Adsorption of Heavy Metal Ions, Dyes and Proteins by Chitosan Composites and Derivatives – A Review

LIU Bingjie1), 2), WANG Dongfeng1), *, YU Guangli2), and MENG Xianghong1)

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Adsorption of Heavy Metal Ions, Dyes and Proteins by Chitosan Composites and Derivatives – A Review

LIU Bingjie, WANG Dongfeng, YU Guangli, and MENG Xianghong

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Chitosan composites and derivatives have gained wide attentions as effective biosorbents due to their low costs and high contents of amino and hydroxyl functional groups. They have showed significant potentials of removing metal ions, dyes and proteins from various media. Chemical modifications that lead to the formation of the chitosan derivatives and chitosan composites have been extensively studied and widely reported in literatures. The aims of this review were to summarize the important information of the bioactivities of chitosan, highlight the various preparation methods of chitosan-based active biosorbents, and outline its potential applications in the adsorption of heavy metal ions, dyes and proteins from wastewater and aqueous solutions.

chitosan composite; chitosan derivative; adsorption; heavy metal; dye; protein

1 Introduction

The biopolymers such as chitin and chitosan are among the choices of materials of removing low concentrations of heavy metal ions, dyes and proteins from wastewater and aqueous solutions (Crini, 2006; Varma., 2004). Chitin, a high molecular weight linear polymer of 2- acetamido-2-deoxy-D-glucopyranose units linked together by 1, 4-glycosidic bonds (Wan Ngah and Isa, 1998; Kumar, 2000), is the second abundant natural fiber, being similar to cellulose, the most abundant natural fiber, in many respects. The most abundant source of chitin is the shell of crab and shrimp (Bhatnagar and Sillanp??, 2009). Chitosan is the partially N-deacetylated derivative of chitin, which has one primary amino and two free hydroxyl groups for each Cand Cbuilding unit (Juang and Shao, 2002; Benavente., 2011).

Chitosan is well known as an excellent biosorbent of heavy metal ions, dyes and proteins. It contains a large number of –NHand –OH groups and is able to remove these stuffs in near–neutral solutions. Such an ability of chitosan can be attributed to 1) its high hydrophilicity generated by a large number of hydroxyl groups of glucose unit; 2) the presence of a large number of functional groups (.., acetamido, primary amino and/or hydroxyl groups); 3) the high chemical reactivity of these groups and 4) the flexible structure of polymer chain (Crini, 2005). The reactive amino groups selectively bind to virtually all group III transition metal ions but not groups I and II ions (alkali and alkaline earth metal ions) (Muzzarelli, 1973). In addition, chitosan shows a cationic behavior in acidic media; the protonation of amine groups leads to adsorption of metal anions by ion exchange (Guibal, 2004; Kunkoro., 2005).

Both chitin and chitosan are becoming important natural polymers due to their unique combination of properties like nontoxicity, biodegradability, biocompatibility, bioactivity, and attractive physical and mechanical performances. These properties promise chitosan a variety of current and potential applications in metal ions removal from aqueous solutions and liquid drinks (Kifune, 1992; Liu., 2011a), dyes (Wang., 2007; Wan Ngah., 2011), protein adsorption (Hoven., 2007), drug delivery (Sinha., 2004), food processing (Suntornsuk., 2002; Dutta., 2009), antibacterial properties (Chen., 2010; Lin., 2009) and among others. Among diverse applications of chitosan, the most attractive is to serve as the metal chelating agents, removing metallic impurities away from wastewaters. The aim of this review was to outline the current and future applications of chitin and chitosan as well as their derivatives.

2 Applications in Metal Ions Adsorption

At least 20 metals are toxic, and half of them (cadmium, Cd; arsenic, As; mercury, Hg; chromium, Cr; copper, Cu; lead, Pb; nickel, Ni; selenium, Se; silver, Ag; and zinc, Zn) can emit into environments reaching harmful amounts to human health. Among these harmful metals, the most toxic are As(III), Cd(II) and Pb(II).

