Regeneration of chitosan-based adsorbents used in heavy metal adsorption: A review

Regeneration of chitosan-based adsorbents used in heavy metal adsorption: A review

Separation and Purification Technology 224 (2019) 373–387 Contents lists available at ScienceDirect Separation and Purification Technology journal ho...

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Separation and Purification Technology 224 (2019) 373–387

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Regeneration of chitosan-based adsorbents used in heavy metal adsorption: A review

T



Mohammadtaghi Vakilia,b, Shubo Dengb, , Giovanni Cagnettab, Wei Wangb,c, Pingping Menga, Dengchao Liub, Gang Yub a

Green Intelligence Environmental School, Yangtze Normal University, Chongqing 408100, China State Key Joint Laboratory of Environment Simulation and Pollution Control, Beijing Key Laboratory for Emerging Organic Contaminants Control, School of Environment, Tsinghua University, Beijing 100084, China c State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xi’ning, Qinghai Province 810016, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Chitosan adsorbent Desorption Desorption mechanism Desorption parameter Heavy metal Regeneration

Adsorption of heavy metal ions on chitosan-based adsorbents has been extensively investigated. However, few studies explored the feasibility of desorbing and regenerating chitosan. Adsorbents used after adsorption of heavy metals are discarded, and this practice exacerbates the solid treatment problem. Regeneration and reuse of exhausted adsorbents should be considered to operate environment-friendly and cost-effective adsorption. This review was performed to summarize the desorption of heavy metal ions and possible regeneration of chitosanbased adsorbents using various desorption agents such as acids, alkalis, salts, and chelating agents. It was found that the highest desorption efficiencies obtained by acidic eluents. The percentage use of desorption agents for desorbing followed the order of acids (49.3%) > chelators (26.9%) > alkalis (14.9%) > salts (8.9%). Moreover, the proper desorption time were estimated to be 0.84 by 1.37 h. Beneficial information is provided for regeneration and recovery of chitosan adsorbents.

1. Introduction Water is an essential and valuable resource for humans' living and development. Water resources are increasingly decreasing and need to be preserved. In recent years, industrialization, agricultural activities, rapid population growth and urbanization have contributed negatively to clean water resources [1,2]. Different pollutants such as pharmaceuticals, pesticides, dyes and heavy metals have contaminated the water resources. Presence of these contaminants in aqueous environments is environmentally hazardous for human beings and animals [3,4]. Among these pollutants, heavy metal is one of the most hazardous species, due to their toxic nature. Heavy metals such as cobalt, mercury, copper, zinc, chromium, cadmium, lead, nickel, arsenic, etc. can enter to the water bodies (directly or indirectly) through various industries (fertilizer, mining, painting, batteries, tanneries, metal plating industries, etc.) [5]. Heavy metals are toxic, non-biodegradable and exist in different oxidation states for long period in the environment. Therefore, it is essential to eliminate heavy metal ions from contaminated wastewater before discharging to environment [6,7]. Adsorption is often the most effective and economically viable



method to remove pollutants from wastewater [8–11]. It is particularly suitable when the target pollutant is non-degradable, like in the case of heavy metals, so that destruction technologies are not efficacious [12–16]. Adsorption can be also economically advantageous compared to other technologies if it is possible to find adsorbents that maximize the elimination of pollutants per unit adsorbent mass [17–20]. In this case, the adsorbing material shows a favorable adsorption behavior, that is, the amount of the pollutant on its surface at equilibrium remarkably greater that the residual amount in the (waste-) water [18,21,22]. However, if such condition is satisfied, it means that the counter-process, i.e. desorption, is unfavorable. An ineffective or slow desorption/regeneration of the adsorbents might be a serious issue for novel sorbents and their applicability on wastewater treatment. Currently, adsorption has a limit that hinders a wider use of such technology: the lack of adequate regeneration methods to recycle the adsorbent. In fact, without regeneration, pollutants may be released into environment by disposal or storage of spent adsorbents. Moreover, storage and dumping of spent adsorbents may lead to explosions, fires, and stinks [23]. Therefore, the spent adsorbent should be stabilized before disposal. The recycling process is performed by repeating the adsorption and desorption cycles and provides substantial economic

Corresponding author. E-mail address: [email protected] (S. Deng).

https://doi.org/10.1016/j.seppur.2019.05.040 Received 27 February 2019; Received in revised form 3 May 2019; Accepted 9 May 2019 Available online 10 May 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Advantages related with regeneration of saturated adsorbents and storage/disposal disadvantages.

discussed accordingly in this review paper. This review is mainly composed of following aspects: (a) adsorption of heavy metal on chitosan-based adsorbents, (b) a highlight on regeneration of saturated chitosan adsorbents (c) a structured review on the use of various eluents for regeneration of chitosan and its derivatives for removal of heavy metal ions and (d) a review on the mechanism of desorption of heavy metal ions from saturated adsorbents and parameters affecting desorption.

and environmental benefits [24]. Indeed, researchers are more inclined toward regeneration and reuse of adsorbents because of the high cost of production, stabilization, and disposal (Fig. 1). Desorption still remains challenging despite the simplicity of adsorption of pollutants from the aqueous solutions because of the high affinity of the adsorbates to the surface of adsorbents. Desorption is primarily performed to remove the reversible adsorbate molecules on the adsorbent and to regenerate the adsorbents [25]. Regeneration is one of the significant characteristics of an appropriate adsorbent for practical applications. In addition, the cost of adsorbent preparation enhances the importance of regeneration. An adsorbent can be applied industrially if it can be used in several regeneration processes (adsorption–desorption cycle). Regeneration of adsorbent generally leads to the recovery of adsorbate molecules, reuse of adsorbent in the adsorption process, reduced secondary waste and cost of adsorption process, and also helps to better understanding of the adsorption process mechanism [26]. Chitosan is the second most abundant naturally available polymer. This polymer has high affinity to adsorb different pollutants from aqueous solution. Chitosan is advantageous because of the following characteristics: low price and abundance, antibacterial property, nontoxicity, biocompatibility, biodegradability, macromolecular structure, hydrophilicity, cationicity, active sites, and high adsorption capability [27]. Chitosan-based adsorbents have been extensively and successfully applied, but the adsorbed pollutants may be leached to the environment after adsorption. Chitosan-based adsorbents eventually become saturated, toxic, and hardly biodegradable after adsorption [28]. Therefore, spent chitosan adsorbents should be recovered completely prior to disposal and release back into the environment. In addition, its good affinity with many inorganic and organic pollutants suggests a slow regeneration process. Hence, regeneration of spent chitosan adsorbents is not only important for an effective and environment-friendly elimination of pollutants from the aqueous media, but also necessary for its economic utilization on large-scale [29]. Numerous studies have focused on the elimination of heavy metals from aqueous solutions using chitosan-based adsorbents. However, few studies have reported on the recycling, regeneration, or disposal of spent chitosan adsorbents after adsorption. Hence, this issue should be investigated properly to explore for an appropriate solution to chitosan disposal. Thus, a review is needed to summarize the research findings in the desorption, regeneration, and recovery of spent chitosan-based adsorbents for heavy metal removal. The main objective of present study is to review the reports on the regeneration of chitosan and its derivatives to recover adsorbents after heavy metal adsorption. Appropriate information is collected and discussed in relation to the efficiencies of different desorption agents and spent adsorbents after regeneration. Furthermore, some of the results about adsorption performance of chitosan and its derivatives have been compared and

2. Metals removal using chitosan-based adsorbents Metal ions are considered as one of the main types of water contaminants due to their high toxicity. Different toxic heavy metals are discharging to the aquatic environment through various industrial processes. Chitosan and its derivatives have gained particular attention as effective adsorbents for adsorption of heavy metal ions from contaminated effluents. High adsorption capability of chitosan-based adsorbents for heavy metal ions might be due (a) specific physico-chemical characteristics, (b) excellent chelation behavior, (c) high reactivity, (b) high selectivity toward metal ions, (d) high hydrophilicity, (e) presence of a large number of adsorption sites and (f) flexible structure of the polymer chain [30,31]. It has been found that NH2 groups are the main reactive groups for adsorption of metal ions. Though, OH groups may also contribute to the adsorption. Different mechanism such as electrostatic attraction and chelation may involve in adsorption of metal ions onto these fictional groups in which the pH is an important factor. The free electron doublet on nitrogen may bind metal cations at pH close to neutrality (or weak acidity). On the other hand, the protonation of amine groups in acidic solutions gives the polymer a cationic behavior and consequently the potential for attracting metal anions [32]. The chelation of metal cations by ligands in solution may result in the formation of metal anions, which therefore turns the chelation mechanism on chitosan to an electrostatic attraction mechanism on protonated amine groups of the polymer. Adsorption of heavy metal ions from water has been investigated by several researchers using chitosan-based adsorbents. Vakili et al. [33] studied the use of diepoxyoctane cross-linked chitosan beads for the removal of Cr(VI) ions. Cross-linked chitosan beads presented excellent Cr(VI) adsorption capacity of 325.2 mg/g. The variation of metal solution pH leads to competition between the coordination of Cr(VI) ions with adsorbent. The optimal pH range for adsorption of Cr(VI) ions onto diepoxyoctane cross-linked chitosan beads was 2. In other study, the performance of poly(methacrylic acid)-grafted chitosan microspheres for removal of Cd(II) ions from aqueous solution was explored by Huang et al. [34]. Initial pH of the metal solution had a major impact on cadmium adsorption capacity of adsorbent. The highest Cd(II) uptake value (158.22 mg/g) was achieved at pH 5. Adsorption of Cu(II) from an aqueous solution via chitosan flakes has been investigated by 374

Cu(II) Cu(II) Cu(II) Cu(II) U(VI) Au(III) Cd(II) Cr(VI) Cr(VI) Cu(II) Co(II) Ni(II) Pb(II) Cd(II) Cu(II) Hg(II) Ag(II) Cr(III) Cr(VI) Cr(III) Cr(VI)

52.6 100 31.20 67.66 72.46 1076.64 158.22 215 325.2 342 232 184 395 269 122 306 107 63 81 63 81

Magnetic chitosan beads

pH

Ref.

