Removal of methylene blue from wastewater using hydrogel nanocomposites: A review

Water pollution by organic dyes continues to pose a serious health and environmental threat to the ecosystem. Although adsorption using biopolymer-based hydrogels has proven to be an ideal technique for the treatment of these dye contaminants from aqueous solutions, these hydrogels suffer from lack of mechanical stability and recovery as compared to synthetic polymers. Herein, we review the low-cost synthesis of hydrogel incorporated with inorganic components mainly focusing on strategies to improve the mechanical stability and separation of the hydrogel in removing methylene blue (MB) dye from aqueous solution. The literature shows that hydrogel nanocomposites are a class of materials that have flourished significant consideration, especially concerning water treatment. In adsorption technology, hydrogel nanocomposites act as absorbents, prominent to enhance their removal efficiency towards contaminants. This review highlights the preparation and use of hydrogel nanocomposites as efficient adsorbents. In-depth discussions on adsorption and diverse synthetic routes of hydrogels have been devoted to applications of these nanocomposites and are compared in this contribution to the removal efficiency of MB dye from wastewater.


Introduction
Water is very important for the survival of living things on earth. 1 Despite the need for this resource, water pollution continues to be a problem in most countries including South Africa, where the mainstream water supplies are underground and surface water. 2 Water pollution may be defined as any water that is unsafe for drinking by humans and animals. 2 There are two classes of water contaminants, namely, point sources and non-point sources in which they are defined as a source of pollution at a fixed location (mines, industries, power stations, water treatment station, etc.) and pollution from moving sources (cars, buses and trains), respectively. 3,4 In point source, water pollutants may be classified as either inorganic (fertilizers and toxic metals), organic (dyes) or microbial (viruses and bacteria). 5 For example, dyes are organic complexes mostly used by textile industries to give colour to fabrics and contribute largely to pollution. 6 Other applications may include use in medical, pharmaceutical, paper, rubber, plastics, leather, food and cosmetics industries. 7 Dyes contain aromatic rings in their structure and can be made up of either chromophores or auxochromes. 7,8 Chromophores are responsible for the production of colour (OH, NH 2 , NHR, NR 2 , Cl, COOH and R =alkyl group), and auxochromes (NO 2 , NO and N = N) improve chromophores, make molecules soluble in water and improve their affinity to bind materials. 8 The discharging of dye effluents into either surface and/or groundwater sources leads to contamination which ultimately results in various health and environmental problems. 8,9 Consumption of contaminated water by humans can lead to vomiting, mutation, cancer, breathing problems, diarrhoea, eyes burn, nausea shock, cyanosis, jaundice and tissue necrosis. 7,[8][9][10] The environmental issues include the death of aquatic organisms, leading to the development of foul smell, 11,12 hence the need to eliminate dyes from waste effluents before discharging them into rivers and other water streams.
Conventional methods used to remove dyes from water include adsorption, membrane filtration, ion-exchange, photochemical method, electrochemical destruction and anaerobic bioremediation systems (Table 1). 13 The membrane filtration technique separates pollutants using a physical barrier that allows permeability by water only such that contaminants are retained on the membrane surface. Factors influencing the separation behaviour of the membrane filtration may include sieving, size of contaminants, adsorption and electrostatic interactions. 14 Its advantage is that it is fast with low space requirement and it removes all dye types. 13 However, the disadvantage is its limited lifetime before fouling, high costs of the membrane and sludge production. 15 During ion-exchange for treatment of dye effluents, the concentration of the dye is reduced through the use of anionic or cationic resins whose ions exchange with the dye molecules to form solid particles that are easy to remove. 16 The advantage of ion-exchange is that it has high removal capacity for dyes, but the major disadvantage is its inability to remove all dye types and high costs. 13,16 The photochemical method uses UV light and most use Fenton reagent. 17 It offers the advantage of no sludge production and odour elimination. 13,16 The main advantage of dye degradation using heterogeneous photocatalysts is the ability to use the sunlight in the production of hydroxyl radicals. Its drawback is the formation of by-products. 18 The electrochemical destruction method applies electrical current for the destruction of dye molecules. 19,20 This method does not generate sludge and does not require the addition of any chemical reagent. 13,19 However, it has the disadvantage of high flow rates, which consequently reduce the removal efficiency of dyes, sludge production and causing a secondary pollution problem. 13,21 It is worth mentioning that above-mentioned conventional methods are also applicable for recovery of metals from wastewater. Apart from the aforementioned processes, cementation processes have been well documented in the literature for recovering metals from wastewater. Several researchers have investigated this process in the recovery of copper (Cu), gold (Au), aluminium (Al) and zinc (Zn) from wastewater. 22 Anaerobic bioremediation involves degradation of organic contaminants under anaerobic conditions in the presence of anaerobic microorganisms. 23 The advantage of this technique is that water-soluble dyes can be decolourized; however, this method results in the production of sulphide and methane. 24 Amongst these techniques, the adsorption technique is most favoured owing to its cheap operation costs, easy design and fast removal of dye. 25,26 During this process, dye molecules are attached on the surface of a solid material (adsorbent) through either physical interactions (hydrogen bonding, weak Van der Waals forces) or chemical bonding. 27 The most used commercial adsorbent is activated carbon (AC) (powder or granular). 6,25 The application of AC as an adsorbent is possible due to its high adsorption capacity, high removal efficiency, large surface area and high porosity. 29 It effectively removes various types of organic micropollutants as well as their secondary oxidation products and inorganic compounds. 29 The main disadvantage of this adsorbent, however, is the high cost and its regeneration problems. 6 To reduce the cost of using commercial AC, agricultural by-products including shells  33 The group reported an increased specific surface area from 680 m 2 /g (spent AC) to 710 m 2 /g (regenerated AC). Despite these improvements, AC still suffers the problem of separation from water bodies and that it produces secondary pollution. 33 In an attempt to improve separation of AC from water bodies, many studies have introduced magnetic nanoparticles into the AC material, to enable recovery through exposure to an external magnetic field. 29,34,35 Considering the evidence that biological materials could be used for dyes removal, many researchers were prompted to investigate the use of biological materials such as polysaccharides, agricultural waste and fruit peels as cheap and effective adsorbents for MB removal. 6 Compared to AC and other materials, polysaccharides (cellulose, chitin, chitosan, sodium alginate, etc.) offer greater advantages for use as adsorbents for removal of MB. 7 The advantages include non-toxicity, cheap cost, and eco-friendliness, sustainability due to their abundant availability, high chemical reactivity, biodegradability, polyfunctionality and biocompatibility. [36][37][38] However, due to their lack of stability, most researchers have explored their application as hydrogels. 7,10,39,40 For example, Zhou et al. 40 prepared cellulose based hydrogels and removed 2197 mg/g of MB.
