Chemosphere 117 (2014) 638–643
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Efficient degradation of rhodamine B using Fe-based metallic glass catalyst by Fenton-like process Xianfei Wang, Ye Pan ⇑, Zirun Zhu, Jili Wu School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
h i g h l i g h t s Fe-based metallic glass was firstly developed to be a Fention-like catalyst. Rhodamine B could be quickly degraded by the catalyst with low dosage of H2O2. Metastable nature of metallic glass accounted for its good degradation ability. The catalyst exhibited considerably good stability and reusability.
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Article history: Received 7 July 2014 Received in revised form 10 September 2014 Accepted 17 September 2014 Available online 25 October 2014 Handling Editor: Y. Liu Keywords: Fenton-like Fe-based metallic glass Rhodamine B Degradation
a b s t r a c t An efficient heterogeneous catalyst, Fe-based metallic glass (Fe–Si–B amorphous ribbon), was successfully prepared for Fenton-like degradation of rhodamine B (RhB) by a melt-spinning method. The catalyst was characterized using XRD and SEM. The effects of various reaction parameters such as H2O2 dosage, temperature, initial pH value, Fe–Si–B dosage and initial RhB concentration on the degradation of RhB were studied. Almost complete degradation of RhB (20 mg L1) was achieved within only 10 min by 0.5 g L1 Fe–Si–B catalyst and 1.6 mM H2O2 at pH 3.0 at 295 K. Kinetic analyses showed that the degradation process could be described by a pseudo-first-order kinetic model. The catalytic stability was also investigated and it was found that the Fe–Si–B catalyst exhibited good structural stability and no loss of performance even after three cycles. It was concluded that the Fe–Si–B amorphous ribbon was a potential heterogeneous Fenton-like catalyst for industrial wastewater treatment. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Dyes have been known to be serious organic pollutants in wastewaters for several years (Pignatello et al., 2006). The treatment of dye waste effluents is, therefore, important so as to minimize their effect on the environment. Conventional removal methods of dyes include flocculation, physical adsorption and biological degradation (Dos Santos et al., 2007; Monteagudo et al., 2008; Eftekhari et al., 2010). These methods usually do not work efficiently as they are non-destructive and merely involve the transfer of pollutants from water to sludge which resulted in the production of secondary waste (Gan and Uau Li, 2013). Thus, there is a need for developing more effective treatment technologies to eliminate dyes from wastewaters. In recent years, advanced oxidation processes (AOPs) which are based on the generation of highly reactive transitory species (i.e., ⇑ Corresponding author. Tel.: +86 25 5209 0681. E-mail address: [email protected] (Y. Pan). http://dx.doi.org/10.1016/j.chemosphere.2014.09.055 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
OH, O2) to initiate non-selective oxidation destruction of organic pollutants have emerged as promising alternatives (Chen et al., 2009; Herney-Ramirez et al., 2010; Hou et al., 2011). Among AOPs, Fenton reaction (H2O2 + Fe2+/Fe3+) has been widely studied. The hydroxyl radical (OH, Eh = 2.80 V) generated in Fenton reaction have powerful oxidizing ability and can cause the degradation and mineralization of many recalcitrant organic contaminants (Herney-Ramirez et al., 2010). However, the application of traditional Fenton reaction is limited by high cost of H2O2, narrow working pH range (less than 3) and difficulty of separation and recovery of the iron species (Ai et al., 2007; Poerschmann et al., 2009). Thus, heterogeneous Fenton systems have received considerable interest and many heterogeneous Fenton-like catalysts have been reported, such as iron cluster (Keenan and Sedlak, 2008; Mylon et al., 2010), iron oxides (Hanna et al., 2008), iron-immobilized zeolites (Navalon et al., 2010) or carbon materials (Wang et al., 2012a). Reactions between these iron-bearing catalysts and H2O2 can effectively oxidize the organic molecules at circumneutral pH and they can easily be separated from the treated wastewater to avoid the secondary
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metal ion pollution. However, the better catalytic performance of these heterogeneous systems is dependent on the presence of ultrasonic and/or UV light irradiation (Chen et al., 2009; Wang et al., 2010). Thus, better heterogeneous Fenton-like catalysts need to be developed. The metallic glasses, unlike the crystalline metals, are formed by far-from-equilibrium process and the constituent atoms do not reside at the thermodynamic equilibrium positions (Greer, 1995; Inoue, 2000). The metastable nature imparts many excellent properties to amorphous alloys that are unachievable for crystalline alloys, such as the good catalytic and chemical properties (Baiker, 1989; Katona and Molnar, 1995; Zhang et al., 2010; Lin et al., 2012; Wang et al., 2012b). Meanwhile, Fe-based metallic glasses have been widely studied due to their low-cost, good corrosion resistance, and excellent soft magnetic properties (Lin et al., 2012). Therefore, the use of Fe-based metallic glasses as a heterogeneous Fenton-like catalyst for dye degradation shows a great potential. In this work, Fe–Si–B amorphous ribbons were prepared by a melt-spinning technique and were used as heterogeneous catalysts. The catalytic performance of the Fe–Si–B amorphous ribbons was investigated for the degradation of rhodamine B used as a model pollutant. Rhodamine B (RhB), one of the important xanthene dye, has become a common organic pollutant and exhibits considerably high resistance to photo and oxidative degradation (Yan et al., 2012). The major objectives of this work are (i) to evaluate the degradation performance of Fe–Si–B amorphous ribbons for RhB; (ii) to investigate the influences of various reaction parameters on the rate of degradation; (iii) to discuss the degradation kinetics and mechanism of RhB and the stability of Fe–Si–B amorphous ribbons.
