Hydrogel applications for adsorption of contaminants in water and wastewater treatment

Abstract

During the last decade, hydrogels have been used as potential adsorbents for removal of contaminants from aqueous solution. To improve the adsorption efficiency, there are numerous different particles that can be chosen to encapsulate into hydrogels and each particle has their respective advantages. Depending on the type of pollutants and approaching method, the particles will be used to prepare hydrogels. The hydrogels commonly applied in water/wastewater treatment was mainly classified into three classes according to their shape included hydrogel beads, hydrogel films, and hydrogel nanocomposites. In review of many recently research papers, we take a closer look at hydrogels and their applications for removal of contaminants, such as heavy metal ion, dyes, and radionuclides from water/wastewater in order to elucidate the reactions between contaminants and particles and potential for recycling and regeneration of the post-treatment hydrogels.

Introduction

Water is the most essential substance to humans and other living organisms. However, some harmful chemical compounds cause water pollution that exerts negative effects not only on species living in water but also on the broader biological community (Guo et al. 2015).

Yearly, heavy metals are released into environments from various industrial production or auxiliary processes as well as other human activities (Chen et al. 2010; Patel et al. 2010). There are more than 40 elements of greater-than 5 g/cm³ mass, which are regarded as heavy metals (Srivastav et al. 1994; Zenk 1996). Heavy metal ions are among the main contaminants found in water. Due to their persistence and toxicity properties, such pollutants, heavy metals, have attracted considerable-attention within the scientific, governmental, and community-activist communities. Toxic metals are very dangerous to all biological organisms and the overall environment, especially when present above the given tolerance levels (Abachi et al. 2013; Ali et al. 2011) and even at low concentrations. Even so, nearly 1 million t of dyes is annually used in the chemical industry worldwide, more than 10% of which are discharged as effluents that seriously pollute the environment and affect humans as well as aquatic organisms (Sethuraman and Raymahashay 1975; Yagub et al. 2014). Generally, the dyes have complex and stable structures, and their degradation in nature is usually very difficult and slow; consequently, they will accumulate (Fu and Viraraghavan 2001; Kadirvelua et al. 2003; Zollinger 2003). Besides heavy metals and dyes, radioactive waste, a waste product containing radioactive materials usually produced in a nuclear process such as nuclear fission, is also one of the common contaminants significantly affecting water resource. Although the radioactivity of such waste will decrease over time, it will do so only at a very slow rate; therefore, it can seriously affect human health and the environment over very long duration (Jing et al. 2013). In addition, in modern agriculture, pesticides have been widely used for protecting crops and livestock, which have also led to a lot of serious health implications. According to experts, these pesticides have been discharged through direct agricultural runoff, leaching, and improper disposal of empty containers (Dwivedi et al. 2014). Besides, the discharge of nitrogen components (nitrate) in the water source is currently harmful for both human and animal health. Nitrates may cause several serious problems when dispersed in water, since they cause the groundwater pollution and the eutrophication of rivers (Rossi et al. 2015). Therefore, the pesticides and nitrate remediation from aqueous solution has been a worldwide problem and challenge.

The investigation and application of various methods to remove these contaminants from water is absolutely essential for protection of the environment. Compared with chemical and physical methods commonly used to remove dyes, heavy metal ions, and others, currently, the adsorption methods stand out as one of the most effective and friendly methods for dye removal from aqueous solution (Azbar et al. 2004; Mohan et al. 1998). Hydrogel is a three-dimensional polymer that can be synthesized by the reaction between one or more monomers. Hydrogel’s three-dimensional network and porous structure typically endows them with a hydrophilic property and corresponding capacity to adsorb large amounts of water or biological fluids (Ahmed 2015). Hydrogels’ high water retention and low cost have attracted great attention as effective adsorbents. The hydrogels commonly applied in water/wastewater treatment were mainly classified into three classes including hydrogel beads, hydrogel films, and hydrogel nanocomposites according to their shape and physicochemical properties (Table 1).

Table 1 The physiochemical properties of three types of hydrogel

In order to improve adsorption efficiency, there are many different and unique nanoparticles utilized for entrapment in hydrogel beads, hydrogel films, or hydrogel nanocomposite as hybrids (Jing et al. 2013). This study will provide an overview of the recent literature’s results obtained in the treatment of wastewater utilizing hydrogels for the removal of heavy-metal ion, dyes, radionuclides, and other pollutants from aqueous solution as well as in the recovery and recycling of used hydrogels. It is evident from the literature survey that nanoparticles into hybrid hydrogels have shown good potential applicability for removal of various aquatic pollutants.

Application of hydrogel beads for adsorption of contaminants

Particles entrapped in hydrogel beads for heavy-metal ion removal

Cellulose is known as the most abundant renewable material in the world. However, its derivatives have usually received more interest in synthesizing hydrogels because they have a better solubility in water than cellulose. One of the most common derivatives is carboxylmethyl cellulose (CMC), which is known as a water-soluble cellulose derivative that has been available on the market. A method commonly used to prepare CMC-based hydrogels is crosslinking method by using the crosslinking agents like polycarboxylic acids, epichlorohydrin (ECH), and N,N′-methylene-bisacrylamide (Fekete et al. 2017). In 2010, typical hydrogel beads were successfully prepared based on CMC by inverse suspension crosslinking using ECH as cross-linker (Fig. 1) (Yang et al. 2010). The adsorption capacity of these hydrogels was evaluated with model wastewater including heavy-metal ions, Pb²⁺, Ni²⁺, and Cu²⁺. The results confirmed that a new bond, CAOAC, was formed by the reaction between ECH and CMC. Compared with pure CMC, the hydrogel beads had lower crystallinity. The hydrogel beads’ adsorption of Pb²⁺, Ni²⁺, and Cu²⁺ were based on the formation of coordination bonds with the oxygen atoms in their carboxyl groups. The formed hydrogel beads thus exhibited a high adsorption capacity for heavy-metal ions.

Fig. 1

Preparation of CMC hydrogel particles with ECH as crosslinker

Recently, a biopolymer based on chitosan (CS) has attracted great interest in various fields, especially environmental biotechnology, because of its very high adsorption capacity of heavy-metal ions and low cost (Jin and Bai 2002; Yoshida and Takemori 1997). Moreover, CS can be easily synthesized at the industrial level by a simple method based on chitin. CS hydrogel beads are regarded as a potential adsorbent for effective removal of heavy metals in industrial wastewater treatment (Chatterjee et al. 2005; Chatterjee et al. 2007; Ngah et al. 2002). CS has also shown a great material in the production process of hydrogel due to the combination ability with various materials. By blending CS with a high concentration of CMC solution, a novel CS-based hydrogel called CM(carboxymethylated)-cellulose/CS in the form of physical hydrogels was successfully synthesized by irradiation-based crosslinking (Zhao and Mitomo 2008). Due to adding CS, the hydrogel’s adsorption capacity for divalent heavy-metal ions and crosslinking extent had been improved. Also, CS could combine with PVA to form a CS/poly(vinyl alcohol) (PVA) hydrogel bead for adsorption of lead ion from an aqueous solution (Jin and Bai 2002). The Pb²⁺ ion adsorption on these hydrogel beads showed a strong dependence on pH, and the mechanism of the adsorption process is based on complexation, ion exchange, and electrostatic interaction. For the electrostatic interaction, the hydrogels possessed a ζ-potential. In the case of pH < 6.3, the ζ-potentials of the CS/PVA hydrogels were positive, and for pH > 6.3, negative. Therefore, the heavy metal ion adsorption occurs even though the interaction between the metal ions and the hydrogels is electrostatically repulsive at pH < 6.3. In an attempt to improve the efficiency of CS-hydrogel beads in metal ion removal applications, a glutamic-CS hydrogel beads (GCS) was synthesized by reactions between CS and different concentrations of glutamic acid (Abdelwahab et al. 2016). It is the presence of glutamic acid in the structure that affected the treatment efficiency of the hydrogels in removing Cu²⁺ and Ni²⁺ ion. It is found that when the concentration of glutamic acid increased, the adsorption of the hydrogels decreased. This phenomenon was explained that the increase of the crosslinking degree caused a decrease in the hydrogel pore size. Especially, it is notable that a combination of CS with ethylenediaminetetra-acetic acid (EDTA), N,N-methylenebis(acrylamide) (MBA), and polyacrylamide (PAM) formed an exceptional type of hydrogel beads which its outstanding properties are a remarkable mechanical strength and recyclability (Ma et al. 2017). The hydrogel preparation process was conducted with a two-step method by which EDTA is cross-linked with CS and MBA is cross-linked with PAM to form a double-network hydrogel (Fig. 2). The hydrogel showed a high adsorption capacity for Cd²⁺, Cu²⁺, and Pb²⁺ ion based on the mechanism of the ion exchange interactions between metal ions and carboxylate groups. This hydrogel was effectively demonstrated for treating practical wastewater with various kinds of heavy metal ions, with an adsorption efficiency of 98%.

Fig. 2

Schematic representation of CTS/PAM gel synthesis (reprinted (adapted) with permission from Ma et al. 2017. Copyright (2017) American Chemical Society)

In preparing a high-strength hydrogel for adsorption of metal ions from industrial lean solvents and reusability, a series of PAM hydrogel beads was successfully synthesized by using three components with different functions (Pal and Banat 2015). To fabricate these hydrogel beads, acrylamide as a monomer was polymerized with ammonium peroxodisulphate as initiator. The acrylamide monomer was then cross-linked to form PAM hydrogels using N,N′-methylenebisacrylamide as the cross-linker. The fabricated hydrogel beads with an entangled 3D network were effectively demonstrated for removal of metal ions. Moreover, the hydrogel beads also showed good reusability: the adsorption capacity had only slightly dropped even after 15 cycles of desorption and reuse. In addition, the hydrogel could easily be desorbed by sodium chloride and the hydrogel beads can be excellent adsorbents offering operation costs and good reusability.

The interaction between two oppositely charged polymers results in the formation of polyelectrolyte complexes (PECs) (Philipp et al. 1989). Recently, PECs have been widely applied as potential flocculent materials to precipitate metal ions in the wastewater treatment industry, owing to their relative cheapness and ease of use. Especially, after simply mixing with other oppositely charged polyelectrolytes according to each experiment condition, PECs will be formed as precipitates or thin films, or dispersed colloids in water, which makes them excellent materials for water treatment (Petzold and Schwarz 2014; Thünemann et al. 2004; Zhang and Sun 2009). Based on PECs’ interaction between positively charged poly(allylamine hydrochloride) (PAH) and negatively charged poly(L-3,4-dihydroxyphenylalanine) (PDopa), new hydrogel beads, namely PDopa-PAH PEC hydrogel beads, were introduced for removal of ionic pollutants in aqueous solution (Yu et al. 2015a). Due to PDopa-PAH PEC loaded inside the hydrogel matrix, this type of hydrogel beads can effectively and efficiently remove various ionic pollutants, both heavy metal ions and ionic organic dyes, from aqueous solution. Because the chemical structure of PDopa contains three types of functional group (catechol group, carboxyl group, and amino group), the formed PDopa-PAH PECs hydrogels possess a larger adsorption capacity of heavy metal ions compared to other adsorbents. Moreover, because both PDopa and PAH are pH-sensitive, the hydrogel beads can be easily recovered for reuse by adjusting the pH in the surrounding media. In addition, given the ease and appropriateness of synthesizing hydrogel beads for large-scale production with little raw material loss and byproduct generation, the synthesis can be extended to various polyelectrolytes.

