Particle-size characterization of flocs and sedimentation volume in electrolyte clay suspensions

Applied Clay Science, 6 (1991) 181-194

181

Elsevier Science Publishers B.V., Amsterdam

Particle-size characterization of flocs and sedimentation volume in electrolyte clay suspensions M. Ohtsubo and M. Ibaraki* Department ofAgriculturalEngineering, Kyushu University,Fukuoka-shi, 812, Japan (Received October 8, 1990; accepted after revision April 16, 1991 )

ABSTRACT Ohtsubo, M. and Ibaraki, M., 1991. Particle-size characterization of flocs and sedimentation volume in electrolyte clay suspensions. Appl. Clay Sci., 6:181-194. The variation of the particle-size of flocs in clay suspension with the salt concentration and the correlation of the particle-size with the sediment behaviour of the suspensions have been studied. The particle-size distribution of flocs was determined by means of the sedimentation balance. Due to the increase of the salt concentration in the suspensions, the particle-size of flocs showed marked increases for the salt concentration range of 0 to 0.01M, followed by smaller increases for the salt concentrations higher than 0.01M. A negative correlation was observed between the mean particle-diameter of flocs in the suspensions and the zeta potential. The sediment volume of the clay suspensions of pH 7 was found to be positively correlated with the mean particle-diameter of flocs. The void ratio of the sediment exhibited a rapid decrease with increasing depth at the surface zone and a subsequent slow decrease at greater depths.

INTRODUCTION

When a stream or a river reaches an estuary, the flow velocity decreases and soil particles in suspension are coagulated into flocs due to the presence of salt water, and deposited onto the bed. The deposits are consolidated as a result of the self-weight of the soil particles, and the flocs in the deposits are brought close to each other. The particle-size of flocs and sedimentation behaviour of the suspensions are influenced by the electro-chemical and physical properties such as the composition and concentration of electrolytes, pH, particle-size distribution, and the solid concentration of the suspensions. Noting that in estuaries the electro-chemical properties of the clay suspensions are of importance for the growth of flocs and sedimentation behaviour *Present address: Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3 G l .

0169-1317/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

182

M. OHTSUBO AND IBARAKI

of clays, the present paper deals with the pH and salt concentration of the suspensions. The role of flocs in sedimentation behaviour of clay suspensions has been investigated previously. Based on the structural model of flocs, Michaels and Bolger ( 1962 ) developed a model of the sedimentation process using flocculated kaolinite suspensions. Melton and Rand ( 1977 ) investigated the sedimentation behaviour of kaolinite suspensions as a function of pH and NaC1 concentration, and explained the variations in sediment volumes in terms of the density of packing of particles within flocs and the average floc size. The effects of the salt concentration on the sediment volume of bentonite suspensions were studied by Akae (1988 ) in an extensive study of the dispersionflocculation behaviour and the rheological properties of bentonite-water systems. In the previous works, however, the particle-size measurements of flocs in clay suspensions have not been made. In the present work, the particle-size distribution of flocs in marine clay suspensions with various salt concentrations was determined by means of the sedimentation balance developed by Mori et al. (1980a, 1985). The correlation of the particle size of flocs with the sedimentation volume of the clay suspensions was then examined. MATERIALS AND METHODS

Materials The marine clay sample was obtained from a paddy field in Ariake-cho, Saga Prefecture reclaimed from the sea bottom. Marine clay deposits of 1520 m depth are widely distributed in Ariake Bay (Ohtsubo et al., 1982). The sample contained 60% clay fraction ( < 2/zm) and smectite as a principal clay mineral with minor amounts of kaolinite, illite, and vermiculite (Fig. 1 ). The smectite content in the < 2#m clay fractions was estimated from the X-ray diffraction patterns to be approximately 46%. The smectite in the marine clay sample is of a low-swelling type (Egashira and Ohtsubo, 1983 ), and because of this, the clay sample responds to salinity and ion saturation in a manner similar to the low-activity minerals, illite and chlorite, rather than in the manner normally associated with smectites. Ohtsubo et al. ( 1982 ) indicated that the liquid limit of this marine clay decreases with a decrease in the salt concentration in the pore water. Methods Salt solutions for preparing clay suspensions were adjusted using NaC1, KC1, MgC12, and CaC12 to be Na: K: Mg: C a = 84:3:10:3 by weight as a cationic composition and 0.5M as a salt concentration simulating sea water. The solutions were then diluted to the various salt concentrations in molarities of

