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VI. Concepts of Point of Zero Charge

VI. Concepts of Point of Zero Charge

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SURFACE CHARGE AND SOLUTE INTERACTIONS



105



lsble V

Definitions of Some Point of Zero Charges“

Symbol



Name



Definition



PZC or

ISP

PZNPC



Point of zero charge

or isoelectric point

Point of zero net

proton charge

Point of zero salt

effect

Point of zero net

charge



pH at which the total net particle charge

vanishes

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

is zero



PZSE

PZNC



Defining

equation

ud= 0



u,=o

(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-



106



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

acetate.

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



107



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,



108



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).



A. POTENTIOMETRIC

TITRATION

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

titration:

Znegative charges = Zpositive charges



(21)



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

positive charges:

uo = positive surface - negative surface



(22)



u0 = (CA - CB)- (H+ - OH-)



(23)



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



(C,



-



CB)- (H+ - OH-)



=0



(24)



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,

then

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 >



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