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III. Development of Surface Charge

III. Development of Surface Charge

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SURFACE CHARGE AND SOLUTE INTERACTIONS



91



soil solution. The variable charge of soils is also affected by other factors, such as

ionic strength (I)of the soil solution and reactions with anions and cations. Important variable-chargeconstituents in soils include oxides and hydroxides of iron (Fe),

aluminum (Al), titanium (Ti), manganese (Mn), and organic matter.

In Table I the nature of charge development in various soil constituents is presented (Gillman and Uehara, 1980). Although clay minerals are considered either

permanently or variably charged, these classifications are idealized end points

(Lewis-Russ, 1991).Isomorphous substitution can occur in oxides and hydrous oxides, thus imparting some permanent charge to these predominantly variable-charge

minerals. Single minerals may exhibit both permanent and variable charge

(Schulthess and Huang, 1990). For example, the broken edges of permanently

charged clay minerals, such as kaolinite and halloysite, are sites of variable charge

(Bolland et al., 1976;Chorover and Sposito, 1995;Schroth and Sposito, 1997).Similarly, the variable-chargedchloritic group of minerals consists of montmorillonite

and vermiculite which formed under acidic conditions and incorporated dissolved

A1 in the interlayers. The interlayers compensate for any permanent charge substitution so the charge character of these clays is variable (Uehara and Gillman, 1980)

Permanent and variable surface charges are developed by three processes

(Stumm and Morgan, 1981; Sposito, 1992):

1. Isomorphous substitution

2. Dissociation and association of protons (H+) (protonation/deprotonation)

3. Specific adsorption of anions and cations

While permanent charge is developed by the first process, variable charge is developed by the last two processes.



Table I

Permanent- and Variable-Charge Constituentsin Soils

Component



Permanent charge



Variable charge



Kaolinite

Halloysi te

Chloritic

Smectite

Illite

Micaceous

Montmorillonite

Vermiculite

Allophane

Fe and Al oxides

Organic matter



J

J

J

J

J

J

J

J



J

J

J



-



-



-



J

J

J



92



N. S. BOLAN ETAL.

Table I1

Ionic Size of Various Cations, Substitution in the Clay Layer, and the Development

of Surface Charge



Ion

Si4+

~13+

Fe3+

Mg2+

Fe2+

Ca2+

Na+



Radius (pm)



Substitution by



Substitution layer



4.2

5.1

6.4

6.6



~ 1 3 +



Tetrahedral

Dioctahedral

Trioctahedral

Trioctahedral

Trioctahedral

Trioctahedral

No substitution

No substitution

Interlayer

Interlayer



7.0

9.9

9.7

13.3



K+

NH:



Mg2+

Mg2+

~ 1 3 +



Fe3+

~ 1 3

+



NH:

K+



Charge

Negative

Negative

Negative

Positive

Positive

Positive



No charge

No charge



A. ISOMORPHOUS

SUBSTTIWTON

Permanent charge is developed by substitution of ions of similar sizes but with

different charges in the lattice structure of clay minerals (Table 11). Permanent negative charge is developed by substitution of higher valence cations with lower valence cations. For example, if in an array of solid SiO, tetrahedra a Si4+is replaced

by an A13+,a negatively charged framework is established. Similarly, isomorphous

replacement of the A13+by a Mg2+in networks of A1,0, dioctahedral sheets of silicate clay minerals leads to a negatively charged lattice (Fig. l). On the other hand,

permanent positive charge is developed by substitution of lower valence cations

with higher valence cations. For example, substitution of a Mg2+by an A13+in the

trioctahedral layer of silicate clay minerals results in net positive charge (Fig. 2)

Isomorphous substitution is more common in 2: 1-type silicate minerals than in

Si4+substituted by A13+



SizOI



SiAlO*-



Figure 1 Developmentof surface negative charge through the substitution of Si4+by A13+ in the

tetrahedral layer of a silicate clay mineral.



SURFACE CHARGE AND SOLUTE INTERACTIONS



OR



93



OR



0



I

MC

I



A$+

I



OH



OK



Figure 2 Development of surface positive charge through the substitution of MgZ+by A13+in the

trioctahedral layer of a silicate clay mineral.



other minerals (Table III). For example, in the common micas, muscovite and biotite, every fourth Si4+is replaced by an A13+, and the resultant charge deficit is

balanced internally by K+ in the interlayer positions. While the permanent negative charge is developed in both tetrahedral and dioctahedral sheets of silicate

clays, the permanent positive charges are developed only in trioctahedral sheets.

Sparingly soluble salts also cany a surface charge because of lattice imperfections.



