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montmorillonite nanosheets: Dye degradation by the synergistic effect of adsorption and photo-Fenton reaction
Chemical Engineering Journal 379 (2020) 122322
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Self-assembled gels of Fe-chitosan/montmorillonite nanosheets: Dye degradation by the synergistic effect of adsorption and photo-Fenton reaction
T
Yunliang Zhaoa,b,c, Shichang Kanga,b, Lei Qinb, Wei Wanga,b, Tingting Zhanga,b, ⁎ ⁎ Shaoxian Songa,b, , Sridhar Komarnenic, a
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China Department of Ecosystem Science and Management and Materials Research Institute, 204 Energy and the Environment Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b c
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of Fe-chitosan/montmorillonite • Gel nanosheets efficiently degraded methylene blue.
effect of adsorption and • Synergistic photo-Fenton reaction led to dye degradation.
had effective reusability due to • Gel continuous reactivation of adsorption sites.
was stable and worked efficiently • Gel under a wide range of pH conditions. blue was decomposed by • Methylene demethylation and oxidation reactions.
A R T I C LE I N FO
A B S T R A C T
Keywords: Synergy Adsorption Photo-Fenton Fe-chitosan/montmorillonite nanosheets gel Methylene blue
Self-assembled gel of Fe-chitosan/montmorillonite nanosheets (Fe-CS/MMTNS) was prepared for elimination of methylene blue (MB) under visible light in the presence of H2O2. The Fe-CS/MMTNS gel was characterized by FTIR, SEM-EDS, XPS and TG. The Fe-CS/MMTNS gel performed well in the removal of MB through the synergistic effect of adsorption and photo-Fenton reaction. Moreover, this gel worked efficiently under a wide range of pH conditions. This composite gel also showed effective reusability because the adsorption sites of the Fe-CS/MMTNS are continually reactivated through photo-Fenton degradation. Additionally, the Fe-CS/MMTNS gel was found to be stable and the iron ions were hardly leached out because of the complexation between iron and chitosan (CS). The MB degradation occurred by two pathways: a part of MB was directly attacked by reactive radicals and gradually converted into inorganic substances while another part of MB was firstly adsorbed by FeCS/MMTNS gel and then degraded by reactive radicals.
⁎ Corresponding authors at: Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China (S. Song). E-mail addresses: [email protected] (S. Song), [email protected] (S. Komarneni).
https://doi.org/10.1016/j.cej.2019.122322 Received 7 June 2019; Received in revised form 20 July 2019; Accepted 23 July 2019 Available online 23 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 379 (2020) 122322
Y. Zhao, et al.
1. Introduction
was introduced via CS on gels for regeneration in this work through the release of hydroxyl radicals to decompose the dye in the presence of H2O2 [33,34], and also visible light was used to photo-reduce Fe3+ into Fe2+ and accelerate the removal of MB [35,36]. Moreover, the Fe-CS/ MMTNS gel prepared in this work intelligently solves the pH sensitivity of metal-CS complex because of the hydration ability of MMTNS surface. The morphological features, elemental distribution and thermal stability of the Fe-CS/MMTNS gels were systematically characterized and the efficiency of MB removal by gels and their regeneration and removal mechanism of MB were also investigated.
With the development of textile, paper, plastic, leather and the other industries, the use of synthetic dyes also increased. The number of commercial species of synthetic dyes increased to 10,000 while their production went up to 7 × 105 tons/year [1,2]. Although dyes brilliantly decorate our world, a lot of serious environmental troubles also emerged. Due to the toxicity, carcinogenicity, and mutagenicity of dyes, people and animals suffer from illness [3,4]. For instance, the symptoms of nausea, vomiting, profuse sweeting and mental confusion can occur upon the inhalation of MB [5,6]. In addition, dyes can hinder the photosynthesis of aquatic organisms. Therefore, many methods including sedimentation [7], biodegradation [8], advanced oxidation [9] and adsorption [10] have been proposed to treat dyes in wastewater before its discharge into the environment. Among all the treatment methods, advanced oxidation process serves as a robust and clean alternative to decompose various dyes. Thanks to mild conditions and easy operation, Fenton process attracts much attention among diverse advanced oxidation processes such as ozonation [11], catalytic wet oxidation [12], electrochemical oxidation [13] and Fenton [14]. Compared to homogeneous Fenton, heterogeneous Fenton attracts a lot of attention owing to its fixation of Fe and adaptability to a wide range of pH conditions. Various materials such as Nafion [15], strongly acidic ion exchange resin (SAIER) [16], molecular sieve [17], carbon fiber [18] and clay [19] were used to synthesize heterogeneous Fenton catalysts. Among these materials, montmorillonite (MMT) clay is an attractive candidate to assemble as a catalyst because of its abundance and cost-effectiveness. More importantly, MMT itself is non-toxic and is able to adsorb many pollutants like MB because of its cation exchange ability and huge specific surface area [20]. However, there are two urgent issues to be solved when MMT needs to be used for practical applications: (1) the interlayer space of untreated MMT is small which hinders the entry of large-sized modifiers or pollutants [21] and (2) the MMT is difficult to separate from liquid [22]. Fortunately, the interlayer space and surface area of MMT can be enlarged by intercalation, exfoliation and pillaring due to the weak electrostatic attraction and Van Der Waals force that bond the MMT units. MMT can be exfoliated into nanosheets to surmount the interlayer space/distance limitation and to expose many more active sites [23]. But an unavoidable fact is that the particle size is very small and most particles reach nano range, which inevitably increases the difficulty in solid-liquid separation [24]. Many methods including granulation, magnetization and gelation are proposed to deal with the difficulty of its separation from liquid [25–27]. Among them, selfassembling the exfoliated MMT nanosheets (MMTNS) into three dimensional gels seems a good option since the gels possess exceptional transport properties of ions as well as water, especially when freezedried because of abundant pores [28]. Chitosan (CS) is an ideal cross-linking agent for MMTNS to prepare self-assembled gels. CS is the N-deacetylated product of chitin, which is merely an inferior type of cellulose in nature. It’s properties such as non-toxicity, hydrophilicity, biocompatibility, biodegradability and adsorption ability are very attractive for many uses [29,30]. In our previous work, we prepared a self-assembled MMTNS-CS gel that was formed as a result of the hydrogen bonds (eOH···+NH3e) and electrostatic interaction between the MMTNS and CS and it performed well in the adsorption of MB [27,31]. Nevertheless, like some other gels [10,32], MMTNS-CS gel did not perform very satisfactorily for recycling as it required a long time for regeneration. However, MMTNS played a primary role in the adsorption of MB on MMTNS-CS gels while amino groups from CS were mostly idle. Hence, it is useful to utilize these idle amino groups of CS for better regeneration of MMTNS-CS gels. As per previous research, CS can serve as a chelating agent for 3d metal ions such as Fe3+, Cu2+ and Ni2+ because of its high hydrophilicity, flexible structure and high chemical reactivity. Hence, Fe3+
2. Experimental 2.1. Materials Raw MMT purchased from Nicheng Tianyu Chemical Industry Co., Ltd (Inner Mongolia, China) was used for preparing MMTNS. 5,5Dimethyl-1-pyrroline N-oxide (DMPO) was purchased from Accelerating Scientific and Industrial Development for analyzing hydroxyl radicals. The other reagents were provided by Sinopharm Chemical Reagent Co. Ltd., China and all of them were of analytical reagent grade. Iron trichloride hexahydrate (FeCl3·6H2O) and chitosan were first dissolved in acetic acid (C2H4O2) and then chitosan was used as a crosslinking agent with MMTNS. Methylene blue (C16H18ClN3S·3H2O) is a prototypical organic pollutant, which was tested here under conventional conditions. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust pH using a concentration of 0.1 mol/L. Trichloromethane (Tri, CHCl3) and tert-Butanol (TBA, C4H10O) were used as the scavengers for hydroxyl radicals and superoxide radicals, respectively. 2.2. Synthesis of Fe-CS/MMTNS self-assembled gels Fe-CS/MMTNS gels were self-assembled by the following three steps: (1) MMTNS with the concentration of 15 g/L was first prepared according to a previous method [37] as follows: Typically, 50 g crude MMT were dispersed in 1 L water by mechanical stirring (IKA RW 20, Germany) for 4 h at a speed of 400 rpm. Then, by centrifuging at 109g for 1 min and 1090g for 5 min, pure MMT was collected by decanting the supernatant. By dispersing pure MMT in 150 mL water, it could be exfoliated to obtain MMTNS. Finally, MMTNS were diluted to a concentration of 15 g/L. (2) To prepare Fe-CS, 1 g CS was dissolved in a 2.5% acetic acid solution (100 mL) and the mixture was stirred at room temperature for 4 h. Subsequently, 0 mL, 2.42 mL, 6.15 mL and 19.88 mL FeCl3 solutions (0.2 mol/L) were injected for making the molar ratios of Fe3+ to amino groups in chitosan as 0, 0.1, 0.25 and 1. Finally, the resultant mixtures were stirred at room temperature for 4 h again. (3) To make Fe-CS/MMTNS gel, 20 mL MMTNS solution was first put into a 50 mL breaker with specific Fe-CS and after that the mixture self-assembled into gels upon heating in a 90 °C oven for 24 h. The products obtained with 0, 0.1, 0.25 and 1 M ratios of Fe/amino groups were named as Fe-CS(0)/MMTNS, Fe-CS(0.1)/MMTNS, Fe-CS(0.25)/ MMTNS and Fe-CS(1)/MMTNS, respectively. The molar ratio of Fe3+ to amino groups in chitosan generally was 0.25 unless otherwise stated and the Fe-CS(0.25)/MMTNS was expressed simply as Fe-CS/MMTNS for convenience. 2.3. Characterization of the Fe-CS/MMTNS gel The gel was coated with platinum for scanning electron microscopy (SEM) analysis using a FEI Nova NanoSEM450 and an energy-dispersive X-ray analysis instrument was utilized to detect the elemental composition of the gel. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 6700 with KBr as the reference sample. X-ray photoelectron spectroscopy (XPS) analysis was determined on an ESCALAB 250Xi instrument and all binding energies were referenced to 2
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the case of Fe-CS/MMTNS gel (Fig. 2d). As for CS (Fig. 2b) and Fe-CS (Fig. 2c), their FTIR spectra are similar and the peaks at 1415 cm−1 and 1383 cm−1 can be attributed to the vibrations of methyl groups and C-N [40]. The peak representing amino groups in CS red-shifted from 1599 cm−1 to 1529 cm−1 in Fe-CS suggesting that amino groups played an important role on the complexation of Fe3+ [41]. Although most of the peaks of CS overlap with MMTNS (Fig. 2d), weak vibrations of the methyl groups and C-N can be seen at 1421 cm−1 and 1390 cm−1, respectively. The vibration of amino groups was observed at 1549 cm−1 (Fig. 2d) indicating that amino groups played an important role in assembling gels [31]. As shown in Figs. 1 and 2, it can be inferred that Fe-CS/MMTNS was successfully synthesized. The wide XPS spectra of Fe-CS/MMTNS revealed the atom weights of N and Fe were 2.55% and 0.68%, respectively as shown in Fig. 3(a). In addition, the high-resolution regions of N 1s, Fe 2p and C 1s were procured to investigate the coordination of iron in CS and MMTNS. When preparing Fe-CS/MMTNS, CS was firstly dissolved in acetic acid solution and CS was protonated which would be attracted by MMTNS as indicated by a peak of N 1s at 402.11 eV as shown in Fig. 3(b) [27,42]. In Fig. 3(b), there was a peak of N 1s at 400.38 eV which conforms to NeC]O and its bonding energy of C 1s was 288.54 eV as shown in Fig. 3(c) [33]. In Fig. 3(b), there was still a peak at 398.91 eV which was assigned to NH2-Fe3+ indicating the combination of CS and Fe3+ [43]. In addition, the peak of Fe3+ emerged at 712.67 eV, 716.14 eV, 726.58 eV accompanied by two satellite peaks at 721.07 eV and 735.1 eV as shown in Fig. 3(d) due to the complexion between CS and Fe3+ [44,45]. Thus far, the above results indicated that amino groups of CS were not only combined with Fe3+ but they also served as cross-linking groups. To further investigate the materials, TG and DTG analyses of FeCS/MMTNS were also obtained and the results are shown in Fig. 4. The TG curve could be divided into four stages of mass loss. The first stage ended at 176 °C with a loss of 8.67% due to the vaporization of adsorbed water. The second stage was assigned to the decomposition of mainly the chains of CS and loss of strongly bonded water. The third stage of mass loss started at 380 °C, which corresponded to the release of volatile products, oxidative degradation and the cross-linking reaction of Fe-CS [46,47]. The fourth stage mass loss was 3.08%, which occurred from 686 °C to 1000 °C. This high temperature mass loss was mainly due to the dehydroxylation of structural OH of MMTNS.
