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Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water
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Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water Kun-Yi Andrew Lin a,∗, Jyun-Ting Lin a, Yi-Feng Lin b,∗ a
Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung, Taiwan Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan Christian University, 200 Chung Pei Rd., Chungli, Taoyuan, Taiwan
b
a r t i c l e
i n f o
Article history: Received 21 March 2017 Revised 12 May 2017 Accepted 15 May 2017 Available online xxx Keywords: Ferrocene Sodium percarbonate Heterogeneous catalysts Amaranth
a b s t r a c t While several iron ions and chelated iron species are employed for activating sodium percarbonate (SPC), heterogeneous iron-based catalysts are more advantageous because they can be easily recovered and separated from solutions. However, existing heterogeneous catalysts for activating SPC are limited and their preparation usually involves complexed procedures and reagents. Thus, there is still a need for developing simple but effective heterogeneous catalyst for activating SPC. Herein, ferrocene (Fc) is proposed as a readily available, non-expensive and Fe2+ -containing catalyst for activating SPC. While SPC and Fc could not degrade amaranth individually, the combination of SPC and Fc rapidly and effectively degrade amaranth, indicating that Fc can activate SPC. The activation of SPC by Fc can be attributed to Fe2+ con• tained in Fc; Fe2+ @Fc reacted with H2 O2 derived from SPC to generate OH , which decolorize amaranth. The amaranth degradation by Fc-activated SPC can be further improved by elevating reaction temperature and lowering solution pH. Through investigating effects of radical scavengers, the mechanism of amaranth • degradation was primarily attributed to OH and other reactive oxygen species to a lesser extent. Fc was also reusable and remained highly effective for activating SPC to degrade amaranth even without regeneration treatments. These features validate that Fc, a readily available Fe2+ -containing organometallic compound, can be a promising heterogeneous catalyst for activating SPC in advanced oxidation processes. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Advanced oxidation processes (AOPs) are the most extensively employed techniques for treating organic contaminants. AOPs typically involve generation of high oxidation potential radicals, such as hydroxyl radicals (OH• ) and sulfate radicals, for degrading organic pollutants. OH• is particularly attractive because it is a nonselective radical with a very high oxidation potential (2.80 eV versus NHE) [1,2], enabling it to degrade refractory pollutants effectively. OH• is conventionally generated from decomposition of hydrogen peroxide (H2 O2 ). While extensive studies have been conducted to facilitate the production of OH• from H2 O2 [3–6], such as “Fenton’s reaction”, several alterative reagents are proposed to substitute H2 O2 for production of OH• , including sodium percarbonate (SPC) (2 Na2 CO3 •3H2 O2 ) [7–13] and calcium peroxide (CaO2 ) [14,15]. These solid H2 O2 carriers can be more advantageous than the traditional H2 O2 solution as they can be handled much eas-
∗
Corresponding authors. E-mail addresses: [email protected] (K.-Y.A. Lin), yfl[email protected] (Y.-F. Lin).
ily and transported safely. In particular, SPC has received increasing attention as it has been successfully adopted for environmental remediation [7–13]. SPC (2Na2 CO3 •3H2 O2 ) is an adduct of 33 wt% of H2 O2 and 67 wt% Na2 CO3 . In comparison with the conventional H2 O2 solution, SPC is a solid reagent which can be considered as a dry carrier of H2 O2 because SPC in water can dissociate into Na2 CO3 and H2 O2 as follows [13] (Eq.(1)):
2Na2 CO3 ·3H2 O2 → 2Na2 CO3 +3H2 O2 .
(1)
The SPC reagent is usually in granulated-form, making it very convenient to use and transport. In addition, while Na2 CO3 would be added to water simultaneously, Na2+ and CO3 2− from Na2 CO3 inherently exist in natural water systems [13]. Thus, SPC is recognized as a non-toxic and environmentally friendly oxidant [13]. Especially, unlike Fenton reaction, which is suitable at acidic conditions [16], SPC can be implemented at a wide range of pH [7–13]. SPC also exhibits an alkaline property, and thus acidification of water environments, such as groundwater, can be prevented [17]. These features of SPC make it attractive for degradation of organic contaminants. For instance, Danish et al. proposed to degrade trichloroethene using SPC activated by zero-valent metals [12], whereas
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Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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Fig. 1. Ferrocene (Fc): (a) chemical structure, (b) SEM images of Fc; (c) SEM image of used Fc (The scale bar is 500 nm).
