Chinese Chemical Letters  2017, Vol. 28 Issue (3): 625-632   PDF    
Fabrication of carboxymethyl chitosan-hemicellulose resin for adsorptive removal of heavy metals from wastewater
Shu-Ping Wu, Xiang-Zi Dai, Jia-Rui Kan, Fang-Di Shilong, Mai-Yong Zhu     
Institute of Polymer Materials, School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
Abstract: Carboxymethyl chitosan-hemicellulose resin (CMCH) was synthesized by thermal cross-linking process and characterized by FTIR, TGA, and SEM. Subsequently, the adsorption properties of CMCH toward Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Hg (Ⅱ), Mn (Ⅶ) and Cr (Ⅵ) were evaluated. Various factors affecting the uptake behavior such as pH, temperature, contact time and the initial concentration of the metal ions were investigated. The results showed that all adsorption processes fit the pseudo-second-order model and Langmuir isotherm equation. Significantly, the regeneration experiments showed CMCH can be used as a potentially recyclable and effective adsorbent for the removal and recovery of metal ions from wastewater.
Key words: Biomass     Carboxymethyl chitosan     Hemicellulose     Heavy metal ions     Adsorbent    
1. Introduction

With the rapid development of economy and the increasing growth of industrialization, water contamination by heavy metals has become a serious environmental issue and has caused widespread concern in the international community [1, 2]. Heavy metals include metals and metalloids with atomic weights and densities exceeding 5 g/cm3. Some heavy metals, such as copper (Cu), zinc (Zn) and iron (Fe), serve as micronutrients at low concentrations, but they are only toxic when in excess, while other heavy metals as lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), manganese (Mn) and nickel (Ni) are highly toxic even at very low concentrations and can accumulate in living organisms, causing several disorders and diseases [3, 4]. For this perspective, it is necessary to explore an efficient method for the removal of heavy metal ions from aqueous environments. There are numerous traditional techniques to remove heavy metals from water, such as chemical precipitation, ion exchange, membrane filtration, and evaporation, etc. [5-7]. However, the adsorption is regarded as one of the most effective and attractive process with no chemical sludge and high removal efficiency in comparison with these conventional technologies [8, 9]. It is a crucial task to develop high performance adsorbent for removing pollutants via adsorption technique. To meet demands of sustainable and low cost, the adsorbent should also exhibit high adsorption capacity toward general pollutants and ease regeneration. Recently, bioadsorbent derived from lignocellulosic biomass has been successfully explored as a renewable and sustainable materials for the removal of metal ions due to its low cost, high efficiency and eco-friendly [10, 11].

Chitosan (CS) is the only natural alkali polysaccharide derived from chitin by deacetylation. CS have many functional groups such as N-acetyl groups, reactive hydroxyl and amino groups, which can be modified to generate various chitosan derivatives [12]. Among them, carboxymethyl chitosan (CMC) is one of the most important derivatives. CMC is the product of the chitosan carboxylation having carboxymethyl substituents on some of both the amino and primary hydroxyl sites of the glucosamine units [13]. Owing to its hydrophilicity, eco-friendly, nontoxicity, biodegradability and metal-chelating ability, CMC is regarded as a promising candidate for bioadsorbents. However, CMC could not be utilized to recover metal ions due to its water-soluble and poor chemical stabilization [14]. In order to overcome this problem, one of the method is to modify CMC by other biopolymers to improve the hydrolysis resistance.

Hemicellulose (HC), heteropolysaccharides composed of different sugar units and branched polymers of low molecular weight with a degree of polymerization of 80-200, is an immense renewable resource of biopolymers accounting for 20%-35% of lignocellulosic biomass [15]. Due to their structural varieties and diversity, HC can be easily dissolved in common solvents such as alkaline solution and H2O, which limits the application of HC in heavy metal contaminated treatment. Nevertheless, HC can be modified with specific functional groups such as carboxyl and hemiacetal to remove metal ions. The hemicellulose-graftedpenetic acid foam biosorbent exhibited good adsorption ability and efficiency for Pb (Ⅱ), Cu (Ⅱ), and Ni (Ⅱ) ions [16]. Hemicellulose based hydrogel with a three-dimensional cross-linked polymer network was demonstrated to have the highly efficient sorption capability for various heavy metals [8, 17]. Therefore, HC has excellent potential application as bioadsorbent for the removal of hazardous metals from the wastewater.