the neutral carbon peak at 284.8 eV. The thermogravimetric (TG and DTG) analysis was performed on a STA449F3 unit as the gel was heated in an atmosphere of nitrogen at 10 °C/min between 32 °C and 1000 °C. 2.4. Removal of MB on Fe-CS/MMTNS gel The photocatalytic degradation of MB was performed under visible light irradiation using PLS-SXE 300 system provided by Beijing Perfectlight Technology co. LTD and no filters were replaced that the spectrum was at 320–780 nm. The center optical power was higher than 2.877 kW/m2. In a typical dye removal experiment, 0.01 g gel was added in a 100 mL of 20 ppm MB solution of a certain pH. First, adsorption-desorption equilibrium was established in 45 min and then 200 µL fresh 30% H2O2 aqueous solution was dispersed in the mixture for 2 min. The reactor was immediately exposed to the light for 2 h and 2.5 mL sample was extracted every 15 min for analyzing the concentration of MB using the absorption at 664 nm (Origin Aquamate 8000 UV–Vis Spectrophotometer). A 13 mm 0.22 μm syringe filter was used to remove the gel powder from liquid before UV–Vis analysis. The stability of gel was tested by repeatedly using the gel for five runs. After each run, the gel was concentrated and washed with distilled water for subsequent reaction. 2.5. Analysis method To reveal the mechanism involved in the decolorization of MB in FeCS/MMTNS + H2O2 + Vis system, hydroxyl radical was identified using DMPO via electron spin resonance (EPR). The parameters of ESR were set as follows: a sweep width at 100 G, a center field at 3500.455 G, a modulation amplitude at 1 G, a microwave frequency at 9.81 GHz, a microwave power at 21.59 and a sweep time at 30.72 s. Trichloromethane (Tri) and Tert-Butanol (TBA) were applied to quench superoxide radicals and hydroxyl radicals, respectively. The intermediate species generated during MB degradation were measured by UV–Vis spectroscopy in the range of 200–800 nm. Moreover, these species were discriminated using a LC/MS system with a DAD detector (220 nm) equipped with ZORBAX Eclipse XDB-C18 column, 4.6 mm × 150 mm, 5 μm. Methanol (A) and water with 0.1% formic acid (B) were used as mobile phase in a gradient mode (t = 0-1530 min, B% = 90-10-10%) at a flow rate of 1.0 mL/min. The MS detection was carried with electrospray ionization in positive mode with 3.5kev for capillary source voltage. The ion source temperature was 320 °C and the desolvation gas flowed at 10 L/min. The mass range of the detector was m/z 20–800.
3.2. Removal of MB by Fe-CS/MMTNS The efficiency of MB removal by Fe-CS/MMTNS was investigated and the results are shown in Fig. 5. As shown in Fig. 5a, MB solution by itself was very stable over time. However, the concentration of MB decreased by about 13% in 45 min through adsorption by Fe-CS/ MMTNS in the absence of light and H2O2. The concentration of MB further decreased but only marginally in 2 h when light was on. However, due to the generation of hydroxyl radicals, the concentration of MB decreased to the extent of 55.81% after 2 h under visible light irradiation in the presence of H2O2 according to the Eqs. (1) and (2).
3. Results and discussion 3.1. Characterization of Fe-CS/MMTNS Optical images of the Fe-CS/MMTNS gels (Fig. 1(a)) show that iron color gradually increased with increasing iron loading. The gels appear to be stacked by layers of MMTNS (Fig. 1(b)) as can be seen from the edge surface. Fig. 1(c) and (d) show the TEM images of Fe-CS/MMTNS at different magnifications. Fig. 1(c) shows stacked MMTNS and Fig. 1(d) at a higher magnification shows that MMTNS was consists of crimpled layers with a thickness of about 6 nm. Based on the distribution of Si (Fig. 1(e)), N (Fig. 1(f)), Fe (Fig. 1(g)), it can be inferred that Fe-CS/MMTNS was successfully assembled. The FTIR spectra of MMTNS, CS, Fe-CS and Fe-CS/MMTNS gel are presented in Fig. 2. Fig. 2a shows a typical FTIR spectrum of MMT with peaks at 3621 cm−1, 914 cm−1 and 841 cm−1, which can be assigned to the eOH bending of Al-Si-OH, Al-Al-OH and Mg-Al-OH, respectively [38]. Furthermore, the peak at 1033 cm−1 can be assigned to the Si-O bending (Fig. 2) while the plane vibrations of OH functional groups from interlayer water molecules can be assigned to the peaks at 519 cm−1 and 464 cm−1 [39]. All the above peaks are also observed in
H2O2 + hv = 2 %OH
(1)
%OH + MB → intermediate + %OH → final product
(2)
Nevertheless, when Fe-CS/MMTNS was added to dye solution in the presence of H2O2 a dramatic change in dye concentration occurred as the mixture became totally colorless in 120 min through synergy of both adsorption and photo-Fenton. The Fe-CS/MMTNS appears to have performed better than previously reported materials such as Fe-PILCs [48], F-400 [49] and LaFeO3/Bentonite Nanocomposite [50]. During the decomposition of MB on Fe-CS/MMTNS, the concentration of MB decreased drastically in the beginning which was mainly controlled by the concentration of H2O2 and Fe3+ on the surface of catalyst, and then it showed a slower disintegration of MB due to the accumulation of 3
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Fig. 1. (a) Optical images of Fe-CS/MMTNS gels with different Fe contents; (b) SEM image of Fe-CS/MMTNS; (c) and (d) TEM images of Fe-CS/MMTNS at two different magnifications; (e) Si element distribution of Fe-CS/MMTNS gel; (f) N element distribution of Fe-CS/MMTNS gel; (g) Fe element distribution of Fe-CS/ MMTNS gel.
due to less effective adsorption sites for MB, which were left behind after the increase in Fe loading. At the same time, however, more reactive radicals could be generated that promoted the removal of MB. Zero-order were also determined on Fe-CS(1)/MMTNS material and the results are shown in Table 1. The degradation kinetic rate constant of MB on Fe-CS(1)/MMTNS was the largest (Table 1) among all the materials tested here. In order to further identify adsorption and photoFenton, Fe-CS(0.25)/MMTNS was selected for the latter experiments since it showed considerable adsorption as well as the photo-Fenton oxidation of MB. Fig. 5c illustrates that MB was discolored on Fe-CS/MMTNS in 2 h under visible light irradiation in a wide range of pH. About 13% of MB was removed by Fe-CS/MMTNS through the adsorption process at both pH = 3.0 and pH = 6.5. However, nearly 25% of MB was adsorbed from 15 mg/L of MB in the mixture at pH = 10.0. In this photocatalytic process, the removal of MB at pH = 3.0 was faster than at pH = 6.5 because photo-Fenton could have been enhanced at the acidic condition [51]. In addition, MB removal was greater at pH = 10.0 than at pH = 6.5 because more MB could be concentrated at pH = 10.0 via
intermediate products of MB and the consumption of H2O2. In order to get more details about the degradation kinetics, zero-order kinetics are applied and the results are displayed in Table 1. It can be seen from Table 1 that the kinetic rate constant of MB degradation on the gel in the presence of H2O2 was 0.2261 mg·L−1·min−1 which is higher than that of the kinetic rate constant of H2O2 alone. Regeneration results by using 0.1 M HCl showed that it was difficult to regenerate Fe-CS(0)/MMTNS without the presence of Fe as shown in Fig. S1(a). The adsorption capacity decreased sharply after five cycles, i.e., the Fe-CS(0)/MMTNS lost its adsorption capacity after five cycles (Fig. S1(a)). H2O2 was also utilized to regenerate Fe-CS(0)/MMTNS as it was possible to produce reactive radicals to oxidize most organics. However, it took a long time to regenerate the gel if only H2O2 was added (Fig. S1(b)). Hence, Fe was introduced to activate H2O2 because it could enhance the generation of reactive radicals through photoFenton. The results of dye removal as a function of iron content are presented in Fig. 5b. Although the adsorption of MB decreased with the increase of iron content, the Fe-rich gels removed MB more quickly (Fig. 5b). The decreased adsorption with increased Fe content may be 4
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Fig. 4. TG and DTG curves of Fe-CS/MMTNS gel.
MMTNS is present, the adaptability of pH is expected to be greatly improved due to the following reasons: there are many hydroxyls on the surface of MMTNS, which can consume H+ under acidic conditions. On the other hand, the edge metal is easily hydrated under alkaline conditions. The zero-order kinetics constants of MB decomposition in solution on Fe-CS/MMTNS at pH = 6.5 and pH = 10.0 are 0.2019 mg·L−1·min−1 and 0.1765 mg·L−1·min−1 respectively (Table 1), which are slightly lower than that at pH = 3.0. The stability and recyclability of catalyst are important for practical
Fig. 2. FTIR spectra of (a) MMTNS, (b) CS, (c) Fe-CS and (d) Fe-CS/MMTNS self-assembled gel.
adsorption on Fe-CS/MMTNS and then it could be oxidized, indicating that both adsorption and photo-Fenton play an important role in the removal of MB. Since the amino group of chitosan is highly protonated and deprotonated, its adaptability to wastewater is greatly limited and the optimum pH for removal is close to neutral [33]. However, when
Fig. 3. (a) XPS spectrum of Fe-CS/MMTNS; (b) N 1s high resolution regions of Fe-CS/MMTNS; (c) C 1s high resolution regions of Fe-CS/MMTNS and (d) Fe 2p high resolution regions of Fe-CS/MMTNS. 5
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Fig. 5. (a) Removal of MB by Fe-CS/MMTNS without H2O2 but with and without light (curve labeled as Fe-CS/MMTNS), Fe-CS/MMTNS with H2O2 only (curve labeled as H2O2) and Fe-CS/MMTNS with H2O2 and light at pH = 3; (b) the effect of iron content on removal of MB at pH = 3 (c) removal of MB by Fe-CS/MMTNS at pH = 3.0, 6.5 and 10.0; (d) the recyclability of Fe-CS/MMTNS. (conditions: MB, 100 mL, 20 mg/L; catalyst, 10 mg; H2O2, 20 mmol/L).
the experiments of adsorptive, synchronous and asynchronous removal of MB on Fe-CS/MMTNS were conducted. In the adsorption experiment, no light from xenon lamp and no H2O2 were used. In the synchronous experiment, the Fe-CS/MMTNS was added in H2O2 + Vis light system directly without adsorption, while for asynchronous experiment, the FeCS/MMTNS was added after adsorption for 45 min. The results shown in Fig. 6 suggest that synchronous removal of MB was quicker than asynchronous removal, indicating that adsorption played an important role in the decolorization/degradation of MB. However, the MB
Table 1 Parameters of zero-order kinetics model in the removal of MB. Reaction type
pH
k0 (mg·L−1·min−1)
R2
H2O2 Fe-CS(0.25)/MMTNS + H2O2 Fe-CS(1)/MMTNS + H2O2 Fe-CS(0.1)/MMTNS + H2O2 Fe-CS(0)/MMTNS + H2O2 Fe-CS(0.25)/MMTNS + H2O2 Fe-CS(0.25)/MMTNS + H2O2 Fe-CS(0.25)/MMTNS + H2O2 + TBA Fe-CS(0.25)/MMTNS + H2O2 + Tri
3 3 3 3 3 6.5 10 3 3
0.0808 0.2261 0.2994 0.1934 0.1752 0.2019 0.1765 0.1035 0.1438
0.9938 0.9755 0.9575 0.9844 0.9957 0.9858 0.9926 0.9514 0.9956
application. Therefore, cycling experiments for the MB degradation with Fe-CS/MMTNS were performed and the results are displayed in Fig. 5d. As can be seen from Fig. 5d, MB decomposition remained very high even after five cycles, indicating that the incorporation of Fe significantly improved the recyclability of the gel compared to the Fe-CS (0)/MMTNS with no Fe. It is well-known that the photo-Fenton can be stimulated when Fe2+ and H2O2 are present during the reaction. The adsorbed MB on the gel could be decomposed through photo-Fenton and the active adsorption sites of the gel are regenerated. Furthermore, the leached amount of iron was only about 0.2 mg/L Fe3+ (Fig. 5d) from Fe-CS/MMTNS during five cycles of the dye removal, demonstrating that the introduced Fe was hardly leached out and therefore, the Fe-CS/MMTNS gel remained stable.
