Adsorption and Synergetic Fenton-like Degradation of Methylene Blue

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Kinetics, Catalysis, and Reaction Engineering

Adsorption and Synergetic Fenton-like Degradation of Methylene Blue by a novel mesoporous #-Fe2O3/SiO2 at Neutral pH Zhengying Wu, Wenjun Zhu, Mengling Zhang, Yan Lin, Nan Xu, Feng Chen, Dongtian Wang, and Zhigang Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00077 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Industrial & Engineering Chemistry Research

Adsorption and Synergetic Fenton-like Degradation of Methylene Blue by a Novel Mesoporous αFe2O3/SiO2 at Neutral pH Zhengying Wua,b,*, Wenjun Zhua, Mengling Zhanga, Yan Lina, Nan Xu a, Feng Chena, Dongtian Wanga and Zhigang Chena,* a

Jiangsu Key Laboratory for Environment Functional Materials, School of Chemistry, Biology

and Material Engineering, Suzhou University of Science and Technology, Suzhou 215009, China. b

International Joint Laboratory of Chinese Education Ministry on Resource Chemistry, Shanghai,

200234, China.

ABSTRACT: In this study, a mesoporous α-Fe2O3/SiO2 composite with highly ordered mesostructure, large surface area and pore volume was feasibly fabricated via a spontaneous infiltration route by employing the template-containing mesoporous SiO2 as supports. Fe species are highly dispersed in the framework of the SiO2 with relatively low Fe content. At high Fe content (rFe:Si = 0.12), α-Fe2O3 nanocrystals with an average size of about 50 nm were incorporated in the matrix of the SiO2, existing as both framework and extra-framework forms. The synthesized α-Fe2O3/SiO2 composite shows a high adsorption capacity (90 mg·g-1) and the superior heterogeneous Fenton-like catalytic activity for the removal of 300 mg·L-1 methylene

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blue from aqueous solutions (removal efficiency = 100 %). The α-Fe2O3/SiO2 composite also has a remarkable adsorption and catalysis performance in a wide pH range of 3.0–11.0, and exhibits an excellent stability and reusability.

KEYWORDS: Mesoporous α-Fe2O3/SiO2; Adsorption; Fenton-like catalysis; Methylene blue


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1. INTRODUCTION Organic dyes are important chemicals widely used in textile, food, paper, printing, and leather industries. During the production and application process, a significant portion of dye (10–15%) is directly discharged into effluent, which leads to various wastewater pollutions.1,2 Moreover, these dyes and their degradation products are toxic and potentially carcinogenic even at low concentrations, which pose serious threat to the human health and the environment.3,4 Therefore, it is essential to find efficient and economical ways of removing dye pollutants from wastewater. Among various physical, chemical and biological techniques, adsorption is one of the conventional methods to remove dyes from aqueous solutions in industry. The high concentration of the dye wastewater could be reduced by utilizing inexpensive and available adsorbents such as activated carbon, zeolites, chitosan, hydrogel and so on.5-9 However, in most cases, the dye wastewater still has a relatively high level of residue, and couldn’t meet the discharge standards after being treated by the simplex adsorption process. Meanwhile, merely removing the pollutants by adsorption cannot fundamentally solve the problem because of the excess residues, which results into secondary pollution and require further treatments. For this reason, advanced oxidation processes (AOPs) have been developed and become a popular method for wastewater treatment, especially for the advanced treatment of non-biodegradable and recalcitrant organic contaminants.1,10,11 AOPs are based on the in situ generation of highly powerful oxidizing hydroxyl radicals, which can mineralize stable organic contaminants into harmless carbon dioxide, water and inorganic ions at mild temperature and constant pressure.1,11 Among them, Fenton process has been proved to be one of the best methods to generate hydroxyl radicals for highly efficient degradation of organic pollutants.12 Besides, Fenton process also has the advantages of easy to


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handle, environmentally benign, fast reaction and low cost.10 However, conventional homogenous Fenton reaction with dissolved ferrous or ferric ions as catalyst also has some limitations in practical applications, such as the narrow and low working pH range (pH = 2~3), generation of ferric hydroxide sludge, as well as the deactivation of iron ions1. In order to overcome the above drawbacks, the heterogeneous Fenton-like catalysts with iron species on supporting materials are considered more practical and efficient.13-16 Activated carbon,17 alginate,18 clay,19 bentonite,20 carbon nanotube,21 graphene oxide,22 and some metal oxides23 can serve as supports to load iron species, forming Fenton-like oxidation catalysts. Moreover, catalytic performance of these iron-based heterogeneous Fenton-like catalysts is strongly affected by the dispersion, particle size, and chemical environment of the active iron species, as well as surface area of the catalyst.24 Consequently, employing a support which can easily disperse the catalytic iron species and provide high surface area seems extremely important to improve activity and stability of the catalyst. Mesoporous silica (SiO2) that has high specific surface area, tunable pore size, narrow pore size distribution, and good hydrothermal stability offers great opportunities to serve as support for the design of effective heterogeneous Fenton-like catalysts.25-28 Metallic species like Fe, Ni and Cu could be easily loaded into mesoporous SiO2 via the simple incipient wetness impregnation method.15,29 Generally, impregnation method needs to in advance remove the template in the mesoporous SiO2 by calcination, then mix metal species with the calcined SiO2 to form a metal precursor/ SiO2 composite. The metal oxide loaded mesoporous SiO2 was finally obtained after a repeated thermal treatment process to converse metal precursors into oxides.15,29 Obviously, the traditional impregnation way is energy and time consuming, and is easily to form metal oxide aggregations in the channels or on the external surface of the supports, resulting in


