Water is the most important substance to humans, by far. Some nations have better access to it than others; those surrounded by the sea in drier climates often use desalinization plants that remove the salt from the water, but this is often slow and energy-intensive. Fortunately, a team of U.S. engineers has come up with a new technique to purify water using “thirsty” filtration devices peppered with “nanopores” which is remarkably energy efficient. Their findings have been reported in the journal Nature Communications.
Nanotechnology is a rapidly developing field, particularly with regards to graphene, the wonder material that can be used to create a multitude of things, from enhanced night vision contact lenses to and filtration devices for water. Sheets of graphene just one atom thick, riddled with small nano-sized holes, allow the water molecules through the barrier while trapping the larger salt molecules on the other side, filtering the water efficiently and using little energy.
This new research has drastically improved on even this highly-efficient method by swapping out the carbon-based graphene with a compound called molybdenum disulfide (MoS2). Using precisely the same nanopore filtering mechanism, they demonstrated that this is able to filter out 70 percent more pure water from salty water than graphene – but how?
It’s not just the nanopores that make molybdenum disulfide an effective water filtration system: It has inherent advantages due to its chemistry. “The molybdenum in the center attracts water, then the sulfur on the other side pushes it away, so we have a much higher rate of water going through the pore,” said Mohammad Heiranian, first author of the study, in a statement. So not only is the water flowing through under pressure, but it’s being “sucked” through by the chemical attraction-then-repulsion mechanism of the filter.
Conventional desalinization plants rely on “reverse osmosis,” the opposite of osmosis, which as you probably remember from school is a process whereby molecules will move from areas of differing concentrations in order to reach a balance.
The removal of pure water from seawater or brackish (salty freshwater) needs the water to move from an area of high salt concentrations to an area of extremely low salt concentrations. This requires high, artificially generated pressures to be applied to the seawater as it encounters a semipermeable membrane, which uses up a relatively high amount of energy. In contrast to this, the molybdenum disulfide method is far less energy intense, and far easier to maintain.
Image credit: A computer model of a nanopore in a single-layer sheet of molybdenum disulfide in action. Salt water is shown on the left (with red and green salt ions); fresh water on the right. Mohammad Heiranian
“Even though we have a lot of water on this planet, there is very little that is drinkable,” said Narayana Aluru, a professor of mechanical science and engineering at the University of Illinois and leader of the study, in a statement. “If we could find a low-cost, efficient way to purify sea water, we would be making good strides in solving the water crisis.”
This new molybdenum disulfide filtration technique is currently only in the developmental stage, but the team appears confident that their findings will be applicable to an industrial scale.