Cellulose-Based Hydrogels for Water Treatment
Lakes, rivers, sea, groundwater, drinking water basins, etc. are the main water sources which can increasingly be polluted by commercial and industrial establishments or human activities. The most existing types of contaminants that pollute these water sources are dye-containing effluents and toxic heavy metals which they affect living being’s life catastrophically. Various methods have been applied to get rid of these kinds of toxic pollutants from water sources such as reverse osmosis, chemical precipitation, membrane filtration, coagulation, ion exchange, electrochemical treatment, and adsorption. Among these methods, adsorption is quite effective and economic method for the removal of toxic pollutants. Hydrogels that can be described as 3D network of hydrophilic polymer chains cross-linked chemically or physically which are able to soak and release a significant amount of water while preserving their network structure from dissolution in aqueous media, and they can be applied in many fields including tissue engineering, drug delivery, wound dressing, food, cosmetics, contact lenses, sensors, and water treatment. Hydrogels are excellent candidate to remove toxic pollutants by adsorption due to their high absorption capacity, porous structure, rich functional groups, and relatively low crystallinity. These hydrogels can be composed of petroleum-derived synthetic polymers, natural occurring materials, or composition of both synthetic and natural materials. Hydrogels that prepared from natural materials are preferred by their low cost and biodegradability and easily available from plenty of resources. To prepare hydrogels, a wide range of synthetic and natural materials have been used, such as cellulose, chitin, and chitosan for natural materials; polyethylene glycol and poly(sodium acrylate) for synthetic materials can be given as an example. Among them, cellulose is a well-known naturally found linear homopolymer having consecutive glucose units connected by glucosidic bond. The use of cellulose-based hydrogels is gaining popularity because of their several advantages such as environmental friendliness, biodegradability, biocompatibility, nontoxicity, easy availability, high abundance, low cost, and thermal and chemical stability for water treatment applications. Therefore, cellulose-based hydrogels have been attracted much attention in both academic and industrial applications including drug delivery, hygiene products, medicine, and water purification technologies. Among these applications, the use of cellulose-based hydrogels for water treatments has been discussed in this chapter.
KeywordsCellulose Hydrogel Heavy metal Dye Water pollution Adsorption
Modern civilization has caused the fast deterioration of our environment due to its rapid increase in the population and industrialization. Water is the most essential and one of the precious natural sources we need on this planet. The pollution of water can be occurred when its quality or compositions are affected by waste disposal, industrial effluents, or human activities. Therefore, its odor, color, taste, or content might be changed, and it becomes hazardous for drinking, domestic, and agricultural usage. Water pollutants which originate from industrial effluents, human activity, sewage disposal, and so forth include almost all kinds of toxic substances ranging from simple organic matter to complex ones such as silt, toxic heavy metals, dyes, pesticides, oils, etc. [1, 2]. Those contaminants are dangerous for marine species, plants, as well as humans and can cause serious health problems such as cancer, organ damage, nervous system damage, diseases of kidneys, and circulatory system and brain damage . The resulting hazardous effects by this kind of polluted waters need to be addressed, and those polluted waters must effectively be cleaned up using appropriate methods.
Gels are cross-linked polymeric network that store large amount of liquids. They are wet and soft and look like a solid material but are capable of undergoing large deformations . When the liquid is an organic solvent, the gel is defined as an organogel [5, 6, 7, 8]. However, the liquid is water, and then it can be defined as hydrogel. Owing to sophisticated features of hydrogels including high water content and the porous network structure, they can be widely used in various applications such as personal care products, agriculture, sensors, drug delivery systems, and sorbents for environmental applications [9, 10]. Hydrogels can be classified as synthetic, semisynthetic, and natural, depending on the source of initial material. A variety of synthetic and natural materials, including polyethylene glycol (PEG), polyacrylamide, poly lactic acid (PLA), and polyvinyl alcohol (PVA) as synthetic and chitosan, hyaluronic acid, cellulose, etc. as natural, have been extensively employed to synthesize hydrogels [11, 12, 13].
Among the natural materials, cellulose which can be produced from plants including cotton, banana, orange peels, and rice husk and from bacteria including Acetobacter, Rhizobium, Agrobacterium, Sarcina, Pseudomonas, Achromobacter, Alcaligenes, Aerobacter, and Azotobacter , most abundant natural polymer on earth, is a natural polysaccharide composed of β-1,4-glucosidic bonds in the polymer chain and is environmentally friendly, biocompatible, green, inexpensive, and sustainable material. Thus, these materials have great potential for applications in wide fields because they can be chemically functionalized with respect to the intended use.
