Researchers in China have
developed a new way to remove bacteria from water that they say is both highly
efficient and environmentally sound. By shining ultraviolet light onto a
two-dimensional sheet of a compound called graphitic carbon
nitride, the team's prototype can purify 10 litres (2.6 liquid gallons) of
water in just one hour, killing virtually all the harmful bacteria
present.
This type of technique for
water cleaning is known as photocatalytic disinfection, and it is often talked
about - and
ardently researched - as an appealing alternative to current water
filtration systems, such as chlorination or ozone disinfection, neither of
which are particularly eco-friendly.
With the world facing a future of climate
change and water
scarcity, finding an environmental way to clean our drinking water is of
the utmost importance.
"The future application of photocatalytic disinfection technology can significantly relieve clean-water scarcity and global energy shortage," explainssenior author Dan Wang.
The way this technology
works is remarkably simple. Various materials can be used as photocatalysts in
water - which means they absorb light and speed up (or catalyse) the oxygen
reactions that happen in water in the presence of certain wavelengths of
light. These reactions produce molecules known as reactive oxygen
species (ROS), which are great at killing microorganisms.
There are plenty of
efficient metallic catalysts one can use for this, but unfortunately they shed
metal residues into the water. Meanwhile non-metal catalysts, such as
nanotubes and graphene, are safer, but less efficient than their metallic
counterparts - they don't produce enough ROS to get the job done. Now,
researchers at the Chinese Academy of Sciences in Beijing and Yangzhou
University (YZU) in Jiangsu think they have at last found a practical,
efficient and non-toxic solution.
The key is a synthetic,
ultra-thin material with all the right properties to absorb light and produce
enough ROS to kill bacteria. Graphitic carbon nitride has attracted ever-increasing
attention over the years because of its potential for energy storage. Using
only light, the researchers found that two-dimensional sheets of this material
can rapidly purify polluted water.
An initial test saw the
deaths of 99.9999 percent of all Escherichia coli bacteria in
a 50 millilitre sample within just 30 minutes. That's five times faster than
any other metal-free photocatalyst we currently know of. Plus, the reaction
only consumes a tenth of the catalyst at a time.
"This remarkably high efficiency is also comparable to that of the best metal-based photocatalysts," the authors write.
The efficiency comes from
the unique structure of this material - the researchers loaded the edges of the
sheets with carboxyl and carbonylmolecules,
which drew additional electrons to these edges.Hit by ultraviolet and visible
light, this whole setup efficiently produced a ROS we all know as hydrogen
peroxide. Once in the water, this antiseptic effectively kills bacteria by
breaking through their cell walls and wreaking havoc on their insides, before
safely dissociating into
water and oxygen.
The best part is that the
authors say a system like this would not be hard to reproduce on a larger
scale. Graphitic carbon nitride can be easily
synthesised at a low cost, and the system itself is simple and inexpensive
to put together.
"The scale-up for both the catalysts and the device is not difficult," says Wang.
It's a good first step, but
the prototype won't be enough on its own to purify water from other
contaminants.
"Purification needs other devices for removing heavy-metal ions, adjusting pH, and removing residue," says Wang. "We need to combine our system with others to meet water-purification requirements."
This study has been published
in Chem.
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