Current World Environment

Introduction

Water is essential for all living organisms, yet, although it covers 71% of Earth's surface, only 3% is freshwater, with just 1% readily usable.¹ Mountains, icebergs, glaciers, and polar snow represent potential locked-in reservoirs of remaining freshwater. Most freshwater is unavailable, and population and industrial expansion exacerbate this scarcity. The type of water pollution resulting from industrial activities depends on the facility's management. Water pollution is primarily due to the release of organic and inorganic pollutants into the aquatic environment. Wastewater from industries such as chemical manufacturing, battery production, metallurgy, and mining releases various coloring compounds (dyes), suspended particles, pesticides, and heavy metal ions.² These contaminants pose a significant risk to the ecosystem. Approximately 10,000 tons of commercial dyes are consumed globally per annum.³ The textile sector accounts for 80% of global dye consumption.⁴ Dyes are also widely used in printing, food production, paper manufacturing, leather treatment, and pharmaceuticals. Their high-water solubility, chemical stability, and resistance to biodegradation allow them to persist in the environment for years. Many dyes are discharged into rivers and lakes without treatment, resulting in 280,000 tons of dye-contaminated wastewater emitted annually.⁵ Heavy metals such as Cu²⁺, Pb²⁺, Cd²⁺, Co²⁺, Mn²⁺, Ni²⁺, Zn²⁺, and Cr⁶⁺ are non-biodegradable, persistent, and pose a serious threat to human health and the environment. Various conventional methods exist, such as chemical coagulation, reverse osmosis, ion exchange (Figure 1), chlorination, and UV treatment, but have drawbacks such as high operating costs, secondary environmental issues, and complexity.⁶

Figure 1: Conventional water treatment methods and their drawbacks. Click here to view Figure

Given the rising threats of dye and heavy metal contamination in water sources, developing effective wastewater treatment systems is critical. Adsorption is the most practical and cost-effective method, offering superior efficacy among existing technologies. Common adsorbent agents, including zeolite, clay, alumina, and activated carbon, can remove various dangerous chemicals.⁷ Hydrogels are effective adsorbents because they can retain large quantities of water within their three-dimensional network, exhibit reversible expansion and contraction, and respond to various stimuli, such as temperature, electric current, and magnetic fields.⁸ Hydrogels are ideal for water treatment due to their biodegradability, ease of production using biobased and synthetic polymers, resistance to rapid thermal change, porous structure, large surface area, high swelling capacity, etc.⁹ Their hydrophilic groups also enable them to hold up to 10-20 times their mass in water, although they do not dissolve because of crosslinking.¹⁰ Hydrogel may be synthesized from either synthetic or natural polysaccharides. Generally, natural polymers such as starch, mucilage, chitosan, pectin, and so forth, are considered preferable because they are biodegradable, abundant, and easy to chemically modify.¹¹ Their hydrophilic groups are capable of removing (heavy metals & dyes) via physical and chemical adsorption. Hydrogel matrices contain additives such as minerals, organic & inorganic molecules, metallic oxides, ions, and carbon-based additives, offering improved physicochemical properties, porous polymeric structure, recyclability, and multifunctionality.¹²

Unlike previous reviews that offer a generalized overview of chitosan hydrogels' performance, this review emphasizes a comprehensive quantitative comparison of adsorption capacities of removal efficiencies, and kinetic-isotherm behaviors across a wide range of pollutant types. By critically evaluating 31 articles from top-tier peer-reviewed publications, this discussion aims to identify the effects of molecular modifications, reinforcing agents, and processing conditions on adsorption performance. The novelty of this review lies in bridging experimental findings with practical applicability, offering an integrated perspective on performance optimization and real-world deployment potential. Moreover, this strategy not only enables a deeper understanding of chitosan hydrogel systems but also identifies areas that require further research to develop future generations of innovative clean water technologies.