Pectin (Pec) is a natural chemical compound with a heterogeneous structure that is basically formed by smooth and hairy regions having various unit of polymers, such as the poly(1 → 4) α-D-galacturonic acid (Pérez et al. 2000; Reis et al. 2009). Pec has been generally found in citric fruits, principally the lemon. This particular polysaccharide has been commonly used in feed compositions (Willats et al. 2006) and the pharmaceutical industry. Its structure includes large numbers of ionic groups, making it an excellent polymer matrix for superabsorbent hydrogel synthesis. Recently, Pec also was applied in synthesis of materials for wastewater treatment through a novel Pec-based hydrogel that can be used to remove Cu²⁺ and Pb²⁺ ions (Guilherme et al. 2010). This hydrogel showed a high adsorption capacity and efficiency for removal of Cu²⁺ and Pb²⁺ from water: 120-mg Cu²⁺ and 130-mg Pb²⁺ per g hydrogel at pH 5.5 with 0.10-g mL⁻¹ adsorbent concentration.

Fly ash (FA) is a waste product produced from thermal power plants, steel mills, and other facilities. Due to its low-cost and abundance, it has been utilized as an adsorbent in many fields (Wang &Wu 2006). With a new approach for preparation of hydrogels, a novel magnetic attapulgite/fly ash/poly(acrylic acid) (ATP/FA/PAA) ternary hydrogel beads was synthesized for removal of Pb²⁺ ion (Jiang and Liu 2014). In this method, magnetic FA particles were used as new cross-linkers (instead of the traditional cross-linkers) via an inverse suspension polymerization process. In this process, acrylic acid (AA) is attached onto an inorganic skeleton obtained by the functionalized ATP and FA to form ternary hydrogel beads. By using inorganic materials, FA, the production cost of the hydrogel beads is significantly lower and the mechanical strength, thermal stability, chemical resistance, and dimensional stability also are improved. These hydrogel beads showed effective treatment efficiency for removal of Pb²⁺ ion via a high adsorption capacity of 38 mg g⁻¹ in 100-mg/L Pb²⁺ solution at pH 5. Besides, the Pb²⁺ ion adsorbed in the hydrogel beads could be completely desorbed in 0.10-mol/L HCl aqueous solution within 90 min.

Particles entrapped in hydrogel beads for dye removal

Crystal violet (CV) is a typical cationic dye commonly used in coloring paper, temporary hair colorant, dyeing cottons, and wools. When discharged into receiving environments, it can pollute the water source and cause some negative effects on human health such as accelerated pulse, vomiting, shock, cyanosis, jaundice, quadriplegia, and others (Kunz et al. 2002). Hence the removal of CV from industrial effluent has attracted the attentions of numerous scientists and researchers. Recently, a semi-IPN (interpenetrating polymer networks) hydrogel bead constituted of poly(acrylic acid-acrylamide methacrylate) and amylose was introduced for adsorption of CV with high adsorption efficiency (Li 2010). However, its adsorption capacity was found to be highly dependent on the amylose content and pH of the solution. As for adsorption mechanism, the results indicated that the CV dye adsorption of hydrogel occurs via swelling combined with electrostatic and hydrophobic interactions. And as for the morphology of the hydrogel, hydrophilic amylose was dispersed in continuous phases of poly(acrylic acid-acrylamide-methacrylate) hydrogel, and many open pores of 100-μm diameter were formed in the hydrogel matrix, which may be favorable for CV adsorption.

In the past decade, graphene oxide (GO) sheets, given their outstanding structure, have become a target of numerous studies for dye removal from wastewater. Due to its two-dimensional structure, GO possesses a high adsorption capacity for different dye molecules through π-π stacking interactions (Singh et al. 2011). Moreover, GO also has negative charges due to the presence of various oxygen-rich functional groups, such as carboxyl, carbonyl, and hydroxyl groups, which enhance electrostatic interactions with dye cations. Therefore, GO has been used as a potential adsorbent in various applications, especially industrial wastewater treatment. By combining GO and polyethylenimine (PEI), a novel hydrogel bead as an efficient dye adsorbent was produced, which they named GO/PEI hydrogel (Guo et al. 2015). As for the preparation process, the hydrogel could be simply formed through a combination of amine-rich PEI and GO sheets achieved by hydrogen bonding and electrostatic interactions. Figure 3 shows that GO sheets possess numerous functional groups (▬COOH and ▬OH) that can form hydrogen bonds with amine molecules of PEI under appropriate conditions. For this type of hydrogel, PEI is added in solution to facilitate the gelation process of GO sheets, as GO sheets are regarded as the most important hydrogel ingredient for maintenance of dye adsorption capacity. Moreover, it is demonstrated that this hydrogel will be a new potential material for the organic dye removal with the high efficiency.

Fig. 3

Schematic depiction of formation of GO/PEI gels. a GO and b amine-rich PEI were combined to yield c GO/PEI hydrogels. (Reprinted (adapted) with permission from Guo et al. 2015. Copyright (2017) Springer Open)

Halloysite is known as an aluminosilicate clay mineral produced by hydrothermal alteration and deposition (Joussein et al. 2005; Levis and Deasy 2002). As halloysite can effectively adsorb both cations and anions, it can be applied in various fields including ceramic raw materials, catalysis, and polymer nanofillers. In its structure, halloysite naturally appears as small cylinders (nanotubes) with a high aspect ratio, for which reason it usually is referred to as “Halloysite nanotubes (HNTs)” (Lvov et al. 2008). Compared with other tubular nanomaterials (such as carbon nanotubes) used in wastewater treatment, natural HNTs are more environmentally friendly and cheaper (Du et al. 2010; Zhixin et al. 2014). By combining the two cheap materials, CS and HNTs, Peng and co-workers fabricated novel hydrogel beads that can be applied for removal of MB and malachite green (MG) from wastewater (Peng et al. 2015). CS–HNTs hydrogel beads were obtained by the dropping and pH-precipitation method. It is found that CS and HNTs are compatible in the preparation of hydrogel adsorbents: (i) HNTs can be immobilized by CS hydrogel and completely incorporated into the hydrogel beads’ structure and (ii) the thermal stability of the hydrogel beads is remarkably enhanced by HNTs. The appearance of hydrogel beads is shown in both wet and dry form (Fig. 4). The average diameter of the hydrogel beads is changed from 2.29 to 2.75 mm in the wet form. It is found that HNTs have an influence on the appearance, diameter, and microstructures. Via the adsorption behavior of the hydrogel beads with MB and MG, it is found that HNTs affected the adsorption capacity of hydrogel beads. It is the presence of HNTs that can considerably increase the adsorption effect of hydrogel beads for both pollutants. Besides, these hydrogel beads can be regarded as an excellent adsorbent, as they can be simply recycled and reused by NaOH solution and acetone. Especially, the high removal ratio of dyes is maintained above 92.0% in all hydrogel beads at the second adsorption.

Fig. 4

Appearance of chitosan and chitosan–HNTs hydrogel beads in wet and dry state (reprinted (adapted) with permission from Peng et al. 2015. Copyright (2017) Elsevier)

Congo red (CR) has been known as one of the dyes that causes an allergic reaction. It is metabolized to benzidine, a human carcinogen, and it is very difficult to biodegrade due to a complex aromatic structure that provides physico-chemical, thermal, and optical stability (Han et al. 2008). Recently, CS has been regarded as an excellent material for wastewater treatment due to the large number of active amine (▬NH₂) groups in its structure. Accordingly, there have been many studies using CS to prepare hydrogel for anionic dyes and heavy metal removal. To enhance adsorption capacity of CS-based hydrogels, certain modification methods, such as cross-linking (Vieira &Beppu 2006) and the addition of other functional groups (Suna et al. 2006), were conducted. By using cetyl trimethyl ammonium bromide (CTAB), a cationic surfactant, CTAB-impregnated CS hydrogel beads were prepared successfully for adsorption of CR (Chatterjee et al. 2009). The addition of CTAB to CS hydrogel beads significantly improved the adsorption capacity of the hydrogel. It is found that the increase of the CTAB concentration led to a significant increase of CR adsorption capacity of the hydrogel beads. Additionally, using cationic surfactant to impregnate CS can enhance the adsorption capacity of hydrogels for CR relative to other materials, and this approach will produce hydrogel adsorbents having a high adsorption efficiency for removal of anionic dyes.

Recently, the development of new adsorbents from feedstock materials, such as starch, cellulose, and lignin, has been of interest to scientists due to the fact that the adsorbents produced form these materials have many unique advantages including low-cost, biodegradability, non-toxicity, and high efficiency (Li and Bai 2006). Lignin is a class of complex organic polymer that is synthesized by the chemical polymerization method from three type of alcohol (Ralph et al. 2004). Offering the outstanding advantages noted above, lignin is regarded as a potential material for preparation of excellent adsorbents. Recently, a type of lignin, lignosulfonate (LS), has been used in the synthesis process of a typical Lignosulfonate-g-acrylic acid (LS-g-AA) hydrogel bead with superabsorbent capacity (Yu et al. 2016). In this preparation method, AA molecules were grafted onto the backbone LS in the presence of N,N′-methylene-bis-acrylamide (MBA) as the cross-linker and laccase/t-BHP as the initiator (Fig. 5). It was found that the hydrogel shows a high adsorption efficiency for MB after 2 h and its adsorption capacity depends on the mass ratio of AA, the pH of solution, and the initial pollutant concentration. Moreover, the LS-g-AA hydrogel also showed that with its desorption and reuse capacities, it is an excellent adsorbent. The hydrogel could be desorbed by using a weak acid solution (pH of 4.0) and reused at least four times but still maintained high MB-adsorption levels. Employing a similar method, using another feedstock material (cellulose), a novel amphoteric hydrogels was introduced for removing three anionic dyes, namely acid red 9 (AR9), acid red 13 (AR13), and acid blue 92 (AB92) (Kono 2015). To prepare the hydrogel, a series of cellulose ampholytes (CAms) was produced by a cationization reaction between sodium carboxymethylcellulose and 2,3-epoxypropyltrimethylammonium chloride. Then, the hydrogels were formed by a crosslinking method according to which CAms is cross-linked by ethyleneglycoldiglycidylether (EGDE). The adsorption capacity of the fabricated hydrogel was revealed to be a function of two factors: the pH of the adsorption solution and the proportions of the cationic groups. The highest adsorption capacity of the anionic dyes could be reached at pH < 3 and with high proportions of cationic groups.

Fig. 5

Schematic of LS-g-AA hydrogel synthesis catalyzed by laccase in presence of hydrogen peroxide (reprinted (adapted) with permission from Yu et al. 2016. Copyright (2017) Elsevier)

Poly(N-vinyl-2-pyrrolidone) (PVP) is a water-soluble polymer that is quite friendly to the environment and to humans due to its non-toxic, biodegradable, and biocompatible properties. Owing to these advantages, PVP is totally suitable for synthesize hydrogel materials. Hydrogels based on the interaction of PVP and other component for removing anionic dyes and metal ions (Lu et al. 2011; Senkal et al. 2006; Wang and Wang 2010) have been successfully prepared. Sodium alginate (NaAlg), a natural polysaccharide derived from brown seaweeds, also possesses the same friendly properties as PVP. Hydrogel beads can be fabricated from NaAlg by cross-linking the guluronic acid component of NaAlg with other cations (Babu et al. 2007). When sodium alginate is dissolved in a neutral and alkaline solution, the carboxylate-functional group has a negative charge, which affects a greater affinity to cations (Rocher et al. 2008). Yet, it has been suggested that anionic hydrogels produced from polysaccharide like alginate have inadequate efficiency for adsorption of anionic dyes from aqueous solution (Blackburn 2004). However, by combining PVP with sodium alginate, that problem was overcome in preparing novel NaAlg/PVP-blend hydrogel beads (Inal and Erduran 2015). The hydrogel beads were fabricated by the gelation method in CaCl₂ solution. They have been effectively demonstrated for adsorption of different dyes such as reactive red-120 (RR), cibacron brilliant red 3B-A (CBR), and remazol brilliant blue R (RBB).

