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Molecular insights into water desalination with graphene oxide membranes

(Nanowerk News) In new work, scientists performed a pressure-driven flow simulation of ionic aqueous solutions across bilayer graphene oxide (GO) membranes to investigate the effects of staggered nanoslits on desalination properties using classical molecular dynamics (MD) simulation (read about What is graphene?.
They performed reverse osmosis (RO) pressure-driven flow to investigate the effect of geometrical parameters and salt species on the performance of GO membranes.
Presenting their findings in ACS Applied Materials & Interfaces ("Molecular Insight into Water Desalination across Multilayer Graphene Oxide Membranes"), a team from China monitored the water flux across the membrane with variation of geometry parameters (i.e., width of gaps and offsetting between two staggered gaps) and made a comparison with the infiltration predicted by the continuum model.
They report good agreement between the MD results and the developed theoretical continuum calculations with modified geometry parameters.
The red, lime, and yellow spheres, respectively, represent the carbon atoms, oxygen atoms, and hydrogen atoms, which make up the graphene oxide sheets
Representative snapshot with different distances of nanoslits. The red, lime, and yellow spheres, respectively, represent the carbon atoms, oxygen atoms, and hydrogen atoms, which make up the GO sheets. The purple, white, blue, and pink spheres correspondingly denote the oxygen atoms, hydrogen atoms, cations, and anions, which make up the ionic aqueous solution. (© ACS)
The synthesized flow model used by the team can be divided into three parts: the entrance of a nanoslit; the conjunction between the gap and passage; and the interlayer inside the capillary channel.
The whole permeation of the GO films is dependent on the series-parallel combination of three resistances in these three parts. The scientists monitored variable configurational parameters to elucidate the effect of different structures.
To eliminate the variance between the classical model and nanoscaled flow, which is caused by the effect of size on the shrunken real flow area, they obtained modified geometrical parameters from density analysis.
MD results show that the water flow rate has a parabolic relation with slit width and decreases for a greater slit alignment, quantitatively in agreement with the theoretical prediction with modified geometrical parameters.
Schematic diagram of the ions with large hydration radii during passing through a nanoslit
Schematic diagram of the ions with large hydration radii during passing through the nanoslit. (© ACS)
In addition, they considered four kinds of solutions (i.e., NaCl, KCl, MgCl2, and CaCl2) in different configurations and recorded the trajectories of these ions.
The team found that different ions have various transport regimes through nanoslits and nanocapillaries because of different hydration structures.
The aim of the results above is to assess the potential of multilayer GO membranes in desalination and help understanding the transport mechanism of water and ions across the tortuous nanonetworks.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny.
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