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Chapter 2. Surface Charge and Solute Interactions in Soils
N. S. BOLAN ETAL.
B. Organic Matter Addition
C. Phosphate and Silicate Addition
XI. Conclusions and Future Research Needs
Many soil physical and chemical properties are controlled by the nature and the
amount of surface charge and the variation of surface charge with soil solution characteristics. These properties include dispersion and flocculation. electrophoretic mobility,
solubility,and the adsorption and movement of solutes. The surfacereactions of charged
particles are essential to the biogeochemicalcycling of nutrients and pollutants and the
pathway of detoxification of the latter when present at hazardous concentrations. Surface charge can be manipulatedto take advantage of solid phase interactions relating to
the movement of nutrient and pollutant ions in soils, the degradation of pesticides, and
the decontaminationof soils.
This chapter brings together fundamental aspects of surface charge and recent developmentson the implicationsof surface charge in relation to other soil properties, particularly solute interactions in soils. We first outline the development of charge on both
permanent- and variable-charge surfaces. Then we discuss the various methods used to
measure surface charge and factors affecting this charge. An attempt has been made to
compare current theories on the nature of the charged solid surface-solution interface.
The manipulation of surface charge can be achieved through liming and the addition of
fertilizers containing specifically adsorbed ions. The practical implications of surface
charge to soil properties have been discussed in relation to the dispersion and the flocculation of soils and the adsorption and leaching of inorganic cations and anions. Future research should focus on the development of methods to measure surface charge
under in situ conditions and to explore further the role of surface charge in remediating
8 1999 Academic Ress
Traditionally, soil scientists have employed the principles of physical, chemical, and biological sciences to understand the properties of soils. In recent times
these basic sciences have contributed much more, especially to an understanding
of the physical, chemical, and biological fertility of soils for plant growth and the
long-term impact of intensive and prolonged agricultural production on soil properties. With increasing awareness of the impact of agricultural activities on the
wider environment, soil is not only considered as a “source” of nutrients for plant
growth but also as a “sink” for the removal of contaminants from industrial and
agricultural waste materials. The “filtering” action of soil is controlled largely by
the reactions of pollutants with soil components carrying surface charge.
The discovery of surface charge on soil components revolutionized both fun-
SURFACE CHARGE AND SOLUTE INTERACTIONS
damental and applied research in soil science (Sposito, 1984; Sparks, 1986).Many
soil physical and chemical properties are directly or indirectly controlled by the
nature and the amount of surface charge and the variation of surface charge with
soil solution characteristics. These properties include dispersion (Sumner, 1993;
Quirk, 1994),flocculation/deflocculation(Suarez et al., 1984;Gregory, 1989;Bolan
et al., 1996b),coagulation (Hohl et al., 1980; Heil and Sposito, 1993),electrophor-etic mobility (Schulthess and Sparks, 1988), solubility (White and Zealazny,
1986; Carroll-Webb and Walther, 1988; Polubesova et al., 1995), and the adsorption and movement of nutrient ions, toxic heavy metals, and pesticides (Barrow,
1989; Evans, 1989; Bolan et al., 1997; Naidu et al., 1996, 1997).
The colloidal behavior of soil particles has been attributed to high surface charge
density resulting from a combination of high charge and very small particle size
(Mattson, 1931a,b). Parks and de Bruyn (1962) reported that a suspension is
stable because of the repulsive effects of the like charges on particles. At the point
of zero charge (PZC; see Section 111) the charge is neutralized, enabling closer approach of the particles due to van der Waals attraction resulting in coagulation or
flocculation and settling. PZC is the point of minimum solubility of a solid phase
in equilibrium with the solution and thus the coagulation and sedimentation rates
are also maximum at the PZC (Parks, 1967; Shanmuganathan and Oades, 1982).
Sposito (1992) indicated that the surface reactions of charged particles are essential to the biogeochemical cycles of trace elements and the pathway of detoxification of these elements when present in aqueous environments at hazardous concentrations.
Surface charge also plays a major role in the interactions of solutes with soils
(Greenland and Hayes, 1981; Banow, 1987, 1996). It can be manipulated to take
advantage of solute interactions relating to the movement of pollutants and nutrient ions in soils, the degradation of pesticides, and the decontamination of soils.
With the greater public awareness of contamination of soil environment there has
been increasing interest among the scientific community in the interactions of pollutants with soils.
In this chapter, we will first outline the development of charge on both permanent- and variable-charge surfaces. Then, we will discuss the various methods
used to measure surface charge. An attempt will be made to compare current theories on charge surface and the solution interface. The practical implications of
surface charge to solute interaction will be discussed in relation to adsorption and
leaching of inorganic cations and anions and organic compounds in soil. The review was undertaken primarily to bring together the fundamental aspects of surface charge and recent developments on the implications of surface charge in relation to solute interactions in soils in a comprehensive manner for readers, other
than surface chemists, including soil scientists, agronomists, and environmental
N. S. BOLAN ETAL.
II. TYPES OF ELECTRICAL SURFACE CHARGE
The electrical charge properties of soil colloids cover a spectrum of behavior
(Arnold, 1978; Barrow, 1983,1985; Barrow etal. 1993).For one end-member, the
charge on the surface is fixed or permanent and remains independent of the solution composition, but the electric potential is sensitive to indifferent electrolyte
concentration.This member is usually termed the constant charge surface or permanent surface charge. For the other end-member of the constant potential surface, the charge varies with changing concentration of indifferent electrolyte. This
charge is known as variable surface charge.
The development of these two types of surface charge is discussed in Section III.
Briefly, permanent charges are developed by isomorphous substitution of ions in the
lattice structures of silicate clay minerals, whereas variable charges are developed
through the dissociationof functionalgroups. Organic surfaces, such as humified organic matter, acquire their charge through the ionization of carboxylic (COOH), phenolic (OH), and possibly other functional groups. In the presence of various functional groups it is difficultto specify single acidic dissociation constants because the
loss of protons (H+)tends to become increasinglydifficultas more functional groups
become negatively charged. Thus, in the dissociation of COOH groups,
The dissociation constant (pKa) is given by:
PKa = PH - 1% (fl(1 - f 1)
wheref is the fraction of COOH groups dissociated. The dissociation constant for
organic compounds is expected to increase with increasingf (Momson and Boyd,
1973).In many soils between 10 and 90% of the total negative charges are derived
from the functional groups of the organic matter.
III. DEVELOPMENT OF SURFACE CHARGE
Soil constituents usually carry both positive and negative charges. Soil contains
both permanent- and variable-charge surfaces. The permanent charges are fixed and
are not altered by soil conditions. The permanent- or fixed-charge constituents develop surface charge as a result of substitution of metal ions in the lattice. In most
silicate clay minerals, the majority of surface charges are fixed and are developed
through substitution. Barrow (1985) suggested that the term “variable charge” be
used to describe those constituents whose charge varies mainly with the pH of the
SURFACE CHARGE AND SOLUTE INTERACTIONS
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.
Permanent- and Variable-Charge Constituentsin Soils
Fe and Al oxides