5 5 4.5 4.5 3 5 5 3 2 7 7 7 7 7 4 4 4 4 4 4 4

[36] [36] [37] [37] [38] [39] [34] [40] [33] [41] [41] [41] [41] [41] [42] [42] [42] [42] [42] [42] [42]

Disadvantages

Verbych et al. [35]. The results showed that the adsorption capacity increased when the solutions contain a high concentration of chloride ions. The maximum Cu(II) adsorption capacity was obtained at pH 5.4–6.0. A summary of adsorption capacities of chitosan-based adsorbent for different metal ions are presented in Table 1.

[46]

[45] concentration • Solvent solubility • Adsorbates properties • Adsorbents • Solution pH

Chitosan beads Chitosan/polyaniline beads Chitosan/glutaraldehyde beads Chitosan/alginate beads Chitosan/epichlorohydrin Chitosan/graphene oxide Chitosan/poly(methacrylic acid) Chitosan/glutaraldehyde Chitosan/diepoxyoctane Chitosan/Glucan

(mg/g)

and type of microorganism and • Nature adsorbents concentration • Adsorbent growth condition • Microbial • Molecular structure of organic pollutants time and temperature • Heating • Type of adsorbate and adsorbent

qmax

for biodegradable contaminants • Suitable of some contaminants to • Toxicity microorganisms the pores on the adsorbents by • Fouling microbial activity regeneration rate • Slow high temperature • Needs cost effective • Not carried out in situ • Not of harmful gases • Release and adsorption capacity losses • Weight of completely regenerating • Incapable on the surface properties of adsorbents • Affect of oxidized sludge/wastage • Production further purification of the solvent • Needs • Incomplete recovery of adsorption capacity

Adsorbate

Affecting factors

Adsorbent

Ref.

Table 1 Adsorption of heavy metal ions onto various chitosan-based adsorbents.

[47,48]

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375

effective • Cost regeneration • Fast • Very short process time Chemical

widely used • Most • Suitable for industrial scale Thermal

recovery of adsorption capacity by biodegradation of adsorbed organic compounds on • Complete the adsorbents through conversion into small ionic toxicants Biological

Regeneration method

Table 2 General classification of regeneration methods of adsorbents.

From environmental, economic and practical points of view, regenerability is an important parameter in evaluating the efficiency of an adsorbent. Regeneration is defined as the rapid recycling or recovery of spent adsorbents using technically and economically feasible methods. For the preparation of new adsorbents, cost is considered as one of the crucial factors. Therefore, the regeneration of spent adsorbents has immense importance for removal of contaminants from aqueous solutions. In the literature, different methods have been adopted for regeneration of saturated adsorbents which are classified into three major groups: biological, thermal and chemical. Some important advantages, disadvantages and affecting factors of these methods are summarized in Table 2. Selecting an appropriate method for regeneration of adsorbents is dependent on type and nature of adsorbent and adsorbate as well as the cost and processing conditions. Therefore, the adopted method for regeneration of spent adsorbent should be eco-friendly, easy to operate, cost effective, efficient, and give the ability to reuse the spent adsorbent in water treatment. Low mechanical stability, low chemical stability and biodegradability of chitosan are some factors affecting regenerability of chitosan-based adsorbents. Beside all disadvantages of thermal and biological methods, some drawbacks of chitosan can hinder the use of these methods for regeneration of chitosan-based adsorbents. Chitosan shows a high sensitivity to high temperature. Thermal analysis of chitosan shows that this polymer is not able to withstand high temperatures [43]. In addition, presence of oxidizable or hydrolyzable bonds in the backbone of chitosan and their accessibility for microorganisms lead to biodegradation of this polymer [44]. Thus, from practical and economical viewpoints, regeneration of saturated chitosan-based adsorbents by chemical treatment method (washing with a suitable desorption agent or eluent) is preferred over other options due to its simplicity, convenience, effectiveness and low cost. In chemical regeneration method, different specific chemicals are applied as desorption agents for desorption of particular species from spent adsorbents [45].

Advantages

3. Regeneration of chitosan-based adsorbents

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3.1. Acid desorption agents

Desorption of adsorbed metal ions from chitosan-based adsorbents is one of the most important advantages of applying these adsorbents during adsorption. Nevertheless, chitosan may not be used in continuous recycling, if the regeneration experiment is performed under unsuitable conditions, because of its biodegradability and dissolution tendency under acidic environment. However, a proper strategy can be designed to achieve an effective desorption and regeneration. Chitosan is a deacetylated form of chitin and composed of D-glucosamine and N-acetyl-D-glucosamine units, with two hydroxyl groups and one amino group in the repeating glucosidic residue. The abundant reactive functional groups on the chitosan structure leads to its high adsorption capability. Interaction of chitosan with metal ions could be the result of chelation interaction, electrostatic attraction, or ion exchange mechanism, in which pH plays a vital role [49]. Chitosan contains high amount of nitrogen in the form of amine groups, which are responsible for the chelation and binding with metal ions. The lone pair of electrons on the amino groups serves as the active adsorption site with high affinity for adsorption [50]. In addition, chitosan is a polymer with high cationicity (pKa 6.2–7) such that in acidic media, chitosan is protonated and exhibits electrostatic characteristics and adsorption capacity through anion exchange mechanisms [51]. Chitosan-based adsorbents could be regenerated by desorption [52]. Adsorption of cationic and anionic heavy metal compounds is feasible in solutions at low and high pH levels, respectively. Thus, changing the pH in adsorption solutions leads to reverse reaction and desorption of the adsorbed ions from the saturated adsorbents. Therefore, after adsorption, regeneration of chitosan might be accomplished by eluting the spent adsorbents with a suitable eluent. Fig. 2 shows the regeneration of a chitosan adsorbent for adsorption of heavy metal ions. A suitable desorption agent for regenerating chitosan-based adsorbent might be able to effectively recover the adsorbent for reuse in adsorption process [53]. In addition, this agent should not be expensive, harmful to the environment, nor destructive to the chitosanbased adsorbents, and can restore close to the original state the physical and adsorption properties of chitosan [54]. Therefore, choosing a suitable eluent with proper desorption properties facilitates a successful desorption and regeneration of spent adsorbents. Various desorption agents, including acids, alkalis, salts, and chelating agents, have been applied to regenerate chitosan adsorbents after adsorption of heavy metals. The capabilities of different desorption agents for desorbing heavy metals and regenerating chitosan-based adsorbents are described as follows.

