Keywords

1 Introduction

A large amount of biomass waste is discarded into the environment without pretreatment. For example, shrimp shell waste is generated in huge quantities from the shrimp industries throughout the world, and it is primarily disposed of into the sea, resulting in intense environmental pollution [1]. India is a major producer of shrimp, which is more than 1,00,000 tons of shrimp bio-waste generated annually and only an insignificant amount of that bio-waste is utilized for the extraction of chitin while the rest is discarded. According to the report in 2012, a total of 250 chitinolytic bacteria from 68 different marine samples were screened by employment enrichment method which utilized native chitin as the sole carbon source. The shrimp shell waste is utilized for the production of natural biopolymers like chitin, chitosan, N-acetyl glucosamine, chitooligosaccharide, other essential amino acids, etc. [2]. Chitin was prepared by a two-step extraction method from shrimp shells using citric acids and deep eutectic solvents. The acids could effectively remove some minerals and proteins followed by deep eutectic solvents with the assistance of microwave and the deproteinization efficiency was more than 88% [3]. It is a linear amino polysaccharide comprised of β-(1,4)-2-deoxy-2-acetamido-d-glucopyranose structure and the deacetylated form of chitin is known as chitosan, i.e., β-(1,4)-2-deoxy-2-amino-d-glucopyranose using NaOH and the chitin with a degree of deacetylation of above 70% is considered as chitosan. Chitosan is insoluble in water but soluble in acetic acid solvents below pH 6. Monoprotic acids such as formic, acetic, hydrochloric, and lactic acids are used for dissolving chitosan, and mostly 1% acetic acid solution used to dissolve chitosan. But it does not dissolve in diprotic and polyprotic acids such as sulfuric and phosphoric acids and it has poor stability over pH 7 due to precipitation/gelation that takes place in the alkali pH range. Chitosan solution forms a poly–ion complex with anionic hydrocolloid to produce a gel.

2 Chemistry of Chitosan

2.1 Covalent Functionalization of Chitosan Without Metal Ions

Chitosan is used to modify chemically to change its structure and properties to meet the requirements of specific applications. The free amine groups are strongly reactive and can be easily modified chemically or physically. One simple modification of chitosan is protonation. Besides, the protonation of amine groups in acidic solutions gives the chitosan a cationic behavior and consequently the potential for attracting the negatively charged species [4]. The protonated form of chitosan is useful to attract negatively charged species like fluoride, chromate ions, and anionic dyes from water through electrostatic interactions [5, 6]. Initially, Viswanathan and Meenaksi have developed protonated chitosan beads (PCB) for fluoride removal from water, in which they found a maximum uptake of fluoride ions of 1664 mg/kg [5], then for copper uptake densities on chemically functionalized chitosan beads were 52, 86, and 126 mg/g for protonated chitosan beads, carboxylated chitosan beads, and grafted chitosan beads, respectively [7] using the same adsorbent was Pokhrel et al. have reported a review article on the chemical functionalization of chitosan and their applications, in which the chemical and physical functionalization of chitosan without altering the degrees of solubilization [8]. Amine groups were responsible to modify chemically with other functional groups rather than hydroxyl groups due to their basic nature for various applications. In 2015, Macquarrie and Hardy have reviewed the chemically bonded groups on chitosan without metal ions for catalysis inorganic transformations [9]. Recently, Mincke et al. [10] have developed chemically functionalized chitosan adsorbents for selective recovery of palladium and platinum groups from acidic aqueous solutions. In this article, the authors have used various functionalities, viz. 1,10-phenanthroline-2,9-dicarbaldehyde (Ch-PDC), [2,2′-bipyridine]-5,5′-dicarbaldehyde (Ch-BPDC), and glutaraldehyde cross-linked chitosan grafted with 8-hydroxyquinoline-2-carbaldehyde (Ch-GA-HQC) for selective recovery of Pd(II) and Pt(IV) metal ions from waters. The obtained maximum adsorption densities based on Langmuir isotherm were 340.3 and 203.9 mg/g for Pb(II) and Pt(IV), respectively, on (Ch-GA-HQC). The sorption mechanism established by external mass transfer is followed by the intra-particle diffusion process and chemisorption [10]. Likewise, a 3D sponge of 11-mercaptoundecanoic acid-functionalized chitosan (MUA) as an effective adsorbent for methyl orange adsorption was also used for antibiotic-free antibacterial activity. The maximum uptake of methyl orange was 388 mg/g, which is much higher adsorption densities compared to many chitosan-based adsorbents in the literature [11]. Interestingly, to improve the adsorption efficiencies of the metals from water, the chitosan moiety was functionalized with more amine groups which tend to be complexed with metal ions easily. Zarghami et al. have developed polyamidoamine (PAMAM)-grafted dendrimer through a divergent growth approach, in which the Pb(II) ions were successful [12]. Moreover, the chemical functionalization of chitosan using different strategies was proposed by Ji et al. for drug delivery systems in detail [13]. Padmaja group has delivered a superabsorbent, namely chitosan–thiobarbituric acid showed maximum uptake capacities of 1,357.69, 2,504.86, and 2,475.38 mg/g for respective Hg°, Hg2+, and CH3Hg+, and the adsorbent could be reused several times using 0.01 N thiourea, perchloric acid, and 0.2 N NaCl solutions [14]. However, this review aims to present the metal ion interaction on the raw and functionalized forms of chitosan (Fig. 1).

