Cellulose-Based Hydrogels for Wastewater Treatment: A Focus on Metal Ions Removal

Citation: Persano, F.; Malitesta, C.; Mazzotta, E. Cellulose-Based Hydrogels for Wastewater Treatment: A Focus on Metal Ions Removal. Polymers 2024,16, 1292. https:// doi.org/10.3390/polym16091292 Academic Editors: Ana-Mihaela Gavrila, Mariana Ionita and Gabriela Toader Received: 9 April 2024 Revised: 24 April 2024 Accepted: 26 April 2024 Published: 5 May 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Review Cellulose-Based Hydrogels for Wastewater Treatment: A Focus on Metal Ions Removal Francesca Persano , Cosimino Malitesta and Elisabetta Mazzotta * Laboratory of Analytical Chemistry, Department of Biological and Environmental Sciences and Technologies (Di.S.Te.B.A.), University of Salento, via Monteroni, 73100 Lecce, Italy; [email protected] (F.P.); [email protected] (C.M.) *Correspondence: [email protected] Abstract: The rapid worldwide industrial growth in recent years has made water contamination by heavy metals a problem that requires an immediate solution. Several strategies have been proposed for the decontamination of wastewater in terms of heavy metal ions. Among these, methods utilizing adsorbent materials are preferred due to their cost-effectiveness, simplicity, effectiveness, and scalability for treating large volumes of contaminated water. In this context, heavy metal removal by hydrogels based on naturally occurring polymers is an attractive approach for industrial wastewater remediation as they offer significant advantages, such as an optimal safety profile, good biodegradability, and simple and low-cost procedures for their preparation. Hydrogels have the ability to absorb significant volumes of water, allowing for the effective removal of the dissolved pollutants. Furthermore, they can undergo surface chemical modifications which can further improve their ability to retain different environmental pollutants. This review aims to summarize recent advances in the application of hydrogels in the treatment of heavy metal-contaminated wastewater, particularly focusing on hydrogels based on cellulose and cellulose derivatives. The reported studies highlight how the adsorption properties of these materials can be widely modified, with a wide range of adsorption capacity for different heavy metal ions varying between 2.3 and 2240 mg/g. The possibility of developing new hydrogels with improved sorption performances is also discussed in the review, with the aim of improving their effective application in real scenarios, indicating future directions in the field. Keywords: cellulose-based hydrogels; heavy metal adsorption; wastewater treatments; adsorption capacity; metal ion removal 1. Introduction Growing industrial activity together with the reduced availability of drinking water sources have made water pollution a serious problem that requires new effective solutions. The industry is responsible for approximately 20% of global water consumption, continuously releasing various pollutants into water systems, thus resulting in irreparable damage to ecosystems. Contaminants, including heavy metals, are continuously released into the environment through wastewater resulting from human and industrial activities [ 1 ]. The presence of heavy metal ions in wastewater inevitably leads to their entry into the surrounding environment and ecological cycles, being subsequently absorbed by animals, microorganisms, and plants. Heavy metal ions, such those of lead, copper, arsenic, cadmium, and nickel, tend to accumulate in living organisms and, due to their toxicity, high permeability, and lack of degradability, lead to a series of physiological disorders, causing alterations in development, constituting a serious threat to human health [ 2 – 6 ]. In fact, evidence accumulated over the years links heavy metals to various diseases that afflict the human population, such as cancer, anemia, and hypertension, as well as damage to the nervous system and kidney Polymers 2024,16, 1292. https://doi.org/10.3390/polym16091292 https://www.mdpi.com/journal/polymers Polymers 2024,16, 1292 2 of 29 failure. It follows that effective removal of heavy metal ions from wastewater before their release into water bodies would bring benefits to both human health and the environment itself [7,8]. Furthermore, although the earth is covered by 70% water, 97% is salty and only 3% is available for human use. The limited availability of water for human use, along with the large volumes of contaminated water resulting from various industrial activities highlight the need for advanced treatments in the decontamination of wastewater. Most conventional strategies used for wastewater treatment, such as chemical precipitation, membrane separation, ion exchange, and electrochemical treatments, present limitations which make their scaling up difficult, thus discouraging their application in real contexts in which large water volumes have to be treated, preferably with simple and low-cost procedures based on reusable materials, allowing for reductions in the required time and costs [9–13]. Chemical precipitation, for example, despite being an economical method, requires additional costs for the disposal of the large quantities of sludge produced during the purification process itself [ 14 ]. Membrane separation is a pressure treatment method whose efficacy strongly depends on the selection of the membrane, and large-scale application is complicated. This process is based on the principle of selective permeability of membranes, which allows for the selective passage of certain components while hindering or preventing the passage of other components. Some membranes may not be able to selectively separate all types of heavy metals present in water. Ion exchange involves the use of solid surfaces that act as ion exchangers and are characterized by high selectivity toward certain ionic species, meaning that only certain types of ions can be adsorbed [ 15 , 16 ]. This can be a disadvantage as multiple steps or different materials are required to effectively treat a mixture of heavy metals, making the process more complex and expensive. Electrochemical treatments, including electrodeposition, electrocoagulation, electroflotation and electrooxidation, have the advantage of possessing a short process time and a simple operation, with the possibility of fully automation with no additional chemicals required. Nonetheless, the side reaction of hydrogen generation and oxygen generation negatively affecting the efficiency of the process have to be taken into account in electrodeposition, as the high energy consumption affects mainly the electrocoagulation process, while most of the processes suffer from electrode passivation, resulting in a subsequent need of periodic replacement [17,18]. In contrast, adsorption-based strategies represent simple, low-cost, easy to implement, and effective methods in the removal of heavy metal ions, which justify the growing research interest toward this approach during recent years. In the adsorption method, the treated aqueous matrix is simply stirred in the presence of an adsorbent able to bind to contaminants due to the presence of surface functional groups acting as active sites, until the saturation of the binding sites is reached [ 19 ]. To achieve an efficient and convenient adsorption process, desired adsorbent properties are a large surface area, numerous active sites, biodegradability, and easy assembly and availability. Traditional adsorbents, such as artificial resin, activated carbon, zeolite, diatomaceous earth, alumina, and silica gel, although widely used in the last few years, are affected by reduced adsorption capacity and poor regeneration performance [20,21]. In this context, porous adsorbents with a three-dimensional structure and large surface area enabling the exposure of numerous active sites are recently emerging, due to their greater adsorption capacity and reduced adsorption time achieved by rapid diffusion kinetics throughout the adsorbent. The efforts of increasing the number of surface functional groups and controlling the surface area for improving the adsorption process have led to the design of tailored porous materials such as metal organic frameworks (MOFs) or covalent organic frameworks (COFs), although their application in water treatment still requires further studies to increase their stability and decrease production costs, along with a conducting critical evaluation of their toxicity. Polymers 2024,16, 1292 3 of 29 Among porous adsorbent materials, hydrogels occupy a key role, combining a high adsorption capacity with reusability, biodegradability, and non-toxicity, reducing environmental