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100 mg Fe–Si–B amorphous ribbons (0.5 g L1) in 200 ml solution of 20 mg L1 RhB. The initial pH of the solution of RhB was adjusted by 1.0 M H2SO4 and 1.0 M NaOH. Typically, the solution of RhB with the desired initial pH value was added with the Fe–Si–B amorphous ribbons and H2O2 under vigorous stirring. For comparison, the degradation of RhB by Fe–Si–B amorphous ribbons in the absence of H2O2 was also investigated. At given time intervals, 2 ml aliquots were collected and then centrifuged to get the supernatant for UV–vis analysis. The UV–vis absorption spectra of the aqueous solutions were recorded from 200 to 800 nm using a UV–vis spectrophotometer (TU1800-PC, Beijing). The RhB concentration was determined based on the constructed calibration curves at maximum absorption wavelength (kmax) of 554 nm. The pH value was detected by a pH meter (pHS-3, Shanghai). When considering the effect of temperature on the degradation rate of RhB, the solution temperature was controlled by a thermostatic water bath (DKS24, Shanghai). Moreover, (Fe2+, Fe3+) concentrations in the solution after reaction were determined by inductively coupled plasma emission spectrometer (ICP) on an Optima 2000 (PerkinElmer, Inc.) instrument. 3. Results and discussion 3.1. Characterization of the Fe–Si–B catalyst The XRD pattern and SEM images of the Fe78Si9B13 ribbons obtained by the melt-spinning technique are presented in Fig. 1. The XRD pattern of the reused Fe78Si9B13 ribbon after 4 cycles in
2. Materials and methods 2.1. Materials RhB (>99%) was procured from Tianjin Chemical Reagent Research Institute, China. H2O2 (30%, w/w), H2SO4 and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All the chemicals were in analytical grade and were used as received without further purification. Deionized water was used for preparing the RhB solutions in the experiments. 2.2. Preparation and characterization of Fe–Si–B amorphous ribbons The master ingot of Fe78Si9B13 was produced by arc melting a mixture of Fe, Si and Fe–B alloy under a Ti-gettered Argon atmosphere. The purity of Fe, Si and Fe–B alloy was higher than 99.9%. The ingot was then remelted in a quartz tube by induction melting, followed by a single roller spinning to obtain amorphous ribbons. The ribbons were 2 mm in width and 30–40 lm in thickness, and they were cut into 10 mm in length to be used in the experiments. The crystal structure of the catalysts was analyzed by X-ray diffraction (XRD). The XRD data were collected on a D8 Bruker diffractometer with Cu Ka radiation (k = 0.1541 nm) under operation conditions of 40 kV and 40 mA. The diffractogram was recorded in the 10–80° 2h range, with a 0.04° step size and a collecting of 0.3 s per point. The surface morphology of the catalysts was observed with a FEI Sirion 200 scanning electron microscope (SEM) at an accelerating voltage of 20 kV. 2.3. Procedure and approach Unless indicated otherwise, batch experiments were carried out in a 250 mL flask at 295 K in dark and reaction mixture contained
Fig. 1. XRD patterns (a) of as-received and reused Fe–Si–B amorphous ribbons, and SEM images (b, c) of as-received Fe–Si–B amorphous ribbon.
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the presence of 1.6 mM H2O2 at pH 3 is also shown in Fig. 1a. The amorphous nature of the as-received ribbon can be verified by the broad peak in the XRD pattern. The result demonstrates that the structure of the ribbon was a short-range ordered and long-range disordered structure, and the zero-valent iron was successfully frozen into amorphous state. Meanwhile, the amorphous nature of Fe–Si–B ribbon was maintained after the use for the degradation of RhB. From Fig. 1b and c, it can be seen that the free surface was nearly perfectly smooth, and the surface in contact with the roller during fabrication of the ribbon was relatively rough.
0.5 M H2O2. However, when H2O2 dosage was further increased to 0.7 and 0.85 M, the degradation rate of RhB decreased and only 40% RhB was degraded within 60 min. H2O2 is an ordinary oxidant and it cannot effectively degrade RhB (Matilainen and Sillanpää, 2010). Being similar to general Fenton-like degradation, the degradation of RhB by Fe–Si–B catalyst and H2O2 is more likely a surface-catalyzed reaction and RhB might be oxidized by near-surface OH (Lin and Gurol, 1998). First of all, RhB was absorbed on the surface of the Fe–Si–B catalyst. Then H2O2 was decomposed at the surface of the Fe–Si–B catalyst to generate OH (Lin and Gurol, 1998; Chen et al., 2009):
3.2. Degradation of RhB by Fe–Si–B catalyst and H2O2 at neutral pH
Fe0 þ H2 O2 ! Fe2þ þ 2OH
ð1Þ
3.2.1. Effect of H2O2 dosage The initial concentration of H2O2 is a key parameter in the Fenton or Fenton-like reactions. Therefore, the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2 was first conducted at neutral pH to determine the effect of H2O2 dosage, and the results are shown in Fig. 2. The absorption intensity of RhB at 554 nm gradually decreased with the increase of reaction time (Fig. 2a), which indicates that the structure of RhB molecules was destroyed during the reaction. From Fig. 2b, it can be seen that the degradation of RhB was significantly affected by the dosage of H2O2. The degradation rate of RhB increased with the increase of H2O2 dosage from 0.05 to 0.5 M and 70% RhB was degraded within 60 min with
