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Construction of Double Network Gel Adsorbent and Application for Pollutants Removal from Aqueous Solution

Liqing Li , Panwang Wu , Jie Ma

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 1010 -1025.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 1010-1025 DOI: 10.7536/PC200691
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Construction of Double Network Gel Adsorbent and Application for Pollutants Removal from Aqueous Solution
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Abstract

In recent years, with the rapid development of industry, the water pollution crisis is one of the major threats facing the world. The development of new environmentally functional materials and technologies to achieve efficient removal of water pollutants is a hot topic in the current research. Double network hydrogels are high molecular polymers with a three-dimensional network structure. They have superior mechanical properties with high strength, can withstand high levels of tensile and compression deformation. The low swelling rate allows the hydrogels to hold a large amount of water, but still maintains the stable morphological and network structures. In addition, due to their unique cross-linking, the hydrogels also have fast self-healing property and significant fatigue performance. The application effect of double network hydrogels as an adsorbent in the removal of heavy metal ions and other pollutants is significantly effective. Therefore, they are adsorption materials with great potential and have attracted widespread attention in the field of water treatment. This paper reviews the physicochemical properties and classifications of the double network hydrogels adsorbents, and their latest application progress to remove heavy metals, antibiotics, dyes and other pollutants from water. Through this review, new ideas, new methods and new technologies are provided for the in-depth development of double network hydrogels adsorbents and engineering applications in water purification.

Contents

1 Introduction

2 Properties

2.1 Mechanical properties

2.2 Swelling properties

2.3 Self-healing properties

3 Classifications

3.1 organic-organic double network gel

3.2 Organic-inorganic double network gel

3.3 Modified double network gel

4 Removal of pollutants in water

4.1 Heavy metals

4.2 Dyes

4.3 Antibiotics

4.4 Other pollutants

5 Conclusion and outlook

Graphical abstract

Keywords

double network hydrogels / adsorption / water treatment / property / network structure

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Liqing Li, Panwang Wu, Jie Ma. Construction of Double Network Gel Adsorbent and Application for Pollutants Removal from Aqueous Solution. Progress in Chemistry, 2021, 33(6): 1010-1025 DOI:10.7536/PC200691

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1 Introduction

Water is the basis for the survival and development of human society. With the acceleration of global modernization, industrial wastewater containing large amounts of toxic dyes and heavy metal ions from papermaking, textile, printing, plastics and electroplating industries is being discharged into the aquatic environment, which seriously threatens the safe use of water resources and human health[1]. To this end, various methods have been developed to treat water pollution, such as chemical precipitation, ion exchange, adsorption, membrane separation, microbial, coagulation-flocculation, flotation, and electrochemical methods[2,3]. Among them, adsorption has become a widely used method to remove pollutants in water because of its high efficiency, low cost and simple operation[4,5]. Conventional adsorbents include activated carbon, clay, polymers, silicon nanomaterials and metal oxides, etc. However, the existing adsorbents have the problems of low adsorption capacity and high difficulty in recovery, so it is urgent to develop more effective and practical adsorbents[6][7][8][9][10].
Hydrogels are hydrophilic materials composed of three-dimensional polymer networks, which swell easily in water and can hold large amounts of water (up to 90 wt%) and maintain the structure unchanged. Due to their unique properties, hydrogels have been used in tissue engineering, drug delivery systems, and adsorption[11⇓⇓~14]. However, the high swelling of the hydrogel may result in low mechanical strength and poor recyclability. In recent years, researchers have constructed a variety of new structural hydrogels with good mechanical properties, such as topological (TP) hydrogels, nanocomposite (NC) hydrogels and double network (DN) hydrogels[15]. In 2003, Gong et al. First prepared PAMPS/PAM double network hydrogel with PAMPS as the first network and PAM as the second network[16]. Traditional bi-network hydrogels consist of two interpenetrating polymer networks with contrasting structural and mechanical properties. The first network consists of a rigid scaffold of highly crosslinked polyelectrolytes, making it hard and brittle; While the second network is filled with a loosely cross-linked hydrophilic neutral polymer into the rigid scaffold, making it soft and stretchable[17]. Under this optimized structure, the DN gel with a water content of about 90 wt% has hardness (elastic modulus of 0.1 to 1.0 MPa), strength (tensile stress at break of 1 to 10 MPa), and toughness (fracture energy 100~1000 J·m-2)[18⇓~20]. As a new type of gel material, double-network hydrogel has attracted much attention in recent years[21⇓~23]. However, the existing research and application are mainly concentrated in the fields of biology, medicine and machinery, and the research direction of pollutant adsorption is relatively few[24⇓~26].
The application effect of the DN hydrogel as an adsorbent in removing heavy metal ions and the like is remarkable. For example, the maximum adsorption capacity of PAA/HS (polyacrylic acid/humic substance) DN hydrogel prepared by Ma et al for Pb (Ⅱ), Cu (Ⅱ) and Cd (Ⅱ) can reach 360.50 mg/G, 151.00 mg/G and 412.76 mg/G respectively, while the adsorption capacity of CMC/PEI (carboxymethyl cellulose/polyethyleneimine) DN hydrogel for Cr (Ⅵ) is 312.46 mg/G[27][28]. In addition to the above ions, Mn (Ⅱ) and Fe (Ⅲ) can also be adsorbed by DN hydrogel[29,30]. The results show that the higher adsorption performance depends to some extent on the 3D porous structure and polar functional groups. The porous structure enables the hydrogel to have a higher specific surface area and provides a large number of active adsorption sites for the adsorption of water pollutants by the hydrogel[31]; Rich active functional groups can combine with metal ions to achieve the goal of efficient removal of pollutants in water[32]. In addition, the anti-swelling performance of the hydrogel can be effectively improved by modification and other methods, so that the hydrogel can maintain structural integrity in water for a long time, and is convenient for separation and multiple recycling after adsorbing pollutants[33]. By converting the irreversible chemical crosslinking (such as crosslinking by covalent bonds) in the gel network into reversible physical crosslinking (such as ionic bonds, hydrogen bonds, electrostatic interactions, and van der Waals forces), the self-healing ability of the DN hydrogel can be improved, and the application of the gel adsorbent in water treatment can be further broadened[34⇓~36].
DN gels include DN hydrogels and DN aerogels, and most of the current research results focus on the field of hydrogels[37⇓~39]. In the past decade, more and more DN hydrogels have been developed and prepared from structural design, preparation methods, modification to environmental applications. DN hydrogels with high water permeability, high mechanical strength, rapid adsorption and high selectivity have been widely used in water treatment and become ideal adsorption materials for water treatment. In this paper, the application progress of dual-network hydrogel adsorbents in removing heavy metals, antibiotics, dyes and other pollutants in water in recent years is reviewed, and their physicochemical properties and main classifications are also summarized.As well as the application prospect in water treatment, which provides a broader solution for the development of double-network hydrogel adsorbent and its large-scale application in water purification.