3.3. Removal mechanism of MB by Fe-CS/MMTNS Fig. 6. Adsorptive, synchronous and asynchronous removal of MB by Fe-CS/ MMTNS. (conditions: MB, 100 mL, 20 mg/L; catalyst, 10 mg; H2O2, 20 mmol/ L).
3.3.1. Synergy between adsorption and photo-Fenton In order to further identify the role of adsorption and photo-Fenton, 6
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removal difference between the synchronous and asynchronous curves is much smaller than the MB removal by adsorption alone. Based on the above results, it appears that the active adsorption sites of the Fe-CS/ MMTNS were renewed after the degradation of the adsorbed MB by photo-Fenton which is similar to the reduction of MB on the surface of MTiCuPd500 as adsorption played an important role in the removal of MB [52]. In the reduction, MB was firstly adsorbed on the surface of MTiCuPb500 and then it accepted electrons transferred from BH4−. Hence, the asynchronous removal followed the synchronous removal except that there was no gap between them at the end. The degradation was not further enhanced with the synchronous removal due to the degradation of all MB on the adsorption sites. This interesting phenomenon implies that there is a synergistic effect between adsorption and photo-Fenton for MB degradation, and the adsorption sites on FeCS/MMTNS are continually reactivated. 3.3.2. Role of hydroxyl radicals and superoxide radicals In a typical photo-Fenton, hydroxyl radicals with a high standard reduction potential of 2.80 V play an important role on the decomposition of organics [53]. At the same time, many other reactive radicals like %OOH, O2%− etc. are generated in the catalytic process. Here, TBA and Tri served as scavengers for hydroxyl radicals and superoxide radicals, respectively and the results are illustrated in Fig. 7a, which demonstrated that more amounts of MB remained in the solution with the addition of TBA and Tri. The zero-order kinetic constants of degradation with TBA and Tri were 0.1035 mg·L−1·min−1 and 0.1438 mg·L−1·min−1, respectively (Table 1), indicating %OH and O2%− played an important role on the decomposition of MB as the degradation kinetic constant of Fe-CS4/MMTNS + H2O2 system was much higher without scavengers. In order to further identify %OH and O2%−, ESR measurements were carried out and the results are shown in Fig. 7b and it can be seen that there are four characteristic peaks (1:2:2:1), which can be assigned to DMPO-%OH [54]. Thus, %OH and O2%− seem to have played substantial role on the decolorization of MB on the FeCS/MMTNS with the presence of H2O2 under visible light irradiation.
Fig. 8. UV–Vis spectra of MB decolored on Fe-CS/MMTNS at different times. (conditions: MB, 100 mL, 20 mg/L; catalyst, 10 mg; H2O2, 20 mmol/L; pH = 3).
transition and one peak at 664 nm attributable to the absorbance of n → π* transition [55]. During the adsorption process, there were no significant changes in the absorbances at 246 nm and 288 nm although the concentration of MB decreased but slightly. At the same time, no matter whether the catalyst adsorbed MB for 15 min or 45 min, the characteristic peak of MB at 664 nm also did not undergo blue shift or red shift, indicating that MB was not decomposed as it maintained the original structure. However, the three peaks changed dramatically during the catalytic reaction in the presence of light and H2O2. Firstly, the characteristic peak at 664 nm blue shifted to 634 nm, illustrating that the chromophore was degraded by reactive radicals. Correspondingly, the other two peaks gradually disappeared attributable to the replacement of N and S as the benzene ring was opened [56]. LCMS and IC were used in order to further identify the intermediate products and describe the decolorization pathway of MB on Fe-CS/ MMTNS and the results are shown in Figs. S2 and S3. From Figs. S2, S3 and S4, the intermediate species of m/z 256, 228, 301 and 279 were found, indicating that the MB underwent demethylation and sulfur oxidation and then the intermediates were oxidized into inorganic substances. Hence, it can be concluded that MB was decolorized in two ways on Fe-CS/MMTNS with the presence of H2O2 under visible light irradiation (Fig. 9). One was that MB directly underwent photo-Fenton degradation and was oxidized into inorganics in aqueous medium, and the other was that MB was adsorbed by Fe-CS/MMTNS and was then oxidized. That is, MB was removed by synergy between adsorption and
3.3.3. Decolorization pathway of MB With the presence of reactive radicals, MB was easily attacked and chromophores were destroyed. In order to reveal the decomposition of MB on Fe-CS/MMTNS, decolorization of MB solution during adsorption and photocatalytic process was determined and the results are shown in Fig. 8. The decolorization of MB during catalytic process could be visually observed. However, a color fading phenomenon was not prominent during the adsorption process alone (see inset in Fig. 8) without visible light irradiation. With the original MB solution, there are two peaks at 246 nm and 288 nm referring to the absorbance of π → π*
Fig. 7. (a) Decomposition kinetics of MB on Fe-CS/MMTNS without scavengers and with scavengers; (b) ESR spectra of reactive radicals during the decolorization. 7
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Fig. 9. Schematic diagram of MB degradation on Fe-CS/MMTNS.
Appendix A. Supplementary data
photo-Fenton. In Fig. 9, MB presented positive charge with an atomic weight of 284 g/mol at the outset since Cl was easily ionized and detached from chromophore. Then part of MB was directly attacked by reactive radicals and gradually converted into inorganic substances like NO3−, SO42−, and CO2 and the other part of MB was firstly adsorbed by Fe-CS/MMTNS gel and S was oxidized into S]O [57] followed by its desorption, which was then captured and degraded by the reactive radicals again. These intermediates were eventually turned into inorganic substances and the adsorption sites were regenerated for adsorbing fresh MB.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122322. References [1] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255, https://doi.org/10.1016/S09608524(00)00080-8. [2] E.N. El Qada, S.J. Allen, G.M. Walker, Adsorption of Methylene Blue onto activated carbon produced from steam activated bituminous coal: a study of equilibrium adsorption isotherm, Chem. Eng. J. 124 (2006) 103–110, https://doi.org/10.1016/ j.cej.2006.08.015. [3] S.J. Yuan, X.H. Dai, Facile synthesis of sewage sludge-derived mesoporous material as an efficient and stable heterogeneous catalyst for photo-Fenton reaction, Appl. Catal. B Environ. 154–155 (2014) 252–258, https://doi.org/10.1016/j.apcatb. 2014.02.031. [4] Y. Liu, W. Jin, Y. Zhao, G. Zhang, W. Zhang, Enhanced catalytic degradation of methylene blue by α-Fe2O3/graphene oxide via heterogeneous photo-Fenton reactions, Appl. Catal. B Environ. 206 (2017) 642–652, https://doi.org/10.1016/j. apcatb.2017.01.075. [5] L. Wang, J. Zhang, A. Wang, Fast removal of methylene blue from aqueous solution by adsorption onto chitosan-g-poly (acrylic acid)/attapulgite composite, Desalination 266 (2011) 33–39, https://doi.org/10.1016/j.desal.2010.07.065. [6] E.A. El-sharkawy, A.Y. Soliman, K.M. Al-amer, Comparative study for the removal of methylene blue via adsorption and photocatalytic degradation, J. Colloid Interface Sci. 310 (2007) 498–508, https://doi.org/10.1016/j.jcis.2007.02.013. [7] J. Dotto, M.R. Fagundes-Klen, M.T. Veit, S.M. Palácio, R. Bergamasco, Performance of different coagulants in the coagulation/flocculation process of textile wastewater, J. Clean. Prod. 208 (2019) 656–665, https://doi.org/10.1016/j.jclepro. 2018.10.112. [8] S.A. Ong, L.N. Ho, Y.S. Wong, Comparison on biodegradation of anionic dye orange II and cationic dye methylene blue by immobilized microorganisms on spent granular activated carbon, Desalin. Water Treat. 54 (2015) 557–561, https://doi. org/10.1080/19443994.2014.880155. [9] Z. Liang, C.F. Yan, S. Rtimi, J. Bandara, Piezoelectric materials for catalytic/photocatalytic removal of pollutants: recent advances and outlook, Appl. Catal. B Environ. 241 (2019) 256–269, https://doi.org/10.1016/j.apcatb.2018.09.028. [10] W. Wang, Y. Zhao, H. Bai, T. Zhang, V. Ibarra-galvan, Methylene blue removal from water using the hydrogel beads of poly (vinyl alcohol) -sodium alginate-chitosanmontmorillonite, Carbohydr. Polym. 198 (2018) 518–528, https://doi.org/10. 1016/j.carbpol.2018.06.124. [11] F. Boudissa, D. Mirilà, V.A. Arus, T. Terkmani, S. Semaan, M. Proulx, I.D. Nistor, R. Roy, A. Azzouz, Acid-treated clay catalysts for organic dye ozonation – Thorough mineralization through optimum catalyst basicity and hydrophilic character, J. Hazard. Mater. 364 (2019) 356–366, https://doi.org/10.1016/j.jhazmat.2018.09.
4. Conclusions The Fe-CS/MMTNS gel performed well in the removal of MB with the presence of H2O2 under visible light through the synergy effect of both adsorption and photo-Fenton. Because of abundant active sites of MMMTNS, the gel was able to photo-degrade MB under a wide range of pH conditions. Moreover, it showed effective reusability because the adsorption sites of the Fe-CS/MMTNS could be continually reactivated. At the same time, the Fe-CS/MMTNS gel was stable as the iron was hardly leached out because of its complexation with CS. MB was degraded through demethylation and oxidation in two pathways: a part of MB was directly attacked by reactive radicals and gradually converted into inorganic substances, and the other part of MB was firstly adsorbed by Fe-CS/MMTNS gel and then it was degraded by reactive radicals upon its desorption.
