Preparation of Ultra-small Goethite Nanorods and Their Application as

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Preparation of Ultra-small Goethite Nanorods and Their Application as Heterogeneous Fenton Reaction Catalysts in Degradating Azo Dyes Zhengxin Liu, Lujie Zhang, Feihong Dong, Jie Dang, Kaile Wang, Dong Wu, Jue Zhang, and Jing Fang ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Applied Nano Materials

Preparation of Ultra-small Goethite Nanorods and Their Application as Heterogeneous Fenton Reaction Catalysts in Degradating Azo Dyes

Zhengxin Liu†, Lujie Zhang†, Feihong Dong†, Jie Dang†, Kaile Wang†, Dong Wu†, Jue Zhang†¶* and Jing Fang†¶

† Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China

¶

*

College of Engineering, Peking University, Beijing 100871, People’s Republic of China

Corresponding authors: Jue Zhang, Email: [email protected]

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The Discharge of azo dyes causes great harm to humans and environment. Azo dyes can be degraded by Heterogeneous Fenton reaction catalyzed by α-FeOOH nanorods. However, due to the large size of conventional goethite nanorods, the reaction rate of heterogeneous Fenton reaction is limited. In this study, we proposed a template method strategy for synthesizing ultra-small α-FeOOH nanorods (SFNs) and investigate their use for catalyzing the degradation of an azo dye, methyl orange (MO). TEM results suggested that the size of SFNs are much smaller than that of the conventional nanorods (LFNs), and the catalytic efficiency of SFNs is 7.4 times higher than that of LFNs. This can be explained by the higher specific surface area and more catalytic sites of the catalyst. The degradation rate of MO increases with the increase concentration of H2O2 and SFNs. Moreover, MO degradation rate can reach 98% under the appropriate parameters after 60 min reaction. In addition, reuse experiments results suggest that SFNs manufactured using our proposed approach achieved satisfied reusability, indicating that SFNs has potential application prospects in the degradation of azo dyes by catalyzing heterogeneous Fenton reaction.

Keywords: Goethite nanorods; Azo dyes; Heterogeneous Fenton reaction; Degradation; Template method


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Introduction Every year textile, leather, plastics and printing industries generates a large number of dyeing wastewater1. An azo dye refers to a colored organic compound containing one or more azo bonds (-N=N-). It is estimated that azo dyes account for 80% of the total organic dyes2. Azo dyes have been identified as potential genotoxic and carcinogenic agents3. Azo bond is the most unstable part of the azo dye molecules and it is the cause of its toxicity and carcinogenicity4. Carcinogenic aromatic amines can be also produced by its degradation5. In addition, the emission of azo dye wastewater is a significant environmental problem. This may lead to serious problems for example damage to aquatic ecosystems6-8. To solve this problem, researchers have proposed a variety of solutions: physical adsorption, condensation, biodegradation, oxidative degradation, and so on. Some chemical and physical methods such as adsorption and coagulation only transfer pollutants and require more processing9. However, many researches indicate that biological treatment is not sufficient to detoxify and degrade azo dyes and in most cases only partial mineralization is obtained10-11. Therefore, it is more appropriate to utilize the advanced oxidation processes (AOP). AOPs are characterized by the production of hydroxyl radicals (HO) with highly reactive. And the HO are able to unselectively degrade organic matters to CO2 and H2O12 ultimately. In particular, the classical Fenton reaction (ferrous iron catalyzing the decomposition of hydrogen peroxide to produce OH) has been shown to be useful for the degradation of organic contaminants13. Although classical Fenton reaction treatment can effectively degrade



