Water-Absorbing Polymers: Review

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Table of Contents

Introduction
Case study
Polymers Classification
Statistics
Harvest Quality
Mechanism of Water-Absorbing Polymers
Conclusion
References

Introduction

Shortage of water in modern society due to increased demand of water has compelled people to devise various ways of conserving water. Increasing population and changing patterns of rainfall due to global warming have made water shortage a common phenomenon in the 21^st century. To conserve water and optimize its uses, the application of water-absorbing polymers has proved to be effective. Water-absorbing polymers conserve water in the soil, and thus reduce excessive wastage during irrigation of crops in arid and semi-arid regions or in areas where sandy soils are dominant. Jhurry (1997) states that, “increasing water-holding capacity of soils, increasing efficiency of water use, enhancing soil permeability and infiltration rates, reducing irrigation frequency, reducing compaction tendency, stopping erosion and water run-off, and increasing plant performance” are the benefits of water-absorbing polymers in agriculture (p. 109). Hence, water-absorbing polymers have significant benefits to farmers. Therefore, the research paper examines a case study of water-absorbing polymers and highlights their mechanisms of function and importance in agriculture.

Case study

Farmers across the world have applied water-absorbing polymers when growing different crops that are in arid and semi-arid regions, which require irrigation. Growth of onions in Australia, Murray-Darling Basin, is a case study that shows the application of water-absorbing polymers in agriculture. In Murray-Darling Basin, sandy soils are favorable for growing onions because they are fertile. Moreover, the sandy soils allow robust penetration of roots and optimum growth of onions. Sandy soils also ease harvesting because they cause minimum damage to the onions. However, in spite of such huge benefits accrued from sandy soils in the growth of onions, leaching of nutrients, soil erosion, and wastage of water increase the cost of producing onions considerably. Philips and Cutting (2008) argue that, “non-wetting soils are difficult to wet up, create preferential flow pathways so that water and nutrients are readily leached and plant production is low” (p. 2). Thus, to improve wetting properties of the sandy soils, prevent leaching of nutrients, and improve the yields of onions, the use of water-absorbing polymers is essential.

In Murray-Darling Basin, agriculturalists use polyacrylamides (PAM) in improving properties of the sandy soils to conserve water, retain nutrients, and sustain growth of onions. PAM is a type of water-absorbing polymer that enhances water retention, prevents leaching, stabilizes soils, and controls soil erosion (Bai, Zhang, Liu, Wu, & Song, 2010). PAM enhances water retention capacity of soils because it improves adsorption and absorption properties of soils. According to Sojka, Bjorneberg, Entry, Lentz, and Orts (2002), since sandy soils have poor retention capacity of water, PAM enhances their retention capacity through adsorption and absorption processes. Consequently, with increased retention capacity of water, PAM prevents leaching of nutrients and pesticides, stabilize soil structure, and prevent surface run-off in irrigated farms. Hence, the use of PAM has proved to be effective in growing onions in Murray-Darling Basin because it increases yields and reduces the cost of production in terms water, fertilizers, and pesticides.

Polymers Classification

Soluble and insoluble polymers are two main classifications of water-absorbing polymers. Agriculturalists developed water-soluble polymers first and applied them to prevent soil erosion, improve percolation of water, and stabilize soils. Sojka, Bjorneberg, Entry, Lentz, and Orts (2002) state that polyacrylamides, hydrolyzed polyacrylonitrile, polyvinyl alcohol, isobutylene maleic acid, vinlyacetate maleic acid, and sodium polycacrylate are some of the common examples of water-soluble polymers used in agriculture. The anionic character of water-soluble polymers supports retention and percolation of water. In contrast, insoluble polymers are gel-forming polymers, which comprise of cross-linked network of polymers (Rigi, Vazirimehr, & Keshtehgar, 2013). Starch-graft copolymers, cross-linked polyacrylamides, and cross-linked polyacrylates are examples of insoluble polymers that agriculturalists use to improve water retention, prevent erosion, stabilize soils, and prevent leaching.

Statistics

Statistics show that the use of water-absorbing polymers has a significant impact on conservation of water in irrigation. For instance, PAM absorbs the amount of water that is about 300 to 400 times its own weight. According to Ekebafe, Ogbeifun, and Okieimen (2011), PAM increases infiltration rate by about 15-50% and prevents surface runoff by approximately 80-99% in furrow irrigation. The insoluble polymers can absorb water that is approximately 1000 times its own weight. Normally, the capacity of a polymer to retain water decreases with the increase in cross-linkages. Jhurry (1997) states that, “cross-linked polyacrylamides hold water up to 400 times their weight and release 95% of the water retained within the granule to growing plants” (p. 111). These figures show that water-absorbing polymers retain a significant amount of water in the soil for the plants to utilize.

