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Abstract
Coarse and fine clay separates from 7-representative soils of Egypt (i.e. alluvial, calcareous and sandy) were subjected to study their charge characteristics. The impact of clay mineralogical composition as well as a amorphous material on the distribution of permanent and variable charges of clay was also investigated. In addition, the effect of P- oxyanion adsorption on surface reactivity and zero points of charge was studied.
1- Potentiometeric titration curves:
Potentiometeric titration (PTC) of clay separates were obtained by plotting the concentration of adsorbed (H + or OH –) (mole/ kg) VS. pH at different ionic strengths (from 0.01 to 1.0 mole / l). The obtained (PTC) curves were remarkably different for clay separates.
Zero point of net proton charge (ZPNPC) was calculated from PTC – curves when H- = zero . In addition, zero point of salt effect (ZPSE) was identified as the intersection point of the suit of titration curves conducted at varying ionic strengths. Total charge, and permanent charges were obtained also from PTC – curves.
A- Charge characteristics of Bentonite and Kaoline:
The data for pure clay materials [i.e. bentonite (90% montmorillonite) and kaolin 90% kaolinite], showed that increasing the ionic strength of the background solution significantly decreased PZNPC values. This is consistent with the theory of the electric double layer (EDL) for variable-charge colloids, which states that increasing solution I yields an increase in the magnitude of the surface charge and decrease pH. Kaolinite (hydroxylic surface) had higher PZNPC values (3.06- 4.55) than for bentonite (1.10-2.49) with dominant siloxanic surface.
Removal of amorphous materials significantly decreased both PZNPC and PZSE, which indicated that amorphous material, provides more hydroxylic surfaces. The maximum total charge of kaolinite was much lower (9.09-10.3 Cmolc/kg) than that of bentonite (92.02-93.90 Cmolc/kg). The permanent charge component is much higher in bentonite compared to kaolinite due to higher isomorphous substitution.