2.1 Chitosan Composites

2.1.1 Chitosan poly (vinyl alcohol) composites

Kumar. (2009) prepared chitosan/PVA blend beads by suspendingchitosan/PVA aqueous solution in a mixture of toluene and chlorobenzene to remove Cd(II) from wastewater. They found that pH had a significant effect on the adsorption capacity of the beads; the adsorption efficiency increased from 50mgLat pH 2 to about 44% and further to 73.75% at pH 6.0 as the maximum. In acidic medium, due to high Hconcentration, active sites of the adsorbent were protonated, leading to the prevention of metal ions adsorption. The maximum adsorption of Cd(II) ions was achieved at pH 6 and 40℃ with an equilibrium time of 8h. The beads could be regenerated with 0.01molLEDTA. Wan Ngah(2004) studied the performance of chitosan/PVA blend beads in adsorbing Cu(II) from aqueous solutions. The adsorption isotherm data could be well interpreted by Langmuir isotherm model. The kinetic experimental data properly correlated with pseudo-second-order kinetic model. The Cu(II) ions can be removed from chitosan/PVA beads rapidly by treating with an EDTA solution. Li. (2011) prepared a novel macroporous bead adsorbents based on poly (vinyl alcohol)/chitosan (PVA/CS beads) to adsorb heavy metal ions from aqueous solutions. The PVA/CS beads they prepared were perfectly spherical in shape and exhibited good mechanical strength and chemical stability. These PVA/CS beads are of macroreticular structures with their easy separation and excellent adsorption of heavy metal ions.

2.1.2 Chitosan montmorillonite composites

Polymer/montmorillonite nanocomposites have better properties in mechanical performance, thermal stability, gas barrier and flame retardation than conventional composites (Zhao., 2010). Assaad(2007) prepared an optimized chitosan-montmorillonite system for the removal of Co(II), Ni(II) and Cu(II). The effects of pH, montmorillonite amount, cation concentration were investigated. It was found that both chitosan and montmorillonite contributed to the metal ion removal when used separately, and that coagulation-flocculation strongly depended on the pH and composition of liquid medium. When used simultaneously, chitosan and montmorillonite displayed a synergy phenomenon, which may be underlined by the interactions among main parameters.

2.1.3 Chitosan clay composites

In order to reduce the cost, many studies have focused on seeking cheap, locally available and effective adsorbents, such as waste biopolymers, clays and clay minerals (Wang., 2009). Among these materials, clay and chitosan are relatively cheap and exhibit higher adsorption capacities (Crini and Badot, 2008; Tang., 2009). Natural clays are low-cost and readily available materials functioning as excellent cation exchangers, which have often been used to adsorb metallic contaminants. Tirtom. (2012) prepared epichlorohydrin crosslinked chitosan-clay composite beads as a biosorbent of Ni(II) and Cd(II) in aqueous solution. The prepared beads were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and thermogravimetry analysis (TGA) to assess the structure and adsorption mechanism. Adsorption study showed that epichlorohydrin crosslinked chitosan-clay composite beads can effectively remove Ni(II) and Cd(II) with a maximal adsorption capacity at pH 6.0 and pH 4.5, respectively. The adsorption process was exothermic for Ni(II) and endothermic for Cd(II). Desorption studies illustrated that adsorbed metal ions could be recovered with EDTA. Polymer-clay composites were designed to adsorb Se(II) from water (Bleiman and Mishael, 2010). The highest adsorption efficiency was obtained for chitosan-montmorillonite composites. These composites were characterized by X-ray diffraction (XRD), zeta potential and FTIR. Adsorption isotherm of Se(II) onto the composite, aluminum oxide and Fe-oxide was in good agreement with Langmuir isotherm model. In addition, adsorption by the composite was not pH-dependent; however its adsorption by the oxides decreased at high pH. Se(II) can be removed from underground water by the composite, bring Se(II) level to

