Introduction

Hydrogels are 3D-structured networks which can be polymerized by physical or chemical crosslinking using water-soluble polymers or monomers as gel solution. In a standard approach, the polymerization reaction is initiated by external stimuli, such as thermal, chemical activation, or irradiation, through initiators and/or catalysts. Hydrogels’ unique characteristics, such as desired functionality, reversibility, and biocompatibility, meet both material and biological requirements; thus, they are commonly used for tissue regeneration, cell-laden, drug release, biosensor, and so on (Jiang et al. 2019; Mahinroosta et al. 2018; Ullah et al. 2015). Also, they can be used to improve electrical conductivity (Lin et al. 2019a), release agrochemicals (Chen and Chen 2019), and reserve water in soil (Gholamhoseini et al. 2018). In recent years, there has been a growing interest in the application of hydrogel adsorbents in pollution control due to their super adsorption capacity.

As is known to all, emerging contaminants (ECs) have raised global concern for their significant threat to aquatic ecosystem and human health. Many ECs have no regulatory standards due to the lack of information on the effects of chronic exposure. Pharmaceuticals and endocrine-disrupting chemicals are some of the most frequently detected ECs in aquatic environments (Tran et al. 2018). Conventional wastewater treatment processes are not currently designed to sufficiently treat and remove ECs from the influent, with the remaining pollutants entering the environment inevitably (Wang et al. 2019b; Zhao et al. 2018a). These contaminants can accumulate and thus cause adverse effects on ecosystems (USEPA 2019, accessed Sep 2019; Zhao et al. 2018a) because they are stable under various conditions such as aerobic digestion, light, and heat. Therefore, adsorption technology has been extensively adopted to treat ECs as it is cost-effective and highly efficient.

Among various adsorbents (Elgarahy et al. 2019; Hai 2016; Kiruba et al. 2014; Neeraj et al. 2016; Senthil Kumar et al. 2018), hydrogel adsorbents show super high adsorption capacity, fast adsorption efficiency, and wide pH independence; thus, they can be chosen as an alternative for ECs’ elimination. However, the poor mechanical strength of hydrogels limits their application in the elimination of pollutants because of the harsh environment during the water treatment process; therefore, it is imperative to develop suitable reinforcing agents or fillers into hydrogel networks to improve their mechanical strength (Alam et al. 2018; Zhao et al. 2015). Some inorganic materials can not only be utilized as fillers to improve physical and chemical properties of hydrogels (Afzal et al. 2018; da Silva et al. 2018; Kang et al. 2019; Rao et al. 2019; Rotbaum et al. 2019; Tong et al. 2019; Yu et al. 2018a), but also satisfy specific roles, for example, the incorporation of TiO2 can provide the hydrogel with photocatalytic properties (Yu et al. 2018b), and Fe3O4 (Liang et al. 2019b; Meng et al. 2019; Tang et al. 2014; Zhang et al. 2019b) and Fe2O3 (Badsha and Lo 2019; Khan and Lo 2017) can afford the hydrogel with magnetic separation and oxidant properties.

A host of review papers is dedicated to the application of hydrogel in adsorption of dyes, metals, and some other inorganic substances (Khan and Lo 2016; Van Tran et al. 2018; Zheng and Wang 2015), but to our knowledge, using hydrogel adsorbents to remove ECs has not been extensively reviewed. Furthermore, while the adsorption property of hydrogels has been commonly investigated, their sustainable regeneration methods are not developed sufficiently. As is known to all, a suitable regeneration method is a major challenge for low-cost and long-term running. Hydrogels have the potential as carriers for catalyst immobilization in order to eliminate pollutants completely for reuse purpose, but that has not received much attention until now. This review aims to provide a coverage of the latest developments in the modification of hydrogels, especially by inorganic fillers, and consequently, their changes in mechanical strength, swelling ratio, and adsorption property. Furthermore, hydrogels as adsorbents for removing ECs and as carriers for immobilizing catalysts are reviewed.

Throughout this article, the name of a hydrogel will be based on its monomer (or polymer)/monomer (or polymer)-filler. For example, a hydrogel composite consisting of alginate sodium (SA), hyaluronic acid (HA), and Fe2O3 will be mentioned as SA/HA–Fe2O3.

Preparation of hydrogel

Hydrogels can be synthesized using natural, synthetic, and/or blend of natural/synthetic materials. The free radical polymerization is the most widely used method to prepare hydrogels (Qi et al. 2019a; Shariatinia and Jalali 2018; Ullah et al. 2015). In a typical hydrogel formulation, the gel precursor reacts with crosslinker(s) to form 3D-crosslinked networks. Examples of some commonly used crosslinking agents include potassium persulfate or ammonium persulfate (Khan and Lo 2016). As shown in Fig. 1, free radical polymerization comprises three main stages: initiation, propagation, and termination (Khan and Lo 2016). In the initiation stage, free radicals (R●) are generated by the dissociation of an initiator. The polymerization reaction is usually initiated using thermal stimulus (Zhuang et al. 2019). Besides, irradiation, e.g., γ-ray or ultraviolet ray, has recently attracted much attention, because it allows rapid conversion of gel solution to solid hydrogel under physiological condition (Aycan and Alemdar 2018; Du and Piao 2018; Sun et al. 2019b). Also, the combination of the two methods is feasible (Yu et al. 2018b). The free radicals then react with molecules of monomer (or polymer) (M) to produce the first radicals M●. Due to the very high reactivity of free radicals, they instantly react on molecules of monomer (or polymer), leading to growth of macroradicals (propagation stage). Termination usually takes place by combination or disproportionation reaction. Various shape of hydrogels, such as bulk (Song et al. 2019), sphere (Liu and Garcia 2016; Mohammadian et al. 2019; Wang et al. 2018b), and film (Getachew et al. 2019; Sun et al. 2019b) can be designed by choosing the appropriate preparation method, raw material, and polymerization condition.

Fig. 1
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Free radical polymerization process

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Preparation of bulk hydrogels

Generally, bulk hydrogel can be easily obtained by solution polymerization. It is a homogeneous polymerization in which all the constituents including the monomer (or polymer), initiator, and crosslinking agent are soluble in the medium. The synthesized bulk hydrogel usually takes the shape of the container in which it is polymerized (Qi et al. 2019a). Song et al. stirred the mixture of 5% carboxymethyl cellulose (CMC) and 4% polyethylenimine (PEI) solution added with 2% crosslinking agent epichlorohydrin (ECH) before pouring them into a closed tube, and placed in a constant temperature drying oven at 60 °C for 4 h to obtain white cylindrical CMC/PEI hydrogel, then it was directly used for removing Cr(VI) (Song et al. 2019). Qi et al. prepared salecan hydrogel for removal of Cu(II). They first mixed all the solution, including 2% salecan, crosslinking agent, and initiator, and then poured them into a Teflon mold and kept at 70 °C for 6 h to facilitate the polymerized reaction (Qi et al. 2019b).

