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VI. Concepts of Point of Zero Charge
SURFACE CHARGE AND SOLUTE INTERACTIONS
Definitions of Some Point of Zero Charges“
Point of zero charge
or isoelectric point
Point of zero net
Point of zero salt
Point of zero net
pH at which the total net particle charge
pH at which the net proton charge is
equal to zero
The pH value that shows no change
with ionic strength
pH at which the total of dissociated and
the outer surface complex charges
(aU,/al), = 0
uos ud= 0
From “The Surface Chemistry of Soils” by Garrison Sposito. Copyright 0 1984 by Garrison Sposito.
Used by permission of Oxford University Press, Inc.
of varying electrolyte concentrations,the curves intersect at a common pH value.
This pH value is often defined as the PZC. However, since the introduction of this
concept, many PZCs have been identified and defined for variable-charge surfaces
(Table V), including PZC or zero point of charge (ZPC), PZNC, point of zero net
pristine charge (PZNPC), isoelectric point (IEP), and point of zero salt effect
(PZSE). According to Polubesova et al. (1993, one of the challenges for soil and
colloid chemists is to understand and apply these myriad “zero point” terminologies. Parker et al. (1979) insisted that terms such as ZPC and IEP were too vague
and preferred terms such as PZSE and PZNC. Sposito (1981) also suggested the
term PZNPC. Bowden et al. (1977) used the term isoelectric point of the solid and
pristine point of zero charge, and Hendershot (1978) used the zero point of titration. Furthermore, the abbreviations ZPC and PZC are used interchangeably.
Parfitt (1980) observed that “isoelectric weathering” (Mattson, 1932) may also
take place in that ZPC approaches the soil pH with time.
Sposito (1 984) indicated that PZCs are pH values at which one or more of the
individual components of the surface charge density specified are equal to zero
(Table V). The PZNPC, which is a pH value at which the net proton surface charge
density is zero (aH= 0), depends on the concentration of the ionizable surface
functional groups and on the composition of the solution phase. The PZNPC is the
most important PZC for soils which contain both permanent and variable charge
because it is the only PZC in which the contribution of aH is considered separately from that of cro. Many others have simply assumed that PZNPC is equal to PZSE
(Bolan er al., 1986b). Equality between PZPNC and PZSE requires the special
condition that the net adsorbed ion charge at the PZNPC is independent of ionic
strength and that a0 is equal to zero. Clearly, there is a need for chemists to iden-
N. S. BOLAN ETAL.
tify the precise nature of their study and the charge measurement conditions prior
to defining the PZC.
In order to estimate either PZPNC or uH in the presence of both variable-charge
and permanent-charge adsorbents, it is necessary to first measure the permanentcharge density accessible to adsorptive ions (effective a,) by an independent
method. One such method is the cesium (Cs+)adsorption method in which the Cs+saturated adsorbent is dried to promote the formation of inner-sphere surface complexes, and then it is washed once with a dilute solution of LiCl. Lithium preferentially displaces Cs+ from variable-charge sites and leaves Cs+ adsorbed to
structural charge sites. Cesium from the latter sites is extracted with ammonium
The PZPNC of organic matter may be well below 3 because the COOH groups
on organic matter are more strongly acidic than simple carboxylic acids (Tipping
and Cooke, 1982); the PZPNC for A1 and Fe oxides is >7 (Parks 1967; Schwartz
et al., 1984), and the PZPNC for kaolinite is between 4 and 5 (Ferries and Jepson,
1975).Both cationic charge and ionic radii may influence the PZNPC values. This
may be attributed to the greater screening effect of the ions with increasing charge
and decreasing hydrated ionic radii.
The PZC values for minerals reported in the literature often vary significantly
with the method used for their estimation. This may be attributed to the nature of
the surfaces and the chemistry at the soil-particle interface which can also be influenced by the method used for the estimation of charge density and the PZC. Adsorbing solids in soils are inorganic and organic polymers bearing surface functional groups whose reactivity determines the operational meaning of surface area
and surface charge. Even in the most oxidic soils, normally particles will be present with appreciable permanent negative charge. Hence, the PZC derived from the
point of intersection of potentiometric titration curves of soil, obtained with different concentrations of electrolyte, is not always the same as the PZC measured
using the direct measurement of ion retention. As a general rule, the PZC measured
using the ion-retention method is lower than that obtained from a potentiometric
titration. This difference is attributed to the fact that some of the negative charge
on the original soil is balanced by strongly adsorbed A13+.Pretreatment of soils
in the ion-retention method will remove most of these A13+ and results in a close
estimate of the PZC.