impact and promoting sustainable practices in water treatment. Hydrogels are soft, elastic materials made of three-dimensional polymer networks with the extraordinary ability to store large amounts of water (up to hundreds of times with respect to their dry weight) thanks to their hydrophilic polymer structure, all features explaining their potential and their increasing applications as adsorbents for water remediation [ 22 , 23 ]. Among the wide panorama of polymers used for hydrogel development, cellulose—a well-known polysaccharide made up of glucose units held together by ß-1,4-glucosidic bonds—occupy a prominent role [ 24 , 25 ], being the most abundant natural polymer on earth (the main component of plant cell walls) with high biocompatibility, biodegradability, low cost, and controllable properties. Related to this, cellulose, its derivatives, and nanocellulose-based hydrogels have been conveniently exploited in a plethora of uses ranging from the biomedical field (administration of therapeutic agents, in tissue engineering, implant materials, and wound dressings) to energy and environmental applications [ 26 – 28 ]. Specifically, the presence of numerous surface functional groups capable of coordinating metal ions in water, together with the possibility of easily obtaining an interconnected porous structure, has promoted the application of cellulose and its derivatives in the production of hydrogels for water decontamination in terms of heavy metals [29,30]. The present review focuses on the main developments in the field, presenting first the main manufacturing methods of cellulose-based hydrogels, then describing the mechanisms involved in heavy metal adsorption processes, and highlighting some recent selected applications. Prospective future trends in the field are also discussed, along with indications of possible directions to take to bridge the gap between laboratory research and practical application and fully exploit the effectiveness of cellulose-based hydrogels and their derivatives in the removal of metal ions from water. 2. Synthesis of Celluloseand Cellulose Derivative-Based Hydrogels 2.1. Structure of Cellulose and Its Derivatives Cellulose hydrogels can be developed from a pure cellulose solution through physical cross-linking thanks to the presence of a many hydroxyl groups (–OH) that allow the polymer network to be connected by hydrogen bonding. The –OH groups can be functionalized with various chemical modifications [ 31 – 39 ], often consisting of partial or total etherification, through reactions with organic species, such as ethyl and methyl units [ 40 ]. Cellulose ethers, based on their substituents, can be classified as single ethers or mixed ethers. Single cellulose ethers involve the substitution of a single functional group in the cellulose unit, while mixed ethers involve the substitution of more than one functional group. Individual ethers include alkyl ethers such as methyl cellulose (MC) and hydroxyalkyl ethers such as hydroxypropyl cellulose, while mixed ethers are mainly represented by carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), and hydroxypropyl methyl cellulose [41,42] (Figure 1). Cellulose ether can be soluble in water depending on the chemical structure of the substituent as well as the type and degree of substitution. Typically, water-soluble cellulose ethers are characterized by a degree of substitution between 0.4 and 2 [ 43 ]. MC is the simplest alkyl ether, soluble in various organic solvents and in water depending on the degree of substitution. Specifically, for a degree of substitution between 1.4 and 2.0, MC is soluble in water [ 44 – 46 ]. Interestingly, MC has thermo-gelling properties which can be effectively exploited in promoting temperature-controlled hydrogel behavior [ 47 ]. CMC is characterized by hydrophilicity, stability in water, elevated chemical stability, biocompatibility, and biodegradability. However, CMC is not soluble in some organic solvents such as ethanol [ 48 ]. HEC turns out to be soluble in water and various organic solvents. In addition, HEC can be blended with cellulose, allowing cellulose to be processed without the use of solvents [49]. Polymers 2024,16, 1292 4 of 29 Polymers 2024, 16, x FOR PEER REVIEW 4 of 31 solvents. In addition, HEC can be blended with cellulose, allowing cellulose to be processed without the use of solvents [49]. One fascinating feature of cellulose-based hydrogels is their biodegradability, being degradable by fungi and bacteria present in water, air, and soil, contrarily to acrylatebased hydrogels, typically used at industrial level, which show slow degradation, are nonbiodegradable, and are therefore considered potential pollutants of water and soil. Storing cellulose in soil for 10 weeks leads to its degradation by 82–95%, depending on the type of soil [50,51]. When applied in decontamination processes, such a feature represents an additional benefit allowing us to reduce the environmental impact of residual materials and thus improving the environmental sustainability of the whole process. Figure 1. General chemical structure of ether cellulose. Functional groups (R) of the main cellulose ethers are reported. The approaches for preparing cellulose-based hydrogels can be essentially divided into two types, namely, physical and chemical cross-linking methods [30,52]. 2.2. Physically Cross-linked Hydrogels Physically cross-linked hydrogels are reversible hydrogels, assembled through the interweaving of polymer chains by means of intraand intermolecular hydrogen bonding, ionic bonding, hydrophobic interaction, and non-permanent forces, such as electrostatic and gravitational forces [53,54]. They are synthesized without the use of chemical crosslinking agents, which, being toxic in most cases, should be completely removed from the system before use. Therefore, the resulting hydrogels possess increased biocompatibility and reduced or even no toxicity [55,56]. One of the most widely used methods of physical cross-linking for the production of cellulose-based hydrogels with elevated absorbency is the cryogenic method (repeated freeze–thaw cycles) [57]. This process leads to the formation of hydrogels due to phase separation in the polymer solution after repeated freezing and thawing cycles. Thus, the formation of polymer crystallites occurs when polymer solutions are exposed to low temperatures, creating insoluble polymer networks. Furthermore, freeze–thaw cycles allow a porous structure to be created in hydrogels thanks to the space left by ice crystals Figure 1. General chemical structure of ether cellulose. Functional groups (R) of the main cellulose ethers are reported. One fascinating feature of cellulose-based hydrogels is their biodegradability, being degradable by fungi and bacteria present in water, air, and soil, contrarily to acrylatebased hydrogels, typically used at industrial level, which show slow degradation, are non-biodegradable, and are therefore considered potential pollutants of water and soil. Storing cellulose in soil for 10 weeks leads to its degradation by 82–95%, depending on the type of soil [ 50 , 51 ]. When applied in decontamination processes, such a feature represents an additional benefit allowing us to reduce the environmental impact of residual materials and thus improving the environmental sustainability of the whole process. The approaches for preparing cellulose-based hydrogels can be essentially divided into two types, namely, physical and chemical cross-linking methods [30,52]. 