This approach can yield a relatively homogeneous hydrogel. However, long equilibrium time is often reported during water treatment (Tang et al. 2010), due to the slow diffusion transport of pollutant to the adsorption sites within hydrogel networks. Thus, for better mass transfer efficiency, bulk hydrogel is often cut into a small-sized pieces manually (Shah et al. 2018) or is transferred to a food blender to produce hydrogel beads having a much smaller size than bulk shape (Thompson et al. 2018). However, the cutting or grinding step has the possibility to yield hydrogel particles with polydispersity and damaged morphology. So, spherical hydrogel is an alternative because grinding or cutting is not required and thus avoids further energy consumption and morphology destruction.

Preparation of spherical hydrogels

Hydrogel bead is one type of spherical hydrogel with a millimeter diameter. It is generally synthesized by dropping the monomer (or polymer) suspension solution into a hardened solution by a syringe; thus, the size of the hydrogel bead is usually dependent on the diameter of the syringe. SA is the most commonly used material for the synthesis of hydrogel bead (Thakur et al. 2018). The interest in the preparation of SA hydrogel comes from its ionic characteristics. It has carboxylic groups along its chains, which can be converted to ionized form, generating electrostatic anion–anion repulsion forces that make it easier for the polymeric chains to expand in water. SA precursor forms to 3D-crosslinked hydrogel networks by contacting with hardened solution of cations, such as Ca(II) (Ohemeng-Boahen et al. 2019; Pathak et al. 2016; Wang et al. 2019a; Yang et al. 2018b; Zhao et al. 2018b) and Ag(I) (Lengert et al. 2019). During the gelation process, the hardened cations bind to two carboxyl groups on the adjacent alginate molecules which can be explained by the egg-box model. Chitosan (CS), a biodegradable, biocompatible, odorless, and nontoxic biopolymer, is another commonly used natural polysaccharide for hydrogel bead preparation. Similarly, CS can be crosslinked by contacting with cations, such as Na ion (Afzal et al. 2019; Ahmad et al. 2019; Kluczka et al. 2018; Ren et al. 2019) and K ion (Bilal et al. 2019a). Other materials are also used for hydrogel bead synthesis. Benhalima et al. mixed the solution of CMC and dextran sulfate, and then dropped the mixture into Al(III) solution to prepare hydrogel bead (Benhalima and Ferfera-Harrar 2019). Polyacrylic acid (PAA) hydrogel bead was obtained by dropping the precursor solution into polyethersulfone/N,N-dimethylformamide (PES/DMF) using a syringe needle with 0.7 mm in diameter, and then applied to remove dyes of MB and MO (Yang et al. 2019a). Boric acid (Mohammadian et al. 2019) and liquid nitrogen (Yu et al. 2015) can also be used as hardening agent to make hydrogel bead.

Although hydrogel bead can be applied directly for pollutant adsorption process, the size is too big to transfer the pollutant effectively (Luo et al. 2019), and usually a long time is required for adsorption equilibrium. In addition to a high adsorption capacity, fast kinetic (short equilibrium time) is another desirable feature of an adsorptive treatment system from a practical viewpoint; thus, a smaller size is favored. Particle form (micro- or nano-sized) hydrogel can be synthesized directly by utilizing heterogeneous polymerization techniques (Liu and Garcia 2016); among them, a liquid–liquid two-phase system is the most common method involving polymerization in bulk solution. Different types of heterogeneous polymerizations, distinguished by the solubility of the monomer (or polymer), initiator, and resulting polymer, have been reviewed by Elbert (2011). Generally speaking, a mixing or homogenization step, by a vortex mixer or a homogenizer, is employed to generate droplets of monomer (or polymer) in one phase, surrounded by another phase (W/O or O/W). The surfactant often serves to prevent re-aggregation of the droplets in the system. After adding the initiator and adjusting the environment correctly (e.g., heating the system to a sufficient temperature for a thermal initiator), the initiator reacts to form free radicals that start the polymerization process. The polymerized product is then collected, washed, and optionally lyophilized for storage. Tuning of particle size can be achieved by applying proper methods, such as emulsion polymerization, suspension polymerization, dispersion polymerization, and precipitation polymerization. In a special method, the particle size can be further controlled by altering certain parameters such as mixing speed and reaction temperature.

The major drawbacks in liquid–liquid two-phase systems include the wide distribution of particle size and uneven distribution of specific functional groups in hydrogel networks. The microfluidic approach can solve these problems. The microfluidic synthesis scheme is continuous as opposed to batch system, and by controlling the pattern of aqueous droplets, uniform distribution of particle size and functional groups can be achieved (Dehli et al. 2019; Liu et al. 2019a; Sun et al. 2018) at the cost of a slower synthesis speed (Liu et al. 2019d). However, the application of a microfluidic device as reactor for mass production of hydrogel is still not practical due to the complexity of the design and operation.

Preparation of hydrogel films

A film is a polymer material with selective function for separating, purifying, or concentrating different components of solution in water treatment. In practice, water filtration membrane can be damaged due to several causes, such as debris that enters the system during installation, operation, and maintenance (Goh et al. 2019). One class of membrane that seems to have a great potential for real application is hydrogel film, especially hydrogel–composite film (Raval et al. 2015; Thakur and Voicu 2016). Many hydrogels exhibit robust and repeatable self-healing in the presence of water, and they can be used as an efficient ion-exchange film for water purification (Mirabedini et al. 2016; Upama Baruah and Chowdhury 2016). Ma et al. reported a simple and fast method with low-energy consumption sunlight polymerization to directly fabricate a poly(amidoxime) hydrogel film for uranium adsorption (Ma et al. 2019).

Hydrogel film is commonly used for sensor in which it is usually employed as an active layer of membrane, primarily to render the surface of the membrane more hydrophilic and less prone to fouling (Erfkamp et al. 2019; Gogoi et al. 2015; Yin and Ma 2019). Wu et al. designed and fabricated a visual volumetric sensor platform with fluorescein-crosslinked polyacrylamide (PAM) hydrogel. The sensor underwent volumetric response to S2−, enabling its visual quantitative detection (Wu et al. 2019b). Nam et al. described a colorimetric hydrogel biosensor for rapid detecting nitrite ions using polyethylene glycol (PEG) hydrogel superimposed glass fiber membrane strips (Nam et al. 2018). Kishore et al. investigated a stimulus-responsive hydrogel encompassed with tetraalkylammonium salts which was specifically sensitive to Cr(VI) (Kishore et al. 2017). For detection of Fe(II), Martínez et al. prepared a PAM hydrogel film first and then the film was immersed in ligand solution which is sensitive to Fe(II) (Martínez et al. 2017). Zhang et al. designed a carboxylated poly(N-isopropylacrylamide) (PNIPAAM) hydrogel and spin coated it on a gold surface by UV light irradiation for the optical sensitive detection of 17β-estradiol (E2) (Zhang et al. 2013). Sun et al. mixed the PAM/cytochrome gel solution and coated it drop by drop on the surface of the retreated GCE for detection of bisphenol A (BPA) (Sun and Wu 2013).