In soil systems, the PZC is rarely equal to the PZSE; various reason have been
advanced for this. First, the presence of permanent negative charge should always
result in an increase in PZSE over PZC (Gillman and Uehara, 1980). Second, the
H+ or OH- added during the potentiometric measurements are sometimes consumed in reactions other than charge balancing and cause a deviation in PZSE from
the PZC (Parker et al., 1979).Third, the selective adsorption of index ions during
the charge measurements by the ion-retention method can displace the PZC from
PZSE (Sposito, 1981).
SURFACE CHARGE AND SOLUTE INTERACTIONS
If there is no variation among the reactive surfaces of the soil in relation to anion adsorption, the PZSE for adsorption should coincide with the PZC of the anionated surface.This has been observed for uniform surfaces such as goethite (Barrow et af., 1980). For heterogeneous surfaces, such as soils, there are many
surfaces that can adsorb anions and many that cannot. The proportion of the surfaces that are reactive in adsorption will vary with the anion and the soil. While
the PZC is the pH at which positive and negative charges of the soil as a whole are
in balance, the PZSE for adsorption is the pH at which the positive and negative
potentials of the surface, which are reactive to that particular anion, are in balance.
The higher value of PZSE for sulfate (SO:-) adsorption than for HPOi- adsorption obtained by Bolan et af. (1986b) in variable-charge soils indicate that the surfaces which are reactive to SO:- have a more positive potential than the surfaces
that are reactive to HPOi-.
Bolan et al. (1986b) examined the effect of HPOi- and SO:- adsorption on
PZSE of allophanic (Patua) and nonallophanic (Tokomaru) soils which vary in
their anion adsorption characteristics. They observed that anion adsorption shifted the PZSE to lower pH values and the extent of shift varied between the soils
and the anions species adsorbed. The commonly observed increase in negative
charge with HPOi- adsorption may explain the movement of the PZSE for adsorption to lower pH values. However, Rajan (1976) suggested that a large amount
of HPOi- must be adsorbed before any addition of negative charge to the surface
occurs. This may explain why for the Tokomaru soil, which adsorbed one-tenth as
much HPOi- as the Patua soil, the PZSE for adsorption was little affected by increasing adsorption. Sulfate, however, is adsorbed onto positive sites, which may
explain why there was little observable effect of increasing SO:- adsorption on
the PZSE for adsorption.
In summary, many PZCs, which give the pH value at which one or more of the
individual components of the surface charge density are equal to zero, have been
proposed for colloidal systems. The definition of a particular PZC depends mainly on the conditions used for charge measurements. Although in soil systems these
PZCs have been measured extensively and often been used interchangeably, their
practical importance in controlling some soil properties has not been well examined.
VII. MEASUREMJZNT OF SURFACE CHARGE
The characterization and modeling of the surface charge behavior of soil and
colloid systems depends on the technique used to measure surface charge. The
method used to measure surface charge enables identification of the specific component of the surface charge, and unless the method of measurement is the same,
N. S. BOLAN ETAL.
it is often difficult to compare the surface charge behavior between different studies. Many techniques have been developed to measure the net total particle surface charge density, intrinsic surface charge density, net structural surface charge
and net proton surface charge in soils, and other geological materials, including
potentiometric titration, ion retention, electrophoretic mobility, salt titration, and
mineral addition (Sposito, 1983; Lewis-Russ, 1991).
Electrometric titration or potentiometric titration methods are used to measure
Qpically, uH is determined for aqueous partithe net proton surface charge (aH).
cle suspensions by electrometric titration as a function of pH for specific conditions of the particle and the aqueous systems. Potentiometric titrations are reserved
for PZC analysis only (Schulthess and Sparks, 1986) in which the pH of a suspension is modified in small steps by adding known concentrations of dilute acid
or base. At each step, the pH is measured to determine the quantity of H+ and OHremaining in solution. These amounts are subtracted from the total H+ and OHadded, and the reminder is assumed to be adsorbed onto the solid particles.
In potentiometric titration electroneutrality is maintained at every point of the
Znegative charges = Zpositive charges
Positive charges are due to the positive surface sites, H+, and other cations in solution. Negative charges are due to the negative surfaces sites, OH-, and other anions in solution. The net surface charge is the sum of both surface negative and
uo = positive surface - negative surface
u0 = (CA - CB)- (H+ - OH-)
When the surface charge is neutral, the negative and positive charges are equal and
the surface is said to have zero charge. However, the charge distribution is such
that the positive and negative charges do not stereochemically cancel each other
(van Raij and Peech, 1972).At the pH of PZC, Eq.(23) simplifies to
CB)- (H+ - OH-)
Combining the Gouy-Chapman equation with Eq.(24) gives Eq.(13). This equa= pH of the suspension, then uo = 0 and (negation indicates that when pH,
tive charge) = (positive charge) for any ionic strength (no).If pH > pH,
uo < 0 and (negative charge) > (positive charge). Conversely, if pH C pH,
then uo > 0 and (negative charge) C (positive charge). At a fixed pH value >