Where Does Our Freshwater Come From?

Though the Earth is predominantly water, the notion of a global water crisis seems paradoxical. Around 71% of the Earth's surface is covered with water, amounting to approximately 332 million cubic miles, enough to fill over half a million Olympic-sized swimming pools. The issue lies in the fact that most of this water is saline, with only 2.5% classified as freshwater; merely 1% of this is readily accessible for human use.

If we could efficiently convert even a small portion of seawater into potable water, it could significantly alleviate the impending global water crisis.

Desalination

Desalination is the technique of extracting salt from seawater to make it suitable for consumption. Traditionally, desalination is prohibitively expensive, making water conservation and recycling more economical alternatives—at least for the time being. Nevertheless, in extremely dry regions like Israel, these alternatives may be insufficient; currently, approximately 80% of Israel's domestic water is sourced from coastal desalination plants. As accessible water becomes increasingly limited, more countries are likely to depend on such facilities.

Most desalination facilities today employ a method known as reverse osmosis. This process utilizes pressure to push saltwater through a semipermeable membrane that permits water molecules to pass while blocking salt and other impurities. As a result, clean freshwater is produced on the opposite side of the membrane.

Introducing Graphene

In 2017, researchers at the Manchester Institute of Science and Technology unveiled an innovative and energy-efficient approach to reverse osmosis utilizing graphene oxide. You may have heard of graphene, a two-dimensional layer of carbon atoms arranged in a hexagonal lattice. Although its theoretical existence was predicted in 1947, it wasn't until 2004 that scientists successfully synthesized it.

This development sparked an era of research into graphene, which boasts remarkable mechanical, electrical, optical, and thermal characteristics. It is considered the strongest material known, with one scientist humorously noting that it could withstand the weight of an elephant balanced on a pencil without breaking. These qualities contribute to its reputation as a "wonder material," prompting scientists to explore its myriad potential applications.

The Manchester research group demonstrated the viability of using graphene oxide, an oxidized variant of graphene, for desalination. Graphene oxide is less expensive to produce than pure graphene, as it can be made by oxidizing inexpensive, abundant graphite. This oxidation yields disordered layers of graphene oxide that can be separated into sheets, facilitating mass production.

So, how does graphene oxide function in desalination? Being hydrophilic, graphene oxide readily allows water to flow through it. The Manchester group created a sieve by stacking graphene oxide layers. When saltwater is passed through, the pores permit water molecules to pass while retaining salt ions. The sieve has pore sizes ranging from 0.64 to 0.98 nanometers, successfully removing 97% of the salt content.

While this is only marginally more effective than conventional reverse osmosis systems, which remove about 90-95% of salts, the significant advantage of the graphene oxide sieve is its lower energy requirement. The water-attracting nature of graphene oxide allows water molecules to flow through with minimal energy input.

This is promising news. By harnessing our oceans, we could potentially avert the looming global water crisis. The water we rely on for drinking, bathing, and agriculture could one day be filtered through a graphene oxide sieve.

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Citations: 1. “How Long Can You Survive without Water?” BBC Future, BBC, https://www.bbc.com/future/article/20201016-why-we-cant-survive-without-water. 2. Desalination by Reverse Osmosis — Organization of American States. https://www.oas.org/dsd//publications/Unit/oea59e/ch20.htm. 3. Weiss, Mark. “How Israel Used Desalination to Address Its Water Shortage.” The Irish Times, 18 July 2019, https://www.irishtimes.com/news/ireland/irish-news/how-israel-used-desalination-to-address-its-water-shortage-1.3959532. 4. “Data.” World Resources Institute, 23 Apr. 2020, https://www.wri.org/aqueduct/data. 5. “How Much Water Is There on Earth?” U.S. Geological Survey, https://www.usgs.gov/special-topic/water-science-school/science/how-much-water-there-earth?qt-science_center_objects=0#qt-science_center_objects. 6. “Affordable Desalination.” The University of Manchester, https://www.manchester.ac.uk/research/beacons/breakthroughs/affordable-desalination/. 7. Abraham, Jijo, et al. “Tunable Sieving of Ions Using Graphene Oxide Membranes.” Nature Nanotechnology, vol. 12, no. 6, 2017, pp. 546–550., https://doi.org/10.1038/nnano.2017.21.