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VIII. Factors Affecting Surface Charge

VIII. Factors Affecting Surface Charge

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SURFACE CHARGE AND SOLUTE INTERACTIONS



113



'Lgble VI

Surface Charges of Various Soil Components

Soil components



Surface charge

(C mol kg-I)



Kaolinite



-0.63 to - 1.36



Kaolinite

Kaolinite

Kaolinite



+0.75 (pH 3.0) to -0.3

(PH 5.5)

-1.7

0

- 13 (pH 7.0)



Origin



Reference



Permanent or

structural



Schroth and Sposito (1997)

Proton or variable



Permanent

Net

Net



Cowan et al. (1992)



Mite

Illite



-2.0 to -5.0

+4.0 (pH 3) to -6.0

(PH 9)

-0.6 (pH 6.2) to

- 1.8 (pH 7.7)

-26

-21.0 (pH 7.0)



Bolland et al. (1976)

Ferris and Jepson (1975)

Hendershot and Lavkulich

(1983)

Wieland and Stumm (1992)

Motta and Miranda (1989)



Permanent

Net



Illite

Illite



-8.Oto -120

-21.0 (pH 7.0)



Permanent

Net



Illite



-69.8



Permanent



Smectite



-72 (pH 4.0) to 80 (pH

9.4)

-98

-90.4 to -127.6 (pH

7.0)

- 10.4



Net



Greenland and Mott (1978)

Hendershot and Lavkulich

(1983)

Madrid er al. (1984)

Hendershot and Lavkulich

(1983)

Anderson and Sposito

(1991)

Cowan er al. (1992)



Permanent

Net



Greenland and Mott (1978)

Bouabid er al. (1991)



Permanent



- 124

- 195.3 (pH 7.0)



Permanent

Net



Muscovite



-80.4t0 -117.1 (pH

7.0)

-22 (pH 7.0)



Anderson and Sposito

(1991)

Greenland and Mott (1978)

Bouabid etal. (1991)

Bouabid er al. (1991)



Net



Microcline



-2 (pH 7.0)



Net



Quartz



-2 (pH7.0)



Net



Allophane (natural)

Allophane (synthetic)

Al hydroxide



+3.3 (pH 5.0)

+3.0 (pH 5.0)

+50.6 (pH 5.0)



Net

Net

Net



Kaolinite

Kaolinite

Kaolini te



Montmorillonite

Montmorillonite

Montmorillonite

Vermiculite

Vermiculite

Smectite



Permanent

Permanent

Net



Hendershot and Lavkulich

(1983)

Hendershot and Lavkulich

(1983)

Hendershot and Lavkulich

(1983)

Rajan (1979)

Rajan (1979)

continues



114



N. S. BOLAN ETAL.

Table VI-Continued



Soil component



Al hydroxide

Al hydroxide

Al oxide



Gi bbsite

Gibbsite

Gibbsite

Gibbsite

Si oxide

Fe hydroxide



Fe hydroxide

Lepidocrocite

Goethite

Goethite

Goethite

Goethite

Goethite

Goethite

Humic acid



Surface charge

(C mol kg-I)

+16.0 (pH 3.6) to

+10.0 (pH 6.0)

+56.0 (pH 5.8)

+24 (pH 2.0) to

-8.0 (pH 11.0)

+7.2 (pH 6.0)

+1.8 (pH 9.0)

-0.88 (pH 9.0)

-0.5 to +0.9 (pH 8.0)

+20 (pH 2) to -220

(PH 10)

+72 (pH 3.8) to

-6.0 (pH 6.5)

+34 (pH 5.8)

+80 (pH 4) to

-40 (pH 9.5)

+4.0 (pH 6.0)

0.0 (pH 9.0)

+18.3 (pH 4.5)

-0.38 (pH 9.0)

-0.9 to 4 . 1 (pH 8.0)

+19.0 (pH 3.5) to

-5.8 (pH 10.5)

