Adsorption of quinoline to kaolinite and montmorillonite

Adsorption of quinoline (pK_a = 4.92) and background electrolyte (CaCl₂) onto specimen kaolinite and montmorillonite were measured as a function of pH (3-7.5) and ionic strength (1-10 mM), and as a function of quinoline concentration (0.2-1.55 mM) at fixed pH values of maximum adsorption. Maximum sorption of quinoline occurred at pH 3.5-4.0 for kaolinite, and pH 3.0-5.0 for montmorillonite. At their respective pH values of maximum adsorption, the sorption capacity for quinoline was 100 times greater with montmorillonite than kaolinite on a mass basis. Selectivity coefficients, which were calculated from pH edge data, indicated selectivity for cationic quinoline (QH⁺) over Ca²⁺ was greater with montmorillonite (K_exc = 27 at pH 4) compared to kaolinite (K_exc = 1.6 at pH 4), and K_exc was not affected by ionic strength for either clay. The results indicate the important role of charged siloxane sites in the adsorption of this N-heterocyclic contaminant.