2.2. Physically Cross-Linked Hydrogels Physically cross-linked hydrogels are reversible hydrogels, assembled through the interweaving of polymer chains by means of intraand intermolecular hydrogen bonding, ionic bonding, hydrophobic interaction, and non-permanent forces, such as electrostatic and gravitational forces [ 53 , 54 ]. They are synthesized without the use of chemical crosslinking agents, which, being toxic in most cases, should be completely removed from the system before use. Therefore, the resulting hydrogels possess increased biocompatibility and reduced or even no toxicity [55,56]. One of the most widely used methods of physical cross-linking for the production of cellulose-based hydrogels with elevated absorbency is the cryogenic method (repeated freeze–thaw cycles) [ 57 ]. This process leads to the formation of hydrogels due to phase separation in the polymer solution after repeated freezing and thawing cycles. Thus, the formation of polymer crystallites occurs when polymer solutions are exposed to low temperatures, creating insoluble polymer networks. Furthermore, freeze–thaw cycles allow a porous structure to be created in hydrogels thanks to the space left by ice crystals that melt during the thawing phase. In a study conducted by Guan et al. [ 58 ], a hemicellulosebased hydrogel with remarkable mechanical properties was developed through repeated freeze/thaw cycles, which led to the formation of a system with a compact structure characterized by high thermal stability and mechanical resistance. The maximum compressive strength was reached after nine freeze/thaw cycles, highlighting the packing of the phys- Polymers 2024,16, 1292 5 of 29 ically cross-linked polymer chains induced by repeated freeze/thaw cycles. The group of Wang et al. applied the cryogenic approach to the synthesis of a composite hydrogel consisting of poly(vinyl alcohol) (PVA) and carboxy methyl cellulose (CMC) (Figure 2). Specifically, a mixed aqueous solution of PVA and CMC was frozen at − 20 ◦ C for 6 h and thawed for 1 h at room temperature, for five freeze/thaw cycles [ 57 ]. Freezing the polymer solutions caused crystals to form, generating insoluble hydrogels. The physical properties and degree of swelling of the gels were clearly influenced by the proportions of PVA and CMC. Pure PVA hydrogels showed a swelling degree of 416%, while those with 1:2 PVA/CMC ratio reached 1437%. The results indicate that an increase in CMC content led to a reduction in gel fraction and an increase in the degree of swelling for PVA/CMC hydrogels with different CMC proportions. This is due to the increasing hydrophilicity of CMC and a lower cross-linking density of the hydrogel as the amount of PVA decreases. Cross-section SEM images of the prepared hydrogels highlight that all PVA/CMC hydrogels exhibited a spongy structure with high porosity (Figure 2). Polymers 2024, 16, x FOR PEER REVIEW 5 of 31 that melt during the thawing phase. In a study conducted by Guan et al. [58], a hemicellulose-based hydrogel with remarkable mechanical properties was developed through repeated freeze/thaw cycles, which led to the formation of a system with a compact structure characterized by high thermal stability and mechanical resistance. The maximum compressive strength was reached after nine freeze/thaw cycles, highlighting the packing of the physically cross-linked polymer chains induced by repeated freeze/thaw cycles. The group of Wang et al. applied the cryogenic approach to the synthesis of a composite hydrogel consisting of poly(vinyl alcohol) (PVA) and carboxy methyl cellulose (CMC) (Figure 2). Specifically, a mixed aqueous solution of PVA and CMC was frozen at −20 °C for 6 h and thawed for 1 h at room temperature, for five freeze/thaw cycles [57]. Freezing the polymer solutions caused crystals to form, generating insoluble hydrogels. The physical properties and degree of swelling of the gels were clearly influenced by the proportions of PVA and CMC. Pure PVA hydrogels showed a swelling degree of 416%, while those with 1:2 PVA/CMC ratio reached 1437%. The results indicate that an increase in CMC content led to a reduction in gel fraction and an increase in the degree of swelling for PVA/CMC hydrogels with different CMC proportions. This is due to the increasing hydrophilicity of CMC and a lower cross-linking density of the hydrogel as the amount of PVA decreases. Cross-section SEM images of the prepared hydrogels highlight that all PVA/CMC hydrogels exhibited a spongy structure with high porosity (Figure 2). Figure 2. SEM images of the cross-section of the different PVA/CMC composite formulations: (A) PVA (1:0, PVA/CMC), (B) P2C1 (2:1, PVA/CMC), (C) P1C1 (1:1, PVA/CMC), and (D) P1C2 (1:2, PVA/CMC) (scale bar: 100 µm), from ref. [57]. The main advantages of hydrogels produced through physical cross-linking over chemically cross-linked ones include ease of fabrication, biodegradability, biocompatibility, reduced toxicity, post-process mass modification, and remodeling. In contrast, in physical cross-linking processes, weak interactions between polymer chains, Figure 2. SEM images of the cross-section of the different PVA/CMC composite formulations: (A) PVA (1:0, PVA/CMC), (B) P2C1 (2:1, PVA/CMC), (C) P1C1 (1:1, PVA/CMC), and (D) P1C2 (1:2, PVA/CMC) (scale bar: 100 µm), from ref. [57]. The main advantages of hydrogels produced through physical cross-linking over chemically cross-linked ones include ease of fabrication, biodegradability, biocompatibility, reduced toxicity, post-process mass modification, and remodeling. In contrast, in physical cross-linking processes, weak interactions between polymer chains, such as hydrogen bonds or physical interactions, allow the hydrogel structure to be formed more easily. Furthermore, physical cross-linking makes hydrogels more biodegradable, as the weak interactions within the polymer network are more susceptible to biodegradation compared to covalent bonds [ 59 ]. On the other hand, physically cross-linked hydrogels usually suffer Polymers 2024,16, 1292 6 of 29 from fast dissolution and reduced stability due to the lack of chemical bonding within the hydrogel network [60]. However, in a recent work, the group of Zhang et al. [ 61 ] proposed the synthesis of a physically cross-linked CMC-Fe 3+ /polyacrylamide dual-network hydrogel via a twostep process. In particular, Fe 3+ cross-linked CMC forms the first network, while the hydrophobic polyacrylamide forms the second network to preserve the integrity of the resulting hydrogel. This system showed excellent mechanical properties. The mechanical properties were evaluated by tensile and compressive tests. Compared with the physically cross-linked single CMC/polyacrylamide hydrogel, the elastic modulus and toughness of the CMC-Fe3+/polyacrylamide hydrogel increased by 420% and 94%, respectively. 2.3. Chemically Cross-Linked Hydrogels Chemical cross-linking involves the formation of covalent bonds between the crosslinking agents and the polymer chains which impart improved stability and great mechanical strength to the resulting hydrogels [ 62 ]. Chemically cross-linked systems do not melt or disintegrate if treated with heat, and do not dissolve in water without first breaking the covalent bonds. For this reason, they are called perpetual systems [ 63 ]. Chemical cross-linking can occur through esterification, radical polymerization, Michael addition, etc. In the case of cellulose and its derivatives, commonly used cross-linking agents are citric acid, succinic acid, epichlorohydrin, polyethylene glycol diacrylate, divinyl sulfone, and glutaraldehyde (EGDE) [30,64]. The group of Zhao et al. prepared a modified cellulose-based hydrogel through an innovative method of simultaneous cross-linking and grafting of acrylamide and acrylic acid (Figure 3) [ 65 ]. The synthesis was carried out by adding ammonium persulfate to an aqueous solution of microcrystalline cellulose to generate hydroxyl radicals under magnetic stirring at 50 ◦ C. Subsequently, a mixed solution of acrylic acid (AA), acrylamide (AM), and N,N ′ -methylene bisacrylamide (MBA) was slowly added, continuing stirring at 50 ◦ C for 2 h until complete polymerization. The physicochemical characterization of the hydrogels successfully confirmed the efficient grafting of acrylate and acrylamide monomers onto the initial cellulosic material. Scanning electron microscopy (SEM) analysis revealed a rough and irregular porous structure. The thermal characteristics of cellulose-based hydrogels were examined through thermogravimetric analysis (TGA). While the unmodified hydrogel showed a high decomposition rate in the temperature range from 240 ◦ C to 360 ◦ C, for the modified hydrogel, the main decomposition stage was significantly prolonged and the rate was reduced. The good thermal stability observed represents a positive element for practical applicability, since it gives them greater resistance to temperature variations. Citric acid (CA), widely used in the food and pharmaceutical sectors, is a highly efficient cross-linking agent. Present in nature, such as in lemon juice where it constitutes approximately 5% of the composition, CA is produced on a commercial scale through the fungal fermentation of glucose. Numerous studies have employed CA as a crosslinking agent in various cellulose-based hydrogels as, compared to other cross-linking agents, CA offers the advantage of being inexpensive and non-toxic [66–69]. For example, Coma et al. [ 66 ] conducted cross-linking of hydroxypropyl methylcellulose with citric acid (CA) in order to reduce the water sensitivity of packaging materials based on watersoluble cellulose derivatives and, above all, to avoid partial solubilization of films in food products. This was achieved by attaching the carboxylic acid moiety to the hydroxypropyl methylcellulose polymer via an esterification reaction of the first cyclic anhydride. Such a