organic contaminants, it has limitations because of its homogeneous system: a large number of dissolved iron must be removed to comply with the statutory emission thresholds specified in European legislation14; the difficulty of catalyst recycling. Besides, we need to keep the solution lower pH value to prevent the generation of iron sludge13. The heterogeneous Fenton reaction can overcome the limitation of the homogeneous Fenton reaction, and has been widely used in recent years15. The key to heterogeneous Fenton reaction is the design of the catalyst. Magnetite16-17, goethite18 and pyrite19 can all be used as catalysts for heterogeneous Fenton reactions. The crystallization, morphology and specific surface area of the catalyst are the main factor determining the Heterogeneous Fenton reaction rate20. How to increase the specific surface area of the catalyst21 and increase the catalytic reaction site is the key to improve the catalytic degradation rate. An effective solution is using nano-catalysts22. Goethite (α-FeOOH), one of the most famous heterogeneous Fenton catalysts, has attracted much attention in recent years due to its high stability, low cost, abundance, nontoxicity, and peroxidase-like activity23-25. Several methods used for the synthesis of α-FeOOH, such as directly room temperature hydrolysis method and reflux method26. The α-FeOOH nanorods produced by this methods are large in size (0.5-1 µm in length and 50-200 nm in diameter)27 and have a small specific surface area which cause a slow catalytic reaction rate. Previous studies have shown that surfactants, as a soft template, can help form one-dimensional nanostructures28. Besides, surfactants can reduce the surface energy of


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system29 and promote the formation of nano grains30. In this study, we try to solve the problem that the lower efficiency of heterogeneous Fenton reaction catalyzed by larger size α-FeOOH nanorods. Therefore, surfactants were used to prepare smaller sized α-FeOOH nanorods using a simple method. And the nanorods were applied as catalysts for heterogeneous Fenton reaction. The characterization of α-FeOOH nanorods was performed by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Then, the efficiency of the catalyst for degrading Methyl Orange (MO) was evaluated by a UV-Vis spectrophotometer.



Experimental Reagents Ferric chloride (FeCl36H2O, AR), sodium hydroxide (NaOH, AR), methyl orange (MO), sodium dodecyl sulfate (SDS, EP) were all bought from Sinopharm Chemical Reagent Ltd. (Shanghai). Ferrous chloride (FeCl24H2O, AR) and hydrogen peroxide (H2O2 ,30%, GR) were purchased from Aladdin Ltd. (Shanghai) and Beijing Chemical Works (Beijing), respectively.

Preparation of α-FeOOH Nanorods 0.1 mol FeCl36H2O, 0.1 mol FeCl24H2O and 0.6mol SDS was dissolved in 20 ml distilled water and then, 1 mol NaOH solution was added drop wisely (to adjust the pH to 7) under stirring for 5 min. The precipitation was ripening for 24h at room temperature (25 ℃). And then the precipitation was washed with DI water three times. Proposed Synthetic Routes for the Synthesis of goethite nanorods is shown in Scheme 1. α-FeOOH precipitation were kept in water at room temperature.


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Scheme 1 Proposed Synthetic Routes for the Synthesis of goethite nanorods

Characterization of α-FeOOH Nanorods The FeOOH nanorods was characterized by X-ray diffractometer (XRD, PERSEE XD-3, Beijing Purkinje General Instrument Co., Ltd., China) using a Cu Kα source at λ = 1.5406 Å. Transmission electron microscope (TEM, Tecnai G2 T20, FEI, USA) were used to identify the morphology and dimensions of nanorods. FTIR spectra were recorded by Fourier transform infrared spectroscopy (FTIR, FTIR-650, Tianjin Gangdong Sci. & Tech. Development Co., Ltd., China) with the KBr pellet. X-ray photoelectron spectroscopy analysis (XPS, Axis Ultra, Kratos Analytical Ltd. UK) with monochromatic Al Kα X-ray radiation at 1486.7 eV was used to



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identify metal oxidation states of the nanorods.