It is undoubtedly that the combination of CMC and HC may provide a mean to prepare materials with unique properties that can increase value and utility of these biopolymers. On this context, carboxymethyl chitosan-crosslinked hemicellulose (CMCH) was explored as a novel bioadsorbent to remove the various metal ions from the aqueous solutions. The structure of the CMCH was confirmed using FTIR, SEM and TGA. The adsorption equilibrium and the kinetics of Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Hg (Ⅱ), Mn (Ⅶ) and Cr (Ⅵ) ions in the solution with CMCH were investigated. The mechanism of interaction between CMCH and metal ions as well as the regenerability of CMCH was also clarified.

2. Results and discussion 2.1. Characterization of CMCH

The schematic representation for the preparation of the CMCH is illustrated in Fig. 1. The amino groups of CMC were functionalized with HC through Schiff base under heating condition, which constructed the framework of the resin (Fig. 1a). Structured bioadsorbent has been prepared from the readily available renewable biopolymer CMC and HC via freezedrying method (Fig. 1b). The pore morphology of CMCH was shown in Fig. 1(c-e). As shown, SEM images revealed the structure of CMCH as being a connected 3D structure with a continuous open pore structure. Under experimental conditions, lamellae with a thickness of a couple of micrometers were formed and separated with a spacing of about 100-200 mm. However, the formation mechanism of pore morphologies during freeze-casting is extremely complex and depends on many factors such as the freezing rate, the interfacial free energy, the ice particle size, distribution and content.

Figure 1. (a) Schematic illustration for the preparation of CMCH, (b) photograph of CMCH and (c–e) SEM images of CMCH with different magnification

Fig. 2 presents the FTIR spectra of HC, CS, CMC and CMCH. A broad intense signal at 1043 cm-1 was reflected the stretching and bending vibration of C-O, C-C or C-OH in the spectrum of HC. The characteristic β-glycosidic linkage between the sugar units gave the sharp band at 897 cm-1 [8]. The peak of amide I band in CS was at 1656 cm-1 and the characteristic peak at 1597 cm-1 was assigned to the primary amino group [18]. Compared to CS, the two new peaks at 1629 cm-1 and 1401 cm-1 in the CMC spectrum were ascribed to the asymmetrical and symmetrical stretching of carboxyl groups which indicated that carboxymethyl groups grafted onto chitosan chain [19]. In the spectrum of CMCH, a new peak at 1644 cm-1 was attributed to the C=N linkage derived from the Schiff base. The existence of these characteristic peaks confirmed that HC and CMC were combined by thermal crosslinking.

Figure 2. FT-IR spectra of HC (a), CS (b), CMC (c), and CMCH (d).

The thermal degradation of HC, CMC and CMCH were carried out in a flowing nitrogen atmosphere. As can be seen from Fig. 3, the TGA curves of pyrolysis process of HC, CMC and CMCH took place mainly in the temperature range of 208-319 ℃, 257-310 ℃ and 178-346 ℃, respectively. This weight loss was mainly due to the releasing of gas products such as CO, CO2, CH4, CH3COOH and HCOOH with some H2O. At 50% weight loss, the decomposition temperature of HC, CMC and CMCH was at 294, 316, and 434 ℃, respectively. These results indicated that the thermostability of CMC decreased in the process of thermal cross-linking.

