Fabrication of a water-retaining, slow-release fertilizer based on nanocomposite double-network hydrogels via ion-crosslinking and free radical polymerization

https://doi.org/10.1016/j.jiec.2020.10.014Get rights and content

Highlights

  • Double-network hydrogel beads were used as water-retaining and slow-release fertilizers.

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

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

Abstract

A new type of water-retaining, slow-release fertilizer (WSF) based on double-network hydrogels was fabricated via the ion-crosslinking of sodium carboxymethyl cellulose and the free radical polymerization of polymerizable β-cyclodextrin (MAH-CD), polyethylene glycol dimethacrylate (PEGDA), acrylamide (AM), and acrylic acid (AA) with urea-loaded halloysite as an additive. The effects of the AM to AA monomer ratio, the halloysite content, the AlCl3 content and the MAH-CD content on the swelling ratio were studied. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were applied to characterize the structure and properties of the WSF. The swelling behavior and water retention capacity of the fertilizer were investigated using a classic gravimetric method. The experimental results indicated that the presence of halloysite nanotubes clearly adjusted the swelling and release properties of the WSF. Kinetic modeling indicated that the swelling mechanism and slow release behavior were consistent with a Fickian diffusion mechanism. Form the considerations of its properties and raw materials, the fertilizer developed here has a good prospect of application and extension.

Introduction

Fertilizers are essential factors in the agricultural production process. To enhance the efficiency of fertilizer utilization and reduce the corresponding environmental problems, slow release fertilizers have been developed that can release nutrients in a slow and controlled way. This is an effective approach to replace traditional fertilizers, reduce the environmental pollution caused by excessive fertilization and improve crop yields [1].

In addition, water is essential to the growth of crops and can also help crops absorb fertilizers. Water and fertilizers interact with and influence each other, jointly promoting crop growth [2]. To fully utilize water sources, hydrogels, which have a three-dimensional polymeric network and are able to absorb and retain a large amount of aqueous solution [3], [4], have been used in agricultural production [5]. Hydrogels not only reduce irrigation frequency and prevent water loss, but also promote the formation of soil aggregates and improve the soil structure [6]. When plants need nutrients for growth, the nutrients absorbed by hydrogels from the soil are released via an exchange interaction [7]. Therefore, water-retention and slow-release fertilizers (WSF), a novel type of hydrogel-based multifunctional fertilizers which not only retain water but also control the release of nutrients, have been developed and attracted significant attention from scientist in agriculture [8].

However, most conventional hydrogels suffer from inherently poor mechanical strength, which has limited their application in certain fields [9]. Therefore, significant efforts have been devoted to developing hydrogels with excellent mechanical properties. Two common approaches to increasing the mechanical strength of hydrogels are forming a double network structure and filling the hydrogel with inorganic nanomaterials [10], [11], [12].

In contrast to single network hydrogels, double network hydrogels have excellent mechanical properties, including high toughness, high failure resistance and high water content, which make them good candidates for drug carriers [13], [14]. Ionic crosslinking is one of the most popular methods used to fabricate double-network hydrogels, and is commonly performed by incorporating multivalent cations. Qiao et al. fabricated a printable dual-network hydrogel by mixing acrylamide-modified hyaluronic acid, folic acid and Fe3+, forming the first layer using metal-carboxylate coordination bonds and forming the second poly(acrylamide) network layer by photopolymerization. They then studied its drug release behavior using acetylsalicylic acid as a model drug [15].

Recently, there have been many reports of using inorganic nano fillers. Among them, natural nanoclay mineralsare cost-effective and biocompatible [16]. Polymer/clay composite hydrogels often exhibit improved physicochemical properties in comparison with pure organic hydrogels. For example, the added clay can adjust the swelling behavior and controlled release behavior of the hydrogels [17]. Among various clays, halloysite nanotubes (HNTs) attracted significant attention due to their attractive properties [18], [19]. HNTs are naturally occurring aluminosilicates with hollow tubular structures which have lengths in the range of 0.2–1.5 μm and inner and outer diameters of 10–30 nm and 40–70 nm, respectively [20]. HNTs are an inexpensive and environmentally-friendly natural material with many surface active groups, including on the outer surface, edges and interior, making them excellent reinforcements for polymer nanocomposites used in a variety of biomedical application [21], [22].

