Recent studies of a separation membrane consisting of chitosan.
Chitosan, known as a most typical marine biological polymer, has a fruitful capability of biocompatible gel formation. Attempts of chitosan have been made to develop it from the multifaceted viewpoint of separation technology. The physicochemical properties of chitosan containing a lot of hydroxyl groups and reactive amino groups help to build the characteristic polymer networks. The deacetylation degree of chitosan is found as the most influential factor to regulate properties of chitosan hydrogels. The antibacterial activity of the chitosan membrane is one of its notable abilities because of its practical application. The chitosan, its derivatives, and the complex formation with other substances has been used for applications in filtration and membrane separation processes. Adsorption processes based on chitosan have been also developed widely. Moreover, complex of chitosan gel helps to immobilize adsorbent particles. The chitosan membrane immobilizing Prussian-Blue for cesium ion removal from the aqueous phase is one of the leading cases. To elaborate the adsorption behavior on the chitosan immobilizing adsorbent, the isothermal equilibrium and mass transfer characteristics can be discussed. The adsorption process using chitosan-based membranes in combination with filtration in a flow process is advantageous compared with the batch process. More advanced studies of chitosan aerogel and chitosan nanofibers have been proceeded recently, especially for adapting to water purification and air filtration.
- deacetylation degree
- water treatment
A separation process, which is often called the “downstream process”, plays a key role of product manufacturing through chemical or biochemical reaction as well as a synthesis process called the “upstream process” . To ensure quality and cost of final products, the separation process is important and has been developed in line with the social demands . For chemical and biochemical industries, the separation process aims to purify objective substances, eliminate undesirable substances, and fractionate each component from their mixture. As environmental awareness around the world increases recently, new separation technologies, such as wastewater treatment , advanced desalination , air cleaning  etc., are in great demand. In addition, materials used in such separation processes are not only expected to be efficient, low cost, easy operation, but also required to be environmentally-friendly.
Chitin and chitosan obtained from crustaceans possess sufficient environmental adaptability as well as an attractive potential to build various types of functional media, e.g., membranes [6, 7, 8, 9, 10], micro/nanoparticles [11, 12], and nanofibers [13, 14, 15, 16]. Many studies have devoted to develop the novel media adapting separation processes using chitosan. The separation performance of such chitosan media should be strongly influenced from chemical properties characterized by deacetylation degree (DD) at amino groups in the chitosan molecular chain. Nevertheless, it was less mentioned that the DD could steer not only the structure of the prepared chitosan gels but also the characteristics as separation media.
This chapter describes the preparation and physicochemical properties of novel chitosan-based media and demonstrates the promising ability of chitosan with focus on principal studies for environmentally-friendly separation processes. The essential factors which regulate the performance of separation media prepared from chitosan, such as DD, molecular weight, and options of cross-linker, are explained. In particular, the notable impacts of DD on the mass transfer mediated by chitosan membrane, the mechanical property, and the antibacterial activity, are introduced based on our previous research [17, 18, 19]. Separation media prepared from chitosan are often combined with various adsorbents [20, 21], carbon nanotubes [22, 23], or other functional materials [24, 25]. In such case, the behavior of mass transfer into the chitosan hydrogel is complicated to quantitatively evaluate. The present chapter shows the determination of effective diffusion coefficient of cesium ions in chitosan membrane immobilizing Prussian Blue particles . Furthermore, the chitosan aerogels with macro-porous structure is proposed for selective separation for anionic dye from aqueous phase. Chitosan nanofibers incorporated with polyethylene terephthalate (PET) non–woven are also covered to describe in an application of air filtration.
2. Physicochemical properties and gelling characteristics
Chitin and chitosan are known as secondary abundant polymers obtained from the external skeleton of crustaceans such as crab and shrimp . Apart from this, chitin is also found in and produced from the exoskeleton of insects or the cell walls of fungi and yeast [6, 27, 28]; however, this contribution is much less than that from the marine resources. In recent years, chitin and chitosan have gained attention instead of raw materials of petroleum origin owing to the inevitable depletion of fossil fuels and the prevention of climate change . This section looks at the physicochemical properties of chitosan and focuses on the gelling characteristics to build environmentally-friendly separation media.
