Recent advances on hydrogels based on chitosan and alginate for the adsorption of dyes and metal ions from water

2.1 Types of water

Natural water is commonly divided into surface water and groundwater. Surface water includes lakes, reservoirs, ponds, rivers, and streams, which each have their own dynamics and are exposed to both the underlying terrestrial surfaces and the atmosphere.

Water can contain toxic compounds such as metals, metalloids, and organic compounds. Pollution is defined by the presence and concentration of these components and depends greatly on the type and size of the water body. Water considered unfit for human drinking can be suitable for other uses, such as irrigation and other works (L. Schweitzer, J. Noblet, 2018).

2.2 Sources of water pollution

Pollution sources are divided and branched into fixed sources and nonspecific sources. Fixed sources are sources with a hard and fast location and an area of permanent pollution that is constantly being polluted, such as power plants, refineries, mines, sewage treatment plants, etc. Unspecified and nonpoint sources are sources that are distributed over a good geographical region and do not have a selected location and an indeterminate pollutant, such as watersheds. Unspecified sources may include mobile sources such as cars, buses, and trains (L. Schweitzer, J. Noblet, 2018).

Wastewater pollution with heavy metals is the largest environmental problem and poses human endangerment worldwide thanks to rapid urbanization and industrialization, such as metallic paint, mining, tanneries, coatings, batteries, paper industries, printing and imaging industries, pesticides, fertilizers, and the manufacturing of automobile radiators. Heavy metal ions such as As(III), As(V), Pb(II), Cd(II), Ni(I), Cr(III), Cr(VI), Zn(II), Cu(II), Hg(I), Hg(II), and Co(II), among others, can be present in wastewater (Mahmud et al. 2016).

Wastewater from textile industries is often a threat to the environment in an exceedingly large part of the planet. From an environmental point of view, the textile industry is one of the foremost polluting industries in the world due to its enormous consumption of water (in some cases up to 3000 m³/day). The use of harmful chemicals, high consumption water and energy, generation of large quantities of large solid and gas waste fuel consumption for transportation to remote locations where textile units are located determine the placement and use of nondegradable packaging materials (Choudhury, A.R., 2014). The effluent from the textile industry is closely associated with the characteristics of the textile product, the standard, the origin of the raw materials (wool, cotton, linen, and artificial fibres), the chemicals that aid the dye, and, additionally because of the inner pollution prevention and process management strategies there are severe fluctuations within the factors (K. Sarayu & S. Sandhya, 2012).

3.1 Interpenetrating polymer networks (IPNs) and semi-interpenetrating polymer network (semi-IPN) chitosan hydrogels

As chitosan is polysaccharide with structure of linear cationic formed by b-(1 / 4)-2-amino-2- deoxy-D-glucopyranose and b-(1 / 4)-2-acetamido-2-deoxy-D-glucopyranose units, while most of hydrogels types containing chitosan are obtained by polymerization or copolymerization of chitosan with other polymer types, such as synthetic polymers or natural polymers, while using crosslinkers or several crosslinkers to form full IPNs or several semi-IPN, while the semi-IPN and full IPN-type hydrogels can be obtained by varying the initiator and crosslinker concentrations, which can differ in the crosslinking degree and chitosan mass ratio (see Fig. 2). Which can be classified to covalent semi-IPN, noncovalent semi-IPN, noncovalent full-IPN, sequential IPN, simultaneous IPN, latex IPN, thermoplastic IPN and gradient IPN.

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Fig. 2

Formation of crosslinked chitosan.

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As the covalent semi-IPN formed unglued to two crosslinked polymeric systems as single polymeric network, while the noncovalent semi-IPN are only one crosslinked polymeric system (see Fig. 3).

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Fig. 3

Formation of the semi-IPN and full-IPN hydrogels starting from chitosan.

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These hydrogels are optimistic for the removal of anionic dyes such as methyl violet and Congo red, and for other industrial dyes from water or solvents, which can be defined by studying the mechanism and isotherm (Maity and Ray, 2014; Silverstein, M.S., 2020; Zoratto, N. and Matricardi, P., 2018).

