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CHEMICAL ENGINEERING

Combating dry land

When combined with soil, super absorbent natural polymers can make agricultural crops viable in arid regions

Hydrogel made from gellan gum, produced by a bacterium, and chitosan, a substance extracted from crustacean shells

Léo Ramos Hydrogel made from gellan gum, produced by a bacterium, and chitosan, a substance extracted from crustacean shellsLéo Ramos

One of the challenges of modern agriculture is to use less water in fields without losing productivity, a goal that can be achieved through the technological contribution of polymers with a high capacity to absorb and retain liquids. Chemist Rodrigo César Sabadini, for his doctorate at the Chemistry Institute in São Carlos, part of the University of São Paulo (IQSC-USP), under the supervision of Agnieszka Joanna Pawlica Maule, developed a super absorbent hydrogel for use on crops in arid soils subject to drought or on irrigated planted ground to reduce water consumption. Mixed with the soil, the product has the ability to absorb large amounts of water—from rain or irrigation—thereby serving as a water reservoir for periods of drought (see Infographic). The hydrogel or water-retaining polymer is made from natural polymers. The research resulted in a patent application, related to synthesis of the product.

Hydrogels were created in the 1950s in the United States and since then have been used in agriculture. “Most of the research and development of these materials has been based on the use of synthetic polymers. The advantage of ours is that they are biodegradable and leave no waste,” says Maule. “Synthetics, in addition to accumulating in the environment, because they do not degrade naturally, can leach into rivers and streams. And their residues of acrylate monomers can be toxic to soil and rivers.” “Leaching is the process of extracting a substance (in this case, acrylates) from a solid medium (crop soil) by its continuous dissolution. Synthetic commercial hydrogels can retain water for a period of three months to one year. According to Sabadini, biodegradable hydrogel durability tests have not yet been done.

While pursuing his doctorate, Sabadini started a new hydrogel project, in which he developed and tested several formulas, including one composed of chitosan and gellan gum. “The process of obtaining and purifying chitosan, which is extracted from crustacean shells, is simple,” says Sabadini. Gellan gum, on the other hand, is generated by bacteria living in the roots of water plants of the genus elodea. Produced in a laboratory culture medium, the gum is available commercially and was chosen for its high water absorption capacity.

At USP’s Chemistry Institute in São Carlos: microscopic and thermogravimetric analysis of the hydrogel to calculate mass loss by temperature

Léo Ramos At USP’s Chemistry Institute in São Carlos: microscopic and thermogravimetric analysis of the hydrogel to calculate mass loss by temperatureLéo Ramos

Wet sponge
Both synthetic and natural hydrogels are similar to those used in the manufacture of infant diapers and feminine hygiene products. In the agricultural version, the gels can have different shapes. When dry, the super absorbent polymer created at USP looks like slightly malleable plastic. When wet, it resembles a soggy sponge. The commercial Hydroplan-EB water-retaining polymer of the French group SNF, the world leader in the manufacture of polyacrylates, is a granular product with different particle sizes, which takes the form of a clear gel after hydration. The French company exports the product to Brazil from one of its factories in the United States. Companies such as Evonik, Sanyo and Basf also manufacture hydrogels worldwide.

“The gel distributed by Hydroplan-EB is an acrylate copolymer of potassium and acrylamide and functions as a reservoir near the roots, storing water and whatever is dissolved there, such as pesticides and fertilizers,” says chemical engineer Loremberg Fernandes de Moraes, sales manager of Hydroplan-EB. The product can absorb 200 – 400 times its weight and increase its size one hundredfold. “The roots of the plant capture water from the gel by osmosis, just as they capture it from the soil.” Moraes points out that the great advantage of the technology is in reducing the frequency and amount of water used in irrigation. “If, under normal conditions farmers need to water their crops every two days, with our gel they only need to do so every three or four days,” says Moraes.

“Polyacrylamides are not degraded biologically, so once applied to the soil they undergo gradual degradation or dissociation by the action of the crop, the ultraviolet rays of the sun and continuous fractionation, which rotates soils about 10% in those continuously cultivated by agricultural implements,” says Moraes. “The polymer’s deterioration occurred in an accelerated manner. In scientific experiments lasting three months, the polymer was placed in solutions containing calcium, magnesium and iron salts. This kind of deterioration can also occur in soils fertilized annually with complete fertilizers.” This conclusion is based on studies published in the 1980s, on scientific articles written by Reda Azzam of Egypt’s Atomic Energy Authority and researchers at Australia’s Horticultural Research Institute and the University of California, Los Angeles.

