Application Progress of Magnetic Chitosan in Heavy Metal Wastewater Treatment

Abstract

Wastewater containing heavy metals can come from a variety of sources and is extremely toxic and hard to break down. Conventional treatment methods can easily result in secondary pollution and are expensive. The research on magnetic chitosan composites, a new adsorbent in the treatment of heavy metal wastewater, is methodically reviewed in this paper. It offers a theoretical foundation for the creation of more environmentally friendly and effective wastewater treatment technology by examining its preparation and modification technology, adsorption mechanism, and application performance. This paper provides a summary of the technology used to prepare and modify magnetic chitosan composites. Both the cross-linking and co-precipitation methods are thoroughly examined. A summary of the fundamental process of heavy metal ion adsorption is provided, along with information on the chemical and physical impacts. Of these, chemical adsorption has been shown to work well with the majority of heavy metal adsorption systems. According to application research, magnetic chitosan exhibits good adaptability in real-world industrial wastewater treatment and has outstanding adsorption performance for various heavy metal ion types and multi-metal coexistence systems (including synergistic/competitive effects). Lastly, the optimization of the material preparation and modification process, the mechanism influencing the various coexisting ion types, and the improvement of regeneration ability should be the main areas of future development.

1. Introduction

Mining, metallurgy, electroplating, the chemical industry, printing and dyeing, agriculture, and other industries are the primary sources of heavy metal wastewater [1]. Lead (Pb), mercury (Hg), cadmium (Cd), chromium (Cr), and copper (Cu) are examples of common pollutants [2]. These heavy metal ions can build up in organisms through the food chain and are extremely toxic, refractory, and bioaccumulative [3,4]. They can also cause ecological imbalances in water and soil, as well as cancer, damage to the nervous system, and other health issues in humans [5]. According to estimates, heavy metal pollution costs the global economy up to tens of billions of dollars annually. Because industrialization is accelerating in developing nations, the issue of heavy metal wastewater discharge is especially severe there [6]. Therefore, coming up with a practical and affordable way to remove heavy metal ions from wastewater has become a hotspot for environmental research. Despite their benefits, conventional heavy metal wastewater treatment technologies (like chemical precipitation, ion exchange, and membrane separation) have drawbacks like high costs, a high risk of secondary pollution, and limited applicability (like low wastewater treatment efficiency for low concentrations) [7,8]. Because of its strong adsorbent designability, low cost, and easy and flexible operation, adsorption is regarded as an effective heavy metal removal technology [9]. Metal oxides, biomass, and activated carbon are examples of common adsorption materials [10]. When compared to conventional adsorption materials (like synthetic resin and activated carbon), chitosan (CS), a natural polysaccharide, offers the advantages of being renewable, highly biocompatible, and having a broad source [11,12]. Through coordination and electrostatic interaction, the molecular chain’s amino (–NH₂) and hydroxyl (–OH) groups can be effectively joined with heavy metal ions.

D-glucosamine and N-acetyl-D-glucosamine units combine to form the common natural polymer compound known as chitosan. It is a byproduct of chitin deacetylation and has β-1,4-glycosidic bonds connecting its molecular chain [13]. The primary instruments for adsorbing aquatic contaminants are the reactive functional groups, such as amino and hydroxyl groups, in the CS structure. It is a research hotspot for eliminating heavy metals, dyes, and fluorides due to its broad sources, biodegradability, and non-toxicity [14]. However, chitosan’s adsorption capacity and large-scale application are restricted because of its poor mechanical qualities, poor pH sensitivity, and poor thermal stability [15,16]. Researchers have created magnetic chitosan (MCS) composites by adding magnetic nanoparticles (like Fe₃O₄) in an effort to overcome the application bottleneck of chitosan-based materials. The external magnetic field can swiftly extract the superparamagnetism found in magnetic materials from the solution [17]. Magnetic chitosan composites are created by combining chitosan with magnetic particles. The two are frequently used to eliminate heavy metal ions from water and can work in tandem. Typical magnetic particle materials include composite ferrite materials like MFe₂O₄ (where M is Mn, Cu, Zn, and Co, etc.) and basic ferrite materials like magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) [18]. Furthermore, through surface modification (such as thiolation or carboxylation), magnetic chitosan can enhance the selective adsorption capacity of particular heavy metals [19], offering specialized solutions for intricate heavy metal wastewater systems.

When conducting bibliometric analysis with visualization of similarities viewer (VOS viewer), we searched all of the Web of Science Core Collection’s literature using the keywords “heavy metal” and “treatment”, and we found 80,980 publications overall, including 31,770 within the last five years. This indicates that the treatment of heavy metals is currently a major area of research interest. And using the keyword “magnetic chitosan”, 3780 articles from the previous five years were found. Figure 1a displays the VOS viewer cloud map. The term “adsorption” has the strongest correlation, according to our findings. In order to find 1316 articles published over the previous five years, we additionally added the keyword “adsorption”. Figure 1b displays the pertinent VOS viewer cloud map. With 66 instances of the term “heavy metal ion” and a range of chemical symbols of heavy metal ions displayed on the VOS viewer cloud map, the study of magnetic chitosan adsorption of heavy metal ions is receiving more and more attention.

Few thorough review papers exist despite the large number of studies on the adsorption of heavy metals by magnetic chitosan. Specifically, there are not many thorough reports on the adsorption mechanism of heavy metal ions or the application evaluation of different magnetic chitosan materials and preparation techniques. Furthermore, a comprehensive analysis of magnetic chitosan’s potential for large-scale practical application is lacking in related research. The structural characteristics, methods for preparing and modifying magnetic chitosan, and potential applications of various preparation methods were all methodically presented in this paper. A thorough analysis of the magnetic chitosan adsorption mechanism was conducted. The impact of a multi-metal coexistence system on magnetic chitosan’s adsorption performance—including synergistic and competitive effects—was investigated further after the adsorption performance of magnetic chitosan on single heavy metal ions was studied. This paper specifically provides an overview of the current state of research on magnetic chitosan’s ability to treat real heavy metal wastewater and uses this analysis to confirm its potential for widespread use. Lastly, a summary of the current state of magnetic chitosan treatment of heavy metal wastewater and the direction of future research is provided.

2. Toxic Effects of Heavy Metals

2.1. Toxic Effects of Lead (Pb)

One of the most prevalent environmental contaminants is lead, which can be found in mineral mining, industrial wastewater, and agricultural fertilizer. Aquatic organisms, including fish and aquatic invertebrates, are particularly vulnerable to the toxic effects of lead because it interferes with their nervous system, which affects their ability to breathe and feed, which in turn affects their growth and ability to reproduce [20]. Long-term exposure to lead-contaminated water causes aquatic species to grow slowly and have a reduced ability to reproduce. Lead poisoning in humans can cause immune system suppression, kidney damage, neurological damage, and cognitive dysfunction, particularly in young children [21]. Lead can enter the human body through the food chain, drinking water, and other sources. In extreme situations, it may result in symptoms of lead poisoning, including hypertension, anemia, and abdominal pain.

2.2. Toxic Effects of Mercury (Hg)

Mercury is a highly toxic metal that finds extensive use in agriculture, industry, and medicine. There are primarily two types of mercury found in water: inorganic and organic. Of these, organic mercury (like methylmercury) has a high biological accumulation, is easily consumed by aquatic life, moves up the food chain, and eventually poses a major risk to human health [22]. Mercury’s toxicity to aquatic life is characterized by immune system and endocrine system damage, as well as nervous system inhibition, particularly in fish. When humans consume aquatic products tainted with mercury, they may harm their nerves, liver, kidneys, and even fetuses and newborns, which can be fatal [23].