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pore blockage and surface area reduction of the catalyst.29 Hence, it is necessary to find a more convenient and efficient methodology to introduce metal species into the mesoporus SiO2. Previously, we developed a novel spontaneous infiltration technique to fast modify mesoporous materials.30,31 Metal species could be easily infiltrated and highly dispersed into mesoporous SiO2 by grinding precursor salts and template-containing mesoporous supports followed by calcinations.25,32 This spontaneous infiltration strategy allows template removal and precursor conversion in one step, saving time and energy. In this study, a novel mesoporous α-Fe2O3/SiO2 composite was prepared through this spontaneous infiltration route using the template-containing mesoporous SiO2 as support. Different amounts of Fe species were introduced into the structure of mesoporous SiO2 by controlling the Fe:Si molar ratio (rFe:Si) of the reaction precursors. The expected α-Fe2O3/SiO2 composite presents a well-ordered mesostructure, large surface area and highly dispersed Fe species in SiO2 matrix. It is fascinating that this α-Fe2O3 modified mesoporous SiO2 shows both of large adsorption capacity and highly catalytic activity for removing hazardous dye from aqueous solutions. This is because that large quantity of mesopores in the composite offers enough space to accommodate the dye molecules, and the incorporated Fe species in it provide effective active sites to generate sufficient hydroxyl radicals and then synergetically degrade the dye.

2. EXPERIMENTAL 2.1 Chemicals Hydrochloric acid (HCl, 35-37 wt.%), ferric trichloride (FeCl3), sodium hydroxide (NaOH), tetraethyl orthosilicate (TEOS), hydrogen peroxide (30 wt.%), ferric sesquioxide (Fe2O3), and


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methylene blue (MB, C16H18ClN3S) were supplied by Sinopharm Chemical Reagent Co. Ltd. of China. Triblock copolymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (Pluronic P123, Mw = 5800, EO20PO70EO20) was purchased from Aldrich. All above chemicals were analytical grade and used without further purification. 2.2 Synthesis of Mesoporous α-Fe2O3/SiO2 Composite Mesoporous SiO2 support was prepared according to literature procedure with a minor modification.33 P123 (2 g) was dissolved in 75 g of H2O, and then 2.41 g of AlCl3·6H2O was added and stirred for 0.5 h. After that, the mixture was heated to 308 K and 4.16 g of TEOS was added. The molar composition of the mixture, TEOS/P123/AlCl3·6H2O/H2O, is 1/0.017/0.5/208. The mixture was further stirred under 308 K for 24 h, aged at 373 K for another 24 h, and then the product was filtered off, thoroughly washed with distilled water, air-dried, labelled as mSiO2As, and then be used as support for the preparation of mesoporous α-Fe2O3/SiO2 composites. During the synthesis, AlCl3·6H2O was used to self-generate the mildly acidic environment for facilitating the hydrolysis and condensation of siliceous species.33 The total amount of template and adsorbed water in the mSiO2-As support is 40 wt.%, which was detected by TG in air (Figure S1). Template in the mSiO2-As was also removed by calcination in air at 823 K for 5 h, and the sample was named as mSiO2 for comparison. The mesoporous α-Fe2O3/SiO2 composites were synthesized via a solvent-free spontaneous infiltration route as follows: 0.6 g of template-containing mesoporous SiO2 (mSiO2-As, which contains about 40 wt.% P123 and adsorbed water, and 60 wt.% (6 mmol) SiO2, as shown in Figure S1) and different amounts (0.12, 0.24, 0.72 and 1.33 mmol) of FeCl3 were mixed together and manually ground in a agate mortar at room temperature for about 20 min, then the homogeneous mixture was calcined at 823 K for 5 h in air to remove the template and generate


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α-Fe2O3. The final obtained samples were denoted FFeGx (x = rFe:Si), where x varies from 0.02, 0.04, 0.12, to 0.22, respectively. 2.3 Characterizations The X-ray diffraction (XRD) patterns were recorded on a D8 Advance (Bruker) diffractometer with a Cu Kα radiation. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F30S-Twin electron microscope. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were performed on a Hitachi S-4800 electron microscope with Oxford EDS system. The surface areas (SBET) and the pore size distributions (Dp) were calculated based on the Brunauer–Emmett–Teller (BET) method and the Barrett–Joyner–Halenda (BJH) model (according to the adsorption branches). UV-Vis diffuse reflectance spectra (DRS) were measured using a T9CS (PERSEE) spectrophotometer with BaSO4 as reference. Fourier Transform Infrared (FT-IR) spectra of powder samples suspended in KBr pallets were recorded on a Bruker Vertex 70 spectrometer. Zeta potentials of the sample were obtained on a Malvern ZEN3690 instrument. The pH of the initial aqueous solution was adjusted by dilute HCl or NaOH solutions. 2.4 Adsorption Tests Batch adsorption test was typically carried out by adding 25 mg of adsorbent (mSiO2 or FFeGx) into a set of 50 mL plastic flasks containing 25 mL of MB solutions with different initial concentrations (20–210 mg·L-1) to get adsorption isotherms. The mixture was shaken at 298 K with the speed of 50 rpm for 24 h to ensure adsorption equilibrium. After the adsorption equilibrium, the mixture was filtered and the MB concentration in filtrate was analyzed using a UV-2450 (Shimadzu) spectrophotometer at a maximum wavelength of 665 nm. The