2 Properties of Double Network Gel

2.1 Mechanical Properties

For a long time, the application range of hydrogels has been severely limited by their mechanical properties, and improving their mechanical properties is an important research direction in the field of gel science[41⇓⇓~44]. In a classic two-network hydrogel, the relatively high Young's modulus of the highly crosslinked first network gives it some stiffness and strength, while the loosely crosslinked second network provides toughness, as shown in fig. 1[16,18]. An understanding of the toughening mechanism is essential for the rational design of DN gels with the desired mechanical properties. In most toughening mechanisms of DN gels, it is generally believed that they are mainly based on the concept of sacrificial bonds: covalent or non-covalent bonds are broken from the inside as sacrificial bonds, so that locally applied stresses are effectively relaxed and crack energy is dissipated[42,45,46][47]. The hydrogel with superior mechanical properties is soft and tough, and can withstand high levels of tensile and compressive deformation. The excellent ductility and stability also make the hydrogel not easy to deform and damage when adsorbed in water, and maintain a certain shape and integrity. Importantly, improvements in mechanical properties are often accompanied by improvements in swelling resistance, self-healing and adsorption properties, which also play a positive role in the application of DN gel adsorbents[17,48,49].
The methods to improve the mechanical properties of DN hydrogels are as follows: (1) To explore more suitable materials and network structures. In the PAMPS/PAA double network hydrogels prepared by Gong et al. For the first time, the molar ratio of the first network monomer to the second network monomer, the crosslinking density of the two networks and the molecular weight of the second network are three important parameters affecting the network structure and mechanical properties of DN hydrogels[16]. However, the first network of conventional double network gels must be a strong polyelectrolyte, which greatly hinders the wide application of DN gels. Nakajima et al. developed a "molecular scaffold" approach to the synthesis of tough DN gels formed from any hydrophilic polymer[50,51]. This is the first reported method of synthesizing tough DN gels using various neutral or weak polyelectrolyte hydrogels as the first network (fig. 3C). The "molecular scaffold" method is similar to the classical two-step polymerization method, in which a first neutral gel network is first synthesized, and then a linear strong polyelectrolyte (molecular scaffold) is introduced into the neutral first network. Through the method, the swelling degree of the gel is remarkably increased, and the gel also has better mechanical properties. With the development of gel research, more and more materials have been used to construct double-network gels[52⇓⇓⇓~56]. (2) Changing the crosslinking mode of the double network: by changing the crosslinking mode, adjustable mechanical properties are provided for the DN gel. Chen et al. Developed a new type of fully physically linked Agar/HPAAm double network gel. By introducing tough HPAAm gel as the second network, it can not only effectively dissipate energy and significantly improve mechanical properties, but also improve the fatigue resistance and self-healing properties of the gel through reversible network reconstruction[34]. At the same time, this study demonstrates that the mechanical properties of gels can be greatly improved by introducing reversible non-covalent (physical) networks into irreversible covalent networks. (3) Adding modified reinforcing fillers: In order to overcome the limitations of many influencing factors on DN gels, the introduction of nano-fillers (such as graphene, graphene oxide, carbon nanotubes and clay) or new cross-linked networks is an effective way to improve the mechanical properties[57⇓⇓~60]. Liu et al. Reported a simple freeze-thaw method to prepare GO-SA/PVA double network hydrogel for the adsorption of methylene blue (MB). The introduction of graphene oxide (GO) into SA/PVA hydrogel not only further enhanced the mechanical properties, but also affected the morphology of DN hydrogel. The small pore size and porous structure increased the specific surface area of the adsorbent, which was conducive to the removal of pollutants[49].

