Anchored Iron Ligands as an Efficient Fenton-Like Catalyst for

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Anchored Iron Ligands as an Efficient Fenton-Like Catalyst for Removal of Dye Pollutants at Neutral pH Yuyuan Yao,*,† Yajun Mao,† Binbin Zheng,†,‡ Zhenfu Huang,† Wangyang Lu,† and Wenxing Chen*,† †

National Engineering Lab of Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, China ‡ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The development of a pH-tolerant Fenton-like catalyst is an active and challenging research front in the area of environmental engineering. In the present work, a novel catalyst iron−γ-aminopyridine ligand (FeAPy) was prepared and immobilized on activated carbon fibers (ACFs) by a covalent bond to obtain a heterogeneous FeAPy-ACFs. FeAPy-ACFs present a pH-tolerant microenvironment for the Fenton-like oxidation process, which exhibits remarkable catalytic ability across a wide pH range from acidic to alkaline. At neutral pH, employing hydrogen peroxide as an oxidant, the FeAPy-ACFs showed enhanced catalytic ability to degrade hazardous environmental pollutants Acid Red 1 (AR1) dyes and avoid secondary pollution with regard to FeAPy. FeAPy-ACFs are stable and remain efficient in repetitive test cycles with no obvious decrease of catalytic activity. Moreover, in comparison to most reported supports for Fenton-like catalyst, the introduction of ACFs contributed specifically to the activity improvement of FeAPy. Probe studies combined with electron paramagnetic resonance experiments were conducted to ascertain the role of several reactive species (•OH, HO2•, and FeIVO) on dye decolorization.

1. INTRODUCTION In recent decades, advanced oxidation processes (AOPs, e.g., H2O2/UV, O3/UV, O3/H2O2, Fenton’s reagent, photo-Fenton, electrochemical oxidation, etc.)1−7 have been widely studied for the treatment of industrial wastewater contaminated with toxic organic pollutants. In particular, Fenton’s reagent (Fe2+/H2O2) has been proved as a promising and attractive treatment due to its high efficiency and simple operation.8−11 However, the current Fenton technology suffers from several environmentally important drawbacks: (i) production of hydroxyl radicals is limited to a narrow pH range (2.5− 3.5),12,13 and thus large quantities of acid are required to produce the optimal pH; (ii) a neutralization step of effluent and ion sludge disposal is required; and (iii) removal of the catalyst from the aqueous system is difficult. Therefore, development of heterogeneous catalysts that expand the working pH range from acidic to neutral or even alkaline and avoid the production of ion sludge has been an active research front in this area. There are many reports describing the use of iron species to be immobilized on diverse supports (such as clay,14 zeolite,15 nafion,16 graphene,17 ion-exchange resin,18 etc.) as heterogeneous catalysts for the Fenton reaction. These heterogeneous Fenton-like catalysts can prevent iron ion leaching and extend the working pH range to a certain extent. Recently, we have reported that Fe@ACFs could be used as an effective heterogeneous Fenton catalyst for the removal of organic dyes.19 Although Fe@ACFs achieved some preliminary results in improvement of catalytic activity and extended the working pH range to a certain extent, its excellent catalytic performance is still restricted to acidic conditions, limiting its further application in neutral and alkaline aquatic environments. © 2014 American Chemical Society

Hence, employing support to immobilize iron ion alone could not completely solve these problems caused by the pH limitation, and it is essential that novel strategies to construct a pH-tolerant microenvironment are proposed. It has been reported that complexation of iron with organic ligands enables an exquisite ligand-to-metal charge transfer (LMCT) process and allows extending the pH range from acidic to alkaline with regard to Fenton processes.20−22 These metal complexes include metallophthalocyanines,23 metalloporphyrins,24 metal coordination complexes with N-based neutral ligands,25 polyoxometalates,26 or metal complexes based on nonporphyrinic nitrogen-containing ligands relying on iron.27 Unfortunately, such catalysts might deteriorate gradually with oxidative destruction of the ligand moiety and induce fragmentation of the structure during the reaction, leading to irreclaimable demetalation of the iron center and secondary contamination.28 Considering that complexation of iron with ligands allows extending the pH range from acidic to alkaline with regard to Fenton processes and the matched support enables various strategies to firmly attach catalytic active species, this provides new possibilities for the application of anchored iron ligands to construct a pH-tolerant microenvironment. First, we chose an appropriate ligand γ-aminopyridine. The robust coordination of iron and γ-aminopyridine ligands enables an exquisite ligand-tometal charge transfer process, and accelerates the transformation in iron valence states, endowing it the potential in Received: Revised: Accepted: Published: 8376

