Bergaya F. Handbook of Clay Science

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Villie

ras, F., Chamerois, M., Bardot, F., Michot, L.J., 2002b. Evaluation of wetting properties

of powders from gas adsorption experiments. In: Mittal, K.L. (Ed.), Contact Angle, Wett-

ability and Adhesion, vol. 2. VSP, Utrecht, pp. 435–447.

Villie

ras, F., Michot, L.J., Bardot, F., Cases, J.M., Franc- ois, M., Rudzinski, W., 1997a. An

improved derivative isotherm summation method to study surface heterogeneity of clay

minerals. Langmuir 13, 1104–1117.

Villie

ras, F., Michot, L.J., Bardot, F., Chamerois, M., Eypert-Blaison, C., Franc-ois, M.,

rard, G., Cases, J.M., 2002a. Surface heterogeneity of minerals. Comptes Rendus

Geosciences 334, 597–609.

Villie

ras, F., Michot, L.J., Cases, J.M., Berend, I., Bardot, F., Franc- ois, M., Ge

rard, G.,

Yvon, J., 1997b. Static and dynamic studies of the energetic surface heterogeneity of clay

minerals. In: Rudzinski, W., Steele, W.A., Zgrablich, G. (Eds.), Equilibria and Dynamics

of Gas Adsorption on Heterogeneous Solid Surfaces. Elsevier, Amsterdam, pp. 573–623.

Chapter 12.9: Surface Area and Porosity978

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

979

Chapter 12.10

CATION AND ANION EXCHANGE

F. BERGAYA

, G. LAGALY

AND M. VAYER

CRMD, CNRS-Universite

d’Orle

ans, F-45071 Orle

ans Cedex 2, France

Institut fu

r Anorganische Chemie, Universita

t Kiel, D-24118 Kiel, Germany

Thomson (1850), Way (1852), Johnson (1859),andvan Bemmelen (1888) pio-

neered the study of cation exchange in soils. Since that time much progress was made

in our understanding of this phenomenon. The upsurge of interest in such issues as

fertilizer efﬁciency or contaminant mobility in the environment contributed to the

large increase in publications on cation exchange in soils, particularly with respect to

the clay fraction of soils. Ion exchange is of fundamental and practical importance to

soil studies and all ﬁelds in which clay materials feat ure. Indeed, the ability of

colloidal particles (including clay minerals) to retai n and exchange positively charged

ions is ‘perhaps the most important chemical property of natural porous media’

(Verburg and Baveye, 1994 ). This property has a controlling inﬂuence on the mo-

bility of positively charged chemical sp ecies in soil, such as potassium (from fer-

tilizers) and heavy metal ions (see Chapter 11.1) as well as on the geochemical cycling

of cations in general.

This chapter gives a brief survey of cation and anion exchange, with particular

reference to the definition and determination of the cation exchange capacity (CEC)

of clay minerals. The main questions are concerned with (i) the best method to

measure CEC and (ii) the real meaning of the measured CEC under speciﬁed con-

ditions.

12.10.1. ION EXCHANGE REACTIONS

A. Cation Exchange Equilibria

+ B

+ A

DOI: 10.1016/S1572-4352(05)01036-6

Cation exchange equilibria are often illustrated in diagrams by plotting the con-

centration ð



Þ or equivalent fraction ð



Þ of A

at the surface vs. the concen-

tration ðc

Þ or equivalent fraction ðw

Þ of this ion in the equilibrium solution. For

monovalent cations the equivalent fractions are given by



=ð



=ð



ð1Þ

where m denotes the concentration (in mol per volume or area unit) of the speciﬁed

cations at the surface and in solution. Fig. 12.10.1 shows the exchange of ammonium

ions on montmorillonite by sodium, potassium, rubidium, and caesium ions (Martin

and Laudelout, 1963). The diagonal of the ‘square plot’ (broken line) indicates no

preference for either cation. In practice, the exchange shows a certain preference of

one cation over another as indicated by the deviation of the curve from the diagonal.

