Functional Polymers in Food Science E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Polymer Science and Plastics Engineering
• Sprache: Englisch

Seitenzahl: 547

Veröffentlichungsjahr: 2015

Beschreibung

Polymers are an important part in everyday life; products made from polymers range from sophisticated articles, such as biomaterials, to aerospace materials. One of the reasons for the great popularity exhibited by polymers is their ease of processing. Polymer properties can be tailored to meet specific needs by varying the "atomic composition" of the repeat structure, by varying molecular weight and by the incorporation (via covalent and non-covalent interactions) of an enormous range of compounds to impart specific activities. In food science, the use of polymeric materials is widely explored, from both an engineering and a nutraceutical point of view. Regarding the engineering application, researchers have discovered the most suitable materials for intelligent packaging which preserves the food quality and prolongs the shelf-life of the products. Furthermore, in agriculture, specific functionalized polymers are used to increase the efficiency of treatments and reduce the environmental pollution. In the nutraceutical field, because consumers are increasingly conscious of the relationship between diet and health, the consumption of high quality foods has been growing continuously. Different compounds (e.g. high quality proteins, lipids and polysaccharides) are well known to contribute to the enhancement of human health by different mechanisms, reducing the risk of cardiovascular disease, coronary disease, and hypertension. This second volume focuses on the importance of polymers and functional food and in food processing

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

Chapter 1: Functional Polymers for Food Processing

1.1 Introduction

1.2 Food Preparation

1.3 Food Processing: Rheology

1.4 Functional Foods and Nutraceuticals

References

Chapter 2: Polyacrylamide Addition to Soils: Impacts on Soil Structure and Stability

2.1 Introduction

2.2 Polyacrylamide (PAM) Properties and Interactions with Soil

2.3 Polymer Effects on Aggregate Stability

2.4 PAM Effects on Soil Saturated Hydraulic Conductivity

2.5 PAM Effects on Infiltration, Runoff and Erosion

2.6 Concluding Comments

References

Chapter 3: Functional Polymeric Membrane in Agriculture

3.1 Introduction

3.2 Principle of Imec

3.3 Imec System

3.4 Plant Cultivation by Imec System

3.5 Comparison between Imec and Hydroponics

3.6 Current Domestic State of Imec Growth

3.7 Imec Vegetables besides Tomato

3.8 Imec Changes Barren Land to Farming Land

3.9 Current State of Overseas Growth of Imec

References

Chapter 4: Enzymes Used in Animal Feed: Leading Technologies and Forthcoming Developments

4.1 Introduction: General Outline and Value Drivers

4.2 Feed Digestive Enzymes

4.3 Actual and Potential Feed Enzyme Market

4.4 Advances in Feed Enzyme Technology

4.5 Conclusions and Future Perspectives

Acknowledgments

References

Chapter 5: Interaction of Biomolecules with Synthetic Polymers during Food Processing

5.1 Introduction

5.2 Basic Biomolecules in Food and Their Interactions with Synthetic Polymers

5.3 Membranes for Food Processing

5.4 Chromatography for Food Processing

5.5 Analogy of Ultrafiltration and Size Exclusion Chromatography

5.6 Future Perspectives of Membranes and Chromatography

References

Chapter 6: Rheological Properties of Non-starch Polysaccharides in Food Science

6.1 Non-starch Hydrocolloids

6.2 Rheological Properties of Non-starch Hydrocolloid Systems

Nomenclature

References

Chapter 7: Polysaccharides as Bioactive Components of Functional Food

7.1 Introduction

7.2 Functional Foods

7.3 Polysaccharides from Seaweed

7.4 Functional Activity of Polysaccharides

7.5 Conclusions

References

Chapter 8: Milk Proteins: Functionality and Use in Food Industry

8.1 Introduction

8.2 Milk Proteins

8.3 Milk Protein Products

8.4 Functional Properties of Milk Proteins

8.5 Conclusions

References

Chapter 9: Bioactive Peptides from Meat Proteins as Functional Food Components

9.1 Introduction

9.2 Generation of Bioactive Peptides in Meat

9.3 Meat-Derived Bioactive Proteins and Peptides

9.4 Conclusion

References

Chapter 10: Antioxidant Polymers: Engineered Materials as Food Preservatives and Functional Foods

10.1 Introduction

10.2 Antioxidant Polymers as Food Additives

10.3 Antioxidant Polymers as Dietary Supplements and Functional Foods

10.4 Conclusion

References

Chapter 11: Biopolymers for Administration and Gastrointestinal Delivery of Functional Food Ingredients and Probiotic Bacteria

11.1 Introduction

11.2 Characteristics of the Gastrointestinal Tract

11.3 Bioencapsulation Techniques for Administration and Gastrointestinal Delivery

11.4 Polymeric Materials for Microencapsulation

11.5 Biopolymers in the Encapsulation of Nonmicrobial Functional Food Ingredients

11.6 Biopolymers in the Encapsulation of Functional Microbes (Probiotics) for Administration and Gastrointestinal Delivery

11.7 Conclusion and Future Trends

References

Chapter 12: Cyclodextrin as a Food Additive in Food Processing

12.1 Introduction

12.2 Inclusion Complex Formation

12.3 Covalent Polymer Networks Containing Cyclodextrins

12.4 Regulatory Issues for CDs as Food Additives and Use in Food Processing

12.5 Applications of CD in Food

12.6 Cholesterol Sequestration

12.7 Taste Modifiers

12.8 Product Stability and Food Preservatives – Improving Shelf Life

12.9 Nutraceutical Carriers – Functional Foods

12.10 Packaging

12.11 Conclusion

References

Chapter 13: Enzymes and Inhibitors in Food and Health

13.1 Introduction

13.2 Traditional Methods of Producing Enzymes

13.3 Biotechnological Methods for Producing Enzyme

13.4 Enzymes in Food Processing

13.5 Endogenous Enzyme Inhibitors from Food Materials

13.6 Concluding Remarks

References

Index

Functional Polymers in Food Science

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-59518-3

Preface

This book is an extensive and detailed overview of recent developments in the application of functional polymeric materials in food science, with an emphasis on the scientific concerns arising from the need to combine the properties of such versatile materials with nutritional needs. Consumers are increasingly conscious of the relationship between diet and health, and thus the request for high quality and safe foods has been continuously growing. This has resulted in tremendous efforts being undertaken in both academia and industry to increase the quality of food composition and storage. By taking advantage of the contribution of researchers in top universities, industrial research and development centers, this book is meant as a link between scientific and industrial research, showing how the development in polymer science can impact the field.

The book is composed of two volumes; the first concerns the application of polymers in food packaging, while the second shows the relationship between polymer properties, functional food and food processing.

The first volume highlights novel insights in the research on the best performing materials for intelligent packaging, capable of preserving food quality and prolonging product shelf life. After an introduction to the field, the volume goes into a detailed evaluation of the key polymeric and composite materials employed in food packaging for eventually addressing regulation issues.

