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X. Manipulation of Surface Charge to Control Solute Interactions

X. Manipulation of Surface Charge to Control Solute Interactions

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(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

Table VIII

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

Surface charge (mmol kg-')




Lime added

(mmol kg-I)

























I .o


"After Bolan eta!. (1988b).










Sulfate adsorbed

(mmol kg-I)











al., 1983): (i) SO:- mineralized from soil organic matter by microorganisms

growing in a more favorable pH environment; (ii) SO:- released from organic

matter by chemical hydrolysis; (iii) adsorbed SO:- released from the soil surface;

or (iv) SO:- release from sparingly soluble Fe and A1 hydroxy sulfates, which become more soluble at higher pH values.

During liming, both the pH of the soil and the concentration of Ca2+in the soil

solution increase. Whereas an increase in soil pH can decrease the adsorption of

anions, such as SO:- and HPOi-, and increase the adsorption of cations such as

K+, an increase in Ca2+concentration has the opposite effect on the adsorption of

both anions and cations. Bolan et al. (1988a) examined the effect of liming on the

adsorption of HPOi- and K+ using both batch and column experiments. In the

case of column experiments, an increase in pH through liming decreased the adsorption of HPOi- but increased the adsorption of K+. This resulted in increased

leaching of added HPOi- but decreased leaching of K+. In batch experiments,

however, an increase in pH through liming increased the adsorption of HPOi- but

decreased that of K+.

Whereas a decrease in HPO,*- adsorption with increasing pH can be attributed

to the decrease in electrostatic potential in the plane of adsorption (Barrow, 1984),

the increase in HPOi- adsorption with increasing Ca2+concentration has been attributed to many mechanisms,including precipitation of calcium phosphate (Freeman and Rowell, 1982), surface complex formation between the sorbed HP0:and solution Ca2+(Helyar ef al., 1976), an increase in ionic strength of the soil

solution (Haynes, 1982),the specific effect of Ca2+on electrostatic potential (Barrow et al. 1980; Curtin et al., 1992) and the adsorption of HP0:- by freshly precipitated Fe and A1 hydroxides following liming (Amarasiri and Olsen, 1973).

In the case of cations such as K+, the concentration of Ca2+in the soil solution

largely influences adsorption. In batch experiments, the decrease in K+adsorption

with liming is mainly due to an increase in the concentration of Ca2+in soil solution (Galindo and Bingham, 1977) and to a decrease in charge density (Goedert et

al., 1975) which results in an increase in selectivity of Ca2+over K+. In column

experiments, however, the Ca2+concentration in soil solution was decreased by

the percolating solution. Thus, in the absence of competition from Ca2+,the increased negative charge at higher pH through liming resulted in an increase in K+


Thus, an increase in pH through liming increases the net negative charge and

thereby increases the adsorption of cations and decreases the adsorption of anions,

whereas an increase in Ca2+in soil solution through liming is likely to increase the

adsorption of anions and decrease the adsorption of cations. Therefore, the resultant effect of liming on the adsorption of cations and anions depends largely on the

concentration of Ca2+in soil solution. Under natural leaching conditions in which

most of Ca2+is lost from soil solution, liming of soils may not necessarily cause

increased leaching of subsequently added K+ (Goedert et al., 1975) or Mg2+ feret



tilizers (Grove et al., 1981). It is possible, however, that liming a soil may lead to

displacement of other cations already present in the soil and hence induce leaching if there is a water flux (Edmeades, 1982).



Porous, electrically charged materials can be used to adsorb nutrients and pollutants from effluents. This can most likely be achieved on farms by infiltration

through constructed soil, sand, or bark filters. Recent research has established that

finely ground, composted Pinus radiata bark is an efficient cation exchanger

(Mahimairaja et af.,1993) which has the potential to trap and remove the bulk of

the NHf and K+from dairy- and piggery-shed effluents and heavy metals from industrial effluents.

Bolan et af. (1996a) examined the potential of I! radiata bark in the retention

and release of various nutrient ions (NH:, HPOi-, and K+) from dairy-shed effluents using batch and column experiments. Bark materials with a size fraction of

1 or 2 mm were treated with Fe and A1 hydroxides and an industrial waste product, fluidized bed boiler ash (FBA), to enhance the cation and anion retention

capacity of the original bark.

Greater retention of HPOi- was obtained for the Fe and A1 hydroxides- and

FEiA-treated bark than for the untreated bark. The retention of NH:, however, increased only for the FBA-treated bark. Fe and A1 hydroxides increased the positive charge of the bark material and thereby increased the retention of HPOi-.

FEiAcontains slacked lime [Ca(OH),] which is likely to precipitate HPOi- as calcium phosphate and increase the pH of the bark materials. The increase in pH

caused an increase in the negative charge (CEC) of the bark and thereby increased

the retention of NH,+and K+ in the dairy-shed effluent.