Titanium dioxide (TiO₂) is a flexible semiconductor that has attracted great interest in the wastewater treatment field. It has many advantages, such as strong oxidizing power, superhydrophilicity, chemical stability, long-term durability, non-toxicity, low cost, and transparency to visible light. Therefore, TiO₂ has seen many applications in water splitting, energy storage, dye-sensitized solar cells, and decomposition of various pollutants. Using TiO₂ as a photocatalyst is regarded one of the most promising applications in terms of the removal of hazardous organic substances, because it shows high photocatalytic activity under UV light irradiation (Nakata and Fujishima 2012; Schneider et al. 2014). According to this approach, a recyclable, recoverable, and reformable hydrogel-based photocatalyst was successfully synthesized (Mai et al. 2017). In the preparation process, the main observations were well-distributed TiO₂ nanoparticles in the agarose hydrogel matrix and the gelation of the agarose in hot water. The study used methyl blue (MB) to examine the photocatalytic activity of the hydrogel and the contents of TiO₂ and agarose in hydrogels as well as hydrogel size, shape, and concentration of a hydrogel are main factors that significantly influenced the photodegradation efficiency. Furthermore, the additional excellent features of this type of hydrogel are its recyclability and the simplicity of the regeneration of pure TiO₂ NPs.

Particles entrapped in hydrogel beads for radioactive waste removal

With the rapid development of nuclear power application around the world, radioactive wastes containing enormous amounts of radionuclides have become a focus of concern, posing serious chemical and radiological toxicity threats to the environment. Among the common radioactive nuclides, ⁶⁰Co is one of the most harmful owning to its strong γ-emission and relatively long half-life of 5.3 years (Wang et al. 2013). The adsorption method is of great interest for removal of ⁶⁰Co, owing to advantages including high efficiency, low cost and operation convenience compared with the alternative methods (chemical precipitation, reverse osmosis, ion exchange, solvent extraction, coagulation, and membrane separation) (Thakura and Thakurb 2015). By using PVA and poly(acrylic acid-co-acrylic amide) P(AA-co-AM), novel PVA-P (AA-co-AM) semi-IPN hydrogels were successfully synthesized via free radical polymerization under the ultrasound-assisted condition for Co²⁺ ion removal (Wang et al. 2016). As for the adsorption capacity of the hydrogel for Co²⁺ ions, it showed an excellent capability, compared with other adsorbents. However, its adsorption efficiency was also influenced by various parameters, such as pH, temperature, adsorption time, and the initial concentration of pollutants. In addition, the hydrogel was also used as an adsorbent owing to the merits of low material cost and operational convenience.

Cesium (¹³⁷Cs) is known as an important radioactive contaminant that can cause serious radiological hazards that affect human health and the environment due to its long half-life and high mobility in aquatic environments (Howard et al. 1991). For the past many decades, development of efficient materials for removal of radioactive cesium from aqueous solutions has been the focus of great attention from researchers worldwide. Inorganic ion exchangers possess many special properties including selectivity, radiation, and thermal stabilities as well as good compatibility with various matrices. Currently, a popular and important inorganic ion exchanger is transition metal hexacyanoferrates, which can be applied in the nuclear industry for removal of cesium. Generally, inorganic ion exchangers can be used to remove cesium in two different ways: (i) co-precipitation of cesium ions and (ii) preparation of metal hexacyanoferrate slurry. In 2015, a new method for synthesis of spherical potassium nickel hexacyanoferrate hydrogel beads that binds the slurry of potassium nickel hexacyanoferrate (KNiHCF) particles in a alginate and polyvinyl alcohol (PVA) matrix was introduced by utilizing KNiHCF as an inorganic ion exchanger (Dwivedi et al. 2015). The hydrogel beads were found to have a high hydrophilicity and a high surface area. The optical microscope (OM) image of the swollen hydrogel beads shown in Fig. 6a reveals a round shape and a relatively uniform size of 2 mm. Figure 6b shows the SEM images of the outer surface of a dried hydrogel bead. As is apparent, it is quite rough and has some large pores. The adsorption of cesium ions onto the KNiHCF particles of the hydrogel beads happens by ion exchange between a potassium ion of KNiHCF and a cesium ion. Also, the hydrogel beads could be effectively applied to adsorb radioactive cesium ions within a wide range of pH (1–12), for a maximum capacity of about ~ 64 mg per g of dry beads. The KNiHCF hydrogel beads are an excellent material for use in cesium removal from low level radioactive waste.

Fig. 6

a OM image and b SEM images of outer surface (reprinted (adapted) with permission from Dwivedi et al. 2015. Copyright (2017) Royal Society of Chemistry)

Taking a similar approach but using another type of inorganic ion exchanger, new hydrogel beads for recovery or removal of cesium from low-level nuclear waste streams were fabricated by encapsulating an inorganic sorbent, potassium zinc hexacyanoferrate (KZnHCF) in hydrogel beads (Pathak et al. 2016). The preparation process of the hydrogel beads is shown in Fig. 7. The KZnHCF-gel beads were prepared using sodium alginate stabilizing PVA to form a binding matrix. The reaction of ZnSO₄ and KHCF will make KZnHCF sol in the presence of PVA. The size of the KZnHCF particles formed can be controlled by the PVA. An aqueous solution of 4% sodium alginate was added to stabilize the KZnHCF sol. Then, hydrogel beads can be formed by dropping this mixture into an aqueous bath containing 4-wt% CaCl₂. The adsorption process of the hydrogel beads was found to be quite efficient in the pH range from 3 to 10 and the adsorption capacity of the hydrogel beads for cesium can approximately reach 95%. Especially, the cesium adsorption efficiency of these hydrogel beads under the pH neutral condition was not affected significantly by the interfering ions (Na⁺, Ca²⁺, and Ba²⁺). This mean that practical applications of such hydrogel beads for treatment of cesium in aqueous nuclear wastes is completely possible.

Fig. 7

Illustration of procedure for preparation of KZnHCF-gel beads (reprinted (adapted) with permission from Pathak et al. 2016. Copyright (2017) Asian Journal of Materials Chemistry). a Preparation of formulation of ZnSO₄ and KHCF in presence of PVA. b Drop wise addition of formulation to CaCl₂ aqueous solution. c Curing of beads for 4 h. d Separation and washing of prepared beads. e Optical microscopy (OM) image of the swollen beads

Particles entrapped in hydrogel beads for other contaminant removal

As has been noted, CS hydrogel beads are commonly used in contaminant removal applications due to their high adsorption capacity. In 2009, a CS hydrogel beads was introduced by Chatterjee and HanWoo 2009 for removal of nitrate. The nitrate adsorption capacity of the hydrogel beads was identified increasing along with decreasing the pH of the solution. Besides, the adsorption process also depends on the temperature: the optimum activity is achieved at 30 °C and the adsorption capacity will decrease with increases in temperature over 30 °C. The maximum adsorption capacity of this hydrogel could reach 92.1 mg g⁻¹ at 30 °C. It is found that the adsorption process of nitrate is a spontaneous, exothermic process that has positive entropy, and that when increasing the pH of the solution to the alkaline condition, desorption of nitrate from hydrogel beads will be accomplished for a desorption ratio of 87% at around pH 12.0.

Methyl parathion (MP) is one of the many types of organophosphorus pesticides used on cotton, soybeans, vegetables, and other crops. When discharged into living systems, the toxic compounds from pesticides will prevent the action of acetyl cholinesterase, which can lead to great harm to living creatures, such as loss of muscle control, paralysis, convulsions, coma, and even death (Agrawal et al. 2010). Therefore, MP’s removal from the water is crucial. A novel hydrogel beads for removal of MP from waste water were introduced by developing a new method for one-pot synthesis of gold nanoparticle-loaded hydrogel beads by using a composite matrix of CS (Dwivedi et al. 2014). These hydrogel beads are a strongly hydrophilic material that can contain up to 96% water and remain stable up to 200 °C. Their high water capacity also enhances the interaction between the MP’s compounds and the active sites of the hydrogel, leading to a higher adsorption efficiency. The study also reveals that a loading of at least 0.5-wt% gold nanoparticles on hydrogel beads is good enough to improve the adsorption capacity of the beads by more than twofold compared with pure beads.

Application of hydrogel films for adsorption of contaminants

Particles entrapped in hydrogel films for metal ion removal

The presence of Ca²⁺ and Mg²⁺ ions in liquid increases water hardness, which is one of the main problems in the water-treatment field. In 2016, a hybrid hydrogel film was successfully prepared by using conjugation of oxidized carbon dots (CDs) produced from 11-mercaptoundecanoic acid with CS (Baruah et al. 2016). Ca²⁺ and Mg²⁺ ions from the solution were removed by using the hydrogel film as a potential platform based on the principles of ion exchange. The removing efficiency of Ca²⁺ and Mg²⁺ ions from pond water by the hydrogel film was 68.01 and 56.35%, respectively. Figure 8 shows the method of synthesis of CS-MUA-SO₃Na-CDs hydrogel thin film. The presence of ▬SH functionalities on the CDs surface was superseded by SO₃Na after oxidizing. Then the hydrogel was formed by mixing the oxidized CDs and CS solution (dissolved in a 3:2 mixture of acetic acid and glycerol). This hydrogel was then spread onto glass slides and dried at 60 °C for 2 h. Typically, the films were of a thickness of ∼ 0.07 mm and the sulphonated CDs–CS hybrid hydrogel nanocomposite film can replace toxic conventional polystyrene crosslinked with divinylbenzene generally used for ion exchange because it is a green material for water-softening applications.

Fig. 8

Schema showing synthesis protocol of thiol functionalized carbon dots (CDs) from MUA followed by their oxidation to form sulphonated CDs and subsequent fabrication of polymeric nanocomposite films via conjugation of sulphonated CDs with biopolymer chitosan (reprinted (adapted) with permission from Baruah et al. 2016. Copyright (2017) Royal Society of Chemistry)

Agarose (Agr) is one of the most promising candidates for generation of thermal reversible hydrogels. It is a linear polymer that is a disaccharide of D-galactose and 3,6-anhydro-L-galactopyranose and is extracted from seaweeds (Golmohamadi et al. 2012). With combination of Agr, CS, and CDs, an Agr/CD hydrogel film as a solid sensing platform was introduced for detection and adsorption of heavy metal (Gogoi et al. 2015). The fabrication of the solid-sensing platform was illustrated in Fig. 9. The generation of the platform is mainly based on a simple electrostatic interaction between the OH⁻ groups present in agarose and the NH₃⁺ group present in the CDs. This hydrogel can detect many different metal ions, specifically Cr⁶⁺, Cu²⁺, Fe³⁺, Pb²⁺, and Mn²⁺. Besides, the hydrogel film is also suitable for use as an efficient filtration membrane for removal of the above-noted heavy metal ions. Especially, the removal of the quintet of heavy-metal ions of the Agr/CD hydrogel was more effective than that of the only-Agr hydrogel. The removal percentage of the Agr/CD hydrogel for Cr6⁺, Cu²⁺, Fe³⁺, Mn²⁺, and Pb²⁺ ion was found to be 27.75, 54.85, 38.48, 35.41, and 83.97%, respectively.

Fig. 9

Schematic illustration of preparation of Agarose/CD (Agr/CD) hydrogel film and its application for removal of heavy metal ions (reprinted (adapted) with permission from Gogoi et al. 2015. Copyright (2017) American Chemical Society)

CS has many advantages for preparation of adsorbents for removal of pollutants from wastewater. However, pure CS materials have several drawbacks, such as low mechanical strength and difficult recovery (Wang and Wang 2008). Moreover, CS is dissolved in acidic solution, which a lot of metal ions are preferentially adsorbed. To improve the mechanical and chemical features of hydrogels synthesized from CS as well as to enhance its stability under acidic conditions, several cross-linking agents (ECH, glutaraldehyde, and others) are combined with CS in the hydrogel-preparation process. Using glyoxal (an organic compound with the chemical formula OCHCHO) as a crosslinked agent, Mirabedini and co-workers formulated a novel magnetic hydrogel via combination of CS and magnetic nanoparticles (Fe₃O₄) (Mirabedini et al. 2017). The outstanding properties of this hydrogel are its easy reusability and high Cr⁶⁺ ion removal rate (80–90%) in water solution with a maximum adsorption capacity of 27.25 mg g⁻¹. Moreover, it can be recycled for at least three adsorption cycles. But most especially, it can be easily desorbed by 0.1-M HCl solution without dissolving CS in the hydrogel. This means that the structure of CS can be significantly enhanced by strong cross-linking with glyoxal.

