Pufferfish-inspired device relies on sunlight to produce clean drinking water
Princeton University
 |
| In a study conducted at Princeton University, researchers placed the gel in lake water where it absorbed pure water, leaving contaminants behind. The researchers then placed the gel in the sun, where solar energy heated up the gel, causing the discharge of the pure water into the container. Image credit: Xiaohui Xu |
A new invention that uses sunlight to drive water purification could help solve the problem of providing clean water off the grid.
The
device resembles a large sponge that soaks up water but leaves contaminants --
like lead, oil and pathogens -- behind. To collect the purified water from the
sponge, one simply places it in sunlight. The researchers described the device
in a paper published this week in the journal Advanced Materials.
The
inspiration for the device came from the pufferfish, a species that takes in
water to swell its body when threatened, and then releases water when danger
passes, said the device's co-inventor Rodney Priestley, the Pomeroy and Betty
Perry Smith Professor of Chemical and Biological Engineering, and Princeton's vice
dean for innovation.
"To me, the most exciting thing about this work is it can operate completely off-grid, at both large and small scales," Priestley said. "It could also work in the developed world at sites where low-cost, non-powered water purification is needed."
Xiaohui
Xu, a Princeton Presidential Postdoctoral Research Fellow in the Department of
Chemical and Biological Engineering and co-inventor, helped develop the gel
material at the heart of the device.
"Sunlight
is free," Xu said, "and the materials to make this device are
low-cost and non-toxic, so this is a cost-effective and environmentally
friendly way to generate pure water."
The
authors noted that the technology delivers the highest passive solar water-
purification rate of any competing technology.
One
way to use the gel would be to place it in a water source in the evening and
the next day place it in the sunlight to generate the day's drinking water, Xu
said.
The
gel can purify water contaminated with petroleum and other oils, heavy metals
such as lead, small molecules, and pathogens such as yeast. The team showed
that the gel maintains its ability to filter water for at least ten cycles of
soaking and discharge with no detectable reduction in performance. The results
suggest that the gel can be used repeatedly.
To
demonstrate the device in real-world conditions, Xu took the device to Lake
Carnegie on the Princeton University campus.
Xu
placed the gel into the cool water (25 degree Celsius, or 77 degrees
Fahrenheit) of the lake, which contains microorganisms that make it unsafe to
drink, and let it soak up the lake water for an hour.
At
the end of the hour, Xu lifted the gel out of the water and set it on top of a
container. As the sun warmed the gel, pure water trickled into the container
over the next hour.
The
device filters water much more quickly than existing methods of passive
solar-powered water purification methods, the researchers said. Most other
solar-powered approaches use sunlight to evaporate water, which takes much
longer than absorption and release by the new gel.
Other
water filtration methods require electricity or another source of power to pump
water through a membrane. Passive filtration via gravity, as with typical
household countertop filters, requires regular replacement of filters.
At
the heart of the new device is a gel that changes depending on temperature. At
room temperature, the gel can act as a sponge, soaking up water. When heated to
33 degrees Celsius (91 degrees Fahrenheit), the gel does the opposite -- it
pushes the water out of its pores.
The
gel consists of a honeycomb-like structure that is highly porous. Closer
inspection reveals that the honeycomb consists of long chains of repeating
molecules, known as poly(N-isopropylacrylamide), that are cross-linked to form
a mesh. Within the mesh, some regions contain molecules that like to have water
nearby, or are hydrophilic, while other regions are hydrophobic or
water-repelling.
At
room temperature, the chains are long and flexible, and water can easily flow
via capillary action into the material to reach the water-loving regions. But
when the sun warms the material, the hydrophobic chains clump together and
force the water out of the gel.
This
gel sits inside two other layers that stop contaminants from reaching the inner
gel. The middle layer is a dark-colored material called polydopamine that
transforms sunlight into heat and also keeps out heavy metals and organic
molecules. With PDA in place, the sun's light can heat up the inner material
even if the actual outdoor temperature is not very warm.
The
final external layer is a filtering layer of alginate, which blocks pathogens
and other materials from entering the gel.
Xu
said that one of the challenges to making the device was to formulate the inner
gel to have the correct properties for water absorption. Initially the gel was
brittle, so she altered the composition until it was flexible. Xu synthesized
the materials and conducted studies to assess the device's ability to purify
water, aided by coauthors Sehmus Ozden and Navid Bizmark, postdoctoral research
associates in the Princeton Institute for the Science and Technology of
Materials.
Sujit
Datta, assistant professor of chemical and biological engineering, and Craig
Arnold, the Susan Dod Brown Professor of Mechanical and Aerospace Engineering
and director of the Princeton Institute for the Science and Technology of
Materials, collaborated on the development of the technology.
The
team is exploring ways to make the technology widely available with the help of
Princeton Innovation, which supports University researchers in the translation
of discoveries into technologies and services for the benefit of society.