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð2Þ
OH is quite reactive and could most probably react with the sorbed species before being able to diffuse to the solution. Finally, adsorbed RhB was degraded by OH and the degradation rate was directly related to the concentration of OH. At low H2O2 concentration, increasing H2O2 dosage would lead to more OH produced and the OH preferentially attacked the RhB molecules, inducing a higher degradation rate. However, further increase in H2O2 dosage resulted in a decrease of degradation rate as the excess H2O2 could be a OH scavenger. The OH may react with H2O2 producing HO2 that is less active than OH (Salem et al., 2009):
OH þ H2 O2 ! HO2 þ H2 O
ð3Þ
As shown in Fig. 2b, the degradation kinetics of RhB could be described according to the pseudo-first-order equation as given below (Yan et al., 2012):
C t ¼ C 0 expðktÞ
ð4Þ
where k denotes the observed first-order reaction rate constant (min1), t is the reaction time (min), C0 is the initial concentration (mg L1) of RhB, and Ct is the concentration (mg L1) of RhB at time t. By nonlinear regression analysis, the maximum rate constant of 0.049 min1 was achieved at the H2O2 dosage of 0.5 M, with the relative coefficient R2 of 0.969. 3.2.2. Effect of temperature Temperature is also a critical parameter to the reaction rate. The effect of temperature on the degradation rate of RhB by Fe–Si–B amorphous ribbons and 0.5 M H2O2 at neutral pH is shown in Fig. 3a. The pseudo-first-order rate constants at different temperatures were also determined and are shown in the inset of Fig. 3a. It can be seen that temperature exerts a strong effect on the degradation rate of RhB. The degradation rate increased by increasing temperature with a rise in rate constant from 0.049 min1 at 295 K to 0.128 min1 at 323 K. Only 70% RhB was degraded within 60 min at 295 K, while nearly 88% RhB could be degraded within 30 min at 323 K. This is because higher temperature could increase the reaction rate between H2O2 and the catalyst, thus accelerating the generation of OH. Based on the kinetic rate constants at different temperatures (T), the activation energy of the degradation process of RhB can be obtained according to the Arrhenius equation (Zhang et al., 2010):
ln k ¼
Fig. 2. UV–vis spectra for RhB degraded with 0.5 M H2O2 (a) and the effect of initial H2O2 dosage on the degradation of RhB (b) by Fe–Si–B amorphous ribbons and H2O2 at neutral pH (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; temperature, 295 K).
Ea þ ln A RT
ð5Þ
where k is the kinetic rate constant, Ea is the activation energy, R is the gas constant, and A is the constant. The Arrhenius plot is presented in Fig. 3b by plotting ln k against 1/T and the activation energy was calculated to be 27.4 kJ mol1. This calculated value of activation energy is much lower than the values reported in
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Fig. 3. Effect of temperature on the degradation of RhB (a) and the corresponding Arrhenius plot of lnk versus 1/T (b) by Fe–Si–B amorphous ribbons and H2O2 at neutral pH, inset in Fig. 3a shows the kinetic rate constants k determined at different temperatures (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; H2O2 dosage, 0.5 M).
literature for the degradation of RhB as well as other organic dyes in the presence of heterogeneous catalytic systems (Chen and Zhu, 2007; Gan and Uau Li, 2013). For example, under similar experimental conditions, an activation energy of 82.5 kJ mol1 has been reported for the degradation of RhB over RHSi-Fe catalyst. Hence, it was deduced that the degradation of RhB in the presence of Fe– Si–B/H2O2 system required a relatively lower energy and this could be attributed to the metastable nature of the metallic glasses. The Fe–Si–B amorphous ribbon could be an excellent heterogeneous Fenton-like catalyst. On the other hand, according to Fig. 3a, the degradation rate of RhB by Fe–Si–B catalyst and H2O2 at neutral pH was a bit slow at room temperature (295 K). In order to increase the degradation rate of RhB using Fe–Si–B catalyst and H2O2 at room temperature and to reduce the dosage of H2O2, the degradation of RhB was then investigated at acidic pH. 3.3. Degradation of RhB by Fe–Si–B catalyst and H2O2 at acidic pH
3.3.1. Effect of initial pH value and H2O2 dosage Fig. 4 shows the effect of initial pH value (Fig. 4a) and H2O2 dosage (Fig. 4b) on the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2 at acidic pH. The experiments were carried out with 1.6 mM H2O2 at different initial pH or with different H2O2 dosage
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Fig. 4. Effect of initial pH value (a) and H2O2 dosage (b) on the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2, inset in Fig. 4b shows the kinetic rate constants k determined at different H2O2 dosage (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; temperature, 295 K; initial pH value of (b), 3).