Thus, in contact with so dium ions the equivalent fract ion of ammonium ions at the

surface is distinctly higher than that in solution, i.e., NH

ions are preferentially

adsorbed over Na

ions. By contrast, Cs

ions are preferred to NH

ions.

Cation exchange reactions may be described by the law of mass action using the

activities of the ions, rather than their concentrations

K ¼



ð2Þ

0.2

0.6

0.4

0.8

0.8 10.60.40.20

Fig. 12.10.1. Exchange of ammonium ions by alkali ions on montmorillonite (Camp Berteau)

in 0.05 M solutions at 25 1C.



and w

are the molar fractions (Eq. 1) of ammonium ions

at the surface and in the equilibrium solution (Martin and Laudelout, 1963). From Jasmund

and Lagaly (1993).

Chapter 12.10: Cation and Anion Exchange980

where a

and a

are the activities of ions A

or B

in solution,



and



the

activities of these ions at the surface of the exchanger, and g is the activity coefﬁcient.

That is, a

¼ c

and



: K repres ents the thermodynamic equilib-

rium constant. If the cation exchange is performed in sufﬁciently dilute solution, the

activity coefﬁcients of the ions in solution g

approach unity but this does not apply

to the activity coefﬁcients of the ions at the surfa ce ð



Þ. This is because the con-

centration of all ions at the surfa ce ði:e:;



c ¼



Þ does not decrease when the

solution is diluted.

Using concentrations and activity coefﬁcients, Eq. (2) becomes

K ¼



¼ K



ð3Þ

where K

is the selectivity coefﬁcien t. This coefﬁcient is often used instead of the true

equilibrium constant, K, because all parameters in K

can be directly measured. As

mentioned above, g

and g

but not



and



approach unity in dilute so-

lutions. It is noteworthy that K

is not constant but changes as the exchange reaction

progresses, i.e., with the ratio



: When B

approaches the surface that is

mainly occupied by A

, steric effects and the free energy of exchange can be dif-

ferent from the situation when B

approaches a surface with B

cations occupying

most exchange positions. Fig. 12.10.2 illustrates the variation of K

as the amount of

sodium ions on the surface increases for a vermiculite and a montmorillonite (Gast

and Klobe, 1971; Gast, 1972).

10.80.60.40 0.2

3.2

2.4

1.6

0.8

ln K

10.80.60.40 0.2

2.4

1.6

0.8

-1.6

-0.8

(a) (b)

Fig. 12.10.2. Variation of the selectivity coefﬁcient K

with the molar fraction of sodium ions,



, on the clay mineral when the Li

cations are exchanged by Na

at 25 1C(Gast and

Klobe, 1971; Gast, 1972). From Jasmund and Lagaly (1993). (a) Vermiculite (Transvaal,

South Africa), total electrolyte concentration 0.01 M. (b) Montmorillonite (Chambers;

Arizona, API No. 23), total electrolyte concentration 0.001 M.

12.10.1. Ion Exchange Reactions 981

To obtain the true equilibrium constant, K, the activity coefﬁcients of the cations

on the surface have to be determined. This procedure and the evaluation of the free

enthalpy of exchange (by integrating the function lnK

) were describ ed by Gast and

Klobe (1971). The same principles earlier used by Theng et al. (1967) to descri be the

exchange of alkylammonium ions with sodium and calci um ions on montmorillonite.

For the heterovalent exchange

+ B

+ 2A

the thermodynamic equilib rium constant is

K ¼



2þ a



2þ

ð4Þ

Often the equivalent fractions of the ions on the surface were introduced



=ð



þ 2



2þÞ



2þ

¼ 2



2þ

=ð



þ 2



2þ

ð5Þ

where m is expressed in mol per volume or area. The (Gaines–Thomas) selectivity

coefﬁcient is given by



2þa



2þ

ð6Þ

Soil scientists often use other coefﬁcients such as the Vanselow selectivity coefﬁcient,

S,V

(Anderson and Sposito, 1991), or the Gapon coefﬁcient, K

S,G

S;V



2þa



2þ

ð7Þ

S;G



2þa



ﬃﬃﬃ

2þ

ð8Þ

The separation factor, a , only considers concentrations

a ¼



2þc



2þ

ð9Þ

Chapter 12.10: Cation and Anion Exchange982

The Vanselow and Gapon coefﬁcients are introduced because in some particular

cases (Shainberg et al., 1987) their values show little change with the degree of

exchange.