Harvest Quality

Water-absorbing polymers improve the quality and quantity of agricultural yields. Sivapalan (2002) states that PAM has improved farming practices in Australia as farmers are able to produce quality rice and soybeans. Since water-absorbing polymers prevent leaching of nutrients, crops do not experience any deficiency of essential minerals. Barihi, Panahpour, and Beni (2013) assert that the use of water-absorbing polymers has enhanced quality of tomatoes, flaxseed oil plant, maize, and kidney beans because the polymers release fertilizers to the plants and encourage the growth of crucial microorganisms. The case study of onions has also proved that water-absorbing polymers improve the quality of onions in Australia.

Mechanism of Water-Absorbing Polymers

The capacity of water-absorbing polymers to retain water depends on the nature of polymers and the degree of cross-linkages that they possess. Regarding the nature of polymers, water-soluble polymers hold more water than polymers that are insoluble in water. Moreover, the degree of cross-linkages determines the water capacity of polymers as a highly cross-linked polymer has low capacity of water, while a lowly cross-linked polymer has high capacity of water (Elliot, 2009). This means that modifications in terms of the nature polymers and degree of cross-linkages determine the capacity of a polymer to hold water. Furthermore, water-absorbing polymers hold water through the mechanism of hydration, hydrogen bonding, and other electrostatic forces. Vashuk, Vorpbieva, Basalyga, and Krutko (2001) argue that, “the combination of intermolecular interactions in water solutions such as hydrogen bonding, hydrophobic, and electrostatic interactions,” enhances the capacity of polymers to hold water (p. 350). The presence of these forces is dependent on the nature of polymers. Overall, the nature of polymers, the degree of linkages, and the intermolecular forces explain the mechanism of water-absorbing polymers.

Conclusion

Water-absorbing polymers are very important in agriculture because they do not only conserve water, but also prevent soil erosion, avert leaching, and improve the quality and quantity of yields. The case study of Murray-Darling Basin in Australia depicts the use of PAM in growing onions. The case study shows that polymers conserve water, prevent leaching, and enhance agricultural yields. The use of soluble and insoluble polymers in agriculture varies according to the nature of soils. Statistics show that PAM increases the infiltration rate of water by 15-50%, prevent surface runoff by 89-99%, and release 95% of their water to plants. Moreover, the use of polymers increases the quality and quantity of crops such as onions, maize, tomatoes, soybeans, and kidney beans amongst others. Fundamentally, the nature of polymers, the degree of cross-linkages, and the intermolecular forces determine water capacity of polymers.

References

Bai, W., Zhang, H., Liu, B., Wu, Y, & Song, J. (2010). Effects of super-absorbent polymers on the physical and chemical properties of soil following different wetting and drying cycles. Soil Use and Management, 26(1), 253-260.

Barihi, R., Panahpour, E., & Beni, M. (2013). Super Absorbent Polymer (hydrogel) and its application in agriculture. World Science Journal, 1(15), 223-228.

Ekebafe, L., Ogbeifun, D., & Okieimen, F. (2011). Polymer Applications in Agriculture. Biochemistry, 23(2), 81-89.

Elliot, M. (2009). Superabsorbent Polymers. Web.

Jhurry, D. (1997). Agricultural Polymers. Web.

Philips, S., & Cutting, M. (2008). The Role of Polyacrylamides (PAM) in Onion Production. Web.

Rigi, K., Vazirimehr, & Keshtehgar, A. (2013). The Effect of Waste Water in Agronomy. International Journal of Agriculture and Crop Sciences, 5(24), 2969-2971.

Sivapalan, S. (2002) Use of PAM in Australian irrigated agriculture. Australian Grain, 12(3), 24-25.

Sojka, R., Bjorneberg, D., Entry, J., Lentz, R., & Orts, W. (2002). Polyacrlamide in Agriculture and Environmental Land Management. Advances in Agronomy, 92(1), 75-162.

Vashuk, E., Vorpbieva, E., Basalyga, I., & Krutko, N. (2001). Water-absorbing properties of hydrogels based on polymeric complexes. Materials Research Innovations, 4(1), 350-352.

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