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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
W1 and W2 represents 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.
References
1.Ahmed EM. Hydrogel: preparation, characterization, and applications. Journal of Advanced Research. 2015;6:105-121. DOI: 10.1016/j.jare.2013.07.006
2.Chen Y. Hydrogels Based on Natural Polymers. 1st ed. Amsterdam, Netherlands: Elsevier; 2019. pp. 1-533. DOI: 10.1016/C2018-0-00171-1
3.Gul K, Gan RY, Sun CX, Jiao G, Wu DT, Li HB, et al. Recent advances in the structure, synthesis, and applications of natural polymeric hydrogels. Critical Reviews in Food Science and Nutrition. 2021:1-6. DOI: 10.1080/10408398.2020.1870034
4.Madduma-Bandarage USK, Madihally SV. Synthetic hydrogels: Synthesis, novel trends, and applications. Journal of Applied Polymer Science. 2021;138:50376. DOI: 10.1002/app.50376
6.Beig B, Niazi MBK, Jahan Z, Hussain A, Zia MH, Mehran MT. Coating materials for slow release of nitrogen from urea fertilizer: A review. Journal of Plant Nutrition. 2020;43:1510-1533. DOI: 101080/0190416720201744647
7.Lipowczan A, Trochimczuk AW. Phosphates-containing interpenetrating polymer networks (ipns) acting as slow release fertilizer hydrogels (srfhs) suitable for agricultural applications. Mater (Basel). 2021;14:2893. DOI: 10.3390/ma14112893
8.Rizwan M, Rubina Gilani S, Iqbal Durani A, Naseem S. Materials diversity of hydrogel: Synthesis, polymerization process and soil conditioning properties in agricultural field. Journal of Advanced Research. 2021;33:15-40. DOI: 10.1016/j.jare.2021.03.007
9.Zhang X, Liu Y, Lu P, Zhang M. Preparation and properties of hydrogel based on sawdust cellulose for environmentally friendly slow release fertilizers. Green Processing and Synthesis. 2020;9:139-152. DOI: 10.1515/gps-2020-0015
10.Cheng D, Liu Y, Yang G, Zhang A. Water- and fertilizer-integrated hydrogel derived from the polymerization of acrylic acid and urea as a slow-release n fertilizer and water retention in agriculture. Journal of Agricultural and Food Chemistry. 2018;66:5762-5769. DOI: 10.1021/acs.jafc.8b00872
11.Rabat NE, Hashim S, Majid RA. Effect of different monomers on water retention properties of slow release fertilizer hydrogel. Procedia Engineering. 2016;148:201-207. DOI: 10.1016/j.proeng.2016.06.573
12.Teodorescu M, Lungu A, Stanescu PO, Neamţu C. Preparation and properties of novel slow-release npk agrochemical formulations based on poly(acrylic acid) hydrogels and liquid fertilizers. Industrial & Engineering Chemistry Research. 2009;48:6527-6534. DOI: 10.1021/ie900254b
13.Al Rohily K, El-Hamshary H, Ghoneim A, Modaihsh A. Controlled release of phosphorus from superabsorbent phosphate-bound alginate-graft-polyacrylamide: Resistance to soil cations and release mechanism. ACS Omega. 2021;5:32919-32929. DOI: 10.1021/acsomega.0c03740
14.Rudzinski WE, Dave AM, Vaishnav UH, Kumbar SG, Kulkarni AR, Aminabhavi TM. Hydrogels as controlled release devices in agriculture. Designed Monomers and Polymers. 2012;5:39-65. DOI: 101163/156855502760151580
15.Sarmah D, Karak N. Biodegradable superabsorbent hydrogel for water holding in soil and controlled-release fertilizer. Journal of Applied Polymer Science. 2020;137:48495. DOI: 10.1002/app.48495
16.Sikder A, Pearce AK, Parkinson SJ, Napier R, O’Reilly RK. Recent trends in advanced polymer materials in agriculture related applications. ACS Applied Polymer Materials. 2021;3:1203-1217. DOI: 10.1021/acsapm.0c00982
