Sustainable Membranes for Water Treatment from Expanded Polystyrene Waste Using Dimethyl Isosorbide as a Green Solvent

This study reports the upcycling of expanded polystyrene (EPS) waste into functional membranes for water treatment through direct sulfonation at 3% and 5% (EPS-3 and EPS-5). Successful incorporation of −SO3H groups was confirmed by FTIR, acid–base titration, and differential scanning calorimetry. Dimethyl isosorbide was identified as the optimal solvent for membrane fabrication due to its balanced viscosity, polymer affinity, and cost-efficiency. The membranes exhibited an asymmetric porous morphology, with the pore size and permeate flux increasing with the sulfonation degree, while total porosity remained close to 74%. Water contact angle decreased from 90.7° for pure EPS to 84.9° for EPS-3 and 71.2° for EPS-5, reflecting higher wettability and water uptake (from 2.9% to 11.6%). At 9 bar, the permeate flux increased from 1.01 to 4.83 L·m–2·h–1 for EPS-5, and Reactive Black 5 rejection increased from 94 to 97% at 5 bar. Mechanical properties were preserved, with Young’s modulus ranging from 146.5 to 127.2 MPa and stable tensile strength. Overall, this study presents a sustainable and cost-effective approach to convert nonbiodegradable EPS waste into valuable membranes for pressure-driven water treatment, simultaneously addressing polymer waste management and water pollution.