Solar hydrogel: the "water cycle master" for greenhouse cultivation

Produced by: Science Popularization China

Author: Shi Chang (PhD in Physical Chemistry)

Producer: China Science Expo

Editor's note: In order to understand the latest developments in cutting-edge science and technology, the China Science Popularization Frontier Science Project has launched a series of articles titled "Understanding Top Science Journals", which selects outstanding papers from authoritative journals and interprets them in plain language as soon as possible. Let us broaden our scientific horizons and enjoy the fun of science through the window of top journals.

With the rapid development of science and technology, fruits and vegetables seem to have broken free from the restrictions of seasons. Watermelon is no longer exclusive to summer, and cabbage no longer dominates the list of winter vegetables. When fresh vegetables and fruits are served on our tables, do we notice that behind this "delicious food" there is an "ecological kingdom" - the greenhouse.

Greenhouse cultivation

(Photo source: veer photo gallery)

Greenhouses, as an important achievement of agricultural technology, not only break the restrictions of seasons and regions, making food supply more stable and diversified, but also greatly enrich our food culture. However, with the continuous increase in the number of greenhouses, a problem that cannot be ignored has also emerged, that is, the waste of water resources.

According to statistics, greenhouse agricultural water accounts for more than 50% of human water use. While greenhouse irrigation water meets the needs of crop roots, it is also quietly lost through soil evaporation and plant transpiration, causing huge waste of the earth's precious water resources. It is also a difficult problem on the road to sustainable agricultural development.

Hydrogel: A magical water-absorbing material

In daily life, children often play with a toy called "Water Baby" that grows bigger by absorbing water. This is a toy with small particles and balls. When you soak the "Water Baby" in water for a while, it will expand, just like "growing". The colorful and bouncy balls make people want to pinch them, but do you know where this bouncy Water Baby comes from?

Colorful "water babies"

(Photo source: veer photo gallery)

This water-absorbing material is called hydrogel, a polymer material with a special three-dimensional network structure that can swell rapidly in water and retain a large amount of water without dissolving itself.

Hydrogels can be divided into natural hydrogels and synthetic hydrogels according to the source of the materials.

Natural polymer hydrogel: It is mainly made of natural polymer materials, such as gelatin, chitosan, sodium alginate, collagen, hyaluronic acid, etc. Most of these materials are derived from animal tissues or marine organisms, so they have good biocompatibility and are degradable.

Synthetic polymer hydrogel: a polymer obtained by polymerization reaction of synthetic materials, such as polyacrylamide, polyethylene glycol, polyacrylic acid, polyvinyl alcohol, etc. This type of hydrogel has the characteristics of good structural controllability, good repeatability and excellent mechanical properties.

Can hydrogels with water-absorbing properties be used for water management in greenhouses?

On July 10, 2024, Chinese scientists published an article in the journal Nature Water about solar-driven hygroscopic hydrogels, which can achieve rapid absorption and release of water. When used in greenhouse cultivation, it can achieve passive recovery of plant transpiration and soil evaporation water, achieving energy-saving and water-saving effects.

The research results were published in the journal Nature Water

(Image source: Nature Water magazine)

The researchers used titanium nitride (TiN), curdlan (CUR), sodium polyacrylate (PAAS) and lithium chloride (LiCl) as raw materials and prepared a porous hydrogel material (TCP-Li) through a "cross-linking-freeze drying" process. Cross-linking reaction is an important chemical reaction, which refers to the reaction in which two or more molecules bond with each other and cross-link into a network structure. This reaction can significantly improve the strength, heat resistance, wear resistance, solvent resistance and other properties of the material. Titanium nitride is a photothermal conversion material that can convert absorbed light energy into heat energy.

Curdlan can form a gel state after heating in an aqueous solution. Sodium polyacrylate is a highly absorbent polymer that plays a role in absorbing and retaining water in hydrogel materials. Lithium chloride can give the material the ability to absorb water vapor, greatly improving the material's water-capturing capacity.

Preparation process of hydrogel; bc. SEM images of hydrogel; d. Element distribution of hydrogel; e. X-ray diffraction pattern of the material; f. Infrared spectrum of the material; g. UV-visible-near infrared absorption spectrum of hydrogel.

(Image source: Reference 1)

The researchers conducted water adsorption and desorption experiments on the prepared TCP-Li hydrogel material to verify its ability to absorb and release water. The experimental results show that the hydrogel material has a strong water absorption capacity. Under the temperature conditions of 30°C and the relative humidity environments of 60% and 90%, the water absorption per gram of the hydrogel material is 2.8 and 3.38 grams respectively.

In order for hydrogel materials to achieve water circulation, it is not enough to have high hygroscopicity, but also the ability to release water quickly. The researchers tested the water release ability of hydrogel materials in an environment simulating sunlight.

The experimental results show that TCP-Li hydrogel material can also release water quickly under the conditions of simulating sunlight. Even under the conditions of 60% sunlight intensity, it can release 95% of water within 60 minutes. At the same time, the material also has a long service life. After completing 15 water molecule absorption and release experiments, the attenuation value of moisture absorption capacity is less than 3%.

Schematic diagram of the structure of the water recovery device TEAD; b. Heat transfer during the TEDA adsorption-desorption process,

Mass transfer analysis; c. TEDA device diagram; d. Infrared temperature diagram of TEAD during desorption.

(Image source: Reference 1)

After testing the performance of TCP-Li hydrogel, the researchers also used the material to prepare a water recovery device (TEAD), which can collect water vapor produced by crop transpiration and soil evaporation at night, and release water for crop irrigation under the action of sunlight during the day. The experimental results show that the TEAD water recovery device can collect 87.1 grams of water from each plant every day and recycle 1890.6 grams of water per square meter, achieving a water-saving effect of 44.9%, greatly reducing the waste of irrigation water.

Other uses of hydrogels

In the field of skin care , hydrogel can be made into a facial mask that fits closely to the skin, effectively locking in and releasing moisture, providing long-lasting deep nourishment to the skin and helping it restore its hydration and radiance.

In the field of environmental protection , hydrogels are also used to develop various highly efficient water treatment materials. They can absorb and remove harmful substances such as heavy metal ions and organic pollutants in water, effectively purify water quality, and ensure the safety of people's drinking water.

Water treatment

(Photo source: veer photo gallery)

In the field of smart materials , hydrogels are widely used due to their unique ability to respond to water. For example, stimuli-responsive hydrogels can deform or generate electrical signals according to changes in the external environment (such as temperature, humidity, etc.), providing new functions and interaction modes for smart wearable devices.

Conclusion

The birth of solar-driven hydrogel is not only a major breakthrough in traditional greenhouses, but also a practice of the concept of sustainable agricultural development. It shows us that with the help of science and technology, humans have the ability to overcome the problem of natural resource shortages and create a greener, more environmentally friendly and efficient production model. We look forward to the application of this high-tech material in agricultural production, and to its valuable contribution to water conservation and the high-tech development of agriculture.

References:

1.Zou, H., Yang, X., Zhu, J. et al. Solar-driven scalable hygroscopic gel for recycling water from passive plant transpiration and soil evaporation[J]. Nat Water, 2024.

2.Achour, Y. et al. Technological progresses in modern sustainable greenhouses cultivation as the path towards precision agriculture[J]. Renew. Sust. Energ. Rev. 12021.

3. Cui Guiguan, Xu Liqin. Discussion on greenhouse irrigation technology[J]. Water-saving irrigation, 2007.

4. Zhai Maolin, Ha Hongfei. Synthesis, properties and applications of hydrogels[J]. University Chemistry, 2001.