Freshwater from thin air
Hydrogels containing a hygroscopic salt can harvest freshwater from dry
air

Date:
February 28, 2022
Source:
Wiley
Summary:
Hydrogels have an astonishing ability to swell and take on water. In
daily life, they are used in dressings, nappies, and more to lock
moisture away. A team of researchers has now found another use:
quickly extracting large amounts of freshwater from air using a
specially developed hydrogel containing a hygroscopic salt. The
study shows that the salt enhances the moisture uptake of the gel,
making it suitable for water harvesting in dry regions.



FULL STORY ========================================================================== Hydrogels have an astonishing ability to swell and take on water. In
daily life, they are used in dressings, nappies, and more to lock moisture away. A team of researchers has now found another use: quickly extracting
large amounts of freshwater from air using a specially developed
hydrogel containing a hygroscopic salt. The study, published in the
journal Angewandte Chemie, shows that the salt enhances the moisture
uptake of the gel, making it suitable for water harvesting in dry regions.


========================================================================== 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.

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.

========================================================================== Story Source: Materials provided by Wiley. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Chuxin Lei, Youhong Guo, Weixin Guan, Hengyi Lu, Wen Shi, Guihua Yu.

Polyzwitterionic Hydrogels for Efficient Atmospheric Water
Harvesting.

Angewandte Chemie International Edition, 2022; DOI: 10.1002/
anie.202200271 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220228114348.htm

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