Sorption of ionizable organic solutes by surfactant modified zeolite

We determined the sorption of ionizable organic solutes on a natural zeolit e modified with hexadecyltrimethyl ammonium (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 cove rages, was affected by solution pH. All of the sorption isotherms were line ar and could be described by a distribution coefficient (K-d) At neutral pH , the K-d values of benzene, phenol, and aniline on SMZ increased with HDTM A loading up to monolayer coverage of 100 mmol/kg. Beyond monolayer coverag e, further increases in HDTMA loading did not increase the K-d values of th e solutes at pH 7.0, where all species exist primarily in their neutral for ms. The K-d values were consistent with the relative octanol-water partitio n coefficients of the three compounds, indicating that sorption of the neut ral species was primarily by partitioning into the bound HDTMA organic pseu dophase. Phenol sorption by SMZ treated to bilayer coverage increased as th e pH, and hence fraction of anionic phenolate, increased. The counterion ba lance indicated that the increased retention of phenol was due partially to anion exchange of phenolate for bromide, the same mechanism responsible fo r sorption of inorganic anions by SMZ. In contrast, decreases in pH resulte d in reduced aniline sorption due to a lower concentration of the neutral s pecies and repulsion of the positively charged anilinium from SMZ treated t o bilayer coverage. The results demonstrate that sorption of target species can be maximized by tailoring the HDTMA surface coverage to account for sp ecies and solution characteristics.