Choline Chloride–Urea Deep Eutectic Solvent as a Dual-Function Additive for Enhancing Membrane Performance in Gas-to-Liquids Wastewater Treatment

Deep eutectic solvents (DES) are increasingly used to modify polysulfone (PSf) ultrafiltration membranes, but almost always alongside a conventional pore former such as polyvinylpyrrolidone (PVP), so the intrinsic pore-forming role of the DES has rarely been isolated. Here, choline chloride–urea at an off-eutectic 1:1 molar ratio was used as the sole additive (1–3 wt%) in PSf membranes, benchmarked against pristine PSf and a 3 wt% PVP control, and evaluated with real gas-to-liquids (GTL) wastewater collected at two seasonal extremes spanning a 6.5-fold change in organic load. All three DES loadings matched the control on porosity, permeability and mean hydraulic pore radius, establishing that the conventional porogen can be removed without structural penalty. The membranes were nevertheless not equivalent in separation. At 2 wt% the membrane was the tightest of the series, with a molecular weight cut-off of 22.3 kDa against 28.7 kDa for the control despite an indistinguishable mean pore radius (16.0 against 17.0 nm), so neither bulk porosity nor mean pore size predicts selectivity. It rejected every organic fraction most effectively and retained the lowest hydraulically irreversible resistance in all four operating regimes, accumulating irreversible resistance 1.8–2.2-fold more slowly per cleaning cycle. The DES acted on both fronts, matching PVP as a pore former while raising surface charge by 36% and lowering contact angle, and difference-FTIR identified a carbohydrate- and humic-rich residual deposit whose ranking followed the resistance data. Choline chloride–urea thus offers a PVP-free route to membranes whose fouling is markedly more reversible on real industrial effluent.