Other study based on polyacrylamide (PAAm) and chitosan (CS) offers an explanation of the strong differences between s-IPN and IPN regarding swelling and interaction with mandaline dyes is provided by the structural changes that occur during formation of the second lattice at high pH, when a fraction is transformed from amide groups into carboxyl groups by primary hydrolysis, thus generating an amphoteric IPN hydrogel. In one step, s-IPN hydrogels were synthesized on the basis of PAAm and CS by cross-linking AAm polymerization in the presence of CS. Adsorption stains on s-IPN1.60 and d-IPN2.60. D-IPNs is higher. Mixing chitosan with other biopolymers has been shown to be an effective way to enhance water resistance and enhance stability within water during the absorption process (Cui, L et al. 2014).

The semi- IPN is defined as follows: an IPN is a polymer containing one or more networks and one or more linear or branched polymer(s), which are characterized by infiltration on a molecular scale and the properties of the networks and linear or branched macromolecules can be determined by spectroscopy (Zoratto, N. and Matricardi, P., 2018). As an example, Ngwabebhoh. F. A. and his research group obtained a superabsorbent chitosan-starch (ChS) hydrogel that includes electrostatically different charge interactions between anionic sulfonyl (-SO₃ ⁻) groups of dissolved DR80 dye and protonated amino (–NH₃⁺) groups of chitosan boosted by hydrophobic/hydrophilic interactions, and the results show an adsorption capacity of 312.77 mg/g, obtaining an economical, inexpensive, nontoxic hydrogel that has an absorbent capacity with a high absorption efficiency of 83.5% and is also environmentally friendly. This hydrogel can be used for absorption in all solutions, wastewater, and water (Ngwabebhoh et al 2016). Wen-Bo Wang et al. synthesized a semi-IPN hydrogel based on chitosan and gelatine, and semi-IPN hydrogels were prepared in situ in an aqueous solution by grafting free radicals and crosslinking reactions between chitosan (CTS), acrylic acid (AA), gelatine (GE), and N,Ń-methylene bis-acrylamide. For Cu (II) ion hydrogels, the results indicate that the hydrogel containing 2 wt% GE has the highest absorption capacity of 261.08 mg/g with a recovery rate of 95.2%. Incorporating 10 wt GE boosted the storage modulus by 103.4% (ω = 100 rad/s) and 115.1% (ω = 0.1 rad/s) and the adsorption rate by 5.67%. Moreover, the absorption capacity of the hydrogel remained as high as 153.9 mg/g after five cycles of adsorption - absorption. It was found that the ion exchange reactions between the functional groups (–COO⁻ and –NH₂) of the aqueous gels are complex and that these hydrogels have the ability to be reused and recovered (Wang et al. 2013).

3.2 Grafted chitosan hydrogels

The hydrogel was prepared by polymerization of polyacrylamide grafted onto chitosan crosslinked with N,N′-methylene bis-acrylamide, whereas the grafting leads to a mild structure and increased molecular weight for the copolymer and an increase in the number of charges (Zhao, N et al 2020). The hydrogel was synthesized by conventional and microwave-assisted methods. The maximum adsorption capacity (255.5 mg/g) of microwave prepared compared with the conventional capacity (151.7 mg/g) and the adsorption efficiency of the microwave synthesized gel was 96.9 ± 0.2% less than that obtained for the synthesized conventional gel (99.1 ± 0.2%) (R. C. da silva et al. 2020).

3.3 Photocatalytic chitosan hydrogels

Photocatalytic processes are inexpensive, environmentally friendly, and suitable for adsorption (MN Chong et al. 2010.). J. Zhou el al. synthesized a nano-TiO₂/chitosan/poly(N-isopropylacrylamide) (nano-TiO2/CS/PNIPAAm) composite hydrogel prepared by a two-step polymerization method; the use of nano-TiO₂, which contains the efficacy photocatalytic activity with the ability to absorb AF dyes, increased the removal rate from 64.02% to 90.80% under UV irradiation from 60 min to 150 min (Zhou et al. 2017). R. Jiang et al. synthesized a nano-ZnO/chitosan hydrogel with photocatalysis under solar light irradiation. The sensitized colloidal CdS nano-ZnO/chitosan (CdS@n-ZnO/CS) hydrogel was designed to realize photocatalytic activity for CdS@n-ZnO/CS and was evaluated with the photodegradation of azo dye (CR). Studies of biofunctionalized CdS@n-ZnO/CS hydrogel that show high photocatalytic activity have even shown that hydrogel has the power to transfer electrons, remove dyes and can be employed in real applications for the ultra-fast treatment and purification of dye-containing wastewater within the presence of solar light (Jiang et al. 2021).