More importantly, Fc is readily available as a commercial and nonexpensive product, making it a practical catalyst for activating oxidants [20,22–24]. However, almost no studies have been conducted to investigate the catalytic activity of Fc for activating SPC. Herein, we aim to evaluate activation of SPC by Fc and select degradation of a toxic acid azo dye, amaranth, as a model reaction. Amaranth dye is extensively used in textiles, foods and cosmetics, and has been proven to be potentially carcinogenic [25,26]. Behaviors of Fc-activated SPC were studied by investigating different factors influencing degradation of amaranth, such as Fc and SPC doses, temperature, pH, and salts as well as inhibitors. The recyclability of Fc was also evaluated by employing used Fc for activating SPC to degrade amaranth. 2. Experimental Fig. 2. (a) XRD patterns and (b) FT-IR of pristine Fc and used Fc after activation of SPC.
Miao et al. also employed SPC to degrade perchloroethylene using Fe2+ as a catalyst [7]. In addition, Lu et al. also developed several methods to activate SPC in order to degrade a number of contaminants in groundwater [7,9–13,18], demonstrating the promising potential of SPC for environmental applications. Nevertheless, as revealed in these previous studies, proper activation of SPC is still required for accelerating production of OH• as for H2 O2 , in order to improve degradation efficiency. As iron appears as the most powerful catalyst for Fenton’s reaction, many iron species have been proposed to activate SPC, such as Fe2+ and Fe3+ [7,10,13]. However, Fe3+ tends to precipitate out under neutral conditions, thereby constraining the effectiveness of SPC. Thus, several studies have proposed to chelate iron species with chelating agents, such as citric acid, ascorbic acid, ethylenediamine tetraacetic acid, glutamate, and ethylenediamineN,N-disuccinic acid [8,9,11]. While activation of SPC by these chelated iron catalysts are successfully demonstrated, a few studies aim to develop heterogeneous iron-based catalysts for activating SPC. These heterogeneous catalysts can be even more advantageous as they can be easily recovered and separated from solutions [12,18]. Nevertheless, the reported heterogeneous catalysts for activating SPC are very limited and preparation of these catalysts usually involves complex procedures and reagents [12,18]. Therefore, there is still a need for developing simple but effective heterogeneous catalysts for activating SPC. To this end, we propose a Fe2+ -containing reagent, Ferrocene (Fc), as a heterogeneous catalyst for activating SPC. Fc is an organometallic compound, which consists of a central Fe2+ atom bound to two cyclopentadienyl (Cp) rings (Fig. 1(a)). As Fc is comprised of an electron donor–acceptor conjugated structure and it exhibits superior redox reversible properties [19–21]. Fc is also non-toxic as well as highly stable in aqueous environments [20,22].
2.1. Materials Chemicals employed in this study were all commercially available and used directly without purification. Sodium percarbonate (SPC), amaranth dye and ferrocene were received from SigmaAldrich (USA). tert-butyl alcohol (TBA), and chloroform were obtained from Alfa Aesar (USA), whereas NaCl was purchased from Showa Chemicals (Japan). Deionized (DI) (<18 MOhm-cm) was used for preparing aqueous solutions. 2.2. Characterization of Fc Although Fc is a readily available chemical reagent, it was essential to characterize Fc to determine its physical and chemical characteristics. The morphology of Fc was revealed by a scanning electron microscopy (SEM) (JEOL JSM-6700, Japan), whereas the crystalline structure of Fc was obtained using an X-ray diffractometer (Bruker D8 Discover, USA). IR spectrum of Fc was measured by a FT-IR spectrophotometer (Perkin-Elmer Spectrum Two, U.S.A.). Surface chemistry of Fc was determined using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, Inc., Japan). 2.3. Degradation of amaranth dye using SPC activated by Fc To evaluate catalytic activity of Fc for activating SPC, degradation of the toxic acidic azo dye, amaranth, by activated SPC was adopted as a model reaction. In a typical degradation experiment, 50 mg of Fc was added to 0.2 L of amaranth solution with an initial concentration (C0 ) of 50 mg/L. The resulting mixture was stirred to disperse Fc and maintained at a desired temperature; a certain dose of SPC (e.g., 300 mg) was then added to the mixture to start degradation experiments. At pre-set times, sample aliquots were taken from the mixture and filtered through syringe disk membranes to separate Fc from amaranth solutions, which were then
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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3.2. Degradation of amaranth dye using Fc-activated SPC
Fig. 3. XPS spectra of Fc: (a) Fe2p core-level and (b) C1s core-level spectra.
