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rubin | international edition 2013
New concept gives rise to hope for
energy-efficient fresh water production
Battery desalinates
seawater
Today, fresh water is a rare commodity in
many places on earth. It is estimated that
humans will have consumed some 90 per
cent of available drinking water by 2025 –
it is therefore essential to explore new water sources. Seawater is available in huge
quantities. However, processing seawater
is immensely energy-consuming – more
so than theoretically necessary. Therefore, RUB chemists are developing a new
process that promises to be much more
energy-efficient.
Concentrated
solution
version” group at the RUB Centre of Electrochemical Science. “Currently, by deploying traditional reverse osmosis technology, some four kilowatt-hours are used up.”
Fifty per cent of that energy is spent prior to the actual desalination for the purification of seawater – biomass and bacteria
have to be removed, as they would otherwise damage the facilities. The other 50 per
cent of the energy input is used for running
the pumps. Those pumps force the seawater through a semipermeable membrane
at high pressure. The membrane works as
“In order to desalinate one cubic me- a filter, letting only specific ions and moltre of seawater, at least 0.6 or 0.7 kilowatt- ecules through. Salt is kept out.
La Mantia and his colleagues have now
hours of energy are required,” explains
Dr Fabio La Mantia, junior group leader tested a brand new seawater desalination
of the “Semiconductor and Energy Con- technology that does not use membranes
and might even work without prior water purification being necessary. What
they do is, they apply the battery principle. In a specific battery cell, seawater is
brought into contact with two electrodes
(fig. 1). One of them contains silver miStep 1
croparticles, the other sodium-mangaDesalination
nese oxide nanorods. When a voltage is
applied, the silver electrode attracts negatively charged chloride ions and “fishes” them out of the water by binding
Seawater
Step 2
Step 4
them chemically; the other electrode
Desalinated does the same to positively charged sodiSeawater
water
um ions (fig. 2). Thus, the common salt
(sodium chloride, NaCl) content in seawater is significantly reduced.
After this stage is completed, the water,
which
now has a lower salt content, is reStep 3
moved from the cell and the cell itself is
refilled with seawater. When voltage is reversed, the electrodes release the chloride
and sodium ions into the water. It is worth
noting that this stage of the process genNa+
Cl–
Other ions
Fig. 2: The battery cell consists of one sodium-manganese oxide nanorod (NMO) and one silver electrode
(Ag). On voltage being applied, the NMO electrode absorbs positively charged sodium ions (Na+) and the silver electrode negatively charged chloride ions (Cl-). Subsequently, the water with the now lower salt content is
replaced by a new batch of seawater, and voltage is reversed. The electrodes release the sodium and chloride
ions back into the water. The salt-enriched water is jettisoned, the cell is ready for the next desalination run.
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Fig. 1: Dr Fabio La Mantia uses a syringe to fill the desalination cell with 0.2 millilitres
of saltwater. The tip of the needle is insulated with Teflon to prevent the cell’s electrodes
short-circuiting on contact with the needle.
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rubin | international edition 2013
Fig. 3: The compact battery boasts the dimensions
3.5 x 2 x 2 centimetres. The electrodes have a surface
of 2 square centimetres each. They each are 300 micrometres thick. The battery is used for desalinating 0.2 millilitres of seawater.
erates small amounts of energy. The seawater, now with a higher salt content, is jettisoned, and the cell is ready for a new desalination run.
Operating under lab conditions, researchers have thus achieved initial success:
their battery cell, boasting a capacity of 0.2
millilitres salt water and an electrode surface of more than two square centimetres
(fig. 3), reduced the salt content of the water by half within an hour (fig. 4). “Extrapolating from that, we are able to desalinate
one litre water pro square metre and hour,”
calculates Dr La Mantia.
However, that alone does not make the
water drinkable. In order to turn seawater
into drinking water, 98 per cent of the salt
content has to be removed. The battery cell
can be used for that purpose, but the pro-
cess would have to involve several stages
– thus resulting in higher time and energy consumption. Consequently, La Mantia
strives to optimise the process, focusing on
new materials for electrode production in
the first place.
