The key to the new process is very precise control over the size of the holes in the graphene sheet. “There’s a sweet spot, but it’s very small,” Grossman says — between pores so large that salt could pass through and ones so small that water molecules would be blocked. The ideal size is just about one nanometer, or one billionth of a meter, he says. If the holes are just a bit smaller — 0.7 nanometers — the water won’t flow through at all.
This blog has covered before, graphene’s amazing properties at water remediation, with the biggest prize of all producing potable water. In a series of computer simulations at MIT, scientists have gone a long way towards making graphene desalination a reality. While the goal of cheap desalination is still pretty far off, their research suggests that it is more this decade than next.
A graphene approach to water desalination
(Nanowerk News) The availability of fresh water is dwindling in many parts of the world, a problem that is expected to grow with populations. One promising source of potable water is the world’s virtually limitless supply of seawater, but so far desalination technology has been too expensive for widespread use. Now, MIT researchers have come up with a new approach using a different kind of filtration material: sheets of graphene, a one-atom-thick form of the element carbon, which they say can be far more efficient and possibly less expensive than existing desalination systems.
—– Grossman and graduate student David Cohen-Tanugi, who is the lead author of the paper, aimed to “control the properties of the material down to the atomic level,” producing a graphene sheet perforated with precisely sized holes. They also added other elements to the material, causing the edges of these minuscule openings to interact chemically with water molecules — either repelling or attracting them. “We were very pleasantly surprised” by how well graphene performed compared to existing systems in computer simulations, Grossman says. One common method of desalination, called reverse osmosis, uses membranes to filter the salt from the water. But these systems require extremely high pressure — and hence, energy use — to force water through the thick membranes, which are about a thousand times thicker than graphene. The new graphene system operates at much lower pressure, and thus could purify water at far lower cost, the researchers say.
—- Because graphene is the subject of research into many different applications, there has been a great deal of work on finding ways of making it inexpensively and in large quantities. And for desalination, because graphene is such a strong material — pound for pound, it’s the strongest material known — the membranes should be more durable than those presently used for reverse osmosis, Grossman says. In addition, the material needed for desalination does not need to be nearly as pure as for electronic or optical uses, he says: “A few defects don’t matter, as long as they don’t open it up” so that salt could pass through.
How much long term graphite demand will come from desalinating water, it’s far too early to tell, but the potential is immense. With our world expected to reach a population of over 9 billion before 2050, desalinating water for drinking, and remediating water for agriculture, will become a big growth industry in the decades ahead.