Mechanisms and membrane fouling properties of dual surfactants coupling nanofiltration for multiple heavy metal rejection

The combination of micelle and ultrafiltration processes has long been considered an excellent strategy for heavy metal separation; however, severe concentration polarization and surfactant leaching pose significant limitations to the application of this process. Based on this, a dual-surfactant system was proposed in this study, confirming the mechanisms and membrane fouling properties of micelle-enhanced nanofiltration in retaining common heavy metals. The study results indicated that the polyethylene glycol trimethylnonyl ether/sodium dodecyl benzene sulfonate (at a 1:1 ratio) effectively eliminated 99 % of Zn(II), Cd(II), Cu(II), and Ni(II), displaying superior micelle loading capacity. Under this condition, the removal efficiency of surfactant was also higher than 96 %, which greatly avoids secondary pollution. Moreover, the Arrhenius relationship and extended Derjaguin-Landau-Verwey-Overbeek theory revealed that the formation of numerous micelles and a gel layer on the membrane surface improved the transmembrane activation energies for heavy metals and surfactants. Analysis of the fouled layer characteristics indicated that surfactants and micelles composed the fundamental structure of the fouled layer, with the porous gel layer preserving a multitude of water molecule channels. This gel membrane still surpassed alone nanofiltration in its ability to retain heavy metals. This study provides an efficient and environmentally friendly approach for utilizing micelle-enhanced nanofiltration in removing heavy metal ions.