Different acidic eluents, such as hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), phosphoric acid (H3PO4), and L-cysteine, have been applied to desorb heavy metal ions from chitosan-based adsorbents. Acidic desorption agents are more suitable for desorbing cationic heavy metal ions because of the adsorption mechanism of these ions on chitosan-based adsorbents. The adsorption of these positively charged ions is favored in basic environment. As pH value increases, the amino groups are free from the protonation for the adsorption of cationic heavy metal ions. Therefore, acidic media is not suitable for adsorbing these ions, and decrease in pH resulted in the desorption of these ions from the surface of adsorbents. Table 3 summarizes the desorption behavior of different acidic desorption agents applied to recover heavy metal ions from chitosan and its derivatives. 3.1.1. HCl HCl as a strong inorganic acid is the most widely used acidic desorption agent for desorption of heavy metal ions from chitosan and its derivatives. The supply of high number of H+ ions can weaken the interaction between adsorption groups and metal ions. Cl− from HCl can easily form a complex with heavy metal ions and then release to the solution [55]. Wan et al. [56] evaluated the desorption of Cu(II) and Pb(II) ions from chitosan coated sand in a batch system using tap water (pH 7) and diluted HCl solutions (pH 1 and 3). The results signified that more metal ions could be recovered under acidic conditions [Cu(II) (99.22%) and Pb(II) (97.91%) at pH 1]. Meanwhile, using tap water (pH 7) resulted in the least desorption level of Cu (II) (0.23%) and Pb(II) 9.12%. Yan et al. [57] also applied HCl solution at different pH levels (1.0 to 12.0) for regeneration of glutaraldehyde (GLA) crosslinked carboxymethylated chitosan beads loaded with Cu(II) ions. Desorption efficiencies were approximately zero at higher pHs (pH > 5.0) and approximately 99% at pH 1. This phenomenon could be due to the relatively weak interactions between metal ions and functional groups on the adsorbents at low pH values and in other hand higher stability of the formed bond between adsorbate and adsorbent under neutral conditions. During the regeneration experiment, increasing the number of adsorption–desorption cycles leads to decrease the adsorption and desorption efficiencies. Wang et al. [58] reported that the Au(III) adsorption capacity of GLA crosslinked carboxymethyl chitosan had 2.8% loss after five cycles (Table 3). In another study, magnetic chitosan composite, loaded with Cu(II) ions, was regenerated using 0.05 M HCl [59]. After six cycles, the adsorption capacity of Cu(II) had 12% loss and decreased

Fig. 2. Schematic representation of regeneration procedure of chitosan adsorbent. 376

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Table 3 Summary of acid desorption agents for recovering heavy metals from chitosan adsorbents. Adsorbent

Chitosan coated sand

Adsorbate

Regeneration

Ref.

Eluent

Adsorption n = no. of cycles

Desorption (%) n = no. of cycles

1.0 N HCl pH 1



99.22, 97.91, 13.06, 55.26, 99.98, 99.93, –

1.0 N HCl pH 3

GLA crosslinked carboxymethyl chitosan beads

Cu(II) Pb(II) Cu(II) Pb(II) Cu(II)

GLA crosslinked carboxymethyl chitosan

Au(III)

0.5 M thiourea–2.0 M HCl

Chitosan/Fe-hydroxyapatite

Pb(II)

0.6 M HCl

0.1 M HCl

Cd(II) Zn(II) Chitosan-Laminaria. japonica

Ni(II)

HCl

Ni(II)

HNO3

Carboxymethyl chitosan

Pb(II)

0.01 N HCl

Chitosan beads Polyaniline/chitosan beads

Cu(II) Cu(II)

0.1 M HCl 0.5 M HCl

GLA crosslinked chitosan/polyaniline

Cd(II) Pb(II) Cu(II)

0.5 M HCl

ECH/triphosphate crosslinked chitosan

100%, n = 1 36%, n = 3 – 83.3 mg/g, n = 1 76.9 mg/g, n = 3 –

EGDE crosslinked chitosan/poly(methacrylic acid)

Cd(II)

1.0 M HNO3 0.1 M HNO3 1.0 M HNO3 0.1 M HNO3 1 M HNO3

ECH crosslinked chitosan beads/crotonic acid

Cu(II)

1 M HNO3

Hg(II)

0.01 M HNO3

Pb(II)/GLA crosslinked dithiocarbamate chitosan beads

Pb(II)

0.2 M HNO3

Magnetic chitosan nanoparticles Chitosan Molybdate/chitosan beads

Hg(II) Cr(VI) As(III)

Cd(II)

GLA crosslinked chitosan/Fe2O4

Molybdate/chitosan beads/ortho-H3PO4 Chitosan/sodium silicate Chitosan/iron oxide nanocomposites

As(V) As(V) As(III)

0.1 M H2SO4 0.1 M H2SO4 0.1 M H2SO4 0.5 M H2SO4 1.0 M H2SO4 0.1 M H2SO4 0.5 M H2SO4 1.0 M H2SO4 0.1 M H3PO4 L-cysteine 0.1 N HCl

Chitosan/Fe3O4/hexadecyl trimethoxysilane

Cu(II)

0.05 M HCl

ECH crosslinked chitosan fiber EGTA–chitosan

Pd(II) Pb(II) Cd(II) Ni(II)

0.5 M thiourea-1.0 M HCl 2 M HNO3

Ni(II)

0.1 M HCl

Cd(II)

0.1 N HNO3 pH 1 0.1 N HNO3 pH 3

As(V)

ECH crosslinked chitosan beads/triethylenetetramine Magnetic chitosan/butyl acrylate Magnetic chitosan/butyl methacrylate Magnetic chitosan/hexyl acrylate Chitosan



0.13 mmol/g, n = 1 0.08 mmol/g, n = 4 0.12 mmol/g, n = 1 0.08 mmol/g, n = 4 56%, n = 1 30%, n = 4 58%, n = 1 48%, n = 4 359.68 mg/g, n = 1 267.08 mg/g, n = 10 82%, n = 2 – –



ECH crosslinked chitosan beads/itaconic acid GLA crosslinked chitosan

130.30 mg/g, n = 1 130.25 mg/g, n = 6 8.32 mmol/g, n = 1 8.08 mmol/g, n = 5 613 mg/g, n = 1 558 mg/g, n = 3 573 mg/g, n = 1 527 mg/g, n = 3 653 mg/g, n = 1 588 mg/g, n = 3 –

87%, n = 3 – 87%, n = 5 105.28 mg/g, n = 1 92.56 mg/g, n = 6 – – ∼90 mg/g, n = 1 ∼50 mg/g, n = 5 –

1 M H2SO4



n=1 n=1 n=1 n=1 n=1 n=6

[56]

[57] [58]



[75]

71.73, n = 1 36.3, n = 5 72.44, n = 1 31.53, n = 5 –

[76]

[77]

94, n = 3 –

[61] [36]

98.94, n = 1 97.5, n = 1 88.7, n = 1 87.9, n = 1 89.9, n = 1 88.5, n = 1 ∼95, n = 1 ∼90, n = 5 –

[62]



[66]



[65]

– 88, n = 1 85, n = 1 86, n = 1 95, n = 1 83, n = 1 96, n = 1 99, n = 1 76, n = 1 100, n = 1 99, n = 1 89.9, n = 5 –

[68] [70] [71]

95.6, n = 3 99.91, n = 10 99.18, n = 10 – 97.5, n = 1 73.8, n = 1 100, n = 1 68, n = 1 44.8, n = 1

[78]

[34] [64]

[73] [74] [79] [59]

[63] [69] [80]

[81]

adsorbates [60]. In addition to reducing the desorption and adsorption efficiencies by increasing the number of cycles, gradual destruction of the binding sites on the adsorbent after each cycle and the biodegradability properties of chitosan, limit continuous recycling. Osifo et al. [61] applied 0.1 M HCl

to 92.56 mg/g (Table 3). In a further study, use of 0.1 M HCl for desorption of Pb(II) ions from the potassium iodate chitosan composite, caused approximately 25–30% loss in desorption efficiency after four regeneration cycles. These reductions could have been caused by the saturation and occupation of adsorption sites with strongly adsorbed 377

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modified with triethylenetetramine using 1 M H2SO4, the Ni(II) adsorption capacity was not reduced significantly and had only 5% loss within four regeneration cycles. However, it decreased rapidly during the fifth cycle and showed 43% reduction (Table 3). It was observed that regeneration conditions can affect the desorption performance of the adsorbents. Bhuvaneshwari et al. [70] found the maximum Cr(VI) desorption efficiency of 88% using 0.1 M H2SO4 under the following optimum conditions: agitation speed, 120 rpm; contact time, 60 min; and temperature, 40 °C. Chen et al. [71] also reported that recovery efficiencies of As compounds enhanced by increasing time and eluent concentration. The maximum desorption efficiencies of As(III) (95%) and As(V) (99%) from the molybdate impregnated chitosan beads were obtained under the following optimum conditions: eluent, 1 M H2SO4; agitation rate, 50 rpm; and contact time 5 h. Desorption was associated with leaving the As ions and formation of complex between sulfate ions and functional groups. Desorption of As compounds from molybdate impregnated chitosan beads and molybdate coagulated chitosan beads was performed using H3PO4. The complete desorption and lowest molybdate release (< 1%) were obtained using 0.1 M H3PO4 at pH 3 [72]. In another experiment, 0.1 M H3PO4 (pH 1.6) also was used for to desorb As(V) ions from molybdate impregnated chitosan beads modified with ortho-H3PO4 in the column system [73]. Three regeneration cycles led to 24.5% and 13.5% reductions in the adsorption and desorption capacities, respectively (Table 3). Desorption of As compounds might have been caused by the high affinity of the beads for adsorption of phosphate ions. This phenomenon resulted in the formation of arsenate–molybdate complex and displacement of arsenate from the adsorbent to the eluent. Boyacı et al. [74] applied two eluents [2.0% (v/v) CH3COOH and 1.0% (w/v), C3H7NO2S] to desorb and recover the As ions in the solution from the surface of chitosan-immobilized sodium silicate. Using acetic acid was suitable for regenerating adsorbents. Acetic acid completely dissolved chitosan, and the adsorbed arsenate was not desorbed even at higher acid concentration. However, using the 1.0% (w/v) Lcysteine (pH 3.0) as eluent was effective and led to desorption efficiency of 100% in both cases.

