Approximately two-thirds of the world's population suffer from varying degrees of potable water shortages, and as many as half a billion of them live in arid regions where there is seemingly nowhere to get clean water. Scientists and engineers at the University of Texas (USA) have developed an inexpensive solution that, for just a couple of dollars, will allow them to extract litres of life-giving water even from dry desert air using super hygroscopic polymer films.
Compared to traditional water treatment technologies that rely on the availability of a nearby reservoir or groundwater source, water extraction from the air appears to be a far more attractive alternative, as the approach does not rely on the geographical or hydrological conditions of the area. Water can be obtained, for example, by fog capture or dew condensation, but this requires high relative air humidity (more than 90%). Obviously, such approaches cannot completely solve the problem, as over a third of the earth's surface area has an average annual humidity of less than 40%.
That is why a team of researchers at the University of Texas at Austin (USA) has decided to develop an inexpensive technology for harvesting atmospheric moisture without regard for the weather phenomena of the potential use site. In a new paper published in the journal Nature Communications, they describe the development of an inexpensive gel superhydroscopic polymer film capable of pulling water out of the air even in the driest climates. The materials needed to create the film cost just two dollars per kilogram, and one kilogram of film can collect up to 5.8 litres of water per day in areas with relative humidity below 15% and up to 13.3 litres in areas with relative humidity up to 30%.
"This new work focuses on practical solutions that people can use to obtain water in the hottest and driest places on Earth," explained co-author Guihua Yu, a professor at the Cockrell School of Engineering at the University of Texas. - It could allow millions of people without regular access to potable water to have simple water generating devices at home, which they could easily control.
To create the hygroscopic polymer matrix (film skeleton), the researchers took hydroxypropyl cellulose and konjac gum, two food additives used as thickeners and stabilizers. When mixed, they created a porous film structure that accelerated the capture of atmospheric moisture. A third component, the hygroscopic salt LiCl, was uniformly distributed in the pores of the film, allowing it to efficiently absorb atmospheric water even at low relative humidity.
The presence in the film composition of heat-sensitive cellulose (hydroxypropylcellulose), which becomes hydrophobic (avoids contact with water molecules) when heated, provides a controlled release of moisture collected by the film within 10 minutes. At the same time, the average water collection efficiency (i.e. the ratio of the amount of collected water to the amount of atmospheric moisture absorbed by the unit) reaches 87%.
The scheme of operation of the device based on the super hygroscopic polymeric film is very simple and elegant: when humid air passes through the pores of the film, it is saturated with atmospheric moisture due to its hygroscopic properties (largely due to the LiCl salt) and increases in size, accumulating water in the polymeric network. The film is then heated to 60 °C to make the hydroxypropylcellulose hydrophobic and to evaporate the water accumulated in the film, which is then condensed on the condenser, collected in the collector and removed from the system.