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chitosan–Fe (III) composite for the removal of anionic azo dyes from wastewater: Equilibrium, kinetics and thermodynamics
Accepted Manuscript Title: Utilization of Diatomite/chitosan-Fe (III) Composite for the Removal of Anionic Azo Dyes from Wastewater: Equilibrium, Kinetics and Thermodynamics Author: Liuchun Zheng Chenggang Wang Yuehong Shu Xiaomin Yan Laisheng Li PII: DOI: Reference:
S0927-7757(14)00934-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.12.015 COLSUA 19595
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
2-10-2014 1-12-2014 8-12-2014
Please cite this article as: Liuchun Zheng, Chenggang Wang, Yuehong Shu, Xiaomin Yan, Laisheng Li, Utilization of Diatomite/chitosan-Fe (III) Composite for the Removal of Anionic Azo Dyes from Wastewater: Equilibrium, Kinetics and Thermodynamics, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.12.015 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.
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*Graphical Abstract (for review)
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*Highlights (for review)
Highlights: Fe(III)/Cs@Dia was confirmed as an excellent absorbent for anionic azo dyes.
Pseudo-second-order kinetic model showed a better-fit to the experimental data.
The equilibrium data fitted better with the Langmuir model.
Adsorption of 2GL dye onto adsorbent was exothermic in nature.
Adsorption mechanisms mainly involved electrostatic interaction and complexation.
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Utilization of Diatomite/chitosan-Fe (III) Composite for the Removal of Anionic Azo Dyes from Wastewater: Equilibrium, Kinetics and Thermodynamics
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Liuchun Zheng**, Chenggang Wang, Yuehong Shu, Xiaomin Yan, Laisheng Li*
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School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
Corresponding author: School of Chemistry and Environment, South China Normal University, Higher Education
Mega Center, Guangzhou 510006, PR China.
Tel: +86 20 39310185/39310250; fax: +86 20 39310187
E-mail address: [email protected] (L.S. Li), [email protected]
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Abstract A novel diatomite/chitosan-Fe(III) composite (Fe(III)/Cs@Dia) was synthesized successfully to use as an adsorbent for the removal of anionic azo dye. The compostie was further
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characterized by SEM, FTIR, XPS, and 13C/29Si NMR, and then the adsorption batch experiments were carried out as a function of initial dye concentrations, pH values, contact time and
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temperatures to determine the adsorption capacities of Fe(III)/Cs@Dia. Compared with several
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typical isotherm models, the adsorption isotherms were all best-fitted by the Langmuir model,
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with a maximal 2GL dye uptake of 1250 mg g-1 at pH 6 and 298K. In addition, kinetic studies followed the pseudo-second-order model and thermodynamic studies revealed that the uptake was
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exothermic in nature. The adsorption mechanisms were proposed with observation that electrostatic interaction and complexation played the key roles in adsorption process. Moreover,
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the composite also showed favorable adsorption properties for other four anionic azo dyes (direct
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yellow R, congo red, methyl orange and direct red 23).
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Keyworks: Anionic azo dye; Composite; Adsorption mechanism; Isotherm; Kinetic.
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1 Introduction
Large amounts of dyes wastewater are produced in many industries, such as textiles, paper, tanneries, clothing, printing, paint, etc. [1]. For example, it is estimated that Guangdong Province
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in China produced 950 million tons of dyeing wastewater in 2012. Unfortunately, most of dyes wastewater contains undesirable color, toxicant and carcinogen. If discharged into the aquatic
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environment above the level that the nature can eliminate, it would pose a great threat to aquatic
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biota and humans [2]. In general, dyes can be classified anionic (direct, acid and reactive dyes),
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cationic (basic dyes) and non-ionic (disperse dyes and vat dyes) [3]. Among them, azo dyes (anionic) with existence of nitrogen-nitrogen double bonds, are considered to be the largest and
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most versatile class of organic dyes [4, 5], but most difficult is to be degraded due to their complicated aromatic structure and poor biodegradability [6]. Therefore, it is s necessary to
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provide suitable technology for the wastewater treatment.
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Conventional technologies, including membrane process, coagulation/flocculation, chemical oxidation and electrolysis, have been employed successfully for the removal of azo dye from wastewaters [5, 7]. In comparison with other techniques, adsorption is considered to be an
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efficient and versatile method, due to various adsorbents having good selectivity toward different types of wastewater [8, 9]. It is well known that agricultural residue [10], clay mineral [11] and waste biopolymer [12] are relatively simple, low-cost, locally available and effective adsorbents. Among these adsorbents, diatomite is mainly accumulation of amorphous hydrated silica, and possesses good adsorptive property for dyes due to their high specific surface area [13]. Chitosan is the product obtained from the deacetylation of the natural biopolymer chitin, and also could
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provide distinctive adsorption functions because of a great number of active hydroxyl (-OH) and amino (-NH2) groups [14]. However, the adsorption capacities of diatomite and chitosan are still unsatisfactory. These could be explained that the surface silanols (Si–OH) of diatomite frustules
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do not have enough strong adsorption affinities, while chitosan as a non-selective adsorbent requires certain modifications to improve its selectivity for target species, and has some chemical
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and mechanical weaknesses. Recently, chitosan-Fe(III) and montmorillonite-chitosan composites
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have been reported to be highly efficient for dye removal [15-17], owing to higher specific surface area, better chemical/mechanical stabilities, and more structural properties. However, to the best
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of our knowledge, little research had been done for diatomite/Chitosan-Fe(III) composite.
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Moreover, the adsorption mechanism of chitosan-Fe(III) composites is still a controversial topic [18], though the chelation between dye and Fe(III) center in chitosan-Fe(III) was proved to be a
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main mechanism [1]. If chitosan-Fe(III) continued to be loaded on diatomite, it would be unclear
exploration.
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how the dye is adsorbed in the composite, and the mechanism also would be require futher
In this study, a diatomite/chitosan-Fe (III) composite was synthesized, characterized, and
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applied for removal of anionic azo dyes, and the effects of several factors, such as initial dye concentrations, pH values, contact time and temperatures were investigated. Thereafter, the equilibrium isotherm data were fitted with typical isotherm models, including Langmuir, Freundlich, Redlich-Peterson, Koble-Corrigan, Tempkin, Dubinin-Radushkevich and Generalized isotherm models. Four kinetic models (pseudo first-order, second-order, Elovich and Double-constant kinetic models) were used to evaluate the mechanism of adsorption. Besides the
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adsorption mechanisms, the thermodynamics of the adsorption processes were also revealed, and regeneration of the adsorbent was used repeatedly up to five cycles for testing its reusability.
2 Materials and methods
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2.1 Chemicals
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Ferric chloride hexahydrate (FeCl3•6H2O), Glutaric dialdehyde (C5H8O2), and ethanol were
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purchased from Damao Chemical Reagent Co. Ltd. (Tianjin, China). Chitosan (deacetylation
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degree≥95%) and diatomite were all analytical grade and obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Direct orange 2GL dye (2GL), direct yellow R (DY R), congo red
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(CR),methyl orange (MO) and direct red 23 (DR 23) were purchased from Guangzhou Chemical
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Reagent Co. Ltd. (Guangzhou, China) (Table 1). All chemicals were used as received.
2.2 Synthesis of Fe (III)/Cs@Dia
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The adsorbent was prepared by a modified literature procedure [1, 19]. First, chitosan powder (1.0 g) was dissolved in 0.2 M FeCl3 aqueous solution (50 mL). After magnetically stirred at room
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temperature for 4 h and dried, the mixture was named as Fe(III)/Cs. Subsequently, 1.5 g diatomite was added into the wet mixture and dispersed sufficiently to obtain a complex precipitation. Then, the complex was washed with ethanol to remove residual FeCl3, and dried at 80 °C. Further, the sample was placed in an ethanol solution in contact with 5% glutaraldehyde for 2 h for the Schiff’s reaction. At last, the mixture was washed with ethanol in order to remove the unreacted glutaradehyde, and dried at 80°C until constant weight to get the final product (labeled as Fe(III)/Cs@Dia). 5 / 25 Page 7 of 41
2.3 Characterization
The general morphology of the products were characterized by the scanning electron microscope (SEM, JSM-6510). FTIR spectra were obtained on an FTTR spectrophotometer using KBr discs to
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prepare the samples. X-ray photoemission spectroscopy was recorded on a Thermo Fisher Scientific standard and monochromatic source (Al Kα) operated at 150 W (15 kV). The solid-state
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CP/MAS 13C and 29Si NMR spectra were recorded on Bruker AVANCE 400 spectrometer at the
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frequency of 100 and 79.49 MHz, respectively. The delay time of 13C MAS NMR was 3 s and the
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proton 90°C pulse time was 6μs. A single-pulse sequence with 90-degree pulse in 29Si MAS NMR was 4μs, and the delay time of 120 s were used. Dye concentrations were analyzed using a
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2.4 Batch adsorption experiments
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UNICO UV-2800H spectrometer.
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For the kinetic study, 1400 mg L-1 dye solutions (50 mL) were agitated with 0.050 g adsorbent at 25 °C for predetermined intervals of time, and the influence of pH values (6, 8, 10 and 12) was investigated for adsorption of 2GL dye. After filtration, the concentrations of 2GL dye in the
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aqueous solutions were measured. For the equilibrium adsorption studies, a fixed mass of Fe(III)/Cs@Dia (0.01 g) was added into a series of 50 mL serum bottles with a 10 mL solution containing a known amount of the respective dye (varying from 600 to 1400 mg L-1). The initial pH value of the dye solution was adjusted by adding 0.1 M HCl or 0.1 M NaOH solution. The serum bottles were agitated for 24 h in a shaking thermostatic bath with the temperature maintained at 25 °C. After centrifugation, the concentration of dye solution was determined. The
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amount of dye adsorbed per unit mass of the adsorbent was evaluated by using the mass balance equation.
qt =
(C0 - Ct )V W
`(1)
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where qt (mg g-1) is the amount adsorbed per gram of adsorbent at time t (min); C0 and Ct are the
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initial concentration and the concentration at time t (mg L-1); W (g) is the mass of the adsorbent
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used, and V (L) is the initial volume of the dye solution.
