- Email: [email protected]
Recent advances in chitosan and its derivatives as adsorbents for removal of pollutants from water and wastewater
Available online at www.sciencedirect.com
ScienceDirect Recent advances in chitosan and its derivatives as adsorbents for removal of pollutants from water and wastewater Changkun Liu1 and Renbi Bai2 This article reviews the recent advances in chitosan and its derivatives as adsorbents for their environmental applications, particularly in pollutant removal from water and wastewater. The adsorbents are discussed in terms of a few distinctive groups, including semi-interpenetrating network (semi-IPN), molecular-imprinting polymer, magnetic nanoparticles, polymer-grafted-chitosan, and cost-effective chitosan composites. The applications of these adsorbents, potentially of great significance, are outlined in several aspects, such as multi-functional adsorption, targeted adsorption for real pollutants and the column adsorption process studies. The primary objective of the paper is to provide a brief summary of the progress in the past five years or so and put forward some possible future perspectives of the related studies. Addresses 1 School of Chemistry and Chemical Engineering, Shenzhen University, 3688 Nanhai Ave., Shenzhen 518060, Guangdong Prov., PR China 2 Department of Civil and Environmental Engineering, Faculty of Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore Corresponding author: Bai, Renbi ([email protected], [email protected])
Current Opinion in Chemical Engineering 2014, 4:62–70 This review comes from a themed issue on Separation engineering Edited by WS Winston Ho and Kang Li For a complete overview see the Issue and the Editorial Available online 14th February 2014 2211-3398/$ – see front matter, # 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2014.01.004
Introduction Chitosan is a biopolymer produced from N-deacetylation of chitin, a major component present in crustacean shells from crabs, shrimps, and insects, and is the second most abundant natural polymer after cellulose [1]. Chitosan and its derivatives have been used in a wide range of applications in the chemical, pharmaceutical, medical, and environmental fields, largely due to their many advantages or unique features such as being readilyavailable, non-toxic, biocompatible, reactive as well as adsorptive. Especially, in the environmental engineering field, chitosan and its derivatives have attained a good reputation as adsorbents for the removal of various contaminants, including heavy metal ions or species, fluorides, dyes, phenol and its derivatives, and many other natural or man-made pollutants. The report on chitosan Current Opinion in Chemical Engineering 2014, 4:62–70
and its derivatives as adsorbents may be dated back to as early as the 1980s [2–5]. For example, Shigeno et al. (1980) published their study in the behaviors of iodine adsorption onto chitosan [2], and Yang and Zall (1984) reported the work in the adsorption of different types of heavy metals (Cu, Zn, Cd, Cr, Pb) with chitosan [3]. These early work opened an interesting research area for chitosan-based materials as adsorbents for the separation or removal of various pollutants in the environmental field. In the literature, there have been a number of review papers summarizing chitosan as adsorbents for the removal of different types of pollutants (i.e. dye, heavy metals, fluoride, among others) from aqueous solutions [1,6,7,8,9,10]. On the basis of those reviews, the areas that are identified for further improvements can be outlined, as given in Table 1. The present paper is therefore in an effort to outline the recent advances that may fill these gaps in chitosan and its derivatives as adsorbents for removal of pollutants from water or wastewater, on the basis of published works mainly in the past five years or so (2009–2013). The paper addresses those outlined concerns mainly from two aspects: the development of novel chitosan-based adsorbents and the application of the adsorbents in areas of potentially practical importance. It is the primary objective of this paper to provide a brief update in the related areas and hopefully some new insights in the current and future practices as well.
Development of novel chitosan-based adsorbents Semi-IPN hydrogel
Chitosan-based hydrogels in the form of water-swollen polymers have often been used as adsorbents for the removal of heavy metal ions, dyes, among others [11]. However, the weak mechanical strength of these hydrogels often posed a hindrance to their applications as adsorbents in the engineering practice, either in the batch or column mode of adsorption operation, resulting from the large shear force or pressure drop to be encountered by these hydrogels. Chemical cross-linking of the hydrogels has often been used to reduce their mechanical fragility to some extent, but this approach usually worked at the sacrifice of a portion of the adsorption capacity of the hydrogels [12]. To provide an alternative solution, a relatively new development in chitosan-based hydrogel adsorbent was to prepare semi-interpenetrating network (semi-IPN) hydrogels, which has attracted some recent research interest. The concept of semi-IPN hydrogels refers to the structure composed of a linear polymer and a cross-linked polymeric network, in which the chains of www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 63
Table 1 Summary of research areas needing improvement from earlier review papers for chitosan and its derivatives as adsorbents for removal of aqueous pollutants. Item 1 2 3 4 5 6 7 8 9
Research gaps
Refs.
Cost factors should be taken into consideration. Adsorption should not be limited to lab-scale batch studies, and column study is highly recommended. Adsorption capacity and selectivity need to be further enhanced. Regeneration studies need to be performed in detail. Adsorption mechanisms need to be explored more clearly by a combination of various modern techniques. It is needed to examine the behaviors of simultaneous removal of co-existed pollutants. Mechanical strength needs to be improved without the sacrifice of adsorption capacity. Adsorption study needs to be extended to real water or wastewater. Specific or selective adsorption of pollutants needs to be investigated and relevant knowledge enhanced.
[1,7,8] [1,7,8,9,10] [6,9,10] [1,8,9] [1,8] [1,8] [6,10] [7,8] [1,6,7,8,9,10]
the linear polymer (usually the functional one) penetrate into the network of the cross-linked polymer without forming chemical bond between the two types of polymers [13]. The advantage of such hydrogels lies in the fact that the properties of each polymer in the semi-IPN hydrogel are maintained while the overall mechanical strength of the hydrogel is greatly enhanced. On the basis of the nature of the structure, the semi-IPN hydrogels are sometimes named as ‘polymer alloy’ as well. Wang et al., for example, reported the preparation of the CTS-g-PAA/GE semi-IPN hydrogels from polyacrylic acid (PAA) grafted chitosan (CTS) as the functional polymer with the gelatin (GE) polymer network, and showed the results on the porous structures of the obtained hydrogels in wet, dewatered and dried states; see Figure 1 [14]. The storage modulus (determined by the rheological method) of the prepared CTS-gPAA/GE semi-IPN hydrogel was 5.94 kPa, more than two folds of that for the CTS-g-PAA functional polymer. The result indicated a significant improvement in the mechanical strength of chitosan-related hydrogel adsorbent in the form of the semi-IPN hydrogels. Experiments conducted for the removal of copper ions from aqueous solutions showed that the adsorption capacity of the semi-IPN hydrogels was also higher (261.08 mg/g) than that of the CTS-gPAA ones (254.42 mg/g) [14]. Zhao et al. also prepared semiIPN hydrogels from chitosan and poly(ethylene glycol) or PEG [12]. They reported that the developed hydrogels achieved rapid adsorption rate and enhanced adsorption capacity in the removal of two types of dyes, that is, Acid Orange 7 (AO7) and Methyl Orange (MO). It however appears that there are still a number of concerns to be addressed further, including, for example, the scalability and cost in the preparation of those semi-IPN hydrogels, the scope and system configuration of their applicability, and the effect of some process operation factors, such as inflow suspended solid content and solution pH, among others. Molecular-imprinted hydrogels
Molecular-imprinting is an emerging material preparation technology that would endow materials with high affinity or excellent selectivity toward the target ions or molecules to be separated or removed. Chitosan-based molecularimprinting polymer (CTS-MIP) has been synthesized with www.sciencedirect.com
cross-linking, in the presence of CTS and another molecule or ion species as the template. The template molecule or ion will subsequently be removed from the CTSMIP to generate a recognition site or ‘cavity’ that is specifically for the same type of molecules or ions as the template one used [15]. Therefore, desired template species can be immobilized onto the CTS-MIP materials to form desired recognition sites for their high selectivity that cannot be easily obtained by other preparation techniques. Hence, the actual significance of the molecularimprinting technology for the removal of pollutants is usually connected with the ‘high selectivity’ generated for the adsorbents, which favors the adsorptive recovery of individual types of pollutants from water or wastewater as a useful resource. A number of recent studies showed the development of CTS-MIP for selective adsorption of pollutants, many of which involved the removal and showed the potential possibility of recovery of heavy metals. For example, Nishad et al. prepared a cobalt(II)imprinted CTS for the selective removal of cobalt(II) from aqueous solutions, for the potential clean-up of the wastewaters containing radioactive cobalt from nuclear power plants [15]. The developed CTS-MIP showed a good selectivity for cobalt over iron, even though the raw CTS had a selectivity of iron over cobalt. Song et al. prepared a silver(I)-imprinted CTS through a phase inversion method, as shown in Figure 2, for the selective removal of silver(I) over copper(II) ions [16]. The selectivity coefficient, k, for the adsorption of silver(I) increased from 0.54 for non-imprinted chitosan to 4.21 for silver(I)imprinted chitosan. Therefore, the molecular-imprinting technique provides an effective means to develop CTSbased adsorbents for specific ions or molecules to be selectively removed or recovered. Other pollutants, such as dyes and fluoro-compounds, have also been investigated for their removal by the CTS-MIP materials [17–19]. Except for those concerns for CTS-based hydrogel adsorbents, another major issue for CTS-MIP will perhaps be in its improvement of the adsorption capacity. Magnetic nanoparticles
As with any other adsorbents, the adsorption capacity and adsorbent reusability (or recovery) are two of the Current Opinion in Chemical Engineering 2014, 4:62–70
64 Separation engineering
Figure 1
(c)
(a)
(b)
(d)
(e) Current Opinion in Chemical Engineering
Photos of formed semi-IPN hydrogels before dewatering (a), after dewatering (b) and after drying (c), as well as SEM images of the semi-IPN hydrogels at 2000 magnification (d) and 5000 magnification (e). Reprinted from Ref. [14] with permission from Elsevier.
important factors in the determination of the performance of CTS-based adsorbents. One approach in this aspect has been the formation of chitosan-based nanoparticles that would enhance the adsorption capacity because large specific surface areas can be obtained when the
adsorbents are made at the nano scale. However, the improved adsorption capacity was usually connected with the sacrifice of the recovery or reusability of the adsorbent because most of the nano-scale adsorbents would suspend in the solution and become very difficult to be separated.