Hydrogels are a class of three-dimensional (3D) soft polymeric materials, which can imbibe a substantial amount of liquids. 41 In this direction, their innate ability to absorb and hold a high volume of liquid qualifies them as ideal materials for the adsorption of aquatic pollutants. 41 Hydrogels attracted a great deal of attention in various fields (gas sensing, 42 drug delivery 43 and cosmetics 44 ) owing to their non-toxicity, low cost, permeability and biocompatibility. 10 However, the applicability of hydrogels has been overshadowed due to their weak mechanical properties and partial adsorption capacity. 10 The yearning for enhancing inherent shortcomings exhibited by hydrogels has instigated tremendous research interest in the development of hydrogel nanocomposite, which is a robust process to improve the disadvantages without losing the advantage of hydrogels. In the past decades, the use of hydrogel nanocomposites for the removal of MB dyes in aqueous environment has increased considerably. In the past 10 years, a developing trend of publications in hydrogel nanocomposites for removal of MB dyes is shown in Figure 1. The literature survey statistics show an exponential increase in the number of publications per year considering hydrogel nanocomposites for the removal of MB dye. This review summarizes the recent advances and developments of hydrogel nanocomposites as adsorbents for wastewater treatment. This is realized by doing a detailed review of the reactions and mechanisms of adsorption including an overview of the adsorption of MB dye. Then, we introduce hydrogel nanocomposite as an adsorbent for MB removal with great emphasis on the structure and synthetic routes.

Background
The attachment or adherence of molecules on the surface of solid material is known as adsorption. The adsorbent material contains active adsorptive sites on its surface to which molecules (gas or liquid) from the bulk solution (adsorbate solution) bind to. 45,46 Adsorption of molecules may occur either physically or chemically. 47 The mass transfer process may occur in the form of migration, diffusion or convection, in which liquid or gaseous molecules are transferred to a solid phase. 48,49 Interactions such as Lewis acid-base, van der Waals and Columbic are characteristic of physisorption 50,51 whereas chemisorption is distinguished through the development of new adsorbate-adsorbent bonds. 52 The adsorptive sites can have the same or different energy, depending on the nature of the material. 53 Owing to its principle, adsorption has gained interest for application in hydrogen storage, sensing, drug delivery, gas capture and water treatment. [54][55][56][57] The recent interest in applying adsorption for the treatment of dyes from wastewater was attracted by its low operation costs, easy design, efficiency and fast removal of dye. 58,59 Before applying an adsorbent material on an industrial scale (column adsorption), it is first optimized in batch mode experiments; then if it possesses a high removal efficiency, it can be employed for column adsorption studies. 60,61 Factors that affect the adsorption process in wastewater treatment include the following 55,56,60 : 1. The adsorbent and adsorbate charges, 2. the solution pH, 3. temperature, 4. dye concentration, 5. adsorbent dose and 6. the surface area of the solid material.
When an adsorption process reaches equilibrium, it means the active sites of the adsorbent are saturated with adsorbate molecules and no further adsorption can take place. 61 Adsorption parameters Adsorption isotherms. The isotherms of adsorption are significant for describing the adsorbate-adsorbent interactions. 62 Additionally, they provide information about the mechanisms of adsorption, adsorption capacity and surface properties. This study employs the Langmuir, Freundlich and Temkin isotherm models. Below are their brief descriptions including their linear and non-linear equations.