In recent years, polymeric gels, especially natural-derived polymers due to its low cost and high abundance, have been widely used in different applications including drug delivery systems, sensors, water treatments, medical technology, agriculture, and oil sorbents [15, 16, 17]. Polysaccharides and their derivatives are one of the most used natural materials because they can be easily obtained from plants or animals. For example, Ummartyotin et al.  have prepared cellulose-based hydrogel, and they have found that the obtained hydrogels presented excellent features including good thermal stability, chemical resistance, and mechanical properties. The swelling ratio of synthesized hydrogels was found to be 400–600%. Essawy et al.  produced a novel bio-based adsorbent by graft copolymerization of acrylic acid onto cellulose in the presence of fulvic acid as interpenetrating agent. The obtained adsorbent is used for the removal of Cu2+. In another study, cellulose which was extracted from rice husk was chemically modified and used for CO2 capture by Einloft et al. .
2 Toxic Pollutants in Water
With rapid population growth and industrialization, the environment is becoming more polluted, and this problem has significant negative effect on economy, human life quality, and environment.
Water pollution by organic contaminants has been one of the most concerning threats needing to be solved by mankind . Oil and petroleum derivatives from oil leakage events, dyes from textile industries, and heavy metals from factory wastewater have become one of the most common pollutants in wastewater. These pollutants affect human life and environment negatively, and many of them are known to be toxic or carcinogenic for human life. For example, mercury is known one of the most toxic heavy metals for environment. Also, heavy metals including mercury and cadmium are not biodegradable which cause various diseases and disorders on the human body . Moreover, the presence of synthetic dyes may cause skin irritation which can cause severe damage to human beings including dysfunction of the kidneys, reproductive system, liver, brain, and central nervous system, and because of that, they are classified as toxic and carcinogenic [22, 23]. Polycyclic aromatic hydrocarbons (PAHs) are organic compounds that are composed of multiple aromatic rings and are persistent in the environment because of their chemically stable structure. Due to that, they are known as carcinogenic, mutagenic, and toxic materials .
The presence of organic contaminants in water is a serious problem. The organic contaminants such as dyes and oils on the water surface can severely impact on the environment of marine life (by inhibiting the penetration of sunlight) as well as on human life (by adversely impacting on fishing, coastal beaches, resorts, etc.) .
Bearing in mind that the mentioned chemicals have adverse effect on human life and environment, the removal of such pollutant from wastewater is urgently needed for living organisms and environments [25, 26].
3 Techniques for the Removal of Toxic Pollutants
Classification of water pollutants 
Suspended or floating matters
Sand, silt, wood chips, paper, etc.
Plastic, tar, pesticides, oil, etc.
Nitrates, phosphates, heavy metals, fluorides, etc.
Bacteria, virus, worms, etc.
Wastewater treatment methods
Advantages and disadvantages of water treatment techniques
Physical and/or chemical methods
Rapid process for toxic pollutant removal, it does not require pre- and posttreatment
High energy costs and formation of by-products
Good removal of a wide range of heavy metals and dyes, energy requirements are minimal
High operating and chemical costs, high sensitive to fouling
Membrane filtration technologies
Good removal of heavy metals and dyes
Concentrated sludge production, expensive, requires periodic cleaning
High sludge production and formation of large particles
Rapid process and effective for certain metal ions
High energy costs and formation of by-products
Applied in gaseous state: alteration of volume
No sludge production
Formation of by-product
Effective at lab scale
Requires a lot of dissolved O2
High sludge production
Effective and capable of treating a variety of wastes and no energy input necessary to activate hydrogen peroxide
Feasible in removing some metals
Technology yet to be established and commercialized
Universal, easy to operate, low cost, can achieve nearly 100% water recovery
Adsorbents require regeneration
While water treatment costs of other technologies including reverse osmosis, ion exchange, and electrodialysis range from 10 to 450 US$ per cubic meter of treated water, adsorption methods need 5–200 US$ per cubic meter of water .
Different types of materials have been used to adsorb pollutants from wastewater. Recently, there are highly increasing interests in low-cost materials to use as sorbents for the removal of organic pollutants from wastewater . For this reason, natural materials from plants and animals have been increasingly studied as adsorbents for the removal of organic and inorganic pollutants.
4 Cellulose-Based Hydrogels for Removal of Pollutants from Wastewater
It was known that hydrogels should have some parameters to use as adsorbents in wastewater applications including high swelling capacity, high adsorption capacity, and high physical, chemical, and mechanical stability and reusability .