at initial pH 3. As it is known to all, zero-valent iron is an effective reducing agent and has been proved to degrade many organic contaminants (Fan et al., 2009). For comparison, the degradation experiments of RhB by Fe–Si–B amorphous ribbons at different initial pH without addition of H2O2 were also conducted and are shown in Fig. 4a. In the absence of H2O2, there was only a little degradation of RhB at pH 4 and 90% RhB was degraded at pH 3 within 60 min. However, when a low dosage of 1.6 mM H2O2 was added, the degradation rate of RhB could be greatly enhanced. Within 10 min, almost complete degradation of RhB could be achieved at pH 3 and 90% RhB could be degraded at pH 3.5 and 4. The pseudo-first-order rate constants were also increased by one order of magnitude to 0.725 and 0.329 min1 for pH 3 and 4, respectively. Therefore, as discussed in Section 3.2, OH is also a significant key to degrade RhB in the presence of Fe–Si–B amorphous ribbons and H2O2 at acidic pH. From Fig. 4b, it can be found that a low dosage of H2O2 was enough for the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2 at pH 3. The pseudo-first-order rate constants at different H2O2 dosage were also determined and are shown in the inset of Fig. 4b. The degradation rate of RhB increased with the increase of H2O2 dosage from 0.5 to 50 mM. The degradation of RhB was up to 70% and 95% within 10 min and then remains constant with 0.5 and 1.0 mM H2O2, respectively. Generally, a very low concentration of H2O2 can be easily depleted, resulting in the difficulty to continuously produce OH, and then the degradation of pollutants was
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suppressed. The degradation of RhB achieved 100% within 10 min when the dosage of H2O2 was between 1.6 and 50 mM. The low dosage of H2O2 is considered for the degradation of RhB with low cost. Therefore, 1.6 mM was chosen as the dosage of H2O2 in the following experiments. Furthermore, according to Fig. 2b, only 40% RhB was degraded at pH 7 within 60 min by Fe–Si–B amorphous ribbon and 50 mM H2O2. The great increase of degradation rate with the decrease of initial pH value from 7 to 3 has also been reported in other Fenton-like systems (Chen and Zhu, 2006). One possible reason was that the production of OH on the surface of Fe–Si–B catalyst was restricted with the formation of oxide or hydroxide complexes at neutral condition, which resulted in a lower degradation rate of RhB (Gan and Uau Li, 2013). In addition, as mentioned in Section 3.2.1, the degradation of RhB by Fe–Si–B catalyst is likely a surface-catalyzed reaction and RhB was oxidized by near-surface OH. At pH below 4, the RhB species are in cationic and monomeric molecular form and this will promote the adsorption of RhB to Fe–Si–B matrix where the net surface charge is negative (Mohammadi et al., 2010). The adsorbed RhB species were then oxidized at the sorbed state and the fast degradation rate at pH 3.0 was observed. Therefore, compared with the traditional Fenton reactions based on Fe(II) and H2O2, the Fenton-like process with Fe–Si–B amorphous ribbons is further confirmed to be a promising technique for degrading organic contaminants because of its high degradation rate, low dosage of H2O2 and wide working pH range. 3.3.2. Effect of Fe–Si–B dosage and initial concentration of RhB The degradation experiments of 20 mg L1 RhB with different Fe–Si–B dosage or RhB of different initial concentration with 0.5 g L1 Fe–Si–B dosage were carried out with 1.6 mM H2O2 at pH 3. The pseudo-first-order kinetic constants are listed in Table 1. The degradation rate of RhB increased with the increase of Fe–Si–B dosage from 0.2 to 0.4 g L1. This is mainly attributed to the availability of more iron active sites that could accelerate the production of OH on the catalyst surface, resulting in an increased rate of reaction. However, the degradation rate was not increased when Fe–Si–B dosage was further increased from 0.4 to 0.5 g L1. This could be attributed to the enough iron active sites for producing OH. The degradation rate of RhB decreased with the increase of its initial concentration. In addition, although the degradation rate was different from each other, complete degradation of RhB could be achieved within 10 min in these experiments. 3.3.3. Stability and reusability of the Fe–Si–B catalyst The stability and reusability of Fe–Si–B amorphous ribbons were evaluated by RhB degradation in the presence of H2O2 at pH 3 under identical experimental conditions. At the end of one experiment, the Fe–Si–B catalyst was easily separated from the reaction solution by a simple filtering procedure. Then it was washed with deionized water, dried in the glovebox under N2 and stored at ambient temperature. As shown in Fig. 5, complete degradation of RhB could still be achieved within 10 min after
Table 1 Rate constants of RhB degradation by Fe–Si–B amorphous ribbons and 1.6 mM H2O2 at initial pH 3 at 295 K. Fe–Si–B dosage (g L1)
Initial RhB concentration (mg L1)
Pseudo-first-order k (min1)
R2
0.2 0.4 0.5 0.5 0.5 0.5
20 20 20 30 40 50
0.372 0.719 0.725 0.515 0.444 0.426
0.995 0.996 0.995 0.990 0.995 0.985
Fig. 5. Stability of the Fe–Si–B catalyst for the degradation of RhB (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; H2O2 dosage, 1.6 mM; temperature, 295 K; initial pH value, 3).
three cycles. It means that the Fe–Si–B amorphous ribbon was able to be reused for at least four cycles and exhibited no significant loss of activity. Meanwhile, there was no significant leaching of Fe ions and the concentration of leached Fe (Fe2+, Fe3+) was 26 mg L1. The amorphous nature of Fe–Si–B catalyst could also be maintained after reuse of the fourth cycle, which was confirmed by XRD measurement as shown in Fig. 1a. Therefore, these results indicated that the Fe–Si–B catalyst has an excellent long-term stability and has great potential application value for its easier separation. 4. Conclusions An efficient heterogeneous Fenton-like catalyst, Fe–Si–B amorphous ribbon, was successfully prepared by a melt-spinning method. Influences of various reaction parameters such as H2O2 dosage, temperature, initial pH value, Fe–Si–B dosage and initial RhB concentration on the degradation of RhB have been investigated and the results show that RhB could be effectively degraded (in 10 min) by the Fe–Si–B catalyst with low dosage of H2O2 at acidic pH. All the degradation process could be described by a pseudo-first-order kinetic model and the activation energy for the RhB degradation by Fe–Si–B catalyst and H2O2 at neutral pH was calculated to be 27.4 kJ mol1, much lower than the values reported for other heterogeneous catalysts. The Fe–Si–B catalyst also exhibited considerably good stability and reusability. In this work, based on the properties of ready availability, low cost, high reactivity and easy separation, Fe–Si–B amorphous ribbon was demonstrated to have great potential to be a green catalyst for heterogeneous Fenton-like degradation of hazardous dye, giving an enhanced treatability of textile wastewater. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities of China (No. 2242014R20012) and the Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1302045B). References Ai, Z.H., Lu, L.R., Li, J.P., Zhang, L.Z., Qiu, J.R., Wu, M.H., 2007. Fe@Fe2O3 core-shell nanowires as iron reagent. 1. Efficient degradation of rhodamine B by a novel sono-Fenton process. J. Phys. Chem. C 111, 4087–4093. Baiker, A., 1989. Metallic glasses in heterogeneous catalysis. Faraday Discuss. Chem. Soc. 87, 239–251.