September 29, 2013 April 27, 2014 April 30, 2014 April 30, 2014 dx.doi.org/10.1021/ie403226v | Ind. Eng. Chem. Res. 2014, 53, 8376−8384

Industrial & Engineering Chemistry Research

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modified by thionyl chloride (SOCl2) at 90 °C for 12 h. Second, 2 g of FeCl3·6H2O and 3.5 g of γ-aminopyridine (APy) were respectively dissolved in N,N′-dimethylformamide (DMF), and then the FeCl3·6H2O solution was slowly added intothe γ-aminopyridine solution, with stirring throughout the process. The SOCl2-modified ACFs were then taken into the mother liquor, keeping at 50 °C for 1 h. The solution temperature increased to 60 °C, and sodium carbonate (2 g) was added into the solution and maintained for 2 h. The FeAPy-ACFs were taken out from the solution, cleaned with alkaline liquid, soaped again by nonionic detergent (2 g/L) under boiling for 15 min, and then rinsed with distilled water many times to remove the unattached FeAPy as well as the other residuals. Calculated according to the iron content in FeAPy-ACFs by atomic adsorption spectrometer (AAS), the content of FeAPy in FeAPy-ACFs was 27.6 μmol/g. For comparison, Fe@ACFs were prepared by absorbing Fe3+ to ACFs directly19 and the content of iron was controlled pretty much the same as FeAPy-ACFs. 2.3. Experimental Procedure. The binding site between activated carbon fiber and FeAPy was investigated using X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRADLD, UK) with Al (mono) Kα irradiation (hν = 1486.6 eV) at a pass energy of 160 eV (wide scan) and 20 eV (narrow scan), respectively. All the binding energy peaks of XPS spectra were calibrated by placing the principal C 1s binding energy peak at 284.7 eV. The decolorization of AR1 was carried out in a 50 mL glass beaker by shaking in a thermostat water bath (SHA-B, Guowang Laboratorial Equipment Factory, China) that was adjusted at 50 °C or other temperature. A typical reaction mixture contained the following concentrations or initial amounts: AR1 (50 μmol/L), FeAPy-ACFs (5 g/L), and H2O2 (50 mM). At given time intervals, the samples were taken from the reaction cell and analyzed immediately by UV− vis absorption spectra (Hitachi U-3010, Japan). The maximum absorption of AR1 was at a wavelength of 531 nm. The procedure used in the repetitive experiments to investigate the sustaining catalytic stability of FeAPy-ACFs is as follows: after the initial AR1 (50 μM) had been removed by the FeAPyACFs/H2O2 system, a definite amount of AR1 and H2O2, which was the same as the initial concentration of AR1 and H2O2, was added again into the system to make the next catalytic cycle carried out under the same conditions as the last cycle. In this way, the repetitive experiments of catalytic decolorization were performed. Electron paramagnetic resonance (EPR) signals of radicals trapped by DMPO were recorded at ambient temperature on a Bruker A300 spectrometer. The settings for the EPR spectrometer were as follows: center field, 3520 G; microwave frequency, 9.77 GHz; modulation frequency, 100 kHz; power, 12.72 mW. Cyclic voltammetric measurement was performed on an IM6ex electrochemical workstation (Zahner, Germany). A conventional three-electrode electrochemical cell was used. A BAS glassy carbon electrode (GCE, 3 mm), platinum wire, and Ag/ AgCl electrode (0.22 V versus standard hydrogen electrode (SHE)) served as the working, counter, and pseudoreference electrodes, respectively.

