Anion Exchange on Cationic Surfactant Micelles, and a Speciation Model for Estimating Anion Removal on Micelles...

Surfactant micelles combined with ultrafiltration can partially, or sometimes nearly completely, separate various ionic and nonionic pollutants from water. To this end, the selectivity of aqueous micelles composed of either cetyltrimethylammonium (CTA⁺) bromide or cetylpyridinium (CP⁺) chloride toward many environmentally relevant anions (IO₃^-, F^-, Cl^-, HCO₃^-, NO₂^-, Br^-, NO₃^-, H₂PO₄^-, HPO₄^2-, SO₄^2-, and CrO₄^2-) was investigated. Selectivity coefficients of CTA⁺ micelles (with respect to Br^-) and CP⁺ micelle (with respect to Cl^-) for these anions were evaluated using a simple thermodynamic ion exchange model. The sequence of anion affinity for the CTA⁺ micelles and for the CP⁺ micelles were the same, with decreasing affinity occurring in the order of: CrO₄^2- > SO₄^2- > HPO₄^2- > NO₃^- > Br^- > NO₂^- > Cl^- > HCO₃^- > H₂PO₄^- ≈ F^-. From the associated component mass balance and ion exchange (i.e., mass action) equations, an overall speciation model was developed to predict the distribution of all anions between the aqueous and micellar pseudophase for complex ionic mixtures. Experimental results of both artificial and real surface waters were in good agreement to model predictions. Further, the results indicated that micelles combined with ultrafiltration may be a potential technology for nutrient and other pollutant removal from natural or effluent waters.