PARTICLE-SIZE CHARACTERIZATIONS IN E L E C T R O L Y T E CLAY SUSPENSIONS

183

Sm

14.7A

18.0A S m : Smectite It

: I Ilite

Kt

: Kaolinite

C h : Chlorite

It

Kt

a a~

i 3].6~

10.0/~

7.,s~

I

]

~/

M g Air-dry

Ch

~qj

Mg

Glycerol

Fig. 1. X-ray diffraction patterns of < 2#m clay fractions.

0.2, 0.1, 0.05, 0.02, 0.01, and 0.005. Clay samples with natural water content of 130% were put into the salt solutions with the respective salt concentrations, and the final solid concentration of the suspensions by weight was adjusted to 0.04% for the particle-size determination of flocs, 0.7% for zeta potential measurement, and 5% for clay sedimentation. For a given salt concentration two sets of clay suspensions were prepared, one being adjusted to pH 4 and the other to pH 7 using HC1 and NaOH. The particle-size distribution of flocs in flocculated and dispersed suspensions was determined by means of the sedimentation balance developed by Moil et al. ( 1980b, 1985 ) shown in Fig. 2. The weighing capacity and sensitivity of the balance are 200 g and 0. l mg, respectively. One thousand milliliters of clay suspensions with 0.04% solid concentration by weight were placed in the sedimentation vessel of 70 m m diameter and 300 m m height followed by stirring with a weighing pan, which was hung at the end of the balance beam. Due to the settlement of clay particles on the weighing pan, the pan moves downward, and the movement is detected with the light beam scanner attached to the end of the balance beam. A very little movement of the balance generates a current, being amplified, which makes the proportional and integral control unit work to bring the balance beam back to the initial equi-

184

M. OHTSUBOAND IBARAKI

b

Compensating bridge for

tare Bridge t a r photocurr ent

from light beam scanner

i

l Ampli tier

i°

1-1

Pr aport ional and integral c o n t r o l unit I

--=--.- =

Ir

i

i

Analogue pen recorder

~_

Digital printer

J~

!

II 11 II Fig. 2. Schematic diagram o f s e d i m e n t a t i o n balance.

librium. Thus the very little movement of the balance caused by the weight of clay particles is successively converted into the current, and the integrated weight of clay particles settled on the weighing pan is recorded on a pen recorder as a function of the sampling time. The measurements required about 24 hours for flocculated suspensions and 40 hours for dispersed suspensions. Figure 3 shows the cumulative weight of the clay solids settled on the weighing pan as a function of time obtained for a clay suspension of 0.01M salt concentration and pH = 6.6. The chart speed of the recorder was changed from initial 60 c m / h to, 6, 0.6, and 0.3 cm/h. From this sedimentation curve, the percentage of the cumulative solid weight of a sampling time, t, against the total weight of solids finally settles on the weighing pan was obtained using the procedure described by Mori et al. ( 1980b ). The relationship between the sampling time, t (s), and the particle diameter, D (cm), can be expressed by the following Stokes equation l=

18r/H

(Ps --Pw)gD

where r/= viscosity coefficient of water, in poise; Ps = density of solid particles,

PARTICLE-SIZECHARACTERIZATIONSIN ELECTROLYTECLAYSUSPENSIONS

185

Time

lOm|n

90mi,n

24hr I

20

A E

40

60 80 1OO 120

60 cm/hr

~

6cm/hr

~

0.6 cm/hr

~ 0.3/hr

Fig. 3. S e d i m e n t a t i o n curve o f clay suspension.