Methodology

In this systematic review, it is imperative to highlight that its methodology is of tremendous significance, as it is intended to provide transparency, replicability, and integrity in the selection of studies, screening, and data extraction. A predetermined procedure for a systematic review is developed to align with its objectives, specifically those related to chitosan hydrogels in wastewater treatment. In this systematic review's methodology, inclusion and exclusion criteria, as well as predetermined data-extraction terms, have been clearly outlined.

Search strategy for data acquisition

Articles were searched across ScienceDirect, SpringerLink, Scopus, Wiley Online Library, and PubMed. This was done by searching for peer-reviewed publications of research studies published between 2019 and 2025. A selection of meticulously designed search terms was used to identify articles containing terms such as “Chitosan Hydrogels,” “Adsorption,” “Heavy Metals,” “Dye Removal,” and “wastewater treatment.” Boolean operators such as (AND/ OR) were utilized to ensure that all sources of articles were searched.

Inclusion and exclusion criteria

A paper qualified for the inclusion in this systematic review if it (i) focused on the synthesis, modification, or application of chitosan-based hydrogel for the adsorption of metal ions and dyes, (ii) provided quantitative data on adsorption capacities or efficiencies, and (iii) employed valid kinetic and isotherm models such as pseudo-first-order kinetic model or pseudo-second-order kinetic model, Langmuir model, or Freundlich model. Studies were excluded if they lacked sufficient quantitative information, were non-peer-reviewed, or focused on unrelated hydrogel systems.

Data extraction and analysis

The data extraction process was conducted meticulously to ensure the findings were accurate and reliable. Points considered in recording data include the type of hydrogel, the method of preparation, the modification of functionality, the adsorption capacity, the removal efficiency, the regeneration ability, and the kinetic or isotherm models applied. Statistics were used to calculate numerical ranges, the average adsorption value, and the kinetic applicability to provide a reliable overall performance assessment.

Quality assessment

The quality of the individual studies included in this systematic review was determined through a fair and objective analysis based on clear experimental designs, data replication, and adequate reporting of detailed study findings. Ideally, inconsistencies across studies or ambiguities in findings were resolved through joint discussion and verification by a team of reviewers, in an effort to maintain objectivity and increase validity within this systematic assessment.

Results

In total, 31 peer-reviewed articles were found that used chitosan hydrogel as an adsorbent for the removal of dyes and heavy metals from contaminated water systems. These studies collectively underline the strong adsorption potential, high removal efficiencies, and versatility of chitosan hydrogels and their composites. The final selection of articles was influenced by predefined criteria that included experimental studies reporting clear adsorption parameters, kinetic and isotherm modeling, and regeneration performance. The data synthesis provides a comprehensive overview of progress in applying chitosan hydrogel systems for wastewater treatment across a range of pollutants, including synthetic dyes and heavy metals.

In the first stage of database search, a total of 122 records were found. Among them, 62 documents were obtained from Scopus and 60 from PubMed. After excluding 65 non-relevant or duplicate records, 57 articles were selected for screening. A total of 26 articles were then excluded for lacking post-quantum adsorption data, being book chapters, not performing kinetic or isotherm modeling, or involving non-related hydrogel systems. A total of 31 qualitative studies met the criteria and were analyzed in detail. The PRISMA graphical illustration is shown in Figure 2.

Figure 2: The PRISMA flow diagram of a systematic review. Click here to view Figure

Natural polysaccharides

Natural polysaccharides are polymeric carbohydrate molecules composed of repeated monosaccharide units connected by glycosidic linkages. These naturally occurring biopolymers are utilized due to their bioresorbable properties, hypoallergenicity, non-immunogenicity, chemical stability, and non-toxicity, making them suitable for various applications.¹³ They are abundantly available in natural resources and can be processed at a relatively low cost. They possess various functional groups, such as hydroxyl, amine, and sulfonate, which facilitate chemical or physical bonding. Biocompatible hydrogels are commonly derived from polysaccharides, including chitin, starch, cassia gum, mucilage, pectin, guar gum, chitosan, and cellulose. The following section will provide a detailed examination of chitosan polysaccharides.