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experimental details were similar as the description in our previous study.34 The amount of dye adsorbed onto FFeGx composite was calculated from the mass balance equation as: =

− 1

where Qe (mg·g-1) is the amount of MB adsorbed per gram of FFeGx at equilibrium; C0 (mg·L-1) and Ce (mg·L-1) are the initial and equilibrium liquid-phase concentration of MB; V (L) is the volume of the solution and M (g) is weight of the dry adsorbent. The adsorption isotherms of MB on FFeGx samples were described by Langmuir-type and Freundlich-type equations.35 Both isotherm models are expressed as follows: Langmuir model: =

2 1 +

Freundlich model: ⁄

=

3

where Qmax (mg·g−1) is the monolayer capacity of adsorbent, KL (L·mg−1) is the Langmuir binding constant. KF [mg·g−1(L·mg−1)1/n]) and 1/nF are Freundlich constants, indicator of adsorption capacity and adsorption intensity,36 respectively. 2.5 Catalytic Experiments Catalytic experiments were conducted in a 250 mL glass conical beaker under magnetic stirring at 298 K. 0.15 g of adsorbent/catalyst (FFeGx) was added into the beaker that contains 150 mL of MB solution with desired concentration for the adsorption and catalytic reactions. When the adsorption equilibrium was achieved at about 6 h, 30 mL of H2O2 was introduced into the system to initiate the catalytic reaction. MB concentration of the reaction solution was then determined by an UV-2450 spectrophotometer after the solution was taken out and high-speed


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centrifuged. Both of adsorption and catalytic reaction were performed under natural light without any other irradiations. The dye removal efficiency was calculated by the equation (4), and the catalytic degradation curve was fitted by a pseudofirst-order kinetic model (equation (5). =

− × 100% 4

− ln !

" = #$ 5

where C0 (mg·L−1) is the initial concentration of MB, C00 (mg·L−1) is the initial dye concentration after the introduction of H2O2, Ct (mg·L−1) represents the dye concentration at different reaction time, k (h−1) is the reaction rate coefficient, and t (h) is the reaction time respectively. MB with different initial concentrations (120–300 mg·L-1) were used to detect the adsorption and catalytic abilities of the FFeG0.12 composite. Acidity of the MB solution was adjusted by HCl or NaOH solution for examining the effect of pH on the adsorption/catalytic performance for the composite. For the recycling test, eight parallel tests were performed for the first run. After that, the used FFeG0.12 catalyst was naturally regenerated, collected together, and dried in atmosphere for the next run. No additional thermal treatment process was needed for the catalyst regeneration. Fe leaching amount in the solution after every catalysis cycle was detected by inductive coupled plasma–atomic emission spectrometry (ICP-AES, Shimadzu ICPE-9000).

3. RESULTS AND DISCUSSION 3.1 Structure and Composition of the Mesoporous Fe2O3/SiO2 Composites Figure 1a shows the low-angle XRD patterns of the pristine mesoporous SiO2 (mSiO2) and the α-Fe2O3/SiO2 composites with different Fe:Si molar ratios (rFe:Si). The long-range highly


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ordered mesostructure of mSiO2 is preserved after the modification even at a high rFe:Si of 0.22 (Fig. 1a). All FFeGx composites possess three well-resolved diffraction peaks at 2θ of 0.5º–2.0º, which can be indexed as (100), (110), and (200) reflections of typical 2-D hexagonal mesostructure (space group p6mm). The unit cell constant value of FFeGx is about 12.0~12.3 nm, which is slightly larger

Figure 1. Low (a) and wide (b) angle XRD patterns of the mesoporous α-Fe2O3/SiO2 composites. than that of mSiO2 support (11.8 nm), indicating that some ferric ions may probably incorporated into the matrix of the mesoporous SiO2. The wide-angle XRD patterns show that FFeG0.02 and FFeG0.04 present only a broad diffraction peak cantered at ca. 23º, which can be attributed to amorphous SiO2 walls (Fig. 1b).25 No XRD peaks assigned to Fe2O3 crystals were observed, which is due to the high dispersion of Fe species in those two composites with relatively lower Fe contents. However, two distinct diffraction peaks at 2θ of 33.1º and 35.6º accompanied with some weak peaks are observed for FFeG0.12, which are indexed to the orthorhombic phase α-Fe2O3 (JCPDS No. 33-0664).3 With more ferric content, sample FFeG0.22 displays stronger and more comprehensive diffraction


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peaks corresponding to α-Fe2O3 (Fig. 1b). The crystallite size of FFeG0.12 and FFeG0.22 calculated by the Debye-Scherrer equation11 based on the (104) peak at 2θ of 33.1º is about 50 and 71 nm.