in 2.6 g/cm3; pw=density of water, in g/cm3; g = acceleration of gravity, in cm/s 2. Thus the cumulative weight percentage for various particle diameters of flocs can be obtained from the sedimentation curve in Fig. 3 and the Stokes equation. The zeta potential measurements were made using electrophoresis apparatus equipped with a flat rectangular cell using the procedure described by Moil et al. (1980a). Measurements were performed by focusing on one of the stationary levels within the cell and timing the motion of a minimum of 5 particles first in one direction and then on reversing the polarity of the applied field. Calculation of the zeta potential is based on the following HelmholtzSmoluchowski's formula (Hiemenz, 1977; Mori et al., 1980a): (_4n~/ l c. SX3002 x 10_~ Dtt where ( = zeta potential in mV; r/= coefficient of viscosity of liquid, in poise; D = dielectric constant of liquid; 1= migration distance of particles, in am; t = t i m e required for particles to migrate distance/, in seconds; c=electric conductance of suspensions, in/~s/cm; i = electric current flowed in the cell, in A; s = cross sectional area of he rectangular cell perpendicular to an electrical gradient, in cm 2. Transparent glass cylinders of 10 cm inside diameter and 51 cm height were used for the sedimentation of clay suspensions. Four thousand milliliters of clay suspensions with 5% solid concentration by weight were placed in the cylinders followed by mixing and agitation. The mixture was allowed to settle until the downward movement of the interface between sediments and clear liquid over a five-day period was reduced to 1 mm, and then the sediment

186

M. OHTSUBOANDIBARAKI

volume of the clay suspensions was determined. The density of the sediment was determined for the specimens withdrawn from the depths of 0.5, 2, 4, 6, 8, and 11 cm by a stainless steel tubing glued to the syringes. The density measurement was made by means of a hand-held density meter (Anton Paar K.G., DMA35) with an accuracy of + 1X 10 - 3 gcm -3. The amount of the specimen used for the measurement was approximately 3 cm 3. PARTICLE-SIZE VARIATION OF FLOCS WITH SALT CONCENTRATION

The particle-size frequency distributions of flocs in the suspensions with various salt concentrations are shown in Fig. 4. The frequency curve of a saltfree suspensions of pH 4 and pH 7 gave the m a x i m u m percentage at the particle-sizes of 1.0 to 1.2tim. With increasing salt concentration, the frequency curves moved toward an increasing particle size, showing a m a x i m u m at the

'° r

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L~ 3 0 = o

q

~2o

glO

i

i

i

i

i

i

i

i

10

O0

40 pH 7

~ 2G o

0

'

10 Particle

Diameter

'

"'

'

100 (~m)

Fig. 4. Particle-size frequency distribution offlocs by weight percentage in the clay suspensions of pH 4 and pH 7 with various salt concentrations.

PARTICLE-SIZE CHARACTERIZATIONS IN E L E C T R O L Y T E CLAY SUSPENSIONS

187

particle-size range of 8 to 10/tm for salt concentrations of 0.02M, 0.2M, and 0.SM. To illustrate the effects of the increase of salt concentration on the growth of flocs in the suspensions, the weight percentage for various particlesize ranges of flocs are presented in Fig. 5 as a function of the salt concentration. The marked changes in the weight percentage of the particle size of flocs took place when the salt concentration increased from 0 to 0.0 IM: the weight

10[.

>37,.m "

10[:

~

~

~

-

~

---'#

I

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10

--

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~

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,

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I

m I

40

i

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i

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0 i

I

~

I

m !

I

I

I

60

o pH4 epH 7

"-" 40 •;

2o

o

0

IS

T , 0.01 0.1 Salt Concentration (M) -

~

, 1.0

Fig. 5. Variation of weight percentagewith salt concentration for various particle-sizeranges of flocs in the clay suspensionsof pH 4 and pH 7.