Langmuir isotherm model. Irving Langmuir 63 established the Langmuir isotherm in 1916. The model theory assumes that the active sites on the solid surface have the same energy, leading to homogeneous adsorption of adsorbate molecules (monolayer coverage). 64 Graphically, adsorption equilibrium is observed by a plateau, which is a point where all active sites are fully occupied and no further adsorption can occur. 65,66 The model is expressed as follows, nonlinear (equation (1)) and linear (equation (2)) forms where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g), K L (L/mg) is the adsorption equilibrium constants of Langmuir, the Langmuir constants q m (mg/g) represent the monolayer adsorption capacity and b relates to the heat of adsorption. The values of qm and K L can be determined from the linear plot of Ce/qe against Ce. The essential features of the Langmuir isotherm can be employed to predict the affinity between the sorbate and the sorbent using the dimensionless equilibrium constant (R L ). The R L is a dimensionless constant denoted as the separation factor. R L is calculated using the following equation (3) If R L > 1, adsorption is not favoured. If R L = 1, adsorption is linear. If R L < 1, adsorption is favoured, and if R L = 0, the reaction is irreversible. 66 Freundlich isotherm. The model assumes multilayer coverage of the adsorbent. The model is expressed by the following equations in a non-linear (equation (4)) and linear (equation (5)) form where q e (mg/g) and C e (mg/L) are the amounts adsorbed and the solution concentration, respectively, at equilibrium. K F is the sorption enthalpy and varies with temperature. K F and n are the sorption capacity and sorption intensity. 67 n determines the non-linear relation between adsorption and the concentration of the solution. 67 Temkin isotherm. The Temkin isotherm theory ignores the concentration values and assumes that the decrease in heat of adsorption concerning the temperature is linear instead of decreasing logarithmically as suggested in the Freundlich equation. The non-linear (equation (6)) and linear (equation (7)) forms of the Temkin model are expressed as follows below where A T is the equilibrium binding constant corresponding to the maximum binding energy (L/g), b T is the Temkin constant related to the heat of adsorption (kJ/mol), R is the universal gas constant (8.314 J/mol/K) and T is the absolute temperature (K). 68 Kinetic isotherms. Adsorption is a process that involves the transfer of mass of a solute from the liquid phase to the surface of the adsorbent. 69 The mechanism of adsorption can be researched using either Lagergren's pseudo-firstorder equation or pseudo-second-order equation. The amount of contaminant adsorbed with time can be analysed provided an appropriate model is used. 70 To identify which model is appropriate, the correlation coefficient (R 2 ) is measured and must be close or equal to 1. The model with a higher R 2 is the most suitable model for the adsorption kinetics of contaminants. The non-linear (equation (8)) and linear (equation (9)) equations for the pseudo-first-order kinetic model are where q e and q t refer to the amount of dye adsorbed (mg/g) at equilibrium and at any time, t (min). K 1 is the equilibrium rate constant of pseudo-first-order sorption (1/min). K 1 will be determined using the slope and intercept of the plot of ln (q e Àq t ) versus t. 71 The non-linear and linear equations for the pseudosecond-order kinetic model are where K 2 is the equilibrium rate constant of pseudo-secondorder adsorption (g/mmol min). K 2 will be determined using the slope and intercept of the plot of t/qt versus t.
Thermodynamics. During the adsorption process, thermodynamic parameters such as enthalpy (ΔH o ), entropy (ΔS o ) and Gibbs free energy (ΔG o ) are required to determine spontaneity and heat change. 72 Thermodynamics parameters may be calculated according to the formulas below ΔG°¼ ÀRTlnK D (13) where K D represents the distribution coefficient of the adsorbate, q e and C e represent the equilibrium concentration (mg/g) and in the solution (mg/L). R represents the universal gas constant (8.314 J/mol K), and T is the temperature (K).
ΔH o and ΔS o may be calculated from the slope and intercept of the plot lnK D versus 1/T. Negative ΔG values at different temperatures are an indication of a spontaneous adsorption process. 71,73 If ΔG decreases when the temperature is increased, it means there is a more efficient interface during adsorption. A positive ΔH value indicates an endothermic adsorption process. 74

Adsorbent materials for MB
Although the adsorption technique is effective for dye removal, the type of adsorbent used limits its efficiency. 75 Various materials have been used for MB dye removal. For example, AC is reported to be the most frequently used nanoadsorbent for removing a variety of inorganic and organic contaminants owing to its high surface area, porous structure, thermal stability and amphoteric nature. 76,77 However, AC has some drawbacks such as intraparticle resistance in industrial application, high production costs and regeneration costs. [77][78][79] Recent research studies have attempted to find ways to improve the regeneration of AC and produce it at a lower cost. 77 On the other hand, alternative adsorbents such as fly ash, graphene, clay, carbon nanotubes and polysaccharides were reported for removal of MB. [80][81][82][83][84][85][86][87] Among these adsorbents, Mittal et al. 80 reported that using polysaccharides-based adsorbents is a promising strategy for MB dye removal from aqueous solutions. This is due to their enhanced adsorption capacity amongst other attractive properties.
Polysaccharides. Polysaccharides can be defined as highly hydrophilic and non-toxic natural polymers made up of multiple small units of saccharides that are connected through glycosidic bonds. [83][84][85] These biopolymers are found and used in plants or animals for structural support and energy storage. 86 Examples of polysaccharides include carboxymethyl cellulose (CMC), alginate, carrageenan, chitosan, guar gum, starch and locust bean gum. [85][86][87] These natural polymers offer the advantages of high absorption capacity, biodegradability and cheap synthesis. 88 However, due to low solubility and other physical drawbacks, synthetic polymers are incorporated into the biopolymeric backbone to improve their properties. 89 Polysaccharides-based materials have recently attracted use for adsorption of dyes and toxic metals because of their non-toxic nature, biodegradability, easy availability and low-cost synthesis. 90 Amongst other polysaccharides, CMC and alginate offer more attractive properties for application as dye adsorbents.
Carboxymethyl cellulose. Cellulose is the main component of most plants. 6 Carboxymethyl cellulose is a highly reactive, hydrophilic, water-dissolving derivative of cellulose with many carboxyl groups on its surface. 82 Commercially it can be obtained as sodium CMC with different degrees of saturation (DS), which may range from 0.6-0.95 depending on how it was prepared, and the solvent used. 91,92 Carboxymethyl cellulose is mostly used in paper, packaging, textile, food, cosmetics and pharmaceuticals. 85,93 Due to its sensitivity to pH, non-toxic nature, hydrophilicity, low cost and ability to form gels, CMC has been applied on its own or in a composite form as an adsorbent for removing inorganic and organic contaminants. 92,94 Carboxymethyl cellulose ( Figure 2) is a cellulose derivative with carboxymethyl groups at carbon 2, 3 and 6. 95  In a study conducted by Kono,96 the resonances of CMC were determined using carbon-13 nuclear magnetic resonance ( 13 C NMR) as shown in Figure 3. In the spectrum of CMC, the carbonyl carbon (C1, C4), overlap of C2, C3, C5, the carboxymethyl groups, and C6 were assigned to broad 13 C resonances at 178, 104, 83, 75 and 63 ppm. 96 The existence of carboxylic groups (COO À ) on the polymer chains of CMC allows for interactions with multivalent metal cations (Al 3+ and Fe 3+ ) which results in the formation of stabilized ionotropic hydrogels. Interactions with metal ions through the hydroxyl groups of the CMC may improve the water insolubility and stability of the polymer aggregates. 95,96 The reaction of cellulose with monochloroacetic acid (MCA) or its sodium salt in the presence of an organic solvent under basic conditions produces NaCMC. 97 There are various materials from which CMC can be derived; for example, Petri 98 prepared CMC from pineapple peels, and in another study, Begum et al. 99 prepared CMC from sugarcane bagasse. The synthesis of CMC occurs in three steps: (1) alkalization, (2) carboxymethylation and (3) neutralization as outlined in the steps below.