3. Results and discussion
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3.1 Characterization
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3.1.1SEMs
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The SEMs of chitosan, Fe(III)/Cs, diatomite, and Fe (III)/Cs@Dia were shown in Fig.1. It illustrated that the surfaces of chitosan and Fe(III)/Cs were amorphous (Fig.1a, b), and the
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diatomite provided high porosity and permeability (Fig.1c). After loaded by Fe(III)/Cs, the diatomite surface was nearly covered, but its pore structure still maintained, which provided a
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good possibility for dyes to be trapped and adsorbed. (Fig.1d).
3.1.2 FTIR
The FTIR spectra of chitosan, Fe(III)/Cs, diatomite, Cs/Dia and Fe(III)/Cs@Dia were presented in Fig.2. The spectrum of diatomite showed the bands at 1085, 786 and 1080 cm-1 , which were identified as Si-O-Si stretching vibrations, Si-OH groups and Si-O-Si groups, respectively [20]. The absorbencies associated with the chitosan were as follow: 3350 cm-1(O-H stretch and N-H stretch), 2880 cm-1(C-H stretch), 1650 cm-1(amide II band, C=O stretch,), 1590 cm-1(N-H 7 / 25 Page 9 of 41
bending), 1390 cm-1 (amide III band, C-N stretch), 1418 cm-1and 1326 cm-1(C-H bending), 1260 cm-1(C-N stretch), 1160cm-1(bridge C-O-C stretch) and 1010 cm-1 (Hydroxyl deformation vibration) [21, 22]. Cs/Dia had a weak band at 3600 cm-1 indicating hydroxyl groups reacted, and
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the weak peak at 1500 cm-1 corresponded to the -NH2 or -NH3+ group of chitosan. Meanwhile, a distinct peak at 1060 cm-1 was identified as the C-H and C-O bending of chitosan, or Si-O-Si
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group asymmetric stretch of diatomite [23]. The broad band of Fe(III)/Cs at 3670 cm-1 was
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ascribed to the multiple overlapping peaks due to O-H and N-H stretching vibration, indicating that complexation occurred among -NH2 group, -OH group and Fe(III). Compared with the
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spectrum of chitosan, N-H bending at 1590 cm-1 moved to 1610 cm-1 and its intensity decreased,
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which further revealed that the -NH2 and -OH groups involved in the coordination reaction with Fe(III) [24-26]. These results were also consistent with Bhatia and Ravi’s view, which suggested
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the Fe(III)/Cs complex was either penta or hexacoordinated, complexed through O and N from the polymer [27]. As a result, the spectrum of Fe(III)/Cs@Dia combined the spectrum of Cs/Dia and
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Fe(III)/Cs, and also confirmed that Fe(III)/Cs was successfully loaded on diatomite.
3.1.3 13C and 29Si NMR
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Fe(III)/Cs@Dia and Cs/Dia were characterized by
13
C and
29
Si NMR (Fig.3). In general, the
chemical shift values of C=O, C1-6 and CH3 in methyl and carbonyl groups of pure chitosan, are 173.8 ppm (C=O), 104.6 ppm (C1), 83.6 ppm (C4), 76.1 ppm (C5), 74.2 ppm (C3), 61.4 ppm (C6), 55.8 ppm (C2) and 23.3 ppm (CH3), respectively [28]. In 13C NMR spectra, the series of peaks of Fe(III)/Cs@Dia and Cs/Dia were also found at 175 ppm (C=O), 104.8 ppm (C1) and 23.6 ppm (CH3), which were similar to those of the monomeric form of pure chitosan. As diatomite was
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added, a very weak splitting of C-4 signal at 83 ppm was showed in Cs/Dia, but it disappeared in Fe(III)/Cs@Dia. Similar results appeared in the C3 and C2 peaks, which indicated that C5 and C6 peaks were enlarged and overlapped with the neighboring C3 and C2. Especially, a sharp peak at
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132 ppm and a wide peak at 148.9 ppm in Fe(III)/Cs@Dia were associated with the double ethylenic bond (C=C), due to the glutaraldehyde reaction [21]. The diatomite spectrum always had
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three signals at -86, -95 and -110 ppm, attributing to Q2 (Cyclic Si-O tetrahedron), Q3 (Layered 29
Si
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Si-O tetrahedron) and Q4 (Shelf-like Si-O tetrahedron) environments, respectively [29]. In
NMR spectra, the only obvious peaks of Fe(III)/Cs@Dia and Cs/Dia were located at 108.8 ppm,
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which implied that both of them only had shelf-like Si-O tetrahedron, whereas other peaks
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disappeared. Moreover, the peak of Fe(III)/Cs@Dia was slightly wider than that of Cs/Dia due to spectral line broadening, which indicated that the magnetic susceptibility increased and nuclear
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relaxation time became shorter as the concentration of Fe(III) increased.
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3.1.4 XPS
The peaks of high-resolution XPS spectrum (Fe2p, O1s and N1s) of Fe(III)/Cs@Dia were provided in Fig.4. The binding energy of Fe2p photoelectrons in the standard spectrum of FeCl3
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was reported at 711.2eV [30], but the peaks of Fe(III) in Fe(III)/Cs@Dia were at 711.5 and 725.1 eV, with shakeup satellite at 715.5 and 732.2 eV, indicating that the complexing reaction occurred between chitosan and Fe(III) [1]. In the spectrum of O1s, there were two broad peaks, one at 531.4 eV and the other one at 532.8 eV. Meanwhile, three lower peak appeared at 399.2 eV, 399.8 eV and 401.7 eV were observed in the spectrum of N1s. These results were consistent with Shen et al’s report for O1s spectrum [1], the peak with a binding energy of 531.4 eV was from -OH
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oxygen in chitosan which was chelated with Fe(III), in contrast to the peak at 532.8 eV for -OH oxygen without chelation. As for N1s spectrum, the peaks at 399.2, 399.8 and 401.7 eV represented the characteristic peaks of the -NH2 nitrogen, the nitrogen in -NH2 and -NH3+ group in
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chitosan, respectively. The lowest peak at 406.6 eV was from the -N=CH- bond, indicating
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successful cross-linking between chitosan and glutaraldehyde.
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3.2 Effect of initial dye concentration
According to FTIR, the functional groups of Fe(III)/Cs@Dia successfully combined those of
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diatomite, chitosan and Fe(III). To determine adsorption properties, 2GL dye uptake of these
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materials was studied under different initial concentrations, ranging from 600-1400 mg L-1. Fig.5 showed that the amount of 2GL dye adsorbed on all the materials increased with an increase of the
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dye concentrations, and then increased relatively slowly from 1000 to 1400 mg L-1 due to the limited number of active sites. Among these adsorbents, Fe(III)/Cs@Dia showed the best
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adsorption capacities. With an initial 2GL dye concentration of 1400 mg L-1, the adsorption capacities were found to be 1222 mg g-1 for Fe(III)/Cs@Dia, 211.9 mg g-1 for Cs/Dia, 86.29 mg g-1 for diatomite, 878.0 mg g-1 for Fe(III)/Cs, and 260.9 mg g-1for chitosan, respectively. Based on
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these results, Fe(III)/Cs@Dia as the best adsorbent was used in the further experiments.
3.3 Effect of initial pH
The effect of pH on 2GL dye removal was studied by varying the initial solution pH at 6, 8, 10 and 12, with a contact time of 11 hours in each batch equilibrium adsorption (Fig.6). It was obvious that the adsorption capacities of Fe(III)/Cs@Dia increased with the contact time, and reached equilibrium at 480, 360, 240 and 60 min, respectively. However, when the initial pH 10 / 25 Page 12 of 41
increased from 6 to 12, the removal capacities declined drastically. In the acidic solution, the protonation of -SO3- groups could shift the -SO3Na of 2GL dye molecule to form -SO3H, resulting in formation of -NH2+SO3- or =NH+SO3- [11, 31]. Moreover, Huang et al. [32] reported that the
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adsorption process was an electrostatic interaction between anionic azo dye and the protonated amine groups of chitosan on the adsorbent [1]. Therefore, more protons on the adsorbent surface,
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the better excellent adsorption capacity in low pH values. However, with pH at 8, 10 and 12, the
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protonation of -SO3- groups and amine groups disappeared. Meanwhile, OH- ions would also compete with the 2GL dye anions for the adsorption sites, resulting in a decrease of 2GL dye
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removal. Under the alkali solution, the complexation between 2GL dye with Fe (III) played the
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dominant role in adsorption [13] (Fig.7).
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3.4 Adsorption kinetics
Kinetic models were used to examine the rate of the adsorption process and propose potential
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rate-controlling step. The batch experiments were carried out using variable initial adsorbate concentration, adsorbent dose, particle size, agitation speed, pH values and temperatures along with different adsorbent and adsorbate types [33]. In this study, the pseudo-first-order (including
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linear and non-linear forms), pseudo-second-order (including linear and non-linear forms), Elovich and Double-constant model were calculated using pH values with the experimental data (Fig.6). The linear and non-linear forms of the pseudo-first-order rate equations are Eq. (2) and (3), respectively. [33,34]:
ln(qep - qt ) = ln qep - k1t
(2)
qt = qep (1 - e- k1t )
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where qep and qt (mg g-1) refer to the adsorbent amount at equilibrium and time t, respectively. k1 is the rate constant. The linear and non-linear forms of the pseudo-second-order rate equations are Eq. (4) and (5),
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respectively. [33,34]:
t 1 t = 2 + qt k2 qep qep
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k2 qep2 t
(5)
1 + k2 qept
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qt =
(4)
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k2 is rate constant.
Elovich equation is a rate equation based on the adsorption capacity commonly and rearranged to
1
ln( )
1
ln t
(6)
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qt
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a linear form Eq. (6) if simplified by assuming αβ ≥ t[35].