17.Pekel N, Yoshii F, Kume T, Güven O. Radiation crosslinking of biodegradable hydroxypropylmethylcellulose. Carbohydrate Polymers. 2004;2:139-147. DOI: 10.1016/j.carbpol.2003.08.015
18.Zohuriaan-Mehr J, Kabiri K. Superabsorbent Polymer Materials: A Review. Iranian Polymer Journal. 2008;17(6):451-447
19.Thombare N, Mishra S, Shinde R, Siddiqui MZ, Jha U. Guar gum based hydrogel as controlled micronutrient delivery system: Mechanism and kinetics of boron release for agricultural applications. Biopolymers. 2021;112:e23418. DOI: 10.1002/bip.23418
20.Michalik R, Wandzik I. A mini-review on chitosan-based hydrogels with potential for sustainable agricultural applications. Polymers. 2020;12:2425. DOI: 10.3390/polym12102425
21.Mohammadi-Khoo S, Moghadam PN, Fareghi AR, Movagharnezhad N. Synthesis of a cellulose-based hydrogel network: Characterization and study of urea fertilizer slow release. Journal of Applied Polymer Science. 2016;133:42935. DOI: 10.1002/app.42935
22.Durpekova S, Di Martino A, Dusankova M, Drohsler P, Sedlarik V. Biopolymer Hydrogel based on acid whey and cellulose derivatives for enhancement water retention capacity of soil and slow release of fertilizers. Polymers. 2021;13:3274. DOI: 10.3390/polym13193274
23.Dhanapal V, Subramanian K. Superabsorbent polymers: A state-of-art review on their classification, synthesis, physicochemical properties, and applications. Reviews in Chemical Engineering. 2021. DOI: 10.1515/revce-2020-0102/html
24.Ramazani-Harandi MJ, Zohuriaan-Mehr MJ, Yousefi AA, Ershad-Langroudi A, Kabiri K. Effects of structural variables on AUL and rheological behavior of SAP gels. Journal of Applied Polymer Science. 2009;113:3676-3686. DOI: 10.1002/app.30370
25.Kabiri K, Faraji-Dana S, Zohuriaan-Mehr MJ. Novel sulfobetaine-sulfonic acid-contained superswelling hydrogels. Polymers for Advanced Technologies. 2005;16:659-666. DOI: 10.1002/pat.637
26.Grzmil BU, Kic B. Pyro- and Tripolyphosphates and Citrates as the Complexing Agents for Micronutrients in Liquid Fertilizers. Industrial & Engineering Chemistry Research. 2021;41:139-144. DOI: 10.1021/ie010223d
27.Behera S, Mahanwar PA. Superabsorbent polymers in agriculture and other applications: A review. Polymer-Plastics Technology and Materials. 2019;59:341-356. DOI: 101080/2574088120191647239
28.Jyothi AN. Starch graft copolymers: Novel applications in industry. Composite Interfaces. 2010;17:165-174. DOI: 10.1163/092764410X490581
29.Samchenko Y, Ulberg Z, Korotych O. Multipurpose smart hydrogel systems. Advances in Colloid and Interface Science. 2011;168:247-262. DOI: 10.1016/j.cis.2011.06.005
30.Khan A, Othman MBH, Razak KA, Akil HM. Synthesis and physicochemical investigation of chitosan-PMAA-based dual-responsive hydrogels. Journal of Polymer Research. 2013;20:1-8. DOI: 10.1007/s10965-013-0273-7
31.Buenger D, Topuz F, Groll J. Hydrogels in sensing applications. Progress in Polym Science. 2012;37:1678-1719. DOI: 10.1016/j.progpolymsci.2012.09.001
32.Koetting MC, Peters JT, Steichen SD, Peppas NA. Stimulus-responsive hydrogels: Theory, modern advances, and applications. Materials Science and Engineering R: Reports. 2015;93:1-49. DOI: 10.1016/j.mser.2015.04.001
33.Achtenhagen J, Kreuzig R. Laboratory tests on the impact of superabsorbent polymers on transformation and sorption of xenobiotics in soil taking 14C-imazalil as an example. Science of the Total Environment. 2011;409:5454-5458. DOI: 10.1016/j.scitotenv.2011.09.021
