This post is part of a series examining the connections between nanotechnology and the top 10 trends facing the world, as described in the Outlook on the Global Agenda 2015. All authors are members of the Global Agenda Council on Nanotechnology.
In the 2015 World Economic Forum’s Global Risks Report survey participants ranked Water Crises as the biggest of all risks, higher than Weapons of Mass Destruction, Interstate Conflict and the Spread of Infectious Diseases (pandemics). Our dependence on the availability of fresh water is well documented, and the United Nations World Water Development Report 2015 highlights a 40% global shortfall between forecast water demand and available supply within the next fifteen years. Agriculture accounts for much of the demand, up to 90% in most of the world’s least-developed countries, and there is a clear relationship between water availability, health, food production and the potential for civil unrest or interstate conflict.
The looming crisis is not limited to water for drinking or agriculture. Heavy metals from urban pollution are finding their way into the aquatic ecosystem, as are drug residues and nitrates from fertilizer use that can result in massive algal blooms. To date, there has been little to stop this accretion of pollutants and in closed systems such as lakes these pollutants are being concentrated with unknown long term effects.
While current solutions such as reverse osmosis exist, and are widely used in the water desalination of seawater, the water they produce is expensive. This is because high pressures are required to force the waster through a membrane and maintaining this pressure requires around 2kWh for every cubic meter of water. While this is less of an issue for countries with cheap energy, it puts the technology beyond the reach of most of the world’s population.
Any new solution for water issues needs to be able to demonstrate precise control over pore sizes, be highly resistant to fouling and significantly reduce energy use, a mere 10% won’t make a difference. Nanotechnology has long been seen as a potential solution. Our ability to manipulate matter on the scale of a few atoms allows scientists to work at the same scale as mot of the materials that need to be removed from water — salts, metal ions, emulsified oil droplets or nitrates. In theory then it should be a simple matter of creating a structure with the correct size nanoscale pores and building a better filter.
Ten years ago, following discussions with former Israeli Prime Minister Shimon Peres, I organised a conference in Amsterdam called Nanowater to look at how nanotechnology could address global water issues. While the meeting raised many interesting points, and many companies proposed potential solutions, there was little subsequent progress.
Rather than a simple mix of one or two contaminants, most real world water can contain hundreds of different materials, and pollutants like heavy metals may be in the form of metal ions that can be removed, but are equally likely to be bound to other larger pieces of organic matter which cannot be simply filtered through nanopores. In fact the biggest obstacle to using nanotechnology in water treatment is the simple fact that small holes are easily blocked, and susceptibility to fouling means that
Fortunately some recent developments in the ‘wonder material’ graphene may change the economics of water. One of the major challenges in the commercialisation of graphene is the ability to create large areas of defect-free material that would be suitable for displays or electronics, and this is a major research topic in Europe where the European Commission is funding graphene research to the tune of a billion euros. Simultaneously there are vast efforts inside organisations such as Samsung and IBM. While defects are not wanted for electronic applications, recent research by Nobel Prize winner Andrei Geim and Rahul Nair has indicated that in graphene oxide they result in a barrier that is highly impermeable to everything except water vapour. However, precisely controlling the pore size can be difficult.
Another approach taken by researchers at MIT involves bombarding graphene sheets with beams of gallium ions to create weak spots and then etching them to create more precisely controlled pore sizes. A similar approach to water transport through defects has been taken by researchers at Penn State University.
While all of the above show that graphene has prospects for use as a filter medium, what about the usual limiting issue, membrane fouling? Fortunately another property of graphene is that it can be hydrophilic, it repels water, and protein absorption has been reported to have been reduced by over 70% in bioreactor tests. Many other groups are working on the use of graphene oxide and graphene nanoplatelets as an anti-fouling coating.
While the graphene applications discussed so far address one or two of the issues, it seems that thin films of graphene oxide may be able to provide the whole solution. Miao Yu and his team at the University of South Carolina have fabricated membranes that deliver very high flux and do not foul. Fabrication is handled by adding a thin layer of graphene to an existing membrane, as distinct from creating a membrane out of graphene, something which is far harder to do and almost impossible to scale up.
Getting a high flux is crucial to desalination applications where up to 50% of water costs are caused by pressurising water for transmission through a membrane. Performance tests reveal around 100% membrane recovery simply by surface water flushing and pure water flux rates (the amount of water that the membrane transmits) are two orders of magnitude higher than conventional membranes. This is the result of the spacing between the graphene plates that allows the passage of water molecules via nanoscale capillary action but not contaminants.
Non-fouling is crucial for all applications, and especially in oil/water separation as most of what is pumped out of oil wells is water mixed with a little oil.
According to G2O Water, the UK company commercialising Yu’s technology, the increased flux rates are expected to translate directly into energy savings of up to 90% for seawater desalination. Energy savings on that scale have the potential to change the economics of desalination with smaller plants powered by renewable energy and addressing community needs replacing the power hungry desalination behemoths currently under construction such as the Carlsbad Project. This opens the possibility of low-cost water in areas of the world where desalination is currently too expensive or there is insufficient demand to justify large scale infrastructure.
While more work is required to build a robust and cost-effective filtration system, the new ability to align sheets of graphene so that water but nothing else is transmitted may be the simple game-changer that allows the world to finally address the growing water crisis.
Author: Tim Harper is Chief Executive Officer of G2O Water.