Figure 2. TEM images of FFeG0.04 (a, b) and FFeG0.12 (d, e). SEM images of FFeG0.04 (c) and FFeG0.12 (f). Insets in Figs. b and e are photographs of FFeG0.04 and FFeG0.12, respectively. TEM images show that Fe species are highly dispersed on the sample with low rFe:Si, and no aggregated nanoparticles can be observed in FFeG0.04 (Fig. 2a and b). However, Fe2O3 nanoparticle aggregates are observed in FFeG0.12 with a relatively higher Fe content (Fig. 2, d and e). Some of the Fe2O3 nanoparticles with the size larger than the pore size of the SiO2 support appear in outer surface of FFeG0.12 (Fig. 2d). Moreover, both of FFeG0.04 and FFeG0.12 have well-ordered hexagonal mesopores, and regular morphologies (Fig. 2, b, c, e and f). Interestingly, color of FFeG0.04 and FFeG0.12 is totally different (Fig. 2, b and e). The


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characteristic color of FFeG0.04 is light yellow, while that of FFeG0.12 is orange-red, indicating the formation of Fe2O3 in FFeG0.12. N2 adsorption-desorption isotherms and corresponding pore size distributions (PSDs) of different samples are presented in Figure 3. The isotherm shape of all FFeGx composites is type

Figure 3. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of mSiO2 and the FFeGx composites. Isotherm curves are offset for clarity. Table 1. Textural properties of mSiO2 and the FFeGx composites Sample

a0

SBET

VP

DBJH

Wd

(nm)

(m2 g-1)

(cm3 g-1)

(nm)

(nm)

rFe:Si*

mSiO2

-

11.75

840

1.05

9.25

2.50

FFeG0.02

0.02

12.03

1222

1.35

8.10

3.93

FFeG0.04

0.04

12.33

1173

1.41

8.10

4.23

FFeG0.12

0.12

12.03

766

0.99

10.58

1.45

FFeG0.22

0.22

12.03

402

0.50

10.58

1.45


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*: the molar ratio of Fe to Si (rFe:Si) was detected by EDS; a0: lattice parameter was calculated by a0 = d100*2/√3; DBJH: pore diameters were calculated from the adsorption branches; Wd: wall thickness was calculated by Wd= a0 –DBJH. IV with an H1 hysteresis loop in the relative pressure range of 0.45~0.85, which belongs to the structure of cylindrical mesopores.25 Adsorption isotherm of the support mSiO2 is slightly changed when a small quantity of ferric species (rFe:Si = 0.02 and 0.04) were incorporated into it. There are tailings in the desorption branches for the isotherms of FFeG0.02 and FFeG0.04 (Fig. 3a). This is attributed to the presence of “plugs” or “constrictions” inside the mesopores, which lead to partially blocked mesopores, more micropores, and then improved mechanical and hydrothermal stabilities for the samples.33,37-38 Consequently, BET surface area of FFeG0.02 and FFeG0.04 is increased to 1222~1173 m2·g-1, which is much larger than that of the pristine mSiO2 support (840 m2·g-1). Simultaneously, the primary pore size of FFeG0.02 and FFeG0.04 decreases to 8.10 nm compared by the support (9.25 nm) due to the plugs in the mesopores (Fig. 3b, Table 1). When the rFe:Si is 0.12, the special tailing desorption isotherm on FFeG0.12 becomes inconspicuous, indicating the disappearance of plugged structures. At the same time, BET surface area of FFeG0.12 slightly declines to 766 m2·g-1. If further increase the incorporated Fe content to rFe:Si = 0.22, the N2 adsorption amount for FFeG0.22 further decreases due to the presence of large amount of Fe2O3 in the composite (Fig. 3a). PSD curves confirm that all composites have narrow pore size distributions, indicating uniform mesopores in them (Fig. 3b). It is interesting to find that the the primary pore size of FFeG0.12 and FFeG0.22 (10.58 nm) is a bit larger than that of mSiO2 (9.25 nm), which is probably caused by the incorporation of Fe in the mesoporous SiO2 matrix. UV-vis diffuse reflectance spectroscopy (DRS) will give more


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information on the chemical environment and coordination nature of the Fe atoms in the SiO2 matrix. Figure 4 presents the UV-vis DRS results of mSiO2 and the FFeGx composites. FFeG0.02 and FFeG0.04 display two adsorption bands centered at 225 and 265 nm (Fig. 4), which can be assigned to the oxygen-to-metal charge-transfer transition involving the isolated Fe3+ in (FeO4) tetrahedral coordination.39,40 The results suggest that Fe3+ ions have been incorporated into the frameworks of FFeG0.02 and FFeG0.04. The absence of adsorption bands above 475 nm reveals

Figure 4. UV-Vis DRS of mSiO2 and the FFeGx composites. that there are no crystalline Fe2O3 particles formed in these two composites with relatively low Fe contents, which is in agreement with the XRD and TEM results (Fig. 1b and Fig. 2a, b).39 New adsorption bands around 350 and 535 nm appear in FFeG0.12 and FFeG0.22 composites. The peak at about 350 nm is assigned to octahedral Fe3+ in small oligomeric FeOx clusters41,42 and the bands at 450-600 nm are characteristics for the Fe2O3 aggregates.40,42,43 It is clear that in FFeG0.22 there is more Fe2O3 crystallites than in FFeG0.12, for the DR adsorption band centered at 535 nm is stronger in FFeG0.22 than in FFeG0.12 (Fig. 4).


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FT-IR spectra for the FFeGx composite with different rFe:Si show characteristic adsorption bands at 802 and 1082 cm-1 (Figure S2), which are ascribed to the symmetric and asymmetric stretching of Si−O−Si [νs(Si-O-Si) and νas(Si−O−Si), respectively. Intensities of those bands are similar to each other in the pristine mSiO2 and four FFeGx composites. However, the 954 cm-1 band ascribed to the bending vibration of silanol groups (Si-OH) slightly declines when Fe species were introduced into SiO2, indicating interactions between Fe and Si species. The adsorption bands at 1632 and 3450 cm−1 assigned to bending and stretching bands of H−O−H [δOH(H−O−H) and νOH(H−O−H)] are also found in all samples. No other bands associated with C-H or C-O vibrations appear in the FFeGx composites, suggesting the completely decomposition of Fe precursors and the template in the as-prepared samples.