2.1 Chemical composition and gelling ability for separation media
Figure 1 shows the chemical conversion between chitin and chitosan. The chemical composition of chitin can be described as a long-chain polymer, poly(β-(1 → 4)-N-Acetyl-D-glucosamine). Chitosan, poly(β-(1 → 4)-D-glucosamine), is obtained mainly by transforming partial deacetylation of chitin in an alkaline condition, such as using sodium hydroxide aqueous solution. It has been reported that chitosan and its oligosaccharides not only possess hydrophilicity, non-toxicity, biodegradability, and biocompatibility, but also possess antimicrobial activity, antioxidant properties, and an affinity for proteins [7, 26].
Chitosan is insoluble in water at neutral pH or in any organic solvent. Consequently, an acidic aqueous solution, such as acetate buffer solution, is usually employed to dissolve chitosan, whereby the acid dissociation constant of chitosan is found as pKa ≈ 6.5 . Chitosan can be dissolved in acidic solutions by protonation of amino groups in glucosamine units.
Deprotonating a chitosan solution through an acid–base neutralization leads to formation of a water-insoluble gel structure without cross-linker due to intermolecular hydrogen bonding . The salt (e.g. NaCl) coexisting with chitosan in an acidic solution acts as counter ions and disrupts intramolecular hydrogen bonding, and then the flexibility of chitosan molecular chains increases . In addition, pH neutralization influences the formation of a polymeric network . Therefore, the neutralization condition should be optimized. From the convenient gelling process, chitosan hydrogels have been developed widely as immobilizing matrices, with enzymes, carbon nanotubes, and electroconductive polymers as typical examples [10, 32, 33].
2.2 Deacetylation degree
Deacetylation degree (DD) is the most important factor to regulate physicochemical properties. The deacetylation degree of chitosan samples was determined using the colloidal titration method-based experimental conditions in previous works [34, 35]. We dissolved chitosan powder (0.5 g) in 5% acetic acid solution, and then increased the total weight of chitosan–acetic acid solution to 100.0 g by adding acetic acid. We mixed a 1 g sample of this chitosan–acetic acid solution to 30 ml of deionized water. The titrant was 0.0025 N potassium polyvinyl sulfate (PVS-K), and the indicator was 1% toluidine blue. The terminal point of titration was clearly indicated by the color changing from blue to claret. The deacetylation degree was calculated using the following equations.
In these equations, X is the equivalent mass of glucosamine contained in a 1 g sample of the chitosan–acetic acid solution, v [ml] is the volume of 0.0025 N PVS-K solution, and f is its concentration factor. Y is the mass of acetyl glucosamine contained in a 1 g sample, calculated as the difference between the mass of the sample and the value of X. We evaluated the deacetylation degree of the chitosan samples as a molar fraction of glucosamine .
2.2.1 Control of the deacetylation degree
A chitosan membrane was prepared by the casting method in combination with N-acetylation reaction . The deacetylation degree (DD) decreased linearly with increasing added amounts of acetic anhydride (Figure 2). Stoichiometric control of the deacetylation degree to the desired level was successfully performed. However, gelation reaction due to excess addition of acetic anhydride inhibited formation of the chitosan membrane.
The gelation behavior of chitosan, which has various degrees of acetylation (DA) of amino groups, was investigated to ensure preparation of the designed membrane structure . The gelation behavior was evaluated by the gelation time and the quantity of syneresis, and useful information not only for preparing a membrane but also for preparing an immobilized carrier or a chemical reaction system was obtained in this work.
2.2.2 Water permeation mechanism of an N-acetylated chitosan membrane
A novel model of the water permeation mechanism in an N-acetyl-chitosan membrane with a cellular structure was proposed . Although the entire membrane structure has a hydrophilic character, the cellular structure incorporates junction zones that practically prevent water permeation.