2.3. Toxic Effects of Chromium (Cr)

Chromium is a common heavy metal in industrial emissions, especially in electroplating, leather treatment and dye production [24]. Hexavalent chromium is more toxic than trivalent chromium, and the two main forms of chromium in water are hexavalent chromium and trivalent chromium. Hexavalent chromium can also penetrate the cell membrane and cause oxidative damage after entering the cell, resulting in DNA damage, mutation, and cancer [25]. Long-term exposure to chromium-containing water bodies can cause abnormal growth and development of aquatic organisms and even death. In humans, hexavalent chromium-contaminated water can increase the risk of cancer, liver damage, and skin ulcers.

2.4. Toxic Effects of Other Heavy Metals

Other common heavy metals found in water include cadmium (Cd), arsenic (As), zinc (Zn), cobalt (Co), and nickel (Ni), in addition to lead, mercury, and chromium. Aquatic life and human health are both negatively impacted by these metals. For instance, liver, kidney, and bone damage can result from cadmium poisoning [26]. Because arsenic is carcinogenic, prolonged exposure can cause lung and skin cancers [27]. The human body needs zinc as a trace element because it plays a role in DNA repair and cell metabolism. On the other hand, chronic overconsumption can harm the kidney, liver, and nervous system, and it can result in symptoms like nausea and vomiting [28]. Even though cobalt is a necessary trace element for all living things, too much of it can cause abnormal growth and development as well as decreased fecundity in aquatic organisms. Humans who are exposed to high levels of cobalt over an extended period of time may develop kidney, heart, and lung damage [29]. However, nickel, a common industrial pollutant, primarily causes lung disease, cancer, and skin allergies through contact with the skin and respiratory tract [30].

In addition to harming aquatic life, heavy metals in water have the potential to upset the ecosystem’s delicate balance. The function of the ecosystem can be impacted by the reduction or extinction of species in the food chain due to the buildup of heavy metals [31]. For instance, the redox process of water will be impacted by the decline in benthic animals, further degrading the water’s quality. In addition, heavy metal pollution will alter water bodies’ biodiversity and weaken aquatic ecosystems’ resilience. As a result, heavy metal pollution may cause a broader ecological crisis in addition to harming species that are directly exposed.

3. Preparation and Modification Technology of Magnetic Chitosan

3.1. Coprecipitation Method

One traditional technique for creating magnetic chitosan composites is co-precipitation [32]. The basic concept is that in alkaline conditions, Fe²⁺/Fe³⁺ (molar concentration ratio of 1:2) hydrolyzes and precipitates to form Fe₃O₄, and chitosan molecular chains coordinate to form and fix magnetic particles in the CS matrix in situ. Ali and associates [33] made magnetic Fe₃O₄ particles using the co-precipitation technique while adjusting the pH in a NaOH solution. Then, using the dispersion precipitation method of cerium nitrate, CeO₂ was loaded onto the surface of Fe₃O₄ particles. Ultimately, CSFe₃O₄@CeO₂ was created by fixing and cross-linking it in CS solution that had been dissolved in oxalic acid using glutaraldehyde. Additionally, the materials made using the co-precipitation method are easily adaptable to various situations. Suo et al. [34] used co-precipitation to create magnetic Fe₃O₄ particles and used composite modification to create magnetic chitosan nanoparticles (CS-Fe₃O₄). Lastly, a carrier (IL-CS-Fe₃O₄) for the immobilization of porcine pancreatic lipase (PPL) was created by modifying imidazole’s functional ionic liquid (IL).

Due to the gradual mixing of iron salts and chitosan, magnetic chitosan materials made using traditional co-precipitation techniques, like the one described above, are susceptible to uneven particle dispersion even though they have strong functional composite capabilities [35]. For the in situ synthesis method, which is based on the co-precipitation method’s optimization, the Fe³⁺ salt is directly dispersed in the chitosan acid solution. A reducing agent is then added to convert some of the Fe³⁺ to Fe²⁺, and the pH is then adjusted to produce Fe₃O₄ [36]. Sathiyaseelan et al. [37] used a modified one-step in situ synthesis method to dissolve CS in acetic acid, directly added it to the prepared K₂TeO₃ solution, and continuously stirred to finally collect CS-Te-NPs with antibacterial and antioxidant properties. The production of porous multi-level magnetic chitosan composites is made simple by the one-step in situ synthesis method and the spatial confinement effect provided by the chitosan molecular chain, which can greatly enhance particle dispersion. Yang et al. [38] created a novel kind of honeycomb-shaped chitosan beads in chitosan solution using a one-step co-precipitation method, that contained evenly distributed manganese dioxide and ferric oxide nanoparticles (MnO₂-Fe₃O₄/CH).

3.2. Crosslinking Method

One of the key technologies for creating magnetic chitosan composites is the cross-linking method. The main idea is to create a composite material with both magnetic responsiveness and functional adsorption by stably combining magnetic nanoparticles (like Fe₃O₄) with CS using a cross-linking agent. Physical and chemical cross-linking are the two types of cross-linking techniques. The chemical cross-linking method combines the cross-linking agent with the active groups (such as amino groups and hydroxyl groups) in the chitosan molecule through covalent bonds to construct a stable three-dimensional network structure and magnetic nanoparticles [39]. The core mechanism includes covalent bond formation, synergy and functional modification. Jiang et al. [40] created a composite material (m-CS-c-PAM) with a broad pH (2–10) adaptation range by first creating magnetic nano-iron particles using the co-precipitation method and then using glutaraldehyde as a cross-linking agent to firmly bind the covalent bonds created by CS, magnetic Fe₃O₄, and polyacrylamide (PAM).

The physical cross-linking method is a green preparation technique based on non-covalent bonds, as opposed to the chemical cross-linking method, which depends on the inflexible structure of covalent bonds. Through ion cross-linking, hydrogen bonds, or electrostatic interaction, the magnetic nanoparticles and chitosan are joined. It is more environmentally friendly than the chemical cross-linking method [41]. Additionally, choosing the right cross-linking agent can increase CS’s solubility in acidic media. Karimi et al. [42] discovered through acid hydrolysis experiments that chitosan and κ-carrageenan cross-linked a magnetic biosorbent by using κ-carrageenan as a cross-linking agent. In addition to effectively preventing chitosan from dissolving in acidic media (weight reduction of roughly 4%), κ-carrageenan can also improve magnetite nanoparticle stability in the cross-linked biosorbent.

Table 1. Types of common crosslinking agents and properties of prepared materials.

Crosslinking Type	Crosslinker	Name of Magnetic Chitosan Adsorbent	Adsorption Object	Adsorbability	Isolation and Regeneration	Reference
Physical crosslinking	Polyphosphates	MCS-PEI	U(VI)	Q = 181.8	n = 4, r = 89.8	[43]
Physical crosslinking	Citric acid	CSC	Cr(VI)	Q = 172	/	[44]
Physical crosslinking	Kappa-carrageenan	Magnetic bionanocomposite adsorbent based on kappa-carrageenan	MB	Q = 123.1	n = 5, r > 90	[45]
Chemical crosslinking	Glutaraldehyde	CHT-GLA/ZnO/Fe3O4	RBBR dye	Q = 176.6	/	[46]
Chemical crosslinking	Silane coupling agent	FS-MC	Emulsified oil	Under the optimal dosage, R = 92.98–94.47; under the optimal pH, R = 69.97–96.3	n = 5, r = 69.97–81.31	[47]
Chemical crosslinking	Epichlorohydrin	EP-DBSB-CS@SrFe12O19	Pb(II), Cd(II)	QPb(II) = 103.5, QCd(II) = 73.5	/	[48]

