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En el presente artículo se presenta una revisión de literatura sobre hidrogeles aplicados a la agricultura, haciéndose mayor énfasis en el uso de fibras vegetales. Los objetivos fueron examinar las tendencias en la investigación, identificar las fibras naturales utilizadas en el desarrollo de hidrogeles, mostrar los métodos de modificación de fibras naturales para la elaboración de hidrogeles, entender los factores que determinan la capacidad de retención de agua de un hidrogel, mostrar algunas metodologías empleadas para evaluar la eficiencia de un hidrogel y recopilar en tablas, información relevante en relación a métodos de modificación de fibras naturales y de síntesis de hidrogeles. Se encontró que las investigaciones se orientan al desarrollo de hidrogeles elaborados a bases de polímeros biodegradables como almidón, quitosano y fibras vegetales modificadas, entrecruzadas con acrilato de potasio y acrilamida. Además, las investigaciones apuntan a la obtención de hidrogeles con propiedades mejoradas, que permitan resistir las variaciones climáticas y los cambios fisicoquímicos del suelo como el pH, la presencia de sales, temperatura y composición. Fibras naturales como caña de azúcar, fibra de agave y fibra de kapoc, modificadas con anhídrido maléico, son una alternativa para la obtención de hidrogeles debido al incremento de las propiedades mecánicas, sumado al incremento de los sitios químicamente activos. Sin embargo, el uso de nanofibras naturales ha sido una propuesta exitosa para mejorar las propiedades mecánicas y de hinchamiento de los hidrogeles, ya que aportan al material elasticidad y rigidez. La eficiencia de un hidrogel aplicado al suelo se mide a través de propiedades como capacidad de hinchamiento resistencia mecánica y retención de agua en el suelo. Se concluyó que los hidrogeles, son una alternativa a las necesidades actuales del sector agrícola.
Introduction
Population growth necessitates an increasing demand for agricultural products in quantity and diversity, which translates into a greater water demand for agricultural activities and a greater soil intensification use available for crops (FAO, 2013). In Colombia, agriculture consumes about 70% of available water (Arévalo, 2012; IDEAM, 2014; IDEAM, 2015). Hydrogels are presented as an alternative to the current needs for the agricultural sector due to most crops requires an additional irrigation. Hydrogels are polymers which have a three-dimensional cross-linked structure that allows them to absorb, store and release water molecules (Mohan, Murthy & Raju, 2006; Pourjavadi & Mahdavinia, 2006). Among the most important applications are as follows: manufacture of personal hygiene products, medical (Arredondo, 2009), environmental (heavy metals removal) (Orozco-Guareno, Hernandez, Gomez-Salazar, Mendizabal & Katime, 2011) and agriculture (Cortés et al., 2007; Liang, Huang, Zhang, Hu & Liu, 2013). The most important agricultural areas are gardening, horticulture and silviculture, respectively (Journal the Business, 2015; Vundavalli, Vundavalli, Nakka & Rao, 2015). In Colombia, hydrogels application is addressed to counteract drought in crops such as mango in the municipality of Magangue-Bolivar, Colombia (Asohofrucol, 2014). In 2014, the superabsorbent hydrogels global production was 3119 million tons. An increase of 11.8% over the previous year. Among the countries with the highest demand for the product are as follows: China, Japan, United States, Germany and countries in the Middle East. Although most of this product is intended for hygiene products manufacture, the agricultural industry occupies the second place in superabsorbent hydrogels consumption (Cannazza, Cataldo, De Benedetto, Demitri, Madaghiele & Sannino, 2014).