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where α (mg g-1 min-1) is the initial adsorption rate and β (g mg-1) is the desorption constant related to the extent of the surface coverage and activation energy for chemisorption. The slope
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and intercept of the plot of qt versus ln t result in the estimation of the kinetic constants.
Double-constant kinetic model is an empirical equation derived from the classical Freundlich isotherm and mainly suitable for more complex reaction kinetics. It is given as Eq. (7):
ln qt k 0 ln t b
(7)
where k0 is the Double-constant kinetic constants, and b is a adsorption rate constant associated with adsorption activation energy.
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The calculated kinetic parameters of Fe(III)/Cs@Dia were given in Table 2. The results showed that the linear form of pseudo-second-order model described the adsorption kinetics better according to their highest correlation coefficient (R2 > 0.996), when compared with the linear
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forms of the pseudo-first-order, Elovich and Double-constant models under four different pH values. Further, for the non-linear forms of pseudo-first-order and pseudo-second-order models, it 2
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was found that the R value for pseudo-second-order model was obviously higher and it is
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inappropriate to apply pseudo-first-order model expression. Moreover, a similar observation was that the qep values obtained from the pseudo-second-order model showed to be nearer the experimental
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data (qexp), which indicated that the overall process was controlled by chemisorption [36], and
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involved valence forces through sharing or exchange of electrons between dye and adsorbent [3]. As can be seen from rate constant k2, 2GL dye was adsorbed by Fe(III)/Cs@Dia faster at higher
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pH, which revealed that the adsorption equilibrium can be attained quickly because of complexation. However, the formation of -NH2+SO3- or =NH+SO3- between 2GL dye molecule
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and adsorbent at pH 6 would make the adsorption process relatively slower [3].
3.5. Adsorption isotherms
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Adsorption isotherm was performed to study the interaction between anionic azo dyes and the adsorbent, and the adsorption uptakes in different initial dye concentrations at given pH values and temperatures [37]. Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich, Redlich-Peterson, Koble-Corrigan and Generalized isotherm models were applied to describe the adsorption isotherm for 2GL dye onto Fe(III)/Cs@Dia. The Langmuir model is valid for a saturated monolayer adsorption onto the homogeneous surface of adsorbent, and the adsorption energy is a
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constant with no migration of adsorbate molecules in the surface plane when maximum adsorption capacity occurs. The Langmuir equation can be represented as Eq. (8) [38]:
qep
=
Cep 1 + bqmax qmax
(8)
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Cep
where qeq and qmax are the equilibrium and maximum uptake capacities (mg g-1), respectively. Ceq
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is the equilibrium concentration (mg L-1), and b is the equilibrium constant.
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The Freundlich isotherm is an empirical equation used to describe heterogeneous systems. It is expressed in the following form Eq. (9) [38]:
1 ln Ceq + ln K F n
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ln qeq =
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where KF and n are the Freundlich constants.
(9)
The Tempkin isotherm model takes into account adsorbent-adsorbate interactions. The form of
qep BT ln AT BT ln Cep
(10)
RT . T is the absolute temperature in K and R is the universal gas constant (8.314 J bT
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where BT =
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the Tempkin equation is given as Eq. (10) [35]:
mol K-1). The constant bT is related to the heat of adsorption, AT is the equilibrium binding
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constant (L min-1) corresponding to the maximum binding energy. The Dubinin-Radushkevich isotherm model is used to describe the adsorption process of microporous adsorbents, and also applied to estimate the mean free energy adsorption (E) using Eqs. (11) - (13) [35].
qep Qm exp( K 2 )
RT ln(1
(11)
1 ) C ep
(12) 14 / 25 Page 16 of 41
where K (mol2 kJ-2) is a constant related to the adsorption energy, and Qm (mg g-1) is the maximum adsorption capacity.
E (kJ mol-1) can be calculated as:
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1 2K
(13)
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E=
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The Redlich-Peterson isotherm is an improvement over the Langmuir and Freundlich isotherms and contains three parameters. It can be described by Eq. (14) [35]:
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AC ep 1 + BC epg
(14)
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qep =
where A, B and g (0 < g <1) are the Redlich-Peterson parameters.
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The Koble-Corrigan model is a combination of the Langmuir and Freundlich isotherm models
[35]:
aCep n
(15)
1 + bCepn
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qep =
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and considered to be more highly nonlinear. It also has three parameters and given by Eq. (15)
where a, b and n are the Koble-Corrigan parameters.
The generalized isotherm is another combination of Langmuir and Freundlich isotherms. It depends on the value of cooperative binding constant (Nb) and described as Eq. (16) [35]:
log(
Qm - 1) = log KG - Nb log Cep qep
(16)
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where KG is the saturation constant (mg L-1); Nb is the cooperative binding constant; and Qm (mg g-1) is the maximum adsorption capacity and obtained from the Langmuir isotherm model.
Isotherm constants of seven adsorption isotherms were listed in Table 3. The highest value of
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coefficient of determination (R2) obtained indicated the Langmuir model displayed the best fit
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compared to other six isotherms, suggesting homogeneous surfaces of the composite and
monolayer coverage of 2GL dye onto the adsorbent. The Langmuir isotherm also could be used to
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estimate the adsorption capacities (qmax), and was found that adsorption capacities of
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Fe(III)/Cs@Dia were 833.3, 1250, 1000 and 1000 mg g-1 in range of 288-318K, respectively. Dubini-Radushkevich, Redlich-Peterson and Koble-Corrigan models also seemed to have good
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applicability due to higher value of coefficient. For Dubini-Radushkevich model, the maximum adsorption capacities (Qm) obtained were 820, 961.4, 989 and 1000 mg g-1, respectively. These
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were less than the value obtained (qmax) by the Langmuir isotherm model. The mean free energy
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adsorption (E) were more than 100 kJ mol-1, which were greater than 16 kJ mol−1, indicating that the adsorption of 2GL dye on adsorbent was by chemisorption. The exponent values (g) in Redlich-Peterson model were close to 1, which signified Langmuir model could explain the
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experimental equilibrium data instead of the Freundlich isotherm. In addition, it could be inferred that homogeneous uptake was the main mechanism in adsorption process from Koble-Corrigan model. In Table 4, The values of maximum adsorption capacities of Fe(III)/Cs@Dia for CR, MO and DR 23 were compared to those values reported for other composites, and Fe(III)/Cs@Dia exhibited very high adsorption abilities for these three dyes. Unfortunately, there were not any published results of DY R and 2GL dyes for the comparison of our research.
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3.6 Adsorption thermodynamics
Based on the constant values obtained using the Langmuir isotherm model, thermodynamic parameters, including enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) changes, were calculated
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using the following equations[33]:
K D = qep / Cep
cr
(17)
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G RT ln K D G H TS
S H R RT
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ln K D
(18) (19)
(20)
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where T is temperature in K. and KD is the distribution coefficient (mL g-1). The enthalpy change (ΔH) and the entropy (ΔS) can be calculated from a plot of ln KD vs. 1/T. The Gibbs free energy
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was determined at 288, 298, 308 and 318K, respectively. ΔG, ΔH and ΔS were obtained from Eqs.
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(17) - (20) and given in Table 5. Negative values of ΔG indicate the adsorption process was spontaneous, and the degree of spontaneity of the reaction decreased with increasing temperature. Negative value of ΔH indicated the exothermic nature of the uptake, reconfirming the previous
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results in the section regarding adsorption isotherm studies. And negative value of ΔS showed that the randomness decreased during the removal of 2GL dye on Fe(III)/Cs@Dia. 3.7 Regeneration/ reusability of Fe(III)/Cs@Dia Many reports suggest that NaOH solution is effective for desorption of the anionic dyes, while HCl or H2SO4 solution is used to recover the adsorption ability of adsorbent [44, 45]. Therefore, 0.5 mol L-1NaOH and 0.1 mol L-1 HCl aqueous solutions were chosen for the desorption and regeneration of 2GL dye-loaded Fe(III)/Cs@Dia, respectively. And the adsorption and desorption 17 / 25 Page 19 of 41
processes were repeated for five times. The results were showed in Table 6, and the recovery efficiency reached 96.32% even after the fifth cycle, which indicated that the adsorbent had the excellent reusability after five repetitions of the adsorption–desorption cycles.
ip t
3.8 Adsorption of other typical anionic azo dyes
cr
Anionic azo dyes, such as direct yellow R (DY R), congo red (CR), methyl orange (MO) and direct red 23 (DR 23) had similar structure with 2GL dye. Therefore, their adsorption mechanisms
us
mainly involved electrostatic interaction and complexation (Fig 7), and Langmuir model was also
an
applied to describe the adsorption isotherm for these dyes, and the parameters of isotherms were also calculated (Table 7). The results showed that the experimental adsorption data fitted very
M
well to the Langmuir isotherm, indicating that their adsorption also were monolayer coverage. The maximum adsorption capacities of MO, CR, DY and DR 23 were 769.2, 1111, 909.1 and 1429 mg
ce pt
4. Conclusions
ed
g-1, respectively. Thus Fe(III)/Cs@Dia was an excellent adsorbent for anionic azo dyes.
Fe(III)/Cs@Dia was proven to be a promising adsorbent for the removal of anionic azo dyes from wastewater. Moreover, electrostatic interaction forming -NH2+SO3- or =NH+SO3- under
Ac
acidic solution, and complexation between 2GL dye and Fe(III) in alkali conditions were found as the adsorption mechanism. The equilibrium sorption data agreed well with Langmuir isotherm model and the maximum adsorption capacities for 2GL dye were 833.3, 1250, 1000 and 1000 mg g-1 in range of 288-318K, respectively. For kinetics measurements, the pseudo-second-order model provides the best correlation, suggesting that the chemisorption was the rate-limiting step. The negative value of enthalpy change (ΔH) and free energy change (ΔG) indicated the exothermic and
18 / 25 Page 20 of 41
spontaneous nature of adsorption. Therefore, Fe(III)/Cs@Dia also showed the excellent adsorption performance for other three dyes, demonstrating the general applicability of the adsorbent to
ip t
anionic azo dyes.