Step 1. Cell-OH + NaOH → Cell-OH.NaOH Step 2. Cell-OH.NaOH + ClCH 2 COONa → Cell-O-CH 2 COO À Na + + NaCl + H 2 O Step 3. NaOH + Cl À CH 2 COONa → HO-CH 2 COONa + NaCl During the synthesis process, carboxymethyl groups replace hydroxyl groups at positions 2, 3 and 6 of cellulose. In the synthesis of CMC, it is important to consider the degree of substitution, which will govern the porosity, and adsorption capacity of CMC. 99 Degree of Substitution (DS) refers to the number of hydroxyl groups replaced by carboxymethyl groups. Each β-glucopyranose unit of cellulose has three hydroxyl groups; therefore, hypothetically DS value will range between 0 and 3.0. 100 Various factors may affect the DS of CMC such as the solvent, temperature and the NaOH concentration. 101 For example, most researchers report that the use of isopropanol as the solvent yields CMC with very high DS. 101 Huang et al. 102 synthesized CMC from the pulp of six different plants; the conditions that yielded the best CMC were 20% NaOH concentration, the temperature of 60°C and 3-5 g of MCA. The group reported that CMC yield increased with increasing MCA content. The degree of saturation influences the application of CMC. 102 The hydrophilicity of CMC increases with an increase in DS. When DS > 0.4 (higher), CMC becomes soluble in water, has better viscosity, enhanced cationic exchangeability and improved covalent crosslinking through radiation. Whereas when DS < 0.4 (lower), CMC is insoluble, and it swells in solution. 102 For example, a study reported the degradation of a polymer with irradiation of 10% CMC (0.7 DS), whereas irradiation of CMC of 1.32 DS resulted in outstanding crosslinking and high gelfraction. 103 For adsorption, the properties of CMC can be modified by crosslinking to form a gel, which will improve the solubility and hydrophilicity of CMC.
Alginate. Alginate is a sentimental term used in the dietetic, cosmetic, pharmaceutical and biotechnological industries. 104 The alginate can be found in salts of magnesium (Mg), potassium (K), calcium (Ca) and sodium (Na). 104 E.C.C Stanford who was a pharmacist 105 discovered alginates in 1929. SA is found in brown algae, marine algae and produced by some bacteria, hence abundant in nature. 106,107 Applications of sodium alginate include drug delivery, thicker, gel-forming agent, binder during the production of tablets, use as a stabilizer and mixing agent. [105][106][107][108] The recent use of polysaccharides as adsorbents for dyes and other pollutants is mostly due to the attractive properties that polysaccharides possess, such as high hydrophilic structures, their non-toxic nature, biodegradability and cheap synthesis. [109][110][111] Sodium alginate (SA, Figure 4) is made up of poly-β-1, 4-D-mannuronic acid and α-1, 4-L-guluronic acid. 112 The structure consists of carboxylic groups and hydroxyl groups, which can react with metal ions or crosslinking agents to form hydrogels. 113 Alginate can be extracted from its salt forms by ion-exchange methods. 114 Because of the presence of divalent ions, monovalent, water-soluble salts of alginates can transform into water-insoluble salts. The COO À and -OH functional groups on the alginate backbone will allow adsorption of cationic dye pollutants. 115 However, sodium alginate just like other polysaccharides suffers from a lack of stability. 111 To enhance the stability of SA and its adsorption capacity, the biopolymer may be cross-linked with synthetic polymers into forming a hydrogel. 116,117 This will allow their use as adsorbents without them dissolving in the adsorbate solution. The major disadvantage of polysaccharides as adsorbents is that they dissolve in water and have low mechanical stability. 116,117 To resolve this problem, polysaccharides are cross-linked to form hydrogels, which do not dissolve in water.

Background
Hydrogels are hydrophilic polymer chains that are crosslinked to form gel structures that swell in aqueous solution and trap fluids for a long period without dissolving. 83,84 Hydrogels may contain carboxylic, amine, imide, hydroxyl and sulfonyl groups in their 3D structure that are responsible for the hydrophilicity and swelling capacity. 118 Depending on the nature of the hydrogel, it can be classified based on various properties. Classification of hydrogels ( Figure 5) can be based on whether they are synthetic (involves the use of synthetic monomers), natural (involves using biopolymers) or a combination of synthetic and natural monomers resulting in a hybrid hydrogel. 119 The polymeric composite classification can be based on the method used to synthesize the hydrogel 119   b. Chemical crosslinking can be achieved by (1) using aldehydes such as acetaldehyde, glutaraldehyde (GA) and formaldehyde and (2) radiation using an electron beam, gamma rays or ultraviolet rays at room temperature in which a free radical is formed, monomers are added to the chain for growth and the gel forms at the critical gelling point. Lastly (3) freeradical polymerization using a cross-linker, for example, MBA. In this method, an initiator such as potassium persulfate (KPS) generates free radicals, the radicals react with other monomers and then MBA is added to the polymer chain to form the hydrogel. 84,121 In addition, the hydrogels may be classified based on the charge on their cross-linked polymer network. 122,123 The charge may be a. ionic (cationic/anionic), b. non-ionic (neutral), c. amphoteric (comprising both basic and acidic groups) and d. zwitterionic (contains anionic and cationic components on each repeating structural unit). The net charge of the gel is zero.
