About 97 percent of the earth’s water is found in seas and oceans, yet only a fraction of the world’s potable water comes from desalination of seawater. The looming water crisis drives the search for efficient, cheap and sustainable desalination techniques to recover potable water from seawater.
In a new study published in Nano Letters, David Cohen-Tanugi and Jeffrey Grossman, reported the use of nanoporous graphene to filter salt ions from seawater. Simulation studies showed this new technique is 2-3 orders of magnitude faster than reverse osmosis (RO), which is currently the best commercial desalination technique.
Nanoporous graphene offers a better alternative to RO because of its superior performance, less energy inputs and use of smaller modules. It is a viable desalination technique worth looking into at an economic cost dependent only in the fabrication of nanoporous graphene. Future improvements in this fabrication process could make this technique cheaper and more sustainable than RO.
Grossman reported that tailored nanostructuring of membranes could filter salt and other impurities by size exclusion. Since salt ions are larger than water molecules, membrane materials could be designed with pore diameters that allow the passage of water molecules only. The use of these specially designed membranes could lead to greater permeability compared to RO.
The RO technique uses high pressure to push water molecules across a porous membrane surface, leaving behind salt ions on the other side of the barrier. In contrast, nanoporous materials work at lower pressures and have well-defined channels where water molecules can pass through, but salt ions are excluded because of their larger size.
There had been previous studies on the use of nanoporous materials for desalination, but this is the first study to use nanoporous graphene. Specifically, this study uses a monolayer of graphene that is only one carbon atom thick. It is the ultimate thin membrane. A great advantage since water permeability across a membrane varies inversely with membrane thickness.
Cohen-Tanugi and Grossman used classical molecular dynamics simulation in their study to examine the permeability of nanoporous graphene with different pore diameters ranging from 1.5 to 62 Å2. These nanopores are introduced into the graphene layer by different methods, such as helium ion beam drilling and chemical etching. These nanopores are then strengthened by passivating or shielding each carbon atom at the pore edge with either hydrogen atoms or hydroxyl groups.
Unshielded carbon atoms at the pore edge are expected to be quite reactive in realistic experimental conditions. These atoms will tend to develop some forms of chemical functionality. According to Grossman, this event can be controlled to an extent by shielding these carbon atoms. They also wanted to explore the influence of hydrophilic versus hydrophobic edge chemistries on the permeability of nanoporous graphene. They did this by shielding the carbon atoms in two ways: using hydrophilic hydroxyl groups, and using hydrophobic hydrogen atoms.
Simulation studies on the permeability of nanoporous graphene, therefore, were conducted with two variables: pore size and pore-edge chemistry. Saltwater, having a salinity of 72 g/L, was passed through the nanoporous graphene. This was twice the salt content of average seawater.
Results showed the graphene layer with the largest nanopore diameter was able to filter water at the fastest rate, but some salt ions were able to pass through. Whereas the graphene layer with intermediate range of nanopore diameter was able to completely exclude salt ions at the most optimum permeability rate.
Pore-edge chemistry results showed hydroxylated graphene had greater permeability because of hydrophilic interactions between the hydroxyl groups and water molecules. It allowed for greater number of hydrogen-bonding configurations inside the pore, resulting to an increase in water flux across the graphene layer. While hydrogenated graphene had a lower permeability because of hydrophobic interactions, the flow of water molecules in the pores were limited to highly ordered configurations.
Comparison of nanoporous graphene and RO showed the former had greater water permeability in terms of liters of output per square centimeter of membrane per day and per unit of applied pressure. High-flux RO had a water permeability of a few tenths, while nanoporous graphene had a water permeability ranging from 39 to 66 for pore diameter that gave complete salt exclusion. This complete salt exclusion was achieved at 23.1 Å2 for hydrogenated pores and 16.3 Å2 for hydroxylated pores.
Presently, there are two main challenges in the use of nanoporous graphene as a viable desalination technique to rival RO. The first challenge is achieving a narrow pore size distribution. Research to improve the fabrication of highly ordered nanoporous graphene is currently underway. The second challenge is the mechanical stability of nanoporous graphene under applied pressure. A possible solution is to use a thin-film support layer similarly used in the RO technique.
Grossman said their research team is continually looking into a range of possible new methods to engineer membranes for desalination and purification of water. They are currently fabricating nanoporous materials, which they will test for desalination performances in the incoming months.