2.2 Swelling Property

Swelling property is one of the most important parameters to evaluate the structure of hydrogels. Conventional hydrogels have a large degree of swelling in water, which makes the stability and mechanical strength of hydrogels decrease sharply[61]. Hydrophilic strong electrolyte polymers, such as poly (2-acrylamido-2-methylpropanesulfonic acid) (PAMPS), are typically used to form a rigid and brittle first network that is highly covalently crosslinked, and due to the high swelling properties, the second network polymer will slowly diffuse into the first gel network, causing the first network to be highly swollen[45]. The high swelling ratio of hydrogels is extremely unfavorable for their application in adsorption treatment. When a hydrogel network is partially destroyed, the balance between elastic energy and osmotic pressure is inevitably broken, which leads to the destruction of the structure and shape of the network[62]. Therefore, it is necessary to explore ways to reduce the swelling ratio of hydrogels.
In 2014, it was reported in Science that Kamata et al. Developed a high-strength hydrogel that would not swell and deform in water, and controlled swelling by introducing heat-responsive segments into the polymer network, which triggered a research hotspot in the research and development of low-swelling and non-swelling gels[62]. This study shows that the mechanical properties of hydrogels are largely dependent on their degree of swelling, and that a low degree of swelling helps to maintain their original shape, so that the mechanical properties remain unchanged. In the same year, Ducrot et al. Also reported the idea of controlling the degree of swelling by changing the crosslinking density of the first network in Science[63]. In general, the swelling ratio of hydrogels depends on the crosslink density of the gel network, and a high crosslink density contributes to a decrease in the swelling ratio[59,64,65]. In addition to the crosslink density, both chemical and physical crosslinks are used to improve the mechanical strength and reduce the swelling ratio of the hydrogel[66]. For example, Zhuang et al. Prepared a new hybrid crosslinked alginate/reduced graphene oxide (RGO) double network (GAD) hydrogel and compared it with alginate/RGO single network (GAS) hydrogel[17]. The results showed that the swelling ratio of GAS and GAD increased with the increase of pH value and decreased with the increase of GO content. Under all conditions, the swelling ratio of GAD was lower than that of GAS, and the low swelling ratio improved the gel stability.

2.3 Self-repairing property

In addition to high mechanical strength, self-repair (self-healing) is also one of the promising properties of hydrogels. Self-healing refers to the ability to self-repair damage and restore relevant mechanical properties, which means that there is no (or little) irreversible damage to the gel, thus prolonging its service life and restoring its mechanical properties. The self-repairing property is closely related to the cross-linking mode of the DN gel. The crosslinking modes of the network structure of the DN gel include complete chemical crosslinking, complete physical crosslinking and hybrid physical-chemical crosslinking[67,68][36,55][46,69]. In conventional DN gels prepared by the two-step process mentioned above, both networks are chemically cross-linked. However, chemical crosslinking is irreversible, and its strengthening mechanism is mainly based on "sacrificial bonds", in which the covalent bonds of the first network are permanently broken at high strain, thus protecting the second network, withstanding stress and storing elastic energy. However, breakage of the first network will result in irreversible and permanent chemical bond breakage, resulting in a severe loss of mechanical properties[35,70]. The chemical crosslinking of hydrogels usually uses toxic crosslinking agents, which are difficult to biodegrade and pose a threat to the environment[22]. In contrast, the physical crosslinking is reversible, which allows the hydrogel to dissipate energy by breaking the physical crosslinking bonds after large deformation or damage, replacing the sacrificial bonds of the first network in the chemical crosslinking, and then re-forming during unloading/quiescence, thus having high fatigue resistance and self-healing properties[45,71][72]. Physically cross-linked networks can be formed by ionic bonding, hydrogen bonding, electrostatic interactions, π-π stacking, hydrophobic interactions, chain entanglement, etc[73][74][75][76][77][78]. The self-healing ability of a hydrogel is generally inversely proportional to its mechanical strength[77]. In general, the mechanical strength of fully chemically cross-linked DN hydrogels is relatively high, while that of fully physically cross-linked DN hydrogels is excellent in self-healing, while hybrid cross-linked hydrogels are between the two[79,80].
Sun et al. Introduced physical crosslinking into the crosslinked structure of DN hydrogels and prepared hybrid crosslinked interpenetrating polymer (IPN) hydrogels, in which Ca2+ crosslinked alginate was the first network and covalently crosslinked PAM was the second network[43]. The gel showed high ductility and toughness, and had a certain degree of self-healing: when it was kept at 80 ℃ for 1 d, the gel recovered 74% of its initial energy from the first loading. Lin et al. prepared a new double-crosslinked hydrogel combining covalent crosslinking and ionic crosslinking of Fe (Ⅲ) -acrylic acid (AAc) coordination, which has a tensile strength of 6 MPa, a toughness of 27 MJ·m-3, and good self-recovery properties (energy recovery rate of 87.6%, stress of 3.5 MPa and strain of 300% at room temperature for 4 H)[73]. The hybrid cross-linked agar/polyacrylamide (agar/PAM) DN hydrogel developed by Chen et al. Has rapid self-healing performance (hardness recovery of about 60% and toughness recovery of about 40% within 2 minutes), significant anti-fatigue performance (40 car crashes, no damage) and significant self-healing performance at room temperature without external stimulation (initial strength recovery of about 40%)[42]. It should be noted that this performance can only be achieved by recovery at 100 ° C. Due to the presence of non-covalently cross-linked networks, all of these hybrid networks exhibit varying degrees of self-healing and fatigue resistance.
Compared with traditional DN hydrogels, self-repairing DN hydrogels have many advantages, such as longer service life and fatigue resistance due to less irreversible damage. However, due to the contradiction between mechanical strength and self-healing properties, it is still a challenge to design hydrogels with excellent self-healing properties and super mechanical strength. Considering the application of DN gel in water treatment adsorbents, it is worth considering the development of more excellent network structure and the method of combining multiple reversible reactions.