-330 to -340



Origin

Variable

Variable

Proton

Net

Net

Net

Variable

Net

Variable

Variable

Variable

Net

Net

Net

Net

Variable

Variable

Variable



Reference

Hendershot and Lavkulich

(1983)

Bolan er al. (1985)

Schulthess and Sparks

( 1987)

Hingston et al. (1974)

Hingston et al. (1974)

Hingston et al. (1974)

Mashali (1977)

Schulthess and Sparks

(1989)

Hendershot and Lavkulich

(1983)

Bolan et al. (1985)

Madrid er al. ( 1984)

Hingston et al. (1974)

Hingston er al. (1974)

Hingston et al. (1974)

Hingston et al. (1974)

Mashali (1977)

Madrid and De

Arambarri (1978)

Posner (1 964)



Courchesne (1991) attributed a decrease in SO:- adsorption above PZNC with increasing ionic strength to an increase in negative charge with increasing ionic

strength. However, in most studies, increasing ionic strength increases anion adsorption above PZC and decreases adsorption below PZC (Ryden et d.,

1977; Barrow er al., 1980; Karen and O’Connor, 1982). This suggests that the effect of ionic strength on adsorption operates through its effect on electrostatic potential in the

plane of adsorption rather than through its effect on surface charge. Barrow et al.

(1980) suggested that if increasing ionic strength decreases anion adsorption, then

this implies that the potential in the plane of adsorption must be positive. In most

studies it has often been observed, depending on the pH, that increasing ionic

strength can either increase or decrease HP0;- adsorption. This suggests that

HP0;- can be adsorbed when the potential in the plane of adsorption is either positive or negative. In contrast, SO:- adsorption always decreased with increasing

ionic strength indicating that SO:- is always adsorbed only when the potential is



SURFACE CHARGE AND SOLUTE INTERACTIONS



11s



positive. Black and Waring (1979) obtained a strong positive correlation between

the adsorption of SO:- and nitrate (NO;) for a range of Australian soils and concluded that both NO; and SO:- are adsorbed only onto positive sites. These observations are supported by the work of Marsh et al. (1 987), who obtained a linear relationship between the amount of SO:- adsorbed and positive charge in a

range of some new Zealand soils.

Bolan et al. (l986b) compared the PZSE on pH and the adsorption of HP0,'and SO:- for two soils which varied in their charge components. The effect of ionic strength of the supporting medium on adsorption varied between HP0,'- and

SO:-. For HPOi-, there was a pH at which adsorption increased with increasing

ionic strength and below which adsorption decreased with increasing ionic

strength. The pH at which ionic strength had no effect on HP0:- adsorption

(PZSE on adsorption) decreased with increasing HPOi- adsorption. In contrast to

HPO:-, SO:- adsorption decreased with increasing ionic strength throughout the

pH range and the adsorption curves converged at above pH 7. At this point the

amount of SO:- adsorbed was very close to zero for both soils. It was thus not

possible to establish the effect of SO:- adsorption on the PZSE for adsorption.

The PZSE for both HP0:- and SO:- adsorption occurred at pH values higher

than that for the PZC for both soils.



C. Son. SOLUTIONCOMPOSITION

Changes in ionic strength indirectly affect the charge distribution on a soil component surface and the surrounding aqueous solution, thereby altering the attractive or repulsive interaction between adsorbing anions and the surface (Hingston,

1981). Changes in ionic strength could also be considered strictly as an increase

in the concentration of competing anions. For example, Neal et al. (1987) observed

that in the case of chloride (Cl-) competition with selenite (SeO?) may be minimal but with other anions, such as SO:- and HPOi-, competition for Se0;- in

aqueous solution may be considerable. Rajan (1979) indicated that SeOg- was retained by soils mostly through mechanisms similar to those for HP0:- and SO:-.

A twofold increase in C1- concentration had no effect on S e q - adsorption. Chloride is considered to be adsorbed by outer-sphere surface complexation and should

therefore have little effect on Se0;- adsorption if it involves inner-sphere surface

complexes such as HPOi-. There was no effect of SO:- on SeOZ- adsorption but

the addition of HP0:- substantially decreased SeOi- adsorption.