the construction of an efficient catalytic active site. Second, the amino group of γ-aminopyridine may interact with function groups in the support to establish a strong link between the iron ligands and the support. Third, the crystal and molecular structures of various metal−pyridine complexes were investigated.29−31 Among them, FeCl2py4+ can catalyze transformation of anilines to nitrosobenzene;32 nickel pyridine complex can catalyze cascade formation of C(sp3)−C(sp3) bonds;33 and bis(imino) pyridine iron can be employed as the catalyst for ethylene polymerization.34 Comparatively, the knowledge about the catalytic performance of metal−pyridine toward oxidants (e.g., H2O2) for the transformation of organic pollutants is rather limited. Hence, the iron−γ-aminopyridine we reported here is promising to expand its application from organic synthesis to environmental remediation. Furthermore, ACFs are chosen as the support of iron−γ-aminopyridine ligands based on the following considerations: (i) as a chemical inert support material, ACFs would impart stability to the iron species without inhibiting their activity; (ii) with the versatility of its texture and surface chemistry, ACFs allow for a wide range of strategies to firmly attach catalytic active species and thus to improve catalytic activity and its possible reuse in the process;35 (iii) with a microporous fibrous structure, ACFs have a large accessible surface area and extremely high adsorption capacity, assuring potential construction of more active sites.36 In the present work, anchored iron−γ-aminopyridine ligands on ACFs were employed as a novel heterogeneous Fenton-like catalyst for degrading dye pollutants. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) were employed, respectively, to characterize the binding site and detect the reactive oxygen species (ROS) during the process. Several model dyes, as well as the factors that may influence the reaction process such as pH, temperature, and hydrogen peroxide concentration were investigated. To the best of our knowledge, a novel iron−γ-aminopyridine ligand (FeAPy) has never been reported previously in the field of environmental engineering, and in the present work, FeAPy has been first reported and anchored to ACFs to achieve an efficient Fenton-like catalyst for removal of dye pollutants. This study paves a new avenue for the development of a simple but highly efficient catalytic system, and has potential value in environmental engineering.

2. EXPERIMENTAL SECTION 2.1. Materials. C. I. Acid Red 1 (AR1), Rhodamine B (RhB), Rhodamine 6G (Rh6G), Orange II, Reactive Red X-3B (RR X-3B), Basic Green 1 (BG1), and Methyl Blue (MB) were used as model dyes without any prior purification. N,NDimethylformamide (DMF), thionyl chloride (SOCl2), FeCl3· 6H 2 O, γ-aminopyridine, methanol, dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO2), o-phenylenediamine (OPDA), 2,3-diamino phenazine (DAPN), and hydrogen peroxide (30 wt %, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were used as analytical reagents. Activated carbon fibers (ACFs) were obtained from Jiangsu Sutong Carbon Fiber Co., Ltd. (Jiangsu, China). The spin trapping reagent 5,5-dimethyl-pyrroline-oxide (DMPO) was supplied by Tokyo Chemical Industry Co., Ltd. Tetrabutylammonuim tetrafluoroborate (TBABF4) was obtained from Acros. Doubly distilled water was used throughout the dye decomposition process. 2.2. Catalyst Preparation. First, ACFs were oxidized by hydrogen nitrate for 24 h, and the oxidized ACFs were

3. RESULTS AND DISCUSSION 3.1. XPS Analysis of FeAPy-ACFs. XPS experiments were used to prove the formation of a covalent bond between FeAPy and oxidized ACFs, as shown in Figure 1. In comparison with the oxidized ACFs, a new band of iron and a marked increase of 8377

dx.doi.org/10.1021/ie403226v | Ind. Eng. Chem. Res. 2014, 53, 8376−8384


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531 nm (attributed to the azo bond) as well as at 312 and 364 nm (attributed to the naphthalene rings) decreased rapidly with time, indicating a dramatic destruction of the azo bond and naphthalene ring in the dye molecule, respectively. Hence, the catalytic oxidation of AR1 in the copresence of FeAPy-ACFs and H2O2 is a chemical decomposition process. Control experiments were used to further investigate the catalytic activity of FeAPy-ACFs. As depicted in Figure 3, the color

Figure 1. XPS spectra of oxidized ACFs and FeAPy-ACFs.

nitrogen were detected in FeAPy-ACFs, suggesting that FeAPy was supported onto oxidized ACFs. Furthermore, Figure 2A indicated that the N 1s peak of oxidized ACFs occurred at 400.0 eV.37 Nevertheless, when FeAPy was supported onto oxidized ACFs, the spectra of N 1s presented a different spectrogram (Figure 2B). Besides the peak of ACFs (399.9 eV), the peaks occurring at 398.3 and 400.7 eV38 were respectively ascribed to pyridine nitrogen atoms and the nitrogen of the amide group (−NH−CO−) being used for bonding between the carboxyl and substituted amino groups. In addition, coincident information about the bonding environment of FeAPy-ACFs was provided by the O 1s peaks (Figure S1, Supporting Information). For the oxidized ACFs (Figure S1A, Supporting Information), the O 1s peaks at 531.7 and 533.5 eV were assigned to oxygen in a CO double bond and oxygen in a single bond of the carboxyl group (CO H), respectively. However, the proportion of COH peaks is smaller in FeAPy-ACFs (Figure S1B, Supporting Information), indicating a part of the COH was consumed by the amidation reaction. For the oxidized ACFs (Figure S2A, Supporting Information), the C 1s peaks at 284.5, 285.2, 285.9, and 288.9 eV were assigned to oxygen in CC/CC, CO, CN, and COOH, respectively. However, when FeAPy was supported onto the oxidized ACFs (Figure S2B, Supporting Information), the peak of the carbon of the COOH group disappeared and a significant peak of the carbon of the amide group (CONH) occurred at 287.8 eV. Therefore, we concluded that FeAPy was supported onto ACFs covalently via amide linkage. 3.2. Decolorization Experiments. Acid Red 1 was selected as the target dye to investigate the catalytic ability of FeAPy-ACFs. Figure S3 (Supporting Information) showed the UV−vis spectra obtained from the AR1 solution at different reaction times (the inset shows the molecular structure of AR1). The characteristic absorption bands of AR1 at 506 and