It is easy to prepare hydrogel film. Usually, filtration substrate is soaked in the hydrogel precursor solution (Yin and Ma 2019), or the hydrogel precursor solution is deposited on top of the filtration substrate (Balasubramanian et al. 2018; Getachew et al. 2019; Nguyen and Liu 2014), then the coated substrate is dried (Mirabedini et al. 2016; Upama Baruah and Chowdhury 2016) or irradiated (Getachew et al. 2019; Sun et al. 2019b) to form hydrogel film. Recently, the electrospinning technique is chosen as a method to directly prepare nanofiber hydrogel film (Zhang et al. 2019a). In order to synthesize a hydrogel composite with a certain special aim, some additive can be loaded into the hydrogel film either by mixing additive with hydrogel precursor before polymerization or being grafted to the hydrogel film after polymerization (Zhang et al. 2018b).

Modification of hydrogels

Materials used for hydrogel synthesis are hydrophilic and flexible, but most of the hydrogels fracture easily, even under mild loading condition, because of their poor mechanical strength and toughness. Therefore, enhancing the strength mechanical plays a key role from a practical view. In recent years, a large effort has been done on tailoring the mechanical properties of these hydrogels to make them more suitable for various applications. Traditional methods, such as physical mixing or chemical grating, can achieve this goal at a considerable degree. Another popular method is the introduction of inorganic materials into hydrogel networks as reinforcing agents to prepare hydrogel composites, which is more effective and simple. Various modification methods are summarized in the following, with emphasis on hydrogel composites and the changes in hydrogels’ properties.

Modification of hydrogels by traditional methods

During preparation of hydrogel, the mixture of more than one type of monomer (or polymer) is a commonly used method to improve the properties of hydrogel. Liu et al. found that the addition of poly(vinyl alcohol) (PVA) into hemicelluloses grafted PAA/bentonite greatly improved the mechanical strength and water absorbency (Liu et al. 2019b). Emami et al. found that increasing the oxidation of SA led to denser structure with smaller pores when SA was incorporated into gelatin hydrogel (Emami et al. 2018). Zhang et al. found that the presence of xanthan gum (XG) visibly improved the swelling ratio of PVA due to the formation of hydrogen bonds between the PVA and XG (Zhang et al. 2019c). The property of hydrogel can also be tuned by controlling the degree of crosslinking through adjusting the synthesis parameters, such as reaction temperature (Baek et al. 2018; Kim et al. 2018), gel time (Bobula et al. 2017), gel pH, crosslinking agent (Xu et al. 2017), and respective amount of different types of ingredients (Liu et al. 2018b; Pettinelli et al. 2019). But it is not sufficient to form polymeric networks if the crosslinking degree is too low, whereas too high results in compact density networks that may prevent the pollutant adsorption (Qi et al. 2018). Besides, the postprocessing technique had a strong correlation with the hydrogels’ properties. Compared with air-dried hydrogels, hydrogels dried by freezing swell significantly because the freezing process can yield micropores (Heimbuck et al. 2019). The freezing (Genevro et al. 2019) and the cyclic freeze–thaw method (Long et al. 2019) are promising and simple techniques for preparing hydrogels with improved mechanical properties.

Chemical modification is another common method to enhance hydrogels’ properties. For examples, HA hydrogel post grafted with tannic acid showed improved mechanical properties compared with the pure one (Lee et al. 2018). The mechanical property, morphology, and swelling behavior of the methacrylated-carboxymethyl chitin hydrogels (Me-CMCH) could be tuned by controlling the degree of methacrylation (Kang et al. 2017). Karbarz modified the N-isopropylacrylamide (NIPAM) hydrogel with diacryloyl derivative of cystine to improve the adsorption of Cd(II) and Pb(II) (Karbarz et al. 2018). By adding sodium lignosulfonate to acrylic acid/maleic anhydride (AA/MA) hydrogel, water adsorption and Ni(II) adsorption were changed (Wang et al. 2018e). Hafezi et al. found that the adsorption of MB could be improved by adding tannic acid into pectin/dopamine hydrogel (Hafezi Moghaddam et al. 2019). CS modified by sebacoyl chloride led to a remarkable improvement in the adsorption of Hg(II), Ni(II), and Co(II) (Kandile and Mohamed 2019). Cellulose/lignin hydrogel post grafted with AA or methacryloxyethyltrimethyl ammonium chloride could increase the Cr(VI) adsorption capacity (Wang et al. 2018f).

Modification of hydrogels by inorganic fillers

Although traditional methods play an important role in modifying hydrogels, most of these methods need sophisticated control procedures. In contrast, hydrogel composites are relatively simpler to design. Compared with organic materials (Huang et al. 2018a; Jung et al. 2019; Shojaeiarani et al. 2019; Xing et al. 2019; Yue et al. 2019a), inorganic components are widely used because they are much cheaper and tougher, and often display high specific surface area, sufficient reactive sites, and fast dissolution; thus, it is a promising method that introduces inorganic materials into hydrogel networks (Wang et al. 2018a). In addition to high mechanical strength and low cost, the merit of inorganic materials is its easy preparation. Two major synthesis routes, solution mixing (Sun et al. 2019a) and impregnation (Dargahi et al. 2018; Wahid et al. 2016), have been shown schematically in Fig. 2. The first technique involves the homogeneous dispersion of inorganic materials in monomer (or polymer) solution followed by polymerization. Inorganic materials can be dispersed in most solvents by imposing an external force ultrasonication or mechanical stirring. Besides the mechanical approaches, chemical approaches such as surface functionalization (Hu et al. 2018; Van den Broeck et al. 2019) and surfactant stabilization (Rassu et al. 2016) are often adopted to disperse inorganic materials uniformly with hydrogel precursor solution. It should be noted that the uniform dispersion of inorganic materials in a solvent does not guarantee a similar dispersion state in the resulting hydrogel composites. Impregnation is such a technique that pure hydrogel synthesized first is immersed into inorganic materials solution to obtain hydrogel composites, or into ion solution followed by being deposited by electrospinning (Im et al. 2010) or precipitation (Li et al. 2019; Pourjavadi et al. 2019) to obtain well-dispersed nanoparticles (NPs) which are embedded in hydrogel networks. This review will mainly focus on the addition of inorganic materials into hydrogels and consequently the property changes in mechanical strength, swelling ratio, and adsorption capability toward aqueous pollutants (Table 1).