2.1.4 Chitosan perlite composites

Perlite is a glassy volcanic rock varying in color from gray to black. Different types of perlite and different origin of perlite have different properties due to their different compositions (Mathialagan and Viraraghavan, 2002). When perlite is heated to high temperatures (850–1100℃), it will expand 4–35 times of its initial volume and is called ‘expanded perlite’ (Shameem., 2008). A new composite chitosan-coated biosorbent was prepared and used for the removal and recovery of heavy metals from aqueous solution (Swayampakula., 2009). Equilibrium adsorption characteristics of Cu(II), Ni(II) and Co(II) from their binary and tertiary solution on newly developed biosorbent chitosan coated perlite beads were evaluated through batch and column studies. These beads were characterized by using FTIR, EDXRF and surface area analysis techniques. The effect of various adsorption parameters like pH value, agitation time, concentration of adsorbate and amount of adsorbent on extent of adsorption was investigated. The adsorption follows Lagergren- first-order kinetic model. The equilibrium adsorption data were fitted to Freundlich and Langmuir isotherm models and the adsorption parameters were evaluated. Both models can represent the experimental data. The sorbent loaded with metal was regenerated with 0.1nmolLNaOH. Furthermore, the column dynamic studies proved re-us-age of the biosorbent.

2.1.5 Chitosan/polyvinyl chloride composites

Poly vinyl chloride (PVC) is one of the most common types of plastic used in the daily life. Consequently, PVC waste has its environmental impact and a suitable method should be established to justify such a waste for environmental basis and for economical reasons. This may be achieved if PVC could be converted into value-added materials (Sobahi., 2011). PVC has high surface area, good physical and chemical stabilities especially in concentrated acidic and basic media and organic solvents for a period of time. Furthermore, the surface of PVC can be modified by sorbent to obtain reversible and efficient enrichment of metal ions (Farzaneh., 2009). Meanwhile, Popuri(2009) developed a new biosorbent by coating chitosan onto PVC beads. Similar to other composite beads, equilibrium and column flow adsorption characteristics of Cu(II) and Ni(II) on the biosorbent were studied. The maximum adsorption capacities of chitosan/PVC composites were 87.9 and 120.5mggfor Cu(II) and Ni(II), respectively, which demonstrated that chitosan coated PVC beads could be used for the removal of Cu(II) and Ni(II) from aqueous solution.

2.1.6 Chitosan/bentonite composite

Uranium (VI) is one of the important resources of energy. The recovery of uranium (VI) from natural sea water and industry wastewater is a challenging problem for chemists. Moreover, based on the fundamental principles of radioactive waste management, uranium (VI) waste minimization should be done in an effective manner (Fayek., 2011). Anirudhan and Rijith (2012) prepared a novel adsorbent, poly (methacrylic acid)-grafted chitosan/bentonite composite, through graft copolymerization reaction of methacrylic acid and chitosan in the presence of bentonite and N, N-methylenebisacrylamide as crosslinking agent. The equilibrium data were correlated with Langmuir isotherm model with an endothermic behavior. The equilibrium uranium (VI) sorption capacity was estimated to be 117.2mggat 30℃.

Thorium (III) is also a naturally occurring radioactive element widely distributed in crust with nuclear significance. The toxic nature of this radionuclide, even at trace amount, has been a public health problem for many years (Rastegarzadeh., 2010). Anirudhan(2010) prepared a novel composite matrix, poly (methacrylic acid)- grafted composite/bentonite, through graft copolymerization reaction of methacrylic acid and chitosan in the presence of bentonite and N, N-methylenebisacryla-mide as crosslinking agent. The equilibrium data were correlated with Langmuir isotherm model. The adsorption process followed pseudo-second-order kinetic model, which indicated that the adsorption process involved chemical reaction besides physical adsorption.

