Gravel sieve is recycling drinking water

Graphene oxide membranes filter out salts dissolved in water

If the distance between graphene layers is small enough, only water molecules will fit through it. Dissolved ions are retained. © University of Manchester
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The future of drinking water treatment? Researchers have for the first time made membranes of graphene oxide that only let water molecules through. Thanks to a special adhesive technique, the individual layers of the membrane keep close together when in contact with water. In this way, the permeability of the fine sieve remains low enough - and it reliably keeps dissolved salt ions back. In the future, it would be possible to transform seawater into drinking water with such membranes without much energy expenditure.

Graphene is a true wonder material. It has exceptional conductivity, is more stable than steel and yet flexible and flexible like rubber. The secret of the material is its particular structure: graphene consists of a single carbon layer, which is only one atom thick.

By stacking graphene oxide layers, researchers have been making membranes for some years that can filter certain salts out of the water. However, very small ions, such as the sodium ion present in the common salt, pass through the membrane largely unhindered. Because when it is in contact with water, the membrane swells up: the distance between the layers becomes larger. Scientists led by Jijo Abraham of the University of Manchester have now found a way to prevent swelling and to filter even the smallest ions out of the water.

Glued layers

To achieve improved filtration performance, the researchers initially stored the membranes at controlled humidity. They observed that the lower the humidity was set, the closer the graphene oxide layers were to each other. At zero percent humidity, the space between each layer was just under seven angstroms - equivalent to about seven atomic diameters.

In the water-soaked state, however, the membrane swelled up until the free spaces were almost twice as large. However, the scientists then prevented this with the help of a decisive trick: they fixed the graphene oxide with epoxy resin, a special type of adhesive. As a result, the individual layers remained in contact with each other at the pre-set distance from each other - and the membrane did not swell as usual. display

No getting through for sodium

Initial experiments with glued-on graphene oxide proved the researchers' success. Reducing the clearance between the layers from 9.8 to 7.4 Angstroms, for example, reduced the permeability of sodium ions by a factor of 100, At the same time, the permeability to water was only halved. Thus, the filter performance for ions has been extremely improved with hardly any deterioration in the water flow, so the team.

Even after five consecutive days of use, the filter performance remained unchanged, as the researchers write in their report: "Membranes with a gap of 6.4 Angstroms showed no detectable ion concentration in the filtrate." This success was due to the prevented swelling of the membrane. Only a minimal increase of one percent was noted by the researchers during their experiments.

Ions are not naked

The fact that ion filtration works at all is amazing at first glance. For a single sodium ion with a diameter of two Angstr m is even smaller than a Wassermolek l, which is about three Angstr m gro . So how does it happen that almost only water molecules pass through the membrane?

This is due to the extra ballast carried by dissolved ions. The ions are not "naked" in the water, but are surrounded by a pool of water molecules. With this case, a sodium ion is then 7.2 Angstr m gro . Thus, it no longer fits through the narrow channels of the maximum compressed graphene oxide membrane.

Although it can strip off the water molecules in order to penetrate between the graphene layers. But this stripping is associated with high energy consumption and is therefore rather avoided. Thus, single, free water molecules remain the only particles that pass through the membrane.

View of clean water

With their improved membranes, researchers have come a long way from their goal of filtering water without much energy and making it drinkable. Previously, however, among other things, the durability of the graph sieves must be improved - especially when very saline and thus aggressive seawater to be cleaned. The inclusion of functional nanoparticles is a promising approach here, the researchers conclude. (Nature Nanotechnology, 2017; doi: 10.1038 / nnano.2017.21)

(University of Manchester, 04.04.2017 - CLU)