34.Rodrigues Sousa H, Lima IS, Neris LML, Silva AS, Santos Nascimento AMS, Araújo FP, et al. Superabsorbent hydrogels based to polyacrylamide/cashew tree gum for the controlled release of water and plant nutrients. Molecules. 2021;26:2680. DOI: 10.3390/molecules26092680
35.Li X, Li Q, Xing X, Yuan S, Yue Q, Gao B. Characterization, swelling and slow-release properties of a new controlled release fertilizer based on wheat straw cellulose hydrogel. Journal of the Taiwan Institute of Chemical Engineers. 2016;60:564-572. DOI: 10.1016/j.jtice.2015.10.027
36.Inukai M, Jin Y, Yomota C, Yonese M. Preparation and characterization of hyaluronate-hydroxyethyl acrylate blend hydrogel for controlled release device. Chemical and Pharmaceutical Bulletin. 2000;48:850-854. DOI: 10.1248/cpb.48.850
37.Lv Q, Min W, Shen Y. Enhanced swelling ratio and water retention capacity for novel super-absorbent hydrogel. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2019;583:123972. DOI: 10.1016/j.colsurfa.2019.123972
38.Yuanyuan B, Baohong C, Feng X, Jinxiong Z, Hong W, Zhigang S. Transparent hydrogel with enhanced water retention capacity by introducing highly hydratable salt. Applied Physics Letters. 2014;105:151903. DOI: 10.1063/1.4898189
39.Wang WB, Wang AQ. Preparation, swelling and water-retention properties of crosslinked superabsorbent hydrogels based on guar gum. Advanced Materials Research. 2010;96:177-182. DOI: 10.4028/www.scientific.net/amr.96.177
40.El-Rehim HAA, Hegazy ESA, El-Mohdy HLA. Radiation synthesis of hydrogels to enhance sandy soils water retention and increase plant performance. Journal of Applied Polymer Science. 2004;93:1360-1371. DOI: 10.1002/app.20571
41.Li Y, Chengxin H, Lan J, Yan B, Zhang Y, Shi L, et al. Hydrogel-based temperature sensor with water retention, frost resistance and remoldability. Polymers. 2020;186:122027. DOI: /10.1016/j.polymer.2019.122027
42.Sharma K, Vijay Kumar BS, Kaith VK, Som S, Kalia S, Swart HC. Synthesis, characterization and water retention study of biodegradable Gum ghatti-poly(acrylic acid–aniline) hydrogels. Polymer Degradation and Stability. 2015;111:20-31. DOI: 10.1016/j.polymdegradstab.2014.10.012
43.Yiming Z, Yonggan Y, Xin C, Xunwei W, Hui W, Jun H, et al. A conductive, self-healing hybrid hydrogel with excellent water-retention and thermal stability by introducing ethylene glycol as a crystallization inhibitor. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2020;607:125443. DOI: 10.1016/j.colsurfa.2020.125443
44.Mingyang C, Zhewei N, Yong S, Guanghong X, Lihui X. Reinforced swelling and water-retention properties of super-absorbent hydrogel fabricated by a dual stretchable single network tactic. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2020;602:125133. DOI: 10.1016/j.colsurfa.2020.125133
45.Shahid SA, Qidwai AA, Anwar F, Ullah I, Rashid U. Improvement in the water retention characteristics of sandy loam soil using a newly synthesized poly(acrylamide-co-acrylic acid)/AlZnFe2O4 superabsorbent hydrogel nanocomposite material. Molecules. 2012;17:9397-9412. DOI: 10.3390/molecules17089397
46.Bakass M, Mokhlisse A, Lallemant M. Absorption and desorption of liquid water by a superabsorbent polymer: Effect of polymer in the drying of the soil and the quality of certain plants. Journal of Applied Polymer Science. 2002;83:234-243. DOI: 10.3390/molecules17089397