Heterogeneous Fenton Process 10 mL of the MO solution with 80 mg/L was reacted in a 25 mL beaker in each run. And different doses of the catalyst and H2O2 were added into reaction system. The pH of solution was adjusted to 5 by adding HCl (0.1 M) and NaOH (0.1 M) determined by a pH meter (Orion Star A221，Thermo Scientific, USA). The reaction temperature was controlled at 60 ℃ by a water bath. During the reaction, a sample (500 µL) was withdrawn at 5,10,20,40,60 min after the start of reaction, following with the filtration through 0.22 µm membranes (Micro PES, Jinteng Membrane Co. China). The absorbance of MO solution whose maximum absorbance wavelength

(λmax =

464 nm)

was

measured

by

an

UV–Vis

spectrophotometer

(Spectrostar Omega, BMG Labtech, Germany) to find the degradation efficiency (DE%). The DE% of MO was then calculated by the following equation: DE%=(C0-Ct)/C0×100 where C0 is the concentrations of MO initially and Ct is the concentrations of MO at time t. Each experiment was run in triplicate, and average values and standard deviations are presented.


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Results and Discussion Characterization of the FeOOH Nanorods The morphology and size of α-FeOOH nanorods which fabricated by previous method31 (large α-FeOOH nanorods, LFNs, synthesizing information of LFNs is described in Supporting Information) and our method (small α-FeOOH nanorods, SFNs) are shown in Fig. 1. Fig. 1a, 1b and Fig. 1c, 1d show the TEM images of SFNs and LFNs sample, respectively. Obviously, the size of SFNs is significantly smaller than that of LFNs. Fig. S1a and S1b show histograms of length and diameter distribution, respectively, based on the method of previous study32. The calculated mean length and diameter of SFNs are 37.9±10.3 nm and 6.8±1.9nm, respectively. In addition, the calculated mean length and diameter of LFNs are 478.3± 146.7 nm and 72.8±32.0 nm, respectively.



Fig. 1. TEM micrographs of SFNs (a, b) and LFNs (c, d).

From the morphological point of view, the size of SFNs is smaller than that of LFNs. As previous study33, the catalytic efficiency is related to size and surface area of catalyst. Catalyst with smaller particle size and higher surface area are more effective than that with larger particle size and lower surface area. Then, why SDS can promote the formation of one-dimensional nanorods? SDS above the critical micelle concentration (CMC) can exist in the form of micelles in the solvent, and micelles can act as a template for the growth of FeOOH nanorods. This is similar to the previous study that α-Fe2O3 nanorods were prepared by L113B as a template28. However, α-FeOOH nanorods structure cannot be generated by the help of the non-ionic surfactant Polyvinylpyrrolidone (PVP) and cationic surfactant cetyltrimethylammonium bromide (CTAB) (See Fig. S2). The possible reason is that addition of alkali to the iron ion solution forms ferric hydroxide colloids. And the ferric hydroxide colloid is positively charged, whereas SDS (anionic surfactant) forms a negatively charged micelle in water. Consequently, the ferric hydroxide will interact with SDS micelles and slowly crystallize to form α-FeOOH nanorods along the surface of the micelles as the ripening time increases. However, the surface of micelles formed by cationic surfactants and nonionic surfactants in water is positively or uncharged, therefore cannot interact with positively charged ferric hydroxide. So that α-FeOOH cannot crystallize along the micellar orientation and only form non-oriented nanoparticles. As a result, cationic surfactant and non-ionic surfactant cannot help to form a one-dimensional rod-like structure.


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The XRD patterns of SFNs and LFNs are shown in Fig. 2a and Fig S3, respectively. The diffraction peaks of SFNs sample match well with the diffraction peaks of indexed goethite (α-FeOOH) structure and consistent with previous study34. In order to prove the crystal structure of SFNs, we analyzed the XRD patterns in detail. The average lattice parameter of SFNs is a=4.6852Å，b=9.6629Å，c=3.0536Å，α=90°，β=90°，γ=90°. The average lattice parameter of α-FeOOH retrieved by standard PDF card (PDF#29-0713) are a=4.608Å， b=9.956Å ， c=3.0215Å ， α=90° ， β=90° ， γ=90°. These two are very close. The diffraction peak of crystal face (110) is strong, indicating the occurrence of preferential orientation, which is consistent with the one-dimensional nanorod structure. Besides, the peak width of SFNs is much wider than that of LFNs, indicating the very small size of the SFNs. Besides, the average grain size of SFNs is 5.5 nm.

