NMR (Bottero et al., 1987), small angle X-ray scattering (SAXS) (Bottero et al., 1982b, .... superficial charge and the stacking of the sheets in the tactoids in the ...
Clay Minerals (1988) 23, 213-224
INTERACTIONS BETWEEN HYDROXYA L U M I N I U M SPECIES A N D H O M O I O N I C N A - A N D C A - M O N T M O R I L L O N I T E P A R T I C L E S , AS M A N I F E S T E D BY ~ P O T E N T I A L , S U S P E N S I O N STABILITY AND X-RAY DIFFRACTION J. Y. B O T T E R O ,
M. B R U A N T
AND J. M. C A S E S
Centre de Recherche sur la Valorisation des Minerais (ENSG)--U.A. 235 (CNRS), B.P. 40, 54501 Vandoeuvre C~dex, France (Received 30 March 1987; revised 26 February 1988)
ABSTRACT : The interactions between AllaO4(OH)24(H20)~~ and higher molecular weight hydroxy-Al species with homoionic Na- and Ca-montmorillonite have been studied by measuring adsorption of the hydroxy-Al on to the clay, turbidity of the resulting suspensions, electrokinetic potential, and d(001) basal spacing. The isolated "Alia" ions are adsorbed according to a cation-exchange process which causes flocculation of the tactoids at low concentrations. At higher concentrations, the adsorption of either isolated "All3" or/and higher molecular weight species is mainly responsible for the dispersion of clay particles with a net positive surface charge (~ ~ + 50 mV). Consequently, the tactoids are destroyed.
In aqueous solution, hydroxy-A1 species have a strong affinity for smectites and formation of Al-interlayers modifies the swelling properties. The specific surface area of Al-clay can be 10 times that of pure Na- or Ca-montmorillonite and then the solid becomes a molecular sieve which has been studied for its catalytic properties (Lahav et aL, 1978; Lahav & Shani, 1978; Occelli & Tindwa, 1983; Pinnavaia et al., 1984; Plee et al., 1985), and agricultural applications (Rengasamy & Oades, 1978; Oades, 1984). Often the interaction mechanisms at the molecular scale have not been described well because the nature of the AI species was not correctly known. Recently, Plee et al. (1985), using 27A1 solid-state high-resolution nuclear magnetic resonance (NMR), showed that the AllaO4(OH) 7+ polycation has a high affinity for the interlamellar space of smectites producing a basal spacing d(001) ~ 1.7-1.8 nm. The polycation is thus protected from the aqueous environment. Recent studies on aluminium chloride hydrolysis products using 27A1liquid N M R (Akitt et al., 1972; Akitt & Farthing, 1978; Bottero et al., 1980, 1982a), 27A1 solid-state high-resolution N M R (Bottero et al., 1987), small angle X-ray scattering (SAXS) (Bottero et al., 1982b, 1987; Axelos et al.,1985, 1986), show that Alia ions have different states of association according to the hydrolysis ratio, r = [OH]/[AIr] where [AIT] is the total A1 concentration. The isolated Al13 polycations predominate in the A1 concentration range 10-3 M - 10-1M when r = 2-2.2. The Al13 polycations are aggregated in high-weight molecular species (molecular weight > 64,000) for r = 2.5. These aggregates possess a fractal geometry (Axelos et al., 1985, 1986). 9 1988 The Mineralogical Society
J . Y . Bottero et al.