Hydrogels for agricultural applications are based on synthesized acrylates. These components have allowed an increasing in the soil available water, which induce to fast growth (Gascue, Aguilera, Ramirez, Prin & Torres, 2006), prolong a plant survival under water stress (Cannazza et al., 2014; Cortés et al., 2007; Hüttermann, Orikiriza & Agaba, 2009; Mo, Shu-quan, Hua-min, Zhan-bin & Shu-qin, 2006; Zhong, Zheng, Mao, Lin & Jiang, 2012) and have allowed a controlled release of fertilizers (Liu et al., 2013). One of the major limitations in hydrogels use for agricultural applications is the low mechanical resistance (Guilherme et al., 2015; Sannino, Demitri & Madaghiele, 2009). A pressure exerted by the plant and soil layer on the hydrogel, influences the loss of swelling capacity, elasticity and rigidity, respectively (Feng, Li & Wang, 2010). To maintain a polymer elasticity, long chain molecules and a suitable crosslinking are required to dissipate the mechanical energy, which is caused by the pressure exerted on the hydrogel (Sannino et al., 2009). One of the alternatives to improve the mechanical properties is the use of natural fibers (Rodrigues et al., 2013), these fibers are characterized by containing cellulose, hemicellulose and lignin, respectively (Bessadok, Marais, Roudesli, Lixon & Métayer, 2008). The lignin layer which covers the cellulose of natural fibers and hinders the reaction with other molecules (Chang & Zhang, 2011). Given these concerns, is necessary to perform pretreatments and chemical modifications to improve the natural fibers reactivity (Feng et al., 2010; Sannino et al., 2009). It is important to note that most commercial hydrogels are based on non-biodegradable acrylic. Therefore, cellulose-based hydrogels fit well in the current trend to develop ecological alternatives addressed to hydrogels production (Sannino et al., 2009). Given these concerns, the aim of this review was to present a research carried out on the methods of hydrogel synthesis based on natural fibers, the characterization methods and their evaluation during soil application. In addition, identify the research trends in this field.
Natural fibers for hydrogels production
Natural fibers are used as a reinforcement in composite materials as an alternative to replace the use of synthetic fibers in order to obtain low-cost and environmentally friendly products (Cuéllar & Muñoz, 2010; Kalia, Kaith & Kaur, 2014; Rodriguez, Jose, Daniel, Viviane & Alves Lavinia, 2014; Thakur & Thakur, 2014). Natural fibers are the most abundant polymers in nature, which are present in leaves, stems, seeds and plant fruits (Thomas, Paul, Pothan & Deepa, 2011). Some examples are flax fibers, jute, pitch, plantain, among others. These are characterized by their strength, flexibility, easy processing and biodegradability (Thakur & Thakur, 2014). The biodegradation rate of natural fibers depends on the environmental conditions and the degradation capacity of the microbial population (Malherbe & Cloete, 2002). Although specific bio-degradability data for natural fibers are not reported in the literature. The biodegradability method of natural fibers in composite materials is reported. For example, Sahoo, Sahu, Rana & Das, (2005), report the biodegradation of jute fiber and jute fiber crosslinked with polybutylacrylate using soil biodegradability method. In fact, the method is to bury a sample of known fiber weight or material to be studied at a specific soil depth under controlled conditions of humidity and temperature, and determine weight loss over time. The authors mentioned that during 12 months of evaluation, the fibers presented greater weight loss (47.1%) (greater degradation) compared to the composite material (31.8%) (jute fibers cross-linked with polybuty- acrylate). On the other hand, Wu (2012), studied the polylactic acid biodegradability reinforced with stay fibers at concentrations between 20 and 40% by weight, using the soil bio-degradability method. He found greater weight loss in reinforced materials with higher stay fiber content during follow-up for six weeks. These fibers are characterized by mainly cellulose, hemicellulose and lignin, respectively (Bessadok et al., 2008). Table 1, shows the composition and major mechanical properties of natural fibers. Cellulose is characterized by the presence of hydroxyl groups in the chain, which improve its reactivity, possibility of chemical modification, compatibility with other polymers and water solubility (Zhou, Fu, Zhang & Zhan, 2013).
Table 1 Composition in cellulose, hemicellulose and lignin (Abdul Khalil, Bhat, & Ireana Yusra, 2012) and mechanical properties of natural fibers (Pickering, Efendy & Le, 2015)
*Source: (Chandramohan & Marimuthu, 2011)
However, a lignin layer which coats the cellulose of natural fibers have achieved a difficulty to react with other molecules (Chang & Zhang, 2011). Given these concerns, is necessary to carry out pretreatments and chemical modifications to improve the reactivity and its interaction with other molecules (Feng et al., 2010; Sannino et al., 2009).