Fig. 2
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Two major synthesis routes of inorganic hydrogel composite. a Solution mixing. b Impregnation

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Table 1 Hydrogel composites based on different inorganic materials and methods
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Mechanical strength

Carbon materials, including graphene, graphene oxide (GO), and carbon nanotubes (CNTs) (Alam et al. 2018), have been the center of intensive research because of their unique structure and technologically useful properties. For example, starch hydrogel presented enhanced mechanical performance, antimicrobial activity, and increased conductivity values by introduction of graphene (Gonzalez et al. 2018). Gelatin–GO hydrogel exhibited a significant increase in mechanical properties by up to 288% in compressive strength, 195% in compressive modulus, 267% in compressive fracture energy, and 160% shear storage modulus with the optimal GO concentration (Piao and Chen 2017). Qin et al. proposed a CS–GO hydrogel composite and it showed 76% increases in the Young’s modulus with the addition of 0.5 wt% GO (Qin et al. 2018). Huang et al. found that addition of GO into CS hydrogel could increase compressive strength and work of compression (Huang et al. 2018b). Not only the mechanical property but also the thermal property was improved by the introduction of GO into PAA/amylose hydrogel (Abdollahi et al. 2018).

Clay is another commonly used material for improving the mechanical property of hydrogels. For example, PAM–calcium hydroxide nanospherulites (CNS) hydrogel exhibited excellent mechanical behavior even considering low CNS content. Such gel could sustain high compressive stress at 97% strain and revert to its original size in 1 s after loading was released (Hu et al. 2019a). The same phenomenon could be found in cellulose–MMT (Peng et al. 2016) and PAM–silica NP hydrogels (Zareie et al. 2019). TiO2 added into SA/PAA (Thakur et al. 2016), poly(N,N-dimethylacrylamide) (PDMAA) (Liao et al. 2014), κ-carrageenan/xanthan/gellan (Balasubramanian et al. 2018), and CS (Liu et al. 2018c; Montaser et al. 2018; Ramos et al. 2016) can strengthen hydrogels’ mechanical properties. TiC (Hu et al. 2019b) and tannic acid-modified Fe3O4 (Fe3O4@TA) (Hu et al. 2018; Liao and Huang 2019a) enhanced both mechanical strength and thermal stability. Of course, the strength of hydrogels is related with the amount of additives. The Young’s modulus of PVA/iota–carrageenan increased by the addition of 0.25% TiO2, but it decreased with a further increase of TiO2. Tensile stress and breaking stress behave similarly (Badranova et al. 2016). Although the fraction of additives is critical, it is usually not proportionate to mechanical properties, so the fraction should be adjusted to obtain a balance.

Swelling ratio

The swelling behavior of hydrogel is influenced by the presence of inorganic materials and usually decreases with the addition of inorganic materials, which means increased crosslinking degree and mechanical strength. Workers found that the GO embedded into gelatin/polyethylene glycol diacrylate (PEGDA) (Mamaghani et al. 2018) or cellulose/PAA/PAM (Dai et al. 2019) decreased their swelling ratios. The addition of MWCNTs also decreased the swelling ratio of hydrogels (Liu et al. 2019c; Mohammadinezhad et al. 2017). By increasing the dosage of carbon dots (CDs) from 0 to ~ 1%, the swelling ratio of keratin/PVA hydrogel was decreased (Lee et al. 2017). The halloysite nanotubes (HNTs) imbedded into CS/cellulose (Kumar et al. 2019) or SA (Huang et al. 2017) could improve the mechanical property and decrease the swelling ratio. Dargahi et al. prepared κ-carrageenan hydrogel first, and the swollen hydrogel was immersed in silver nitrate solution followed by transferring to sodium chloride solution, and thus, AgCl NPs were introduced, leading to the decreased swelling ratio compared with pure hydrogel (Dargahi et al. 2018). By introduction of Fe3O4 NPs into salecan, water uptake was decreased also (Hu et al. 2018).

Generally, the inorganic material plays a reverse role in the swelling ratio that usually decreases with the addition of inorganic material as presented above. However, there are exceptions, for example, the swelling ratio of PVA hydrogel increased by 56.3% with only 1 wt% boron nitride nanofibers added (Li et al. 2018). E-glass fiber (Martin and Youssef 2018), acid-activated MMT (Vieira et al. 2018), ZnO, and PEG-functionalized CNTs (Van den Broeck et al. 2019) increased the swell ratio of hydrogels. Qin et al. found that the average pore diameter and porosity was increased by adding GO into CS, leading to the reduction of swelling ratio (Qin et al. 2018). But the opposite results were obtained by Saravanan et al. who revealed that the addition of GO into CS decreased the porosity and increased the swelling ratio (Saravanan et al. 2018). The addition of TiO2 into κ-carrageenan, xanthan gum, or gellan gum (Balasubramanian et al. 2018) and PEGylated CS (Liu et al. 2018c) reduced the swelling ratio, whereas it was increased when embedding TiO2 into collagen/CS (Zazakowny et al. 2016). These different results may be caused by different interactions between water and various hydrogel composites. It is important to note that the swelling characteristics are related to the concentration of additives. Ding et al. prepared PAM–Fe(III) hydrogels by immersing the PAM hydrogel in various concentration of FeCl3 solutions based on coordination complex. The water content of PAM–Fe(III) hydrogels first decreased and then increased as CFe(III) increased from 0.01 to 0.25 M achieving the minimum value at CFe(III) = 0.05 M (Ding et al. 2019).

Adsorption capability

The addition of inorganic materials into hydrogels usually improves the adsorption capability. The amount of Cu(II) adsorbed by PAM/SA–MWCNTs was 1.28 times higher than that of PAM/SA owing to its multiwalled hollow nanostructure (Yue et al. 2019b). Adding MWCNTs into hydrogels also showed better adsorption behavior toward Pb(II) (Mohammadinezhad et al. 2017) and MB (Makhado et al. 2018) than pure hydrogels. The dyes of MB (Dai et al. 2019; Geng 2018; Kong et al. 2019b; Liu et al. 2018a; Salzano de Luna et al. 2019; Zhuang et al. 2019) and Nile Red (Sun et al. 2018) adsorbed by hydrogels were largely increased by adding GO. Metal ions, such as Cu(II) (Geng 2018) and Hg(II) (Zhuang et al. 2019) adsorbed by cellulose or selenocarrageenan hydrogels, were also improved due to the presence of GO.

