Graphene Membrane Turns Seawater Into Drinking Water

With the advent of poor water conditions plaguing the world, several groups of researchers have entered a quest to secure clean sources of water. A seemingly infinite amount of water exists in the ocean, however purifying salt water is whole separate issue to tackle. Among the technologies available to purify water, the design and development of relatively-new graphene membrane systems suggests a promising future.

Graphenes are a class of mechanically robust, ultrathin, high-flux, high selectivity, and fouling resistant separation membranes that are able to advance water desalination technologies. These next-generation membranes are a cost effective and sustainable alternative for water purification applications. Graphene membranes have been observed offer ultrafast permeation, excellent mechanical strength and precise ionic sieving (Mahmoud, Mansoor, Mansour, & Khraisheh, 2015).

Researchers have observed the properties of a silica-crosslinked graphene oxide (GO) membrane, and highlighted it's unique capability to remove neutral organic molecules from water, such as glucose and sucrose. The silica-crosslinked GO membrane was created by immersing a layer-stacked GO film in a solution of saturated silica, which remained stable under various test conditions. The negatively charged GO membrane was found to remove the neutral organic molecules 84% to 90% more efficiently than the a negatively charged ionic species, trisodium citrate (22%), a saline-possessing chemical. This suggests that the interactions between the GO membrane and the tested neutral organic species indicates the GO membrane may better remove charged molecules compared to nanofiltration or reverse osmosis membranes (Zheng & Mi, 2016).

GO membranes have been observed to carry exceptional molecular permeation properties, with promise for many applications. However, their use in filtering ions and removing salt is limited, since the the diameters of the sieve is larger than that of the ions in salts. Achieving a smaller diameter for the GO membrane immersed in water has proved to be a challenge. Researchers have described and demonstrated how to control the diameter of such membranes. However,  permeation rates decrease exponentially with decreasing sieve size but the transportation of water is weakly affected (by a factor of <2). Based on these findings, the researchers were able to demonstrate that the graphene-based membranes exhibit a 97% rejection for NaCl (Abraham et al., 2017).

---

References

Abraham, J., Vasu, K., Williams, C., Gopinadhan, K., Su, Y., & Cherian, C. et al. (2017). Tunable sieving of ions using graphene oxide membranes. Nature Nanotechnology. http://dx.doi.org/10.1038/nnano.2017.21

Mahmoud, K. , Mansoor, B. , Mansour, A. , & Khraisheh, M. (2015). Functional graphene nanosheets: The next generation membranes for water desalination. Desalination, 356, 208-225.

Zheng, S. , & Mi, B. (2016). Emerging investigators series: Silica-crosslinked graphene oxide membrane and its unique capability in removing neutral organic molecules from water. Environmental Science: Water Research & Technology, 2(4), 717-725.