Figure 2
(I)
(II)
phase inversion Chitosan solution Ag(I)
(III)
Elution with HNO3
crosslinking silver oxide
pore space after removal of silver oxide
Ag ion template
chitosan interconnected pores
specific space for Ag(I) Current Opinion in Chemical Engineering
Schematic diagram for the preparation of Ag(I)-imprinted CTS hydrogel. Reprinted with permission from Ref. [16]. Copyright (2012) American Chemical Society. Current Opinion in Chemical Engineering 2014, 4:62–70
www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 65
Figure 3 6
(a)
(b)
(c)
Magnetization (emu/g)
4
2
0
-2 2:5 Mγ-Fe2O3/CSCs -4
-6 -1500
1:10 Mγ-Fe2O3/CSCs
magnet -1000
-500
0
500
1000
1500
Hext (Oe) Current Opinion in Chemical Engineering
Magnetization curves of (a) Mg-Fe2O3/chitosan and photographs showing the different clarity of the adsorption systems before (b) and after (c) magnetic separation of the nano-adsorbent in the solution. Reprinted from Ref. [20] with permission from Elsevier.
Although centrifuge may be used as a proper method for the recovery in the laboratory, the high energy and capital costs of the centrifuge devices may greatly limit their uses for the separation or recovery of these nano-scaled adsorbents in practical engineering applications. Recently, some research interest has been directed to the preparation of chitosan-based magnetic nano-adsorbents or micro-adsorbents (CMA) for applications in the removal of pollutants such as heavy metal ions and dyes [20– 23,24]. The CMA was often prepared in the composite forms by chitosan with one or two other components, such as g-Fe2O3 or clay/magnetite materials [20,21]. The distinctive advantage of the CMA lies in its easiness of recovery of the nano-scaled adsorbent with the use of a magnet field; see Figure 3 [20], while retains the advantages in preparing adsorbents into small sizes such as nanoparticles and microbeads. Cho et al. reported the fabrication of nano-scaled chitosan/clay/magnetite composite adsorbent and showed its good adsorption capacities in the removal of copper(II) and arsenate(V) ions [21]. Fan et al. also showed the preparation of magnetic b-cyclodextrin–chitosan/graphene oxide nanoparticle adsorbent for the removal of methylene blue (MB) dye and the high adsorption capacity [22]. In both cases, a magnet field was used to achieve the facile recovery of the magnetic nanoparticle adsorbents. The development of magnetic adsorbents to certain extent has advanced the prospect of using chitosan-based adsorbents for practical engineering applications, especially in where the recovery of nano-scaled or micro-scaled adsorbents has been difficult or energy-intensive. However, there www.sciencedirect.com
still are some outstanding issues to be addressed, such as the scalability of the magnetic process or system, and the stability of those magnetic particles in relation to the magnetic field under various operation conditions and with interferential components in the feed. Polymer-grafted-chitosan
In recent years, many studies have shown the interest and prospect in polymer-grafted-chitosan for pollutant removal from water and wastewater [25–27,28,29]. The ‘graft’ method endows the chitosan substrate with a wide variety of other functions or properties resulting from the nature of the grafted polymers. Therefore, the polymer-grafted-chitosan can be designed for adsorption of various specific pollutants, with good adsorption capacity and enhanced adsorption selectivity. Chatterjee et al. grafted polyethyleneimine (PEI) onto chitosan and formed adsorbents in the bead form [25]. It was observed that the PEI grafted chitosan gained significantly improved adsorption capacity for Reactive Black 5, being about 3–5 times higher than that of chitosan. Often, the polymer-grafted-chitosan was prepared by polymerization of functional monomers onto the surfaces of chitosan beads or particles. For example, surface-initiated polymerization can be easily carried out through irradiation-induced, redox-induced or surface-initiated atom transfer radical polymerization (SI-ATRP) reactions [26,27,28,29]. In the literature, Li et al. reported the preparation of polyacrylamide grafted chitosan beads via the SI-ATRP technique (see Figure 4 [28]) for selective removal of mercury from lead ions in aqueous solutions Current Opinion in Chemical Engineering 2014, 4:62–70
66 Separation engineering
Figure 4
OH
Br
O
CH3
C
C
Br
O
O
CH3
C
C
CH3
NH2
O
O C Br
NH
C
C
O
CH3
O CH2
CH3 CH3
Et3N, dry THF
NH2
Br
O
CH3
C
C
C
n
CH3 CH3
CuBr:CuBr2(10:1) Me6TREN/DMF
NH
C
C
O
CH3
Br
CH
CH2
CH
NH2
CH2
CH C
Br n NH2
O
Chitosan
Surface-initiated chitosan
Chitosan-g-polyacrylamide Current Opinion in Chemical Engineering
Schematic diagram showing the preparation of polyacrylamide grafted chitosan bead via the SI-ATRP method. Reprinted with permission from Ref. [28]. Copyright (2005) American Chemical Society.
and showed both high adsorption capacity and selectivity [28]. Huang et al. also reported a recent study in the preparation of poly(methacrylic acid) grafted chitosan microspheres for cadmium(II) removal via the SI-ATRP method, showing a high adsorption capacity of 1.3 mmol/ g and fast adsorption kinetics [29]. The research results reported in the literature so far have well demonstrated that polymer-grafted-chitosan in general can be an effective method to provide the prepared adsorbents with greatly enhanced adsorption capacity, in addition to their possibly achieved unique selectivity. In view of the large bank of functional monomers or polymers that may be explored, there can exist a great prospect in the range of new product development and separation technology advancement in this area.
of chitosan or the fly ash, respectively; as shown in Figure 5. The desorption and repeated adsorption processes were repeated for 3 times, with some slight decrease in the adsorption capacity observed. Kumar et al. reported the adsorption of phenolic compounds with chitosan-coated perlite bead composite, and found the adsorption capacity reaching as high as 192, 263 and 322 mg/g for phenol, 2-chlorophenol and 4-chlorophenol, respectively [32]. Hence, the preparation of chitosan composite as adsorbent provides the prospect for their cost effective large scale practical applications.
Figure 5
35
Cost effective chitosan composites
Current Opinion in Chemical Engineering 2014, 4:62–70
chitosan-coated fly ash 28
qe(mg/g)
Chitosan possesses many functions and can be used as an effective material for the adsorption of various pollutants. However, the applicability of chitosan sometimes has been greatly limited due to its solubility in acidic solutions and its weak mechanical strength. Recently, considerable research attention has been paid to the fabrication of chitosan composite containing a ‘cheap’ material, including clay, cotton, perlite and fly ash [30– 35], to provide improved physical and chemical stabilities. Chitosan was coated onto these materials or blended with them to form cost effective chitosan composites. These composites were found to have increased mechanical strength, enhanced insolubility in highly acidic solutions, and importantly be cost effective for potentially large-scale industrial applications. For example, Wen et al. developed a chitosan-coated fly ash composite, and used it for Cr(VI) adsorption [31]. The composite was successfully applied in the acidic solution with pH as low as 2, and the achieved adsorption capacity of the composite for Cr(VI) was higher than that
HO
OH
HCrO4-
NH3+
NH3+
Cr (III) Cr(OH)2+ Cr3(OH)45+
3HCrO4-
Al3+
Al3+
Reduction
Cr (VI)
21
Electrostatic attraction 14
7
0
15
30
45
60
75
Ce(mg/L) fly ash
chitosan
chitosan-coated fly ash
Current Opinion in Chemical Engineering
Equilibrium adsorption isotherm of Cr(VI) on different adsorbents and the mechanism of chromium sorption by chitosan-coated fly ash composite.Reprinted from Ref. [31] with permission from Elsevier. www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 67
Applications of great potential significance
Figure 6
Multi-functions
Targeted adsorptions for real pollutants
As summarized in the previous sections, chitosan-based adsorbents can be prepared with different methods and of various functions to removal a wide range of pollutants, including heavy metal ions, dyes, phosphates, among others, in their adsorption applications. However, most of the adsorption experiments carried out in the past was at the lab-scale with simulated water or wastewater. Recently, a number of research papers have reported the applications of removing real pollutants in drinking water or industrial wastewater treatment with chitosanbased adsorbents [26,40,41,42]. For example, Kyzas et al. showed the removal of dye from industrial dyeing effluent with chitosan-based adsorbent [40]. They commented that as the real effluent contained other compounds besides the dye, other factors, such as swelling ratio and desorption efficiency, should also be considered for the adsorption process. Bleiman and Mishael used a chitosan-clay composite for the adsorption of selenium in the treatment of drinking water extracted from a well [41]. Their results showed that the adsorbent was able to www.sciencedirect.com
350
Fluorescence Intensity (a.u.)
(a)
(b)
(c)
300 250 200 150 100 50 0 540
560
580
600
620
640
660
680
700
Wavelength (nm) Current Opinion in Chemical Engineering
Fluorescence spectra of a rhodamine–chitosan material in aqueous solution in the presence of different amount of Hg(II) (0–0.4 mM). Insert: color changes for isolated rhodamine–chitosan material (a) without, (b) with Hg(II) and (c) with other cations. Reprinted from Ref. [36] with permission from Elsevier.
reduce the selenium concentration to below the WHO limit, and selectively remove selenium in the presence of sulfur from the well water; see Figure 7. These adsorption studies targeted the removal of real pollutants in actual water or wastewater and provided useful support in the prospect of extending the past lab-scale experimental results into future real or large scale actual applications for water and wastewater treatment.