At present, researchers apply silver microparticles with a diameter of two micrometres resp. sodium-manganese oxide
nanorods onto carbon fibre fabric using
a bonding agent (fig. 5). “We utilise particles and rods rather than smooth surfaces in order to enhance the electrodes’ surface,” explains the chemist. “Thus, ions
contained in the water are bonded more
quickly.” However, the surface must not be
too large: otherwise the ions are “captured”
more quickly, but at the same time, electrolysis sets in more quickly as well; that
means water is split into its components
oxygen and hydrogen at the electrodes – an
effect that is all but desirable when attempting to achieve desalination.
“Silver goes about capturing ions very
slowly, though,” says Dr La Mantia. The
other electrode might be quicker than
silver, but here, too, is still room for improvement. Therefore, researchers have set
out to find new materials that will absorb
ions more efficiently, whilst at the same
time will not trigger electrolysis too soon;
in addition, they must not be hazardous
to health. The materials should, moreover,
affect sodium and chloride ions alone, without extracting other nutrients such as magnesium and calcium that are vital components of drinking water. If reverse osmosis
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Fig. 5: Sodium-manganese oxide nanorods under the microscope. To manufacture them, scientists dissolve
sodium and manganese in water and absorb the solution with a cotton pad. The cotton is subsequently dried
and burned. What remains are sodium-manganese oxide nanorods in powder form. They are then applied to
carbon fibre fabric with the help of a bonding agent.
is applied, those crucial nutrients are extracted from water in the same way as salt
is, and they have subsequently be re-added
to the drinking water. Deploying electrode
materials best suited for the task, this step
would not be necessary. The work group is
looking for suitable materials among those
substances that are applied in modern
lithium-ion batteries.
“Compared with reverse osmosis that
has been deployed for the last 40 years, our
process is a very recent one. Reverse osmosis has been fully exploited in science, it is
unlikely the process will undergo any major changes. Our process, however, is still
in the development stage, and we are definitely going to achieve considerable optimisation,” as he confidently states. The scientist also wishes to enhance energy efficiency; at 33 per cent, it is currently lower than that achieved by reverse osmosis
(about 50 per cent).
Should the researchers prove to be suc-
cessful in “relieving” seawater of 80 per
cent of its salt content, the process might
be used in combination with reverse osmosis. Thus, the last remaining salt would be
extracted from the water – requiring a lower energy input than today.
The scientists hope that the battery process will not require biomass being filtered
out of seawater prior to deployment. This
would result in significant energy savings.
However, no detailed data are available
yet, as, to date, the researchers have been
conducting short experiments rather than
long-term tests in the lab. What they do
know is that the process that involves using a syringe to manually fill battery cells
with tiny amounts of water and extract that
water in the same way can in future be automated. “The system could conceivably be
set up in the shape of a long, winding tube,
with seawater being poured through and
desalinated in the process,” as Dr La Mantia elaborates his vision.
md
IonNa+K+Mg2+Ca2+Cl-SO42Seawater (mg/L)
11250
450
1400
450
18500
2750
Extraction at 25% of the maximum voltage (mg/L)
9840
= 47%
430
<1%
1130
= 9%
280
= 3%
14470
= 87%
2750
(Control)
Extraction at 50% of the maximum voltage (mg/L)
7860
= 57%
390
<1%
860
= 9%
180
= 3%
11430
= 76%
2750
(Control)
Fig. 4: At 25 per cent of the maximum voltage, the battery extracts 87 per cent of the theoretically feasible
amount of chloride ions (theoretically feasible amounts not listed in the table), at 50 per cent of the maximum
voltage 76 per cent – the total efficiency of the process thus reaches 50 resp. 40 per cent. It is not possible to
apply a higher voltage as that would trigger electrolysis. In addition to sodium and chloride ions, other ions
are extracted, as well – but only to a much smaller degree.