
5
0
(a)
0
2
3
4
5
6
7
8
9
10
11
12
pH
80
Original aged FeC:l3 eonce,ntralion (raM)
70
0 0.33
[]
3.3
"~ 6o
a
50
E
--D
[330
20
<
A
A
10
(b)
00
I,
1
~
2
3
,
4
5
6
I,
7
I,
$
i O,
9
i
10
|
11
pll
e
12
Figure 4. Amount of Fe deposited on the kaolinite particles from FeC13, at different pH: a) from unaged FeC13; b) from
aged FeC13.
that Fe concentration in the liquid had decreased from
0.5 to 0.33 M. In clay suspensions mixed with aged
FeC13 solution, ferric hydroxides and ferric oxides deposited on the clay particles. The amount of Fe deposited on the kaolinite particles, as a function of pH
and concentration of unaged FeC13 or aged FeCI3, is
reported in Figure 4. It appears that the amount of Fe
deposited on kaolinite particles from unaged FeC13 increased with the initial Fe concentration and pH. On
the contrary, the amount of deposited Fe from aged
FeCI 3 was almost independent of the pH.
When the pH and Fe 3+ concentrations in clay suspensions were changed, the association mode of clay
particles changed. Figure 5 shows the SEM observations after a supercritical drying of kaolinite sediments
made with different concentrations of unaged FeCI3 at
different pH. Without any FeC13 at pH 9.5 (Figure 5a),
clay particles were compacted. Most of the particles
were connected randomly. Without any FeCI 3 at pH
4.0 (Figure 5b), the sediment was relatively dense.
Some kaolinite particles appeared to be randomly connected. However, FF and EE associations can also be
observed. With the unaged FeC13 concentration of 0.67
m M at pH 4.0 (Figure 5c), the sediment was very
loose. Most clay particles were associated in the EE
mode. With a higher unaged FeC13 concentration (3.33
mM) at pH 4.0, Figure 5d shows a very densely
packed structure. Almost all of the kaolinite particles
were associated in the FF mode.
Figures 6a and 6b show the dependence of zeta potentials on the pH and concentrations of unaged and
aged FeC13 in kaolinite suspensions. The zeta potentials increased with the concentration of unaged or
aged FeC13 at all pH values. However, the addition of
unaged FeC13 resulted in a faster increase of the zeta
potential than aged FeCI 3 did.
738
Ma and Pierre
Clays and Clay Minerals
Figure 5. SEM micrographs of kaolinite sediments made at 10 kV: a) the accumulated kaolinite sediment without FcCI 3 at
pH 9.5; b) the accmnulated kaolinite sediment without FeCI.~ at pH 4; c) the flocculated kaolinite sediment made at pH 4
with unaged FeCl~ at a concentration of 0.67 mM; d) the accumulated kaolinite sediment made at pH 4 with unaged FeCI 3
at a concentration of 3.33 raM.
The o b s e r v e d s e d i m e n t a t i o n b e h a v i o r o f kaolinite
s u s p e n s i o n s with u n a g e d FeC13 is s u m m a r i z e d in Figure 7a, while that for aged FeC13 is s u m m a r i z e d in
Figure 8a. D a s h e d lines w e r e d r a w n to outline the
range o f c o n d i t i o n s w h e r e e a c h type o f s e d i m e n t a t i o n
occurred. F r o m the d e t e r m i n e d values o f zeta potential, a d i a g r a m s h o w i n g 3 d o m a i n s c o u l d b e established a c c o r d i n g to the f o l l o w i n g standards: 1) The
Vol. 45, No. 5, 1997
Colloidal properties of a fine kaolinite with Fe3+ ions
739
3~I
20
g
_
/
/
/ /
//
d
/, r
^
7 -2
-302~
--~
0.0
I
0.5
,
I
1.0
,
~
1.5
,
t
2.0
~
: 2.5
I
~
3.0
I (al
3.5
4.0
Unaged Feel 3 Concentration (mM)
40
30
~7~'~
-3(]
9i ~
1
0
I
1
1
]
2
1
I
3
I
I
4
I
]
I
5
I
6
I
|
7
9
pH=9.4
A
pHffill
I
]
8
I
]
9
(b)
I
!
10
'
I
11
9
12
Aged FeCI 3 Concentration (mM)
Figure 6. Variation of the zeta potential with the concentration of FeC13 at different pH values in 0.5% kaolinite suspensions
treated with Na4P207: a) unaged FeC13; b) aged FeCI3.
first domain, with data points represented by open
dots, corresponds to a zeta potential absolute value [El
< 10 inv. 2) The second domain, with data points
represented by solid dots, corresponds to I~] > 15 mV.
3) The third domain, represented by semi-open dots,
corresponds to data points such that 10 mV --< I~1 --15 mv. These diagrams from the zeta potential are
reported in Figure 7b for kaolinite suspensions with
unaged FeCI3 and in Figure 8b with aged FeC13. When
comparing Figure 7a with 7b or Figure 8a with 8b, it
appears that the diagrams constructed from the zeta
potential are similar to those from the experimental
sedimentation behaviors.
