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Synthetic polymer hydrogels and modified natural polymer hydrogels are widely and increasingly used in agriculture, health care textiles, effluent treatment, drug delivery, tissue engineering, civil concrete structure, etc. Among them, the use of hydrogels in agricultural and horticultural sectors as matrices for the controlled release of water, various primary and secondary nutrients has drawn significant attraction from researchers, scientists, and industry persons due to their smartness with reference to controlled release characteristics based on plant requirement. Since the use of these hydrogels for controlled release application ensures the minimum utilization of water and plant nutrients in fields. Besides, this will bring down the overloading of fertilizer, soil contamination, and water pollution such as eutrophication, nitrate pollution, and micronutrient imbalance. This chapter is focused on the class of hydrogels that are used for the controlled release application in the agricultural and horticultural sectors as matrices, the possible methods of fine-tuning their structures for improving their fertilizer uptake and release behavior, safety aspects, and environmental issues.
Department of Chemistry, Sri Ramakrishana Mission Vidyalaya College of Arts and Science, India
Subhapriya Pushparaju
Department of Chemistry, Bannari Amman Institute of Technology, India
*Address all correspondence to: vdhanachemist@gmail.com
1. Introduction
The smart polymers hydrogels are the class of functional polymers, which finds extensive use [1, 2] in diverse areas like agricultural, medical, pharmaceutical, effluent treatment, textile, etc. They have physicochemically crosslinked three-dimensional network, which are derived from water-soluble acrylic monomers, crosslinkers and natural pre-polymers. These smart hydrogels are capable of imbibing and retaining water or aqueous fluids such as urine, blood, electrolyte solution, etc. to the extent of 200 g to 1–2 kg of fluids without dissolving [3, 4, 5]. This hydrophilic nature of hydrogel leads to managing drought conditions in arid and semi-arid regions as a matrix for the controlled release of water and fertilizers [5]. To serve this, polymers with different chemical architecture are essential for diverse soil characteristics [5].
Agrochemicals such as primary and secondary fertilizers are used to hike crop yield with substantial quality foodstuff [6]. However, the traditional method of growing foodstuffs using synthetic fertilizers will not ensure a high-quality environment [6]. Depending on the method of application and climatic conditions, about 90% of conventionally applied fertilizers never reach their objectives to realize the desirable biological response at the precise time and in the quantities required [6]. Such a mode of application provides a higher initial concentration than required for quick results. The conventional method of fertilizer amendment provides an initial concentration far above that required for immediate results to ensure the availability of sufficient nutrients. But such overdosing will result in waste of fertilizers [6] and produce undesirable side effects in the environment. Hence, there is a need for more controlled application of fertilizer, affording lower amounts of active ingredients without diminishing the efficacy. Controlled-release formulations were used to maintain an effective local concentration of active ingredients in the soil and to reduce runoff [6]. Besides, the application frequency required in the growing season could be minimized through controlled release technology. The controlled release was defined [6] as a technique or a method by which water or active chemicals were made available to a specified target at a definite rate and duration designed to accomplish an intended effect [7, 8, 9, 10, 11, 12, 13, 14, 15]. The method of choice to achieve controlled release in a particular application depends on the cost, release rate, potency and properties of the active compounds [14, 15, 16]. This chapter addresses the synthesis, characterization and controlled release applications of synthetic and natural polymer modified hydrogels in agriculture as matrices [16], different types of hydrogel used for controlled release, advantages, limitations and challenges.
The smart hydrogels with controlled release characteristics have been prepared either from water-soluble acrylic monomers, crosslinkers and modified natural polymer by grafting.
2.1 From hydrophilic monomers
The hydrogels with good swelling ability are synthesized from water-soluble hydrophilic acrylic monomers such as acrylamide, acrylic acid, acrylates, itaconic acid, etc. using suitable initiators and crosslinkers through radical or photochemical polymerization methods [17]. This will be achieved either by solution or suspension or emulsion or bulk polymerization methods [17]. Free-radical polymerization mechanism is predominantly employed to synthesize hydrogel using olefinic monomers. The initiation of monomers is carried out by the initiators such as peroxides (benzoyl or t-butyl peroxides), azo-compounds (azobisisobutyronitrile) and persulphates. Peroxides and peroxy compounds can facilitate ambient temperature polymerization under the influence of tetramethylene diamine, sodium metabisulfite/ferrous salts, triethylamine, etc. [17, 18]. Benzyl alcohol, ethanol, water, and ethanol-water mixtures are commonly used solvents to achieve solution polymerization. The monomers (Table 1), cross-linkers (Table 2) and natural polymers (Table 3) that are used for hydrogel synthesis are given in the respective Tables.