Fig. 1
figure 1

Schematic overview of the (a) synthesis of Ch-PDC, Ch-BPDC, and Ch-GA-HQC and (b) the chelating and cross-linking compounds. The figure was adapted with permission from Ref. [10]

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2.2 Functionalization of Chitosan with Metal Ions

Functionalization of chitosan in its amine and hydroxyl groups with various toxic-free metal ions is used for adsorption of toxic ions removal from water, which provides higher adsorption densities, hydrophilic character, and faster adsorption kinetics. Different types of metal ions were immobilized on the surface functionalities of chitosan through in situ and ex situ synthetic methods. These reactive functional groups may interact with metal ions through different mechanisms depending on the metal, the pH, and the matrix of the solution. The free-electron doublet on nitrogen may bind metal cations at pH close to neutrality (or weak acidity). The donation of lone paired electrons plays an important role in the amine–metal interaction. The interaction of metals on amine moiety present in the chitosan was studied using Raman spectroscopy and density functional theory (DFT). The most obvious change of the surface Raman spectra pattern arises from the amino group. It’s interesting to note that NH2 vibration modes show a significant frequency shift and an abnormal Raman enhancement in the measurements. Zhao et al. [15] have explained well about the aniline adsorption on the metal centers via Raman spectroscopy with the substantial aid of DFT calculations. In this paper, the amine–metal interaction mainly arises from the donation of amino lone pair orbital to metal unoccupied s band as described well [15]. According to Pearson’s hard and soft acid-based (HSAB) concept, the hard acid metals tend to bind with hard base moieties to form a stable adduct and the same phenomenon for soft-based metals. Based on this concept, the NH2 groups of chitosan considered as hard base the loading of metal ions should be hard acid behavior which leads to forming a stable interaction between them (Fig. 2).

Fig. 2
figure 2

Alumina cross-linked chitosan with HPMC (hydroxypropyl methylcellulose). The figure was adapted from Ref. [16]

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3 Applications of Chitosan for Environmental Remediation