Acknowledgments
cr
The authors are grateful for the financial support from the National High-Tech Research and
us
Development Program (No. 2013AA06A305), the National Natural Science Foundation of China (No. 21247004), the Science & Technology Office of Guangdong Province (No.
an
2012B031000016), and Foundation for Distinguished Young Talents in Higher Education of
M
Guangdong, China (2013LYM0018).
ed
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Figure captions Fig.1. SEM microgaraphs of the chitosan (a), Fe(III)/Cs (b), diatomite (c), and Fe(III)/Cs@Dia (d) Fig.2. FTIR spectra of the Cs/Dia, Diatomite, Fe(III)/Cs, Fe(III)/Cs@Dia and Chitosan
ip t
Fig.3. CP/MAS 13C and 29Si solid-state NMR spectra of Cs/Dia and Fe(III)/Cs @Dia
cr
Fig.4. XPS spectra of the Fe(III)/Cs@dia survey and high resolution XPS spectra of Fe2p, O1s
us
and N1s.
Fig.5. Effect of initial 2GL dye concentration on uptake capacities by different adsorbents
an
(adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K)
Fig.6. Kinetic adsorption of Fe(III)/Cs@Dia for 2GL dye in different pH values (adsorbent dosage:
M
1 g L-1, T: 298K)
ed
Fig.7. The proposed adsorption mechanism of 2GL dye onto Fe (III)/Cs@Dia Table 1 The general data of several types of anionic azo dyes used in this work
ce pt
Table 2 Calculated parameters for four kinetic models for the adsorption of 2GL dye on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, T: 298K) Table 3 Isotherm constants of various adsorption isotherms for the uptake of 2GL dye onto
Ac
Fe(III)/Cs@Dia at different temperatures
(adsorbent dosage: 1 g L-1, pH: 6.0)
Table 4 Maximum adsorption capacities for different adsorbents and dyes Table 5 Thermodynamic parameters for the uptake of 2GL dye on Fe(III)/Cs@Dia
Table 6 Batch adsorption-desorption cycles of 2GL dye Table 7 Parameters of Langmuir Isotherms for the adsorption of other anionic azo dyes on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K) 25 / 25 Page 27 of 41
Table(s)
Table 1 The general data of several types of anionic azo dyes used in this work
Molecular
Dye
Chemical structure
volume
Formula
λmax(nm)
ip t
(g mol -1)
327.3
CR
696.7
DY R
452.4
DR23
790.69
cr
MO
C32H22N6Na2O6S2
C16H10N2Na2O7S2
464
498
500
739.6
498 C35H25N7Na2O10S2
C26H16N5Na3O11S3
Ac
2GL
ce pt
ed
M
an
us
C14H14N3O3SNa
414
1/7 Page 28 of 41
Table 2 Calculated parameters for four kinetic models for the adsorption of 2GL dye on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, T: 298K)
Pseudo first-order (linear form)
Pseudo second-order (linear form)
qeq (mg g-1)
k1
R2
qeq (mg g-1)
6
1042
658.2
0.0085
0.9412
1111
8
826.0
364.5
0.0104
0.9511
833.3
10
572.5
230.3
0.0101
0.8353
588.2
12
185.6
35.49
0.0046
0.6880
pH
Experimental
k1
1042
914.7
0.0499
8
826.0
782.5
10
572.5
521.3
12
185.6
0.00004
0.9983
0.00008
0.9998
0.00011
0.9992
0.00096
0.9985
Pseudo second-order (non-linear form)
qeq (mg g-1)
k2
R2
985.8
0.00007
0.9185
0.0483
0.9198
836.4
0.00009
0.9728
0.0622
0.7482
555.3
0.00017
0.8788
0.1300
0.8815
182.3
0.00013
0.9131
ce pt 174.4
0.8122
R2
ed
6
R2
M
qeq (mg g-1)
181.8
an
Pseudo first-order (non-linear form)
k2
us
(mg g-1)
ip t
Experimental
cr
pH
Elovich
Double-constant
β
R2
k0
b
R2
367.9
0.0070
0.9908
0.2115
5.6550
0.9618
343.8
0.0085
0.9593
0.2129
5.4826
0.9269
10
449.8
0.0139
0.9746
0.1747
5.3026
0.9740
12
10741
0.0681
0.7221
0.1033
0.1033
0.6928
6 8
Ac
α
2/7 Page 29 of 41
Table 3 Isotherm constants of various adsorption isotherms for the uptake of 2GL dye onto Fe(III)/Cs@Dia at different temperatures
(adsorbent dosage: 1 g L-1, pH: 6.0)
Temperature 288K
298K
308K
318K
Langmuir qmax (mg g-1) B (L mg-1) R2
833.3 0.1875 0.9997
1250 0.1600 0.9823
1000 0.1563 0.9991
1000 0.1538 0.9996
Freundlich KF 1/n R2
469.8 0.0996 0.8586
828.6 0.0451 0.0705
551.0 0.0988 0.4942
Tempkin AT BT bT R2
391.0 71.04 33.71 0.8771
1.180E7 51.25 48.34 0.1198
Dubini-Radushkevich K Qm E R2
2×10-5 820.0 158.1 0.9410
Redlich-Peterson A B g R2
us
cr
ip t
Model and parameter
524.4 0.1113 0.5373 250.1 87.70 30.15 0.5622
9×10-8 961.4 2357 0.9540
2×10-5 989.0 158.1 0.9334
2×10-5 1000 158.1 0.9555
110.5 0.1263 0.9002 0.9980
1521 1.2012 0.9549 0.9714
58.95 0.0135 0.9011 0.9878
55.33 0.0135 0.8886 0.9866
Koble-Corrigan a b n R2
119.72 0.1398 0.9818 0.9924
1583.5 1.2708 0.9982 0.9302
0.1977 0.0002 3.2259 0.9798
0.1956 0.0002 3.2375 0.9735
Generalized kG Nb R2
3.9902 0.8528 0.2357
1.6990 0.4965 0.3034
1.5382 0.6376 0.4981
2.8438 0.8122 0.6441
M
ed
ce pt
Ac
an
918.6 77.01 33.25 0.5111
3/7 Page 30 of 41
Table 4 Maximum adsorption capacity for different adsorbents and dyes adsorbents
qmax mg g-1
references
CR
Mg-Fe-CO3-LDH composite
104.6
[39]
CR
chitosan hydrogel beads impregnated with
450.4
[40]
carbon nanotubes
ip t
Dyes
Fe(III)/Cs@Dia
1111
MO
chitosan/kaolin/nanosized γ-Fe2O3 composite
14.2
[15]
MO
chitosan-based semi-IPN hydrogel composite
202.0
[41]
MO
chitosan/Al2O3/magnetite nanoparticles
[42]
769.2
This study
172.4
[43]
Fe(III)/Cs@Dia
1429
This study
Fe(III)/Cs@Dia
909.1
This study
Fe(III)/Cs@Dia
1250
This study
M
Fe(III)/Cs@Dia
DR 23
magnetic multi-walled carbon nanotubes-Fe3C
Ac
2GL*
ce pt
nanocomposite
ed
MO
DY R*
us
an
417.0
composite
DR 23
This study
cr
CR
*: none published results for the comparison of our research
4/7 Page 31 of 41
Table 5 Thermodynamic parameters for the uptake of 2GL dye on Fe(III)/Cs@Dia
ΔG (KJ mol-1)
298K
308K
318K
(KJ mol-1)
-5.10
-5.02
-4.94
-4.86
-7.41
(J mol-1 K-1)
-8.03
Ac
ce pt
ed
M
an
us
cr
Fe(III)/Cs@Dia
288K
ΔS
ip t
adsorbent
ΔH
5/7 Page 32 of 41
Table 6 Batch adsorption-desorption cycles of 2GL dye
qep mg g-1
Recovery %
1
1027.8
98.26
2
1023.7
97.87
3
1022.2
97.72
4
1011.1
96.66
5
1007.5
96.32
Ac
ce pt
ed
M
an
us
cr
ip t
Cycles
6/7 Page 33 of 41
Table 7 Parameters of Langmuir Isotherms for the adsorption of other anionic azo dyes on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K)
Langmuir isotherm model
b (L mg-1)
R2
MO
769.2
0.1204
0.9988
CR
1111
2.2500
DY R
909.1
0.0233
DR23
1429
0.4375
us
cr
qmax (mg g-1)
ip t
Dyes
0.9999
an
0.9953
Ac
ce pt
ed
M
0.9999
7/7 Page 34 of 41
ed
M
an
us
cr
ip t
Figure(s)
Ac
ce pt
Fig.1. SEM microgaraphs of the chitosan (a), Fe(III)/Cs (b), diatomite (c), and Fe(III)/Cs@Dia (d)
1/7 Page 35 of 41
ip t cr us an M
Ac
ce pt
ed
Fig.2. FTIR spectra of the Cs/Dia, Diatomite, Fe(III)/Cs, Fe(III)/Cs@Dia and Chitosan
2/7 Page 36 of 41
ip t cr us an M ed ce pt Ac
Fig.3. CP/MAS 13C and 29Si solid-state NMR spectra of Cs/Dia and Fe(III)/Cs @Dia
3/7 Page 37 of 41
ip t cr us an M
ed
Fig.4. XPS spectra of the Fe(III)/Cs@dia survey and high resolution XPS spectra of Fe2p, O1s
Ac
ce pt
and N1s
4/7 Page 38 of 41
ip t cr us an M
ed
Fig.5. Effect of initial 2GL dye concentration on uptake capacities by different adsorbents
Ac
ce pt
(adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K).
5/7 Page 39 of 41
ip t cr us an M
Ac
ce pt
dosage: 1 g L-1, T: 298K)
ed
Fig.6. Kinetic adsorption of Fe(III)/Cs@Dia for 2GL dye in different pH values (adsorbent
6/7 Page 40 of 41
ip t cr us an M ed
Ac
ce pt
Fig.7. The proposed adsorption mechanism of anionic azo dyes onto Fe (III)/Cs@Dia
7/7 Page 41 of 41
S0927-7757(14)00934-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.12.015 COLSUA 19595
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
2-10-2014 1-12-2014 8-12-2014
Please cite this article as: Liuchun Zheng, Chenggang Wang, Yuehong Shu, Xiaomin Yan, Laisheng Li, Utilization of Diatomite/chitosan-Fe (III) Composite for the Removal of Anionic Azo Dyes from Wastewater: Equilibrium, Kinetics and Thermodynamics, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.12.015 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.
