Synthesis and characterization of double-network hydrogels based on sodium alginate and halloysite for slow release fertilizers

Highlights

• Alginate-based double-network hydrogel beads were used as fertilizers.

• The beads were fabricated via ion-crosslinking and the free radical polymerization.

• The fertilizers contain sodium alginate, halloysite nanotubes and β-cyclodextrin.

• The fertilizers have the advantages of slow-release and water-retention.

Abstract

In this work, novel sodium alginate–based double-network hydrogel beads were prepared and applied for the water-retention and slow release of fertilizers (WSF). The WSF beads were prepared by ion-crosslinking and the free radical polymerization of acrylic acid, acrylamide, and polymerizable β-cyclodextrin in the presence of urea-loaded halloysites. The WSF beads were characterized using SEM, FTIR, and TGA. Their swelling capacity and water retention were measured by a weighing method and their slow-release behavior was studied by spectrophotometry. The water retention and slow release results showed that the fertilizer displayed improved urea release and water retention properties, indicating that halloysite nanotubes clearly improved the performance of fertilizers. In addition, the experimental data of the slow release of urea in water and soil better fitted the Korsmeyer-Peppas model compared with the Higuchi model, a zero-order model or a first order mode. The release behavior of the fertilizer beads followed a Fickian diffusion mechanism.

Introduction

Hydrogels are three-dimensional polymers that can absorb and retain large amounts of water or aqueous solution. Due to their unique properties, including flexibility and good biocompatibility, hydrogels are widely applied in the drug delivery, agricultural, food, chemical and environmental industries [[1], [2], [3], [4], [5], [6]]. With the continuous growth of the world population and incessant reduction of cultivated area and water resources, improving the production of agricultural crops is an essential goal. The viability of agricultural crops is improved by the effective utilization of fertilizer and water. According to the report [7], fertilization has contributed 56.81% to the crop yield in China. However, 30–70% of the nutrients in conventional fertilizers are released into the environment, resulting in serious environmental pollution. Therefore, developing slow-release fertilizers (WSF) is an effective way to maintain fertilizer concentration, mitigate the loss of nutrients and promote steady crop growth. Compared with conventional fertilizers, WSF slowly release nutrients into the soil, improving the utilization efficiency of fertilizer [8]. Some materials have been developed to provide more control over the release of fertilizer nutrients into the soil [9,10]. Hydrogels have been applied to prevent water loss in the desert and improve water retention for cultivated plants [11,12]. Base on this, hydrogels for use as slow-release fertilizers have been prepared using several different methods and materials. Bortolin et al. prepared a methyl cellulose/montmorillonite hydrogel for sorption and desorption study of a nitrogenated fertilizer, urea (CO(NH₂)₂) [8]. Olad et al. studied a maize bran-based superabsorbent nanocomposite for use as a slow-release fertilizer and tested its swelling and kinetic properties [9]. Essawy et al. synthesized chitosan-cellulose-grafted poly(acrylic acid) hybrid hydrogels for the controlled release of soil nutrients [12].

To reduce the cost of fertilizers as well as their influence on environment, scientists have been paid more and more attention on the fertilizer prepared from non-toxic, biocompatible, abundantly available and relatively cheap natural materials such as natural polysaccharides and clays [[8], [9], [10], [11]].

Sodium alginate (SA), a natural polysaccharide macromolecule composed of β-d-mannose uronic acid and α-l-guluronic acid, is extracted from brown algae and is non-toxic, biodegradable and has good biocompatibility. SA has been widely studied and applied in textiles [13,14], food [15,16], medicine [17,18], and fertilizer [19] because the structure of SA contains two secondary hydroxyl groups (-OH) at C-2 and C-3 positions and a carboxyl group (-COOH) at C-6 position [20], which make SA hydrophilic and modifiable and give it varying properties at different solution pH values. SA can be used to prepare porous membranes, microspheres or beads via crosslinking with ions such as Ca²⁺ and Ba²⁺ [21].

However, SA hydrogels have poor swellabilities, and their mechanical properties remain to be improved [22]. Clay particles have shown promise for several biological and industrial applications due to their physico-chemical properties. Among clays, halloysite nanotubes (HNTs) are considered to be the most promising for application in many fields due to their hollow tubular morphology, large specific area and tunable surface chemistry as well as their hydrophilicity resulted from Al-OH and Si-OH groups [[23], [24], [25]]. In aqueous solution, HNTs have a negatively charged outer surface and positively charged inner surface, and can easily form electrostatic interactions with different charged molecules [26]. Owing to their special structure, HNTs are studied widely in pharmaceutics and medicine as carriers of active agents such as drugs, proteins, etc. [27]. Recently, our group developed a kind of slow-release and water-retention fertilizer (WSF) based on starch and HNTs by free radical copolymerization [10]. More recently, Assimi et al. fabricated coating materials by PCL grafted on guar gum and HNTs based on in situ ring opening polymerization and used them as coatings for slow-release diammonium phosphate fertilizer [28]. The research results showed that HNTs have wide application prospects in fertilizer field.

To broaden the application of hydrogels, numerous work recently have been carried out to improve the mechanical properties of hydrogels. Among them, double network hydrogel with a combination of physical, covalent or ionic bonds have drawn extensive attention in recent years [29]. The double network hydrogel comprises a rigid network and a flexible network, which can effectively conduct and disperse pressure and give excellent mechanical properties to the hydrogel under the synergistic effects of two networks [30,31]. Bi et al. prepared a double network hydrogel with superior compressive, tensile, recoverability and anti-swelling properties by using freezing-heating alternate treatment to the chitosan-poly(vinyl alcohol) alkaline solution, which is a hopeful material for application in the tissue engineering repair [32]. Ma et al. prepared a double network from the waste cotton fabrics and polyacrylamide and studied its heavy metal ion absorption characteristics [33]. Sarıyer et al. fabricated pH-responsive alginate/kappa (κ)-carrageenan double network hydrogel beads and studied its release behavior to Bovine Serum Albumin (BSA) in simulated gastric fluid and intestinal fluid. The result indicated the double network could slow down the release of BSA and eliminated burst release [34]. Some studies on the double network hydrogel used for drug delivery have been published recently, there are few reports on its use for slow release fertilizer [35,36];

Based on this context, a novel double network hydrogel was prepared for application as a slow-release fertilizer using ion-crosslinking of SA and the free radical polymerization of polymerizable β-cyclodextrin, acrylic acid, and acrylamide in the presence of HNTs. Urea was preloaded into the cavities of the HNTs under vacuum. The swelling capacity of the WSF was investigated in water and in different salt solutions. The urea release behavior and release kinetics in soil and water were also studied, and the release mechanism was determined. To our knowledge, there have been few reports of using a new double network hydrogel based on SA, HNTs and polymerizable β-cyclodextrin to prepare the carrier for a slow-release fertilizer. Considering the advantages of the double network hydrogel, this fertilizer has potential for application in agriculture and horticulture.