Figure 3. Concentration changes of AR1 under various conditions: (a) Fenton (pH 7); (b) ACFs; (c) ACFs + H2O2; (d) Fenton (pH 3); (e) FeAPy-ACFs; (f) FeAPy + H2O2; (g) FeAPy-ACFs + H2O2. Conditions: [FeAPy-ACFs] = 5 g/L (containing 138 μM FeAPy), [FeAPy] = 138 μM, [AR1]0 = 50 μM, [H2O2] = 50 mM, pH 7 (except part d), T = 50 °C.

removal was negligible in the presence of Fenton’s reagent (Fe2+/H2O2) under neutral pH conditions, which was attributed to the precipitation of iron ion. When the ACFs were present, about 18% of AR1 was eliminated after 40 min. In the copresence of ACFs and H2O2, the AR1 removal increased to only 21%, indicating that ACFs itself almost cannot activate H2O2 to catalytically decolorize dye. When the FeAPy-ACFs were present, about 25% of AR1 was removed. In the presence of FeAPy and H2O2, approximately 35% of dyes was removed, significantly higher than that of Fenton’s reagent at its optimum pH of 3 (21% removal in Figure 3, curve d), suggesting that the chelation of iron to γ-aminopyridine ligands enables a relatively obvious enhancement of the catalytic activity. However, when AR1 in aqueous solution was exposed to FeAPy-ACFs and H2O2 together, more than 99% of AR1 was removed in 30 min. Hence, compared with Fenton reagent and Fenton-like catalyst FeAPy, FeAPy-ACFs are more highly efficient to catalytically decolorize dyes, suggesting that the APy ligands and ACFs displayed a synergetic effect in terms of catalytic activity for the iron catalyst. 3.3. Sustaining Catalytic and Regenerative Stability. To investigate the sustainable catalytic stability of FeAPy-ACFs, several oxidation processes to catalyze AR1 were carried out

Figure 2. Curve fit of the N 1s peak of oxidized ACFs (A) and FeAPy-ACFs (B). 8378



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chelate iron (FeAPy) can remove about 40% of dye AR1 in 60 min at pH 7, which is obviously superior to the catalytic ability of Fenton’s reagent (the Fenton’s reagent was almost completely ineffective for the removal of dyes at pH 7, as shown in Figure 3, curve a). However, FeAPy also showed poor regenerative stability because its catalytic ability decreased obviously in the following three cyclic utilizations (the dye removal dropped from 40% in the first cycle to 10% in the fourth cycle). Therefore, to avoid the disadvantage (poor regenerative stability) associated with Fe@ACFs and FeAPy, the development of efficient catalysts with superior regenerative stability to treat dye pollutants is highly desired. To our surprise, FeAPy-ACFs, the simple incorporation of the two strategies (Fe@ACFs and FeAPy), brought more remarkable and stable catalytic ability than that of FeAPy and Fe@ACFs, successfully solving this problem (Figure 5). This result suggests that the APy ligands and ACFs displayed a synergetic effect in terms of catalytic stability for the iron catalyst. These findings indicate that FeAPy-ACFs not only provide sustainable catalytic oxidation but also have an improved ability to regenerate in situ with respect to Fe@ACFs or FeAPy. Moreover, the effect of dissolved iron (leaching test) on the dye decolorization catalyzed by FeAPy-ACFs under different pH conditions was also studied. The concentration of the leaching iron ion at different pH values was detected by atomic absorption spectrometer (AAS). As shown in Figure S5 (Supporting Information), the highest amount of iron ion leached from FeAPy-ACFs after the catalytic reaction at different pH values (3, 5, 7, 9, 11) was only 0.04 ppm, much lower than the amount of 2 ppm in water for European standard.39 And the effect of dissolved iron on decolorization of the dye was further investigated. As shown in Figure S6 (Supporting Information), the highest dye removal by the dissolved iron is only 14% at pH 3 within 40 min, much lower than that of 99% by FeAPy-ACFs. These results demonstrated that the enhanced catalytic decolorization of dye was not attributed to the leaching iron ion from FeAPy-ACFs, as the dissolved iron has little influence on the catalytic decolorization of the dye. It was obvious that the FeAPy-ACFs are relatively stable for decolorizing dyes, which is in agreement with the above results. 3.4. Oxidative Removal of Other Organic Dyes. We further investigated the FeAPy-ACFs/H2O2 catalytic system for the oxidative decolorization of other organic dyes including Rhodamine B, Rhodamine 6G, Orange II, Reactive Red X-3B, Basic Green 1, and Methyl Blue. As depicted in Table 1, when FeAPy-ACFs and H2O2 were present together, all of these dyes were effectively removed. As for the dye Orange II, more than