Table 1.
Typical monomers used for hydrogel synthesis.
Table 2.
Typical crosslinkers used for hydrogel synthesis.
Table 3.
The representative natural pre polymers used for hydrogel synthesis.
2.2 Modification of natural pre-polymers
Water swellable hydrophilic hydrogel polymer can also be synthesized by performing appropriate chemical modification of natural polymers such as gelatin, starch, alginate, cellulose, chitosan, pectin, etc. via grafting using acrylic monomers. In-situ incorporation of micronutrient (boron) on acrylic acid grafted guar gum-based hydrogel [19], acrylic monomers grafted chitosan hydrogel [20], urea loaded cellulose [21], carboxymethylcellulose-hydroxyethylcellulose cross-linked with citric acid [22], etc. can also be used as matrices for the controlled release of fertilizers and water in agricultural field.
The potential applicability [23] of smart polymers are gauged based on their chemical structure, the extent of chemical and physical crosslinking, crosslink density, mechanical properties, degrees of swelling (hydrophilicity), release characteristics, hydrophobicity, surface morphology, biodegradability, biocompatibility, glass-transition temperature, thermal stability, photo-stability, bio-resorbability, interaction with biological fluids, environmental sensitivity, dielectric properties, toxicity, the toxicity of the degraded products, etc. For instance, the nature of functional groups, crystallization deformation of polymers, biodegradation, moisture uptake properties, nature of interactions between components are evaluated using Fourier Transform Infrared Analysis (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy. The modification after polymerization such as chemical composition, grain size, the extent of crosslinking, pore size, pore volume are evaluated using Atomic Force Microscopy (AFM) or Scanning Electron Microscopy (SEM) and X-ray diffraction analysis. The oxidative thermal degradation, glass transition temperature, lifetime prediction, melting point, etc. are assessed through Thermal Analysis (TGA and DSC). The mechanical characteristics such as tensile strength and elastic moduli and strain are evaluated using a tensile-compressive tester.
3.2 Swelling measurements
The swelling ability of hydrogel is a significant characteristic for field application. The absorption capacity of the hydrogel can be evaluated [23] gravimetrically at successive time intervals using tea-bag, sieves, centrifugal, volumetric, microwave, gravimetric, NMR, DSC methods based on the required precision. The extent of swelling (DS) was measured using Eq. (1) by performing triplicate measurements.
DS=Wt−W0W0E1
The weight of dried (W0) and swollen polymers Wtat a particular time are measured gravimetrically.
3.3 Absorption under load (AUL)
The extent of water absorption under load is determined by performing AUL of hydrogel samples [24] using the Eq. (2). The AUL test will display the absorption capacity of smart polymer hydrogel under stressed conditions (load) and ionic strength.
AULg/g=W2−W1W1E2
W1and W2represents weight of dry and swollen hydrogel respectively.
3.4 Fertilizer uptake and release studies
The quantum of fertilizer absorption and release characteristics of smart hydrogels are measured based on Eq. (3). The percentage release of fertilizer from the loaded hydrogels are measured gravimetrically [12]. This procedure was followed for every two-day interval to ensure maximum fertilizer release. The percentage of urea/potash release was calculated [12] using Eq. (3).
Percentage of fertilizer released=∆Wn×100−n−1×2/2+∑i=1n−1∆WiWoE3
The amount of fertilizer released from the hydrogel in 2 and ith ml are represented by W0 and (ΔW)i respectively. The number of nutrient releases at different time intervals for the single experiment is denoted by the term “n”.
3.5 Transport kinetics
The rate of nutrient absorption by the plants depends on various parameters such as plant age, nature of fertilizers, and the concentration of fertilizers. However, the micronutrients are supplied as chelates or complexes (using synthetic complexing agents such as salicylic, lactic, formic, citric, succinic, propionic, ascorbic, tartaric and gluconic acids and their sodium, potassium and ammonium salts. Amino acids such as glutamine cysteine, glycine, and lignosulfonates can also be used as complexing agents [25]. The water, nutrients uptake and release behavior of hydrogels are regulated by their chemical constituents namely sulfonic acid, amide, hydroxyl, amine, carboxylic acid, carboxylate groups, etc.