In recent decades, many studies have begun considering the effective and economic aspect of heavy-metal ion removal. The biosorption method, a common approach, uses the synthesis of raw materials, such as cellulose, starch, or CS for metal ion adsorption; they are abundant, low-cost, biodegradable, and environmentally friendly (Zhou et al. 2012).

Cellulose, abundant natural polymer with excellent biocompatibility properties, is one of the most commonly employed adsorption materials in wastewater treatment. However, it still has not reached its potential applications in many areas because it is difficult to process in common solutions (Zhou et al. 2004). Several efficient solvents have been studied and applied for the development of the production of cellulose-based adsorbents such as lithium chloride/N,N-dimethylacetamide (Zhou et al. 2004), N-methylmorpholine-N-oxide (Cai et al. 2004), and ionic liquid (Zhu et al. 2006). By using phosphoric acid as an efficient solvent for cellulose, several recent studies successfully developed a potential hydrogel, cellulose-graft-acrylic acid (C-g-AA) hydrogel, through the free-radical polymerization method (Zhou et al. 2011; Zhou et al. 2012). This type of hydrogel showed outstanding adsorption behavior for both dyes MB and metal ions (Cd²⁺, Pb²⁺, and Ni²⁺) in aqueous solutions. In case of adsorbing metal ions, the adsorption capacity of C-g-AA hydrogel depends on the pH of adsorbent solution, the most effective value being reached at pH 5.0.

By combining cellulose and GO, a simple and novel GO/cellulose hydrogel as a potential material was introduced for adsorption of metal ions (Chen et al. 2016). In this method, GO nanosheets consisting of carboxyl and hydroxyl groups are linked into the surface of the cellulose hydrogel, while the gel structure and its nanoporous property are maintained. Due to the linking with GO, the GO/cellulose hydrogel films possess good compressive strength. Increasing the GO/cellulose ratio, furthermore, significantly improved the adsorption capacity for Cu²⁺ ions in aqueous solution. The GO/cellulose hydrogel showed a high adsorption capacity, compared with the pure cellulose hydrogels. The GO/cellulose hydrogel also showed other distinguishing features, such as high-efficiency regeneration and high adsorption capacities for other metal ions (Zn²⁺, Fe³⁺, and Pb²⁺).

Starch (St) is a raw material that is abundant, easy to produce, recoverable, and naturally biodegradable. Many published studies have used chemically modified starch via vinyl graft copolymerization to enhance the properties of starch, expand the range of its application (Khalil et al. 1993; Mostafa and El-Sanabary 2003), and provide for the biodegradability and biocompatibility of corresponding hydrogels. Combining starch with sodium humates in a polymer hydrogel made possible a series of starch-g-poly(acrylic acid)/sodium humate (St-g-PAA/SH) hydrogels that were tested for their Cu²⁺ ion adsorption capacity in aqueous solution (Zheng et al. 2010). This hydrogel displayed an excellent ability in adsorbing Cu²⁺ within a wide pH range in an acidic environment (2.7–5.0). Moreover, the most remarkable point respecting the hydrogel was that it could be regenerated by using NaOH solution as the regenerating agent, and it was found that the regeneration ability depends on the ratio of sodium humates to hydrogel.

Particles entrapped in hydrogel films for dye removal

For removal of dyes from aqueous solutions, various techniques have been investigated and developed, among which adsorption is a widely used process within the field of advanced wastewater treatment. Particularly, nanoscale materials, such as hydrogel films, are typical adsorbents offering great utility in dye removal (Wang et al. 2015; Zhang et al. 2011; Zhang et al. 2012b). For hydrogel films, the properties of high flexibility and porosity along with the ability to interact with a number of different nanoscale adsorbents are particularly important in the purification field (Zhang et al. 2016).

Compared with synthetic polymers, microorganisms possess many more advantages, among which are maintainable, friendly and unvarying scaffolds with biomacromolecules (Moradi et al. 2016) (Zhou et al. 2016; Zhu et al. 2016). Filamentous fungus, a member of microorganism family, has a large aspect ratio and remarkable chemical characteristics, for example, high flexibility and ductility. These properties can make their efficient formation more stable in macroscopic monoliths (e.g., a porous membrane) (Chai et al. 2015; Wang et al. 2015; Zhu et al. 2016). Therefore, fungus hyphae have been trialed as great reinforcement agents with friendly, low-cost properties for improved mechanical stability of formed macroscopic structures by interaction with graphene. Based on interaction between the GO and fungus hyphae at a temperature of 130 °C, a porous and flexible hydrogel film was synthesized successfully and the primary mechanism for interaction between the fungus hyphae and GO is based on the reduction of GO by the biomacromolecules on the cell of the microorganism (Zhang et al. 2016). Notably, during this interaction process, nanoscale building blocks can be effectively loaded into the film by addition of a nanoparticle. The film-loading nanoparticles could be assembled in an adsorption setup to eliminate dyes from aqueous solution, which is useful for nanoparticles in water-purification applications. In addition, the hydrogel film is an excellent reusable and recycled material that could still maintain a high removal proportion after ten adsorption-desorption cycles.

Hydrogels based on PAM, polyacrylonitrile (PAN), methacrylic acid, CS, polyvinylpirrolidone (PVP), and their derivatives have been shown to form complexes with dyes (Jeon et al. 2008; Reis et al. 2012). These types of hydrogels usually have been used as adsorbents in water treatment owing to advantages including high swelling and wettability, easy loading, creation of chelated complex, and a capacity for semi-continuous operation (Oladipo and Gazi 2015; Wang et al. 2012). With high swelling and wetting, adsorption of the target molecules or ions of the hydrogel can be enhanced due to the production of more precise surface area and contact with additional functional groups for adsorption (Wang et al. 2012). In order to develop these approaches, a number of studies have combined natural CS and synthetic PAN. When combined with CS, PAN has a role as an active component of an ion exchanger. Because of its possessing a nitrile group, PAN is able to form hydrogel bonds and complexes with other materials of positive charge. Besides, due to its hydrophilicity, porosity, and mechanical strength, PAN is a potential material for hydrogel preparation in the water treatment field (Sebesta et al. 1995). For improved stability and swelling ratio, Fahad S. Al-Mubaddel’s group fabricated a novel hydrogel in two steps for adsorption of rhodamine B in aqueous solution (Al-Mubaddel et al. 2015). Firstly, CS was interacted with PAN to form CS/PAN blend films. Then, the CS in these films was cross-linked with ECH to make the hydrogel films. PAN was found to significantly enhance the stability of the fabricated hydrogel film. For instance, with more than 50% PAN content, the stability reached a good degree, while it sharply decreased with less than 50% PAN content. As for the rhodamine B adsorption behavior, the hydrogel film’s adsorption ratio rapidly increased during the initial time (~ 200 min), but in the equilibrium state, after ~ 240 min, the adsorption and desorption of the dyes attained a balance.

Particles entrapped hydrogel nanocomposites for adsorption of contaminants

Proteins are large biomolecules or macromolecules, and they are broadly distributed in nature. Proteins can be synthesized in nature from two main sources, animals and plants. For instance, proteins derived from animals are collagen, keratin, and gelatin and proteins derived from plants are soya. Basically, proteins are chain polymers of high molecular weight that are not soluble or only very slightly soluble in water. There are two ways to solubilize proteins in water: alkaline and enzymatic hydrolysis (Pourjavadi et al. 2006). Proteins have the ability to bind thoroughly with heavy-metal because they possess a number of electric arrays of chemical functionality such as ▬COOH, ▬NH, ▬SH, ▬OH, and imidazole groups. Therefore, it is expected that proteins will enhance the adsorption capacity of hydrogel (Hwang and Damodaran 1997). However, based on our search data, up to now there has been little research relating to the preparation of protein-based hydrogels for removal of heavy-metal ions. In 1997, soy protein, which is a sustainable, low cost, and abundant plant protein, was applied in preparing a soy-protein-based hydrogel by crosslinking an ethylenediaminetetraacetic acid dianhydride (EDTAD)-modified soy protein isolate for adsorption of divalent calcium, zinc, mercury, and lead ions from aqueous solution (Hwang and Damodaran 1997). The equilibrium adsorption capacities of the hydrogel for these metal ions were 0.70, 0.65, 0.95, and 0.70 mmol per gram of dry hydrogel, respectively. In 2017, soy protein was also introduced in new nanocomposite hydrogels synthesized by the crosslinking method using soy protein isolate as a matrix and polyethyleneimine (PEI) as a functional component to replace EDTAD (Liu et al. 2017). These nanocomposite hydrogels were effectively demonstrated for adsorption of Cu²⁺ ions from aqueous solution, the results indicating a Cu²⁺ adsorption capacity that reached 136.2-mg Cu²⁺/g hydrogel. In addition, the nanocomposite hydrogel showed an excellent selectivity for the removal of Cu²⁺ ions co-existing with competitive heavy-metal ions, including Zn²⁺, Cd²⁺, and Pb²⁺. The two above-noted studies suggest that such a soy protein isolate-based hydrogel is a good candidate for selective adsorption and recycling of metal ions in the wastewater treatment industry. Other protein, cottonseed protein, was also used in synthesis process of a cottonseed protein-poly(acrylic acid) hydrogel nanocomposite for removal of copper and lead ions in aqueous solution. The hydrogels showed high adsorption capacities that were influenced by various changes such as the content of cottonseed protein, the initial concentration of metal ions, and pH. Moreover, the hydrogel could be easily regenerated in HCl solution and reused while maintaining over 90 and 57% of its original adsorption capacity for Cu²⁺ and Pb²⁺ ions, respectively, after four cycles of reuse (Zhang et al. 2012a). Gelatin, another protein derived from collagen, is a natural protein present in the tendons, ligaments, and tissues of mammals. Gelatin has been applied to the preparation of many hydrogels for drug delivery through crosslinking reactions with monomers (Guo et al. 2015). In 2003, a gelatin hydrogel was introduced for removal of Cu²⁺ and Cr⁶⁺ ions from aqueous solution (Chauhan et al. 2003). The hydrogels were synthesized by crosslinking reaction of gelatin with three acrylamides monomers (Acrylamide (AAm), 2-acrylamido-2-methyl-1-propanesulfonic acid (AAmPSA), and N-iso-propylacrylamide (N-i-PAAm)) in the presence of a crosslinker (N,N-methylene bisacrylamide) at different temperatures. The structural aspects of the formed hydrogels depend on the nature of the amine group and the water uptake capacity. The hydrogels also effectively showed adsorption of metal ions (Fe²⁺, Cu²⁺, and Cr⁶⁺) based on the mechanism of ion exchange.

GO is graphite oxide monolayer having a two-dimensional structure. GO is able to be dispersed in polar solvents such as water or organic solvents because of its possession of many functional hydroxyl, epoxy, and carboxyl groups (Haubner et al. 2010). Hyperbranched polymers are highly branched three-dimensional polymers. They have a globular and dendritic structure, which endows them with many interesting properties, such as abundant groups and numerous end groups, low viscosity, and high solubility. Owing to the end groups in their structure, hyperbranched polymers are used to synthesize, by the interaction of end groups and the hydrogel matrix, polymers or hydrogel materials with customizable structures (Carlmark et al. 2013). Through a novel cost-effective and environmentally friendly method, a poly(acrylic acid) (PAA) hydrogel nanocomposite was successfully synthesized by using hyperbranched polymers and GO (Yu et al. 2015b). The PAA hydrogel showed a high water-adsorption ability and a high, 209 mg g⁻¹ (at pH = 5) adsorption capacity for Cu²⁺ ions in water solution. In addition, the hydrogel nanocomposite was demonstrated to be suitable for use as a low-cost, environmentally friendly adsorbent for copper adsorption and recovery, because the Cu²⁺ ions adsorbed in the hydrogel can be eluted and the hydrogel nanocomposite recovered by using nitric acid solution. The adsorption capacity of the hydrogel, moreover, was maintained at over 90%.