Chitosan membranes with a controlled degree of deacetylation (DD) were prepared using a casting method. Changes in the total water content and the pressure driven water flux of the membrane were observed with a change in DD (Figure 3). The membrane properties were analyzed and evaluated using water permeability measurements, scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). SEM observations indicated that the membrane structure was an individual cellular structure and that this cellular structure grew with decrease in DD (Figure 4). From XRD measurements, the intensity in the range from 10° to 20° were detected in the chitin (DD = 1.1%) and the chitosan membranes (71.3% < DD < 92.2%), which indicated that the crystal structure of the membrane was amorphous regardless of DD. The free water content (Wf), the freezable bound water (Wfb), and the bound water not able to freeze (Wb) were evaluated by DSC. The total water content and the sum of free water content ratios (Wf +
Pressure driven water flux was measured using the ultrafiltration apparatus; it was dependent on the operational pressure, membrane thickness, and the feed solution viscosity, and obeyed the Hagen-Poiseuille flow. At a higher DD, water permeation proceeded due to degradation of the cellular structure; the amount of water in permeation channels was greater than that for lower DD membranes even though the total water content in the membrane was less. The water flux of the chitosan membrane was determined by the water content constructing channels through the membrane and not by the total water content in the membrane.
2.2.3 Antibacterial activity
The antibacterial activity is also explained, because of its long-time practical application. The antibacterial activity of chitosan membranes was investigated by a conductimetric assay using a bactometer . The growth of the gram-positive sample (S. aureus) was more strongly inhibited by chitosan than the gram-negative sample (E. coli). This inhibitory effect was recognized as a bactericidal effect. Antibacterial activity was also observed and was dependent on the shape and specific surface area of the powdered chitosan membrane. The influence of the DD of the chitosan on inhibiting the growth of S. aureus was investigated by two methods: incubation using a mannitol salt agar medium and a conductimetric assay (Figure 6). In both methods, chitosan with a higher DD successfully inhibited growth of S. aureus. Our findings regarding the dominant role of the DD of chitosan will be useful for designing lasting, hygienic, membrane-based processes.
2.3 Molecular weight
The molecular weight of chitosan also plays a significant role in the properties of a prepared membrane. It was found that the tensile strength and elongation of membranes prepared from high molecular weight chitosan were higher than those prepared from low molecular weight chitosan; however, the permeability of membranes from high molecular weight chitosan is lower than those prepared from low molecular weight ones . For convenient determination, the viscometric average molecular weight (M [g mol−1]) can be calculated from the intrinsic viscosity (η [mL g−1]) using the Mark-Houwink-Sakurada relationship as follows:
For a case, chitosan is dissolved in an acetate buffer composed of 0.2 M acetic acid +0.2 M sodium acetate at 25 ̊C. The parameters in the above relationship were found as K = 7.9 × 10−2 [mL/g] and α = 0.796 [−] [27, 39, 40].
Various types of cross-linkers are often employed for the fabrication of suitable chitosan membranes adapted to separation processes. Glutaraldehyde (GA) is usually used as cross-linker, because it is extremely reactive in cross-linking chitosan polymer chains via the Schiff reaction between aldehyde groups and amino groups to form covalent imine bonds [41, 42, 43]; however, GA is also toxic. Genipin, which is produced from the hydrolysis of geniposide extracted from the fruits of Gardenia jasminoides Ellis, is attracted as a biocompatible and low biohazardous cross-linker [44, 45]. Tetraethyl orthosilicate (TEOS) is also used as a covalent cross-linker, which serves to cross-link with chitosan polymers at their hydroxyl groups, to immobilize adsorbent particles, and to hydrophobize membrane surfaces [22, 46, 47, 48]. Jóźwiak and coworkers widely investigated the effect of the cross-linker type occurring covalent bond or ionic bond on the chitosan hydrogel prepared for ionic dye adsorption. In case of covalent agents, it is suggested that epoxide functional groups prefer to attack hydroxyl groups of chitosan during cross-linking, and the free amine groups formed are responsible for anionic dye adsorption .
3. Filtration processes with sieving to molecules
Table 1 summarizes the recent studies of a filtration membrane consisting of chitosan or chitosan derivatives. In several membranes, the functional materials are additionally composed to improve the separation ability. The relevant studies are picked up and described below.