Besides carbon-based materials, SiO2 NPs can also enhance hydrogels’ adsorption capacity for removal of MB (Panão et al. 2019) and metal ions such as Co(II), Cu(II), Pb(II), and Zn(II) (Pourjavadi et al. 2015). The addition of MMT into PVA/SA/CS (Wang et al. 2018d) or cellulose (Wang et al. 2019c) could enhance the adsorption of MB. Also, the adsorption of Pb(II) was enhanced by adding MMT into CS hydrogel (Wang et al. 2019d). The addition of HNTs into the PAA hydrogel showed better adsorption of rhodamine B (RhB) than the bare PAA hydrogel (Bethi et al. 2018). Metal compound NPs can also be used as hydrogel fillers. The addition of AgCl NPs into κ-carrageenan hydrogel showed better adsorption toward crystal violet (Dargahi et al. 2018). By imbedding Fe3O4 NPs into chitin (Liao and Huang 2019b) or PVA/CS (Wu et al. 2019a), the Cu(II) adsorption capacity was enhanced.

These inorganic fillers are responsible for the significant improvement of mechanical and functional properties. It is worth noting that some inorganic materials not only provide a strong support to hydrogels and change the swelling ratio and adsorption capability, but also help to form hydrogel networks, such as GO, which acted as multifunctional crosslinking sheets and reduced the gelation time by nearly 20% only with a small amount of GO (0.5% weight ratio of CS into CS) (Qin et al. 2018). The same phenomenon was found in MMT which can also reduce the cellulose gelation time (Peng et al. 2016). Layered double hydroxide (Yang et al. 2019b) and Al2O3 NPs (Xu et al. 2018) could also be used as crosslinking agents. This phenomenon was more evident for TiO2 which can be used as light initiator to totally replace conventional chemical crosslinking agents (Glass et al. 2018; Kangwansupamonkon et al. 2018). These studies confirm that some inorganic materials can act as an effective physical crosslinker for hydrogel synthesis, reduce the gel time, and improve the properties of hydrogels in the meanwhile.

Application of hydrogels

Hydrogels used as adsorbents

Adsorption capacity

The reported adsorption capacities for ECs are summarized in Table 2. The adsorption process strongly depends on the type of ECs as well as the hydrogel adsorbents. The maximum adsorption capacity of various hydrogels for BPA in water at the best conditions was between several and hundred milligrams per gram of adsorbents after tens of minutes of reaction. But much time was necessary if the size of hydrogels was too big to mass transfer quickly (Luo et al. 2019). The maximum adsorption capacities of ciprofloxacin (CIP) were 100, 154.89, and 200 mg/g for SA–GO bead, CS–biochar bead and κ-carrageenan/SA bead (Zhao et al. 2018b; Afzal et al. 2019; Yu et al. 2019), respectively. The PNIPAAM/CS–Fe3O4 hydrogel showed maximum adsorption capacities of 164.3 mg/g for BPA, while that for sulfamethoxazole (SMZ) was 15.7 mg/g (Zhou et al. 2019). However, SMZ adsorbed by poly(2-hydroxyethyl methacrylate)/poly(N-methyl maleic acid)–CuS (PHEA/PNMMA–CuS) hydrogel was 3.86 mg/g (Yang et al. 2017). Diclofenac sodium (DS) adsorbed by CS–Fe3O4 and SA/cellulose/PVA were 469.48 mg/g (Liang et al. 2019a) and 418.41 mg/g (Fan et al. 2019), respectively. It is worth mentioning that the environmental parameters such as pH, initial pollutant concentration, and adsorbent dose are critical factors which show significant effects on ECs’ adsorption capacity.

Table 2 Adsorption behavior of ECs from aqueous solution by hydrogel adsorbents
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Removal mechanism

As presented in Table 2, the adsorption data of ECs on hydrogels can be well interpreted by the Freundlich model or Langmuir model, with the kinetic model being pseudo-second-order in most cases. Hydrogel-based adsorbents have strong affinity for ECs, mainly due to the diverse interactions of these adsorbates with the adsorbents, including hydrophobic interaction, electrostatic interaction, hydrogen bonding, and π–π interaction, which varies according to the adsorbent, chemical nature of the adsorbate, pH, the ionic strength of the solution, and so on.

Hydrophobic interaction

Hydrophobic interaction is the main mechanism responsible for the hydrophobic organic compounds. The maximum amount of CIP adsorbed by SA–GO hydrogel occurred at neutral pH of 7.0, when CIP was in zwitterionic form. At this pH, SA–GO can provide more hydrophobic sites, which in turn increased the hydrophobic interaction between SA–GO and CIP molecules (Zhao et al. 2018b). It also played a crucial role in CIP adsorption on the surface of humic acid-modified CS–biochar (Afzal et al. 2019). Due to the hydrophobicity of BPA, hydrophobic interaction between BPA and the hydrophobic group isopropyl of PNIPAAM/CS–Fe3O4 might occur. With an increase in pH, BPA molecules were dissociated, which led to the destruction of hydrophobic interaction and, thus, a decrease in BPA adsorption (Zhou et al. 2019).

Electrostatic interaction

Electrostatic interaction is another mechanism for hydrogel materials to adsorb ECs. When the solution pH is different from the isoelectric point (pHpzc) of hydrogel adsorbents, the surface of hydrogel adsorbents will carry either a positive or negative charge. At the same time, ionizable ECs can also be protonated or deprotonated and become charged at different pH values, and the resulting electrostatic interactions between them may influence the adsorption property of ECs. The amount of DS adsorbed by humic acid-modified CS–Fe3O4 hydrogel decreased with increasing pH, owing to repulsive electrostatic interactions between the negative surface charge of hydrogel and the negatively charged anionic form of DS (–COO−1 and –O−1). When pH < pHzpc, the CS–Fe3O4 surface had a positive charge and can adsorb negatively charged DS. However, when pH > pHzpc, the CS–Fe3O4 surface had a negative charge, which cannot attract DS with a negative charge on the surface (Liang et al. 2019a). Fan et al. found that the SA/CNC/PVA hydrogel surface charge was positive when pH < 5.3, and when pH > 5.3, the surface charge was negative. The –COOH in DS was ionized when pH > 4, mainly in the form of anions. Therefore, when the pH was between 4.2 and 5.3, the positively charged surface of SA/CNC/PVA had electrostatic interactions with anionic DS. At pH > 5.3, the adsorption capacity decreased sharply, mainly because the repulsion force between SA/CNC/PVA and DS in anionic form increased (Fan et al. 2019). For SMZ, the adsorption capacity by CS hydrogel reached maximum at an equilibrium pH of around 6 and decreased on either side of this range. The amino groups of CS were protonated under acidic condition, which caused the adsorbent to be positively charged, and thus, anionic SMZ− as the predominant species under pH 6 formed the electrostatic attraction with positively charged adsorbent. With the pH increasing, the positive charge of the hydrogels decreased, leading to the reduction of electrostatic attraction and, thus, a decrease in the adsorption capacity (Zhou et al. 2019).

