2020-04-02_Science_Illustrated

and other larger molecules. If only water
molecules get through, pure water results.

Efficient graphene membrane
Reverse osmosis is not as energy consuming
as thermal desalination, but the method still
requires electricity to power the pumps that
put the ocean water under pressure. Scien-
tists are working on using renewable energy
in the desalination process. Dubai’s plans
will see reverse osmosis powered by solar-
cell electricity producing more than 1.1
billion litres of drinking water a day in 2030.
Another solution to the energy problem
is to produce new types of membranes that
allow water molecules to pass more easily,
more efficiently blocking out salt and impu-
rities. One of the most promising materials
is graphene – 2D carbon sheets that are only
one atom thick. Scientists from the Univer-
sity of Manchester have developed a method
by which a graphene-oxide membrane lets
scientists control the size of the pores of the
membrane very accurately. Salt molecules

The waste product from ocean water desalination is highly concentrated brine.

Substances such as caustic soda and lithium can be extracted from the brine.

Graphene membranes make

reverse osmosis more eco-friendly

1

Reverse osmosis membranes now block

out 90-99% of the salt of ocean water,

but they require lots of energy, as the

water must be forced through the membrane

under high pressure. Chinese scientists have

developed a graphene membrane that directs

the water 100 times more efficiently, so requiring

lower pressure, and less power to drive it.

Wooden membrane

efficiently filters out salt

2

Scientists from Princeton University

have created a 500-micrometre-thick

wooden membrane that is water-

repellent, yet allows vapour to pass. One side

of the wood is heated, making the water

evaporate and pass through the pores to the

opposite cold side, where it condenses. The

salt is left behind on the warm side.

Waves put water under pressure

3

The Atmocean company has developed

a pump that is mounted on a buoy

and powered by waves. In step with

the wave movements the pump sucks in water,

forcing it out again under high pressure. A system

of 15 pumps causes a water pressure which is so

high that it can power a reverse osmosis plant

without any extra energy consumption.

Heat

Cold water

Membrane

Water vapour

Buoy

Desalination plant

Outflow

Pump

Inflow

Water molecule

Salt ion

Carbon nanotubes

Graphene sheet

SOLUTION

SHU

TTE

RST

OCK

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