Q/Q_e: the maximum capacity of adsorbent (mg/g), e represents different adsorption objects; R: adsorption object removal rate (%); n: adsorbent cycle times (times); r: the removal rate of the adsorption object after the adsorbent cycle n times (%); i: after n cycles, the adsorption rate of the adsorbed object (%).

currently lists the most popular kinds of crosslinking agents along with the characteristics of the materials that are produced. It is evident that magnetic chitosan adsorption materials made by physical and chemical crosslinking are highly effective at removing heavy metal ions from wastewater as well as treating wastewater that contains oils and dyes. They also have the capacity to regenerate and reuse themselves. The use of chemical crosslinking agents, primarily synthetic ones, can result in the introduction of toxic residues (like glutaraldehyde’s cytotoxicity), and the reaction conditions (temperature, pH) have greater requirements for controlling the degree of crosslinking. The physical crosslinking technique, which primarily uses natural crosslinking agents, is gentle and does not require the use of hazardous chemicals. Although physical crosslinking materials have low mechanical strength and are prone to dissociating in harsh environments (such as strong acid/alkali), biocompatibility offers notable advantages. Chemical crosslinking is appropriate for situations requiring high material stability (like heavy metal adsorption), whereas physical crosslinking is better suited for biomedical applications (like drug carriers). This illustrates the two techniques’ synergistic complementarity. To strike a balance between functionality and safety, future research can concentrate on the creation of intelligent responsive composite systems or green crosslinking agents (like enzyme-catalyzed crosslinking).

3.3. Other Preparation Modification Methods

Two types of very traditional technologies for the production of magnetic chitosan materials are the co-precipitation method and the cross-linking method. Both procedures have some flaws, despite the fact that they are stable and mature. In recent years, researchers have come up with a number of creative ways to overcome the drawbacks of conventional approaches. Bai et al. [49] suggested a brand-new electrodeposition technique. The CS was in situ coordinated with Cu and Ni ions to promote the synthesis reaction after the suspended nanoparticles (NP) were first coordinated with the regulated surface dissolution of the acidic electroplating solution. Ultimately, a Cu and Ni-based magnetic chitosan independent film (MCFF) was created. The embedding of NP during the electrodeposition process generates the magnetism. The preparation of magnetic chitosan using the electrodeposition method exhibits the potential for high precision and environmental friendliness, particularly in situations requiring specialized film materials. However, the issues of process optimization and equipment cost still need to be resolved for its industrial promotion. The preparation of magnetic chitosan using the electrodeposition method exhibits the potential for high precision and environmental friendliness, particularly in situations requiring specialized film materials. However, the issues of process optimization and equipment cost still need to be resolved for its industrial promotion. The “electric” preparation method is also the foundation of electrospinning. A high voltage electric field is used to stretch the polymer solution or melt into nano/microfibers. These fibers are then joined with magnetic nanoparticles and CS to create a material that has both an adsorption function and magnetic responsiveness. Philippou et al. [50] used electrospinning to create superparamagnetic polyvinylpyrrolidone/chitosan/Fe₃O₄ nanofibers. At pH 6, the uranium (U) adsorption capacity in aqueous solution peaked at 183.3 mg/g.

While the ion imprinting method can remove specific ions and exhibit unique limiting ability, the aforementioned methods are part of the comprehensive adsorption filtration method [51]. The fundamental idea is to create an adsorption material with particular recognition sites by combining the functional monomer with the template molecule (target ion). The introduction of magnetic cores, cross-linking polymerization, template elution, and the combination of template ions with functional monomers are the primary processes. Hajri et al. [52] created a modified chitosan polymer ligand (PBCS) by grafting an amide bond with chitosan after first synthesizing a Schiff base ligand. It was then crosslinked by glutaraldehyde and eluted to create mercury ion imprinted adsorption material (Hg-PBCS) after complexing with Hg (II) ions to form a polymer complex. The experiment demonstrated that the adsorbent’s maximum capacity for Hg (II) adsorption could be 315 mg/g. Through the “template-polymerization-elution” mechanism, the ion imprinting method, despite its complicated and expensive preparation, gives magnetic chitosan high selectivity, rapid separation, and strong stability, making it particularly well-suited for the targeted removal of organic pollutants and heavy metals. It is generally advised to use chitosan nanoparticle derivatives with enhanced magnetic and electrostatic capabilities, which are better suited to handling complex wastewater systems [53]. In contrast to other industrial uses, wastewater treatment favors ultrafine nanoparticles smaller than 100 nm because these particles have a higher surface area and can absorb more contaminants. The modified chitosan magnetic nanoparticles made using various synthesis techniques had different properties, according to an analysis of the materials’ characterization results using standard characterization techniques (such as Fourier transform infrared spectroscopy and scanning electron microscopy). For instance, some electron microscopy pictures revealed that the modified chitosan magnetic nanoparticles made using the co-precipitation method had a rougher surface, which was better for CS’s amino exposure. As a result, the adsorption of heavy metal ions frequently uses the magnetic chitosan materials made using this technique. In real-world applications, the adsorption target and material characterization should be examined alongside the optimal preparation technique. Figure 2 depicts the common methods for preparing and modifying magnetic chitosan materials today.

Table 2. Advantages and disadvantages of different preparation methods and application potential.

Method	Dominance	Defects	Application Potential	Cost
Coprecipitation method	The process is simple, easy to modify, controllable porous structure, compatible with multi-functional components.	Stepwise mixing is easy to cause uneven dispersion of particles.	High—process mature, mild conditions.	Low
Crosslinking method	It has stable structure, high mechanical strength and long-term cycle use resistance.	There are many steps, high requirements for crosslinking conditions, and toxic residues may be introduced.	Moderate—there are many steps in the middle-reaction, and batch production is feasible.	More than middle
Electro chemical deposition	The film layer is uniform and controllable.	High equipment requirements, small output.	Low—limited by production and equipment size.	Moderate
Electrospinning method	It has large specific surface area, rich pore structure and easy functionalization.	Precise control of solution and spinning conditions is required.	Moderate—it can be scaled but requires professional equipment.	More than middle
Ion imprinting technology	High selectivity, strong anti-interference	Template removal is complex and site stability is limited.	Low—template cost and regeneration limitations.	More than middle

displays the analysis of application potential as well as a comparison of the benefits and drawbacks of various preparation techniques. The co-precipitation method has the greatest potential for scale, a mature process, mild conditions, and continuous production. Despite its exceptional stability, the cross-linking method has a lot of steps, delicate conditions, and a marginally lower scale efficiency. Both the electrodeposition and electrospinning techniques are appropriate for small and medium-sized batch production, but they depend on the equipment and process stability. The cost of the template and the effectiveness of regeneration limit the ion imprinting method, and industrialization is challenging. The co-precipitation method is the most cost-effective because it uses inexpensive raw materials and is a straightforward process; the crosslinking, electrospinning, and ion imprinting methods are medium to expensive because they require a crosslinking agent, a special solvent, or a template. The electrodeposition method has a low material cost, but it requires a significant equipment investment. From the standpoint of industrial applicability, the crosslinking method’s high stability makes it appropriate for industrial applications requiring high structural strength and long-term cycle conditions. The co-precipitation method is popular in the fields of environmental treatment and catalysis because of its adaptability and affordability. Water treatment applications that call for specialized functional membranes are better suited for electrodeposition and electrospinning. The highly selective separation of metal ions is one of the special benefits of the ion imprinting method; however, in order to accommodate large-scale industrialization, the process and cost must be optimized.