The uptake and release mechanisms are clearly understood by analyzing the transport kinetics. The movement of solvent and solute either into or out of hydrogel is also regulated by the shrinking and swelling of hydrogels. The second-order kinetic model [Eq. (4)] was used to explain the swelling of hydrogel [23].
dMdt=ksM∞−M2E4
where, M: uptake at time t, M∞: uptake at equilibrium condition, and ks: kinetic rate constant.
The swelling rate (SR), and swellability (St) and(St+∆t) at time ‘t’ and ‘t+Δt’ respectively are measured using the Eq. (5).
SR=St+∆t−St∆tE5
3.6 Diffusion
A random molecular process causes the movements of solvent or solute molecules from one part to another part of hydrogels. Further, this movement is also influenced by temperature, pressure, solute size and viscosity. Generally, in hydrogel water molecules diffusion is connected to the extent of polymer-solvent interactions. Based on hydrogel relaxation rate, the diffusion is categorized as non-Fickian and Fickian [26], and the power-law Eq. (6) is used to evaluate the penetration characteristics of solvent into the hydrogel [26].
Mt=ktnE6
The value of diffusion exponent (n) is ranged from 0.5 to 1 and the parameter k represents the rate constant.
3.7 Fickian and non-Fickian
The diffusion mechanism [26] of solution in the hydrogel during network collapse or swelling was analyzed using Fick’s law. Fickian diffusion was noticed when the operating temperature of the system was greater than the glass transition temperature (Tg) of the hydrogel. Fickian type diffusion was also predicted if the solvent diffusion rate (Rdiff) was slower than hydrogel relaxation rate (Rrelax) i.e., (Rdiff <<Rrelax). Besides, the diffusion distance and the square root of time were found to have a direct relationship [Eq. (7)]
Mt=kt1/2E7
The value of ‘n’ provides the diffusion characteristics, for instance, if n = 0.5 in Eq. (6) Fickian diffusion is followed, and the ‘n’ values lie between 1 and 0.5 non-Fickian (anomalous) transport mechanism is followed. Further, non-Fickian model was noticed below glass transition of the hydrogel.
The substantial foodstuff production requires an adequate amount of primary and secondary nutrients [6] along with water during cultivation. To achieve expected yield farmers used to feed an additional amount of fertilizers than the required quantity [6] during each amendment. However, 90% of the applied fertilizers are going as waste due to different climatic conditions and the application method [6]. An excess dose of fertilizers leads to economic losses, toxicity problems and effects on aquatic organisms [6] which cause uninvited effects such as water and soil pollution. Hence, there is a necessity to adopt the method, which facilitates the controlled release of fertilizers without affecting efficacy. An execution of controlled release using polymer based matrix is being used for a long time [6]. The loaded fertilizers have been released through chemical cleavage of the polymer-active agents or by depolymerization reaction (originated other factors) [6]. However, the implementation of a controlled release technique for the particular application depends on the factors namely release rate, cost, effectiveness and properties of synthetic fertilizers.
4.1 Advantages
In agricultural field, smart hydrogels have discharged numerous applications [27] and the notable merits are minimum use of fertilizers and water through controlled a release mechanism. The list of noteworthy advantages of hydrogel amendment in the soil is displayed in Figure 1. However, hydrogels used for the controlled release of fertilizers and water in the field must have
High water retaining ability with slow-release behavior
Excellent efficiency
Appreciable permeability and infiltration rate
Highly stable enough under various environmental conditions for the prolonged use
Reduced frequency of irrigation
Ability to undergo biodegradation without affecting soil fertility
Enhanced plant growth in arid and semiarid conditions
Figure 1.
Advantages of hydrogel in field.
The use of smart hydrogels in agricultural sector have attracted great attention as water management material in soil and matrices for the controlled release of primary and secondary fertilizers. The release rates of hydrogels [23, 27] are depends on the functional groups that are present in the polymer, functionality of crosslinker, pH, temperature, ionic strength of the medium, etc. Besides, the incorporation of natural pre-polymers in synthetic polymer hydrogel will bring down the operation cost, since they are readily available at a low cost and highly biodegradable. Nevertheless, natural polymer incorporation may induce a few limitations such as the lack of solubility of monomers in aqueous and non-aqueous solvents during hydrogel synthesis [16]. This characteristic behavior will result in excess utilization of pre-polymers to enhance agricultural yield.