214
Results are presented of an investigation of the affinity of these two A1 structures (isolated and aggregated A113 polycatiGns) for Na- or Ca-homGionic montmGrillonite. The subsequent effects on the stability of the suspensions are analysed with a view to optimizing water treatment by coagulation. MATERIALS
AND EXPERIMENTAL
METHODS
Clay and aluminium preparation The clay used was montmGrillonite (Clay Spur 26 from Ward's Natural Science). The impurities were removed by centrifugation and the clays made homoionic by treatment with 1 M NaC1 or 0-5 M CaCI2 solutions. The clay suspensions were washed with distilled water until the equilibrium solution was free of chloride ions, and then the solid was dried at 110~ for 24 h. The cation exchange capacity (CEC) measured by the classical ammonium exchange method at pH = 7-0 was 95 mEq/100 g. The specific surface area, calculated from N2 adsorption on samples dried at ll0~ for 24 h at 10-2 torr, was 44 m 2 g-1 for Namontmorillonite, and 39 m E g-1 for Ca-montmorillonite. The All 3 solutions were prepared by hydrolysing aluminium chloride salt (BotterG et al., 1980). The OH/A1 ratios were r = 2.2 and 2.5, only one-day Gld solutions were used, and the A1 concentrations varied from l0 -4 M to 10 -1 M. The Alia content in A1 solutions was determined by the solution state 27A1 N M R technique (Fig. 1). For r = 2.2, the A l i a concentrations were 92% ([AIT] = 10 -1 M and 10-2 M) and 85% ([AIT] = 10 -3 M). The other ions were essentially the monomers A1 (H20) 3+, AI
I [Ati]/[All]
•
lOOp,~.
r
T
",~\. o ....... ~k
8O1
,;/D! ,J" ii\!
Ar 0 o.::
,,7/ ill '/,i
/ ,
'\" ,7 i ',~'~\ V,'/ 9i
6oh/
/
'
|.-"
l-" 0
\
I 05
',~" I \1
',\'\ I 1
.
I 15
i','~ \.
I \ \N\ 2 2.5
I 3
r=[o,,,/E,,~,
FIG. 1. Distribution of the different aluminium species (Ali) from solution state 27A1 N M R vs. the total AI concentration (AIT) and the hydrolysis ratio r = (OH)/(AIT). - . . . . (AIT) = 10-1tool 1-1 ; (AIT) = 10 -2 tool 1-1 ; . . . . . . . (A1T) = 10 -3 tool 1-1.
Interactions between h y d r o x y - A l species and montmorillonite
215
(H20)5 OH 2+ and A1 (H20)4(OH)~. For r = 2.5, the isolated Al13 polycation concentration is 40~ ([AIT] = 10-1 M), 62~ ([A1T]= 10-2 M) and 70~ ([A1T]= 10-3 M) (Bottero et al., 1980, 1982b). The remaining A1 ions formed aggregates which could not be detected by liquid 27A1 NMR. With 27A1 solid-state N M R (Bottero et al., 1987) it has been shown that these aggregates are composed of about 70~o polymerized All 3 species and 30~o layered octahedral A1 after aging for 24 h, and possess a fractal structure with a dimension DI ~ 1.9 (Axelos, 1984), and a radius of gyration of 14 nm (Bottero et al., 1982a).
Flocculation and adsorption
The concentration of solids for all the suspensions used was 20 g 1-1 with an ionic strength of 1.2 10-3 M (NaC1) or 0.6 10-3 M (CaC12). 90 ml of A1 solution (r = 2.2 or r = 2-5) were added to 10 ml of Na- or Ca-montmorillonite suspension and stirred (100 r.p.m, at 25~ for 2 h. The ionic strength depends on the initial AI concentration at high values. An aliquot of each suspension was allowed to settle for 30 min, and the turbidity of the supernatant was measured with a H A C H 2100 A turbidimeter, the values being expressed in relative units. The other part of the suspension was used to measure the electrokinetic potential, adsorption and basal spacing. The electrokinetic potential (mV) was measured with a model 501 A Laser Zee Meter (Pen Kem Inc) at the pH and ionic strength of the solutions. The A1 equilibrium concentration was determined by atomic absorption, after centrifuging (73,000 G for 30 min). The adsorption isotherms were constructed by plotting the adsorbed amount, Q, (mmol g-l), calculated from the difference between the initial and equilibrium quantities of A1 in solution, vs. equilibrium concentration. The (001) reflection was obtained by X-ray diffraction after drying for 24 h at 110~ and storage under atmospheric conditions, using Jobin-Yvon Sigma 2000 apparatus and Cu-K~ radiation.
