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 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 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.