InfoChem
Water for life
Tom Westgate investigates the chemistry and chemists
helping more people to access clean water
ISSUE 129 | JUNE 2011
In this issue
Glutamate
The source of delicious
savoury sensations
The Last Castle
Can a water cannon fire a
hook and chain?
Backyard chemistry
The science of Space Dust
A day in the life
Julian Huppert MP
Plus ...
ANDREA SCHAFER
Prize puzzles
How can you sum up the importance of water to life
on Earth? It covers 70% of the planet’s surface, makes
up 50 to 70% of the human body, and adults need to
drink around 2.5 litres every day. But still more than one
billion people, mostly in developing countries, cannot
access safe water for drinking, cooking, washing,
growing crops or rearing livestock, according to UK
charity Practical Action. Meanwhile one in eight people
in the world drink water that contains pollutants,
bacteria or viruses that are likely to make them ill,
according to US-based group charity Water. So how can
chemistry help to make water safe to drink, anywhere in
the world?
Safe supply
When you turn on the tap, the water that comes out has
come a long way since falling as rain and accumulating
in a reservoir or in groundwater (water contained
underground in soil and rocks). Depending on the
source, your glass of water may have been through
several purification steps to remove solid particles,
dissolved minerals and ions, chemical pollutants,
bacteria and viruses before it is safe to drink.
At a water purification plant, the first step is often to
clarify the water by removing microscopic (0.1 µm or
smaller) particles of dirt suspended in the water which
make it murky or cloudy. The dirt particles carry a
negative electrostatic charge on their surface, which
means they repel each other and remain suspended
instead of settling at the bottom of a container. To
remove the particles, chemicals called flocculants
are added to the water. Flocculants contain soluble
cations, which attract the negatively-charged particles,
neutralising their charge and allowing them to stick
together to form larger and larger clumps (flocs) which
Editor
Karen Ogilvie
Assistant editor
David Sait
Layout
Scott Ollington
Publisher
Bibiana Campos-Seijo
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eventually sink to the bottom of the flocculation tank.
The most common flocculant is potassium aluminium
sulfate, or alum, but other cations like iron, calcium
or magnesium, or positively charged polymers like
polydiallyldimethylammonium chloride can be used.
on the pore size). These unwanted substances are
trapped, and purified water can be collected on the
other side of the membrane.
‘Anything above 10 µm can be separated in a filter’
explains Professor Nidal Hilal, an expert on membrane
separation at Swansea University. ‘Anything smaller
Filtration
Filtration through sand, gravel and charcoal removes the can be separated with a membrane, from 10 µm to the
atomic level.’ Nanofiltration (using a membrane with
smallest particles, and chlorine is used to kill bacteria
pores of 1–5 nm) is used in Bangladesh, for example,
and viruses (decontamination), so the water that is
where some groundwater is contaminated with the
pumped to your tap is safe to drink. Even so, there are a
poisonous metal arsenic, and in Paris, to remove
range of products available to purify it even further.
pesticides from the water in the River Seine.
Water filter jugs can be bought in most supermarkets.
The filter cartridges contain activated carbon, a highly
porous form of carbon that has a very large surface
area capable of absorbing chlorine, pesticides and
other organic pollutants. The filter cartridges also often
contain an ion exchange resin that can reduce the
concentration of metal ions like calcium and magnesium
(which cause limescale when they precipitate out of
the water) as well as harmful heavy metals like copper,
lead, arsenic, chromium, radium, and uranium. The ion
exchange resin is made up of beads of a porous polymer
with sodium or potassium bonded to a negative group
such as carboxylate (NaOOC or KOOC). When water
passes through the bead, strongly positive metal ion
contaminants displace the sodium or potassium ions
and bond to the carboxylate group (COO–) on the resin.
The harmless sodium or potassium ions take the place
of the other metal ions in the water that drips out of the
filter.
A cross-section of a water
filter cartidge, showing
the activated carbon and
ion exchange resin
2
Atomic force microscope image of membrane pores,
showing a pore blocked with biocolloid.
NIDAL HILAL
Andrea Schäfer, a professor of environmental
engineering at the University of Edinburgh, designs
membrane purification systems and believes they are
particularly well suited for places where getting drinking
water is difficult. ‘For example in natural disaster zones,
where the infrastructure has been destroyed or was
never there, a membrane plant can be set up from a
container to start producing clean water straight away’
Schäfer said. Her research group has also set up a solar
powered membrane filtration plant in the Australian
outback. Closer to home, nanofiltration is used in
remote communities in Scotland where providing clean
water is a ‘major challenge’ according to Schäfer, due to
organic material from peat contaminating supplies.
Sea water and salty groundwater can also be turned
into a source of drinking water, thanks to membranes.