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Efficient degradation of rhodamine B using Fe-based metallic glass catalyst by Fenton-like process Xianfei Wang, Ye Pan ⇑, Zirun Zhu, Jili Wu School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
h i g h l i g h t s Fe-based metallic glass was firstly developed to be a Fention-like catalyst. Rhodamine B could be quickly degraded by the catalyst with low dosage of H2O2. Metastable nature of metallic glass accounted for its good degradation ability. The catalyst exhibited considerably good stability and reusability.
a r t i c l e
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Article history: Received 7 July 2014 Received in revised form 10 September 2014 Accepted 17 September 2014 Available online 25 October 2014 Handling Editor: Y. Liu Keywords: Fenton-like Fe-based metallic glass Rhodamine B Degradation
a b s t r a c t An efficient heterogeneous catalyst, Fe-based metallic glass (Fe–Si–B amorphous ribbon), was successfully prepared for Fenton-like degradation of rhodamine B (RhB) by a melt-spinning method. The catalyst was characterized using XRD and SEM. The effects of various reaction parameters such as H2O2 dosage, temperature, initial pH value, Fe–Si–B dosage and initial RhB concentration on the degradation of RhB were studied. Almost complete degradation of RhB (20 mg L1) was achieved within only 10 min by 0.5 g L1 Fe–Si–B catalyst and 1.6 mM H2O2 at pH 3.0 at 295 K. Kinetic analyses showed that the degradation process could be described by a pseudo-first-order kinetic model. The catalytic stability was also investigated and it was found that the Fe–Si–B catalyst exhibited good structural stability and no loss of performance even after three cycles. It was concluded that the Fe–Si–B amorphous ribbon was a potential heterogeneous Fenton-like catalyst for industrial wastewater treatment. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Dyes have been known to be serious organic pollutants in wastewaters for several years (Pignatello et al., 2006). The treatment of dye waste effluents is, therefore, important so as to minimize their effect on the environment. Conventional removal methods of dyes include flocculation, physical adsorption and biological degradation (Dos Santos et al., 2007; Monteagudo et al., 2008; Eftekhari et al., 2010). These methods usually do not work efficiently as they are non-destructive and merely involve the transfer of pollutants from water to sludge which resulted in the production of secondary waste (Gan and Uau Li, 2013). Thus, there is a need for developing more effective treatment technologies to eliminate dyes from wastewaters. In recent years, advanced oxidation processes (AOPs) which are based on the generation of highly reactive transitory species (i.e., ⇑ Corresponding author. Tel.: +86 25 5209 0681. E-mail address: [email protected] (Y. Pan). http://dx.doi.org/10.1016/j.chemosphere.2014.09.055 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
OH, O2) to initiate non-selective oxidation destruction of organic pollutants have emerged as promising alternatives (Chen et al., 2009; Herney-Ramirez et al., 2010; Hou et al., 2011). Among AOPs, Fenton reaction (H2O2 + Fe2+/Fe3+) has been widely studied. The hydroxyl radical (OH, Eh = 2.80 V) generated in Fenton reaction have powerful oxidizing ability and can cause the degradation and mineralization of many recalcitrant organic contaminants (Herney-Ramirez et al., 2010). However, the application of traditional Fenton reaction is limited by high cost of H2O2, narrow working pH range (less than 3) and difficulty of separation and recovery of the iron species (Ai et al., 2007; Poerschmann et al., 2009). Thus, heterogeneous Fenton systems have received considerable interest and many heterogeneous Fenton-like catalysts have been reported, such as iron cluster (Keenan and Sedlak, 2008; Mylon et al., 2010), iron oxides (Hanna et al., 2008), iron-immobilized zeolites (Navalon et al., 2010) or carbon materials (Wang et al., 2012a). Reactions between these iron-bearing catalysts and H2O2 can effectively oxidize the organic molecules at circumneutral pH and they can easily be separated from the treated wastewater to avoid the secondary
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metal ion pollution. However, the better catalytic performance of these heterogeneous systems is dependent on the presence of ultrasonic and/or UV light irradiation (Chen et al., 2009; Wang et al., 2010). Thus, better heterogeneous Fenton-like catalysts need to be developed. The metallic glasses, unlike the crystalline metals, are formed by far-from-equilibrium process and the constituent atoms do not reside at the thermodynamic equilibrium positions (Greer, 1995; Inoue, 2000). The metastable nature imparts many excellent properties to amorphous alloys that are unachievable for crystalline alloys, such as the good catalytic and chemical properties (Baiker, 1989; Katona and Molnar, 1995; Zhang et al., 2010; Lin et al., 2012; Wang et al., 2012b). Meanwhile, Fe-based metallic glasses have been widely studied due to their low-cost, good corrosion resistance, and excellent soft magnetic properties (Lin et al., 2012). Therefore, the use of Fe-based metallic glasses as a heterogeneous Fenton-like catalyst for dye degradation shows a great potential. In this work, Fe–Si–B amorphous ribbons were prepared by a melt-spinning technique and were used as heterogeneous catalysts. The catalytic performance of the Fe–Si–B amorphous ribbons was investigated for the degradation of rhodamine B used as a model pollutant. Rhodamine B (RhB), one of the important xanthene dye, has become a common organic pollutant and exhibits considerably high resistance to photo and oxidative degradation (Yan et al., 2012). The major objectives of this work are (i) to evaluate the degradation performance of Fe–Si–B amorphous ribbons for RhB; (ii) to investigate the influences of various reaction parameters on the rate of degradation; (iii) to discuss the degradation kinetics and mechanism of RhB and the stability of Fe–Si–B amorphous ribbons.
