Membranes made from carefully arranged graphene layers could help transform seawater desalination from an energy intensive process into one that is significantly more energy efficient. Such a transformation would reduce the carbon footprint and power demand of countries that rely on desalination to meet their freshwater needs, like the UAE, but current technological limitations make large-scale, low-cost fabrication of graphene membranes challenging.
Now, researchers from Masdar Institute have come up with an innovative synthesis technique that could help take graphene-based filters a step closer towards becoming competitive with their conventional polymer membrane counterparts.
The technique uses a “bottom-up” approach, which involves layering many small graphene sheets together, in contrast to the more common “top-down” approach, which involves fabricating a single large graphene sheet and subsequently poking tiny holes in it to construct pores. The new approach and findings are described in the Journal of Membrane Science, in a paper by Dr. Linda Zou, Professor of Chemical and Environmental Engineering, Zhongshen Zhang, Research Engineer, Dr. Mustapha Jouiad, Principal Research Scientist in Mechanical and Materials Engineering Department and Microscopy Facility Manager, and two others.
The technique is “a novel but extremely challenging approach to fabricating graphene laminate films to be used as a membrane’s top layer,” Dr. Zou says. In contrast to other fabrication techniques, she adds that this one “has more potential to be scaled up.”
“This research addresses several important challenges limiting the development of graphene water filtration membranes. This includes a lamination process that controls the layer separation within tight limits, especially well-suited to water molecule passage, and which better stabilizes reduced graphene oxide in water. Long term, these membranes have potential for both water softening and desalination,” said Dr. John H. Lienhard, the Abdul Latif Jameel Professor of Water and Director of the Abdul Latif Jameel World Water and Energy Security Lab at the Massachusetts Institute of Technology (MIT), who was not involved in the research.
Graphene is a one atom-thick sheet of carbon often regarded as the lightest, strongest, thinnest, best heat- and electricity-conducting material ever discovered. Immediately recognized as a ‘wonder’ material after it was first isolated in 2003, it is graphene’s unique two-dimensional structure, chemical stability and superior strength that makes it a potential candidate as a membrane for energy-efficient desalination. In particular, its two-dimensional structure makes it easy to form a barrier or have good permeability if perforated, enabling water to pass through relatively easy compared to conventional polymeric membranes.
“Conventional polymeric membranes have shortcomings. While they have relatively high ability to filter out salt ions, they are susceptible to damage caused by high temperatures and the chemicals, such as chlorine and others, that are used to clean the membrane and disinfect the water,” explained Dr. Zou.
These factors cause polymeric membranes to require frequent replacement. Graphene, on the other hand, is significantly more resistant to the damaging effects of chemicals, temperature, and high pressure, which makes them much less vulnerable to damage and fouling – which is the buildup of filtered material on the membrane.
In response to the need for low-cost, high-quality graphene, Dr. Zou’s team uses an affordable chemical approach to fabricate tiny graphene sheets, that when layered together, form a large graphene sheet with fine gaps. When the tiny graphene sheets are stacked in layers, water molecules pass through the channels that are formed between the layered sheets. A finely controlled condition is required to keep the channels wide enough to let water molecules pass, while blocking the salt ions.
“We used a chemical oxidation and reduction process to produce large numbers of small reduced graphene oxide sheets as the building blocks,” explained Dr. Zou. The team generated thin sheets of graphene oxide from graphite particles and then removed some of the oxygen groups to produce reduced graphene oxide, which are then assembled into a laminate film as the top layer of a membrane.
“Our approach does not need to fabricate any large graphene sheets to be used as a membrane. Instead, we got around this by fabricating countless numbers of tiny sheets and layering them together,” Dr. Zou added. This technique is more affordable and in turn, more easily scalable.
Dr. Hongwei Zhu, Professor of Materials Science and Engineering from Tsinghua University, who was not involved in the study, said, “Dr. Zou’s research is of great significance in advancing graphene oxide films to a further level of practical application. Meanwhile, the subsequent research on improving the salt rejection of sodium chloride is still worth waiting, and this will be of great value in promoting the application of graphene oxide membranes in seawater desalination and thus help alleviate the freshwater scarcity in the Middle East.”
The project led by Dr. Zou is exemplary of the type of research needed to bolster graphene’s commercial potential as a desalination membrane. The team’s research on this topic is on-going and the graphene-assisted membrane’s potential will be further fine-tuned for energy-efficient desalination.
News and Features Writer
09 November 2016