Hydrogen bonding

Hydrogen bonding is a vital aspect for the successful adsorptive removal of ECs by hydrogel adsorbents. pH is an important factor affecting adsorption mechanism. When the pH exceeded the pHpzc of SA/CNC/PVA, acid–base interactions decreased, and negatively charged SA/CNC/PVA and DS mainly reacted via hydrogen bonding interactions (Fan et al. 2019). During the adsorption of SMZ by PNIPAAM/CS–Fe3O4, neutral molecules were the predominant form of SMZ at pH < 6; thus, electrostatic attraction was less likely to occur. However, a little adsorption capacity still existed, indicating that there might be hydrogen bonds formed by the sulfonamide groups of SMZ as well as the –NH and –OH of CS (Zhou et al. 2019). The BPA can be adsorbed by SA/CS–MOF due to the formation of hydrogen bonds between oxygen atoms in the −COO of SA/CS–MOF and −OH of BPA, which was confirmed by full-scale XPS pattern, because the proportion of −COO in SA/CS–MOF was detected lower after adsorption (Luo et al. 2019). During the adsorption of CIP by CS–biochar hydrogel, some functional groups in CIP, such as −NH and −OH, acted as hydrogen bond donors and some acted as hydrogen bond acceptors, such as the benzene ring. Another possible hydrogen bond can occur between the −OH present on the surface of CS–biochar and polar CIP functional groups, such as F, N, and O, which acted as hydrogen bond acceptor (Afzal et al. 2019).

π–π interaction

Considering the presence of benzene rings in both BPA molecules and the SA/CS–MOF, π–π interactions may occur during the process of BPA adsorption (Luo et al. 2019). A similar mechanism was reported for CS/biochar for adsorption of CIP which also contains a benzene ring (Afzal et al. 2019).

Desorption studies

Desorption is important in the field of actual application, and the choice of elution solvent is very significant. Under a given desorption condition, the interaction of ECs with the binding sites on the polymer surface could be disrupted, and subsequently, EC molecule was released from the polymer into the desorption medium. In order to strengthen the mass transfer, ultrasonic or shaking is usually needed during the desorption process.

As one of the most detected ECs in aquatic environment, BPA did not dissolve completely in pure water due to its low water solubility. So, organic solvents are often used as regeneration liquid. Methanol is the most common option. The desorption of cellulose hydrogel bead was investigated by Lin and coworkers. After adsorption, the cellulose hydrogel bead saturated with BPA was added into methanol solution in a thermostatic water bath shaker at 25 °C for 2 h, and the adsorption capacity was 68.11% of the initial after four adsorption/desorption cycles (Lin et al. 2019b). When BPA was adsorbed by PNIPAAM/CS–Fe3O4, the adsorption capacity gradually decreased to 94% of the initial in five cycles of adsorption/desorption when methanol was used as the eluent (Zhou et al. 2019). The regeneration of SA/CS–MOF was studied by eluting using methanol, and the adsorption efficiency of BPA on SA/CS–MOF decreased by about 2.4% after the first adsorption/desorption cycle and then remained almost unchanged until the end of the fifth cycle, indicating the high stability and excellent reusability of the SA/CS–MOF (Luo et al. 2019). It is worth noting that different eluents own different elution efficiencies. The authors revealed that the highest desorption efficiency (100%) was obtained by using methanol/dimethylsulfoxide as eluting solution (v:v = 9:1), and the efficiencies were 97 and 98% for ethanol and methanol, respectively (Du and Piao 2018). Adding water to pure organic solvents can reduce regeneration cost without affecting the desorption effect, for example, the cellulose hydrogel kept 90% of the original capacity of BPA after five adsorption/desorption cycles by immersing the BPA-loaded hydrogel in the mixed solvent of ethanol:water (v:v = 3:1) (de Souza and Petri 2018). Zhou et al. have studied the desorption experiment to remove the adsorbed SMZ from PNIPAAM/CS–Fe3O4 hydrogel and the spent adsorbent was treated with methanol. The adsorption capacity decreased to 55% of the initial capacity after five cycles of adsorption/desorption (Zhou et al. 2019).

Unlike BPA and SMZ, which often use organic solvent as regeneration liquid due to its low solubility in water, certain ECs can be solved in inorganic solvent perfectly such as acid/alkaline solution or distilled water. To desorb CIP, used humic acid CS–biochar was shaken with 1.0 M NaOH for 30 min in the steam bath vibrator at 250 rpm and then separated for repeated use. The adsorption capacity remained 39.53% as the original capacity after four regeneration cycles (Afzal et al. 2019). For the desorption experiment of DS, saturated CS–Fe3O4 was treated with 0.1 M NaOH for 5 min by ultrasonication and stirring. Then the adsorption capacity was reduced to 45.77% of the original capacity after four cycles (Liang et al. 2019a). In another study, DS-adsorbed SA/cellulose/PVA was subjected to desorption with 0.1 M HCl. After five adsorption/desorption cycles, SA/cellulose/PVA hydrogel showed only a slight loss (11.44%) in adsorption capacity (Fan et al. 2019).

Overall, it was concluded that after several adsorption/desorption cycles, the adsorption capacity of adsorbent declined gradually. Reduction in the surface area and number of functional groups on the adsorbent may result in a decrease in adsorption capacity of hydrogel adsorbent. Besides, the degree of capacity loss is also dependent on the substrate and the eluent.

Hydrogel templates for the synthesis of nanomaterials

During the adsorption process, the pollutants continuously accumulate onto hydrogels until they are fully saturated. These exhausted materials can be either burnt or disposed of in landfills, but this entails environmental concerns. An alternate option which is rather environmentally safe and economically attractive is the reuse of exhausted hydrogels. Recent trends have demonstrated that hydrogels can be used as nanoreactor for in situ synthesis of NPs as the following steps: adsorbents for metal removal, templates for metal NP synthesis, and reactors for pollutant removal. The adsorbed metal ions can be transformed to metal elemental NPs usually by the reduction process, including chemical synthesis (Ilgin et al. 2019; Pourbeyram and Mohammadi 2014; Tang et al. 2018), electrochemical synthesis (Nešović et al. 2018), and photo crosslinking (Ai and Jiang 2013). The oxidation process is (Natkański et al. 2016; Ojeda et al. 2017) often used for metal oxidant NP synthesis. Such findings are encouraging because if hydrogels are highly reusable, it makes the overall process more economical and sustainable.