B. PROTONATION/DEPROTONA~ON

When metal oxides are suspended in water, the metal ions near the surface or

broken edges tend to coordinate with water molecules rather than with the hydroxyl (OH-) groups which make up the bulk of the material. These surface water molecules then tend to lose or gain protons (H+) depending on the H+ concentration in solution.When the pH is low (with a higher H+concentration)the surface

gains a H+ resulting in an excess of positive charge. Similarly, when the pH is high



lbble III

Examples of Isomorphow Substitutionin Clay Layers and the Amount of Charge Developed

Substitutionand net charge

Mineral



Vermiculite

Muscovite

Vermiculite



Octahedral sheet



Tetrahedral sheet



2: 1-type dioctahedral minerals

AI,,,M&.3 (-0.3)

Siz.6Al,, (-0.4)

Si,AI (- 1.00)

A4

2: I-type trioctahedral minerals

Mg,,Fe,,;

(+0.3)

Siz,5(Al.Fe),,5(- 1.00)



Charge per unit formula



-0.70

- 1.00

-0.70



94



N. S. BOLAN ETAL.

AI(OH);



-



AlOH'



A10



High pH (net negative)



Low pH (net positive)



Figure 3 Development of variable surface positive charge through the dissociation or association

of H+on a mineral surface.



the water molecules tend to lose H+,resulting in an excess of negative charge (Fig.

3).

Alternatively,depending on the pH, dissociation of H+ (deprotonation)or OH-,

(protonation) from the exposed surfaces of minerals and organic matter results in

the the creation of surface charge (Fig. 4). Most oxides and hydrous oxides exhibit

such amphoteric behavior. Thus, the charge is strongly pH dependent; at low pH

a positively charged surface prevails, and at high pH a negatively charged surface.

The mechanism by which H+ is lost or gained by soil organic matter differs in

that it involves functional groups, such as COOH and OH groups. Again, the

process of charge development by organic matter through dissociation of H+ depends on the pH. The pH at which the dissociation of H+ occurs varies between

the different functional groups on the organic matter (Morrison and Boyd, 1973).

The development of net negative or net positive charge on organic matter depends

on the relative distribution of the various functional groups (Harter and Naidu,

1995) and this can influence the metal-binding capacity of soils.

Dissociation of H+ from COOH or OH functional groups in soil organic matter

results in net negative charge (Fig. 4). Similarly,on a bacterium surface the charge

results from protolysis of functional amino (NH,) and COOH groups (Fig. 5 )

(Stumm and Morgan, 1981;Huysman and Verstraete, 1993).



C. SPECIFICADSORPTION

OF ANIONSAND CATIONS

Charge can also originate by a process in which a solute becomes coordinately

bound to solid surface. For example, the enrichment of iron hydrous oxides with

phosphate (HPOi-) results in net negative charge:

FeOOH,,,



+ HPOi- @ FeOHPO& + OH-



(3)



This phenomenon, which is frequently referred to in the literature as specific adsorption, causes charge reversal (Parfitt, 1978; Barrow, 1985). Specific adsorption



COOH



COO



+



OH



0



+



H'

H+



(pH>3)

@H>9)



Figure 4 Development of variable surface positive charge through the dissociation or association

of H+on an organic matter surface.



SURFACE CHARGE AND SOLUTE INTERACTIONS



95



Figure 5 Developmentof variable surface positive charge through the dissociation or association

of H+on a bacterium surface.



of anions and cations conveys negative (Hingston et al., 1972; Ryden and Syers,

1975; Wann and Uehara, 1978a; Naiduef al., 1990a) and positive charge (Parks,

1967; Bolan et al., 1993), respectively, to the surface.

Many studies have shown that specific adsorption of anions and cations increases the net surface charge of variable-charge surfaces (Table IV). The amount

and the nature of surface charge acquired through specific adsorption depends on

the nature of anion and cation adsorbed and the pH and electrolyte concentration

of the solute (Bowden et al., 1980; Yu, 1997). Addition of HPOi- and silicate

(SO:-) to soils has been done in an attempt to increase the negative charge [or

cation exchange capacity (CEC) of variable-charge soils (Wann and Uehara,

1978a,b;Naidu et al., 1990b).The mechanisms involved in the development of surface charge through specific adsorption will be discussed in detail in Section IX.

In summary, permanent charge in soils originates from isomorphous substitution of metal ions in layer silicate minerals. This charge develops over a long period during pedogenic weathering and cannot easily be altered by soil management

Table IV

Increase in Surface Charges Due to SpecificAdsorption of Anions and Cations

Soil constituent



Solute



pH



Iron hydrous oxide

Allophane

Soil



Phosphate

Phosphate

Phosphate

Sulfate

Phosphate



6.5

5.1

6.5

6.5



Soil



Soil

Soil

Aluminum oxide

Soil

Soil

Soil



Phosphate

Phosphate

Sulfate

Sulfate

Calcium

Calcium



5.0

6.5

7.5

7.0

5.8



5.0

5.6

5.8

5.8



Charge added

(mol mol-' ion)

1.25



0.5

0.65

0.26

0.38

0.47

0.77

0.35-0.7



0.52

1.06

0.25

0.35-0.58

0.52



Reference

Bolan et al. (1985)

Rajan et al. (1974)

Bolan et al. (1986b)

Sawhney ( 1974)



Schalscha et al. (1974)

Naidu et al. (199Ob)

Rajan (1978)

Curtin and Syers ( 1990)

Bolan et a[. (1993)

Ryden and Syers (1975)



96



N. S. BOLAN ETAL.



practices. Variable charge in soils originates from the dissociation of H+or OHfrom functional groups of soil organic matter and from the specific adsorption of

anions and cations. This charge can be altered through soil management practices.