4. Adsorption Mechanism of Magnetic Chitosan

4.1. Physical Adsorption

The multi-scale structural properties of magnetic chitosan (pore filling effect, van der Waals force, and electrostatic interaction) are primarily responsible for its physical adsorption. High-density adsorption sites are made up of the numerous polar amino and hydroxyl groups found in chitosan’s molecular chain. For instance, the addition of Fe₃O₄ nanoparticles controls the material’s pore structure and surface charge distribution through interface interaction in addition to giving it magnetic response properties [54]. According to studies, MCS’s specific surface area can range from 80 to 150 m²/g, and its average pore size is typically between 2 and 50 nm. For the physical adsorption of heavy metal ions, this hierarchical pore structure offers the perfect mass transfer channel and enrichment area. Guo et al. [55] discovered that the magnetic chitosan-graphene oxide material’s pore size distribution ranged from 2 to 20 nm, confirming its mesoporous nature. Notable peaks were located at 3.75 nm, 10.04 nm, 11.57 nm, 13.20 nm, and 14.38 nm. The size of mesoporous material can efficiently create the transport pathway for adsorbate diffusion to the pore surface, which will encourage Pb(II) ion adsorption.

Dispersion, induction, and orientation forces are all included in the van der Waals force. All molecules experience this type of force. Although the range of action is broad, the strength is weak. Scanning electron microscopy was used to observe the adsorbent’s morphological properties. Yuan et al. [56] discovered that the prepared inorganic clay modified magnetic chitosan adsorbent (ICMMCA) had cracks of varying sizes dispersed across its surface. These fissures improve the adsorption of Cr(VI) by increasing the adsorbent’s specific surface area and strengthening the Van der Waals force. In acidic media, chitosan will simultaneously undergo protonation to form -NH³⁺ groups, which will result in electrostatic attraction with the solution’s negatively charged heavy metal complex ions. The adsorption capacity of magnetic graphene oxide-chitosan hybrid (MGOCS) for Zn(II) exhibits a trend of first increasing and then decreasing with increasing pH [57]. This is because the amino group on the surface of MGOCS is highly protonated at low pH values, making it more likely to bind H⁺. Meanwhile, Zn(II) will not be able to reach the adsorbent’s active center due to the electrostatic repulsion produced by H⁺. Repulsion gives way to attraction in the electrostatic interaction. High pH causes the surface of MGOCS to form a complex Zn(II) anion, which transforms electrostatic attraction into repulsion.

4.2. Chemical Action

The chemical mechanism of magnetic chitosan adsorption of heavy metals mainly includes the following three aspects: coordination and chelation, redox reaction and ion exchange.

(1) Coordination and chelation

Coordination is the process by which lone pair electrons from the empty orbitals of heavy metal ions and single amino or hydroxyl groups on the chitosan molecular chain form coordination bonds. It possesses the qualities of reversibility, neutral pH dependence, and quick reaction. Bulin et al. [58] using N1s spectra, found that the –C(=O)NH- group, which is created by the amidation reaction between the hydroxyl groups of graphene oxide and the –NH₂ of chitosan, showed a blue shift in its sub-peak from 399.24 eV to 399.89 eV after adsorption, while its peak proportion dropped from 94.74% to 38.57%. This suggests that the lone pair electrons of N in –C(=O)NH- transferred to Hg(II) during the adsorption processes. A five- or six-membered cyclic chelate must be formed by the combination of the same metal ion with several functional groups. It has high selectivity, structural rigidity requirements (the degree of crosslinking and the flexibility of the CS molecular chain influence the spatial arrangement of chelating sites), irreversible properties, and thermodynamic stability that is noticeably higher than that of monodentate coordination.

(2) Redox reaction

The –NH₂ and –OH on the chitosan molecular chain can be used as electron donors to directly participate in the reduction reaction. At the same time, the valence state conversion of magnetic nanoparticles such as Fe²⁺/Fe³⁺ in Fe₃O₄ can also indirectly reduce heavy metals through electron transfer. The process can be summarized as three steps: electron transfer → adsorption and complexation → co-precipitation. Zhang et al. [59] found that there were two bands belonging to Cr(III) by XPS photoelectron spectroscopy, indicating that Cr(VI) was partially reduced during the adsorption process. Compared with CS and chitosan/graphene oxide composites (CS/GO), chitosan/magnetite-graphene oxide (CS/MGO) composites had stronger reduction ability (86.74%), which confirmed that not only the oxygen/nitrogen-containing groups in CS were involved in the reduction process, but that the magnetic nanoparticles could also facilitate charge transfer to further improve the reduction ability.

(3) Ion exchange

Through ion exchange adsorption, the –NH₂ of CS is protonated in an acidic environment to create –NH₃⁺, which is then coupled with heavy metal cations (like Pb²⁺, Cd²⁺, and Cu²⁺). Protonation and charge attraction → replacement reaction → coordination complexation are the two stages that make up the process. Eltaweil et al. [60] discovered that following the adsorption of Cr(VI) by O1s spectra, the peak intensity of –OH decreased somewhat, confirming the presence of an ion exchange mechanism between –OH and Cr(VI).

The surface morphology of the adsorbent, magnetism, and adsorption parameters like pH, time, adsorbent concentration, wastewater temperature, and initial pollutant dose all affect the mechanism and kinetics of heavy metal adsorption by magnetic chitosan [61]. Figure 3 illustrates the chemical and physical process by which magnetic chitosan adsorbs heavy metal ions.

4.3. Mechanism Model and Equation

Given how well magnetic chitosan removes different metal pollutants, merely stating the removal rate or adsorption capacity is insufficient to capture its essence and inform material design. the various metal ions’ chemical forms as well as the system’s parameters (pH, temperature, coexisting ions, etc.). it will work together to ascertain the energetic characteristics, rate-determining steps, and site occupation. Thus, under a single theoretical framework, a quantitative correlation that can be tested between the experimental data and the adsorption/exchange/complexation/reduction mechanism must be established. This enhances the results’ interpretability, comparability, and transferability. The kinetic model can be used to differentiate between the surface reaction and diffusion control of materials, while the isothermal model can explain the site characteristics and saturation behavior [62]. The spontaneous and endothermic nature of the reaction can be ascertained through thermodynamic analysis [63]. In order to make the physical and chemical mechanism of the adsorption of heavy metal ions by magnetic chitosan easier to understand, we also provide some representative reaction formulas. The chemical reaction formulas and representative models used in this study are listed below.

(1) Isothermal adsorption model

① Langmuir (single layer/uniform point)

$$q_e={\displaystyle \frac{Q_{max}{K}_L{C}_e}{1+K_L{C}_e}}, R_L={\displaystyle \frac{1}{1+K_L{C}_0}}$$

(1)

where q_e is the equilibrium adsorption capacity (mg/g); Q_max is the Langmuir theoretical saturation capacity (mg/g); K_L is the Langmuir adsorption constant (L/mg); C_e is the equilibrium liquid phase concentration (mg/L), C₀ is the initial liquid phase concentration (mg/L); R_L is the separation factor (dimensionless). When R_L = 0, it indicates irreversible adsorption, 0 < R_L < 1 indicates favorable adsorption, and R_L = 1 indicates unfavorable adsorption.

② Freundlich (multilayer/non-uniform surface)

$$q_e=K_F{C}_e^{1/n}$$

(2)

where K_F is the Freundlich adsorption constant ((mg/g)/(L/mg)^1/n); n is the empirical constant (dimensionless) related to the adsorption strength. When n > 1, the adsorption is favorable.

③ Redlich-Peterson (combined model)

$$q_e={\displaystyle \frac{P{C}_e}{1+\alpha C_e^{\beta }}}$$

(3)

where P is the adsorption constant of Redlich-Peterson; α is related to the surface heterogeneity of the adsorbent; β is used as an index (dimensionless).

(2) Kinetic model

① Pseudo-first-order (often related to liquid film diffusion/weak binding)

$$q_t=q_e(1-e^{-k_{1}t})$$

(4)

where q_t is the adsorption capacity of time t (mg/g); k₁ is the rate constant of Pseudo-first-order (1/min); t is the contact time (min).

