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IX. Effect of Surface Charge on Soil Properties
SURFACE CHARGE AND SOLUTE INTERACTIONS
Comparison between Nonspecific and Specific Anion Adsorption Procesws
Electrostatic attraction between the negatively
charged anions and the positive sites on the
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
In variable-charge soils, the adsorption is high
at low pH and decreases with an increase in
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
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-
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
B. ANION- AND CATION-INDUCED
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
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.
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
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
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
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-
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
(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.
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
The Effect of Liming on Surface Charge and the Adsorption of Sulfate"
Surface charge (mmol kg-')
"After Bolan eta!. (1988b).