Figure 3. TGA curves of HC, CMC and CMCH

2.2. Adsorption mechanism

FTIR technique is a useful tool to analyze the adsorption mechanism of metal ions by adsorbents. As can be seen from Fig. 4a, the FTIR spectrum of CMCH after adsorption of various metal ions exhibits many alterations from that of CMCH before metal ions adsorption. The major differences are the absorption band at 3448 cm-1 corresponding to the stretch vibration of -NH2 group shifting to the lower wavenumber (about 3419 cm-1). After adsorption of Cr (Ⅵ), Cu (Ⅱ), Ni (Ⅱ), Hg (Ⅱ), Cd (Ⅱ) and Mn (Ⅶ) [CMCH-metal ions], new bands were observed at 611-1110 cm-1, which are assigned to the stretching vibration of N-Metal and O-Metal. In the spectrum of CMCH-Cr (Ⅵ), the absorption band at 1629 cm-1 and 1401 cm-1 assigned to the stretching vibration of C=O of -COOH group was be significantly enhanced. The reasonable molecule structures of CMCH-metal ions complexes are shown in Fig. 4b-c. There are two models to elucidate the structure of CMCH and metal ions [20]. One is the pendant pattern (Fig. 4b), in which metal ions were bound to one -NH2, -OH and -COOH of one CMCH molecular chain. The other is the bridge pattern (Fig. 4c), in which metal ions bond to two or more -NH2, -OH and -COOH of more CMCH chains as a bridge.

Figure 4. FTIR spectra (a) of CMCH before and after adsorption of Cr (Ⅵ), Cu (Ⅱ), Ni (Ⅱ), Hg (Ⅱ), Cd (Ⅱ) and Mn (Ⅶ), the reasonable structure of CMCH-Metal ions complexes, pendant pattern (b) and bridge pattern (c)

2.3. Effect of pH

Results of Cr (Ⅵ), Cu (Ⅱ), Ni (Ⅱ), Hg (Ⅱ), Cd (Ⅱ) and Mn (Ⅶ) uptake by CMCH at different initial pH are presented in Table 1. The uptake capacities of metal ions increased as the initial pH of metallic solution increases from 3.0 to 4.0. Decreased uptake capacities of metal ions at lower pH may be due to the protonation of the amino groups in low pH environment, which resulted in the reduction of number of binding sites available for the adsorption of metal ions. However, the adsorption capacities of metal ions decreased with an increase of pH owing to the interaction between OH-and heavy metal ions to form hydroxide precipitate. Here, the optimum pH value for the adsorption capacity of metal ions appeared to be about 6.0 for Cu (Ⅱ) and 4.0 for other metal ions.

Table 1
Effect of pH on the uptake of metal ions

2.4. Adsorption thermodynamics

To conclude whether the process is spontaneous or not, the experiments were performed at 298, 308 and 318 K. The effect of temperature on metal ions adsorption of CMCH is shown in Fig. 5. The equilibrium constant KD for the adsorption process is obtained as follows [8, 21]:

Figure 5. Plot of the equilibrium constant at different temperature for the metal ions adsorption onto CMCH


where αs is the activity of metal ions adsorbed onto CMCH, αe is the activity of metal ions in solution at equilibrium, υs is the activity coefficient of the adsorbed metal ions and ne is the activity coefficient of metal ions in solution at equilibrium, Cs is the concentration of metal ions adsorbed on CMCH (mg/g); Ce is the equilibrium concentration of metal ions in the solution (mg/L). KD can be obtained by plotting a straight line of ln (Cs/Ce) versus Cs and extrapolating Cs to zero. The intercept gives the values of KD.