Lastly, hydrogels can also be categorized according to what stimulates their response. These hydrogels are sometimes called smart hydrogels. 119,121 There are two categories of stimulus, namely, physical stimuli and chemical stimuli as stipulated in Table 2. The swelling or deswelling of hydrogels caused by changes in the environment can be too much to a point that the hydrogel changes phase (volume collapse).

Application of hydrogels
Hydrogels have gained much attention for use in various applications owing to their outstanding qualities; these include use in the making of disposable diapers, absorbent pads, use in hydrogen storage, sensing, CO 2 capture, biomedical field (drug and cell delivery systems), immobilization of enzymes, wastewater treatment and agriculture for trapping water. 11,48,49 The ideal properties of a hydrogel depend on its specific application. For application in water treatment, the hydrogel must have the following properties below 98 : 1. High absorption capacity; 2. low residue monomer and soluble content; 3. cost-effective; 4. reusable; 5. high biodegradability and not produce harmful byproducts; 6. must retain neutral pH after swelling in water and 7. easy recoverability.
The swelling capacity of hydrogels is indicative of their absorptive capacity which may be affected by factors such as the intermolecular spaces in the 3D network, the existence of hydrophilic groups on the hydrogel polymer backbone and the pore size of the hydrogel surface. 10 Hence, parameters such as the amount of initiator, crosslinker, polymer, monomer and solvent volume are optimized during the preparation of the hydrogel to obtain optimum conditions for preparing an ideal hydrogel. 122

Preparation methods
As stated previously, biopolymers suffer poor mechanical, thermal stability and they dissolve in water. 89 Most researchers have reported that crosslinking, grafting and modification with inorganic constituents such as metal oxides and clay improves solubility and stability problems associated with biopolymers. 3,121 The type of method used to prepare the hydrogel, therefore, affects its structural make-up or physical properties. Amongst several physical and chemical techniques used for hydrogel synthesis, the most widely used methods are discussed below.
Grafting. Grafting is the modification of the polymer backbone using synthetic polymers such as acrylamide, acrylic acid, methacrylamide and vinyl alcohol as support materials. 123 During grafting, an initiator creates a free radical site, and then a monomer unit is added to the generated free radical site. 124 Grafting can occur via either a chemical or radiation. Chemical grafting involves using chemical reagents such as ammonium persulfate, KPS or other chemical initiators. 125 Radiation grafting involves initiating free radicals using UV visible or microwave radiation. 126 It was shown in the literature that using a microwave radiation method for synthesizing hydrogels produces sterilized hydrogels. 126 Naturally, polysaccharides suffer from poor chemical and physical stability. 89 Grafting has been reported to solve these issues including enhancing the performance of hydrogels through the introduction of new functionalities from grafted monomers. 127 For example, in a  86 it could be observed from the thermogravimetric analysis (TGA) curves of (a) SA and (b) SA-g-P (AA-co-AM)/PVP semi-IPN SAP ( Figure 6) that grafting with synthetic polymers enhanced the thermal stability of hydrogels by more than 20% weight loss at above 200°C. A study of the modification of CMC by grafting with poly (3-sulfopropyl methacrylate), P (SPMA), in the presence of KPS initiator for MB removal was carried out. 93 The main vibrational peaks at C =O (1718 cm À1 ), S =O (1085 cm À1 ) and S-O (626 cm À1 ) were observed in Figure 7(a) of the Fourier transform infrared (FTIR) of the CMC-g-P (SPMA) hydrogel. 93 The scanning electron microscopy (SEM) image showed the well-defined pores and 3D interconnections of CMC-g-P (SPMA) hydrogel, for which the hydrogel could allow easy penetration of dye molecules in solution through its pores as observed in Figure 7(b). 93 The adsorption of MB by the hydrogel was characterized by an absorption peak at 664 nm corresponding to the characteristic absorption band of MB (Figure 7(c)). From the X-ray diffraction (XRD) (Figure 7(d)), the partially crystalline structure of CMC was characterized by sharp peaks at 2θ = 9.4 and 20.1°resulting from the intermolecular and intramolecular hydrogen bonding between carboxylic groups and hydroxyl groups of the biopolymer. Upon grafting on the CMC-g-P (SPMA), it could be observed that the peaks were broader, indicating the hydrogen bonds were destructed, allowing grafting to take place. 93 Crosslinking. The process of crosslinking can occur through physical or chemical interactions. Physical interactions are irreversible and include electrostatic interactions, hydrogen bonding and the van der Waals forces. 128 Chemical crosslinking involves forming irreversible covalent bonds in the hydrogel polymer chain usually through the reaction of complementary groups. 129 Chemically cross-linked hydrogels are mostly used in medical applications. For example, Liu et al. 130 synthesized chitosan-based hydrogels from PEG for application in drug delivery. In another study by Kumar et al., 131 cross-linked hydrogels were produced from GA as a covalent cross-linker (Scheme 1).
