Quinoline sorption on kaolinite-humic acid complexes

Adsorption of quinoline (pK(a) = 4.92) and background electrolyte (LiCl) on to specimen kaolinite was measured as a function of surface-bound humic aci d (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 bg 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 p K(2), quinoline sorption increases with increasing pH and decreasing proton competition. Above the pK(a), sorption is reduced in parallel with (but of fset to a higher pH from) the ionized fraction. Competition with Li+ for su rface sites is apparent from diminished quinoline adsorption with increasin g ionic strength, but sorption of the ionized form of quinoline is always f avored and kaolinite exhibits selectivity for cationic quinoline over Li+ ( K-exc = 65 at pH 5), However, increasing f(OC) diminishes quinoline sorptio n and selectivity (K-exc = 32 at pH 5) and increases sorption reversibility relative to uncoated kaolinite. Humic acid alone exhibits Lower selectivit y for quinoline (K-exc = 4 at pH 5). The results indicate that mineral-sorb ed humic substances can diminish retention of cationic quinoline despite an increase in overall cation-exchange capacity.