Figure 7 100
Selenium removal (%)
Multi-functional adsorbents are a recently developed adsorption technology in which at least two functions to the removal of pollutants are realized by the same adsorbent. Several types of multi-functional adsorption applications with the chitosan-based adsorbents were reported in the literature. Firstly, a pollutant may be detected and simultaneously removed by the adsorbent (Two different functions for one pollutant). Meng et al. prepared a rhodamine–chitosan material which can detect the presence of and simultaneously remove Hg(II) through adsorption in the adsorption process [36]. The color of the material would change from yellow to dark red when Hg(II) is present and adsorbed; as shown in Figure 6. Secondly, simultaneous adsorption of two different pollutants can be effectively achieved with the same adsorbent (One material functioned for two dissimilar pollutants). Kyzas et al. prepared a poly(ethyleneimine)-grafted-chitosan that could remove effectively both a reactive dye and hexavalent chromium at the same time [37]. Thirdly, spent adsorbents with one pollutant adsorbed on the surface can be used as an effective adsorbent for the removal of another pollutant (Two sequential functions). For example, Dai et al. reported the successful application of copper-adsorbed spent chitosan hydrogels for the removal of phosphate from aqueous solutions [38]. Fourthly, the developed adsorbents can degrade one pollutant and adsorptively remove another pollutant (Two functions for two pollutants). Li et al. synthesized an ion-imprinted chitosan-TiO2 adsorbent which was shown not only degrading methyl orange but also adsorbing nickel(II) ions [39]. These multifunctional applications have the potential to increase treatment process efficiency as well as save cost.
80
60
40 Selenate Selenate+Sulfate Well Water (5g/L) Well Water (10g/L)
20
0 0.0
0.1
0.2
0.3
0.4
chitosan (g/g clay) Current Opinion in Chemical Engineering
Selenium removal by a chitosan-clay composite from selenite solution, selenite and sulfate solution, and Shimron well water.Reprinted from Ref. [41] with permission from Elsevier. Current Opinion in Chemical Engineering 2014, 4:62–70
68 Separation engineering
Figure 8
Column study
The column study for the adsorption process is one of the essential steps, which paves a progress closer to many industrial adsorption applications. There have been numerous publications reported in the literature on the adsorptive removal of pollutants with chitosan-based adsorbents in the batch process mode, while the adsorption in the column mode for possibly continuous operation is relatively new, with a very limited number of publications available [32,41,43,44]. Osifo et al. investigated chitosan bead adsorbent packed into a fixed-bed column for the adsorption of copper ions; as schematically shown in Figure 8 [43]. It was found that an improved adsorption for copper occurred at the 2nd and 3rd cycle, despite there was a mass loss of 14% and 20%, respectively, for the chitosan, and the chitosan beads were found being damaged after the fifth adsorption cycle. Auta and Hameed studied the column adsorption with chitosanactivated carbon adsorbents, for the removal of methylene blue dye (MB) and acid blue 29 dye (AB29) [44]. It was found that the adsorption was influenced by many factors including initial dye concentrations, flow rate and column bed depth. There is no doubt that still a great gap exists in the data base for chitosan-based adsorbents in their adsorption applications for column mode based operations.
Sampling point
Adsorbent packed column
Stirrer Pump To vaccum pump Influent tank
Effluent tank Current Opinion in Chemical Engineering
Schematic diagram of the continuous flow column arrangement used for Cu(II) removal with chitosan bead adsorbent. Reprinted from Ref. [43] with permission from Elsevier.
Table 2 Summary and comparison of the strength and weakness or improvement needed of each approach mentioned in the paper regarding the chitosan-based adsorbents and adsorption applications, together with the existing commercial technologies for the removal of pollutants. No.
Approach
Chitosan-based adsorbents 1 Semi-IPN hydrogel 2 3 4 5 6
Molecular-imprinted hydrogels Magnetic nanoparticles Polymer-grafted-chitosan Cost effective chitosan composites Activated carbon (C)
Pollutant-removal applications 7 Multi-functions 8 9
Targeted adsorptions for real pollutants Column study
10
Chemical precipitation (C)
11
Reverse Osmosis (C)
Strength Enhanced mechanical strength
Weakness/improvement needed
High selectivity toward pollutants
Scalability; cost factors and system configuration to be considered Adsorption capacity to be improved
Ease of separation; feasible for nano-sized adsorbents Enhanced adsorption capacity and selectivity Cost-effective
Scalability of the process; the stability of the magnetic particles to be examined Cost factor to be considered Possible disintegration of the composites
Industrialized, high production; applied in large-scale
Higher cost; difficult to regeneration; non-selective
Enhanced treatment process efficiency, lower cost Prepared for actual water and wastewater treatment applications Paving a progress closer to industrial adsorption applications Ease of operation
Adsorbents to be tailor-made; regeneration methods to be considered Scalability and system configuration of the applicability to be addressed Not mature enough, improvement needed on both adsorbents and adsorption process Higher operation cost, difficult to remove pollutants at low concentrations High operation cost, High material replacement cost
High pollutant removal efficiency, applicable to a wide range of pollutant removal applications
Note: (C) means the commercialized technologies.
Current Opinion in Chemical Engineering 2014, 4:62–70
www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 69
Concluding remarks and future work perspectives The present paper outlined a variety of recent approaches or developments in chitosan and its derivatives as adsorbents for the removal of pollutants from water or wastewater. Table 2 summarized their strengths and weakness or possible areas needing improvement for the various approaches reviewed in this paper. It can be found that over the past five to ten years, there have been considerable research progresses made to the chitosan-based adsorbents for environmental applications, especially in pollutant removal from water or wastewater, including new development in the structure, function, and performance of the adsorbents. However, there are still various areas that need to be addressed in the future, including the scalability and cost in the preparation of the adsorbents; the scope and system configuration for their applicability; the effect of some process operation factors, such as inflow suspended solid content, solution pH, and interferential components; the improvement of the adsorption capacity, selectivity and kinetic rate. In addition, the regeneration and reusability of the various prepared adsorbents have not been well studied. Compared to the batch adsorption process, the column adsorption study has far been behind, which may limit the scope and prospect of the chitosan-based adsorbents in real applications on a large industrial scale in the future.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Wan Ngah WS, Teong LC, Hanafiah MAKM: Adsorption of dyes and heavy metal ions by chitosan composites: a review. Carbohydr Polym 2011, 83:1446-1456.
2.
Shigeno Y, Kondo K, Takemoto K: Functional monomers and polymers. LXX. On the adsorption of iodine onto chitosan. J Appl Polym Sci 1980, 25:731-738.
3.
Yang TC, Zall RR: Absorption of metals by natural polymers generated from seafood processing wastes. Ind Eng Chem Prod Res Dev 1984, 23:168-172.
4.
McKay G, Blair HS, Gardner JR: Adsorption of dyes on chitin. I. Equilibrium studies. J Appl Polym Sci 1982, 27:3043-3057.
5.
Peniche-Covas C, Alvarez LW, Argu¨elles-Monal W: The adsorption of mercuric ions by chitosan. J Appl Polym Sci 1992, 46:1147-1150.
6.
Wu FC, Tseng RL, Juang RS: A review and experimental verification of using chitosan and its derivatives as adsorbents for selected heavy metals. J Environ Manage 2010, 91:798-806. This short review paper provides a summary of adsorption of seven types of heavy metal ions with chitosan and its derivatives.
7.
Crini G, Badot PM: Application of chitosan, a natural aminopolysaccharide, for dye removal from aqueous solutions by adsorption processes using batch studies: a review of recent literature. Prog Polym Sci 2008, 33: 399-447. This paper provides a comprehensive review of dye adsorption with chitosan-based materials. Readers who are interested in understanding the basics and having a more complete picture of dye adsorption with chitosan-based adsorbents may refer to this paper for details.
www.sciencedirect.com
Bhatnagar A, Sillanpa¨a¨ M: Applications of chitin- and chitosanderivatives for the detoxification of water and wastewater — a short review. Adv Colloid Interface Sci 2009, 152:26-38. This paper provides a summary on the types of pollutants that can be adsorptively removed from aqueous solutions with chitosan-based materials. Readers who are unfamiliar with chitosan’s adsorption capability for various types of pollutants may refer to this paper.
8.
9.
Miretzky P, Cirelli AF: Fluoride removal from water by chitosan derivatives and composites: a review. J Fluorine Chem 2011, 132:231-240.