The second volume opens with an overview of how polymers can be used to improve the quality of food by affecting agricultural processes, and subsequently the food rheology and nutritional profile of novel functional foods and nutraceuticals are extensively developed.

Chapter 1

Functional Polymers for Food Processing

Giuseppe Cirillo*, Umile Gianfranco Spizzirri and Francesca Iemma

Department of Pharmacy, Health and Nutritional Sciences, University of Calabria, Rende, Italy

*Corresponding author: [email protected]

Abstract

Polymeric materials can be used as functional elements for reaching an efficient food production and processing, with considerable advantages for the whole food industry. Among others, the applicability of polymers involves their use in agriculture, animal feed, modification of food rheology, and the development of functional food and nutraceutics.

Keywords: Functional polymers, agriculture, animal feed, food rheology, functional foods, nutraceutics

1.1 Introduction

Living a long healthy life is the desire of every human on earth. This basic desire is affected by almost every activity of human beings, with nutrition acting as one of the key elements, since it provides the essential elements for the cell cycle, such as carbohydrates, fats, proteins, vitamins and minerals [1].

Food, basically of plant or animal origin, is defined as any substance consumed to provide nutritional support for the human body. The rapid development of economies allows consumers to access foodstuff by the food industry, while the direct production has sensibly been reduced over the last century. The main challenge of the food industry is to address the growing global emphasis and attention on food quality and safety concerns, which are related not only to the consumers’ rising and persistent demand for requirements for safe food and better quality of food and beverage, but also the strict norms of government regulations.

“Food quality” refers to the quality characteristics of food that are acceptable to consumers, including such external factors as appearance (size, shape, color, gloss and consistency), texture, and flavor, and other internal factors (chemical, physical and microbial contamination).

The improvement of food quality can be related to one of the foodstuff production steps, namely production, manufacturing and storage. Each of these industrial activities acts as a key determining step for the final food quality assessment, and several different technologies have been developed for a substantial and sustainable quality improvement.

Polymers, from both natural and synthetic origin, are practically indispensable for everyday life in modern society, representing one of the main classes of compounds within the global chemical market. Almost every human activity in life highly depends on polymers, which are used in technological fields such as communications, transportation, electronics, as well as in the pharmaceutical, medical and food industries [2]. One of the reasons for the great popularity exhibited by polymers is their ease of processing. Polymer properties can be tailored to meet specific needs by varying the “atomic composition” of the repeat structure and by varying molecular weight. The flexibility can also be varied through the presence of side-chain branching and according to the lengths and the polarities on the side chains. The degree of crystallinity can be controlled through the amount of orientation imparted to the polymers during processing, through copolymerization, by blending with other polymers, and via the incorporation (via covalent and noncovalent interactions) of an enormous range of compounds [3].

1.2 Food Preparation

Providing for the health and welfare of its population with abundant, safe, and affordable food has long been the goal of food systems all around the world. This is related to the production of foodstuffs of both plant and animal origins, where the use of polymeric materials in the production step is explored differently.

1.2.1 Functional Polymers in Agriculture

As a consequence of the impressive technological progress of the last decades, agriculture is becoming an industrial sector with complex supply chains and electronically aided information and logistics systems [1].

In the agricultural field, polymers are widely used for many applications [4]. Although they were first used just as structural materials for creating a climate beneficial to plant growth (inhert polymers), in the last decades functionalized polymers have revolutionized the agricultural and food industries with new tools for the molecular treatment of diseases, rapid disease detection, enhancing the ability of plants to absorb nutrients, etc. [5].

Smart polymeric materials and smart delivery systems help the agricultural industry combat viruses and other crop pathogens. Functionalized polymers are used to increase the efficiency of pesticides and herbicides, allowing lower doses to be used and to protect the environment indirectly through filters or catalysts to reduce pollution and clean up existing pollutants [6].

The first application of polymeric materials is related to the enhancement of the soil stability, including aridity remediation. The use of polymeric materials with good water absorption and retention capacities even under high pressure or temperature represents a valuable approach to these aims. An important class of systems with this behavior is composed of the Superabsorbent polymers (SAPs), organic materials with lightly cross-linked three-dimensional structure possessing high to very high swelling capacity in aqueous media [7].

Generally, the SAP materials used in agriculture are polyelectrolyte gels often composed of acrylamide, acrylic acid, and potassium acrylate. Therefore, they swell much less in the presence of monovalent salt and can collapse in the presence of multivalent ions [8,9]. These ions naturally exist in the soil or are introduced through fertilizers and pesticides.

Interesting base elements for the preparation of highly engineered SAPs are natural polymers such as starch [10], chitosan [11], guar gum [12] and poly (amino acid)s [13], since they are environmentally friendly, biodegradable, and independent of soil resources.

A further development in the use of polymers in agriculture for soil protection is related to the use of plastic mulch [4], which offers the advantages of increased soil temperature, reduced weed pressure, moisture conservation, reduction of certain insect pests, higher crop yields, and more efficient use of soil nutrients.

Nanotechnology represents another area holding significant promise in the agricultural scenario [14]. Polymeric nanomaterials hold great promise regarding their application in plant protection and nutrition due to their size-dependent qualities, high surface-to-volume ratio and unique optical properties, making them suitable for developing agrochemical carriers for pesticides (inhibitors, antibiotics and toxins), biopesticides (bacteria, viruses and fungi enzymes), fertilizers, and biofertilizers (live formulations of beneficial microorganisms) [15]. Furthermore, they are suitable to be used for the assisted delivery of genetic material for crop improvement and as nanosensors for plant pathogen and pesticide detection.

In recent years, the removal of hazardous heavy metals from water and soil environments and industrial waste streams has attracted considerable attention. Enhanced metal separation techniques that require less energy with minimal impact on the environment are desirable [16,17]. When soils are contaminated with heavy metals, the clean-up is one of the most difficult tasks for environmental engineering. For remediating sites contaminated with inorganic pollutants, several techniques have been developed. An efficient technique for the removal of metal ions from wastewaters is the use of functionalized water-soluble polymers combined with the membrane-based separation method of ultrafiltration [18]. It consists of making heavy metals react with a water-soluble macromolecular ligand to form a macromolecular complex. Solution containing macromolecular complex is pumped through an ultrafiltration membrane. Unbound chelates pass through the membrane, while metal-loaded polymers are of sufficient molecular size to be retained.

1.2.2 Functional Polymers and Animal Feed

Animal feed is an industrial field that has drastically increased in the last decades, since efforts have been made to improve its production to obtain a substantial reduction in production costs and to improve the quality of animal-based foodstuffs [19].

In animal feed, key issues are related to the safety of the components. Directive 2002/32/EC of the European Parliament and of the Council of 7 May 2002 on undesirable substances in animal feed is the Directive governing the measures on undesirable substances in feed. “Undesirable substance” is defined as any substance or product, with the exception of pathogenic agents, which is present in and/or on the product intended for animal feed and which presents a potential danger to animal or human health or to the environment or could adversely affect livestock production [20].

