Insights of CMNPs in water pollution control

Abstract

The various toxic contaminants such as dyes, heavy metals, pesticides, rare‐earth elements, and hazardous chemicals are the major threats to all the flora and fauna. Owing to the harmful ill effects caused by the toxic contaminants, it is necessary to eliminate these compounds from the authors’ ecosystem. The chitosan magnetic nanomaterials (CMNPs) are one of the superior materials used in the wastewater treatment through various conventional technologies. The chitosan is a natural source obtained from the crustacean shells of crabs, prawns etc. The magnetic nanomaterial prepared by the reinforcement of chitosan is highly effective in the removal of heavy metals, dyes, organic matter, and harmful chemicals. It is used in various technologies such as adsorption, flocculation, immobilisation, photocatalytic technology, and bioremediation. This possesses unique surface and magnetic characteristics, Moreover, it is simple, economically feasible, and eco‐friendly material used efficiently in wastewater treatment. This review paper depicts the overview of CMNP in the industrial effluent treatment.

1 Introduction

Water is one of the most essential components of our mother nature and all the living organism in this universe depend on water for their survival. In the current era, the tremendous growth of science and technology on the other side has negatively influenced our environment [1]. The progressive explosion of population growth, industrialisation, and urbanisation lead to the admittance of various harmful contaminants into the environment [2]. Water pollution is an augmenting environmental issue faced globally due to the expulsion of environmental pollution and deterioration of water assets [3]. Among the various environmental problems, water deficit has become one of the predominant problems faced by human beings in our everyday life. Meanwhile, the continuous growth of human populace has directly resulted in a deficiency of fresh water sources which remain as the basic necessity for every agricultural, household, and industrial sector. According to the survey conducted by the World Health Organization, it is inferred that half of the global population will face severe water scarcity before the year 2025 [1]. Water contamination due to the addition of toxic components such as heavy metals, dyes, and pesticides becomes a genuine problem globally [4]. All kinds of living organisms are dependent on water for their survival, most predominantly human beings and aquatic organisms get affected vigorously due to water pollution [4]. Owing to the high toxicity of the harmful components present in the wastewater, it is mandatory to get rid of those contaminants for the betterment of society [3]. Owing to the harmful effects of water contamination, Government has enunciated several rules and regulations for the discharge of industrial effluents into the environment; also, it has set certainly permissible for the various toxic contaminants discharged into the ecosystem [1].

To remove and treat the harmful contaminants abided in the wastewater, various advanced techniques such as ultrafiltration, biological treatment, chemical oxidation, chemical precipitation, reverse osmosis, electrochemical treatment, photocatalytic oxidation, bioremediation, and adsorption are implemented in the treatment of industrial effluents [2, 3, 4]. Even though all these techniques are capable of treating the noxious contaminants, they cannot be used often due to some of its disadvantages such as high cost, type of operation, low efficacy, and high‐energy consumption. In spite of all the new technologies introduced, nanomaterials have gained immense attention in the wastewater treatment sector and they have made an evolution in the engineering and material science fields [2]. Introduction of innovative advancements in the field of nanoscience has always led to the implementation of nanotechnology in water and wastewater treatment. Wastewater treatment utilising nanotechnology has gotten adequate considerations as a potential treatment method when compared with the customary treatment techniques [1]. In recent times, nanomaterials have been applied in various processes of wastewater treatment such as adsorption, catalytic oxidation, membrane process, sensing, and disinfection [1].