10. Miretzky P, Cirelli AF: Hg(II) removal from water by chitosan and chitosan derivatives: a review. J Hazard Mater 2009, 167:10-23. 11. Li N, Bai RB: Copper adsorption on chitosan-cellulose hydrogel beads: behaviors and mechanisms. Sep Purif Technol 2005, 42:237-247. This paper showed the preparation of chitosan/cellulose composite hydrogel and its applications in heavy metal removal, with a high citation of up to 150 times. The paper illustrated the adsorbent preparation, characterization, adsorption behaviors (adsorption influential factors, isotherms, kinetics, among others) and adsorption mechanism in great details. The work is of great importance for readers who would have a comprehensive knowledge in their research using chitosan materials as the adsorbent in heavy metal removal. 12. Zhao S, Zhou F, Li L, Cao M, Zuo D, Liu H: Removal of anionic dyes from aqueous solutions by adsorption of chitosan-based semi-IPN hydrogel composites. Composites Part B 2012, 43:1570-1578. 13. Myung D, Waters D, Wiseman M, Duhamel PE, Noolandi J, Ta CN, Frank CW: Progress in the development of interpenetrating polymer network hydrogels. Polym Adv Technol 2008, 19: 647-657. This paper provides summaries in the definition, preparation, application and advantages concerning the interpenetrating polymer networks (IPN). Readers who are interested in the IPN area may refer to this paper for useful information. 14. Wang WB, Huang DJ, Kang YR, Wang AQ: One-step in situ fabrication of a granular semi-IPN hydrogel based on chitosan and gelatin for fast and efficient adsorption of Cu2+ ion. Colloids Surf B 2013, 106:51-59. 15. Nishad PA, Bhaskarapillai A, Velmurugan S, Narasimhan SV: Cobalt (II) imprinted chitosan for selective removal of cobalt during nuclear reactor decontamination. Carbohydr Polym 2012, 87:2690-2696. 16. Song X, Li C, Xu R, Wang K: Molecular-ion-imprinted chitosan hydrogels for the selective adsorption of silver(I) in aqueous solution. Ind Eng Chem Res 2012, 51:11261-11265. This work is one of the few studies showing selective adsorption of silver(I) with the molecular-imprinted chitosan-based materials. It demonstrated the potential of practical importance to recover silver from wastewaters for reuse. 17. Kyzas GZ, Lazaridis NK, Bikiaris DN: Optimization of chitosan and b-cyclodextrin molecularly imprinted polymer synthesis for dye adsorption. Carbohydr Polym 2013, 91:198-208. 18. Yu Q, Deng S, Yu G: Selective removal of perfluorooctane sulfonate from aqueous solution using chitosan-based molecularly imprinted polymer adsorbents. Water Res 2008, 42:3089-3097. 19. Liu B, Wang D, Gao X, Zhang L, Xu Y, Li Y: Removal of arsenic from Laminaria japonica Aresch juice using As(III)-imprinted chitosan resin. Eur Food Res Technol 2011, 232:911-917. 20. Zhu HY, Jiang R, Xiao L, Li W: A novel magnetically separable gFe2O3/crosslinked chitosan adsorbent: preparation, characterization and adsorption application for removal of hazardous azo dye. J Hazard Mater 2010, 179:251-257. 21. Cho DW, Jeon BH, Chon CM, Kim Y, Schwartz FW, Lee ES, Song H: A novel chitosan/clay/magnetite composite for adsorption of Cu(II) and As(V). Chem Eng J 2012, 200–202: 654-662. 22. Fan L, Luo C, Li X, Lu F, Qiu H, Sun M: Fabrication of novel magnetic chitosan grafted with graphene oxide to enhance adsorption properties for methyl blue. J Hazard Mater 2012, 215–216:272-279. Current Opinion in Chemical Engineering 2014, 4:62–70
70 Separation engineering
23. Huang G, Zhang H, Shi JX, Langrish TAG: Adsorption of chromium(VI) from aqueous solutions using cross-linked magnetic chitosan beads. Ind Eng Chem Res 2009, 48:26462651.
35. Swayampakula K, Boddu VM, Nadavala SK, Abburi K: Competitive adsorption of Cu(II), Co(II) and Ni(II) from their binary and tertiary aqueous solutions using chitosan-coated perlite beads as biosorbent. J Hazard Mater 2009, 170:680-689.
24. Ren Y, Abbood HA, He F, Peng H, Huang K: Magnetic EDTA modified chitosan/SiO2/Fe3O4 adsorbent: preparation, characterization, and application in heavy metal adsorption. Chem Eng J 2013, 226:300-311. This paper reported the preparation of a novel magnetic chitosan-based adsorbent, considering the effect of various factors during the preparation process. It solved the problem of the dissolution of both chitosan and Fe3O4 under acidic conditions. The preparation method merits an attention.
36. Meng Q, He C, Su W, Zhang X, Duan C: A new rhodamine chitosan fluorescent material for the selective detection of Hg2+ in living cells and efficient adsorption of Hg2+ in natural water. Sens Actuators B 2012, 174:312-317. This paper reported the development of a dual-functional chitosan-based adsorbent for the selective detection and adsorptive removal of Hg2+. The adsorbent integrated two distinctive functions onto one material, expanded the usefulness and prospect of the prepared adsorbent for monitoring as well as remediation in the environmental field.
25. Chatterjee S, Chatterjee T, Woo SH: Influence of the polyethyleneimine grafting on the adsorption capacity of chitosan beads for Reactive Black 5 from aqueous solutions. Chem Eng J 2011, 166:168-175.
37. Kyzas GZ, Lazaridis NK, Kostoglou M: On the simultaneous adsorption of a reactive dye and hexavalent chromium from aqueous solutions onto grafted chitosan. J Colloid Interface Sci 2013, 407:432-441.
26. Sokker HH, El-Sawy NM, Hassan MA, El-Anadouli BE: Adsorption of crude oil from aqueous solution by hydrogel of chitosan based polyacrylamide prepared by radiation induced graft polymerization. J Hazard Mater 2011, 190:359-365.
38. Dai J, Yang H, Yan H, Shangguan Y, Zheng Q, Cheng R: Phosphate adsorption from aqueous solutions by disused adsorbents: chitosan hydrogel beads after the removal of copper(II). Chem Eng J 2011, 166:970-977. This paper showed a new approach in adsorbent applications. It utilized the copper-loaded waste chitosan hydrogel as a good adsorbent for another pollutant — phosphate — removal.
27. Konaganti VK, Kota R, Patil S, Madras G: Adsorption of anionic dyes on chitosan grafted poly(alkyl methacrylate)s. Chem Eng J 2010, 158:393-401. 28. Li N, Bai RB, Liu CK: Enhanced and selective adsorption of mercury ions on chitosan beads grafted with polyacrylamide via surface-initiated atom transfer radical polymerization. Langmuir 2005, 21:11780-11787. This paper investigated the preparation of a novel polymer-graftedchitosan adsorbent via the SI-ATRP method and its capability of selective adsorption of Hg ions over Pb ions in the same solutions. Cited over 80 times, this paper merits the attention using the SI-ATRP method for grafting of functional polymers on chitosan to achieve unique adsorption selectivity. 29. Huang L, Yuan S, Lv L, Tan G, Liang B, Pehkonen SO: Poly(methacrylic acid)-grafted chitosan microspheres via surface initiated ATRP for enhanced removal of Cd(II) ions from aqueous solution. J Colloid Interface Sci 2013, 405: 171-182. 30. Zhang J, Wang A: Adsorption of Pb(II) from aqueous solution by chitosan-g-poly(acrylic acid)/attapulgite/sodium humate composite hydrogels. J Chem Eng Data 2010, 55:2379-2384. 31. Wen Y, Tang Z, Chen Y, Gu Y: Adsorption of Cr(VI) from aqueous solutions using chitosan-coated fly ash composite as biosorbent. Chem Eng J 2011, 175:110-116. 32. Kumar NS, Suguna M, Subbaiah MV, Reddy AS, Kumar NP, Krishnaiah A: Adsorption of phenolic compounds from aqueous solutions onto chitosan-coated Perlite beads as biosorbent. Ind Eng Chem Res 2010, 49:9238-9247. 33. Qu R, Sun C, Wang M, Ji C, Xu Q, Zhang Y, Wang C, Chen H, Yin P: Adsorption of Au(III) from aqueous solution using cotton fiber/ chitosan composite adsorbents. Hydrometallurgy 2009, 100: 65-71. 34. Tirtom VN, Dinc¸er A, Becerik S, Aydemir T, C¸elik A: Comparative adsorption of Ni(II) and Cd(II) ions on epichlorohydrin crosslinked chitosan-clay composite beads in aqueous solution. Chem Eng J 2012, 197:379-386.
Current Opinion in Chemical Engineering 2014, 4:62–70
39. Li Q, Su H, Tan T: Synthesis of ion-imprinted chitosan-TiO2 adsorbent and its multi-functional performances. Biochem Eng J 2008, 38:212-218. This work took a new approach to expend the traditional separation function of the adsorbent. It prepared a chitosan-TiO2 composite combining the advantages of chitosan for adsorption of heavy metal ions and TiO2 for degradation of dye in the solution. This paper introduced a more flexible idea to prepare multi-functional adsorbents. 40. Kyzas GZ, Kostoglou M, Vassiliou AA, Lazaridis NK: Treatment of real effluents from dyeing reactor: experimental and modeling approach by adsorption onto chitosan. Chem Eng J 2011, 168:577-585. 41. Bleiman B, Mishael YG: Selenium removal from drinking water by adsorption to chitosan-clay composites and oxides: batch and columns tests. J Hazard Mater 2010, 183:590-595. 42. Prado AGS, Pescara IC, Evangelista SM, Holanda MS, Andrade RD, Suarez PAZ, Zara LF: Adsorption and preconcentration of divalent metal ions in fossil fuels and biofuels: gasoline, diesel, biodiesel, diesel-like and ethanol by using chitosan microspheres and thermodynamic approach. Talanta 2011, 84:759-765. 43. Osifo PO, Neomagus HWJP, Everson RC, Webster A, vd Gun MA: The adsorption of copper in a packed-bed of chitosan beads: Modeling, multiple adsorption and regeneration. J Hazard Mater 2009, 167:1242-1245. This paper examined the fix-bed column adsorption process with chitosan-based adsorbents. Although the used adsorbents showed certain drawbacks in the column study, the work is of great research interest to explore the potential of the chitosan-based adsorbents for industrial column-based applications. 44. Auta M, Hameed BH: Coalesced chitosan activated carbon composite for batch and fixed-bed adsorption of cationic and anionic dyes. Colloids Surf B 2013, 105:199-206.