Ilgin et al. prepared the PAM/N-methacrylamido-thiomorpholine hydrogel for selectively adsorbing Au(III) and then they were reduced to Au NPs by NaBH4 reducing agent for degrading 4-NP (Ilgin et al. 2019). The same method was also suitable for reducing Ni, Co (Sahiner et al. 2012), and Pd ions (Yao et al. 2018) to their corresponding NPs. Cu NPs could also be synthesized by reducing adsorbed Cu(II) with NaBH4 (Godiya et al. 2018, 2019; Su et al. 2018) or hydrazine (Pourbeyram and Mohammadi 2014). Besides NaBH4, the metal ions could be reduced by special functional groups which existed in hydrogels themselves. For example, AgNO3 solution was mixed with PVA/CS–graphene or PVA/lignin hydrogel presolution and Ag NPs were synthesized based on the antioxidant properties of CS (Nešović et al. 2018) or lignin (Li et al. 2019). The cellulose with hydroxyl groups could also reduce the Ag ions into Ag NPs (Han et al. 2016) or Ag@AgCl NPs (Tang et al. 2018). The NPs can also be obtained without reducing agents. As examples, Ai et al. dropped SA into Ag solution and then the hydrogel was irradiated with a lamp for 90 s to prepare Ag NPs and used for reduction of 4-NP (Ai and Jiang 2013). CdS was obtained by mixing PAA/PAM hydrogel solution with CdCl2 and Na2S2O3, and then irradiated by 60Co gamma rays at room temperature (Yang et al. 2018a). The synthesized PAA–GO hydrogel served as an efficient adsorbent for Cd(II) and as a suitable matrix for the CdS quantum dot formation by in situ precipitated and further used for photocatalyzing MB (Kong et al. 2019a). As for metal oxidant NPs, such as Fe2O3 (Natkański et al. 2016) or TiO2 (Ojeda et al. 2017), the oxidation process is the proper method.

To sum up (Table 3), the hydrogel used as matrix to synthesize metal NPs is just suitable for metal pollutants because the adsorbed metal ions could be converted to metal NPs which can be further used as catalyst for pollutant remediation. But as for organic compounds, hydrogels can hardly be recycled in the same way. So, it is necessary to combine the hydrogel and the catalyst, such as chemical catalyst and enzyme, to mineralize pollutants completely and realize the regeneration of hydrogels after organic compound adsorption.

Table 3 Hydrogel templates for the synthesis of nanomaterials
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Hydrogel matrixes as chemical catalyst carriers

In order to promote the recovery of hydrogels after organic pollutant adsorption, the solvent extraction method is widely used, which may destroy the structure of hydrogels during the regeneration process and bring about secondary pollution. Advanced oxidation processes (AOPs) are promising technologies for the treatment of wastewater containing noneasily removable organic compounds, because they can effectively mineralize the contaminants or transform them into harmless products. By taking advantage of the mechanical property and oxidation property, some inorganic materials can be used in AOPs to remove organic pollutants completely. Hydrogels are not only ideal adsorbents for pollutant removal, but also suitable carriers for catalyst immobilization for providing hydrogels with catalytic performances.

Photocatalysis can not only remove the chemically stable and nonbiodegradable organic pollutants, but also efficiently degrade them under mild conditions. According to the summarized data provided in Table 4, the most commonly used photocatalyst is TiO2. Hydrogels acted as matrixes to immobilize TiO2 for photodegradation of pollutants such as MO (Badranova et al. 2016), MB (Mai et al. 2017), azo, anthraquinone (Dhanya and Aparna 2016), and 4-NP (Xia et al. 2019). Some hydrogels not only acted as matrixes for catalyst immobilization, but also could adsorb substrates first, and then take advantage of the synergistic effect between adsorption and catalysis. For example, some researchers entrapped TiO2 into various hydrogels for adsorption and photodegradation of dyes such as reactive blue 4 (Yu et al. 2018b), RhB (Wang et al. 2018c), acid fuchsin (Zhou et al. 2017), Congo red (Thomas et al. 2016), and MB (Moon et al. 2013). Im et al. immobilized TiO2 into PVA hydrogel by the electrospinning method for removing dyes (procion blue and acridine orange), and dyes were adsorbed first and then guided to the TiO2 photocatalyst for degradation further (Im et al. 2010). Lučić et al. prepared the CS–TiO2 hydrogel by immersing the CS hydrogel into TiO2 NP solution for photodegrading dyes and found that the adsorption of dyes could be improved by increasing the amount of TiO2 (Lučić et al. 2014). Apart from TiO2, other photocatalysts are also applied. Xia et al. prepared SA–Ba hydrogel where Ba ion was used as both crosslinking agent and photocatalyst for degrading 4-NP (Xia et al. 2019). A simple chemical precipitation method was applied to load BiOI onto PHEA/APTM hydrogel to develop PHEA/APTM–BiOI to effectively eliminate methylparaben (MP) by visible light (Hu et al. 2019c). Zhu et al. introduced CdS into PHEA/poly(N-hydroxymethyl acrylamide) (PHEA/PHAM) by precipitation method and found that the swelling ratio and adsorption of BPA by PHEA/PHAM–CdS was higher than PHEA/PHAM, and showed the synergy between adsorption and photocatalytic oxidation (Zhu et al. 2018). Yang et al. reported that the adsorption capacity of SMZ by PHEA/PNMMA–CuS was higher than PHEA/PNMMA due to the presence of CuS which immobilized to PHEA/PNMMA by precipitation, and exhibited the synergy between adsorption and photocatalytic degradation (Yang et al. 2017). Hao et al. entrapped graphitic carbon nitride (g-C3N4) into PAM–bentonite hydrogel and found that the enhanced removal of total organic carbon (TOC) is attributed to the synergistic effect of adsorption and photocatalytic degradation (Hao et al. 2018). Mu et al. showed that BPA could be 100% removed in 12 min by the synergy of adsorption and photocatalysis using graphene hydrogel (rGH) which precipitated with Ag3PO4 (Mu et al. 2017). Kaur and Jindal entrapped zirconium (IV) in CS/gelatin hydrogel and the results showed that the adsorption and photodegradation of MB was enhanced compared with the pure CS/gelatin or zirconium (IV), showing the synergy between adsorption and photocatalytic degradation (Kaur and Jindal 2019). The addition of lanthanide into CS–graphite provided the hydrogel photocatalytic ability and increased the adsorption capacity for MB, as well as suppressed electron-hole recombination rates during the process of photocatalytic MB (Sirajudheen and Meenakshi 2019). Also, photocatalysts can be used as regeneration agent. For example, Chen et al. imbedded MoS into cellulose to prepare cellulose–MoS hydrogel, and the photoregeneration experiments after adsorption of MB showed that the adsorbent can be activated easily by illumination with little loss of dye adsorption capacity (Chen et al. 2017).