|Membrane body||Additive functionalizing material||Cross-linker||Target substances||Expected application||Ref.||Year|
|chitosan||phosphotungstic acid||NA||proton, methanol||DMFCsa||||2016|
|chitosan/PSFb||MOFsc||NA||NaCl, MgCl2, CaCl2, Na2SO4||nanofiltration||||2017|
|chitosan||MWNTd||glycerin, PEGDEe||NaCl, MgCl2MgSO4||nanofiltration|||
|carboxymethyl chitosan/PVDFf||NA||NA||humic acid||nanofiltration|||
|chitosan/PVAi||NA||adipic acid||NaCl aerosol||air filtration|||
|phosphorylated chitosan/PANj||NA||GAk||MgCl2Na2SO4MBlMOm, AYRn||nanofiltration|||
|chitosan||polyester nonwoven fabric||sulfuric acid||H2O (vapor)||membrane drier||||2019|
|PVAi||microparticles of chitosan, phosphorylated chitosan, glycidol-modified chitosan, or sulphated chitosan||GAk||ethanol||pervaporation|||
|phosphorylated chitosan||graphene oxide||GAk||DB38o, PSp, XOq||nanofiltration|||
|chitosan/methoxy pectin||cutin||glycerol||H2O (vapor)||packaging film|||
|chitosan||NA||succinic acid||H2O (vapor)||packaging film||||2021|
As a common procedure, many types of chitosan-based membranes are prepared using the casting method. Chitosan membranes prepared by the casting method usually has a highly compacted gel structure since hydrogen bonding derived from a lot of hydroxyl groups. The dense membranes are beneficial for the nanofiltration process separating small substances, for instance, salts, organic acids, or organic dyes [41, 43, 50, 51, 52, 53, 54]. For improving the separation ability, combination of organic–inorganic polymeric hybrid membrane is an innovative approach. Metal–organic frameworks (MOFs) were incorporated into the chitosan polymeric matrix to obtain a positively charged membrane surface for cation removal . Montmorillonite clay, which is dispersed uniformly in a porous matrix, enhances chromium removal . Also, carbon nanotubes are combined with the chitosan membrane for improving solution permeability and salt rejection [51, 55].
3.2 Gas separation
The chitosan membranes with dense polymeric structures can also be used for gas separation processes. It is reported that a chitosan/polyvinyl alcohol (PVA)-blended membrane exhibited high air filtration with antibacterial activity . Nowadays, CO2 separation technology has caught attention due to the increase in concerns related to climate change because of greenhouse gases. Chitosan-based membranes for CO2 separation membranes are developed to immobilize active sites, such as copper-1,3,5-benzenetricarboxylic acid (Cu-BTC)  and hydrotalcite .
3.3 Hydrophilicity-based process
Due to a high hydrophilicity derived from abundant hydroxyl groups in glucosamine units, the polymeric membranes consisting of chitosan are suitable for separation between water and organic solvents using pervaporation processes [42, 44, 46, 59]. From its hydrophilicity, the chitosan membrane has also been used as a separation membrane in direct methanol fuel cells (DMFCs) requiring blocking of methanol permeation as well as proton conductivity . Regarding the hydrophilicity of chitosan, removal or blocking of water vapor was tested using a chitosan membrane for a part of a membrane drier apparatus  and packaging membrane [62, 63]. As a very recent issue, the interest in biodegradable films for packaging has recently been steadily increasing due to significant concerns on environmental pollution caused by nonbiodegradable packaging materials [64, 65].
4. Adsorption processes
Adsorption is frequently used in separation processes to recover worthy substances as well as water treatment due to its advantages, such as easy operation, high selectivity, and low operational costs [66, 67]. Tables 2 and 3 summarize the recent studies of adsorption processes, which employed chitosan or chitosan derivatives as either main body of adsorbent or immobilizing media for other adsorbents (mainly fine particles).