reaction exposes a new carboxylic acid unit in the CA, which has the chemical connectivity necessary to form a new intramolecular anhydride moiety with the adjacent carboxylic acid unit. Subsequently, further reaction with cellulose ether led to cross-linking. This process resulted in limiting the affinity of the films with water, since the –OH groups of hydroxypropyl methyl cellulose are involved in the ester bonds. The study demonstrated that cross-linking offers significant benefits in reducing the water solubility of ether cellulose films while simultaneously improving moisture barrier properties by approximately 34%. Polymers 2024,16, 1292 7 of 29 Demitri et al. [ 68 ] synthesized a biodegradable HEC/CMC hydrogel using CA as a crosslinking agent. Their results highlighted that the concentration of CA and the reaction time influence the degree of swelling of cellulose-based hydrogels. In particular, the hydrogels showed a high degree of swelling at low concentrations of CA (1.75%, 2.75%, and 3.75% by weight polymer); a swelling degree of 900 was achieved in the presence of 3.75% CA (Figure 4A), while CA concentrations lower than 1.75% resulted in weak cross-linking associated with insufficient mechanical properties. On the other hand, higher CA concentrations (10% and 20% by weight polymer) produced hydrogels with a low degree of swelling (Figure 4B), indicating high cross-linking. It was also observed that this type of hydrogel, after swelling, had good rigidity and retained the original shape of the tub of synthesis. Subsequently, CA as a cross-linking agent was successfully used in another study for the synthesis of CMC and HEC hydrogels. In particular, chemical treatment with CA induced –COOH functional groups, with an increase in hydrophilicity and surface roughness of the resulting materials [ 69 ], as revealed by contact angle and Atomic Force Microscopy (AFM) measurements. The results indicated a significant decrease in contact angle, approximately 50% and 67%, after cross-linking between HEC/HEC-CA and CMC/CMC-CA hydrogels, respectively. Furthermore, AFM profiles evidenced that the addition of CA resulted in an increase in roughness, measured by root mean square (RMS), with average values of 31.82, 39.72, 9.98, and 16.56 nm for CMC, CMC-CA, HEC, and HEC-CA, respectively. Yan et al. produced a hydrogel based on hydroxypropyl cellulose cross-linked with epichlorohydrin and ammonia in an aqueous sodium hydroxide solution [ 70 ]. A temperature-induced phase separation (TIPS) process was applied to synthesize a microporous cross-linked hydrogel which involves the creation of a gel-like structure through the phase transition of the polymer. Typically, the process starts with a solution containing hydroxypropyl cellulose in water or a compatible solvent. Next, the system is heated or cooled to a specific temperature, often above or below the critical solution point of the polymer. This temperature change induces phase separation, leading to the formation of a three-dimensional network of interconnected polymer chains which trap water or the solvent within the gelatinous structure. Epichlorohydrin as a cross-linking agent was also used for the synthesis of a new superabsorbent system based on native cellulose and quaternized cellulose (cellulose that has been chemically modified by adding quaternary ammonium groups to its structure) in a NaOH/urea aqueous solution to overcome the limitations of reduced biocompatibility, poor mechanical properties, and a lack of antimicrobial activity associated with the superabsorbent hydrogel normally used in the production of disposable diapers [ 71 ]. Increasing the density of the cross-linking network using epichlorohydrin led to an improvement in the mechanical properties of the hydrogel. In addition, the introduction of quaternized cellulose within the hydrogel networks not only improved the swelling ratio due to the electrostatic repulsion of the quaternary ammonium groups, but also endowed the hydrogel with antimicrobial properties. The presence of antimicrobial activity was due to the attraction of the anionic moieties of the microbial membranes toward the internal pores of the polycationic hydrogel, which caused its rupture. In this regard, cellulose-based hydrogels with interesting antibacterial properties (for Gram-positive bacteria) were developed in a very recent study, in which borax (sodium tetraborate) was used as a cross-linking agent [ 72 ]. Borax is commonly used as a nontoxic food additive, being cheap and soluble in water. Cellulose-based hydrogels were synthesized using cellulose extracted from water hyacinth via a dissolution process in the presence of urea and sodium hydroxide, with borax employed to generate cross-linking between the –OH groups of the cellulose molecules (Figure 5). The use of borax gave the cellulose-based hydrogel products extraordinary superabsorbent properties, with a swelling ratio of 325% for the non-cross-linked hydrogels and a swelling ratio of approximately 900% for the cross-linked ones. Polymers 2024,16, 1292 8 of 29 Polymers 2024, 16, x FOR PEER REVIEW 7 of 31 Figure 3. A schematic of the synthesis reaction of bioadsorbents from the modified cellulose through simultaneous cross-linking and grafting of both acrylamide and acrylic acid, from ref. [65]. Aam: acrylamide; AA: acrylic acid; MBA: N,N’-methylene bisacrylamide. Citric acid (CA), widely used in the food and pharmaceutical sectors, is a highly efficient cross-linking agent. Present in nature, such as in lemon juice where it constitutes approximately 5% of the composition, CA is produced on a commercial scale through the fungal fermentation of glucose. Numerous studies have employed CA as a cross-linking agent in various cellulose-based hydrogels as, compared to other cross-linking agents, CA offers the advantage of being inexpensive and non-toxic [66–69]. For example, Coma et al. [66] conducted cross-linking of hydroxypropyl methylcellulose with citric acid (CA) in order to reduce the water sensitivity of packaging materials based on water-soluble cellulose derivatives and, above all, to avoid partial solubilization of films in food products. This was achieved by attaching the carboxylic acid moiety to the hydroxypropyl methylcellulose polymer via an esterification reaction of the first cyclic anhydride. Such a Figure 3. A schematic of the synthesis reaction of bioadsorbents from the modified cellulose through simultaneous cross-linking and grafting of both acrylamide and acrylic acid, from ref. [ 65 ]. Aam: acrylamide; AA: acrylic acid; MBA: N,N′-methylene bisacrylamide. Polymers 2024,16, 1292 9 of 29 Polymers 2024, 16, x FOR PEER REVIEW 9 of 31 Figure 4. Swelling ratio versus reaction time of CMC/HEC (weight ratio 3:1) hydrogels with 1.75%, 2.75%, and 3.75% (w/w) of CA cross-linker concentration (A). Swelling ratio versus reaction time of CMC/HEC (weight ratio 3:1) hydrogels with 10% and 20% (w/w) of CA cross-linker concentration (B), from ref. [68]. In this regard, cellulose-based hydrogels with interesting antibacterial properties (for Gram-positive bacteria) were developed in a very recent study, in which borax (sodium tetraborate) was used as a cross-linking agent [72]. Borax is commonly used as a non-toxic food additive, being cheap and soluble in water. Cellulose-based hydrogels were synthesized using cellulose extracted from water hyacinth via a dissolution process in the presence of urea and sodium hydroxide, with borax employed to generate cross-linking between the ─OH groups of the cellulose molecules (Figure 5). The use of borax gave the cellulose-based hydrogel products extraordinary superabsorbent properties, with a swelling ratio of 325% for the non-cross-linked hydrogels and a swelling ratio of approximately 900% for the cross-linked ones. Figure 4. Swelling ratio versus reaction time of CMC/HEC (weight ratio 3:1) hydrogels with 1.75%, 2.75%, and 3.75% (w/w) of CA cross-linker concentration (A). Swelling ratio versus reaction time of CMC/HEC (weight ratio 3:1) hydrogels with 10% and 20% (w/w) of CA cross-linker concentration (B), from ref. [68]. Polymers 2024, 16, x FOR PEER REVIEW 10 of 31 Figure 5. A schematic representation of the cross-linking mechanism between the hydroxyl groups of the cellulose chains and borax as a cross-linking agent, from ref. [72]. 