RESULTS The adsorbed hydroxy-A1 species at the clay-aqueous solution interface modified the solid superficial charge and the stacking of the sheets in the tactoids in the c-direction.
Na-saturated montmorillonite AI solution; r = 2.2 (Fig. 2).
Three regions can be defined according to the equilibrium concentration Ce: (i) for Ce ~ 4.10 -5 mol 1-1 and < 1.4.10 -3 mol g-l, Qa increases sharply to 2 mmol g-a, increases from - 3 7 mV to +48 mV. The suspension is flocculated (turbidity = 2 FTU) and d(001) increases to 1.56 nm (Fig. 2, Table 1A). For Ce ~ 1-3.10 -4 mol 1-1 or Qa = 1.25 mmol g-a, the isoelectric point (IEP) is attained (flocculation is at a maximum) and the stacking order decreases (Fig. 3); (iii) for Ce > 10-2 mol 1-1, the latter suspension is re-dispersed and a high stacking disorder is observed (Fig. 3e). The ~ potential is + 55 mV (Fig. 2).
216
J . Y . Bottero et al. C(mv)
"'~176
.,,,,,Ill
.
T(FTU) 9
600t
~00~[--i
,
i I IIIIII
~.....~
I I
.-
~.-
1.8~d001(nm)
3~ Oa(mm~
2~
(o)~__--.--J~ z 3~7B9
10_s
z ~Ls6"~lg
10-~
(')
z 3~s6789
10-3
z ~s6"~ ~
10-2
(mot/t)
FIG. 2. Adsorption on to Na-montmoriUonite. Plot of the adsorbed amount Q= (O), zeta
potential ~ (O), d(00t) ( 2.2.10 -4 and < 1-25.10 -3 mol 1-1, the I E P is a t t a i n e d for Qa ~ 0 . 7 5 m m o l g-1 or Ce ~ 3.10 -4 mol 1-1 w h e n ( increases to + 4 5 m V and a higher s t a c k i n g disorder is o b s e r v e d (Fig. 5). T h e dispersion appears f r o m Ce ~ 10 -3 mol 1-1 (Fig. 4). (iii) F o r Ce > 1.25.10 -3 mol 1-1, Qa is ~ 2 m m o l g - l , but ~ increases again to + 55 mV, the stacking disorder increases (Fig. 5) and the suspension is highly dispersed.
Interactions between hydroxy-Al species and montmorillonite (00=)
Q,,x 0.25 m molg-1
(a) Q--O.35mmolg-1
(b) -- 1.67 m tool g-1
(c)
:1.85
mmolg
-1
(d) 2.07m
tool
g-1
(e)
r 0.33
; 0.4
I 0.5
I 0.66
I 1
I 2
d(nm)
FIG. 3. X-ray diffraction patterns of Na-montmorillonite/Al(OH)2.2 sample after drying at 110~ for 24 h. The (001) and hk bands are indicated. The position of the (001) peaks varies with the a m o u n t of All3 adsorbed.
C(mv) o--oo~-~
"'~
FU.E
.... ,111 T(FTU)
M
600 400
20~ 0
II] d001(nm)
.
.
.
.
,Ill
.
1.8
1., 1,0
, , I
-3
I
3! O.a(mmot./g)
,
i i
,
(d)
2
i
(e)
,%
(f)
1 10-5
2 3t. 56789 2 3 ~.56759 2 3/.56789 2 3LSG78 10-L 10-3 10-2 Ce(mot/t)
FIG. 4. A d s o r p t i o n on to Ca-montmorillonite. Plot o f the adsorbed a m o u n t Qa ( O ) , zeta potential ~ (C)), d(001) (~-) and t u r b i d i t y ( ~ ) vs. A I e q u i l i b r i u m concentration in r = 2.2 solution.
217
218
J.Y.
Bottero et al.
