The charge of 1:1 clay minerals is a pH dependent charge results from ionization of the o-Al-oH surfaces of the aluminum octahydra.
Under zero point of charge, deionization of the oxy-hyroxide surfaces occur and the positive charge is dominant
o-Al-oH + H+------o-Al+ + H2O
The charge or mineral 1:1 Kaolinite is due variable charge developmet firts for pH.
The positive charge developed by kaolinite is mainly due to 'protonation' , this charge is pH dependent charge occurs at low pH
In amphoteric soils, i.e. those in which most of the colloidal particles have variable surface charges, there are a few mechanisms capable of changing the net surface charge. Roughly, we can say that the net charge willl be determined by sorption-desorption reactions and the balance will be given by the ion most sorbed (the most common ions being H+ and OH-). Then you should be very acquinted with the main parameters involved in soil sorption-desorption dynamics, as mineralogy, SOM, pH, water content, ionic strenght, metal burden, and so on. Each colloid has a peculiar response to those parameters. That makes the term "pH dependent charges" inadequate, once the charge balance relies on quite a number of variables. For general purposes we can assume that kaolinite will be electropositive only below pH 3.5.
Also, the kaolinite (1:1) sheets bond to each other in the "c" axis due to hydrogen bonds so that its basal growth pattern explains why this mineral is predominantly amphoteric. The bulk charge sites exposed to soil solution are those at the edges of the 1:1 sheets. These charges are variable, and as there is virtually no isomeric substitution in this mineral group, they are the main charges of kaolinite in soil.
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