www.sciencedirect.com
ScienceDirect Recent advances in chitosan and its derivatives as adsorbents for removal of pollutants from water and wastewater Changkun Liu1 and Renbi Bai2 This article reviews the recent advances in chitosan and its derivatives as adsorbents for their environmental applications, particularly in pollutant removal from water and wastewater. The adsorbents are discussed in terms of a few distinctive groups, including semi-interpenetrating network (semi-IPN), molecular-imprinting polymer, magnetic nanoparticles, polymer-grafted-chitosan, and cost-effective chitosan composites. The applications of these adsorbents, potentially of great significance, are outlined in several aspects, such as multi-functional adsorption, targeted adsorption for real pollutants and the column adsorption process studies. The primary objective of the paper is to provide a brief summary of the progress in the past five years or so and put forward some possible future perspectives of the related studies. Addresses 1 School of Chemistry and Chemical Engineering, Shenzhen University, 3688 Nanhai Ave., Shenzhen 518060, Guangdong Prov., PR China 2 Department of Civil and Environmental Engineering, Faculty of Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore Corresponding author: Bai, Renbi ([email protected], [email protected])
Current Opinion in Chemical Engineering 2014, 4:62–70 This review comes from a themed issue on Separation engineering Edited by WS Winston Ho and Kang Li For a complete overview see the Issue and the Editorial Available online 14th February 2014 2211-3398/$ – see front matter, # 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2014.01.004
Introduction Chitosan is a biopolymer produced from N-deacetylation of chitin, a major component present in crustacean shells from crabs, shrimps, and insects, and is the second most abundant natural polymer after cellulose [1]. Chitosan and its derivatives have been used in a wide range of applications in the chemical, pharmaceutical, medical, and environmental fields, largely due to their many advantages or unique features such as being readilyavailable, non-toxic, biocompatible, reactive as well as adsorptive. Especially, in the environmental engineering field, chitosan and its derivatives have attained a good reputation as adsorbents for the removal of various contaminants, including heavy metal ions or species, fluorides, dyes, phenol and its derivatives, and many other natural or man-made pollutants. The report on chitosan Current Opinion in Chemical Engineering 2014, 4:62–70
and its derivatives as adsorbents may be dated back to as early as the 1980s [2–5]. For example, Shigeno et al. (1980) published their study in the behaviors of iodine adsorption onto chitosan [2], and Yang and Zall (1984) reported the work in the adsorption of different types of heavy metals (Cu, Zn, Cd, Cr, Pb) with chitosan [3]. These early work opened an interesting research area for chitosan-based materials as adsorbents for the separation or removal of various pollutants in the environmental field. In the literature, there have been a number of review papers summarizing chitosan as adsorbents for the removal of different types of pollutants (i.e. dye, heavy metals, fluoride, among others) from aqueous solutions [1,6,7,8,9,10]. On the basis of those reviews, the areas that are identified for further improvements can be outlined, as given in Table 1. The present paper is therefore in an effort to outline the recent advances that may fill these gaps in chitosan and its derivatives as adsorbents for removal of pollutants from water or wastewater, on the basis of published works mainly in the past five years or so (2009–2013). The paper addresses those outlined concerns mainly from two aspects: the development of novel chitosan-based adsorbents and the application of the adsorbents in areas of potentially practical importance. It is the primary objective of this paper to provide a brief update in the related areas and hopefully some new insights in the current and future practices as well.
Development of novel chitosan-based adsorbents Semi-IPN hydrogel
Chitosan-based hydrogels in the form of water-swollen polymers have often been used as adsorbents for the removal of heavy metal ions, dyes, among others [11]. However, the weak mechanical strength of these hydrogels often posed a hindrance to their applications as adsorbents in the engineering practice, either in the batch or column mode of adsorption operation, resulting from the large shear force or pressure drop to be encountered by these hydrogels. Chemical cross-linking of the hydrogels has often been used to reduce their mechanical fragility to some extent, but this approach usually worked at the sacrifice of a portion of the adsorption capacity of the hydrogels [12]. To provide an alternative solution, a relatively new development in chitosan-based hydrogel adsorbent was to prepare semi-interpenetrating network (semi-IPN) hydrogels, which has attracted some recent research interest. The concept of semi-IPN hydrogels refers to the structure composed of a linear polymer and a cross-linked polymeric network, in which the chains of www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 63
Table 1 Summary of research areas needing improvement from earlier review papers for chitosan and its derivatives as adsorbents for removal of aqueous pollutants. Item 1 2 3 4 5 6 7 8 9
Research gaps
Refs.
Cost factors should be taken into consideration. Adsorption should not be limited to lab-scale batch studies, and column study is highly recommended. Adsorption capacity and selectivity need to be further enhanced. Regeneration studies need to be performed in detail. Adsorption mechanisms need to be explored more clearly by a combination of various modern techniques. It is needed to examine the behaviors of simultaneous removal of co-existed pollutants. Mechanical strength needs to be improved without the sacrifice of adsorption capacity. Adsorption study needs to be extended to real water or wastewater. Specific or selective adsorption of pollutants needs to be investigated and relevant knowledge enhanced.
[1,7,8] [1,7,8,9,10] [6,9,10] [1,8,9] [1,8] [1,8] [6,10] [7,8] [1,6,7,8,9,10]
the linear polymer (usually the functional one) penetrate into the network of the cross-linked polymer without forming chemical bond between the two types of polymers [13]. The advantage of such hydrogels lies in the fact that the properties of each polymer in the semi-IPN hydrogel are maintained while the overall mechanical strength of the hydrogel is greatly enhanced. On the basis of the nature of the structure, the semi-IPN hydrogels are sometimes named as ‘polymer alloy’ as well. Wang et al., for example, reported the preparation of the CTS-g-PAA/GE semi-IPN hydrogels from polyacrylic acid (PAA) grafted chitosan (CTS) as the functional polymer with the gelatin (GE) polymer network, and showed the results on the porous structures of the obtained hydrogels in wet, dewatered and dried states; see Figure 1 [14]. The storage modulus (determined by the rheological method) of the prepared CTS-gPAA/GE semi-IPN hydrogel was 5.94 kPa, more than two folds of that for the CTS-g-PAA functional polymer. The result indicated a significant improvement in the mechanical strength of chitosan-related hydrogel adsorbent in the form of the semi-IPN hydrogels. Experiments conducted for the removal of copper ions from aqueous solutions showed that the adsorption capacity of the semi-IPN hydrogels was also higher (261.08 mg/g) than that of the CTS-gPAA ones (254.42 mg/g) [14]. Zhao et al. also prepared semiIPN hydrogels from chitosan and poly(ethylene glycol) or PEG [12]. They reported that the developed hydrogels achieved rapid adsorption rate and enhanced adsorption capacity in the removal of two types of dyes, that is, Acid Orange 7 (AO7) and Methyl Orange (MO). It however appears that there are still a number of concerns to be addressed further, including, for example, the scalability and cost in the preparation of those semi-IPN hydrogels, the scope and system configuration of their applicability, and the effect of some process operation factors, such as inflow suspended solid content and solution pH, among others. Molecular-imprinted hydrogels
Molecular-imprinting is an emerging material preparation technology that would endow materials with high affinity or excellent selectivity toward the target ions or molecules to be separated or removed. Chitosan-based molecularimprinting polymer (CTS-MIP) has been synthesized with www.sciencedirect.com
cross-linking, in the presence of CTS and another molecule or ion species as the template. The template molecule or ion will subsequently be removed from the CTSMIP to generate a recognition site or ‘cavity’ that is specifically for the same type of molecules or ions as the template one used [15]. Therefore, desired template species can be immobilized onto the CTS-MIP materials to form desired recognition sites for their high selectivity that cannot be easily obtained by other preparation techniques. Hence, the actual significance of the molecularimprinting technology for the removal of pollutants is usually connected with the ‘high selectivity’ generated for the adsorbents, which favors the adsorptive recovery of individual types of pollutants from water or wastewater as a useful resource. A number of recent studies showed the development of CTS-MIP for selective adsorption of pollutants, many of which involved the removal and showed the potential possibility of recovery of heavy metals. For example, Nishad et al. prepared a cobalt(II)imprinted CTS for the selective removal of cobalt(II) from aqueous solutions, for the potential clean-up of the wastewaters containing radioactive cobalt from nuclear power plants [15]. The developed CTS-MIP showed a good selectivity for cobalt over iron, even though the raw CTS had a selectivity of iron over cobalt. Song et al. prepared a silver(I)-imprinted CTS through a phase inversion method, as shown in Figure 2, for the selective removal of silver(I) over copper(II) ions [16]. The selectivity coefficient, k, for the adsorption of silver(I) increased from 0.54 for non-imprinted chitosan to 4.21 for silver(I)imprinted chitosan. Therefore, the molecular-imprinting technique provides an effective means to develop CTSbased adsorbents for specific ions or molecules to be selectively removed or recovered. Other pollutants, such as dyes and fluoro-compounds, have also been investigated for their removal by the CTS-MIP materials [17–19]. Except for those concerns for CTS-based hydrogel adsorbents, another major issue for CTS-MIP will perhaps be in its improvement of the adsorption capacity. Magnetic nanoparticles
As with any other adsorbents, the adsorption capacity and adsorbent reusability (or recovery) are two of the Current Opinion in Chemical Engineering 2014, 4:62–70
64 Separation engineering
Figure 1
(c)
(a)
(b)
(d)
(e) Current Opinion in Chemical Engineering
Photos of formed semi-IPN hydrogels before dewatering (a), after dewatering (b) and after drying (c), as well as SEM images of the semi-IPN hydrogels at 2000 magnification (d) and 5000 magnification (e). Reprinted from Ref. [14] with permission from Elsevier.
important factors in the determination of the performance of CTS-based adsorbents. One approach in this aspect has been the formation of chitosan-based nanoparticles that would enhance the adsorption capacity because large specific surface areas can be obtained when the
adsorbents are made at the nano scale. However, the improved adsorption capacity was usually connected with the sacrifice of the recovery or reusability of the adsorbent because most of the nano-scale adsorbents would suspend in the solution and become very difficult to be separated.
