A systematic evaluation on the performance and mechanism of surfactants as additive of draw solution in forward osmosis

In this study, the performances of six different types of surfactants, each with a series of concentrations around its critical micelle concentration, were systematically tested as additives of draw solution (DS) to improve the performance of DS in forward osmosis. The influence mechanism of surfactants on DS characteristics, membrane fouling and characteristics of ideal surfactants were discussed comprehensively. The six surfactants showed big differences in considering their effects on the characteristics of DS and their performances, which could be explained by the combined effects of thin dense layer formed by surfactant molecules adsorbed on the surface of supporting layer (SL), blocked pores of SL by micelles aggregation, increased viscosity of DS, improved hydrophilicity of membrane SL and ionization of the hydrophilic groups. The nonionic surfactants could significantly reduce the reverse solute flux, accompanied by sharply decreased water flux and heavy membrane fouling. Compared with nonionic surfactants, the anionic surfactants with their Hydrophile-Lipophilic Balance around 10 could significantly reduce the reverse solute flux while with no or low decrease of water flux and reversible membrane fouling. In our work, Sodium dodecyl benzene sulfonate, at 5 mmol/L concentration, had the best performance on the improvement of DS.