sequentially by adding the same amount of AR1 with H2O2. As shown in Figure 4, FeAPy-ACFs can remove 99.6% of dye in

Figure 4. Sustaining catalytic stability of FeAPy-ACFs for the decomposition of AR1. Conditions: [FeAPy-ACFs] = 5 g/L), [H2O2] = 50 mM (addition of AR1, 50 μM/run, H2O2, 50 mM/ run, pH 7.0, T = 50 °C).

the first run and 98.3% in the 15th run, respectively, demonstrating FeAPy-ACFs could be utilized 15 times without any obvious decrease of catalytic activity. With respect to bare FeAPy, the sustainable catalytic ability of FeAPy-ACFs are obviously superior, as the bare FeAPy gradually lost its activity due to the oxidative self-destruction in reuse tests (with dye removal of 35.7% in the first run and 20.1% in the third run, respectively, as shown in Figure S4, Supporting Information). These findings indicate that the catalyst is stable and remains efficient in repetitive test cycles with no obvious decrease of catalytic activity, which may be attributed to the fact that the chemically inert ACFs impart stability to the iron species without inhibiting their activity. Regeneration is another important concern for the catalyst to be utilized in practical applications. The evaluation of the regenerative stability of FeAPy-ACFs was carried out, compared with the heterogeneous Fenton catalyst Fe@ACFs and the heterogeneous Fenton-like catalyst FeAPy through four repetitive experiments. For every run, the FeAPy-ACFs, Fe@ ACFs, and FeAPy were taken out, rinsed with distilled water, and dried for 60 min. As shown in Figure 5, although employing ACFs to anchor iron (Fe@ACFs) can efficiently remove 99% of dye AR1 in 60 min at pH 7, which breaks the pH limitation (2.5−3.5) of Fenton’s reagent, its catalytic ability decreased obviously in the following three cyclic utilizations (the dye removal dropped rapidly from 99% in the first cycle to 21% in the fourth cycle). And using APy ligands alone to

Table 1. Catalytic Oxidation of Different Dyesa entry

dye

reaction time (min)

C/C0b (%)

1 2 3 4 5 6

Rhodamine B Rhodamine 6G Orange II Reactive Red X-3B Basic Green 1 Methyl Blue

40 90 23 110 50 140

4.3 8 0.3 6.0 0.22 5.5

Standard conditions: the dye solution (50 μM) was treated in the presence of FeAPy-ACFs (5 g/L) with H2O2 (50 mM), pH 7, T = 50 °C. bThe remaining rate (C/C0) of dyes in aqueous solution was measured by UV−visible spectrometer. a

Figure 5. Regenerative stability of different catalysts for the decomposition of AR1. Conditions: [FeAPy-ACFs] = 5 g/L (containing 138 μM FeAPy), [FeAPy] = 138 μM, [Fe@ACFs] = 5 g/L, [AR1]0 = 50 μM, [H2O2] = 50 mM, pH 7, T = 50 °C. 8379



