Quinoline sorption on kaolinite-humic acid complexes

Adsorption of quinoline (pK(a) = 4.92) and background electrolyte (LiCl) onto specimen kaolinite was measured as a function of surface-bound humic acid (HA) concentration (f(oc) = 0-0.5%), pH (3-10), and ionic strength (1-10 mM). Complexation of HA on the kaolinite surface (4.5 mg C kg^-1) reduced the point of zero net charge (pznc) for kaolinite by more than one pH unit and resulted in a significant increase in negative surface charge. Maximum sorption of quinoline occurred near its pK(a) for all sorbents. Below the pK(a), quinoline sorption increases with increasing pH and decreasing proton competition. Above the pK(a), sorption is reduced in parallel with (but offset to a higher pH from) the ionized fraction. Competition with Li⁺ for surface sites is apparent from diminished quinoline adsorption with increasing ionic strength, but sorption of the ionized form of quinoline is always favored and kaolinite exhibits selectivity for cationic quinoline over Li⁺ (K(exc) = 65 at pH 5). However, increasing foe diminishes quinoline sorption and selectivity (K(exc) = 32 at pH 5) and increases sorption reversibility relative to uncoated kaolinite. Humic acid alone exhibits lower selectivity for quinoline (K(exc) = 4 at pH 5). The results indicate that mineral-sorbed humic substances can diminish retention of cationic quinoline despite an increase in overall cation-exchange capacity.