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100 mg Fe–Si–B amorphous ribbons (0.5 g L1) in 200 ml solution of 20 mg L1 RhB. The initial pH of the solution of RhB was adjusted by 1.0 M H2SO4 and 1.0 M NaOH. Typically, the solution of RhB with the desired initial pH value was added with the Fe–Si–B amorphous ribbons and H2O2 under vigorous stirring. For comparison, the degradation of RhB by Fe–Si–B amorphous ribbons in the absence of H2O2 was also investigated. At given time intervals, 2 ml aliquots were collected and then centrifuged to get the supernatant for UV–vis analysis. The UV–vis absorption spectra of the aqueous solutions were recorded from 200 to 800 nm using a UV–vis spectrophotometer (TU1800-PC, Beijing). The RhB concentration was determined based on the constructed calibration curves at maximum absorption wavelength (kmax) of 554 nm. The pH value was detected by a pH meter (pHS-3, Shanghai). When considering the effect of temperature on the degradation rate of RhB, the solution temperature was controlled by a thermostatic water bath (DKS24, Shanghai). Moreover, (Fe2+, Fe3+) concentrations in the solution after reaction were determined by inductively coupled plasma emission spectrometer (ICP) on an Optima 2000 (PerkinElmer, Inc.) instrument. 3. Results and discussion 3.1. Characterization of the Fe–Si–B catalyst The XRD pattern and SEM images of the Fe78Si9B13 ribbons obtained by the melt-spinning technique are presented in Fig. 1. The XRD pattern of the reused Fe78Si9B13 ribbon after 4 cycles in
2. Materials and methods 2.1. Materials RhB (>99%) was procured from Tianjin Chemical Reagent Research Institute, China. H2O2 (30%, w/w), H2SO4 and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All the chemicals were in analytical grade and were used as received without further purification. Deionized water was used for preparing the RhB solutions in the experiments. 2.2. Preparation and characterization of Fe–Si–B amorphous ribbons The master ingot of Fe78Si9B13 was produced by arc melting a mixture of Fe, Si and Fe–B alloy under a Ti-gettered Argon atmosphere. The purity of Fe, Si and Fe–B alloy was higher than 99.9%. The ingot was then remelted in a quartz tube by induction melting, followed by a single roller spinning to obtain amorphous ribbons. The ribbons were 2 mm in width and 30–40 lm in thickness, and they were cut into 10 mm in length to be used in the experiments. The crystal structure of the catalysts was analyzed by X-ray diffraction (XRD). The XRD data were collected on a D8 Bruker diffractometer with Cu Ka radiation (k = 0.1541 nm) under operation conditions of 40 kV and 40 mA. The diffractogram was recorded in the 10–80° 2h range, with a 0.04° step size and a collecting of 0.3 s per point. The surface morphology of the catalysts was observed with a FEI Sirion 200 scanning electron microscope (SEM) at an accelerating voltage of 20 kV. 2.3. Procedure and approach Unless indicated otherwise, batch experiments were carried out in a 250 mL flask at 295 K in dark and reaction mixture contained
Fig. 1. XRD patterns (a) of as-received and reused Fe–Si–B amorphous ribbons, and SEM images (b, c) of as-received Fe–Si–B amorphous ribbon.
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the presence of 1.6 mM H2O2 at pH 3 is also shown in Fig. 1a. The amorphous nature of the as-received ribbon can be verified by the broad peak in the XRD pattern. The result demonstrates that the structure of the ribbon was a short-range ordered and long-range disordered structure, and the zero-valent iron was successfully frozen into amorphous state. Meanwhile, the amorphous nature of Fe–Si–B ribbon was maintained after the use for the degradation of RhB. From Fig. 1b and c, it can be seen that the free surface was nearly perfectly smooth, and the surface in contact with the roller during fabrication of the ribbon was relatively rough.
0.5 M H2O2. However, when H2O2 dosage was further increased to 0.7 and 0.85 M, the degradation rate of RhB decreased and only 40% RhB was degraded within 60 min. H2O2 is an ordinary oxidant and it cannot effectively degrade RhB (Matilainen and Sillanpää, 2010). Being similar to general Fenton-like degradation, the degradation of RhB by Fe–Si–B catalyst and H2O2 is more likely a surface-catalyzed reaction and RhB might be oxidized by near-surface OH (Lin and Gurol, 1998). First of all, RhB was absorbed on the surface of the Fe–Si–B catalyst. Then H2O2 was decomposed at the surface of the Fe–Si–B catalyst to generate OH (Lin and Gurol, 1998; Chen et al., 2009):
3.2. Degradation of RhB by Fe–Si–B catalyst and H2O2 at neutral pH
Fe0 þ H2 O2 ! Fe2þ þ 2OH
ð1Þ
3.2.1. Effect of H2O2 dosage The initial concentration of H2O2 is a key parameter in the Fenton or Fenton-like reactions. Therefore, the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2 was first conducted at neutral pH to determine the effect of H2O2 dosage, and the results are shown in Fig. 2. The absorption intensity of RhB at 554 nm gradually decreased with the increase of reaction time (Fig. 2a), which indicates that the structure of RhB molecules was destroyed during the reaction. From Fig. 2b, it can be seen that the degradation of RhB was significantly affected by the dosage of H2O2. The degradation rate of RhB increased with the increase of H2O2 dosage from 0.05 to 0.5 M and 70% RhB was degraded within 60 min with
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð2Þ
OH is quite reactive and could most probably react with the sorbed species before being able to diffuse to the solution. Finally, adsorbed RhB was degraded by OH and the degradation rate was directly related to the concentration of OH. At low H2O2 concentration, increasing H2O2 dosage would lead to more OH produced and the OH preferentially attacked the RhB molecules, inducing a higher degradation rate. However, further increase in H2O2 dosage resulted in a decrease of degradation rate as the excess H2O2 could be a OH scavenger. The OH may react with H2O2 producing HO2 that is less active than OH (Salem et al., 2009):