② Pseudo-second-order (often directed to coordination/ion exchange dominant)

$$q_t={\displaystyle \frac{k_2{q}_e^{2}t}{1+k_2{q}_{e}t}}$$

(5)

where k₂ is the rate constant of Pseudo-second-order ((g/(mg·min)).

③ Weber-Morris (related to intraparticle diffusion)

$$q_t=k_p{t}^{0.5}+C$$

(6)

where k_p is the rate constant of Weber-Morris ((mg/(g·min^0.5)); C is the constant of the reaction boundary layer effect (mg/g).

(3) Thermomechanical analysis

$$∆G^0=-RT\ln K_c, \ln K_c={\displaystyle \frac{{∆S}^0}{R}}-{\displaystyle \frac{{∆H}^0}{RT}}$$

(7)

where K_c is the thermodynamic equilibrium constant (dimensionless); R is the ideal gas constant (8.314 J/(mol·K)); T is the thermodynamic temperature (K); ΔG₀ is the standard quasi-Gibbs free energy (J/mol); ΔH₀ is the enthalpy change (J/mol); ΔS₀ is the entropy change (J/(mol·K)). ΔG₀ < 0 indicates that the reaction is spontaneous, and ΔH₀ > 0/< 0 is often related to the release of solvated water and the decrease in interfacial order.

(4) Chemical equation

① Metal coordination/chelation (-NH₂/-OH sites)

$$R-N{H}_2+M^{2+}\rightleftharpoons R-N{H}_2·M^{2+}$$

(8)

$$2R-OH+M^{2+}\rightleftharpoons {\left(R-O\right)}_{2}M+2H^{+}$$

(9)

② Ion exchange (protonated amine sites)

$$R-N{H}_3^{+}+M^{2+}\rightleftharpoons R-N{H}_2·M^{2+}+H^{+}$$

(10)

$$R-N{H}_3^{+}+A^{-}\rightleftharpoons R-N{H}_3^{+}·A^{-}$$

(11)

③ Metal oxide surface complexation (taking As as an example)

$$\equiv M_e-OH+H_{2}As{O}_4^{-}\rightleftharpoons M_e-HAs{O}_4^{-}+H_{2}O$$

(12)

④ Redox (taking Cr as an example)

$$HCr{O}_4^{-}+3F{e}^{2+}+7H^{+}\to C{r}^{3+}3F{e}^{3+}+4H_{2}O$$

(13)

$$C{r}^{3+}+3R-CO{O}^{-}\rightleftharpoons R-[COO{]}_{3}Cr$$

(14)

where M is some cationic metal; R–NH₂, R–OH, R–COO⁻ and R–NH₃⁺ are amine/hydroxyl/carboxylic acid/protonated amine sites on chitosan and its derivatives, respectively; A⁻ is a metal anion form; ≡M_e–OH is a hydroxyl site on the surface of the oxide (M_e = Fe, Zr, etc.).

5. Study on the Application of Magnetic Chitosan in Heavy Metal Wastewater Treatment

5.1. Study on the Adsorption Properties of Single Heavy Metal Ions

5.1.1. Lead (Pb²⁺)

One common toxic heavy metal that can seriously harm both human health and the ecological environment of water is lead ion (Pb²⁺). Its sources are primarily classified as human activities, such as heavy metal mining, smelting, and battery manufacturing, and natural sources, such as dust from volcanic eruptions, flying ground dust particles, and sea salt aerosols, among other natural phenomena released into the environment. Inorganic lead can be methylated by bacteria, and it typically occurs in the environment as divalent inorganic, though it can also exist as tetravalent [64]. There are numerous amino and hydroxyl groups in the magnetic chitosan molecule. These functional groups have the ability to readily adsorb lead ions and coordinate with them. Rusmin et al. [65] used palygorskite as the skeleton and Fe₃O₄ as the magnetic particles to create magnetic chitosan-palygorskite (MCP) nanocomposites in a single step. The adsorbent’s maximum capacity to adsorb Pb²⁺ in water was 58.5 mg/g. It was monolayer adsorption, according to the Langmuir isotherm model (R² = 0.98). Pb²⁺ was eliminated by surface complexation (binding to –NH₂, –OH of CS, and Si-O groups of palygorskite) and chemical reduction (Pb²⁺ was reduced to Pb⁰ by Fe²⁺), according to XPS and FTIR analysis. Modified magnetic chitosan materials have given people new options as the need for heavy metal wastewater environmental remediation and water quality detection grows. Wu et al. [66] successfully created renewable ion-imprinted magnetic biocomposites (IIMB) by using Fe₃O₄ as the magnetic component, Pb²⁺ as the template ions, and serratia marcescens and carboxymethyl chitosan (CMC) as carriers. The adsorbent’s average adsorption capacity can exceed 70 mg/g, and its adsorption affinity for Pb²⁺ is noticeably higher than that of other heavy metal ions. FTIR and XPS confirm that physical adsorption, electrostatic attraction, and complexation are the primary methods of Pb²⁺ adsorption.

5.1.2. Cadmium (Cd²⁺)

The primary sources of cadmium ions (Cd²⁺) in the water environment are industrial processes like mining, smelting, electroplating, and battery manufacturing. In an acidic environment, it exists in the divalent state, and in neutral or alkaline conditions, it readily forms precipitates, such as cadmium hydroxide. In addition to destroying aquatic ecosystems and suppressing microbial activity, this common heavy metal can also seriously impair human health by affecting nerve and liver tissues. Symptoms like lightheadedness, coughing, vomiting, diarrhea, pneumonia, and bone pain can be brought on by its biological buildup [67]. As a result, eliminating Cd²⁺ from wastewater is crucial. Pure CS has poor mechanical qualities and dissolves readily in acidic environments. Following magnetic modification, Cd²⁺’s adsorption capacity and practicability can be greatly increased. Rahmi et al. [68] created magnetic PEDGE-MCh microspheres by crosslinking them with the non-toxic crosslinking agent polyethylene glycol diglycidyl ether (PEDGE) and embedded Fe₃O₄ into chitosan using the reverse phase dispersion method. According to FITR analysis, the Redlich-Peterson isothermal model (R² = 0.9996) was followed by the adsorption mechanism of Cd²⁺, which was dominated by electrostatic interaction and O/N functional groups. It demonstrates that the surface adsorption is multilayer heterogeneous. Relevant reports state that when chitosan reacts with cross-linking agents, it loses its amine active sites, which lowers the amount of pollutant it adsorbs [69]. The synergy between groups, particularly those containing O and N, is crucial to the adsorption of heavy metals by magnetic chitosan. Grafting modification technology is currently an effective solution to this issue. Rahmi et al. [70] increased the variety of functional groups by introducing –COOH through glycine grafting to create bonds with MCS’s –NH₂ and –OH. The formation of Cd–O bonds was confirmed by FTIR analysis, which also revealed an increase in O/N functional groups. When the pH was 5 and the contact time was 30 min, the Cd²⁺ removal rate was greater than 98%. The material could be used three times, according to the regeneration experiment (efficiency > 80%).