The most commonly used chemical cross-linkers include ethylene glycol dimethyl-acrylate, N, N-methylene-bisacrylamide (MBA), tetra-ethylene glycol dimethyl-acrylate (EGDMA) or tri-propyleneglycol diacrylate (TPGDA). 132 However, these cross-linkers are usually toxic and result in non-biodegradable hydrogels. 133 In addition, because of the lack of internal structural homogeneity and an effective mechanism for energy dissipation, chemically cross-linked hydrogels are very brittle. 134 This prompted many researchers to consider introducing nanocomposites, ionic interactions, hydrogen bonding and hybrid hydrogel systems to improve the hydrogel properties. 135-137 A hybrid system consists of physical interactions that will aid in energy dissipation and chemical interaction, which will mainly improve structural properties. 138 Figure 8 illustrates the preparation of hydrogel beads through ionic interactions by Rahmani et al. 139 Briefly, a mixture containing gum tragacanth (GT), graphene oxide (GO) and calcium carbonate (CaCO 3 ) was added dropwise into a concentrated solution of Ca 2+ which interacted with -OH and COO À groups on the GT and GO to form hydrogel beads. Ionic crosslinking offers the advantage of less toxicity. 133 In a study by Eftekhari-sis et al., 140 hybrid hydrogels constructed from poly (N-isopropyl acrylamide-co-itaconic acid) and non-toxic octa-vinyl polyhedral oligomeric silsesquioxane (OV-POSS) cross-linker were prepared. Their obtained SEM (Figure 9), XRD and TGA ( Figure 10) results are discussed. The rough non-homogeneous surface was observed on the SEM image (a) of poly (NIPAM-co-IA). Upon hybridization with 8% POSS (b), 12% POSS (c) and 12% POSS (d), the morphology changed to a honeycomb like structure with homogeneous pores of varying pore sizes as POSS content was increased. The study reported that at 12% POSS the honey-comb pattern was disrupted, indicating that the degree of uniformity in the hydrogel hybrid can be controlled by changing the crosslinking content. 140 For example, the XRD patterns in Figure 10(a) showed a successful hybridization and a uniform dispersion of the cross-linker as confirmed by the disappearance of characteristic crystalline diffraction peaks of OV-POSS at 2θ = 9.8, 23.1 and 23.9°. 140 The TGA thermogram (Figure 10(b)) showed that the crosslinking with POSS in the hybrid hydrogels increased the thermal stability, wherein the weight loss at temperatures 340-500°C was less than weight loss obtained for poly (NIPAM-co-IA) at the same temperature. 140 From the obtained literature, it could be observed that the chemical and physical properties of the hydrogels were improved, including creating porous structures that enable penetration of adsorbate molecules.
Free-radical polymerization. Free-radical polymerization is a technique in which polymerization is achieved by generating free radicals to which chain growth occurs in succession. 141 This method involves a combination of grafting and crosslinking. The decomposition of the initiator may occur using temperature, light or photon, which then forms a free radical. 142 Decomposition by photon offers advantages such as low costs; it allows better spatial and time-based control of the reaction process and does not use chemical solvent. 143 Scheme 2 shows a schematic representation of an overview of free-radical polymerization for synthesizing hydrogels. Firstly, there is the generation of free radicals by an initiator (initiation). Secondly, the free radicals react with the monomer to generate vacant active sites (propagation) and lastly, the formation of a polymer network through crosslinking (termination). 144 Free-radical polymerization is the most frequently used procedure in the polymer industry. 1 The advantages of using free-radical polymerization are that it is easy to carry out, convenient, it is not easily affected by impurities and it is suitable for designing and preparing polymers that can be used for a variety of applications. Free-radical polymerization allows well-characterized reaction kinetics and achievement of in situ properties. 145 Modification with inorganic materials. Depending on the intended application of the hydrogels, their physical properties can be further enhanced through the incorporation of inorganic materials. For example, for use in removing contaminants from aqueous solutions, the hydrogel must be mechanically and thermally stable for application especially for removing effluents from industries that utilize water for cooling reactions, 146,147 wherein the contaminants may be introduced at that point. In addition, the hydrogels must be easy to recover. Many recent studies have incorporated nanofillers such as clay-based materials, carbon-based materials and metal oxides into the hydrogel matrix during the polymerization process to improve stability and recoverability. The blended product is then called a hydrogel nanocomposite.
Carbon-based. Carbon-based hydrogel nanocomposites are hydrogels synthesized by incorporating materials such as graphene oxide (GO), biochar, AC and carbon nanotubes. 148,149 GO is a carbon material prepared by oxidizing graphene through chemical or thermal reduction processes. It contains highly hydrophilic groups such as hydroxyl, carboxylic and epoxy groups that are essential for adsorbing dyes and toxic metals. 150 GO may interact with contaminants through pi-to-pi interactions, electrostatic interactions or hydrogen bonding. 151 Owing to its outstanding mechanical, electrical and thermal properties, GO has attracted its use in the biomedical, energy and environmental field. 152 Carbon nanotubes are simply graphene sheets rolled up in cylinders of 1 nm in diameter. 153 As a result of their porous structure, large surface area, high tensile strength   (0.15 TPa) and elastic modulus (0.91 TPa), CNTs have attracted interests in use for adsorption of pollutants such as dyes, dichlorobenzene, ethylbenzene and some heavy metals. 154,155 CNTs can be categorized into two forms, namely, single-walled carbon nanotubes (SWCNTs), which are made up of single layers of graphene sheets, and multiwalled carbon nanotubes (MWCNTs), which are made up of multiple layers of concentric cylinders. 153,156 For example, the incorporation of CNTs in gels improves the mechanical properties; however, the rate of degradation decreases. This is because carbon-based materials have high hydrothermal stability, which makes them resistant to harsh environments. 154 Scheme 3 shows various interactions or applications that carbon-based materials can take part in such as surface complexation, physical adsorption through its surfaces and electrostatic interactions. 157 Amongst recent studies that have used carbon-based materials to modify properties of the hydrogel, the incorporation of MWCNTs into XG/PAA hydrogel improved the surface hydrophilicity and specific area of xanthan gum. 10 Makhado et al. 119 conducted studies on xanthan gum polymer cross-linked with polyacrylic acid and incorporated with reduced graphene oxide (rGO); the successful synthesis was confirmed by FTIR. Various carbon-based nanocomposite hydrogels and their adsorption properties for removing contaminants from aqueous solutions are listed in Table 3.