Specific protocols have been developed for the the detection of contaminants (e.g., heavy metals, botanical species, alkaloids, toxins, bacteria and fungi), some of them involving the use of polymer-based biosensors.

Furthermore, various attempts have been made to develop efficient delivery system for animals based on pH-sensitive polymers [21].

1.3 Food Processing: Rheology

The quality and desirability of food products depends on their flavor and texture [22]. Food texture has historically been considered those properties that are not covered in the classical definitions for taste and flavor compounds. This includes the mechanical properties evaluated from force-deformation relationships, tactile sensations such as adhesion, in addition to visual and auditory stimuli [23]. Textural properties are most accurately measured by sensory analysis techniques that use panelists trained to detect and evaluate specific textural attributes such as “hardness” and “stickiness.” Indeed, a case has been made that texture is a sensory property that cannot be simply measured by analytical tests [24].

The modern approach of viewing foods as soft condensed materials has offered new avenues for probing the complex molecules and structures that provide the appearance, flavor and texture of foods [25].

Proteins and polysaccharides are widely employed as valuable elements to modulate the food rheology. The use of complex structures that are based on emulsions (single or multiple, fluid gels and air-filled emulsions) seems to offer an attractive range of tools to engineer healthier foods without compromising the organoleptic properties of the product [26].

1.4 Functional Foods and Nutraceuticals

Researchers generally agree that a growing body of evidence from epidemiology, clinical trials and modern nutritional biochemistry underlines the connection between diet and health. This impact is not only in the short term, but also in the development and management of chronic diseases [27]. In this regard, in the last decades, the terms “functional food” and “nutraceuticals” have been widely used to indicate products associated with foods which are proved to have physiological benefits and/or reduce the risk of chronic disease. Nevertheless, there is no universally accepted definition of functional food and nutraceuticals. According to a recent definition by Health Canada, a functional food is similar in appearance to, or may be, a conventional food that is consumed as part of a usual diet, and is demonstrated to show healthy benefits beyond basic nutritional functions, i.e. they contain bioactive compound. A nutraceutical is a product isolated or purified from foods that is generally sold in medicinal forms not usually associated with foods. A nutraceutical is demonstrated to have a physiological benefit or provide protection against chronic disease.

The main components of food are polymeric materials (e.g., polysaccharides and protein), and thus several different functional polymers have been proposed as functional foods. Researchers have also considered developing new functional foods containing bioactive phytochemicals and plant extracts to enhance food safety or incorporate the bioactive molecules to confer biological properties including antibacterial, antifungal, antiviral, antigenotoxic, anti-inflammatory, antiulcerogenic, cardioprotective, antiallergic, anticancer, chemopreventive, radioprotective, antioxidant, hepatoprotective, antidiarrheal, hypoglycemic and antidiabetic properties [28–30].

Another interesting research field is related to the use of biopolymers of natural origin for the delivery of active molecules to the gastrointestinal tract, in an effort to couple the nutritional properties of food with the bioactivity of released molecule [31].

In summary, it is possible to distinguish a wide range of effective approaches in steps taken by the food industry. In this volume we have summarized the key findings of the use of polymeric materials as functional elements for enhancing food production and processing.

References

1. Kinsey, J. Expectations and realities of the food system. In US Programs Affecting Food and Agricultural Marketing, Natural Resource Management and Policy, eds. W. J. Armbruster, R. D. Knutson, Springer Science + Business Media: New York, USA, 2013.

2. Kobayashi, S., and Makino, A. Enzymatic polymer synthesis: An opportunity for green polymer chemistry. Chemical Reviews 109:5288–5353, 2009.

3. Puoci, F., Iemma, F., Spizzirri, U. G., Cirillo, G., Curcio, M., Picci, N. Polymer in agriculture: A review. American Journal of Agricultural and Biological Sciences 3:299–314, 2008.

4. Kasirajan S., and Ngouajio, M. Polyethylene and biodegradable mulches for agricultural applications: A review. Agronomy for Sustainable Development 32:501–529, 2012.

5. Vroman, I., and Tighzert, L., Biodegradable polymers. Materials 2:307–344, 2009.

6. Poole, A. J., Church, J. S., Huson, M. G. Environmentally sustainable fibers from regenerated protein. Biomacromolecules 10:1–8, 2009.

7. Zohuriaan-Mehr, M. J., Omidian, H., Doroudiani, S., Kabiri, K. Advances in non-hygienic applications of superabsorbent hydrogel materials. Journal of Materials Science 45:5711–5735, 2010.

8. Zohuriaan-Mehr, M. J., and Kabiri, K. Superabsorbent polymer materials: A review. Iranian Polymer Journal 17:451–477, 2008.

9. Liu, M., Liang, R., Zhan, F., Liu, Z., Niu, A. Preparation of superabsorbent slow release nitrogen fertilizer by inverse suspension polymerization. Polymer International 56:729–737, 2007.

10. Wu, J. H., Wei, Y. L., Lin, J. M., Lin, S. B. Study on starch-graft-acrylamide/mineral powder superabsorbert composite. Polymer 44:6513–6520, 2003.

11. Mahdavinia, G. R., Pourjavadi, A., Hosseinzadeh, H., Zohuriaan, M. Modified chitosan 4: Superabsorbent hydrogel from poly (acrylic acid-co-acylamide) grafted chitosan with salt- and pH-responsiveness properties. European Polymer Journal 40:1399–1407, 2004.

12. Wang W., and Wang, A. Preparation, characterization and properties of superabsorbent nanocomposites based on natural guar gum and modified rectorite. Carbohydrate Polymers 77:891–897, 2009.

13. Kunioka, M., Biodegradable water absorbent synthesized from bacterial poly (aminoacid)s. Macromolecular Bioscience 4:324–329, 2004.

14. Kuzma, J. Moving forward responsibly: Oversight for the nanotechnology-biology interface. Journal of Nanoparticle Research 9:165–182, 2007.

15. Wu, S. C., Cao, Z. H., Li, Z. G., Cheung, K. C., Wong, M. H. Effects of biofertilizer containing N-fixer, P and K solubilizers and Am fungi on maize growth: A greenhouse trial. Geoderma 125:155–166, 2005.

16. Tokuyama, H., Hisaeda, J., Nii, S., Sakohara, S. Removal of heavy metal ions and humic acid from aqueous solutions by co-adsorption onto thermosensitive polymers. Separation and Purification Technology 71:83–88, 2010.

17. Iemma, F., Cirillo, G., Spizzirri, U. G., Puoci, F., Parisi, O. I., Picci, N. Removal of metal ions from aqueous solution by chelating polymeric microspheres bearing phytic acid derivatives. European Polymer Journal 44:1183–1190, 2008.

18. Canizares, P., Perez, A., Camarillo, R., Linares, J. J. Simulation of a continuous metal separation process by polymer enhanced ultrafiltration. Journal of Membrane Science 268:37–47, 2006.

