PZNPCPoint of Zero Net Proton Charge
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The PZNPC calculated by Eqns 7 and 8 and that calculated by Eqn 9 were very similar, for the Humic Podosol-organic, Ferrosol, and Vertosol soils only (Table 3), and therefore satisfy surface charge balance principles (Anderson and Sposito 1992; Chorover and Sposito 1995; Polubesova et al.
kaolinite) have a PZNPC of <5, and would typically exhibit a net negative surface charge, whereas soils dominated by sesquioxides have a PZNPC of >7 (Table 3) and could still exhibit a net positive surface charge after leachate application.
Consequently, for pH values greater than their PZNPC (pH >7; Table 3), hydrous Fe and Al oxides are repelled from the surfaces of negatively charged clays, which can encourage dispersion of soil colloids and reduce soil hydraulic conductivity (Suarez et al.
Unfortunately this aspect of the effect of landfill leachate on surface charge characteristics was not investigated here, but it is possible that DOC adsorption could cause a decline in the PZNPC, particularly in those soils dominated by hydrous Fe oxides such as Ferrosols.