Fig. 2. XRD pattern (a), FTIR spectra (b), XPS spectra (c) and XPS Fe 2p spectra (d) of



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SFNs.

The FTIR spectra of the SFNs (Fig. 2b) and LFNs (Fig. S4) showed features typical of α-FeOOH35. On account of the existence of the OH stretching vibration in α-FeOOH, there is a very strong IR band at 3168 cm-1. Moreover, Owing to the envelope of OH groups on the surface of hydrogen bonds or to stretching vibration of surface H2O molecules, there is a IR band at 3388 cm-1 36. The bands at 1630, 1385 cm-1 can be attributed to the Fe–O vibrational mode in α-FeOOH37. And three bands at 1063, 885, 795 cm-1 which are the typical bands of goethite can be due to Fe-O-H bending vibrations in α-FeOOH26, 38. The band at 1230 cm-1 can be due to the stretching vibration of C-O. The bands at 2854 and 2924 cm-1 are related to the symmetric and asymmetric C-H bonds, respectively. In addition, the bands at 2360, 2341 cm-1 may be owing to the stretching vibration of O = S = O. These may be due to the remaining SDS that is not completely washed off.

To further characterize the SFNs, Fig. 2c and Fig. 2d shows the XPS spectra of SFNs sample. The peaks of O and Fe can be clearly distinguished in the figure. The binding energies are consistent with the results of previous study39. Besides, satellite peaks are induced by charge transfer screening, which can be entirely due to the existence of Fe (III) ions40.

Comparison

and

Kinetic

Study

of

two

Catalysts

in

Heterogeneous Fenton Process SFNs and LFNs are both the catalysts for heterogeneous Fenton reactions. To compare


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the differences in the ability of SFNs and LFNs to catalyze heterogeneous Fenton reactions, the Fenton reactions is applied for the degradation of MO in aqueous solution using SFNs or LFNs. As shown in Fig. 3, the DE% of MO is 98% after 60 min in SFNs group. In the LFNs group, the DE% is 38% after 60 min. In order to observe the discoloration process of MO, pseudo-first-order kinetics model was proposed. ln(Ct/C0)=-k1×t where k1 is the pseudo-first-order rate constant. The kinetic constant k1 for the SFNs and LFNs were tabulated in Table S1. The k1 of the SFNs group is approximately 7.4 times that of LFNs group (i.e., the catalytic activity of SFNs is much higher than that of LFNs). This may be due to the fact that the SFNs with larger specific surface area can provide more catalytic sites33.

Fig. 3. Comparison of LFNs and SFNs in heterogeneous Fenton process. (a) shows the change of DE% over time, (b) was the variation of ln (C0/Ct) vs. time. Experimental parameters: [MO]0 = 80 mg/L, [H2O2]0 = 3 g/L, [LFNs or SFNs] = 100 mg/L, and pH = 5.

Fig. 4a shows the change of UV-Vis spectra of MO over time. The results show that the



characteristic peak of MO at 464 nm disappeared gradually, indicating the degradation of MO. The functional groups of SFNs after use were identified through FTIR (Fig. 4b). Compared with the FTIR of SFNs (Fig. 2b), the bands of SFNs after use at 2360 and 2341 cm-1 disappear. It indicates that the O=S=O bonds disappears. This may be due to that the unwashed SDS were degraded by heterogeneous Fenton reaction. Besides, we realized residual SDS in SFNs might affect the heterogeneous Fenton reaction catalyzed by SFNs. We compared the difference between the unwashed SFNs and washed SFNs for catalyzing the heterogeneous Fenton reaction. The results show that MO only decolorized 31.2% within 60 min catalyzed by unwashed SFNs (Fig. S5). Under the same conditions ([MO]0 = 80 mg/L, [SFNs]0 = 100 mg/L, [H2O2]0 = 3 g/L, and pH = 5), MO could decolorize 96.1% within 60 min catalyzed by washed SFNs. As a result, the washing step of SFNs are indispensable. More SDS remaining in the unwashed SFNs may attach to the surface of the SFNs. These SDS can block the reaction of SFNs with MO and H2O2, thereby hindering the heterogeneous Fenton reactions. In addition, a new band at 1129 cm-1 can be owing to stretching vibration of C-N, which may be due to partial MO or degradation intermediates adsorbed on SFNs.