Nanomaterials are commonly portrayed as the materials in the size range of 1–100 nm. They are used in enormous application in various engineering, industrial, and medical sectors. Nanomaterials are highly preferred rather than other materials due to its outstanding properties such as low capital, high potent, and greater environmental friendliness. These materials possess phenomenal physical, chemical, thermal, optical, and electrical properties. Owing to its uniqueness, nanomaterials have been used in various sectors such as in drug delivery, sensors, catalytic reactions, electronics, industries, engineering processes, and also in wastewater treatment [5]. The nanomaterials generally possess a larger surface area with more number of active sites. They also possess higher surface free energy which is the main reason behind its noteworthy surface effectiveness. The size of the nanomaterial is an important characteristic because of which these nanomaterials are used in various applications. Owing to its outstanding properties, it is also implemented in the wastewater treatment process for the removal of harmful contaminants [1]. Mostly, the nanomaterials used in the wastewater treatment process are utilised in the form of nanoparticles (NPs). These NPs, when coming in contact with the wastewater, show some practical difficulties such as arduous separation, precipitation, cross‐reaction, harmful health disorders etc. To overcome the above problems, one of the successful strategy followed in the wastewater treatment is the implementation of nanocomposites. Nanocomposites are generally defined as the nanomaterials embedded in some particular supporting material which enhances the surface activity of the nanomaterials. Nanocomposites are of various forms such as gels, colloids, porous structures, and copolymers. The type of material on which the nanomaterial is embedded is one of the striking characteristic features in the synthesis of nanocomposites [1]. The nanomaterial is of various types depending on the type of material embedded in it; some of the well known NPs are carbon‐based NPs, titanium dioxide (TiO₂) based, iron oxide Ferric oxide (Fe₃ O₄), aluminium oxide, magnesium oxides, ferric oxide, cerium oxides etc. embedded NPs [1, 2]. The chitosan magnetic NPs (CMNPs) are new class of nanomaterials introduced effectively in the wastewater treatment sector and it must be reviewed in detail.

This review paper depicts the striking properties of the CMNPs. It highlights the appliance of CMNP in the wastewater treatment sector. The primary objective is to outline the various synthesis procedure, properties, and characterisation of the CMNP and their utilisation in sequestering various contaminants such as heavy metals, dyes, organic compounds, harmful chemicals etc., This paper also points out the main features of the CMNP to be taken into account while using it in a large‐scale industrial application.

2 Chitosan magnetic NPs

Chitosan is a natural polymer synthesised during the deacetylation process of chitin. It is chemically known as β ‐(1→4)‐linked D‐glucosamine (or 2‐amino‐2‐deoxy‐D‐glucose). The chitosan polymer is composed of random units of polyaminosaccharide arranged in a linear structure [6]. The chitosan polymer is derived from the cell wall of some organisms such as fungi, prawns, crabs, and arthropods [7]. Chitosan is extracted from the most prevalent polysaccharide called chitin which is used globally next to the cellulose polymer. Chitosan is an eco‐friendly organic polymer possessing good compatibility, hydrophilicity, degradability, and antibacterial activity [3, 7]. The NPs in common have a major drawback of getting aggregated in wastewater, so it is strengthened using some reinforcements. Chitosan is one of the major stabilising agent utilised along with the metal NPs to defeat the aggregation issue [8]. Chitosan polymer is found to be a promising agent for expanding the durability of the magnetic NPs in the aqueous solution. The presence of amine and hydroxyl groups in the chitosan polymer contributes to the enhanced removal of heavy metals and dyes present in the industrial effluent [9]. The chitosan polymers are not directly practised in the sequestration of heavy metal ions due to its feeble intransigence in acid media. To strengthen this chitosan in the acidic medium, the primitive crosslinking agents such as glutaraldehyde are used during the heavy metal adsorption process [10]. Owing to its simplicity in the segregation of the adsorbent from the aqueous solution, the polymer‐capped NPs are considered to be the best solution for sequestering various contaminants such as heavy metal ions and dyes. In the midst of several NPs, the Fe₃ O₄ magnetic NPs are observed to be the finest and efficient pollutant segregating tool due to some of its features such as simpler synthesis and outstanding magnetic properties [11]. The primary procedure to develop the CMNPs is divided into three parts such as impregnation, lamination, and diffusion of magnetic NPs with chitosan solution [12].

Chemical precipitation is a simple and fast technique used to deliver Fe₃ O₄ NPs. This precipitation method is considered to be a promising technique for the synthesis of magnetic NPs [7]. Even though the chemical precipitation method shows an effective sorption capacity, it is not preferred during the bulk production of CMNP. With the support of modern technological advancements, the limitations of the chemical precipitation are overcome. Fan et al. [12] have suggested the preparation of CMNP using a novel technique known as high‐gravity reactive precipitation method. This method is the perfect combination of rapid precipitation and high‐gravity process, also this method has been used in the preparation of various nanomaterials such as zinc sulphide, zinc oxide (ZnO), silicon dioxide (SiO₂), TiO₂ etc. Fan et al. is the first crew to implement this high‐gravity reactive precipitation method in the production of CMNP.