19. Greiner, R., and Farouk, A.-E. Purification and characterization of a bacterial phytase whose properties make it exceptionally useful as a feed supplement. Protein Journal 26:467–474, 2007.

20. Verstraete, F. Risk management of undesirable substances in feed following updated risk assessments. Toxicology and Applied Pharmacology 270:230–247, 2013.

21. Curcio, M., Altimari, I., Spizzirri, U. G., Cirillo G., Vittorio, O., Puoci, F., Picci, N., Iemma, F. Biodegradable gelatin-based nanospheres as pH-responsive drug delivery systems. Journal of Nanoparticle Research 15:1581, 2013.

22. Foegeding, E. A. Rheology and sensory texture of biopolymer gels. Current Opinion in Colloid & Interface Science 12:242–250, 2007.

23. Lawless, H. T., and Heymann, H. Sensory Evaluation of Food, Principles and Practices. Chapman and Hall: New York, USA, 1998.

24. Murray, B. S. Rheological properties of protein films. Current Opinion in Colloid & Interface Science 16:27–35, 2011.

25. Norton, I. T., Frith, W. J., Ablett, S. Fluid gels, mixed fluid gels and satiety. Food Hydrocolloid 20:229–239, 2006.

26. Le Révérend, B. J. D., Norton, I. T., Cox, P. W., Spyropoulos, F. Colloidal aspects of eating. Current Opinion in Colloid & Interface Science 15:84–89, 2010.

27. www.agr.gc.ca

28. Iemma, F., Puoci, F., Curcio, M., Parisi, O. I., Cirillo, G., Spizzirri, U. G., Picci, N. Ferulic acid as a comonomer in the synthesis of a novel polymeric chain with biological properties. Journal of Applied Polymer Science 115:784–789, 2010.

29. Cirillo, G., Kraemer, K., Fuessel, S., Puoci, F., Curcio, M., Spizzirri, U. G., Altimari, I., Iemma, F. Biological activity of a gallic acid-gelatin conjugate. Biomacromolecules 11:3309–3315, 2010.

30. Wang, S., Marcone, M. F., Barbut, S., Lim, L.-T. Fortification of dietary biopolymers-based packaging material with bioactive plant extracts. Food Research International 49:80–91, 2012.

31. Cirillo, G., Iemma, F., Spizzirri, U. G., Puoci, F., Curcio, M., Parisi, O. I., Picci, N. Synthesis of stimuli-responsive microgels for in vitro release of diclofenac diethyl ammonium. Journal of Biomaterials Science, Polymer Ed 22:823–844, 2011.

Chapter 2

Polyacrylamide Addition to Soils: Impacts on Soil Structure and Stability

Guy J. Levy* and David N. Warrington

Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet-Dagan, Israel

*Corresponding author: [email protected]

Abstract

Maintaining adequate soil structure and stability, especially in soils from semiarid and arid regions that suffer from poor structural stability, is essential for sustaining productive agriculture and protecting the environment. This chapter considers the potential of polyacrylamide (PAM, a synthetic organic polymer), as a soil-stabilizing agent. Addition of PAM to aggregates at the soil surface leads to their stabilization and to improved bonding between adjacent aggregates that, in turn, improves soil physical and hydraulic properties. The results reported within this chapter clearly demonstrate that addition of PAM to soil beneficially modifies soil properties associated with the degradation of its stability. Therefore, amending soils with PAM can be considered as a viable option for enhancing and/or maintaining soil-structural stability.

Keywords: Polyacrylamide, soil amendments, soil-structure stability, soil aggregates, electrolyte concentration, soil hydraulic conductivity, infiltration rate, soil erosion

2.1 Introduction

Soil structural stability describes the ability of soil to retain its arrangement of solid and void space when exposed to different stresses (e.g., tillage, traffic, wetting and drying cycles), and can be expressed quantitatively by numerous indicators, such as aggregate stability, infiltration rate, soil erosion, etc. From an agricultural point of view, soil structure strongly affects the ability of soil to support plant growth, cycle nutrients and carbon, hold and transmit water and prevent soil erosion. This chapter concentrates on soils from semiarid and arid zones because these soils commonly exhibit poor structural stability, which often leads to runoff, soil erosion and loss of soil productivity. Poor structural stability is mainly related to elevated levels of sodicity and low organic matter content. Soils in these regions are widely cultivated (e.g., western US, the Mediterranean region and large areas in India and Australia) despite the scarcity of water. Safeguarding the stability of soil structure is therefore critical to maintaining sustainable agriculture and conserving soil and water in cultivated lands in semiarid and arid regions.

Traditional strategies for preventing degradation of soil structure and its stability, mainly by rainwater and/or overhead irrigation water, include measures that (i) protect the soil surface from raindrop impact to prevent seal formation and soil detachment (e.g., mulching, cover crops), (ii) increase surface depression storage and soil roughness to reduce runoff volume and velocity, and (iii) alter slope-length gradient and direction of surface runoff flow.

An alternative approach to maintaining and possibly even improving soil-structure stability is one that advocates modification of those soil properties that are associated with the processes that control the degradation of soil stability. By increasing the stability of surface aggregates and preventing clay dispersion, it is possible to maintain soil hydraulic conductivity (HC) and to reduce soil susceptibility to sealing, thereby reducing runoff and erosion. This chapter discusses the potential contribution of polyacrylamide (PAM, a synthetic, environmentally friendly and nontoxic organic polymer), to the stabilization of soil structure. The contribution of this amendment to soil-structure stability is demonstrated via its effects on aggregate stability, soil permeability and soil erosion.

2.2 Polyacrylamide (PAM) Properties and Interactions with Soil

Use of synthetic organic polymers as soil conditioners began in the 1950s. Numerous reviews have discussed the use of organic polymers as soil conditioners in general [1,2] and for improving soil structure and physical properties in particular [3–9]. Of the various polymers studied over the years, water-soluble (i.e., non-crosslinked) polyacrylamide (PAM) has received considerable attention with respect to its impact on various facets associated with soil structure and stability. Furthermore, in the last two decades PAM has been used intensively in commercial fields for controlling irrigation-induced erosion and enhancing infiltration [9].

In general, polymers are small recognized repeating units (monomers) coupled together to form extended chains. Polymers are characterized mainly by their molecular weight, molecular conformation (coiled or stretched), type of charge, and charge density. PAM is a homopolymer compound formed by the polymerization of identical acrylamide and related monomers [10]. Its molecular weight varies in the range of 10 × 104 Da (medium molecular weight) to 20 × 106 Da (high molecular weight). In the latter case, PAM comprises 1–2 × 105 repeating units, each with a molecular weight of 71 Da [10]. PAM can be cationic, nonionic or anionic, with the anionic form being most commonly used for soil conditioning. PAM formulations with proportions of charged comonomer of <10%, 10%–30%, and >30% are considered to be of low- medium- and high-charge density, respectively.