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Fig. 4. (a) UV–vis spectra of MO degradation by heterogeneous Fenton process with SFNs within different time intervals. (b) FTIR spectra of SFNs after use. Experimental parameters: [MO]0 = 80 mg/L, [H2O2]0 = 3 g/L, [SFNs]0 = 100 mg/L, and pH = 5.

Effect of the Experimental Parameters on the Heterogeneous Fenton Process



The nanorods we prepared have very high catalytic activity. Fig. 5a shows the degradation rate of MO at various temperatures. The activation energy of this process can be obtained according to the Arrhenius equation41: In k = In A - E a /RT , where k is the reaction rate constant, A is a constant, Ea is the activation energy, R is the gas constant, and T is the reaction temperature. Fig. 5c shows the Arrhenius plot by plotting In k against 1/T. The Ea for the MO degradation reaction is 41.2 kJ/mol.

Fig. 5. The effect of reaction temperature on DE% of MO in heterogeneous Fenton process. (a) shows the change of DE% at different temperature over time, (b) was the variation of ln (C0/Ct) vs. time at different temperature, (c) was the Arrhenius plot. Experimental parameters: [MO]0 = 80 mg/L, [H2O2]0 = 3 g/L, [SFNs] = 100 mg/L, and pH = 5.


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SFNs is catalyst of heterogeneous Fenton reaction for the degradation of MO in this work. The dosage of SFNs has a significant effect on the rate of the degradation of MO. Therefore, the effect of the SFNs dosage on the degradation of MO solution was investigated, and the results are demonstrated in Fig. 6. As shown in Fig. 6a, when the dosage of the SFNs is 0 mg/L (i.e., without the addition of SFNs), the removal of MO is -6.5% after 60 min reaction. It may be due to the increase of MO concentration caused by the evaporation of solvent water. More importantly, this also shows that SFNs is indispensable as catalyst in this heterogeneous Fenton reaction. As shown in Fig. 6c, with the increase of SFNs dosage, the DE% of MO increased but the slope decreased, which is similar to previous results9. It is well known that the catalytic rate of nano-catalysts is positively related to the surface area of catalyst. Nanostructured materials tend to agglomeration, and the conformation and fraction of the agglomerates formed depend on particle concentration42. The higher the concentration of nanoparticles, the more severe the agglomeration. Therefore, as the concentration of nanoparticles increases, the amount of increase in specific surface area decreases. And accordingly, the increase in DE% is less. Therefore, when the concentration reaches 200 mg/L, more particles agglomerate, and the specific surface area is only a little larger than that of 100 mg/L. As a result, the DE% of 200 mg/L is only a little larger than that of 100 mg/L. Therefore, a suitable SFNs dosage is 100 mg / L.



Fig. 6. The effect of SFNs dosage on DE% of MO in heterogeneous Fenton process. (a) shows the change of DE% with different SFNs dosage over time, (b) was the variation of ln (C0/Ct) vs. time with different SFNs dosage, (c) shows the DE% of MO with different SFNs dosage after 20 min reaction. Experimental parameters: [MO]0 = 80 mg/L, [H2O2]0 = 3 g/L, and pH = 5. Fig. 7 demonstrates the effect of initial H2O2 concentration on the DE%. As shown in Fig. 7a, when the concentration of the H2O2 is 0 g/L (i.e., without the addition of H2O2), the removal of MO is 7.6% after 60 min reaction. And with the extension of time, its removal efficiency reduces slowly. This is probably due to the desorption of MO adsorbed on SFNs. SFNs would agglomerate due to their small size after a few minutes. And this could result in surface area getting smaller. Some of the MO adsorbed on SFNs will desorb. More importantly, this also


