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Adsorptive removal of Congo red from aqueous solutions using crosslinked chitosan and crosslinked chitosan immobilized bentonite
Accepted Manuscript Title: Adsorptive removal of Congo red from aqueous solutions using crosslinked chitosan and crosslinked chitosan immobilized bentonite Author: Ruihua Huang Lujie Zhang Pan Hu Jing Wang PII: DOI: Reference:
S0141-8130(16)30084-8 http://dx.doi.org/doi:10.1016/j.ijbiomac.2016.01.083 BIOMAC 5773
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International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
19-8-2015 15-1-2016 22-1-2016
Please cite this article as: Ruihua Huang, Lujie Zhang, Pan Hu, Jing Wang, Adsorptive removal of Congo red from aqueous solutions using crosslinked chitosan and crosslinked chitosan immobilized bentonite, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2016.01.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Adsorptive removal of Congo red from aqueous solutions using crosslinked chitosan and crosslinked chitosan immobilized bentonite Ruihua Huang a* a
*
Lujie
Zhanga
Pan Hu a
Jing Wang a
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, China
Corresponding author：Ruihua Huang College of Science, Northwest A&F University, Yangling, Shaanxi 712100, China; Tel.: +86 029 87092226 E-mail address: [email protected] (R.H. Huang)
Graphical abstract
Chitosan
Crosslinked chitosan biobenbbbbbbbenbenc
Bentonite
Crosslinked chitosan/bentonite composite biobenbbbbbbbenbencobentonitecomposite
obentonitecomp
osite They were characterized by FTIR and XRD. Adsorption of Congo red (CR) using them was investigated.
The adsorption data were analyzed by the isotherm and kinetic models. The adsorption capacities of crosslinked chitosan and crosslinked chitosan/bentonite composite at 298 K and natural pH value were 405 and 500 mg/g, respectively.
Highlights 1. CCS and CCS/BT composite was prepared and characterized. 2. The adsorption of CR onto the CCS and CCS/BT composite followed the Langmuir isotherm model. 3. The pseudo-second-order model described the adsorption kinetic behavior. 4. The CCS/BT composite showed a better adsorption capacity for CR over CCS.
Abstract: Batch experiments were executed to investigate the removal of Congo red (CR) from aqueous solutions using the crosslinked chitosan (CCS) and crosslinked chitosan immobilized bentonite (CCS/BT composite). The CCS and CCS/BT composite were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) techniques. The removal of CR was examined as a function of pH value of CR solution, contact time, and inorganic sodium salt and ionic strength. The equilibrium data of CCS and CCS/BT composite agreed well with the Langmuir model. The adsorption capacities of CCS and CCS/BT composite at 298 K and natural pH value were 405 and 500 mg/g, respectively. The kinetic data correlated well with the pseudo- second-order model. The adsorption of CR onto the CCS was mainly controlled by chemisor- ption while the adsorption of CR onto the CCS/BT composite was controlled by chemisorption and the electrostatic attraction. Keywords: Adsorption isotherm; Bentonite; Chitosan
1. Introduction The treatment and disposal of dye-contaminated wastewater is one of the most serious environmental problems faced by the textile, papermaking, printing, and related industries. Dyes are among the most hazardous materials found in industrial effluents, which needs to be treated, because its presence in water bodies prevents sunlight and oxygen penetration and then has a derogatory effect on photosynthetic activity in aquatic systems [1]. Various treatment processes including biological treatment [2-3], membrane filtration [4], photocatalytic degradation [5] and adsorption [6], have been developed to remove these comp- ounds from colored effluents. Among these methods, adsorption is considered as one of the most competitive methods due to its low-cost, simplicity of operation, as well as the availability of a wide range of adsorbents [7−8]. In recent years, a considerable number of studies have focused on low-cost alternative materials including bentonite [1], agricultural wastes [9-10] and zeolites [11], for the removal of dyes by adsorption method. Bentonite, which is mainly composed of montmorillonite, is a layered mineral with a crystalline structure and net negative surface charge. bentonite application in wasterwater treatment has received much attention in the past decades. However, bentonite in natural form often shows a low adsorption capacity for anionic dyes due to the net negative surface charge. Modification of bentonite through acid treatment, calcinations, functionalization, compositing with other material and pillaring is employed to to enhance its adsorption capacity. Chitosan is a product of the partial N-deacetylation of chitin, which has several desirable properties like biodegradability, hydrophilicity, anti-bacterial property, and non-toxicity [12−13]. It contains amino (−NH2) and hydroxyl (−OH) groups that can serve as adsorptive sites for heavy
metals or dyes. However, chitosan has weak mechanical and chemical properties where it easily dissolves in dilute acids, and agglomerates to form a gel in aqueous solution [14−15]. In order to overcome these limitations, the physical and chemical modification needs to be carried out on chitosan. Chemical cross- linking is an effective method to improve its mechanical strength and its chemical stability in acidic media. However, these improvements were at the cost of the loss of amino (−NH2) or hydroxyl (−OH) groups, which often resulted in the decreased adsorption capacity [16]. In this study, bentonite would be composited with cross-linked chitosan. Cross-linked chitosan was intercalated in bentonite, resulting in an increase in the layer spacing of bentonite. This increase would allow an easy access to its binding sites. Therefore, it was expected that the combination of chitosan with bentonite may create a more efficient adsorbent as compared with single crosslinked chtosan and bentonite. Congo red (CR) [-naphthalene sulfonic acid, 3, 3’-(4, 4’-biphenylenebis (azo)) bis (4-amino-) disodium salt] (Fig. 1) was chosen as the model anionic dye in this study. CR is seriously hazard to aquatic living organisms and can cause human carcinogen [16]. Removal of CR from aqueous solutions was investigated using crosslinked chitosan (CCS) and crosslinked chitosan immobilized bentonite composite (CCS/BT composite). Effects of pH value of CR solution, the presence of other anions, and contact time on the CR adsorption by CCS and CCS/BT composite were examined. The equilibrium data were analyzed using Langmuir and Freundlich isotherm models. The kinetic data were determined using the pseudo-first order and pseudo second order equations. Surface structural analysis of CCS and CCS/BT composite was done by XRD and FTIR techniques.