8380

82% of dye was removed within 180 min

90 and 99% of dye removal in 60 min under visible light (kobs = 0.0413 min−1) and sunlight (kobs = 0.122 min−1), respectively about 98 and 34% of dye was removed within 240 min in the first and fourth run, respectively pH 7, about 92% of dye was removed within 120 min, with a decoloration rate constant of kobs = 0.02858 min−1; the RhB removal decreased obviously from about 90% at a lower pH of around 3 to 35% at a higher pH of around 11. 30, 60, and 90% of dye was removed at pH 3, 6.4, and 10 within 6 h, respectively

pH 3, 98.4% of dye was removed in 150 min; pH 4, less than 20% of dye was removed in 180 min

Disperse Orange 3 (62.5 μM in ACN/H2O (1:2)) Acid Red 73 (10 mg/L)

Acid Red 1 (50 mg L−1) Rhodamine B (20 μM)

[Cat.] = 0.4 g/L, [H2O2] = 10 mM, pH 3 and 4, T = 26 °C, under visible light [Cat.] = 0.4 g/L, [H2O2] = 6 mM, pH 3, T = 25 °C, under visible light or sunlight [Cat.] = 4 g/L, [H2O2] = 4 mM, pH 3, T = 30 °C [Cat.] = 100 mg/L, [H2O2] = 2 mM, at room temperature, visible light irradiation (λ > 420 nm) [Cat.] = 62.5 μM and [H2O2] = 0.375 mM in ACN/H2O (1:2), pH 3, 6.4, and 10, at room temperature [Cat.] = 3 g/L and [H2O2] = 100 mM, pH 7, at room temperature CoTSPc@MCM-4145

Fe/kaolin42 iron(II) bipyridine complex-clay43 FeP-K1044

Fe−SiO2

iron-modified rectorite

41

Acid Blue 29 (50 mg L−1) Rhodamine B (80 μM)

reaction conditions dye catalyst

40

Table 2. Comparison of Dye Removal by Different Heterogeneous Fenton or Fenton-Like Catalysts Based on Various Supports

97% of the dye was eliminated from the dye solution within 23 min by FeAPy-ACFs, greatly superior to the catalyst of iron(II) bipyridine supported on clay that almost cannot remove this dye.43 These results suggest that the FeAPy-ACFs/H2O2 system can remove most common organic dyes, such as acid dyes, basic dyes, reactive dyes, etc. The nonselective catalytic decolorization process is of greater importance to practical applications in removing organic dyes. Some literature data on the removal of dyes by different heterogeneous Fenton or Fenton-like catalysts based on various supports are listed in Table 2. As observed in Table 2, almost all of the parameters tested for the FeAPy-ACFs/H2O2/dye system used in this work are more effective than those of the previously reported systems: a superior sustaining catalytic oxidation and in situ regeneration, broader pH range (3−11) (Figure S16, Supporting Information), nonselective catalytic decolorization for various dyes, being efficient without any additional light irradiation and with almost complete color removal of AR1 achieved in 30 min at 50 °C and a low activation energy (Ea = 44.02 kJ mol−1) (Figure S17, Supporting Information), indicating that the ACFs were more effective in enhancing the catalytic ability of iron species compared with other supports. Such an enhancement of the catalytic activity provided by FeAPy-ACFs may be through the involvement of the following: (i) accelerating the generation of highly reactive species by electron transfer from ACFs to FeAPy due to the rich free electrons in ACFs;46 (ii) making the diffusion of the substrate to the catalytic centers much easier due to the excellent adsorption performance and high affinity to dyes of ACFs, which may be beneficial in the binding interactions between dye molecules and the catalyst. Notably, the presence of ACFs conferred remarkable sustained catalytic stability that avoids the serious disadvantage associated with bare FeAPy, poor stability due to the oxidative self-destruction in reuse tests. Further, our findings suggest that this practical catalyst can operate continuously and efficiently in actual implementations treating organic pollutants. 3.5. Investigation of Active Species in the Catalytic System. The generation of hydroxyl radicals (•OH) is a predominant catalytic mechanism in the homogeneous Fenton reaction. Isopropanol (IPA), a scavenger of •OH,47 was employed to investigate the reactive oxygen species (ROS) involved in the FeAPy-ACFs/H2O2/AR1 system. As shown in Figure S7 (Supporting Information), when IPA was added into the dye solution, the decolorization efficiency decreased obviously, indicating that •OH radicals may be the active species in this catalytic process. However, these results also demonstrated that IPA did not completely inhibit the decomposition, even though excess IPA was added, suggesting that ROS besides •OH radicals were also involved in this catalytic process. It has been reported that sulfoxides (e.g., dimethyl sulfoxide, methyl phenyl sulfoxide, and methyl p-tolyl sulfoxide) can react with Fe(IV) species to generate corresponding sulfones, which differed markedly from their •OH-involved products.48 In the present study, dimethyl sulfoxide (DMSO) was employed to probe the existence of Fe(IV). As shown in Figure 6, significant enhancement of the peak at 207 nm was attributed to the generation of dimethyl sulfone (DMSO2), demonstrating that a high-valent iron (Fe(IV)) was formed in the presence of FeAPy-ACFs and H2O2. These findings suggested that Fe(IV) might participate in the catalytic decomposition of dyes by FeAPy-ACFs.