DISCUSSION
In the present study, the sedimentation behaviors of
kaolinite suspensions were classified into several
types. Since kaolinite particles have a platelike shape,
these particles can be associated in the 3 basic fashions: EE EE and FE, which were summarized by van
Olphen (1977). The SEM observations of sediments
clearly show that the different sedimentation behaviors
were related to the different architectures of clay aggregates, depending on the concentration of Fe 3§ and
pH (Figure 5). The present important result is that the
sedimentation behaviors of kaolinite suspensions appeared to be predictable from the value of the zeta
740
Clays and Clay Minerals
Ma and Pierre
12
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Unaged FeCI 3 concentration (mM)
(a)
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10
9
$
\
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es.
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Unaged FeCI 3 concentration (mM)
(b)
Figure 7. Diagram of the sedimentation behavior in 0.5% kaolinite suspensions mixed with unaged FeC13: a) observed
results; b) predictions from the measured zeta potential, 4. The open dots are for I~l < 10 mV, the solid dots for I~l > 15 mV,
the semi-open dots for 10 m V -< IX[ <- 15 mV.
Vol. 45, No. 5, 1997
Colloidal properties of a fine kaolinite with Fe 3§ ions
9 9 i/o'~ i
ii ~Accumulate~'
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9
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(a)
12
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10
9
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l
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4
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I
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1O 11
(raM)
(b)
Figure 8. D i a g r a m of the sedimentation behavior in 0.5% kaolinite suspensions m i x e d with aged FeC13: a) o b s e r v e d results;
b) predictions f r o m the m e a s u r e d zeta potential, 4. The open dots are for
< 10 mV, the solid dots for
> 15 mV, the
semi-open dots for 10 m V -<
-< 15 mV.
141
Ir
141
742
Ma and Pierre
Clays and Clay Minerals
Figure 9. Schematic illustration of variations in the sedimentation behavior of diluted fine kaolinite suspensions with the Fe
concentration.
potential. This is to say, the dispersion or flocculation
behavior of clay particles was globally consistent with
the DLVO theory.
In the present study, kaolinite was treated with
NaaP207 so that the platelike particles were negatively
charged both on their edges and faces. Hence, Fe 3+
acted as counterions to the clay particles, and it is possible to argue that several types of sedimentation behavior, due to a changing mode of particle association,
should occur as the concentration of Fe 3§ increases.
These types of sedimentation behaviors are illustrated
in Figure 9.
Without Fe 3§ or with a low Fe 3+ concentration in
clay suspensions at a high pH, clay particles had a
relatively high surface potential (in absolute magnitude) and a thick electrical double layer (Figure 6a).
That is to say, a strong repulsion between the clay
particles operated. No aggregation occurred and the
clay suspensions were stable. Only accumulation sedimentation occurred (Figure 9a). During sedimentation, the heaviest particles settled first under gravity
and then the smaller particles took a long time to settie. The clay particles formed an accumulated sediment at the bottom of a cylinder. The sediment was
relatively dense. In terms of the particle association
mode, random packing occurred (Figure 5a).
As the Fe 3§ concentration increased, the Fe 3§ counteflon compressed the electrical double layer around
the clay particles. Thus, the magnitude of the zeta potential was significantly reduced. When - 1 0 mV <
< 0 mV, the EE association of the clay particles was
favored according to theoretical calculations based on
the DLVO theory (Ma 1995). The clay suspensions
were unstable and only flocculation sedimentation oc-
curred (Figures 6a and 7a). During flocculation sedimentation, as suggested by Michaels and Bolger
(1962), clay particles formed very porous flocs, and
these flocs joined to each other, from wall to wall in
the glass cylinder, so as to constitute an apparently
uniform sediment, separated by a sharp interface from
the clear supernatant liquid. The sediment kept shrinking during sedimentation, but it would still occupy a
significantly large space after a long time, in agreement with the present experiment (Figures 2 and 5c).
However, the transition from flocculation sedimentation to accumulation sedimentation was progressive.
In some Fe 3§ concentrations and pH levels, accumulation sedimentation was in competition with flocculation sedimentation so that a mixed sedimentation
was observed (Figure 9b). The heaviest kaolinite particles settled quickly at the beginning so that the sedimentation process appeared by accumulation. After a
while, the remaining particles had time to focculate.
They formed a flocculated sediment, so that the second
step of sedimentation was by flocculation. Finally, the
sediment was composed of a randomly accumulated
sediment at the bottom of the glass cylinder and a
flocculated sediment over it.
Fe 3+ ions are strongly hydrolyzed in aqueous media.