Q :0.08mmo|,g -1
(a) Q =0.33mmol.g-1
(b) Q =1,69retool.g-1
(c) Q~:2.075 retool.g-1
(d) (e) Q:2
m mol.g-1
(f)
1 0.33
0.4
0.5
1
0.66
d(nm)
2
FIG. 5. X-ray diffraction patterns of Ca-montmorillonite/Al(OH)2.2 sample after drying at 110~ for 24 h. See FIG. 3 for allocation of the peaks. TABLE lB. Data for the Ca-saturated montmorillonite/hydroxy-A1 (r = (OH/AI) = 2.2) system. Alequilibrium 2.10 -5 3-4.10 -s concentration Ce (tool 1-1) Adsorbed amount 0-08 0-25 Qa (mmol g-l) Turbidity 10 15 T (FTU) Zeta potential - 18 - 16 (mV) pH 5.3 5.3 Interlamellar 1.42 1-23 distance d(001) (nm)
4.2.10 -5
2.10-4
2.2.10 -3
1-25.10-3 3.25.10-3
0.34
0-35
0-39
2.07
14
6
5
- 17
-6
5-1 1.46
4.9 1.48
5.10-3
5.10-2
1.97
2.03
2.05
36
18
25
720
- 10
45
50
55
55
4.4 1.58
4.4 1.84
4.3 1-66
4.3 1660
4.1 1.25-1-60
N a - s a t u r a t e d m o n t m o r i l l o n i t e / h y d r o x y - A l ; r = 2"5 (Fig. 6, 7, T a b l e 2A). (i) F o r Ce < 7 . 1 0 -5 m o l 1-1, Qa is low ( < 0 . 1 5 m m o l g - l ) b u t d ( 0 0 1 ) i n c r e a s e s slightly to 1.25 n m . T h e s u s p e n s i o n is s t a b l e a n d ~ is n o t m o d i f i e d (Fig. 6). (ii) F o r Ce > 7 . 1 0 -5 m o l 1-1 a n d < 2 . 1 0 -~ m o l 1-1, f l o c c u l a t i o n o c c u r s a n d t u r b i d i t y falls, b e i n g a t a m i n i m u m at t h e I E P (Ce ~- 1.3.10 -~ m o l 1-1 or Qa ~ 2 . 6 m m o l g - l ) . T h e s t a c k i n g d i s o r d e r also i n c r e a s e s (Fig. 7c).
Interactions between hydroxy-Al species and montmorillonite ~(mv)
-40'~ o-o-c-o-oo\RIE 0
~b'
T(FTU)
I 600 r
/~-~---
,oo~ .oo~,,Hl
~I'~176176! o
~.~
,.,,HI ,,,,,j .
:..,'~..--_~,,,'/'E.,,,HI, , , , ,
I0~2 a~s67~9 z 3~S6mO 3, 3,s6~9 2 3~s67 Ce~moL/t) 10-~ 10-2
FIG. 6. Adsorption on to Na-montmorillonite. Plot of the adsorbed amount Q, (ll), zeta potential ~ (O), d(001) g~-), and turbidity (:)~)vs. AI equilibrium concentration in r = 2.5 solution.
.i,'~
.f
(a)
j/t;, .....
I
d (nm> I 0.33
I 0.4
I 0.5
I 060
I 1
I 2
FIG. 7. X-ray diffraction patterns of Na-montmorillonite/Al(OH)2.5 sample after drying at ll0~ for 24 h. See FIG. 3 for allocation of the peaks.
219
220
J.Y.
Bottero et al.
