RUDN University agronomist checked whether hydrogels can save agriculture from water shortage

RUDN University agronomists have studied the thermodynamics of hydrogels, which must absorb water from the air and hold it in the ground to prevent evaporation. It turned out that this approach is unlikely to help save agriculture from drought – hydrogels retain water too well and give it poorly. The results are published in the Journal of Composites Science.

Sustainable development requires 500 cubic meters of water per person per year. Therefore, the issue of water shortage remains one of the most acute problems. They try to solve the problem with the help of special composite materials – hydrogels, which can retain moisture. This can be useful, for example, in agriculture – it is on it that the most fresh water is spent. If these hydrogels are placed in the soil, they can “absorb” moisture from the air and keep it in the ground. This is to prevent excess evaporation. RUDN University agronomists measured the ability of such materials to retain moisture, hygroscopicity, in an arid climate.

“The highest consumption of clean water is in agriculture, especially in arid climates with high daily evaporation. Innovative materials can be used both to purify water and to preserve it in the soil by reducing evaporation. We used a thermodynamic approach to evaluate the hygroscopicity of gelling materials. Our goal was to investigate how hygroscopicity changes in a wide range of temperatures and relative air humidity typical of an arid climate,” Andrey Smagin, Doctor of Science in Biology, Leading Researcher at the Laboratory “Smart Technologies for the Sustainable Development of the Urban Environment in the Conditions of Global Changes” of RUDN University.

Agronomists have proposed a simple and accessible method for any laboratory based on thermal desorption. Its essence is to measure the specific heat of evaporation of water under different conditions. The technique was applied on a composite material of Aquapastus brand. It consists of a hydrophilic acrylic substrate that is filled with by-products from the synthesis of polyacrylamide.

The hydrogel showed high hygroscopicity – up to 80-130%. This is higher than that of natural materials. For example, in clay soils, hygroscopicity is 15-40%, and in a tree – no more than 60%. However, there are examples of artificial materials in which the water content can reach up to 150%. However, RUDN researchers concluded that it may not make sense to use such hydrogels in practice to retain water in the soil. The fact is that the water that “got” into such a composite material is too hard to “pull out” from there. The authors estimate that under normal conditions, this would require more than 2,700 joules per kilogram. Plant roots can’t handle this – their “strength” usually does not exceed 2,000 joules per kilogram. It turns out that such hydrogels will be even more useless than sea water.

“The hygroscopicity of the investigated composite materials in humid air reaches 80-130%. However, this water has too high a retention energy and the plants simply cannot consume it. This casts doubt on the very technology of obtaining water from the air using hygroscopic materials ,” Andrey Smagin, Doctor of Science in Biology, Leading Researcher at the Laboratory “Smart Technologies for the Sustainable Development of the Urban Environment in the Context of Global Changes” at RUDN University.