Thermal decomposition method is a common method utilised in the synthesis of Ferric oxide (Fe₃ O₃) NPs. In this method, the oxygen‐containing metal salts such as carbonates, acetates, and nitrates are disintegrated by heating at higher solvent temperatures until the bubbling occurs. Meanwhile, this method is an effective method because it is maintained at monitored and disciplined environmental conditions. The NPs with an appropriate size, distribution, and structure are better achieved in the chemical decomposition technique [7].

Crosslinking method is one of the effective methods for binding the magnetic NPs and chitosan; this method uses various chelating agents for the formation of a stronger bonding [12]. The chitosan materials are chosen as best reinforcing material for adsorption of industrial contaminants due to their outstanding surface properties and size. Basically, these chitosan‐embedded adsorbents are of micron and sub‐micron level in size, but they are not efficient when compared with the removal efficacy of nanosized chitosan adsorbents. The nanosized absorbents are highly preferred due to their higher surface area and lower diffusion resistance [13].

In present times, some of the innovative nanosized adsorbents such as NPs, magnetic NPs, carbon nanotubes, mesoporous NP, and metal ions reinforced nanomaterials are used in efficient wastewater treatment [14]. The evolution of the NPs in the wastewater treatment sector has paved the way to the amalgamation of various efficient NPs such as Fe₃ O₄ NPs. Owing to their ease of separating the Fe₃ O₄ NPs are very much preferred in treating a huge amount of industrial effluent within a limited time period. As a result of the various unique features of magnetic NPs, it is widely used now in various industrial applications and researches [15]. Sun et al. [8] have depicted the methodology to synthesise the chitosan‐stabilised Ferrous sulphide (FeS) NP and also it is characterised using various analyses such as Fourier‐transform infrared spectroscopy, X‐ray photoelectron spectroscopy (XPS), and X‐ray diffraction. In this work, it is inferred that the size of the synthesised NP was found to be 20 nm, whereas the external surface area was 21.3 m² /g. The XPS analysis of the chitosan–FeS NP confirmed the adsorption of mercury ions on the adsorbent. Esmaeili and Farrahi [11] have researched the ability of the CMNP combined with bacteria in eliminating the nickel (Ni) toxic metal ions present in the synthetic and industrial wastewater using a two‐stage reactor. They observed that the heavy metal removal capacities of synthetic and industrial wastewater using this CMNP combined bacterial adsorbent are 83 and 92.1%.

Nasirimoghaddam et al. [15] have investigated the productive removal of the mercury heavy ions from the industrial effluent and oil sludges using the chitosan‐coated Fe₃ O₄ NPs. In this work, the Fe₃ O₄ NPs are prepared using the co‐precipitation method and the prepared NPs were seemed to be narrowly dispersed particles of 10 nm in diameter. Fan et al. [16] have investigated the performance evaluation of the magnetic chitosan beads used in the elimination of heavy metal ions. In this paper, the Fe₃ O₄ magnetic particles are synthesised in two distinct proportions and evaluated using two approaches such as adsorption recovery index and effort vector data visualisation for determining the efficient method. Sullivan et al. [17] have conducted a study on the amalgamation of monodisperse chitosan NP using chitosan and tripolyphosphate and the impact of parameters such as molecular weight, pH, mass ratio, and initial concentration on the synthesis of monodisperse chitosan NPs are discussed. All the above factors are seemed to be affecting the particle size of the NP.