4.1 Removal of Heavy Metals
In comparison to the materials mentioned above, hydrogels are considered as promising materials for the effective removal of metal ions from aqueous solutions for the environmental purpose. Hydrogels are three-dimensional cross-linked hydrophilic network materials that can absorb and desorb a large amount of water, and they generally have porous structure and high surface area which are favorable spaces for the adsorption process . When the hydrogels absorb water, they also uptake the water-soluble species and hold them inside their structure. This phenomenon is an excellent feature of the hydrogels which can be used for the wastewater treatment and the removal of heavy metal ions from the aqueous solutions. Heavy metals can create coordination complexes with electron-rich atoms such as oxygen, nitrogen, sulfur, etc. Therefore, hydrogels having electron-rich atoms in their backbone structure possess excellent property for heavy metals to have suitable places in order to form a chelate, and the removal of heavy metals from aqueous solutions can be operated effectively by hydrogels . To constitute such hydrogels, biopolymers are challenging materials than synthetic ones due to their outstanding biocompatibility and biodegradation . As a natural biopolymer, cellulose is the most abundant, versatile, inexpensive, renewable, and widely studied polymer which is usually used for paper industries, fabrics, packaging, binders, and adhesives. Cellulose-based hydrogels have great attraction in the adsorption of heavy metals from aqueous solutions by combining the feature of cellulose and hydrogels’ network structure such as possibility of the incorporation of different functional groups that are capable of chelating like hydroxyl, amine, carboxyl, phosphate, etc., internal porous structure, high surface area, cost-effectivity, and eco-friendly construction [43, 52]. However, natural polymer-based hydrogels generally have low mechanical strength than the hydrogels based on synthetic polymers. For this reason, scientists’ attention is driven to create composite hydrogels using natural and synthetic polymers or inorganic clays and the chemical modification of the natural polymers to obtain superior hydrogel networks . Chemical modification of cellulose can improve the heavy metal adsorption capacity by introducing suitable functional groups into cellulose. Hydrogels composed of modificated cellulose have higher adsorption capacity than unmodified cellulose-based hydrogels for heavy metal ions .
A great deal of research on cellulose-based hydrogels owing to their promising advantages has been applied for the removal of heavy metals from aqueous solutions. A study for the removal of heavy metal ions from aqueous solutions was accomplished using chitosan-carboxymethylated cellulose hydrogel . Carboxymethyl cellulose (CMC) which is a derivative of cellulose is a well-known, inexpensive cellulose ether with biodegradable feature. It composed of an anionic linear polymer in which hydrogen atoms of cellulose hydroxyl groups are interchanged by carboxymethyl group. CMC is an excellent natural polymer owing to its anionic groups which is ready to collect counterions such as toxic heavy metal ions in aqueous solutions. Chitosan has also amine and hydroxyl groups in its structure as chelating zone for heavy metal ions. Blending both chitosan and highly carboxymethylated cellulose by irradiation technique to form chitosan-CMC physical hydrogels combines the properties of two natural polymers to increase the heavy metal adsorption capacity from aqueous solution. Radiation cross-linking is the best alternative synthesis approach for such hydrogels instead of using toxic chemical cross-linkers by the advantage of free additive process and high purity product generation. Water swelling of chitosan-CMC hydrogels decreases by increasing chitosan content in the hydrogel structure due to increasing cross-linking degree. However, when the swelling degree of chitosan-CMC hydrogel decreases by the increment of chitosan content, metal (Cu2+, Cd2+, Zn2+) adsorption from aqueous solution increases due to the increase of amino groups on chitosan and cross-linking which provides denser arrangements of functional groups (carboxyl, amino) for chelation in the network. Three possible mechanisms were offered for the adsorption of the divalent metal ions: (1) the adsorption by ionic interaction between each divalent metal ions and one carboxyl group of CMC, (2) the adsorption of each divalent metal ion by two or more carboxyl groups through chelation, and (3) the chelation of divalent metal ions with amino groups of chitosan content. Furthermore, adsorption capacity of the cellulose-based hydrogel significantly increases at higher initial Cu2+ concentrations. The adsorption of Cu2+ ions at lower concentrations increases linearly, indicating that hydrogel surface has enough active units for adsorption and the level of adsorption depends on the amount of Cu2+ that were moved from solution to sample surface.