Fig. 1 goes here. 2. Materials and methods 2.1. Materials Chitosan (CTS) was purchased from the Sinopharm Group Chemical Reagent Limited Company (China). Bentonite powder with a particle size of 200-mesh was acquired from the chemical factory of Shentai, Xinyang, Henan, China. Congo red (CR) was supplied by Sigma chemical company, and used as adsorbate in this study. Congo red concentrations were measured at 497 nm using a UV–vis spectrometer (754N) which was provided by Shanghai precision &scientific instrument Co., Ltd (China). All other reagents were of analytical grade. pH value of CR solutions was adjusted by adding 0.1M HCl or NaOH solutions. 2.2. Preparation of crosslinked chitosan (CCS) The crosslinked chitosan particles were prepared by dissolving 2.000 g of chitosan in 100.0 mL of 2% (v/v) acetic acid and were continuously stirred for 30 min. 25 wt% glutaraldehyde (GLA) solution was dropped slowly into the viscous solution of chitosan and the ratio of GLA to chitosan was approximately 5 mL/g of chitosan. Cross-linking reaction occurred for 22 h at 60 ℃ and then the cross-linked chitosan was washed with distilled water to remove any free GLA and dried in an oven at 60 ℃ for 24 h. After drying, the dry crosslinked chitosan was ground to obtain 100-mesh size particles, which were used for adsorption studies. 2.3. Preparation of crosslinked chitosan immobilized bentonite composite (CCS/BT composite) This composite was prepared by adding 2.000 g of chitosan to 5.000 g of bentonite dispersed in 100 mL of 2% (v/v) acetic acid, and the solution was continuously stirred for 2 h. 25 wt% GLA solution was dropped slowly into the
viscous chitosan-bentonite solution. The following opera- tions were similar to the preparation of CCS. 2.4. Adsorption studies For studying the effect of pH value on CR adsorption by CCS and CCS/BT composite, 40 mg of the power samples (CCS and CCS/BT composite) were added in 50 mL 200 mg/L CR solution. The samples were agitated on a thermostated shaker for 100 min at 298K. Adsorption kinetics experiments at different concentrations (100, 200, 300 and 500 mg/L) were performed by the batch method, where approximately 40 mg of power samples (CCS and CCS/BT composite) were added in 50 mL CR solution. The samples were agitated for times varying from 5 to 400 min. At the end of the each adsorption period, the samples were collected by filtration, and the concentrations of the residual CR in the filtrates were then determined. The removal toward CR, R, and the adsorption capacity, qe (mg/g), at equilibrium, were calculated as follows: R
(C0 Ce ) 100 C0
(1)
qe
(C0 Ce ) V M
(2)
where C0 and Ce are the concentrations of CR in the initial solution and at equilibrium, respectively; V is the volume of CR aqueous solution (L) and M is the mass of adsorbent (g). Adsorption equilibrium isotherm experiments at different temperatures (298, 308, 318 and 328 K) were carried out by varying the concentrations of CR solution from 100 to 800 mg/L. 40 mg of power samples (CCS and CCS/BT composite) were added in 50 mL CR solution. The samples were agitated for 360 min.. After equilibrium, the samples were separated by filtration, and the final concentrations of CR in the filtrates
were analyzed similarly. 2.5. Characterization of the CCS and CCS/BT composite The X-ray diffraction (XRD) patterns were obtained using a Shimadzu XD3A diffrac- tometer equipped with a monochromatic Cu Kα source operating at 40 kV and 30 mA. The diffraction patterns were recorded from 5◦ to 50◦ with a scan rate of 0.02 ◦/s. The FTIR spectra of samples dispersed in KBr disks were recorded at room temperature using a FTIR spectrometer (Shimadzu 4100) over a range of 4000−400 cm-1. The pH value at zero point charge (pHzpc) of the adsorbent was determined by the solid addition method [17]. Initial pH of 0.1 mol/L NaCl solutions (pHi) was adjusted from pH 2 to 11 by adding either 0.1 mol/L HCl or 0.1mol/L NaOH. Adsorbent dose (40 mg) was added to 50 mL of 0.1 mol/L NaCl solution in 150 mL conical flasks and stirred for 300 min and final pH (pHf ) of solution was measured. The difference between the initial and final pH (pHf − pHi) was plotted against the initial pH (pHi) and the point at which pHf − pHi = 0 was taken as the pHzpc.
3. Results and discussion 3.1. Adsorbent characterization 3.1.1 XRD analysis Figure 2 shows the XRD patterns of bentonite, CCS, CCS/BT composite as well as CCS and CCS/BT composite samples after CR adsorption. The XRD pattern of CCS showed the charac- teristic crystalline peak at around 19.88◦. The XRD pattern of bentonite exhibited a typical reflection of montmorillonite at 6.56◦. Upon addition of CCS, this peak was shifted to a lower diffraction angle 5.62◦. This indicates that the intercalation of CCS into the silicate layers of the clay mineral had occurred. For the XRD pattern of the CCS after CR adsorption, the charac- teristic peak of chitosan
became relatively broader and less intense, indicating the deceased crystallinity of CCS. For the XRD pattern of the CCS/BT composite after CR adsorption, a shift from 5.62◦ to 5.70◦ was observed. This shift could be due to the compression of layers after CR adsorption. A similar result was also reported for Rhodamine-6G adsorption onto chitosan- nanoclay composite [6]. Fig. 2 goes here.
3.1.2 FTIR spectroscopy FTIR spectra of the CCS, CCS/BT composite as well as CCS and CCS/BT composite samples after CR adsorption were presented in Fig. 3. The characteristic peaks assignment of chitosan are as follows: 3424 cm-1 (N−H stretch overlapped with O−H stretch), 2932 (C−H stretching vibration), 1655 cm-1 (amide band I, N−H bending vibrations), 1564 cm-1 (amide II band, N−H stretching vibration) and 1387 cm-1 (CH3 symmetrical angular deformation) [17]. In the FTIR spectrum of the CCS after CR adsorption, the peak at 1564 cm-1 (amide II band, N−H stretching vibration) almost disappeared, suggesting the −NH2 groups were mainly involved in the adsorption of CR onto the CCS. Meanwhile, some new peaks were observed at 1505 and 1320 cm-1 attributed to the C−C stretching vibration of benzene and −SO3− in CR, respectively, confirming the presence of CR molecules on the adsorbent surface. In the spectrum of the CCS/BT composite, the characteristic peaks of bentonite were observed. The bands corresponding to the phyllosilicate structure occurred between 466 and 1100 cm−1 and were associated with the stretching and angular deformations of the Si−O−Si and Si−O−Al bonds in bentonite [18]. The intensity of this peak at 1564 cm−1 was reduced as compared with that of CCS, indicating the interaction between −NH2 groups in CCS and bentonite had occurred. After the CCS/BT
composite was loaded by CR, the peak at 1564 cm-1 attributed to N−H stretching vibration almost disappeared, and the peak at 3440 cm-1 relative to O−H stretching vibration was shifted to 3429 cm-1. Meanwhile, the intensity of the peak at 794 cm−1 due to the symmetric stretching vibration of Si−O−Si in bentonite was weakened and other characteristic peaks of bentonite almost had no changes in the positions. These results suggested the involvement of amino and hydroxyl groups in the CCS/BT composite in the adsorption of CR Fig. 3 goes here.
3.2. Effect of pH value of CR solutions and the pHzpc of the CCS and CCS/BT composite The initial pH of the dye solutions is one of the most important parameters affecting the adsor- ption process. In this study, the pH dependence of CR adsorption was investigated and the results were given in Fig. 4a. Increasing the pH value of CR solutions decreased the adsorption of CR onto the CCS and CCS/BT composite. Above pH 7, the CCS/BT composite exhibited a consider- able decrease in removal as compared with the CCS. To evaluate better the effect of pH of CR solutions on adsorption, the zero point charge (pHzpc) of the CCS and CCS/BT composite were measured. The values for the pHzpc of the CCS and CCS/BT composite were obtained at 5.93 and 6.08, respectively (Fig. 4b). The surface of the adsorbent will be negatively charged above the pHzpc and positively charged below the pHzpc. Since CR is an acid dye, it ionizes to form negative ions which were liable to be adsorbed by the positive charge of the adsorbent surface due to the electrostatic attraction at lower pH values (pH < pHzpc). Increasing the pH values of dye solutions weakened the positively charge of the CCS and CCS/BT composite, thus the electrostatic
attraction between CR and the positively charge of the CCS and CCS/BT composite decreased accordingly, resulting in the decreasing removal. At higher pH values, a decline in removal may be ascribed to the repulsive force between the negatively charged surface of the CCS and CCS/BT composite and anionic CR as well as the competitive adsorption of abundant OH- ions. However, the CCS/BT composite exhibited much higher pH-dependence than CCS. Based on this trend, we can speculate that although the electrostatic interaction between CCS/BT composite (CCS) and CR existed, it may not be the only pathway for CR adsorption. Due to multiple amino and hydroxyl groups in the CCS, the chemisorption between CCS and CR may be one of the principle pathways for CR adsorption, because these amino and hydroxy groups in the CCS can form hydrogen bonds with the O, N, and S atoms in CR. As confirmed by FT-IR study, a part of the amino groups in CCS were mainly involved in the CR adsorption, they were no longer available for response to pH change. Therefore, the pH change only had slight effect on CR adsorption. However, for CCS/BT composite, although the chemisorption between CCS/BT and CR may occur, as illustrated in FT-IR study, the CCS amount in the CCS/BT composite was obviously lower than pure CCS. At the moment, the electrostatic attraction predominated, thus the CCS/BT composite showed a higher pH-dependence. Owing to the high removal of CCS and CCS/BT composite at natural pH value, there is no need to adjust pH value for further studies. Fig. 4 goes here.
