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X. Manipulation of Surface Charge to Control Solute Interactions
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).
N. S. BOLAN ETAL.
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
SURFACE CHARGE AND SOLUTE INTERACTIONS
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
N. S. BOLAN ETAL.
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).
XI. CONCLUSIONS AND FUTURERESEARCH NEEDS
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-
SURFACE CHARGE AND SOLUTE INTERACTIONS
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.