Cationic PAM is actually the most effective form of PAM for soil stabilization due to the ease with which it binds to soil particles (which are mostly negatively charged), but has known issues with water contamination. Since PAM may contain residual amounts of, or degrade to produce, acrylamide, which is a nerve agent, only food-grade anionic PAM is recommended for use in agricultural soils [9]. This type of PAM has very little residual acrylamide (<0.05%). Furthermore, the relatively small quantities used present little risk of contamination to the environment especially as the anionic PAM is usually not desorbed from the soil and would not likely enter the water system [11]. Studies have also shown that PAM itself does not pose a threat to human health [4] and that even when subject to thermal degradation while in liquid form does not release the harmful acrylamide in levels that would cause drinking water to exceed the US Environmental Protection Agency standards of 0.5 ppb [12]. Therefore, anionic PAM soil stabilizers are considered to be safe and environmentally friendly.

2.2.1 Polymer-Clay Interactions

The relationship between polymer treatment and soil physical and hydraulic properties is greatly affected by the adsorptive behavior of the polymer molecules on soil particles. The extensive studies of polymer adsorption to clay minerals were motivated by the understanding that clays are the reactive fraction of the soil; their data have been summarized in several reviews [13–15]. Of the various polymer properties (molecular weight and conformation, charge type and density), it is the type of charge that largely determines the mechanisms controlling polymer adsorption and thus it has received a great deal of attention [14,15].

Negatively charged polymers tend to be repelled from the clay surface, and little adsorption occurs. In addition, anionic polymers do not tend to enter the interlayer space of expanding minerals [16]. Adsorption is promoted by the presence of polyvalent cations, which act as “bridges” between the anionic groups on the polymer and the negatively charged sites on the clay [17]. Increasing the ionic strength of the soil or polymer solution reduces the electrostatic repulsion between the polyanion and the clay surfaces [16,18], and may also lead to decreased polymer charge and size [18], both of which enhance adsorption of polyanions [19,20]. Acidic conditions, associated with an increase in positive edge sites on clays, also favor polyanion adsorption [15].

The amount of polymer adsorbed by clay depends not only on the type of polymer charge but also on the type of clay mineral. Only small quantities of anionic PAM are adsorbed on Na-montmorillonite, 2 to 3 g kg−1[19,23]. By comparison, Wyoming Na-montmorillonite adsorbs 45 g kg−1 of nonionic PAM [19]. Stutzmann and Siffert [23] postulated that the small quantity of PAM adsorbed on clay results from the fact that PAM is adsorbed only on the external surfaces of the clay particles (rather than over the total surface area of the clay particles), and that PAM adsorption therefore depends on the cation exchange capacity of the external surfaces of the clay mineral. Ben-Hur et al. [24] and Deng et al. [22] observed a considerably larger adsorption of anionic PAM by illite than by montmorillonite under nonacidic conditions; no differences in adsorption of the anionic PAM by the two clays were noted for pH < 7 [22]. It was postulated that polymer adsorption depends on the external charge density of the adsorbing clay rather than on the adsorbing clay’s cation exchange capacity [24].

Polymer added to a colloidal suspension can act as a dispersant or flocculent, depending on its properties and the electrolyte concentration in the suspension [25]. Anionic PAM enhanced clay suspension stability in solutions having electrical conductivity <0.05 dS m−1[19,22]; however, it promoted flocculation in solutions with electrical conductivity of 0.7 dS m−1 for polymer concentrations >5 g m−3 [19]. Flocculation by anionic PAM was explained by cation bridging and osmotic attraction [19]. Molecular properties (molecular weight and molecular charge density) of PAM may interact in affecting its efficacy at flocculating soil clay. Increasing PAM molecular weight increases the length of the polymer chain; the longer the molecules, the more strongly the molecules adsorb on the mineral surfaces and thus the more effective the PAM molecules are as a flocculent [26]. Moreover, Heller and Keren [27], who studied the rheological behavior of Na-montmorillonite suspensions, reported that the higher the molecular weight of PAM, the more effective its ability to stabilize flocs of clay in a clay suspension that was free of electrolytes. Similarly, PAM with 20% hydrolysis provided the greatest degree of charge and chain extension, facilitating adsorption [10]. Thus, it could be expected that PAM with high molecular weight would be effective at stabilizing soil surface aggregates and thus would reduce seal formation, runoff and erosion.

2.2.2 Polymer-Bulk Soil Interactions

Polymer adsorption on clay material was considered an accurate representation of polymer adsorption on soil. However, a review of studies devoted to polymer adsorption on soils [28] pointed out that the aforementioned supposition was not necessarily valid. Nadler and Letey [29] studied the adsorption of three anionic polymers by a coarse loamy soil. Similar to anionic polymer-clay systems [19,20], Ca soils and initial high pH resulted in increased polymer adsorption; the observed increase, however, was very small. The adsorption levels were in the range of mg of polymer per kg of soil, being two to three orders of magnitude lower than those reported by Aly and Letey [19] for the same polymers on montmorillonitic clay. It was postulated that (i) higher specific surface area and amount of charge available for adsorption associated with the clay size fraction of the montmorillonite, and (ii) smaller accessible and active surfaces in the soil due to the presence of organic matter and aggregation are responsible for the lower adsorption of the polymers by the soil material as compared with the clay [29].

Some uncertainty exists regarding the issue of whether PAM penetrates into aggregates or adsorbs only on the aggregates’ exterior surfaces, and thus stabilizes merely those surfaces. Malik and Letey [26] studied adsorption of high-molecular-weight anionic PAM (107 Da) by soil, the clay size fraction extracted from soil and washed quartz. Adsorption of PAM on soils and washed quartz was similar and three orders of magnitude lower than that on the extracted clay size fraction. Malik and Letey [26] and Mamedov et al. [30], who used both high- and medium-molecular-weight PAM (12 × 106 and 2 × 105 Da, respectively), observed that PAM adsorption on soils is mostly on exterior surfaces of soil material, and that PAM does not penetrate into aggregates. Conversely, the study of Shaviv et al. [31] on low-molecular-weight PAM and those of Miller et al. [32] and Levy and Miller [33] on high-molecular-weight PAM showed that the PAM does penetrate into pores within aggregates. Lu and Wu [34] suggested that the depth of PAM penetration into soil aggregates depends on its properties, method of application and the properties of the soil and water used. Levy and Miller [33] emphasized the aspect of scale: when high-molecular-weight PAM is used, the narrow pores in small-size aggregates (<1 mm) may not allow penetration of the large PAM molecules into the aggregates, while the opposite is true for large aggregates having greater macro-porosity and/or intra-aggregate porosity.

Desorption of polymers from soils rarely occurs. Nadler et al. [11] measured desorption of PAM from soil material. Very little or no desorption occurred if the soil was kept wet. Moreover, upon drying, most or all of the polymer initially left in the solution became irreversibly bonded to the soil [11]. It was considered unlikely that all segments of the polymer can be simultaneously detached from the soil surface, and remain detached long enough for the polymer to move away from the surface to the bulk solution [29].