Table 4 Hydrogel matrixes as chemical catalysts carriers
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Recently, metal-binding hydrogels are applied as Fenton reagents because of the high metal-binding ability with hydrogels’ abundant functional groups. Fe(II) was immobilized into CS/SA by Kang et al. via a simple crosslinking reaction to activate persulfate to degrade BPA. The CS/SA–Fe(II) system demonstrated a much higher removal efficiency of BPA (99.5%) than the conventional Fe(II) system (69.2%) with a negligible amount of Fe ions released into the solution, indicating that the CS/SA hydrogel played a positive role in protecting Fe(II) (Kang et al. 2018). Ben Hammouda et al. dropped SA solution into Fe(II) solution to prepare SA–Fe(II) composite for Fenton oxidation of indole, and the SA–Fe(II) catalyst present good stability and can be used for at least five times without an obvious decrease in activity (Ben Hammouda et al. 2016). Rashid et al. prepared CS–Cu–Fe complex by mixing CS powder, Fe(III), and Cu(II) together, and the mixture was precipitated by the addition of ethanol. The results indicated that discoloration of C.I. Reactive Black 5 by the hydrogel composite achieved 96.5% in a short reaction time, demonstrating the combination of adsorptive enrichment and catalytic degradation (Rashid et al. 2015). Zhang et al. immobilized Fe2O3 to rGO/polypyrrole (rGO/Ppy) by precipitation, and the rGO/Ppy–Fe2O3 hydrogels could be easily separated via a magnet during Fenton degradation of MB, and showed superior reusability in recycling experiments (Zhang et al. 2017).

Hydrogel matrixes as enzyme carriers

Enzyme catalysis is another method to remove contaminants thoroughly, and it is more specific and mild than AOPs. But its application is challenged due to its unstable nature and low stability against variation in pH and temperature. Immobilization of enzyme is a widely used approach to enable repeated use and enhance the stability and durability of the enzyme which leads to economical operations.

Hydrogels can be applied as suitable carriers to restrict enzymes within the confined polymer spaces or networks (Falcone et al. 2019). Saoudi and Ghaouar entrapped laccase in PEGDA hydrogel, and the laccase kept 95% of its initial activity at 60 °C (Saoudi and Ghaouar 2019). Han et al. entrapped carbonic anhydrase, lysozyme, or xylanase in silk fibroi hydrogel and found that the three kinds of enzyme were increased in protein rigidity against pH denaturation (Han et al. 2018). Martín prepared SA/pectinase by entrapment, and the storage stability was improved (Martín et al. 2019). Olajuyigbe et al. entrapped laccase in SA for degradation of BPA, and the storage, pH, and temperature stability were slightly improved by immobilization (Olajuyigbe et al. 2019). Bilal et al. immobilized laccase in CS bead by crosslinking, and the immobilized biocatalyst showed good operational stability, retaining 71.24% of its original activity after 10 repeated catalytic cycles. Storage stability was improved also. In addition, the CS-based biocatalytic system achieved almost complete removal of BPA from the aqueous solution after 150 min of the transformation process (Bilal et al. 2019a).

Besides improved stability, reusability is another essential property for enzyme immobilization. PVA hydrogel protected protease from damage, helped maintain enzymatic activity at high levels, exhibited better reusability, and was easy to separate from the product (Zhang et al. 2018a). Naghdi et al. encapsulated laccase onto CS–biochar composite, with only 2% of laccase leaked during a 5-day period, and the encapsulated laccase exhibited 30% of the initial activity after five regeneration cycles (Naghdi et al. 2018). Sato et al. found that poly(ethylene glycol) methyl ether acrylate (PEGMEA)/lipase could catalyze the hydrolysis of triacetin without leakage of lipase or loss of activity during repeated use, and the efficiency was the same with free lipase (Sato et al. 2014). Bilal et al. entrapped horseradish peroxides in agarose/CS hydrogel, and 6-fold and 4-fold greater catalytic activity was achieved at 50 and 70 °C compared with free enzyme during dye degradation and the immobilized enzyme sustained above 90 and 60% of original activity after 5 and 10 continuous cycles of use (Bilal et al. 2019b). It can be concluded that immobilizing appropriate enzymes into hydrogels can catalyze target contaminants effectively and economically due to improved stability and reusability.

As has been discussed above (Table 5), entrapment is the commonly used approach for immobilizing enzymes into hydrogels, by which enzymes mixed with the aqueous monomer (or polymer) solution of hydrogels to obtain a homogeneous dispersion of enzymes throughout hydrogel networks. That is to say, enzyme immobilization is a one-step synthesis. Besides entrapment, hydrogels can be prepared first and then immobilize enzymes into solid hydrogels by adsorption or chemical crosslinking (Wolf and Paulino 2019). Liu et al. prepared cellulose–Fe3O4 hydrogel first and then papain was immobilized in it by adsorption, and the immobilized papain showed higher thermal stability than free papain (Liu et al. 2017). Liao et al. obtained chitin/PVA–Fe3O4/flavourzyme by immobilizing enzymes to solid hydrogels through adsorption and crosslinking, displaying preferable thermal stability and great reusability (Liao and Huang 2019a).

Table 5 Hydrogel matrixes as enzyme carriers
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Challenges

Although hydrogels have exhibited superior performance in the adsorptive removal of a wide range of aqueous pollutants, including dyes, heavy metals, and ECs, their selective adsorption is rarely explored; thus, research efforts are needed to develop hydrogels with the desired structure and functionality possessing higher selectivity toward a specific pollutant. Many hydrogels have obtained satisfactory strength and adsorption property, but their chemical and biological stability needs to be taken into consideration in wastewater treatment for economic viability and sustainability, which are always ignored in the current research. In addition, although the smaller hydrogel particle size can improve the adsorption efficiency, the subsequent separation and recovery from the solution becomes a challenge. So how to reach a balance between the adsorption effect and their facility to recovery must be compromised.

Conclusion

This review has provided a general overview on hydrogel preparation and modification and the changes in mechanical strength, swelling ratio, and adsorption capacity of hydrogels due to the introduction of inorganic materials into networks, revealing that the hydrogel composites have better mechanical property and adsorption capacity compared with pure hydrogels. While a majority of studies have devoted to dye and metal adsorption by various hydrogels, there is clearly increased interest in EC removal afterwards. Moreover, hydrogels have potential applications in immobilizing catalysts to removal of pollutants totally by the synergic function of adsorption and catalytic reaction, so hydrogel/catalyst composites should be developed as many as possible for reusability and sustainability concerns.