New approaches to graphene membranes
Graphene continues to be the focus of many new research projects that attempt to identify a better material for filtering and desalting water. This week, WDR looks at two new approaches to using graphene in membrane applications.
Graphene coated UF membrane
The membrane was developed by Miao Yu at the University of South Carolina, and employs graphene as a UF membrane surface coating for oil/water separation. According to Dr Yu, small graphene oxide (GO) flakes are laid on a polyamide fiber substrate. The flakes wrap tightly around the polyamide fibers and act as a functional coating for modifying the surface properties of the polyamide. A pi-pi interaction between the flakes and fibers also ensures stability, even with the shear stresses resulting in cross-flow operation.
Yu told WDR, “In the oil/water separation process, water preferentially permeates through our GO coated membranes, forming numerous water pockets inside the structure, which serve as a repulsive support for oil droplets. The hydrophilic GO coating helps trap the water and reduces the effective permeation pore size, introducing additional nano-scaled roughness for smaller oil droplets’ repulsion. Oil droplets are repelled to the membrane surface and accumulate to form bigger droplets. Then, the oil is released from the super-oleophobic GO membrane surface and floats on top of the oil/water mixture.”
For the lab tests, 3cm to 9cm membrane sheets were produced using vacuum filtration to pull GO from a bulk solution onto the polyamide support. Yu said that he expects to use variations on the technique to also produce semipermeable membranes with smaller pore sizes for NF and RO applications.
The technology has a patent pending and has been licensed by the UK-based G2O Water for commercialization. G2O’s Tim Harper told WDR that the group is now gearing up to conduct a funding round in order to continue testing and to develop membranes for specific customer requirements and in sufficient quantities to allow pilot testing.
Graphene RO membrane
The 23 March online issue of Nature Nanotechnology describes a new technique for fabricating nanoporous graphene membranes in which nanoscale pores were created in a single layer of graphene. Most of the work was done by a team of researchers at the Oak Ridge National Laboratory (ORNL) in Tennessee.
After decomposing methane into carbon and hydrogen in a 1,000°C furnace, the carbon atoms are deposited on a copper foil where they self-assemble into adjoining hexagons to form a one-atom thick sheet of graphene. The sheet is then transferred to a silicon nitride support and exposed to an oxygen plasma that knocks some of the atoms from the carbon lattice to create pores. The longer the exposure, the bigger the pores.
Ivan Vlassiouk, a study co-leader, told WDR that even though they worked with microscale graphene sheets, they believe scale-up is possible. He said that with a pore size of 1nm or less and a density of one pore per 100nm2, the team was able to achieve nearly 100 percent salt rejection, adding, “Pore functionalization occurred as a result of the manufacturing process; it seems that our pores are terminated by silicon atoms, although other termination groups may work even better.”
Shannon Mark Mahurin, Dr Vlassiouk’s project co-leader, said that a patent application has been filed, but it’s too early to consider commercialization. “Our next step is to obtain additional government funding to do more experimental work, including to understand how pore functionalization affects performance,” he said.
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