3 CMNPs in wastewater treatment

3.1 Adsorption

Adsorption is one of the highly preferred physical method utilised for the treatment of industrial wastewater. The substances such as mud, agricultural wastes, activated carbon, polymeric substances etc. are some of the commonly used adsorbents for removing hazardous heavy metal and dye compounds from the industrial effluents [3]. Chitosan has a few vital preferences that make it a powerful biosorbent. The main preferred standpoint is its minimal effort contrasted with business adsorbent [5]. The chitosan is considered to be one of the best reinforcement agents for increasing the stability of the magnetic NPs during the adsorption process in aqueous solution. The amine and hydroxyl functional groups abided in the chitosan enhance the heavy metal and dye removal capacity of the magnetic NPs [10]. The chelating conduct of the chitosan polymer is one of the most promising characteristics which effectively adsorb toxic contaminants such as dyes and heavy metal ions [3]. The CMNPs are highly preferred in the adsorption studies because of their effortless separation from the aqueous solution using an external magnetic field after adsorption. The Fe₃ O₄ magnetite NPs are considered to be the initial magnetic NP used as adsorbent. As a result of its admirable magnetic characteristics and greater surface area, it is highly preferred as adsorbents. Later, these magnetite particles are coated with various strengthening agents and successfully implemented in the wastewater treatment. The surface functionalisation is one of the limiting factors for the durability of the magnetic NPs in aqueous solution [10]. As the reusing of the adsorbent utilised in adsorption forms is of extraordinary significance monetarily in industries, the chitosan polymers are highly preferred in treating the industrial effluents due to its high reusability rate [3].

3.1.1 CMNP as nanosorbents for removal of heavy metals

Sun et al. [8] have investigated the adsorption performance of mercury‐contaminated aqueous solution using chitosan‐stabilised magnetic ferrous sulphide NPs. In this work, the adsorption capacity of the chitosan‐stabilised magnetic ferrous sulphide NPs are compared with the Fe₃ O₄ reinforced ferrous sulphide NPs; the results infer that the chitosan‐embedded NPs showed greater mercury sequestering capacity than the other NPs. Then, the separation of the chitosan NPs from the aqueous solution was found to be quite simpler than the other NPs during the application of external magnetic force. Mi et al. [10] have explored the adsorption capability of the magnetic chitosan beads on the copper heavy metal ions. In this paper, the N, O‐carboxymethyl chitosan‐capped magnetic NPs are assimilated in the gel beads made out of chitosan–citrate complex. The mingled chelation of the magnetic chitosan beads accredited to the greater adsorption capacity of 294.11 mg/g of copper heavy metal ions.

Fan et al. [12] have inspected the effective method for the synthesis of a magnetic NP embedded on to the chitosan polymer. In this work, the CMNPs are produced in bulk using the high‐gravity reactive precipitation method in a stream rotating packed bed. Then, the removal efficiency of the prepared CMNP is examined in the Pb(II) and Cd(II) heavy metal ions. Ngoc et al. [13] explored the efficient removal of chromium metal using the magnetic chitosan NPs and in this paper; they have used co‐precipitation technique to effectively produce the CMNP. Nasirimoghaddam et al. [15] have investigated the adsorption of mercury heavy metal ions of chitosan‐reinforced Fe₃ O₄ magnetic NPs. The higher mercury removal efficiency of 92.4% was ascertained while maintaining the adsorption conditions at pH 3 at room temperature. These NPs seemed to possess a good desorption capacity and they are easily rejuvenated using various eluents such as ethylenediaminetetraacetic acid and sulphuric acid. Fan et al. [16] have conducted adsorption studies for various heavy metals such as Cu²⁺, Ag⁺, Cr³⁺, and Cr⁶⁺ using the magnetic chitosan beads. Malwal and Gopinath [18] have inspected the efficiency of arsenic elimination from wastewater using chitosan magnetic beads coated with silica. In this research work, the chitosan magnetic beads are synthesised using the precipitation method; subsequently, the silica is coated on the magnetic beads. The adsorption capacity of the chitosan magnetic beads and the silica‐modified beads on the arsenic heavy metal ion are estimated to be 0.082 and 1.699 mg/g. Hosseinzadeh and Ramin [19] have investigated the copper heavy metal removal capacity of the chitosan‐reinforced graphene oxide nanocomposites. The graphene oxide material is altered using the chitosan, Fe₃ O₄, and ethylenediamine. This research paper reported that the maximum adsorption capacity obtained seemed to be 217.4 mg/g. Song et al. [20] have discussed the elimination of heavy metal ions such as mercury, cadmium, zinc, copper, and lead using the magnetic thiolated/quaternised‐chitosan composite. The composite reported in this work has been synthesised using the inverted suspension method. Among the various heavy metals used, the greater removal efficiency of 235.63 mg/g was observed in the neutral pH and also good rejuvenation rate of 93% is observed in the adsorption of lead ions even after five consecutive cycles. Bai et al. [21] have inspected the effective removal of the lead heavy metal ions released from the rare‐earth industry with the help of chitosan Fe₃ O₄ beads stabilised with the diglycolamic acid. It is reported from the paper, that the stronger affinity between the diglycolamic acid and chitosan enhances the lead metal adsorption with an ultimate adsorption capacity of about 70.57 mg/g. The study on the elimination of the toxic contaminants such as cadmium heavy metal ion and phenols using the titanium oxide nanocomposite stabilised using the chitosan polymer is carried out by Alizadeh et al. [22] and it is inferred that the cadmium is adsorbed with a superlative adsorption capacity of about 209.205 mg/g. Yan et al. [23] have successfully implemented the Fe₃ O₄ NPs stabilised using polyvinyl alcohol for the elimination of chromium heavy metal ions from the wastewater. The polyethylenimine‐coated chitosan magnetic particles are used in the treatment of more noxious heavy metals such as manganese, copper, and cobalt. It is reported that the copper is said to possess higher removal efficiency than the other two metals [24] (Table 1).

