Water harvesting gel works at low humidity levels

Researchers in the US have designed a sustainable polymer gel that can harvest large quantities of water from the surrounding air, even in low-humidity conditions. Created by Youhong Guo and colleagues at the University of Texas at Austin, the low-cost material combines water-absorbing plant fibres with cellulose, which expels water when heated.

Many parts of the world experience some degree of water scarcity and the problem is expected to grow with increasing global warming. Extracting moisture directly from the atmosphere could provide millions of people with vital access to clean water. Researchers have already developed a variety of different porous materials that can capture and release moisture on demand – but these often require humid conditions. In dryland regions, now home to over a third of the world’s population, existing techniques suffer from low water uptake and high energy demand.

To address this challenge, Guo’s team developed a new polymer material, containing a hybrid of konjac gum (KGM) – a plant-based fibre widely used in Asian cuisine — and hydroxypropyl cellulose (HPC). This polymer matrix also contains a uniformly-dispersed solution of lithium chloride – a moisture-retaining salt.

Large surface area

Within the material, hydrophilic KGM has a hierarchically porous structure. This provides a large water-collecting surface area, while also allowing water vapour to rapidly pass through the structure. When heated,  HPC undergoes a phase change, and its previously straight polymer fibres contract into amorphous, twisted shapes. In the process, any moisture in the KGM fibres is rapidly released.

Guo and colleagues have shown that in 14–24 cycles of water uptake and release in arid conditions, 1 kg of the gel can produce more than 6 l of water per day in 15% relative humidity. At 30% relative humidity, up to 13 l per day can be produced.

The researchers also showed that the polymer can be easily produced through a user-friendly casting method, where a gel precursor containing all three ingredients is mixed and poured into a mould. After 2 min, the mixture is freeze-dried and peeled from the mould, ready to be used straight away.

On top of this, the material’s three ingredients are abundant and can be sourced sustainably. Altogether, the ingredients cost just $2 per kilogram. Guo’s team hope that the low cost and simplicity of production will mean that the gel can be produced commercially. They predict that far larger quantities of water could be readily harvested by fabricating thicker films and introducing absorbent beds to the gel.

The research is described in Nature Communications.

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