Clay-based hydrogels. Natural clays and their improved forms have recently been used for removing pollutants from water. 161 The most commonly used clays, especially for treating toxic metals and dyes, are modified kaolinite and montmorillonite. 162 However, these clays are difficult to regenerate and reuse because of their colloidal dimensions. 163 This prompted the functionalization of these clays, for example, the modification of montmorillonite to form Cloisite 30B clay. In a study where C30B clay was incorporated onto polypropylene grafted with maleic anhydride (PP-g-MA) and thermoplastic starch (TPS), it was reported that the biodegradability of the materials was improved. 164 Modification of adsorbents by introducing functionalized clay components may improve both the physical and chemical properties of adsorbents. 163,164 Other modifications with clay results improve the number of adsorbing active sites, enhanced porosity and low levels of mineral impurities. 164 In a recent study, C30B was mixed with a culture obtained from an anaerobic sludge to remove hexavalent chromium, where the removal capacity of nearly 100% was obtained. 165 Peng et al. 166 prepared cellulose-MMT hydrogels for removing MB. The hydrogels had a maximum removal capacity of 1065 mg/g (Table 4). Their viscoelasticity studies from the sweep measurements (Figure 11(a)) showed that when G 0 < G 00 the cellulose-MMT systems were in a viscous liquid form; however with Table 3. Carbon-based adsorbent hydrogels and their adsorption properties in removing contaminants from aqueous solution.

Adsorbent
Cross-linker Dye increasing time when G 0 > G 00 , the materials were in a gel form. Figure 11(b) in this case revealed that increasing clay content decreased the gel formation. Figure 11(c) revealed that the storage moduli (G 0 ) of hydrogels containing clay was higher than that of unmodified hydrogel and was proportional to the clay content from 10 to 15 wt.%. The incorporation of clay, however, increased the gel strength from 0.3 KPa to 4.7 KPa as shown in Figure 11(d) and (e), which was approximately 16 times higher. Furthermore, the study reported that an increase in clay content from 10-15 wt.% reduced the swelling capacity of the hydrogel ( Figure  9(f)), reportedly due to a high degree of crosslinking. Increasing the crosslinking density of the hydrogel enhanced mechanical properties. 166 Metal oxide-based hydrogels. Metal oxide-based nanoparticles have been reported to have a high density and restricted size, which are responsible for their very interesting and unique chemical and physical properties. 170,171 Examples of metal oxides include titanium dioxide (TiO 2 ), iron oxide (Fe 3 O 4 ), magnesium oxide (MgO) and aluminium oxide (Al 2 O 3 ). Owing to their non-toxic nature, high surface area, high chemical stability and economical friendliness, Fe 3 O 4 nanoparticles are widely used for the removal of toxic metals and organic pollutants from water. 170,171,172,173 Ion-oxide and zinc oxide nanoparticles are some of the most frequently used metal oxides in treating dyes from aqueous solutions. 174 Magnetic (Fe 3 O 4 ) nanocomposite hydrogels. Magnetite (Fe 3 O 4 ) also known as black iron oxide amongst other transition metals has the strongest magnetism and is stable at ambient temperatures. 175 Magnetite is prepared from the coprecipitation of iron oxide salts that result in an inverse spinel crystal structure consisting of half of the Fe 3+ in tetrahedral coordination and the other half Fe 2+ ions in octahedral coordination ( Figure 12). 176,177 The co-precipitation method is the most useful and suitable technique in preparing magnetite at both lab-scale and industrial scale. 178 Due to their cheap synthesis costs, susceptibility, stability, high porosity, magnetic properties, biocompatibility and easy chemical modification, MNPs have attracted much use in the wastewater treatment field. [173][174][175][176][177] The introduction of MNPs into polymer structures to produce nanocomposite hydrogels leads to a hybrid hydrogel with advantages of both components, thus improving chemical and physical properties. An example of such hybridized hydrogel is Fe 3 O 4 -g-pAA hydrogel synthesized via radical polymerization in the presence of MBA cross-linker, which resulted in a highly adsorptive hydrogel of 507.7 mg/g removal capacity for MB 179 as shown in Table 5. However, according to Flory's theory, the swelling degree of a hydrogel depends on the density of crosslinking, the ionic osmotic pressure and the attraction of the gel for the liquid. 180 When the crosslinking density is high, the space between the polymer chains decreases making the gel to be stiff due to small nonexpendable pores. 180 Additionally, an increase for MNPs increases the thermal properties but consequently reduces the swelling capacity as the Fe 3+ from the incorporated MNPs may act as a physical crosslinking agent. 179 The greatest advantage of incorporating MNPs into the hydrogel matrix is the improved stability and easy recovery of the material after application. 181 Zinc oxide-based hydrogels. Zinc oxide (ZnO) can be found in nature as a zincite mineral; however, the majority of it is prepared synthetically. Its crystal structure can be found in a hexagonal wurtzite form or cubic zinc blende form ( Figure 13). The form that is most stable and commonly found at ambient temperature is the wurtzite structure. 188 ZnO is commonly used for treating skin-related problems such as nappy rash, dandruff and incorporation in ointments used in wound dressing. 188 Other applications of ZnO include use in catalysis, batteries, sensors and adsorption of contaminants. [189][190][191][192] Khan et al. 193 reported their application as adsorbent material in a study, where a guar gum adsorbent hydrogel incorporated with ZnO nanoparticles was used for removing chromium (VI) from water. The group reported that incorporating ZnO nanoparticles improved the recovery of the adsorbent from the aqueous solution after the removal of Cr (VI). In another study, CMC hydrogel was modified with ZnO for antimicrobial activity, which was influenced by their inexpensiveness and lacks colour. 194 Therefore, looking at these properties, the ZnO would be ideal for preparing hydrogels for adsorbing dyes.