Table 1.

Removal of heavy metals using various chitosan magnetic adsorbents

Heavy metal	Synthesis method	Adsorbent	Adsorption capacity, mg/g	pH	Time, min	Kinetic model	Isotherm study	Reference
mercury	chemical precipitation	polythiophene CMNP	—	7	60	pseudo‐second‐order kinetics	Freundlich isotherm	[25]
chromium	crosslinking method	magnetic chitosan microspheres	233.1	2.5	—	pseudo‐second‐order kinetics	Langmuir isotherm	[26]
Ni	precipitation	alginate‐coated chitosan NPs	—	3	30	pseudo‐second‐order kinetics	Freundlich isotherm	[27]
cadmium	in situ chemical precipitation	chitosan crosslinked maghemite magnetic composite	15.2	5	60	pseudo‐second‐order kinetics	Langmuir isotherm	[28]
lead	solvothermal method	magnetic chitosan anaerobic granule sludge composite	97.97	6	30	pseudo‐second‐order kinetics	Langmuir isotherm	[29]
copper	solvothermal method	magnetic chitosan anaerobic granule sludge composite	83.65	3	30	pseudo‐second‐order kinetics	Langmuir isotherm	[29]
Ni	—	chitosan metal ion NPs	315	5.2	30	pseudo‐second‐order kinetics	Langmuir isotherm	[30]
copper	—	chitosan metal ion NPs	405	5.2	30	pseudo‐second‐order kinetics	Langmuir isotherm	[30]
arsenic	encapsulation	chitosan Fe3 O4 NPs	147	6–8	—	film diffusion	Langmuir–Freundlich isotherm	[31]
Ni	co‐precipitation	alkyl acrylate–magnetic chitosan Sodium formaldehyde sulfoxylate (SFS) composite	121.96	7	1440	pseudo‐first order	Freundlich isotherm	[32]
lead	co‐precipitation	magnetic chitosan‐4‐[(pyridin‐2‐ylimino)methyl] benzaldehyde Schiff's base	104.16	5	105	pseudo‐second‐order kinetics	Langmuir isotherm	[33]
copper	chemical crosslinking	thiourea‐modified chitosan microsphere	60.6	5.5	960	pseudo‐second‐order kinetics	Langmuir isotherm	[34]
cadmium	sol–gel method	hydroxyapatite nanorods	92	5.6	90	pseudo‐second‐order kinetics	Freundlich isotherm	[35]
cadmium	sol–gel method	nano‐hydroxyapatite chitosan composites	122	5.6	90	pseudo‐second‐order kinetics	Freundlich isotherm	[35]
chromium	co‐precipitation	chitosan–magnetic zeolite composite	—	2	240	—	—	[36]
lead	crosslinking	poly acrylic acid–glutaraldehyde‐crosslinked chitosan nanoadsorbent	734.3	5	—	pseudo‐second‐order kinetics	Langmuir isotherm	[37]
chromium	conventional hydrothermal method	chitosan/Poly vinyl alcohol (PVA)/Polyether sulfone (PES) Fe3 O4 magnetic NP	509.7	2	30	pseudo‐first order	Langmuir isotherm	[38]
lead	conventional hydrothermal method	chitosan/PVA/PES Fe3 O4 magnetic NP	525.8	6	30	pseudo‐first order	Langmuir isotherm	[38]
lead	thermal decomposition	chitosan–manganese dioxide biocomposite	—	7	120	—	—	[39]
lead	micro‐emulsion method	chitosan magnetic microspheres	154.4	6	120	pseudo‐second order	Langmuir model	[40]
lead	micro‐emulsion method	chitosan magnetic semi‐microspheres	133.4	6	120	pseudo‐second order	Langmuir model	[40]
chromium	crosslinking	chitosan–citric acid NPs	106.5	3	60	pseudo‐second order	Redlich–Peterson	[41]
copper	co‐precipitation	Saccharomyces cerevisiae chitosan‐coated magnetic NPs	144.9	4.5	120	—	Langmuir model	[42]