5.1.3. Chromium (Cr³⁺ and Cr⁶⁺)

As one of the most dangerous sources of heavy metal pollution, chromium ions have long been a hot topic in environmental protection [71]. Statistics show that over half of the chromium-containing wastewater released annually from the metallurgy, tanning, electroplating, and other industries surpasses the World Health Organization’s (WHO) recommended drinking water safety level of 0.05 mg/L. Trivalent chromium [(Cr(III)] and hexavalent chromium [(Cr(VI)] are the two valence states of chromium found in wastewater. Because of its high toxicity and persistence, Cr(VI) has been designated as a priority pollutant in comparison to Cr(III). The creation of magnetic CS adsorbents that simultaneously reduce Cr(VI) to Cr(III) in order to achieve simultaneous removal of both has currently emerged as a development trend in addition to the investigation of traditional adsorption techniques for the removal of Cr(VI) or Cr(III) [72]. Huang et al. [73] used Fe₃O₄ nanoparticles as the core, CS coating, and amino-functionalized modification (APTMS) to create a novel magnetic amino-grafted chitosan composite (Fe₃O₄@CS-APTMS). The issue of traditional chitosan dissolving in an acidic environment was resolved, and the maximum adsorption capacity of Cr(VI) was 269.54 mg/g (pH = 2, T = 298 K). The partial reduction of Cr(VI) [of which 65% was accounted for by Cr(III)] and its fixation by chelation were confirmed by XPS and density functional theory (DFT). The environmental scope and capacity of various grafting modification materials to extract Cr(VI) from MCS vary. Li et al. [74] created Fe₃O₄ nanoparticles in situ using CS matrix, and added –SH and –SO₃H groups using epichlorohydrin (ECH) crosslinking and L-cysteine grafting. MCB–ECH–SH/SO₃H, a functionalized magnetic chitosan bead, was created. The adsorbent’s maximum total chromium adsorption capacity was 293.46 mg/g at pH 5 and 60 °C, and the removal rate of Cr(VI) exceeded 89% in the pH 1–7 range. And in just one hour, the low concentration of Cr(VI) (8 mg/L) was 95% removed.

5.1.4. Mercury (Hg²⁺)

A highly hazardous heavy metal, mercury can be found in nature in a variety of forms, including elemental, inorganic, and organic forms. As a prevalent form of mercury, mercury ions exhibit potent bioaccumulation and biomagnification properties. Mercury ions harm aquatic ecosystems by preventing photosynthesis and respiration in aquatic plants, affecting aquatic animals’ reproductive, immune, and neurological systems, and seriously harming the human body’s vital organs, including the kidneys, liver, and nervous system. According to WHO regulations, drinking water cannot contain more inorganic mercury than 6 parts per billion (30 nM) [75]. The scientific community has made mercury removal technology a primary area of study due to the significant health risks associated with wastewater containing mercury. It has been demonstrated that adsorption is an easy and efficient way to remove mercury from wastewater. Fu et al. [76] used tannic acid to cross-link poly (m-aminothiophenol) and CS to create a novel magnetic network polymer composite (MCTP) based on magnetic-mesoporous silica nanoparticles. The network structure of MCTP is abundant in functional groups for sulfur, nitrogen, and oxygen. Within 15 min, the capacity and adsorption rate of Hg(II) were 245 and 49 mg/g and 98 and 16 percent, respectively. According to the Langmuir model, the maximum theoretical adsorption capacity was 515−46 mg/g, and the material stability was high. After five uses, the adsorption capacity stayed between 173 and 44 mg/g. The adsorbent made from naturally extracted CS exhibits strong adsorption performance and regeneration cycle ability when compared to commercial CS. Azari et al. [77] created a magnetic glutaraldehyde crosslinked chitosan (MCS–GA) nanocomposite by removing chitosan from shrimp shell, loading Fe₃O₄ nanoparticles using the co-precipitation method, and then crosslinking with glutaraldehyde. When applied to chlor-alkali industrial wastewater (Hg²⁺ concentration of 345.21 mg/L), the removal rate was 91.03 percent, and the maximum adsorption capacity for Hg(II) was 96 mg/g at pH 5.0 and 25 °C. After 12 cycles, the adsorption capacity can still surpass 75%.

5.1.5. Copper (Cu²⁺), Nickel (Ni²⁺) and Other Metals

Magnetic chitosan has been extensively employed in the adsorption of various heavy metal ions, including copper (Cu²⁺), manganese (Mn²⁺), zinc (Zn²⁺), uranium (U⁶⁺), arsenic (As⁵⁺) and antimony (Sb³⁺), in addition to the common heavy metal ions mentioned above. Specifically, nickel (Ni²⁺), cobalt (Co²⁺), and gold (Au³⁺) are valuable resources with crucial tactics. As indicated in

Table 3. Adsorption of other heavy metal ions by magnetic chitosan materials.

Name of Magnetic Chitosan Adsorbent	Adsorption Object	Adsorption Condition	Adsorbability	Isolation and Regeneration	Reference
IIMCD	Cu(II)	pH = 5, T = 25 °C, t = 2 h, C = 800–900	Q = 78.1	n = 10, q decreased 8%	[78]
Ch/g-hnts@ZnγM	Mn(II)	pH = 9, T = 30 °C, t = 60 min, C = 40	R = 87.1	/	[79]
C-Fe2O3 NPs	Zn(II)	pH = 6, T = 25 °C, t = 60 min, C = 3	R = 99.8	n = 3, r = 65.1	[80]
MP@[Chi-CPTMS(1/4)-SiO2]	Au(III)	pH = 5, D = 20 mg, t = 2 h.	Q = 112	n = 5, A = 39.2	[81]
P-MCS	Co(II)	T = 298 K, t = 25 min, D = 1 g/L, C = 100	Q = 46.1	n = 5, i = 85.26	[82]
MIIPs	Ni(II)	pH = 7, t = 1 h, T = 298 K, D = 0.05 g	Q = 18.5	n = 15, q decreased about 10%	[83]
Fe3O4@CHT@p(GMA)	U(VI)	pH = 6, t = 120 min, T = 25 °C, C = 1–400	Q = 328.4–434.7	n = 5, q > 87% of the initial adsorption capacity	[84]
MCMB	As(V)	pH = 7, T = 298 K, D = 1 g/L	Q = 21.63	n = 5, i > 76	[85]
MCC	Sb(III)	pH = 3–10, T = 25 °C, D = 0.4 g/L	Q = 38.234	n = 3, i > 75.54	[86]

T: reaction temperature; t: reaction time; D: adsorbent dosage; C: adsorption object concentration (mg/L); Q: maximum capacity of adsorbent (mg/g); R: adsorption object removal rate (%); n: adsorbent cycle times (times); r: the removal rate of the adsorption object after the adsorbent cycle n times (%); i: after n cycles, the adsorption rate of the adsorption object (%); q: after n cycles, the adsorption capacity of the adsorption object (%), A: recovery rate (%).

, numerous investigations into the use of magnetic chitosan for adsorption and recycling have also been conducted by researchers.

It is evident from the aforementioned research that the magnetic chitosan composite material, which can be made by varying the raw materials (such as CS source, various magnetic particles, and crosslinking agents, etc.), has been widely used in the field of removing heavy metal ions in wastewater. The adsorption performance of magnetic chitosan composite materials is improved by the introduction of modified grafting groups, strengthening of the CS structure, and use of synergistic effects through preparation techniques (e.g., ion imprinting technology to strengthen the removal of special ions). On the one hand, adding various functional groups (like carboxyl, amino, and phosphate groups) can improve the material’s binding force and selectivity by specifically enhancing its coordination, complexation, or electrostatic interaction with heavy metal ions. For instance, carboxyl and phosphate groups greatly boost the adsorption capacity through multidentate coordination, whereas amino groups help form stable complexes with soft acidic metal ions. However, functionalization can also enhance the material’s mechanical strength, chemical stability, and dispersibility, allowing it to retain high recycling efficiency in intricate water environments. On the other hand, unmodified magnetic chitosan frequently has limited active sites and poor structural stability, making it challenging to strike a balance between the demands of effective adsorption and real-world uses. Thus, in addition to being the primary method of performance enhancement, functional modification is also the crucial component that makes industrial application feasible.