Itaconic acid (IA) is a water-soluble monomer having two carboxylic acid groups. Due to their double functionality, IA-based polymers are regarded as functional polymers and potential materials for wastewater treatment. Besides, in recent years, researches using 2-hydroxyethyl acrylate (HEA) in the preparation of hydrogels have been reported and, indeed, HEA-based hydrogels have been shown to be promising adsorbents for removal of heavy-metal ions from aqueous solutions (Antić et al. 2015; Li et al. 2013; Wu and Li 2013). By combination of IA with 2-hydroxyethyl acrylate, pH-sensitive hydrogels were introduced for use as adsorbents to remove Pb²⁺ from aqueous solution (Antić et al. 2016). Because both HEA and IA are materials having a good adsorption capacity, HEA/IA hydrogels relative to corresponding homopolymers also have shown high adsorption capacities for Pb²⁺ ions. A comparison of the adsorption capacities between the reported adsorbents indicated that it is the combination between IA and HEA that significantly enhanced the adsorption capacity of the HEA/IA hydrogels relative to most of the previously investigated adsorbents. Moreover, the hydrogel nanocomposite could be easily regenerated, and desorption of the adsorbed Pb²⁺ ions could be conducted in HNO₃, HCl, and CH₃COOH acid solutions. After three cycles, significantly, nearly 90% of the initial adsorption capacity had been maintained. IA also has been used in some other studies for synthesis of hydrogels, specifically in combination with alginate, a high-molecular-weight polysaccharide extracted from various species of brown algae. In 2012, sodium alginate/itaconic (NaAlg/IA) hydrogels were also successfully synthesized by a free-radical polymerization in which an aqueous solution of 20% IA was mixed with 10-wt% NaAlg at room temperature (Mahmoud and Mohamed 2012). This hydrogel was effectively demonstrated for removal of Pb²⁺ ions to a maximum capacity of 1.23 mmol/L. The same NaAlg/IA-type hydrogel was synthesized and introduced for removal of dyes, MB (Mahmoud et al. 2014). In MB adsorption process, parameters affecting the MB-adsorption capacity in wastewater including a concentration of NaAlg and temperature. The presence of NaAlg in the hydrogel changed the adsorption efficiency; in particular, when increasing the NaAlg percentage in the hydrogel, the adsorption capacity also increased. Especially, for reuse of the hydrogel after MB adsorption, acid fast red dye (AFR) was used to examine the adsorption capacity for the NaAlg/IA hydrogel-loaded MB in a second adsorption from aqueous solution. The results showed that after adsorbing MB, the surfaces of the NaAlg/IA hydrogel were covered with a layer of canionic dye and that in the second adsorption, anionic AFR was adsorbed on the hydrogel-loaded MB through an electrostatic interaction. The AFR removal ratio of the hydrogel-loaded MB could reach 62 to 74%, depending on the amount of MB loaded onto the surface of the hydrogel.

Clay shows a strong affinity for both cationic and anionic dyes, whereas the adsorption capacity for basic dye is much higher than that for acidic dye due to the negative charge on the structure of the clay (Crini 2006). Clay minerals commonly have layered structures with a large surface area and high ionic exchange capacity. Besides, its current market price is cheaper than that of active carbon and some other materials (Babel and Kurniawan 2003). Numerous clays including kaolinite, laponite, montmorillonite, attapulgite, diatomite, vermiculite, alumina, mica, potassium humate, sodium humate, glass, smectite, vermiculite, sercite, rectorite, and sodium silicate have been added to the polymeric matrix to synthesize nanocomposite hydrogel materials (Kabiri et al. 2011; Magalhães et al. 2013). According to Zhao group’s review (Zhao et al. 2015), clay mineral-loading nanocomposite hydrogels have possessed exceptional composition and properties. Consequently, it has attracted numerous interests in the synthesis of adsorbents over the last decades. Clay-mineral can be widely used to add into the hydrogel matrix to form novel superabsorbent nanocomposite hydrogels with high absorption capacities up to several hundred times for their dried weight. It was investigated that a successful combination between montmorillonite clay and cellulose in the design of superabsorbent cellulose/clay nanocomposite hydrogels is illustrated (Peng et al. 2016). In preparation method, these hydrogels were formed by crosslinking method, whose cellulose and CMC will make chemical crosslinking with ECH in the presence of clay nanosheets. The authors revealed that the hydrogels exhibit very high equilibrium swelling ratio in distilled water and the maximum ratio can reach 1443 g/g, which indicates their superabsorbent properties. Due to the superabsorbent properties, the hydrogels incorporated with intercalated clays exhibited superior MB removal capacity. Furthermore, the characteristic properties of hydrogels including the mechanical and thermal properties are able to be largely affected by the content of added clays (Haraguchi et al. 2003). These properties can be significantly enhanced when clay minerals are incorporated into the polymer matrix (Bitinis et al. 2011) because the increasing the clay amount will lead to an increase in the crosslink density. In some studies of Haraguchi’s group, for instance, a modified clay was used as the cross-linker to synthesize poly (N-isopropylacrylamide)/clay (Haraguchi et al. 2002) and poly(N,N-dimethylacrylamide)/clay nanocomposite hydrogels (Haraguchi et al. 2003) with higher mechanical properties. They revealed that these hydrogels showed very high elongation at break, close to or greater than 1500% and their tensile properties strongly depended on the content of clay. The increase of modulus and strength values is proportional to the content of clay. In addition, clays can improve the higher toughness of hydrogels based on the formation of an intercalated architecture with more cross-linking points (Peng et al. 2016). For the enhancement of thermal properties, it was demonstrated that the use of bentonite clay can increase the thermal stability of a β-cyclodextrin/bentonite clay nanocomposite hydrogel (Heydari and Sheibani 2015). It was concluded that the clay plays a role in the prevention of the polymer thermal degradation. It is the presence of the clay nanolayer inside the polymer matrix that led to the improvement and the clays act as barriers to maximizing the heat insulation and minimizing the permeability of volatile degradation products to the hydrogels.

A novel hydrogel nanocomposite was achieved based on combining CS, AA, and organically modified nanosilica by 3-aminopropyl triethoxysilane (APTS) (Fig. 10) (Pourjavadi et al. 2015). The applicability of this hydrogel nanocomposite in water treatment was demonstrated for adsorption of metal ions (Co²⁺, Cu²⁺, Pb²⁺, and Zn²⁺) from aqueous solution. According to the result, the study concluded that the nanosilica played an important role in the properties and adsorption capacity of this hydrogel. The nanosilica might play the role of a physical cross-linker. It is certain, in any case, that the presence of modified nanosilica significantly contributed to the enhanced porosity of the hydrogel network. In addition, the nanosilica functionalized by APTS increased the porosity of the hydrogel and created extensive network domains capable of permeating water. The hydrogel showed a high adsorption capacity that was owed to the presence of amine groups on the nanosilica surface as well as the highly porous structure. Also, the effects of different parameters (Pb²⁺ concentration, adsorbent content, pH value, and contact time) on the adsorption capacity of the hydrogel were low. On the other hand, the electrostatic interaction between the hydrogel’s carboxylate groups and the Co²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions lead to unselectivity for removal of these metal ions from aqueous solution (Pourjavadi et al. 2015). According to Table 2’s comparison, the CS-based hydrogel crosslinked with AA exhibited a much higher Cu²⁺ uptake than most of the other adsorbents under the same condition.

Fig. 10

Synthesis route of hydrogel nanocomposite based on chitosan, acrylic acid and modified nanosilica particles (reprinted (adapted) with permission from Pourjavadi et al. 2015. Copyright (2017) Springer)

Table 2 Comparison of metal ion adsorption capacities of different hydrogels (reprinted (adapted) with permission from Pourjavadi et al. 2015. Copyright (2017) Springer)

Adsorption kinetics and mechanisms of adsorbates on hydrogel

Adsorption kinetics

To qualitatively evaluate adsorbents, determination of their kinetics is one of the most commonly used methods currently utilized and indeed, adsorption kinetics, are important characteristics in any evaluation of adsorption efficiency (Zhao and Mitomo 2008). The purpose of adsorption kinetics is to establish a period of time for adsorption of adsorbates on the hydrogels and to describe the adsorption process by means of a theoretical model. Generally, to express the adsorption kinetics of hydrogel, most studies have used three models, including the (1) pseudo-first order, (2) pseudo-second order, and (3) intra-particle diffusion models (Yagub et al. 2014). Besides, some other models have been applied to describe the mechanism of the kinetics for the adsorption, such as Elovich model (Bhattacharyya and Ray 2013; Mittal et al. 2015) and Bangham model (Bhattacharyya and Ray 2013).

• (1) Pseudo-first-order model: This model describes the proportion of the adsorbate adsorption with time to the concentration of adsorbents (Table 3).

The pseudo-first-order model generally is represented by Formula (1) (Lagergren 1898)

$$ \frac{d{q}_{\mathrm{t}}}{dt}={\mathrm{k}}_1\left({q}_{\mathrm{e}}-{q}_{\mathrm{t}}\right) $$

(1)

Integrating this for the boundary conditions t = 0 to t = t and q_t = 0 to q_t = q_t, the Formula (1) may be rewritten for linearized data plotting as the Formula (2):

$$ \log \left({q}_{\mathrm{e}}-{q}_{\mathrm{t}}\right)=\log \left({q}_{\mathrm{e}}\right)-\frac{k_1}{2.303}t $$

(2)

where q_t and q_e are the amounts of pollutants adsorbed at time t and at equilibrium (mg g⁻¹) and t is the contact time (min) and k₁ is the rate constant of pseudo-first-order adsorption (min⁻¹). Values of k₁ were calculated from the plots of ln(q_e − q_t) versus t for different concentrations of pollutants and q_e being determined from the slope and intercept of the plot.

• (2) Pseudo-second-order model (Ho and McKay 1999).

According to Ho and McKay’s theory, the adsorption capacity of hydrogels depends on the number of active sites attached on the hydrogels. The pseudo-second-order model is expressed as Formula (3):

$$ \frac{dq_{\mathrm{t}}}{dt}={k}_2{\left({q}_{\mathrm{e}}-{q}_{\mathrm{t}}\right)}^2 $$

(3)

Integrating this for the boundary conditions t = 0 to t = t and q_t = 0 to q_t = q_t gives:

$$ \frac{1}{q_{\mathrm{e}}-{q}_{\mathrm{t}}}=\frac{1}{q_{\mathrm{e}}}+{k}_2t $$

(4)

where k₂ (g/(mg min)) is the second-order rate constant of adsorption. The constants can be determined by the plot of t/q_t versus t.

• (2) Intra-particle diffusion model:

According to Weber and Morris, the adsorption capacity of an adsorbent (q_t) will be proportional to t^0.5 in adsorption process and can be described as Formula (5) (Weber and Morris 1963):

$$ {q}_{\mathrm{t}}={K}_{\mathrm{id}}{t}^{0.5}+I $$

(5)

where q_t is the adsorption capacity at time t, t^0.5 is the half-life time in seconds, K_id (mg g⁻¹ min^0.5) is the rate constant of intra-particle diffusion, and I is the intercept (mg/g). To determine the rate constants, q_t is plotted versus t^0.5 to yield a linear relationship, and K_id is calculated from the slope of the plot.

(4) Bangham kinetic model:

The linear form of this model is given by the following Formula (6) (Bhattacharyya and Ray 2013):

$$ \ln {q}_{\mathrm{t}}=\ln {k}_{\mathrm{t}}+\frac{1}{m}\ln t $$

(6)

where $\frac{1}{m}$ measures the intensity of sorption and k_t is the rate constant for sorption. The values of rate constant k_t and m can be obtained by plot versus of lnq_t against lnt.

