Concentrate of surfactant-based draw solutions in forward osmosis by ultrafiltration and nanofiltration

The exploration of coupling surfactants with highly charged phosphate as novel draw solutions for forward osmosis (FO) to minimize salt leakage and enhance draw solute recovery at low energy is presented for the first time. In this work, anionic (sodium dodecyl sulfate (SDS)), cationic (cetyltrimethylammonium bromide) and non-ionic (polyethylene glycol tert-octylphenyl ether, TX-100) surfactants were used to couple with Na₃PO₄ for enlarging draw solution particle size that could improve the solute retention. Three kinds of nanofiltration (NF) membranes (HL, DL and TS80 with molecular weight cutoff (MWCO) of 100–500) and three kinds of ultrafiltration (UF) membranes (GE, GH and GK with MWCO of 1,000–3,000) were used to evaluate the draw solution recovery. The formed second layer, micelles, MWCO and charged valent were determined to be the dominant mechanisms for diluted draw solution recovery. The results show that coupling non-ionic surfactant with 0.02 M Na₃PO₄ can be regenerated with recoveries as high as 60% and 96% when using UF-GE and NF-TS80, respectively. The overall performance demonstrates that coupling surfactant with highly charged phosphate is promising as draw solutes for application to FO.