to desorb Cu(II) loaded on GLA crosslinked chitosan beads and found desorption efficiency of 94% during the third cycle. However, desorption efficiency decreased significantly after the fifth cycle because of the physical damage in chitosan (25% mass loss). Igberase et al. [36] applied 0.5 M HCl as desorbing agent to recover Cu(II) ions from polyaniline graft chitosan beads. After two regeneration cycles, mass loss of 4.4% was observed in the beads, but the adsorption behavior was not affected. This improvement might be due to the opening of the pores on the bead surface during the acid treatment. However, at third cycle, mass loss of the beads increased to 8.9%, and the maximum adsorption capacity decreased to 76.9 mg/g (Table 3). However, Igberase and Osifo [62] reported 98.94% and 97.5% desorption of Cd(II) and Pb(II) ions, respectively, from GLA crosslinked chitosan beads grafted with polyaniline using 0.5 M HCl. After four regeneration cycles, adsorption capacity remained constant, and only 3% loss in the mass of the beads was observed in the fifth cycle. This phenomenon might be due to the crosslinking and high stability of the prepared beads in an acid environment. 3.1.2. HNO3 Desorption of heavy metal ions adsorbed onto chitosan-based adsorbents by means of HNO3 as a strong inorganic acid have been performed by many researchers. In HNO3 solution, H+ can replace adsorbed ions on the surface of adsorbent. Zhao et al. [63] successfully regenerated chitosan functionalized with ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA–chitosan) using 2 M HNO3 and recovered 99.91% and 99.18% of Pb(II) and Cd(II) ions, respectively, within 10 cycles (Table 3). Decrease in adsorption and desorption efficiencies of the adsorbents after regeneration have been reported by some resrechers. Huang et al. [34] used 1 M HNO3 to recover Cd(II) ions from chitosan microspheres modified with ethylene glycol diglycidyl ether (EGDE) and poly(methacrylic acid) and reported 5% and 3% loss in adsorption capacity and desorption efficiency, respectively, after five cycles. These slight reductions might be associated with some strong metal ion and adsorption group complexes and incomplete desorption. Epichlorohydrine (ECH) crosslinked chitosan beads grafted with crotonic acid and itaconic acid loaded with Cu(II) ions were regenerated using 1 M HNO3 [64]. After four cycles, both ECH/chitosan/crotonic acid and ECH/ chitosan/itaconic acid showed approximately 55% and 68% of their yield, respectively, which could be due to the crosslinked structure of beads resulting in incomplete regeneration rate. The recyclability of Pb (II) imprinted GLA dithiocarbamate chitosan beads by 0.2 M HNO3 solution was investigated after 10 times of reuse, and the Pb(II) adsorption capacity decreased from 359.68 mg/g at first cycle to 267.08 mg/g at tenth cycle (25.7% loss) [65]. Kyzas and Deliyanni [66] analyzed the elimination of Hg(II) from GLA crosslinked chitosan and Fe2 O4 functionalized chitosan using 0.01 M HNO3 (pH 2). After four regeneration cycles, adsorption efficiency of 56% for GLA crosslinked chitosan in first cycle decreased to 30% at the 4th cycle, compared with the 10% reduction for GLA crosslinked chitosan/Fe2O4 (58 by 48%). This phenomenon could be due to the more complex network and structure of GLA crosslinked chitosan/Fe2O4 compared with GLA crosslinked chitosan which resulted in higher stability behavior.

3.2. Chelating desorption agents Many studies have investigated the desorption behavior of different chelating desorption agents, such as EDTA and EDTA-disodium (Na2EDTA) to regenerate different forms of chitosan adsorbents. The applied chelator agents are very effective in desorbing heavy metal ions from adsorbents. EDTA is a chemical consists of nitrogen atoms and short chain carboxylic groups and usually marketed as its sodium salts. These agents are the most common and very strong chelating agents for many heavy metals. These chemicals have high affinity constant to form metal-EDTA complexes and can replace the functional groups on the adsorbents complexed with ions and complexate with ions [82]. A summary of the chelating desorption agents used for desorption of heavy metal ions from chitosan-based adsorbents are discussed briefly in present section and results are tabulated in Table 4. Cui et al. [83] applied diethylene triamine pentacetate acid (DTPA), EDTA, NaNO3, H2SO4, and NaOH solutions to recover Pb(II) ions loaded on carboxymethyl chitosan modified with Aliquat 336 ([A336] [CMCTS]). NaNO3 and H2 SO4 were not effective for desorption, and NaOH solution presented only desorption efficiency of 36%. However, 0.1 M of DTPA and EDTA solutions had highest desorption efficiencies (90%), probably because of their high affinity with Pb(II) ions. Triphosphate (TPP) crosslinked chitosan beads saturated with Pb(II) and Cu(II) were regenerated by agitating the spent adsorbents into HCl, EDTA, H2SO4 and HNO3 for 1 h [84]. All eluents, except EDTA solution, led to the deformation of the beads and then reduction in their adsorption capacities (Table 4). A number of investigations have been studided effect of EDTA concentration on desorption efficiencies of the metal ions. It has been

3.1.3. H2SO4, H3PO4 and L-cysteine In addition to the above-mentioned acidic eluents, H2SO4, H3PO4 and L-cysteine have been applied as desorption agents for regeneration of chitosan-based adsorbents saturated with heavy metal ions. Chang et al. [67] successfully recovered the Hg(II) ions from GLA alginate chitosan beads using (1 N H2SO4). The used beads were reused thrice, and adsorption capacity showed a negligible decrease (up to 95% recovery). In another investigation, H2SO4 was reported to be as an effective eluent with adsorption efficiency of 82% for Hg(II) ions (up to 2 cycles) from magnetic chitosan nanoparticles [68]. Liao et al. [69] showed that after regeneration of the ECH crosslinked chitosan beads 378

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Table 4 Summary of the chelator desorbing agents for recovering heavy metals from chitosan adsorbents. Adsorbent

TPP crosslinked chitosan

Adsorbate

Pb(II)

Regeneration

Ref.

Eluent

Adsorption n = no. of cycles

Desorption (%) n = no. of cycles

0.01 M EDTA

2.42 mg/g, n = 1 2.24 mg/g, n = 3 2.38 mg/g, n = 1 2.14 mg/g, n = 3 2.19 mmol/g, n = 1 2.03 mmol/g, n = 5 –

97.60, n = 1 97.49, n = 3 92.26, n = 1 96.25, n = 3 90, n = 5

[84]

65.24, n = 1 95.38, n = 1 97.67, n = 1 82.30, n = 1 – 97.2, n = 1 88.5, n = 5 96.8, n = 1 87.4, n = 5 97.3, n = 1 88.3, n = 5 –

[87]

Cu(II) Aminated chitosan beads

Hg(II)

0.01 M EDTA

Chitosan beads GLA crosslinked chitosan beads ECH crosslinked chitosan beads EGDE crosslinked chitosan beads Chitosan/maleic anhydride/titania Magnetic/2-aminopyridine glyoxal Schiff’s

Cu(II)

0.0001 M EDTA 0.01 M EDTA

Pb(II) Cu(II)

0.01 M EDTA EDTA

92%, n = 3 –

48.1 mg/g, n = 1 32.8 mg/g, n = 5 ∼55%, n = 4 ∼65%, n = 4 91%, n = 3 – –

Cd(II) Ni(II) Chitosan-Fe(III)/carbon disulfide

Cd(II)

0.1 M EDTA

Magnetic chitosan/rectorite composite microspheres

0.1 M Na2EDTA

Magnetic chitosan beads/cystein glutaraldehyde’s schiff Magnetic chitosan nanoparticles/α-ketoglutaric acid TPP crosslinked chitosan/montmorillonite beads

Cu(II) Cd(II) Cu(II) Cu(II) Cu(II)

0.01 M Na2EDTA 0.1 M Na2EDTA 0.005 M EDTA

Chitosan/rhodamine ethylenediamine

Hg(II)



Chitosan/perlite beads Chitosan/SiO2/Fe3O4

Cu(II) Cu(II)

0.01 M EDTA 0.01 M Na2EDTA

Chitosan/SiO2/Fe3O4/EDTA

Pb(II)

93%, n = 3 85%, n = 4 – > 95%, n = 5 25%, n = 12 > 95%, n = 5 25%, n = 12

[85]

[89] [88]

[94]



[91]

– 91.5, n = 1 86, n = 1 84, n = 3 –

[90] [93] [95] [96]