In other study, nanocomposite hydrogels were obtained by γ-radiation-induced copolymerization, known as the gamma irradiation technique, which involve crosslinking of chitosan biopolymer (CS), acrylic acid (AAc) and TiO₂ nanoparticles (CS-PAAc/TiO₂). Theis hydrogel was used to remove methylene blue (MB) dye from aqueous solution, and the results showed that CS-PAAc/TiO₂ nanocomposites ranked with high adsorption capacity towards MB reaching 0.20 g per liter (Mahmoud et al. 2020).

3.4 Ion exchanger chitosan hydrogels

The hydrogel based on the chitosan-gelatine ion exchanger has high selectivity, and the addition of metallic elements such as zirconium (IV) leads to a positive charge that is affected by the anionic dyes. The addition of selenophosphate leads to selectivity for cationic dyes. As the MB absorption efficacy of the materials increased (99% from 12%), the adsorption process followed Langmuir adsorption (Q_o = 10.46 mg⁻¹) and a nonlinear PFO kinetic model with k1, qe (calculated), R2, RMSE = 0.011, 1.02 (mg.g⁻¹), 0.996, and 0.01709, respectively, with significant degradation (99%) of MB within 3 h of photoirradiation with a maximum adsorption capacity of 10.46 (mg g⁻¹) (K. Kaur, R. Jindal, 2019).

3.5 Crosslinked chitosan hydrogels

Graphene is usually derived from coal and methods for preparing the graphene oxide film are reviewed, including vacuum suction filtration, spray coating, spin coating, dip coating and the layer-by-layer method, and the oxidized graphite (GO) can be obtained by heating and chemical reduction. H. Mittal et al. prepared graphene oxide, chitosan and carboxymethyl cellulose (CS/CMC-NCH)-crosslinked nanocomposite hydrogels that were synthesized as adsorbents for the remediation of water. The hydrogels were made to absorb anionic (methyl orange, MO) and cationic dyes (methylene blue, MB) contaminated in wastewater or water. Approximately 99% of the dye was adsorbed from a 50 mg/L MB dye solution at 0.4 g/L by using the CS/CMC-NCH hydrogel at pH 7, and the MO absorption efficacy was approximately 82% as the dye was adsorbed at 0.6 g/L by using the CS/CMC-NCH hydrogel at pH 3 (Mittal et al. 2021).

In other study R. R. Mohamed successfully synthesized physically crosslinked hydrogels N-quaternized chitosan/poly(acrylic acid) manufactured from the following materials, N,N,N-trimethyl chitosan chloride (N-quaternized chitosan) and poly(acrylic acid), with different weight ratios (3:1), (1:1) and (1:3). These hydrogels are denoted with the following codes: Q3P1, Q1P1, and Q1P3, respectively. When using chitosan, the maximum adsorption capacity for the metals Cr(III), Fe(III), Ni(II), Cu(II), Cd(II) was only 332.0 mg/g, 355.4 mg/g, 647.2 mg/g, 677.2 mg/g and 1095.2 mg/g, when using the Q1P3 hydrogel, the maximum adsorption capacity was 275.9 mg/g, 599.5 mg/g, 398.0 mg/g, 503.2 mg/g and 1261.7 mg/g, when using the Q1P1 hydrogel, the maximum adsorption capacity was 269.5 mg/g, 315.0 mg/g, 452.0 mg/g, 521.5 mg/g and 1198.5 mg/g, and when using Q3P1 hydrogel, the maximum adsorption capacity was 233.5 mg/g, 115.0 mg/g, 398.8 mg/g, 507.6 mg/g and 1197.7 mg/g, respectively (Mohamed et al. 2017).