3 Classification of Double Network Gel

According to the different components, it can be divided into organic-organic DN gel, organic-inorganic DN gel, in addition, there are modified DN gels loaded with other materials on the basis of the first two.

3.1 Organic-organic double network gel

In order to solve the problem that most natural or synthetic hydrogels lack mechanical strength, Gong et al. First prepared PAMPS/PAM double network gels with high mechanical properties in 2003[16]. The two-layer network of the gel is an organic polymer, which is a very widely used DN gel material at present. Organic-organic hydrogels are mainly composed of cheap and easily available polymers or natural macromolecules. Polymers include Polyacrylamide (PAM), Poly (2-acrylamide-2-methylpropanesulfonic acid) (PAMPS), Polyacrylic acid (PAA), Polyvinyl alcohol (PVA), and Polyethyleneimine (PEI)[14][16][81][82][28]. The common organic polymers have the advantages of flexibility, biocompatibility, hydrophilicity, low cost, easy availability and the like, and can provide more adsorption sites due to rich carboxyl groups and strong hydrophilicity, so that the organic polymers are widely used in the field of environmental water treatment[83⇓~85][31].
High molecular polymers usually form double networks by covalent crosslinking, but irreversible chemical crosslinking is difficult to achieve a good balance between mechanical properties and self-healing. Physically crosslinked hydrogels prepared from natural biomacromolecules have good adsorption properties for heavy metals and other pollutants due to their rich hydroxyl, carboxyl and amino functional groups, and physical crosslinking provides excellent fatigue resistance and self-repair characteristics for hydrogels. Natural macromolecules such as Alginate, Gelatin, Agar, Chitosan (CTS), Cellulose, κ-Carrageenan, starch and so on have been widely used in DNA gel adsorbents due to their low cost, good biocompatibility and biodegradability[17][86][42][87][88][89][90]. For example, alginate, a natural polysaccharide carbohydrate, can bind to metal ions to form various alginates, and the most common way of crosslinking sodium alginate is to crosslink with Ca2+ to form gels (Fig. 3A). In addition, gelatin was also cross-linked by ionic bonds in DN-gels with an organic matrix (fig. 3B).
Chen et al. Synthesized physical-chemical cross-linked Agar/polyacrylamide (Agar/PAM) DN hydrogel by a simple one-pot method, which is composed of two interpenetrating networks: the hydrogen bond cross-linked Agar neutral network and the covalent cross-linked PAM polymer network[42]. The DNG shows excellent mechanical properties, including stiffness (elastic modulus of 123 kPa), strength (maximum compressive stress of 38 MPa and maximum tensile stress of 1.0 MPa), toughness (dissipation energy of 9 MJ·m-3 and fracture energy of 102~103J·m-2), excellent ductility (fracture strain of 1500% to 2000%), rapid recovery (energy recovery rate of 65% in 10 min) and unique free-forming characteristics (formation of many complex geometric shapes) (Fig. 3D). Chu et al. prepared a high-strength polyvinyl alcohol/acrylic acid double network gel (PVA/PAA) adsorbent through a simple two-step method. The adsorbent has the characteristics of simple operation, fast separation, low cost and low swelling ratio, and can effectively remove heavy metals in actual industrial wastewater without causing secondary pollution, and has excellent reusability[82]. In 2019, Li et al. Prepared sodium alginate/κ-carrageenan (SA/κ-CG) with low swelling, high mechanical properties and good adsorption capacity for ciprofloxacin (CIP) by a simple ionic crosslinking method[91]. Organic-organic DN hydrogels have been widely studied as DN gel adsorbents because of their low cost, high mechanical properties and certain adsorption properties for heavy metal ions and other water pollutants.