3.3. Effect of contact time and kinetics The effect of contact time on CR adsorption was investigated at 298 K by using 50 mL of the CR solutions (100, 200, 300, and 500 mg/L) and 40 mg of CCS and
CCS/BT composite. As it can be seen in Fig. 5, the adsorption of CR by the CCS and CCS/BT composite showed three stages: a rapid adsorption at initial stage, a slower stage, and adsorption equilibrium. At the start of the adsorption, CR removal can be rapid surface adsorption (external surface adsorption). In the later slower stage, adsorption mainly occurred via transportation of surface-adsorbed dye to the internal adsorption sites of the adsorbent (internal surface adsorption) [19]. Meanwhile, part of external sites were released and cycled for next adsorption. After 360 min, the adsorption reached equilibrium. Fig. 5 goes here. In order to understand the CR adsorption process on the CCS and CCS/BT composite, the kinetic data were analyzed using the pseudo-first-order and pseudo-second-order models. The pseudo-first- order model can be expressed according to Eq. (3)
log(qe qt ) log qe k1t
(3)
where qe (mg/g) and qt (mg/g) are the amounts of CR adsorbed at equilibrium and at different time intervals, respectively. k1 (1/min) is the rate constant of pseudo-first-order model. Linear plots of “log(qe-qt) vs t ” were used to predict k1 and qe from the slope and intercept, respectively (Fig. 6a and 6a’).Only initial kinetic data are used for fitting the pseudo-first-order model because in many cases the pseudo-first-order rate model does not fit well to the whole range of contact time, and is generally applicable over the rapid initial stage of adsorption [21−22]. The equation of pseudo-second-order model can be expressed according to Eq. (4): t 1 1 t 2 qt k2 qe qe
(4)
where qe and qt are the same as described above, and k2 (g/mg.min) is the rate constant of the pseudo-second order adsorption process. The values of k2 and qe can be determined from the
slope and intercept of the “ t/qt vs t ” plots, respectively (Fig.
6b and 6b’). The parameters as obtained from the linearized plots are given in Table 1. As observed, the calculated adsorption capacity qe(cal) for pseudo-second-order model was close to the experimental adsorption capacity qe(exp) for various CR concentrations confirming the applicability of pseudo- second-order model compared to pseudo-first-order model. These results were also confirmed by the higher regression coefficient (R2) values for pseudo-second-order model compared to pseudo-first order model (Table 1). Thus, it further verified that the chemisorption between both adsorbents and CR may be one of the principle pathways for CR adsorption..The adsorption process happened due to the valence forces between the CR molecules and adsorbent, ie, these amino and hydroxy groups in the CCS or CCS/BT composite can form hydrogen bonds with the O, N, and S atoms in CR molecules.
Fig. 6 goes here. 3.4. Effects of inorganic sodium salt and ionic strength Sodium salt is often used as a stimulator in dying industries. Effect of inorganic sodium salt (0.1 mol/L) on CR adsorption was presented in Fig.7a. The presence of HCO3- and PO43- ions almost had no influence on CR adsorption by CCS, while decreased CR adsorption by the CCS /BT composite. These results can be explained according to their pH-dependence. The addition of HCO3- and PO43- enhanced the pH values of CR solutions. As presented in the results of pH effect in Section 3.2, the CCS exhibited a low pH-dependence, CCS still showed high removal toward CR
although the pH value of CR solution increased. However, the increased pH value weakened the removal toward CR due to the high pH-dependence of CCS/BT composite, as mentioned in Section 3.2. For both CCS and CCS/BT composite, the presence of SO42- and Cl- almost had no influence on CR adsorption. This trend may be attributed to the constant pH value. To further investigate the effect of inorganic salts, like SO42- and Cl- on CR adsorption, various NaCl concentrations were applied to conduct CR adsorption experiments. The results in Fig.7b showed increasing NaCl concentration (ionic strength) almost had no effect on CR adsorption by CCS and CCS/BT composite. Previous studies have shown that if the electrostatic attraction is the main adsorption mechanism, the ionic strength has a significant negative effect on the adsorption process [20−21]. Thus, our results indicated that the electrostatic interaction between CR and CCS (or CCS/BT composite) was not the main interaction in CR adsorption, and other interactions, ie, chemisorpton exhibited in CR adsorption. Fig. 7 goes here.
3.5. Adsorption isotherm In the present study, in order to investigate the adsorption isotherms of CR on the CCS and CCS/ BT composite, the Langmuir and the Freundlich models were employed to simulate the experimental data. The Langmuir isotherm is expressed as follow: Ce C 1 e qe Qb Q
(5)
where Q is the maximum amount of adsorption with complete monolayer coverage on the adsorbent surface (mg/g) and b is the Langmuir constant, which is related to the
energy of adsorption (L/mg). From the linear plots of Ce/qe against Ce, Q and b (Fig. 8a and 8a’) values can be calculated from the slope and intercept, respectively. Besides, the essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter, RL, that is defined by the following equation: RL
1 (1 bC0 )
(6)
where b and C0 are the same as defined before. After calculation of RL from the above equation, the nature of the adsorption process could be evaluated to be either unfavorable (RL > 1), linear (RL = 1), or favorable (0 < RL < 1) [22]. All RL values obtained were listed in Table 2. The Freundlich isotherm equation is expressed in Eq. (7):
log qe log K f
1 log Ce n
(7)
where Kf [(mg/g)(L/mg)1/n] and n are Freundlich constants related to adsorption capacity and heterogeneity factor, respectively. Kf and n values can be calculated from the intercept and slope of the linear plots between logCe and logqe (Fig. 8b and 8b’). The isotherm constants and correlation coefficients (R2) were calculated and listed in Table 2. By comparing R2, it was found that the CR adsorption using CCS and CCS/BT composite was described by the Langmuir isotherm model due to higher R2 in comparison with Freundlich isotherm model. The applicability of Langmuir isotherm suggested the monolayer coverage of the dye on the surface of CCS and CCS/BT composite. And it was found that increasing temperature enhanced the CR adsorption onto CCS and CCS/BT composite, suggesting the CR adsorption was favored at higher temperatures. The maximum adsorption capacity (Q) of the CCS/BT composite was obtained at 500 mg/g at natural pH value and 298 K. Although the CS
amount in the CCS/BT composite was obviously lower than pure CCS, it showed higher adsorption capacity than CCS (405 mg/g), thus the immobilization of crosslinked chitosan in low-cost clay can reduce the cost of adsorbents effectively. The higher adsorption capacity for the CCS/BT composite may be attributed to the intercalation of CCS in bentonite. The increased layer spacing facilitated the CR adsorption onto the CCS/BT composite, resulting in an increasing adsorption capacity. Besides, the RL values were between 0 and 1 (Table 2), indicating that CCS/BT composite and CCS were favorable adsorbents for CR adsorption. When compared with previously reported adsorbents [23-28], the proposed CCS/BT composite gave a relatively high adsorption capacity for CR (Table 3). The above results also further confirmed that CCS/BT composite was favorable for CR removal. Fig. 8 goes here.
3.6. Thermodynamic parameters Thermodynamic parameters including change in enthalpy (△H ), gibbs free energy (△G ), and entropy (△S ) of the adsorption were determined from the experimental data (500 mg/L) by the following equations: KL G RTLnK L LnK L
S H R RT
Cs Ce
(8)
(9) (10)
where KL is the adsorption equilibrium constant, Cs is the amount of CR adsorbed per mass of CCS/BT composite or CCS (mg/g ), C e is the CR concentration in solution at equilibrium (mg /L), R is the universal gas constant, T is temperature (K). The values of △H calculated from the slope and intercept of the plot of LnKL vs. 1/T were
summarized in Table 4. Negative △G means spontaneous adsorption of CR by CCS/BT composite and CCS in the 25–55℃ range. Positive △H suggested that the adsorption of CR by CCS/BT composite and CCS was endothermic, which was consistent with the increased adsorption of CR at high temperatures. The positive value of △S indicated the increased randomness at solid/solution interface during the adsorption process. 3.7. Possible adsorption mechanism Based on the results described above including FTIR analysis, kinetics, and the effects of CR solution as well as inorganic sodium salt and ionic strength, we propose that the mechanism controlling CR adsorption onto the CCS was mainly the chemisorption between CR and CCS. The mechanism controlling CR adsorption onto the CCS/BT composite included the chemisorption, ie, the formation of hydrogen bonds between CR and CCS/BT composite and the electrostatic interaction between CR and CCS/BT composite, but the chemisorption predominated during the process of CR adsorption.