Acknowledgements The financial supports to this work from the National Natural Science Foundation of China under the projects No. 51874220 and 51674183 are gratefully acknowledged. This research is also supported by the financial supports for this work from Natural Science Foundation of Hubei Province of China (2018CFB468). Dr. Yunliang Zhao gratefully acknowledges financial support from China Scholarship Council (File No. 201806955006). One of us (SK) was supported by the College of Agricultural Sciences under Station Research Project No. PEN04566. 8
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[35] P.V. Nidheesh, R. Gandhimathi, S.T. Ramesh, Degradation of dyes from aqueous solution by Fenton processes: a review, Environ. Sci. Pollut. Res. 20 (2013) 2099–2132, https://doi.org/10.1007/s11356-012-1385-z. [36] M.C. Pereira, L.C.A. Oliveira, E. Murad, Iron oxide catalysts: Fenton and Fentonlike reactions–a review, Clay Miner. 47 (2012) 285–302, https://doi.org/10.1180/ claymin.2012.047.3.01. [37] H. Yi, W. Zhan, Y. Zhao, S. Qu, W. Wang, P. Chen, S. Song, A novel core-shell structural montmorillonite nanosheets/stearic acid composite PCM for great promotion of thermal energy storage properties, Sol. Energy Mater. Sol. Cells 192 (2019) 57–64, https://doi.org/10.1016/j.solmat.2018.12.015. [38] Y. Fang, A. Zhou, W. Yang, T. Araya, Y. Huang, P. Zhao, Complex formation via hydrogen bonding between rhodamine B and montmorillonite in aqueous solution, Sci. Rep. 229 (2018) 1–10, https://doi.org/10.1038/s41598-017-18057-8. [39] S. Bensalem, B. Hamdi, S. Del Confetto, M. Iguer-Ouada, A. Chamayou, H. Balard, R. Calvet, Characterization of chitosan/montmorillonite bionanocomposites by inverse gas chromatography, Colloids Surf. A Physicochem. Eng. Asp. 516 (2017) 336–344, https://doi.org/10.1016/j.colsurfa.2016.12.051. [40] L. Wang, A. Wang, Adsorption characteristics of Congo Red onto the chitosan/ montmorillonite nanocomposite, J. Hazard. Mater. 147 (2007) 979–985, https:// doi.org/10.1016/j.jhazmat.2007.01.145. [41] M.H.M. Hussein, M.F. El-Hady, W.M. Sayed, H. Hefni, Preparation of some chitosan heavy metal complexes and study of its properties, Polym. Sci. Ser. A. 54 (2012) 113–124, https://doi.org/10.1134/S0965545X12020046. [42] C. Liu, W. Zhang, S. Song, H. Li, A novel method to improve carboxymethyl cellulose performance in the flotation of talc, Miner. Eng. 131 (2019) 23–27, https:// doi.org/10.1016/j.mineng.2018.11.003. [43] X. Lin, L. Wang, S. Jiang, L. Cui, G. Wu, Iron-doped chitosan microsphere for As(III) adsorption in aqueous solution: kinetic, isotherm and thermodynamic studies, Korean J. Chem. Eng. 36 (2019) 1102–1114, https://doi.org/10.1007/s11814-0180117-6. [44] L.L. Min, L.M. Yang, R.X. Wu, L. Bin Zhong, Z.H. Yuan, Y.M. Zheng, Enhanced adsorption of arsenite from aqueous solution by an iron-doped electrospun chitosan nanofiber mat: preparation, characterization and performance, J. Colloid Interface Sci. 535 (2019) 255–264, https://doi.org/10.1016/j.jcis.2018.09.073. [45] T. Zhang, Y. Zhao, S. Kang, Y. Li, Q. Zhang, Formation of active Fe(OH)3 in situ for enhancing arsenic removal from water by the oxidation of Fe(II)in air with the presence of CaCO3, J. Clean. Prod. 227 (2019) 1–9, https://doi.org/10.1016/j. jclepro.2019.04.199. [46] J. Qu, Q. Hu, K. Shen, K. Zhang, Y. Li, H. Li, Q. Zhang, J. Wang, W. Quan, The preparation and characterization of chitosan rods modified with Fe3+ by a chelation mechanism, Carbohydr. Res. 346 (2011) 822–827, https://doi.org/10.1016/j. carres.2011.02.006. [47] R.B. Hernández, A.P. Franco, O.R. Yola, A. López-Delgado, J. Felcman, M.A.L. Recio, A.L.R. Mercê, Coordination study of chitosan and Fe3+, J. Mol. Struct. 877 (2008) 89–99, https://doi.org/10.1016/j.molstruc.2007.07.024. [48] A. De Leo, Catalytic activity of an iron-pillared montmorillonitic clay mineral in heterogeneous photo-Fenton process, Catal. Today 133 (2008) 600–605, https:// doi.org/10.1016/j.cattod.2007.12.130. [49] S. Juan, A. De Leo, Iron-pillared clays as catalysts for dye removal by the heterogeneous photo-Fenton technique, React. Kinet. Mech. Catal. 110 (2013) 101–117, https://doi.org/10.1007/s11144-013-0593-y. [50] S. Janaki, K. Punithamurthy, L. Renuk, A novel approach for synthesis of LaFeO3/ bentonite nanocomposite for degradation of methylene blue with enhanced photocatalytic activity, Mater. Res. Exp. 6 (2019) 035013, , https://doi.org/10.1088/ 2053-1591/aaf333. [51] M. Bobu, A. Yediler, I. Siminiceanu, S. Schulte-hostede, Degradation studies of ciprofloxacin on a pillared iron catalyst, Appl. Catal. B Environ. 83 (2008) 15–23, https://doi.org/10.1016/j.apcatb.2008.01.029. [52] A. Joseph, K. Vellayan, B. González, M.A. Vicente, A. Gil, Effective degradation of methylene blue in aqueous solution using Pd-supported Cu-doped Ti-pillared montmorillonite catalyst, Appl. Clay Sci. 168 (2019) 7–10, https://doi.org/10. 1016/j.clay.2018.10.009. [53] Y. Zhang, M. Zhou, A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values, J. Hazard. Mater. 362 (2019) 436–450, https://doi.org/10.1016/j.jhazmat.2018.09.035. [54] S. Gazi, R. Ananthakrishnan, Semi-quantitative determination of hydroxyl radicals by benzoic acid hydroxylation: an analytical methodology for photo-Fenton systems, Curr. Anal. Chem. 8 (2012) 143–149, https://doi.org/10.2174/ 157341112798472297. [55] J. Sun, S. Dong, J. Feng, X. Yin, X. Zhao, Enhanced sunlight photocatalytic performance of Sn-doped ZnO for Methylene Blue degradation, J. Mol. Catal. A Chem. 335 (2011) 145–150, https://doi.org/10.1016/j.molcata.2010.11.026. [56] J. Juhui, L. Xiangdong, C. Huajun, X. Guoxi, Synergistic degradation of methylene blue by ultrasound/H2O2, J. Henan Norm. Univ. Nat. Sci. 34 (2006) 106–109. [57] Y. Hu, L. Zhou, H. Liu, X. Guo, Visible light photocatalytic degradation of methylene blue over N-doped TiO2, Key Eng. Mater. 609–610 (2014) 141–146, https://doi. org/10.4028/www.scientific.net/KEM.609-610.141.
070. [12] S.C. Kim, D.K. Lee, Preparation of Al-Cu pillared clay catalysts for the catalytic wet oxidation of reactive dyes, Catal. Today 97 (2004) 153–158, https://doi.org/10. 1016/j.cattod.2004.03.066. [13] P.V. Nidheesh, M. Zhou, M.A. Oturan, An overview on the removal of synthetic dyes from water by electrochemical advanced oxidation processes, Chemosphere 197 (2018) 210–227, https://doi.org/10.1016/j.chemosphere.2017.12.195. [14] J. Feng, X. Hu, P.L. Yue, Effect of initial solution pH on the degradation of Orange II using clay-based Fe nanocomposites as heterogeneous photo-Fenton catalyst, Water Res. 40 (2006) 641–646, https://doi.org/10.1016/j.watres.2005.12.021. [15] D. Gumy, P. Fernández-Ibáñez, S. Malato, C. Pulgarin, O. Enea, J. Kiwi, Supported Fe/C and Fe/Nafion/C catalysts for the photo-Fenton degradation of Orange II under solar irradiation, Catal. Today 101 (2005) 375–382, https://doi.org/10. 1016/j.cattod.2005.03.036. [16] J. Feng, X. Hu, P.L. Yue, Degradation of salicylic acid by photo-assisted Fenton reaction using Fe ions on strongly acidic ion exchange resin as catalyst, Chem. Eng. J. 100 (2004) 159–165, https://doi.org/10.1016/j.cej.2004.01.031. [17] Q. Wu, H. Wang, C. Yi, Preparation of photo-Fenton heterogeneous catalyst (Fe-TS1 zeolite) and its application in typical azo dye decoloration, J. Photochem. Photobiol. A Chem. 356 (2018) 138–149, https://doi.org/10.1016/j.jphotochem. 2017.12.041. [18] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Mineralization of an azo dye Acid Red 14 by electro-Fenton’s reagent using an activated carbon fiber cathode, Dye. Pigment. 65 (2005) 227–233, https://doi.org/10.1016/j.dyepig.2004.07.019. [19] Q. Wang, S. Tian, J. Long, P. Ning, Use of Fe(II)Fe(III)-LDHs prepared by co-precipitation method in a heterogeneous-Fenton process for degradation of Methylene Blue, Catal. Today 224 (2014) 41–48, https://doi.org/10.1016/j.cattod.2013.11. 031. [20] C.A.P. Almeida, N.A. Debacher, A.J. Downs, L. Cottet, C.A.D. Mello, Removal of methylene blue from colored effluents by adsorption on montmorillonite clay, J. Colloid Interface Sci. 332 (2009) 46–53, https://doi.org/10.1016/j.jcis.2008.12. 012. [21] M. Aflaki Jalali, A. Dadvand Koohi, M. Sheykhan, Experimental study of the removal of copper ions using hydrogels of xanthan, 2-acrylamido-2-methyl-1-propane sulfonic acid, montmorillonite: kinetic and equilibrium study, Carbohydr. Polym. 142 (2016) 124–132, https://doi.org/10.1016/j.carbpol.2016.01.033. [22] F. Bergaya, G. Lagaly, General Introduction: Clay, Clay Minerals, and Clay Science, Elsevier, 2006 doi: 10.1016/S1572-4352(05)01001-9. [23] Y. Zhao, H. Yi, F. Jia, H. Li, C. Peng, S. Song, A novel method for determining the thickness of hydration shells on nanosheets: a case of montmorillonite in water, Powder Technol. 306 (2017) 74–79, https://doi.org/10.1016/j.powtec.2016.10. 045. [24] W. Wang, Y. Zhao, H. Yi, T. Chen, S. Kang, T. Zhang, F. Rao, S. Song, Pb(Ⅱ) removal from water using porous hydrogel of chitosan-2D montmorillonite, Int. J. Biol. Macromol. 128 (2019) 85–93, https://doi.org/10.1016/j.ijbiomac.2019.01.098. [25] G.L. Visavale, R.K. Nair, S.S. Kamath, M.R. Sawant, B.N. Thorat, Granulation and drying of modified clay incorporated pesticide formulation, Dry. Technol. 25 (2007) 1369–1376, https://doi.org/10.1080/07373930701438980. [26] Y.-N. Zhang, Y.-X. Yu, Adsorptive removal of Cr3+, Cu2+ and Ni2+ ions by magnetic Fe3O4@alkali-treated coal fly ash, Desalin. Water Treat. 123 (2018) 277–287, https://doi.org/10.5004/dwt.2018.22778. [27] W. Wang, Y. Zhao, H. Yi, T. Chen, Preparation and characterization of self- assembly hydrogels with exfoliated montmorillonite nanosheets and chitosan, Nanotechnology 29 (2018) 025605. [28] L. Ai, L. Li, Efficient removal of organic dyes from aqueous solution with ecofriendly biomass-derived carbon@montmorillonite nanocomposites by one-step hydrothermal process, Chem. Eng. J. 223 (2013) 688–695, https://doi.org/10.1016/j.cej. 2013.03.015. [29] W.S. Wan Ngah, L.C. Teong, M.A.K.M. Hanafiah, Adsorption of dyes and heavy metal ions by chitosan composites: a review, Carbohydr. Polym. 83 (2011) 1446–1456, https://doi.org/10.1016/j.carbpol.2010.11.004. [30] C. Gerente, V.K.C. Lee, P. Le Cloirec, G. Mckay, Application of chitosan for the removal of metals from wastewaters by adsorption-mechanisms and models Review, Crit. Rev. Environ. Sci. Technol. (2007) 41–127, https://doi.org/10.1080/ 10643380600729089. [31] S. Kang, Y. Zhao, W. Wang, T. Zhang, T. Chen, H. Yi, F. Rao, S. Song, Removal of methylene blue from water with montmorillonite nanosheets/chitosan hydrogels as adsorbent, Appl. Surf. Sci. 448 (2018) 203–211, https://doi.org/10.1016/j.apsusc. 2018.04.037. [32] X. Hu, R. Liang, G. Sun, Super-adsorbent hydrogel for removal of methylene blue dye from aqueous solution, J. Mater. Chem. A (2018) 17612–17624, https://doi. org/10.1039/c8ta04722g. [33] S. Rashid, C. Shen, X. Chen, S. Li, Y. Chen, Y. Wen, J. Liu, Enhanced catalytic ability of chitosan-Cu-Fe bimetal complex for the removal of dyes in aqueous solution, RSC Adv. 5 (2015) 90731–90741, https://doi.org/10.1039/c5ra14711e. [34] R. Hrdina, Oxidation of azo textile soluble dyes with hydrogen peroxide in the presence of Cu (II) e chitosan heterogeneous catalysts, Dye. Pigment 73 (2007) 19–24, https://doi.org/10.1016/j.dyepig.2005.10.004.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Self-assembled gels of Fe-chitosan/montmorillonite nanosheets: Dye degradation by the synergistic effect of adsorption and photo-Fenton reaction
T
Yunliang Zhaoa,b,c, Shichang Kanga,b, Lei Qinb, Wei Wanga,b, Tingting Zhanga,b, ⁎ ⁎ Shaoxian Songa,b, , Sridhar Komarnenic, a
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China Department of Ecosystem Science and Management and Materials Research Institute, 204 Energy and the Environment Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b c
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of Fe-chitosan/montmorillonite • Gel nanosheets efficiently degraded methylene blue.
effect of adsorption and • Synergistic photo-Fenton reaction led to dye degradation.
had effective reusability due to • Gel continuous reactivation of adsorption sites.
was stable and worked efficiently • Gel under a wide range of pH conditions. blue was decomposed by • Methylene demethylation and oxidation reactions.