analyzed using a UV–vis spectrophotometer (ChromTech CT-2200, Taiwan) for determining the remaining concentration of amaranth at a given time t, (Ct ). The effect of co-existing ions on amaranth degradation by Fc-SPC was investigated by adding NaCl to amaranth solutions. The concentrations of NaCl adopted in this study were selected based on a few previous studies in which the concentration of NaCl ranged from a few hundreds to a few thousands mg/L [27,28]. A hot filtration test was also conducted by removing Fc from the reaction solution at the half of the reaction time (i.e., 60 min) and the concentration of amaranth was continuously monitored until the end of the reaction. In addition, the recyclability of Fc for activating SPC to degrade amaranth was evaluated by re-using Fc for activating SPC over multiple times. The used Fc was recovered via centrifugation and added to a subsequent batch experiment of amaranth degradation without regeneration treatments.
3. Results and discussion 3.1. Characterization of Fc Fig. 1(b) reveals the morphology of commercially available Fc, which exhibited very fine configurations, including filamentlike and ribbon-like nanoscale structures. This indicates that even though Fc was used as received, Fc was inherently nanoscale, facilitating its contact with other reactants. The crystalline structure of Fc is shown in Fig. 2(a), which agrees with the reported pattern of Fc (JC-PDS # 29-1711) [29]. On the other hand, chemical functionalities of Fc were determined using IR and are shown in Fig. 2(b). Several significant peaks were observed: the peak at 475 cm−1 attributed to the Fe-Cp stretching mode; the peak at 815 cm−1 derived from the C–H bending mode; the peak at 10 0 0 cm−1 owing to the C–H in-plane bending mode; the peaks at 1103 and 1407 cm−1 ascribed to the C=C stretching mode; and the peak at 3094 cm−1 attributed to the C–H stretching mode. Furthermore, the surface chemistry of Fc was analyzed using XPS and is shown in Fig. 3. According to the reported studies [23,30], the peak at 709 eV can be attributed to Fe2+ , whereas the peak at 712.1 eV is derived from Fe3+ . The peak at 715.1 eV is the satellite peak, and the peaks at 721.8 and 724.8 eV can be assigned to Fe2+ and Fe3+ , respectively. The previous studies have indicated that a small fraction of Fe2+ in Fc can be oxidized to Fe3+ which thus can be still detected in Fc [31]. However, the XPS analysis reveals that Fe2+ is still the dominant species of Fc. In addition, Fig. 3(b) displays the core-level spectrum of C1s, in which a tall peak was observed at 284.3 eV, attributed to C–C bonds of Cp rings in Fc.
As degradation of amaranth was selected as a model reaction for examining activation capability of Fc for SPC, amaranth was possibly decolorized via adsorption to Fc. To elucidate this possibility, Fc alone was present in an amaranth solution and no amaranth was removed even after 120 min (Fig. 4(a)). This demonstrates that Fc itself could not decolorize or degrade amaranth. Fig. 4(a) also reveals that when SPC alone was present, very negligible amount of amaranth was removed (Ct /C0 ∼ 0.98), validating that SPC, without activation, was ineffective. However, once Fc was combined with SPC, amaranth was rapidly degraded and Ct /C0 approached zero within 80 min. The spectral variation of amaranth during degradation was determined and is displayed in Fig. S1. When the spectral pattern of amaranth remained consistent, the intensity of maximum absorbance at 520 nm was reduced with the reaction time, demonstrating that amaranth was degraded. Since SPC and Fc were incapable of degrading amaranth individually, this result indicates that SPC was activated in the presence of Fc. Furthermore, a hot filtration test was conducted to examine whether the activation of SPC by Fc was attributed the leaching of iron from Fc. In the hot filtration test, Fc was separated at the half of the reaction time (i.e., 60 min). Fig. 1(a) shows that amaranth degradation in the hot filtration was slowed down and almost stopped as a small fraction of amaranth still remained in the end of the reaction. This suggests that no significant amount of iron leached out from Fc and left in the solution for further activation of SPC. To further investigate respective roles of Fc and SPC during amaranth degradation, effects of Fc and SPC doses were further examined. Fig. 4(b) shows amaranth degradation by the combination of Fc and SPC at different Fc doses. As the Fc dose decreased significantly from 500 mg/L to 125 and 50 mg/L, Ct /C0 still reached below 0.2 after 120 min, showing that the degradation extents remained quite similar with much less Fc doses. However, the degradation kinetics was noticeably influenced. In order to distinguish the effect of Fc dose on degradation kinetics, the pseudo first order equation was adopted to interpret the kinetic data via the following equation (Eq. (2)):