Figure 2
(I)
(II)
phase inversion Chitosan solution Ag(I)
(III)
Elution with HNO3
crosslinking silver oxide
pore space after removal of silver oxide
Ag ion template
chitosan interconnected pores
specific space for Ag(I) Current Opinion in Chemical Engineering
Schematic diagram for the preparation of Ag(I)-imprinted CTS hydrogel. Reprinted with permission from Ref. [16]. Copyright (2012) American Chemical Society. Current Opinion in Chemical Engineering 2014, 4:62–70
www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 65
Figure 3 6
(a)
(b)
(c)
Magnetization (emu/g)
4
2
0
-2 2:5 Mγ-Fe2O3/CSCs -4
-6 -1500
1:10 Mγ-Fe2O3/CSCs
magnet -1000
-500
0
500
1000
1500
Hext (Oe) Current Opinion in Chemical Engineering
Magnetization curves of (a) Mg-Fe2O3/chitosan and photographs showing the different clarity of the adsorption systems before (b) and after (c) magnetic separation of the nano-adsorbent in the solution. Reprinted from Ref. [20] with permission from Elsevier.
Although centrifuge may be used as a proper method for the recovery in the laboratory, the high energy and capital costs of the centrifuge devices may greatly limit their uses for the separation or recovery of these nano-scaled adsorbents in practical engineering applications. Recently, some research interest has been directed to the preparation of chitosan-based magnetic nano-adsorbents or micro-adsorbents (CMA) for applications in the removal of pollutants such as heavy metal ions and dyes [20– 23,24]. The CMA was often prepared in the composite forms by chitosan with one or two other components, such as g-Fe2O3 or clay/magnetite materials [20,21]. The distinctive advantage of the CMA lies in its easiness of recovery of the nano-scaled adsorbent with the use of a magnet field; see Figure 3 [20], while retains the advantages in preparing adsorbents into small sizes such as nanoparticles and microbeads. Cho et al. reported the fabrication of nano-scaled chitosan/clay/magnetite composite adsorbent and showed its good adsorption capacities in the removal of copper(II) and arsenate(V) ions [21]. Fan et al. also showed the preparation of magnetic b-cyclodextrin–chitosan/graphene oxide nanoparticle adsorbent for the removal of methylene blue (MB) dye and the high adsorption capacity [22]. In both cases, a magnet field was used to achieve the facile recovery of the magnetic nanoparticle adsorbents. The development of magnetic adsorbents to certain extent has advanced the prospect of using chitosan-based adsorbents for practical engineering applications, especially in where the recovery of nano-scaled or micro-scaled adsorbents has been difficult or energy-intensive. However, there www.sciencedirect.com
still are some outstanding issues to be addressed, such as the scalability of the magnetic process or system, and the stability of those magnetic particles in relation to the magnetic field under various operation conditions and with interferential components in the feed. Polymer-grafted-chitosan
In recent years, many studies have shown the interest and prospect in polymer-grafted-chitosan for pollutant removal from water and wastewater [25–27,28,29]. The ‘graft’ method endows the chitosan substrate with a wide variety of other functions or properties resulting from the nature of the grafted polymers. Therefore, the polymer-grafted-chitosan can be designed for adsorption of various specific pollutants, with good adsorption capacity and enhanced adsorption selectivity. Chatterjee et al. grafted polyethyleneimine (PEI) onto chitosan and formed adsorbents in the bead form [25]. It was observed that the PEI grafted chitosan gained significantly improved adsorption capacity for Reactive Black 5, being about 3–5 times higher than that of chitosan. Often, the polymer-grafted-chitosan was prepared by polymerization of functional monomers onto the surfaces of chitosan beads or particles. For example, surface-initiated polymerization can be easily carried out through irradiation-induced, redox-induced or surface-initiated atom transfer radical polymerization (SI-ATRP) reactions [26,27,28,29]. In the literature, Li et al. reported the preparation of polyacrylamide grafted chitosan beads via the SI-ATRP technique (see Figure 4 [28]) for selective removal of mercury from lead ions in aqueous solutions Current Opinion in Chemical Engineering 2014, 4:62–70
66 Separation engineering
Figure 4
OH
Br
O
CH3
C
C
Br
O
O
CH3
C
C
CH3
NH2
O
O C Br
NH
C
C
O
CH3
O CH2
CH3 CH3
Et3N, dry THF
NH2
Br
O
CH3
C
C
C
n
CH3 CH3
CuBr:CuBr2(10:1) Me6TREN/DMF
NH
C
C
O
CH3
Br
CH
CH2
CH
NH2
CH2
CH C
Br n NH2
O
Chitosan
Surface-initiated chitosan
Chitosan-g-polyacrylamide Current Opinion in Chemical Engineering
Schematic diagram showing the preparation of polyacrylamide grafted chitosan bead via the SI-ATRP method. Reprinted with permission from Ref. [28]. Copyright (2005) American Chemical Society.
and showed both high adsorption capacity and selectivity [28]. Huang et al. also reported a recent study in the preparation of poly(methacrylic acid) grafted chitosan microspheres for cadmium(II) removal via the SI-ATRP method, showing a high adsorption capacity of 1.3 mmol/ g and fast adsorption kinetics [29]. The research results reported in the literature so far have well demonstrated that polymer-grafted-chitosan in general can be an effective method to provide the prepared adsorbents with greatly enhanced adsorption capacity, in addition to their possibly achieved unique selectivity. In view of the large bank of functional monomers or polymers that may be explored, there can exist a great prospect in the range of new product development and separation technology advancement in this area.
of chitosan or the fly ash, respectively; as shown in Figure 5. The desorption and repeated adsorption processes were repeated for 3 times, with some slight decrease in the adsorption capacity observed. Kumar et al. reported the adsorption of phenolic compounds with chitosan-coated perlite bead composite, and found the adsorption capacity reaching as high as 192, 263 and 322 mg/g for phenol, 2-chlorophenol and 4-chlorophenol, respectively [32]. Hence, the preparation of chitosan composite as adsorbent provides the prospect for their cost effective large scale practical applications.
Figure 5
35
Cost effective chitosan composites
Current Opinion in Chemical Engineering 2014, 4:62–70
chitosan-coated fly ash 28
qe(mg/g)
Chitosan possesses many functions and can be used as an effective material for the adsorption of various pollutants. However, the applicability of chitosan sometimes has been greatly limited due to its solubility in acidic solutions and its weak mechanical strength. Recently, considerable research attention has been paid to the fabrication of chitosan composite containing a ‘cheap’ material, including clay, cotton, perlite and fly ash [30– 35], to provide improved physical and chemical stabilities. Chitosan was coated onto these materials or blended with them to form cost effective chitosan composites. These composites were found to have increased mechanical strength, enhanced insolubility in highly acidic solutions, and importantly be cost effective for potentially large-scale industrial applications. For example, Wen et al. developed a chitosan-coated fly ash composite, and used it for Cr(VI) adsorption [31]. The composite was successfully applied in the acidic solution with pH as low as 2, and the achieved adsorption capacity of the composite for Cr(VI) was higher than that
HO
OH
HCrO4-
NH3+
NH3+
Cr (III) Cr(OH)2+ Cr3(OH)45+
3HCrO4-
Al3+
Al3+
Reduction
Cr (VI)
21
Electrostatic attraction 14
7
0
15
30
45
60
75
Ce(mg/L) fly ash
chitosan
chitosan-coated fly ash
Current Opinion in Chemical Engineering
Equilibrium adsorption isotherm of Cr(VI) on different adsorbents and the mechanism of chromium sorption by chitosan-coated fly ash composite.Reprinted from Ref. [31] with permission from Elsevier. www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 67
Applications of great potential significance
Figure 6
Multi-functions
Targeted adsorptions for real pollutants
As summarized in the previous sections, chitosan-based adsorbents can be prepared with different methods and of various functions to removal a wide range of pollutants, including heavy metal ions, dyes, phosphates, among others, in their adsorption applications. However, most of the adsorption experiments carried out in the past was at the lab-scale with simulated water or wastewater. Recently, a number of research papers have reported the applications of removing real pollutants in drinking water or industrial wastewater treatment with chitosanbased adsorbents [26,40,41,42]. For example, Kyzas et al. showed the removal of dye from industrial dyeing effluent with chitosan-based adsorbent [40]. They commented that as the real effluent contained other compounds besides the dye, other factors, such as swelling ratio and desorption efficiency, should also be considered for the adsorption process. Bleiman and Mishael used a chitosan-clay composite for the adsorption of selenium in the treatment of drinking water extracted from a well [41]. Their results showed that the adsorbent was able to www.sciencedirect.com
350
Fluorescence Intensity (a.u.)
(a)
(b)
(c)
300 250 200 150 100 50 0 540
560
580
600
620
640
660
680
700
Wavelength (nm) Current Opinion in Chemical Engineering
Fluorescence spectra of a rhodamine–chitosan material in aqueous solution in the presence of different amount of Hg(II) (0–0.4 mM). Insert: color changes for isolated rhodamine–chitosan material (a) without, (b) with Hg(II) and (c) with other cations. Reprinted from Ref. [36] with permission from Elsevier.
reduce the selenium concentration to below the WHO limit, and selectively remove selenium in the presence of sulfur from the well water; see Figure 7. These adsorption studies targeted the removal of real pollutants in actual water or wastewater and provided useful support in the prospect of extending the past lab-scale experimental results into future real or large scale actual applications for water and wastewater treatment.
