Effect of acid and alkali treatments on surface-charge properties of selected minerals

Bentonite, biotite, illite, kaolin, vermiculite and zeolite were treated with 0.1, 1.0 and 5.0 mol. dm(-3) HCl or NaOH. Suspensions of Na homoionic forms of the initial and the treated minerals were back titrated with 0.1 mol. dm(-3) NaOH. From back-titration data variable surface charge, Q(V), vs. pH dependencies and apparent surface dissociation constant distribution functions were estimated. Variable charge vs. pH curves were scaled against cation exchange capacity at pH 7.2 to obtain actual charge, Q(A), vs. pH plots. In general both treatments led to an increase of Q(V), while the Q(A) value increased and decreased depending on the mineral and the treatment. Products of 5 mol dm(-3) NaOH action on zeolite and of 1 and 5 mol. dm(-3) NaOH on kaolin apparently developed a positive surface charge below pH similar to3.7 and below similar to4.3, respectively. The heterogeneity of charge-generating surface groups was observed in natural minerals. During acid treatment, the number of weakly-acidic surface functional groups increased while the number of groups of stronger acidic character decreased. The opposite was found for alkaline treatment: the number of surface groups of intermediate acidity increased and that of low acidity decreased.