3. Hydrogels Based on Cellulose and Cellulose Derivatives for Water Decontamination of Heavy Metals In recent years, gels based on cellulose and its derivatives have increasingly attracted research interest for their possible application as heavy metal absorbents in wastewater decontamination processes, thanks to several advantageous characteristics such as a porous structure, numerous functional groups, and high absorption capacity [73–81]. Hydrogels, similar to ion exchange resins, can subtract contaminants from water bodies thanks to the presence of functional groups, including carboxyl, hydroxyl, and amino groups [82–88], which can establish different interactions with metal ions [89]. In addition, through the modification of the functional groups, the properties of the hydrogel can be suitably modified to further improve its interaction ability with metal ions, for example introducing chelating functional groups or opportunely charged moieties able to bind to metal ions through electrostatic interaction. Furthermore, the unique threedimensional structure of the hydrogel allows for a uniform distribution of metal ions throughout the system, preventing aggregation and oxidation [62]. Indeed, metal ions uniformly distributed within the hydrogel matrix are less susceptible to interaction with oxygen or other oxidizing chemical species present in the surrounding environment, helping to prevent their oxidation. If metal ions are oxidized during the removal process, they may lose their ability to be effectively captured by adsorbents, thus reducing the overall treatment efficiency. Also, the water present in the hydrogel plays a role in this sense, not only providing support to the hydrogel itself, allowing it to maintain a certain shape, but also ensuring the presence of nanotransport channels for the free diffusion of metal ions and participating in the formation of hydrogen bonds, thus constituting additional active sites for the interaction with the target ionic species [90]. In cellulose-based hydrogels, electrostatic attraction usually occurs between unprotonated carboxyl groups (–COO−) and metal cations [79,80]. Other mechanisms, such as complexation and microprecipitation, have been suggested for the absorptive removal of heavy metals by using cellulose-based hydrogels. Complexation can occur between the lone pair of O electrons and metal ions. Under some circumstances, for example when the temperature, pH, or concentration of other substances vary, the stability of the metal complex may decrease, leading to metal recrystallization as solid particles with the formation of small solid particles near the metal complexation sites (microprecipitation) [81,91]. In the next sections, brief descriptions will be provided on the adsorption kinetics and isotherms that govern the adsorption processes of heavy metal ions by hydrogels based on cellulose and its derivatives. Finally, a broad overview of the application of such materials as adsorbents for heavy metal ions, both in non-composite and composite systems, will be presented. Figure 5. A schematic representation of the cross-linking mechanism between the hydroxyl groups of the cellulose chains and borax as a cross-linking agent, from ref. [72]. Polymers 2024,16, 1292 16 of 29 initial concentration of the metal ions in the aqueous solutions, revealing an adsorption capacity of 90 mg/g, 69 mg/g, and 60 mg/g for Cu2+, Cr6+, and As5+, respectively. Hydroxyalkyl ether CMC has also found widespread use as a composite hydrogel in wastewater decontamination systems for removing heavy metal ions. The addition of polyacrylamide (PAM) increased the ability of the CMC-based hydrogels to capture heavy metals, such as Cu 2+ , Pb 2+ , and Cd 2+ , by 2.3, 3.6, and 1.5 times compared to the pristine CMC hydrogel, due to the presence of –C=O and –NH2active in PAM [94]. Baiya et al. proposed the assembly of a CMC-based hydrogel (25%) mixed with PVA, employing glutaraldehyde as a cross-linking agent for the selective adsorption of Cu 2+ ions [ 109 ]. Cellulose for CMC production was extracted from sugarcane bagasse, a waste product deriving from the crushing and pressing of sugar cane, with the aim of recycling waste materials in an eco-sustainable approach. To minimize the synthesis time and improve the reaction efficiency, the hydrogel was developed through a microwaveassisted irradiation process. The composite hydrogel revealed an adsorption capacity for Cu2+ at room temperature and pH 5.0 of 2.3 mg/g. An excellent adsorbent material for the selective removal of Cr 6+ ions was developed with the addition of the polyethylenimine (PEI) polymer to CMC, characterized by a notable quantity of imine groups [ 110 ]. The CMC/PEI composite hydrogel was produced with a simple one-step procedure, consisting of mixing an aqueous solution of CMC with an aqueous solution of PEI, to which the cross-linking agent epichlorohydrin was added. The protonated imine groups (at a slightly acidic pH) are able to adsorb Cr 6+ ions through electrostatic interaction, while the oxidizing hydroxyl groups of CMC act by reducing Cr 6+ ions to Cr 3+ . The in situ produced Cr 3+ is immobilized through coordination with N and O and then removed through precipitation. The CMC/PEI composite hydrogel can be thus regenerated and reused, exhibiting a Cr 6+ adsorption capacity higher than 2.0 mmol/g even after four cycles. A composite hydrogel was assembled for the decontamination of aqueous matrices by removing cadmium and nickel based on CMC, microcrystalline cellulose (MCC), and xylan, using ethylene glycol diglycidyl ether as the cross-linking agent [ 111 ]. The reported maximum adsorption capacity was 61.44 for Cd 2+ and 55.85 mg/g for Ni 2+ . The adsorption of metal ions can occur at the level of the carboxyl groups of the CMC, or even at the level of the hydroxyl groups of the MCC and xylan through ionic bonding. Batch adsorption tests of heavy metal ions from an aqueous solution were carried out by the group of Astrini et al., using a CMC-graft-poly(acrylic acid)/montmorillonite composite hydrogel as the adsorbent [ 112 ]. Clay minerals such as montmorillonite are introduced into the lattice structure of the polymer hydrogel in order to improve its mechanical properties. Their inclusion is motivated by their abundance, chemical and mechanical stability, and low cost, which makes them an attractive cellulose immobilization material. The impact of various parameters (such as pH and the presence of competing ions) on the adsorption capacity of the composite system was evaluated. The obtained results confirmed that the adsorption efficiency of metal ions by the composite hydrogel is significantly influenced by pH, with the maximum adsorption at pH 5.0 (Figure 7A). At this pH value, an adsorption capacity for the composite systems of 146.19 mg/g and 286.67 mg/g was obtained for Pb 2+ and Zn 2+ ions, respectively. Fourier transform infrared spectroscopy (FTIR) spectra before and after adsorption demonstrated that the complex formation between metal ions and the –COOH groups of the hydrogel is the main adsorption mechanism of heavy metal ions. Furthermore, competitive adsorption tests revealed that the adsorption capacity of the hydrogel for Zn 2+ ions is higher than that for Pb 2+ ions (Figure 7B). Desorption studies confirmed the effective metal ion desorption from the hydrogel, which could be reused twice through washing with a low pH solution. Polymers 2024,16, 1292 17 of 29 Polymers 2024, 16, x FOR PEER REVIEW 18 of 31 Figure 7. Influence of pH on heavy metal ion adsorption by CMC-graft-poly(acrylic acid)/montmorillonite hydrogel after 6 h of treatment (A). Competitive adsorption study at pH 5.0 of Pb 2+ and Zn 2+ from mixed solution with concentration of each ion fixed at 250 ppm or 500 ppm (B), from ref [112]. A novel acrylic acid/ethylenediaminetetraacetic acid (EDTA) biochar-based chitosan (CH) composite hydrogel was developed through radical polymerization from degreased waste cotton [113], mainly consisting of cellulose, hemicellulose, and lignin. The resulting hydrogel was able to adsorb Pb 2+ and Cu 2+ ions efficiently, with an adsorption capacity of 1105.78 and 678.04 mg/g, respectively. As shown in Figure 8, the impact of several factors on the adsorption capacity of the composite hydrogel was evaluated, finding that the recovery of Pb 2+ and Cu 2+ ions from the adsorbent was at an optimal level even after five cycles. The composition of the adsorbent plays a crucial role in the effectiveness of adsorption. As shown in Figure 8A, the adsorption performance of CH/EDTA was significantly improved compared to that of the CH hydrogel, due to the addition of EDTA which enhances the chelating effect of contaminants. In parallel, an increase in the amount of porous cotton biochar (CBC) increases the adsorption capacity of the hydrogel. This occurs due to the addition of numerous oxygenand nitrogen-containing functional groups on the surface of the CBC after its acid–base modification, thus providing more anchoring sites for pollutants. However, once the CBC content reaches 0.03 