MECHANISMS FOR ADSORPTION OF ORGANIC BASES ON HYDRATED SMECTITE URFACES

摘要

Mechanisms by which organic bases are adsorbed on hydrated smectite surfaces were investigated. Three Ca-saturated reference smectites (Otay, SPV, and Panther Creek) were dispersed in distilled water containing 5 umol of pyridine or 3-butylpyridine. The pH was adjusted to between 7.5 and 3 using 0.01 M HCl. After a 2-h equilibration, the amount of pyridine or 3-butylpyridine adsorbed on the clay and the amount of Ca desorbed from the clay were determined. Negligible amounts of pyridine were adsorbed by the Ca-smectites in the neutral systems (pH > 7); however, most of the added pyridine was adsorbed on the smectites in the acidified systems (pH < 5). Equivalent amounts of Ca~(2+) were desorbed from the clays, indicating that pyridine was adsorbed as a protonated species by cation exchange. By contrast, 40 to 90 of added 3-butylpyridine was adsorbed on the smectites at neutral pHs, whereas only small amounts of Ca~(2+) were desorbed. The results suggest that 3-butylpyridine is initially retained by hydrophobic bonding between the alkyl side chain of the molecule and hydrophobic nanosites located between the charge sites on smectite surfaces. Surface acidity catalyzed protonation 1 to l.5 pH units above the pK_a of the bases.