OH þ H2 O2 ! HO2 þ H2 O
ð3Þ
As shown in Fig. 2b, the degradation kinetics of RhB could be described according to the pseudo-first-order equation as given below (Yan et al., 2012):
C t ¼ C 0 expðktÞ
ð4Þ
where k denotes the observed first-order reaction rate constant (min1), t is the reaction time (min), C0 is the initial concentration (mg L1) of RhB, and Ct is the concentration (mg L1) of RhB at time t. By nonlinear regression analysis, the maximum rate constant of 0.049 min1 was achieved at the H2O2 dosage of 0.5 M, with the relative coefficient R2 of 0.969. 3.2.2. Effect of temperature Temperature is also a critical parameter to the reaction rate. The effect of temperature on the degradation rate of RhB by Fe–Si–B amorphous ribbons and 0.5 M H2O2 at neutral pH is shown in Fig. 3a. The pseudo-first-order rate constants at different temperatures were also determined and are shown in the inset of Fig. 3a. It can be seen that temperature exerts a strong effect on the degradation rate of RhB. The degradation rate increased by increasing temperature with a rise in rate constant from 0.049 min1 at 295 K to 0.128 min1 at 323 K. Only 70% RhB was degraded within 60 min at 295 K, while nearly 88% RhB could be degraded within 30 min at 323 K. This is because higher temperature could increase the reaction rate between H2O2 and the catalyst, thus accelerating the generation of OH. Based on the kinetic rate constants at different temperatures (T), the activation energy of the degradation process of RhB can be obtained according to the Arrhenius equation (Zhang et al., 2010):
ln k ¼
Fig. 2. UV–vis spectra for RhB degraded with 0.5 M H2O2 (a) and the effect of initial H2O2 dosage on the degradation of RhB (b) by Fe–Si–B amorphous ribbons and H2O2 at neutral pH (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; temperature, 295 K).
Ea þ ln A RT
ð5Þ
where k is the kinetic rate constant, Ea is the activation energy, R is the gas constant, and A is the constant. The Arrhenius plot is presented in Fig. 3b by plotting ln k against 1/T and the activation energy was calculated to be 27.4 kJ mol1. This calculated value of activation energy is much lower than the values reported in
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Fig. 3. Effect of temperature on the degradation of RhB (a) and the corresponding Arrhenius plot of lnk versus 1/T (b) by Fe–Si–B amorphous ribbons and H2O2 at neutral pH, inset in Fig. 3a shows the kinetic rate constants k determined at different temperatures (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; H2O2 dosage, 0.5 M).
literature for the degradation of RhB as well as other organic dyes in the presence of heterogeneous catalytic systems (Chen and Zhu, 2007; Gan and Uau Li, 2013). For example, under similar experimental conditions, an activation energy of 82.5 kJ mol1 has been reported for the degradation of RhB over RHSi-Fe catalyst. Hence, it was deduced that the degradation of RhB in the presence of Fe– Si–B/H2O2 system required a relatively lower energy and this could be attributed to the metastable nature of the metallic glasses. The Fe–Si–B amorphous ribbon could be an excellent heterogeneous Fenton-like catalyst. On the other hand, according to Fig. 3a, the degradation rate of RhB by Fe–Si–B catalyst and H2O2 at neutral pH was a bit slow at room temperature (295 K). In order to increase the degradation rate of RhB using Fe–Si–B catalyst and H2O2 at room temperature and to reduce the dosage of H2O2, the degradation of RhB was then investigated at acidic pH. 3.3. Degradation of RhB by Fe–Si–B catalyst and H2O2 at acidic pH
3.3.1. Effect of initial pH value and H2O2 dosage Fig. 4 shows the effect of initial pH value (Fig. 4a) and H2O2 dosage (Fig. 4b) on the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2 at acidic pH. The experiments were carried out with 1.6 mM H2O2 at different initial pH or with different H2O2 dosage
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Fig. 4. Effect of initial pH value (a) and H2O2 dosage (b) on the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2, inset in Fig. 4b shows the kinetic rate constants k determined at different H2O2 dosage (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; temperature, 295 K; initial pH value of (b), 3).
at initial pH 3. As it is known to all, zero-valent iron is an effective reducing agent and has been proved to degrade many organic contaminants (Fan et al., 2009). For comparison, the degradation experiments of RhB by Fe–Si–B amorphous ribbons at different initial pH without addition of H2O2 were also conducted and are shown in Fig. 4a. In the absence of H2O2, there was only a little degradation of RhB at pH 4 and 90% RhB was degraded at pH 3 within 60 min. However, when a low dosage of 1.6 mM H2O2 was added, the degradation rate of RhB could be greatly enhanced. Within 10 min, almost complete degradation of RhB could be achieved at pH 3 and 90% RhB could be degraded at pH 3.5 and 4. The pseudo-first-order rate constants were also increased by one order of magnitude to 0.725 and 0.329 min1 for pH 3 and 4, respectively. Therefore, as discussed in Section 3.2, OH is also a significant key to degrade RhB in the presence of Fe–Si–B amorphous ribbons and H2O2 at acidic pH. From Fig. 4b, it can be found that a low dosage of H2O2 was enough for the degradation of RhB by Fe–Si–B amorphous ribbons and H2O2 at pH 3. The pseudo-first-order rate constants at different H2O2 dosage were also determined and are shown in the inset of Fig. 4b. The degradation rate of RhB increased with the increase of H2O2 dosage from 0.5 to 50 mM. The degradation of RhB was up to 70% and 95% within 10 min and then remains constant with 0.5 and 1.0 mM H2O2, respectively. Generally, a very low concentration of H2O2 can be easily depleted, resulting in the difficulty to continuously produce OH, and then the degradation of pollutants was