Ct = C0 exp (−kobs t )
(2)
where kobs is the observed rate constant and calculated kobs values are summarized in Table S1. When Fc was 50 mg/L, kobs was 0.014 min−1 , which increased to 0.018 and 0035 min−1 at Fc = 125 and 500 mg/L, respectively. Even though Fc decreased significantly from 500 to 50 mg/L (i.e., 90% reduction), kobs merely changed from 0.035 to 0.014 min−1 . This suggests that Fc played a catalytic role during amaranth degradation as only small doses of Fc are needed to facilitate amaranth degradation. We also examined the effect of SPC does and Fig. 5(a) shows amaranth degradation by the combination of Fc and SPC at various doses of SPC. When SPC decreased from 1500 to 750 mg/L, the degradation extent and kinetics were both adversely influenced as Ct /C0 merely approached 0.4 and kobs reduced from 0.035 to 0.007 min−1 . When SPC was even further reduced to 150 mg/L, the corresponding Ct /C0 only reached 0.75. This indicates that degradation extent of amaranth was highly dependent on SPC dose because oxidation of amaranth was attributed to SPC instead of Fc. On the other hand, when SPC dose was doubled (i.e., 30 0 0 mg/L), the degradation extent remained the same but kobs increased to 0.040 min−1 , demonstrating that higher doses of SPC also facilitated the degradation kinetics. As SPC is a solid carrier of H2 O2 , adding SPC to water leads to generation of H2 O2 . It is well known that H2 O2 can be decomposed to form radicals (i.e., OH• ) in the presence of iron species, including Fe2+ . Since Fc consists of Fe2+ , Fe2+ is expected to react with
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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Fig. 4. Activation of SPC by Fc for degradation of amaranth: (a) a comparison between adsorption to Fc, SPC, Fc-activated SPC and hot filtration test (i.e., Fc was removed from the reaction solution at half of the reaction time); (b) effect of Fc dosage (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, T = 30 °C, Fc = 250 mg/L).
Fig. 5. Effects of (a) SPC dosage (T = 30 °C) and (b) temperature on degradation of amaranth by Fc-activated SPC (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, Fc = 250 mg/L).
H2 O2 to produce OH• , and Fe2+ is oxidized to Fe3+ simultaneously as follows (Eq.(3)) [4,7–13,32,33]:
Fe2+ @Fc+H2 O2 → Fe3+ @Fc + OH• +OH− .
(3)
The resulting Fe3+ then reacts with H2 O2 to return to Fe2+ via the following reaction (Eq. (4)):
Fe3+ @Fc+H2 O2 → Fe2+ @Fc+H+ +HO2 • . •
H+
(4) •−
HO2 possibly evolves into and O2 , which can react with Fe3+ to form O2 , while Fe3+ transforms back to Fe2+ [34]. Therefore, the transformation between Fe2+ and Fe3+ leads to the catalytic decomposition of H2 O2 and production of high oxidation potential radicals, such as OH• , for attacking amaranth molecules. This potential mechanism for activation of SPC by Fc can be schematically illustrated in Fig. 6. 3.3. Effects of temperature and pH In addition to the effects of Fc and SPC doses, we further investigated the effect of temperature. Fig. 5(b) shows amaranth degradation by Fc-activated SPC at different temperatures. While amaranth was fully degraded at all tested temperatures, degradation kinetics was substantially accelerated at elevated temperatures. As kobs was 0.035 min−1 at 30 °C, it increased to 0.078 min−1 at 40 °C and even further 0.198 min−1 at 50 °C, validating the enhancing effect of higher temperatures on degradation kinetics. This is possibly because higher temperatures facilitated mass transfer of reactants. Since kobs increased with temperature, the relationship between kobs and temperature was further correlated through the Arrhenius equation as follows (Eq. (5)):
In kobs = ln A − −Ea /RT
(5) (min–1 );
where A is the pre-exponential factor R is the universal gas constant; and T is the solution temperature in Kelvin (K). Based
Fig. 6. A proposed mechanism for activation of SPC by Fc.