• (5) Elovich kinetic model:

The assumption of various activation energies and heterogeneous active sites of hydrogels for adsorption of organics can be represented by Elovich model as the following formula (Chien and Clayton 1980):

$$ {q}_{\mathrm{t}}=\frac{1}{\beta}\ln \left(\alpha \times \beta \right)+\frac{1}{\beta}\ln t $$

(7)

where α and β are the initial rate of adsorption (mg g⁻¹ min⁻¹) and desorption rate constant, respectively. The values of α and β are determined through plot versus of q_t against lnt.

As suitable for the adsorption kinetics of an adsorbent, most of the literatures have based their models on the liner regression correlation coefficient (R²). A higher value of R² will indicate a better fitness of model. According to the summarized data provided in Table 4, the pseudo-first-order and pseudo-second-order model are the suitable models for describing the kinetic adsorption data. It is notable that, with intra-particle diffusion, Elovich, and Bangham kinetic models, only linear regression may carry out since non-linear regression will give the same statistical parameters, while with pseudo-first-order and pseudo-second-order models, both linear and nonlinear regression can carry out with experimental adsorption data (Bhattacharyya and Ray 2013).

Table 3 Comparison of the maximum adsorption capacity q_max (mg/g) of hydrogels and other adsorbents

Table 4 Summary of hydrogel adsorption kinetics and mechanisms of pollutants for various pollutants

Adsorption isotherm

The feasibility of the adsorption process and interaction between the adsorbents and adsorbates at a particular temperature might be evaluated and described using an adsorption isotherm. In addition, it is able to predict the distribution of adsorbate molecules between the liquid and solid phase at equilibrium (Mittal et al. 2015). There are several isotherm models that can be used to describe adsorption isotherm such as (i) two parameter isotherm including Langmuir, Freundlich, Dubinin–Radushkevich, Temkin, Flory–Huggins, and Hill isotherm models and (ii) three parameter isotherm like Redlich–Peterson, Sips, Toth, Koble, Khan, and Radke–Prausnitz isotherm models (Foo and Hameed 2010; Jana et al. 2017; Mittal et al. 2015).

Two parameter isotherm models

Langmuir and Freundlich models are the most two parameter isotherms widely used because they have usually shown the best fitness to the experimental data than the other isotherms.

• (1) Langmuir adsorption isotherm model

The Langmuir isotherm model is built based on the assumptions of monolayer adsorption. According to the presumption of this model, the adsorbent surface is homogeneous and has only one type of binding site. The linear form of the Langmuir isotherm can be represented as formula (8) (Langmuir 1916):

$$ \frac{C_{\mathrm{e}}}{q_{\mathrm{e}}}=\frac{1}{q_{\mathrm{m}}b}+\frac{C_{\mathrm{e}}}{q_{\mathrm{m}}} $$

(8)

where C_e (mg/L) and q_e (mg/g) are the adsorbate concentration and the amount of adsorbates adsorbed at the equilibrium, respectively. b is the Langmuir constant (L/mg) and q_m the maximum adsorption capacity of the adsorbent (mg/g) and the values of q_m and b can be obtained through the plot of C_e/q_e vs C_e.

The separation factor, R_L (also known as the equilibrium parameter) is a constant that can be calculated as Formula (9):

$$ {R}_{\mathrm{L}}=\frac{1}{1+{bC}_0} $$

(9)

where C₀ is the initial concentration of adsorbates in solution (mg/L).

The value of R_L determines the shape and feasibility of isotherms and indicates if the Langmuir process is unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1) or irreversible (R_L = 0) (Bhattacharyya and Ray 2013).

• (2) Freundlich adsorption isotherm

Contrary to Langmuir model, the Freundlich isotherm model can be applied to assume adsorption isotherm in case of heterogeneous surface as well as multilayer adsorption (Jana et al. 2017). The linear form of this isotherm is given by the following equation (Freundlich 1906).

$$ \log {q}_{\mathrm{e}}=\log {K}_{\mathrm{F}}+\frac{1}{n}\log {C}_{\mathrm{e}} $$

(10)

where K_F is the Freundlich isotherm constants and n is the adsorption intensity.

Three parameter isotherm models

• (1) Sips isotherm model

Sips isotherm is a combination of the Langmuir and Freundlich models deduced for predicting the heterogeneous adsorption systems, and it is expressed as (Sips 1948):

$$ {q}_{\mathrm{e}}=\frac{q_{\mathrm{max}}{K}_{\mathrm{e}\mathrm{q}}{C}_{\mathrm{e}}^{\mathrm{n}}}{1+{K}_{\mathrm{e}\mathrm{q}}{C}_{\mathrm{e}}^{\mathrm{n}}} $$

(11)

where K_eq (L/mg) is the equilibrium constant of the Sips equation, and q_max (mg/g) is the maximum adsorption capacity. The heterogeneity factor (n) of the Sips isotherm model represents the nature of the adsorption process. It is interesting that Sips will reduce to Freundlich isotherm at low adsorbate concentrations, while it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm at high concentrations (Foo and Hameed 2010).

• (2) Redlich–Peterson isotherm model

The Redlich–Peterson isotherm (Redlich and Peterson 1959) is a hybrid isotherm featuring both Langmuir and Freundlich isotherms. This model assumes that monolayer formation and multi-site adsorption can occur simultaneously.

The non-linear and linear forms of the Redlich–Peterson isotherm are described as Formulas (12) and (13), respectively (Paulino et al. 2011).

$$ {q}_{\mathrm{e}}=\frac{AC_{\mathrm{e}}}{1+{BC}_{\mathrm{e}}^{\mathrm{g}}} $$

(12)

$$ \ln \left(A\frac{C_{\mathrm{e}}}{q_{\mathrm{e}}}-1\right)=g\ \ln {C}_{\mathrm{e}}+\ln B $$

(13)

where A, g, and B are the Redlich–Peterson constants, C_e (mg/L) is the adsorbate concentration remaining in the solution, and q_e (mg/g) is the adsorption capacity of the hydrogels at equilibrium.

This model can be applied both homogeneous or heterogeneous systems due to its versatility. In the limitation, it only approaches Freundlich isotherm model at high concentration and Langmuir isotherm model at low concentration (Foo and Hameed 2010).

The applicability of isotherm models to the removal of contaminants by hydrogels was evaluated in terms of regression coefficient (R²) and non-linear x². In the good fitness, R² will be close to unity; x² will be very low (Bhattacharyya and Ray 2013).

Adsorption mechanism

One of the biggest challenges to the development of new adsorbents for adsorption of pollutants in wastewater is to clearly define and identify the adsorption mechanism. Especially, it is really important to represent the interaction between adsorbates and adsorbents at the interface. To determine the adsorption mechanism, most studies have used two isotherm models, namely the Langmuir and Freundlich models, which are expressed the above section (Jiang and Liu 2014). Moreover, in order to better understand the mechanism of pollutant sorption on hydrogels, a number of studies have used FTIR, XPS scans, and X-ray diffraction to examine the functional groups and structure of the adsorbent before and after adsorption of pollutants. XPS is a commonly used technique to realize the mechanism of heavy-metal adsorption on hydrogels, as it can indicate the change of the distribution of the electrons around the corresponding atoms caused by the interaction between metal ion and functional groups (Deng et al. 2003).

According to Table 4’s summarized data, electrostatic interactions have been regarded the main mechanisms in the adsorption of pollutants by hydrogels. The functional groups in unique particles or monomers of hydrogels, such as the ▬COOH group in AA (Jiang and Liu 2014; Li 2010; Pourjavadi et al. 2015; Yu et al. 2015b), GO (Guo et al. 2015), IA (Antić et al. 2016), cellulose (Peng et al. 2016) and cellulose ampholytes (Kono 2015), the amine group in CS (Jin and Bai 2002, Mirabedini et al. 2017), the imine group in poly(m-phenylenediamine) (Zhang et al. 2016), the tertiary amine group in PVP (Inal and Erduran 2015), and PEI (Guo et al. 2015), will interact with pollutants because of their charge oppositeness. In the adsorption mechanism by electrostatic interaction, the functional groups on the surface of hydrogels will protonate to form anions attracting cationic pollutants (metal ions, cationic dyes) or deprotonate to form cations interacting with anionic pollutants. For cationic pollutants, such as metal ions and cationic dyes, most studies in the literature have used the ▬COOH group as a functional group for adsorption of pollutants through electrostatic interactions. The ▬COOH group deprotonate as negatively charged ▬COO⁻ groups under the optimum condition of pH and the negatively charged functional groups attract the positively charged ions. In order to increase the adsorption efficiency and prevent the hydrolysis of cationic pollutants, the selection of the optimum pH, where hydrogels will reach the highest adsorption capacity, is very important (Jiang and Liu 2014). Generally, the pH value commonly selected for adsorption of metal ions and cationic dyes will be higher than the pKa of unique particles or monomers containing ▬COOH groups (Chatterjee et al. 2009, Li 2010) and that value commonly is neutral or basic. On the other hand, in the case of anionic pollutants, such as anionic dyes (Congo red (CR), AR9, AR13, and AB92), adsorption usually occurs under the acidic condition. For example, the tertiary amine groups of PVP molecules in the hydrogel protonates and possesses a positive charge in acidic pH, to which charge dye anions of negative charge are attracted (Inal and Erduran 2015). Especially, in acidic solution, a metal cation, Cr⁺⁶, also can be adsorbed on the surface of hydrogel, because it exists as HCrO₄⁻ with a pH < 4. The electrostatic interactions in the adsorption of HCrO₄⁻ by Fe₃O₄ NPs/CS/glyoxal hydrogel film are illustrated in Fig. 11. Under acidic condition, the amino groups (▬NH₂) of CS protonated to form the ▬NH₃⁺ group and the surface of the hydrogel were loaded with a positive charge. The adsorption of Cr⁺⁶ ion was accomplished through the electrostatic interaction between HCrO₄⁻ ions (negative charge) and the ▬NH₃⁺ group (positive charge) (Mirabedini et al. 2017).

Fig. 11

Adsorption of Cr⁶⁺ ion on Fe₃O₄ NPs/CS/glyoxal hydrogel film by electrostatic interactions under optimum condition (pH 4)

Beside electrostatic interaction, ion exchange is also an important mechanism in the adsorption of pollutants from aqueous solution. Several studies have indicated that some metal ions can be effectively removed from solution by the ion exchange mechanism. Charu Dwivedi et al. (2015) and Pathak et al. (2016) used potassium nickel hexacyanoferrate (KNiHCF) and potassium zinc hexacyanoferrate (KZnHCF) loaded onto hydrogel beads to remove cesium ions according to the principle of ion exchange. The adsorption of cesium ions onto KNiHCF or KZnHCF particles takes place by the exchange between a potassium ion of KNiHCF or KZnHCF and a cesium ion (Dwivedi et al. 2015; Pathak et al. 2016). According to the hypothesis of L. Vrtoch and co-workers, this ion exchange mechanism can be achieved by a cation-exchange Reaction (1) (Vrtoch et al. 2011).

$$ {\mathrm{K}}_{2-\mathrm{X}}{\mathrm{Ni}}_{\mathrm{X}/2}^{\mathrm{II}}\left[{\mathrm{Ni}}^{\mathrm{II}}{\mathrm{Fe}}^{\mathrm{II}}{\left(\mathrm{CN}\right)}_6\right]+{\mathrm{Cs}}^{+}\leftrightarrow {\mathrm{K}}_{1-\mathrm{X}}{\mathrm{Cs}\mathrm{Ni}}_{\mathrm{X}/2}^{\mathrm{II}}\left[{\mathrm{Ni}}^{\mathrm{II}}{\mathrm{Fe}}^{\mathrm{II}}{\left(\mathrm{CN}\right)}_6\right]+{\mathrm{K}}^{+} $$

(1)

Baruah’s group specifically explained, as in Fig. 12, the ion exchange mechanism in the adsorption process of Ca²⁺ and Mg²⁺ from solution on CS-MUA-SO₃Na-CDs hydrogel film (Baruah et al. 2016). The scheme indicates that the electrostatic interaction between the positively charged amine groups of CS and the negatively charged sulphonate groups on the surface of the CDs is maintained under acidic condition. In the left-hand scheme, when hydrogel films are dipped in sodium hydroxide solution, the sulphonate groups of the CDs interact with Na⁺ ions to form ion-pairs (Na⁺ SO₃⁻) and thereby, the Na⁺ ions adhere to the CDs embedded in the hydrogel film. In the right-hand scheme, because of the presence of M²⁺ (Ca²⁺/Mg²⁺) metal ions in the aqueous solution, the M²⁺ ions are able to replace Na⁺ ions in the hydrogel film through ion exchange interactions. In this way, Na⁺ ions from the hydrogel film are released into the solution and M²⁺ ions in the solution are adsorbed into the hydrogel film.