TABLE 2A. Data for the Na-saturated montmorillonite/hydroxy-A1 (r = (OH/A1) = 2.5) system. Alequilibrium concentration Ce (tool 1-1) Adsorbed amount
1.25.10-5 3-28.10-5 5.32.10-5 1-26.10-4 6.32.10-4 1.58.10-3 5-15.10-3 1.9.10-2 3.5.10-2
0.08
0.09
0.15
2.64
3.28
3.71
6'43
8-5
18-5
Turbidity
130
--
130
15
425
425
425
520
650
Zeta potential (my) pH Interlamellar distance d(001) (nm)
- 34
- 34
- 35
11
40
40
42
52
50
6.8 1.12
6-5 I. 19
6.7 1.22
4-7 --
4"8 1"70
4-8 1"84
Qa(mmolg-l)
T (FTU)
4-8 4.8 4-8 high high high disorder disorder disorder
r.(mv) - 4 0~--o--,.. o
RI.E
0
T(F.TU)
600 400
~
~
~---o--~
J .
,.2
~
..,,,ll]
..,,,iLl
.. ,,,ill
, , ,,,I
.,,,HII
,,,,,ll
,,,,H
16tQa(mm~) ,0, ,~ i0_s z 3~s6n~ ~ ~SS~O_3~ ~s67~s 2 ~ss7 10-4 I0-2
C~mo~.IL)
FIG. 8. Adsorption on to Ca-montmorillonite. Plot of the adsorbed adsorb~ amount Q, (O), zeta potential { (O), d(001) (-(~-), ('(~-), and turbidity I S ) vs. A1 eq equilibrium concentration in r = 2.5 Ly (~t:) solution. (iii) F o r Ce > 2 . 1 0 -4 m o l 1-1, t h e s u s p e n s i o n is d i s p e r s e d ( t h e t u r b i d i t y is 620 F T U for Ce = 3 . 1 0 -2 m o l 1-1), ~ = + 5 2 m V , a n d t h e s t a c k i n g d i s o r d e r is a t a m a x i m u m (Fig. 7d a n d 7e). Ca-saturated m o n t m o r i l l o n i t e / h y d r o x y - A l ; r = 2.5 (Fig. 8, 9 a n d T a b l e 2B). (i) F o r Ce < 2-27. l 0 -5 m o l 1-1, Qa i n c r e a s e s slightly to 0.08 m m o l g-1 b u t ~ i n c r e a s e s a n d f l o c c u l a t i o n occurs. (ii) F o r Ce > 2 . 2 7 . 1 0 - s a n d < 2 . 1 0 -4 m o l 1-1, Qa i n c r e a s e s to 2.6 m m o l g - l , a n d t h e I E P is a t t a i n e d for Ce ~ 5.7. l 0 -5 m o l 1-1 o r Qa ~ 1.25 m m o l g-1. T h e s u s p e n s i o n is c o m p l e t e l y
Interactions between hydroxy-Al species and montmorillonite
221
' t~QE 0.034m tool.g-1 (a) ./~
/ / ' ~~ O~ (b)
v
-
'
~
/
"
(e)
d
0.33
I
I
0.4
0.5
I
0.06
I
I
I
.,
(n m )
2
FIG. 9. X-ray diffraction patterns of Ca-montmorillonite/Al(OH)2.5 sample after drying at 110~ for 24 h. See FIG. 3 for allocation of the peaks.
TABLE 2B. Data for the Ca-saturated montmorillonite/hydroxy-A1 (r = (OH/A1) = 2.5) system. AI equilibrium 1.25.10-s 2.27.10 -s 6.10-s 2.10-4 1.45.10-3 concentration Ce (mol 1-1) Adsorbed amount 0-03 0.08 1.00 2.60 2.88 Qa (mmol g-l) Turbidity 6 3 17 820 900 T (FTU) Zeta potential - 15 - 11 50 47
3.10-3
9.7.10-3
4.10 -2
4.93.10-2
3.01
4.14
7.00
11.40
850
850
850
850
53
52
52
51
4.8
4.7
4.8
4.8
4.8
1.72
1.70
1.60
1.72
1.70
(mV) pH Interlamellar distance d(001) (nm)
6"0 1.47
6-0 1.50
4.7
flocculated for Ce < 6 . 1 0 -5 mol 1-1 . W h e n the floes are re-dispersed, ~ is positive ( + 50 mV), and d(001) increases to 1-7 n m associated w i t h a stacking disorder (Fig. 9c). (iii) F o r Ce > 2 . 1 0 -4 mol 1-1, Qa increases to 11-4 m m o l g - l , ~ ( + 52 m V ) and d(001) are constant, and the stacking disorder increases (Fig. 9d and 9e).
