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IX. Effect of Surface Charge on Soil Properties

IX. Effect of Surface Charge on Soil Properties

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SURFACE CHARGE AND SOLUTE INTERACTIONS



121



Table VII

Comparison between Nonspecific and Specific Anion Adsorption Procesws

Nonspecificadsorption



Specific adsorption



Electrostatic attraction between the negatively

charged anions and the positive sites on the

soil particles

Balances the positive charges on the surface and

hence no new charges are added to the surface



Chemical bond formation between the anions

and the ions on the soil surface



Significant adsorption occurs only when the soil

is net positively charged

Adsorption depends on the number of positive

charges (anion exchange capacity) on the

surface

In variable-charge soils, the adsorption is high

at low pH and decreases with an increase in

soil pH

Adsorption is weak and reversible



Add negative charge to the surface and the

number of negative charges added is

generally less than the anion charge

Adsorption occurs even when the surface is

net negatively charged

Adsorption exceeds the anion exchange

capacity of the soils

Adsorption occurs over a wide range of soil

pH values

Adsorption is strong and less reversible



and distribution of the charge between the tetrahedral sheets are also important

characteristics which influence the extent of K+ fixation by phyllosilicates (Goulding, 1983; Inoue, 1983).

Horvath and Novak (1975) and Ruhlicke (1985) found that the amount of K+

fixed by vermiculite and smectites is related to the total charge density. Weir and

White (195 1) and van Olphen (1966) stated that when the charge is concentrated

in the tetrahedral sheets, K+ is bound by stronger electrostatic forces because of

the proximity of charge to the interlayer K+. Barshad and Kishk (1970) and Ristori (1979) found that for smectite with similar total charge densities, those with

higher tetrahedral charge fixed more K+ than those with higher octahedral charge.

A good correlation is generally observed between K+ fixation and total CEC and

tetrahedral CEC. Although octahedral CEC is not correlated with K+ fixation, it

does contribute to total interlayer charge density and thus to K+ fixation. Bouabid

et al. (1991) observed that tetrahedral and octahedral charges contribute to 64 and

36% of K' fixation, respectively. This effect of the former charge is due mainly to

the proximity of tetrahedral charge to the interlayer space in 2:l phyllosilicate

clays. The fact that the intercept of the relationship between K+ fixation and total

CEC is close to zero indicates that total CEC accounts for most of the K+ fixed.

The effect of pH values >6 in lowering free metal ion activities in soils has been

attributed to the increase in pH-dependent surface charge on oxides of Fe, Al, and

Mn (Stahl and James, 1991), chelation by organic matter, or precipitation of metal hydroxides (Lindsay, 1971). The larger the CEC of the soil, the lower the satu-



122



N. S. BOLAN ETAL.



ration of the exchange sites by a given Zn2+concentration.The effect of pH on the

activity of Zn2+ in solution in naturally acidic soils is found to decrease with increasing pH. The gradual decrease in Zn2+activity with increasing pH is attributed to increasing CEC (Shuman, 1986). Similarly, Stahl and James (1991) observed that with an increase in surface charge there was an increase in Zn2+

retention but nonexchangeable Zn2+sorption was favored over exchangeable Zn2+

retention. In general, both the CEC and the total amount of Zn2+ removed from

soil solution increased with an increase in soil pH.

Thus, the extent of nonspecific adsorption of cations and anions by soils depends largely on the amount of negative and positive charge, respectively, whereas the specific adsorption of cations and anions generally exceeds the amounts of

charges in soils. The effect of soil solution composition (ionic strength, pH, etc.)

on the adsorption of cations and anions operates through its effect on surface

charge.



B. ANION- AND CATION-INDUCED

ADSORPTION

The increase in surface charge due to the specific adsorption of anions and

cations (see Section VIII) induces the adsorption of other ions. Anion-induced

cation adsorption has been reported for many cations (Ryden and Syers, 19676;

Bolland et al., 1977; Wann and Uehara, 1978a; Shuman, 1986; Kamewada and

Takahashi, 1996). Specific adsorption of inorganic anions onto variable-charge

components has often been shown to increase the surface negative charge (Bolan

and Barrow, 1984; Barrow, 1987). Ryden and Syers ( 1976) concluded that the retention of Ca2+in response to HP0;- sorption by soils results from the increase

in negative charge induced by HPOi- sorption. Similarly, Bolland er al. (1977)

and Shuman (1986) observed that the specific adsorption of anions, such as

HP0:- and SO:-, increases the adsorptionof Zn2+by variable-chargesoils. Wann

and Uehara (1978b) observed that K+ added in the presence of HP0:- is less susceptible to leaching than that added in the presence of other anions. Kuo and

McNeal(l984) reported that HPOi- adsorption increases the adsorption of Cd2+

by hydrous ferric oxide. Anion-induced cation adsorption depends on the variablecharge components of the soils. Addition of SO:- has been shown to increase A13+

adsorption possibly due to a SO:--induced increase in negative surface charge

(Gibson et al., 1992).