In heterovalent exchange reactions, the more highly charged cations are preferred

over monovalent ions, as indicated by Eq. (4) (Laudelout et al., 1968). This is

illustrated by Fig. 12.10.3 showing the equilibrium (



vs. w

) and selectivity

coefﬁcient for the K

/Ca

exchange on montm orillonite (Inoue and Minato,

1979).

Nir and co-workers (Nir et al., 1986, 1994; Margulies et al., 1988; Hirsch et al.,

1989; Rytwo et al., 1996) evaluated the cation-binding coefﬁcients from the exchange

isotherms (Table 12.10.1 ). The procedure consists in solving the electrostatic

Gouy–Chapman equations and calculating the amounts of cations adsorbed as the

sum of the cations residing in the double-layer region and those bound to the sur-

face. The model also accounts explicitly for cation complex ation in solution. By

taking into account the adsorption of CaCl

and MgCl

, the apparent increase in

CEC with divalent cation concentration is eliminated.

An alternative thermodynamic model was developed by Kraepiel et al. (1999) who

consider the clay mineral particles as a porous solid bearing permanent charges. In

this model, the Gouy–Chapman diffuse ionic layer is extended to the interlayer

space, while the high concentration of cations and their (differ ent) distribution in the

interlayer space (Kjellander, 1996; Quirk, 2001) are ignored.

0.8

10.80.60.40.20

0.2

0.4

0.6

10.80.60.40.20

-2

(a) (b)

Fig. 12.10.3. Exchange of Ca

ions by K

ions (

) and of K

ions by Ca

ions ()at

35 1C. Montmorillonite Aterazawa (Yamagata Pref., Japan) (Inoue and Minato, 1979). From

Jasmund and Lagaly (1993). (a) Equivalent fractions of K

ions on montmorillonite,



; vs.

the equivalent fraction of K

ions in the equilibrium solution, w

: (b) Variation of the

selectivity coefﬁcient with



12.10.1. Ion Exchange Reactions 983

The exchangeable cations occupy sites of different energy in the interlayer space

(Goulding and Talibudeen, 1980; Brouwer et al., 1983). The formation of ion-ex-

change sites with very high selectivity for caesium ions, induced by (i) repeated

wetting–drying cycles of potassium montmorillonite and (ii) layer charge reduction

using the Hofmann–Klemen effect, was described by Maes et al. (1985).

B. Selectivity

Clay minerals show a preference for larger over smaller inorganic cations. This

tendency, referred to as ‘ﬁxation’ in the soil science lite rature, becomes more pro-

nounced as layer charge increases (Klobe and Gast, 1967; Laudelout et al., 1968;

Gast and Klobe, 1971 ; Sawhney, 1972; Maes and Cremers, 1977; Jasmund and

Lagaly, 1993 ). For smectites, this preference (for larger cations) follows the order:

4Rb

4Na

4Li

and

2þ

4Sr

2þ

4Ca

2þ

4Mg

2þ

At higher layer charge (vermiculites) the preference is

2þ

4Ca

2þ

 Sr

2þ

 Ba

2þ

As a consequence of the particular interaction between the large alkali ions and the

siloxane surface, these surface sites show the preference for caesium ions over lithium

ions whereas the ionizable surface groups at the edges have a much lower selectivity

for caesium ions (Anderson and Sposito, 1991).

The selectivity of the preferentially adsorbed cations decreases with increasing

degree of coverage by this cation (Gast and Klobe, 1971; Sawhney, 1972; Inoue and

Minato, 1979; McBride, 1979; Bergseth, 1980; Maes et al., 1985; Siantar and Fripiat,

1995; Staunton and Roubaud, 1997).

