Novel desalination method with nearly perfect salt rejection and high water flux
(Nanowerk Spotlight) Nanofluidic membranes based on two-dimensional (2D) materials are promising materials for next-generation water desalination and purification. For instance, pristine and chemically modified graphene oxide membranes (GOMs) effectively block organic dyes and nanoparticles as small as 9 Å.
However, these nanomembranes fail to exclude smaller inorganic salt ions, which would be required to extract pure potable water from unconventional water sources such as, salt water, industrial wastewater, and rainwater.
Researchers have proposed a number of strategies to address the sieving of small inorganic salt ions, for instance by reducing the interlayer spacing down to merely several angstrom. However, one critical challenge for such physically or chemically compressed GOMs is the extremely low water flux (<0.1 Lm-2h-1bar-1) that prevents them from being used in real-world applications.
"With our novel approach, which we termed planar heterogeneous interface desalination, we can achieve high-performance water desalination with 97% rejection rate for NaCl and extraordinarily high water flux of 1529 Lm-2h-1bar-1," Professor Wei Guo from the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, in Beijing, tells Nanowerk. "We achieved this via an inverted T-shaped water extraction mode that does not require a compression of the interlayer spacing of the graphene oxide membrane."
Fabrication and characterization of planar heterogeneous graphene oxide membrane (PHGOM). a-c) Scheme and photographs of the dual-flow filtration setup. n-GO (dark yellow) and p-GO solutions (brown) are put into the two-compartment filtration vessel separated by a plastic clapboard. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"Assisted by a forward electric field, the bi-unipolar ion transport behavior in the oppositely charged graphene oxide multilayers generates a remarkable salt concentration depletion zone in the transition area, and consequently offers an opportunity for extracting deionized water," Guo points out. "Our proposed inverted T-shaped water extraction mode is completely different from conventional membrane filtration processes and it is more effective for harvesting fresh water from 2D layered materials."
Whereas in conventional filtration membranes the water flow proceeds through the membrane in a vertical direction, the team's novel approach absorbs the feed solution into the membrane from the lateral ends, and clean water is then extracted out of the membrane in a vertical direction.
The researchers point out that their novel and unconventional dual-flow (or multi-flow) filtration method allows simultaneously patterning of two or more different types of colloidal 2D materials into laterally connected planar multi-layer configuration.
According to Guo, this method is easily extendible to other colloidal 2D materials in order to achieve new functionality and better performance.
Scheme of the working mechanism of planar heterogeneous graphene oxide membrane (PHGOM)-based desalination. Forward electric field generates a salt concentration depletion state in the transition zone. Under a negative pressure (-ΔP), deionized water can be extracted from the PHGOM. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"With a high salt rejection rate close to 97%, our PHGOM-based desalination presents the highest water flux among existing inorganic, polymeric, and bio-hybrid membrane based systems so far," says Guo. "It is more than two orders of magnitude higher than polymeric reverse osmosis membranes and mixed matrix membranes (<10 Lm-2h-1bar-1), and even exceeds the aquaporin-based membranes (601 Lm-2h-1bar-1) and porous monolayer graphene (252 Lm-2h-1bar-1)."
Furthermore, the total energy consumption per one cubic meter of fresh water of the team's device is ∼0.28 kWh, which is lower than traditional desalination processes (>0.5 kWh).
The thermodynamic efficiency of the PHGOM-based desalination device reaches 50.0%, ahead of the state-of-the-art seawater reverse osmosis (SWRO) and brackish water reverse osmosis (BWRO) desalination process (<40%).
"The minimum working pressure is 0.4 bar in our test; this means that even an adult human can suction out water from the membrane," Guo concludes. "This planar heterogeneous interface desalination method is completely different from any previous desalination methods. It holds great promise for portable and low-energy-consumption water drawing appliance."