Naidu ef al. (1994b) studied the effect of inorganic ligands on Cd2+adsorption

by an Oxisol and a Xeralf. They found that adsorption increased markedly in the

presence of SO:- and HPO:-. A subsequent detailed study using two soils which

varied in variable-charge components showed that there was only a small effect

of increasing HP0:- adsorption on Cd2+adsorption by a soil dominated by permanent-charge silicate clay minerals (Bolan er al., 1997). However, increasing



SURFACE CHARGE AND SOLUTE INTERACTIONS



123



adsorption of HPOi- caused a significant increase in the adsorption of Cd2+ by

a soil dominated by variable-charge components. The increase in Cd2+ adsorption per unit increase in HPOi- adsorption decreased with increasing HPOi- adsorption.

Several mechanisms can be advanced for the positive effect of HPOi- on Cd2+

adsorption: (i) precipitation of Cd2+as CdJPO,),, (ii) coadsorption of HPOi- and

Cd2+ as an ion pair, (iii) surface complex formation of Cd2+ onto the adsorbed

HPOi-, and (iv) HPOi--induced Cd2+adsorption.

Precipitation of Cd,(PO,), was ruled out because the concentrations of HPOiand Cd2+were deliberately kept below 0.1 M and the soil systems were unsaturated with respect to Cd3(P0,),. Furthermore, the Cd2+adsorbed with an increasing concentration of HPOi- was completely recovered in MgSO,. Similarly,

many authors have shown that the solubility of Cd3(P0,)2 is too high to control

the concentration of Cd2+ in suspensions involving Fe and A1 oxides and soils

(Bolland et al., 1977; Street et al., 1978; Soon, 1981; Kuo and McNeal, 1984;

Naidu et al., 1994b).

Marcano-Martinezand McBride (1989) observed an increase in the adsorption

of Ca2+in the presence of SO;- which was attributed to cooperative adsorption

of Ca2+and SO;- as an ion pair. Equimolar adsorption of Ca2+and SO:- at a high

concentration of CaSO, has been taken as evidence for coadsorption of Ca2+and

SO:- as an ion pair (Aha et al., 1990).

It has been suggested that specifically adsorbed anions such as HPOi- form

complexes with the soil surface so that cations are adsorbed on the adsorbed anion. Helyar et al. ( 1976)and Bolland er al. ( 1977) proposed a similar complex formation for the increased sorption of Ca2+onto HPOi--enriched gibbsite and Zn2+

onto HPOi--enriched goethite.

It has been shown that Zn2+,Cd2+,or Cu2+sorption by A1 and Fe oxides can be

increased by low or moderate enrichment of oxides with HPOi- (Bolland et al.,

1977; Kuo, 1986).This may be due to the increased surface negative charge or potential (or reduced surface positive charge) after HPOi- adsorption. Similarly,the

increases in the adsorption of Ca2+(Ryden and Syers, 1976) and Cd2+(Bolan et

al., 1997; Fig. 10) by soils with HPOi- adsorption have been attributed to an increase in surface negative charge.

Cation-induced anion adsorption by variable-charge soils has also been reported (Katou er al., 1996). For example, Bolan er al. (1993) observed that the adsorption of SO;- ions by variable-charge soils was higher in the presence of Ca2+

than K+. Various mechanisms have been suggested for the increase in SO;- adsorption in the presence of the Ca2+.First, the increase in the adsorption of anions

such as H2PO; and SO:- in the presence of Ca2+has been related to the formation of a surface complex between the anion and Ca2+(Helyar et al., 1976). This

involves coordination of one Ca2+to two adsorbed anion groups, reducing the repulsive force between two adjacent anion groups and thereby enhancing further



N. S. BOLAN ETAL.



124



0



20



40



80



80



100



Increase In negatlve charge (mmol (4kg-I)



Figure 10 Relationship between the increase in Cd2+ adsorption and the increase in negative

charge. The increase in negative charge was achieved through phosphate adsorption when phosphate

was added as calcium phosphate (0)

or potassium phosphate ( 0 )