Particles smaller than 1 nm, such as salt, require a
process called reverse osmosis to separate them from
water. In this technique, salty water and clean water
are separated by a membrane. Normally, osmosis
would cause pure water to flow through the membrane
into the salty water, in order to dilute the salt further,
reducing the difference in concentration between the
two sides of the membrane. If enough pressure is
applied to the salty side, however, reverse osmosis
takes place: ‘you can push clean water back through
the membrane, leaving contaminants behind,’
explains Hilal.
Marvellous membranes
In some places, water supplies are contaminated with
specific compounds and require another purification
step. A common solution to this problem is to use a
membrane, a polymer layer with tiny pores that let water When water is scarce, waste water or sewage from
through but are too small to let through contaminants
the bath, shower or even toilet can be put to use
like organic molecules, metals, or bacteria (depending
and turned into drinkable water using a membrane
InfoChem
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bioreactor. These systems, which have been used
to treat drinking water in Singapore and Australia,
make use of the bacteria that are naturally present in
the sewage. ‘The bacteria eat everything available,’
explains Professor Schäfer, before a membrane is used
to filter out the bacteria and solid waste products.
‘The problem is people don’t like drinking it because it
started as sewage, but it can be better than tap water’
said Schäfer. Professor Hilal also believes we need to
re-think how we use water, by being less wasteful and
making the most of under-used sources like waste
water, flood water and rain water. ‘This is an extremely
important message, especially for young people,’ he said.
One way to remove organic compounds from water is
to use the highly reactive hydroxyl radical (HO•), which
is one of the most powerful oxidising agents known.
The so-called advanced oxidation technologies rely
on generating hydroxyl radicals in dirty water, which
then react with any contaminants, converting them
into harmless molecules. ‘Hydroxyl radicals will destroy
practically all organic compounds, forming carbon
dioxide and water’ explains Professor Gianluca Li Puma, a
chemical engineer at the University of Loughborough.
Hydroxyl radicals are formed when one of the covalent
O-H bonds in H2O breaks and the oxygen and hydrogen
atoms take one each of the bonding pair of electrons.
The resulting unpaired electron on the oxygen atom is
what makes the hydroxyl radical so reactive. The O-H
bond can be broken in a number of ways: by vibrating
it with ultrasound, by electrolysis, by reacting water
with ozone, or with hydrogen peroxide in the presence
of ultraviolet light. Li Puma’s team are developing a
method that uses solar energy to power the bondbreaking reaction. ‘The idea is to use photons to
activate a catalyst that can generate the hydroxyl
radical’ said Li Puma. The catalyst, made up of titanium
dioxide nanoparticles, is a semiconductor, which means
it has electrons available to absorb energy from light
(in this case ultraviolet light). These electrons become
‘excited’ and can move around the surface of the
nanoparticles, in turn leaving behind a positive charge
(or ‘hole’) which can oxidise water to form the allimportant hydroxyl radical.
Li Puma says the TiO2 catalyst reactor is most suited
to industrial applications, because of the cost of the
catalyst. But if more efficient catalysts that absorb
more light (especially in the visible spectrum) can be
developed the technology might become cheaper. This
ANDREA SCHAFER
Nice nanotechnology
In industry, treating waste water streams to remove
organic compounds such as pesticides and unwanted
pharmaceutical by-products is vital to ensure they
don’t enter the water supply. Research into solving
this problem has led to some interesting new
decontamination methods that could be applied in the
wider world.
could help to produce clean drinking water, especially
in developing countries where there is a lot of sunlight.
His group is currently working on a project to design
a reactor that goes one step further than destroying
organic pollutants, and could solve two environmental
problems at the same time. ‘The hydroxyl radicals
destroy the pollutants, but the electrons from the
catalyst can reduce water too, and produce hydrogen
that could be used to generate electricity in fuel cells,’
Li Puma explains. This could be a source of ‘green,
renewable energy, using only sunlight,’ said Li Puma,
who hopes to power the reactor’s water pumps using
solar panels, making it totally self-sufficient.
A solar-powered reverse
osmosis plant in the
Australian outback
As the population grows, the global water crisis will
become worse. The world will need to find new ways
of keeping drinking water clean and safe wherever it is
needed, and research by chemists will hold the key to
solving this problem.
Try it yourself
You can measure for yourself the quality of your local water, and compare
your results with those from around the globe by taking part in the world’s
biggest chemistry experiment. The global water experiment (part of the
International Year of Chemistry 2011) includes measurements of the salt
content and pH of a local water source, as well as a challenge to create the
most efficient solar-powered water purification still.
For more information go to http://water.chemistry2011.org/web/iyc.
InfoChem
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