Hydrogels can be prepared from synthetic or natural materials. The properties of synthetic polymers can be adjusted easily, but they are usually not biodegradable and consume non-renewable resources [23]. Therefore, hydrogels made from polysaccharides have attracted significant attention due to the abundance, non-toxicity and good biocompatibility of the polysaccharide source materials [24], [25], [26]. However, when used as drug carriers, pure polysaccharide beads have low mechanical strength and disintegrate, which can lead to a burst drug release profile. One important and efficient technique is the incorporation of inorganic nanoparticles into hydrogels [27]. Among the various natural polysaccharides, carboxymethyl cellulose (CMC) has been frequently used. CMC is one of the most important anionic polysaccharides and has many carboxyl and hydroxyl groups, which enables easy crosslinking with metal ions to form spherical hydrogel beads. Due to its unique properties, CMC is extensively used in the biomedical, food and agricultural industries [28], [29], [30], [31]. Recently, it has become a popular candidate for hydrogel preparation via metal coordination interactions due to its unique chemical structure and versatile biological function [32], [33], [34].

Considering that double network hydrogels incorporate the advantages of each network [35], [36], [37] and the release behavior of fertilizers and other properties can be tuned via changing the component and content of each network, double network hydrogels are especially suitable for the preparation of release-controlled fertilizers. In addition, natural materials have a promising application in agricultural fields due to their biodegradability, abundance and renewability [38], [39], [40]. Therefore, developing fertilizers based on double network hydrogel and natural materials is of significance.

Inspired by these previous results, we have fabricated a kind of double network fertilizer via the ionic crosslinking of sodium alginate and CaCl2 and the free radical polymerization of polymerizable β-cyclodextrin (MAH-CD), AA, AM and PEGDA (used as a crosslinking agent) in an aqueous solution of urea, in which HNTs loaded with urea were pre-suspended [7]. In this study, to further delay the release of urea, we replaced sodium alginate and CaCl2 with CMC and Al3+ to fabricate a double network hydrogel WSF. The water retention, swelling capacity and release behavior of the WSF were investigated. In best of our knowledge, there have been few previous reports of using this method for the preparation of a WSF. This design strategy opens a new approach to fabricating WSFs.

Section snippets

Materials

Acrylamide (AM), ammonium persulfate (APS), and sodium carboxymethyl cellulose (CMC) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Aluminum chloride (AlCl3), halloysite nanotubes (HNTs), and ascorbic acid (VC) were obtained from Aladdin Biochemical Co., Ltd. Polyethylene glycol dimethacrylate (PEGDA, Mn = 750) was obtained from Sigma Aldridge Reagent Co., Ltd. Urea was supplied by Tianjin Kaitong Chemical Reagent Co., Ltd. Acrylic acid (AA) was obtained from the Tianjin Damao

SEM analysis

Fig. 2 shows the surface, cross-section and interior morphologies of the WSF observed by SEM. It can be seen that the surface of the WSF sample (Fig. 2a) is smooth and dense, which hinders the rapid entry of water in the internal part of the WSF bead and help slow the release rate of urea. Fig. 2b and c shows that the outer layer of the WSF bead is dense while its internal structure is loose and porous. This is probably due to rapid ion crosslinking that formed a dense outer layer on the WSF

Conclusions

In this study, novel WSF beads containing double-network hydrogel were fabricated via the ionic crosslinking and free radical polymerization of CMC, PEGDMA, AA, AM, and MAH-CD and HNTs were incorporated. The results showed that the time to reach swelling equilibrium increased when the WSF was formed from a double-network hydrogel and the swelling process became stable and slow. One-factor experiments were used to optimize the experimental conditions for swelling and the optimal conditions were

Declaration of interests

None.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The work was supported by Key Scientific Research Project of Colleges and Universities in Henan Province, China [grant number 20A430008]; and the Key Science and Technology Project of Henan Province, China [grant numbers: 202102310008; 202102210040]; Henan University of Technology [grant number: HAUTZX202003].

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