3.1 Fluoride Retention Using Metal–Chitosan Based Composites

Fluoride is one of the essential microelements required in trace amounts for the normal growth and development of various organs in our body more prominently bone and teeth. Fluoride is thus considered beneficial in drinking water at levels of about 0.5 mg/L but it is harmful once it exceeds 1.5 mg/L which is the World Health Organization (WHO) limit being followed in most of the nations. The Bureau of Indian standards has fixed a permissible limit of fluoride in drinking water as 1 mg/L. The excess of fluoride content in the groundwater sources is one of the major environmental health issues with the continuing growth of population and human intervention in alteration of water quality being one of the prominent causes. In India, 230 districts of 20 states are at risk of high levels of fluoride in drinking water. There are two types of fluorides: (1) ionizable/non-ionizable and (2) organic/inorganic, in which organic fluoride does not dissolve rapidly in water until fluoride ion is released via any chemical reaction. Mostly the inorganic fluoride ions come from rock phosphates, minerals, and earth crust (950 mg/L) in its ionic form. Excessive intake of fluoride contained water may pose serious health concerns like dental, skeletal, and non-skeletal forms of fluorosis depending upon the concentration of the fluoride contents in water [17]. There are several technologies used in the defluoridation of water: reverse osmosis, nanofiltration, dialysis and electro-dialysis, and adsorption. Among them, adsorption appears to be the most appropriate solution because it is cost-effective, simple to operate, and produces high-quality treated water [18]. Recently, chitosan-based composites using metals, metal oxides, and bimetals have been receiving large attention as alternative sorbents in water treatment processes as these kinds of materials yield high adsorption densities. To date, several successful attempts made toward the development of new materials with the maximum adsorption densities have been undertaken by employing the following strategies: (1) metal ions encapsulations, (2) magnetic nanoparticle coatings, and (3) combination of both (1) and (2) in the chitosan matrix. Viswanathan and Meenakshi have started working on chitosan and modified forms of the chitosan for fluoride removal from waters. The chemically modified forms of the chitosan, i.e., protonated and carboxylated forms beads showed excellent fluoride adsorption densities were 1.66 and 1.33 mg/L at the initial concentration of 10 mg/L [5, 19]. Then, to enhance the adsorption performance of fluoride, metal-loaded chitosan beads were prepared which showed higher adsorption densities than the protonated and carbolated forms of beads. The fluoride adsorption capacities on La- and Fe-loaded chitosan beads were 4.71 and 4.23, respectively. To continue this, La–Zr bimetallic chitosan beads were prepared by Prabhu and Meenakshi later [20], where the adsorption capacities were ~3.8 times higher than the protonated forms. Also, the material was regenerated with 0.1 M NaOH about 6 times for maximum reuse of the materials. Further, the authors have synthesized metals (Al3+, Ce3+, La3+ and Zr4+) incorporated chitosan grafted polyamidoamine dendrimers for the adsorption of fluoride from water. The results showed that Zr-loaded PAAGCB was more selective with the maximum adsorption densities of 17.47 mg/g than the other metal ions loaded PAAGCB. The thermodynamic study results indicated that the adsorption process was spontaneous and endothermic. The Zr-PAAGCB material could be effectively utilized as an adsorbent for fluoride adsorption [21]. Similar adsorption densities were obtained for magnetic materials on the chitosan matrix. The order of the magnetic materials prepared by several authors and the capacities was arranged in decreasing order as Fe3O4/chitosan/Al(OH)3 beads [22] > magnetic hydrocalumite–chitosan [23] > magnetic hydrotalcite–chitosan [24] > magnetic hydroxyapatite [25]. The main advantages of these magnetic materials are easy separation of materials from solution and it suppresses separation process time and the cost of the process. Chitosan modified with highly electropositive metals and a supported material delivered high adsorption capacities. For example, Ce–chitosan (153 mg/g), Aloe vera supported Nano Zr chitosan [26] and Al–chitosan–hydroxypropyl methylcellulose [27]. The Aloe vera supported Zr nanoparticles were prepared by a simple method. The extract of Aloe vera was used as a reducing agent to develop Zr nanoparticles and further the nanoparticles were embedded in the chitosan matrix. Recently, a record high adsorption densities of fluoride were obtained by Affonso et al. [28] using carbon nanotube functionalized with chitosan sponge from effluent of fertilizer production industry. These adsorbents have shown a record high maximum adsorption density of 975.4 mg/g based on the kinetic and freundlich isotherm analysis results. The high adsorption caapacity was attributed to the chitosan functional groups and the high interaction area promoted by sponge form and the carbon nanotube. Additionaly, the material was reused for about 5 times with the aid of 0.1 M NaOH solution as an eluent [28]. Mostly the developed materials for fluoride adsorption were regenerated with NaOH with the cycles range between 5 and 10 (Fig. 3). As a result, the metal-incorporated chitosan composites proved to be more efficient adsorbents than their chemically modified counterparts. The other chitosan-based materials reported for fluoride sorption are displayed in Table 1.

Fig. 3
figure 3

A schematic illustration of fluoride adsorption mechanism onto the magnetic MgFe2O4–chitosan–CaAl composite. The figure was adapted with permission from Ref. [27]

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Table 1 Functionalized forms of chitosan for fluoride removal from water
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3.2 Newly Developed Chitosan-Derivatives for Arsenic Adsorption