4. Conclusion In this study, the adsorption of CR by the CCS and CCS/BT composite from aqueous solution was investigated. Effects of pH, contact time and ionic strength, as well as the kinetics and isotherm of the CR adsorption, were determined. Adsorptive behavior of CR using the CCS and CCS/BT composite can be well described by the Langmuir model and pseudo-second-order model. The CCS/BT composite showed a better adsorption capacity for CR over CCS although the CCS/ BT composite exhibited 30% of CCS weight content. The adsorption of CR by the CCS was mainly controlled by chemisorption while the adsorption of CR by the CCS/BT composite
was controlled by chemisorption and the electrostatic attraction. As an effective and low-cost adsorbent for CR, the CCS/BT composite was expected to have a promising future for the dye-polluted water purification.
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Figure captions Fig. 1 Molecular structure of Congo red (CR) Fig. 2 XRD patterns of (a)CCS, (b) CR loaded CCS, (c) CCS/BT composite and (d) CR loaded CCS/BT composite Fig. 3 FTIR spectra of (a) CCS, (b) CR loaded CCS, (c) CCS/BT composite and (d) CR loaded CCS/BT composite Fig. 4 Effect of pH value of CR solution on CR adsorption and the pHzpc of the CCS and CCS/ BT composite Adsorption conditions: adsorbent dosage: 40 mg; at 298 K and pH (2–11); Contact time: 100 min; Dye concentration: 200 mg/L. For the study about the pHzpc of adsorbents Adsorption conditions: adsorbent dosage: 40 mg; at 298 K and pH (2–11); Contact time: 300 min; NaCl concentration: 0.1mol/L. Fig. 5 Effect of contact time on CR adsorption by (a) CCS and (b)CCS/BT composite Adsorption conditions: adsorbent dosage: 40 mg; at 293 K and natural pH; Contact time: 5–400 min; Dye concentration: 100, 200, 300 and 500 mg/L. Fig. 6 Adsorption kinetic models of CR onto (a and b) CCS and (a’ and b’) CCS/BT composite Adsorption conditions:adsorbent dosage: 40 mg; at 293K and natural pH; Contact time: 5–400 min; Dye concentration: 100, 200, 300 and 500 mg/L. Fig .7 Effects of (a) inorganic sodium salt and (b) ionic strength on CR adsorption
Adsorption conditions:adsorbent dosage: 40 mg; at 293 K and natural pH; Contact time: 360 min; Dye concentration: 100 mg/L. Fig. 8 Langmuir and Freundlich isotherm plots for CR adsorption by (a and b) CCS and (a’ and b’) CCS/BT composite Adsorption conditions: adsorbent dosage: 40 mg; at natural pH; Contact time: 360 min; Dye concentration: 100–800mg/L.
Fig.1
Fig.2
Fig.3
100
2
(a)
CCS CCS/BT composite
1
80
CCS CCS/BT composite
60
pHf-pHi
Removal/%
(b) 0
40
-1 -2 -3
20
-4 -5
0 4
5
6
7
8
9
pH value of CR solution
Fig.4
10
11
2
4
6
8
pHi
10
12
100
80
80
Removal/%
Removal/%
100
60 100 mg/L 200 mg/L 300 mg/L 500 mg/L
(a) 40
60
100 mg/L 200 mg/L 300 mg/L 500 mg/L
(b)
40
20
20
0
0 0
50
100
150
200
250
Contact time/min
Fig.5
300
350
400
450
0
50
100
150
200
250
Contact time/min
300
350
400
450
2.4 2.5
(a) 2.0
100 mg/L 200 mg/L 300 mg/L 500 mg/L
(a')
100 mg/L 200 mg/L 300 mg/L 500 mg/L
2.0
log(qe-qt)
log(qe-qt)
1.6 1.5
1.2
1.0
0.8
0.5
0.4
0.0
0.0 0
50
100
200
150
250
0
40
80
120
160
t/min
240
280
t/min 3.5
3.5
100 mg/L 200 mg/L 300 mg/L 500 mg/L
3.0
2.5
2.0
1.5
(b')
100 mg/L 200 mg/L 300 mg/L 500 mg/L
3.0
(b)
t/qt/min/mg/g
2.5 t/qt/min/mg/g
200
2.0
1.5
1.0
1.0
0.5
0.5
0.0
0.0 0
100
200
300
t/min
Fig.6
400
0
100
200
300
t/min
400
320
100
100
CCS CCS/BT composite
(b)
(a)
80
80
CCS CCS/BT composite Removal/%
Removal/%
60
40
20
60
40
20
0
0
Zero
NaCl
Na2SO4
Na2CO3
Na3PO4
Type of inorganic sodium salt
Fig.7
0.0
0.1
0.2
0.3
0.4
NaCl concentration/mg/L
0.5
0.8
1.2
298K 308K 318K 328K
1.0
(a)
(a')
0.6 Ce/qe/g/L
0.8 Ce/qe/g/L
298K 308K 318K 328K
0.6
0.4
0.4
0.2 0.2
0.0
0.0 0
100
200
300
400
500
0
100
200
Ce/mg/L 3.0
298K 308K 318K 328K
2.8
(b)
300
400
Ce/mg/L 298K 308K 318K 328K
2.8
(b')
2.6
logqe
logqe
2.6
2.4
2.4
2.2
2.2
2.0
2.0 -2
-1
0
logce
Fig.8
1
2
3
-1
0
1
logce
2
3
Table 1 Pseudo-first-order and Pseudo-second-order models parameters for CR adsorption by the CCS and CCS/BT composite Pseudo-first order model Adsorbent
C0/ mg/L
qe,exp/mg/g
Pseudo-second order model
R2
k1/min-1
qe,cal/mg/g
R2
k2/g/mg/min
qe,cal/mg/g
100
124.7
0.8537
0.0011
6.6
1.000
0.0142
124.2
200
241.7
0.9768
0.0066
93.6
0.9997
0.0003
253.2
300
325.1
0.9796
0.0044
133.8
0.9990
0.0002
336.7
500
408.3
0.9805
0.0042
177.3
0.9988
0.0001
416.7
100
123.9
0.9719
0.0064
18.4
0.9999
0.0022
125.3
CCS/BT
200
245.3
0.9899
0.0047
75.2
0.9988
0.0003
249.4
composite
300
313.8
0.9943
0.0033
109.4
0.9990
0.0002
319.5
500
410.8
0.9830
0.0031
171.7
0.9975
0.0001
423.7
CCS
Table 2
Langmuir and Freundlich models parameters for CR adsorption by the CCS and CCS/BT composite Langmuir
Adsorbent
Temperature/K
model
Freundlich model
R2
RL
b/L/mg
Q/mg/g
R2
Kf [(mg/g)(L/mg)1/n]
n
298K
0.9995
0.0067-0.0514
0.1847
404.9
0.9846
199.5
6.474
308K
0.9984
0.0084-0.0637
0.1469
485.4
0.9122
245.2
8.489
318K
0.9995
0.0035-0.0271
0.3596
625.0
0.8397
284.1
6.515
328K
0.9962
0.0046-0.0358
0.2695
740.7
0.9523
364.5
7.348
298K
0.9945
0.0241-0.1647
0.0507
500.0
0.9652
145.7
4.864
CCS/BT
308K
0.9937
0.0175-0.1250
0.0700
578.0
0.9901
186.0
5.116
composite
318K
0.9976
0.0071-0.0541
0.1747
641.0
0.9632
233.0
5.115
328K
0.9976
0.0015-0.0117
0.8411
787.4
0.8228
366.6
5.647
CCS
Table 3 Comparison of maximum monolayer adsorption capacity of various absorbent for CR Adsorbent
Adsorption capacity/ mg/g
References
Hydroxyapatite/chitosan composite
769
[23]
CS/carbon nanotubes
450.4
[24]
Activated carbon/surfactant (DDAC)
769.23
[25]
Chitosan hydrogel core–shell
beads(CSBN1a)
199.98
[26]
Graphene oxide/chitosan fibers
144.93
[27]
PCNT/Mg(Al)O nanocomposites
1250
[28]
CCS
404.9
This study
CCS/BT composite
500.0
This study
Table 4
Thermodynamic parameters for CR adsorption onto CCS and CCS/BT composite
Adsorbents
CCS
CCS/BT composite
T/K
△G /kJ/mol
298
-1.70
308
-2.73
318 328
-6.33
298
-2.62
308
-3.20
318 328
-5.54
△H /kJ/mol
△S /KJ/mol/K
66.9
0.228
49.1
0.172
-14.0
-7.65
S0141-8130(16)30084-8 http://dx.doi.org/doi:10.1016/j.ijbiomac.2016.01.083 BIOMAC 5773
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
19-8-2015 15-1-2016 22-1-2016
Please cite this article as: Ruihua Huang, Lujie Zhang, Pan Hu, Jing Wang, Adsorptive removal of Congo red from aqueous solutions using crosslinked chitosan and crosslinked chitosan immobilized bentonite, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2016.01.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Adsorptive removal of Congo red from aqueous solutions using crosslinked chitosan and crosslinked chitosan immobilized bentonite Ruihua Huang a* a
*
Lujie
Zhanga
Pan Hu a
Jing Wang a
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, China
Corresponding author：Ruihua Huang College of Science, Northwest A&F University, Yangling, Shaanxi 712100, China; Tel.: +86 029 87092226 E-mail address: [email protected] (R.H. Huang)
Graphical abstract
Chitosan
Crosslinked chitosan biobenbbbbbbbenbenc
Bentonite
Crosslinked chitosan/bentonite composite biobenbbbbbbbenbencobentonitecomposite
obentonitecomp
osite They were characterized by FTIR and XRD. Adsorption of Congo red (CR) using them was investigated.