Fig. 12

Scheme illustration of adsorption mechanism of Ca²⁺, Mg²⁺ on Ch-SO₃Na-CD hydrogel nanocomposite based on principle of ion exchange (reprinted (adapted) with permission from Baruah et al. 2016. Copyright (2017) Royal Society of Chemistry)

Other studies have demonstrated that the adsorption of pollutants by hydrogels also can take place by several other mechanisms: hydrogen bonding (Al-Mubaddel et al. 2015; Pal and Banat 2015), coordination bonds (Yang et al. 2010), chelating interaction (Gogoi et al. 2015; Yu et al. 2015a), or a combination of interactions, such as ion exchange/electrostatic interaction (Jin and Bai 2002; Liu et al. 2017; Mirabedini et al. 2017; Yu et al. 2015b), chelating/electrostatic interaction (Zhang et al. 2016), ion-exchange/chelating interaction (Zheng et al. 2010; Zhou et al. 2012), electrostatic interaction/chelation (Jiang and Liu 2014; Zhang et al. 2016; Zhao and Mitomo 2008), or electrostatic/hydrophobic interaction (Li 2010; Peng et al. 2016).

Adsorption capacity and affecting factors

Adsorption capacity

The adsorption process of hydrogels can be divided into two stages: first stage (or rapid stage) at initial time of adsorption and second stage (or slow stage) at later time of adsorption (Kasgoz et al. 2003; Tu et al. 2017). During the first stage, adsorption process will be so fast due to the high concentration of ions and also the high number of free adsorptive sites, while at second stage, adsorption process will slow down and then reach an equilibrium state because of a concentration reduction and exhaustion of the free adsorptive sites (Pour and Ghaemy 2015). Adsorption capacity of a hydrogel may be determined as an equilibrium adsorption capacity through mentioned adsorption isotherm models. It will be determined when the contact time between adsorbents and adsorbates is sufficient, and the adsorption rates and the desorption rates are equal (Foo and Hameed 2010). There are many factors affecting adsorption capacity, such as contact time, pH of medium, initial adsorbate concentration, mechanical properties, and competition, and some of the factors are discussed in the next section. Optimization of such conditions will define the maximum adsorption capacity of hydrogels.

Compared with other conventional adsorbents, it is notable that hydrogels possess a remarkable adsorption capacity. It presents a comparison of the maximum adsorption capacity q_max (mg/g) between hydrogel materials and other adsorbents for dyes and metal ion from aqueous solution (Table 3). Among the adsorbents, hydrogels loaded unique particles show a highest q_max value for both dyes and metal ions. Especially, in case of lignosulfonate-g-acrylic acid hydrogels and cellulose-graft-acrylic acid hydrogels (C-g-AA), these hydrogels have a threefold greater q_max value to active carbon, a popular material for pollutant adsorption in water and wastewater. This demonstrates that hydrogels are one of the most potential material for water/wastewater treatment in the present and future.

pH of solution

The pH of the adsorption media is one of the most important external parameters that can affect the adsorption capacity of hydrogel (Kono 2015). Generally, the pH of a solution can cause the protonation or deprotonation of functional groups, such as carboxylic (Yang et al. 2010) or amine (Abdelwahab et al. 2016; Zhao and Mitomo 2008) groups and the precipitation of metal ions in hydrogels. In most of kinetic studies in adsorption of heavy metals, the adsorption capacity of hydrogels for metal ions will increase towards the basic pH of the solution; in fact, there is a range of optimal pH values wherein the highest adsorption capacity can be reached. Some of the literature has explained this phenomenon in terms of the fact that at low pH (acidic solution), the concentration of H⁺ is high and the functional groups in hydrogels are easily protonated to form an electrostatic repulsion of cationic metal ions; therefore, there is a completion between H⁺ and cationic metal ions, which causes a decrease of adsorption capacity. When increasing the pH of solution, the H⁺ concentration declines, and consequently, deprotonation of functional groups occurs and electrostatic repulsion decreases. As a result, the adsorption capacity of metal ions increases (Abdelwahab et al. 2016; Jiang and Liu 2014; Yang et al. 2010; Zhao and Mitomo 2008). However, a further increase in pH leads to the precipitation of metal hydroxide, thereby inhibiting the adsorption process. At pH > 7.0, precipitation will occur, and then a pH of 5.0–6.0 usually is chosen as the optimal value for the adsorption process of metal ions (Chen et al. 2016; Liu et al. 2017; Yu et al. 2015b; Zhao and Mitomo 2008; Zheng et al. 2010; Zhou et al. 2012). For Cr⁶⁺ ions, because they exist predominantly as anions (HCrO₄⁻) in solution, so the maximum capacity of Cr⁶⁺ adsorption is attained at the pH of 4.0 (Mirabedini et al. 2017). In the case of the adsorption of cesium, Pathak et al. and Charu Dwivedi et al. introduced the types of hydrogel that can be effectively adsorb cesium with a wide range of pH (3–10) because transition metal hexacyanoferrates are able to effectively bind cesium under both acidic- and alkaline-pH conditions (Dwivedi et al. 2015; Pathak et al. 2016).

In the dye-adsorption process, the adsorption efficiency also is strongly pH-dependent, because pH can change the chemical structure of dye molecules and functional groups on the surface of hydrogels (Inal and Erduran 2015). In the case of cationic dyes MB or CV, the adsorption capacity will increase with increasing pH, which is the same phenomenon as is operative in the adsorption process of metal ions (Li 2010; Peng et al. 2016). Peng and co-workers explained this phenomenon based on the electrostatic interaction of positively charged MB with negatively charged hydrogels, which interaction is increased at high pH values and decreased at low pH values (Peng et al. 2016). Besides electrostatic interaction, an electro-osmotic expanding network-like ultrastructure in the hydrogels can be produced by electrostatic repulsive forces among adjacent ionized groups in the hydrogel matrix, which forces are generated by the increased levels of ionized groups at high pH values, which further results in greater uptake of MB (Yu et al. 2016). By contrast, for adsorption of anionic dyes (congo red, AR9) into hydrogels, the process is inversely proportional to the change of the solution pH: when the pH of the adsorption solution is increased, the adsorption capacity of hydrogels usually decreases. This mechanism is indicated by the change in the electrostatic interaction between the dye anions and the hydrogel cations under different conditions of pH (Chatterjee et al. 2009; Inal and Erduran 2015; Kono 2015).

Initial concentration of pollutant and adsorbent dosage

The initial concentration of pollutants and the adsorbent dosage are parameters that play important roles in the adsorption efficiency of hydrogels. The effects of the initial adsorbate concentration and the adsorbent weight mainly depend on the direct relation between the concentration of adsorbates and the available sites on an adsorbent surface (Peng et al. 2015). Several studies have indicated that the increase of adsorbate concentration leads to enhancement in the adsorption capacity represented by the value of q_e (adsorption amount per unit adsorbent weight), but decreases the adsorbate removal efficiency (Abdelwahab et al. 2016; Pourjavadi et al. 2015; Yang et al. 2010; Zhou et al. 2012). However, the increase of adsorbent weight showed opposite results which gradually increase the removal ratio of pollutants and decrease the adsorption capacity (Antić et al. 2016; Chatterjee et al. 2009; Chen et al. 2016; Dwivedi et al. 2015; Kono 2015; Mirabedini et al. 2017; Peng et al. 2015). These phenomena are explained in the following way: at low adsorbate concentrations, the ratio of the initial number of moles of adsorbate ions to the available surface area of hydrogel is large, whereas at higher concentrations, the available sites of adsorption become fewer, and the percentage removal of pollutants decreases in turn (Abdelwahab et al. 2016). In the case of an increasing amount of hydrogel, there are two explanations of those phenomena. First, the higher adsorbent dosage offers a larger number of active sites and a larger surface area for adsorption (Dwivedi et al. 2015); second, a decrease in the total surface area of the adsorbent and the correspondingly high adsorbent concentration results in particle interactions, such as aggregation (Chen et al. 2016).

Existence of other ions and the selectivity of hydrogel

The existence of multiple ionic species in aqueous solution can influence the adsorption efficiency of hydrogel for each ion species (Antić et al. 2016). Some of the pollutants in the effluent might decrease the adsorption of others through competition or enhance the adsorption together through cosorption (Zheng et al. 2010). Moreover, there are always several co-existing ions in wastewater in real applications (Liu et al. 2017); thus, it is necessary to examine the adsorption selectivity performance of hydrogel under typical multi-component conditions. According to Ma and co-workers, the presence of interference ions (K⁺, Na⁺, Mg²⁺, and Ca²⁺) in solution can cause competition around the same active sites of the adsorbent between these ions and the target ions (Cd²⁺, Cu²⁺, and Pb²⁺), which decreases the removal efficiency of heavy-metal ions. In addition, the inhibitory effect of these co-ions on the removal of Cd²⁺, Cu²⁺, and Pb²⁺ ions follow the order: Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ (Ma et al. 2017). The reason for the adsorption decrease through competition in the vicinity of chelation sites on hydrogels or the sharing of the same binding sites was also explained (Antić et al. 2016; Hwang and Damodaran 1997; Zheng et al. 2010).

Besides, the functional groups of unique particles entrapped in hydrogel, such as carboxyl and amine groups have shown adsorption selectivity to specific ions based on ion properties including radius, electronegativity, and ionization potential. For instance, the carboxyl group in ATP/FA/PAA nanocomposite hydrogel beads has a higher adsorption selectivity to Pb²⁺ ions than to Cu²⁺, Cd²⁺, Ni²⁺, or Zn²⁺ ions, owing to their different properties (Jiang and Liu 2014). If ions have a larger ionic radius, they will possess a higher affinity with hydrogels than with other ions. In other words, the larger the ionic radius, the better the binding capacity (Antić et al. 2016; Hwang and Damodaran 1997; Zhou et al. 2012). Therefore, the selectivity of hydrogels for these ions also is better than for other ions. Besides ionic radius, the selectivity of hydrogels for ions can be affected by ion hydration radius as well. Dwivedi’s group demonstrated that the removal percentage and adsorption capacity of KNiHCF-gel hydrogel beads for cesium ions had no significant change in the presence of interfering cations (Na⁺, Ca²⁺, and Ba²⁺), because the cesium ion has a smaller hydration radius than that of the other interfering ions (Dwivedi et al. 2015).

The selectivity of an adsorbent for a target adsorbate can be altered by changing the types and structures of the polymeric matrix. However, it is found that the functional groups play a more critical role in changing the selectivity because the functional groups are where the adsorbates are adsorbed (Fritz 2005). It was demonstrated that the metal ion adsorption selectivity of a series of PAM hydrogels prepared by Mannich reaction using different amine compounds, such as ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), and sulfomethylation reaction, largely depends on the functional groups (Kasgoz et al. 2003). These hydrogel’s selectivity is displayed in (Fig. 13). The M8 hydrogel, a Mannich product which was prepared with EDA, was not significantly selective to any metal ion, while the other Mannich product prepared with DETA (M10) possesses a distinct selective towards Cu²⁺ ions. This difference is mainly due to the presence of the secondary amine groups in the hydrogel matrix. Compared to M10, however, M11 hydrogels prepared with TETA have the lower selectivity towards Cu²⁺, although TETA has higher amount of secondary amine groups than that of DETA. This is because of the result of lower amine value and a decrease in swelling degree due to crosslinking during the Mannich reaction. Contrary to amine group, the presence of carboxylate group in hydrogel matrix (S2) showed a very distinct selectivity towards Pb²⁺.