on the Arrhenius equation, a plot of ln kobs versus 1/T is shown in Fig. S2, in which the data points are well fitted by linear regression with R2 = 0.996. This suggests that the relationship between kobs and temperature can be properly interpreted by Arrhenius equation and the corresponding activation energy was thus calculated as 70.4 kJ/mol. Furthermore, the effect of pH was also examined as pH has been considered as the most important factor for Fenton-like reactions. Fig. 7(a) reveals amaranth degradation by Fc-activated SPC at pH = 3, 6, 9 and 11. While degradation extents remained similar at various pH values, the corresponding kinetics were noticeably influenced. As kobs was 0.035 min−1 at pH = 6 (i.e., the pH-unadjusted case), it increased to 0.041 min−1 at pH = 9 and 0.061 min−1 at
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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Fig. 7. Effects of (a) pH and (b) NaCl on degradation of amaranth by Fc-activated SPC (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, Fc = 250 mg/L, T = 30 °C).
Fig. 8. (a) Effect of inhibitors on degradation of amaranth by Fc-activated SPC; and (b) recyclability of Fc for activating SPC (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, Fc = 250 mg/L, T = 30 °C, TBA = chloroform = DMSO = 1 M).
pH = 3 but decreased to 0.024 min−1 at pH = 11. This demonstrates that amaranth degradation by Fc-activated SPC was more favorable under relatively acidic conditions. It has been reported that OH• generated from Fenton-like reaction exhibits the highest oxidizing potential at pH = 3 [35]. Thus, the amaranth degradation by Fc-activated SPC at the acidic condition was the most effective. On the contrary, the oxidizing potential of OH• is greatly diminished under alkaline conditions [36]; therefore amaranth degradation became even less effective at pH = 11. 3.4. Effects of salts and inhibitors Since salts, especially NaCl, are usually added to enhance dyefixation performance [37–39], the effect of NaCl was further investigated. While amaranth concentration in this study was merely 50 mg/L, we particularly added much higher concentrations of NaCl to amaranth solutions to reveal its effect. Fig. 7(b) shows the effect of NaCl on amaranth degradation by Fc-activated SPC. When the concentration of NaCl was relatively low (i.e., 500 and 10 0 0 mg/L), no obvious effect was observed and the degradation extent remained the same. However, once the concentration of NaCl increased to 50 0 0 mg/L, the degradation extent and kinetics both substantially hindered. A previous study reveals that when the concentration of NaCl was lower than 10 mM (i.e., 580 mg/L), the adverse effect of NaCl on degradation using SPC is insignificant. Nevertheless, once the concentration of NaCl is higher than 10 0 mM (i.e., 580 0 mg/L), the degradation kinetics is remarkably slowed because the as-generated OH• was consumed by reaction with the excessive amount of Cl− [10]. Additionally, radical scavengers were also evaluated for their effects on amaranth degradation by Fc-activated SPC in order to elucidate degradation mechanism. As discussed earlier, OH• should be generated from SPC, and therefore a particular OH• scavenger, TBA,
was evaluated. Fig. 8(a) shows that amaranth degradation was substantially affected in the presence of TBA as kobs decreased from 0.035 to 0.014 min−1 , validating that OH• certainly involved in amaranth degradation. To validate the existence of OH• , another common OH• scavenger, DMSO, was also evaluated for its scavenging effect. The presence of DMSO indeed inhibited amaranth degradation (Fig. 8(a)) as kobs also lowered to 0.019 min−1 , confirming that the amaranth degradation by Fc-SPC involved with OH• . On the other hand, as discussed earlier, HO2 • can be also produced when H2 O2 reacts with Fe3+ @Fc. Since HO2 • can be further decomposed to H+ and O2 •− , chloroform, a radical scavenger specifically for O2 •− was also evaluated. Fig. 8(a) shows that the presence of chloroform slightly influenced the degradation kinetics as kobs decreased from 0.035 to 0.023 min−1 , suggesting the existence of O2 •− . Based on the effects of these radical scavengers, the mechanism of amaranth degradation can be primarily attributed to OH• and other reactive oxygen species to a lesser extent. 3.5. Recyclability of Fc for activating SPC As a heterogeneous catalyst, Fc should be more conveniently recovered after reactions. However, it is essential to investigate whether the recovered Fc can be reused for activating SPC. Thus, the long-term recyclability of Fc for SPC activation was evaluated by reusing Fc for activating SPC to degrade amaranth. Fig. 8(b) shows that amaranth was still fully degraded using SPC activated by used Fc and the degradation kinetics remained similar without significant delay. This demonstrates that Fc can be reused for activation of SPC even without regeneration treatments. On the other hand, reused Fc was characterized using XRD and FT-IR to examine whether the multi-cycle activation of SPC changed characteristics of Fc. Fig. 2(a) and (b) show that the reused Fc exhibited the consistent XRD pattern and IR spectrum to those