The common magnetic chitosan composite adsorbent conforms to the Langmuir or Freundlich isotherm model, suggesting that the adsorption of heavy metal ions by magnetic chitosan may be single-layer adsorption or multi-layer adsorption. The adsorption process of heavy metal ions by magnetic chitosan typically follows the pseudo-second-order kinetic model, which indicates that the adsorption process is primarily controlled by chemical adsorption. Furthermore, variables like solution pH, temperature, and initial heavy metal ion concentration also have an impact on the adsorption capabilities of magnetic chitosan. Additionally, a number of variables, including solution pH, temperature, competitive ions, and example strength, influence the adsorption performance of magnetic chitosan. The amino group is easily protonated in acidic environments, which reduces the number of adsorption sites. The deprotonated amino and hydroxyl groups are more likely to coordinate with metal ions in the weakly acidic to neutral environment, greatly increasing the adsorption efficiency. The behavior of adsorption is also influenced by temperature. The stability of the chitosan matrix may be destroyed by an excessively high temperature, but heating typically aids in improving the diffusion of metal ions on the material’s surface. Conversely, the presence of competitive ions may occupy adsorption sites and decrease the rate at which target metals are removed, whereas single-component systems can achieve higher capacity. Furthermore, by shielding charge, ionic strength and background electrolyte concentration also influence the binding capacity between metal ions and functional groups. Thus, a thorough analysis of these variables can more accurately assess magnetic chitosan’s potential for use in wastewater treatment.

5.2. Multi-Metal Ion Coexistence System

Compared to synthetic wastewater, industrial wastewater has a far more complex composition. Consequently, when using adsorbents in an industrial setting, the impact of coexisting ions in wastewater should be carefully taken into account. These coexisting ions may have the following effects on the removal of heavy metal ions in actual water bodies: (1) Synergistic effect: Certain cations may interact with the active groups on the surface of magnetic chitosan to alter their chemical characteristics and surface charge distribution, increasing the adsorption capacity of heavy metal ions. Furthermore, certain anions may combine with heavy metal ions to improve their stability in solution and, consequently, the magnetic chitosan’s adsorption efficiency. (2) Competitive role: This situation is more common than synergy. On the one hand, coexisting ions may reduce the adsorption of heavy metal ions by competing with them for the active adsorption sites on the magnetic chitosan surface [87]. For example, a high concentration of alkali metal ions (like potassium and sodium ions) in the solution will reduce the adsorption efficiency of heavy metal ions by competing with them for amino and hydroxyl groups on the surface of magnetic chitosan. However, coexisting ions have the potential to alter the solution’s chemical characteristics, including its pH level and ionic strength. which consequently influences how well magnetic chitosan adsorbs heavy metal ions. For instance, the adsorption capacity of magnetic chitosan will be reduced in acidic conditions due to hydrogen ions competing with heavy metal ions for the active groups on its surface; in a solution with high ionic strength, the electrostatic interaction between ions will be reduced, which will also impact the adsorption effect of magnetic chitosan on heavy metal ions.

The radius of the hydrated ion and the ion charge are significant variables influencing the competitive adsorption outcomes in the multi-metal ion coexistence system. The metal ion has the advantage of preferential adsorption because, generally speaking, the higher its electronegativity, the stronger its attraction to the adsorbent, and the smaller its hydrated ion radius, the lower its mass transfer resistance [88,89]. Tang et al. [90] investigated the impact of coexisting divalent metal ions [Ca(II), Mg(II), Cu(II), and Zn(II)] on Pb(II) adsorption by biochar modified with iron-manganese oxide. It was discovered that other metal ions had minimal impact on its adsorption, while Cu(II) had the biggest effect. This is because Cu(II) (1.90) has the largest electronegativity value and the smallest hydrated ion radius (4.19 Å) when compared to the other three metal ions. It is also the closest to Pb(II) (electronegativity value 2.33, hydrated ion radius 4.01 Å). This outcome clearly shows that cations with a smaller hydration radius and higher electronegativity can readily occupy the adsorption site through chelation or coordination reactions. Numerous factors, including the type and concentration of coexisting ions, the pH of the solution, and the temperature, influence the synergistic/interfering effects of coexisting ions [91]. The interference effect on the adsorption of heavy metal ions is generally more noticeable at higher concentrations of coexisting ions. The charge characteristics of the magnetic chitosan surface and the presence of heavy metal ions are influenced by the pH level of the solution. The synergistic/interfering effect of coexisting ions will be impacted by the temperature change since it will alter the kinetic and thermodynamic characteristics of the adsorption process. Figure 4 illustrates the interference/synergistic effect of magnetic chitosan on heavy metal adsorption in the multi-metal coexistence system.

5.3. Actual Industrial Wastewater

For the treatment of heavy metal wastewater, magnetic chitosan’s triple benefits of “multi-functional group chelating + magnetic separation + renewable design” demonstrate its high efficiency, economy, and environmental friendliness. In terms of adsorption capacity, selectivity, cycle stability, and operation efficiency, it is currently better than conventional materials, according to a wealth of research data, and it may find use in industry. A summary of how certain chitosan materials are used in real industrial wastewater is provided in

Table 4. Application of magnetic chitosan in actual industrial wastewater.

Name of Magnetic Chitosan Adsorbent	Adsorption Object	Adsorption Condition	Adsorbability	Isolation and Regeneration	Reference
MC@CS	Cr(VI)	pH = 3, D = 60 or 70, C0 = 55.67 mg/L	D = 60 mg: C < 0.01, D = 70 mg: R = 100	In the actual wastewater experiment: n = 10, q = 68	[92]
Fe3O4-MoS2@CS	Cr(VI)	pH = 4, D = 1 g/L, C0 = 10 ppm	R = 97	n = 3, r = 89	[93]
MCM	Hg(II)	C0 = 21.65 mmol/L	C < 0.05	n = 3, q = 96.89% of the initial adsorption capacity	[94]
Nano-Fe3O4/chitosan-acrylamide hydrogel	Pb(II)	pH = 5, t = 8 min, D = 0.02 g, C0 = 0–15 μg/L	A > 97	n = 3, i > 90	[95]
Fe3O4@UiO-66-NH2/CTS-PEI	Cu(II), Ag(I)	pH = 4.93, C0(Cu) = 23.83 mg/L, C0(Ag) = 37.82 mg/L	C(Cu) = 0.64, R(Cu) = 97.32; C(Ag) = 0.82 mg/L, R(Ag) = 97.7	n = 5, r(Cu) = 82, r(Ag) = 79	[96]
TMBC	Cd(II)	pH = 3.86, D = 0.25 g, t = 6 h, C0 = 4.9 mg/L	R ≈ 100	n = 5, i = 84.83	[97]
Fe3O4-CSN	V(V), Pd(II)	pH = 5(V), pH = 6(Pd), C0 = 10 mg/L, D = 1.5 g/L, t = 10 min, T = 20 ± 1 °C	R(V) = 99.99, R(Pd) = 92.3	/	[98]

T: reaction temperature; t: reaction time; D: adsorbent dosage; C₀: adsorption object concentration; C: the effluent concentration of the adsorption object (mg/L); R: adsorption object removal rate (%); n: adsorbent cycle times (times); r: the removal rate of the adsorption object after the adsorbent cycle n times (%); i: after n cycles, the adsorption rate of the adsorption object (%); q: after n cycles, the adsorption capacity of the adsorption object (%), A: recovery rate (%).