A R T I C LE I N FO
A B S T R A C T
Keywords: Synergy Adsorption Photo-Fenton Fe-chitosan/montmorillonite nanosheets gel Methylene blue
Self-assembled gel of Fe-chitosan/montmorillonite nanosheets (Fe-CS/MMTNS) was prepared for elimination of methylene blue (MB) under visible light in the presence of H2O2. The Fe-CS/MMTNS gel was characterized by FTIR, SEM-EDS, XPS and TG. The Fe-CS/MMTNS gel performed well in the removal of MB through the synergistic effect of adsorption and photo-Fenton reaction. Moreover, this gel worked efficiently under a wide range of pH conditions. This composite gel also showed effective reusability because the adsorption sites of the Fe-CS/MMTNS are continually reactivated through photo-Fenton degradation. Additionally, the Fe-CS/MMTNS gel was found to be stable and the iron ions were hardly leached out because of the complexation between iron and chitosan (CS). The MB degradation occurred by two pathways: a part of MB was directly attacked by reactive radicals and gradually converted into inorganic substances while another part of MB was firstly adsorbed by FeCS/MMTNS gel and then degraded by reactive radicals.
⁎ Corresponding authors at: Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China (S. Song). E-mail addresses: [email protected] (S. Song), [email protected] (S. Komarneni).
https://doi.org/10.1016/j.cej.2019.122322 Received 7 June 2019; Received in revised form 20 July 2019; Accepted 23 July 2019 Available online 23 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
was introduced via CS on gels for regeneration in this work through the release of hydroxyl radicals to decompose the dye in the presence of H2O2 [33,34], and also visible light was used to photo-reduce Fe3+ into Fe2+ and accelerate the removal of MB [35,36]. Moreover, the Fe-CS/ MMTNS gel prepared in this work intelligently solves the pH sensitivity of metal-CS complex because of the hydration ability of MMTNS surface. The morphological features, elemental distribution and thermal stability of the Fe-CS/MMTNS gels were systematically characterized and the efficiency of MB removal by gels and their regeneration and removal mechanism of MB were also investigated.
With the development of textile, paper, plastic, leather and the other industries, the use of synthetic dyes also increased. The number of commercial species of synthetic dyes increased to 10,000 while their production went up to 7 × 105 tons/year [1,2]. Although dyes brilliantly decorate our world, a lot of serious environmental troubles also emerged. Due to the toxicity, carcinogenicity, and mutagenicity of dyes, people and animals suffer from illness [3,4]. For instance, the symptoms of nausea, vomiting, profuse sweeting and mental confusion can occur upon the inhalation of MB [5,6]. In addition, dyes can hinder the photosynthesis of aquatic organisms. Therefore, many methods including sedimentation [7], biodegradation [8], advanced oxidation [9] and adsorption [10] have been proposed to treat dyes in wastewater before its discharge into the environment. Among all the treatment methods, advanced oxidation process serves as a robust and clean alternative to decompose various dyes. Thanks to mild conditions and easy operation, Fenton process attracts much attention among diverse advanced oxidation processes such as ozonation [11], catalytic wet oxidation [12], electrochemical oxidation [13] and Fenton [14]. Compared to homogeneous Fenton, heterogeneous Fenton attracts a lot of attention owing to its fixation of Fe and adaptability to a wide range of pH conditions. Various materials such as Nafion [15], strongly acidic ion exchange resin (SAIER) [16], molecular sieve [17], carbon fiber [18] and clay [19] were used to synthesize heterogeneous Fenton catalysts. Among these materials, montmorillonite (MMT) clay is an attractive candidate to assemble as a catalyst because of its abundance and cost-effectiveness. More importantly, MMT itself is non-toxic and is able to adsorb many pollutants like MB because of its cation exchange ability and huge specific surface area [20]. However, there are two urgent issues to be solved when MMT needs to be used for practical applications: (1) the interlayer space of untreated MMT is small which hinders the entry of large-sized modifiers or pollutants [21] and (2) the MMT is difficult to separate from liquid [22]. Fortunately, the interlayer space and surface area of MMT can be enlarged by intercalation, exfoliation and pillaring due to the weak electrostatic attraction and Van Der Waals force that bond the MMT units. MMT can be exfoliated into nanosheets to surmount the interlayer space/distance limitation and to expose many more active sites [23]. But an unavoidable fact is that the particle size is very small and most particles reach nano range, which inevitably increases the difficulty in solid-liquid separation [24]. Many methods including granulation, magnetization and gelation are proposed to deal with the difficulty of its separation from liquid [25–27]. Among them, selfassembling the exfoliated MMT nanosheets (MMTNS) into three dimensional gels seems a good option since the gels possess exceptional transport properties of ions as well as water, especially when freezedried because of abundant pores [28]. Chitosan (CS) is an ideal cross-linking agent for MMTNS to prepare self-assembled gels. CS is the N-deacetylated product of chitin, which is merely an inferior type of cellulose in nature. It’s properties such as non-toxicity, hydrophilicity, biocompatibility, biodegradability and adsorption ability are very attractive for many uses [29,30]. In our previous work, we prepared a self-assembled MMTNS-CS gel that was formed as a result of the hydrogen bonds (eOH···+NH3e) and electrostatic interaction between the MMTNS and CS and it performed well in the adsorption of MB [27,31]. Nevertheless, like some other gels [10,32], MMTNS-CS gel did not perform very satisfactorily for recycling as it required a long time for regeneration. However, MMTNS played a primary role in the adsorption of MB on MMTNS-CS gels while amino groups from CS were mostly idle. Hence, it is useful to utilize these idle amino groups of CS for better regeneration of MMTNS-CS gels. As per previous research, CS can serve as a chelating agent for 3d metal ions such as Fe3+, Cu2+ and Ni2+ because of its high hydrophilicity, flexible structure and high chemical reactivity. Hence, Fe3+
2. Experimental 2.1. Materials Raw MMT purchased from Nicheng Tianyu Chemical Industry Co., Ltd (Inner Mongolia, China) was used for preparing MMTNS. 5,5Dimethyl-1-pyrroline N-oxide (DMPO) was purchased from Accelerating Scientific and Industrial Development for analyzing hydroxyl radicals. The other reagents were provided by Sinopharm Chemical Reagent Co. Ltd., China and all of them were of analytical reagent grade. Iron trichloride hexahydrate (FeCl3·6H2O) and chitosan were first dissolved in acetic acid (C2H4O2) and then chitosan was used as a crosslinking agent with MMTNS. Methylene blue (C16H18ClN3S·3H2O) is a prototypical organic pollutant, which was tested here under conventional conditions. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust pH using a concentration of 0.1 mol/L. Trichloromethane (Tri, CHCl3) and tert-Butanol (TBA, C4H10O) were used as the scavengers for hydroxyl radicals and superoxide radicals, respectively. 2.2. Synthesis of Fe-CS/MMTNS self-assembled gels Fe-CS/MMTNS gels were self-assembled by the following three steps: (1) MMTNS with the concentration of 15 g/L was first prepared according to a previous method [37] as follows: Typically, 50 g crude MMT were dispersed in 1 L water by mechanical stirring (IKA RW 20, Germany) for 4 h at a speed of 400 rpm. Then, by centrifuging at 109g for 1 min and 1090g for 5 min, pure MMT was collected by decanting the supernatant. By dispersing pure MMT in 150 mL water, it could be exfoliated to obtain MMTNS. Finally, MMTNS were diluted to a concentration of 15 g/L. (2) To prepare Fe-CS, 1 g CS was dissolved in a 2.5% acetic acid solution (100 mL) and the mixture was stirred at room temperature for 4 h. Subsequently, 0 mL, 2.42 mL, 6.15 mL and 19.88 mL FeCl3 solutions (0.2 mol/L) were injected for making the molar ratios of Fe3+ to amino groups in chitosan as 0, 0.1, 0.25 and 1. Finally, the resultant mixtures were stirred at room temperature for 4 h again. (3) To make Fe-CS/MMTNS gel, 20 mL MMTNS solution was first put into a 50 mL breaker with specific Fe-CS and after that the mixture self-assembled into gels upon heating in a 90 °C oven for 24 h. The products obtained with 0, 0.1, 0.25 and 1 M ratios of Fe/amino groups were named as Fe-CS(0)/MMTNS, Fe-CS(0.1)/MMTNS, Fe-CS(0.25)/ MMTNS and Fe-CS(1)/MMTNS, respectively. The molar ratio of Fe3+ to amino groups in chitosan generally was 0.25 unless otherwise stated and the Fe-CS(0.25)/MMTNS was expressed simply as Fe-CS/MMTNS for convenience. 2.3. Characterization of the Fe-CS/MMTNS gel The gel was coated with platinum for scanning electron microscopy (SEM) analysis using a FEI Nova NanoSEM450 and an energy-dispersive X-ray analysis instrument was utilized to detect the elemental composition of the gel. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 6700 with KBr as the reference sample. X-ray photoelectron spectroscopy (XPS) analysis was determined on an ESCALAB 250Xi instrument and all binding energies were referenced to 2
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the case of Fe-CS/MMTNS gel (Fig. 2d). As for CS (Fig. 2b) and Fe-CS (Fig. 2c), their FTIR spectra are similar and the peaks at 1415 cm−1 and 1383 cm−1 can be attributed to the vibrations of methyl groups and C-N [40]. The peak representing amino groups in CS red-shifted from 1599 cm−1 to 1529 cm−1 in Fe-CS suggesting that amino groups played an important role on the complexation of Fe3+ [41]. Although most of the peaks of CS overlap with MMTNS (Fig. 2d), weak vibrations of the methyl groups and C-N can be seen at 1421 cm−1 and 1390 cm−1, respectively. The vibration of amino groups was observed at 1549 cm−1 (Fig. 2d) indicating that amino groups played an important role in assembling gels [31]. As shown in Figs. 1 and 2, it can be inferred that Fe-CS/MMTNS was successfully synthesized. The wide XPS spectra of Fe-CS/MMTNS revealed the atom weights of N and Fe were 2.55% and 0.68%, respectively as shown in Fig. 3(a). In addition, the high-resolution regions of N 1s, Fe 2p and C 1s were procured to investigate the coordination of iron in CS and MMTNS. When preparing Fe-CS/MMTNS, CS was firstly dissolved in acetic acid solution and CS was protonated which would be attracted by MMTNS as indicated by a peak of N 1s at 402.11 eV as shown in Fig. 3(b) [27,42]. In Fig. 3(b), there was a peak of N 1s at 400.38 eV which conforms to NeC]O and its bonding energy of C 1s was 288.54 eV as shown in Fig. 3(c) [33]. In Fig. 3(b), there was still a peak at 398.91 eV which was assigned to NH2-Fe3+ indicating the combination of CS and Fe3+ [43]. In addition, the peak of Fe3+ emerged at 712.67 eV, 716.14 eV, 726.58 eV accompanied by two satellite peaks at 721.07 eV and 735.1 eV as shown in Fig. 3(d) due to the complexion between CS and Fe3+ [44,45]. Thus far, the above results indicated that amino groups of CS were not only combined with Fe3+ but they also served as cross-linking groups. To further investigate the materials, TG and DTG analyses of FeCS/MMTNS were also obtained and the results are shown in Fig. 4. The TG curve could be divided into four stages of mass loss. The first stage ended at 176 °C with a loss of 8.67% due to the vaporization of adsorbed water. The second stage was assigned to the decomposition of mainly the chains of CS and loss of strongly bonded water. The third stage of mass loss started at 380 °C, which corresponded to the release of volatile products, oxidative degradation and the cross-linking reaction of Fe-CS [46,47]. The fourth stage mass loss was 3.08%, which occurred from 686 °C to 1000 °C. This high temperature mass loss was mainly due to the dehydroxylation of structural OH of MMTNS.