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shows that the degradation of MO is certainly because of heterogeneous Fenton process, rather than the physical adsorption of SFNs. In order to study whether MO can be adsorbed by SFNs, the FTIR spectra of SFNs soaked in the MO for 1 h is shown in Fig. S6. By comparing the FTIR of SFNs (Fig. 2b), the bands intensities at 2854 and 2924 cm-1 become stronger after the SFNs were soaked in the MO, which are owing to the asymmetric and symmetric C-H bonds, respectively. It may be due to the MO adsorbed on SFNs. Besides, the band at 1129 cm-1 can be related to stretching vibration of C-N, which may be due to partial MO adsorbed on SFNs. The results of fitting the relationship between reaction rate constant k and H2O2 concentration (as shown in Fig. 7c) are consistent with previous results19. This may be owing to the fact that the production and consumption of OH is not the major factor in the degradation of MO. As discussed later, the oxide species that cause the decolorization of MO may be Fe(IV) instead of OH.43



Fig. 7. The effect of H2O2 concentration on DE% of MO in heterogeneous Fenton process. (a) shows the change of DE% with different H2O2 concentration over time, (b) was the variation of ln (C0/Ct) vs. time with different H2O2 concentration, (c) shows the change of kinetic rate constants for the degradation of MO with different H2O2 concentration. Experimental parameters: [MO]0 = 80 mg/L, [SFNs]0 = 100 mg/L, and pH = 5. One of the most significant parameters in the heterogeneous Fenton process is initial pH44-45. Therefore, the reactions that occur at different initial pH were investigated (Fig. 8). The results are very different with previous study45. Interestingly, the catalytic efficiency of SFNs is not sensitive to initial pH. The zeta potential of SFNs at different pH values were measured (Fig. 8c). The result shows that the zeta potential of SFNs was between -30 and +30 mV when the pH value was between 2.3 and 11. This indicates that the surface charge of


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SFNs is approximately zero under pH range from 2.3-1146. Therefore, the MO adsorption of SFNs is affected little by pH under pH range from 2.3-11. In addition, the heterogeneous Fenton reaction mainly occurs on the surface of the catalyst47, so only the MO adsorbed on the surface of the SFNs can be degraded. This may be the reason why the decolorization rate of MO is not sensitive to initial pH. The catalytic efficiency of SFNs is consistent and high over a wide range of initial pH. This may indicate that the catalytic mechanism of SFNs is different from traditional heterogenous Fenton reaction and needs to be further explored in the future.



Fig. 8. The effect of initial pH on DE% of MO in heterogeneous Fenton process. (a) shows the change of DE% with different initial pH over time, (b) was the variation of ln (C0/Ct) vs. time with different initial pH, (c) shows the Zeta potential of SFNs at different pH values. Experimental parameters: [MO]0 = 80 mg/L, [SFNs]0 = 100 mg/L, and [H2O2]0 = 3 g/L.


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In all experiments, the degradation rate are also consistent with the pseudo-first order kinetics. The kinetic constant k1 were tabulated in Table S2.

Degradation Mechanism The influence of scavengers on the degradation of MO was determined to investigate the oxide species mediated in the process. n-butanol can remove all OH in the system, including OH in solution and OH on the surface of nanoparticles48-49. We added excess OH radical scavenger n-butanol to the reaction system and the decolorization of MO was not greatly affected (as shown in Fig. 9), indicating that the OH is not the oxide species of degrading MO. Previous results50 also showed that the addition of OH radical scavenger had little effect on the heterogeneous Fenton reaction when pH>5. As we all know, the heterogeneous Fenton reactive species recognized by the academic community currently have two kinds51-53, one is OH and the other is Fe(IV). We realized that the main oxide species in our reaction was not the OH but Fe(IV)43. Obviously, the generation of Fe(IV) occurs on the surface of the catalyst. Although the mechanism of surface Fe(IV) production in heterogeneous Fenton systems is still unknown, Eq. (1)

47, 54

is sometimes used to illustrate

its mechanism. ≡FeⅡ + H2O2 → ≡FeⅣ=O + H2O

(1)

Therefore, the larger the surface area of the catalyst, the more Fe(IV) is produced on the surface and the higher the degradation rate. Thus, smaller goethite (SFNs) has a larger specific surface area and a higher catalytic activity than larger goethite (LFNs).