96, n = 1 –

[86] [92]

ions from saturated chitosan-based adsorbents. Cu(II) ions were desorbed from magnetic chitosan beads modified with cystein glutaraldehyde’s schiff by 0.01 M Na2EDTA solution. The results from three consecutive three regeneration cycles also showed that adsorption efficiency did not decrease considerably but reached approximately 91% in last cycle (Table 4). Thus, the prepared modified beads were stable and recyclable [90]. Magnetic chitosan-organic rectorite composite microspheres loaded with Cu(II) and Cd(II) ions were regenerated by HCl, EDTA, and Na2EDTA [91]. Among the eluents, Na2EDTA solution was the most effective eluent because of its stronger electronic supply ability resulting in stronger capability of complexation with metal ions. By contrast, HCl presented the lowest desorption performance because of the increased Fe3O4 dissolution rate. This trend could be due to the different desorption mechanisms. Desorption mechanisms using Na2EDTA and EDTA solutions could be attributed to the formation of stable complex with metal ions. Ion exchange is involved in the desorption with HCl solution. Cd(II) presented higher adsorption capacity than Cu(II) in the fourth regeneration cycle using 0.1 M Na2EDTA. This phenomenon could have been caused by the stronger affinity of Cu(II) for the complex formation with adsorbent. This characteristic resulted in lower desorption quantity and consequently lower adsorption efficiency in the following cycles. Cu(II), Pb(II), and Cd(II) loaded on chitosan/SiO2/Fe3O4 and EDTA-modified chitosan/SiO2/Fe3O4 were desorbed using Na2EDTA solution [92]. The results showed that increasing the eluent concentration enhanced desorption ratios. After repeated adsorption-desorption cycle for five times using 0.01 M Na2EDTA , the adsorption efficiencies of both adsorbents were still beyond 90% but decreased to 75% after the 12th cycle. These reductions could be due to the occupation of adsorption sites and also adsorbent mass loss (11–14%) during the regeneration cycles. Cu(II) ions from chitosan-coated magnetic nanoparticles modified with α-ketoglutaric acid were recovered using 0.1 M and 0.025 M Na2EDTA solutions [93].

found that an increase in eluent concentration enhances the desorption efficiency. Jeon and Ha Park [85] studied the desorption of Hg(II) from aminated chitosan beads by different concentrations of EDTA solution. Desorption efficiency increased from 48% to 95% as EDTA concentration increased from 0.001 M to 0.1 M. Hasan et al. [86] also found that an increase in eluent concentration enhanced the desorption efficiency. In this study, EDTA solutions with concentrations ranging from 0.01 M to 0.05 M were applied to desorb Cu(II) from chitosan-coated perlite beads, and maximum desorption efficiency was obtained by 0.01 M EDTA. The results revealed that desorption efficiency was almost complete (96%) in first cycle. However, an increase in eluent concentration led to a decrease in the adsorption efficiency of the beads. Ngah et al. [87] reported that treatment of saturated chitosan beads with high concentration of EDTA was not successful because chitosan beads were soluble in higher concentration of EDTA. However, best desorption results of crosslinked beads obtained at higher EDTA concentration. It has been reported by some researchers that desorption and adsorption efficiencies decreased by increase in the adsorption-desorption cycle’s number. This could be due to the incomplete desorption of the ions and saturation of the adsorption sites on the adsorbent. Regeneration using 0.01 M EDTA solution revealed that the adsorption capacity decreased by 7.3% after fifth cycle. Metal ions from glyoxal crosslinked magnetic chitosan modified with 2-aminopyridine glyoxal Schiff’s were desorbed using EDTA solution. The results showed that the eluent efficiencies of Cu(II), Cd(II), and Ni(II) ions in first cycle decreased from 97.2%, 96.8% and 97.3% to 88.5%, 87.4% and 88.3%, respectively, in fifth cycle [88]. However, Shaker and Yakout [89] used 0.01 M EDTA solution to regenerate chitosan modified by maleic anhydride and titania loaded with Pb(II) ions and reported only 2% reduction in adsorption efficiency after three consecutive cycles (92%). Na2EDTA is another chelating agent applied for desorption of metal 379

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using NaOH solutions because of their high mechanical stability at alkali conditions.

The results showed that an increase in eluent concentration enhanced quantity of desorbed metal ions. Maximum desorption percentage of 91.5% was obtained by 0.1 M Na2EDTA. Repeating the adsorption–desorption process for 7 times did not affect adsorption performance. The results of chitosan and its derivatives regeneration studies using chelating desorption agents are presented in Table 4.

3.3.2. NH4OH Using ammonia solution (NH4OH) to elute Pb(II) from CS supported on porous glass beads showed that adsorption capacity of the beads did not change after five cycles and remained stable [103]. In another study, 5% v/v NH4OH solution was applied as an eluent to regenerate phosphated chitosan impregnated with ethyl hexadecyl dimethyl ammonium bromide contaminated with Cr(VI) ions. After 10 regeneration cycles, desorption efficiency had approximately 30% loss, but removal efficiency was still approximately 70% (Table 5). Therefore, the NH4OH basic solution showed good desorption performance, and no change was observed in the structure of the adsorbents [104]. Table 5 shows the desorptions of the different adsorbates from used chitosan adsorbents by alkali desorption agents.

3.3. Alkali desorption agents Sodium hydroxide (NaOH) and ammonia solution (NH4OH) are the most frequently used basic solutions to desorb heavy metals from chitosan and its derivatives. NaOH is a strong inorganic base, contains the sodium (Na+) and the hydroxide (OH−) and NH4OH is an alkali possess NH+4 and OH− in aqueous solutions. These alkali agents can effectively desorb the anion heavy metal compounds such as HAsO42−, H2AsO3−, CrO42−, HCrO4−, HCr2O7−, and Cr2O72−, from adsorbents. This could be due to the higher affinity of heavy metal ions to react with Na+ or NH+4 than that of adsorption sites and the weakening of the adsorbate and adsorbent bonds in basic conditions. Thus, the application of basic eluents in desorption and regeneration studies of chitosan adsorbents saturated with heavy metal ions is discussed.

3.4. Salt desorption agents These eluents can be considered as proper desorption agents to recover the adsorbed heavy metal ions because of their ability to weaken the interaction between metal ions and binding sites on the adsorbent surface to form stable complexes. The adsorption–desorption performances of various chitosan derivatives regenerated by salt desorption agents, such as sodium chloride (NaCl), potassium nitrate (KNO3), sodium carbonate (Na2CO3), and sodium sulfate (Na2SO4) to remove the different heavy metal ions from aqueous solutions are discussed. The desorption efficiencies and other parameters affecting the regeneration of chitosan adsorbents using salt desorption agents are summarized in Table 6.

3.3.1. NaOH As mentioned earlier, due to the dissolution tendency of chitosan in acidic environment, use of acidic eluents may leads to destruction of adsorbent’s structure and decrease the adsorption capacity. However, basic eluents such as NaOH as alternative desorption agents can present better desorption performance. This is in agreement with the findings suggested by Sankararamakrishnan et al. [97]. In this study, regeneration of GLA crosslinked chitosan beads saturated with Cr(VI) ions by 0.01 N of HCl and NaOH solution revealed that basic solution was more effective as desorption agent, because acidic solution led to the deformation of chitosan beads. In the first two cycles, adsorption rate was approximately 65%. In the third cycle, the rate decreased to 40% and then remained constant until the last cycle (10th). Padilla-Rodríguez et al. [98] investigated the desorption of As(III) and As(V) ions from ferric hydroxide chitosan beads by 0.1 M and 1.0 M of NaOH, H2SO4, HCl, and HNO3. Elution of the beads by acidic solutions led to leaving the Fe(III) on the surface of the beads. Thus, the spent adsorbents could not be reused. Consequently, NaOH solutions that maintain the integrity of the prepared beads were chosen as proper eluents for further adsorption–desorption experiments. During the regeneration experiments, for both NaOH concentrations (0.1 M and 1.0 M) desorption efficiencies of As(III) and As(V) were complete (100%) except at 0.1 M NaOH in which desorption percentage decreased to 82% in the fifth cycle (Table 5). This reduction could be due to the partial loss of iron from the prepared adsorbent. Liu et al. [99] also reported that the As adsorption capacity of the magnetic chitosan nanoparticle remained stable during the 10 regeneration cycles using 0.15 M NaOH solution. In first and tenth cycles, the corresponding adsorption efficiencies were 98.1% and 97.5% for As(V) and 95.6% and 95.3% for As(III). Kumar and Jiang [100] evaluated the regeneration behavior of chitosan functionalized with graphene oxide. In this study, 1.0 M NaOH solution was used to elute adsorbed As(III/V), and regeneration experiments were repeated for five cycles. Adsorption efficiency after three cycles was > 99% and then gradually declined to approximately 85% in the last cycle. The Cu(II) ions adsorbed on chitosan magnetic composites modified with polystyrene and polyethylenimine were recovered by 0.1 M NaOH solution [101]. The results showed that the Cu (II) removal efficiency decreased slowly and in last regeneration cycle (6th) reached 85%. This reductions caused by repeating the regeneration cycles could be a result of the decline in the effective adsorbateadsorbent interaction, saturation, and unavailability of adsorption sites [102]. Moreover, it was found that the shape and mass of the prepared chitosan adsorbent remained stable during the regeneration cycles