Adsorption of Se0:- was found to be higher in the presence of calcium (Ca2+)

than sodium (Na+), and the effect of Ca2+on Se0;- adsorption was slight at pH

values where Ca adsorption is expected to be least (Neal er al., 1987). At higher

pH values, Ca2+adsorption increases, and this is where there is increased SeOiadsorption by the solid phases. Bolland er al. (1977) and Curtin er al. (1992) ob-



116



N. S. BOLAN ETAL.



served an increase in the adsorption of HPOi- in the presence of divalent cations

such as CaZ+.An increase in surface charge by Ca2+adsorption could enhance the

adsorption of Se0;- and HP0:- as observed by Bolan er al. (1993) for SO:-. Ryden and Syers (1976) have also observed a similar effect of Ca2+adsorption on

HPO$- adsorption. Naidu er al. (1995) and Naidu and Harter (1998) investigated

the effects of organic ligands on adsorption of Cd2+by soils at varying pH values.

They found that at low pH values changes in solution composition controlled Cd2+

adsorption through changes in the surface charge of soils. At low pH values, adsorption of organic ligands by soils enhanced Cd2+retention, whereas nonspecifically sorbing anions such as NO; had little effect on Cd2+adsorption. The effect

of specifically sorbing anions on electrophoretic mobility and the surface charge

density of soil colloids was also discussed by Harter and Naidu (1995).



D. PH OF SOILSOLUTION

Soil solution pH is one of the major factors controlling surface properties of

variable-charge components (Barrow, 1984; Mora and Barrow, 1996). pH affects

the surface charge through the supply of H+ for adsorption onto the metal oxides

and the dissociationof the functional groups in the soil organic matter. An increase

in pH increases the net negative charge and a decrease in pH increases the net positive charge. Thus, change in surface charge has been considered as one of the reasons for the effect of pH on anion and cation adsorption.

Studying anion adsorption at a range of pH values, Hingston (198 1) obtained a

relationshipbetween the apparent Langmuir maxima for a range of anions and pH.

This was termed the “adsorption envelope” and an attempt was made to relate the

characteristics of the envelope to properties of the adsorbent and the adsorbate.

Apparent maxima in the envelope were found at the pK, values for anions with

monoprotic conjugate acids and the breaks of slope were found at pKu values for

anions of polyprotic conjugate acids. A good correlation was found between points

of inflection in the adsorption envelope and pKa values for conjugate acids. The

correlation between the tendency for anions to react at oxide surfaces through ligand exchange and the pKu of their conjugate acids is analogous to the correlation

between the log of stability constants for complex formation with metal ions and

pKu value of the acids correspondingwith various ligands. Indeed, chemical bonding for anions specifically adsorbed on oxides would be expected to be similar to

the bonding in complexes and crystalline compounds. The characteristic used to

distinguish between adsorption and compound formation is that adsorption occurs

only at the interfaces.

Bolan er al. (1 997) observed that the adsorption of Cd2+increased with an increase in pH, consistent with the findings of Tiller er al. (1979), Basta and Tabatabai

( 1992), and Naidu er al. ( 1994a). Three possible reasons have been advanced for



SURFACE CHARGE AND SOLUTE INTERACTIONS



117



the increase in Cd2+adsorption with increasing pH (Naidu et al., 1994a). First, in

variable-charge soils, an increase in pH causes an increase in surface negative

charge resulting in an increase in cation adsorption. Second, an increase in soil pH

is likely to result in the formation of hydroxy species of metal cations which are

adsorbed preferentially over the metal cation (Hodgson et al., 1964). Naidu et al.

(1994a) observed that CdOH+species are formed above pH 8 which have a greater

affinity for adsorption sites than do Cd2+species. Third, precipitation of Cd2+as

Cd(OH), is likely to result in greater retention at pH values above 10.