Another nanocomposite hydrogel based on cellulose was synthesized by grafting method using carboxymethyl cellulose (CMC), N-isopropyl acrylamide (NIPAm), and acrylic acid (AA) . Researchers took advantages of the carboxymethyl group of CMC to collect heavy metal ions and NIPAm which can show great changes in pH, ionic strength, or temperature of the environment to prepare stimuli-responsive hydrogels. Adding nanoscale materials to create a layer inside the hydrogel structure is another approach to improve the mechanical strength and thermal stability of the cellulose-based hydrogels. Graft copolymerization of NIPAm and AA on CMC was carried out in Na-montmorillonite (MMT)/water suspension media which provides the mechanical and thermal strength to obtain hydrogel for the removal of Cu2+ and Pb2+ ions from water. Swelling degree of the hydrogels increases by increasing pH values which provides the ability of taking more metal ions inside the network structure. High amount of metal ion adsorption by hydrogels was observed, and in the presence of MMT, there was a slight increase compared to the pure hydrogel. Metal ion adsorption capacity of the cellulose-based hydrogels higher for Cu2+ than Pb2+ ions and pseudo-second-order kinetic model well fitted with the adsorption mechanism. Also, CMC-based hydrogels are selective for Cu2+ than Pb2+ ions. For nanocomposite hydrogels, two main mechanisms were suggested: (1) adsorption of metal ions by carboxyl groups on the CMC polymer and (2) ion exchange feature of MMT. Cation exchange capacity of the clays as MMT is effective in the removal of heavy metals. The ability of cation exchange of MMT plays an important role in the adsorption capacity of CMC-based nanocomposite hydrogels.
CMC have been mostly used by researchers to prepare hydrogels for the removal of heavy metal ions. Epichlorohydrin (ECH) is a common cross-linker which can react with the hydroxyl groups of CMC in alkaline conditions. Yang et al. prepared CMC-based hydrogel beads using ECH as a cross-linker in aqueous alkaline conditions for the removal of Pb2+, Ni2+, and Cu2+ . The cross-linking between CMC and ECH destroyed the crystalline regions of the CMC and increased the amorphous zones in the CMC/ECH hydrogel beads regarding X-ray diffraction measurements understood by disappearing characteristic diffraction signals of CMC. This situation leads to easy penetration of metal ions because of the reducing crystallinity of CMC/ECH hydrogel beads and demonstrates positive impact on adsorption of heavy metal ions. CMC/ECH hydrogel beads have porous structure due to the numerous carboxylate anions which expand the pore sizes of the hydrogel beads by electrostatic repulsions between the carboxylate anions in the hydrogels. Because of the porous structure of the CMC/ECH hydrogel beads, metal ions could easily diffuse into the hydrogel network that could increase the adsorption capacity of the CMC-based hydrogels. SEM photograph proved that the size of the hydrogel’s opened pores reduces after the loading of heavy metal ions by diminishing the electrostatic repulsions of carboxylate anions in the network due to the captured cationic metal ions by carboxyl groups.
pH values of the metal ion solutions could affect the adsorption capacity of the hydrogels. When the pH values increase to the alkaline region, the carboxyl groups of CMC/ECH hydrogel beads turn into carboxylate ions (COO−) which have better electrostatic affinity to the heavy metal ions than carboxyl groups. Therefore, when pH value of the metal ion solution becomes higher than 4.6, the pKa of the carboxylic groups leads the hydrogel highly ionized and results to higher heavy metal adsorption. Amounts of adsorbed heavy metal ions are 6.23, 3.02, and 4.82 mmol/g for Cu2+, Ni2+, and Pb2+, respectively. Cu2+ ions are more adsorbed than the others because of the stronger attraction to the lone pair of electrons in the oxygen atoms of carboxyl groups, which leads to prompt more stable complexes with CMC/ECH hydrogel beads.
2-Acrylamido-2-methyl propane sulfonic acid (AMPS) which has electron donor atoms such as N, O, and S that have the ability to form coordinate bonds with heavy metal ions was copolymerized and cross-linked with carboxymethyl cellulose applying γ-irradiation method. Several CMC/AMPS hydrogels with different compositions were synthesized using versatile irradiation dose to generate composite hydrogel for the removal of Co2+, Cu2+, Fe3+, and Mn2+ from aqueous solutions. The increase of the AMPS content in the hydrogel causes better cross-linked structure for the absorption of water and the adsorption of the metal ions. However, the increase in the irradiation dose may result the cleavage of CMC chains which leads to lose cross-linked network structure. The synthesized copolymer hydrogels have the following order of affinity for the metal ions, Fe3+>Cu2+>Co2+>Mn2+, and can be used at least five times without losing their efficiency . pH is a critical factor for the adsorption process which can affect the chelation of metal ions with the hydrogel’s functional groups and swelling properties of the networks. The adsorption of heavy metals typically depends on the pH values, which the adsorption capacity of the CMC/AMPS hydrogels increases when the pH values become higher than pKa values of AMPS. Generally, adsorption of heavy metals on CMC/AMPS hydrogel increases when the pH moves from 1 to 5. At lower pH values, excess of hydrogen atoms competes with metal ions to bind active sites of the hydrogel that causes lower metal ion uptake. The heavy metal ion adsorption of CMC/AMPS hydrogels mainly depends on the content of AMPS inside the hydrogel structure. The increase of the AMPS content stimulates the higher heavy metal adsorption of the hydrogel as well as the increase in pH values and initial metal ion concentration.