Titanium dioxide-based hydrogels. Titanium dioxide is a naturally occurring oxide of titanium. It exists in three crystalline phases, namely, brookite, anatase and rutile. 195 The rutile phase is thermodynamically stable compared to the anatase and brookite phases, which are metastable. 195 The advantages of TiO 2 are that it is non-toxic, chemically stable, highly reactive, biocompatible, cost-effective, contains electrochemical properties and is safe to produce. 195,196 In addition, TiO 2 possesses antimicrobial and UV-protection properties. 196 Applications of titanium dioxide include the use of a photocatalyst for water-dye degradation, in food and pharmaceuticals as a white colourant, in sensors, ceramic industries, inorganic membranes and biological implants. 195,196 For example in a study by Kangwansupamonkon et al.,197 TiO 2 /poly [acrylamideco-(acrylic acid)] hydrogel was reported to have 91.6% removal for MB using 5% TiO 2 as an inorganic initiator. Another study by Ari et al. 195 used P (AMPS)-TiO 2 composite hydrogel for photocatalytic degradation of MB and reported high catalytic performance and high thermal stability due to the incorporated TiO 2 nanoparticles. The group also reported over 80% removal capacity for MB after repeated cycles of more than five, hence improved reusability and regeneration. Another study by Han et al. 198 also reported excellent performance in terms of regeneration, self-cleaning and recovery of the prepared hydroxyethyltitanium dioxide-CMC (HEC-TiO 2 -CMC) hydrogel cage for the removal of MB. 198 Adsorption of MB onto polyacrylamide-grafted gum ghatti (PAAm-g-Gg)/TiO 2 hydrogel nanocomposite prepared by Mittal and Ray was reported to be greatly dependent on the adsorbent dose, the  Figure 13. Crystal structure of zinc oxide. 162 ionic strength, pH and temperature of the solution. 199 The pair reported a removal capacity of 1305.5 mg g À1 dominated by pseudo-second-order rate model and Langmuir adsorption isotherm. 199 The incorporation of TiO 2 NPs onto sodium alginate cross-linked polyacrylic acid (SA-cl-poly (AA) hydrogel) was reported to have improved removal capacity from 80% using hydrogel with no TiO 2 to 94.1% and 99.4% for nanocomposites containing 0.05 and 0.2 g of TiO 2 NPs, respectively. 200 XRD patterns from this study showed the disappearance of some TiO 2 peaks after MB absorption indicating a reduction in the crystallinity of the nanocomposite that is said to be caused by the trapping of dye molecules within the micropores and macropores of the absorbent. 200 In recent years, efforts have been made to confirm the effectiveness of using NCHs as adsorbent for wastewater treatment. In this context, Malatji and co-workers fabricated clay/magnetic hydrogel nanocomposites via free-radical polymerization method; their material exhibited improved thermal stability and mechanical strength in comparison to the pure hydrogel. However, their adsorption studies showed that the incorporation of Fe 3 O 4 and Cloisite 30B in the polymeric hydrogel networks reduced the adsorption capacity. The prepared clay/magnetic hydrogel nanocomposites exhibited an adsorption capacity of 1081.60 mg/ g, which is low when compared to the adsorption capacity (1109.55 mg/g) of pure hydrogel. 201 Surprisingly, Zhang et al. 202 prepared PAAm/CS/Fe 3 O 4 composite hydrogel via in situ polymerization for the removal of MB dye from aqueous solution, and the obtained results demonstrated that the incorporation of Fe 3 O 4 magnetic nanoparticles in the hydrogel matrices improved both swelling and adsorption capacity. 202 Another study by Makhado et al., 182 synthesized sodium alginate/ZnO hydrogel nanocomposites by the in situ free-radical polymerization method. They applied the as-prepared material as an adsorbent for MB dye removal from the aqueous phase. According to their results, sodium alginate/ZnO hydrogel nanocomposites showed high adsorption capacity (1529.6 mg/g) for MB in contrast to adsorption capacity (1129.0 mg/g) exhibited by the pure hydrogel. In addition, sodium alginate/ZnO hydrogel nanocomposites displayed relatively high thermal stability and excellent recycling performance than pure hydrogel. Nonetheless, the incorporation of ZnO NPs in the SA-poly  (AA) hydrogel matrix increased the swelling capacity, which sequentially led to a decrease in the mechanical strength. 182 Table 6 shows the advantages and disadvantages of variously reported hydrogel nanocomposites in the removal of MB from wastewater. [199][200][201][202][203][204][205]

Conclusions
This review discusses recent studies applied for removing MB from aqueous solution using biopolymer-based hydrogel nanocomposites. Owing to various advantages such as low cost and easy design, the adsorption method is recognized as the most promising treatment technique for the removal of MB dye. This work discussed the principle behind the adsorption method, common adsorbent materials used and its advantages. We also briefly explained the hydrogels and methods to improve their properties. Three isothermal models: Langmuir, Freundlich and Temkin models have been thoroughly explained with their respective principles. From this review, it was observed that the preparation of hydrogels can be affected by various factors and its removal capacity is affected by pH, temperature and dye concentration. This contribution emphasized the importance of incorporating metal oxide nanoparticles in hydrogels for the removal of dyes and also their advantages and disadvantages towards the gel structure and properties. Concerning the research up to date, it has been observed that the realization of hydrogels for practical purposes needs more research, especially the preparation of hydrogel nanocomposites with the ability to remove organic contaminants such as dyes. It was seen that both magnetic nanoparticles and ZnO nanoparticles have contributed greatly to improving stability and enabling easy separation of adsorbent materials after application. Although the actual wastewaters are complex to be treated because they possess various ions and pollutants, it is more pivotal to conduct analysis on wastewater for adsorption studies using the actual industrial water samples. This is mainly because the adsorbent performance may be noticeably different from the one carried out in batch mode in the laboratory.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.