Sorption of Ionizable Organic Solutes by Surfactant-Modified Zeolite

Abstract

We determined the sorption of ionizable organic solutes on a natural zeolite modified with hexadecyltrimethylammonium (HDTMA), a cationic surfactant. The sorption of benzene and its ionizable analogues phenol and aniline by surfactant-modified zeolite (SMZ), prepared at different HDTMA surface coverages, was affected by solution pH. All of the sorption isotherms were linear and could be described by a distribution coefficient (Kd). At neutral pH, the Kd values of benzene, phenol, and aniline on SMZ increased with HDTMA loading up to monolayer coverage of 100 mmol/kg. Beyond monolayer coverage, further increases in HDTMA loading did not increase the Kd values of the solutes at pH 7.0, where all species exist primarily in their neutral forms. The Kd values were consistent with the relative octanol−water partition coefficients of the three compounds, indicating that sorption of the neutral species was primarily by partitioning into the bound HDTMA organic pseudophase. Phenol sorption by SMZ treated to bilayer coverage increased as the pH, and hence fraction of anionic phenolate, increased. The counterion balance indicated that the increased retention of phenol was due partially to anion exchange of phenolate for bromide, the same mechanism responsible for sorption of inorganic anions by SMZ. In contrast, decreases in pH resulted in reduced aniline sorption due to a lower concentration of the neutral species and repulsion of the positively charged anilinium from SMZ treated to bilayer coverage. The results demonstrate that sorption of target species can be maximized by tailoring the HDTMA surface coverage to account for species and solution characteristics.