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Figure 6. UV/vis spectroscopy of (a) DMSO, (b) oxidation products of DMSO by H2O2, (c) oxidation products of DMSO by FeAPyACFs/H2O2. The inset shows the absorption spectrum of 1 mM DMSO2 aqueous solution. Conditions: [DMSO] = 1 mM, [FeAPyACFs] = 5 g/L, [H2O2] = 20 μM, after 10 min of reaction at ambient temperature.

Figure 8. DMPO spin-trapping ESR spectra of •OH radicals during repetitive experiments of the different catalytic systems.

systems above (Figure 9). When OPDA instead of dye was added into the FeAPy/H2O2 system, the oxidation of OPDA by

An EPR spin-trapping technique was adopted to further convince the •OH radicals involved in the FeAPy/H2O2/AR1 and FeAPy-ACFs/H2O2/AR1 systems, respectively. As shown in Figure 7, the 4-fold characteristic peak with an intensity ratio of 1:2:2:1 is ascribed to the DMPO−•OH adduct, whereas the sextet characteristic peak is attributed to the DMPO−HO2• adduct. Moreover, FeAPy-ACFs could catalyze the H2O2 decomposition to produce a significantly greater amount of •OH but a slightly greater amount of HO2• species with respect to the FeAPy/H2O2/AR1 system. These findings suggested that enhanced formation of •OH due to the interference of ACFs may be responsible for the promoted catalytic activity of FeAPy-ACFs. In addition, effects of γaminopyridine ligands and ACFs on the •OH formation during repetitive experiments was further explored. As shown in Figure 8, the Fenton’s reagent almost cannot generate •OH owing to precipitation of iron ions at pH 7, while the coordination of γaminopyridine ligands to iron (FeAPy) avoids this problem and effectively catalyzes the H2O2 decomposition to produce •OH. With respect to FeAPy, although Fe@ACFs exhibits a higher ability to activate H2O2 to generate •OH, it shows poorer stability as the signal intensity of the DMPO−•OH adduct decreased obviously in the second time for utilization. In contrast, incorporation of the two strategies using FeAPy-ACFs as catalyst presents constant signal intensity of the DMPO− •OH adduct in two cyclic utilizations, thereby fixing the shortcomings for FeAPy and Fe@ACFs, which is consistent with the robust and constant catalytic activity of FeAPy-ACFs. In addition, a probe of •OH radicals, o-phenylenediamine (OPDA), was employed to quantitatively assess the •OH radical formation49 from H2O2 catalyzed by the various catalytic

Figure 9. Absorbance of DAPN at 532 nm for repetitive experiments with different catalysts. Conditions: [OPDA] = 3 mM, [FeAPy-ACFs] = 5 g/L (containing 138 μM FeAPy), [FeAPy] = 138 μM, [Fe@ ACFs] = 5 g/L, [H2O2] = 50 mM, pH 7, at 50 °C.

•OH radicals produced 2,3-diaminophenazine (DAPN), leading to the increase of the absorption peak at 453 nm. The absorbance of DAPN at 453 nm for the FeAPy-ACFs/ H2O2 system almost remained constant at the same reaction time (5 and 10 min) in two cyclic utilizations, indicating that FeAPy-ACFs showed remarkable stability due to the constant production of •OH radicals in repetitive experiments, whereas, for FeAPy and Fe@ACFs, the corresponding absorbance showed a slight decrease and an obvious decrease, respectively. These findings were in good agreement with the ESR results, suggesting that, at neutral pH conditions, iron catalyst works in the most efficient way through the cooperative effect of ligands and support. 3.6. Working Hypothesis Based on Electrochemistry. To explain this enhancement of activity by the interference of

Figure 7. DMPO spin-trapping ESR spectra of (A) •OH radicals and (B) HO2• in systems of FeAPy/H2O2/AR1 and FeAPy-ACFs/H2O2/AR1. 8381



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complete exhaustion of H2O2. Hence, the remarkable and promoted catalytic ability of FeAPy-ACFs might be attributed to a strong electronic interaction between FeAPy and ACFs.