The adsorption data were analyzed by the isotherm and kinetic models. The adsorption capacities of crosslinked chitosan and crosslinked chitosan/bentonite composite at 298 K and natural pH value were 405 and 500 mg/g, respectively.
Highlights 1. CCS and CCS/BT composite was prepared and characterized. 2. The adsorption of CR onto the CCS and CCS/BT composite followed the Langmuir isotherm model. 3. The pseudo-second-order model described the adsorption kinetic behavior. 4. The CCS/BT composite showed a better adsorption capacity for CR over CCS.
Abstract: Batch experiments were executed to investigate the removal of Congo red (CR) from aqueous solutions using the crosslinked chitosan (CCS) and crosslinked chitosan immobilized bentonite (CCS/BT composite). The CCS and CCS/BT composite were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) techniques. The removal of CR was examined as a function of pH value of CR solution, contact time, and inorganic sodium salt and ionic strength. The equilibrium data of CCS and CCS/BT composite agreed well with the Langmuir model. The adsorption capacities of CCS and CCS/BT composite at 298 K and natural pH value were 405 and 500 mg/g, respectively. The kinetic data correlated well with the pseudo- second-order model. The adsorption of CR onto the CCS was mainly controlled by chemisor- ption while the adsorption of CR onto the CCS/BT composite was controlled by chemisorption and the electrostatic attraction. Keywords: Adsorption isotherm; Bentonite; Chitosan
1. Introduction The treatment and disposal of dye-contaminated wastewater is one of the most serious environmental problems faced by the textile, papermaking, printing, and related industries. Dyes are among the most hazardous materials found in industrial effluents, which needs to be treated, because its presence in water bodies prevents sunlight and oxygen penetration and then has a derogatory effect on photosynthetic activity in aquatic systems [1]. Various treatment processes including biological treatment [2-3], membrane filtration [4], photocatalytic degradation [5] and adsorption [6], have been developed to remove these comp- ounds from colored effluents. Among these methods, adsorption is considered as one of the most competitive methods due to its low-cost, simplicity of operation, as well as the availability of a wide range of adsorbents [7−8]. In recent years, a considerable number of studies have focused on low-cost alternative materials including bentonite [1], agricultural wastes [9-10] and zeolites [11], for the removal of dyes by adsorption method. Bentonite, which is mainly composed of montmorillonite, is a layered mineral with a crystalline structure and net negative surface charge. bentonite application in wasterwater treatment has received much attention in the past decades. However, bentonite in natural form often shows a low adsorption capacity for anionic dyes due to the net negative surface charge. Modification of bentonite through acid treatment, calcinations, functionalization, compositing with other material and pillaring is employed to to enhance its adsorption capacity. Chitosan is a product of the partial N-deacetylation of chitin, which has several desirable properties like biodegradability, hydrophilicity, anti-bacterial property, and non-toxicity [12−13]. It contains amino (−NH2) and hydroxyl (−OH) groups that can serve as adsorptive sites for heavy
metals or dyes. However, chitosan has weak mechanical and chemical properties where it easily dissolves in dilute acids, and agglomerates to form a gel in aqueous solution [14−15]. In order to overcome these limitations, the physical and chemical modification needs to be carried out on chitosan. Chemical cross- linking is an effective method to improve its mechanical strength and its chemical stability in acidic media. However, these improvements were at the cost of the loss of amino (−NH2) or hydroxyl (−OH) groups, which often resulted in the decreased adsorption capacity [16]. In this study, bentonite would be composited with cross-linked chitosan. Cross-linked chitosan was intercalated in bentonite, resulting in an increase in the layer spacing of bentonite. This increase would allow an easy access to its binding sites. Therefore, it was expected that the combination of chitosan with bentonite may create a more efficient adsorbent as compared with single crosslinked chtosan and bentonite. Congo red (CR) [-naphthalene sulfonic acid, 3, 3’-(4, 4’-biphenylenebis (azo)) bis (4-amino-) disodium salt] (Fig. 1) was chosen as the model anionic dye in this study. CR is seriously hazard to aquatic living organisms and can cause human carcinogen [16]. Removal of CR from aqueous solutions was investigated using crosslinked chitosan (CCS) and crosslinked chitosan immobilized bentonite composite (CCS/BT composite). Effects of pH value of CR solution, the presence of other anions, and contact time on the CR adsorption by CCS and CCS/BT composite were examined. The equilibrium data were analyzed using Langmuir and Freundlich isotherm models. The kinetic data were determined using the pseudo-first order and pseudo second order equations. Surface structural analysis of CCS and CCS/BT composite was done by XRD and FTIR techniques.
Fig. 1 goes here. 2. Materials and methods 2.1. Materials Chitosan (CTS) was purchased from the Sinopharm Group Chemical Reagent Limited Company (China). Bentonite powder with a particle size of 200-mesh was acquired from the chemical factory of Shentai, Xinyang, Henan, China. Congo red (CR) was supplied by Sigma chemical company, and used as adsorbate in this study. Congo red concentrations were measured at 497 nm using a UV–vis spectrometer (754N) which was provided by Shanghai precision &scientific instrument Co., Ltd (China). All other reagents were of analytical grade. pH value of CR solutions was adjusted by adding 0.1M HCl or NaOH solutions. 2.2. Preparation of crosslinked chitosan (CCS) The crosslinked chitosan particles were prepared by dissolving 2.000 g of chitosan in 100.0 mL of 2% (v/v) acetic acid and were continuously stirred for 30 min. 25 wt% glutaraldehyde (GLA) solution was dropped slowly into the viscous solution of chitosan and the ratio of GLA to chitosan was approximately 5 mL/g of chitosan. Cross-linking reaction occurred for 22 h at 60 ℃ and then the cross-linked chitosan was washed with distilled water to remove any free GLA and dried in an oven at 60 ℃ for 24 h. After drying, the dry crosslinked chitosan was ground to obtain 100-mesh size particles, which were used for adsorption studies. 2.3. Preparation of crosslinked chitosan immobilized bentonite composite (CCS/BT composite) This composite was prepared by adding 2.000 g of chitosan to 5.000 g of bentonite dispersed in 100 mL of 2% (v/v) acetic acid, and the solution was continuously stirred for 2 h. 25 wt% GLA solution was dropped slowly into the
viscous chitosan-bentonite solution. The following opera- tions were similar to the preparation of CCS. 2.4. Adsorption studies For studying the effect of pH value on CR adsorption by CCS and CCS/BT composite, 40 mg of the power samples (CCS and CCS/BT composite) were added in 50 mL 200 mg/L CR solution. The samples were agitated on a thermostated shaker for 100 min at 298K. Adsorption kinetics experiments at different concentrations (100, 200, 300 and 500 mg/L) were performed by the batch method, where approximately 40 mg of power samples (CCS and CCS/BT composite) were added in 50 mL CR solution. The samples were agitated for times varying from 5 to 400 min. At the end of the each adsorption period, the samples were collected by filtration, and the concentrations of the residual CR in the filtrates were then determined. The removal toward CR, R, and the adsorption capacity, qe (mg/g), at equilibrium, were calculated as follows: R
(C0 Ce ) 100 C0
(1)
qe
(C0 Ce ) V M
(2)
where C0 and Ce are the concentrations of CR in the initial solution and at equilibrium, respectively; V is the volume of CR aqueous solution (L) and M is the mass of adsorbent (g). Adsorption equilibrium isotherm experiments at different temperatures (298, 308, 318 and 328 K) were carried out by varying the concentrations of CR solution from 100 to 800 mg/L. 40 mg of power samples (CCS and CCS/BT composite) were added in 50 mL CR solution. The samples were agitated for 360 min.. After equilibrium, the samples were separated by filtration, and the final concentrations of CR in the filtrates
were analyzed similarly. 2.5. Characterization of the CCS and CCS/BT composite The X-ray diffraction (XRD) patterns were obtained using a Shimadzu XD3A diffrac- tometer equipped with a monochromatic Cu Kα source operating at 40 kV and 30 mA. The diffraction patterns were recorded from 5◦ to 50◦ with a scan rate of 0.02 ◦/s. The FTIR spectra of samples dispersed in KBr disks were recorded at room temperature using a FTIR spectrometer (Shimadzu 4100) over a range of 4000−400 cm-1. The pH value at zero point charge (pHzpc) of the adsorbent was determined by the solid addition method [17]. Initial pH of 0.1 mol/L NaCl solutions (pHi) was adjusted from pH 2 to 11 by adding either 0.1 mol/L HCl or 0.1mol/L NaOH. Adsorbent dose (40 mg) was added to 50 mL of 0.1 mol/L NaCl solution in 150 mL conical flasks and stirred for 300 min and final pH (pHf ) of solution was measured. The difference between the initial and final pH (pHf − pHi) was plotted against the initial pH (pHi) and the point at which pHf − pHi = 0 was taken as the pHzpc.