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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of pristine Fc. Fig. 1(c) also reveals that used Fc retained its morphology as the filament-like and ribbon-line nanostructures were still maintained. This suggests that Fc remained intact and no obvious damage on Fc occurred even after the multi-cycle activation of SPC, validating that Fc can be a recyclable and stable heterogeneous catalyst for SPC activation. 4. Conclusion In this study, Fc was employed for the first time for activating the solid H2 O2 carrier, SPC, for degrading the toxic acidic azo dye, amaranth. While SPC and Fc could not degrade amaranth individually, the combination of SPC and Fc rapidly and effectively degrade amaranth, indicating that Fc can activate SPC. The activation of SPC by Fc can be attributed to Fe2+ contained in Fc; Fe2+ @Fc reacted with H2 O2 to generate OH• , which decolorize amaranth. The amaranth degradation by Fc-activated SPC can be further improved by elevating reaction temperature and lowering solution pH. Through investigating effects of radical scavengers, the mechanism of amaranth degradation was primarily attributed to OH• and other reactive oxygen species to a lesser extent. Fc was also reusable and remained highly effective for activating SPC to degrade amaranth even without regeneration treatments. These features validate that Fc, a readily available Fe2+ -containing compound, can be a promising heterogeneous catalyst for activating SPC in AOPs. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.05.017. References [1] Andreozzi R, Insola A, Caprio V, Marotta R, Tufano V. The use of manganese dioxide as a heterogeneous catalyst for oxalic acid ozonation in aqueous solution. Appl Catal A 1996;138:75–81. [2] Legube B, Karpel Vel Leitner N. Catalytic ozonation: a promising advanced oxidation technology for water treatment. Catal Today 1999;53:61–72. [3] Khaksar M, Amini M, Boghaei DM, Chae KH, Gautam S. Mn-doped ZrO2 nanoparticles as an efficient catalyst for green oxidative degradation of methylene blue. Catal Commun 2015;72:1–5. [4] Ali MEM, Gad-Allah TA. Badawy MI heterogeneous Fenton process using steel industry wastes for methyl orange degradation. Appl Water Sci 2013;3:263–70. [5] Velichkova F, Julcour-Lebigue C, Koumanova B, Delmas H. Heterogeneous Fenton oxidation of paracetamol using iron oxide (nano)particles. J Environ Chem Eng 2013;1:1214–22. [6] Amini M, Khaksar M, Ellern A, Keith WL. A new nanocluster polyoxomolybdate [Mo36O110(NO)4(H2O)14]•52H2O: synthesis, characterization and application in oxidative degradation of common organic dyes. Chin J Chem Eng 2017. doi:10.1016/j.cjche.2017.03.031. [7] Miao Z, Gu X, Lu S, Zang X, Wu X, Xu M, et al. Perchloroethylene (PCE) oxidation by percarbonate in Fe2+ -catalyzed aqueous solution: PCE performance and its removal mechanism. Chemosphere 2015;119:1120–5. [8] Fu X, Gu X, Lu S, Xu M, Miao Z, Zhang X, et al. Enhanced degradation of benzene in aqueous solution by sodium percarbonate activated with chelated-Fe(II). Chem Eng J 2016;285:180–8. [9] Cui H, Gu X, Lu S, Fu X, Zhang X, Fu GY, et al. Degradation of ethylbenzene in aqueous solution by sodium percarbonate activated with EDDS–Fe(III) complex. Chem Eng J 2017;309:80–8. [10] Fu X, Gu X, Lu S, Sharma VK, Brusseau ML, Xue Y, et al. Benzene oxidation by Fe(III)-activated percarbonate: matrix-constituent effects and degradation pathways. Chem Eng J 2017;309:22–9. [11] Fu X, Gu X, Lu S, Miao Z, Xu M, Zhang X, et al. Enhanced degradation of benzene by percarbonate activated with Fe(II)-glutamate complex. Environ Sci Pollut Res 2016;23:6758–66. [12] Danish M, Gu X, Lu S, Brusseau ML, Ahmad A, Naqvi M, et al. An efficient catalytic degradation of trichloroethene in a percarbonate system catalyzed by ultra-fine heterogeneous zeolite supported zero valent iron-nickel bimetallic composite. Appl Catal A: Gen 2017;531:177–86.