Fig. 13

The metal ion selectivity properties of various hydrogels at pH = 4.5. M8—hydrogel which was prepared by Mannich reaction with EDA. M10—hydrogel which was prepared Mannich reaction with DETA. M11—hydrogel which was prepared Mannich reaction with TETA. S2—hydrogel which was prepared by sulfomethylation reaction (reprinted (adapted) with permission from Kasgoz et al. 2003. Copyright (2018) Elsevier)

The swelling degree of hydrogel

Swelling behavior is one of the most important characteristics of the hydrogel as well as adsorbents and the formation of porous structure may lead to the high swelling ratio of hydrogel. According to Flory’s theory, swelling degree of a gel depends on the ionic osmotic pressure, crosslinking density and affinity of the gel for water (Flory 1953). Many studies indicated that the pH value in solution has a significant impact on the swelling degree of the hydrogel in term of increase in swelling ratio along with increasing the pH of medium (Mahmoud et al. 2014; Souda and Sreejith 2014; Zhou et al. 2011). At pH < pK_a of functional groups (carboxylic and amine groups), these functional groups are protonated (▬COOH), which result in a decrease of swelling ratios due to the reduction of the electrostatic repulsion between the molecular chains of the hydrogel (Mahmoud et al. 2014). At pH > pK_a, these functional groups are deprotonated (▬COO⁻) and this increases amount of ionized group, which causes electrostatic repulsion forces among the adjacent ionized groups of polymer matrix. This produces an expansion network-like ultrastructure of the hydrogels, which will improve the swelling ratio (Zhou et al. 2011). In the relation between the swelling ratio and the adsorption capacity of hydrogels, it is found that the higher swelling capacities means the higher number of ionic groups in hydrogels (Flory 1953), which will enhance the adsorption capacity of hydrogels. Therefore, the adsorption capacity of hydrogels will be proportional to the swelling degree. It was found that because the high swelling capacity of the hydrogel networks is likely to give high specific surface area and more functional groups exposed are readily approachable for adsorption, the high swelling ratio can facilitate the adsorption of metal ions (Trakulsujaritchok et al. 2011).

Also, the swelling ability can largely depend on the content of nanoparticle entrapped on the hydrogels. It was found that when amount of added clays can be uniformly dispersed into hydrogel networks, the hydrogel’s swelling ratio will be enhanced due to their surface containing negative charges (Peng et al. 2016). However, the swelling ratio will decrease as the clay contents is so high. This phenomenon can be explained that a stable structure will be formed to inhibit traversing of water molecules based on hydrogen bonds formed between polymers and the crosslinking agent when increasing the content of crosslinking agents (Tu et al. 2017).

Recycling and recovery of used hydrogel

To economically and usefully evaluate the effect of an adsorbent, one of the most important factors is the recyclable and recoverable ability. This means that effective hydrogels must be easily reused for more than a few cycles without incurring any significant change in its adsorption capacity. The regeneration and reuse ability is also an essential factor for broad, practical assessment of the applicability of a material. A novel hydrogel based on polydopamine (PDA)-functionalized graphene (PDA-GH) was successfully synthesized, and this hydrogel effectively demonstrated its adsorption capacity for different contaminants, such as heavy-metal ions (Pb²⁺ and Cd²⁺) and organic pollutants (rhodamine B and ρ-nitrophenol) (Gao et al. 2013). Notably, the hydrogel could be easily regenerated by a simple process using low-cost reagents, and it maintained a high adsorption capacity even after multiple adsorption-desorption cycles. According to the authors, to recover the ions (Pb²⁺, Cd²⁺) from the adsorbents of PDA-GH, they used HCl solution (0.1 M) as the desorption agent. The reduced stability of the PDA covering in a strong acid possibly can cause the decline of the adsorption capacity of PDA-GH. Nevertheless, the regenerated PDA-GH still maintained a high adsorption capacity (more than 80%) after ten adsorption and desorption cycles. Hence, PDA-GH has been highly regarded as a potential adsorbent, owing especially to its easy recyclability and reusability in the process of heavy-metal ion removal. However, to regenerate the hydrogel in the case of the adsorption of organic pollutants, they used ethanol as a desorption agent instead of HCl, as a consequence of which, the high adsorption capacities for removal of organic pollutants (rhodamine B and ρ-nitrophenol) were not significantly decreased.

In another study, a chelator-mimetic multi-functionalized hydrogel possessing a high adsorption capacity for metal ions (Cd, Pb, and As) and excellent reusability was introduced (Mohammadi et al. 2017). By using a low concentration of HCl acid, the hydrogel can be regenerated in its functionality, and also, the ions adsorbed by the hydrogel can be eluted. This particular hydrogel could maintain a better-than 60% removal ratio after five consecutive sorption-wash-reuse cycles. Employing a similar method, Pourjavadi and co-workers performed recycling experiments for a novel hybrid hydrogel based on CS, AA, and amine-functionalized nanosilica (Pourjavadi et al. 2015). HCl (1 M) solution was used as a solvent to recover the Pb²⁺-loaded CS-g-acrylic hydrogel. Then, the adsorbent was regenerated by filtering and washing with distilled water and reused in the next cycle of the adsorption process. Significantly, the desired capacity of the regenerated adsorbent remained at 685–715 (mg g⁻¹) after three cycles. Thus, due to its good recycling efficiency, the hydrogel nanocomposite was judged to be suitable for practical applications.

Vivek and Prasad described the regeneration and reusability process of a self-healing hydrogel prepared by a novel method using in situ polymerization of AA and acrylamide (Vivek and Prasad 2015). The hydrogel performed with remarkable efficiency (90–98%) in adsorbing organic dyes (MB and methyl orange) and metal ions (Cd²⁺, Pb²⁺, and Hg²⁺). The hydrogel was recovered by using methyl orange as the adsorbent. The methyl orange adsorbed from the hydrogel was extracted by ethanol solution, and the hydrogel was separated when reaching the equilibrium condition. According to the result of that study, it took approximately 20 min to conduct the desorption process, and the dye-recovery efficiency was rather high, about 97%. Besides, the authors indicated that with five cycles of regeneration process, the hydrogel still performed at an excellent adsorption capacity, maintaining an adsorption rate of 90–98%.

Magnetic hydrogels represent one of the most effective techniques widely used for removal of metal ions from flowing streams (Yavuz et al. 2009). Recently, a novel magnetic hydrogel possessing advantages in fast Cr(IV) removal and a high adsorption capacity of about 200 mg g⁻¹ was developed (Tang et al. 2010). Another advantage of this hydrogel is that it shows an easy recoverability by regeneration in sodium chloride solution (Tang et al. 2014). The application utility of any magnetic hydrogel in wastewater depends on two factors: a reduced amount of recovery solution and an increased metal-ion concentration in that solution. Tang’s group’s method, schematized in Fig. 14, satisfies these requirements. Briefly, treated wastewater is collected in a magnetic separation unit that separates the hydrogel; sodium chloride at various concentrations is added for the regeneration process; the recovery solution is collected by separating the magnetic hydrogel. Tang et al. conducted a set of regeneration tests with stepwise additions of NaCl solution. They collected the recovery solution, followed by another addition of NaCl solution. Their results suggested that the Cr(VI) removal and recovery performance of the magnetic hydrogel were likely to be maintained for 20 cycles for a 97–98% Cr(VI)-removal efficiency. According to their data, the Cr concentration in the recovery solution reached 500–600 mg/L with the stepwise adsorption and regeneration process, representing a 40:1 wastewater- to-recovery-volume ratio.

Fig. 14

Schematic illustration of wastewater treatment prototype with magnetic separation unit (reprinted (adapted) with permission from Tang et al. 2014. Copyright (2017) American Chemical Society)

Easy separation and recycling without additional costs are important factors impacting on the practical applicability of hydrogel-based photocatalysts. In 2017, a hydrogel-based smart photocatalyst, TiO₂/agarose hydrogel, which has an excellent recyclability was introduced (Mai et al. 2017). Because the prepared hydrogel is much larger than TiO₂ NPs, separation is very convenient. The recyclability of this hydrogel was evaluated using MB and some other dyes. The results indicated that the degradation of MB can be maintained until the fifth cycle with high photodegradation efficiency, and that it gradually decreases beginning with the sixth cycle. In addition, the recyclability of the hydrogel for various dyes was demonstrated using acidic dyes (MB, MO) and an alkaline dye (RhB). There were two different trends that were noted with respect to the photocatalytic efficiency. The photodegradation ability with the MB and MO dyes increased until the second cycle and then decreased slightly in the third cycle, whereas that with RhB decreased progressively after the third cycle. The reason for the difference was that various dye parameters, such as pH and molecular structure, effected different diffusion mechanisms and adsorption behaviors. More importantly, with this type of hydrogel, pure TiO₂ NPs can be recovered by the simple process shown in Fig. 15. In this process, after recycling, the hydrogel is re-melted at high temperature into a diluted TiO₂/agarose aqueous solution (Fig. 15a, b). Then, centrifugation is conducted to separate TiO₂ NPs, the result being that higher-purity TiO₂ NPs can be obtained (Fig. 15c, d). For further-increased purity, the purification process is repeated several times.

Fig. 15

Recovery of pure TiO₂ NPs from a TiO₂/agarose. a Melting, b dilution, c centrifugation, and d separation (reprinted (adapted) with permission from Mai et al. 2017. Copyright (2017) Royal Society of Chemistry)

Reusability is one of the most essential properties for most applications for hydrogels; however, it is a huge challenge for conventional hydrogels because they are commonly poor strength and unrecyclable due to the lost mechanical toughness after swelling in water (Kamata et al. 2014). Therefore, enhancing the strength mechanical plays a key role in maintaining the adsorption capacity of pollutant-adsorbed hydrogels. If hydrogels possess strength mechanical properties, their reusability in practical applications is available to be greatly enhanced (Gong et al. 2016). Hydrogels with desirable mechanical properties is available to maintain their service life due to their self-healing ability when they suffer the swollen state. Cui’s group also supported these when they introduced a graphene oxide (GO)/hydrophobically associated PAM nanocomposite hydrogels (GHA gels) as a reusable adsorbent for the removal of dye pollutants by incorporating GO with hydrophobically associated PAM (Cui et al. 2015). This hydrogel has superior mechanical strength and high toughness due to strong hydrogen bonds between GO sheets and polymer chains. They also indicated that incorporation of GO highly increases the wall thickness of GHA gels and the higher GO concentration lead to thicker pore walls. Therefore, GHA gels can go through the swollen state in water over a long period of time without breakdown. It is the preservation of desired mechanical properties of GHA gels after swelling that significantly enhances their reusability and makes it promising to use them as recyclable adsorbents in pollutant removal. Also, Gong’s group has the same point of view. They prepared double network hydrogels exhibiting high mechanical strength compared with single physical network and physically chemically hybrid network hydrogels (Gong et al. 2016). These hydrogels combine PVA and PAM-co-PAA to form a dual physically cross-linked network. The hydrogels have excellent mechanical properties, which improved the self-healing ability and reusability of hydrogels.

Conclusion

In this review, we have summarized the recent research into the development of hydrogels for water-treatment applications, and we have also emphasized the unique particles that have been encapsulated into those hydrogels. Three classes of hydrogel are commonly employed for removal of pollutants in aqueous solutions: hydrogel bead, hydrogel film, and hydrogel nanocomposite. The particles encapsulated into hydrogel are very diverse, each particle having its unique characteristics and advantages. According to the hydrogel-synthesis method and the contaminant type, particles are chosen for improved adsorption efficiency and enhancement of hydrogel properties including mechanical strength, swelling ratio, pH sensitivity, recyclability, and regeneration. Many hydrogels have already been effectively synthesized and successfully applied for removal of pollutants from water, with more, no doubt, to come in the near future.