Graphene frameworks ‘can make desalination 100x faster’

New computational research on graphene sheets is showing that, when oxidized sheets are linked together as graphene oxide frameworks (GOFs), they can be used as a tunable desalination membrane.

Researchers at Oak Ridge National Laboratory (ORNL) and Rensselaer Polytechnic Institute (RPI) in the USA used supercomputer simulations to explore the purification potential of GOFs.

After developing computational models to describe the interactions among the material’s atoms, Dr Bobby G Sumpter, director of ORNL’s Nanomaterials Theory Institute, set out with RPI’s Prof Vincent Meunier and Dr Adrien Nicolaï to compute the ideal configuration for a GOF desalination membrane. They used high-performance computers to simulate how layer thickness, the density of the linking pillars and applied pressure affect the material’s performance.

The simulations revealed that fine-tuning the GOF structure results in the ability to remove all the ions from saltwater at a much quicker rate — approximately 100 times faster than the materials currently used as reverse osmosis membranes. The use of water-repellent graphene as part of the porous membrane contributes to the increased performance by forcing the water into channels.

The coupling between water permeability and salt rejection of GOF membranes is studied as a function of linker concentration n, thickness h and applied pressure ΔP. The simulations reveal that water permeability in GOF- (n,h) membranes can be tuned from 5 (n = 32 and h = 6.5 nm) to 400 L cm−2 day −1 MPa−1 (n = 64 and h = 2.5 nm) and follows a Cnhαn law.

For a given pore size (n = 16 or 32), water permeability of GOF membranes increases when the pore spacing decreases, whereas for a given pore spacing (n = 32 or 64), water permeability increases by up to two orders of magnitude when the pore size increases.

Furthermore, for linker concentrations n ≤ 32, the high water permeability corresponds to a 100% salt rejection, elevating this type of GOF membrane as an ideal candidate for water desalination, the authors claim in their paper Tunable water desalination across graphene oxide framework membranes in the journal Physical Chemistry Chemical Physics.

The researchers also observe on the ORNL website that, as well as for salt ions, the GOF material could be used as filtration membranes for other contaminants such as bacteria. Since GOFs are made with abundant, inexpensive materials through a standard fabrication process, the researchers also believe that the GOF-based membranes could help make desalination more economically viable.