222
J . Y . Bottero et al.
DISCUSSION The suspensions of montmorillonite in water are formed of tactoids consisting of elemental sheets stacked in a turbostratic arrangement along the c-axis, and therefore disoriented in the ab plane. The Na-saturated montmorillonite has a specific surface area of 44 m 2 g-i (N2 adsorption method), whereas the specific surface area measured by the Harkins & Jura method in which the clay is immersed in water (Cases & Francois, 1982; Harkins, 1952) is 103 m 2 g-1. Thus, it is possible to calculate the average size of the sheets, the average number of sheets per tactoid, and the values of the surface area of the three parts of the total surface i.e. the internal, exposed-basal, and broken-edges surface area (Cases, 1985). For the Nasaturated montmorillonite, the average number of sheets per tactoid is ~ 30 in the dry state, and 8 in aqueous solution. The latter value has also been obtained by Fripiat et al. (1982). The total basal specific surface area is 800 m 2 g-X and is independent of the dimensions of the sheet in the ab plane. The average surface occupancy by a N a + ion, considering the value of the CEC, is ~ 1.40 nm 2. This is confirmed if a typical montmorillonite unit cell is assumed: Nao.67 [A12.33 Mgo.67] (Si)s 020 (OH)4, corresponding to 4 hexagonal cavities/0.67 N a + or 6 hexagonal cavities/Na +. The cross sectional area for one hexagonal cavity is 0-23 nm 2. The Na-saturated montmorillonite suspensions, prepared with a r = 2.2 A1 solution, are well dispersed. The Al13 ions, for Ce < 4.10 -5 mol 1-1 are adsorbed to a small extent on exposed faces of the tactoids, and the ~ potential and d(001) remain constant. The destabilization of the suspension occurs when Ce >~4.10 -5 mol l-X, i.e. for ( > - 35 mV and ~< + 4 0 mV (the IEP is located at Ce = 1.3.10 -4 mol 1-1). In this region, the Alx3 ions adsorb not only on the exposed faces, but also in the internal basal faces as shown by the increase in d(001). The Na + ~ "Al13" exchange can be calculated from the Qa value on the plateau (2 mmol g-l). It can be written: CEC = ~da.q. 100
(1)
n
where n is the number of A1 ions in the polycation, and q is the charge. The Alx3 ions are preferentially adsorbed (Plee et al., 1985) and the concentration of monomers is low (r = 2.2) (Fig. 1). For the "Al~7~'' ion the CEC is ~ 107 mEq/100 g, and for "A16~-'' the C E C is 92.3 mEq/100 g, which is in good agreement with the experimental value. This suggests that on the plateau region of the adsorption isotherm, complete exchange of N a + ~ "Alx3" is achieved. When Ce is > 1.10 -3 mol 1-x, the suspension is dispersed, as shown by the increase in turbidity which represents destruction of the tactoid texture, with an increase of the projected cross section of the scattering particles (Allen, 1974). After drying, there is no stacking order. This phenomenon seems to be related to a change in the superficial nature of the adsorbed phase, as ~ increases from + 40 mV to + 50 mV whereas Qa remains constant at 2-0 mmol g-X. This means that isolated "Al13" adsorbed on to the external surfaces can be polymerized to form locally higher electric charge. In the interlamellar space, the average surface area occupied by the ,,A17 ,,,13 ,, ion is 9.8 nm 2 which is seven times that of N a +. If the average interpillar distance taken from edge to edge is about 2-5 nm (Pinnavaia et al., 1984), the calculated " A I ~ " ion radius is 0.51 nm which is approximately the radius in the crystal state (0.55 nm) (Johansson et al., 1960), or 0.4 nm from the results reported by Pinnavaia et al. (1984). In solution, the average spherical radius of the hydrated "^17+"rxlx3ion is 1-26 nm (Bottero et al., 1982a). It can be concluded that the ,,AI~+,, 9~1x3 ion is totally desolvated in the interlamellar space.