(Bolan et al., 1997).



adsorption. Second, increased adsorption of SO$- and H2PO; at higher levels of

Ca2+addition has been attributed to precipitation reactions occurring at high pH

(>7.0) (Adams and Rawajfih, 1977; Freeman and Rowell, 1982). Third, specific

adsorption of Ca2+ by hydrous oxides has been shown to increase the positive

charge on the surface (Kinniburgh et al., 1975; Kinniburgh, 1983) and thereby increase the adsorption of anions. Marcano-Martinez and McBride (1989) proposed

a mechanism involving CaSOt ion pair adsorption on mineral surface in which the

presence of one ion of high concentration facilitates the formation of an ion pair

and increases the adsorption of the other ion.

Bolan et al. (1993) observed that the increase in positive charge with Ca2+adsorption accounted for most of the increase in SO:- adsorption at low levels of

Ca2+(<0.002 M) in solution. The role of positive charge in SO:- adsorption by

soils has been documentedwell (Marsh etal., 1987,1988; Curtin and Syers, 1990).

Thus, the specific adsorption of anions and cations increases the net negative and

positive charge, respectively,and thereby increases the adsorption of ionic solutes.

Addition of specifically adsorbed anions has been attempted to increase the CEC

of variable-charge soils (see Section X).



SURFACE CHARGE AND SOLUTE INTERACTIONS



125



C. DISPERSION

AND FLOCCULATION

Dispersion and flocculation of colloid particles are often manifested through

changes in surface potential and charge densities. As discussed earlier, when two

colloid particles approach each other, their diffuse layers overlap and repulsion is

experienced between them. This interaction of the diffuse layer and the surface potential of colloid particles contributes to the overall stability of colloids. Thus, manipulation of particle charge densities assists management of dispersive soils. Such

charge manipulation has often been obtained by the use of inorganic salts. Added

salts can reduce both the effective surface potential and the extent of the diffuse

layer, giving a lower colloid stability. Generally, indifferent electrolytes are not

used as flocculating agents, probably because they do not allow the formation of

strong aggregates which can withstand the shear forces encountered in most flocculation studies. Salts with specifically adsorbing counter ions are much more

effective.

Dispersion is caused by mutual repulsion of soil particles because of surface

charge. If the repulsive forces are dominant, soil becomes dispersed and virtually

unmanageable in an agronomic sense. It is the balance of attractive and repulsive

forces which determines whether a soil is flocculated or dispersed. Double-layer

theory is remarkably successful in explaining the flocculation and deflocculation

behavior of soils.

Factors which affect the surface charge of soil particles determine the extent of

dispersion, including electrolyte concentration of the soil solution, the valence of

the dominant cation occupying the exchange sites, and pH (Arora and Coleman,

1979; Shainberg et al., 1989; Itami and Kyuma, 1995). Suarez et al. (1984) and

Chiang er al. (1987) indicated that pH is one of the important factors affecting dispersion, and the sensitivity of hydraulic conductivity to pH changes depends on

the quantity of variable-charge minerals and organic matter present in the soil.

Soils with large amounts of variable charge are likely to be most susceptible to pH

effects.

Arora and Coleman (1979) found that increasing the pH of a Georgia kaolinite,

which showed variable-charge properties, increased dispersion more than in any

of the permanent-charge clays, including smectites, illites, and vermiculites. Also,

Chiang et al. (1987) observed that whereas an increase in the pH of Cecil soil,

which contained a significant amount of variable-charge components, caused a decrease in K,,a similar increase in the pH of Davidson and Iredell soils, which contained no variable-charge components, resulted in no change in the K,.

Bolan et al. (1 996b) examined the effect of pH on dispersion and saturated hydraulic conductivity (K,)in two soils with different variable-charge components.

Dispersion remained constant between pH 4.4 and 6.4 and increased on either side

of these pH values. The relationship between pH and K, was the mirror image of

that between pH and dispersion.At the lowest pH value (pH 2 or 3), there was ev-



126



N. S. BOLAN ETAL.



idence for the dissolution of Fe and A1 compounds. Such compounds can act as

binding agents (Deshpande et aZ., 1964; El-Swaify, 1976) and their dissolution

may be one of the reasons for the sharp increase in dispersion and consequent decrease in K,.However, the main effect of pH on dispersion and Ks is generally attributed to its effect on charge (Rengasamy and Naidu, 1994).