3.4.1. NaCl NaCl is an ionic compound contains Na+ and Cl−. The Na+ can interact with heavy metal ions and make a complex. The Cl− also can take their place and interact with adsorption sites on chitosan adsorbents and release to the solution. Several researchers have investigated the desorption performance of NaCl for recovery of heavy metal ions from chitosan-based adsorbents. Futalan et al. [109] analyzed the recovery of Pb(II), Ni(II), and Cu(II) ions from chitosan immobilized on bentonite (ChB) using 0.1 M of HCl, NaCl, and NaOH solutions. Among all adsorbates, the Pb(II) ions presented the highest desorption capacity, probably due to the fact that more functional groups on ChB could be occupied by Pb(II) ions. Thus, more metal ions could recover during desorption. NaOH solution showed the lowest metal recovery level probably because of the deprotonation of amine groups on the ChB and interaction of adsorption sites on bentonite (eSiOH) with excess hydroxyl ions in a basic solution. Compared with the other eluents, HCl provided higher concentration of H+in the solution which could replace the metal ions and achieve the highest desorption efficiencies of 81%, 79%, and 70% for Pb(II), Cu(II) and Ni (II), respectively. However, this solution resulted in the most damage to ChB. Therefore, regeneration of ChB by NaCl presented the best result over HCl and NaOH, which could be attributed to the formation of stable chloro complexes with metal ions [110]. Heidari et al. [111] applied 1.0 M NaCl and 0.1 M EDTA solutions to recover Pb(II), Cd(II), and Ni(II) ions from chitosan methacrylic acid nanoparticles. The results exhibited that the NaCl solution was more favored than EDTA solution for desorbing metal ions suggesting that electrostatic interaction was more effective than chelation. In addition, after three regeneration cycles, adsorption of metal ions did not change considerably (approximately 5% loss). Javanbakht et al. [112] also studied the desorption performance of CaCl2 , HNO3 and NaCl solution to recover Pb (II) ions from magnetite chitosan clinoptilolite nanocomposite and reported that 0.01 M NaCl solution showed better desorption performance followed by CaCl2 and HNO3 solutions. This phenomenon might be due to the strong electronic supply ability of NaCl resulting in the 380

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Table 5 Summary of the alkali desorbing agents for recovering heavy metals from chitosan adsorbents. Adsorbent

Adsorbate

Regeneration

Ref.

Eluent

Adsorption n = no. of cycles

Desorption (%) n = no. of cycles 65, n = 2 40, n = 10 100, n = 5 100, n = 5 82.4, n = 5 100, n = 5 –

[105]



[100]



[106]



[107]

70, n = 10 –

[104] [99]

– –

[101] [108]

GLA crosslinked chitosan beads

Cr(VI)

0.01 M NaOH



Ferric hydroxide chitosan beads

As(III)

88.9%, n = 5 79.8%, n = 5 72.5%, n = 5 43.9%, n = 5 91.13%, n = 1 75.73%, n = 10 > 99%, n = 1 85%, n = 5 99.5%, n = 1 88.2%, n = 5 97.9%, n = 1 76.02%, n = 5 83%, n = 4 85%, n = 4 – 98.1%, n = 1 97.5%, n = 10 95.6%, n = 1 95.3%, n = 10 85%, n = 6 92.5%, n = 1 87.5%, n = 7

Ethylenediamine/GLA crosslinked chitosan

Cr(VI)

0.1 M NaOH 1.0 M NaOH 0.1 M NaOH 1.0 M NaOH 0.1 N NaOH

Chitosan/graphene oxide

As(III/V)

1.0 M NaOH

Magnetic chitosan beads

As(V)

0.1 M NaOH

As(V)

As(III) Fe-Mn binary oxide/chitosan beads Phosphated chitosan/ethyl hexadecyl dimethyl ammonium bromide Magnetic chitosan nanoparticle

As(V) As(III) Cr(VI) As(V)

0.5 M NaOH 5% NH4OH 0.15 M NaOH

As(III) Magnetic chitosan/polystyrene/polyethylenimine Chitosn/graphene oxide/Na2EDTA

Cu(II) Cr(VI)

0.1 M NaOH 1.0 M NaOH

[97] [98]

chitosan for adsorption of U(VI). The Na2CO3 solution was recommended as the more effective eluent to recover adsorbed uranium, and desorption efficiency was 94% (Table 6).

formation of a steady complex with Pb(II) ions. 3.4.2. KNO3 and Na2CO3 Ahamed et al. [113] used 2.0 M KNO3 solution (pH 10.5) to desorb silver and gold from aminoguanidyl imprinted GLA crosslinked chitosan. In the first regeneration cycle, extraction of adsorbed adsorbates was above 95%. After five consecutive regeneration cycles, the adsorption capacities of both silver and gold decreased and remained at approximately 85%. The possible desorption mechanism could be the deprotonation of aminoguanidyl groups and subsequent release of metal ions into the solution. High concentration of KNO3 formed a driving force for the elution and replace the loaded ions on the adsorbent. Yu et al. [114] studied the recyclable availability of magnetic hydrothermal cross-linking chitosan contaminated with U(VI) ions by 1.0 M of Na2CO3, NaOH, EDTA, H2SO4, and H2O solutions and found 91.2% desorption efficiency with 1.0 M Na2CO3. After five regeneration cycles, the removal efficiency was found to be over 88% in the last cycle. Stopa and Yamaura [115] performed the desorption experiment by Na2CO3 and sodium oxalate as eluents to regenerate a magnetic

3.4.3. Na2SO4 Gylienë and Nivinskienë [28] investigated the adsorption and desorption of Ni(II) and citrate through the batch adsorption–desorption process. Different solutions (NaCH3COO, Na2SO4 and NH4Cl) were applied to recover spent adsorbents. Investigations showed that desorption proceeded slowly, and maximum desorption (5–10%) was achieved after 50–60 min and then it became constant. Therefore, electrolysis process was applied to regenerate the spent adsorbent because of the low desorption performance. The results revealed that electrolysis enhanced desorption and led to the deposition of Ni(II) and oxidation of citrate onto the cathode and anode, respectively. Regenerated adsorbent in Na2SO4 solutions presented adsorption properties similar to those in the first adsorption after 10 adsorption–desorption cycles. In other investigation, Gylienė et al. [116] evaluated the recovery of the Cu(II)–EDTA from chitosan using electrolysis. The results showed

Table 6 Summary of the salt desorbing agents for recovering heavy metals from chitosan adsorbents. Adsorbent

Adsorbate

Chitosan

Cu(II)

Chitosan/bentonite

Cu(II) Ni(II) Pb(II) Cd(II) Ni(II) Pb(II) Ag(II)

Chitosan/methacrylic acid nanoparticles

GLA crosslinked chitosan/aminoguanidyl

Regeneration

Ref.

Eluent

Adsorption n = no. of cycles

Desorption (%) n = no. of cycles

0.1 M Na2SO4/electrolysis 0.2 (200 mA for 8 h) 0.1 M NaCl

1.2 mmol/g, n = 1



[117]



[109]

1.0 M NaCl

2.0 M KNO3

95%, n = 3 95%, n = 3 95%, n = 3 –

36%, n = 3 47%, n = 3 13%, n = 3 –

[113]

1.0 M Na2CO3 1.1 g/L Na2CO3

88%, n = 5 –

95%, 85%, 95%, 85%, – 94%

Au(II) Magnetic hydrothermal cross-linked chitosan Magnetic chitosan

U(VI) U(VI)

381

n=1 n=5 n=1 n=5

[111]

[114] [115]

Separation and Purification Technology 224 (2019) 373–387

As(V)

Chelators (26.9%) Alkalis (14.9%)

Co(II)

Ag(II)

U(VI)

Na SO /Electrolysis

As(III)

KNO

Cr(VI)

Na CO

l-cysteine

Hg(II)

NaCl

H PO

Acids (49.3%)

Ni(II)

NH OH

Zn(II)

NaOH

Cd(II)

Na EDTA

Au(II)

EDTA

Pb(II)

HCl/Thiourea

HNO

HCl

Cu(II)

H SO

M. Vakili, et al.

Salts (8.9%)

Fig. 3. Scheme of the regeneration of a saturated chitosan adsorbent by an acid desorption agent.