Modifying methods of natural fibers for hydrogels production
The modification methods of natural fibers usually are divided in two stages: a pretreatment, whose objective is the lignin removal throughout mechanical or chemical treatments by alkaline treatment (Shi, Wang, Zheng & Wang, 2014) and chemical modification, consisting of molecules insertion into the active sites (hydroxyl groups) of cellulose (Thomas et al., 2011). These modifications increase the water absorption and retention capacity throughout an interaction with modifying agents and active sites generation (Liang et al., 2013; R. F. Rodrigues, Trevenzoli, Santos, Leão & Botaro, 2006). Table 2, shows modifying agents used for different natural fibers, pretreatment conditions and the most significant effect.
Modification of sugarcane bagasse fibers using maleic anhydride, acetic anhydride, acrylic acid and styrene, increases the fibers hydrophobic capacity and decreasing mechanical tensile properties. Similarly, as indicated in Table 1, agave fibers modified with maleic anhydrous and styrene showed an increasing tensile strength forces (Bessadok et al., 2008). Therefore, modified fibers with maleic anhydride are an alternative for obtaining hydrogels due to an increasing in mechanical properties, which have allowed an increasing in chemical active sites of a modified chain due to a double link site, which allows a maleic anhydride adhesion to cellulose chain. Mechanical modification of plant fibers is another way of increasing the degree of gel swelling. In fact, the mechanical activation affects the destruction of a molecular lignin structure and an increasing of active sites in the cellulose throughout destruction of its crystalline structure (Huang et al., 2009; Liang et al., 2013). Similarly, Liang et al. (2013), found that increasing the mechanical modification time, increases the degree of swelling of a synthesized hydrogel from sugar cane bagasse and acrylic acid.
Nevertheless, the chemical modification methods for natural fibers present disadvantages as reactive use at high concentrations and high amount of solvent to carry out the reaction. In addition, reaction times are high, generally times vary between 1, 3, 24 and 48 hours. The number of stages is increased due to a separation, washing and neutralization operations involved with the used corrosive reagents nature and the amount of generated waste water (Chen et al., 2011; Liu et al., 2007). Therefore, other methods have been proposed to minimize the use of solvents. Alternatives such as assisted microwave modification, which is characterized by using short reaction times and less solvent. Lie et al., (2009), performed the optimization of assisted cellulose modification by microwave using acetic anhydride as modifying agent, during the reaction tests the temperature, time, radiation potency were varied and iodine (I2) was used as the catalyst. The substitution degree of cellulose hydroxyl groups was evaluated by finding that substitution is increased with an increasing concentration of the catalyst. In this sense, modification were carried out using ultrasound, which becomes as a strategy to reduce the use of solvents and lignin removal. Ultrasonic pretreatment increased the lignin removal from sugarcane bagasse, which translates into a greater exposure of the cellulose hydroxyl groups, which may react with the modifying agent (Liu et al., 2007).
Hydrogels synthesis from plant fibers and effect on swelling ability
In general, the methods of hydrogels synthesis are carried out by mass polymerization, solution and reverse suspension, using an initiator and a cross-linker. The hydrogels synthesis based on plant fibers usually uses the solution polymerization method. In Table 3, some methods of hydrogels synthesis based on natural fibers are presented.
In tests carried out by Shi et al. (2014), described in Table 3, it was found that an increasing in the amount of fiber during the hydrogels synthesis, increased the swelling ability and the elastic modulus. However, natural fiber had achieved an increasing above 10% from a decreasing gel formation due to the fiber is insoluble in water and the active sites are decreased due to the initiator inefficiency.
In the study reported in Table 3, Liang et al. (2013), showed the swelling is modified by the effect of pH change, the presence of salts (NaCl and CaCl2) and temperature. In acidic media, the presence of hydronium ions interacts with the hydroxyl groups of cellulose chain, generating a greater presence of hydrogen linking forces, increasing the chain cross-links and a decreasing in the absorption capacity. In basic media, the present cation, interacts with the carboxyl group of the polyacrylate and polyacrylamide chain, neutralizing the electrostatic attraction active sites, which causes a decreasing in the swelling ability at higher saline concentration. With reference to temperature, the swelling ability was increased at temperatures between 0 and 50°C, at higher temperatures, decreases due to the surface water had achieved a higher energy compared to the attraction energy exerted by the polymer chain on the molecule. Similar results were obtained in hydrogels from kapok fibers (Shi et al., 2014).
