Pinning Down the Water Transport Mechanism in Graphene Oxide Pervaporation Desalination Membranes

Abstract

Graphene oxide (GO) based membranes have attracted considerable interest in desalination and organic dehydration. However, the concern on their stability in water medium still hampers their practical application. In this work, we fabricated a series of GO composite membranes via filtration coating on different substrates, and tested these membranes for their pervaporation desalination performance. We discovered the water productivity was independent of the GO layer thickness (in the GO loading range of 0.034 to 136.1 μg/cm 2 membrane) for the pervaporation desalination process. The composite pervaporation membrane exhibited stable desalination performance with challenging brines, showing an ultrahigh salt rejection over 99.99% over 50 h of continuous operation. We further proved that the stable performance was originated from the presence of cation within the laminates, which cross-linked the separated nanosheets to maintain the stacked structure with confined nanochannels. Finally, the water transport mechanism was detailed, confirming the liquid to vapor phase transition mainly occurred at the top few layers within the GO laminates, and water was mainly transferred through the membrane in their vapor form. Understanding the mechanism can enlighten further fabrication, application, and optimization of the pervaporation desalination membranes for practical applications. In this work, the optimized pervaporation membrane exhibited the highest water flux of 67 L/m 2 h with only 70 °C feed. With both permeation flux and salt rejection significantly higher than the commercial reverse osmosis membranes, the GO pervaporation membranes provide a new route of water treatment toward zero-liquid discharge with the possibility of harnessing secondary heat.

Advantages, limitations, and future suggestions in studying graphene-based desalination membranes

“…The literature suggests the emerging of other less explored applications. Recent GO membranes provide better performances in MD 16,17 and pervaporation 145,146 desalination process, as well as in fouling reduction when combined with silver nanoparticles 68 and in thin-lm nanocomposite membranes. 88 Applications in MD and pervaporation membranes and the role of graphene and GO in improving these processes is a promising direction, which certainly deserves more in-depth consideration.…”

“…Pervaporation is an alternative process for liquid mixture separation by using nonporous membranes that could have benets from the use of graphene or GO membranes. 145 GO composite membranes fabricated by ltration on different substrates were tested relatively to pervaporation desalination performance, 146 showing no dependence on the GO layer thickness, a stable desalination performance with 99.99% salt rejection for over 50 hours of continuous brine desalination, and 67 L m À2 h À1 with 70 C feed. It was determined that the transition of a liquid to vapor mainly took place at the top few layers of the GO laminates, and water transfer across the membrane occurred in their vapor form.…”

“…Fig.5Schematic diagram of the brine desalination process with the GO pervaporation membranes Reprinted (adapted) with permission from (Lin Li, Jingwei Hou, and Vicki Chen Industrial & Engineering Chemistry Research, 2019, 58(10), 4231-4239. DOI: 10.1021/acs.iecr.8b06081) 146. Copyright (2019) American Chemical Society.…”

Molecular dynamics study of transport mechanisms in desalination membranes

“…4b) to match the high water fluxes obtained in experiments, otherwise water evaporation would be abnormally slow. However, Li, et al120 proposed a four-stage transport mechanism as shown in Figure2.4c where the phase transition took place inside the GO nanochannels, so that it explained the unchanged flux with different number of GO layers. A more recent MD study (Figure2.4d) showed that the phase transition occurred at the permeate surface of GE membrane for the separation of ethanol and water.…”

“…Figure 2.4. (a and b) Schematic illustrations of water transport mechanisms inside GO membranes for pervaporation; 119 (c) Schematic diagram of water transport and salt rejection in GO membranes for pervaporation;120 (d) Snapshot of MD simulations for ethanol/water mixture separation using pervaporation through GE nanochannels, where while, red, and green sticks represent hydrogen, oxygen, and carbon atoms respectively 121. …”