The increased preference of clay minerals for potassium (and also rubidium and

caesium) ions as layer charge increases (Schwertmann, 1962) is related not only to

the decrease in hydration energy (K

¼ 2314 kJ=mol; Li

¼ 2508 kJ=mol) but also

Table 12.10.1. Binding coefﬁcients for different cations on montmorillonite (in M

–1

)

TFT

0.6



200



4–40



¼ methylene blue; TFT

¼ thioﬂavin.



Nir et al. (1986).

Rytwo et al. (1996).

Hirsch et al. (1989).

Margulies et al. (1988).

Chapter 12.10: Cation and Anion Exchange984

to the strongly enhanced van der Waals energy because K

can ﬁt closely into the

ditrigonal cavity of the silicate layer. When Li

is displaced by K

in Wyoming

montmorillonite, the entropy change is negative (DS1 ¼ 2 3 :12 J=K), the (decisive)

value of DH1 is –2.98 kJ/mol and DG1 is –2.05 kJ/mol (Gast and Klobe, 1971; Gast,

1972; Maes and Cremers, 1977; Eberl, 1980; Goulding and Talibudeen, 1980).

The preference of clay minerals for certain cations is caused by several effects.

These include hydration of the cations at the surface and in solution (entr opy!),

electrostatic cation–surface and cation–cation inter actions, interaction between the

water molecules and the surface, and the polarizability or hard and soft acid–base

(HSAB) character of the cations (Xu and Harsh, 1992; Auboiroux et al., 1998).

Entropic effects are also often decisive (Laudelout et al., 1968; Gast and Klobe,

1971; Gast, 1972; Maes and Cremers, 1977; Inoue and Minato, 1979; McBride, 1979;

Jasmund and Lagaly, 1993). An inﬂuence of the oxidation state of the structural iron

was also observed (see Chapter 8).

The structure of the silicate layers (localization of charges, orientation of OH

dipoles, rotation of tetrahedra) should have a strong e ffect when the layer separation

is small as in the case of micas (Kodama et al., 1974). Potassium ions in micas can be

exchanged by other cations only under particular conditions, e.g., by complexing the

displaced K

in very dilute solutions (Scott et al., 1973), or reacting with barium

ions at 80–120 1C(Reichenbach and Rich, 1969; Reichenb ach, 1973). The latter

process is accompanied by a decrease in layer charge (Beneke and Lagaly, 1982).

The cation exchange reaction in micas often proceeds as a co-operative process

(Reichenbach and Rich, 1969; Reich enbach, 1973).

C. Hysteresis

Exchange reactions on clay minerals typically show hysteresis effects. This is illus-

trated in Fig. 12.10.4 for the K

/Ca

exchange on montmorillonite induced by

drying. The curves A, B, and C (dotted) represent the exchange of Ca

by K

three different total concentrations. For K

-montmorillonite that was dried at

80 1 C, the exchange of K

by Ca

follows the solid curves indicating that the K

ratios on the surface are higher than those obtained during the exchange of

by K

(Inoue and Minato, 1979 ).

The thermodynamics of binary exchange of cations on clay minerals and soils was

reviewed by Verburg and Bav eye (1994). The model proposed takes into account the

clay mineral type, state of hydration, and electrolyte concentration, at least qua l-

itatively. The exchangeable cation s are classiﬁed into three groups (Table 12.10.2). If

exchange occurs between cations of different groups, hysteresis appears due to one of

several mechanisms: heterogeneous distribution of charges or surface sites, site in-

accessibility caused by coagulation or ﬂocculation (formation of aggregates), clay

mineral dehydration, differences in cation hydration, and osmotic or extensive in-

terlayer swelling (see Chapters 5 and 13.2). A satisfactory explanation of hysteresis

effects is still lacking.