Brazil, according to Moraes, is the second largest consumer of hydrogels used for agriculture in the world, behind only the United States, where the product is also used in gardens. “Here, super absorbent gels are primarily used on eucalyptus plantations, which require a lot of water. We estimate that agricultural demand for the product is about 500 tons per year. On a eucalyptus plantation with  about 1,200 plants, for example, uses 1.5 kilos per hectare. The price of Hydroplan-EB varies according to the volume ordered by the grower, but it costs between R$25 and R$30 per kilo,” says Moraes.

The high cost of the natural hydrogel based on chitosan and gellan gum is, according to Maule of IQSC-USP, the biggest disadvantage of the innovation. “Our raw material is expensive compared to synthetic polymers – about 100 times higher. We expect the natural raw materials to come down in price as demand for them increases,” she says. But before commercial manufacturing begins, the product still needs to undergo small-scale testing. “Normally, the tests are developed in partnership with companies interested in production or with technology institute laboratories.”

Hydrogel_248Lettuce and coffee
During product development to evaluate its effectiveness, Sabadini tested the hydrogel on lettuce plants. Seeds were placed in small pots to germinate in a coconut shell substrate. Some containers were mixed with dry hydrogel samples and others with fertilizer. A third batch represented the control samples and contained just seeds. All pots received the same amount of water in a single watering. “After a few days, we observed the germination of the seeds that received only the hydrogel and the hydrogel with the fertilizer. The control samples did not germinate,” says Sabadini.

The first research in Brazil on the technology began around 2000. In principle, hydrogels can be used in any type of agricultural or forestry crop and in different types of soil. “There is no limitation on applying the product, since the idea is to prolong the presence of water in the soil or to promote its fertilization. But the product’s best use would be in arid and sandy soils, which do not have the capacity to retain water,” says Maule.

The Federal University of Lavras (UFLA) in Minas Gerais State has been doing research on this technology since 2009. The agronomist-engineer Rubens José Guimarães is investigating the benefits of synthetic hydrogels for coffee plantations. The study is supported by the Coffee Research Consortium, coordinated by Embrapa Café, which is based in Brasília, and has been the subject of four master’s theses and a doctoral dissertation, which, according to Guimarães, confirms the effectiveness of the product. Although hydrogels are a well known technology, it is important to establish parameters for their use, which varies according to the region’s culture, soil, climate and meteorological characteristics. The study also sought to identify the ideal granulometry of the gel, which is the Hydroplan-EB, to be used on coffee plantations, as well as the amount of product used on each coffee plant, among other variables.

“Water optimization with this product can occur both in rainfed coffee regions [not irrigated and with low rainfall] as well as in irrigated areas. In the case of rainfed crops, using the polymer can ensure the ‘sticking’ of the seedlings when it does not rain. Yet on irrigated plantations, the hydrogels allow more time between irrigations,” says Guimarães. “The product can be an ally of coffee growers in periods of prolonged drought, reducing losses.”

At USP, an experiment with lettuce plants showed that seeds germinated in the pots with hydrogel. They did not germinate in pots without hydrogels

Rodrigo César Sabadini At USP, an experiment with lettuce plants showed that seeds germinated in the pots with hydrogel. They did not germinate in pots without hydrogelsRodrigo César Sabadini

UFLA’s studies showed that coffee plants with the added water-retaining polymer showed a 10% higher rate of development than the others. Another surprising result was discovered in the roots, which grew 40% more than in the plants that did not receive the product. The hydrogel product, according to Guimarães, should be used so that farmers do not lose the seedlings and seeds planted during times of drought. Thus, the product guarantees that the plants will thrive even in the absence of rainfall. “Gel use is a viable option because plants are able to extract the water necessary for their survival from the polymer. This was shown in studies highlighting the root development inside the hydrated polymer granules, by promoting greater surface contact between the roots, the water and the nutrients essential to the plant’s growth,” says Guimarães.

Guimarães also notes that the technology reduced the mortality rate of the coffee plants and, consequently, the required amount of replanting, a practice that increases production costs. “But other technologies should also be encouraged, such as proper management of invasive plants, green manuring, and localized irrigation, in addition to other practices.”

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