3.1.2 CMNP as nanosorbents for removal of dyes

Dyes are one of the most toxic classes of contaminant emitted from the industrial sources and these dyes cause life‐threatening health disorders to human beings and living organisms. The dyes are not easily degradable compounds and they also possess high toxicity even when it is used in very mild concentrations. Some of the industries such as leather, textile, food, cosmetics, and rubber industries utilise dye as one of the main ingredients in their manufacturing process. These industrial effluents contain dye as one of their main components and it needs to be treated well before its liberation into the environment. Around 100,000 varieties of dyes are used in the industrial sectors for various processes and nearly 7 × 10⁵ of dye‐related compounds are manufactured every year in the industries. Owing to the discharge of dye‐containing effluents into nearby water sources, it spoils the surface water sources and also it seeps into the underground causing damage to the groundwater sources. The treatment of dye‐containing wastewater is an arduous process because of their lethal chemical composition and complexity aromatic structure. Out of the various types of dyes, the azo dyes are notable highly toxic dye used predominantly in various industrial sectors [6]. A major phenolic compound called 4‐nitrophenol which is used in the synthesis of various toxic dyes is effectively reduced using the silver‐coated chitosan microcapsules in the study conducted by Xu et al. [43]. This paper resulted in an effective reduction of 4‐nitrophenol to p‐aminophenol with 98% of removal efficiency in a limited period of time. Reza and Nasab [6] have investigated the removal of calcon dye using the CMNP as the adsorbent. He stated that the presence of the NH₂ group in the CMNPs enhances the probability of the adsorbent to trap the dye molecules and also the reaction rate of the adsorption. Xu et al. [44] have attempted the elimination of methylene blue using the magnetic chitosan microspheres polymerised by poly(2‐acrylamido‐2‐methylpropane sulphonic acid). In this paper, the higher dye adsorption capacity was seemed to be 1000, 1250, and 1428 mg/g at various temperatures such as 30, 40, and 50°C, and also this adsorbent can easily be rejuvenated. The magnetic chitosan fluid synthesised using the precipitation method is one of the unique adsorbent used in the adsorption of a toxic anionic Congo dye. In addition, this adsorption adheres to the Langmuir isotherm model possessing an adsorption capacity of 1700 mg/g at pH 7 in the study conducted by Ma and Pu [45]. The dyes such as methylene blue and malachite green are removed using a peculiar CMNP adsorbent coated with polydopamine in the research work carried out by Wang et al. [46]. They prepared this adsorbent using Schiff base reaction; moreover, it eliminates both the dyes with higher adsorption capacity and removal efficiency. Zhou et al. [47] have investigated the adsorption of dyes such as FD&C Blue 1 and FD&C Yellow which are used in the food industry. In this paper, the CMNP reinforced along with glutaraldehyde is used to treat both the food dyes with higher adsorption capacities of 475.61 and 292.07 mg/g (Table 2).

Table 2.