2.3 Polymer Effects on Aggregate Stability

Soil aggregates may be subjected to stresses related to tillage, traffic, abrasion by flowing water, and wetting and drying cycles. The ability of aggregates to resist stresses when wet (i.e., wet aggregate stability), was originally used to characterize soil erodibility [35]. However, aggregate stability has since been increasingly used to evaluate the cohesion of aggregates and the dynamics and nature of bonding between particles [36], all of which are major contributors to soil structure stability.

Polymer addition to soil leads to stabilization of existing aggregates and improved bonding between, and aggregation of, adjacent soil particles [37,38]. The stabilizing efficacy of polymers is greatly affected by the adsorptive behavior of their molecules [26,29], and by their charge density in the case of PAM [39]. Shainberg et al. [40] noted that addition of PAM effectively stabilized the aggregates in three Israeli semiarid soils with low (~2) and high (>10) exchangeable sodium percentage (ESP). Nadler et al. [41] studied low rates of PAM application (25–75 mg polymer kg−1 soil) on the stability of both dry and wet aggregates of a semiarid, sandy loam soil. Improvement in stability was observed for both the dry and wet aggregates, the magnitude of which depended upon polymer charge density, soil moisture content, and type of exchangeable ion (Na vs Ca). In predominantly kaolinitic soils of varying texture [32] and aggregate size [33], addition of PAM significantly increased the percentage of stable aggregates compared with untreated aggregates.

Green et al. [42] evaluated the effects of PAM molecular weight on its ability to stabilize aggregates from three soils, and found only minimal differences in aggregate stability among the various PAM formulations studied (Table 2.1). Conversely, Mamedov et al. [30], who compared the impact of high-molecular-weight PAM (12 × 106 Da) to that of a medium-molecular-weight PAM (2 × 105 Da) on the stability of four smectitic soils, concluded that neither of the two polymers could be singled out as preferable since their effects varied among the soils used and depended on initial aggregate size and solution ionic strength. Furthermore, these authors concluded that in order to enhance aggregate stability it is enough to stabilize the exterior surfaces of the aggregates with PAM; PAM molecules that entered into the aggregates’ pores did not appear to have any significant impact on aggregate stability. In an additional study, Mamedov et al. [43] observed that the effectiveness of PAM in improving aggregate stability in soils varying in clay mineralogy followed in the order of kaolinitic < illitic < smectitic soils. Conversely, for the non-treated aggregates, aggregate stability decreased in the order of kaolinitic > illitic > smectitic soils. Mamedov et al. [43] concluded that the efficacy of PAM in improving aggregate stability is inversely related to the inherent stability of the aggregates (as dictated by clay mineralogy). However, unlike clay mineralogy, soil texture (represented by changes in clay content) did not affect the ability of PAM to stabilize aggregates, particularly in illitic and kaolinitic soils [43].

Table 2.1 Effects of polyacrylamide treatments (three levels of charge density, 20%, 30% and 40%; three molecular weights, 106, 1012 and 1018 Da) on slaking index (SI) and aggregate stabilization index (ASI) of three soils (from Green et al. [42]).

2.4 PAM Effects on Soil Saturated Hydraulic Conductivity

The hydraulic conductivity (HC) of a soil is a measure of the soil’s ability to transmit water when submitted to a hydraulic gradient. Hydraulic conductivity is defined by Darcy’s law that, for one-dimensional vertical flow, can be written as follows [44]:

(4.1)

where q is the flux (or the mean velocity of the soil fluid through a geometric cross-sectional area within the soil; LT−1), Δh is the hydraulic head (L), and z is the vertical distance in the soil (L). The coefficient of proportionality, K, in Equation 4.1, is called the hydraulic conductivity (LT−1). The HC depends, among other things, on the relative amount of soil fluid (i.e., degree of saturation) present in the soil matrix. Herein, the term HC refers to HC under saturated conditions.

Leaching an inert medium (e.g., sand) with a PAM solution leads to a decrease in its HC; this phenomenon was postulated to be due to the greater viscosity of PAM solutions in comparison to water [45]. It was further noted that (i) even PAM concentrations as low as 50 mg L−1 can lead to a substantial decrease in HC, and (ii) the finer the medium, the greater the impact of the PAM solution on the HC [45]. Conversely, Gardiner and Sun [46] observed that leaching soils with wastewater containing PAM at rates of ≤ 40 mg L−1 results in higher HC values than leaching the soils with wastewater alone. This apparent discrepancy can be explained by the fact that in the presence of electrolytes (e.g., in wastewater), the adverse effects of PAM are alleviated, especially when the electrolytes contain Ca salts [47]. It was further suggested that in the presence of electrolytes, the PAM molecules coil and form short loops that are less effective at clogging the soil pores than the long polymer chains in their uncoiled conformation and subsequently at reducing water movement through them [47–50]. Shainberg et al. [50] proposed that the configuration of the unadsorbed segments of the PAM chains in the soil is not rigid and may vary according to the quality of the leaching solution, thus dictating the effects of PAM on the HC.

In general, three different types of studies have been conducted to evaluate the impact of PAM on soil HC: (i) initially treating the soils with a PAM solution, and only after that determining the HC during leaching with water; (ii) evaluation of the HC of the soil based on actual leaching of the soil with PAM solutions; and (iii) PAM was added to the soil in the form of dry grains prior to leaching the soil.

In the first type of experiments, the effects of PAM have been found to be inconsistent. El-Morsy et al. [51] observed, for a sandy loam, that the efficiency of the PAM treatment in maintaining high HC increases with the increase in the electrolyte concentration of the pretreatment solution. They suggested that the addition of PAM promotes aggregate stability during the drying phase, which subsequently contributes to higher HC values [51]. For a heavy-textured high shrink-swell soil, PAM was useful in increasing the soil HC if added to a dry cracked soil; if the PAM was added to a soil without cracks, little or no increase in HC was noted [52]. These authors concluded that in the case of a cracked soil, addition of PAM stabilizes the cracks and prevents them from closing during water flow, thereby enabling faster water flow through the soil. Furthermore, Zahow and Amrhein [53], who evaluated the contribution of PAM addition to reclaiming the HC of saline-sodic soils, showed that PAM is effective in increasing the HC only in soils with ESP < 15. Combined application of PAM and gypsum also increased the HC of a soil with an ESP of 32. Zahow and Amrhein [53] postulated that at low ESP, PAM prevents aggregate slaking and thus improves HC. At high ESP, however, PAM alone is ineffective at maintaining high HC because clay swelling controls the reduction in HC. Some additional studies highlighted the importance of drying of the soil between the application of PAM and the subsequent leaching (Figure 2.1). It was observed that pretreating the soil with a PAM solution (3 20 mg L−1) decreased its HC during immediate subsequent leaching with deionized water [47,49,50]. Yet, when the soil was allowed to dry between PAM application and leaching with deionized water, the obtained HC values were similar to, or higher than, those of a soil that had not been treated with PAM [47,50]. These findings suggest that when the PAM solution is not allowed to dry, its presence in the form of a concentrated solution in the soil hinders water flow through the pores. Conversely, upon drying the degree of PAM adsorption on the soil particles increases, thus leaving fewer segments of the polymer chain free and available to take up pore space; consequently, water flow is less restricted and a higher HC is maintained [50].