Depending on the pH, a FeC13 solution may contain
monomers such as [Fe(OH)(OH)2)5] 2+ or polycations
such as [Fe403(OH2)412~ +. The precipitates that can be
formed include c~-FeO(OH) and [3-FeO(OH), and the
oxide a-Fe203 (Livage et al. 1988; Henry et al. 1990),
depending on the pH. As mentioned by Blackmore
(1973), these hydrolysis products can themselves adsorb onto clay particles (Figure 4) and coat their surface (Figure 3). Since the point of zero charge (p.z.c.)
Vol. 45, No. 5, 1997
Colloidal properties of a fine kaolinite with Fe 3+ ions
of Fe minerals is in a range f r o m 7 to 9 (Schwertmann
and Taylor 1989), the sign o f electric charges of clay
particles can reverse w h e n the Fe 3+ concentration increases, in agreement with our result (Figure 6a). In
this case, the Fe 3+ hydrolysis species are no longer
counterions to the clay particles. Rather, they can act
as a bonding agent to bridge kaolinite particles to each
other. Our observations are consistent with this v i e w
as they showed that the kaolinite particles were associated mostly according to the F F mode. Consequently, dense aggregates formed and they easily settled under gravity. The formation o f a v e r y dense or packed
accumulated sediment was observed. In the sedimentation test, a n e w transition f r o m flocculation sedimentation to accumulation sedimentation (Figure 9e) was
o b s e r v e d as the FeC13 concentration increased. Again,
competition b e t w e e n flocculation sedimentation and
accumulation sedimentation was o b s e r v e d for intermediate conditions (Figure 9d). In this case, the sedim e n t was c o m p o s e d of a packed accumulated sedim e n t at the bottom of the glass cylinder and a flocculated sediment o v e r it.
The above evolution in the architecture of clay sediments as the F e concentration increased was confirmed w h e n aged FeC13 was used. If the sedimentation
behaviors with unaged and aged FeC13 are compared,
it appears that the field of conditions where accumulation sedimentation occurred was enlarged with aged
FeC13 toward low Fe concentration and high p H (Figures 7a and 8a). In fact, the Fe amount adsorbed onto
the clay particles was l o w e r f r o m unaged than f r o m
aged FeC13 (Figure 4). Also, the Fe amount adsorbed
by kaolinite from unaged FeC13 d e p e n d e d on the pH,
because an increasing p H is k n o w n to accelerate the
hydrolysis of Fe 3+ ions. However, the hydrolyzed process of Fe 3+ was c o m p l e t e d after aging so that the
amount of Fe-adsorbed Fe on kaolinite was eventually
independent of the pH. Partially, the population of
highly charged Fe species in solution was less with
aged Fe electrolyte solutions than with u n a g e d solution. Consequently, the electrical double layer was less
c o m p r e s s e d with the aged Fe electrolytes. In turn, the
v e r y porous floc with the E E association was less frequent with the aged Fe electrolytes. As a result of this
difference, the final sediment v o l u m e was always
higher with u n a g e d than with aged FeC13 (Figure 2).
However, since the aged Fe electrolytes coated the kaolinite particles m o r e easily (Figure 4), the electrical
charge on the particles changed sign for a l o w e r
amount of aged than unaged FeC13. Also, the zeta potential increased faster with the aged Fe electrolytes,
which explains that packed accumulation sedimentation occurred with a lower concentration of aged than
unaged FeC13 (Figures 7a and 8a).
In summary, the sedimentation behavior of the kaolinite suspensions could first be qualitatively explained in terms of the D L V O theory, as a function of
743
the Fe concentration. Secondly, the data o f zeta potentials could be used to predict the type o f sedimentation
behaviors. In practice, flocculation sedimentation occurred w h e n I~1 < 10 mV, accumulation sedimentation
w h e n 141 > 15 mV, and m i x e d f l o c c u l a t i o n - a c c u m u lation sedimentation w h e n 10 m V --< 10 --- 15 mV.
CONCLUSION
T h e present study showed that F& + and the hydrolysis products acted as counterions to flocculate clay
particles w h e n the particles were charged negatively
on both their edges and faces. These F e products also
acted as a bonding agent to connect the clay particles.
The sedimentation in kaolinite suspensions can o c c u r
according to 1 o f the following 3 behaviors: 1) accumulation; 2) flocculation; or 3) m i x e d fiocculation-accumulation. The different types o f clay particle associations o b s e r v e d under the electron m i c r o s c o p e were
consistent with their sedimentation behaviors. This
sedimentation b e h a v i o r could be explained according
to the D L V O theory and related to the value of the
zeta potentials o f the clay particles.
The current results can be used to i m p r o v e the castability and sinterability o f ceramics products. Also,
they are applicable in environmental science and engineering, such as in the treatment o f oil sand tailing
sludge.
ACKNOWLEDGMENTS
The authors are very grateful to the Natural Sciences and
Engineering Research Council of Canada (NSERC) for funding this research on an operating grant.
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(Received 16 August 1996; accepted 25 January 1997; Ms.
2805)