A molecular to macro level assessment of direct contact membrane distillation for separating organics from water

Abstract

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 a warm feed and a cold permeate, prevent mixing due to the robust entrapment of air inside the (membranes') pores. Thus, the membranes allow only pure water vapor to transport across, following the thermal gradient. Here, we assessed 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 the 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, the membranes’ hydrophobicity could not be recovered by rinsing them with pure water and blow-drying; a heat treatment at 363 K for 1 h proved to be successful, however. Our molecular dynamics simulations revealed that ethanol molecules in water got preferentially stabilized at the interfaces of water and hydrophobic surfaces. Furthermore, this stabilization is significantly enhanced at higher alcohol concentrations due to the emergence of II-D interfacial networks comprising adsorbed alcohol molecules. Thus, this micro to macro-scale assessment demonstrates that DCMD with hydrophobic membranes is not suitable for separating organics from water, even at low alcohol concentrations. We also compare the efficacies of apparent advancing and receding contact angles towards a reliable characterization of fouled surfaces.