the neutral carbon peak at 284.8 eV. The thermogravimetric (TG and DTG) analysis was performed on a STA449F3 unit as the gel was heated in an atmosphere of nitrogen at 10 °C/min between 32 °C and 1000 °C. 2.4. Removal of MB on Fe-CS/MMTNS gel The photocatalytic degradation of MB was performed under visible light irradiation using PLS-SXE 300 system provided by Beijing Perfectlight Technology co. LTD and no filters were replaced that the spectrum was at 320–780 nm. The center optical power was higher than 2.877 kW/m2. In a typical dye removal experiment, 0.01 g gel was added in a 100 mL of 20 ppm MB solution of a certain pH. First, adsorption-desorption equilibrium was established in 45 min and then 200 µL fresh 30% H2O2 aqueous solution was dispersed in the mixture for 2 min. The reactor was immediately exposed to the light for 2 h and 2.5 mL sample was extracted every 15 min for analyzing the concentration of MB using the absorption at 664 nm (Origin Aquamate 8000 UV–Vis Spectrophotometer). A 13 mm 0.22 μm syringe filter was used to remove the gel powder from liquid before UV–Vis analysis. The stability of gel was tested by repeatedly using the gel for five runs. After each run, the gel was concentrated and washed with distilled water for subsequent reaction. 2.5. Analysis method To reveal the mechanism involved in the decolorization of MB in FeCS/MMTNS + H2O2 + Vis system, hydroxyl radical was identified using DMPO via electron spin resonance (EPR). The parameters of ESR were set as follows: a sweep width at 100 G, a center field at 3500.455 G, a modulation amplitude at 1 G, a microwave frequency at 9.81 GHz, a microwave power at 21.59 and a sweep time at 30.72 s. Trichloromethane (Tri) and Tert-Butanol (TBA) were applied to quench superoxide radicals and hydroxyl radicals, respectively. The intermediate species generated during MB degradation were measured by UV–Vis spectroscopy in the range of 200–800 nm. Moreover, these species were discriminated using a LC/MS system with a DAD detector (220 nm) equipped with ZORBAX Eclipse XDB-C18 column, 4.6 mm × 150 mm, 5 μm. Methanol (A) and water with 0.1% formic acid (B) were used as mobile phase in a gradient mode (t = 0-1530 min, B% = 90-10-10%) at a flow rate of 1.0 mL/min. The MS detection was carried with electrospray ionization in positive mode with 3.5kev for capillary source voltage. The ion source temperature was 320 °C and the desolvation gas flowed at 10 L/min. The mass range of the detector was m/z 20–800.
3.2. Removal of MB by Fe-CS/MMTNS The efficiency of MB removal by Fe-CS/MMTNS was investigated and the results are shown in Fig. 5. As shown in Fig. 5a, MB solution by itself was very stable over time. However, the concentration of MB decreased by about 13% in 45 min through adsorption by Fe-CS/ MMTNS in the absence of light and H2O2. The concentration of MB further decreased but only marginally in 2 h when light was on. However, due to the generation of hydroxyl radicals, the concentration of MB decreased to the extent of 55.81% after 2 h under visible light irradiation in the presence of H2O2 according to the Eqs. (1) and (2).
3. Results and discussion 3.1. Characterization of Fe-CS/MMTNS Optical images of the Fe-CS/MMTNS gels (Fig. 1(a)) show that iron color gradually increased with increasing iron loading. The gels appear to be stacked by layers of MMTNS (Fig. 1(b)) as can be seen from the edge surface. Fig. 1(c) and (d) show the TEM images of Fe-CS/MMTNS at different magnifications. Fig. 1(c) shows stacked MMTNS and Fig. 1(d) at a higher magnification shows that MMTNS was consists of crimpled layers with a thickness of about 6 nm. Based on the distribution of Si (Fig. 1(e)), N (Fig. 1(f)), Fe (Fig. 1(g)), it can be inferred that Fe-CS/MMTNS was successfully assembled. The FTIR spectra of MMTNS, CS, Fe-CS and Fe-CS/MMTNS gel are presented in Fig. 2. Fig. 2a shows a typical FTIR spectrum of MMT with peaks at 3621 cm−1, 914 cm−1 and 841 cm−1, which can be assigned to the eOH bending of Al-Si-OH, Al-Al-OH and Mg-Al-OH, respectively [38]. Furthermore, the peak at 1033 cm−1 can be assigned to the Si-O bending (Fig. 2) while the plane vibrations of OH functional groups from interlayer water molecules can be assigned to the peaks at 519 cm−1 and 464 cm−1 [39]. All the above peaks are also observed in
H2O2 + hv = 2 %OH
(1)
%OH + MB → intermediate + %OH → final product
(2)
Nevertheless, when Fe-CS/MMTNS was added to dye solution in the presence of H2O2 a dramatic change in dye concentration occurred as the mixture became totally colorless in 120 min through synergy of both adsorption and photo-Fenton. The Fe-CS/MMTNS appears to have performed better than previously reported materials such as Fe-PILCs [48], F-400 [49] and LaFeO3/Bentonite Nanocomposite [50]. During the decomposition of MB on Fe-CS/MMTNS, the concentration of MB decreased drastically in the beginning which was mainly controlled by the concentration of H2O2 and Fe3+ on the surface of catalyst, and then it showed a slower disintegration of MB due to the accumulation of 3
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Fig. 1. (a) Optical images of Fe-CS/MMTNS gels with different Fe contents; (b) SEM image of Fe-CS/MMTNS; (c) and (d) TEM images of Fe-CS/MMTNS at two different magnifications; (e) Si element distribution of Fe-CS/MMTNS gel; (f) N element distribution of Fe-CS/MMTNS gel; (g) Fe element distribution of Fe-CS/ MMTNS gel.
due to less effective adsorption sites for MB, which were left behind after the increase in Fe loading. At the same time, however, more reactive radicals could be generated that promoted the removal of MB. Zero-order were also determined on Fe-CS(1)/MMTNS material and the results are shown in Table 1. The degradation kinetic rate constant of MB on Fe-CS(1)/MMTNS was the largest (Table 1) among all the materials tested here. In order to further identify adsorption and photoFenton, Fe-CS(0.25)/MMTNS was selected for the latter experiments since it showed considerable adsorption as well as the photo-Fenton oxidation of MB. Fig. 5c illustrates that MB was discolored on Fe-CS/MMTNS in 2 h under visible light irradiation in a wide range of pH. About 13% of MB was removed by Fe-CS/MMTNS through the adsorption process at both pH = 3.0 and pH = 6.5. However, nearly 25% of MB was adsorbed from 15 mg/L of MB in the mixture at pH = 10.0. In this photocatalytic process, the removal of MB at pH = 3.0 was faster than at pH = 6.5 because photo-Fenton could have been enhanced at the acidic condition [51]. In addition, MB removal was greater at pH = 10.0 than at pH = 6.5 because more MB could be concentrated at pH = 10.0 via
intermediate products of MB and the consumption of H2O2. In order to get more details about the degradation kinetics, zero-order kinetics are applied and the results are displayed in Table 1. It can be seen from Table 1 that the kinetic rate constant of MB degradation on the gel in the presence of H2O2 was 0.2261 mg·L−1·min−1 which is higher than that of the kinetic rate constant of H2O2 alone. Regeneration results by using 0.1 M HCl showed that it was difficult to regenerate Fe-CS(0)/MMTNS without the presence of Fe as shown in Fig. S1(a). The adsorption capacity decreased sharply after five cycles, i.e., the Fe-CS(0)/MMTNS lost its adsorption capacity after five cycles (Fig. S1(a)). H2O2 was also utilized to regenerate Fe-CS(0)/MMTNS as it was possible to produce reactive radicals to oxidize most organics. However, it took a long time to regenerate the gel if only H2O2 was added (Fig. S1(b)). Hence, Fe was introduced to activate H2O2 because it could enhance the generation of reactive radicals through photoFenton. The results of dye removal as a function of iron content are presented in Fig. 5b. Although the adsorption of MB decreased with the increase of iron content, the Fe-rich gels removed MB more quickly (Fig. 5b). The decreased adsorption with increased Fe content may be 4
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Fig. 4. TG and DTG curves of Fe-CS/MMTNS gel.
MMTNS is present, the adaptability of pH is expected to be greatly improved due to the following reasons: there are many hydroxyls on the surface of MMTNS, which can consume H+ under acidic conditions. On the other hand, the edge metal is easily hydrated under alkaline conditions. The zero-order kinetics constants of MB decomposition in solution on Fe-CS/MMTNS at pH = 6.5 and pH = 10.0 are 0.2019 mg·L−1·min−1 and 0.1765 mg·L−1·min−1 respectively (Table 1), which are slightly lower than that at pH = 3.0. The stability and recyclability of catalyst are important for practical
Fig. 2. FTIR spectra of (a) MMTNS, (b) CS, (c) Fe-CS and (d) Fe-CS/MMTNS self-assembled gel.
adsorption on Fe-CS/MMTNS and then it could be oxidized, indicating that both adsorption and photo-Fenton play an important role in the removal of MB. Since the amino group of chitosan is highly protonated and deprotonated, its adaptability to wastewater is greatly limited and the optimum pH for removal is close to neutral [33]. However, when
Fig. 3. (a) XPS spectrum of Fe-CS/MMTNS; (b) N 1s high resolution regions of Fe-CS/MMTNS; (c) C 1s high resolution regions of Fe-CS/MMTNS and (d) Fe 2p high resolution regions of Fe-CS/MMTNS. 5
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Fig. 5. (a) Removal of MB by Fe-CS/MMTNS without H2O2 but with and without light (curve labeled as Fe-CS/MMTNS), Fe-CS/MMTNS with H2O2 only (curve labeled as H2O2) and Fe-CS/MMTNS with H2O2 and light at pH = 3; (b) the effect of iron content on removal of MB at pH = 3 (c) removal of MB by Fe-CS/MMTNS at pH = 3.0, 6.5 and 10.0; (d) the recyclability of Fe-CS/MMTNS. (conditions: MB, 100 mL, 20 mg/L; catalyst, 10 mg; H2O2, 20 mmol/L).
the experiments of adsorptive, synchronous and asynchronous removal of MB on Fe-CS/MMTNS were conducted. In the adsorption experiment, no light from xenon lamp and no H2O2 were used. In the synchronous experiment, the Fe-CS/MMTNS was added in H2O2 + Vis light system directly without adsorption, while for asynchronous experiment, the FeCS/MMTNS was added after adsorption for 45 min. The results shown in Fig. 6 suggest that synchronous removal of MB was quicker than asynchronous removal, indicating that adsorption played an important role in the decolorization/degradation of MB. However, the MB
Table 1 Parameters of zero-order kinetics model in the removal of MB. Reaction type
pH
k0 (mg·L−1·min−1)
R2
H2O2 Fe-CS(0.25)/MMTNS + H2O2 Fe-CS(1)/MMTNS + H2O2 Fe-CS(0.1)/MMTNS + H2O2 Fe-CS(0)/MMTNS + H2O2 Fe-CS(0.25)/MMTNS + H2O2 Fe-CS(0.25)/MMTNS + H2O2 Fe-CS(0.25)/MMTNS + H2O2 + TBA Fe-CS(0.25)/MMTNS + H2O2 + Tri
3 3 3 3 3 6.5 10 3 3
0.0808 0.2261 0.2994 0.1934 0.1752 0.2019 0.1765 0.1035 0.1438
0.9938 0.9755 0.9575 0.9844 0.9957 0.9858 0.9926 0.9514 0.9956
application. Therefore, cycling experiments for the MB degradation with Fe-CS/MMTNS were performed and the results are displayed in Fig. 5d. As can be seen from Fig. 5d, MB decomposition remained very high even after five cycles, indicating that the incorporation of Fe significantly improved the recyclability of the gel compared to the Fe-CS (0)/MMTNS with no Fe. It is well-known that the photo-Fenton can be stimulated when Fe2+ and H2O2 are present during the reaction. The adsorbed MB on the gel could be decomposed through photo-Fenton and the active adsorption sites of the gel are regenerated. Furthermore, the leached amount of iron was only about 0.2 mg/L Fe3+ (Fig. 5d) from Fe-CS/MMTNS during five cycles of the dye removal, demonstrating that the introduced Fe was hardly leached out and therefore, the Fe-CS/MMTNS gel remained stable.
