Surfactant-induced wetting dynamics in the context of hypersaline desalination for membrane distillation

Abstract

Amongst different desalination technologies to tackle freshwater demand, membrane distillation (MD) is promising in that it can effectively treat hypersaline feed or reverse osmosis reject and further improve freshwater recovery while simultaneously reducing the amount of liquid discharge. However, wetting of the membrane pores by surfactant compromises the separation efficiency since MD relies on maintaining a stable air gap in the membrane pore. The kinetics of surfactant-induced wetting for a hydrophobic membrane applied in MD technology have been shown to depend only on bulk surfactant concentration and vapour flux. In this study, we examine the decoupled effect of salt concentration and bulk surfactant concentration and its relation to surfactant-induced wetting. Even at low surfactant concentration (0.1 mM sodium dodecyl sulphate), the concentration of salt (sodium chloride) can significantly affect the wetting dynamics. In particular, high salt concentrations (above 1.2 M or 70 g L⁻¹ NaCl) can notably accelerate wetting, and thereby render MD unsuitable for such feeds. On the other hand, surfactant concentrations well above the critical micelle concentration (CMC) are tested with low salt concentration, and the results reveal that hydrophobic PVDF membranes perform quite stably without any significant loss in salt removal efficiency. A mathematical framework that captures ionic strength and surfactant activity is also proposed to predict different membrane wetting regimes. These findings point to the need for coupling bulk surfactant concentration with salt concentration to predict surfactant-induced wetting more accurately. These results also open an avenue for an alternative mechanism that complements the existing understanding of surfactant-induced wetting.