A Molecular to Macro-scale Assessment of Direct Contact Membrane Distillation for Separating Organics from Water

by S. Pillai, A. Santana, R. Das, B. R. Sreshta, E. Manalastas, H. Mishra
Year: 2019

Bibliography

A Molecular to Macro-scale Assessment of Direct Contact Membrane Distillation for Separating Organics from Water. S. Pillai, A. Santana, R. Das, B. R. Shrestha, E. Manalastas, H. Mishra*. ACS Applied Materials & Interfaces, 2019 (under review).

Abstract

Numerous industries, such as bioresource processing, necessitate the removal of amphiphilic molecules such as ethanol from aqueous feeds. To this end, direct contact membrane distillation
(DCMD) has been proposed as a solution. DCMD utilizes hydrophobic membranes, typically comprising perfluorocarbons, that robustly entrap air inside their pores, thereby preventing the mixing of aqueous feed and permeate solutions; the application of heat on the feed-side drives the separation. Here, we assess DCMD for separating small amphiphilic molecules from aqueous feeds in light of organic fouling at water-hydrophobe interfaces. Specifically, we investigate the
 loss of hydrophobicity of perfluorinated surfaces exposed to ethanol-water solutions in the volumetric ratios: 0%, 0.1%, 0.5%, 1% , 18%, and 100%. The extent of the physisorption of ethanol is evaluated by (i) the measurement of hydrophobic surface forces, and (ii) the characterization of hydrophobic surfaces withdrawn from those alcohol-water solutions. The results reveal significant and rapid organic fouling of perfluorinated and hydrocarbon surfaces even in 0.1% solutions. Similarly, perfluorinated ultrafiltration membranes separating a 0.1% ethanol-water solution and salty water failed within four hours due to the time-dependent loss of hydrophobicity. We investigated the efficacy of standard diagnostic and cleaning protocols for fouling, including advancing/receding/apparent contact angles, backwashing, blow drying, and heating. To gain insights into the molecular origins of organic fouling, potential of mean force calculations using molecular dynamics simulations were performed. They revealed that at dilute concentrations (0.4%), ethanol molecules are stabilized at the water-hydrophobe interface by a free energy of -2.5 kBT in comparison to the bulk solution. At higher concentrations (22%), the speciated alcohol molecules create a percolation network, which stabilizes them further. These findings demonstrate that DCMD is not suitable for separating amphiphilic solutes from water even at dilute concentrations, and they also inform on stricter monitoring, characterization, and cleaning protocols for fouled surfaces.