Cellulose can be used to improve mechanical strength of the hydrogels and as a binding substrate for heavy metal ions. Collagen which is an animal protein was blended with cellulose to form collagen/cellulose hydrogels for the removal of Cu2+ ions from wastewaters . Hydrogen bond interactions between cellulose and collagen units could be increased Young’s modulus and tensile strength to provide more robust hydrogels. Composition of the hydrogel affects the surface area of collagen/cellulose hydrogels, which reaches its higher level when the collagen/cellulose hydrogel ratio becomes 2/1. Mainly, amine groups on collagen units were responsible to capture Cu2+ ions by chelation, and the maximum Cu2+ adsorption capacity of the collagen/cellulose (2/1) hydrogels is 1.06 mmol/g.
In many researches, combining two or more polymers has took place for improving the adsorption properties of the hydrogels. Chitosan-cellulose-based hydrogels have been prepared by an instantaneous gelation method with carboxylated cellulose nanofibril, polyvinyl alcohol (PVA) blended chitosan, and amine-functionalized magnetite nanoparticles, for the removal of Pb2+ from aqueous solutions . Carboxylated cellulose nanofibrils (CCNFs) were used as a reinforcing material by their excellent mechanical strength, dispersion stability, and ability to form nanosize networks. Magnetite nanoparticles were introduced into the hydrogel structure for recycling manner which is promising system to clean up environmental problems. Prepared magnetite hydrogel has better Pb2+ adsorption at higher pH values (pH >5.5) due to the OH− ion concentrations in the adsorption medium. At lower pH values, hydrogel demonstrates lower adsorption because of the competition between H+ and Pb2+ ions. The metal ion adsorption capacity of the hydrogels containing CCNFs is higher than those hydrogels without CCNFs regarding carboxylate groups on the backbone of CCNFs which is playing an important role on adsorption. m-CS/PVA/CCNF hydrogels show quite higher Pb2+ adsorption by the value of 171.0 mg/g, and the adsorption capacity can stay at 90% even after four recycling processes.
Graphene oxide (GO) and cellulose were cross-linked using ECH in NaOH/urea aqueous solution to create hydrogels for the heavy metal ion adsorption applications . GO improves the mechanical strength as well as the adsorption capacity of GO/cellulose hydrogels. GO/cellulose hydrogels demonstrate 94.34 mg/g Cu2+ uptake capacity which was more than pure cellulose. The GO/cellulose hydrogel also has high adsorption capacity for Zn2+, Fe3+, and Pb2+ ions.
Different amounts of PVA and CMC were cross-linked using freeze-thaw method to have composite hydrogels for the removal of heavy metal ions from aqueous solutions . Adsorption studies of obtained porous PVA/CMC hydrogels were conducting for Ag+, Ni2+, Cu2+, and Zn2+ in noncompetitive and competitive conditions. The highest adsorption capacity was observed by P2C1 (containing two-thirds of PVA and one-third of CMC) hydrogel toward Ag+ ions. PVA/CMC hydrogels show higher selectivity to Ag+ than other heavy metal ions. Ni2+ possesses lower adsorption due to the weaker attraction between Ni2+ and functional groups of PVA/CMC hydrogels.
Acrylic acid was grafted on cellulose to obtain cross-linked C-g-AA hydrogels using N,N′-methylene bisacrylamide (MBA) as a cross-linker for Cu2+ and Ni2+ removal from aqueous solutions . The adsorption capacity of C-g-AA hydrogels increased by increasing the initial metal concentrations and the pH value of the solution. Maximum adsorption capacity of C-g-AA hydrogels was found as 182 and 200 mg/g for Cu2+ and Ni2+, respectively.