188

M. OHTSUBO AND IBARAKI

10

0

8

o

e

~4

o

0

i

2

I

0 0

0

• IF-

• ' 0.01

Salt

Concentration

l 0.1

pH4

pH7

1.0

(M)

Fig. 6. Mean particle diameter of flocs, Dso, in the clay suspensions of pH 4 and pH 7 as a function of salt concentration.

percentage increased for the particle-size ranges of 13.1-9.25 and 9.256.54#m, and 6.54-4.63#m while it decreased for the particle-size ranges smaller than 2.3 l#m. For samples exhibiting wide particle-size distributions, the mean particle diameter, Dso, whose size is greater than that of 50% of the particles by weight has been used (Iitani et al., 1965 ). The mean particle diameters of flocs, Dso, obtained from the distribution curves for the suspensions of pH 4 and pH 7 in Fig. 4 are plotted against the salt concentrations in Fig. 6. This figure indicates that for suspensions of both pH 4 and pH 7, the particle diameter Ds0 increased markedly with increasing salt concentration from 0 to 0.01M, showing slight increases at the salt concentration ranges higher than 0.01M. The growth of flocs in the suspensions with increasing salt concentration shown in Figs. 5 and 6 can be explained in terms of the net energy of the Van der Waals attraction and electrostatic repulsion between clay particles. At salt concentrations ( < 0.01 M) where the net interaction is repulsive, the suspensions are dispersed for both the suspensions of pH 4 and pH 7, leading to a smaller particle diameter Dso (Fig. 6). At salt concentrations above 0.01M, where the repulsive energy barrier does not exist, particles are drawn into close proximity and the particle diameter Dso increased. PARTICLE SIZE OF FLOCS VERSUS ZETA P O T E N T I A L

Taking note of the fact that the zeta potential is directly related to the total interaction energy between particles, the weight percentage is plotted against the zeta potential for various particle-size ranges of flocs in the suspensions of pH 4 and pH 7 (Fig. 7 ). The weight percentage for the particle-size ranges larger than 18.5/tm are smaller than 10% and independent of the zeta poten-

PARTICLE-SIZECHARACTERIZATIONSIN ELECTROLYTECLAYSUSPENSIONS

,of

,ills

iO

9

iP

189

>37,um

aDO

i

,

37-26.2pm

10[L •~;~

1° I

'° I 30

, ~,0~

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•

,

26.2--18.5pm O~

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i

0 0~10 •0~18"5-13"1#m• I 0

0

13.1-9.25~m O011D •O0

@

I

On

•

I

40 o •OB o

20

ooe

o

•

|

I

30

o o

.o

eo

i

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o

I

i

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•e 0

0

.¢

o%.o •.. • I

i

l

O

o oo;o ,1'; i 20

°I 0

10

I

4.63 -3.27~m I

• 3.27- 2.31~m

o• pH4 • pH7 o • (~b IG 01~10 00 t<2.31'um n t 20 30 Zeta

Potential

40

(-mV)

Fig. 7. Variation of weight percentage with zeta potential for various particle-size ranges of floes in the clay suspensions o f p H 4 and pH 7.

tial. For the particle-size ranges of 13.1-9.25 and 9.25-6.54pm, the weight percentage was constant in the zeta potential range of - 1 6 m V to a r o u n d - 26mV although the data were scattered, and decreased in the higher zeta potential range. On the other hand, the weight percentage for the particle-size ranges of 4.63-3.27, 3.27-2.31, and < 2.31/tm was unchanged in the zeta potential of - 16mV to around - 2 6 m V , showing increases for further increases in the zeta potential. This indicates that the floes which are smaller than 4.63/tm change to the floes ranging from 13.1-6.54/tm. This critical

190

M. OHTSUBOAND IBARAKI

10 ~ , ~ _

i

=

"q~O

¢3

2

• pH 7

(3D(3D~O Y=-O.24x +12.46 r=0.77"*

v 6

**Sil~nificant lat the 1% level

10

15 Zeta

01 p H 4

20

~ 25

Potential

(-mV)

•

n 30

•

35

Fig. 8. Mean particle diameter of flocs, Dso, in the clay suspensions of pH 4 and pH 7 as a function of zeta potential.

change in floc size at a zeta potential of around - 2 6 m V corresponds to a salt concentration of 0.01M. To correlate the overall particle size of flocs with the zeta potential, the mean particle diameter Ds0 of flocs in the suspensions is shown in Fig. 8 as a function of the zeta potential. A negative correlation was observed between the mean particle diameter Dso and the zeta potential for the suspensions of pH 4 and pH 7. PARTICLE SIZE O F FLOCS VERSUS S E D I M E N T V O L U M E