In the second type of experiments, where the HC of the soil was measured during leaching of the soil with PAM solutions, the reported effects of PAM on soil HC varied. Santos and Serralheiro [54] observed a 168% increase in the saturated HC of Mediterranean, loamy sand soil during leaching with a solution containing low concentrations of PAM (1 and 10 mg L−1). Kim [55] observed that addition of PAM to waste drilling fluid improves its HC, and consequently affects the speedy leaching of excessive salts; the addition of PAM therefore enabled the safe land disposal of this waste fluid. However, the favorable effect of PAM on increasing HC seemed to diminish quickly in subsequent applications of water/solution that did not contain PAM [46]. Unlike the former studies, Ajwa and Trout [47] and Shainberg et al. [50] reported that leaching soils with dilute PAM solutions (≤25 mg L−1) prepared with deionized water, resulted in a two- to three-fold decrease in the soil’s HC.

In the third type of experiments, addition of PAM to the surfaces of sand or soil columns in the form of dry grains (5.6–44 kg ha−1) and leaching with either a dilute CaSO4 solution [56] or deionized water [50] (thus mimicking conditions for natural rain), resulted in a significant decrease in the HC of both sand and soil columns compared with the HC obtained with no addition of PAM to the soil. However, when the added dry PAM grains were mixed with a dry source of electrolytes (e.g., phosphogypsum grains) the resultant HC was higher than that obtained when the mixture of PAM and phosphogypsum was not added (Figure 2.1) [50]. These authors maintained that in the absence of electrolytes, the sharp decrease in the HC resulted from unadsorbed segments of the dissolved PAM that extended from the solid particles to the pores and thus hindered flow of water. In the presence of a source of electrolytes, the unadsorbed segments of the dissolved PAM were coiled and therefore the interference with water movement in the soil pores was reduced [50].

2.5 PAM Effects on Infiltration, Runoff and Erosion

Soil infiltration rate is defined as the volume flux of water flowing into the soil profile per unit surface area, and has dimensions of velocity (LT−1). In many cultivated soils worldwide, and in particular in semiarid and arid soils, a decrease in soil infiltration rate is a common phenomenon often resulting from gradual deterioration of soil surface structure and the formation of a seal at the soil surface [57]. Seal formation is the result of two complementary mechanisms [58,59]: (i) physical disintegration of surface soil aggregates and subsequent compaction of the disintegrated aggregates by raindrop impact, and (ii) physicochemical dispersion of soil clays. Surface seals are thin (<2 mm) and are characterized by greater density, higher shear strength, finer pores and lower saturated hydraulic conductivity than the underlying soil [58,60]. Consequently, seals have major effects on numerous soil phenomena, e.g., decreased infiltration and increased runoff and erosion. Concerning the latter, water erosion begins when a water drop strikes the bare soil, detaching soil particles from the surface with subsequent transport downslope by raindrop splash or by overland flow (runoff). If runoff is in thin sheets, sheet flow and sheet (or interrill) erosion occurs. When water velocity increases in excess of 0.30 m s−1[61], flow becomes turbulent and causes rills to form.

Use of PAM to maintain high infiltration rates and to control runoff and soil erosion has been studied extensively under conditions of both rain and irrigation (furrow and overhead). PAM can be applied via the irrigation water [62–64], by spraying a PAM solution onto the soil surface [65–67], or as a dry powder that is spread on the soil surface and subsequently dissolved by rain or irrigation water [9,68–71]. Spreading dry PAM on the soil surface has the advantages of low shipping costs and long shelf life, and it avoids the difficulty involved in dissolving dry PAM in irrigation water and eliminates the need to handle the resultant viscous PAM solution [10]. In the case of rain or overhead irrigation, addition of PAM to the soil mitigates the adverse effects of the beating action of the water drops on the surface aggregates. Conversely, in furrow irrigation, PAM is added mainly to enhance the stability of the surface aggregates against the shearing force of the flowing water, thereby preventing furrow erosion. For clarity, the following discussion is divided according to these two distinct conditions for which PAM is used.

2.5.1 Furrow Irrigation

Although surface irrigation is the most commonly used irrigation practice worldwide, its water-use efficiency is low [72]. Furthermore, erosion caused by surface/furrow irrigation is a severe problem in millions of hectares of irrigated cropland [73]. Since the early 1990s, wide attention has been given to the use of polymers, mainly PAM, to control erosion and enhance infiltration in furrow irrigation [7,74,75]. Laboratory and field tests, mostly in the US, have clearly indicated that use of PAM in furrow irrigation leads to a nearly complete prevention of erosion and to a significant increase in water infiltration [64,74].

The beneficial effects of PAM application in furrow irrigation are attributed to higher infiltration, less runoff, slower stream flow, less soil detachment and reduced sediment-transport capacity [76,77]. The effectiveness of PAM, coupled with its relative ease of application, particularly in furrow irrigation, has resulted in rapid acceptance of the technology in the US, with over 400,000 ha of irrigated land employing PAM for erosion and/or infiltration management [78]. The decrease in runoff water and sediment load following PAM addition also greatly reduces the removal of nutrients, pesticides, and oxidizable organic substances from the field, and thus reduces the contamination of surface water bodies [79,80].

The rapid acceptance of this technology can be attributed to several reasons. The amount of PAM required to control erosion in furrow irrigation is <1.0 kg ha−1[64], with effective concentrations of PAM in irrigation water being 2–10 mg L−1[81]. These amounts are significantly smaller than the amounts of PAM required to control sealing and erosion in dryland farming. The difference in the amounts of PAM needed may possibly stem from the fact that the shearing force of water flowing in furrows is 200-fold less than the kinetic energy of raindrops [82]. Versatility in the method of PAM application also adds to its appeal. The favorable effects of adding dry grains of PAM to the gated irrigation pipe in order to prevent erosion and increase filtration are comparable to those achieved by adding a stock solution of PAM to the furrow inflows [64].

For PAM application to be effective in controlling furrow erosion, it has to be added with the first irrigation on a newly tilled field. Following initial PAM treatment, erosion in subsequent irrigations can usually be controlled with much less than 10 mg L−1 (1 to 5 mg L−1 PAM) if the soil has not been disturbed between irrigations. If the soil remains undisturbed between irrigations and PAM is not reapplied, erosion control is typically reduced by half [78]. On the other hand, application of PAM to surfaces containing aggregates that have undergone slaking (e.g., by raindrops) will also minimize its effectiveness [7].

The effects of PAM on water infiltration in furrow irrigation are somewhat complex, and seem to be affected by soil texture. Presence of PAM in the irrigation water flocculates suspended fine particles (clay and silt particles) that enter the water, and causes them to settle at the bottom of the furrow [77]. In medium- to fine-textured soils, PAM application was noted to enhance infiltration, while in coarse-textured soils, a decrease in infiltration was noted upon use of PAM [77]. Conversely, a study in a Mediterranean loamy sand soil showed that furrow irrigation with a 10 mg L−1 PAM solution improved infiltration by 14% to 20% [54].