Chitosan

The chitosan is a carbohydrate extracted from algae & fungi and consists of an N-acetyl-D-glucosamine monomer attached to each other through a (1–4) linkage¹⁴ as shown in Figure 3. The white, powdery, flaky, iridescent, and non-toxic linear cationic polysaccharide is chitosan.¹⁵ The primary process for producing chitosan involves the deacetylation of chitin using an alkaline chemical. However, it is also a naturally occurring carbohydrate found in green algae and fungi. After cellulose, chitin is the most prevalent biopolymer. It is an intractable, weakly biodegradable polymer.  Nonetheless, chitin's derivative, called chitosan, is a well-known biopolymer because it is non-toxic, biocompatible, less expensive, and soluble in acidic solutions. Eukaryotes such as fungi, insects, and crabs produce chitin.¹⁶ Chitin generally makes up about 20% to 30% of the weight of crustacean shell waste. Nonetheless, specific lobster genera have the highest chitin proportion of any chitin-containing animal, with chitin contents ranging from 60% to 75% by weight. The main characteristics of chitosan are changed by its molecular weight and degree of acetylation.  These significantly impact functional and practical properties, such as dissolvability, biodegradability, the ability to impart the material structure, and other bioactive properties. Functional groups such as hydroxyl groups, secondary hydroxyl groups (OH) and primary amines are present in the chitosan molecule. These functional groups can be chemically altered without altering the degree of polymerization. Also, they allow chitosan to form hydrogen bonds and electrostatic interactions with heavy metals and dyes. Chitosan offers the advantages of a high recovery rate and reasonable cost compared to well-known materials like activated carbon for adsorption.¹⁷ Besides its antioxidant, fungistatic, and antimicrobial activity, it can be used as a substrate to produce films. The properties render it applicable in agriculture, biomedicine, and the environment. It can be used in the agricultural industry as a seed treatment agent and biopesticide to help plants resist fungal diseases. It can be used to safeguard the wine in the course of production, and it can also be used as a fining agent.¹⁸

Figure 3: Structure of Chitosan has been adapted from Oyekunle et al (2024).19 Click here to view Figure

Other polysaccharides

Several polysaccharides, including cellulose, mucilage, starch, pectin, sodium alginate, glucan, guar gum, agar, and their derivatives, as well as chitosan, have enormous potential for adsorbing dyes and heavy metal ions, making them suitable for environmental remediation. Such polysaccharides are like mucilage in that they possess many natural origins, such as plants, algae, and microorganisms, which makes them sustainable and biodegradable.²⁰ Starch is a polysaccharide made of glucose and is mainly obtained in fruits, pulses, roots, tubers (e.g., potatoes), and cereals. It consists of units of amylose and amylopectin.²¹ Cellulose is the most widespread polysaccharide, being derived from the extraction of cell walls of plants (e.g., cotton, wood), and containing B-1,4-linked glucose units. Pectin is a heteropolysaccharide of galacturonic acid produced by extracting the peel of citrus and apples.²² Sodium alginate is extracted using brown algae, i.e., Laminaria and Macrocystis, which contain mannuronic and guluronic acid units. B-glucan is derived from fungi, yeast, and cereals, i.e., oats and barley.²³ Guar gum is a polysaccharide galactomannan of non-ionic nature extracted from the seeds of guar bean (Cyamopsis tetragonoloba).²⁴ The agar is a polysaccharide comprising galactose and is obtained from red algae, such as Gelidium and Gracilaria. Enteromorpha polysaccharide is a sulfate polysaccharide that is derived from green algae Enteromorpha prolifera.²⁵ The polysaccharides are divided into homopolysaccharides (e.g., cellulose, starch, glucan, agar) along with heteropolysaccharides (e.g., pectin, sodium alginate, guar gum, Enteromorpha polysaccharide). The functional groups give their adsorption properties as they contain hydroxyl (-OH), carboxyl (-COOH), and sulfate (-SO₃H) groups (Table 1). The cellulose and starch contain hydroxyl groups that facilitate the hydrogen bonding with dyes, while pectin and sodium alginate contain carboxyl groups that enable the exchange of ions with heavy metal ions like Pb²⁺ and Cd²⁺.²⁶ Sulfate groups in the Enteromorpha polysaccharide enhance the electrostatic interactions with cationic dyes, and the non-ionic features of guar gum enable the van der Waals interactions.²⁷ The structural diversity and the large variety of functional groups in these polysaccharides make them useful platforms for hydrogel synthesis, which will boost the performance of mucilage-based systems in environmental applications. This is because of their broad accessibility from agricultural, algal, and microbial sources, which accentuates their efficacy in providing a renewable mode of removal.

