S. Pillai, A. Santana, R. Das, B.R. Shrestha, E. Manalastas, H. Mishra
Journal of Membrane Science, p. 118140, (2020)
Direct contact membrane distillation, Organic fouling, Hydrophobic membranes, Hydrophobic interactions, Organics
The removal of water-soluble organics from aqueous feeds is required in numerous practical applications, including bioresource processing, fermentation, and wastewater treatment. To this end, direct contact membrane distillation (DCMD) has been proposed as a separation technology. DCMD utilizes hydrophobic membranes – typically, comprising perfluorocarbons – which, when placed between the warm feed and cold permeate reservoirs, prevent their mixing due to the entrapment of air inside the (membranes’) pores, allowing only vapor to transport across. Here, we assess DCMD for separating organics from aqueous feeds in light of organic fouling by utilizing ethanol and perfluorodecyltrichlorosilane (FDTS) as the surrogate organic and hydrophobic coating, respectively. We investigated the adsorption of ethanol onto FDTS-grafted surfaces and membranes exposed to alcohol-water mixtures. Using a Surface Force Apparatus, we found that the magnitude of hydrophobic forces between ultra-smooth FDTS-grafted mica surfaces in water-alcohol mixtures decreased with the increasing alcohol content. To simulate a practical DCMD scenario, we utilized FDTS-grafted polycarbonate membranes to separate a pure water reservoir from another containing 0.6 M NaCl and alcohol. For the 0% alcohol case, the membranes robustly separated the reservoirs for over a week, whereas even for 0.1% ethanol content, the membranes leaked within < 5 h. After the leakage, membranes’ hydrophobicity did not recover even after rinsing with pure water and drying; a heat treatment at 363 K for 1 h was able to regenerate their hydrophobicity. Our molecular dynamics simulations revealed that ethanol molecules in water are preferentially stabilized at the water-hydrophobe interface, and this stabilization is significantly enhanced at higher concentrations due to the formation of a II-D interfacial network. Taken together, these findings demonstrate that hydrophobic membranes and hence DCMD are not suitable for separating organics from water even at low concentrations, and they also inform on monitoring, characterizing, and cleaning protocols for fouling.