It is important to note that most soils contain both permanent- and variable-charge

components.



Iv. COMPONENTS OF SURFACE CHARGE

Based on the structure of the solid soil components and their reactions with

aqueous species, five principal charge components have been identified (Breeuwsma and Lyklema, 1971, 1973; Sposito, 1981): structural (uo),net proton (aH),

inner-sphere complex (uis),outer-sphere complex (uos),and diffuse layer (ud).The

first four charge components are related to the solid component and the last component is related to the aqueous suspension. The first four surface charge components are grouped into intrinsic surface charge density (structural and net proton)

and Stem layer surface charge density (inner-sphere and outer-sphere or specific

and nonspecific).

Although solid particles may carry electrical charge, aqueous suspensions of

particles are always electrically neutral. To maintain the surface charge balance,

the sum of the previously mentioned five charge components must be equal to zero

(Sposito, 1984):

uo + U H + UiS + uos + Ud = 0



(4)



All the charge components are measured either in Coulombs or moles charge and

are expressed either on a unit mass (C or mol kg- ') or surface area (C or mol mP2)

basis. When surface charge is expressed per unit surface area, it gives the surface

charge density, which is the measure of the resultant surface charge within the heterogeneous mixture of reactive solid surfaces of soils (Sposito, 1984).

Structural surface charge density is created by isomorphous substitution in the

crystal structures of clay minerals. The structural surface charge is constant and is

not affected by the solid-solution interface. Although isomorphous substitution

results in both positive and negative surface charge, invariably the net uo in silicate minerals is negative (see Section 111). This is mainly due to the more extensive substitution of Si4+by A13+in the tetrahedral layers of silicate clay minerals,

resulting in net negative charge.

Net proton surface charge density is proportional to the difference between the

amounts of H+and OH- adsorbed or complexed by surface functional groups:

uH = F(q,+



- qOH-)/as



(5)



where qH+or qoH- is the specific adsorbed charge (mol kg-') of H+or OH- cornplexed by surface groups, F is Faraday's constant, and as is the specific surface



SURFACE CHARGE AND SOLUTE INTERACTIONS



97



area. Conceptually, diffuse-layer H+ is not included in the definition of uH The

values of uH can be negative, zero, or positive, depending on pH, ionic strength,

etc.

Inner-sphere complex surface charge density is contributed by the net total

charge of the ions, other than the potential determining ions (PDIs), such as H+or

OH-, which are bound into inner-sphere surface complexes. Outer-sphere surface

complexes are contributed by the net total charge of the ions, other than the PDIs,

which are bound into outer-sphere surface complexes. The inner-sphere and outer-spherecomplex charge components are also known as specifically adsorbed and

nonspecifically adsorbed charge components, respectively (Bowden et al., 1980).



V. SOLUTION-SURFACE INTERFACE

A. DIFFUSE

DOUBLE

LAYER

Most particles in aqueous media are charged for various reasons, such as the

ionization of surface groups and specific adsorption of ions. In a solution, the distribution of ions around a charged particle is not uniform and gives rise to an electric double layer (Hunter, 1981).The behavior of charged soil and colloidal particles in soil water suspension is similar to that of charged particles in an electric

field (Lee,similar to that of electrophoresis).During electrophoresisthe charge distribution in soil solution relative to the immobile capillary surface leads to the formation of an electrical double layer (Li, 1992). Similarly, in a soil with predominantly negatively charged particles there is an accumulation of cations and a deficit

of anions in the vicinity of the solid surfaces relative to the equilibrium solution.

The thermal motion of the ions counteracts the electrostatic interaction. Thus, as

cations are being attracted and anions repelled, the cation concentration increases

as the surface is approached, whereas the anion concentration decreases (Fig. 6).

The concentration of ions near the soil particle surface is high and it decreases with

increasing distance from the surface. This diffuse character of the counter ion

“atmosphere” was first noticed by Gouy ( 1910, 1917) and Chapman ( 1913), who

presented a theoretical relationship describing the diffuse layer.

According to the Gouy model, if the double layer is created by the adsorption

of PDIs such as H+or OH-, the electric potential at the double-layer surface is

solely determined by the concentration (or activity) of these ions in solution since

the particles act as a reversible electrode toward these ions. If this was so then the

potential is given by the Nernst equation:

Qo = (kT/ve)Zn(c/co)



(6)



where Q is the electric potential at the surface, k is the Boltzman constant, Tis the

absolute temperature, e is the electric charge, v is the valence of the PDIs, c is the



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