Arsenic exists as oxides in soil, sediments, and water in many parts of the world and originates from both natural and anthropogenic processes. Arsenic existed in four different types of states as −3, +3, 0, and +5, in which +3 form, called arsenite (H3ASO3, As(III)) and arsenate (HAsO42−, As(V)) occurs naturally in predominant forms. Arsenite is more toxic (50 times) than arsenate due to the less mobility in the water. The chemistry of arsenic and its toxicity was well explained in the previous review article by Hao et al. [42]. The maximum permissible limit of arsenic in groundwater is 10 μg/L, described by WHO. The removal mechanisms of the toxic inorganic pollutants on chitosan adsorbents are controversial with theories including chemisorption and complexation. It is reported that the uptake of arsenic on amine moiety in chitosan through coordination with unprotonated amine groups and the protonated form of amine groups would tend to adsorb arsenic via an electrostatic interaction at different pH conditions [43]. Chitosan in its unmodified form had lower adsorption capacities than the metal incorporated chitosan moiety due to its lack of active sites for adsorption as listed out in Table 2. Kwok et al. have reported the mechanisms of arsenic removal on chitosan and nano chitosan in detail at different pH conditions and the adsorption–desorption behavior for arsenic at various eluents [43]. Further, the unmodified chitosan was used for the removal of arsenite and arsenate ions from water at pH 5 and found that the maximum adsorption densities were 1.83 and 1.94 mg/g, respectively. The chitosan was regenerated with 0.5–0.1 M H2SO4 for about 15 cycles, which is far preferable to the mere three cycles afforded by coconut husk carbon in arsenic recovery [44]. To improve its adsorption capacities, the chitosan was modified with chelating resins, and the densities were improved drastically to 62.42 mg/g with pseudo-second-order kinetic models for arsenate removal, and the materials were regenerated using 0.2 M NaOH and utilized materials for further five times with maximum adsorption densities. Metal impregnation is proposed to increase arsenic uptake capacity and favor chitosan selectivity over other metal ions and co-ions. The metals such as lanthanum, zirconium, iron, molybdenum, aluminum, and manganese are known to form complexes with arsenic ions and for their strong affinity for chitosan [45]. Salih et al. [54] have recently reported chitosan/diatomaceous composite beads as an effective adsorbent for the removal of toxic ions from water. The binary maximal adsorption densities of As(III) and As(V), as determined from the Langmuir isotherms were 87.81 and 44.07 at a pH 6, respectively. The presence of phosphate as co-existing anions has an obvious impact on the adsorption of arsenic species due to isomeric forms of phosphate and arsenic. In the desorption experiments, the chitosan/diatomaceous composite beads were examined using 0.2 M NaOH as an eluent and the adsorbents were reused for five consecutive cycles and the losses in bead mass after 5 regeneration cycles were measured as approximately 8% and 11% for As(III) and As(V) loaded beads, respectively. Likewise, a microsphere using iron-loaded chitosan was prepared by Lin et al. (2019) for the removal of toxic As(III) ions from water [46]. The As(III) ions tend to bind with the Fe-rich surface of chitosan through electrostatic interaction at acidic pH conditions and over pH 8, the result was contrary, the excess of –OH ions would compete with arsenite ions and the densities were decreased. The mechanisms of interaction was confirmed by FTIR and XPS studies, in which the FTIR studies confirmed that the presence of NH stretching vibration mode was shifted to 1,371 cm−1 from 1,387 cm−1 and a new peak was observed at 819 cm−1 corresponded to Fe-O-As after adsorption. The magnetic chitosan bead and composie were made by two different groups and found that the adsorption densities were higher at compoiste form rather the beads, due to higher surface area and more active sites existed in the composite form. The respective adsorption densities of arsenite and arsenate were 73.69 and 79.49 mg/g on magnetic chitosan composite which is higher adsorption densities than the magnetic chitosan beads (65.5 mg/g). Lanthanum and molybdenum ions are considered as hard acids which tend to bind with arsenate, a hard base, by well understood HSAB principle.

Table 2 Adsorbents used for the removal of arsenite and arsenate from waters
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Bimellaic assembled chitosan materials were utilized to maximize the adsorption efficiencies of both As(III) and As(V) from waters. Recently, Priya et al. [47] have developed a material consisting of Fe–Al layered double hydroxides on chitosan with graphene oxide materials. The nanocomposite was successfully prepared by hydrothermal and ex situ polymerization process with an enhanced surface area which provides more number of active sites. The LDH moieties were attached with GO groups through electrostatic interaction and the negatively charged arsenic species interacted with this nanocomposite through coordination, ligand exchange, and electrostatic interaction mechanisms. The adsorption capacities of the materials were found to be 167.80 mg/g, which is higher than the pristine forms of the materials [47]. Another group from China has recently developed goethite/graphene oxide/chitosan nanocomposite materials for the removal of As(III) from water. They found that the multifunctional groups such as NHCO, C-O, O-H, and Fe-O were responsible for adsorption of arsenite ions from water. The maximum adsorption density of 289.42 mg/g was calculated using the Langmuir isotherm model and the material was used five consecutive times with the minimal loss of its efficiencies [48].