Thermodynamic parameters including Gibbs free energy changes (ΔG), enthalpy change (ΔH), and entropy change (ΔS) are calculated using the following equations [22, 23]:


where R is the universal gas constant (8.314J/(molK)) and T is the temperature (K) in Kelvin. The values obtained are presented in Table 2. Results showed that the values of KD arranged in the following order Ni (Ⅱ)>Cd (Ⅱ)>Cu (Ⅱ)>Hg (Ⅱ)>Mn (Ⅶ)>Cr (Ⅵ) which indicated that the affinity of Ni (Ⅱ) onto CMCH was higher than that of the other metal ions. The ΔG is negative at all temperatures and decreases as temperature rises, indicating that the adsorption was spontaneous and the spontaneity increased as temperature increases.Positivevalues of ΔH furtherconfirmed the endothermic nature of the adsorption process. The positive ΔS suggested the increase in randomness at CMCH-solution interface during the adsorption process.

Table 2
Thermodynamic parameters for the adsorption of metal ions onto CMCH.

2.5. Adsorption kinetics

Fig. 6 shows the relationship between the contact time and adsorption capacities of CMCH for Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Hg (Ⅱ), Mn (Ⅶ) and Cr (Ⅵ). The adsorption capacities were significantly increased within 7h at 298K. Obviously, the adsorption capacities reached equilibrium beyond 7h. To interpret the adsorption kinetics of metal ions onto CMCH, the pseudo-first-order equation and the pseudo-second-order equation were employed to explain the adsorption mechanism. The pseudo-first-order equation was represented by the following equation [24]:

Figure 6. Effect of contact time on the uptake of metal ions by CMCH


The pseudo-second-order kinetic rate equation can be expressed as follows [25]:


where qe and qt (mg/g) were the amounts of metal ions adsorbed on CMCH at equilibrium and at any time t (h), respectively, and k1 was the pseudo-first-order rate constant (1/h). The value of k1 can be calculated from the slopes of the linear plot of ln (qe -qt) vs t. k2 was the pseudo-second-order rate constant (g/(mgh)). The value of k2 and qe, cal value can be calculated fromthe intercept and slopes of the linear plots of t/qt vs t.

The corresponding kinetic parameters from different models are summarized in Table 3. The correlation coefficient (R2) for the pseudo-second-order adsorption model had higher value, and the theoretical qe value (qe, the) calculated from pseudo-second-order model was close to the experimental qe values (qe, exp) than those calculated from pseudo-first-order model. This indicated that the adsorption processes best followed the pseudo-second-order model and suggested chemical sorption being the rate-controlling step.

Table 3
Kinetic parameters for the metal ions adsorption by CMCH

2.6. Adsorption isotherms

Adsorption isotherms were fundamental to describe the interactive behaviors between the adsorbate and adsorbent and illuminate the properties and affinity of the adsorbent [26]. Langmuir and Freundlich adsorption isotherm models were used to analyze the adsorption equilibrium data. Langmuir model is based on the assumption that adsorption binding sites had equal affinity and energy, only monolayer adsorption occurs in a homogenous surface [27]. Furthermore, there is no interaction between adsorbed molecules. It can be represented as follows:


where Ce was the equilibrium concentration of metal ions in solution (mg/L), qe was the amount of metal ions adsorbed at equilibrium (mg/g), qm represented the maximum adsorption capacity of CMCH (mg/g) and KL was the Langmuir constant (L/mg) related to the affinity of binding sites and the energy of adsorption. The values of qm and KL were determined from the slope and intercept of the plots of Ce/qe versus Ce.

The Freundlich isotherm expresses adsorption at multilayer and on energetically heterogeneous surface which can be expressed as follows [28]:


where KF (mg/g) was the Freundlich constant, and n was the heterogeneity factor. The values of KF and 1/n were determined from the intercept and slope of the linear plot of lnqe versus lnCe. Fig. 7a illustrates that the experimental data of metal ions were in good agreement with the Langmuir model while there were some differences between the Freundlich plots and the trend lines (Fig. 7b). The isotherm parameters and correlation coefficients are tabulated in Table 4. The maximum adsorption capacities obtained from Langmuir model were close to the experimental data, which indicated that the metal ions adsorption on the surface of CMCH was monolayer biosorption. The maximum adsorption capacity for Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Mn (Ⅶ), Hg (Ⅱ) and Cr (Ⅵ) was 362.3, 909.1, 333.3, 42.0, 28.2, and 49.0 mg/g, respectively. Freundlich parameters (KF and n) indicated whether the nature of sorption was either favorable or unfavorable [8]. The values of n were greater than 1 indicating that the metal ions were favorable adsorbed by CMCH at high concentrations. A comparison of the maximum adsorption capacities of various metal ions onto different adsorbents is given in Table 5. It can be seen that the qm value varied considerably for different adsorbents. By contrast, CMCH exhibited a good capacity to adsorb Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Mn (Ⅶ), Hg (Ⅱ) and Cr (Ⅵ) from aqueous solutions.

Figure 7. Langmuir (a) and Freundlich (b) isotherm model fitted for the adsorption of metal ions by CMCH

Table 4
Langmuir and Freundlich isotherm parameters for the metal ions adsorption onto CMCH

Table 5
Comparison of the maximum adsorption capacities of various metal ions onto different adsorbents reported in the literatures

2.7. Regeneration of CMCH

Fig. 8 shows the desorption efficiencies of CMCH on various metal ions after adsorption-desorption five cycles at 0.1 mol/L EDTA solution. It was found that metal ions adsorbed on CMCH were easily desorbed. The desorption efficiency of Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Mn (Ⅶ), Hg (Ⅱ) and Cr (Ⅵ) reach about 95%, 93%, 90%, 99%, 99% and 89%, after the first cycle, respectively. The desorbed CMCH was still kept highly effective for the re-adsorption of metal ions after five repetitions of the adsorption-desorption cycles. This result indicated that CMCH was an effective and reusable bioadsorbent.

Figure 8. Desorption efficiencies of heavy metal ions onto CMCH

3. Conclusion

The adsorption of Ni (Ⅱ), Cd (Ⅱ), Cu (Ⅱ), Mn (Ⅶ), Hg (Ⅱ) and Cr (Ⅵ) was investigated using CMCH as a novel heavy metal adsorbent. CMCH exhibited a highly macroporous structure, pH-sensitivity behavior and highly efficient sorption capabilities with metal ions. The adsorption kinetics indicated that the adsorption processes best followed the pseudo-second-order model. The adsorption isotherms illustrated metal ions adsorption on the surface of CMCH followed the Langmuir model, and Freundlich parameter n indicated that the adsorption was favorable at high concentrations. The thermodynamics of metal ions adsorption onto CMCH confirmed the spontaneous and endothermic nature of the adsorption process. The regeneration studies indicated that CMCH can be used repeatedly without significant loss of the adsorption capacity. Therefore, CMCH is recommended as a potentially recyclable and effective adsorbent for the removal and recovery of metal ions from wastewater.

4. Experimental 4.1. Chemicals and reagents

HC isolated by alkaline extraction form corncobs was purchased from Shanghai Hanhong Ltd. (China). CS and the other chemicals used were of analytical grade obtained from Sinopharm Chemical Reagent Co., Ltd. All aqueous solutions were prepared using purified water with a resistance of 18.2 MΩ cm.

4.2. N-Acetyl chitosan synthesis

N-Acetyl chitosan was synthesized according to the literatures [39, 40]. CS (20 g) was dissolved in 400 mL 10% (w/w) acetic acid and filtered to remove insoluble residue. The solution was then diluted with 1600 mL methanol, and acetic anhydride was added with stirring at room temperature. The mixture was stored overnight at room temperature to obtain a rigid gel. The gel was stirred with 0.5 mol/L NaOH in ethanol at 25 ℃ for 24 h. The solution was precipitated by adding concentrated NH4OH solution and collected by filtration. The precipitate was washed neutral with 75% ethanol, and vacuum dried to give N-acetyl chitosan.