2.2 Chitosan Derivatives

Modifications of chitosan can make it more selective and effective for adsorbing several metal ions, especially heavy metal ions. Grafting new functional groups such as thiourea (Zhou., 2009; Wang., 2010), thiocarbamoyl (Bratskaya., 2011), L-lysine (Fujiwara., 2007), α-ketoglutaric acid (Zhou., 2009), glycine (Ramesh., 2008), ethylenediamine (Zhou., 2010a; Hu., 2011) onto cross-linked chitosan materials, including chitosan resin and films, can also improve its selectivity and adsorption ability.

Wang(2011) developed ethylenediamine-modi- fied magnetic chitosan (EMMC) complex as a novel magnetic adsorbent for the removal of uranyl ions. The magnetic particles were pure FeOwith a spinel structure, and the binding of chitosan did not result in a phase change. FeOparticles were successfully bounded by chitosan and more amino groups appeared in EMMC. EMMC was very efficient in adsorbing uranyl ions at pH 2–7. Equilibrium was established within 30min, and the kinetic experimental data properly correlated with the pseudo-second-order kinetic model. These parameters indicated that the chemical adsorption was the rate-limiting step. The adsorption data could be best interpreted by the Sips model with the maximum adsorption capacity of 82.83mgg. EMMC can be regenerated with high efficiency, suggesting that this adsorbent can be employed to recover uranyl ions. Zhou(2010b) prepared ethylenediamine-modified magnetic chitosan nanoparticles (EMCN) by adding the basic precipitant of NaOH solution into a W/O microemulsion system. The transmission electron microscope (TEM) showed that the diameter of EMCN ranged from 15 to 40nm. The adsorption experiments indicated that the maximum adsorption capacity occurred at around pH 2.0 for both Pt(IV) and Pd(II). Due to the small diameter and high surface reactivity, the adsorption equilibrium of Pt(IV) and Pd(II) onto EMCN reached very quickly. The maximum adsorption capacity of EMCN for Pt(IV) and Pd(II) was determined to be 171 and 138mgg, respectively. Sorption isotherm models were determined both in single component with pure metal solutions and bicomponent system with different Pd/Pt mass ratios. The results showed that the sorbents had a greater affinity for Pt(IV) than Pd(II). The total sorption capacity was comparable to that of each metal individually, which indicates that the metals compete for the same sorption sites. It was found that 0.4molLHNO–1.0molLthiourea solution provided effectiveness of desorption for Pt(IV) and Pd(II) from EMCN, while 5molLammonia exhibited the highest selectivity for the tested metal ions. Crosslinked chitosan resin chemically modified with L-lysine has been used to investigate the adsorption of Pt(IV), Pd(II) and Au(III) from aqueous solutions (Fujiwara., 2007). The maximum adsorption capacity was found at pH 1.0 for Pt(IV), pH 2.0 for Au(III) and Pd(II). Langmuir and Freundlich isotherm models were applied to analyze the experimental data. The thermodynamic study indicated that the adsorption process is spontaneous and exothermic in nature. Xiao and Zhou, (2008) also prepared a novel crosslinked chitosan resin modified by L-lysine. The novel chitosan derivative that was produced through the reaction between glutaraldehyde semi-crosslinked chitosan and L-lysine, could be considered either as the reactive and functional polymer or macroporous weak anion exchanger (Chi., 2008). Cárdenas. (2001) developed a good method to prepare chitosan mercaptanes derivatives using mercaptoacetic acid and 1-chloro-2, 3-epoxy propane propionic acid. They also tested the evaluation of retention capacities using Cu(II) and Hg(II) solutions with concentrations ranging from 10 to 104mgLat pH 2.5 and 4.5. However, theis lower and similar at pH 2.5 and 4.5 for Cu(II) ions, which indicated that the selectivity was not decreased with acidic environment. In order to increase the Cu(II) adsorption capacity, the raw chitosan beads were chemically modified into protonated chitosan beads (PCB), carboxylated chitosan beads (CCB) and grafted chitosan beads (GCB), which showed a significant SC of 52, 86 and 126mgg, respectively, while the raw chitosan beads (RCB) displayed only 40mgg. Among the studied sorbents, GCB experienced a higher SC than RCB, PCB and CCB. Sorption experiments were performed by varying contact time, pH, presence of co-anions, different initial Cu(II) concentrations and temperature for optimization (Gandhi., 2011). In another research, chitosan was used for adsorption of Cu(II), Zn(II), Cd(II) and Pb(II) complexes with the ‘green’ chelating agent-polyaspartic acid from waste water effluents (Ska, 2011). The most important factor affecting the effectiveness of sorption is pH and concentration. The adsorption capacity of chitosan for Cu(II), Zn(II), Cd(II) and Pb(II) complexes was equal to 85.21, 46.82, 89.58 and 79.05mgg, respectively. The data were best described by Langmuir isotherm model.

