Pznpc

The net charge at the hydrous oxide surface is established by the proton balance (adsorption of H or OH" and their complexes at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidimetric titration curve and from a measurement of the extent of adsorption of specifically adsorbed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). 

The point of zero charge is the pH at which net adsorption of potential determining ions on the oxide is zero. It is also termed the point of zero net proton charge (pznpc). It is obtained by potentiometic titration of the oxide in an indifferent electrolyte and is taken as the pH at which the titration curves obtained at several different electrolyte concentrations intersect (Fig. 10.5). It is, therefore, sometimes also termed the common point of intersection (cpi). The pzc of hematite has been determined directly by measuring the repulsive force between the (001) crystal surface and the (hematite) tip of a scanning atom force microscope, as a function of pH the pzc of 8.5-8.S was close to that found by potentiometic titration (Jordan and Eggleston, 1998). This technique has the potential to permit measurement of the pzc of individual crystal faces, but the authors stress that the precision must be improved. 

Specific adsorption of ions other than protons causes the pzc and the iep to shift along the pH scale (Stumm, 1992). Specifically adsorbed cations (anions) shift the titration curve and the point of zero proton condition at the surface (pznpc) to lower (higher) pH values, whereas the iep is moved to higher (lower) pH values. The shift of the iep of hematite to a lower pH by adsorbed EDTA and Cl" is shown in Figure 10.8. 

This is often referred to as the isoelectric point. It is the condition where particles do not move in an applied electric field. If one wants to specify that the pzc is established solely due to binding of or OH", one may specify point of zero net proton charge (or condition) (pznpc). Furthermore we can define a point of zero salt effect (pzse)... 

Figure 10.4 Schematic distribution of charged surface species (denoted by the triple dashes) on (a) silica gel and (b) ferrihydrite as a function of pH, showing for both the predominance of positive, neutral, and negatively charged surface species with increasing pH. The pH of the PZNPC is found where the net surface charge is zero (i.e., i=SiOHJl = [sSiO J and [sFeOHj l = IsFeO ]). Based on Healy (1974).

TABLE 10.3 The pH of the point of zero net proton charge PZNPC) of some minerals and organic substances... 

The pH at which the surface of the oxyhydroxides is uncharged in pure water is called the point of zero net proton chaige (PZNPC) (see Chap. 10). For the ferric oxyhydroxides in pure water the PZNPC corresponds to the pH of minimum solubility, which is near pH 8 (Fig. 12.4). Below this pH, consistent with the charge of the predominant Fe(III)-OH aqueous complex, solid surfaces are OH deficient (relative to Fe(III)/OH = 1/3) and so have a net positive charge. Above pH 8 the surface has... 

Figure 12.6 Experimentally derived stability ratio, of a hematite suspension plotted as a function of pH for different ionic strengths. The pH of the PZNPC is indicated. Dashed lines are drawn through the experimental points as a guide. The solid line has been model-calculated. From Ae/uaric Sciences 52(1) 32-55, L. Liang and J. J. Morgan, Chemical aspects of iron oxide coagulation in water Laboratory studies and implications for natural systems, Copyright 1990 by Birkhauser Verlag, Basel, Switzerland. Used by permission.

As the pH of an oxyhydroxide suspension is varied, the ccc concentration of strongly adsorbed species also changes. For example, near pH = 3, 10 mg/L of goethite flocculates most effectively (log W = 0) for ZPO4 = lO"" M, whereas when 2PO4 is essentially zero (10 M), the optimum flocculation pH is about 8 near the pH of the PZNPC (Liang and Morgan 1990). 

The most common procedure to estimate (underlined by MK) the PZNPC is to equate... 

Figure 3.19, Effect of ionic strength, /, on the point of zero net charge (PZNC) (a) and the point of zero net proton charge (PZNPC) (b) of a soil containing both permanent (P) and variable (V) charge minerals.

The point of zero net proton charge (PZNPC) is the pH value of the soil solution at which ctjj, defined in Eq. 1.24, is equal to zero. As can be inferred from Eq. 1.25, the PZNPC can be measured by potentiometric titration, provided only proton-selective surface functional groups on the soil solids are titrated. 

Figure 3.1. Experimental examples of the PZC of > Al203, the PZNPC of goethite, the PZSE of hydroxyapatite, and the PZNC of kaolinite. (u is the electrophoretic mobility.)...

The relationship between the PZNPC and the PZNC for a soil when the experimental conditions of rule 1 are met can be deduced from Eq. 3.3b and Table 3.1. Consider the PZNPC. Equation 3.3b becomes... 

It should be noted that the relation between the sign of oq and that of PZNPC-PZNC does not depend on equality between the PZC and the PZNC but only on the condition that = 0. 

The intrinsic equilibrium constants have thermodynamic significance, in that they determine the PZNPC through the condition Xso- sohj (pH = PZNPC) applied to Eq. 5.42 ... 

Figure 5.2, with its two lines of unequal slope (in absolute value), illustrates another typical feature of applications of the constant capacitance model. The value of the capacitance parameter C inferred from each slope is not the same above and below the PZNPC. This result is in conflict with the nonspecific, coulombic nature of indicated in... 

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