g, the Figure 7. Influence of pH on heavy metal ion adsorption by CMC-graft-poly(acrylic acid)/montmorillonite hydrogel after 6 h of treatment (A). Competitive adsorption study at pH 5.0 of Pb 2+ and Zn 2+ from mixed solution with concentration of each ion fixed at 250 ppm or 500 ppm (B), from ref [ 112 ]. A novel acrylic acid/ethylenediaminetetraacetic acid (EDTA) biochar-based chitosan (CH) composite hydrogel was developed through radical polymerization from degreased waste cotton [ 113 ], mainly consisting of cellulose, hemicellulose, and lignin. The resulting hydrogel was able to adsorb Pb 2+ and Cu 2+ ions efficiently, with an adsorption capacity of 1105.78 and 678.04 mg/g, respectively. As shown in Figure 8, the impact of several factors on the adsorption capacity of the composite hydrogel was evaluated, finding that the recovery of Pb 2+ and Cu 2+ ions from the adsorbent was at an optimal level even after five cycles. The composition of the adsorbent plays a crucial role in the effectiveness of adsorption. As shown in Figure 8A, the adsorption performance of CH/EDTA was significantly improved compared to that of the CH hydrogel, due to the addition of EDTA which enhances the chelating effect of contaminants. In parallel, an increase in the amount of porous cotton biochar (CBC) increases the adsorption capacity of the hydrogel. This occurs due to the addition of numerous oxygenand nitrogen-containing functional groups on the surface of the CBC after its acid–base modification, thus providing more anchoring sites for pollutants. However, once the CBC content reaches 0.03 g, the adsorption capacity stops increasing significantly, as a state of supersaturation of the CBC content is reached. Polymers 2024,16, 1292 18 of 29 Polymers 2024, 16, x FOR PEER REVIEW 19 of 31 adsorption capacity stops increasing significantly, as a state of supersaturation of the CBC content is reached. Figure 8. The impact of different parameters on the adsorption capacity: the composition of the adsorbent gel (A), pH (B), the amount of the adsorbent (C), the treatment time (D), and the initial concentration of the contaminant (E), from ref. [113]. CS: CH (chitosan); CBC: porous cotton biochar. A new CH/cellulose composite sponge adsorbent system for the selective and rapid removal of Hg2+ ions from aqueous solutions was successfully developed through simple cross-linking with glutaraldehyde and a freeze-drying procedure [114]. The CH/cellulose composite system showed in the first two minutes an adsorption of more than 97% of the maximum adsorption capacity (equal to 495 mg/g with an initial concentration of 598 mg/L) in aqueous solutions. In addition, the composite system showed excellent selectivity for the adsorption of Hg2+ ions, with a selectivity coefficient 35, 277, and 461 times higher than that for Cd2+, Pb2+, and Cu2+ ions, respectively. The selectivity coefficient was determined using the separation factor, considering the distribution coefficients of Hg2+ ions and interfering metal ions. XPS and FTIR studies confirmed that the main mechanism of Hg2+ ion adsorption is a chelation reaction with N ligand and O-containing functional groups. In addition, washing with 1 M nitric acid allows the CH/cellulose Figure 8. The impact of different parameters on the adsorption capacity: the composition of the adsorbent gel (A), pH (B), the amount of the adsorbent (C), the treatment time (D), and the initial concentration of the contaminant (E), from ref. [ 113 ]. CS: CH (chitosan); CBC: porous cotton biochar. A new CH/cellulose composite sponge adsorbent system for the selective and rapid removal of Hg 2+ ions from aqueous solutions was successfully developed through simple cross-linking with glutaraldehyde and a freeze-drying procedure [ 114 ]. The CH/cellulose composite system showed in the first two minutes an adsorption of more than 97% of the maximum adsorption capacity (equal to 495 mg/g with an initial concentration of 598 mg/L) in aqueous solutions. In addition, the composite system showed excellent selectivity for the adsorption of Hg 2+ ions, with a selectivity coefficient 35, 277, and 461 times higher than that for Cd 2+ , Pb 2+ , and Cu 2+ ions, respectively. The selectivity coefficient was determined using the separation factor, considering the distribution co- Polymers 2024,16, 1292 19 of 29 efficients of Hg 2+ ions and interfering metal ions. XPS and FTIR studies confirmed that the main mechanism of Hg 2+ ion adsorption is a chelation reaction with N ligand and O-containing functional groups. In addition, washing with 1 M nitric acid allows the CH/cellulose composite system to be effectively regenerated, with slightly lower mechanical strength preserving an 85% adsorption capacity after five cycles. Another lignocellulose-based composite hydrogel system was developed by dispersing TEMPO-oxidized cellulose nanofibrils (TOCNs) in an aqueous solution of 7% NaOH and 12% urea by weight at room temperature [ 115 ]. In the same system, dissolved cellulose was produced at subzero temperatures. The TOCN dispersion and cellulose solution were mixed at different temperatures based on their temperature-dependent solubility. The composite hydrogels were obtained by mixing a cellulose solution, TOCN dispersion, and alkaline lignin solution in an aqueous NaOH/urea solution. The resulting hydrogel showed an excellent adsorption capacity for Cu 2+ , equal to 540.7 mg/g (achieved after 45 min with an initial Cu 2+ concentration of 250 mg/mL) (Figure 9). According to authors, the adsorption process of Cu 2+ ions by composite hydrogels can be divided into two distinctive phases. Initially, Cu 2+ ions diffuse onto the surface of the composite hydrogels from the surrounding solution. Subsequently, Cu 2+ ions further bind to the active adsorption sites (phenolic and carboxyl groups from TOCNs and lignin) present on the composite hydrogels via complexation. In addition, the 3D porous network structure, which provides the maximum surface area and an abundant pore structure, leads to the exposure of many active sites for the complexation of Cu2+ ions. TEMPO negatively charged oxidized cellulose nanofibers (TOCNFs) were used for the synthesis of a composite system through the self-assembly of nanocellulose and nanochitin [ 116 ]. Electrostatic forces guided the self-assembly between TOCNFs and the positively charged partially deacetylated chitin nanofibers at room temperature without the use of cross-linking agents. The resulting hybrid hydrogel was thus physically cross-linked through electrostatic interactions and H-bonding between TOCNFs and chitin nanofibers. Subsequent freeze-drying of the hybrid hydrogel resulted in an aerogel with a tightly interconnected porous structure and extraordinary adsorption capacity of As(III) ion (217 mg/g) at neutral pH. TEMPO-oxidized nanocellulose was used also to produce a composite system with sodium alginate and carboxymethyl chitosan, using Ca 2+ as a cross-linking agent [ 117 ]. Carboxymethyl chitosan is a derivative of chitosan characterized by a high number of free –OH, –COOH, and –NH 2 groups, thus offering several binding sites for heavy metal ions. The gel produced was used in adsorption tests of Cu 2+ and Pb 2+ from aqueous solutions, showing an excellent adsorption efficiency for both ions, with an adsorption capacity of 169.94 mg/g for Cu 2+ and 472.59 mg/g for Pb 2+ . In addition, the composite system maintained a high adsorption efficiency even after five adsorption–desorption cycles. Interestingly, FTIR analysis before and after the adsorption of Cu 2+ and Pb 2+ confirmed the involvement of –OH, –COOH, –NH 2 groups, as well as methyl and methylene groups in the adsorption, demonstrating the key role of different components in determining the adsorption performances of the composite hydrogel. A novel composite hydrogel based on microcrystalline cellulose (MCC) and CH was recently proposed, with the integration of polydopamine (PDA) and polyethyleneimine (PEI) for effective adsorption of ions from wastewater [ 118 ]. Thanks to the presence of several active adsorption sites, the MCC-PDA-PEI/CH-PDA-PEI composite system showed excellent adsorption capacities, equal to about 434.8, 277.7 and 261.8 mg/g for Cu 2+ , Zn 2+ , and Ni 2+ ions, respectively, in a single ion experiment. In a multi-ion adsorption study, the composite hydrogel showed slight selectivity in the removal of Cu 2+ ions. Furthermore, the results of FTIR studies suggested metal ion chelation as a possible adsorption mechanism. Repeated adsorption/desorption tests revealed that the hydrogel maintained an adsorption capacity of more than 40% after four cycles. Polymers 2024,16, 1292 20 of 29 Polymers 2024, 16, x FOR PEER