Solubility-normalized Freundlich isotherm for the prediction of sorption of phenols in HDTMA modified montmorillonite

摘要

Single- and bisolute competitive sorption of phenols (2-chlorophenol, 3-cyanophenol and 4-nitrophenol) onto montmorillonite modified with cationic surfactant (hexadecyltrimethylammonium, HDTMA cation) was investigated. In single-solute sorption, sorption affinity increased in the order of 2-chlorophenol>4-nitrophenol>3-cyanophenol, as expected from the magnitude of the octanol-water partition coefficient (K ow). The difference in affinity is mainly attributed to the hydrogen bonding with the water molecules incorporated in the sorbed phase. The sorption affinity of the phenolic compounds onto the HDTMA-montmorillonite was in the order of pH 7>pH 3≫pH 11.5. Compared to uptake at pH 3 and 7, the uptake at pH 11.5 was quite low. The reduced uptake at pH 11.5 is attributed to lower solubility of anions in the core of the organic medium due to the unfavorable hydrophobic interaction between hydrated anions and nonpolar organic medium and thus anions adsorbing near the surface of the nonpolar organic medium. Solubility-normalized Freundlich model fitted the single-solute sorption data well. Competition between the solutes in bisolute sorption reduced the sorbed amount of each solute compared with that in the single-solute system. The ideal adsorbed solution theory (IAST) coupled with the solubility-normalized Freundlich model predicted the bisolute competitive sorption data successfully. The solubility-normalized model was analyzed to characterize sorption mechanism of phenols onto HDTMA-montmorillonite at very low (Henry's law resion) and at high concentration (Ralout's law region).