3.2 Organic-inorganic double network gel

Polymer materials have good hydrophilicity, biocompatibility and toughness, but most organic hydrogels can not have excellent mechanical properties and self-healing at the same time. In order to solve this problem, it is a promising idea to combine inorganic and organic materials. Taking sodium alginate as an example, it is a natural polysaccharide, which contains a large number of carboxyl and hydroxyl groups, has good gel-forming properties, and can cross-link with divalent salt ions (such as Ca2+, Ba2+, etc.) To gel. However, the unavoidable swelling and ion exchange between the divalent ions in alginate and the monovalent ions in the solution seriously hinder the practical application of alginate hydrogels to remove dyes from wastewater, resulting in poor mechanical strength and low stability, and to some extent limiting their application[62]. Inorganic materials such as alginate and graphene oxide are used as double networks to prepare gels, which can greatly enhance mechanical strength and adsorption characteristics[17,93].
In organic-inorganic hydrogels, the organic materials are also polymers or natural macromolecules, while the inorganic materials are composed of graphene and its derivatives graphene oxide and reduced graphene oxide, in addition to reduced graphene oxide nanoparticles[76][93][17][94]. In recent years, carbon nanomaterials such as carbon nanotubes (CNTs) and graphene have become common adsorbents for the removal of dyes and heavy metal ions from water[95,96]. The production cost of graphene is lower than that of carbon nanotubes, but the adsorption capacity of graphene and carbon nanotubes is similar[97]. Compared with other adsorbents, graphene oxide (GO), as an important derivative of graphene, has a high specific surface area (2630 m2/g) and many oxygen-containing functional groups (including C = O, — O —, — OH and — COOH), which are conducive to further hybridization and crosslinking with other materials and provide more active adsorption sites for metal ions, thus becoming an excellent candidate for adsorption materials[98]. Introducing graphene into hydrogel not only combines the advantages of graphene (numerous functional groups and excellent mechanical properties) and hydrogel (high water content, porous structure, processability and expandability), but also combines the advantages of graphene and hydrogel.At the same time, because of the high specific surface area and the strong interfacial interaction between GO and polymer molecular chains, the adsorption and swelling properties of DN gels can be significantly improved[17].
In 2013, Huang et al. First developed an inorganic-organic DN hydrogel synthesized by a two-step method, which uses 3D graphene architecture as the first network and polyacrylic acid (PAA) as the second network, and has good stretchability and conductivity[99]. In the same year, Cong et al. prepared a novel graphene oxide (GO)/poly (N-acryloyl-6-aminocaproic acid) (PAACA) hydrogel that was fully physically crosslinked without using any conventional organic crosslinking agent[64]. On the one hand, GO nanosheets form a loose 3D network, which is cross-linked by hydrogen bonds between oxygen-containing groups in GO and polar groups in PAACA side chains. On the other hand, GO nanosheets and calcium ions were used as crosslinkers to make PAACA form another flexible polymer network through coordination interaction. The introduction of GO nanosheets increases the crosslinking density of the hydrogel, reduces the swelling ratio, and makes the double network more firm and stable. Due to its strong hydrogen bonding network, the mechanical properties of the DN hydrogel are significantly enhanced relative to the pure PAACA hydrogel, and it has rapid self-healing properties. In 2012, Lin et al. Prepared Ca2+ cross-linked GO/PAA double network hydrogels for methylene blue (MB) removal[100]. With the help of GO, the gel provides a simple and low energy solution for dye removal (Fig. 4A), since dyes usually have a complex aromatic molecular structure and are therefore stable and difficult to remove by traditional wastewater treatment techniques. 3D GO/PAA double network gel (Fig. 4B) and PSA/GO double network gel (Fig. 4D) for removal of Cd (II) and Mn (II) ions were also used for dye adsorption. In recent years, the introduction of non-covalently cross-linked graphene three-dimensional networks into hydrogels has become a hot [100~102] in hydrogel science.
Graphene and its derivatives are the inorganic components of most of these hydrogels, while other inorganic materials such as hydroxyapatite (Fig. 4C) have little application research. In 2017, Kamio et al. Reported a silica nanoparticle/poly N, N-dimethylacrylamide (silica/PDMAAm) double network ionic gel synthesized by one-pot method[103]. In the gel, the physically crosslinked inorganic component forms a brittle network and the chemically crosslinked organic component forms a tough network, so the gel has excellent mechanical strength, good thermal stability and self-healing performance. In 2002, Li et al prepared P (AA-co-AM)/WG gels, in which water glass solution reacted with sodium bicarbonate as a coagulant to form silicic acid in the network structure, and then the silicic acid molecules polymerized into silicic acid polymers.The polysilicic acid network (WG) can be connected with the polymer network through entanglement and intermolecular hydrogen bonding to form an inorganic/organic double-network gel system, and the structure enhances the toughness, strength and elasticity of the gel[104].