Specifically in flax fiber, Feng et al. (2010), found a decreasing swelling ability in saline solutions at acid (1-4) and basic (10-14) pH. The material biodegradation in 52 days was 40% at 40°C and performed an increasing in microorganism attack, since the material porosity and temperature promote the activation thereof.
Zhong et al. (2012), found that phosphoric rock inclusion in the polymer matrix, have allowed an improvement in the swelling ability and the rate of water release. In addition, this compound becomes an alternative for fertilizers inclusion in the polymer network whose advantage is a controlled release in the soil.
Nanofibers in hydrogels
Hydrogels with high swelling ability are usually fragile due to the lack of energy dissipation mechanisms and an uneven distribution of cross-linking points (C. Zhou & Wu, 2011). However, the use of natural fibers at nanoscale, improves the hydrogel mechanical properties due to they act as matrix reinforcement (Aouada, de Moura, Orts & Mattoso, 2011; Guilherme et al., 2015; Krishnan K, Jose, K. R & George, 2015; Missoum, Belgacem & Bras, 2013) without affecting the swelling ability (Rodrigues et al., 2013). In fact, the nanofibers have allowed to improve the swelling ability due to the capacity of forming hydrogen bridges, elasticity and available natural fiber surface (Nitta et al., 2015; Zhou et al., 2013; Spagnol, Rodrigues, Pereira et al., 2012). The presence of hydrophylic groups (hydroxyl groups - OH) in the nanofibers, facilitate a more precise liquids diffusion into the hydrogel matrix more efficient and faster. Therefore, have allowed an increasing in the swelling rate (Spagnol, Rodrigues, Pereira et al., 2012a, 2012b). Spanol, Rodrigues, Neto et al. (2012a), Spagnol & Rodrigues, (2012b), who synthesized hydrogels from cotton cellulose nanofibers. They found the swelling ability was increased with an increasing in nanofiber content. Conversely, the swellability decreased for concentrations greater than 10% by weight in nanofibers. Similar results found by Zhou et al. (2013), at concentrations greater than 5% by weight in cellulose nanofibers content. In fact, an excessive increasing in the nanofibers content, which have allowed an increasing in the physical crosslinking and nanofiber stacking in polymer material networks, in which the water molecules are stored and results in a decreasing swelling ability (Spagnol & Rodrigues, 2012b).
The addition of nanofibers in hydrogels increases the density of cross-linking points, which improves the resistance to fracture, flexural strength and compression (Kabiri, Omidian, Doroudiani & Zohuriaan, 2011). Also, they promote the formation of a porous morphology (Wen, Zhu, Gauthier & An, 2015), which improves biodegradability, thermal stability and water retention capacity due to temperature changes in the medium (Aouada et al., 2011). The resistance to compression and the elastic modulus is increased by chitosan nanofibers addition in polyethylene glycol acrylate based on hydrogels compared to hydrogels without nanofibers addition (Nitta et al., 2015). As indicated in Table 2, Zhou & Wu (2011), evaluated the addition of chitosan nanofibers in hydrogels from acrylamide. The resistance to compression was increased for hydrogels based on chitosan nanofibers. This increase was attributed to a good nanofibers dispersion in the matrix, and the nanofibers interfacial adhesion to polyacrylamide chain. However, there was a decreasing in swelling ability due to a low chitosan affinity with water and the presence of amino (-NH2) functional groups, which increased the crosslinking points density in the matrix, due to possible reactions that could be presented with the cross-linker.
Characterization of hydrogels
Swelling ability
In hydrogels, the swelling mechanism occurs by the solvent diffusion towards the hydrogel polymeric networks due to the hydrophilic capacity of the present functional groups (Guilherme et al., 2015). The process causes the expansion of the cross-linked chains, hence an increasing in mass and volume. In fact, the swelling ability is determined by the gravimetric method (Guilherme et al., 2015) Initially, dry hydrogel (xerogel) is weighed and immersed into an excess of distilled water or solvent of interest. The amount of water absorbed is determined by weight difference (Cortés et al., 2007; Orozco-Guareno et al., 2011). Generally, the test is performed in different periods of time until the maximum swelling ability is achieved, which have allowed the swelling kinetics determination. The swelling ability depends on the degree of hydrogel crosslinking. For hydrogels applied to agriculture, is necessary to determine properties that simulate soil conditions (Agaba, Orikiriza, Obua, Kabasa & Worbes, 2011; Ekebafe, Ekebafe, Ogbeifun & Okieimen, 2011). pH, the presence of salts and soil temperature are the major factors that influence the swelling ability (Chang, Han & Zhang, 2009; Delgado-Pelayo, Gallardo-Guerrero & Hornero-Méndez, 2014; Ullah, Othman, Javed, Ahmad & Akil, 2015). The presence of salts, acidic or basic pH and temperatures above 50°C decrease the swelling ability (Guilherme et al., 2015; Shi et al., 2014; Ullah et al., 2015).