Chitosan-supported mixed metal oxyhydroxides (ZrxAl1−xOOH) were prepared by Prabhu and Sasaki for the simultaneous removal of toxic fluoride and arsenite ions from single and binary solutions. A higher amount of arsenite was adsorbed in binary solutions than the single solutions, through electrostatic interaction and ligand exchange mechanisms at different pH conditions. Further, the material was supplied to remove the three-component system of natural co-existing anions and found no significant decrement in the adsorption densities. The material could be reused about four times using 0.1 M NaOH as an eluent. Again bimetallic rich chitosan–alginate biopolymeric nanocomposite was developed by Zeng et al. (2020) that materials showed a surface area of 128.16 m2/g and showed maximum efficiencies of 24.06 mg/g [63]. The mechanism of attraction of arsenic by complexation and the desorption by NaOH could be used for four adsorption and desorption cycles (Fig. 4). The other materials used for the removal of toxic arsenite and arsenate using chitosan-based materials are listed in Table 2.

Fig. 4
figure 4

A schematic view of mechanism of intercalation and ex situ polymerization of chitosan and its adsorption of arsenate from water. The figure was adapted with permission from Ref. [47]

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3.3 Metal and Metal-Free Chitosan Materials for Phosphate and Nitrate Adsorption

The nutrients pollution in a water body such as nitrogen and phosphorus are serious environmental issues worldwide. The dissolved nutrients species ammonium, nitrate, and phosphate are particularly important pollutants, resulting in aquatic eutrophication, which is an abundance of aquatic plants, growth of algae, and depletion of dissolved oxygen [67]. The nitrate ion in drinking water is 40 mg/L set by WHO, when it exceeds causes public health hazards including infant methemoglobinemia called Blue baby syndrome and the other forms of carcinogenic nitrosamines and nitrosamides. Agricultural fertilizers, aquaculture, agri-food industries, municipal wastewaters, and detergent manufacturing industries are the main sources of nitrate and phosphate release in the waterbody. Also, thermal power plants discharge their wastes into water that contains large amounts of nitrate ions [68]. A lot of materials were tried to remove the maximum amount of nitrate and phosphate ions through adsorption method. A few review articles explained well the materials used for the removal of those ions from water. This review is currently updating the chitosan-based materials used for the removal of phosphate and nitrate ions from water. Kumar et al. [69] investigated the NO3− and PO43− adsorption properties of chitosan encapsulated lanthanum oxide admixed Aegle marmelos (La2O3AM@CS) composite beads. The hydrothermal supported La2O3AM@CS composite beads possess the NO3− and PO43− adsorption capacity of 27.84 and 34.91 mg/g, respectively [69]. The pH influences the NO3− and PO43− adsorption capacity of La2O3AM@CS composite beads, and SO42− ion mainly competes with NO3− and PO43− adsorption due to charge and size effects. They also reported that the chitosan-supported metal/bentonite composite beads viz. Zr@CSBent, La@CSBent, and Ce@CSBent for NO3− adsorption from water. Among the studied materials, Zr@CSBent composite beads showed a higher adsorption capacity of 23.89 mg/g [70]. Banu et al. reported the Zr4+ ions implanted chitosan–soybean husk activated bio-char composite beads (Zr-CS-SAC) for NO3− and PO43− adsorption and possesses the maximum adsorption capacity of 90.09 and 131.29 mg/g at 30°C [71]. Jiang et al. [72] synthesized the Fe3O4/ZrO2/chitosan composite using a low-cost method at the mild condition. The developed Fe3O4/ZrO2/chitosan composite has an adsorption capacity of 89.3 mg/g and 26.5 mg P/g for NO3− and PO43−, respectively [72]. Karthikeyan et al. developed the lanthanum-loaded chitosan membrane (La@CS) by casting technique, and the La@CS membrane showed the PO43− and NO3− adsorption capacity of 76.6 and 62.6 mg/g, respectively [73]. The same authors also developed Fe3+ cross-linked chitosan/alginate (Fe-CS-Alg) hybrid beads for PO43− and NO3− from an aqueous solution and having the maximum adsorption capacity of 84.74 and 69.10 mg/g, respectively [74]. Further, they moved to prepare the cerium loaded chitosan-β-cyclodextrin (Ce-CS-β-CD) microspheres and applied for the removal of PO43− and NO3−, which exhibited the excellent adsorption capacity of 88.54 and 72.12 mg/g for PO43− and NO3−, respectively [75]. They also prepared zirconium fixed chitosan–starch membrane (Zr-CS-ST) for PO43− and NO3− removal, which possesses the adsorption capacities of 86.28 and 70.88 mg/g, respectively [76]. An environmentally friendly Al3+ incorporated chitosan and gelatin (Al@CS-Ge) microspheres were fabricated by Karthikeyan et al. [77] and utilized as an effective adsorbent for the removal of PO43− and NO3−. The main interaction between the two biopolymeric materials entrapped with metal ions would tend to adsorb more nitrate and phosphate ions due to the availability of the active sites for adsorption. The electrostatic interaction, complexation, and ion-exchange mechanisms were the mainly governed factors for adsorption of nitrate and phosphate ions from waters [77]. Kim et al. [78] have applied iron oxide nanoparticle–chitosan (ION-chitosan) composites for PO43− removal from the natural water. The ION-chitosan composite is not pH sensitive between the pH values of 5.0 to 9.0 and has the PO43− removal capacity of 0.059 mg P/g [78]. Kumar and Viswanathan prepared multivalent metal ions like Zr4+, Fe3+, and Ca2+ cross-linked chitosan bentonite composites Zr@CSBent, Fe@CSBent, and Ca@CSBent, respectively. The developed composites possess the efficient PO43− adsorption capacities of 40.86, 22.15, and 13.44 mg/g than the specific CSBent composite [79]. The Zr4+-imprinted chitosan (CCP-Zr) was prepared by forming-membrane and cross-linking method by Liu et al. [80]. The CCP-Zr have the maximum monolayer adsorption capacity of 71.68 mg/g at pH 3 and 30°C [80]. The chitosan beads modified with zirconium (ZCB) were developed by Liu and Zhang for PO43− removal from an aqueous medium, which possesses the maximum monolayer adsorption capacity of 60.6 mg/g [81]. Mahaninia and Wilson cross-linked the chitosan beads with glutaraldehyde and epichlorohydrin and used for the removal of phosphate dianion (HPO42−) from an aqueous solution at pH 8.5 and 22°C and possesses adsorption capacity of 22.4–52.1 mg/g [82]. Nthumbi et al. [83] prepared chitosan/polyacrylamide nanofibers and used them for the removal of PO43−. The nanofibers had an adsorption capacity of 392 mg/g and removal efficiencies of 97.4% in synthetic and field water samples [83]. Sowmya and Meenakshi have developed various types of chitosan-based adsorbent materials including quaternized chitosan beads (QCB) [84], hydrotalcite/chitosan (HT/CS) composite, quaternized chitosan–melamine–glutaraldehyde resin (QCMGR) [85], protonated a carboxylated cross-linked chitosan bead (PCB), (CCB) [86], La(III)-loaded silica-chitosan composite (LSCC) [87], Zr(IV) cross-linked chitosan beads (ZrCB) [88], zinc-loaded cross-linked chitosan beads (ZnCB) and zinc-loaded carboxylated cross-linked chitosan beads (ZnCCB) for the removal of both PO43− and NO3− from water [89]. The reported the adsorbent materials possess very good adsorption capacity for the removal of PO43− and NO3− from synthetic and real-water samples. The LSCC material exhibited the highest affinity for phosphate removal and nitrate removal from water out of all the developed materials. Based on this scenerio, Banu et al. (2018) [90] synthesized the La3+-incorporated chitosan–montmorillonite composite (La-CS-MMT) for the effective removal of PO43− from aqueous solution, which successfully removed 92% of the PO43− within 30 min. By electrostatic attraction, the phosphate ions were adsorbed on La-CS-MMT to form an outer-sphere complex, as confirmed by the D-R isotherm, and the adsorbed material was then desorbed with 0.1 M NaOH. In regard to MMT and CS, hydrogen bonding is a common mechanism for their interaction with phosphate and nitrate. Zavareh et al. [91] reported the magnetic Cu–chitosan/Fe3O4 nanocomposite for selective and effective removal of PO43−. The synthesized magnetic adsorbent has the maximum adsorption capacity of 88 mg P2O5/g, which is higher than the neat chitosan and chitosan/Fe3O4 according to the Langmuir isotherm [91]. Kumar and Viswanathan have prepared the tetra amine copper(II) salt grafted chitosan (TAC@CS) composite beads for PO43− adsorption, and the TAC@CS composite beads posses the enhanced adsorption capacity of 41.42 ± 0.071 mg/g compared to other prepared adsorbent materials [92]. Kumar and Viswanathan have developed magnetic adsorbents like amine-grafted magnetic gelatin (AGMGel) composite, magnetic-chitosan-assisted hydrotalcite (MCSHT), and magnetic-alginate-assisted hydrotalcite (MAlgHT) composite beads for the adsorption of NO3− and PO43− from water. The researchers also synthesized zirconium oxyhydroxide (Zr@CSKN) composite by hydrothermal method for enhanced nitrate and phosphate adsorption [93,94,95].

