Developing Quantitative Models for the Removal of Quaternary Ammonium Ions as Representative Cationic Emerging Contaminants by Cation Exchange Resins
Understanding the sorption mechanism of organic contaminants on cation exchange resins (CXRs) will enable application of these resins for the removal of cationic organic compounds from contaminated water. In this study, sorption of a diverse set of 12 organic cations and 8 neutral aromatic solutes on two polystyrene CXRs, MN500 and Amberlite 200, was examined. MN500 showed higher sorption capacities due to its microporous structure. The sorption capacities followed the same trend of aromatic cations > aliphatic cations > neutral solutes for both resins. Generally, solute-solvent interactions, nonpolar moiety of the solutes, and resin matrix can affect selectivity of the cations. Sorption capacities of the neutral compounds were significantly less than those of the cations, indicating a combined effect of electrostatic and non-electrostatic interactions. By conducting multiple linear regression between Gibbs free energy of sorption and Abraham descriptors for all 20 compounds, polarity/polarizability (S), H-bond acidity (A), induced dipole (E), and electrostatic (J+) interactions were found to be involved in the sorption of the cations by the resins. After converting the aqueous sorption isotherms to sorption from the ideal gas-phase by water-wet resins, a more significant effect of J+ was observed. Predictive models were then developed based on the linear regressions and validated by accurately estimating the sorption of different test-set compounds with a root mean square error range of 0.91 to 1.1 and 0.76 to 0.85 for MN500 and Amberlite 200, respectively. The models also accurately predicted sorption behavior of aniline and imidazole between pH 3 and 10.