Removal of toxic dyes using various chitosan magnetic adsorbents

Dyes	Synthesis method	Adsorbent	Adsorption capacity, mg/g	pH	Time, min	Kinetic model	Isotherm study	Reference
Congo red	ionotropic gelation	CMNPs	5107	6	5	—	—	[48]
methyl orange	freeze thawing	chitosan–polyvinyl alcohol–hydrogel beads	6.936	4	30	pseudo‐first order	Langmuir model	[49]
Acid Green 25 (AC 25)	—	magnetic chitosan microspheres	1111.1	2	90	pseudo‐second order	Langmuir model	[50]
Reactive Blue 19 (RB 19)	—	magnetic chitosan microspheres	769.2	2	90	pseudo‐second order	Langmuir model	[50]
methylene blue	solvothermal method	sulphonated chitosan–NPs	—	—	30	—	—	[51]
Acid Red 2	precipitation method	glutaraldehyde–chitosan magnetic nanocomposites	90.06	3	—	—	Redlich–Peterson model	[52]
Congo red	crosslinking method	polyethylenimine–chitosan magnetic composite	1876	7	160	pseudo‐second order	Redlich–Peterson model	[53]
reactive brilliant red X‐3B	reduction–precipitation method	chitosan magnetic Fe3 O4 NPs	490.3	2	300	pseudo‐second order	Langmuir model	[54]
methylene blue	traditional heating	chitosan rectorite‐reinforced Fe3 O4 composite microspheres	24.69	6	60	pseudo‐second order	Langmuir model	[55]
methyl orange	traditional heating	chitosan rectorite‐reinforced Fe3 O4 composite microspheres	5. 56	6	60	pseudo‐second order	Langmuir model	[55]
Congo red	crosslinking method	diammonium tartrate‐modified chitosan	1447	—	480	pseudo‐second order	Sips isotherm model	[56]
Congo red	crosslinking method	urea diammonium tartrate‐modified chitosan	1597	—	48	pseudo‐second order	Sips isotherm model	[56]
methylene blue	co‐precipitation method	cyclodextrin chitosan NPs	2.87	5	50	pseudo‐second order	Langmuir isotherm	[57]
Acid Green 25	crosslinking method	glutamine–chitosan magnetic microspheres	698.95	2	20	pseudo‐second order	Langmuir isotherm	[58]

3.1.3 CMNP as nanosorbents for removal of organic compounds

Dong et al. [59] have inspected the adsorption properties for humic acid using CMNPs which is prepared using the co‐precipitation technique. In addition, the chitosan NPs showed higher attraction toward humic acid resulting in maximum adsorption capacity of 29.3 mg/l. The hydrogen bonding prevailing between the surface groups such as hydroxyl and amine groups plays a vital role in the humic acid adsorption. An attempt was made by Wang et al. [60] to inspect the removal of salicylic acid and humic acid using adsorption and coagulation methodologies. The CMNPs are used as the adsorbent and aluminium sulphate is utilised as the coagulant in this paper. The results showed that the removal efficiencies of both the humic acid and salicylic acid are higher in the adsorption rather than the results of coagulation. The oil spill is one of the more hazardous pollutions and it is more complicated to treat. In the experiment conducted by Soares et al. [61], the chitosan silica nanosorbents are used to remove the oil spill from water. In this paper, the nanosorbents are prepared using the sol–gel–emulsion technique. Rather than the biosorbents used before for treating the oil spill, the magnetic chitosan adsorbents showed higher removal efficiency and higher affinity toward the organic compounds present in the oil. Even the chitosan magnetic nanosorbents have been applied in the treatment of organic compound such as bovine serum albumin in the study carried out by Wang et al. [62]. It is found that the carboxy methyl chitosan magnetic nanosorbents helped effectively in eliminating the bovine serum albumin with higher removal efficiency and also this adsorption adhered to the pseudo‐kinetic‐order model and Langmuir model. Samadi et al. [63] have investigated the removal of an oil spill from water. In this paper, the oils such as pump oil, motor oil, and lubricating oil are removed using the chitosan‐polyacrylonitrile‐magnetic zeolite nanofibres coated onto the sponges. The study resulted in higher oil removal capacities of about 99.4, 95.3, and 88.1 g/g, and also these sponges are reusable several times effectively for oil adsorption. Fan et al. [64] have inspected the adsorption of hydroquinol using CMNPs surface modified with β ‐cyclodextrin. The adsorption capacity of about 1.75 mmol/g of hydroquinol, along with considerable reusability is determined in this paper. Ayad et al. [65] have conducted a study on the reduction of 4‐nitrophenol with the support of palladium‐coated polyaniline–chitosan magnetic composites effectively. Lincosamides antibiotic is removed from the aqueous solution using silver sulphide‐coated chitosan nanocomposites and nanohybrids with effective adsorption capacities of about 153.21 and 181.28 mg/g [66]. Wan et al. [67] have successfully implemented the zirconium reinforced chitosan magnetic hydrogels for the removal of phosphorus from the aqueous solution. The study revealed that the adsorption mechanism in this study is based on the inner‐sphere complex and ligand exchange.

