Sulfonated Poly(styrene-isobutylene-styrene) Membranes with Counter-Ion Substitution for the Separation of Water from Urea

The development of a low energy alternative for the separation of hydrophilic components has been studied for the purpose of recycling wastewater, such as urine, in situations in which resources like energy and clean water are limited. The other main component in urine besides water is urea, which forms strong hydrogen bonds with water making it a very challenging separation. In this study, the separation process of urea and water was completed using a polymer nanocomposite membrane (PNM) made from sulfonated poly(styrene-isobutylene-styrene) (SIBS) (81.52% sulfonation) with and without Mg⁺², Ca⁺² and Ba⁺² counter-ions. The counter-ions cross-linked into the ionic domains of the PNM, while the membrane in its acid form was used as control. Characterization techniques were applied to corroborate the presence of Mg⁺², Ca⁺² and Ba⁺², and to identify the changes in their properties in comparison to the control. Thermogravimetric Analyses (TGA) demonstrated a major thermal stability and less water absorption in our samples, it also reduced the weight loss up to 15% and produced more temperature resistant membranes (up to 150^oC higher). Atomic Force Microscopy (AFM) was performed, and morphological changes in the arrangement of hydrophilic and hydrophobic domains were observed resembling channels that could facilitate the transport of water. The permeability of urea through these membranes was studied as well. For this, the identification and quantification of urea within large water quantities was accomplished utilizing Fourier-Transform Infrared Spectroscopy (FTIR) in a range of 0.5 to 3.0% concentration. A calibration curve was developed between the absorbance and concentration of urea, which allowed us to calculate permeabilities using a diffusion cell. The permeability values were 1x10^-6 for Mg⁺², 2x10^-5 for Ca⁺² and 4x10^-7 for Ba⁺² cm²/s suggesting that these PNMs can be excellent candidates for the separation of urea in water. Further studies will focus on the understanding of the transport mechanism of urea, as well as the study of the permeability of water.