3.2 Adsorption Performance of the FFeGx Composites to MB

Figure 5. Adsorption isotherms of MB on mSiO2 and the FFeGx composites (T = 298 K, Cad. = 1 g·L-1, pH = 7.0).


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Figure 5 displays the adsorption isotherms of MB on the mesoporous SiO2 and the α-Fe2O3 modified FFeGx composites. The amount of adsorbed MB grows steeply when its initial concentrations increase from 20 to 210 mg·L-1, and then reach maximums. The MB uptakes at 24 h by FFeG0.02 and FFeG0.04 are 112 and 107 mg·g-1, which are close to that by the mSiO2 (115 mg·g-1). It is already known that FFeG0.02 and FFeG0.04 have relatively larger surface areas than that of the mSiO2 support (Table 1), but the MB uptakes for FFeG0.02 and FFeG0.04 are not larger than mSiO2 (Fig. 5, Table 2). This is because that adsorption property of a material is not only depends on the surface area but also effective adsorption sites and pores in the material. For example, the micropores (< 1 nm) in in FFeG0.02 contribute much to the high surface area of the composite, but which is not in favor of adsorbing and accommodating MB that has a molecular size of 1.7 nm. The adsorption capacities of the composites with the higher Fe species (FFeG0.12 and FFeG0.22) decrease to 98 and 92 mg·g-1, respectively (Fig. 5). This is primarily due to the reduced surface areas and pore volumes of FFeG0.12 and FFeG0.22 that caused by the presence of α-Fe2O3 crystals in the mesopores (Table 1). The adsorption data for MB onto mSiO2 and the FFeGx materials were analyzed by Langmuir and Freundlich equations to investigate the relationship between the adsorbate molecules and adsorbent in solutions.44 Sorption isotherms for mesoporous mSiO2 support, FFeG0.02, FFeG0.04 and FFeG0.12 fit the Langmuir model (RL = 0.94−0.98) well (Figure 5, Table 2). It implies that the surfaces of those materials are more homogeneous than heterogeneous, and the accessible sorption sites are energetically equivalent to the MB molecules during adsorption. However, the isotherm of MB adsorption on the sample with the highest Fe content (FFeG0.22) can be fitted by


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Table 2. Langmuir and Freundlich parameters for the adsorption of MB onto mSiO2 and FFeGx composites Langmuir model Qmax -1

KL -1

Freundlich model RL2

KF (mg g-1) -1 1/n

1/nF

RF2

(mg·g )

(L mg )

(L mg )

mSiO2

106.39

1.22

0.9359

53.90

0.1733

0.90964

FFeG0.02

101.37

1.32

0.9369

52.52

0.1655

0.90196

FFeG0.04

101.27

0.89

0.9469

50.92

0.1666

0.85588

FFeG0.12

92.35

0.52

0.98257

43.53

0.171

0.81500

FFeG0.22

84.12

2.32

0.87831

48.76

0.1393

0.88981

two isotherm models but the linear regression correlations (RL2 = 0.88, and RF2 = 0.89) are not as high as those for other FFeGx composites (Table 2). The Langmuir monolayer capacity (Qmax) of FFeGx material reduces according to the enhancement of rFe:Si in the sample. Qmax of FFeG0.02, FFeG0.04, FFeG0.12 and FFeG0.22 are 101.4, 101.3, 92.4 and 81.4 mg·g-1, respectively, which are 5%, 5%, 13% and 21% less than that of mSiO2 (Table 2). This phenomenon indicates that the Fe species in the mSiO2 matrix are not beneficial for improving the adsorption performance of the material. Nevertheless, the introduced Fe species in FFeGx will promote the catalytic ability of the material, which will be discussed subsequently.

3.3 Synergetic Catalytic Degradation of MB by the FFeGx Composites. 3.3.1. Adsorption and Catalytic Performance of the FFeGx Materials Figure 6 displays the adsorption and catalytic performance of mSiO2 and FFeGx materials on the removal of MB in aqueous solutions (120 mg·L-1) at different time. There is a very fast


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uptake of MB during the initial 1 h for all samples. The MB removal percentage is 75% for mSiO2, about 66–70% for FFeG002, FFeG004 and FFeG012, and 59% for FFeG022, respectively. The adsorption rate of MB slows down in the subsequent hours and adsorption equilibrium is established at approximately 6 h for the materials (Fig. 6a). During this period, the mesoporous mSiO2 has the largest MB adsorption capacity among all materials, while the FFeGx composite that modified by α-Fe2O3 have relatively smaller MB uptakes than mSiO2. Furthermore, the MB adsorption amount decreases with the increasing of Fe content in the FFeGx composites, which is similar to the results of adsorption isotherm experiments (Fig. 5, Fig. 6a). H2O2 was introduced into the system to initiate the catalytic reaction after the adsorption. There is an instant decline of MB concentration in the system with mSiO2, which is due to the solution dilution by the addition of H2O2. After that, no more obvious change on the MB concentration is


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Figure 6. Adsorption and catalytic degradation of MB by mSiO2 and the FFeGx composites (T = 298 K, CMB = 120 mg·L-1, Cad./cat. = 1 g·L-1, CH2O2 = 1.76 mmol·L-1, pH = 7.0): relative concentration of MB at different reaction time (a), removal efficiency (b), and kinetic curves (c).


