Effect of Acid and Alkali Treatments on Surface-Charge Properties of Selected Minerals

Abstract

Abstract 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 ∼3.7 and below ∼4.3, 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.