Cross-linked oxalic acid/chitosan hydrogels have been manufactured and studied for their azo dye absorbing properties Reactive Red 195 (RR195), wherein a difference in the charge leads to interactions between protonated amino groups of chitosan and oxalate ions, where the surface properties are studied for adsorption/desorption operations to analyse specific areas. The difference in charge leads to electrostatic interactions between protonated amino groups of chitosan and oxalate ions, leading to an increase in the absorption and an increase in the absorption efficiency as the percentage of dye removal by the biosorbent shows a maximum value of removal of 90.6% with an adsorption capacity of 114 mg/g at pH = 4 (Pérez-Calderón et al. 2020).

In another study, porous material adsorbents are formed in the presence of salicylaldehyde linked to chitosan to remove toxic dyes, showing excellent surface porosity. These porous properties contribute to the absorption of dyes such as Rose Bengal in acidic aqueous solution (pH 4) and crystal violet in neutral solution (pH 6). Chitosan-salicylaldehyde has an absorbent efficacy that ranges from 98% and 99% at room temperature. In addition to its porous properties, the dye absorption depends on stronger interactions, i.e., H-bonding, electrostatic interaction, and π-π interaction between the hydrogel and the functional groups of the dye (Parshi et al. 2019).

3.6 Magnetic chitosan hydrogels

Magnetic chitosan/poly(vinyl alcohol) hydrogel beads (m-CS/PVA HBs) were synthesized by using an instantaneous gelation method without prejudice and loss of crystal structure shape of Fe₃O₄. The addition of metallic Fe₃O₄ nanoparticles can increase the saturation magnetization (21.96 emu g⁻¹) of m-CS/PVA HBs compared to other magnetic bioadsorbents and can increase the adsorption capacity of Congo red onto m-CS/PVA HBs to 470.1 mg g⁻¹ (Zhu et al. 2012).

The addition of elements such as magnetic graphene oxide nanoparticles (GO/Fe₃O₄) with magnetic properties increases the selectivity and increases the removal efficiency in aqueous media, as these materials also result in an increase in chemical stability that leads to a steady state. This element (GO/Fe₃O₄) was added to create magnetic chitosan hydrogel beads based on chitosan (75–85% degree of deacetylation, Mw = 200 kDa) and isophthaloyl chloride via interfacial polymerization to study the absorption and removal properties of cationic and anionic dyes from aqueous solution, and the results showed that the maximum adsorption capacities of cationic MB and anionic EBT were 289 mg/g and 292 mg/g, respectively (Mahsa Jamali & Ahmad Akbari, 2021).

In addition, chitosan hydrogels can be grafted; for example, chitosan-based hydrogel, graft co-polymerization with methylene bisacrylamide and poly(acrylic acid), (for example, CS-co-MMB-co-PAA), demineralization pH between 4.5 and 5.5, and initial mineral concentration of 300 mg dm⁻³ with 100 mg of dried hydrogel mass, achieving a recovery of 97% for Cd (II) and 94% for Pb (II) or Cu (II) ions with a Pb(II), Cd(II) and Cu(II) ion adsorption capacity of 163.90 mg/g, 135.51 mg/g and 152.42 mg/g, respectively (Paulino et al. 2011).

Activated carbon, which is usually derived from charcoal but is now derived from chitosan hydrogel coated with tea saponin due to its own surface, permits the removal and adsorption of dyes using methylene blue as a model dye and reaches a maximum adsorption capacity of 604.10 mg/g. The total pore volume of the hydrogel reaches 6.039 cm³/g, and a maximum adsorption capacity of only 123.52 mg/g (Ma et al. 2021).

3.7 Natural materials to prepare chitosan hydrogels

This hydrogel is made with sugar and polysaccharide natural ingredients, such as glucose and chitosan hydrogel (GC hydrogel), using an initiator and a crosslinking agent by an ultrasonic-assisted method. This hydrogel was successfully synthesized and applied to adsorb Co(II) in wastewater. The results showed that GC hydrogels had adsorption capacities of 202 mg g⁻¹ for Co(II) at 20 °C, pH = 7 and an adsorbent dosage of 0.01 g (Liu et al. 2019).