Zhao and Feng reported the modified chitosan beads for PO43− and NO3− adsorption with different pH, the initial concentration, contact time, and adsorbent dosage. The results displayed that modified chitosan beads are an excellent adsorbent for the removal of NO3− and PO43− with an adsorption capacity of 32.15 and 33.90 mg/g, respectively [96]. Golie and Upadhyayula have synthesized chitosan/bentonite, chitosan/titanium oxide, and chitosan/alumina (ChBT, ChTi, and ChAl, respectively) for NO3− adsorption from aqueous solution by batch biosorption experiments, and the composites possess the adsorption capacity of 35.68 and 43.62 and 45.38 mg/g, respectively [97]. Banu and Meenakshi reported the one-pot synthesis of chitosan grafted quaternized resin for the removal of NO3− and PO43− from water. The removal efficiency of NO3− and PO43− on chitosan quaternized resin was 78% and 90%, respectively, with 0.1 g of adsorbent and the initial concentration as 100 mg/L [98]. A granular chitosan–Fe3+ complex with high chemical stability and good environmental adaptation was developed by Hu et al. and used for the adsorption of NO3− and possesses the adsorption capacity of 8.35 mg NO3–N/g based on Langmuir–Freundlich model [99]. Zhang et al. (2018) and Zhao et al. (2020) have developed the composites like Fe(III)-doped chitosan (CTS-Fe) [100] and cross-linked Fe(III)-chitosan (CTS-Fe-CL) [101] for the removal of phosphorus. The maximum phosphate adsorption capacity for CTS-Fe and CTS-Fe-CL was 15.7 and 10.2 mg P/g at 30°C, respectively. Lanthanum-chitosan hydrogel coated with polydopamine (La-CS@PDA) was synthesized with abundant amino groups, and the composite followed the Langmuir isotherm model with an adsorption capacity of 195.3 mg/g. Chitosan/Zeolite Y/Nano ZrO2 nanocomposite adsorbent was synthesized by Teimouri et al. [102] and applied for the removal of NO3− from the aqueous solution. The NO3− adsorption process was described by Langmuir isotherm model with the adsorption capacity of 23.58 mg/g [102].

The nanochitosan-graphene oxide composite (NCS@GO) was prepared by Salehi and Hosseinifard, which is examined for adsorption of PO43− and NO3− from aqueous solutions. The zirconium loading was optimized in NCT@GO composite to make it selective for the adsorbate anions, and NCS@GO/H-Zr established a very good PO43− and NO3− uptake of 172.41 mg P/g and 138.88 mg N/g, respectively. The removal capacity of P and N anions was also assessed in bi-component systems and the Freundlich isotherm fitted well, suggesting the multilayer formation. Real samples analysis indicated that the prepared material works well for the removal of P and N anions from contaminated waters [103]. Yazdani et al. [104] reported the zinc(II)−chitosan complexes as a bio-sorbent for PO43− removal from aqueous solutions. The zinc(II) ions into chitosan expand its performance towards PO43− removal from 1.45 to 6.55 mg/g. Using functionalized nanochitosan/clinoptilolite (Nano-CS/Clino) composites, nitrate ions were removed from water. The Nano-CS/Clino@PEHA composite was found to have a higher nitrate adsorption capacity of 277.77 mg/g than Nano-CS/Clino@H (227.27 mg/g) and Nano-CS/Clino (185.18) [104, 105]. More recently, Fu et al. [106] developed the polyethylenimine-grafted chitosan core-shell (Fe3O4/CS/PEI) magnetic nanoparticles and utilized them for adsorbing PO43− in water. The adsorption isotherms of PO43− on Fe3O4/CS/PEI particles were well fit by the Langmuir equation, and its maximum adsorption was 48.2 mg/g at an equilibrium pH of 3.0 and 25°C [106]. Non-cross-linked lanthanum-chitosan (La-CTS-0X) and cross-linked lanthanum-chitosan (La-CTS-1X/2X) composites were prepared as new complex biosorbents for effective PO43− removal from wastewater by Liu et al. [107]. The maximum PO43− adsorption capacity for La-CTS-0X/1X/2X was 47.28, 57.84, and 31.01 mg/g at pH 6, respectively [107].

