MIT engineers have synthesized a superabsorbent material that can soak up a record amount
of moisture from the air, even in desert-like conditions.
As the material absorbs water vapor, it can swell to make room for more moisture.
Even in very dry conditions, with 30 percent relative humidity, the material can pull vapor
from the air and hold in the moisture without leaking.
The water could then be heated and condensed, then collected as ultrapure water.
The transparent, rubbery material is made from hydrogel, a naturally absorbent material
that is also used in disposable diapers.
The team enhanced the hydrogel’s absorbency by infusing it with lithium chloride — a
type of salt that is known to be a powerful dessicant.
The researchers found they could infuse the hydrogel with more salt than was possible
in previous studies.
As a result, they observed that the salt-loaded gel absorbed and retained an unprecedented
amount of moisture, across a range of humidity levels, including very dry conditions that
have limited other material designs.
If it can be made quickly, and at large scale, the superabsorbent gel could be used as a
passive water harvester, particularly in the desert and drought-prone regions, where the
material could continuously absorb vapor, that could then be condensed into drinking
water.
The researchers also envision that the material could be fit onto air conditioning units as
an energy-saving, dehumidifying element.
In MIT’s Device Research Lab, researchers are designing novel materials to solve the
world’s energy and water challenges.
In looking for materials that can help to harvest water from the air, the team zeroed
in on hydrogels — slippery, stretchy gels that are mostly made from water and a bit
of cross-linked polymer.
Hydrogels have been used for years as absorbent material in diapers because they can swell
and soak up a large amount of water when it comes in contact with the material.
The researchers dug through the literature and found that others had experimented with
mixing hydrogels with various salts.
Certain salts, such as the rock salt used to melt ice, are very efficient at absorbing
moisture, including water vapor.
And the best among them is lithium chloride, a salt that is capable of absorbing over 10
times its own mass in moisture.
Left in a pile on its own, lithium chloride could attract vapor from the air, though the
moisture would only pool around the salt, with no means of retaining the absorbed water.
So, researchers have attempted to infuse the salt into hydrogel — producing a material
that could both hold in moisture and swell to accommodate more water.
But the MIT team found that others reached a limit to the amount of salt they could load
into their gels.
The best performing samples to date were hydrogels that were infused with 4 to 6 grams of salt
per gram of polymer.
These samples absorbed about 1.5 grams of vapor per gram of material in dry conditions
of 30 percent relative humidity.
In most studies, researchers had previously synthesized samples by soaking hydrogels in
salty water and waiting for the salt to infuse into the gels.
Most experiments ended after 24 to 48 hours, as researchers found the process was too slow,
and not very much salt ended up in the gels.
When they tested the resulting material’s ability to absorb water vapor, the samples
soaked up very little, as they contained little salt to absorb the moisture in the first place.
What would happen if the material synthesis was allowed to go on, say, for days, and even
weeks?
Could a hydrogel absorb even more salt, if given enough time?
For an answer, the MIT team carried out experiments with polyacrylamide (a common hydrogel) and
lithium chloride (a superabsorbent salt).
After synthesizing tubes of hydrogel through standard mixing methods, the researchers sliced
the tubes into thin disks and dropped each disk into a solution of lithium chloride with
a different salt concentration.
They took the disks out of solution each day to weigh them and determine the amount of
salt that had infused into the gels, then returned them to their solutions.
In the end, they found that, indeed, given more time, hydrogels took up more salt.
After soaking in salty solution for 30 days, hydrogels incorporated up to 24, versus the
previous record of 6 grams of salt per gram of polymer.
The team then put various samples of the salt-laden gels through absorption tests across a range
of humidity conditions.
They found that the samples could swell and absorb more moisture at all humidity levels,
without leaking.
Most notably, the team reports that at very dry conditions of 30 percent relative humidity,
the gels captured a “record-breaking” 1.79 grams of water per gram of material.
Any desert during the night would have that low relative humidity, so conceivably, this
material could generate water in the desert.