A xanthate-modified chitosan/poly(N-isopropylacrylamide) compound hydrogel was synthesized in an easy two-step method for selective absorption of Cu(II), Pb(II), and Ni(II) metal ions from aqueous solutions and manufactured for selective adsorption of Cu(II), Pb(II) and Ni(II) from aqueous solutions. The maximum absorption capacities for Co(II), Pb(II), and Ni(II) at 293 K were approximately 115.1 mg.g⁻¹, 172.0 mg g⁻¹ and 66.9 mg g⁻¹, respectively (Wu et al. 2018).

Polydopamine-modified chitosan (CS-PDA) aerogels were synthesized through dopamine self-polymerase and glutaraldehyde binding reactions to enhance adsorption capacity and acidic chitosan resistance. The maximum adsorption capacities of CS-PDA for Cr(VI) and Pb(II) were 374.4 mg/g and 441.2 mg/g, respectively. After eight cycles, the CS-PDA absorption capacity showed no apparent reduction, excellent economic feasibility and reusability (Guo et al. 2018).

3.8 Clays/chitosan hydrogels

Montmorillonite (Mt) is a phyllosilicate mineral that contains a layered nanostructure (layers with a thickness of 1 nm) with each layer consisting of one O-Al(Mg)-O bond and two O-Si-O bonds and composed of tetrahedral sheets and octahedral sheets (approximately 100 nm × 100 nm in width and length) that impart a porous structure with high adsorption and selective absorption of dyes and metal ions. The treatment proceeds via by Van der Waals forces and electrostatic forces, and the adsorption proceeds through an ion exchange reaction. As wastewater contains the mineral elements Na⁺, Ca²⁺, Br⁻, Cl⁻ and organic cations (Zhou et al., 2019). W. Wang et al. synthesized poly(vinyl alcohol)-sodium alginate-chitosan-montmorillonite nanosheets (MMTNS), where the structure is based on hydrogen bonds and electrostatic interactions at high pH values due to the high electronegativity resulting from the deprotonation of hydroxyl groups. These hydrogels, due to the presence of clay and chitosan, lead to stability and reusability, and the MB removal rate for the hydrogel beads was 90% after 250 min when using MMTNS at an amount of 29.70% (W. Wang et al. 2018).

W. Wang et al. synthesized a carboxymethyl cellulose-chitosan system to synthesize an absorbent material with porous properties that is thermodynamically stable and has the ability to absorb dyes thanks to the porosity of the structure. These materials was successfully synthesized via hydrogen bonding of the montmorillonite with –NH₂ of the chitosan, as amidation and chains produce interactions to reveal a high MB removal rate of 97%, which was achieved via use of 0.2 g/L hydrogel within 360 min (Wang et al. 2020).

Chitosan/acid-activated montmorillonite composites and chitosan-based hydrogels were synthesized by copolymerization of radical chitosan, acrylic acid and N,N′-methylene bis-acrylamide. Absorption was studied for metal ions such as Pb(II) and Ni(II), and the results showed that chitosan-based hydrogels had adsorption capacities ranging from 41.06 mg/g and 30.20 mg/g. After adding montmorillonite, the adsorption capacity increased to 42.38 mg/g and 36.45 mg/g for dried hydrogels in the pH range of 5.5–3.5, respectively (Vieira et al. 2018).

Bentonite clay, chitosan, and acrylic copolymer gel hydrogels were successfully synthesized by free radical polymerization in the presence of N,Ń-methylene bis-acrylamide as a crosslinker, which has been separately reported and used for the adsorption of dyes. Bentonite is an inorganic material adsorbent. Montmorillonite is the main ingredient, which consists of two layers of sandwiching tetrahedral silica sheets, and the hydrogel adsorption capacity reaches 93–492 mg/g for malachite green and 91.6–482 mg/g for methyl violet dye with an efficient removal efficiency of 75–96.5% for malachite green dye and 73.5–93% for methyl violet dye (Bhattacharyya and Ray, 2014).

Table 1 shows the classification of chitosan hydrogels and their adsorption conditions and performances.