Figure 7 100
Selenium removal (%)
Multi-functional adsorbents are a recently developed adsorption technology in which at least two functions to the removal of pollutants are realized by the same adsorbent. Several types of multi-functional adsorption applications with the chitosan-based adsorbents were reported in the literature. Firstly, a pollutant may be detected and simultaneously removed by the adsorbent (Two different functions for one pollutant). Meng et al. prepared a rhodamine–chitosan material which can detect the presence of and simultaneously remove Hg(II) through adsorption in the adsorption process [36]. The color of the material would change from yellow to dark red when Hg(II) is present and adsorbed; as shown in Figure 6. Secondly, simultaneous adsorption of two different pollutants can be effectively achieved with the same adsorbent (One material functioned for two dissimilar pollutants). Kyzas et al. prepared a poly(ethyleneimine)-grafted-chitosan that could remove effectively both a reactive dye and hexavalent chromium at the same time [37]. Thirdly, spent adsorbents with one pollutant adsorbed on the surface can be used as an effective adsorbent for the removal of another pollutant (Two sequential functions). For example, Dai et al. reported the successful application of copper-adsorbed spent chitosan hydrogels for the removal of phosphate from aqueous solutions [38]. Fourthly, the developed adsorbents can degrade one pollutant and adsorptively remove another pollutant (Two functions for two pollutants). Li et al. synthesized an ion-imprinted chitosan-TiO2 adsorbent which was shown not only degrading methyl orange but also adsorbing nickel(II) ions [39]. These multifunctional applications have the potential to increase treatment process efficiency as well as save cost.
80
60
40 Selenate Selenate+Sulfate Well Water (5g/L) Well Water (10g/L)
20
0 0.0
0.1
0.2
0.3
0.4
chitosan (g/g clay) Current Opinion in Chemical Engineering
Selenium removal by a chitosan-clay composite from selenite solution, selenite and sulfate solution, and Shimron well water.Reprinted from Ref. [41] with permission from Elsevier. Current Opinion in Chemical Engineering 2014, 4:62–70
68 Separation engineering
Figure 8
Column study
The column study for the adsorption process is one of the essential steps, which paves a progress closer to many industrial adsorption applications. There have been numerous publications reported in the literature on the adsorptive removal of pollutants with chitosan-based adsorbents in the batch process mode, while the adsorption in the column mode for possibly continuous operation is relatively new, with a very limited number of publications available [32,41,43,44]. Osifo et al. investigated chitosan bead adsorbent packed into a fixed-bed column for the adsorption of copper ions; as schematically shown in Figure 8 [43]. It was found that an improved adsorption for copper occurred at the 2nd and 3rd cycle, despite there was a mass loss of 14% and 20%, respectively, for the chitosan, and the chitosan beads were found being damaged after the fifth adsorption cycle. Auta and Hameed studied the column adsorption with chitosanactivated carbon adsorbents, for the removal of methylene blue dye (MB) and acid blue 29 dye (AB29) [44]. It was found that the adsorption was influenced by many factors including initial dye concentrations, flow rate and column bed depth. There is no doubt that still a great gap exists in the data base for chitosan-based adsorbents in their adsorption applications for column mode based operations.
Sampling point
Adsorbent packed column
Stirrer Pump To vaccum pump Influent tank
Effluent tank Current Opinion in Chemical Engineering
Schematic diagram of the continuous flow column arrangement used for Cu(II) removal with chitosan bead adsorbent. Reprinted from Ref. [43] with permission from Elsevier.
Table 2 Summary and comparison of the strength and weakness or improvement needed of each approach mentioned in the paper regarding the chitosan-based adsorbents and adsorption applications, together with the existing commercial technologies for the removal of pollutants. No.
Approach
Chitosan-based adsorbents 1 Semi-IPN hydrogel 2 3 4 5 6
Molecular-imprinted hydrogels Magnetic nanoparticles Polymer-grafted-chitosan Cost effective chitosan composites Activated carbon (C)
Pollutant-removal applications 7 Multi-functions 8 9
Targeted adsorptions for real pollutants Column study
10
Chemical precipitation (C)
11
Reverse Osmosis (C)
Strength Enhanced mechanical strength
Weakness/improvement needed
High selectivity toward pollutants
Scalability; cost factors and system configuration to be considered Adsorption capacity to be improved
Ease of separation; feasible for nano-sized adsorbents Enhanced adsorption capacity and selectivity Cost-effective
Scalability of the process; the stability of the magnetic particles to be examined Cost factor to be considered Possible disintegration of the composites
Industrialized, high production; applied in large-scale
Higher cost; difficult to regeneration; non-selective
Enhanced treatment process efficiency, lower cost Prepared for actual water and wastewater treatment applications Paving a progress closer to industrial adsorption applications Ease of operation
Adsorbents to be tailor-made; regeneration methods to be considered Scalability and system configuration of the applicability to be addressed Not mature enough, improvement needed on both adsorbents and adsorption process Higher operation cost, difficult to remove pollutants at low concentrations High operation cost, High material replacement cost
High pollutant removal efficiency, applicable to a wide range of pollutant removal applications
Note: (C) means the commercialized technologies.
Current Opinion in Chemical Engineering 2014, 4:62–70
www.sciencedirect.com
Recent development in chitosan-based adsobents Liu and Bai 69
Concluding remarks and future work perspectives The present paper outlined a variety of recent approaches or developments in chitosan and its derivatives as adsorbents for the removal of pollutants from water or wastewater. Table 2 summarized their strengths and weakness or possible areas needing improvement for the various approaches reviewed in this paper. It can be found that over the past five to ten years, there have been considerable research progresses made to the chitosan-based adsorbents for environmental applications, especially in pollutant removal from water or wastewater, including new development in the structure, function, and performance of the adsorbents. However, there are still various areas that need to be addressed in the future, including the scalability and cost in the preparation of the adsorbents; the scope and system configuration for their applicability; the effect of some process operation factors, such as inflow suspended solid content, solution pH, and interferential components; the improvement of the adsorption capacity, selectivity and kinetic rate. In addition, the regeneration and reusability of the various prepared adsorbents have not been well studied. Compared to the batch adsorption process, the column adsorption study has far been behind, which may limit the scope and prospect of the chitosan-based adsorbents in real applications on a large industrial scale in the future.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Wan Ngah WS, Teong LC, Hanafiah MAKM: Adsorption of dyes and heavy metal ions by chitosan composites: a review. Carbohydr Polym 2011, 83:1446-1456.
2.
Shigeno Y, Kondo K, Takemoto K: Functional monomers and polymers. LXX. On the adsorption of iodine onto chitosan. J Appl Polym Sci 1980, 25:731-738.
3.
Yang TC, Zall RR: Absorption of metals by natural polymers generated from seafood processing wastes. Ind Eng Chem Prod Res Dev 1984, 23:168-172.
4.
McKay G, Blair HS, Gardner JR: Adsorption of dyes on chitin. I. Equilibrium studies. J Appl Polym Sci 1982, 27:3043-3057.
5.
Peniche-Covas C, Alvarez LW, Argu¨elles-Monal W: The adsorption of mercuric ions by chitosan. J Appl Polym Sci 1992, 46:1147-1150.
6.
Wu FC, Tseng RL, Juang RS: A review and experimental verification of using chitosan and its derivatives as adsorbents for selected heavy metals. J Environ Manage 2010, 91:798-806. This short review paper provides a summary of adsorption of seven types of heavy metal ions with chitosan and its derivatives.
7.
Crini G, Badot PM: Application of chitosan, a natural aminopolysaccharide, for dye removal from aqueous solutions by adsorption processes using batch studies: a review of recent literature. Prog Polym Sci 2008, 33: 399-447. This paper provides a comprehensive review of dye adsorption with chitosan-based materials. Readers who are interested in understanding the basics and having a more complete picture of dye adsorption with chitosan-based adsorbents may refer to this paper for details.
www.sciencedirect.com
Bhatnagar A, Sillanpa¨a¨ M: Applications of chitin- and chitosanderivatives for the detoxification of water and wastewater — a short review. Adv Colloid Interface Sci 2009, 152:26-38. This paper provides a summary on the types of pollutants that can be adsorptively removed from aqueous solutions with chitosan-based materials. Readers who are unfamiliar with chitosan’s adsorption capability for various types of pollutants may refer to this paper.
8.
9.
Miretzky P, Cirelli AF: Fluoride removal from water by chitosan derivatives and composites: a review. J Fluorine Chem 2011, 132:231-240.