3.2 Photocatalytic technology

Photocatalytic technology is one of the most important techniques used in the removal of toxic environmental pollutants from industrial effluents. Various photocatalytically active materials such as ZnO, TiO₂, and tungsten trioxide are used in this process for effluent treatment [68]. Abdelwahab and Morsy [69] have inspected the degradation of methylene blue dye using the photocatalytic technique. In this research work, three various types of functionalised materials such as TiO₂ Fe₃ O₄ material, TiO₂ chitosan‐reinforced Fe₃ O₄ material, and methylpyrazolone‐functionalised TiO₂ chitosan‐reinforced Fe₃ O₄ nanomaterials are synthesised. The results declared that out of all the photocatalytic materials, the methylpyrazolone‐modified Fe₃ O₄ materials seemed to show degeneration rate of about 98% of methylene blue dye in the presence of visible light at a time period of 40 min. Then, the degradation rate of TiO₂ magnetic material was about 96.7% at a time period of about 100 min. Correspondingly, the higher removal rate of about 98.9% at 60 min is obtained using the TiO₂ chitosan Fe₃ O₄ photocatalytic material. Kazemi et al. [68] have studied the photocatalytic elimination of toxic chromium heavy metal ion using the tungsten oxide nanomaterials reinforced with iron and chitosan polyethersulphone layers. The ultrafiltration membrane used in this study showed enhanced removal of Cr(VI) ions from the aqueous solution under the visible light environment.

3.3 Flocculant

In the midst of various techniques used in the wastewater treatment, flocculation is one of the effective methods used for removing the toxic contaminants. This process is very simple as it is based on the functional groups present in the surface and molecular weight of the compound. As the chitosan compounds possess good compatibility, easy gel formation ability, hydroxyl, and amino functional groups [70]. Lü et al. [71] have utilised the magnetic NPs reinforced with quaternised chitosan to segregate the emulsified oil from aqueous solution. The results declared that the magnetic chitosan particles demulsified the diesel oil from water with higher segregation efficiency of 105 mg and it also inferred that the chitosan particles used in this paper retain the same separation efficiency even after seven cycles of recycling. The polyacrylamide‐modified CMNPs are used as a flocculant in the removal of copper heavy metal ions in the study conducted by Maa et al. [72]. In this paper, the polyacrylamide CMNPs are grafted under the assistance of ultraviolet rays. The experiment resulted in a separation capacity of about 90.38% of copper metal ions and it seemed to be an effective treatment for copper‐influenced industrial effluents. Liu et al. [70] have studied the separation of toxic Ni heavy metal and methylene green dye from the aqueous environment utilising chitosan magnetic flocculant enhanced with a carboxylate group. This flocculant effectively removed the Ni(II) and methyl green dye with separation capacities of about 98.3 and 87.4%.

4 Conclusion and future perspectives

As mentioned in the review, the chitosan NPs are utilised in the industrial effluent treatment for removing the harmful contaminants from the aqueous solution. The unique features such as simple synthesis, low cost, and easy separation mechanism attract the researchers toward the chitosan nanomaterials. Owing to the compatibility of the chitosan nanomaterials, it is used in various applications such as drug delivery, immobilisation, biosensor, medicinal, and engineering applications. The chitosan nanomaterials are employed as adsorbents, flocculants, and photocatalytic materials in the treatment of harmful contaminants. The chitosan‐assisted effluent treatment has been a breakthrough in the wastewater treatment field. It is employed in the effective elimination of various contaminants such as dyes, heavy metals, organic matter, pesticides etc. The eco‐friendliness and the easy rejuvenation of the chitosan materials decrease the cost and secondary pollution. In the various synthesis methods employed in the preparation of chitosan, nanomaterials are chemical methods, the alternative and innovative ideas should be implemented in the preparation of chitosan magnetic particles in an eco‐friendly way and this material should be made compatible to treat highly hazardous chemicals and organic matter. Further research in this material can result in the enhancement of remarkable characteristics and application of this material in various other sectors such as medicine, engineering, agriculture, and material science.