Bolan et al. (1997) attempted to relate the pH-induced increases in surface

charge to an increase in Cd2+adsorption. Approximately 50% of the increase in

surface negative charge was found to be occupied by Cd2+.The remaining surface

negative charge was expected to be occupied by the H+ and K+added in acid and

alkali to alter the soil pH. This indicates that the increase in Cd2+adsorption with

an increase in pH is attributable to an increase in negative charge (Fig. 8). Simi-



0



0

0



A



A

A



0



50



0



I



I



I



I



100



150



200



250



increase in negative charge (mmoi kg-')



Figure 8 Relationshipbetween the increase in Cd2+adsorption and the increase in negative charge

for the Egmont ( 0 )and the Manawatu (A) soils. The dotted line indicates the stoichiometric relationship between the amount of Cd2+ adsorption and the amount of negative charge required (2 mol of

charge per mole of CdZ+).The increase in negative charge was achieved by increasing the pH of the

soils (Bolan et al., 1997).



118



N. S. BOLAN ETAL.



larly, Naidu et al. (1994a) demonstratedthat the ionic strength effects on Cd2+adsorption operate through the effect of Cd2+on surface charge.



E. SPECIFICADSORPTIONOF ANIONS AND CATIONS

It has often been observed that the specific adsorption of anions and cations contributes charge to the surface (TableW,Fig. 9). The increase in surface charge with

specific adsorption of anions and cations can be explained both by double-layer

theory and by empirical reaction equations. Using empirical equations (White,

1980) it has been demonstratedthat, in the case of HF'Oi- adsorption, the charge

conveyed to the surface is large when the HPOz- displaces the water molecules

coordinated with the surface and small when it displaces the OH-. The same description is achieved by focusing attention on the way the charge on the HPOa- is

balanced. In the reaction involving displacement of water molecules, the charge

on the €PO:- must be balanced by changes in the electrolyte ions associated with

the surface,either an increasedsurface concentrationof cations or a decreasedconcentration of anions. That is, charge balance is outside the boundary of the surface

and the net change in the charge on the surface would be equal to the charge on

the anion. In the second reaction the charge balance occurs by displacement of

OH-. That is, the charge balance is inside the boundary of the surface and the net

change in charge of the surface would be zero. However, neither of these equations by itself an adequate representation of the behavior. There is always some

negative charge conveyed to the surface and the amount is always less than the

charge on the anion (Hingston 1970; Bowden et al., 1980; Naidu et al., 1990b).

Similarly, the output of Bowden's variable-charge model also shows that the

charge added to the surface per unit molecule of HPOZ- adsorbed (5) is always

less than 2 (Bowden et al., 1980).

The amount of charge added to the surface through anion adsorption varies depending on the net charge on the surface. At pH values below the PZC, the surface

is positively charged. In the absence of HF'Oa- adsorption, this positive charge on

the surface is balanced by the electrolyte anions (e.g., C1-). At low adsorption, the

negative charge on the adsorbed HF'0:- is largely balanced by displacement of

the C1-. Therefore, the decrease in positive charge on the surface would be rapid

initially and produce large 5 values at low surface coverage. With increased

HF'Oi- adsorption there will be fewer C1- to be displaced and there will be a

greater tendency for the negative charge on the adsorbed €€PO:- to be balanced

by adsorption of H+ onto (or release of OH- from) the surface plane. The net

charge conveyed to the surface is therefore smaller and 5 is decreased. When the

surface becomes negative (at high pH or at high levels of adsorption) the charge

on the adsorbed €€PO:- is balanced by the adsorption of electrolyte cations (e.g.,

Na+).Thus, the charge balance moves increasinglyto the region outside the bound-



SURFACE CHARGE AND SOLUTE INTERACTIONS



119



\

n



a



1

I

0



50



'



I

100



'



I

160



'



I

200



'



I

260



Phosphats adsorbsd (mmol P kg-')



0.40



0.00



I

0



60



'



I

100



'



I

150



'



I

200



'



I

250



Phosphats sdsorbod (mmol P kg-')