When a soil suspension is allowed to stand, soil particles settle to a fixed volume under gravity. The volume of the sediment is designated as the sediment volume of a soil, and this has been indicated to be a measure of particle interactions changing with pH and salt concentration of the soil suspensions (Van Olphen, 1977; Melton and Rand, 1977; Egashira and Ohtsubo, 1983 ). The variation in sediment volume, brought about by changes in the electrolyte environment in the suspension has been explained in terms of the size of floes in the suspensions. Melton and Rand ( 1977 ) calculated the ratio of floc to kaolinite volume concentrations (a measure of the floc size ) with the equation of Michaels and Bolger ( 1962 ) and showed that the higher floe-kaolinite volume ratio gave the higher sediment volumes. For the suspensions of the present study, a positive correlation was observed between the mean particle diameter Dso and the sediment volume for the suspensions o f p H 7, whereas the sediment volume did not change against the mean particle diameter Ds0 for the suspensions of pH 4 (Fig. 9). The results could be explained in terms of the clay particle configuration as a function of the salt concentration. For both suspensions o f p H 4 and pH 7 the clay

PARTICLE-SIZECHARACTERIZATIONSIN ELECTROLYTECLAYSUSPENSIONS i

!

I

191

I

OpH4

E

4

o

3

o

OpH

7

o

Si/~anb Y = O ' 3 2 x + 1 . 7 2

E "13

, , e t n t r ~ ' ~ ; i % ,.v., 0

i

I

I

I

2

4

6

8

D50

(~m]

Fig. 9. Sediment volume of the clay suspensions of pH 4 and pH 7 as a function of the mean particle diameter of floes. Dso.

panicles in the large flocs which developed at high salt concentrations form face-face aggregates linked up in edge-face and edge-edge fashion (Van O1phen, 1977; Rand and Melton, 1977 ), leading to the large sediment volume. The separation of floes into thinner flakes due to the decrease in the salt concentration would result in the parallel orientation of the particles for the suspensions of pH 7, and thereby producing the closely packed sediments with small sediment volume. For the suspensions of pH 4, the sediment volume does not change even after the separation of flocs into thinner flakes because the edge-face and edge-edge configuration is preserved. VOID RATIO PROFILE OF SEDIMENT

Some experiments have shown that the sediments are not uniform and that their void ratio and porosity decreases rapidly with depth (Been and Sills, 1981; Tiller and Khatib, 1984). This is confirmed in Fig. 10 where the void ratio is plotted against the distance from the sediment surface and the effective stress in the sediment. The values of the void ratio for both pH 4 and pH 7 decreased rapidly down to a depth of 2 cm and subsequently much more slowly with depth. The void ratio profile of Fig. 10 can be explained in terms of the structural model of floes and their networks (Michaels and Bolger, 1962). The most probable floc shape is a sphere, the shape most capable of resisting deformation by surface forces. These floes have a certain amount of mechanical strength due to the electrostatic forces between particles, and so are able to retain the spherical shapes at the mild stresses encountered during sedimentation under normal gravity. Near the top of the sediments, floes are present as incompressible rigid spheres because the overburden weight is smaller than

192

M. OHTSUBO AND IBARAKI Depth

60

0

2 &

(cm)

4

6

8

10

i

i

!

I

pH4 )

•" O . O O S M

o

•

o 0.01M .o

4O

O.02M

" O.05M • 0.1M

gE

o 0.2M o

> 20

0

I

0

0.05 Effective

I

O.1 S t r e s s ( K N / m 2)

0.15

Depth ( c m )

0

2

4 ,

60 pH

6

8

,

,

10 ,

7

40 .o t~

20

0] 0

i

i

0.05

0.1

Effective

Stress

0.15

(KN/m2)

Fig. 10. Void ratio vs. depth a n d effective stress o f the clay sediments of pH 4 a n d pH 7.

the mechanical strength of the flocs. In this zone flocs are brought into more packed arrays with increasing effective stress while preserving the flocs sphere, leading to a marked decrease in void ratio. Further increases in effective stress would change the particle configurations in the flocs into more parallel orientations and the flocs tend to become flat, resulting in a decreased void ratio. SUMMARY AND CONCLUSIONS