Table 1: Polysaccharides, their parent groups, sources, and functional groups.

Polysaccharide hydrogel for dye and heavy metal removal

The characteristics of biomaterial-based hydrogels mean they can be used in environmental regeneration programs. They could be beneficial in removing different pollutants from water bodies as they are able to absorb water. The most common type of biopolymer hydrogel is natural polysaccharides. Such materials are malleable and can undergo drastic changes to enhance their application.³⁵ Consequently, they are the best at eliminating pollutants. Hydrogels based on polysaccharides have attracted the interest of many researchers. This section evaluates the use of chitosan-based hydrogels, reinforced with other materials as adsorbents for the removal of contaminants (heavy metals & dyes), along with their manufacturing processes and properties. The adsorption properties for dyes and heavy metals removal using natural polysaccharide-based hydrogel adsorption interactions that have already been examined will be discussed.

Removal of dyes using Chitosan hydrogel

Chitosan is a cationic macromolecule present in abundance in nature and can also be degraded by living things. It is often used to produce hydrogels.³⁶ It has exceptional biological characteristics and degradability, which have led to its widespread application in tissue engineering, drug release, and skin restoration. It is also possible to consume in powdered or other edible forms, as -NH₂ and OH groups are present in chitosan, this substance has a decent adsorption and chelation capacity toward metallic ions or anionic dyes.³⁷ The pH level affects the amino group because it is protonated and deprotonated in basic and acidic environments. As per, the pendant group exhibits swelling behaviors when ionized, primarily when pH is below its pKa value. Chitosan's use in wastewater treatment is restricted by its low porosity, insolubility in water, lack of active sites, and low mechanical strength.³⁸ Moreover, the mechanical durability of hydrogels derived from simple chitosan is low. To overcome constraints, chitosan-based hydrogels are synthesized using several organic and inorganic compounds, along with synthetic polymers, as additives. Wang et al., created a pH-CS interpenetrating hydrogel and CMC by cross-linking it with epichlorohydrin.³⁹ They synthesized an amphoteric hydrogel, demonstrating swelling and adsorption in both acidic and basic environments. Several chitosan (CS) and carboxymethyl cellulose (CMC) solution ratios (8/2, 7/3, 6/4, and 5/5) were prepared using the solution casting process, mixed, and constantly swirled before the cross-linker was added. Investigations were conducted on removing Acid Orange II (ve) and MB (+ve). High swelling capacity carboxymethyl cellulose/chitosan (CMC5/CS5) hydrogel absorbed, 120 mg/g of MB, and 100 mg/g of Acid Orange II (AO II) at pH 11 & 2, respectively (Table 2). The adsorption process demonstrated that at an alkaline pH, CMC's COOH group ionizes to COO. & interacts in electrostatic interactions with MB. On the other hand, the chitosan NH₂ NH₃⁺ group demonstrated the most excellent affinity for AO II when protonated at pH levels above the acidic range. The hydrogel's potential uses across a broad pH spectrum are limited by its effectiveness, which is limited to specific pH ranges.³⁹