47.Bajpai S, Bajpai M, Sharma L. Investigation of water uptake behavior of superabsorbent polymers composed of N-Vinyl-2-pyrrolidone and partially neutralized acrylic acid. Journal of Macromolecular Science—Part A. 2006;43:1323-1337. DOI: 10.1080/10601320600814648
48.Bajpai S, Bajpai M, Sharma L. Investigation of water uptake behavior and mechanical properties of super-porous hydrogels. Journal of Macromolecular Science—Part A. 2006;43:507-524. DOI: 10.1080/10601320600575249
49.Nnadi F, Brave C. Environmentally friendly superabsorbent polymers for water conservation in agricultural lands. Journal of Soil Science and Environmental Management. 2011;2:206-211
50.Zhang J, Sun MW, Zhang L, Xie XM. Water absorbency of poly (sodium acrylate) superabsorbents crosslinked with modified poly (ethylene glycol). Journal of Applied Polymer Science. 2003;90:1851-1856. DOI: 10.1002/app.12844
51.Chen Y, Tan H. Crosslinked carboxymethylchitosan-g-poly (acrylic acid) copolymer as a novel superabsorbent polymer. Carbohydrate Research. 2006;341:887-896. DOI: 10.1016/j.carres.2006.01.027
52.Yudi W, Colten B, Simeng L, Gang C. Synthesis of microwave-mediated biochar-hydrogel composites for enhanced water absorbency and nitrogen release. Polymer Testing. 2021;93:106996. DOI: 10.1016/j.polymertesting.2020.106996
53.Elbarbary AM, Ghobashy MM. Controlled release fertilizers using superabsorbent hydrogel prepared by gamma radiation. Radiochimica Acta. 2017;105:865-876. DOI: 10.1515/ract-2016-2679
54.Helaly FM, Essawy HA, El-Nashar DE, Maziad NA. Slow Release of urea as a source of nitrogen from some acrylamide and acrylic acid hydrogels. Polymer-Plastics Technology and Engineering. 2005;44:253-263. DOI: 10.1081/PTE-200048712
55.Noppakundilograt S, Pheatcharat N, Kiatkamjornwong S. Multilayer-coated NPK compound fertilizer hydrogel with controlled nutrient release and water absorbency. Journal of Applied Polymer Science. 2014;132:41249. DOI: 10.1002/app.41249
56.Daniel T, Gungula FP, Andrew JJ, Semiu AK, Jeffery TB, Elizabeth FA, et al. Formulation and characterization of water retention and slow-release urea fertilizer based on Borassus aethiopum starch and Maesopsis eminii hydrogels. Results in Materials. 2021;12:100223-100232. DOI: 10.1016/j.rinma.2021.100223
57.Mohamed MG, Mousaa IM, Gharieb SE. Radiation synthesis of urea/hydrogel core shells coated with three different natural oils via a layer-by-layer approach: An investigation of their slow release and effects on plant growth-promoting rhizobacteria. Progress in Organic Coatings. 2021;151:106022. DOI: 10.1016/j.porgcoat.2020.106022
58.Kiran RT, Krishnamoorthi S, Kumar K. Synthesis of cross-linker devoid novel hydrogels: Swelling behaviour and controlled urea release studies. Journal of Environmental Chemical Engineering. 2019;7:103162. DOI: 10.1016/j.jece.2019.103162
59.Vudjung C, Saengsuwan S. Biodegradable IPN hydrogels based on pre-vulcanized natural rubber and cassava starch as coating membrane for environment-friendly slow-release urea fertilizer. Journal of Polymers and the Environment. 2018;26:3967-3980. DOI: 10.1007/s10924-018-1274-8
60.Guohua D, Zhonghua M, Dongni L, Luwen S, Wenzhi Z, Yueyue G, et al. Starch phosphate carbamate hydrogel based slow-release urea formulation with good water retentivity. International Journal of Biological Macromolecules. 2021;190:189-197. DOI: 10.1016/j.ijbiomac.2021.08.234
61.Guo M, Liu M, Zhan FWL. Preparation and properties of a slow-release membrane-encapsulated urea fertilizer with superabsorbent and moisture preservation. Industrial & Engineering Chemistry Research. 2005;44:4206-4211. DOI: 10.1021/ie0489406