3.3. Removal mechanism of MB by Fe-CS/MMTNS Fig. 6. Adsorptive, synchronous and asynchronous removal of MB by Fe-CS/ MMTNS. (conditions: MB, 100 mL, 20 mg/L; catalyst, 10 mg; H2O2, 20 mmol/ L).
3.3.1. Synergy between adsorption and photo-Fenton In order to further identify the role of adsorption and photo-Fenton, 6
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removal difference between the synchronous and asynchronous curves is much smaller than the MB removal by adsorption alone. Based on the above results, it appears that the active adsorption sites of the Fe-CS/ MMTNS were renewed after the degradation of the adsorbed MB by photo-Fenton which is similar to the reduction of MB on the surface of MTiCuPd500 as adsorption played an important role in the removal of MB [52]. In the reduction, MB was firstly adsorbed on the surface of MTiCuPb500 and then it accepted electrons transferred from BH4−. Hence, the asynchronous removal followed the synchronous removal except that there was no gap between them at the end. The degradation was not further enhanced with the synchronous removal due to the degradation of all MB on the adsorption sites. This interesting phenomenon implies that there is a synergistic effect between adsorption and photo-Fenton for MB degradation, and the adsorption sites on FeCS/MMTNS are continually reactivated. 3.3.2. Role of hydroxyl radicals and superoxide radicals In a typical photo-Fenton, hydroxyl radicals with a high standard reduction potential of 2.80 V play an important role on the decomposition of organics [53]. At the same time, many other reactive radicals like %OOH, O2%− etc. are generated in the catalytic process. Here, TBA and Tri served as scavengers for hydroxyl radicals and superoxide radicals, respectively and the results are illustrated in Fig. 7a, which demonstrated that more amounts of MB remained in the solution with the addition of TBA and Tri. The zero-order kinetic constants of degradation with TBA and Tri were 0.1035 mg·L−1·min−1 and 0.1438 mg·L−1·min−1, respectively (Table 1), indicating %OH and O2%− played an important role on the decomposition of MB as the degradation kinetic constant of Fe-CS4/MMTNS + H2O2 system was much higher without scavengers. In order to further identify %OH and O2%−, ESR measurements were carried out and the results are shown in Fig. 7b and it can be seen that there are four characteristic peaks (1:2:2:1), which can be assigned to DMPO-%OH [54]. Thus, %OH and O2%− seem to have played substantial role on the decolorization of MB on the FeCS/MMTNS with the presence of H2O2 under visible light irradiation.
Fig. 8. UV–Vis spectra of MB decolored on Fe-CS/MMTNS at different times. (conditions: MB, 100 mL, 20 mg/L; catalyst, 10 mg; H2O2, 20 mmol/L; pH = 3).
transition and one peak at 664 nm attributable to the absorbance of n → π* transition [55]. During the adsorption process, there were no significant changes in the absorbances at 246 nm and 288 nm although the concentration of MB decreased but slightly. At the same time, no matter whether the catalyst adsorbed MB for 15 min or 45 min, the characteristic peak of MB at 664 nm also did not undergo blue shift or red shift, indicating that MB was not decomposed as it maintained the original structure. However, the three peaks changed dramatically during the catalytic reaction in the presence of light and H2O2. Firstly, the characteristic peak at 664 nm blue shifted to 634 nm, illustrating that the chromophore was degraded by reactive radicals. Correspondingly, the other two peaks gradually disappeared attributable to the replacement of N and S as the benzene ring was opened [56]. LCMS and IC were used in order to further identify the intermediate products and describe the decolorization pathway of MB on Fe-CS/ MMTNS and the results are shown in Figs. S2 and S3. From Figs. S2, S3 and S4, the intermediate species of m/z 256, 228, 301 and 279 were found, indicating that the MB underwent demethylation and sulfur oxidation and then the intermediates were oxidized into inorganic substances. Hence, it can be concluded that MB was decolorized in two ways on Fe-CS/MMTNS with the presence of H2O2 under visible light irradiation (Fig. 9). One was that MB directly underwent photo-Fenton degradation and was oxidized into inorganics in aqueous medium, and the other was that MB was adsorbed by Fe-CS/MMTNS and was then oxidized. That is, MB was removed by synergy between adsorption and
3.3.3. Decolorization pathway of MB With the presence of reactive radicals, MB was easily attacked and chromophores were destroyed. In order to reveal the decomposition of MB on Fe-CS/MMTNS, decolorization of MB solution during adsorption and photocatalytic process was determined and the results are shown in Fig. 8. The decolorization of MB during catalytic process could be visually observed. However, a color fading phenomenon was not prominent during the adsorption process alone (see inset in Fig. 8) without visible light irradiation. With the original MB solution, there are two peaks at 246 nm and 288 nm referring to the absorbance of π → π*
Fig. 7. (a) Decomposition kinetics of MB on Fe-CS/MMTNS without scavengers and with scavengers; (b) ESR spectra of reactive radicals during the decolorization. 7
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Fig. 9. Schematic diagram of MB degradation on Fe-CS/MMTNS.
Appendix A. Supplementary data
photo-Fenton. In Fig. 9, MB presented positive charge with an atomic weight of 284 g/mol at the outset since Cl was easily ionized and detached from chromophore. Then part of MB was directly attacked by reactive radicals and gradually converted into inorganic substances like NO3−, SO42−, and CO2 and the other part of MB was firstly adsorbed by Fe-CS/MMTNS gel and S was oxidized into S]O [57] followed by its desorption, which was then captured and degraded by the reactive radicals again. These intermediates were eventually turned into inorganic substances and the adsorption sites were regenerated for adsorbing fresh MB.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122322. References [1] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255, https://doi.org/10.1016/S09608524(00)00080-8. [2] E.N. El Qada, S.J. Allen, G.M. Walker, Adsorption of Methylene Blue onto activated carbon produced from steam activated bituminous coal: a study of equilibrium adsorption isotherm, Chem. Eng. J. 124 (2006) 103–110, https://doi.org/10.1016/ j.cej.2006.08.015. [3] S.J. Yuan, X.H. Dai, Facile synthesis of sewage sludge-derived mesoporous material as an efficient and stable heterogeneous catalyst for photo-Fenton reaction, Appl. Catal. B Environ. 154–155 (2014) 252–258, https://doi.org/10.1016/j.apcatb. 2014.02.031. [4] Y. Liu, W. Jin, Y. Zhao, G. Zhang, W. Zhang, Enhanced catalytic degradation of methylene blue by α-Fe2O3/graphene oxide via heterogeneous photo-Fenton reactions, Appl. Catal. B Environ. 206 (2017) 642–652, https://doi.org/10.1016/j. apcatb.2017.01.075. [5] L. Wang, J. Zhang, A. Wang, Fast removal of methylene blue from aqueous solution by adsorption onto chitosan-g-poly (acrylic acid)/attapulgite composite, Desalination 266 (2011) 33–39, https://doi.org/10.1016/j.desal.2010.07.065. [6] E.A. El-sharkawy, A.Y. Soliman, K.M. Al-amer, Comparative study for the removal of methylene blue via adsorption and photocatalytic degradation, J. Colloid Interface Sci. 310 (2007) 498–508, https://doi.org/10.1016/j.jcis.2007.02.013. [7] J. Dotto, M.R. Fagundes-Klen, M.T. Veit, S.M. Palácio, R. Bergamasco, Performance of different coagulants in the coagulation/flocculation process of textile wastewater, J. Clean. Prod. 208 (2019) 656–665, https://doi.org/10.1016/j.jclepro. 2018.10.112. [8] S.A. Ong, L.N. Ho, Y.S. Wong, Comparison on biodegradation of anionic dye orange II and cationic dye methylene blue by immobilized microorganisms on spent granular activated carbon, Desalin. Water Treat. 54 (2015) 557–561, https://doi. org/10.1080/19443994.2014.880155. [9] Z. Liang, C.F. Yan, S. Rtimi, J. Bandara, Piezoelectric materials for catalytic/photocatalytic removal of pollutants: recent advances and outlook, Appl. Catal. B Environ. 241 (2019) 256–269, https://doi.org/10.1016/j.apcatb.2018.09.028. [10] W. Wang, Y. Zhao, H. Bai, T. Zhang, V. Ibarra-galvan, Methylene blue removal from water using the hydrogel beads of poly (vinyl alcohol) -sodium alginate-chitosanmontmorillonite, Carbohydr. Polym. 198 (2018) 518–528, https://doi.org/10. 1016/j.carbpol.2018.06.124. [11] F. Boudissa, D. Mirilà, V.A. Arus, T. Terkmani, S. Semaan, M. Proulx, I.D. Nistor, R. Roy, A. Azzouz, Acid-treated clay catalysts for organic dye ozonation – Thorough mineralization through optimum catalyst basicity and hydrophilic character, J. Hazard. Mater. 364 (2019) 356–366, https://doi.org/10.1016/j.jhazmat.2018.09.
4. Conclusions The Fe-CS/MMTNS gel performed well in the removal of MB with the presence of H2O2 under visible light through the synergy effect of both adsorption and photo-Fenton. Because of abundant active sites of MMMTNS, the gel was able to photo-degrade MB under a wide range of pH conditions. Moreover, it showed effective reusability because the adsorption sites of the Fe-CS/MMTNS could be continually reactivated. At the same time, the Fe-CS/MMTNS gel was stable as the iron was hardly leached out because of its complexation with CS. MB was degraded through demethylation and oxidation in two pathways: a part of MB was directly attacked by reactive radicals and gradually converted into inorganic substances, and the other part of MB was firstly adsorbed by Fe-CS/MMTNS gel and then it was degraded by reactive radicals upon its desorption.
