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Mechanisms for adsorption of organic bases on hydrated smectite surfaces
Author(s) -
Laird David A.,
Fleming Pierce D.
Publication year - 1999
Publication title -
environmental toxicology and chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.1
H-Index - 171
eISSN - 1552-8618
pISSN - 0730-7268
DOI - 10.1002/etc.5620180809
Subject(s) - chemistry , pyridine , adsorption , protonation , clay minerals , alkyl , inorganic chemistry , distilled water , organic matter , organic chemistry , ion , mineralogy , chromatography
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 μmol 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 1.5 pH units above the p K a of the bases.