3.3 Modified double-network gel

The addition of reinforcing fillers and the introduction of new cross-linked networks are both considered to be effective ways to improve mechanical properties[14]. Modification refers to loading organic compounds (such as polyvinyl alcohol, chitosan, ethylenediaminetetraacetic acid, polyethyleneimine) and nano-fillers (such as graphene oxide, nano-oxide, carbon nanotubes and nano-clay) on the basis of the original DN gel to improve mechanical properties and adsorption properties[48][32][87][108][58][109][110][85].
Inorganic nanomaterials are ideal candidates for enhancing the properties of polymers due to their high specific surface area and unique physicochemical properties[95,96,111]. As a widely used carbon nanomaterial, GO can form graphene-based hydrogels through self-assembly process, and many chemical functional groups on GO also provide considerable space for the modification or functionalization of hydrogels[112]. In 2013, Fan et al. First introduced GO nanosheets into the DN network to prepare a new ionic and covalently crosslinked graphene oxide/sodium alginate/polyacrylamide (GO/SA/PAM) ternary nanocomposite hydrogel.The double network is formed by alginate and polyacrylamide, and the GO nanosheet is used as a modified reinforcing filler, so that the hydrogel can recover most of the elongation at break and show good elasticity[14]. Due to the introduction of GO, the GO/SA/PAM composite hydrogel showed good adsorption capacity for both cationic and anionic dyes. Nano-oxides are also introduced to enhance the crosslink density and porous structure of the gel. Since then, studies on the addition of GO as a modifier to DN gels have become more extensive, including agar-PAM/GO double network hydrogel (Figure 5A), Gelatin/PAA/GO gel (Figure 5B), SA/PVA/GO gel (Figure 5C), etc. In 2015, Tang et al. synthesized a new calcium alginate/polyacrylic acid/graphene oxide (CA/PAA/GO) double-network composite hydrogel adsorbent with good mechanical properties, and studied the adsorption performance and mechanism of Cd (II) on the hydrogel (Fig. 5d)[33]. Besides GO, other inorganic nanomaterials have also been used in DN hydrogels. Wang et al. Used silica particles grafted with vinyl groups on the surface as macromolecular crosslinking agents to introduce PAMPS/PAM networks, and the obtained DN hydrogels showed strong compressive toughness and excellent ductility compared with traditional DN gels[113].
Organic compounds can provide more functional groups and binding sites for metal ions[8]. For example, Zhuang et al. Used polyvinyl alcohol (PVA) as a modifier in the Alginate/GR double network hydrogel. Because PVA contains abundant hydroxyl groups, which have a positive effect on the adsorption of antibiotics, the addition of PVA in the double network can improve the adsorption capacity[48]. The results showed that the maximum adsorption capacity of tetracycline and ciprofloxacin on the double network hydrogel was significantly higher than that on the single network hydrogel in this study. Ma et al prepared chitosan/polyacrylamide (CTS/PAM) double network hydrogel crosslinked and modified by ethylenediaminetetraacetic acid (EDTA), whose porous structure is conducive to the diffusion of heavy metal ions to the active sites inside the gel, thus promoting the adsorption of heavy metal ions[87].