A composite hydrogel using both natural polymer sugarcane bagasse cellulose (CB) and gelatin (GT) by incorporation into the copolymer network which comprise of acrylamide (AM) and acrylic acid (AA) cross-linked with MBA . Experimental results revealed that an increase in the GT or CB content enhanced the adsorption capacity of the hydrogels. An increase in the amount of AM/AA causes a reduction in the adsorption capacity. The adsorption mechanism of the optimized hydrogel was confirmed as chemisorption of Cu2+ to the hydrogel by second-order kinetics.
Sodium carboxymethyl cellulose (CMC-Na) and sodium styrene sulfonate (SSS) were cross-linked by γ-irradiation to form a hydrogel for the removal of heavy metal ions from contaminated waters . Water uptake ability of the CMC/SSS hydrogels is higher than CMC hydrogel due to the -COOH and -SO3H groups on the polymer backbone, and also adsorption of metal ions was becoming more effective with these functional groups.
Removal of heavy metals by cellulose-based hydrogels
CMC/AMPS (CAM 6)
CMC/chitosan physical hydrogels
4.2 Removal of Dyes
Since ancient times, humanity have used pigments for artistic and decorative purposes . These pigments were obtained from plants, insects, animals, and minerals. With the development of industrial process since the nineteenth century, natural dyes left its place to synthetic dyes which have reduced the cost and facilitate to produce them .
Dyes have been extensively used (about 7 × 105 ton per year [67, 68]) in many industries including textiles, food, plastics, paper, leather, pharmaceutical, etc. Among them, textile industries are the main pollution factor for water resources which they are responsible for 20% of total water pollution in the world according to the World Bank report . Dye pollution in wastewater affects marine life negatively by inhibiting penetration of sunlight and reduces the dissolved oxygen (DO) level in water.
All dyes are toxic, carcinogenic, and mutagenic, even at low concentrations; therefore the existence of dye in the environment could cause health problems to human beings and aquatic living organisms . Moreover, dyes in wastewater have become crucial issue because they have high chemical oxygen demand, strong color, high toxicity, poor biodegradability, and persistent bioaccumulation . Due to their negative effects into the environments, dye pollution must be solved immediately.
Dye removal from wastewater can be achieved by three main process, including physical separation, chemical process, and biological degradation. Different methods such as electrocoagulation [72, 73, 74], oxidation , membrane separation [76, 77], and adsorption [78, 79, 80, 81] have been mostly used for the removal of such dyes from wastewater. Although each method has advantages and disadvantages, adsorption has been known to be mostly used in the effective process because of its low cost, effectiveness, ease of use, and environmentally friendliness .
Although activated carbon is the most used material for the removal of dyes from wastewater, however, its high costs, nonreusable, and low efficiency restrict its use as sorbents. An ideal adsorbent should have some criteria such as easily accessible, fast sorption rate, high absorption capacity, reusable, and low cost. One way to obtain good and effective sorbents for the removal of these kinds of pollutants from water is to prepare cross-linked hydrogel. Considering the abovementioned criteria, cellulose-based polymers have gaining much interest, and many efforts have been also conducted to investigate the cellulose-based materials as sorbent for the removal of dyes from wastewater.
For example, Zhang et al. have prepared an adsorbent using acrylic acid and carboxymethyl cellulose as dye removal material. The dye removal efficiency was investigated using methyl orange, disperse blue 2BLN, and green chloride and was found to be 84.2%, 79.6%, and 99.9%, respectively .
Salama et al. have synthesized a superabsorbent hydrogel using carboxymethyl cellulose and 2-(dimethylamino) ethyl methacrylate. The effect of contact time, pH of solution, and initial dye concentration on the adsorption of methyl orange onto cellulose hydrogel were studied, and the maximum adsorption capacity was reported to be 1825 mg for 1 g of superabsorbent hydrogel .
Mahdavinia et al.  have produced nanocomposite hydrogels from grafting of acrylamide onto hydroxypropyl methylcellulose (HPMC) with the incorporation of the natural sodium montmorillonite. The final nanocomposite hydrogels were used for the removal of cationic crystal violet from wastewater, and the maximum adsorption capacity of nanocomposite hydrogels is 67.2 mg/g.
Deng et al.  prepared novel high-strength and highly cost-effective hydrogel using chitosan and cellulose. The obtained material showed good elasticity, high strength, excellent resilience, and high interest toward Congo red. It was found that the saturated adsorption amount was found to be 166.10 mg/g of hydrogel. It was also found that the removal rate was ≈100% when the initial concentration was less than 100 mg/L.