Li et al. (2012), studied the retention and re-swelling ability of hydrogels based on wheat straw. Water retention was determined using hydrogel samples with their maximum swelling ability using distilled water and saline solutions (NaCl, CaCl2, Na2SO4, MgCl2) and water release was studied by immersing the swollen hydrogel in saline solutions (NaCl 0.9%). It should be taken into account that hydrogel re-swelling is determined by performing swelling and drying cycles to the sample.
Resistance to compression and elastic modulus
When a material is subjected to an increasing tension in a single direction, a rupture occurs due to an increasing in microscopic material defects, the maximum tension that resists the material when it arrives at the point of fracture, is denominated resistance to the compression (Dianney et al., 2010). In this sense, the resistance to compression is determined by a vertical force on the hydrogel surface in its maximum state of swelling. Nevertheless, the applied pressure and material displacement resulting from the compression, determine the mechanical properties, these are calculated throughout a theoretical model (Ahearne, Yang & Liu, 2008). Mechanical properties such as elastic modulus and resistance to compression are of great interest due to hydrogels for agricultural applications are affected by an exerted pressure of the soil layer and the pressure of the plant root system on the material (Sannino et al., 2009).
The hydrogel samples are brought to their maximum swellability, which are subjected to a resistance to compression at a rate of 0.1N.min-1 in a DMA analyzer (mechanical dynamic analysis) where a deformation-tension curve is obtained (Chang et al., 2009). However, the resistance to compression and the elongation is increased in hydrogels obtained from pineapple leaf fiber pretreated with NaOH and acrylamide using NN-methylene bisacrylamide as compared to hydrogels obtained with untreated fiber due to an increasing chemical treatment in the fiber surface area, which improves the interfacial fiber adhesion into the acrylamide. In addition, the increase of active sites facilitates the initiator and crosslinker action (Ghazali, Adrus & Jamaluddin, 2014).
The elastic modulus in hydrogels is related to a hydrogel ability to return to its original state even were subjected to a force (Guilherme et al., 2015). Alternatively, Nitta et al. (2015), determined the elastic modulus of hydrogels based on chitosan nanofibers throughout resistance to compression test using a rheometer. Previously, Cortés et al. (2007), evaluated the swelling ability at different mechanical loads and determined the elastic modulus of hydrogels throughout an application of a mechanical load using a piston.
The elastic modulus is increased by the addition of cotton cellulose nanofibers from a concentration of 10% by weight in hydrogels synthesized from cassava starch and sodium acrylate (Spagnol et al., 2012b). Similar results were found in hydrogels synthesized based on acrylamide reinforced with chitosan nanofibers. In fact, the results of the resistance to compression, indicates an increasing (50.2KPa) measurement, compared to hydrogels synthesized only with acrylamide (6.5KPa) and hydrogels cross-linked with acrylamide and chitosan (21.3KPa). In addition, the results indicate that hydrogels with nanofibers are more flexible, recover up to 80% of their original height compared to hydrogels synthesized only with acrylamide (recovery of 50% of the original height) (Zhou & Wu, 2011).
in vitro methods to determine the water retention and release capacity of a hydrogel
Cortés et al. (2007), evaluated the release of water from acrylic based on hydrogels using a pressure plate equipment, which is used to determine soil moisture retention curves. For instance, the equipment is provided with a pressure chamber in which the wet samples are located and a porous plate, which have allowed the passage of water, the hydrogel is brought to its maximum swelling ability when is placed into the chamber, a specific pressure is applied and maintained until there is no flow of water from the chamber (Singh, Sarkar, Kumar Singh, Parsad & Singh-Parmar, 2010). The pressure values are set according to the thermodynamic soil suction potential value (average pressure exerted by a plant on the soil to absorb water)(Demitri, Scalera, Madaghiele, Sannino & Maffezzoli, 2013). It was found that the greater amount of water is released at low pressures, due to plant present greater availability to a lesser effort of the roots.