3. Results and discussion 3.1. Adsorbent characterization 3.1.1 XRD analysis Figure 2 shows the XRD patterns of bentonite, CCS, CCS/BT composite as well as CCS and CCS/BT composite samples after CR adsorption. The XRD pattern of CCS showed the charac- teristic crystalline peak at around 19.88◦. The XRD pattern of bentonite exhibited a typical reflection of montmorillonite at 6.56◦. Upon addition of CCS, this peak was shifted to a lower diffraction angle 5.62◦. This indicates that the intercalation of CCS into the silicate layers of the clay mineral had occurred. For the XRD pattern of the CCS after CR adsorption, the charac- teristic peak of chitosan
became relatively broader and less intense, indicating the deceased crystallinity of CCS. For the XRD pattern of the CCS/BT composite after CR adsorption, a shift from 5.62◦ to 5.70◦ was observed. This shift could be due to the compression of layers after CR adsorption. A similar result was also reported for Rhodamine-6G adsorption onto chitosan- nanoclay composite [6]. Fig. 2 goes here.
3.1.2 FTIR spectroscopy FTIR spectra of the CCS, CCS/BT composite as well as CCS and CCS/BT composite samples after CR adsorption were presented in Fig. 3. The characteristic peaks assignment of chitosan are as follows: 3424 cm-1 (N−H stretch overlapped with O−H stretch), 2932 (C−H stretching vibration), 1655 cm-1 (amide band I, N−H bending vibrations), 1564 cm-1 (amide II band, N−H stretching vibration) and 1387 cm-1 (CH3 symmetrical angular deformation) [17]. In the FTIR spectrum of the CCS after CR adsorption, the peak at 1564 cm-1 (amide II band, N−H stretching vibration) almost disappeared, suggesting the −NH2 groups were mainly involved in the adsorption of CR onto the CCS. Meanwhile, some new peaks were observed at 1505 and 1320 cm-1 attributed to the C−C stretching vibration of benzene and −SO3− in CR, respectively, confirming the presence of CR molecules on the adsorbent surface. In the spectrum of the CCS/BT composite, the characteristic peaks of bentonite were observed. The bands corresponding to the phyllosilicate structure occurred between 466 and 1100 cm−1 and were associated with the stretching and angular deformations of the Si−O−Si and Si−O−Al bonds in bentonite [18]. The intensity of this peak at 1564 cm−1 was reduced as compared with that of CCS, indicating the interaction between −NH2 groups in CCS and bentonite had occurred. After the CCS/BT
composite was loaded by CR, the peak at 1564 cm-1 attributed to N−H stretching vibration almost disappeared, and the peak at 3440 cm-1 relative to O−H stretching vibration was shifted to 3429 cm-1. Meanwhile, the intensity of the peak at 794 cm−1 due to the symmetric stretching vibration of Si−O−Si in bentonite was weakened and other characteristic peaks of bentonite almost had no changes in the positions. These results suggested the involvement of amino and hydroxyl groups in the CCS/BT composite in the adsorption of CR Fig. 3 goes here.
3.2. Effect of pH value of CR solutions and the pHzpc of the CCS and CCS/BT composite The initial pH of the dye solutions is one of the most important parameters affecting the adsor- ption process. In this study, the pH dependence of CR adsorption was investigated and the results were given in Fig. 4a. Increasing the pH value of CR solutions decreased the adsorption of CR onto the CCS and CCS/BT composite. Above pH 7, the CCS/BT composite exhibited a consider- able decrease in removal as compared with the CCS. To evaluate better the effect of pH of CR solutions on adsorption, the zero point charge (pHzpc) of the CCS and CCS/BT composite were measured. The values for the pHzpc of the CCS and CCS/BT composite were obtained at 5.93 and 6.08, respectively (Fig. 4b). The surface of the adsorbent will be negatively charged above the pHzpc and positively charged below the pHzpc. Since CR is an acid dye, it ionizes to form negative ions which were liable to be adsorbed by the positive charge of the adsorbent surface due to the electrostatic attraction at lower pH values (pH < pHzpc). Increasing the pH values of dye solutions weakened the positively charge of the CCS and CCS/BT composite, thus the electrostatic
attraction between CR and the positively charge of the CCS and CCS/BT composite decreased accordingly, resulting in the decreasing removal. At higher pH values, a decline in removal may be ascribed to the repulsive force between the negatively charged surface of the CCS and CCS/BT composite and anionic CR as well as the competitive adsorption of abundant OH- ions. However, the CCS/BT composite exhibited much higher pH-dependence than CCS. Based on this trend, we can speculate that although the electrostatic interaction between CCS/BT composite (CCS) and CR existed, it may not be the only pathway for CR adsorption. Due to multiple amino and hydroxyl groups in the CCS, the chemisorption between CCS and CR may be one of the principle pathways for CR adsorption, because these amino and hydroxy groups in the CCS can form hydrogen bonds with the O, N, and S atoms in CR. As confirmed by FT-IR study, a part of the amino groups in CCS were mainly involved in the CR adsorption, they were no longer available for response to pH change. Therefore, the pH change only had slight effect on CR adsorption. However, for CCS/BT composite, although the chemisorption between CCS/BT and CR may occur, as illustrated in FT-IR study, the CCS amount in the CCS/BT composite was obviously lower than pure CCS. At the moment, the electrostatic attraction predominated, thus the CCS/BT composite showed a higher pH-dependence. Owing to the high removal of CCS and CCS/BT composite at natural pH value, there is no need to adjust pH value for further studies. Fig. 4 goes here.