|Membrane body||Membrane type||Additional adsorbent||Cross-linker||Target substances||Ref.||Year|
|chitosan/PETe||fibrous membrane||NA||GAd||Cu(II), Pb(II), Cd(II), Cr(VI)||||2017|
|chitosan||affinity membrane||multilayered molecularly imprinted||TEOSc||artemisinin||||2019|
|chitosan/ polyacrylonitrile||fibrous membrane||zirconium MOFf||NA||Pb(II), Cd(II), Cr(VI)|||
|chitosan||dense membrane immobilizing adsorbents||PBg||NA||Cs|||
|oxidized chitosan/PVAa||fibrous membrane||PHMGj||NA||Cu(II)|||
|chitosan/PVAa||dence membrane immobilizing adsorbents||ZIF-8k||NA||MGl||||2020|
|octyl-trimethoxysilane modified chitosan||dense membrane||NA||TEOSc||impurities from extracted artemisinin|||
|chitosan/PVAa||dence membrane immobilizing adsorbents||ZIF-8k||NA||MGl|||
|chitosan/PVDFn||fibrous membrane||ZIF-8k||NA||BSAo, Cr(VI)|||
|chitosan/polyamide6||fibrous membrane||NA||NA||Acid Fuchsin dye|||
|Matrix body||Additional adsorbent||Cross-linker||Type of adsorbent media||Adsorption process||Target substances||Ref.||Year|
|chitosan||NA||GAa, EGDEb||spherical beads||through packed bed||Cu(II), Pb, Zn(II), E. coli, S. aureus||||2016|
|chitosan/PAMc||EDTAd||MBAe||cylindrical tablets||batch adsorption||Cu(II), Pb(II), Cd(II)||||2017|
|Rayon fibers coated with chitosan||NA||PBf||rayon fibers||batch adsorption||Cs|||
|chitosan||NA||CITg, TPPh, SSAi, OAj, ECHk, GAa, TTEl, EGDEb||spherical beads||batch adsorption||RB5m|||
|chitosan||graphene nanopalates||GAa||spherical beads||batch adsorption||MOn, AR1o||||2018|
|chitosan||PDMAEMAp||NA||magnetic spherical beads||batch adsorption||AG25q, RB19r|||
|chitosan||CA-CDs||EDCt, NHSu||particles||through packed column||RB49v|||
|chitosan||CBw||NA||powder||batch adsorption||Pb, RO5x, AB25y, RY145z||||2019|
|chitosan||bentnite, cobalt oxide||NA||powder||batch adsorption||CRA, Cr(VI)|||
|N-allylthiourea chitosan||NA||NA||powder||batch adsorption||AS(III)B|||
|chitosan||Fe3O4||NA||magnetic spherical beads||batch adsorption||ABC|||
|chitosan||EDTAd-silane/mGOE||GAa||magnetic spherical beads||batch adsorption||Pb(II), Cd(II)|||
|oleoyl chitosan||NA||NA||nanoparticles||batch adsorption||Fe(II)|||
|chitosan||DESF(choline chloride + urea or chline chloride + glycerol)||NA||spherical beads||batch adsorption||MGG||||2020|
As shown in Figure 1, chitosan has abundant functional groups. Thereby, chitosan and its derivatives also show adsorbent capabilities with various metal ions and organic substances depending on the pH and the concentration of ionic substances . Hence, their adsorption properties are being widely researched . A large portion of the study employed chitosan and its derivatives for adsorption processes devoted to remove heavy metal from polluted water [11, 13, 14, 15, 16, 28, 69, 70, 71]. Considering organic compounds, chitosan can adsorb anionic dyes due to amine groups in a chitosan polymer chain [72, 73, 74]. It has been reported that nonionic compounds like naphthalene  are also adsorbed on chitosan. For improving the adsorption ability, fibrous membranes are fabricated to remove ionic substances or heavy metal ions [12, 13, 14, 15, 16, 71]. Fibrous membranes with the adsorption ability are beneficial due to not only large surface area but also high permeation flux. Chitosan gel have shown brilliant abilities as immobilizing media to various adsorbent substances not only to enhance absorption performance but also to enlarge coverage of adsorbates [20, 21, 23, 25, 66, 73, 75, 76, 77, 78, 79].
Adsorption processes are liberally categorized into two types which are adsorption conducted in a batch process and adsorption in combination with filtration in a flow process.
4.1 Membrane-type absorbents adapting with filtration process
Chitosan have been focused as the media immobilizing various adsorbents since it is able to form stable gel structure through pH responsible [15, 20, 24, 71, 80]. From the advantages in both the adsorption process with selectivity and filtration with continuous operation, the membrane-type adsorbents have been recently developed to adopt the flow process. Table 2 summarizes the studies of membrane-type adsorbents consisted of chitosan or chitosan derivatives.