The effect of pH on dispersion was investigated by obtaining a relationship between dispersion and pH relative to PZNC. Except for the lowest pH values, at

which dissolution of the soil surface occurred, dispersion increased as the difference between the PZNC and pH increased. In other words, when the pH of the soil

is close to PZNC, the net charge is less and hence there is less repulsion between

the particles, resulting in flocculation. When the pH is further away from the

PZNC, the net charge increases and hence particles repel each other, causing dispersion. Gillman (1974) also observed that when the pH of a soil is close to its

PZNC, the amount of water-dispersibleclay becomes small. Shanmuganathanand

Oades (1982) reported that the addition of Fe polycations increased the PZNC of

soil and complete flocculation occurred when a sufficient amount of polycation

had been added to increase its PZNC to the pH of the soil.

Bolan et al. (1996b) observed that the effect of pH on dispersion varied between

the Na+- and the Ca2+-saturatedsoils. At the same value of net charge, the Ca2+saturated soils exhibited less dispersion than the Na+-saturated soils. This can partly be explained by the increased surface charge screening mechanism of Ca2+compared to Na+, developed from the Derjaguin, Landau, Verwey, and Overbeek

theory (Greene et aZ., 1973). It has often been observed that the dispersion of clay

decreases as the percentage of Ca2+-saturation increases (Rengasamy, 1983),

which has been related to the decrease in charge density (van Olphen, 1977).The

thickness of the DDL of the Ca2+-saturatedsoil samples is likely to be smaller than

that of the Na+-saturated soils. As the DDL becomes smaller, the soil particles are

attracted to each other resulting in increased flocculation and greater Ks.

In summary, dispersion is caused by mutual repulsion of soil particles because

of surface charge and dispersion can be reduced by reducing the surface charge

and/or by decreasing the thickness of the DDL. When the pH of a soil is brought

close to the PZNC, the net surface charge will decrease, resulting in flocculation.

Similarly, when the soil is saturated with polyvalent cations, such as Ca2+,the

thickness of the DDL will decrease, resulting in flocculation.



X. MANIPULATION OF SURFACE CHARGE

TO CONTROL SOLUTE INTERACTIONS

Various attempts have been made to control the retention of anions and cations

by manipulating the surface charge of variable-charge soils. Wann and Uehara



SURFACE CHARGE AND SOLUTE INTERACTIONS



127



(1978a) suggested that, unlike soils with constant surface charge mineralogy, the

surface charge density of soils with variable surface charge (or constant surface

potential) mineralogy should be treated as a management variable. Because variable charge largely depends on the pH of the soil solution, treatment of soils with

pH amendments has frequently been used to control the reactions of nutrient ions

and toxic heavy metals. Similarly, addition of specifically adsorbed anions can increase the CEC of soils. Addition of electrically charged porous materials, such as

exchange resins, bark materials, and other organic materials, enhances the retention of cations and anions in soils.



A. LIMING

Liming of soils has often been shown to decrease the retention of anions, such

as SO:- (Marsh er al., 1987; Bolan et al., 1988b) and HPO$- (Naidu et al.,

1990b), and increase the retention of cations, such as nutrient ions (Adams, 1984)

and toxic heavy metals (Alloway and Jackson, 1991; Helmke and Naidu, 1996).

Bolan et al. (1988b) and Naidu ef al. (1 990b) observed that the addition of liming

materials increases soil pH and thereby decreases the positive charge and the adsorption of SO:- (Table VIII) and HPOi-, respectively.

The decrease in adsorption of anions increases their uptake by plants and their

loss by leaching (Bolan et al., 1986a;Motavalli et al., 1993).Addition of lime has

often been observed to increase the concentration of anions such as SO:- in soil

solution (Probert, 1976; David et al., 1982; Bolan et al., 1988b), and several reasons have been proposed to explain this (Freney and Stevenson, 1966; Korentager



Table VIII

The Effect of Liming on Surface Charge and the Adsorption of Sulfate"

Surface charge (mmol kg-')

Soil

Patua



Tokomaru



Lime added

(mmol kg-I)



PH



+ve



0

160

320

600

0

40

80

160



4.7

5.6

6.4

7.0

4.9

5.9

6.5

6.8



11.9

8.0

3.9

3.0

3.9

2.0

I .o

0



"After Bolan eta!. (1988b).



-ve

53.9

157.5



236.2

277.6

74.6

83.9

96.3

109.8



Sulfate adsorbed

(mmol kg-I)

4.67

2.52

1.88

1.44

0.66

0.43

0.38

0.37



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