A nanocomposite hydrogel containing lignocellulose-g-poly(acrylic acid), MMT has been utilized as adsorbent for the removal of methylene blue (MB) from aqueous solution. The effect of MMT content, contact time, initial concentration and pH of the dye solution, and adsorption temperature were investigated. It was found that the adsorption capacity of MB increased with the increasing contact time, initial dye concentration, and pH value but decreased with increasing MMT content and temperature. The maximum capacity of obtained hydrogels was reported as 1994.38 mg/g for methylene blue .
Juang et al.  have used cellulose-based wastes and banana and orange peels, for the adsorption of different dyes including methyl orange (MO), methylene blue (MB), rhodamine B (RB), Congo red (CR), methyl violet (MV), and amino black 10B (AB) from wastewater. It was found that the banana peel was more effective than the orange peel. The amount of adsorption for banana peel was found to be 17.2 for MO, 15.9 for MB, 13.2 for RB, 11.2 for CR, 7.9 for MV, and 7.9 mg/g for AB under the conditions tested (C0 = 100 mg/L, dosage of adsorbent 1 g/L).
Varaprasad et al.  used carboxymethyl cellulose (CMC) together with acrylamide (AM) and graphene oxide (GO) for the design of hydrogel via free-radical polymerization method to obtain cross-linked structures. The CMC-AM-GO hydrogels were used to adsorb dye from an acid blue 113 solution, and the removal capacity of hydrogels was reported as 185.45 mg/g.
Novel interpenetrating polymer networks (IPNs) were prepared by the copolymerization of cellulose, polymethacrylic acid (PMAA), and bentonite as a dye adsorbent. N,N′-methylenebisacrylamide (MBA) and potassium peroxydisulfate (K2S2O8) were also used as a cross-linker and an initiator, respectively. It was found that the hybrid hydrogels can be reusable for the removal of MB with the capacity of 371.67 mg/g .
Tam et al. investigated the preparation of hydrogel beads comprising of cellulose nanocrystals and the alginate . The effect of various parameters including adsorbent dosage, pH, temperature, ionic strength, contact time, cross-linking ratio, and bead size on the dye removal efficiency was investigated. The maximum capacity was found as 256.41 mg for a gram of hydrogel beads.
Wang et al.  developed a low-cost bio-adsorbent by mixing corn stover hemicellulose with polyethylene glycol diglycidyl ether under alkaline conditions, and also clay nanosheets have been used to prepare hybrid hydrogels. The synthesized materials were investigated for the adsorption of methylene blue. The adsorption capacity of hydrogels with and without clay was 148.8 and 95.6 mg/g, respectively. It was found that the addition of clay into the prepared material improved both mechanical strength and the adsorption capacity of hydrogels.
Liu et al.  synthesized a cellulose-based adsorbent via free-radical polymerization methods. The adsorption kinetics (pseudo-first-order, pseudo-second-order, and intraparticle diffusion models) was also studied, and the maximum adsorption capacity of sorbent was reported as 1734.816 mg/g at pH 9.
Atrei et al. reported  the preparation of magnetic hydrogels based on carboxymethyl cellulose using magnetite nanoparticles functionalized with 3-aminopropyltrimethoxysilane as a cross-linker. The developed hydrogels were investigated as adsorbents for organic and inorganic pollutions including methylene blue and cadmium chloride and found to be 620 ± 100 mg/g for methylene blue and Cd (100 ± 15 mg/g), respectively.
A novel adsorbent was prepared via cross-linking graft copolymerization of 2-dimethylamino) ethyl methacrylate (DMAEMA) onto the carboxymethyl cellulose backbone. The synthesized hydrogels are used to adsorb methyl orange from wastewater, and its capacity is 1825 mg for 1 g of hydrogels .
Adsorption capacities of the cellulose-based hydrogels
Initial concentration (mg/L)
Adsorption capacity (mg/g)
Malachite green chloride
Hemicellulose/clay hybrid hydrogels
Pineapple peel cellulose-based hydrogels
Acid blue 133
Cellulose nanocrystal-alginate hydrogel beads
Interpenetrating polymer network
Carboxymethyl cellulose hydrogels
Macroporous cellulose-based cryogels
Acid blue 93
Cellulose-based porous adsorbent
CMC-based superabsorbent hydrogel
Lignocellulose-based nanocomposite hydrogel
4.3 Removal of Other Pollutants
Cellulose-based hydrogels have been used mainly to remove toxic heavy metal and dyes from waters. Matter such as oil, pesticides, bacteria, drug, etc. are other contaminants for water resources that can cause serious water pollution and affect human health. Cellulose-based hydrogels were involved for the removal of those kinds of pollutants as well as heavy metals and dyes from waters.