4.1.1 Characterization of chitosan membrane immobilizing adsorbent in batch process
For a large portion of such studies, adsorption abilities are evaluated from isothermal adsorption in batch process to reveal adsorption mechanisms [24, 72]. As a typical case of an adsorption membrane immobilizing adsorbent particles, chitosan membrane incorporating Prussian Blue (PB) was developed for cesium removal from the aqueous phase . The maximum adsorption capacity can be evaluated from the equilibrium adsorption amount of absorbate on absorbent (qe [mol g−1]) and the equilibrium concentration of absorbate in aqueous phase (Ce [mol g−1]) using the Langmuir adsorption isotherm (Eq. 5). The Langmuir isotherm explains both monolayer and homogeneous adsorption.
Where, Qmax and K are the maximum adsorption capacity [mol g−1] and the equilibrium adsorption constant [L mol−1], respectively. Figure 7a displays the effect of the mass fraction of immobilized PB in chitosan membrane (MFPB) on the maximum adsorption capacity for the membrane and for the PB immobilized in membranes. Immobilization in the chitosan membrane achieved to improve cesium adsorption without inhibition of the adsorption ability of PB.
The diffusivity of adsorbate molecules in adsorbent media strongly influences the adsorption rate . The effective diffusion coefficient (Deff [m2 s−1]) of cesium ions in the chitosan membrane immobilizing PB was determined according to the mass transfer theory. Figure 7b depicts the effect of MFPB on the Deff of cesium ions in the initial period of isothermal adsorption. The obtained values of Deff were lower than that of the diffusion coefficient in the bulk aqueous phase, which was previously reported as 2.17 × 10−9 m2 s−1 . The structure of the membrane consisting of a chitosan polymer chain and PB were observed to suppress the diffusion of cesium ions into the membrane. The diffusion of cesium ions was inhibited considerably by the immobilized PB, which became dominant in comparison to the mass transfer resistance by the chitosan polymer chain .
4.1.2 Adsorbed separation in flow process
The adsorbed separation in flowing process through the membrane immobilizing adsorbents is more efficiently than equilibrium adsorption in flask. Owing to the adsorption ability collaborated with molecular size screening ability, selective separation is exhibited higher than without adsorbent system. Moreover, adsorption capability of adsorbents is fully utilized in the adsorption in combination with filtration in a flow process whereas the batch adsorption is up to equilibrium by the decrease of solutes concentration in liquid phase .
Li and co-workers have prepared chitosan fibrous membrane and investigated the dynamic flow adsorption using filtration apparatus for removal of Cr (VI) from aqueous phase . The study revealed the flow adsorption is advantageous than static batch adsorption for small concentration of feed Cr (VI). In addition, the flow rate of feed solution during the flow process directly influenced on the adsorption behavior. Lower flow rate increases the adsorbed amount of Cr (VI) nearly up to maximum adsorption capacity since the contact time between adsorbent and adsorbate increases.
Separation performance is strongly depended on the membrane structure as well as selection of absorbent. Khajavian and co-workers have demonstrated the removal of Malachite Green known as a cationic dye via flow process using chitosan/poly(vinyl alcohol) incorporating metal–organic frameworks (ZIF-8) . The physical factors including membrane thickness and porous structure generated by pore generator (polyethylene glycol) inside membrane regulate water flux and dye rejection. The chitosan membrane adopting flow processes should be optimized for each system and target substances.
4.2 Other types of absorbent media composed of chitosan
A spherical bead is the basis of the adsorbent media. Chitosan beads can be obtained easily via droplet method which a small portion of chitosan dissolved in acid solution is dropped to alkaline solution for neutralization. Bead-type absorbents can be applied to packed bed adsorption operated in a flow process .
Adsorbents composed of chitosan and its derivatives incorporated with inorganic substances have been developed to enhance adsorption performance [21, 75, 78, 84]. In addition, it is a notable trend that the studies about the removal of organic dye from aqueous are clearly increasing.
As a very recent approach, Sadiq and coworkers developed chitosan beads incorporating with deep eutectic solvents (DES) which are prepared from choline chloride with urea or glycerol for removing organic dye . The DES is paid attention to possess biocompatibility and low toxicity regardless of similar characteristics with ionic liquids [79, 85, 86].
