Hydrogels containing a hygroscopic salt can harvest freshwater from dry air
Hydrogels can absorb and store many times their weight in water. In
so doing, the underlying polymer swells considerably by incorporating
water. However, to date, use of this property to produce freshwater from
atmospheric water has not been feasible, since collecting moisture from
the air is still too slow and inefficient.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article at https://doi.org/10.1002/anie.202200271
On the other hand, moisture absorption could be enhanced by adding
hygroscopic salts that can rapidly remove large amounts of moisture from
the air. However, hygroscopic salts and hydrogels are usually not
compatible, as a large amount of salt influences the swelling capability
of the hydrogel and thus degrades its properties. In addition, the salt
ions are not tightly coordinated within the gel and are easily washed
away.
The materials scientist Guihua Yu and his team at the University of
Texas at Austin, USA, have now overcome these issues by developing a
particularly “salt-friendly” hydrogel. As their study shows, this gel
gains the ability to absorb and retain water when combined with a
hygroscopic salt. Using their hydrogel, the team were able to extract
almost six liters of pure water per kilo of material in 24 hours, from
air with 30% relative humidity.
The basis for the new hydrogel was a polymer constructed from
zwitterionic molecules. Polyzwitterions carry both positive and negative
charged functional groups, which helped the polymer to become more
responsive to the salt in this case. Initially, the molecular strands in
the polymer were tightly intermingled, but when the researchers added
the lithium chloride salt, the strands relaxed and a porous, spongy
hydrogel was formed. This hydrogel loaded with the hygroscopic salt was
able to incorporate water molecules quickly and easily.
In fact, water incorporation was so quick and easy that the team were
able to set up a cyclical system for continuous water separation. They
left the hydrogel for an hour each time to absorb atmospheric moisture,
then dried the gel in a condenser to collect the condensed water. They
repeated this procedure multiple times without it resulting in any
substantial loss of the amount of water absorbed, condensed, or
collected.
Yu and the team say that the as-prepared hydrogel “should be
optimal for efficient moisture harvesting for the potential daily water
yield”. They add that polyzwitterionic hydrogels could play a
fundamental role in the future for recovering atmospheric water in arid,
drought-stricken regions.
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About the Author
Dr. Guihua Yu is a Professor at the
Materials Science and Mechanical Engineering Department at the
University of Texas at Austin, Austin, USA. His highly interdisciplinary
research group explores nano-architected functional materials for
energy storage and conversion, novel organic nanomaterials for solar
water purification and atmospheric water harvesting, self-assembled
nanostructures and bio-inspired materials for advanced energy,
environment, and sustainability technologies.