[13] Fu X, Gu X, Lu S, Miao Z, Xu M, Zhang X, et al. Benzene depletion by Fe2+-catalyzed sodium percarbonate in aqueous solution. Chem Eng J 2015;267:25–33. [14] Xue Y, Gu X, Lu S, Miao Z, Brusseau ML, Xu M, et al. The destruction of benzene by calcium peroxide activated with Fe(II) in water. Chem Eng J 2016;302:187–93. [15] Zhang X, Gu X, Lu S, Miao Z, Xu M, Fu X, et al. Application of calcium peroxide activated with Fe(II)-EDDS complex in trichloroethylene degradation. Chemosphere 2016;160:1–6. [16] Yap CL, Gan S, Ng HK. Fenton based remediation of polycyclic aromatic hydrocarbons-contaminated soils. Chemosphere 2011;83:1414–30. [17] Viisimaa M, Goi A. Use of hydrogen peroxide and percarbonate to treat chlorinated aromatic hydrocarbon-contaminated soil. J Environ Eng Landscape Manage 2014;22:30–9. [18] Danish M, Gu X, Lu S, Ahmad A, Naqvi M, Farooq U, et al. Efficient transformation of trichloroethylene activated through sodium percarbonate using heterogeneous zeolite supported nano zero valent iron-copper bimetallic composite. Chem Eng J 2017;308:396–407. [19] Tsai F-C, Chang C-C, Liu C-L, Chen W-C, Jenekhe SA. New thiophene-linked conjugated poly(azomethine)s: theoretical electronic structure, synthesis, and properties. Macromolecules 2005;38:1958–66. [20] Wang Q, Tian S, Cun J, Ning P. Degradation of methylene blue using a heterogeneous Fenton process catalyzed by ferrocene. Desalin Water Treat 2013;51:5821–30. [21] Li Y, Xue L, Li H, Li Z, Xu B, Wen S, et al. Energy level and molecular structure engineering of conjugated donor−acceptor copolymers for photovoltaic applications. Macromolecules 2009;42:4491–9. [22] Nie Y, Hu C, Qu J, Hu X. Efficient photodegradation of Acid Red B by immobilized ferrocene in the presence of UVA and H2O2. J Hazard Mater 2008;154:146–52. [23] Ye X, Cui Y, Wang X. Ferrocene-modified carbon nitride for direct oxidation of benzene to phenol with visible light. ChemSusChem 2014;7:738–42. [24] Lin K-YA, Lin J-T, Jochems AP. Oxidation of amaranth dye by persulfate and peroxymonosulfate activated by ferrocene. J Chem Technol Biotechnol 2016 n/a-n/a. [25] Gupta VK, Jain R, Mittal A, Saleh TA, Nayak A, Agarwal S, et al. Photo-catalytic degradation of toxic dye amaranth on TiO2 /UV in aqueous suspensions. Mater Sci Eng C 2012;32:12–17. [26] Sudrajat H, Babel S. Rapid photocatalytic degradation of the recalcitrant dye amaranth by highly active N-WO3. Environ Chem Lett 2016;14:243. [27] Wang Z, Yuan R, Guo Y, Xu L, Liu J. Effects of chloride ions on bleaching of azo dyes by Co2+ /oxone regent: kinetic analysis. J Hazard Mater 2011;190:1083–7. [28] Anipsitakis GP, Dionysiou DD, Gonzalez MA. Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions. Environ Sci Technol 20 06;40:10 0 0–7. [29] Chen P, Wu Q-S, Ding Y-P. Preparation of ferrocene nanocrystals by the ultrasonic-solvent-substitution method and their electrochemical properties. Small 2007;3:644–9. [30] Ye X, Zheng Y, Wang X. Synthesis of ferrocene-modified carbon nitride photocatalysts by surface amidation reaction for phenol synthesis. Chin J Chem 2014;32:498–506. [31] Song Y, Kang X, Zuckerman NB, Phebus B, Konopelski JP, Chen S. Ferrocene– functionalized carbon nanoparticles. Nanoscale 2011;3:1984–9. [32] Kasiri MB, Aleboyeh H, Aleboyeh A. Degradation of Acid Blue 74 using Fe-ZSM5 zeolite as a heterogeneous photo-Fenton catalyst. Appl Catal B: Environ 2008;84:9–15. [33] Rahim Pouran