4 Removal of contaminants from water

4.1 Heavy metal ion

A heavy metal is a metallic element with an atomic mass of 63.5 to 200.6 and a specific gravity greater than 5.0[2,115]. With the rapid development of mining, metal plating, tanning, battery, fertilizer, pesticide and paper industries, heavy metal ions are directly or indirectly discharged into rivers, lakes or marine environment[116]. Some toxic heavy metals such As nickel (Ni), arsenic (As), chromium (Cr), zinc (Zn), copper (Cu), cadmium (Cd), cobalt (Co) and antimony (Sb) remain a serious public health problem in wastewater due to their mobility in natural aquatic ecosystems, long-term accumulation of toxicity in humans and the environment, and difficulty in degradation[117,118]. Adsorption is considered to be the most effective technology for the removal of heavy metals from wastewater among various heavy metal treatment methods[5]. However, the traditional adsorbents have many shortcomings, so it is urgent to develop adsorbents with better adsorption performance.
Compared With other commonly used adsorbents (activated carbon, clay and polymer materials, etc.), the DN gel adsorbent has higher theoretical maximum adsorption capacity (412.76 mg/G for Cd (Ⅱ), 690 mg/G for Cu (Ⅱ) and 542.9Cr (Ⅵ) can reach 312.46mg/G), excellent regenerability (most of the adsorption capacity basically does not change or decreases little after 5 cycles of adsorption-desorption) and selective adsorption (strong selective adsorption for Pb (Ⅱ), Cd (Ⅱ), etc.). In 2012, Fei et al. Prepared polyacrylic acid-silica (PAA/SiO2) amphiphilic hybrid hydrogel with double network (DN) structure by two-step method to remove Cu2+ and Cr2 O 7 2 -[31]. Wherein the PAA is the first network and the SiO2 is the second network. This is the first time to use DN gel to adsorb heavy metal ions, and the gel adsorbent has a certain ability to remove heavy metals from wastewater. Zhou et al. Developed an amino-functionalized starch/polyacrylic acid gel, which has an adsorption capacity of 256.4 mg/G for Cd (Ⅱ) due to the presence of a large number of amino functional groups[90]. However, the equilibrium adsorption capacity of Starch/PAA double network gel was only half of that of NH2-Starch/PAA double network gel due to the absence of bonding sites.
Adsorption is a complex process that may involve different adsorption mechanisms, including ion exchange, electrostatic interactions, chelation, and complexation[119]. Due to the presence of specific functional groups in the monomer, whose charge is opposite to that of the contaminant ion, many studies have hypothesized that electrostatic interaction is the primary mechanism [120~122] for contaminant ion removal using hydrogels. In the case of hydrogels with acidic or basic functional groups, the adsorption mechanism is believed to be ion exchange[82,87,123]. In addition, the coordination interactions between heavy metal ions and functional groups containing O and N are often mentioned[27,32,124]. For the kinetic, thermodynamic and isotherm models in the adsorption process, the pseudo-second order kinetic model (pseudo-second order model, abbreviated as P2 in the table) is most consistent with the adsorption kinetic data in most studies (Table 1), that is, the adsorption process is mainly chemical adsorption, and a small part can be fitted by the pseudo-first order kinetic model (pseudo-first order model, abbreviated as P1 in the table); The Langmuir adsorption isotherm model best fits most of the studies (Table 1), indicating monolayer surface adsorption for many DN gels, and thus predicts the maximum adsorption capacity of the adsorbent; Among the thermodynamic parameters, the negative values of the ΔG0 and the positive values of the ΔH0 and ΔS0 indicate that the adsorption is spontaneous and endothermic.
Selective adsorption of certain heavy metal ions is an important property for the treatment of target molten wastewater. Zhou et al. Prepared Jute/polyacrylic acid (Jute/PAA) double network hydrogel by one-pot method, which was used to adsorb heavy metals such as Pb (II) and Cd (II) in molten wastewater. The maximum adsorption capacity of Pb (II) was 542.9 mg/G, and that of Cd (Ⅱ) was 401.7 mg/G. The adsorbent had high adsorption performance, which was much higher than the [125~127] of most hydrogel adsorbents[81]. The DN gel preferentially adsorbs Pb2+ and Cd2+ with an adsorption efficiency of about 97% and 89%, respectively, and the adsorption efficiency is about Pb2+>Cd2+>Cu2+>Zn2+>Mn2+. The adsorption selectivity may be caused by the difference of electronegativity and mass to charge ratio. The electronegativity will lead to the different chelating ability of metal ions with the adsorbent, thus showing different adsorption effects[128].

4.2 Dye

Dyes are widely used in many technical fields, including the leather and textile industries, the paper industry, food technology, pharmaceuticals, and cosmetics[132]. Dyes are classified as cationic (all basic dyes), anionic (direct, acid, and reactive dyes), and nonionic (disperse dyes) [133~135] based on the charge of the particles after dissolution in aqueous media. Common organic dyes include methylene blue (MB) and rhodamine (Rh B). The release of dyes into the environment is of great concern worldwide due to their toxic effects on aquatic organisms and humans, including organ damage, mutagenesis, carcinogenesis, and developmental abnormalities[136]. At the same time, printing and dyeing wastewater has become a global problem because of its complex composition, deep chromaticity and large discharge. Adsorption is the most widely used method to remove dyes[105].
In 2013, the GO/SA/PAM composite hydrogel prepared by Fan et al showed good adsorption capacity for cationic dyes (R6G, MB, MG and BG) and anionic dyes (CA, MO, BR and RB)[14]. Zhuang et al. Prepared a novel porous alginate/graphene double-network nanocomposite gel (GAD) and compared it with alginate/graphene single-network gel (GAS)[93]. Compared with GAS, GAD has more porous structure, higher specific surface area and higher NaCl tolerance. Based on the Langmuir model, the maximum adsorption capacities of GAS and GAD were 1. 84 G/G and 2. 30 G/G, respectively. In the column adsorption, after 10 adsorption-desorption cycles, the removal rate of MB decreased slightly, but still maintained comparable values (GAS 47.0%, GAD 60.2%). The results obtained indicate that GAD has a better potential for use as an adsorbent than GAS. Based on the physicochemical properties of the dye and the hydrogel, three adsorption mechanisms were mainly reported: (1) electrostatic interaction between the functional groups of opposite charges of the dye and the hydrogel; (2) hydrogen bond; (3) Hydrophobic interaction between the nonpolar component of the dye molecule and the hydrogel (Table 2)[23,93,119].