Ac
ce pt
ed
M
an
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cr
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*Graphical Abstract (for review)
Page 1 of 41
*Highlights (for review)
Highlights: Fe(III)/Cs@Dia was confirmed as an excellent absorbent for anionic azo dyes.
Pseudo-second-order kinetic model showed a better-fit to the experimental data.
The equilibrium data fitted better with the Langmuir model.
Adsorption of 2GL dye onto adsorbent was exothermic in nature.
Adsorption mechanisms mainly involved electrostatic interaction and complexation.
Ac
ce pt
ed
M
an
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cr
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Page 2 of 41
Utilization of Diatomite/chitosan-Fe (III) Composite for the Removal of Anionic Azo Dyes from Wastewater: Equilibrium, Kinetics and Thermodynamics
ip t
Liuchun Zheng**, Chenggang Wang, Yuehong Shu, Xiaomin Yan, Laisheng Li*
Ac
ce pt
ed
M
an
us
cr
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
Corresponding author: School of Chemistry and Environment, South China Normal University, Higher Education
Mega Center, Guangzhou 510006, PR China.
Tel: +86 20 39310185/39310250; fax: +86 20 39310187
E-mail address: [email protected] (L.S. Li), [email protected]
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Abstract A novel diatomite/chitosan-Fe(III) composite (Fe(III)/Cs@Dia) was synthesized successfully to use as an adsorbent for the removal of anionic azo dye. The compostie was further
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characterized by SEM, FTIR, XPS, and 13C/29Si NMR, and then the adsorption batch experiments were carried out as a function of initial dye concentrations, pH values, contact time and
cr
temperatures to determine the adsorption capacities of Fe(III)/Cs@Dia. Compared with several
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typical isotherm models, the adsorption isotherms were all best-fitted by the Langmuir model,
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with a maximal 2GL dye uptake of 1250 mg g-1 at pH 6 and 298K. In addition, kinetic studies followed the pseudo-second-order model and thermodynamic studies revealed that the uptake was
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exothermic in nature. The adsorption mechanisms were proposed with observation that electrostatic interaction and complexation played the key roles in adsorption process. Moreover,
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the composite also showed favorable adsorption properties for other four anionic azo dyes (direct
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yellow R, congo red, methyl orange and direct red 23).
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Keyworks: Anionic azo dye; Composite; Adsorption mechanism; Isotherm; Kinetic.
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1 Introduction
Large amounts of dyes wastewater are produced in many industries, such as textiles, paper, tanneries, clothing, printing, paint, etc. [1]. For example, it is estimated that Guangdong Province
ip t
in China produced 950 million tons of dyeing wastewater in 2012. Unfortunately, most of dyes wastewater contains undesirable color, toxicant and carcinogen. If discharged into the aquatic
cr
environment above the level that the nature can eliminate, it would pose a great threat to aquatic
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biota and humans [2]. In general, dyes can be classified anionic (direct, acid and reactive dyes),
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cationic (basic dyes) and non-ionic (disperse dyes and vat dyes) [3]. Among them, azo dyes (anionic) with existence of nitrogen-nitrogen double bonds, are considered to be the largest and
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most versatile class of organic dyes [4, 5], but most difficult is to be degraded due to their complicated aromatic structure and poor biodegradability [6]. Therefore, it is s necessary to
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provide suitable technology for the wastewater treatment.
ce pt
Conventional technologies, including membrane process, coagulation/flocculation, chemical oxidation and electrolysis, have been employed successfully for the removal of azo dye from wastewaters [5, 7]. In comparison with other techniques, adsorption is considered to be an
Ac
efficient and versatile method, due to various adsorbents having good selectivity toward different types of wastewater [8, 9]. It is well known that agricultural residue [10], clay mineral [11] and waste biopolymer [12] are relatively simple, low-cost, locally available and effective adsorbents. Among these adsorbents, diatomite is mainly accumulation of amorphous hydrated silica, and possesses good adsorptive property for dyes due to their high specific surface area [13]. Chitosan is the product obtained from the deacetylation of the natural biopolymer chitin, and also could
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provide distinctive adsorption functions because of a great number of active hydroxyl (-OH) and amino (-NH2) groups [14]. However, the adsorption capacities of diatomite and chitosan are still unsatisfactory. These could be explained that the surface silanols (Si–OH) of diatomite frustules
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do not have enough strong adsorption affinities, while chitosan as a non-selective adsorbent requires certain modifications to improve its selectivity for target species, and has some chemical
cr
and mechanical weaknesses. Recently, chitosan-Fe(III) and montmorillonite-chitosan composites
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have been reported to be highly efficient for dye removal [15-17], owing to higher specific surface area, better chemical/mechanical stabilities, and more structural properties. However, to the best
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of our knowledge, little research had been done for diatomite/Chitosan-Fe(III) composite.
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Moreover, the adsorption mechanism of chitosan-Fe(III) composites is still a controversial topic [18], though the chelation between dye and Fe(III) center in chitosan-Fe(III) was proved to be a
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main mechanism [1]. If chitosan-Fe(III) continued to be loaded on diatomite, it would be unclear
exploration.
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how the dye is adsorbed in the composite, and the mechanism also would be require futher
In this study, a diatomite/chitosan-Fe (III) composite was synthesized, characterized, and
Ac
applied for removal of anionic azo dyes, and the effects of several factors, such as initial dye concentrations, pH values, contact time and temperatures were investigated. Thereafter, the equilibrium isotherm data were fitted with typical isotherm models, including Langmuir, Freundlich, Redlich-Peterson, Koble-Corrigan, Tempkin, Dubinin-Radushkevich and Generalized isotherm models. Four kinetic models (pseudo first-order, second-order, Elovich and Double-constant kinetic models) were used to evaluate the mechanism of adsorption. Besides the
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adsorption mechanisms, the thermodynamics of the adsorption processes were also revealed, and regeneration of the adsorbent was used repeatedly up to five cycles for testing its reusability.
2 Materials and methods
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2.1 Chemicals
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Ferric chloride hexahydrate (FeCl3•6H2O), Glutaric dialdehyde (C5H8O2), and ethanol were
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purchased from Damao Chemical Reagent Co. Ltd. (Tianjin, China). Chitosan (deacetylation
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degree≥95%) and diatomite were all analytical grade and obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Direct orange 2GL dye (2GL), direct yellow R (DY R), congo red
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(CR),methyl orange (MO) and direct red 23 (DR 23) were purchased from Guangzhou Chemical
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Reagent Co. Ltd. (Guangzhou, China) (Table 1). All chemicals were used as received.
2.2 Synthesis of Fe (III)/Cs@Dia
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The adsorbent was prepared by a modified literature procedure [1, 19]. First, chitosan powder (1.0 g) was dissolved in 0.2 M FeCl3 aqueous solution (50 mL). After magnetically stirred at room
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temperature for 4 h and dried, the mixture was named as Fe(III)/Cs. Subsequently, 1.5 g diatomite was added into the wet mixture and dispersed sufficiently to obtain a complex precipitation. Then, the complex was washed with ethanol to remove residual FeCl3, and dried at 80 °C. Further, the sample was placed in an ethanol solution in contact with 5% glutaraldehyde for 2 h for the Schiff’s reaction. At last, the mixture was washed with ethanol in order to remove the unreacted glutaradehyde, and dried at 80°C until constant weight to get the final product (labeled as Fe(III)/Cs@Dia). 5 / 25 Page 7 of 41
2.3 Characterization
The general morphology of the products were characterized by the scanning electron microscope (SEM, JSM-6510). FTIR spectra were obtained on an FTTR spectrophotometer using KBr discs to
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prepare the samples. X-ray photoemission spectroscopy was recorded on a Thermo Fisher Scientific standard and monochromatic source (Al Kα) operated at 150 W (15 kV). The solid-state
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CP/MAS 13C and 29Si NMR spectra were recorded on Bruker AVANCE 400 spectrometer at the
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frequency of 100 and 79.49 MHz, respectively. The delay time of 13C MAS NMR was 3 s and the
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proton 90°C pulse time was 6μs. A single-pulse sequence with 90-degree pulse in 29Si MAS NMR was 4μs, and the delay time of 120 s were used. Dye concentrations were analyzed using a
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2.4 Batch adsorption experiments
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UNICO UV-2800H spectrometer.
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For the kinetic study, 1400 mg L-1 dye solutions (50 mL) were agitated with 0.050 g adsorbent at 25 °C for predetermined intervals of time, and the influence of pH values (6, 8, 10 and 12) was investigated for adsorption of 2GL dye. After filtration, the concentrations of 2GL dye in the
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aqueous solutions were measured. For the equilibrium adsorption studies, a fixed mass of Fe(III)/Cs@Dia (0.01 g) was added into a series of 50 mL serum bottles with a 10 mL solution containing a known amount of the respective dye (varying from 600 to 1400 mg L-1). The initial pH value of the dye solution was adjusted by adding 0.1 M HCl or 0.1 M NaOH solution. The serum bottles were agitated for 24 h in a shaking thermostatic bath with the temperature maintained at 25 °C. After centrifugation, the concentration of dye solution was determined. The
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amount of dye adsorbed per unit mass of the adsorbent was evaluated by using the mass balance equation.
qt =
(C0 - Ct )V W
`(1)
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where qt (mg g-1) is the amount adsorbed per gram of adsorbent at time t (min); C0 and Ct are the
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initial concentration and the concentration at time t (mg L-1); W (g) is the mass of the adsorbent
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used, and V (L) is the initial volume of the dye solution.