2.3 Chitosan Imprinted Beads

Molecular imprinting technology (Claude., 2010) has been widely used for synthesizing selective sorbents for various molecules and metal ions. These are highly crosslinked polymers synthesized in the presence of a chosen molecule or metal ion as template. The template is subsequently removed to get the molecularly/metal ion imprinted polymer. It has demonstrated that reducing the volume of radioactive resin bed generated during decontamination using cobalt imprinting on organic polymers was possible (Bhaskarapillai., 2009; Ahmadi., 2010). Designing biosorbents with similar selectivity can make the process more economic and environment friendly. In order to improve the selectivity of chitosan, molecular imprinting technology was applied in the preparation of chitosan biosorption materials (Liu., 2011c).

Nishad(2012) prepared a Co(II) imprinted chitosan resin using epichlorohydrin as the crosslinking agent to adsorb Co(II) from Co(II) and Fe(II) mixed solution. The imprinted chitosan showed selective sorption of Co(II) over Fe(II), while the raw chitosan was selective to Fe(II) over Co(II). The imprinted chitosan was found to retain the enhanced selectivity towards Co(II) in various solutions, including typical nuclear reactor decontamination formulations containing strong complexants. An interpenetration network (IPN) ion-imprinting hydrogel (IIH) was synthesized using uranyl ions as template for adsorption and removal of uranyl ions from aqueous solutions (Liu., 2010). The IIH was obtainedcrosslinking of blended chitosan/polyvinyl alcohol using ethylene glycol diglycidyl ether. The ability of IIH to adsorb and remove uranyl ions from aqueous solutions was assessed using a batch adsorption mode. The adsorption process could be well described by both Langmuir and Freundlich isotherm models. The selectivity coefficient of uranyl ions and other metal cations onto IIH indicated an overall preference for uranyl ions, which was much higher than that of the non-imprinted hydrogel. This suggested that IIH is a promising sorbent material for selective removal of uranyl ions from aqueous solutions. Liu. (2011b) prepared a novel, bio-based α- FeOimpregnated chitosan beads (As-IFICB) using As(III) as imprinted ions for adsorption and removal of As(III) ions from aqueous solutions. Batch adsorption experiments were performed to evaluate the adsorption conditions, selectivity and reusability. The selectivity coefficiency of As(III) ions and other metal cations onto As-IFICB indicated an overall preference for As(III) ions, which was much higher than non-imprinted chitosan beads. Fan. (2011) successfully synthesized a novel, thiourea-chitosan coating on the surface of magnetite using Ag (I) as imprinted ions for adsorption and removal of Ag(I) ions from aqueous solutions. The selectivity coefficiency of Ag(I) ions and other metal cations onto Ag- TCM indicated an overall preference for Ag(I) ions, which was much higher than non-imprinted thiourea- chitosan beads. Moreover, the biosorbent was stable and could be easily recovered; the adsorption capacity was about 90% of the initial saturation adsorption capacity after being used five times. Huo(2009) prepared an Ag(I)-imprinted biosorbent to adsorb Ag(I) from wastewater using the surface molecular imprinting technology. The adsorption affinity and selectivity for the imprinting Ag(I) ions were higher than those for other non-im- printing metal ions. The SEM images showed that the noble metal silver could be well adsorbed by the biosorbent. The silver nanoparticles were of regular geometry shape and relatively uniform.