4.3 Antibiotics

In the past few decades, antibiotics have been widely used in clinical medicine and animal husbandry[139,140]. The heavy use of antibiotics may result in relatively high environmental concentrations and increase the risk of antibiotic resistance in the environment[141]. Although the harmfulness of antibiotics is not as intuitive as other environmental pollution, the abuse of antibiotics is becoming more and more serious, and antimicrobial resistance has become a very urgent health problem[107]. In addition, antibiotics persist and persist in aquatic ecosystems even at low concentrations[142]. Among the main treatment methods of antibiotic contamination, the adsorption method has universal applicability and [143~146]. At present, there are relatively few studies on the treatment of antibiotics in water by double-network gel adsorbent, mainly focusing on the adsorption of tetracycline and ciprofloxacin (Table 3).
Zhuang et al. Developed PVA-modified alginate/graphene double network hydrogel in 2017. Due to its excellent porous structure and abundant functional groups, its maximum adsorption capacity for tetracycline (TC) and ciprofloxacin (CIP) is 290.70 mg/G and 344.83 mg/G[48]. This is the first DN gel used to adsorb antibiotics, and it has the largest adsorption capacity at present. It also has a very high reproducibility, and its adsorption capacity remains above 200 mg/G after 10 cycles. The adsorption mechanism of the gel is electrostatic interaction. Tetracycline and ciprofloxacin both change from positive charge to negative charge with the increase of pH value. At high pH value, the electrostatic attraction between antibiotics and adsorbent changes to electrostatic repulsion, so the adsorption capacity decreases. Sun et al. Prepared amino-functionalized graphene/alginate double network hydrogel with excellent mechanical properties and adsorption properties, and its maximum adsorption capacity for ciprofloxacin reached 301. 36 mg/G, and its adsorption mechanism was electrostatic interaction, hydrogen bonding and π-π interaction[76].

4.4 Other pollutants

In addition to the above pollutants, the DN gel adsorbent can also remove phenol, Cs+ and Al3+. Phenolic compounds are bioaccumulated in nature due to their low biodegradability and are listed as priority pollutants by the United States Environmental Protection Agency due to their toxicity and adverse effects on the environment[147,148]. Due to its relatively long half-life, high diffusivity and solubility, the isotope Cs-137 is considered the most critical radionuclide[149]. Aluminum, on the other hand, causes toxicity in the body's hematopoietic and nervous systems (Alzheimer's disease, Lou Gehrig's disease (ALS))[150]. Therefore, wastewater containing these pollutants must be properly treated before it is released into the environment.
Wang et al. Developed a double network composite hydrogel based on hydroxypropyl cellulose (HPC) and graphene oxide (GO) and used it to adsorb phenol[108]. Polyethyleneimine (PEI) was introduced into the composite hydrogel to provide a large number of active groups (such as —NH2) to promote the adsorption and binding of phenol. On the other hand, PEI played an important role in promoting the self-assembly of GO sheets and the formation of cross-linked networks, which provided a high specific surface area and facilitated the adsorption of phenol. The maximum adsorption capacity of the gel for phenol was 213. 5 mg/G, and no significant loss of adsorption capacity was observed after 6 adsorption-desorption cycles. The adsorption mechanism involves electrostatic, hydrogen bonding and π-π interactions. Zhao et al. Prepared PAM/PAMPS double network hydrogel. Due to the existence of sulfonate groups, the hydrogel has cation selectivity. The resulting composite material shows high Cs+ adsorption capacity (397 mg/G in 10 min)[151]. Özkahraman et Al. prepared a GG-MA/PAA hydrogel with high mechanical properties and studied its potential as an adsorbent for the removal of Al (III)[150]. Gellan Gum was functionalized with methacrylic acid groups to obtain GG-MA, which was then immersed in an acrylic acid solution to obtain a bi-network hydrogel. The DN gel has good regenerability, and the adsorption capacity does not decrease significantly after 5 adsorption-desorption cycles.

5 Conclusions and outlook

Over the past decade, significant progress has been made in the development of dual-network hydrogel adsorbents, which have become promising materials for many industrial and basic applications due to their excellent mechanical properties, high water content and biocompatibility. The main purpose of this review is to explore the physicochemical properties of DN hydrogels as adsorbents, as well as their application and adsorption mechanism in water treatment, so as to help researchers to briefly understand the latest progress of DN hydrogel adsorbents. In fact, DN hydrogels provide a new but complex platform for researchers to understand the internal structure and interaction mechanism between the two networks, as well as to control mechanical properties, swelling, self-healing and adsorption properties. Compared with some mature hydrogels, the research of DN hydrogels is still in the initial stage. Although the efficiency and practicability of DN hydrogels in wastewater treatment are not as good as those of mature technologies, they still have considerable potential, and many in-depth and systematic studies need to be carried out in the following aspects in the follow-up research: 1) The mechanical properties of DN hydrogels are inversely proportional to their self-healing properties.If we want to balance the two under the premise of meeting the needs, we need to consider new strategies to adjust the network structure of chemically or physically crosslinked DN gels, or to introduce new reinforcement materials. 2) physically cross-linked gels generally do not require additional cross-linking agents to aid in gelation, and they generally do not exhibit significant volume changes, physically cross-linked DN gels provide a fairly promising indicator of mechanical and adsorption properties, thereby minimizing the decrease in mechanical properties due to swelling; 3) To further develop the construction and synthesis of multi-network gels and develop tough hydrogels with good adsorption properties. The design of Triple-network gels should focus on the formation of three independent but mutually permeable networks; 4) The composition of polluted water is very complex, so it is very important to improve the wide adaptability or high selectivity of DN gel to pollutants. Compared with traditional adsorbents such as activated carbon and zeolite, DN gel is not economical enough, and new processes need to be developed to reduce production costs. Therefore, it is still a huge challenge to explore hydrogels with high adsorption capacity.

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