62.Liang R, Liu M, Wu L. Controlled release NPK compound fertilizer with the function of water retention. Reactive and Functional Polymers. 2007;67:769-779. DOI: 10.1016/j.reactfunctpolym.2006.12.007
63.Liu M, Liang R, Zhan F, Liu Z, Niu A. Synthesis of a slow-release and superabsorbent nitrogen fertilizer and its properties. Polymers for Advanced Technologies. 2006;17:430-438. DOI: 10.1002/pat.720
64.Li A, Zhang J, Wang A. Preparation and slow-release property of a poly (acrylic acid)/attapulgite/sodium humate superabsorbent composite. Journal of Applied Polymer Science. 2007;103:37-45. DOI: 10.1002/app.23901
65.Bortolin A, Aouada FA, De Moura MR, Ribeiro C, Longo E, Mattoso LH. Application of polysaccharide hydrogels in adsorption and controlled-extended release of fertilizers processes. Journal of Applied Polymer Science. 2012;123:2291-2298. DOI: 10.1002/app.34742
66.Sharma J, Sukriti KBS, Bhatti MS. Fabrication of biodegradable superabsorbent using RSM design for controlled release of KNO3. Journal of Polymers and the Environment. 2018;26:518-531. DOI: 10.1007/s10924-017-0959-8
67.León O, Soto D, Muñoz-Bonilla A, Marta F. Amylose modified starches as superabsorbent systems for release of potassium fertilizers. Journal of Polymers and the Environment. 2021. DOI: 10.1007/s10924-021-02352-7
68.Jamnongkan T, Kaewpirom S. Potassium release kinetics and water retention of controlled-release fertilizers based on chitosan hydrogels. Journal of Polymers and the Environment. 2010;18:413-421. DOI: 10.1007/s10924-010-0228-6
69.Chen Y-C, Chen Y-H. Thermo and pH-responsive methylcellulose and hydroxypropyl methylcellulose hydrogels containing K2SO4 for water retention and a controlled-release water-soluble fertilizer. Science of The Total Environment. 2019;655:958-967. DOI: 10.1016/j.scitotenv.2018.11.264
70.Zonatto F, Muniz EC, Tambourgi EB, Paulino AT. Adsorption and controlled release of potassium, phosphate and ammonia from modified Arabic gum-based hydrogel. International Journal of Biological Macromolecules. 2017;105:363-369. DOI: 10.1016/j.ijbiomac.2017.07.051
71.Shaon Kumar D, Goutam KG. Hydrogel-biochar composite for agricultural applications and controlled release fertilizer: A step towards pollution free environment. Energy;242:122977. DOI: 10.1016/j.energy.2021.122977
72.Abuchenari A, Hardani K, Abazari S, Naghdi F, Ahmady Keleshteri M, Jamavari A, et al. Clay-reinforced nanocomposites for the slow release of chemical fertilizers and water retention. Journal of Composites and Compounds. 2020;2:85-91. DOI: 10.29252/jcc.2.2.4
73.Rozo G, Bohorques L, Santamaría J. Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel. Polímeros. 2019;29:1-7. DOI: 10.1590/0104-1428.02719
74.Calcagnile P, Sibillano T, Giannini C, Sannino A, Demitri C. Biodegradable poly(lactic acid)/cellulose-based superabsorbent hydrogel composite material as water and fertilizer reservoir in agricultural applications. Journal of Applied Polymer Science. 2019;136:47546. DOI: 10.1002/app.47546
75.Lan W, Mingzhu L. Slow-release potassium silicate fertilizer with the function of superabsorbent and water retention. Industrial & Engineering Chemistry Research. 2007;46:6494-6500. DOI: 10.1021/ie070573l
76.Singh A, Sarka DJ, Mittal S, Dhaka R, Maiti P, Singh A, et al. Singh SB Zeolite reinforced carboxymethyl cellulose-Na+-g-cl-poly(AAm) hydrogel composites with pH responsive phosphate release behavior. Journal of Applied Polymer Science. 2019;136:47332. DOI: 10.1002/app.47332