Due to the increase of salt concentration, the particle size of flocs in clay suspensions shows marked increases for salt concentrations ranging from 0 to 0.01 M, and only small increases for salt concentrations higher than 0.01M. This critical salt concentration of 0.01M corresponds to the zeta potential of

PARTICLE-SIZECHARACTERIZATIONSIN ELECTROLYTECLAYSUSPENSIONS

193

- 2 5 to -30mV. A negative correlation exists between the mean particle diameter of flocs and the zeta potential. A positive correlation is observed between the sediment volume and mean particle diameter of flocs for the suspensions of pH 7, while the sediment volume does not change against the mean particle diameter of flocs at pH 4. This could be explained in terms of the particle configuration within the flocs. The void ratio of the sediment rapidly decreases with increasing depth at the surface zone and subsequently more slowly at further depths. ACKNOWLEDGMENT

This study was supported in part by the Grant-in-Aid for Scientific Research (C), the Ministry of Education, Japan, Grant No. 63560232. The experimental work of particle-size analysis was carried out in the Mining Department of Kyushu University. The aid for the experimental work given by Dr. S. Mori and Mr. K. Hara is gratefully acknowledged.

REFERENCES Akae, T., 1988. Influence of added salt concentration on dispersion-flocculation behavior and rheological properties of bentonite-water systems. Trans. Japan Soc. Irr., Drainage Reel. Eng., 133: 43-50. Been, K. and Sills, G.C., 1981. Self-weight consolidation of soft soils: an experimental and theoretical study. Geotechnique, 31: 519-535. Egashira, K. and Ohtsubo, M., 1983. Swelling and mineralogy of smectites in paddy soils derived from marine alluvium, Japan. Geoderma, 29:119-127. Hiemenz, P.C., 1977. Principles of Colloid and Surface Chemistry. Marcel Dekker, New York, 516 pp. Iitani, G., 1965. Funtai Ryudo Sokuteihou, Funtai Kougaku Kenkyukai, Youkendou: 3-7. Melton, I.E. and Rand, B., 1977. Particle interaction in aqueous kaolinite suspensions - Sedimentation volume. J. Colloid Interface Sci., 60:331-336. Michaels, A.S. and Bolger, J.C., 1962. Settling rates and sediment volumes of flocculated kaolin suspensions. Ind. Eng. Chem. Fundamentals, 1:14-20. Mori, S., Okamoto, H., Hara, T. and Aso, K., 1980a. An improved method of determining the zeta-potential of mineral particles by micro-electrophoresis. In: Proc. Int. Symp. on Fine Particles Processing 1: 632-651. Mori, S., Hara, T. and Aso, K., 1980b. Particle size distribution analysis of powder by the chain system sedimentation balance. In: Proc. 4th Joint Meeting of the Am. Inst. of Mining, Metallurgical, and Petroleum Engineers (AIME) and the Mining and Metallurgical Inst. of Japan (MMIJ), Tokyo, Tech. Session C-1: 47-60. Mori, S., Okamoto, H., Hara, K. and Aso, K., 1985. Flocculation and dispersion properties of quartz and fluorite particles in suspension. Colloids and Surfaces, 14:109-120. Ohtsubo, M., Takayama, and Egahsira, K., 1982. Marine quick clays from Ariake Bay area, Japan. Soils Foundations, 22 (4): 71-80. Rand, B. and Melton, I.E., 1977. Particle interactions in aqueus kaolinite suspensions I. Effect

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M. OHTSUBOANDIBARAKI

of pH and electrolyte on the mode of particle interaction in homoionic sodium kaolinite suspensions. J. Colloid Interface Sci., 60: 308-320. Tiller, F.M. and Khatib, Z., 1984. The theory of sediment volumes of compressible, particulate structures. J. Colloid Interface Sci., 100:55-67. Van Olphen, H., 1977. An Introduction to Clay Colloid Chemistry. (2nd ed. ). Wiley, New York, pp. 92-149.