4.1 Interpenetrating networks (IPNs) and semi-IPNs of alginate

B. Mandal and S. K Ray., successfully synthesized IPN hydrogels using a free radical in situ method with crosslink copolymerization for acrylic acid and hydroxyethyl methacrylate in an aqueous solution of sodium alginate in the presence of N,N-methylenebisacrylamide as a comonomer crosslinker to obtain crosslinked hydrogels to remove congo red and methyl violet from water (Mandal and Ray, 2013). E. S. Dragan and his collaborator obtained cryogels with a full cationic/ionic IPN containing two different opposite charge networks that were crosslink independent. The difference between the preparation of semi-interpenetrating network cryogels and a full IPN is that for the full IPN, one must first add cross-linking polymerization of acrylamide with N,Ń-methylene bis-acrylamide, and chitosan and then add NaOH with ECH during freezing conditions, as the main big difference between a semi and full IPN is the parameters such as the cross-linker ratio, pH of the chitosan solution, and the chitosan molar mass for the full-IPN as a function of pH and the gel morphology. Additionally, the MB adsorption capacity using the full-IPN cryogel reaches approximately 750 mg/g, which is higher than the adsorption capacity obtained using a semi-IPN (Dragan et al., 2012a; 2012b). Hydrogels were synthesized from sodium alginate, and the copolymer of acrylic acid and acrylamide provided the highest adsorption at almost the highest Q_m (mg/g) due to the structure of acrylamide. The semi-IPN (semi-interpenetrating network) is a crosslinked system with linearly forming hydrogels, and Sharma, A. K and his collaborators in 2019 fabricated semi-IPN nanocomposite hydrogels adsorbents for removal of biebrich scarlet and crystal violet dyes with a maximum adsorption capacity of 100 mg and 500 mg and absorption efficacy of 93.43% and 89.1%, respectively (Sharma et al., 2019).

4.2 Carbon-derived materials incorporated with alginate hydrogels

The fabricated sodium alginate crosslinked acrylic acid/graphite hydrogel showed unique thermal properties and was used to remove malachite green dye from aqueous solution under the batch adsorption method, showing that the maximum adsorption tendency using the Langmuir model reached 628.93 mg g⁻¹ (Verma et al. 2020). Other study shows an acrylamide bonded sodium alginate hydrogel and acrylamide/graphene oxide bonded sodium alginate nanocomposite hydrogel showed a maximum adsorption capacity of 62.07 mg/g and 100.30 mg/g for crystal violet dye, respectively, as the graphene oxide led to higher adsorption due to its porous structure (Pashaei-Fakhri et al., 2021).

Composite hydrogels were successfully synthesized from modified graphite carbon nitride by a facile cross-linking polymerization method. This improved the adsorption ability of each component to target metal ions and solved the recycling and separation problem of g-C₃N₄, which is an absorbent powder that is difficult to recycle after adsorption. The maximum absorption capacities for Pb(II), Ni(II), and Cu(II) were calculated to be 383.4 mg/g, 306.3 mg/g, and 168.2 mg/g, respectively (Wei Shen et al. 2020).

4.3 Alginate/synthetic polymer hydrogels

The octaminopropyl polyhedral oligomeric silsesquioxane hydrochloride salt based alginate/polyacrylamide hybrid is a new hydrogel type with mechanical properties suitable for the absorption process and was synthesized using the combination of a double network and POSS nanoparticle DN gels to increase the crosslinking and toughness to show great mechanical properties. DN hydrogels with an increased OA-POSS ratio (N3%), Alg/PAAm/OA-POSS1, show an absorption capacity of more than 20 mg OAAlg/PAAm/OA-POSS1 for methylene blue at pH 9 and 11 (Bahrami et al. 2019).

Another example is sodium alginate polyacrylic acid hydrogel which was synthesized by free radical in situ polymerization to obtain a hydrogel and poly sodium alginate (acrylic acid)/zinc oxide) hydrogel nanocomposite and was used to isolate toxic MB dye from an aqueous solution. A maximum absorption capacity of 1129 mg/g and 1529.6 mg/g was found for the sodium alginate polyacrylic acid and poly sodium alginate (acrylic acid)/zinc oxide) hydrogel nanocomposite solution in 0.25 g/L of solution at a pH of 6.0 over 40 min (Makhado et al. 2020).