3 Application in Dyes Adsorption

Dyes have become one of main sources of severe water pollution as a result of rapid development of textile industries. The release of colorant effluent has triggered a major concern on the human health as well as marine lives. Dyes consist of two main groups of compounds, chromophores and auxochromes. Chromophores determine the color of dye while the auxochromes determine the intensity of color (Moussavi and Mahmoudi, 2009).

3.1 Chitosan Composites

Zhu. (2010) prepared a new chitosan bead which was blended with maghemite (γ-FeO) and kaolin. From the SEM and TEM images, it was found that there were many pores and pleats on the surface of the composites, which provided active sites for dyes entrapment. The composites exhibited good adsorption ability as they could adsorb up to 70% methyl orange at pH 6. The effect of competitive anions on the adsorption efficiency was studied, which involved Cl, NO, SO, COand PO. The results for the decolorization of dye are no addition >Cl>NO>SO>CO>PO. Therefore, the addition of anions had caused the adsorption of methyl orange to decrease. This could be explained by the preference of POadsorption over methyl orange due to the higher negative charge of phosphate anion.

Bentonite contains a high proportion of swelling clays. It mainly consists of montmorillonite with a composition of SiO, AlO, CaO, MgO, FeO, NaO, and KO (Holzer., 2010; Kang., 2009). Bentonite is a 2:1 type aluminosilicate, the unit layer which consists of one Al(III) octahedral sheet between two Si(IV) sheets (Li., 2010; Wei., 2009). Wan Ngah(2010) prepared crosslinked chitosan/bentonite composites to adsorb tartrazine, a dye which contains azo group that is harmful to living things. The chitosan composites, crosslinked with epichlorohydrin were able to improve the chitosan performance as an adsorbent. According to the literature, the pH of tartrazine plays a crucial role in the adsorption process. At pH lower than pH, the surface of the adsorbent would carry positive charge and tartrazine molecules would be attracted. With pH increasing, the surface of the adsorbent carried more negative charges.

3.2 Chitosan Derivatives

Zhou. (2011) prepared the ethylenediamine- modified magnetic chitosan nanoparticles (EMCN) to adsorb Acid Orange 7 (AO7) and Acid Orange 10 (AO10) from aqueous solutions. EMCN were essentially monodispersed and had a main particle size distribution of 15–40nm and saturated magnetization of 25.6emug. The adsorption experiments indicated that the maximum adsorption capacity occurred at pH 4.0 for AO7 and pH 3.0 for AO10, respectively. Due to the small diameter and high surface reactivity, the adsorption equilibrium of AO7 and AO10 onto EMCN reached very quickly. Equilibrium experiments fitted well with Langmuir isotherm model, and the maximum adsorption capacity of EMCN at 25℃ was determined to be 3.47mmolgfor AO7 and 2.25mmolgfor AO10, respectively. Chitosan was cross- linked with ethylenediamine to prepare an outstanding sorbent for the removal of anionic dye eosin Y from aqueous solution (Huang., 2011). FTIR, DTG and zeta potential analysis were used to characterize the biosorbent. The effect of particle size, solution pH, agitation rate, temperature, adsorbent dosage (50–500mgL), contact time (10min–24h) and initial concentration of dye (50–300mgL) on the adsorption process was investigated. Langmuir and Freundlich adsorption isotherm models were applied to describe the isotherm and isotherm constants, and the data fitted well with Langmuir isotherm model with the maximum adsorption capacity of 294.12mggat 25℃. Kinetic studies followed the pseudo second-order rate model, which indicated that chemisorption is the rate-limiting step. Chitosan-graft poly(methyl- methacrylate) was synthesized to adsorb anionic azo dyes (Procion Yellow MX, Remazol Brilliant Violet and Reactive Blue H5G) from aqueous solution over a wide pH range of 4–10, being most at pH 7 (Singh., 2009). The adsorbent was also found efficient in decolorizing the textile industry wastewater and was much more efficient than chitosan. The experimental equilibrium data for each adsorbent-dye system were successfully fitted to Langmuir and Freundlich sorption isotherm models. In another research, cross-linked chitosan derivatives were synthesized as biosorbents to adsorb a reactive (Remazol Yellow Gelb 3RS) and a basic (Basic Yellow 37) dye from aqueous solutions through grafting with carboxyl and amide groups (Kyzas and Lazaridis, 2009). Chitosan grafted with amide groups was found superior sorbent for reactive dye at pH 2 (=1211mgg), while chitosan grafted with carboxyl groups for basic dye at pH 10 (=595mgg). Shen. (2010) developed a new efficient catalyst by immobilizing water soluble Co(II) tetrasulfophthalocyanine onto adsorbent chitosan microspheres covalently for the heterogeneous catalytic oxidation of C. I. Acid Red 73 with HO. It offered the capabilities and advantages of: 1) using of hydrogen peroxide as an environmentally friendly oxidant and chitosan as a cost-effective support; 2) efficient removal of the C. I. Acid Red 73 in aqueous solution; 3) being active in a wide pH range and of outstanding performance in neutral conditions; 4) being efficient without light source; 5) easy separation of catalyst from reaction mixture and possibility of recycling.