10. Miretzky P, Cirelli AF: Hg(II) removal from water by chitosan and chitosan derivatives: a review. J Hazard Mater 2009, 167:10-23. 11. Li N, Bai RB: Copper adsorption on chitosan-cellulose hydrogel beads: behaviors and mechanisms. Sep Purif Technol 2005, 42:237-247. This paper showed the preparation of chitosan/cellulose composite hydrogel and its applications in heavy metal removal, with a high citation of up to 150 times. The paper illustrated the adsorbent preparation, characterization, adsorption behaviors (adsorption influential factors, isotherms, kinetics, among others) and adsorption mechanism in great details. The work is of great importance for readers who would have a comprehensive knowledge in their research using chitosan materials as the adsorbent in heavy metal removal. 12. Zhao S, Zhou F, Li L, Cao M, Zuo D, Liu H: Removal of anionic dyes from aqueous solutions by adsorption of chitosan-based semi-IPN hydrogel composites. Composites Part B 2012, 43:1570-1578. 13. Myung D, Waters D, Wiseman M, Duhamel PE, Noolandi J, Ta CN, Frank CW: Progress in the development of interpenetrating polymer network hydrogels. Polym Adv Technol 2008, 19: 647-657. This paper provides summaries in the definition, preparation, application and advantages concerning the interpenetrating polymer networks (IPN). Readers who are interested in the IPN area may refer to this paper for useful information. 14. Wang WB, Huang DJ, Kang YR, Wang AQ: One-step in situ fabrication of a granular semi-IPN hydrogel based on chitosan and gelatin for fast and efficient adsorption of Cu2+ ion. Colloids Surf B 2013, 106:51-59. 15. Nishad PA, Bhaskarapillai A, Velmurugan S, Narasimhan SV: Cobalt (II) imprinted chitosan for selective removal of cobalt during nuclear reactor decontamination. Carbohydr Polym 2012, 87:2690-2696. 16. Song X, Li C, Xu R, Wang K: Molecular-ion-imprinted chitosan hydrogels for the selective adsorption of silver(I) in aqueous solution. Ind Eng Chem Res 2012, 51:11261-11265. This work is one of the few studies showing selective adsorption of silver(I) with the molecular-imprinted chitosan-based materials. It demonstrated the potential of practical importance to recover silver from wastewaters for reuse. 17. Kyzas GZ, Lazaridis NK, Bikiaris DN: Optimization of chitosan and b-cyclodextrin molecularly imprinted polymer synthesis for dye adsorption. Carbohydr Polym 2013, 91:198-208. 18. Yu Q, Deng S, Yu G: Selective removal of perfluorooctane sulfonate from aqueous solution using chitosan-based molecularly imprinted polymer adsorbents. Water Res 2008, 42:3089-3097. 19. Liu B, Wang D, Gao X, Zhang L, Xu Y, Li Y: Removal of arsenic from Laminaria japonica Aresch juice using As(III)-imprinted chitosan resin. Eur Food Res Technol 2011, 232:911-917. 20. Zhu HY, Jiang R, Xiao L, Li W: A novel magnetically separable gFe2O3/crosslinked chitosan adsorbent: preparation, characterization and adsorption application for removal of hazardous azo dye. J Hazard Mater 2010, 179:251-257. 21. Cho DW, Jeon BH, Chon CM, Kim Y, Schwartz FW, Lee ES, Song H: A novel chitosan/clay/magnetite composite for adsorption of Cu(II) and As(V). Chem Eng J 2012, 200–202: 654-662. 22. Fan L, Luo C, Li X, Lu F, Qiu H, Sun M: Fabrication of novel magnetic chitosan grafted with graphene oxide to enhance adsorption properties for methyl blue. J Hazard Mater 2012, 215–216:272-279. Current Opinion in Chemical Engineering 2014, 4:62–70
70 Separation engineering
23. Huang G, Zhang H, Shi JX, Langrish TAG: Adsorption of chromium(VI) from aqueous solutions using cross-linked magnetic chitosan beads. Ind Eng Chem Res 2009, 48:26462651.
35. Swayampakula K, Boddu VM, Nadavala SK, Abburi K: Competitive adsorption of Cu(II), Co(II) and Ni(II) from their binary and tertiary aqueous solutions using chitosan-coated perlite beads as biosorbent. J Hazard Mater 2009, 170:680-689.
24. Ren Y, Abbood HA, He F, Peng H, Huang K: Magnetic EDTA modified chitosan/SiO2/Fe3O4 adsorbent: preparation, characterization, and application in heavy metal adsorption. Chem Eng J 2013, 226:300-311. This paper reported the preparation of a novel magnetic chitosan-based adsorbent, considering the effect of various factors during the preparation process. It solved the problem of the dissolution of both chitosan and Fe3O4 under acidic conditions. The preparation method merits an attention.
36. Meng Q, He C, Su W, Zhang X, Duan C: A new rhodamine chitosan fluorescent material for the selective detection of Hg2+ in living cells and efficient adsorption of Hg2+ in natural water. Sens Actuators B 2012, 174:312-317. This paper reported the development of a dual-functional chitosan-based adsorbent for the selective detection and adsorptive removal of Hg2+. The adsorbent integrated two distinctive functions onto one material, expanded the usefulness and prospect of the prepared adsorbent for monitoring as well as remediation in the environmental field.
25. Chatterjee S, Chatterjee T, Woo SH: Influence of the polyethyleneimine grafting on the adsorption capacity of chitosan beads for Reactive Black 5 from aqueous solutions. Chem Eng J 2011, 166:168-175.
37. Kyzas GZ, Lazaridis NK, Kostoglou M: On the simultaneous adsorption of a reactive dye and hexavalent chromium from aqueous solutions onto grafted chitosan. J Colloid Interface Sci 2013, 407:432-441.
26. Sokker HH, El-Sawy NM, Hassan MA, El-Anadouli BE: Adsorption of crude oil from aqueous solution by hydrogel of chitosan based polyacrylamide prepared by radiation induced graft polymerization. J Hazard Mater 2011, 190:359-365.
38. Dai J, Yang H, Yan H, Shangguan Y, Zheng Q, Cheng R: Phosphate adsorption from aqueous solutions by disused adsorbents: chitosan hydrogel beads after the removal of copper(II). Chem Eng J 2011, 166:970-977. This paper showed a new approach in adsorbent applications. It utilized the copper-loaded waste chitosan hydrogel as a good adsorbent for another pollutant — phosphate — removal.
27. Konaganti VK, Kota R, Patil S, Madras G: Adsorption of anionic dyes on chitosan grafted poly(alkyl methacrylate)s. Chem Eng J 2010, 158:393-401. 28. Li N, Bai RB, Liu CK: Enhanced and selective adsorption of mercury ions on chitosan beads grafted with polyacrylamide via surface-initiated atom transfer radical polymerization. Langmuir 2005, 21:11780-11787. This paper investigated the preparation of a novel polymer-graftedchitosan adsorbent via the SI-ATRP method and its capability of selective adsorption of Hg ions over Pb ions in the same solutions. Cited over 80 times, this paper merits the attention using the SI-ATRP method for grafting of functional polymers on chitosan to achieve unique adsorption selectivity. 29. Huang L, Yuan S, Lv L, Tan G, Liang B, Pehkonen SO: Poly(methacrylic acid)-grafted chitosan microspheres via surface initiated ATRP for enhanced removal of Cd(II) ions from aqueous solution. J Colloid Interface Sci 2013, 405: 171-182. 30. Zhang J, Wang A: Adsorption of Pb(II) from aqueous solution by chitosan-g-poly(acrylic acid)/attapulgite/sodium humate composite hydrogels. J Chem Eng Data 2010, 55:2379-2384. 31. Wen Y, Tang Z, Chen Y, Gu Y: Adsorption of Cr(VI) from aqueous solutions using chitosan-coated fly ash composite as biosorbent. Chem Eng J 2011, 175:110-116. 32. Kumar NS, Suguna M, Subbaiah MV, Reddy AS, Kumar NP, Krishnaiah A: Adsorption of phenolic compounds from aqueous solutions onto chitosan-coated Perlite beads as biosorbent. Ind Eng Chem Res 2010, 49:9238-9247. 33. Qu R, Sun C, Wang M, Ji C, Xu Q, Zhang Y, Wang C, Chen H, Yin P: Adsorption of Au(III) from aqueous solution using cotton fiber/ chitosan composite adsorbents. Hydrometallurgy 2009, 100: 65-71. 34. Tirtom VN, Dinc¸er A, Becerik S, Aydemir T, C¸elik A: Comparative adsorption of Ni(II) and Cd(II) ions on epichlorohydrin crosslinked chitosan-clay composite beads in aqueous solution. Chem Eng J 2012, 197:379-386.
Current Opinion in Chemical Engineering 2014, 4:62–70
39. Li Q, Su H, Tan T: Synthesis of ion-imprinted chitosan-TiO2 adsorbent and its multi-functional performances. Biochem Eng J 2008, 38:212-218. This work took a new approach to expend the traditional separation function of the adsorbent. It prepared a chitosan-TiO2 composite combining the advantages of chitosan for adsorption of heavy metal ions and TiO2 for degradation of dye in the solution. This paper introduced a more flexible idea to prepare multi-functional adsorbents. 40. Kyzas GZ, Kostoglou M, Vassiliou AA, Lazaridis NK: Treatment of real effluents from dyeing reactor: experimental and modeling approach by adsorption onto chitosan. Chem Eng J 2011, 168:577-585. 41. Bleiman B, Mishael YG: Selenium removal from drinking water by adsorption to chitosan-clay composites and oxides: batch and columns tests. J Hazard Mater 2010, 183:590-595. 42. Prado AGS, Pescara IC, Evangelista SM, Holanda MS, Andrade RD, Suarez PAZ, Zara LF: Adsorption and preconcentration of divalent metal ions in fossil fuels and biofuels: gasoline, diesel, biodiesel, diesel-like and ethanol by using chitosan microspheres and thermodynamic approach. Talanta 2011, 84:759-765. 43. Osifo PO, Neomagus HWJP, Everson RC, Webster A, vd Gun MA: The adsorption of copper in a packed-bed of chitosan beads: Modeling, multiple adsorption and regeneration. J Hazard Mater 2009, 167:1242-1245. This paper examined the fix-bed column adsorption process with chitosan-based adsorbents. Although the used adsorbents showed certain drawbacks in the column study, the work is of great research interest to explore the potential of the chitosan-based adsorbents for industrial column-based applications. 44. Auta M, Hameed BH: Coalesced chitosan activated carbon composite for batch and fixed-bed adsorption of cationic and anionic dyes. Colloids Surf B 2013, 105:199-206.
www.sciencedirect.com