Based on results from laboratory studies, Bhardwaj et al. [83] proposed that in medium- to fine-textured soils, PAM is effective in improving soil permeability in furrows because it leads to the flocculation of the suspended material into larger particles, which subsequently settle to form a less dense and more permeable layer on the soil surface compared with that forming in non-treated furrows. In coarse-textured soils, the possible lack of success of PAM in improving the permeability of the soil may stem from probable accumulation of the flocculated material in the pores in the upper few mm of the coarse-textured soil, thus forming a layer with a permeability that is lower than that of non-treated furrows.

2.5.2 Rain and Overhead Irrigation Conditions

Studies on the efficacy of surface application of polymers (whether in the form of dry grains or by spraying a concentrated PAM solution) have shown that in the case of the commonly used anionic high-molecular-weight PAM, addition of 10 to 40 kg PAM ha−1 is required to maintain a significantly higher infiltration rate and lower levels of runoff and erosion in soils exposed to rain as compared with non-treated soils [65,84–86]. For the control of runoff and erosion in sprinkler irrigation, required amounts are somewhat lower, less than 5 kg PAM ha−1[87], than those reported for rain conditions. The difference in the required amount of PAM may stem from the fact that the kinetic energy of raindrops is usually higher than that of waterdrops in sprinkler irrigation.

However, it is important to note that under rain conditions, PAM addition to the soil surface as dry grains was effective in controlling soil erosion, but was ineffective in controlling runoff (Figure 2.2) [69,70]. Yu et al. [70] suggested that, upon dissolution of the PAM grains by the rainwater, PAM molecules adsorb on the surface aggregates, binding them together to form a cohesive surface that better resists particle detachment by the raindrops and thus reduces soil erosion. At the same time, PAM molecule segments that do not adsorb on the surface particles instead block the water conducting pores, resulting in the observed lower infiltration rates (leading to higher runoff levels), which is therefore due to a reduction in soil hydraulic conductivity rather than to seal formation [70].

Figure 2.2 Soil loss and total runoff for untreated (control) or PAM-treated (20 kg ha−1) soil samples of a silty loam and a sandy clay. Within a soil type and type of measurement, bars labeled with the same letter do not differ significantly at the p < 0.05 level.

(Modified from Yu et al. [70]).

Studies in which a surface application of PAM (irrespective of the method of application) was supplemented with gypsum showed a significantly higher final infiltration rate, and lower runoff and erosion, than those in which either no amendments (control) or an application of only one of either of the amendments were used [62,65,68–71,88,89]. In the presence of electrolytes (e.g., gypsum solution), the negative charge and the thickness of the diffuse double layer at the clay and polymer surfaces are suppressed, resulting in decreased repulsion forces and greater adsorption of soil particles to the anionic polymer [28,90]. In addition, the dissolved gypsum increases the electrolyte concentration in the soil solution to values above the flocculation value of the soil clay [91]. The latter has been reported to be effective in enhancing the cementing and stabilization of aggregates at the soil surface by PAM [92–95]. Moreover, it was reported that application of PAM in combination with gypsum is very effective at increasing infiltration and reducing runoff and erosion in smectitic soils having high levels of exchangeable sodium, i.e., highly dispersable soils [71], as well as in a kaolinitic acidic soil [96]. It can, therefore, be concluded that the effectiveness of treating the soil with a combination of PAM and gypsum in controlling seal formation and runoff is related to the reduction of both the physical disintegration of surface aggregates by PAM and the chemical dispersion of the soil clay by gypsum [62,65].

The kinetic energy of water drops also influences the efficacy of PAM in controlling seal formation, runoff and erosion [62,92]. For water drops with high kinetic energy (>10 kJ m−3), the use of PAM to prevent aggregate breakdown by the beating impact of water drops was found to be essential. Conversely, for drops having low kinetic energy (<5 kJ m−3), effects of gypsum and PAM + gypsum on decreasing runoff were similar, suggesting that under conditions of water drops with low kinetic energy, clay dispersion (counteracted by gypsum) is more important in the sealing process than aggregate disintegration by the drops’ energy (counteracted by PAM).

Contrary to the studies on clay material, which showed a preference for the use of high-molecular-weight PAM, investigations into the response of soil to applications of PAMs with different molecular weights have yielded varying results. Vacher et al. [66] noted that PAM with a high molecular weight (>106 Da) was more effective than its low-molecular-weight counterpart (<105 Da) in controlling soil erosion. Green et al. [42,84], who compared PAMs with various molecular weights (6–18 × 106 Da), and Levy and Agassi [93], who compared two PAMs with different molecular weights (2 × 105 and 2 × 107 Da), found that the molecular weight of the PAM either has no effect on the ability of the various PAMs to maintain high infiltration and low erosion levels, or has an effect that is dependent on soil type. Unlike these two studies, in which PAM was added to the soil surface in the form of a solution, a study comparing the effects of surface application of grains of two PAMs (2 × 105 Da and 1.2 × 107 Da) together with phosphogypsum on runoff and erosion from three smectitic soils [97] showed that the effects of the two PAM formulations on runoff, relative to each other, depended on soil type; however, dry PAM of moderate molecular weight was more effective at reducing soil loss than dry PAM of high molecular weight, irrespective of soil type (Figure 2.3). It seems that the inconsistencies regarding the effects of PAM molecular weight may arise from differences in the methods of PAM application (dry vs dissolved) and soil type.

Figure 2.3 Soil loss for three different soil types as a function of the following treatments: control (no addition of polyacrylamide or phosphogypsum); 4 Mg ha−1 phosphogypsum (PG); 20 kg ha−1 polyacrylamide with high molecular weight (PAM(H)); 20 kg ha−1 polyacrylamide with moderate molecular weight (PAM(M)). Within a soil type, bars labeled with the same letter are not significantly different at p < 0.05.

(Modified from Mamedov et al. [97]).

Conflicting evidence exists regarding the residual effects of polymers, especially when added via the irrigation water. Aase et al. [87] reported that addition of PAM via irrigation water at rates of 10 to 30 mg L−1 was effective at maintaining high infiltration rates and low levels of soil erosion in a few subsequent irrigations, with the residual effect of PAM being more pronounced in reducing erosion than runoff. Conversely, Levy et al. [98] observed that addition of low concentrations (10–20 mg L−1) of PAM in the irrigation water had no effect on infiltration rate or soil erosion in subsequent irrigations with water alone. Gardiner and Sun [46] also noted that the benefits incurred from a single PAM application disappear shortly thereafter. They suggested that an alternate PAM application (i.e., every second irrigation) might be a practical approach for improving infiltration, since its addition to every irrigation is not feasible due to poor infiltration resulting from the high viscosity of the PAM-enriched water.

2.6 Concluding Comments