2.1. Synthesis Sulfonated Polysulfone and Membrane Preparation

Commercial polysulfone (Lasulf from Lati SPA) was sulfonated according to the procedure reported elsewhere [14]. Briefly, 2 g of PSU was first dried under vacuum at 80 °C for 24 h and then dissolved in anhydrous chloroform under vigorous magnetic stirring at room temperature until a homogeneous solution was obtained. Therefore, trimethylsilyl chlorosulfonate (Aldrich, St. Louis, MO, USA) was added as sulfonating agent (molar ratio sulfonating agent/repetitive units equal to 2.5) and the reaction left for 7 h at 50 °C under reflux to produce a silyl sulfonate polysulfone. Sodium methoxide (Aldrich) was then used to cleave the silyl sulfonate moieties over 1 h to yield the sulfonated polysulfone (sPSU). After precipitation in a bath of ethanol, the sPSU in fine powder was recovered by filtration, vigorously washed with ethanol, and rinsed several times with distilled water prior to be heated at 60 °C in an oven until dry. Robust transparent membrane of sulfonated polysulfone was prepared via solution casting method. Two hundred grams of polymer were completely dissolved in 10 mL of N,N-dimethylacetamide (DMAc, Aldrich) at room temperature. The resulting homogeneous solution was cast on a petri dish and dried in an oven at 60 °C until complete evaporation of the solvent (dry thickness = 80 μm). The morphological investigation, carried out by Scanning electron microscopy (SEM), revealed the sPSU membrane had dense, compact and homogeneous structure. Also, the absence of any cracks and/or defects indicates good film quality (see Figure S1 of the supporting information). Finally, the membrane was converted into the acid form with 1 M H₂SO₄ solution (7 h at 50–60 °C), followed by washing several times with boiling deionized water to remove any residual acid. For the as-prepared sulfonated polysulfone, the ion exchange capacity (IEC), determined via titration [7], was 1.56 meq g⁻¹, which corresponds to a sulfonation degree (DS) of 80%.