Interactions between hydroxy-Al species and montmorillonite
223
For the Na-saturated montmorillonite A1 solution (r = 2-5) system, the A1 species have a weak affinity for the clay surface up to Ce ~ 7.10 -5 mol 1-1. The various parameters do not vary. For Ce slightly higher than 7.10 -5 tool 1-1, the suspension flocculates, ~ increases from - 3 4 mV to the IEP, and Q, is then about 2.6 mmol g-1 due to adsorption of the isolated "Alx3" ions on the exposed and internal faces. The equilibrium concentration corresponding to the IEP (1.3.10 -4 tool 1-1) is the same as that relative to the adsorption from a r = 2.2 A1 solution. The range of concentration within which flocculation occurs is narrow, and redispersion occurs as soon as Ce is > 2.10 -4 mol 1-1. From this value, Qa increases until 18 mmol g-a, the zeta potential reaches + 52 mV and the turbidity is 650 FTU. This condition represents the partial destruction of the tactoid (stacking disorder increases), and indicates a strong affinity of the hydroxy-A1 aggregates for the clay surfaces. Thus, the adsorption is not due to an ionic exchange mechanism only. It is interesting to note that the value of the zeta potential at high concentration is always in the same range as noted earlier. For the Ca-saturated montmorillonite, r = 2.2 system, the suspensions are unstable for Ce < 5.10 -3 mol 1-1. At the IEP the turbidity is 3.3 F T U for Ce = 3.10 -4 mol 1-1 or Q, = 0-75 mmol g-X. On both sides of the IEP, the turbidity is 25 F T U which is lower than that of pure Ca-saturated montmorillonite (170 FTU). The adsorption mechanism of the "Alx3" ions corresponds to the scheme previously described: (i) the adsorption begins on the exposed faces and the suspension is destabilized for low Ce values because the basal exposed faces are partly saturated by the Ca 2+ ions; (ii) the interlamellar space is partly occupied for Ce > 2.10 -4 mol 1-t, and the Ca z+ ~ " A I ~ " exchange is total for Ce > 1.3.10 -3 mol 1-~ ; (iii) the tactoids are then destroyed for Ce >~ 5.10 -3 mol 1-x. The number of sheets per tactoid decreases as the adsorbed amount increases. The zeta potential is constant at + 55 mV, and X-ray diffraction shows total stacking disorder (Fig. 5e,f). For the Ca-saturated montmorillonite, r = 2.5 system, it can be seen that the affinity of hydroxy-A1 species is the highest because the IEP is obtained at the lowest Ce value, i.e. 5.7.10 -5 mol 1-x (Fig. 8). However, the adsorbed amounts per g are lower than for Namontmorillonite.
CONCLUSIONS The adsorption of isolated "AI~-" polycations (r = 2.2) on to Na- or Ca-montmorillonite is due to an ion-exchange mechanism that provokes flocculation and is limited initially to the exposed faces. At higher equilibrium concentration, the polycation Alx3 polymerizes on the surface (Bottero et al., 1987), and the suspension is stabilized. The superficial phase, at high concentration, is characterized by a zeta potential of ~ + 55 mV. The number of sheets per tactoid for Al-montmorillonite is less than that for Na- or Ca-montmorillonite. For the r = 2.5 solutions, the affinity of the isolated ,,^17+-,x113ion for the adsorbent seems to be the cause of the flocculation of the suspension at low equilibrium concentration, whereas the adsorption of higher molecular weight hydroxy-A1 species seems to be responsible for dispersion of the suspension and destruction of the tactoids. This superficial phase is also characterized by a ~ value ~ + 55 mV which is similar to that obtained with the r -- 2-2 solution.
224
J.Y.
Bottero et al.
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