4.3 Highly porous aerogels
Highly porous media from chitosan gel which have large surface area contributing high adsorption ability have been developed recently [87, 88, 89, 90, 91]. Such highly porous media are frequently called aerogel. Chitosan aerogels can be prepared via freeze-drying of chitosan aqueous solution and stabilization using cross-linker, such as glutaraldehyde (GA). Yi and coworkers reported chitosan aerogel silylated with methyl trimethoxysilane which has spring-like structure leading to high oil absorption . Yu and coworkers revealed preparation of chitosan aerogel incorporating graphene oxide and montmorillonite without cross-linker exhibited efficiently Cr(VI) adsorption .
Chitosan aerogel prepared with GA has highly porous structure showed by a field emission scanning electron microscopy (FE-SEM) (Figure 8a). The chitosan aerogel prepared with GA adsorbed anionic dye (Methyl Orange: 327 Da) rapidly although cationic dye of similar molecular weight (Methylene Blue: 320 Da) is not removed from their aqueous solutions (Figure 8b). Selective removal of anionic substrates in high adsorption rate can be achieved by static attraction derived abundant amine and sufficient surface area. Moreover, chitosan aerogel can immobilize various functional particles hence it is expected to develop as new adsorbent media [88, 90].
5. Chitosan nanofiber
Recently, along with the expanding area of nanotechnologies, the area of nanofibers has been gaining interest . Nanofibers with diameter in the range from several micrometers down to tens of nanometers have useful properties such as high specific surface area, porosity, as well as biocompatibility [93, 94]. Nanofibers made from chitosan have not yet been fully established as compared with other nanofibers [95, 96, 97]. Chitosan nanofibers have diameter around 10 nm and amino groups on its surface with positive charge . Accordingly, chitosan nanofibers possess unique characteristics and advantages that other nanofiber does not have, that has been expected to be used in various sorts of industrial fields, for instance, filtrations, recovery of metal ions, adsorption of proteins, drug release, enzyme carriers, wound healing, cosmetics, and biosensors have been developed [99, 100, 101, 102, 103, 104, 105, 106].
In particular, chitosan nanofiber can be expected as an alternative to air filter media (Figure 9). The particle collection performance across chitosan nanofiber media decreased with increase in the amount of chitosan nanofibers on the polyethylene terephthalate (PET) non–woven, as shown in Figure 10. In addition, it was found that the particle collection performance of filter media with high weight of chitosan nanofibers (Weight of sheet (Ws) = 6.0 g/m2) is compatible to commercial electret filter media.
This chapter described the promising development of utilizing chitosan and its derivatives with the focus on separation processes, and overviewed principal investigations. Reactive molecular chains of chitosan derived from hydroxyl and amino groups serve to form an attractive polymeric gel structure. In the filtration process, dense chitosan membranes separate small molecules of solvents, organic dyes, and toxic ions. The deacetylation degree is found to be the most notable factor for water permeation through membranes and its antibacterial ability. The complexity of chitosan-immobilizing systems with functional materials are found to be available in water purification processes. In recent years, adsorption combined with filtration is being developed for various types of water treatment. In this new type process, both the equilibrium adsorption capability and diffusivity in the absorbent matrix should be mentioned as describe in Section 4.1. Due to the easy operation, high selectivity, and low operational costs, adsorption process with chitosan gel is widely studied. As advanced separation media, highly porous chitosan aerogels and nanofibers, which possess large surface area that contribute to improved separation ability, have been developed recently.
Great demands of a novel biocompatible material and environmentally-friendly processes will continue to increase in future. Innovative separation technology utilizing chitosan obtained from bioresources is promising. The multifaceted development of chitosan-based system will lead to environmentally-friendly processes.
The authors sincerely appreciate the financial support from Mukai Science and Technology Foundation (Japan). Prussian Blue used in this study was kindly provided from Dainichiseika Color & Chemicals Mfg. Co., Ltd. (Japan). Authors are also grateful for laboratory students in National Institute of Technology, Oyama College and their great efforts on the studies presented in this chapter: Mr. Tomoyuki Fujisaki for the study on chitosan membrane immobilizing Prussian Blue particles and Mr. Haruki Koya for preparation of chitosan aerogel.
The authors sincerely thank Prof. Norikazu Namiki of Kogakuin University, who provide the experimental set up for evaluating particle collection performance of test filter media. Ms. Yuki Suto of laboratory student in Kogakuin University, who for the study on chitosan nanofibers filter media.