Thermodynamics of Anion Exchange on Surfactant-Modified Zeolite

We determined the anion selectivity sequence of Br^-, Cl^-, NO₃^-, and ClO₄^-, with surfactant-modified zeolite (SMZ). This information is important for understanding the stability of the modified zeolite and its selectivity for removal of anions from contaminated waters. Previous studies demonstrated that the surfactant counterion controls the maximum sorption of surfactant and of anionic contaminants; with the surfactant hexadecyltrimethylammonium (HDTMA), the surfactant sorption capacity trend follows HDTMA-Br>HDTMA-Cl>HDTMA-HSO₄.

We prepared the SMZ by circulating 10-mM solutions of either HDTMA-Cl or HDTMA-Br through packed beds of clinoptilolite-rich tuff from the St. Cloud mine in Winston, New Mexico, until the HDTMA concentration in the effluent became constant. After saturation with HDTMA, the zeolite was washed with distilled water to remove excess surfactant and air dried.

Ion-exchange equilibrium data were obtained by allowing 2.5 g of SMZ-X (X = Br^-, Cl^-) in 50-mL polyallomer centrifuge tubes to react with 30-mL solutions containing different amounts of Br^- (or Cl^-) and a competing anion (Cl^-, Br^-, NO₃^-, or ClO₄^-). The total anion concentration in each case was 100 mM. The samples were shaken at 25°C for 24 h. The solid and liquid were separated by centrifugation and the supernatant analyzed for anions and HDTMA via HPLC.

The results show that both Br^- and NO₃^- exchange selectively for Cl^- in SMZ-Cl, with a somewhat higher affinity shown for Br^-; ClO₄^- shows incomplete exchange in SMZ-Cl. NO₃^- exchanges effectively with Br^- in SMZ-Br, but Cl^- and ClO₄^- are much less selectively exchanged.

We determined thermodynamic exchange parameters for all the anions with SMZ-Cl and SMZ-Br using a procedure developed for cation exchange in zeolites.