Image: The colors of Fall can be seen reflected in a waterfall along the Blackberry River in Canaan, Connecticut REUTERS/Jessica Rinaldi
Summary: For faster, longer-lasting water filters, some scientists are looking to graphene –thin, strong sheets of carbon — to serve as ultrathin membranes, filtering out contaminants to quickly purify high volumes of water.
Graphene’s unique properties make it a potentially ideal membrane for water filtration or desalination. But there’s been one main drawback to its wider use: Making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material — creating defects through which contaminants can leak. Now engineers have devised a process to repair these leaks.
Graphene’s unique properties make it a potentially ideal membrane for water filtration or desalination. But there’s been one main drawback to its wider use: Making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material — creating defects through which contaminants can leak.
Now engineers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals (KFUPM) have devised a process to repair these leaks, filling cracks and plugging holes using a combination of chemical deposition and polymerization techniques. The team then used a process it developed previously to create tiny, uniform pores in the material, small enough to allow only water to pass through.
Combining these two techniques, the researchers were able to engineer a relatively large defect-free graphene membrane — about the size of a penny. The membrane’s size is significant: To be exploited as a filtration membrane, graphene would have to be manufactured at a scale of centimeters, or larger.
In experiments, the researchers pumped water through a graphene membrane treated with both defect-sealing and pore-producing processes, and found that water flowed through at rates comparable to current desalination membranes. The graphene was able to filter out most large-molecule contaminants, such as magnesium sulfate and dextran.
Rohit Karnik, an associate professor of mechanical engineering at MIT, says the group’s results, published in the journal Nano Letters, represent the first success in plugging graphene’s leaks.
“We’ve been able to seal defects, at least on the lab scale, to realize molecular filtration across a macroscopic area of graphene, which has not been possible before,” Karnik says. “If we have better process control, maybe in the future we don’t even need defect sealing. But I think it’s very unlikely that we’ll ever have perfect graphene — there will always be some need to control leakages. These two [techniques] are examples which enable filtration.”
Sean O’Hern, a former graduate research assistant at MIT, is the paper’s first author. Other contributors include MIT graduate student Doojoon Jang, former graduate student Suman Bose, and Professor Jing Kong.
A delicate transfer
“The current types of membranes that can produce freshwater from saltwater are fairly thick, on the order of 200 nanometers,” O’Hern says. “The benefit of a graphene membrane is, instead of being hundreds of nanometers thick, we’re on the order of three angstroms — 600 times thinner than existing membranes. This enables you to have a higher flow rate over the same area.”
O’Hern and Karnik have been investigating graphene’s potential as a filtration membrane for the past several years. In 2009, the group began fabricating membranes from graphene grown on copper — a metal that supports the growth of graphene across relatively large areas. However, copper is impermeable, requiring the group to transfer the graphene to a porous substrate following fabrication.
However, O’Hern noticed that this transfer process would create tears in graphene. What’s more, he observed intrinsic defects created during the growth process, resulting perhaps from impurities in the original material.
Plugging graphene’s leaks
To plug graphene’s leaks, the team came up with a technique to first tackle the smaller intrinsic defects, then the larger transfer-induced defects. For the intrinsic defects, the researchers used a process called “atomic layer deposition,” placing the graphene membrane in a vacuum chamber, then pulsing in a hafnium-containing chemical that does not normally interact with graphene. However, if the chemical comes in contact with a small opening in graphene, it will tend to stick to that opening, attracted by the area’s higher surface energy.
The team applied several rounds of atomic layer deposition, finding that the deposited hafnium oxide successfully filled in graphene’s nanometer-scale intrinsic defects. However, O’Hern realized that using the same process to fill in much larger holes and tears — on the order of hundreds of nanometers — would require too much time.
Instead, he and his colleagues came up with a second technique to fill in larger defects, using a process called “interfacial polymerization” that is often employed in membrane synthesis. After they filled in graphene’s intrinsic defects, the researchers submerged the membrane at the interface of two solutions: a water bath and an organic solvent that, like oil, does not mix with water.
In the two solutions, the researchers dissolved two different molecules that can react to form nylon. Once O’Hern placed the graphene membrane at the interface of the two solutions, he observed that nylon plugs formed only in tears and holes — regions where the two molecules could come in contact because of tears in the otherwise impermeable graphene — effectively sealing the remaining defects.
Using a technique they developed last year, the researchers then etched tiny, uniform holes in graphene — small enough to let water molecules through, but not larger contaminants. In experiments, the group tested the membrane with water containing several different molecules, including salt, and found that the membrane rejected up to 90 percent of larger molecules. However, it let salt through at a faster rate than water.
The preliminary tests suggest that graphene may be a viable alternative to existing filtration membranes, although Karnik says techniques to seal its defects and control its permeability will need further improvements.
“Water desalination and nanofiltration are big applications where, if things work out and this technology withstands the different demands of real-world tests, it would have a large impact,” Karnik says. “But one could also imagine applications for fine chemical- or biological-sample processing, where these membranes could be useful. And this is the first report of a centimeter-scale graphene membrane that does any kind of molecular filtration. That’s exciting.”
- Sean C. O’Hern, Doojoon Jang, Suman Bose, Juan-Carlos Idrobo, Yi Song, Tahar Laoui, Jing Kong, Rohit Karnik. Nanofiltration across Defect-Sealed Nanoporous Monolayer Graphene. Nano Letters, 2015; 150427124208006 DOI: 10.1021/acs.nanolett.5b00456