Le et al. tested the removal of Congo red (CoR) and Brilliant Blue FCF (BBF) with a novel chitosan hydrogel activated with carbon dioxide. It was prepared using carbon dioxide gas passed through an adsorption tank containing a dye solution and chitosan. CO₂-activated chitosan showed a greatly increased absorption capacity of the dye compared to chitosan in pure water.⁴⁰ Both high temperatures and the initial adsorbate concentration had an impact on the dye adhesion on the hydrogel design. It was demonstrated that BBF bonded more rapidly than Congo Red (CoR) because the chitosan and -NH₂ groups in CoR are electrostatically repelled. At temperatures ranging from 25 to 35 degrees Celsius, this hydrogel dissolves in an aqueous solution when the pH is lowered. Remarkably, when heated to 45 and 55 degrees Celsius, the hydrogel remained inflated and did not disintegrate in the solution. The hydrogel is stable at high pH values but unstable at low ones, and chitosan crosslinking is likely responsible for its increased stability between 45°C and 55°C.⁴⁰ Hydrogel networks utilize clays to leverage their flexibility and wide adsorption capacity. Ferfera-Harrar et al., utilized Triton X-100 as a surfactant to develop a hydrogel nanocomposite (clay-based) for the removal of Basic Red 46.⁴¹ The matrix was hydrolyzed polyacrylamide, the polymer was chitosan, and the nanofiller was montmorillonite. By performing radical graft copolymerization of PAAm monomers, its stability and physicochemical properties improved, making it pH and temperature-sensitive. In a NaOH solution, hydrolysis of the hydrogel occurred, and it produced active groups, including amide anions, carboxylates (COO–), amines (NH2), and hydroxyl groups (OH) in the hydrogel. This procedure is convenient for the preparing of an ampholytic adsorption-like matrix. The nanocomposite's porosity and swelling were enhanced by adding a small amount of Montmorillonite (MMT) into the grafted matrix while applying a pore-generating agent, Triton X-100. With a 2-weight percent MMT addition, the hydrogel's adsorption ability rose from 1813 mg/g to 1550 mg/g. Physisorption was demonstrated by the isotherm data's perfect agreement with the Langmuir model (LM). The QM of the hydrogel has increased to 2 weight percent with the addition of MMT clay. Excess clay content lowered adsorption parameters (qe and R%). MMT particles occupy more network pores and interstitial spaces, reducing available space. MMT concentration increased network crosslinking due to hydrogen bonding interactions between clay hydroxyl groups and matrix components, such as chitosan (CTS) and hydrolyzed polyacrylamide (PAAm). Reduced network flexibility reduced water and dye penetration.⁴¹ Sharma et al., developed a superabsorbent composite hydrogel film consisting of citric acid-crosslinked Beta-cyclodextrin, chitosan, and polyvinyl alcohol (B-CD/Ch/PVA) that can effectively remove the dyes tartrazine (TG) and CoR from aqueous solutions. Fourier-transform infrared spectroscopic analysis verified that the functional groups -OH & -NH₂ were present, while Field emission scanning electron microscopy demonstrated a rough, porous surface morphology.⁴² BET analysis confirmed the mesoporous characteristics of the hydrogel, revealing a mean pore diameter of 4.12 nm along with a surface area (SA) of 17.34 m²/g. The adsorption study was carried out using batch experiments & the Box-Behnken design model was used to optimize it. At a pH of neutral to slightly acidic, the hydrogel showed 95.38% and 96.3% removal of CoR & TG dye under ideal circumstances. The adsorption data supported monolayer adsorption, as evidenced by the matching of the Langmuir isotherm and the PSO kinetic model. The highest adsorption capabilities for CoR and TG have been identified as 1436.04 mg/g and 366.34 mg/g, respectively (Table 2). Endothermic or spontaneous adsorption is a product in a thermodynamic study, primarily driven by hydrogen bonds and electrostatic interactions between the dye molecules and the hydrogel matrix. B-CD/Ch/PVA hydrogel demonstrated significant reusability, retaining up to 85% of its removal efficiency across five adsorption–desorption cycles.⁴²

Table 2: Utilization of Chitosan-based hydrogels for dyes and heavy metals adsorption. Click here to view Table

Removal of heavy metals using Chitosan hydrogel