3), although the IEP was lower than PZNC and PZSE
of the soil due to the use of soil clay fraction in zeta potential determination.
The absence of a similar effect in Zeolite indicates that much of the added protons were consumed in neutralising the initially high alkalinity rather than being involved in surface charge reactions, as suggested above for PZSE measurements.
Furthermore, values for the PZSE, PZNC, and PZNPC generally were consistent with charge balance principles defined by Sposito (1981, 1992) for soils containing a mixture of both permanent and variable charge colloids.
Permanent surface charge density ([[sigma].sub.p]), point of zero net charge (PZNC), point of zero salt effect (PZSE), and point of zero net proton charge and apparent proton charge at the PZNC ([([sigma].sub.Happ]).sub.PZNPC]) calculated using either Eqns 7 and 8, or Eqns 9 and 8 [[sigma] PZNPC Soil .sub.p] PZNC PZSE Eqns 7 & 8 Eqn 9 Zeolite -32.85 n.d.
We used new samples that we equilibrated with KCl solutions but adjusted the pH to the point where the variable-charge components had no net proton charge (PZNPC), which has been assumed to be equal to PZSE (Sposito 1984; Bolan et al.
The PZSE occurred between pH 3.7 and 5.0 and was always less than pH([H.sub.2]O).
Variable charge, quantified as the value of -([DELTA][H.sup.+] - [DELTA]O[H.sup.-]) at PZSE, yielded values of -2.9 to -5.0.
The seasonal pattern occurred because soil acidified and PZSE increased between December and April, resulting in soil with less net negative charge as pH approached PZSE.
Indeed, Gillman (1974) showed that dispersion increased as the difference between PZSE and pH increased.