3.3. Effect of contact time and kinetics The effect of contact time on CR adsorption was investigated at 298 K by using 50 mL of the CR solutions (100, 200, 300, and 500 mg/L) and 40 mg of CCS and
CCS/BT composite. As it can be seen in Fig. 5, the adsorption of CR by the CCS and CCS/BT composite showed three stages: a rapid adsorption at initial stage, a slower stage, and adsorption equilibrium. At the start of the adsorption, CR removal can be rapid surface adsorption (external surface adsorption). In the later slower stage, adsorption mainly occurred via transportation of surface-adsorbed dye to the internal adsorption sites of the adsorbent (internal surface adsorption) [19]. Meanwhile, part of external sites were released and cycled for next adsorption. After 360 min, the adsorption reached equilibrium. Fig. 5 goes here. In order to understand the CR adsorption process on the CCS and CCS/BT composite, the kinetic data were analyzed using the pseudo-first-order and pseudo-second-order models. The pseudo-first- order model can be expressed according to Eq. (3)
log(qe qt ) log qe k1t
(3)
where qe (mg/g) and qt (mg/g) are the amounts of CR adsorbed at equilibrium and at different time intervals, respectively. k1 (1/min) is the rate constant of pseudo-first-order model. Linear plots of “log(qe-qt) vs t ” were used to predict k1 and qe from the slope and intercept, respectively (Fig. 6a and 6a’).Only initial kinetic data are used for fitting the pseudo-first-order model because in many cases the pseudo-first-order rate model does not fit well to the whole range of contact time, and is generally applicable over the rapid initial stage of adsorption [21−22]. The equation of pseudo-second-order model can be expressed according to Eq. (4): t 1 1 t 2 qt k2 qe qe
(4)
where qe and qt are the same as described above, and k2 (g/mg.min) is the rate constant of the pseudo-second order adsorption process. The values of k2 and qe can be determined from the
slope and intercept of the “ t/qt vs t ” plots, respectively (Fig.
6b and 6b’). The parameters as obtained from the linearized plots are given in Table 1. As observed, the calculated adsorption capacity qe(cal) for pseudo-second-order model was close to the experimental adsorption capacity qe(exp) for various CR concentrations confirming the applicability of pseudo- second-order model compared to pseudo-first-order model. These results were also confirmed by the higher regression coefficient (R2) values for pseudo-second-order model compared to pseudo-first order model (Table 1). Thus, it further verified that the chemisorption between both adsorbents and CR may be one of the principle pathways for CR adsorption..The adsorption process happened due to the valence forces between the CR molecules and adsorbent, ie, these amino and hydroxy groups in the CCS or CCS/BT composite can form hydrogen bonds with the O, N, and S atoms in CR molecules.
Fig. 6 goes here. 3.4. Effects of inorganic sodium salt and ionic strength Sodium salt is often used as a stimulator in dying industries. Effect of inorganic sodium salt (0.1 mol/L) on CR adsorption was presented in Fig.7a. The presence of HCO3- and PO43- ions almost had no influence on CR adsorption by CCS, while decreased CR adsorption by the CCS /BT composite. These results can be explained according to their pH-dependence. The addition of HCO3- and PO43- enhanced the pH values of CR solutions. As presented in the results of pH effect in Section 3.2, the CCS exhibited a low pH-dependence, CCS still showed high removal toward CR
although the pH value of CR solution increased. However, the increased pH value weakened the removal toward CR due to the high pH-dependence of CCS/BT composite, as mentioned in Section 3.2. For both CCS and CCS/BT composite, the presence of SO42- and Cl- almost had no influence on CR adsorption. This trend may be attributed to the constant pH value. To further investigate the effect of inorganic salts, like SO42- and Cl- on CR adsorption, various NaCl concentrations were applied to conduct CR adsorption experiments. The results in Fig.7b showed increasing NaCl concentration (ionic strength) almost had no effect on CR adsorption by CCS and CCS/BT composite. Previous studies have shown that if the electrostatic attraction is the main adsorption mechanism, the ionic strength has a significant negative effect on the adsorption process [20−21]. Thus, our results indicated that the electrostatic interaction between CR and CCS (or CCS/BT composite) was not the main interaction in CR adsorption, and other interactions, ie, chemisorpton exhibited in CR adsorption. Fig. 7 goes here.
3.5. Adsorption isotherm In the present study, in order to investigate the adsorption isotherms of CR on the CCS and CCS/ BT composite, the Langmuir and the Freundlich models were employed to simulate the experimental data. The Langmuir isotherm is expressed as follow: Ce C 1 e qe Qb Q
(5)
where Q is the maximum amount of adsorption with complete monolayer coverage on the adsorbent surface (mg/g) and b is the Langmuir constant, which is related to the
energy of adsorption (L/mg). From the linear plots of Ce/qe against Ce, Q and b (Fig. 8a and 8a’) values can be calculated from the slope and intercept, respectively. Besides, the essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter, RL, that is defined by the following equation: RL
1 (1 bC0 )
(6)
where b and C0 are the same as defined before. After calculation of RL from the above equation, the nature of the adsorption process could be evaluated to be either unfavorable (RL > 1), linear (RL = 1), or favorable (0 < RL < 1) [22]. All RL values obtained were listed in Table 2. The Freundlich isotherm equation is expressed in Eq. (7):
log qe log K f
1 log Ce n
(7)
where Kf [(mg/g)(L/mg)1/n] and n are Freundlich constants related to adsorption capacity and heterogeneity factor, respectively. Kf and n values can be calculated from the intercept and slope of the linear plots between logCe and logqe (Fig. 8b and 8b’). The isotherm constants and correlation coefficients (R2) were calculated and listed in Table 2. By comparing R2, it was found that the CR adsorption using CCS and CCS/BT composite was described by the Langmuir isotherm model due to higher R2 in comparison with Freundlich isotherm model. The applicability of Langmuir isotherm suggested the monolayer coverage of the dye on the surface of CCS and CCS/BT composite. And it was found that increasing temperature enhanced the CR adsorption onto CCS and CCS/BT composite, suggesting the CR adsorption was favored at higher temperatures. The maximum adsorption capacity (Q) of the CCS/BT composite was obtained at 500 mg/g at natural pH value and 298 K. Although the CS
amount in the CCS/BT composite was obviously lower than pure CCS, it showed higher adsorption capacity than CCS (405 mg/g), thus the immobilization of crosslinked chitosan in low-cost clay can reduce the cost of adsorbents effectively. The higher adsorption capacity for the CCS/BT composite may be attributed to the intercalation of CCS in bentonite. The increased layer spacing facilitated the CR adsorption onto the CCS/BT composite, resulting in an increasing adsorption capacity. Besides, the RL values were between 0 and 1 (Table 2), indicating that CCS/BT composite and CCS were favorable adsorbents for CR adsorption. When compared with previously reported adsorbents [23-28], the proposed CCS/BT composite gave a relatively high adsorption capacity for CR (Table 3). The above results also further confirmed that CCS/BT composite was favorable for CR removal. Fig. 8 goes here.
3.6. Thermodynamic parameters Thermodynamic parameters including change in enthalpy (△H ), gibbs free energy (△G ), and entropy (△S ) of the adsorption were determined from the experimental data (500 mg/L) by the following equations: KL G RTLnK L LnK L
S H R RT
Cs Ce
(8)
(9) (10)
where KL is the adsorption equilibrium constant, Cs is the amount of CR adsorbed per mass of CCS/BT composite or CCS (mg/g ), C e is the CR concentration in solution at equilibrium (mg /L), R is the universal gas constant, T is temperature (K). The values of △H calculated from the slope and intercept of the plot of LnKL vs. 1/T were
summarized in Table 4. Negative △G means spontaneous adsorption of CR by CCS/BT composite and CCS in the 25–55℃ range. Positive △H suggested that the adsorption of CR by CCS/BT composite and CCS was endothermic, which was consistent with the increased adsorption of CR at high temperatures. The positive value of △S indicated the increased randomness at solid/solution interface during the adsorption process. 3.7. Possible adsorption mechanism Based on the results described above including FTIR analysis, kinetics, and the effects of CR solution as well as inorganic sodium salt and ionic strength, we propose that the mechanism controlling CR adsorption onto the CCS was mainly the chemisorption between CR and CCS. The mechanism controlling CR adsorption onto the CCS/BT composite included the chemisorption, ie, the formation of hydrogen bonds between CR and CCS/BT composite and the electrostatic interaction between CR and CCS/BT composite, but the chemisorption predominated during the process of CR adsorption.