3. Results and discussion
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3.1 Characterization
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3.1.1SEMs
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The SEMs of chitosan, Fe(III)/Cs, diatomite, and Fe (III)/Cs@Dia were shown in Fig.1. It illustrated that the surfaces of chitosan and Fe(III)/Cs were amorphous (Fig.1a, b), and the
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diatomite provided high porosity and permeability (Fig.1c). After loaded by Fe(III)/Cs, the diatomite surface was nearly covered, but its pore structure still maintained, which provided a
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good possibility for dyes to be trapped and adsorbed. (Fig.1d).
3.1.2 FTIR
The FTIR spectra of chitosan, Fe(III)/Cs, diatomite, Cs/Dia and Fe(III)/Cs@Dia were presented in Fig.2. The spectrum of diatomite showed the bands at 1085, 786 and 1080 cm-1 , which were identified as Si-O-Si stretching vibrations, Si-OH groups and Si-O-Si groups, respectively [20]. The absorbencies associated with the chitosan were as follow: 3350 cm-1(O-H stretch and N-H stretch), 2880 cm-1(C-H stretch), 1650 cm-1(amide II band, C=O stretch,), 1590 cm-1(N-H 7 / 25 Page 9 of 41
bending), 1390 cm-1 (amide III band, C-N stretch), 1418 cm-1and 1326 cm-1(C-H bending), 1260 cm-1(C-N stretch), 1160cm-1(bridge C-O-C stretch) and 1010 cm-1 (Hydroxyl deformation vibration) [21, 22]. Cs/Dia had a weak band at 3600 cm-1 indicating hydroxyl groups reacted, and
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the weak peak at 1500 cm-1 corresponded to the -NH2 or -NH3+ group of chitosan. Meanwhile, a distinct peak at 1060 cm-1 was identified as the C-H and C-O bending of chitosan, or Si-O-Si
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group asymmetric stretch of diatomite [23]. The broad band of Fe(III)/Cs at 3670 cm-1 was
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ascribed to the multiple overlapping peaks due to O-H and N-H stretching vibration, indicating that complexation occurred among -NH2 group, -OH group and Fe(III). Compared with the
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spectrum of chitosan, N-H bending at 1590 cm-1 moved to 1610 cm-1 and its intensity decreased,
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which further revealed that the -NH2 and -OH groups involved in the coordination reaction with Fe(III) [24-26]. These results were also consistent with Bhatia and Ravi’s view, which suggested
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the Fe(III)/Cs complex was either penta or hexacoordinated, complexed through O and N from the polymer [27]. As a result, the spectrum of Fe(III)/Cs@Dia combined the spectrum of Cs/Dia and
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Fe(III)/Cs, and also confirmed that Fe(III)/Cs was successfully loaded on diatomite.
3.1.3 13C and 29Si NMR
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Fe(III)/Cs@Dia and Cs/Dia were characterized by
13
C and
29
Si NMR (Fig.3). In general, the
chemical shift values of C=O, C1-6 and CH3 in methyl and carbonyl groups of pure chitosan, are 173.8 ppm (C=O), 104.6 ppm (C1), 83.6 ppm (C4), 76.1 ppm (C5), 74.2 ppm (C3), 61.4 ppm (C6), 55.8 ppm (C2) and 23.3 ppm (CH3), respectively [28]. In 13C NMR spectra, the series of peaks of Fe(III)/Cs@Dia and Cs/Dia were also found at 175 ppm (C=O), 104.8 ppm (C1) and 23.6 ppm (CH3), which were similar to those of the monomeric form of pure chitosan. As diatomite was
8 / 25 Page 10 of 41
added, a very weak splitting of C-4 signal at 83 ppm was showed in Cs/Dia, but it disappeared in Fe(III)/Cs@Dia. Similar results appeared in the C3 and C2 peaks, which indicated that C5 and C6 peaks were enlarged and overlapped with the neighboring C3 and C2. Especially, a sharp peak at
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132 ppm and a wide peak at 148.9 ppm in Fe(III)/Cs@Dia were associated with the double ethylenic bond (C=C), due to the glutaraldehyde reaction [21]. The diatomite spectrum always had
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three signals at -86, -95 and -110 ppm, attributing to Q2 (Cyclic Si-O tetrahedron), Q3 (Layered 29
Si
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Si-O tetrahedron) and Q4 (Shelf-like Si-O tetrahedron) environments, respectively [29]. In
NMR spectra, the only obvious peaks of Fe(III)/Cs@Dia and Cs/Dia were located at 108.8 ppm,
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which implied that both of them only had shelf-like Si-O tetrahedron, whereas other peaks
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disappeared. Moreover, the peak of Fe(III)/Cs@Dia was slightly wider than that of Cs/Dia due to spectral line broadening, which indicated that the magnetic susceptibility increased and nuclear
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relaxation time became shorter as the concentration of Fe(III) increased.
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3.1.4 XPS
The peaks of high-resolution XPS spectrum (Fe2p, O1s and N1s) of Fe(III)/Cs@Dia were provided in Fig.4. The binding energy of Fe2p photoelectrons in the standard spectrum of FeCl3
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was reported at 711.2eV [30], but the peaks of Fe(III) in Fe(III)/Cs@Dia were at 711.5 and 725.1 eV, with shakeup satellite at 715.5 and 732.2 eV, indicating that the complexing reaction occurred between chitosan and Fe(III) [1]. In the spectrum of O1s, there were two broad peaks, one at 531.4 eV and the other one at 532.8 eV. Meanwhile, three lower peak appeared at 399.2 eV, 399.8 eV and 401.7 eV were observed in the spectrum of N1s. These results were consistent with Shen et al’s report for O1s spectrum [1], the peak with a binding energy of 531.4 eV was from -OH
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oxygen in chitosan which was chelated with Fe(III), in contrast to the peak at 532.8 eV for -OH oxygen without chelation. As for N1s spectrum, the peaks at 399.2, 399.8 and 401.7 eV represented the characteristic peaks of the -NH2 nitrogen, the nitrogen in -NH2 and -NH3+ group in
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chitosan, respectively. The lowest peak at 406.6 eV was from the -N=CH- bond, indicating
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successful cross-linking between chitosan and glutaraldehyde.
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3.2 Effect of initial dye concentration
According to FTIR, the functional groups of Fe(III)/Cs@Dia successfully combined those of
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diatomite, chitosan and Fe(III). To determine adsorption properties, 2GL dye uptake of these
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materials was studied under different initial concentrations, ranging from 600-1400 mg L-1. Fig.5 showed that the amount of 2GL dye adsorbed on all the materials increased with an increase of the
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dye concentrations, and then increased relatively slowly from 1000 to 1400 mg L-1 due to the limited number of active sites. Among these adsorbents, Fe(III)/Cs@Dia showed the best
ce pt
adsorption capacities. With an initial 2GL dye concentration of 1400 mg L-1, the adsorption capacities were found to be 1222 mg g-1 for Fe(III)/Cs@Dia, 211.9 mg g-1 for Cs/Dia, 86.29 mg g-1 for diatomite, 878.0 mg g-1 for Fe(III)/Cs, and 260.9 mg g-1for chitosan, respectively. Based on
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these results, Fe(III)/Cs@Dia as the best adsorbent was used in the further experiments.
3.3 Effect of initial pH
The effect of pH on 2GL dye removal was studied by varying the initial solution pH at 6, 8, 10 and 12, with a contact time of 11 hours in each batch equilibrium adsorption (Fig.6). It was obvious that the adsorption capacities of Fe(III)/Cs@Dia increased with the contact time, and reached equilibrium at 480, 360, 240 and 60 min, respectively. However, when the initial pH 10 / 25 Page 12 of 41
increased from 6 to 12, the removal capacities declined drastically. In the acidic solution, the protonation of -SO3- groups could shift the -SO3Na of 2GL dye molecule to form -SO3H, resulting in formation of -NH2+SO3- or =NH+SO3- [11, 31]. Moreover, Huang et al. [32] reported that the
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adsorption process was an electrostatic interaction between anionic azo dye and the protonated amine groups of chitosan on the adsorbent [1]. Therefore, more protons on the adsorbent surface,
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the better excellent adsorption capacity in low pH values. However, with pH at 8, 10 and 12, the
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protonation of -SO3- groups and amine groups disappeared. Meanwhile, OH- ions would also compete with the 2GL dye anions for the adsorption sites, resulting in a decrease of 2GL dye
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removal. Under the alkali solution, the complexation between 2GL dye with Fe (III) played the
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dominant role in adsorption [13] (Fig.7).
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3.4 Adsorption kinetics
Kinetic models were used to examine the rate of the adsorption process and propose potential
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rate-controlling step. The batch experiments were carried out using variable initial adsorbate concentration, adsorbent dose, particle size, agitation speed, pH values and temperatures along with different adsorbent and adsorbate types [33]. In this study, the pseudo-first-order (including
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linear and non-linear forms), pseudo-second-order (including linear and non-linear forms), Elovich and Double-constant model were calculated using pH values with the experimental data (Fig.6). The linear and non-linear forms of the pseudo-first-order rate equations are Eq. (2) and (3), respectively. [33,34]:
ln(qep - qt ) = ln qep - k1t
(2)
qt = qep (1 - e- k1t )
(3) 11 / 25 Page 13 of 41
where qep and qt (mg g-1) refer to the adsorbent amount at equilibrium and time t, respectively. k1 is the rate constant. The linear and non-linear forms of the pseudo-second-order rate equations are Eq. (4) and (5),
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respectively. [33,34]:
t 1 t = 2 + qt k2 qep qep
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k2 qep2 t
(5)
1 + k2 qept
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qt =
(4)
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k2 is rate constant.
Elovich equation is a rate equation based on the adsorption capacity commonly and rearranged to
1
ln( )
1
ln t
(6)
ed
qt
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a linear form Eq. (6) if simplified by assuming αβ ≥ t[35].
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where α (mg g-1 min-1) is the initial adsorption rate and β (g mg-1) is the desorption constant related to the extent of the surface coverage and activation energy for chemisorption. The slope
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and intercept of the plot of qt versus ln t result in the estimation of the kinetic constants.