The additional expected physicochemical and mechanical properties from the synthesized hydrogel for field applications are good stability during swelling (without dissolving), photostability, ability to uptake and hold maximum water with good swelling rate, particle size, maximum fertilizer uptake, porosity, odorless, neutral pH, colorless, low residual monomer content, non-toxicity, biodegradability without yielding toxic reside, and low cost [28, 29]. However, it should be remembered that the synthesis of hydrogel with all these features is difficult to achieve. However, some of its features namely porosity, stimuli responsiveness (pH and temperature), residual monomer content and swellability [30, 31, 32] are fine-tunable. The extent of hydrogel swellability, which are amended in the soil can be fine-tuned based on the requirement by making modification in the functional groups such as −NH2, −COOH, −OH, −CONH2, −CONH− and –SO3H. Besides, osmotic pressure, movable counter ions and capillary effect have also influenced swelling and release phenomena [33]. During swelling, the process of water uptake by the hydrogel will follow multiple steps that include hydration of polar hydrophilic and hydrophobic groups leading to the formation of primary and secondary bound water respectively. Meanwhile, infinite dilution of the hydrogel network will be resisted by the formation of either chemical or physical cross-links. Hence, the water molecules that are entering into the network during the initial and equilibrium stages are known as total bound and bulk water/free water respectively. During swelling these water molecules shall occupy the gaps available between chains and the midpoint of pores. The quantum of water uptake by the hydrogel networks is influenced by various parameters such as temperature, pH, nature of interactions, etc. that exist between networks and water molecules [33]. The list of representative hydrogels that are used as water-retaining agents and matrices for the controlled release of nitrate, potash, phosphate fertilizers are presented in Tables 4–7.
Representative hydrogels used for the controlled release of phosphate fertilizer.
4.2 Effects of hydrogel amendment
Smart hydrogel amendment in the soil during cultivation process will alter the hydraulic conductivity and pore size of soil to some extent due to water absorption [85, 86]. However, it will improve residual and saturated water content, which results in the reduction of subsequent water loss and infiltration due to percolation, this will facilitate aeration in soil due to expansion and contraction of hydrogel through absorption and evaporation [85]. The suitability of hydrogel for semi-arid and arid regions was due to the release of water and fertilizers with reference to environmental temperature, which results in increased survival [85] of plants. Besides, the hydrogel amendment has reduced the uptake of toxic metals and soil salinity by plants [87, 88].
4.3 Safety aspect and environmental concern
The practical applicability of hydrogel in field applications is dependent on safety, toxicity and eco-friendly degradability under soil conditions after its service and other environmental issues. Most of the hydrogels used in agricultural sector have stable service life (5–7 years), but their degradability is suspected. Hydrogels amended in the soil will experience stress from various factors such as microbes, light, pH, temperature, etc. The degradability of hydrogels depends on their structures and other environmental factors such as intensity of light, soil microbes, heat, pH, etc. The degradability of hydrogels could be attained by incorporating favorable functional groups such as ester, amide, urethane, anhydride, glycocidic (ether), urea, ortho-ester, carbonate, etc. in the backbone. The degradation sequence of polymers have predicted as anhydride > ester> orthoester> carbonate> urea>urethane> ether [89].
The monomers of hydrogels are known to be toxic and carcinogenic, but the polymer derived from the same monomers are proved to be non-toxic [18]. This characteristic behavior could be attributed to low boiling point and the low molecular weight of acrylic monomers and crosslinkers, which may effortlessly enter into the human body through skin absorption and inhalation [90, 91]. The studies have also recorded that these acrylic monomers imposed wide a range of health effects such as skin and eye irritation, allergic action, asthma, nerves problem, internal organ toxicity and impacts on fertility [90, 91]. The contentious exposures of acrylates will yield acrylic acid [90, 91] in the human body during metabolic activity. However, the crosslinked hydrogels will not cause any harmful effects on living organisms due to their insolubility and non-volatile nature [90, 91].
The chapter is focused on the development of smart hydrogels derived from synthetic monomers and natural pre-polymer for agricultural application as water retaining material and matrices for the controlled release of fertilizers. However, in the majority of the report, the mechanical properties of those hydrogels are not good enough for prolonged application in the field. Hence, this chapter addressed the route in which the mechanical properties of such hydrogel are fine-tuned. Besides, it focused on the typical hydrogels that are used for the controlled release of water, urea, potash and phosphate fertilizers, their advantages in the field, effects on the hydraulic conductivity of soil and their safety aspects.
The authors would like to thank Sri Ramakrishana Mission Vidyalaya College of Arts, and Science, Coimbatore and Bannari Amman Institute of Technology, Sathyamangalam, for encouraging this work.
The authors declare that no conflicts of interests.
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