Figure 9 Relationship between (left) the total negative surface charge and the amount of phosphate adsorbed (0,Egmont soil; A, Manawatu soil) and (right)the negative charge added per unit phosphate adsorbed (0,Egmont soil; A, Manawatu soil) and the amount of phosphate adsorbed (Bolan et

al., 1997).



ary of the surface and the value for 5 increases when the surface charge becomes

net negative. The negative charge added to the surface by adsorption of H P O has been found to increase with the ionic strength of the background electrolyte

(Atkinson eral., 1967; Hingston el al., 1972; Ryden and Syers, 1975) because with

increasing ionic strength the increased availability of electrolyte ions causes the

charge balance to increasingly move to the outside boundary of the surface.

The charge added to the surface during anion adsorption depends on the charge

on the anion adsorbed (Hingston, 1970), but it also differs between the anions of

the same charge. The 5 values for SO:- adsorption on gibbsite were found to be

larger than those for HPO:-, with both anions having the same charge (Hingston,

1970).This difference can be explained by the position of the plane of adsorption.

If the mean plane of adsorption is further away from the solid surface the charge

balance is mainly outside the boundary of the surface. Hence, when the surface is

positive the charge on the adsorbed anion is more likely to be balanced by the displacement of electrolyte anions; when the surface is negative, it is likely to be balanced by the adsorption of electrolyte cations. As a result, the net charge conveyed

to the surface will be larger. This may suggest that the mean plane of adsorption

of SO:- is further away from the surface than that of HPO:-. On the other hand,

if the mean plane of adsorption is close to the surface the charge on the adsorbed



120



N. S. BOLAN ETAL.



anions would be mainly balanced inside the boundary of the surface by uptake of

H+ (or release of OH-). As a result, the net charge conveyed to the surface would

be small. This is probably the situation for fluoride (F-), for which the value of 5

has been found to be very small (Hingston, 1970). It is reasonable to assume that

the small F- is adsorbed very close to the surface.

In summary, while the nature and quantity of soil components affect the permanent and variable charge, soil solution composition affects mainly the variable

charge. In general, soils containing noncrystalline materials, such as organic matter and short-range order Fe and A1 oxides and hydrous oxides, carry large net surface charge. The nature and the concentration of cations and anions in the soil solution affect variable charge through specific adsorption and electrostatic charge

balance mechanisms. In general, an increase in pH increases the net negative

charge and a decrease in pH increases net positive charge, resulting in increased

adsorption of anions and cations at high and low pH values, respectively.



IX. EFFECT OF SURFACE CHARGE

ON SOIL PROPERTIES

A. SOLUTEINTERACTIONS

Charged solute species (ions) are attracted to the charged soil surface by electrostatic attraction and/or through the formation of specific bonds (Mott,1981).

Retention of charged solutes by charged surfaces is broadly grouped into specific

and nonspecific retention. In general, nonspecific anion adsorption is a process in

which the negatively charged anions balance the positive charges on the soil particles through electrostaticattractions, whereas specific adsorption involves chemical bond formation between the anions and the ions in the soil surface. Nitrate and

Cl- are considered to be adsorbed by a nonspecific process and the adsorption of

HPOZ- and SO:- involves both specific and nonspecific adsorption processes.

The main differences between the specific and nonspecific adsorption processes

are presented in Table VII. Marsh et al. (1988) suggested that SO:- was adsorbed

electrostatically, although the amounts of SO:- adsorbed exceeded the measured

positive charge.

Cation adsorption is largely determined by the amount of surface negative

charge (Bouabid et al., 1991; Kookana et al., 1994). It is well established that the

mechanism of K+ fixation in 2: 1 phyllosilicates is the entrapment of K+ in the interlayer space. Entrapment is due to the collapse of adjacent silicate layers and the

associated dehydration of the interlayer cations (Grim, 1968). The low hydration

energy of K+and its size, similar to that of ditrigonal holes in the tetrahedral sheets,

explain why K+ is preferentially fixed by 2: 1 phyllosilicates. Total charge density



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-



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