SA/polyvinyl alcohol/graphene oxide hydrogel microspheres were also prepared in a simple manner. Sodium alginate was physically crosslinked by calcium; GO was encapsulated in the compound to harden the hydrogels; polyvinyl alcohol played an important role in well-dispersed graphene oxide in sodium alginate. The hydrogels were used as an effective adsorbent for removing Cu(II) and U(VI) from aqueous solutions, and the maximum absorption capacities for the hydrogel microspheres for Cu(II) and UO₂(II) were 247.16 mg/g and 403.78 mg/g, respectively (Yi et al. 2018).

In another case, sodium alginate/polyethylene amine compound hydrogel was designed and manufactured to remove heavy metal ions from wastewater. For the absorption of Cu(II) and Pb(II) ions in single and bilateral aqueous solutions and for the absorption of Cu(II) and Cd(II) ions in wastewater, the adsorption capacity was 322.6 mg/g and 344.8 mg/g, respectively. However, in both the alginate and polyethyleneimine segments, in addition to the 3D network structure and a large number of –OH, –COOH and –NH₂ groups provide abundant active sites for heavy metal ion absorption (Godiya et al., 2019a; 2019b). Chirag B. Godiya et al. synthesized sodium alginate combined with polyethyleneimine and used it to remove MB in aqueous medium. The sodium alginate/polyethyleneimine hydrogel showed excellent MB removal performance, that is, approximately 99% of dye was removed from water within approximately 30 min with a 0.5 g/L sodium alginate/polyethyleneimine hydrogel at an initial concentration of 100 mg/L. The absorption was carried out by functional groups with chelation between the –NH₂, –OH and –COOH groups with MB, with a maximum MB absorption capacity of 400.0 mg/g (Godiya et al., 2020a; Godiya et al., 2020b).

4.4 Alginate incorporated with metals

The microparticles are very small spheres formed by synthesizing calcium alginate microparticles that are small in size and uniformly shaped and obtained using a flow-focusing microfluidic device combined with external gelation in a CaCl₂ solution. Due to their size, the microspheres showed efficacy in absorbing Cu(II) and Ni(II) ions from solutions in record time with good heavy metal ion absorption. Since the tiny size of these nanoparticles and within the size of the micro assist in determining the mechanism and evidence established on the exchange of charges and action mechanism between the metal ions from solution, and, therefore the free carboxylic groups of the calcium alginate hydrogels. Where the absorption is bigger, the capacities of the calcium alginate microparticles towards Cu(II) and Ni(II) ions were 0.36 mg/mg and 0.81 mg/mg (Duc et al. 2021).

4.5 Alginate/natural polymer hydrogel

A heat responsive (thermoresponsive) hydrogel containing sodium alginate and 2-hydroxy-3 isopropoxypropyl starch was developed to remove Cu(II) from an aqueous solution. The hydrogel showed a lattice and porous structure, as well as abundance of carboxyl groups within the structure, giving it sufficient binding sites for Cu(II) absorption, and the maximum absorption capacity was 25.81 mg/g. In addition, the hydrogel could be successfully absorbed with only small amounts of dilute HCl within a short period of time due its heat-responsive property, and it also showed the feasibility of regeneration, because the adsorption capacity of Cu (II) remained higher than 15.23 ± 0.27 mg/g even after five cycles. For example, the absorption capacities of the hydrogel gel beads and sodium at pH 4.5 were 22.37 ± 0.18 mg/g and 15.01 ± 0.21 mg/g, respectively (Dai et al. 2019).

4.6 Sodium alginate hydrogel membranes

4.7 Hydrogels as photocatalyst functionalization

4.8 MOF-incorporated Cu-based alginate/PVA hydrogel

(MOF)-incorporated single-network alginate (MOF-Alg(Cu)) beads were used to adsorb ions in seawater. MOF-incorporated Cu-based alginate/PVA hydrogel (MOF-Alg(Cu)/PVA) beads were fabricated to enhance the ion adsorption desalination technique. Cu-based MOFs were successfully synthesized in situ on an interpenetrating polymer network (IPN). Given that the IPN hydrogel beads have high stability, the amount of MOF particles extracted during the adsorption of ions is reduced as the concentration of seawater is reduced to 1500 ppm at stage six with an ion removal efficiency of 95.7% (Lee et al. 2021).