4. Conclusion In this study, the adsorption of CR by the CCS and CCS/BT composite from aqueous solution was investigated. Effects of pH, contact time and ionic strength, as well as the kinetics and isotherm of the CR adsorption, were determined. Adsorptive behavior of CR using the CCS and CCS/BT composite can be well described by the Langmuir model and pseudo-second-order model. The CCS/BT composite showed a better adsorption capacity for CR over CCS although the CCS/ BT composite exhibited 30% of CCS weight content. The adsorption of CR by the CCS was mainly controlled by chemisorption while the adsorption of CR by the CCS/BT composite
was controlled by chemisorption and the electrostatic attraction. As an effective and low-cost adsorbent for CR, the CCS/BT composite was expected to have a promising future for the dye-polluted water purification.
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Figure captions Fig. 1 Molecular structure of Congo red (CR) Fig. 2 XRD patterns of (a)CCS, (b) CR loaded CCS, (c) CCS/BT composite and (d) CR loaded CCS/BT composite Fig. 3 FTIR spectra of (a) CCS, (b) CR loaded CCS, (c) CCS/BT composite and (d) CR loaded CCS/BT composite Fig. 4 Effect of pH value of CR solution on CR adsorption and the pHzpc of the CCS and CCS/ BT composite Adsorption conditions: adsorbent dosage: 40 mg; at 298 K and pH (2–11); Contact time: 100 min; Dye concentration: 200 mg/L. For the study about the pHzpc of adsorbents Adsorption conditions: adsorbent dosage: 40 mg; at 298 K and pH (2–11); Contact time: 300 min; NaCl concentration: 0.1mol/L. Fig. 5 Effect of contact time on CR adsorption by (a) CCS and (b)CCS/BT composite Adsorption conditions: adsorbent dosage: 40 mg; at 293 K and natural pH; Contact time: 5–400 min; Dye concentration: 100, 200, 300 and 500 mg/L. Fig. 6 Adsorption kinetic models of CR onto (a and b) CCS and (a’ and b’) CCS/BT composite Adsorption conditions:adsorbent dosage: 40 mg; at 293K and natural pH; Contact time: 5–400 min; Dye concentration: 100, 200, 300 and 500 mg/L. Fig .7 Effects of (a) inorganic sodium salt and (b) ionic strength on CR adsorption
Adsorption conditions:adsorbent dosage: 40 mg; at 293 K and natural pH; Contact time: 360 min; Dye concentration: 100 mg/L. Fig. 8 Langmuir and Freundlich isotherm plots for CR adsorption by (a and b) CCS and (a’ and b’) CCS/BT composite Adsorption conditions: adsorbent dosage: 40 mg; at natural pH; Contact time: 360 min; Dye concentration: 100–800mg/L.
Fig.1
Fig.2
Fig.3
100
2
(a)
CCS CCS/BT composite
1
80
CCS CCS/BT composite
60
pHf-pHi
Removal/%
(b) 0
40
-1 -2 -3
20
-4 -5
0 4
5
6
7
8
9
pH value of CR solution
Fig.4
10
11
2
4
6
8
pHi
10
12
100
80
80
Removal/%
Removal/%
100
60 100 mg/L 200 mg/L 300 mg/L 500 mg/L
(a) 40
60
100 mg/L 200 mg/L 300 mg/L 500 mg/L
(b)
40
20
20
0
0 0
50
100
150
200
250
Contact time/min
Fig.5
300
350
400
450
0
50
100
150
200
250
Contact time/min
300
350
400
450
2.4 2.5
(a) 2.0
100 mg/L 200 mg/L 300 mg/L 500 mg/L
(a')
100 mg/L 200 mg/L 300 mg/L 500 mg/L
2.0
log(qe-qt)
log(qe-qt)
1.6 1.5
1.2
1.0
0.8
0.5
0.4
0.0
0.0 0
50
100
200
150
250
0
40
80
120
160
t/min
240
280
t/min 3.5
3.5
100 mg/L 200 mg/L 300 mg/L 500 mg/L
3.0
2.5
2.0
1.5
(b')
100 mg/L 200 mg/L 300 mg/L 500 mg/L
3.0
(b)
t/qt/min/mg/g
2.5 t/qt/min/mg/g
200
2.0
1.5
1.0
1.0
0.5
0.5
0.0
0.0 0
100
200
300
t/min
Fig.6
400
0
100
200
300
t/min
400
320
100
100
CCS CCS/BT composite
(b)
(a)
80
80
CCS CCS/BT composite Removal/%
Removal/%
60
40
20
60
40
20
0
0
Zero
NaCl
Na2SO4
Na2CO3
Na3PO4
Type of inorganic sodium salt
Fig.7
0.0
0.1
0.2
0.3
0.4
NaCl concentration/mg/L
0.5
0.8
1.2
298K 308K 318K 328K
1.0
(a)
(a')
0.6 Ce/qe/g/L
0.8 Ce/qe/g/L
298K 308K 318K 328K
0.6
0.4
0.4
0.2 0.2
0.0
0.0 0
100
200
300
400
500
0
100
200
Ce/mg/L 3.0
298K 308K 318K 328K
2.8
(b)
300
400
Ce/mg/L 298K 308K 318K 328K
2.8
(b')
2.6
logqe
logqe
2.6
2.4
2.4
2.2
2.2
2.0
2.0 -2
-1
0
logce
Fig.8
1
2
3
-1
0
1
logce
2
3
Table 1 Pseudo-first-order and Pseudo-second-order models parameters for CR adsorption by the CCS and CCS/BT composite Pseudo-first order model Adsorbent
C0/ mg/L
qe,exp/mg/g
Pseudo-second order model
R2
k1/min-1
qe,cal/mg/g
R2
k2/g/mg/min
qe,cal/mg/g
100
124.7
0.8537
0.0011
6.6
1.000
0.0142
124.2
200
241.7
0.9768
0.0066
93.6
0.9997
0.0003
253.2
300
325.1
0.9796
0.0044
133.8
0.9990
0.0002
336.7
500
408.3
0.9805
0.0042
177.3
0.9988
0.0001
416.7
100
123.9
0.9719
0.0064
18.4
0.9999
0.0022
125.3
CCS/BT
200
245.3
0.9899
0.0047
75.2
0.9988
0.0003
249.4
composite
300
313.8
0.9943
0.0033
109.4
0.9990
0.0002
319.5
500
410.8
0.9830
0.0031
171.7
0.9975
0.0001
423.7
CCS
Table 2
Langmuir and Freundlich models parameters for CR adsorption by the CCS and CCS/BT composite Langmuir
Adsorbent
Temperature/K
model
Freundlich model
R2
RL
b/L/mg
Q/mg/g
R2
Kf [(mg/g)(L/mg)1/n]
n
298K
0.9995
0.0067-0.0514
0.1847
404.9
0.9846
199.5
6.474
308K
0.9984
0.0084-0.0637
0.1469
485.4
0.9122
245.2
8.489
318K
0.9995
0.0035-0.0271
0.3596
625.0
0.8397
284.1
6.515
328K
0.9962
0.0046-0.0358
0.2695
740.7
0.9523
364.5
7.348
298K
0.9945
0.0241-0.1647
0.0507
500.0
0.9652
145.7
4.864
CCS/BT
308K
0.9937
0.0175-0.1250
0.0700
578.0
0.9901
186.0
5.116
composite
318K
0.9976
0.0071-0.0541
0.1747
641.0
0.9632
233.0
5.115
328K
0.9976
0.0015-0.0117
0.8411
787.4
0.8228
366.6
5.647
CCS
Table 3 Comparison of maximum monolayer adsorption capacity of various absorbent for CR Adsorbent
Adsorption capacity/ mg/g
References
Hydroxyapatite/chitosan composite
769
[23]
CS/carbon nanotubes
450.4
[24]
Activated carbon/surfactant (DDAC)
769.23
[25]
Chitosan hydrogel core–shell
beads(CSBN1a)
199.98
[26]
Graphene oxide/chitosan fibers
144.93
[27]
PCNT/Mg(Al)O nanocomposites
1250
[28]
CCS
404.9
This study
CCS/BT composite
500.0
This study
Table 4
Thermodynamic parameters for CR adsorption onto CCS and CCS/BT composite
Adsorbents
CCS
CCS/BT composite
T/K
△G /kJ/mol
298
-1.70
308
-2.73
318 328
-6.33
298
-2.62
308
-3.20
318 328
-5.54
△H /kJ/mol
△S /KJ/mol/K
66.9
0.228
49.1
0.172
-14.0
-7.65