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observed in the reaction system (Fig. 6a). On the contrary, concentration of MB in the system containing FFeGx and H2O2 continuously decreases along with the reaction time. Degradation of MB is very rapid within the first 3 h after the introduction of H2O2. The MB degradation rate slows down at catalytic reaction time of 6 h (total time is 12 h), and equilibrium was established at about 24 h (total time is 30 h) for the system using FFeG0.02, FFeG0.04 and FFeG0.12 as catalysts (Fig. 6a). MB was completely removed by these three materials at total reaction time of 30 h (Fig.6a, b). Sample FFeG0.22 with the highest Fe content displays a relatively slower catalytic rate than the other FFeGx composites. The pseudo-first-order reaction rate coefficient (k) of FFeG0.22 is smaller than those of the others (Fig. 6c), which is primarily due to that the αFe2O3 nanocrystals in mesopores of FFeG0.22 hinders the mass transfer of MB and the product molecules. Even so, the MB removal efficiency for FFeG0.22 also reaches 99% after 60 h (Fig. 6b). It is obvious that the α-Fe2O3 modified FFeGx material presents its superiority in the removal of MB from aqueous solutions through combining the adsorption and catalytic degradation. Although the MB adsorption capacity of all FFeGx materials is slightly lower than that of mSiO2, the total removal of MB by FFeGx (99~100%) is much larger than those by mSiO2 (75%) and Fe2O3 (28%), as shown in Fig. 6a, b. In fact, the MB adsorption capacity for mSiO2 is limited to 106 mg·g-1 and this value changes very small when CMB is increased (Fig. 5). However, the removal number of MB by the FFeGx composites will continuously increase if further increasing the initial concentration of MB. 3.3.2. Effect of Initial Concentration for MB. Figure 7 shows the adsorption and following catalytic degradation of MB with different initial concentrations by FFeG0.12. The adsorption amount of MB is 81 mg·g-1 at C0 = 150 mg·g-1, and it maintains in 88−91 mg·g-1 at C0 = 180−300 mg·L-1. Consequently, the adsorption efficiency is


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gradually diminished along with the increase of the C0 (MB). However, the catalytic degradation efficiency still enhances when C0 (MB) is raised (Fig. 7a, b). For the systems with C0 (MB) change from 150 to 300 mg·L-1, the concentrations of MB are all dramatically reduced within 6 h after the introduction of H2O2, then the degradation rates slow down within subsequent time. After the catalytic reactions were proceed for 18 h, about 95% of MB are removed from the solutions at C0 (MB) = 150−300 mg·L-1, indicating the excellent catalytic performance of the αFe2O3/SiO2 composite. Moreover, the reaction rate of FFeG0.12 in the higher C0 (MB) system is similar to that in the C0 (MB) of 120 mg·L-1 (Fig. 7a). At reaction time of 60 h, the high concentrated MB solutions become clear. The MB removal efficiency is 100% for the system with C0 (MB) of 180−300 mg·L-1 (Fig. 7b).

Figure 7. Effect of initial concentration on the removal of MB in the adsorption and catalytic degradation process (T = 298 K, Ccat. = 1 g·L-1, CH2O2 = 1.76 mmol·L-1, pH = 7.0): relative concentration of MB at different reaction time, insert figure show the kinetic results (a), removal efficiency at different C0 (MB) (b). 3.3.3. Effect of Solution pH.


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Figure 8. Effect of solution pH on the removal of MB in the adsorption and catalytic degradation process (T = 298 K, CMB = 300 mg·L-1, Ccat. = 1 g·L-1, CH2O2 = 1.76 mmol·L-1): relative concentration of MB at different reaction time (a), removal efficiency (b), and Zeta potential of FFeG0.12 at different solution pH (c).


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Figure 8 depicts the effect of solution pH on the removal of MB by FFeG0.12. The residual MB in the solution decreases with the increase in solution pH at the adsorption stage. The removal efficiency in the adsorption process raises from 26% to 57% when solution pH changes from 3 to 11 (Fig. 8a, b). This is due to that the surface of FFeG0.12 in aqueous solution is negatively charged, and the Zeta potential of FFeG0.12 becomes more and more negative following with the solution pH increasing (Fig. 8c). The improvement of the surface negative charges for FFeG0.12 is beneficial for enhancing the electrostatic interactions between the FFeG0.12 adsorbent and the positively charged dye molecule (MB+). Moreover, H+ ions in reaction solutions will compete with the MB+ cations for the adsorption sites in FFeG0.12.34 Consequently, the MB removal amount grows along with solution pH increasing. After the adsorption, H2O2 was introduced into the system to initiate the further catalytic degradation of MB. It shows that FFeG0.12 has a high efficiency in the removal of MB over a wide pH range from 3 to11. At total reaction time of 36 h, MB in the solution with a wide pH value (3−11) was completely removed by FFeG0.12, and the high concentrated MB (300 mg·L-1) aqueous solution became clear again, even at neutral pH of 7 (Fig. 8a). As a result, FFeG0.12 shows a 100% MB removal efficiency for solutions with different pH values (pH = 3~11) (Fig. 8b), which is similar to the photo-Fenton system using α-Fe2O3/graphene oxide as catalyst,3 suggesting the superiority of the mesoporous FFeG0.12 over the conventional Fenton-like catalysts.45 FFeG0.12 also shows a different dye removal performance from that of the directly hydrothermal synthesized mesoporous Fe2O3/SiO2 composite (Si:Fe =10), whose decolonization ability for methyl orange dramatically decreases when solution pH is above 3.46 In addition, solution pH affects the adsorption and degradation rate of MB. The relative concentration of MB (C/C0) in the solution with pH of 3 and 11 is near zero at total reaction time of 12 h, suggesting