. As a novel form of water treatment material, it is evident that it performs exceptionally well in eliminating heavy metal ions from water. However, in the detailed investigation, coexisting ions have a major impact on its removal effect. The intricacy of the ion composition in water must be thoroughly taken into account in practical applications due to the presence of interference and synergy. Furthermore, whether magnetic chitosan materials can be promoted in real-world applications is determined by their recycling performance. From one perspective, materials made of magnetic chitosan that can be recycled can drastically lower the cost of treating industrial wastewater, as well as lessen waste production and its detrimental effects on the environment. However, one of the most important factors in ensuring the material’s stability and dependability over time is its capacity for recycling.

At the same time, magnetic chitosan composites exhibit special structural and functional advantages over conventional adsorbents like activated carbon and biochar. Because of their large specific surface area, variety of raw materials, and affordability, activated carbon and biochar have found extensive use in industrial applications. Nevertheless, their selectivity is low, and their adsorption mechanism primarily relies on physical effects. Furthermore, issues like high energy consumption and secondary pollution are present, and the regeneration process typically calls for high temperatures or chemical agents [99]. Activated carbon typically has an adsorption capacity of 40–120 mg/g for heavy metal ions, with a higher peak value. Its regeneration, however, typically calls for a strong oxidant or high temperature heat treatment, which uses a lot of energy and quickly causes pore structure collapse and subpar recycling. On the other hand, biochar typically has adsorption capacities of 40–100 mg/g and 15–60 mg/g for certain heavy metal ions, such as Cr(VI) and Cu(II) [100]. Specifically, it has been reported that modified biochar materials containing MgO prepared by adding modifiers can reach 1624.96 mg/g and 479.87 mg/g, respectively, for the maximum adsorption capacity of Pb(II) and Cd(II) [101]. This is greater than that of the majority of magnetic chitosan materials; however, in acidic environments, its adsorption performance dramatically declined. While the adsorption capacity of Cd(II) was nearly zero at pH = 2, that of Pb(II) dropped to 553.26 mg/g. Furthermore, biochar’s limited surface functional groups make it incapable of chelating multivalent metals effectively. On the other hand, magnetic chitosan not only offers a porous structure but also uses the hydroxyl and amino groups on the molecule to bind to metal ions in a specific way. By functionally altering the molecule, it can further increase its adsorption capacity and selectivity. According to the aforementioned research, MCS’s maximum adsorption capacity for heavy metal ions is typically between 80 and 120 mg/g, though some can reach 200–300 mg/g. Additionally, it exhibits high removal efficiency across a broad pH range, and after 5–10 cycles, the majority of them still maintain an adsorption rate of over 60%, demonstrating exceptional stability and repeatability. Furthermore, MCS can effectively remove heavy metal ions and quickly separate them using an external magnetic field, preventing secondary pollution and drastically lowering sludge treatment costs when compared to conventional chemical precipitation or ion exchange methods. Although MCS generally has a lower peak capacity for some metals than traditional adsorption materials, its benefits—such as cyclic regeneration performance, adaptability to changing working conditions, and ease of separation and recovery—make it more promising for widespread use in resource recovery and wastewater treatment.

Despite their exceptional adsorption capacity, selectivity, and recyclability, magnetic chitosan composites still face two major obstacles in their engineering applications. On the one hand, it is the material’s stability. The chitosan matrix may swell or degrade, the active sites are competitively occupied, and the surface charge environment is disrupted under extreme conditions like high salinity, strong acid/alkali, complexing agent, and multi-metal ion coexistence. These conditions attenuate adsorption efficiency and reduce cycle performance. In a strong oxidation/reduction environment, the magnetic core may also experience valence changes that compromise structural integrity and magnetic separation. These risks necessitate the introduction of hierarchical pores and interface engineering to shorten the mass transfer path and reduce site shielding, the expansion of selectivity through specific functional modification, and the improvement of skeleton tolerance through stable crosslinking. However, the stability of the chitosan source, the price of the crosslinking/modifier and magnetic nanoparticles, energy use, and equipment investment are all factors in large-scale preparation. Simultaneously, consideration should be given to the regeneration process’s reagent consumption and secondary pollution control. MCS is biodegradable and renewable in large-scale wastewater treatment applications, and appropriate recovery and regeneration following use can lower secondary pollution. MCS offers advantages over conventional adsorbents in terms of environmental friendliness. However, there may be an environmental cost associated with using metal salts or organic cross-linking agents in large-scale synthesis processes. To lower possible risks, low-toxicity modification techniques and green synthesis must be developed. Although MCS is beneficial in increasing wastewater treatment efficiency and encouraging the sustainable use of water resources, its industrialization is also constrained by factors like cost, energy consumption, and equipment investment. Low-toxic/bio-based crosslinking agents and recyclable modifiers can be developed, modular and continuous preparation and regeneration processes can be enhanced, and green solvents and mild processes can be given priority in order to lower overall costs and increase sustainability. Although the magnetic chitosan composite material generally exhibits high selective adsorption and good environmental friendliness, it still requires improvement and breakthroughs in cost control and industrialization.

6. Conclusions and Perspective

A range of adsorption mechanisms and preparation and modification methods are available for magnetic chitosan adsorption materials. Because of their effective performance and benefits for the green cycle, they are currently widely used in the treatment of heavy metal wastewater. Co-precipitation and cross-linking are the two most widely used techniques in the technology of magnetic chitosan preparation. A stable core–shell structure can be formed using the co-precipitation method. The dispersion of particles can be enhanced by the in situ synthesis technique. Physical and chemical cross-linking are the two categories of cross-linking techniques. Although the preparation material for chemical cross-linking is more stable, it may leave harmful residues behind. Although the physical cross-linking is non-toxic, it can easily separate in harsh conditions. Their individual flaws must be compensated for using a composite cross-linking technique. Additionally, there are cutting-edge preparation modification techniques like ion imprinting, electrospinning, and electrodeposition. Different situations call for different approaches. Both chemical and physical adsorption are components of the magnetic chitosan’s heavy metal adsorption mechanism. The material’s multi-scale structural properties, including pore filling, van der Waals force, and electrostatic interaction, are the source of physical adsorption. Coordination, chelation, redox, and ion exchange are all examples of chemical action. The adsorbent’s surface morphology, magnetism, and adsorption conditions all affect the adsorption process. According to application research, the magnetic chitosan adsorbent exhibits good recycling capabilities in addition to a good adsorption effect on a variety of heavy metal ions. In real wastewater treatment, it exhibits good adaptability and can continue to operate steadily in specific environmental circumstances. According to application research, a range of heavy metal ions can be effectively adsorbed by the magnetic chitosan adsorbent. About 80–120 mg/g of adsorption capacity can be sustained, and some can even reach 200–300 mg/g. After five to ten cycles, the magnetic chitosan material can still retain over 60% of the heavy metal adsorption rate, demonstrating its good recycling ability. Furthermore, in real wastewater treatment, it exhibits good adaptability and can continue to operate steadily in specific environmental circumstances. In a broad pH (2–7) range, the majority of magnetic chitosan materials can continue to exhibit good adsorption performance. Chemical adsorption dominates the adsorption process, which typically follows the Freundlich or Langmuir model. Additionally, the coexisting metal ions in the wastewater will interfere with or have a synergistic effect on the adsorption process. Competitive adsorption will result from the difference in ion electronegativity and hydration radius, and coexisting ions may interfere with selectivity. Despite their promising future in wastewater treatment, magnetic chitosan composites still face obstacles like low stability, high cost, and water complexity in large-scale industrial applications. To overcome these challenges and advance its development to useful production applications, it will be necessary in the future to optimize the low-cost green synthesis process, improve material stability, and integrate regeneration technology. Future research should focus on improving the preparation of magnetic chitosan and its composites, examining the ways in which different coexisting ion types impact heavy metal removal, and determining the stability and efficacy of magnetic chitosan’s secondary use in a range of water quality conditions.