4 Application in Proteins and Amino Acids Adsorption

Many authors studied the preparation and performance of dedicated molecular imprinting polymers for separation of various mixtures of amino acids (Yu and Mosbach, 2000; Chen., 2001; Huang., 2003), peptides (Rachkov and Minoura, 2001) and proteins (Rachkov and Minoura, 2001; Guo., 2004). Guo. (2005a) prepared two kinds of molecularly imprinted polymers using hemoglobin as the imprinting molecule, acrylamide as the functional monomer, chitosan beads and maleic anhydride-modified chitosan beads as the matrixes, respectively. Static adsorbing results showed that an equal class of adsorption was formed in the imprinted polymers and the adsorption equilibrium constant. Chromatographic characteristics showed that the column bedded with the hemoglobin imprinted beads could separate hemoglobin and bovine serum albumin effectively from their mixture, which indicated that the imprinted beads have very higher selectivity for hemoglobin than the non-imprinted with the same chemical composition. Guo. (2005b) also prepared a simple molecularly imprinted polymer using hemoglobin (Hb) as imprinted molecule and acrylamide as functional monomer. The molecularly imprinted polymer was achieved by grafting the selective soft polyacrylamide gel to maleic anhydride- modified chitosan beads with monomers and protein diffusing into pores of chemically modified chitosan matrix before starting the polymerization. The chitosan beads were free from the surrounding polyacrylamide gel by washing. The molecularly imprinted polymer has much higher adsorption capacity and selectivity for hemoglobin than non-imprinted polymer with the same chemical composition. Guo. (2004) prepared a simply Hb molecularly imprinted polymer using Hb as the imprinted molecule, acrylamide as the functional monomer and cross-linked chitosan beads as the supporting matrix. The molecularly imprinted polymer was achieved by entrapment of the selective soft polyacrylamide gel in the pores of cross-linked chitosan beads by letting acrylamide monomer and the protein diffuse into pores of chitosan beads before starting the polymerization. The chitosan beads were free from the surrounding polyacrylamide gel by washing. The molecularly imprinted polymer has much higher adsorption capacity for Hb than non-imprinted polymer with the same chemical composition, and the molecularly imprinted polymer also has a higher selectivity for imprinted molecule. According to Monier’s research, L-glutamic acid imprinted cross-linked chitosan (LGIC) was prepared by cross-linking of chitosan using glutaraldehyde as crosslinking agent to separate L-glutamic acid from dilute aqueous solution by solid-phase extraction based on molecular imprinting technique (Monier, 2010). Non- imprinted cross-linked chitosan (NIC) was also prepared as control by the same procedure in the absence of template molecules. The maximum adsorption capacities of L-and D-glutamic acid on LGIC were 42mgg±1mggand 26mgg±1mgg, respectively, while in case of NIC, both L- and D-glutamic acid present the same maximum adsorption capacity of 7mgg±1mgg, which confirmed that the molecular imprinting technique created an enantioselectivity of LGIC toward L-glutamic acid. A graft copolymerization of acrylamide with N, N-methylenebisacrylamide on chitosan in aqueous medium was utilized in the synthesis of molecularly imprinted polymer gels using bovine serum albumin as template (Fu., 2007). The resultant molecularly imprinted polymer gels based on chitosan-g-polyacrylamide showed significantly higher imprinting efficiency than those only consisting of polyacrylamide, and also better than those only composed of chitosan/PAM semi-inter- penetrating polymer network.