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the very fast MB removal rate of these two reaction systems. C/C0 reaches zero at about 24 h for the systems with pH = 5 and 9, and it takes 30 h to be zero for the system with pH =7 (Fig. 8a). This phenomenon indicates that MB can be completely removed by FFeG0.12 composite through adsorption combining the synergetic degradations, but the removal rate in the neutral pH aqueous solution is slightly lower than that in the acidic or alkaline solutions. 3.3.4. Recycling Tests Stability and reusability of the α-Fe2O3 modified FFeGx composite are significant for its application. Therefore, the adsorption and catalytic performance of FFeG0.12 in recycling experiments were evaluated. The initial concentration of MB is kept constant (300 mg·L-1) at neutral solution pH (pH =7). After the adsorption for 6 h and degradation for another 54 h, the catalyst (FFeG0.12) was naturally regenerated, then filtrated, dried in atmosphere and reused. It is observed that the catalyst is very stable within the first two runs. The MB removal efficiency is 100%, and the reaction solution was clear after the adsorption and degradation for 60 h. After the

Figure 9. Recycling abilities of the mesoporous FFeG0.12 composite (T = 298 K, CMB = 300 mg·L-1, Ccat. = 1 g·L-1, CH2O2 = 1.76 mmol·L-1, pH = 7.0, adsorption time = 6 h, degradation time = 54 h).


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five runs, the FFeG0.12 composite is still active, and the total MB removal percentage slightly decreases from 100% to 94% (Fig. 9). In fact, the regeneration time for FFeG0.12 is prolonged with the increase in the recycling time. The reaction solution is clear after the 1st and 2nd runs, and it became clear after 2 days after the 3rd run. After the 4th and 5th runs, it needs about one week to remove all MB in the aqueous solutions and to regenerate FFeG0.12, indicating a slight activity decrease for FFeG0.12 to a certain extent. The activity decrease of FFeG0.12 is possibly due to the Fe species leaching during the reactions. About 6.38 mg L-1 of Fe was detected in the reaction solution after the 1st run, and 3.31, 1.65, 0.337 and 0.157 mg L-1 of Fe were detected in the next 2nd, 3rd, 4th, and 5th runs, respectively. It should be pointed out that no any thermal treatments are needed to regenerate the catalyst (FFeG0.12) in this experiment. This is related to the corresponding mechanism for the removal of MB in the system using FFeG0.12 as catalyst, and will provide high potential for the FFeGx composite in real applications.

3.4 Proposed Mechanisms for the Adsorption/degradation of MB by FFeGx Composite In order to observe the molecular features and the transformation of structural of MB during the degradation process, the evolution of the UV–vis spectra of the reaction solution (CMB = 300 mg·L--1, FFeG0.12 as catalyst) over time was investigated (Fig. 10). Two characteristic absorption bands at around 665 and 612 nm appear in the visible range, which are attributed to chromophores functional groups of MB monomers and dimers. The peaks at 291 and 245 nm in the ultraviolet region is ascribed to the π → π* transitions related to unsaturated conjugate aromatic rings of MB.47


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Figure 10. Temporal UV–vis absorption spectra of MB solution during the degradation process in the FFeG0.12 + H2O2 system (a), and UV-vis diffuse reflectance spectra of the FFeG0.12 powder collected at different reaction time (Initial reaction conditions: CMB = 300 mg·L-1, Ccat. = 1 g·L-1, CH2O2 = 1.76 mmol·L-1, pH = 7.0, T = 298 K, H2O2 was added after adsorption for 6 h, and MB solution was diluted 20 times for UV–vis detections). Intensities of these peaks decrease with reaction time after the introduction of H2O2 in the system. The primary UV–vis absorption peaks of MB slightly blue shift from 665 nm to 654 nm with reaction time varies from 6 to 36 h (Fig. 10a), which is attributed to the N-demethylation involved in MB degradations.47 At reaction time of 48 h, the absorbance peaks at 665 and 291 nm totally disappear. The decay of the peak at 665 nm is caused by the destruction of the conjugate structure in MB molecules, and the disappearance of absorbance at 291 nm is