4.3. Carboxymethylation of CS

CMC was also prepared according to the methods described by [39, 40]. N-Acetyl chitosan (10 g) was dispersed in 50% (w/w) NaOH and kept at -20 ℃ overnight. The frozen alkali chitosan was transferred to 100 mL 2-propanol containing 50% (v/v) ClCH2CO2H, and then stirred for 4 h at 60 ℃. The product was dialyzed against deionized water for 3 days and vacuum dried at room temperature.

4.4. Preparation of CMCH

CMC (1.5 g) was prepared in 50 mL 2% (v/v) glacial acetic acid and stirred for 2 h at room temperature. HC (6.0 g) was dissolved into the CMC solution and refluxed under oil bath at 100 ℃ for 2 h to obtain a hydrogel. Then, the hydrogel was heated in drying oven for complete crossing curing at 40 ℃ under atmospheric conditions for 7 h and washed with purified water several times to remove the redundant acetic acid. The products was finally freezedried for 24 h at -50 ℃.

4.5. Characterizations

FTIR spectra of the samples were recorded with KBr discs in the range of 4000-400 cm-1 on Nicolet-170 SX spectrophotometer. The fracture section of the CMCH was observed using scanning electron microscopy (SEM, Hitachi X-650 microscope, Japan). Thermogravimetric analysis (TGA) of CMCH was carried out on Pris TGA linked to a Pyris diamond TA Lab System (PerkinElmer Co., USA) at a heating rate of 10 K/min from 30 ℃ to 600 ℃ under a nitrogen atmosphere.

4.6. Adsorption studies

The adsorption capacities of heavy metals were investigated in batch experiments using atomic absorption spectrophotometer (AAS, TAS-990F). The adsorption isotherm experiments were performed by adding 0.1 g CMCH into conical flasks containing 100 mL with different heavy metals concentrations (10-250 mg/L) under 100 rpm shaking at 298 K for 12 h. In order to study the effect of pH on the heavy metal ions removal capacities by CMCH, the experiments were carried out at pH 3, 4, and 6 with equilibration for 7 h. Effect of contact time was conducted by placing 0.1 g CMCH in a flask containing 100 mL metal ions (100 mg/L) solution at pH 6.0 for Cu (Ⅱ) and pH 4.0 for Cr (Ⅵ), Hg (Ⅱ), Ni (Ⅱ), Cd (Ⅱ) and Mn (Ⅶ). The thermodynamic experiments were performed at the solution temperatures of 298 K, 308 K and 318 K, respectively. The absolute amount adsorbed and removal efficiency of metal ions were calculated by the following equations [26, 41]:


where qe was the amount of metal ions adsorbed per unit amount of CMCH (mg/g); C0 and Ce were the initial concentrations of metal ions and the final or equilibrium concentrations of metal ions, respectively (mg/mL); V was the volume of metal ions solution (mL), and W was the weight of CMCH (g).

4.7. Regeneration of CMCH

The desorption experiment of CMCH loaded with metal ions was performed using 0.1 mol/L EDTA solution. Briefly, 0.1 g CMCH was added into 100 mL metal ions solution of 100 mg/L, and the adsorption conditions were kept at 100 rpm and 298 K for 7 h. CMCH loaded with metal ions was filtered and gently washed with purified water to remove the unabsorbed metal ions. The resin was then treated by 0.1 M EDTA solution. Five cycles of consecutive adsorption-desorption-regeneration were carried out to validate the reusability of CMCH. The desorption percentage (D) was calculated as follow:


where CEDTA was the metal ion desorbed to the EDTA solution (mg/ L) and Cad was the metal ion adsorbed onto the CMCH (mg/L).


This work was supported by the National Natural Science Foundation of China (No. 21403091), the Natural Science Foundation of Jiangsu Province, China (No. BK20130486), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1601066B) and a Project Funded by Jiangsu University for Senior Intellectuals (Nos. 14JDG128 and 12JDG093). The authors want to express their gratitude to Jiangsu Province for supporting this project under the innovation/entrepreneurship program (Surencaiban[2015]26).

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