The materials used for the removal of phosphate and nitrate were mostly regenerated with NaCl and NaOH at different concentrations depending upon the metal concentrations of composites/hybrids and the maximum re-useable cycles are between 5 and 10. The metal-loaded chitosan-based composites showed extraordinary performance than the pristine forms due to the hydroxide ions and the metal affinity towards the nitrate and phosphate ions from water. The quarternized forms of chitosan materials attract phosphate and nitrate ions through a ligand-exchange mechanism which can be confirmed by an ion-chromatogram, where the Cl− ions concentrations can measure periodically and the phosphate and nitrate ions concentration can be measured simultaneously (Fig. 5 and Table 3).

Fig. 5
figure 5

Mechanism of phosphate adsorption using Fe(III)-doped chitosan and cross-linked Fe(III)-doped chitosan. The figure was adapted with permission from Ref. [100]

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Table 3 Removal of phosphate and nitrate ions from water using chitosan-based sorbents
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4 Summary and Outlooks

Recently, studies have proposed the synthesis of novel, new, and eco-friendly nanomaterials that use adsorbent technologies to remove toxic ions from waters and wastewaters. Overall, the reported materials with the background of chitosan showed ultra-high adsorption densities toward the removal of fluoride, arsenic, phosphate, and nitrate ions from water under batch experiments. Mostly, the materials used for this review were prepared by conventional method and a few of them were synthesized by in situ and ex situ synthetic methods which significantly improved the stability of the materials. Besides, the surface area of the materials was also enhanced due to the synergistic interaction of the active material with the chitosan background. At different pH levels, the mechanisms of interactions with toxic ions include electrostatic interactions, inner-and outer-sphere complexation, and ligand exchange. Despite the wide applications were adopted for new generation material for the removal of fluoride, arsenic, phosphate, and nitrate from water, some drawbacks still exist.

  1. 1.

    The chitosan-based materials can be easily dissolved in 1–2% acetic acid and they can be modified in various forms by adding the metal ions in it. But the adsorption takes place at acidic conditions and most of the metal ions may undergo dissolution which leads to taking the secondary treatment to recover the metal nanoparticles from water. Also, the excess loading of metal ions might block the active sites and the metal hydroxide is the only one responsible for the removal of toxic ions from water. The water quality parameters before and after the treatment were analyzed using standard methods for the determination of residual toxicity.

  2. 2.

    The regeneration of the chitosan-based materials is mandatory to reduce the cost factor of the process and the feasibility of the studies. Most of the studies were focused on batch adsorption and only a few were reported with column experiments. Regeneration studies of the materials were rarely reported in this study. In order to develop the model for domestic and community models, the column study can be applied to promote the next step of development. Only a few of the reported studies were focused on the recovery of adsorbed material and desorbed metals. Newly developed desorption techniques for metallic ions using recyclable chemicals reduce the consumption of concentrated acids and alkalis.

  3. 3.

    Real-water analysis is the fundamental factor in deciding the applicability of the adsorbent. Naturally, the groundwater contains several anions, cations that interfere with the adsorption of toxic ions from water. It is important to choose adsorbents that are able to withstand even higher concentrations of interference without affecting their adsorption capacities. After the ions were adsorbed, the adsorbents were disposed in landfills or ponds, easily contaminating water, soil, and the air is referred to as secondary pollution. The arsenic and fluoride-sorbed materials were carefully stabilized before disposed of. There are few reports about the stabilization of Cs+, Sr2+ using fly-ash-based zeolite materials, which helps to avoid releasing them into the environment again.

  4. 4.

    Adsorbents cost factors are the important parameter for practical feasibility. This will be done by comparing the cost of the method developed in the laboratory and that of other commercially available models.