Double-constant kinetic model is an empirical equation derived from the classical Freundlich isotherm and mainly suitable for more complex reaction kinetics. It is given as Eq. (7):
ln qt k 0 ln t b
(7)
where k0 is the Double-constant kinetic constants, and b is a adsorption rate constant associated with adsorption activation energy.
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The calculated kinetic parameters of Fe(III)/Cs@Dia were given in Table 2. The results showed that the linear form of pseudo-second-order model described the adsorption kinetics better according to their highest correlation coefficient (R2 > 0.996), when compared with the linear
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forms of the pseudo-first-order, Elovich and Double-constant models under four different pH values. Further, for the non-linear forms of pseudo-first-order and pseudo-second-order models, it 2
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was found that the R value for pseudo-second-order model was obviously higher and it is
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inappropriate to apply pseudo-first-order model expression. Moreover, a similar observation was that the qep values obtained from the pseudo-second-order model showed to be nearer the experimental
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data (qexp), which indicated that the overall process was controlled by chemisorption [36], and
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involved valence forces through sharing or exchange of electrons between dye and adsorbent [3]. As can be seen from rate constant k2, 2GL dye was adsorbed by Fe(III)/Cs@Dia faster at higher
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pH, which revealed that the adsorption equilibrium can be attained quickly because of complexation. However, the formation of -NH2+SO3- or =NH+SO3- between 2GL dye molecule
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and adsorbent at pH 6 would make the adsorption process relatively slower [3].
3.5. Adsorption isotherms
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Adsorption isotherm was performed to study the interaction between anionic azo dyes and the adsorbent, and the adsorption uptakes in different initial dye concentrations at given pH values and temperatures [37]. Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich, Redlich-Peterson, Koble-Corrigan and Generalized isotherm models were applied to describe the adsorption isotherm for 2GL dye onto Fe(III)/Cs@Dia. The Langmuir model is valid for a saturated monolayer adsorption onto the homogeneous surface of adsorbent, and the adsorption energy is a
13 / 25 Page 15 of 41
constant with no migration of adsorbate molecules in the surface plane when maximum adsorption capacity occurs. The Langmuir equation can be represented as Eq. (8) [38]:
qep
=
Cep 1 + bqmax qmax
(8)
ip t
Cep
where qeq and qmax are the equilibrium and maximum uptake capacities (mg g-1), respectively. Ceq
cr
is the equilibrium concentration (mg L-1), and b is the equilibrium constant.
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The Freundlich isotherm is an empirical equation used to describe heterogeneous systems. It is expressed in the following form Eq. (9) [38]:
1 ln Ceq + ln K F n
an
ln qeq =
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where KF and n are the Freundlich constants.
(9)
The Tempkin isotherm model takes into account adsorbent-adsorbate interactions. The form of
qep BT ln AT BT ln Cep
(10)
RT . T is the absolute temperature in K and R is the universal gas constant (8.314 J bT
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where BT =
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the Tempkin equation is given as Eq. (10) [35]:
mol K-1). The constant bT is related to the heat of adsorption, AT is the equilibrium binding
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constant (L min-1) corresponding to the maximum binding energy. The Dubinin-Radushkevich isotherm model is used to describe the adsorption process of microporous adsorbents, and also applied to estimate the mean free energy adsorption (E) using Eqs. (11) - (13) [35].
qep Qm exp( K 2 )
RT ln(1
(11)
1 ) C ep
(12) 14 / 25 Page 16 of 41
where K (mol2 kJ-2) is a constant related to the adsorption energy, and Qm (mg g-1) is the maximum adsorption capacity.
E (kJ mol-1) can be calculated as:
ip t
1 2K
(13)
cr
E=
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The Redlich-Peterson isotherm is an improvement over the Langmuir and Freundlich isotherms and contains three parameters. It can be described by Eq. (14) [35]:
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AC ep 1 + BC epg
(14)
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qep =
where A, B and g (0 < g <1) are the Redlich-Peterson parameters.
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The Koble-Corrigan model is a combination of the Langmuir and Freundlich isotherm models
[35]:
aCep n
(15)
1 + bCepn
Ac
qep =
ce pt
and considered to be more highly nonlinear. It also has three parameters and given by Eq. (15)
where a, b and n are the Koble-Corrigan parameters.
The generalized isotherm is another combination of Langmuir and Freundlich isotherms. It depends on the value of cooperative binding constant (Nb) and described as Eq. (16) [35]:
log(
Qm - 1) = log KG - Nb log Cep qep
(16)
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where KG is the saturation constant (mg L-1); Nb is the cooperative binding constant; and Qm (mg g-1) is the maximum adsorption capacity and obtained from the Langmuir isotherm model.
Isotherm constants of seven adsorption isotherms were listed in Table 3. The highest value of
ip t
coefficient of determination (R2) obtained indicated the Langmuir model displayed the best fit
cr
compared to other six isotherms, suggesting homogeneous surfaces of the composite and
monolayer coverage of 2GL dye onto the adsorbent. The Langmuir isotherm also could be used to
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estimate the adsorption capacities (qmax), and was found that adsorption capacities of
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Fe(III)/Cs@Dia were 833.3, 1250, 1000 and 1000 mg g-1 in range of 288-318K, respectively. Dubini-Radushkevich, Redlich-Peterson and Koble-Corrigan models also seemed to have good
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applicability due to higher value of coefficient. For Dubini-Radushkevich model, the maximum adsorption capacities (Qm) obtained were 820, 961.4, 989 and 1000 mg g-1, respectively. These
ed
were less than the value obtained (qmax) by the Langmuir isotherm model. The mean free energy
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adsorption (E) were more than 100 kJ mol-1, which were greater than 16 kJ mol−1, indicating that the adsorption of 2GL dye on adsorbent was by chemisorption. The exponent values (g) in Redlich-Peterson model were close to 1, which signified Langmuir model could explain the
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experimental equilibrium data instead of the Freundlich isotherm. In addition, it could be inferred that homogeneous uptake was the main mechanism in adsorption process from Koble-Corrigan model. In Table 4, The values of maximum adsorption capacities of Fe(III)/Cs@Dia for CR, MO and DR 23 were compared to those values reported for other composites, and Fe(III)/Cs@Dia exhibited very high adsorption abilities for these three dyes. Unfortunately, there were not any published results of DY R and 2GL dyes for the comparison of our research.
16 / 25 Page 18 of 41
3.6 Adsorption thermodynamics
Based on the constant values obtained using the Langmuir isotherm model, thermodynamic parameters, including enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) changes, were calculated
ip t
using the following equations[33]:
K D = qep / Cep
cr
(17)
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G RT ln K D G H TS
S H R RT
an
ln K D
(18) (19)
(20)
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where T is temperature in K. and KD is the distribution coefficient (mL g-1). The enthalpy change (ΔH) and the entropy (ΔS) can be calculated from a plot of ln KD vs. 1/T. The Gibbs free energy
ed
was determined at 288, 298, 308 and 318K, respectively. ΔG, ΔH and ΔS were obtained from Eqs.
ce pt
(17) - (20) and given in Table 5. Negative values of ΔG indicate the adsorption process was spontaneous, and the degree of spontaneity of the reaction decreased with increasing temperature. Negative value of ΔH indicated the exothermic nature of the uptake, reconfirming the previous
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results in the section regarding adsorption isotherm studies. And negative value of ΔS showed that the randomness decreased during the removal of 2GL dye on Fe(III)/Cs@Dia. 3.7 Regeneration/ reusability of Fe(III)/Cs@Dia Many reports suggest that NaOH solution is effective for desorption of the anionic dyes, while HCl or H2SO4 solution is used to recover the adsorption ability of adsorbent [44, 45]. Therefore, 0.5 mol L-1NaOH and 0.1 mol L-1 HCl aqueous solutions were chosen for the desorption and regeneration of 2GL dye-loaded Fe(III)/Cs@Dia, respectively. And the adsorption and desorption 17 / 25 Page 19 of 41
processes were repeated for five times. The results were showed in Table 6, and the recovery efficiency reached 96.32% even after the fifth cycle, which indicated that the adsorbent had the excellent reusability after five repetitions of the adsorption–desorption cycles.
ip t
3.8 Adsorption of other typical anionic azo dyes
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Anionic azo dyes, such as direct yellow R (DY R), congo red (CR), methyl orange (MO) and direct red 23 (DR 23) had similar structure with 2GL dye. Therefore, their adsorption mechanisms
us
mainly involved electrostatic interaction and complexation (Fig 7), and Langmuir model was also
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applied to describe the adsorption isotherm for these dyes, and the parameters of isotherms were also calculated (Table 7). The results showed that the experimental adsorption data fitted very
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well to the Langmuir isotherm, indicating that their adsorption also were monolayer coverage. The maximum adsorption capacities of MO, CR, DY and DR 23 were 769.2, 1111, 909.1 and 1429 mg
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4. Conclusions
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g-1, respectively. Thus Fe(III)/Cs@Dia was an excellent adsorbent for anionic azo dyes.
Fe(III)/Cs@Dia was proven to be a promising adsorbent for the removal of anionic azo dyes from wastewater. Moreover, electrostatic interaction forming -NH2+SO3- or =NH+SO3- under
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acidic solution, and complexation between 2GL dye and Fe(III) in alkali conditions were found as the adsorption mechanism. The equilibrium sorption data agreed well with Langmuir isotherm model and the maximum adsorption capacities for 2GL dye were 833.3, 1250, 1000 and 1000 mg g-1 in range of 288-318K, respectively. For kinetics measurements, the pseudo-second-order model provides the best correlation, suggesting that the chemisorption was the rate-limiting step. The negative value of enthalpy change (ΔH) and free energy change (ΔG) indicated the exothermic and
18 / 25 Page 20 of 41
spontaneous nature of adsorption. Therefore, Fe(III)/Cs@Dia also showed the excellent adsorption performance for other three dyes, demonstrating the general applicability of the adsorbent to
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anionic azo dyes.
Acknowledgments
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The authors are grateful for the financial support from the National High-Tech Research and
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Development Program (No. 2013AA06A305), the National Natural Science Foundation of China (No. 21247004), the Science & Technology Office of Guangdong Province (No.