Acknowledgements The financial supports to this work from the National Natural Science Foundation of China under the projects No. 51874220 and 51674183 are gratefully acknowledged. This research is also supported by the financial supports for this work from Natural Science Foundation of Hubei Province of China (2018CFB468). Dr. Yunliang Zhao gratefully acknowledges financial support from China Scholarship Council (File No. 201806955006). One of us (SK) was supported by the College of Agricultural Sciences under Station Research Project No. PEN04566. 8
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[35] P.V. Nidheesh, R. Gandhimathi, S.T. Ramesh, Degradation of dyes from aqueous solution by Fenton processes: a review, Environ. Sci. Pollut. Res. 20 (2013) 2099–2132, https://doi.org/10.1007/s11356-012-1385-z. [36] M.C. Pereira, L.C.A. Oliveira, E. Murad, Iron oxide catalysts: Fenton and Fentonlike reactions–a review, Clay Miner. 47 (2012) 285–302, https://doi.org/10.1180/ claymin.2012.047.3.01. [37] H. Yi, W. Zhan, Y. Zhao, S. Qu, W. Wang, P. Chen, S. Song, A novel core-shell structural montmorillonite nanosheets/stearic acid composite PCM for great promotion of thermal energy storage properties, Sol. Energy Mater. Sol. Cells 192 (2019) 57–64, https://doi.org/10.1016/j.solmat.2018.12.015. [38] Y. Fang, A. Zhou, W. Yang, T. Araya, Y. Huang, P. Zhao, Complex formation via hydrogen bonding between rhodamine B and montmorillonite in aqueous solution, Sci. Rep. 229 (2018) 1–10, https://doi.org/10.1038/s41598-017-18057-8. [39] S. Bensalem, B. Hamdi, S. Del Confetto, M. Iguer-Ouada, A. Chamayou, H. Balard, R. Calvet, Characterization of chitosan/montmorillonite bionanocomposites by inverse gas chromatography, Colloids Surf. A Physicochem. Eng. Asp. 516 (2017) 336–344, https://doi.org/10.1016/j.colsurfa.2016.12.051. [40] L. Wang, A. Wang, Adsorption characteristics of Congo Red onto the chitosan/ montmorillonite nanocomposite, J. Hazard. Mater. 147 (2007) 979–985, https:// doi.org/10.1016/j.jhazmat.2007.01.145. [41] M.H.M. Hussein, M.F. El-Hady, W.M. Sayed, H. Hefni, Preparation of some chitosan heavy metal complexes and study of its properties, Polym. Sci. Ser. A. 54 (2012) 113–124, https://doi.org/10.1134/S0965545X12020046. [42] C. Liu, W. Zhang, S. Song, H. Li, A novel method to improve carboxymethyl cellulose performance in the flotation of talc, Miner. Eng. 131 (2019) 23–27, https:// doi.org/10.1016/j.mineng.2018.11.003. [43] X. Lin, L. Wang, S. Jiang, L. Cui, G. Wu, Iron-doped chitosan microsphere for As(III) adsorption in aqueous solution: kinetic, isotherm and thermodynamic studies, Korean J. Chem. Eng. 36 (2019) 1102–1114, https://doi.org/10.1007/s11814-0180117-6. [44] L.L. Min, L.M. Yang, R.X. Wu, L. Bin Zhong, Z.H. Yuan, Y.M. Zheng, Enhanced adsorption of arsenite from aqueous solution by an iron-doped electrospun chitosan nanofiber mat: preparation, characterization and performance, J. Colloid Interface Sci. 535 (2019) 255–264, https://doi.org/10.1016/j.jcis.2018.09.073. [45] T. Zhang, Y. Zhao, S. Kang, Y. Li, Q. Zhang, Formation of active Fe(OH)3 in situ for enhancing arsenic removal from water by the oxidation of Fe(II)in air with the presence of CaCO3, J. Clean. Prod. 227 (2019) 1–9, https://doi.org/10.1016/j. jclepro.2019.04.199. [46] J. Qu, Q. Hu, K. Shen, K. Zhang, Y. Li, H. Li, Q. Zhang, J. Wang, W. Quan, The preparation and characterization of chitosan rods modified with Fe3+ by a chelation mechanism, Carbohydr. Res. 346 (2011) 822–827, https://doi.org/10.1016/j. carres.2011.02.006. [47] R.B. Hernández, A.P. Franco, O.R. Yola, A. López-Delgado, J. Felcman, M.A.L. Recio, A.L.R. Mercê, Coordination study of chitosan and Fe3+, J. Mol. Struct. 877 (2008) 89–99, https://doi.org/10.1016/j.molstruc.2007.07.024. [48] A. De Leo, Catalytic activity of an iron-pillared montmorillonitic clay mineral in heterogeneous photo-Fenton process, Catal. Today 133 (2008) 600–605, https:// doi.org/10.1016/j.cattod.2007.12.130. [49] S. Juan, A. De Leo, Iron-pillared clays as catalysts for dye removal by the heterogeneous photo-Fenton technique, React. Kinet. Mech. Catal. 110 (2013) 101–117, https://doi.org/10.1007/s11144-013-0593-y. [50] S. Janaki, K. Punithamurthy, L. Renuk, A novel approach for synthesis of LaFeO3/ bentonite nanocomposite for degradation of methylene blue with enhanced photocatalytic activity, Mater. Res. Exp. 6 (2019) 035013, , https://doi.org/10.1088/ 2053-1591/aaf333. [51] M. Bobu, A. Yediler, I. Siminiceanu, S. Schulte-hostede, Degradation studies of ciprofloxacin on a pillared iron catalyst, Appl. Catal. B Environ. 83 (2008) 15–23, https://doi.org/10.1016/j.apcatb.2008.01.029. [52] A. Joseph, K. Vellayan, B. González, M.A. Vicente, A. Gil, Effective degradation of methylene blue in aqueous solution using Pd-supported Cu-doped Ti-pillared montmorillonite catalyst, Appl. Clay Sci. 168 (2019) 7–10, https://doi.org/10. 1016/j.clay.2018.10.009. [53] Y. Zhang, M. Zhou, A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values, J. Hazard. Mater. 362 (2019) 436–450, https://doi.org/10.1016/j.jhazmat.2018.09.035. [54] S. Gazi, R. Ananthakrishnan, Semi-quantitative determination of hydroxyl radicals by benzoic acid hydroxylation: an analytical methodology for photo-Fenton systems, Curr. Anal. Chem. 8 (2012) 143–149, https://doi.org/10.2174/ 157341112798472297. [55] J. Sun, S. Dong, J. Feng, X. Yin, X. Zhao, Enhanced sunlight photocatalytic performance of Sn-doped ZnO for Methylene Blue degradation, J. Mol. Catal. A Chem. 335 (2011) 145–150, https://doi.org/10.1016/j.molcata.2010.11.026. [56] J. Juhui, L. Xiangdong, C. Huajun, X. Guoxi, Synergistic degradation of methylene blue by ultrasound/H2O2, J. Henan Norm. Univ. Nat. Sci. 34 (2006) 106–109. [57] Y. Hu, L. Zhou, H. Liu, X. Guo, Visible light photocatalytic degradation of methylene blue over N-doped TiO2, Key Eng. Mater. 609–610 (2014) 141–146, https://doi. org/10.4028/www.scientific.net/KEM.609-610.141.
070. [12] S.C. Kim, D.K. Lee, Preparation of Al-Cu pillared clay catalysts for the catalytic wet oxidation of reactive dyes, Catal. Today 97 (2004) 153–158, https://doi.org/10. 1016/j.cattod.2004.03.066. [13] P.V. Nidheesh, M. Zhou, M.A. Oturan, An overview on the removal of synthetic dyes from water by electrochemical advanced oxidation processes, Chemosphere 197 (2018) 210–227, https://doi.org/10.1016/j.chemosphere.2017.12.195. [14] J. Feng, X. Hu, P.L. Yue, Effect of initial solution pH on the degradation of Orange II using clay-based Fe nanocomposites as heterogeneous photo-Fenton catalyst, Water Res. 40 (2006) 641–646, https://doi.org/10.1016/j.watres.2005.12.021. [15] D. Gumy, P. Fernández-Ibáñez, S. Malato, C. Pulgarin, O. Enea, J. Kiwi, Supported Fe/C and Fe/Nafion/C catalysts for the photo-Fenton degradation of Orange II under solar irradiation, Catal. Today 101 (2005) 375–382, https://doi.org/10. 1016/j.cattod.2005.03.036. [16] J. Feng, X. Hu, P.L. Yue, Degradation of salicylic acid by photo-assisted Fenton reaction using Fe ions on strongly acidic ion exchange resin as catalyst, Chem. Eng. J. 100 (2004) 159–165, https://doi.org/10.1016/j.cej.2004.01.031. [17] Q. Wu, H. Wang, C. Yi, Preparation of photo-Fenton heterogeneous catalyst (Fe-TS1 zeolite) and its application in typical azo dye decoloration, J. Photochem. Photobiol. A Chem. 356 (2018) 138–149, https://doi.org/10.1016/j.jphotochem. 2017.12.041. [18] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Mineralization of an azo dye Acid Red 14 by electro-Fenton’s reagent using an activated carbon fiber cathode, Dye. Pigment. 65 (2005) 227–233, https://doi.org/10.1016/j.dyepig.2004.07.019. [19] Q. Wang, S. Tian, J. Long, P. Ning, Use of Fe(II)Fe(III)-LDHs prepared by co-precipitation method in a heterogeneous-Fenton process for degradation of Methylene Blue, Catal. Today 224 (2014) 41–48, https://doi.org/10.1016/j.cattod.2013.11. 031. [20] C.A.P. Almeida, N.A. Debacher, A.J. Downs, L. Cottet, C.A.D. Mello, Removal of methylene blue from colored effluents by adsorption on montmorillonite clay, J. Colloid Interface Sci. 332 (2009) 46–53, https://doi.org/10.1016/j.jcis.2008.12. 012. [21] M. Aflaki Jalali, A. Dadvand Koohi, M. Sheykhan, Experimental study of the removal of copper ions using hydrogels of xanthan, 2-acrylamido-2-methyl-1-propane sulfonic acid, montmorillonite: kinetic and equilibrium study, Carbohydr. Polym. 142 (2016) 124–132, https://doi.org/10.1016/j.carbpol.2016.01.033. [22] F. Bergaya, G. Lagaly, General Introduction: Clay, Clay Minerals, and Clay Science, Elsevier, 2006 doi: 10.1016/S1572-4352(05)01001-9. [23] Y. Zhao, H. Yi, F. Jia, H. Li, C. Peng, S. Song, A novel method for determining the thickness of hydration shells on nanosheets: a case of montmorillonite in water, Powder Technol. 306 (2017) 74–79, https://doi.org/10.1016/j.powtec.2016.10. 045. [24] W. Wang, Y. Zhao, H. Yi, T. Chen, S. Kang, T. Zhang, F. Rao, S. Song, Pb(Ⅱ) removal from water using porous hydrogel of chitosan-2D montmorillonite, Int. J. Biol. Macromol. 128 (2019) 85–93, https://doi.org/10.1016/j.ijbiomac.2019.01.098. [25] G.L. Visavale, R.K. Nair, S.S. Kamath, M.R. Sawant, B.N. Thorat, Granulation and drying of modified clay incorporated pesticide formulation, Dry. Technol. 25 (2007) 1369–1376, https://doi.org/10.1080/07373930701438980. [26] Y.-N. Zhang, Y.-X. Yu, Adsorptive removal of Cr3+, Cu2+ and Ni2+ ions by magnetic Fe3O4@alkali-treated coal fly ash, Desalin. Water Treat. 123 (2018) 277–287, https://doi.org/10.5004/dwt.2018.22778. [27] W. Wang, Y. Zhao, H. Yi, T. Chen, Preparation and characterization of self- assembly hydrogels with exfoliated montmorillonite nanosheets and chitosan, Nanotechnology 29 (2018) 025605. [28] L. Ai, L. Li, Efficient removal of organic dyes from aqueous solution with ecofriendly biomass-derived carbon@montmorillonite nanocomposites by one-step hydrothermal process, Chem. Eng. J. 223 (2013) 688–695, https://doi.org/10.1016/j.cej. 2013.03.015. [29] W.S. Wan Ngah, L.C. Teong, M.A.K.M. Hanafiah, Adsorption of dyes and heavy metal ions by chitosan composites: a review, Carbohydr. Polym. 83 (2011) 1446–1456, https://doi.org/10.1016/j.carbpol.2010.11.004. [30] C. Gerente, V.K.C. Lee, P. Le Cloirec, G. Mckay, Application of chitosan for the removal of metals from wastewaters by adsorption-mechanisms and models Review, Crit. Rev. Environ. Sci. Technol. (2007) 41–127, https://doi.org/10.1080/ 10643380600729089. [31] S. Kang, Y. Zhao, W. Wang, T. Zhang, T. Chen, H. Yi, F. Rao, S. Song, Removal of methylene blue from water with montmorillonite nanosheets/chitosan hydrogels as adsorbent, Appl. Surf. Sci. 448 (2018) 203–211, https://doi.org/10.1016/j.apsusc. 2018.04.037. [32] X. Hu, R. Liang, G. Sun, Super-adsorbent hydrogel for removal of methylene blue dye from aqueous solution, J. Mater. Chem. A (2018) 17612–17624, https://doi. org/10.1039/c8ta04722g. [33] S. Rashid, C. Shen, X. Chen, S. Li, Y. Chen, Y. Wen, J. Liu, Enhanced catalytic ability of chitosan-Cu-Fe bimetal complex for the removal of dyes in aqueous solution, RSC Adv. 5 (2015) 90731–90741, https://doi.org/10.1039/c5ra14711e. [34] R. Hrdina, Oxidation of azo textile soluble dyes with hydrogen peroxide in the presence of Cu (II) e chitosan heterogeneous catalysts, Dye. Pigment 73 (2007) 19–24, https://doi.org/10.1016/j.dyepig.2005.10.004.
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