5 Future Works and Perspectives

The adsorption of heavy metals, dyes and proteins by chitosan composites and derivatives had been compiled and reviewed in this paper. From the literatures, adsorption of heavy metals, dyes and proteins using chitosan composites and derivatives is expected to have improved performance in the future (Wan Ngah., 2011). However, the comparison between the composites and derivatives is just a guideline. It is almost impossible to make a comparison among composites and derivatives due to the different materials used during the formation of chitosan composites. Different experimental conditions, scarcity of information provided and the inconsistencies in data presentation have also added to the difficulty in making the comparison (Dash., 2011). Here, we would like to highlight some points which might help future research such as chitosan nanoparticle as the protein delivery carrier (Gan and Wang, 2007), hydrolysis (Saeed., 2011; Chang and Chen, 2009) and others. The cost factor should be taken into consideration as low production cost with high removal efficiencies are much preferred. Therefore, reuse is also an important aspect for reducing cost. It is one of key issues to determine whether the selection of adsorbent is appropriate to be implemented in large scale (Kumar, 2000). Chitosan composites and derivatives, one of chitosan-based materials are economically feasible because they are easy to prepare and involve inexpensive chemical reagents (Muzzarelli, 2011).

6 Conclusions

Chitosan composites and derivatives are becoming promising alternatives of conventional adsorbents of heavy metal ions, dyes and proteins in various media. Chitosan dissolve in acidic media, making it inconvenient in application; however, cross-linking agents such as glutaraldehyde, epichlorohydrin and ethylene glycol diglycidyl ether improve its property obviously, allowing chitosan composites and derivatives to be applicable under acidic conditions. Amino groups of chitosan composites and derivatives can be protonated, making chitosan adsorb heavy metal ions, dyes and proteins molecules through various types of interaction mechanisms such as electrostatic attractions, chelation and among others. There are a variety of chitosan based adsorbents (raw chitosan, chitosan derivatives, chitosan composites and among others). It is necessary to determine an adsorbent appropriate for an ion. Comparison among adsorbents is almost impossible; the parameters and the adsorbate are different each other. Extensive works are appreciated in order to commercialize chitosan composites and derivatives.

Acknowledgements

This work was financially supported by Ocean Public Welfare Scientific Research Special Appropriation Project (201005020), Fundamental Research Funds for the Central Universities and Program for Changjiang Scholars and Innovative Research Team in University (IRT1188). We are grateful to thank the editors and anonymous reviewers for their suggestions and comments, which significantly improved the quality of manuscript.

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(Edited by Qiu Yantao)

10.1007/s11802-013-2113-0

ISSN 1672-5182, 2013 12 (3): 500-508

. Tel: 0086-532-82031575 E-mail: wangdf@ouc.edu.cn

(July 18, 2012; revised January 10, 2013; accepted April 3, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2013