Organic pollutions such as pesticides, petroleum products, and polyaromatic hydrocarbons (PAHs) have toxic, carcinogenic, and mutagenic features for human life . For this reason, the removal of such pollutants is highly essential. Adsorption is the most commonly used technique to remove different kinds of organic pollutants including PAHs, pesticides, oils, etc. [98, 99]. Despite having restricted studies of cellulose-based hydrogels for other pollutants, several scientific works are summarized in this section.
Ghaffar et al. produced carboxymethyl cellulose hydrogels by copolymerization of acrylamide and methacrylic acid onto carboxymethyl cellulose via direct radiation grafting technique . It was shown that the obtained polymeric hydrogels have interest not only toward dyes and heavy metals but also toward pesticides such as 4-chlorophenol and 2,4-dichlorophenoxy acetic acid. The adsorption capacity of carboxymethyl cellulose hydrogels for 4-chlorophenol and 2,4-dichlorophenoxy acetic acid is reported as ≈6 and ≈14 g/g, respectively.
Mattaso et al. fabricated polyacrylamide- and methylcellulose-based hydrogels as an adsorbent for the removal of pesticide paraquat from aqueous solution . The maximum adsorption capacity was found to be 14.3 mg/g for the obtained hydrogels.
Berry et al. synthesized biodegradable ethanethiol-cellulose bead hydrogels by a novel method using 1,1′-carbonyldiimidazole followed by the reaction with aminoethanethiol . The adsorption of metolachlor from aqueous solution was examined batch reaction conditions and fixed-bed column technique. The maximum adsorption capacity of cellulose beads was found to be 1300 μmol/g metolachlor.
Cellulose-based hydrogels were prepared from carboxymethyl cellulose sodium salt and β-cyclodextrin . The swelling capacities in water were 70–200 mL/g of hydrogel beads. It is also found that the prepared hydrogel beads have also interest toward bisphenol A (BPA) in water. Batch adsorption experiments were analyzed using Langmuir isotherm models, and the maximum BPA adsorption capacity of polymer hydrogels was 167 μmol g−1.
Nano-fibrillated cellulose (NFC) hydrogel was used to modify a hydrated regular cellulose filter for the separation of oil from waters . NFC was cross-linked using citric acid to add stability and strength to the coating, avoiding breakdown of the coating during filtration process and providing long lifetime for filter. Hydrated regular cellulose is modified by dipping and drying process using NFC which creates a hydration layer over regular cellulose by absorbing water. Oleophobic behavior of modified cellulose filter was improved by triple layer of hydrogel, water, and oil. The high difference surface energy between water and oil keeps the oil away from penetrating through filter. Without the modification of filter by NFC hydrogels, both water and oil could penetrate through filter and cause fouling and clogging. NFC hydrogel creates a hydrated layer when it reaches maximum swelling degree and provides a big surface energy difference between oil and filter. Modified filter prevents the oil clogging inside filter by the help of NFC hydrogels and permits the flow of water while repels the oil penetration. Cellulose-based hydrogel was used in a nanoscopic scale to improve an eco-friendly water/oil separation method.
This chapter summarizes the recent progress in the application of cellulose-based hydrogels for water treatments. With the increasing industrializations, population growth, urbanization, and technological improvements worldwide, water pollution from heavy metals, dyes, oils, and other organic contaminants has become a significant problem for human life and animals even at very low concentrations. Aquatic pollutants such as heavy metals, dyes, oils, etc. are very dangerous pollutants which once enter the water make water no longer safe for drinking purposes, and removal of organic pollution from wastewater is very important from the point of view of the environment and health. Various approaches such as membrane separation, oxidation, reverse osmosis, etc. have been utilized for the removal of aquatic pollutions. Among them, adsorption has gained importance for the removal of such pollutants from wastewater due to their fascinating properties such as low cost, reusability, and high and quick adsorption abilities.
Cellulose-based hydrogels have many favorable properties such as hydrophilicity, biodegradability, biocompatibility, thermal and chemical stability for water treatment applications, low cost, and nontoxicity. Therefore, cellulose-based hydrogels have wide application in many areas including sensors, tissue engineering, drug delivery, agriculture, and water purification. Several reports have indicated that cellulose-based hydrogels are considered to be an ideal material for the removal of different inorganic or organic pollutants such as heavy metals, organic solvents/oils, antibiotics, dyes, and other contaminants. With ongoing research in cellulose-based hydrogels, the properties of cellulose-based materials will be much better and applicable than in current form, and cellulose which is environmentally friendly and low cost will replace with the petroleum-based materials.
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