12.10.1. Ion Exchange Reactions 985

D. Heavy Metal Ion Adsorption

The exchange adsorption of heavy metal cations by clay minerals was widely studied

because of environmental concerns about heavy metal pollution (see Chapter 11.1)

(Nagy and Ko

nya, 1988; Siantar and Fripiat, 1995; Altin et al., 1998; Bereket et al.,

1997; Kodama and Komarneni, 1999; Schlegel et al., 1999a, 1999b; Strawn and

Sparks, 1999; Abollino et al., 2003). The amount of multivalent heavy metal cations

adsorbed often exceeds the CEC of the clay mineral. One reason is that multivalent

cations can be bound by equimolar, rather than equivalent, exchange (Weiss, 1958c;

0.8

0.6

0.4

0.2

10.80.60.40.20

Fig. 12.10.4. Hysteresis of the Ca

exchange on montmorillonite after drying at 80 1 C.

Dotted curves denote exchange of Ca

by K

, while full curves describe exchange of K

. Total electrolyte concentrations (eq/L): A, 0.1; B, 0.05; C, 0.01. Montmorillonite as in

Fig. 3 (Inoue and Minato, 1979). From Jasmund and Lagaly (1993).

Table 12.10.2. Classiﬁcation of cations in three groups. Hysteresis was documented for binary

reactions involving cations from different groups (Verburg and Baveye, 1994)

Group 1 Group 2 Group 3

Chapter 12.10: Cation and Anion Exchange986

Rytwo et al., 1996; Tournassat et al., 2004.):

+ 2B

+2X

+ 2A

In addition , the heavy metal ions can be precipitated at the surface in the form of

(hydr)oxides, hydroxy carbonates, or other basic salts (Siantar and Fripiat, 1995)

(for surface precipitation see Chapter 5). In analysing cation exchange data it is

therefore important to separate the exchange reaction from accompanying processes

such as adsorption, surface precipitation, and dissolution (Nagy and Ko

nya, 1988;

Schlegel et al., 199 9b ; Tournassat et al., 2004).

The presence of complexing compounds promotes the exchange of interlayer

cations in clay minerals (Ko

ster et al., 1973; Maes et al., 1982; Rytwo et al., 1996;

nya and Nagy, 1998; Ko

nya et al., 1998; Abollino et al., 2003). The inﬂuence

of solvents on the exchange process was investigated by Hanna et al. (1995) and

El-Batouti et al. (2003).

E. Anion Exchange

Clay minerals also have a certain capacity for anion exchange. For anions, such as

chloride and nitrate, the anion exchange capacity amounts to a few cmol()/kg. By

contrast, up to 20–30 cmol()/kg of phosphate and arsenate can be adsorbed by

kaolinite and montmorillonite (Muljadi et al., 1966a, 1966b, 1966c; Grim, 1968;

Parﬁtt, 1978; Bergseth, 1985; Violante and Pigna, 2002) and distinctly more by

allophanes (Theng et al., 1982). Kaolinite and montmorillonite can also adsorb

10–20 and 20–30 cmol()/kg of ﬂuoride ions, respectively (Weiss et al., 1956).

Clay minerals can adsorb anions by three different mechanisms:

(1) By electrostatic interaction with particle edges when these are positively charged

(see Chapter 5) (van Olphen, 1951b; Schoﬁeld and Samson, 1954; Ferris and

Jepson, 1975).

(2) By exchanging structural OH groups at the edges, and also on basal (planar)

surfaces of kaolinite (Mulja di et al., 1966a, 1966b, 1966c; Parﬁtt, 1978; Theng

et al., 1982). This process is referred to as ‘ligand exchange’ or ‘speciﬁc adsorp-

tion’ although the latter term is outmoded (see Fig. 5.2 in Chapter 5).

(3) By accompanying multivalent cations at exchange positions (see Section D).

In mechanism (1) the extent of anion exchange decreases with increasing pH and

approaches zero at the point of zero net pro ton charge, p.z.n.p.c. (see Chapter 5).

The amount of anions bound by ligand exchange should increase to a maximum

around the p.z.n.p.c. However, the adsorption of anions as counterions in an acidic

medium may decrease with increasing pH as it is generally observed for phosphate

12.10.1. Ion Exchange Reactions 987