100 80

DE%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40

MO + 350 mM n-Butanol MO

20 0 0

10

20

30

40

50

60

Time (min) Fig. 9. The effect of addition n-Butanol on the degradation efficiency of MO in heterogeneous Fenton process.

To verify the mineralization and degradation of MO, the FTIR test was applied55. The FTIR spectra of SFNs soaked in the MO for 1 h and FTIR spectra of SFNs after use for 1 h (catalyzing the degradation of MO for 1h) was compared. As shown in Fig. S6 and Fig. 4b, after the heterogeneous Fenton reaction, the bands at 2926 and 2851 cm-1 decrease, indicating that the C-H bond has been greatly destroyed. Besides, the deceasing of the band at 1230 cm-1 may be due to the destroying of C-N bond. As a result, MO did mineralize and degrade after heterogeneous Fenton reaction catalyzing by SFNs55-56. In order to better verify the degradation of the benzene ring in MO, the change of UV-Vis spectra of MO over time


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was investigated. As shown in Fig.S7(Supporting Information), at 0 min, the absorbance peaks from 230-270 nm attributed to benzene ring structure. As the reaction proceeded, a strong absorbance peak appeared at 220-250 nm. This illustrates that the benzene ring is opened and produces a large number of small molecule degradation products such as conjugated dienes. When the reaction reached 60 min, only a weak absorbance peak at 225 nm indicated that only a small amount of small molecule degradation products such as conjugated diene remained.

SFNs Reusability One of the advantages of heterogeneous Fenton reactions over homogeneous Fenton reactions is their ability to be recycled for multiple uses. To investigate the multiple use of SFNs, the catalyst was recovered and reused 5 times. After each run, the catalyst was centrifuged from the solution, washed and reused in the next experiment. The DE% for these five runs is shown in Fig. S8. The attenuation of the active catalytic sites caused by a small amount of iron leaching from the catalyst, the reduction of catalyst specific surface area and the poisoning of the adsorbed organic substances on the active catalytic sites may result in the loss of catalyst activity. To analyze the leaching of Fe, we used Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES, Prodigy 7, Leeman Labs, USA) to measure the concentration of Fe in the solution before and after the reaction. The Fe content in the solution before the reaction was 0.062 μg/mL, and the Fe content in the solution after the reaction was 1.311 μg/mL, indicating that part of Fe was leached from the catalyst into the solution. Consistent results have also been observed by previous studies57-59. The results



reveal that after many uses, DE% did not drop significantly. This indicates that the catalyst can be reused, which have great significance in reducing the cost and pollution caused by the emission of catalyst.

In summary, the α-FeOOH nanorods with 37.9 ± 10.3 nm length and 6.8 ± 1.9 nm diameter were synthesized by adding SDS, which confirmed by TEM, XRD, XPS and FTIR analyses. By comparing the catalytic efficiency of heterogeneous Fenton reaction catalyzed by SFNs and LFNs, the catalytic efficiency of SFNs is 7.4 times higher than that of LFNs. The enhancement of catalytic efficiency can be explained by the increase of specific surface area and catalytic sites. It is found that degradation efficiency is positively correlated with H2O2 concentration and SFNs concentration. Under the conditions of pH = 5, H2O2 concentration of 3 g / L, SFNs concentration of 100 mg / L, initial dye concentration of 80 mg / mL after 60 min of process, the degradation rate of MO could reach 98%. Moreover, SFNs has excellent reusability. In summary, SFNs has the advantages of high catalytic efficiency, insensitivity to initial pH, reusability and so on. It has potential application prospects in the degradation of azo dyes by catalyzing heterogeneous Fenton reaction.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Preparation method of LFNs, Table S1, S2 and Fig. S1-S8.

Acknowledgement This research is sponsored by the Peking University Biomed-X Foundation.



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Preparation of Ultra-small Goethite Nanorods and Their Application as

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