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originated from the degradation of aromatic fragments of MB.3,48 No other absorption peaks appear, indicating no new intermediates are generated during the degradation of MB. The catalyst (FFeG0.12) was also separated and collected from the reaction solutions to give more information on the catalytic process. The UV-vis diffuse reflectance (DR) spectra of the catalyst (FFeG0.12) at different reaction time are shown in Fig. 10b. The primary adsorption band of aqueous MB at 665 nm becomes weak, and a new adsorption band centered at 564 nm appears in the FFeG0.12 powder at reaction time 6 to 36 h. This new band in the DRS is assigned to the trimers and higher aggregates of MB [(MB)nn+, n ≥ 3] on the surface of FFeG0.12.49 The absorbtion peak of aqueous MB at 291 nm shifted to 284 nm in FFeG0.12, which is probably related to the interactions between MB molecules and the FFeG0.12 catalyst, and detailed mechanisms still need to be studied in the future. After the reaction for 48 h, the adsorption peaks corresponding to MB disappeared from the DR spectra of FFeG0.12 (Fig. 10b, curve in dark yellow). At the same time, we observed from the experiments that the original orange-red color reappeared with the disappearance of the blue color in FFeG0.12. This indicates that the MB adsorbed on FFeG0.12 were thoroughly degraded at 48 h, which is in accord with the UV-vis spectra of the residual MB in the reaction solutions (Fig. 10a). Based on the UV-vis absorption of residual MB solution and DRS results of FFeG0.12 in the reaction process, a possible mechanism is proposed for the degradation of MB using FFeGx composite as heterogeneous catalyst. Firstly, Fe(III) species in the FFeGx composite likely reacted with H2O2 to generate hydroperoxyl and hydroxyl radicals involving Equations (69).15,48,50 Secondly, the produced hydroxyl radicals (OH) attacked the MB molecules that absorbed on the surface of the catalyst, and produced reaction intermediates. Then, the reaction


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intermediates degraded to H2O and CO2 (Equation (10)).48,51 Finally, the catalytic sites were regenerated and () *+ + ,- .- → () -+ + ,.- ∙ + , +

(6)

() *+ + ,.- ∙ → () -+ + .- + , +

(7)

() -+ + ,- .- → () *+ + 2., ∙

(8)

() -+ + ., ∙ → () *+ + ., 1

(9)

., ∙ + 2 ↔ 45$)67)849$): ↔ .- + ,- .

(10)

again available for occupation by the MB molecules when those products (H2O and CO2) were desorbed from the catalytic sites.52 Experimental results also show that the initial neutral solution (pH = 7.0) changed to be acidic (pH = 3.0) after the catalytic reaction for the FFeG0.12 + H2O2 system, which is consistent with the generation of H+ in the above reactions. The reduction of pH during the Fenton reaction was also observed in the system using Cu and Fe oxides modified mesoporous SiO2 as catalyst.15 It is obvious that the novel mesoporous FFeGx composite presents both high adsorption capacity and superior catalytic activity in this heterogeneous Fenton reaction system. The adsorption capacity is originated primarily from the mesostructure of the composite, because the mSiO2 support can adsorb 106 mg·g-1 of MB, and all FFeGx composites also have considerable MB uptakes in the batch tests (Fig. 5, Table 2). The catalytic activity is own to the Fe species in FFeGx,


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Figure 11. Schematic diagram of MB adsorb on and be degraded by FFeGx composite. for mSiO2 shows negligible while FFeGx presents remarkable catalytic activity in the degradation of MB (Fig.6 a and b). In fact, heterogeneous Fenton reaction normally combines the surface adsorption and catalytic oxidation, and the surface adsorption supplies a basic reaction platform for the sequence catalytic oxidation.52 However, in conventional conditions, the active species (HO2 and OH) and MB molecules will compete for the same absorption sites during the reaction, which easily leads to the reduced activity of the catalyst. Nevertheless, in this system using mesoporus FFeGx as catalyst, the mesopores in the FFeGx composite are beneficial for the adsorption of MB and consequently enhance the catalytic performance of the material. The high number mesopores and silanol groups in the composite provide enough adsorption space and active sites to catch and save MB molecules. Then, the generated hydroxyl radicals (OH) can easily attack and degrade the MB molecules adsorbed on the catalyst (Fig. 11). As a result, the FFeGx composite shows a quite excellent catalytic performance for the removal of MB from aqueous solutions.

4. Conclusions


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In summary, a mesoporous α-Fe2O3/SiO2 composite (FFeGx) as both adsorbent and catalyst has been successfully prepared via a spontaneous infiltration route. Different from the traditional wetness impregnation approaches, this synthesis process using template-containing SiO2 as support provides an energy- and time- saving way for developing novel mesoporous catalysts. Due to the presence of large number of mesopores and silanol groups for enough adsorption sites, and sufficient Fe species for active catalysis sites, FFeGx composite can completely remove the high concentrated MB (300 mg·L-1) through adsorption and the synergetic degradations over a wide pH range of 3.0–11.0. In addition, this composite can be naturally regenerated without any thermal treatments, and it exhibits an excellent stability and reusability. The present study exploits a versatile technique for the design and synthesis of mesoporous metal oxide modified SiO2 materials, which can be served as both adsorbents and Fenton-like catalysts in removing organic pollutants from aqueous solutions.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI. Further characterizations and experimental data (TG-DSC of the FFeG0.12-As, and FTIR spetra of the FFeGx composites) (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +86-512-67374120. Fax: +86-512-67374120; *E-mail: [email protected]. Phone: +86-512-68083175. Fax: +86-512-67374120; Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation (NSF) of China (51478285, 21777110 and 51778392), NSF of Jiangsu Province (BK20151198). Financial supports from Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, the Open Projects of the International Joint Laboratory of Chinese Education Ministry on Resource Chemistry (A-2017-002) and the State Key Laboratory of MaterialsOriented Chemical Engineering (KL17-06), and the Jiangsu Innovation Project for Graduate Education (KYCX17_2064) are also gratefully acknowledged.


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TOC

A novel α-Fe2O3/SiO2 composite was feasibly fabricated and shows both of high adsorption capacity and excellent Fenton-like catalytic activity.


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Adsorption and Synergetic Fenton-like Degradation of Methylene Blue

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