REVIEW 20 of 31 composite system to be effectively regenerated, with slightly lower mechanical strength preserving an 85% adsorption capacity after five cycles. Another lignocellulose-based composite hydrogel system was developed by dispersing TEMPO-oxidized cellulose nanofibrils (TOCNs) in an aqueous solution of 7% NaOH and 12% urea by weight at room temperature [115]. In the same system, dissolved cellulose was produced at subzero temperatures. The TOCN dispersion and cellulose solution were mixed at different temperatures based on their temperature-dependent solubility. The composite hydrogels were obtained by mixing a cellulose solution, TOCN dispersion, and alkaline lignin solution in an aqueous NaOH/urea solution. The resulting hydrogel showed an excellent adsorption capacity for Cu 2+ , equal to 540.7 mg/g (achieved after 45 min with an initial Cu 2+ concentration of 250 mg/mL) (Figure 9). According to authors, the adsorption process of Cu 2+ ions by composite hydrogels can be divided into two distinctive phases. Initially, Cu 2+ ions diffuse onto the surface of the composite hydrogels from the surrounding solution. Subsequently, Cu 2+ ions further bind to the active adsorption sites (phenolic and carboxyl groups from TOCNs and lignin) present on the composite hydrogels via complexation. In addition, the 3D porous network structure, which provides the maximum surface area and an abundant pore structure, leads to the exposure of many active sites for the complexation of Cu 2+ ions. Figure 9. Synthesis schematic diagram and amount of Cu 2+ adsorbed on the alkaline lignin/TOCN/dissolved cellulose composite hydrogel (10/10/80) at different initial Cu 2+ ion concentrations, adapted from ref. [115]. Figure 9. Synthesis schematic diagram and amount of Cu 2+ adsorbed on the alkaline lignin/TOCN/dissolved cellulose composite hydrogel (10/10/80) at different initial Cu 2+ ion concentrations, adapted from ref. [115]. In a recent study, MCC was used for the synthesis of a biodegradable MCC–gelatin adsorbent for the adsorption of Cd 2+ , Pb 2+ , and Cr 3+ ions from aqueous solutions [ 119 ]. The presence of several functional groups, such as –COOH groups, –OH groups, and amide groups (–CONH 2 ), could facilitate the adsorption of heavy metal ions. The characterization of the composite hydrogel with FTIR analysis, scanning electron microscopy analysis (SEM), and energy dispersive X-ray analysis confirmed that the gelatin was intercalated within the microcrystalline cellulose matrix. Performing batch adsorption studies, the impact of several parameters including pH, adsorbent dosage, and initial metal ion concentration on the removal capacity of heavy metal ions was evaluated. Specifically, it was observed that there was a maximum adsorption efficiency for Cd 2+ , Pb 2+ , and Cr 3+ of 95%, 88%, and 70%, respectively, at pH 6 with an initial concentration of each metal ion of 60 ppm and an adsorbent dosage of 1.0 g L−1. Rodriguez et al. [ 120 ] developed free radical cross-linking composite chitosan-gpoly(acrylic acid) hydrogels filled with cellulose nanowhiskers (CNWs) and evaluated Polymers 2024,16, 1292 21 of 29 the absorption capacity for Pb 2+ and Cu 2+ ions from water. In the synthesis of the composite hydrogel by free radical cross-linking, three key components are involved such as N,N ′ -methylenebisacrylamide (MBA), which acts as a cross-linker, helping to stabilize the structure of the hydrogel, potassium persulfate (KPS), a polymerization initiator, and N,N,N ′ ,N ′ -tetramethylethylenediamine (TEMD), which acts as an accelerator of the polymerization reaction. For these composite systems, the maximum adsorption for Pb 2+ (818.4 mg/g) and Cu 2+ (325.5 mg/g) was obtained within the first 30 min, at pH 4.0 and using 20 mg of the composite matrix with a CNW concentration of 10% (w/w). FTIR analysis revealed that functional groups (mainly –COOH and –OH) in composite hydrogels act as coordination sites for the metal ions. Desorption studies showed that the composite system can be regenerated (flushing with a 0.1 M HCl solution) and reused for up to five cycles without significant loss of efficiency. Sphere-shaped composite systems have recently been proposed as possible adsorbents for the treatment of wastewater contaminated with heavy metal ions. In a recent work [ 121 ], a composite adsorbent based on cellulose nanofibers and sodium alginate was prepared through a simple cross-linking process for the efficient removal of Pb 2+ . The spheres were produced by mixing an aqueous solution of sodium alginate with an aqueous solution of cellulose nanofibers under stirring. A suspension of calcium phosphate (CaP) was then added and left stirring until a uniform dispersion was obtained, after which a CaCl 2 solution was added. Overnight stirring resulted in the production of solidified cellulose nanofiber/sodium alginate/CaP beads. Subsequent stirring of the spheres in HCl (pH = 2.0) resulted in the disintegration of the CaP within the spheres, thus obtaining composite spheres with a porous structure, as revealed by scanning electron microscopy. XPS analysis confirmed the involvement of -OH and -COOH groups in the adsorption process involving electrostatic attraction and complexation. Also, the effect of temperature on the adsorption of Pb 2+ ions was studied, revealing an increase in the limiting adsorption capacity as the treatment temperature increased, with a limiting adsorption capacity of 280.90 mg/g at 291 K, 307.69 mg/g at 297 K, and 318.47 mg/g at 303 K. Regeneration experiments with 1 M HCl showed that the adsorption capacity of Pb 2+ on the beads remained satisfactory, with an adsorption above 80% after five cycles. More recently, Aljar et al. [ 122 ] proposed composite beads made from PVA, chitosan, and cellulose for the removal of Pb 2+ , Cd 2+ , Zn 2+ , and Co 2+ from water. The spheres were fabricated via a simple physical cross-linking method, in which an aqueous solution of PVA was mixed with an acidic aqueous solution of chitosan under continuous stirring at 70 ◦ C, followed by cellulose powder addition. Subsequent dropwise addition of the obtained solution to a NaOH solution led to the formation of the hydrogel spheres. The morphology and major element composition of the composite spheres were analyzed before and after heavy metal ion adsorption via SEM and energy dispersive X-ray (EDX). SEM images revealed that the hydrogels possessed a spherical or slightly oval shape with a compact surface and a well cross-linked structure (Figure 10). In general, after adsorption, it was observed that the surface structure of the spheres remained as a well-cross-linked structure, even if the channels were more compact and the pores had a significantly smaller diameter. EDX spectra (Figure 10) clearly showed the characteristic peaks of heavy metal ions, confirming their adsorption. Adsorption studies also showed the efficient removal of Pb 2+ , Cd 2+ , Zn 2+ , and Co 2+ ions, with 99, 95, 92, and 84% removal within 60 min. After regeneration of the spheres with 1 M HCl, the composite hydrogel maintained a high removal efficiency, which remained above 80% even after five adsorption–desorption cycles. Table 3reports the capacity adsorption values for the above presented cellulose-based hydrogels and composite adsorbents based on cellulose derivatives. Polymers 2024,16, 1292 22 of 29 Table 3. Composition and adsorption capacity of different cellulose-based hydrogels used for heavy metal absorption. Cellulose or Cellulose Derivative Additional Polymer Other Components or Modifications Adsorption Capacity Ref. Noncomposite hydrogels CMC — epichlorohydrin as cross-linking agent Cu2+: 6.49 mmol/g Pb2+: 5.15 mmol/g Ni2+: 4.06 mmol/g [96] CMC — aluminum nitrate as cross-linking agent Pb2+: 550 mg/g Ni2+: 620 mg/g Co2+: 760 mg/g [95] Cellulose from bamboo fibers — titanate nanotubes Cu2+: 176.4 mg/g Pb2+: 613.5 mg/g Sr2+: 113.3 mg/g Fe2+: 158.2 mg/g Cd2+: 294.6 mg/g Zn2+: 187.8 mg/g Cr3+: 97.8 mg/g [99] Cellulose fiber from pineapple leaves — modification with carboxymethyl group (CM) and ethylenediaminetetraacetic acid (EDTA) group to produce Cell-CM and Cell-EDTA, respectively For Cell-CM Pb2+: 63.4mg/g Cd2+: 23 mg/g For Cell-EDTA Pb2+: 41.2mg/g Cd2+: 33.2 mg/g [93] HEC — EDTA dianhydride (EDTAD) as cross-linking agent Pb2+: 405.37 mg/g Cu2+: 265.47 mg/g Cd2+: 203.36 mg/g Zn2+: 108.36 mg/g [49] Ginger fibers — citric acid (C6H8O7) and hydrochloric acid (HCl) Cu2+: 45.053 mg/g [ 103 ] Jute — sodium nitrite and nitric acid Pb2+: 2270 mg/g [ 104 ] Composite hydrogels CMC and microcrystalline cellulose Xilan ethylene glycol diglycidyl ether as a crosslinking agent Cd2+: 61.44 mg/g Ni2+: 55.85 mg/g [ 111 ] CMC Polyacrylic