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2012B031000016), and Foundation for Distinguished Young Talents in Higher Education of
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Guangdong, China (2013LYM0018).
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ce pt
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24 / 25 Page 26 of 41
Figure captions Fig.1. SEM microgaraphs of the chitosan (a), Fe(III)/Cs (b), diatomite (c), and Fe(III)/Cs@Dia (d) Fig.2. FTIR spectra of the Cs/Dia, Diatomite, Fe(III)/Cs, Fe(III)/Cs@Dia and Chitosan
ip t
Fig.3. CP/MAS 13C and 29Si solid-state NMR spectra of Cs/Dia and Fe(III)/Cs @Dia
cr
Fig.4. XPS spectra of the Fe(III)/Cs@dia survey and high resolution XPS spectra of Fe2p, O1s
us
and N1s.
Fig.5. Effect of initial 2GL dye concentration on uptake capacities by different adsorbents
an
(adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K)
Fig.6. Kinetic adsorption of Fe(III)/Cs@Dia for 2GL dye in different pH values (adsorbent dosage:
M
1 g L-1, T: 298K)
ed
Fig.7. The proposed adsorption mechanism of 2GL dye onto Fe (III)/Cs@Dia Table 1 The general data of several types of anionic azo dyes used in this work
ce pt
Table 2 Calculated parameters for four kinetic models for the adsorption of 2GL dye on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, T: 298K) Table 3 Isotherm constants of various adsorption isotherms for the uptake of 2GL dye onto
Ac
Fe(III)/Cs@Dia at different temperatures
(adsorbent dosage: 1 g L-1, pH: 6.0)
Table 4 Maximum adsorption capacities for different adsorbents and dyes Table 5 Thermodynamic parameters for the uptake of 2GL dye on Fe(III)/Cs@Dia
Table 6 Batch adsorption-desorption cycles of 2GL dye Table 7 Parameters of Langmuir Isotherms for the adsorption of other anionic azo dyes on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K) 25 / 25 Page 27 of 41
Table(s)
Table 1 The general data of several types of anionic azo dyes used in this work
Molecular
Dye
Chemical structure
volume
Formula
λmax(nm)
ip t
(g mol -1)
327.3
CR
696.7
DY R
452.4
DR23
790.69
cr
MO
C32H22N6Na2O6S2
C16H10N2Na2O7S2
464
498
500
739.6
498 C35H25N7Na2O10S2
C26H16N5Na3O11S3
Ac
2GL
ce pt
ed
M
an
us
C14H14N3O3SNa
414
1/7 Page 28 of 41
Table 2 Calculated parameters for four kinetic models for the adsorption of 2GL dye on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, T: 298K)
Pseudo first-order (linear form)
Pseudo second-order (linear form)
qeq (mg g-1)
k1
R2
qeq (mg g-1)
6
1042
658.2
0.0085
0.9412
1111
8
826.0
364.5
0.0104
0.9511
833.3
10
572.5
230.3
0.0101
0.8353
588.2
12
185.6
35.49
0.0046
0.6880
pH
Experimental
k1
1042
914.7
0.0499
8
826.0
782.5
10
572.5
521.3
12
185.6
0.00004
0.9983
0.00008
0.9998
0.00011
0.9992
0.00096
0.9985
Pseudo second-order (non-linear form)
qeq (mg g-1)
k2
R2
985.8
0.00007
0.9185
0.0483
0.9198
836.4
0.00009
0.9728
0.0622
0.7482
555.3
0.00017
0.8788
0.1300
0.8815
182.3
0.00013
0.9131
ce pt 174.4
0.8122
R2
ed
6
R2
M
qeq (mg g-1)
181.8
an
Pseudo first-order (non-linear form)
k2
us
(mg g-1)
ip t
Experimental
cr
pH
Elovich
Double-constant
β
R2
k0
b
R2
367.9
0.0070
0.9908
0.2115
5.6550
0.9618
343.8
0.0085
0.9593
0.2129
5.4826
0.9269
10
449.8
0.0139
0.9746
0.1747
5.3026
0.9740
12
10741
0.0681
0.7221
0.1033
0.1033
0.6928
6 8
Ac
α
2/7 Page 29 of 41
Table 3 Isotherm constants of various adsorption isotherms for the uptake of 2GL dye onto Fe(III)/Cs@Dia at different temperatures
(adsorbent dosage: 1 g L-1, pH: 6.0)
Temperature 288K
298K
308K
318K
Langmuir qmax (mg g-1) B (L mg-1) R2
833.3 0.1875 0.9997
1250 0.1600 0.9823
1000 0.1563 0.9991
1000 0.1538 0.9996
Freundlich KF 1/n R2
469.8 0.0996 0.8586
828.6 0.0451 0.0705
551.0 0.0988 0.4942
Tempkin AT BT bT R2
391.0 71.04 33.71 0.8771
1.180E7 51.25 48.34 0.1198
Dubini-Radushkevich K Qm E R2
2×10-5 820.0 158.1 0.9410
Redlich-Peterson A B g R2
us
cr
ip t
Model and parameter
524.4 0.1113 0.5373 250.1 87.70 30.15 0.5622
9×10-8 961.4 2357 0.9540
2×10-5 989.0 158.1 0.9334
2×10-5 1000 158.1 0.9555
110.5 0.1263 0.9002 0.9980
1521 1.2012 0.9549 0.9714
58.95 0.0135 0.9011 0.9878
55.33 0.0135 0.8886 0.9866
Koble-Corrigan a b n R2
119.72 0.1398 0.9818 0.9924
1583.5 1.2708 0.9982 0.9302
0.1977 0.0002 3.2259 0.9798
0.1956 0.0002 3.2375 0.9735
Generalized kG Nb R2
3.9902 0.8528 0.2357
1.6990 0.4965 0.3034
1.5382 0.6376 0.4981
2.8438 0.8122 0.6441
M
ed
ce pt
Ac
an
918.6 77.01 33.25 0.5111
3/7 Page 30 of 41
Table 4 Maximum adsorption capacity for different adsorbents and dyes adsorbents
qmax mg g-1
references
CR
Mg-Fe-CO3-LDH composite
104.6
[39]
CR
chitosan hydrogel beads impregnated with
450.4
[40]
carbon nanotubes
ip t
Dyes
Fe(III)/Cs@Dia
1111
MO
chitosan/kaolin/nanosized γ-Fe2O3 composite
14.2
[15]
MO
chitosan-based semi-IPN hydrogel composite
202.0
[41]
MO
chitosan/Al2O3/magnetite nanoparticles
[42]
769.2
This study
172.4
[43]
Fe(III)/Cs@Dia
1429
This study
Fe(III)/Cs@Dia
909.1
This study
Fe(III)/Cs@Dia
1250
This study
M
Fe(III)/Cs@Dia
DR 23
magnetic multi-walled carbon nanotubes-Fe3C
Ac
2GL*
ce pt
nanocomposite
ed
MO
DY R*
us
an
417.0
composite
DR 23
This study
cr
CR
*: none published results for the comparison of our research
4/7 Page 31 of 41
Table 5 Thermodynamic parameters for the uptake of 2GL dye on Fe(III)/Cs@Dia
ΔG (KJ mol-1)
298K
308K
318K
(KJ mol-1)
-5.10
-5.02
-4.94
-4.86
-7.41
(J mol-1 K-1)
-8.03
Ac
ce pt
ed
M
an
us
cr
Fe(III)/Cs@Dia
288K
ΔS
ip t
adsorbent
ΔH
5/7 Page 32 of 41
Table 6 Batch adsorption-desorption cycles of 2GL dye
qep mg g-1
Recovery %
1
1027.8
98.26
2
1023.7
97.87
3
1022.2
97.72
4
1011.1
96.66
5
1007.5
96.32
Ac
ce pt
ed
M
an
us
cr
ip t
Cycles
6/7 Page 33 of 41
Table 7 Parameters of Langmuir Isotherms for the adsorption of other anionic azo dyes on Fe(III)/Cs@Dia (adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K)
Langmuir isotherm model
b (L mg-1)
R2
MO
769.2
0.1204
0.9988
CR
1111
2.2500
DY R
909.1
0.0233
DR23
1429
0.4375
us
cr
qmax (mg g-1)
ip t
Dyes
0.9999
an
0.9953
Ac
ce pt
ed
M
0.9999
7/7 Page 34 of 41
ed
M
an
us
cr
ip t
Figure(s)
Ac
ce pt
Fig.1. SEM microgaraphs of the chitosan (a), Fe(III)/Cs (b), diatomite (c), and Fe(III)/Cs@Dia (d)
1/7 Page 35 of 41
ip t cr us an M
Ac
ce pt
ed
Fig.2. FTIR spectra of the Cs/Dia, Diatomite, Fe(III)/Cs, Fe(III)/Cs@Dia and Chitosan
2/7 Page 36 of 41
ip t cr us an M ed ce pt Ac
Fig.3. CP/MAS 13C and 29Si solid-state NMR spectra of Cs/Dia and Fe(III)/Cs @Dia
3/7 Page 37 of 41
ip t cr us an M
ed
Fig.4. XPS spectra of the Fe(III)/Cs@dia survey and high resolution XPS spectra of Fe2p, O1s
Ac
ce pt
and N1s
4/7 Page 38 of 41
ip t cr us an M
ed
Fig.5. Effect of initial 2GL dye concentration on uptake capacities by different adsorbents
Ac
ce pt
(adsorbent dosage: 1 g L-1, pH: 6.0, T: 298K).
5/7 Page 39 of 41
ip t cr us an M
Ac
ce pt
dosage: 1 g L-1, T: 298K)
ed
Fig.6. Kinetic adsorption of Fe(III)/Cs@Dia for 2GL dye in different pH values (adsorbent
6/7 Page 40 of 41
ip t cr us an M ed
Ac
ce pt
Fig.7. The proposed adsorption mechanism of anionic azo dyes onto Fe (III)/Cs@Dia
7/7 Page 41 of 41