77.Liu S, Wu Q, Sun X, Yue Y, Tubana B, Yang R, et al. Novel alginate-cellulose nanofiber-poly(vinyl alcohol) hydrogels for carrying and delivering nitrogen, phosphorus and potassium chemicals. International Journal of Biological Macromolecules. 2021;172:330-340. DOI: 10.1016/j.ijbiomac.2021.01.063
78.Lohmousavi SM, Sharif Abad HH, Noormohammadi G, Delkhosh B. Synthesis and characterization of a novel controlled release nitrogen-phosphorus fertilizer hybrid nanocomposite based on banana peel cellulose and layered double hydroxides nanosheets. Arabian Journal of Chemistry. 2020;13:6977-6985. DOI: org/10.1016/j.arabjc.2020.06.042
79.Khadiga A, Adel G, Azza E, Hany EH, Mohamed HE. Controlled release of phosphorous fertilizer bound to carboxymethyl starch-g-polyacrylamide and maintaining a hydration level for the plant. International Journal of Biological Macromolecules. 2018;116:224-231. DOI: 10.1016/j.ijbiomac.2018.04.182
80.Ali O, Hamed G, Abdolreza M, Ahmad B. Synthesis, characterization, and fertilizer release study of the salt and pH-sensitive NaAlg-g-poly(AA-co-AAm)/RHA superabsorbent nanocomposite. Polymer Bulletin. 2017;74:3353-3377. DOI: 10.1007/s00289-016-1899-5
81.Tyliszczak B, Polaczek J, Pielichowski J, Pielichowski K. Preparation and properties of biodegradable slow-release paa superabsorbent matrixes for phosphorus fertilizers. Macromolecular Symposia. 2009;279:236-242. DOI: 10.1002/masy.200950534
82.Ali O, Hamid Z, Dariush S, Abdolreza M, Adel RT. Slow-release NPK fertilizer encapsulated by carboxymethyl cellulose-based nanocomposite with the function of water retention in soil. Materials Science and Engineering: C. 2018;90:333-340. DOI: 10.1016/j.msec.2018.04.083
83.Zhong K, Zheng XL, Mao XY, Lin ZT, Jiang GB. Sugarcane bagasse derivative-based superabsorbent containing phosphate rock with water-fertilizer integration. Carbohydrate Polymers. 2012;90:820-826. DOI: 10.1016/j.carbpol.2012.06.006
84.Zhan F, Liu M, Guo M, Wu L. Preparation of superabsorbent polymer with slow-release phosphate fertilizer. Journal of Applied Polymer Science. 2004;92:3417-3421. DOI: 10.1002/app.20361
85.Andry H, Yamamoto T, Irie T, Moritani S, Inoue M, Fujiyama H. Water retention, hydraulic conductivity of hydrophilic polymers in sandy soil as affected by temperature and water quality. Journal of Hydrology. 2009;373:177-183. DOI: 10.1016/j.jhydrol.2009.04.020
86.Abedi-Koupai J, Sohrab F, Swarbrick G. Evaluation of hydrogel application on soil water retention characteristics. Journal of Plant Nutrition. 2008;31:317-331. DOI: 10.1080/01904160701853928
87.Hüttermann A, Orikiriza LJ, Agaba H. Application of superabsorbent polymers for improving the ecological chemistry of degraded or polluted lands. Clean–Soil, Air, Water. 2009;37:517-526. DOI: 10.1002/clen.200900048
88.Lentz RD, Sojka RE. Long-term polyacrylamide formulation effects on soil erosion, water infiltration, and yields of furrow-irrigated crops. Agronomy Journal. 2009;101:305-314. DOI: 10.2134/agronj2008.0100x
89.Woodard LN, Grunlan MA. Hydrolytic degradation and erosion of polyester biomaterials. ACS Macro Letters. 2018;7:976-982. DOI: 10.1021/acsmacrolett.8b00424
90.Gosavi SS, Gosavi SY, Alla RK. Local and systemic effects of un-polymerised monomers. Dental Research Journal. 2010;7:82-87
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