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Graphene is a unique form of carbon that is one atom thick; so thin that it is essentially considered two dimensional.
Graphene has shown significant promise for water filtration applications and a team of scientists from Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia recently developed an extremely efficient water purification system that appears to be simpler and faster to use than conventional filtration systems.
The novel filtration system is based on a unique form of graphene called ‘Graph Air’, which produces clean drinking water from extremely polluted water. The single-step Graph Air system comprises a unique form of graphene, a heat source, a membrane filter, and a water pump.
Scientists have known for years that the water permeability of graphene is multiple orders of magnitude greater than industry-standard reverse osmosis membranes. With extremely narrow channels, Graph Air was able to screen out the larger molecules of highly toxic compounds to yield pure water. The researchers also used their system to filter high concentrations of acids and alkalines. They were able to generate water with a neutral pH from these samples.
Perhaps most importantly, the Australian research team found their system was able to address one major hang-up that most conventional systems have: a phenomenon known as fouling. Over the course of time, a water filtration system can build up various types of pollutants that can reduce effectiveness. These pollutants are removed periodically so the filtration system can function properly again. The Graph Air system was able to operate without fouling.
Making Graph Air
Highly sophisticated developments in graphene processing have enabled the creation of graphene that is well-suited to water filtration. However, these cutting-edge processes involve multiple intricate and resource-intensive steps that are challenging to implement at scale.
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The Graph Air team was able to develop their unique form of graphene using an ‘ambient air vapor deposition process’ that is much easier, more cost-effective, quicker and more eco-friendly than sophisticated graphing fabrication processes used in pursuit of better water filtration systems. This process does not require the costly, volatile gases that are necessary for some graphene production methods. The Australian researchers also used a safe, cheap and renewable raw material to produce their graphene: soybean oil.
The CSIRO researchers found their production process allowed for the continuous growth of graphene films with highly desirable vapor-permeable qualities that are useful for water filtration.
Making a Filter
Growing in popularity, membrane distillation (MD) is a heat-powered water purification approach that appears very promising for the filtration of seawater and industrial waste. MD involves water purification through a vapor pressure gradient that occurs across a hydrophobic membrane. An MD system is based on parallel flows of a ‘hot’ feed side and a ‘cold’ permeate side, with water vapor forming on the feed side. Filtration occurs when the purified water vapor is transported over to the cold permeate side.
One of the biggest advantages of MD is the ability to yield purified water regardless of the source solution’s salinity. The system is also able to reject most non-volatile impurities, including salt, organic compounds and colloids. Many MD systems are capable of using low-grade heat waste to drive their process.
The Graph Air system is an MD system that makes use of graphene’s nanochannels, which permit the passage of water but not large undesirable molecules. The CSIRO team used Graph Air as part of the active layer in an MD system, and found it produced desirable high levels of water vapor flux. It also has significant durability and resistance to fouling, and a superior ability to reject salt.
Throughout the standard MD process, the filtration membrane normally becomes coated with contaminants. The CISRO scientists found that when their system filtered polluted water from Sydney Harbor, it was twice as effective as a normal water filter.
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The team also discovered the Graph Air filter removed more contaminants, screening out 99% of impurities faster than the standard filters. The Graph Air filter did become coated with impurities but continued to function as normal.
This finding essentially eliminates the standard step of taking out contaminants from the water by sending it to a membrane filter.
In their tests on unprocessed seawater from Sydney Harbor, the team also used a control filter, which is a commercially available MD membrane. As expected, this membrane fouled while processing the harbor water, resulting in a constant decrease in vapor flux and a small decrease in salt removal. On the other hand, the Graph Air membrane showed 100% effective performance with respect to salt removal, all while sustaining a high vapor flux and long-term stability over the course of 72 hours. The graphene-based membrane was capable of yielding around 0.4 to 0.5 liters of seawater per day without any deterioration in performance.
The study team said their results showed Graph Air to be a promising material for using in MD filtration, adding it could address some issues that have been affecting this type of system.
No Need for Chlorine?
In a more recent study, researchers from Russia's National University of Science and Technology (MISiS) revealed how graphene might be used to purify water to the degree it does not need chlorination.
In the study, scientists removed E. coli from a solution by injecting it with a graphene oxide saline solution. The study team said the injected graphene was able to "capture" the toxic bacteria by forming flakes around it. The flakes were then removed to yield drinkable water and a graphene byproduct that could be reused.
According to laser confocal microscopy, scanning electron microscopy and fluorescent testing, a 2.5 g/L solution of graphene oxide was several times more efficient at removing E. coli compared to the control.
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While the researchers were not able to identify precisely how bacteria were neutralized, the scientists said graphene oxide likely triggers the development of free radicals that are toxic to bacteria.
The researchers went on to say that a purification system based on their method would eliminate the need for chlorination. Low-cost and easy to scale, a graphene oxide system would be well-suited to use in large wastewater treatment facilities.
References and Further Reading
MISIS National University of Science and Technology (2019) How to Purify Water With Graphene. Available at: https://en.misis.ru/university/news/science/2019-04/6103/ (Accessed on 13 May 2020).
Seo, D.H., Pineda, S., Woo, Y.C. et al. Anti-fouling graphene-based membranes for effective water desalination. Nat Commun 9, 683 (2018). https://doi.org/10.1038/s41467-018-02871-3
CSIRO. Tiny membrane makes Sydney Harbour "drinkable". Available at: https://www.csiro.au/en/News/News-releases/2018/Tiny-membrane-makes-Sydney-Harbour-drinkable (Accessed on 13 May 2020).