Sulfonated Poly(Styrene-Isobutylene-Styrene) Membranes with Counter-Ion Substitution for Brackish Water Desalination

In this study, sulfonated poly(styrene-isobutylene-styrene) (SIBS) was used to validate its performance as a novel desalination media for brackish water through ion exchange. The effect of counter-ion substitution in the ionic domains (e.g., ${\mathrm{Ba}}^{2+}$ and ${\mathrm{Mg}}^{2+}$) was used to change the nanostructure and the sodium removal percentage. Material characterization techniques were employed to evaluate the chemical structure and physical properties of the polymer membranes depending on sulfonation levels and counter-ion substitution. Infrared spectroscopy was used to confirm the presence of sulfonic groups and interactions with ${\mathrm{Ba}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ counter-ions in the polymer nanostructure. An experimental design and ANOVA were executed to assess the effect on sodium removal percentage of (1) sulfonation level, (2) counter-ion substitution, and (3) sodium chloride concentration. The results show that high values of ion-exchange capacity and water absorption were directly related to the degree of sulfonation in the polymer. Maximum values obtained from ion-exchange capacity and water absorption were $1.75\mathrm{meq}/\mathrm{g}$ and 581.3%, respectively. The incorporation of ${\mathrm{Ba}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ counter-ions considerably reduced these two properties due to the ionic cross-linking formed in the sulfonated polymer nanostructure, confirming the suitability of this polymer for water desalination.