Conversely, Singh et al. (2010), evaluated the retention ability and pressure plate release of hydrogels synthesized from acrylamide, commercial amorphous resin (heteropolysaccharide derived from Cochlospermum species) and clay nanoparticles. Therefore, to carry out tests, mixtures of hydrogel (0.50 and 0.75% by weight) with sandy loam soil, a soil conditioner (coconut peat, vermiculite, and perlite at rate of 3: 1: 1 per volume proportions) and control sample (without hydrogel), were used. The results showed that the retained water content was higher compared to control sample with respect to the applied pressure. The water release capacity is higher for 0.75% from soil hydrogel mixtures compared to control. In this sense, a higher release capacity indicates a greater water availability of wate for the plant under conditions of water stress.
Performance evaluation of the hydrogels applied to soil
Various techniques are used to determine the performance of the hydrogel applied to soil, such as moisture retention curves, instant irrigation tests, wilt tests and plant growth tests for fast growing species (Cortés et al., 2007).
More recently, Cannazza et al. (2014), studied the hydrogels performance with clay soils and soils with high iron content. For evaluation purpose, seedlings of chicory and tomato were used. Soil samples were prepared using hydrogel percentages between 0 and 1.5%, followed for 78 days evaluating the loss of soil moisture. In fact, hydrogels were found to improve the soil moisture retention capacity.
Liu, Liang, Zhan, Liu & Niu, (2006), used soil samples from arid regions. Mixing 1g of hydrogel with 100g of soil, which was added with 250g of distilled water and the soil moisture loss was monitored for 30 days. They also carried out water infiltration tests where they used mixtures of 1g of hydrogel per 100g of dried soil, which were added with 250g of water, maintained the samples at room temperature, and the sample weight loss was assessed for nine days. Respectively, they found the evaporation percentage and soil infiltration samples based on hydrogel was lower than 41 and 55.7%, respectively compared to normal soil. Similar results were found in hydrogels synthesized from waste fiber from flax (F. Wu, Zhang, Liu & Yao, 2012). Moisture levels were higher in sandy loam soils compared to control sample (soil without hydrogel) at temperatures of 25 and 45°C, respectively, by the addition of hydrogels obtained from clay nanoparticles. Due to an increasing water availability in the soil by hydrogels addition, the wilting point in plants was delayed up to 3.6 days (Singh et al., 2010).
Given these concerns, hydrogels are an alternative to the current needs of the agricultural sector, due to majority of crops require an additional irrigation. In this sense, hydrogels have allowed an increasing in available water in the soil, which is responsible to induce a rapid plant growth, prolong the survival of plants under water stress and had achieved a controlled release of fertilizers. It was found that the trend in hydrogels research for use in agriculture is the search for methods that allows to obtain hydrogels with high swelling ability in short times, in such a way, they increase the water available in the soil. Similarly, biodegradable hydrogels and high mechanical resistance are required, presumably, hydrogels are affected by a pressure exerted by the soil layer and the plant root system pressure on the material. In fact, this provides more accurate and reliable estimates of the use of nano particles obtained from plant fibers, which have allowed to improve the swelling ability due to presence of hydrophilic groups in nanofibers, carry out the diffusion of liquid into the hydrogel matrix more efficient and fast. In addition, the addition of nanofibers in hydrogels allows an increasing in the crosslinking point density, which had achieved an improved fracture, flexural force and resistance to compression, which promotes the formation of a porous morphology, improve biodegradability, thermal stability and water retention capacity to changes of temperature in the middle.
It should be taken into account that the use of non-environmental friendly solvents used to modify natural fibers is one of the disadvantages in the hydrogels synthesis, which is why there are some literature citations on the influence of the use of microwaves and ultrasound to mitigate the impact caused by solvents. This review approached in the efficiency of hydrogels applied to the soil, which depends on soil properties such as swelling ability, mechanical resistance, soil water retention and that the fiber proportions, crosslinking point and the modifying agent, determine the hydrogel performance.