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suppressed. The degradation of RhB achieved 100% within 10 min when the dosage of H2O2 was between 1.6 and 50 mM. The low dosage of H2O2 is considered for the degradation of RhB with low cost. Therefore, 1.6 mM was chosen as the dosage of H2O2 in the following experiments. Furthermore, according to Fig. 2b, only 40% RhB was degraded at pH 7 within 60 min by Fe–Si–B amorphous ribbon and 50 mM H2O2. The great increase of degradation rate with the decrease of initial pH value from 7 to 3 has also been reported in other Fenton-like systems (Chen and Zhu, 2006). One possible reason was that the production of OH on the surface of Fe–Si–B catalyst was restricted with the formation of oxide or hydroxide complexes at neutral condition, which resulted in a lower degradation rate of RhB (Gan and Uau Li, 2013). In addition, as mentioned in Section 3.2.1, the degradation of RhB by Fe–Si–B catalyst is likely a surface-catalyzed reaction and RhB was oxidized by near-surface OH. At pH below 4, the RhB species are in cationic and monomeric molecular form and this will promote the adsorption of RhB to Fe–Si–B matrix where the net surface charge is negative (Mohammadi et al., 2010). The adsorbed RhB species were then oxidized at the sorbed state and the fast degradation rate at pH 3.0 was observed. Therefore, compared with the traditional Fenton reactions based on Fe(II) and H2O2, the Fenton-like process with Fe–Si–B amorphous ribbons is further confirmed to be a promising technique for degrading organic contaminants because of its high degradation rate, low dosage of H2O2 and wide working pH range. 3.3.2. Effect of Fe–Si–B dosage and initial concentration of RhB The degradation experiments of 20 mg L1 RhB with different Fe–Si–B dosage or RhB of different initial concentration with 0.5 g L1 Fe–Si–B dosage were carried out with 1.6 mM H2O2 at pH 3. The pseudo-first-order kinetic constants are listed in Table 1. The degradation rate of RhB increased with the increase of Fe–Si–B dosage from 0.2 to 0.4 g L1. This is mainly attributed to the availability of more iron active sites that could accelerate the production of OH on the catalyst surface, resulting in an increased rate of reaction. However, the degradation rate was not increased when Fe–Si–B dosage was further increased from 0.4 to 0.5 g L1. This could be attributed to the enough iron active sites for producing OH. The degradation rate of RhB decreased with the increase of its initial concentration. In addition, although the degradation rate was different from each other, complete degradation of RhB could be achieved within 10 min in these experiments. 3.3.3. Stability and reusability of the Fe–Si–B catalyst The stability and reusability of Fe–Si–B amorphous ribbons were evaluated by RhB degradation in the presence of H2O2 at pH 3 under identical experimental conditions. At the end of one experiment, the Fe–Si–B catalyst was easily separated from the reaction solution by a simple filtering procedure. Then it was washed with deionized water, dried in the glovebox under N2 and stored at ambient temperature. As shown in Fig. 5, complete degradation of RhB could still be achieved within 10 min after
Table 1 Rate constants of RhB degradation by Fe–Si–B amorphous ribbons and 1.6 mM H2O2 at initial pH 3 at 295 K. Fe–Si–B dosage (g L1)
Initial RhB concentration (mg L1)
Pseudo-first-order k (min1)
R2
0.2 0.4 0.5 0.5 0.5 0.5
20 20 20 30 40 50
0.372 0.719 0.725 0.515 0.444 0.426
0.995 0.996 0.995 0.990 0.995 0.985
Fig. 5. Stability of the Fe–Si–B catalyst for the degradation of RhB (initial RhB concentration, 20 mg L1; Fe–Si–B dosage, 0.5 g L1; H2O2 dosage, 1.6 mM; temperature, 295 K; initial pH value, 3).
three cycles. It means that the Fe–Si–B amorphous ribbon was able to be reused for at least four cycles and exhibited no significant loss of activity. Meanwhile, there was no significant leaching of Fe ions and the concentration of leached Fe (Fe2+, Fe3+) was 26 mg L1. The amorphous nature of Fe–Si–B catalyst could also be maintained after reuse of the fourth cycle, which was confirmed by XRD measurement as shown in Fig. 1a. Therefore, these results indicated that the Fe–Si–B catalyst has an excellent long-term stability and has great potential application value for its easier separation. 4. Conclusions An efficient heterogeneous Fenton-like catalyst, Fe–Si–B amorphous ribbon, was successfully prepared by a melt-spinning method. Influences of various reaction parameters such as H2O2 dosage, temperature, initial pH value, Fe–Si–B dosage and initial RhB concentration on the degradation of RhB have been investigated and the results show that RhB could be effectively degraded (in 10 min) by the Fe–Si–B catalyst with low dosage of H2O2 at acidic pH. All the degradation process could be described by a pseudo-first-order kinetic model and the activation energy for the RhB degradation by Fe–Si–B catalyst and H2O2 at neutral pH was calculated to be 27.4 kJ mol1, much lower than the values reported for other heterogeneous catalysts. The Fe–Si–B catalyst also exhibited considerably good stability and reusability. In this work, based on the properties of ready availability, low cost, high reactivity and easy separation, Fe–Si–B amorphous ribbon was demonstrated to have great potential to be a green catalyst for heterogeneous Fenton-like degradation of hazardous dye, giving an enhanced treatability of textile wastewater. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities of China (No. 2242014R20012) and the Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1302045B). References Ai, Z.H., Lu, L.R., Li, J.P., Zhang, L.Z., Qiu, J.R., Wu, M.H., 2007. Fe@Fe2O3 core-shell nanowires as iron reagent. 1. Efficient degradation of rhodamine B by a novel sono-Fenton process. J. Phys. Chem. C 111, 4087–4093. Baiker, A., 1989. Metallic glasses in heterogeneous catalysis. Faraday Discuss. Chem. Soc. 87, 239–251.
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