adsorbents. Filipkowska [119] analyzed the desorption performance of chitosan adsorbents and found that adjusting the pH value enhanced the adsorption and desorption performances of adsorbents. The highest adsorption and desorption efficiencies were observed at pH levels of 5 and 11, respectively. Temperature increase may also intensify the positive charge of heavy metal (Me) compounds and consequently break down the binding of adsorbate and adsorbent [120]. At high temperature, the breakdown of adsorbate and adsorbent binding also increases the release of adsorbed ions and improves desorption performance. With regard to the mass transfer phenomena from the solid phase (i.e. adsorbate) to the liquid one that determine the regeneration kinetics, it is a complex process that involves several diffusion mechanisms. In general, metal desorption from a porous adsorbent may be limited by intra-particle and extra-particle diffusion. The intra-particle diffusion includes metal transfer to liquid-filled macro-pores (with a diameter larger than 50 nm); solid diffusion in micro-pores (> 2 nm), where interactions between pore walls and the adsorbate are stronger than those of large-pores; and the reaction kinetics of the chemisorption processes. The extra-particle diffusion is limited by the external mass transfer between the surface of particles and the liquid phase; and the fluid phase mixing, or the lack of it. Some studies identified the intraparticle diffusion in small pores as the main rate-limiting step of the adsorption-desorption process [121,122]. Nonetheless, the weight of the extra-particle diffusion may become relevant under non-optimal conditions, like employment of unfavorable desorption agent [121]. Concerning regeneration effectiveness, the majority of chitosanbased adsorbents used in adsorption presented an acceptable desorption performance when an efficient and effective desorption agent is applied. Desorption by acidic eluents is more effective than that of other eluents. In acidic environments, the high number of H+ ions in the solution desorbs adsorbed ions. Compared with heavy metal ions, H+ ions have stronger affinity to adsorb on chitosan functional groups and higher diffusivity coefficient because of their smaller radii [123]. Consequently, the cationic exchange between H+ and adsorbates, the protonation of adsorption sites, and the replacement of H+ ions with heavy metal ions in adsorbent–adsorbate complexes release adsorbed ions into the desorption solution and reduce the heavy metal ions that bind on adsorbents (Fig. 3) [56]. This reaction is shown in the following equation [Eq. (1)]:

that the desorption in acidic or alkaline conditions was not successful, and the highest desorption efficiencies of both adsorbates did not exceed 3% for Cu(II) and 10% for EDTA. This phenomenon could be due to the strong bonds and electrostatic interactions between the adsorbent and adsorbates. However, regeneration using electrolysis in the presence of 0.1 M Na2SO4 solution facilitated the regeneration of chitosan after adsorption in both acidic and basic solutions. The results showed that, after regeneration, the adsorption behavior of chitosan for removing adsorbates did not change and remained stable. However, the molecular weight of chitosan decreased slightly because of the prolonged electrolysis and destruction of chitosan. Regeneration of chitosan loaded with Cu(II) in 0.1 M Na2SO4 showed that higher current strength led to reduced deacetylation degree, molecular weight loss, and destruction of chitosan structure. These phenomena resulted in insufficient Cu(II) desorption [117]. The best results were achieved at longer electrolysis with low current strength. Regeneration of chitosan at optimum condition (200 mA current intensity for 8 h) resulted in 1.2 mmol/g of Cu(II) adsorption ability. In addition, no physicochemical change was observed in the properties of the adsorbent, and the adsorption performances of chitosan before and after regeneration under optimum conditions were similar.

4. Desorption mechanism Adsorption and desorption processes are determined by several physico-chemical factors. Since chitosan can bind metal ions by forming chemical bonds, the chemical parameters play major role in such process mechanism. The pH level of a solution plays effectively facilitates the adsorption process. The optimum pH for adsorption depends on the types of adsorbate and adsorbent. The free electron pair of nitrogen on amine groups is responsible for the adsorption of adsorbates on chitosan adsorbents. Decreased pH (acidic media) results in the protonation of amine groups and enhances the cationicity potential of chitosan, thereby, improving the adsorption process [26]. By contrast, the deprotonation of protonated amine groups in basic solutions may affect the adsorption behavior of chitosan adsorbents and reduce adsorption capacity [118]. The adsorption and binding of adsorbate with adsorbent is favored at a specific pH. Thus, changing the pH value might reverse the reaction. Furthermore, the variation in pH level is crucial in the adsorption/desorption of adsorbed ions in chitosan-based 382

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Chitosan-NH2 Me + H+ → Chitosan-NH3+ + Me

Chitosan-NH2 Me + NaCl → Chitosan-NH2 Na + Me Cl

(3)

acidic desorption agents may leads to desorption of adsorbed metal ions from the surface of adsorbents. Moreover, the highest desorption efficiencies are obtained by acidic eluents, and thus, desorption efficiencies finish after 10 cycles (∼100%) in some cases. Other eluents can be used for desorption of heavy metal ions, although they are less effective with lower desorption efficiencies compared with acidic solutions. Some researchers have found that acidic eluents also reduce the metal ion adsorption capabilities of regenerated chitosan-based adsorbents even though acidic desorption agents are extremely highly capable of desorbing metal ions. This condition may be attributed to the dissolution tendency of chitosan-based adsorbents in acidic environments. The use of acidic eluents can affect the adsorption and desorption properties of chitosan-based adsorbents, which affect functional groups in chitosan. In acidic solutions, more hydrogen ions react to break the chains of chitosan and reduce the chitosan molecular weight, which changed the pore network and reduced adsorption capacity. In addition, increasing the protonated amino groups of chitosan in the acidic solution increased its solubilization [125]. Nevertheless, changes in the network and structure of chitosan adsorbents may be extremely poor and cannot be considered through regeneration using other desorption agents. Therefore, appropriate desorption agent should be used to ascertain the effective adsorption performance of regenerated chitosan adsorbents.

Chitosan-NH2 Me + EDTA → Chitosan-NH2 + Me EDTA

(4)

6. Desorption parameters

(1)

Among the alkali solutions, NaOH and NH4OH solutions are the most effective and frequently used to desorb heavy metal ions from adsorbents. Desorption mechanism by basic treatments involves deprotonation and negatively charged adsorption sites in adsorbents. These characteristics weaken the existing electrostatic interactions between chitosan adsorbent functional groups and heavy metal ions and the subsequent separation of adsorbed ions from adsorption sites [100]. The reaction responsible for the desorption process by NaOH, which is the most frequently applied basic eluent, is given as follows [Eq. (2)]: Chitosan-NH3+ Me− + NaOH → Chitosan-NH2 + Me− Na+ + H2O (2) The desorption mechanism using salt eluents and chelating agents may be the deprotonation of functional groups of adsorbents, which releases heavy metal ions. Moreover, the strong electronic supply capability of these desorption agents easily desorbs adsorbed metal ions and forms a steady complex with desorption agents [90,112]. The desorption mechanism in NaCl and EDTA solutions could be explained through the following equations [Eqs. (3) and (4)]:

During electrolysis desorption, ions move toward the electrode of the opposite charge. The electrolysis of chitosan adsorbents saturated with adsorbates converts amide groups of chitosan into amine groups and deposits adsorbates onto the electrode with the opposite charge [Eq. (5)] [124]. Chitosan-NH2 Me + e → Chitosan-NH2 + e Me

Data from the literature show that the chitosan-based adsorbent performance in the desorption by eluents depends on different factors such as the physical and chemical structures of adsorbents and the chemistry of eluents and heavy metal compounds. The experimental variables such as stirring rate, contact time, and temperature considerably affected the desorption and regeneration of chitosan-based adsorbents. Studies have demonstrated that increasing the stirring and agitation speed in desorption enhances system mobility and increases desorption rate. This result is attributed to the appropriate distribution of eluents and contacts among the adsorbed metal ions on the adsorbent. Thus, desorption agent can promote the effective desorption and release of metal ions to the desorption solution. Temperature is an important parameter controlling the desorption process. Temperature can affect the desorption process by altering molecular activity of adsorbate and interfering with interactions between the adsorbent and adsorbate. Increasing temperature can increase the desorption of metal ions from adsorbents. The attractive forces between adsorbent surface and metal ions are weakened, and desorption increases as temperature rises [126]. The increase in the time of desorption process also increases the desorption of metal ions from adsorbents. Extended desorption process time provides more opportunities for the desorption agent to contact and react with adsorbed metal ions, thereby increasing desorption rate. In desorption process, when saturated adsorbent is exposed to the desorption agent, some of the adsorbed adsorbates are desorbed and released to eluent, while the other remain. Initially, the desorption rate is high since the surface of adsorbent is covered by large amount of adsorbate and thus the chance of reaction greatly increases. As desorption process time passes, desorption rate decreases and an equilibrium will be established between the amount of desorbed adsorbate and the amount of adsorbate on the adsorbent. After reaching desorption equilibrium, no more adsorbates are released to the solution and further increase in contact time does not have significant effect on desorption. From practical and economical points of view, further increase in contact time after equilibrium seems to be not suitable. Therefore, to keep the process cost low, analysis of equilibrium data helps to develop mathematical models that could be used to estimate the suitable time for effective desorption. Table 7 presents the comparison of the estimated and experimental desorption equilibrium time for different metal ions. The desorption times are estimated using a numerical solution for mass transfer equations for adsorption/

