Researchers at the University of Texas at Austin, USA, have developed hydrogel containing a hygroscopic salt that quickly extracts large amounts of fresh drinking water from the air in relatively low humidity.
Hydrogels have the ability to absorb and store many times their weight in water. They have an astonishing ability to swell and take on water. In daily life, their excellent absorbency makes them useful in dressings, nappies, and more to lock moisture away. However, to date, the 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.
The moisture absorption could be enhanced by adding hygroscopic salts that can rapidly remove large amounts of moisture from the air. But these 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. Also, the salt ions are not tightly coordinated within the gel and are easily washed away.
Now, the researchers have overcome these issues by developing a particularly “salt-friendly” hydrogel. Their study shows that the new gel gains the ability to absorb and retain water when combined with a hygroscopic salt.
The basis for the new hydrogel was a polymer constructed from zwitterionic molecules. These polyzwitterions carry both positive and negative charged functional groups, which helped the polymer become more responsive to the salt in this case. The resulting hydrogel was very efficient at absorbing water from the air around it.
The molecular strands in the polymer were initially 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 hygroscopic salt was able to incorporate water molecules quickly and easily.
In fact, water incorporation was so quick and easy that the team was able to set up a cyclical system for continuous water separation. First, the material was left for an hour each time to absorb atmospheric moisture, then dried the gel in a condenser to collect the condensed water. They repeated this cycle multiple times without it, resulting in any substantial loss of the amount of water absorbed, condensed, or collected.
In tests using the hydrogel, the team was able to extract 5.87 liters of freshwater per kilogram of material per day from air with 30% relative humidity. Researchers say their 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.