Addition of specifically adsorbed anions, such as HPOi- and SiOi-, has been

attempted to increase the CEC of soils (Blair et al., 1990). Fox (1978) showed that

the addition of SiOi- reduced HPOi- sorption in a Typic Gibbsihumox soil in

Hawai. Application of HPOi- has often been shown to increase the retention of

specifically adsorbed cations, such as Zn2+ and Ca2+(see Section IX),through an

increase in the surface negative charge. Ayers and Hagihara (1953) and Wann and

Uehara (1 978b) showed that leaching losses of K+ in variable-charge soils could

be reduced by prior application of P fertilizer to the soil. Wann and Uehara (1978a)

suggested that HPO$- fertilizers added to soils not only serve as a nutrient but also

as an amendment to increase CEC of the soil. The most frequently cited causes for



HPO:--induced CEC include (i) a shift in the ZPC to lower pH values (Hingston

et al., 1972; Breeuwsma and Lyklema, 1973; Wann and Uehara, 1978a) (ii) neutralization of positive charge (Hingston et al., 1972; Schalscha et al., 1974),

(iii) and electrolyte inhibition (Thomas, 1960).

Although P fertilizer application has been considered a management tool to increase the CEC of variable-charge soils, large quantities of fertilizer are required

to cause a significant increase in CEC. At a maintenance application rate of 40 kg

P ha-' it can be estimated that the increase in CEC ranges from 0.07 to 0.18 C mol

kg-' soil (assuming a bulk density of soil = 1.0 Mg m-3, depth of incorporation

of fertilizer = 50 mm, and the increase in surface charge due to HPOi- adsorption = 0.31-0.70 mol(-) mol P-I).


Soils carry both permanent- and variable-charge surfaces. The permanent

charge is developed through isomorphous substitution of cations with similar size

but different valencies. The variable charge is developed through dissociation/

association of H+from mineral surfaces and the functional groups of organic matter. Specific adsorption of anions and cations also results in surface charge. While

specific adsorption of anions increases the negative charge, the specific adsorption

of cations increases the positive charge.

Surface charge in soils is measured mainly by potentiometric and ion-retention

methods. Potentiometric methods are suitable for the measurement of PZC and the

ion-retention method is suitable for the measurement of both variable and permanent charges. Improved ion-retention methods, involving ions which are accessible to permanent-charge sites, have been developed to differentiate between permanent and variable charges in soils carrying both these surface charges.

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

aqueous species, five charge components have been identified: structural, proton,

inner-sphere complex, outer-sphere complex, and diffuse-layer charges. Many

PZCs have been identified which measure the pH values at which one or more of

the individual components of the surface charge density are equal to zero.

In an aqueous solution containing soil particles, the distribution of ions around

a charged particle is not uniform and gives rise to an electric double layer. Many

models have been developed to describe the relative distribution of ions close to

the soil surface and in the soil solution. In these models the ions are assumed to

distribute at different distances (planes) from the charged surfaces and such models have been used successfully to describe the adsorption of anions and cations

by charged surfaces.

Although both the nature and the quantity of soil constituents affect the perma-



nent and variable charge on soil particles, the soil solution composition affects

mainly the variable-charge component. Generally, soils containing the silicate clay

minerals carry mostly permanent charge and in soils containing Fe, Al, and Mn oxides and organic matter they carry mostly variable charge. Soil pH is considered

to be the most important property which influences variable charge in soils. An increase in pH increases the net negative charge and a decrease in pH increases the

net positive charge.

Surface charge is involved in the retention and movement of cations and anions

and in flocculation and dispersion in soils. Surface charges in soils can be manipulated to enhance the retention of solutes and to improve the hydraulic conductivity of soil. Liming has often been shown to increase the negative surface charge

and thereby increase the retention of nutrient ions and toxic heavy metals. This is

likely to result in reduced leaching of these ions and thereby minimize the risk of

contamination of groundwater.

Although much work has been done on the assessment of surface charge characteristics of soils there remains a need to develop techniques which enable quantification of permanent- and variable-charge components and in siru measurement

of charge. Almost all techniques currently used expose soil surfaces to solutions

of varying composition and concentration. Such solutions invariably interact with

the surfaces of colloids, thereby altering their charge characteristics. Thus, much

of the published information provides at best estimates rather than real charge

values as exhibited by colloid particles under field conditions.

The effect of particle charge density on contaminant interactions in soils and the

implications to contaminant transport and remediation are areas which lack

detailed research. As we increasingly protect the natural resource base there is a

need to study the role of surface charge on solute-colloid interactions at the soilparticle interface in relation to both nutrient dynamics and remediation. The role

of charge and surfactants in remediation studies is just beginning to be realized by

soil scientists, and to gain a better understanding of these phenomena soil scientists must interact with scientists from other disciplines.


Adams, E. (1984). “Soil Acidity and Liming.” Soil Sci. Soc.Am., Madison, WI.

Adams, F., and Rawajfih. Z. (1977). Basaluminite and alunite: A possible cause of sulfate retention by

acid soils. Soil Sci. Soc. Am. J. 41,686-692.

Alloway, B. J., and Jackson, A. P. (1991). The behavior of heavy metals in sludge amended soils.

Sci. Total Environ. 100,151-176.

Aha. A. K., Sumner, M. E., and Miller, W. P.(1990). Reactions of gypsum and phosphogypsum in

highly weathered acid subsoils. Soil Sci. SOC.Am. J. 54,993-998.

Arnarasiri, S. I., and Olsen, S. R. (1973). Liming as related to solubility of P and plant growth in an

acid tropical soil. SoilSci. Soc. Am. Proc. 37,716-721.

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