Solar-Powered Hydrogel Harvests Clean Water From Arid Desert Air

Water is a precious resource, often taken for granted. But for millions of people around the world, access to drinkable fresh water is no certainty.

The areas where water scarcity and sanitation problems are at their worst also happen to be some of the sunniest places on Earth. As a result, many have wondered whether this sunlight could be harnessed to help produce drinkable, clean water in these regions.

In a new paper published in Applied Physics Reviews, researchers from China’s Shanghai Jiao Tong University present a solar-powered atmospheric water harvesting technology that can pull enough water out of arid desert air to help provide drinking water to those in difficult, dryland areas. Their technology is based on an improved type of hydrogel that overcomes previously noted problems with the gels operating in low humidity.

Ending water stress worldwide

According to figures from the United Nations, upwards of 733 million people today are living in countries with high or critical levels of water stress – when the ratio of withdrawn freshwater to total renewable water resources is above 75%.

This water stress is highest in Northern Africa and Western Asia, where a combination of the naturally dry climate with little rainfall and the rise of fast-growing, densely populated cities exert additional pressure on the region’s freshwater resources.

Given the intensity of the sun in these regions, there is a strong interest in developing solar-powered technologies that could be used to harvest and produce fresh water.

Hygroscopic hydrogels are a unique material made up of three-dimensional networks of polymer chains that can swell and absorb large volumes of water. This property has attracted the attention of individuals looking to develop a passive, energy-efficient water harvesting solution. Water harvesting technologies using hydrogels composed of polyacrylamide (PAM) or poly(N-isopropylacrylamide) (PNIPAM) networks have been developed, but they come with one major limitation.

“PAM and PNIPAM are commonly used adsorption-based atmospheric water harvesting materials, and they usually work at high relative humidity, e.g., > 80% relative humidity. However, regions with water scarcity usually have low relative humidity, so adding hygroscopic salts [to the hydrogel] is a common practice,” explained first author Chengjie Xiang, a postdoctoral fellow in the Engineering Research Center of Solar Power & Refrigeration (MOE China) at Shanghai Jiao Tong University.

“Unfortunately, these hydrogel matrices suffer from the salting-out effect, namely the reduced water solubility of polymers in response to the presence of salt,” Xiang said. “Thus, the salting-out effect leads to the unwanted aggregation of the polymer chains and suppresses the swelling of these hydrogels, which limits their water transfer, water storage ability and ultimately water vapor sorption capacity.”

Improved hygroscopic gels for clean water harvesting

The new super hygroscopic gel developed by Xiang and colleagues is made using plant derivatives, the hygroscopic salt lithium chloride and titanium nitride as a photothermal component that helps to drive the solar-powered release of any absorbed water. Analysis showed that one kilogram of dry gel was able to absorb up to 6.4 kg of water in humid environments and 1.18 kg in arid environments.

The research team incorporated their new hydrogel into a proof-of-concept prototype device, equipped with parallel desorption and condensation chambers.

In an outdoor demonstration, the prototype was able to simultaneously collect and release water during peak daylight hours, with release still possible even during the early mornings and late afternoons when the sun is weaker. A turbofan added to the condensation chamber was also found to increase the recovery of desorbed water to more than 90%.

“Our prototype with separated desorption and condensation chambers has the best liquid water collection capability to date during the daytime, and still works even at low sunlight intensity,” said Xiang.

Beyond its use in drinking water harvesting, Xiang says that this new hydrogel technology could have even further reaching impacts. The fast adsorption–desorption kinetics of the hydrogel, plus its simple and inexpensive synthesis from natural plant derivatives, could make it of interest in other sectors.

“In addition to obtaining drinking water, researchers in this field have carried out fruitful exploration in desert farmland, sea farmland and other places lacking fresh water, verifying the feasibility of using atmospheric water resources to grow food,” explained Xiang. “In addition, traditional fields such as adsorption heat storage and adsorption refrigeration will also be revitalized with the development of new adsorbents. In the energy field, the simultaneous capture of water and carbon dioxide will make liquid sunlight (using solar energy to convert water and carbon dioxide into methanol, etc.) come true.”

Reference: Xiang C, Yang X, Deng F, Chen Z, Wang R. Daytime air–water harvesting based on super hygroscopic porous gels with simultaneous adsorption–desorption. Appl Phys Rev. 2023;10(4):041413. doi: 10.1063/5.0160682

Dr. Chengjie Xiang was speaking to Alexander Beadle, Science Writer for Technology Networks.