(5)

5. Strengths and limitations of the desorption agents The desorption of heavy metals and regeneration of chitosan-based adsorbents are compiled and reviewed in the present research. From the literature, desorption of heavy metals from chitosan adsorbents and their reuse in the adsorption process is expected to improve in the future. The recovery of chitosan and its derivatives using desorption agents is preferred because this method can effectively remove heavy metal ions from the adsorbent. This method is simple, easily controlled, and can be used to regenerate large amounts of adsorbents. Moreover, recovery does not require an extremely long period, and thus, one step (desorption) or two steps (desorption and then activation of adsorbent for next adsorption cycle) of this method can be easily conducted, that is, chitosan and its derivatives can be recovered by changing the pH level of a medium. However, pH should be changed after desorption prior to another adsorption process. Thus, a large volume of acid or alkali solution followed by washing with distilled water is required to stabilize pH in the neutral range. Tables 3–6 summarize the adsorption and desorption efficiencies of chitosan-based adsorbents using different desorption agents. However, the comparison between eluents is only a guideline. Owing to the paucity of data in related literature, different chitosan adsorbent properties, eluents, and operating conditions, such as, adsorbate concentration, temperature, pH value, eluent concentration, and adsorbent dose, render the comparison of desorption performances of eluents difficult. Considering these issues, some important points should be highlighted. The percentages of research studies related to the regenerating of chitosan-based adsorbents for the removal of heavy metal ions by using different eluents are shown in Fig. 4. It can be inferred from the results of the reported data that acidic solutions are the most widely applied desorbing agent for desorption of heavy metal ions. Adsorption of metal cations such as Ni(II), Hg(II), Cd(II), Pb(II) and Cu (II) is favored at alkaline condition. Therefore, decreasing pH using 383

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Desorption H Me

2+

+

+

(NH2)

NH3

Me 2+

Fig. 4. Percentage use of different desorption agents for regenerating chitosan-based adsorbents.

+

acidic solutions. Therefore, cross-linked chitosan adsorbents can be used in numerous recycling cycles. Moreover, the ratio of the metal concentration in the eluent to the metal concentration in the adsorbent loading [concentration factor (CR)] and the mass of the loaded adsorbent to the volume of the eluent [solid to liquid ratio (S/L)] are two important factors in the desorption process [134]. For efficient desorption, the fundamental goal is to optimize S/L and CR to attain a high degree of metal extraction and a highly concentrated eluent. A high CR is required for the overall performance of the adsorption process, leading to a high capacity to concentrate metal ions. A high S/L ratio indicates a similarly high concentration of desorbed metal ions in the eluent and a small volume of eluent required for desorption. However, a high concentration of metal released into eluent can reduce desorption efficiency by leaving some residual metal still sorbed after a new equilibrium is attained, given that the adsorption of metal ions is a reversible process [135]. In desorbing with eluents, a new equilibrium is achieved between adsorbate molecules. The equilibrium proceeds toward adsorbate release into the eluent, but some adsorbates may be retained by adsorbents with various degrees [136]. In the adsorption process, the affinities of adsorbate molecules for the main functional groups lead to the sorption selectivity of adsorbents. Similarly, another selectivity pattern may be observed in desorbing with eluents [137]. This selectivity in desorption helps separate adsorbate molecules when necessary. The capacity of adsorbents to concentrate adsorbate molecules is critical in the adsorption process. High adsorbate concentration in adsorbent indicates a high performance of the adsorption process. In this case, desorbing and recovering adsorbate molecules also become further feasible because of the high concentration in the small volume of eluent solute ion [138].

desorption of binary ion-exchange. Such model is based on the simplification of considering spherical adsorbent particles and infinite volume of regenerating solution [127]. Desorption times are calculated considering 99% of desorbed metal respect to the maximum (i.e. equilibrium) molar amount. As can be observed in this table, there is a significant difference between estimated and experimental desorption time. It must be noted that all reviewed works aimed to determine the metal partition amounts (between liquid and solid phase) at equilibrium, hence desorption times were kept intentionally long. In fact, the range of the estimated time varied between 0.848 h and 1.37 h which is very far from the experimental desorption time (3 h to 29..9 h). It noteworthy that, among desorption agents, desorption of heavy metal ions using acidic solutions needs shorter time to reach the equilibrium point. However, regarding experimental conditions, in the works that deals with the acidic desorption agents, longer desorption times was chosen to surely reach the equilibrium point. It should be considered that exposure of the chitosan-based adsorbents to the acidic condition for a long period of time, a dissolution phenomenon caused by the protonation of NH2 groups in chitosan molecular structure, may affect their adsorption properties. Therefore, the experimental desorption experiment needs to be perform in a period as accurately as possible by the estimated processing time in order to preserve absorbent properties and avoid waste of time and save energy and costs. Here, it is important to notice that such information is fundamental for a correct estimation of desorption cost. The lack of this kind of data hinders a proper estimation of regeneration costs related to novel adsorbents and, in the end, possible application at large-scale. Spent chitosan-based adsorbents are regenerated by washing with an appropriate solution, and thus, the strength, concentration, and type of this solution is essential in the desorption process. The use of eluents at high concentrations is significant for the desorption of adsorbed metal ions. The increase in eluent concentration enhances the driving force for the desorption and release of adsorbed ions in the chitosanbased adsorbents [113]. However, eluents with extremely high concentrations are ineffective to regenerate chitosan adsorbents. Desorption wash can affect the structure of the chitosan-based adsorbents. Chitosan adsorbents suffer from decomposition and mass loss during lengthy processes, which can be attributed to the dissolution tendency and inadequate mechanical properties of chitosan-based adsorbents in acid solutions. Acidic eluents may increase the solubilization of chitosan-based adsorbents by affecting the functional groups and increasing the protonated amino groups in chitosan. These drawbacks can severely limit the regeneration of chitosan adsorbents. To overcome such problem, chitosan can be modified using cross-linking agents. Cross-linking can stabilize chitosan in acid solutions, improve mechanical resistance, prevent swelling, and reinforce the chemical stability of chitosan in

7. Conclusion and future perspectives Regeneration is an important factor determining the reusability and effectiveness of adsorbents. This factor also contributes to the economic feasibility and efficiency of the adsorption technology. Elution by an eluent is the most common approach in the literature to regenerate chitosan-based adsorbents. The percentage application of eluents followed the order of salts < alkalis < chelators < acids. The HCl, EDTA, NaOH, and then NaCl solutions are the most commonly used desorption agents. Increase in regeneration cycle decreases adsorption and desorption efficiencies. The increase of eluent concentrations enhances desorption efficiency. However, high concentration of eluents may affect adsorbent structure and decrease desorption efficiency. At high temperature, the breakdown of adsorbate and adsorbent molecules binding also increases the release of adsorbed ions and improves the

Table 7 Comparison between the estimated desorption time and the average experimental desorption time in reviewed literature. Metal ion

Estimated desorption time (h) +

Pb(II) Cu(II) Cd(II) Zn(II) Ni(II) Hg(II)

Experimental desorption time (h) +

+

Acid (H )

Base, salt (Na )

Acid (H )

Ref.

Base, salt (Na+)

Ref.

0.848 1.120 1.112 1.137 1.210 0.945

1.023 1.284 1.276 1.300 1.370 1.117

24.6 29.9 8.04 4.16 24 24

[56,62,81,128] [36,56,57,64,129] [34,62,75,129,132] [75,132] [69,80] [67]

9.5 8.8 9.25 – 3 12.83

[84,92,103,111] [84,92,95,130,131] [92,102,111,133] – [111] [85,110]

384

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desorption performance. Points that may be useful for future research are highlighted hereafter. According to the literature, studies have rarely focused on the regeneration of chitosan-based adsorbents. In addition, few studies have investigated desorption mechanism and parameters. Nonetheless, such information is necessary to carry out process design and economic assessment. Thus, further studies should further elucidate the desorption of adsorbates. The desorption and regeneration of chitosan adsorbents using eluents transfer heavy metal ions from effluent to eluent in concentrated form, which may require further treatment to recover metals. Thus, the preparation and application of an effective desorption method to recover the adsorbate from aqueous solutions with less byproducts are required. This process will facilitate the recovery of metals for reuse as well as the reduction of secondary pollution. The properties and characterization of regenerated adsorbents should be studied using different techniques because it would provide a more complete picture and more useful data for full-scale-application. In this sense, studies should not be limited to determine equilibrium points but should provide desorption kinetics and any other useful information for scaling up the process. Pilot-scale investigations are obviously precious to produce such kind of data. Specifically, regeneration experiments should be optimized similar to actual conditions and large scale uses.

[15]

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[22]

Acknowledgments [23]

We thank the funding of National Key Research on Water Environment Pollution Control in China (2017ZX07202-001), Tsinghua University Initiative Scientific Research Program (20141081174), Collaborative Innovation Center for Regional Environmental Quality and Yangtze Normal University for financial support.

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