ACFs essentially, a working hypothesis based on electrochemistry was tried out. First of all, the cyclic voltammogram of FeAPy in 50 mM tetrabutylammonium tetrafluoroborate (TBABF4) aqueous solutions was tested to investigate the electrochemical behavior of FeAPy (Figure 10). The oxidation

4. CONCLUSIONS The usefulness of the current Fenton technology is limited by a narrow pH range in which it can be used (pH 2.5−3.5), and this work has explored novel Fenton-like catalysts (FeAPyACFs) of iron-amino pyridine ligands (FeAPy) supported covalently onto activated carbon fibers (ACFs). Compared to bare FeAPy, the as-prepared catalyst exhibits more excellent catalytic activity, stability, and pH tolerance. Interestingly, the APy ligands and ACFs displayed a synergetic effect in terms of catalytic activity and stability. In comparison with the traditional Fenton reagent, the FeAPy-ACFs/H2O2 system offers the advantages of (i) bringing no secondary pollution owing to the stability of this supported catalyst, (ii) presenting a novel heterogeneous Fenton-like catalyst with improved stability and excellent catalytic activity, (iii) showing remarkable enhancement of catalytic activity across a wider pH range from 3 to 11, and (iv) improved recycling of Fe(III)/Fe(IV) with enhanced •OH production due to electron transfer from ACFs to FeAPy. Hence, this study provides a viable strategy for designing anchored iron-ligand catalysts for efficient treatment of pollutants at neutral pH.

Figure 10. Cyclic voltammogram (CV) of FeAPy in DMF containing 50 mM tetrabutylammonium tetrafluoroborate (TBABF4).

wave with an onset at 0.87 V occurred at the central metal iron in FeAPy, which was ascribed to Fe(IV)/Fe(III). As illustrated in Figure 11, the work function of extended π-conjugated

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing O1s and C1s peaks in XPS, spectral changes during catalytic oxidation of AR1, the sustaining catalytic stability of FeAPy, the dissolved iron during AR1 degradation under different pH conditions, the comparison of homogeneous Fenton and heterogeneous Fenton-like catalysis on AR1 decoloration at various pH values, the effect of isopropanol on the decomposition of AR1, first- and second-order kinetics fitting of the dye decomposition, removal of AR1 dyes at various pHs, and linear regressions of ln kobs versus 1/T and of ln(kobs/T) versus 1/T. Table showing the removal rate constants and correlation coefficients at different operational parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 11. Schematic illustration of electron transfer in the FeAPyACFs/H2O2 system.

AUTHOR INFORMATION

Corresponding Authors

structural ACFs can been determined to be approximately 4.7 eV, which is related to the Fermi energy (EF).50 The oxidation potential of FeAPy (Eox) was +1.09 V vs SHE based on E vs SHE = E vs Ag/AgCl + 0.22 V. Furthermore, the HOMO level of FeAPy (EHOMO(FeAPy)) was −5.49 eV according to the empirical formula EHOMO = −e (Eox + 4.4) (eV).51 In the FeAPy-ACFs/H2O2 system, since the reduction potential of H2O2 was as high as +1.8 V vs SHE,52 which enables the oxidation of FeAPy (FeIII) to the oxidation state (FeIVO) via axial coordination between H2O2 and the central iron ion, the H2O2 itself was reduced to •OH (oxidation potential: 2.8 V).53 By accepting electrons from ACFs, the oxidation state FeIVO could be converted to FeIII. Finally, the ACFs can be rapidly reduced by H2O2 (oxidation potential: 0.695 V52) after losing electrons, leading to the generation of •OOH and the evolution of O 2. As the decomposition of H2 O2 belongs to a nonequilibrium redox process, these reaction steps can be continuously driven by the relative potential difference until

*Tel.: +86-571-86843810. Fax: +86-571-86843255. E-mail: [email protected]. *Tel.: +86-571-86843005. Fax: +86-571-86843255. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the State Key Program of National Natural Science of China (No. 51133006), the National Natural Science Foundation of China (No. 51103133), Zhejiang Provincial Natural Science Foundation of China (No. LY14E030013, LY14E030015), and the Young Researchers Foundation of Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University (No. 2012QN02). 8382



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Anchored Iron Ligands as an Efficient Fenton-Like Catalyst for

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