acid montmorillonite Pb2+: 146.19 mg/g Zn2+: 286.67 mg/g [ 112 ] CMC PVA — Ag+: 8.4 mg/g [57] Degreasing cotton Chitosan ethylenediaminetetraacetic acid (EDTA) Pb2+: 1105.78 mg/g Cu2+: 678.04 mg/g [ 113 ] Cellulose Chitosan glutaraldehyde (EGDE) as crosslinking agent Hg2+: 495 mg/g [ 114 ] TEMPO-oxidized negatively charged (unidimensional) cellulose nanofibers (TOCNFs) Partially deacetylated chitin nanofibers — As(III) ion: 217 mg/g [ 116 ] TEMPO-oxidized nanocellulose Carboxymethyl chitosan and sodium alginate Ca2+ as cross-linking agent Pb2+: 472.59 mg/g Cu2+: 169.94 mg/g [ 117 ] Microcrystalline cellulose Chitosan, polydopamine (PDA), and polyethyleneimine (PEI) — Cu2+: 434.8 mg/g Zn2+: 277.7 mg/g Ni2+ 261.8 mg/g [ 118 ] Cellulose nanowhiskers (CNWs) Chitosan-g-poly (acrylic acid) MBA as cross-linking agent and TEMED as catalyst Pb2+: 818.4 mg/g Cu2+: 325.5 mg/g [ 120 ] Polymers 2024,16, 1292 23 of 29 Polymers 2024, 16, x FOR PEER REVIEW 23 of 31 Figure 10. SEM images of surface of PVA/chitosan/cellulose hydrogel spheres before and after adsorption of heavy metal ions (left). EDX spectra of hydrogel spheres before and after adsorption of heavy metal ions (right), from ref. [122]. Table 3 reports the capacity adsorption values for the above presented cellulosebased hydrogels and composite adsorbents based on cellulose derivatives. Table 3. Composition and adsorption capacity of different cellulose-based hydrogels used for heavy metal absorption. Cellulose or Cellulose Derivative Additional Polymer Other Components or Modifications Adsorption Capacity Ref. Non-composite hydrogels CMC ― epichlorohydrin as cross-linking agent Cu2+: 6.49 mmol/g Pb2+: 5.15 mmol/g Ni2+: 4.06 mmol/g [96] CMC ― aluminum nitrate as cross-linking agent Pb2+: 550 mg/g Ni2+: 620 mg/g Co2+: 760 mg/g [95] Cellulose from bamboo fibers ― titanate nanotubes Cu2+: 176.4 mg/g Pb2+: 613.5 mg/g [99] Figure 10. SEM images of surface of PVA/chitosan/cellulose hydrogel spheres before and after adsorption of heavy metal ions (left). EDX spectra of hydrogel spheres before and after adsorption of heavy metal ions (right), from ref. [122]. 4. Conclusions Polymer-based hydrogels, due to their ability to absorb large volumes of water, have emerged as a promising alternative material for the development of water decontamination processes. Herein, we have provided an overview of recent studies exploring the use of cellulose or cellulose derivative-based hydrogels for wastewater processing to remove heavy metal contaminants. Several properties have made cellulose and its derivatives an interesting class of materials for the remediation of contaminated water, including high biodegradability, abundance, low cost, optimal stability in a wide variety of conditions, and the large number of surface functional groups. Furthermore, since non-covalent interactions, such as hydrogen bonds and ion pairs, are commonly involved in the removal of heavy metal contaminants, cellulose-based systems can be potentially reused for several cycles. Polymers 2024,16, 1292 24 of 29 Although they are the subject of numerous research studies and some pilot trials, their large-scale adoption in real water bodies is still far from actual implementation. This is due to the possible effect of several additional factors when moving from laboratory-scale applications to the real environment, which can significantly affect metal ion adsorption processes. Along with metal ions, real wastewater contains a variety of contaminants, including microplastics, dyes, drugs, oils, detergents, pesticides, and fertilizers. Therefore, rather than merely concentrating on the investigation of a single metal ion, future adsorption research should be expanded not only to solutions containing different metal ions for evaluating possible cross-interference, but above all, to multipollutant evaluation. Other physical/chemical characteristics of real water bodies, namely, pH, temperature, salinity, dissolved oxygen, and light exposure, can affect metal ion adsorption processes due to their natural variability which possibly represents an additional factor to be considered. Nonetheless, it is rare to find in the literature the testing of these hydrogels for real water samples or under simulated environmental conditions. The effectiveness of hydrogels under challenging environmental conditions must therefore be prioritized. Environmental conditions can severely affect also cellulose-based hydrogel stability. As mentioned above, their biodegradability represents an additional value for their environmental applications as it means complete and safe degradation without toxic products being released. At the same time, their stability in real water naturally hosting different microorganisms must be considered and optimized to find a reasonable compromise between adsorption process kinetics and biodegradation timescales. Also in this sense, hydrogel tests in real environments are mandatory. Cost evaluation represents a key aspect for promoting a technology transfer to the real world. In the case of hydrogels for environmental applications, costs are related not only to fabrication processes, but also to their regeneration capacity that determines their economic viability. However, although the hydrogel regeneration capacity is evaluated in a number of investigations, in most cases, the adsorption process is mostly focused on, with little or no attention given to effective hydrogel reuse. More in-depth studies should be conducted in this context to comprehend the regeneration capability of hydrogels during various cycles. Furthermore, the developed hydrogel regeneration processes must be sustainable not only in terms of costs but also in terms of environmental impact: safe post-treatment techniques for heavy metal desorption should be preferred for guaranteeing environment and operator protection during the whole process. Importantly, the practical applicability of hydrogels in wastewater treatment can be eased by their integration with flowing beds, adsorption columns, and other devices that can be easily handled by simply exposing the system firstly to contaminated water and then to regeneration solutions. This further technological step should be considered in the design of hydrogels as it could be very favorable for their large-scale application, especially in the case of bead-shaped hydrogels. As discussed in the presented literature survey, such hydrogel format has been successfully applied to wastewater treatment studies, taking advantage of high surface area, which results in better contact with water and a larger adsorption area. Their packing within properly designed devices allows for their exposure to uniform waterflow and makes their handling easier, speeding up the whole process. These aspects, if critically evaluated in studies focused on the development of cellulosebased adsorbents, can surely help bridge the gap between laboratory research and practical application, providing critical information for the success of cellulose-based hydrogels in removing metal ions in real environmental conditions. Author Contributions: Conceptualization, F.P. and E.M.; methodology, E.M.; investigation, F.P.; resources, C.M. and E.M.; data curation, F.P. and E.M.; writing-original draft preparation, F.P.; writingreview and editing, C.M. and E.M.; supervision, C.M. and E.M.; project administration, E.M.; funding acquisition, E.M. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially funded by the European Union Horizon Europe program under grant agreement No 101046946 (RESORB). Polymers 2024,16, 1292 25 of 29 Conflicts of Interest: The authors declare no conflicts of interest. Abbreviations CMC carboxy methyl cellulose –COOH carboxyl groups CCNF carboxylated cellulose nanofiber CM carboxymethyl group CH chitosan CA citric acid EDTA ethylenediaminetetraacetic acid group EGDE glutaraldehyde HEC hydroxyethyl cellulose –OH hydroxyl groups MC methylcellulose MCC microcrystalline cellulose CNF nanofibrillated cellulose NOCNF nitro-oxidized carboxycellulose nanofibers PVA poly(vinyl alcohol) TOCNF TEMPO-oxidized negatively charged (unidimensional) cellulose nanofiber XRD X-ray diffraction TEMED N,N,N′,N′-tetramethylethylenediamine MBA N,N′-methylenebisacrylamide References 1. Soliman, N.K.; Moustafa, A.F. Industrial solid waste for heavy metals adsorption features and challenges; A review. J. Mater. Res. Technol. 2020,9, 10235–10253. [CrossRef] 2. Diaconu, L.I.; Covaliu-Mierlă, C.I.; Păunescu, O.; Covaliu, L.D.; Iovu, H.; Paraschiv, G. Phytoremediation of Wastewater Containing Lead and Manganese Ions Using Algae. Biology 2023,12, 773. [CrossRef] [PubMed] 3. 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