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Graphene and nanotube mesh filters salt from water.

A graphene-based desalination membrane with the potential to be scaled-up for practical applications has been created by a team of physicists in China and the US. Yanbing Yang and Xiangdong Yang at Wuhan University and colleagues created the material by combining a single sheet of graphene with a mesh of carbon nanotubes to create a centimetre-sized membrane that can remove salt from seawater.

As the demand for fresh water increases worldwide, large-scale, new technologies for the desalination of seawater are becoming increasingly sought after. Removing salt from water is easily done by evaporation, but this requires large amounts of energy. Today, most modern plants pump seawater through a membrane that blocks the passage of salt ions. Called reverse osmosis, this process requires less energy than evaporation but could benefit from better membranes.

Freshwater: a new graphene-based membrane could someday be used in commercial desalination plants.

(Courtesy: Quan Yuan)

The answer could be to use graphene, which is a sheet of carbon just one atom thick. Graphene is very strong, and sheets can be punctuated with sub-nanometre-sized pores that let water through while blocking salt. While this works well for micrometre-sized membranes it is very difficult to make larger graphene sheets without defects, which act as large pores that let salt through. Defects also reduce the mechanical strength of the graphene, making it difficult to create larger membranes.

Another approach is to create a membrane from a patchwork of small overlapping sheets of graphene oxide (also just one atom thick). Water can move through the membrane by permeating the gaps between the sheets – but the larger salt ions cannot.  While scientists have already made centimetre-sized membranes this way, this material tends to swell-up when wet and let more salt through.

Mesh-like network

In this latest research, Yang, Yang and colleagues have devised a way to create centimetre-sized sheets of porous graphene that do not suffer from the effects of defects. This was done by depositing a mesh-like network of single-walled carbon nanotubes on top of a graphene sheet, which essentially reinforces the material and blocks the spread of cracks and tears. Then the pores are etched in the material to create a desalination membrane.

A possible next step in creating practical large-scale membranes could be to stack several layers of nanotube-reinforced graphene membranes on top of each other. Molecules would enter the membrane via a pore but would then have to travel some distance between layers to find a pore in the next layer. This would mitigate problems associated with the widening of pores in single layers, and the presence of pores would minimize the swelling of the membrane.

The membrane is described in Science.

By Sam Jarman. Physics World. Posted: June 24, 2019.

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