S, Abdul Raman AA, Wan Daud WMA. Review on the application of modified iron oxides as heterogeneous catalysts in Fenton reactions. J Cleaner Prod 2014;64:24–35. [34] Kang N, Hua I. Enhanced chemical oxidation of aromatic hydrocarbons in soil systems. Chemosphere 2005;61:909–22. [35] Safarzadeh-Amiri A, Bolton JR, Cater SR. The use of iron in advanced oxidation processes. J Adv Oxid Technol 1996;1:18–26. [36] Gogate PR, Pandit AB. A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions. Adv Environ Res 2004;8:501–51. [37] Adil MR, Qadir MA, Mahmood K. Effects of various buffers and salt on color strength of reactive dye sumifix 3RF. J Chem Soc Pak 2009;31:7–11. [38] Xiang J, Yang X, Chen C, Tang Y, Yan W, Xu G. Effects of NaCl on the J-aggregation of two thiacarbocyanine dyes in aqueous solutions. J Colloid Interface Sci 2003;258:198–205. [39] Muthukumar M, Sargunamani D, Selvakumar N, Nedumaran D. Effect of salt additives on decolouration of Acid Black 1 dye effluent by ozonation. Indian J Chem Technol 2004;11:612–16.
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Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water Kun-Yi Andrew Lin a,∗, Jyun-Ting Lin a, Yi-Feng Lin b,∗ a
Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung, Taiwan Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan Christian University, 200 Chung Pei Rd., Chungli, Taoyuan, Taiwan
b
a r t i c l e
i n f o
Article history: Received 21 March 2017 Revised 12 May 2017 Accepted 15 May 2017 Available online xxx Keywords: Ferrocene Sodium percarbonate Heterogeneous catalysts Amaranth
a b s t r a c t While several iron ions and chelated iron species are employed for activating sodium percarbonate (SPC), heterogeneous iron-based catalysts are more advantageous because they can be easily recovered and separated from solutions. However, existing heterogeneous catalysts for activating SPC are limited and their preparation usually involves complexed procedures and reagents. Thus, there is still a need for developing simple but effective heterogeneous catalyst for activating SPC. Herein, ferrocene (Fc) is proposed as a readily available, non-expensive and Fe2+ -containing catalyst for activating SPC. While SPC and Fc could not degrade amaranth individually, the combination of SPC and Fc rapidly and effectively degrade amaranth, indicating that Fc can activate SPC. The activation of SPC by Fc can be attributed to Fe2+ con• tained in Fc; Fe2+ @Fc reacted with H2 O2 derived from SPC to generate OH , which decolorize amaranth. The amaranth degradation by Fc-activated SPC can be further improved by elevating reaction temperature and lowering solution pH. Through investigating effects of radical scavengers, the mechanism of amaranth • degradation was primarily attributed to OH and other reactive oxygen species to a lesser extent. Fc was also reusable and remained highly effective for activating SPC to degrade amaranth even without regeneration treatments. These features validate that Fc, a readily available Fe2+ -containing organometallic compound, can be a promising heterogeneous catalyst for activating SPC in advanced oxidation processes. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Advanced oxidation processes (AOPs) are the most extensively employed techniques for treating organic contaminants. AOPs typically involve generation of high oxidation potential radicals, such as hydroxyl radicals (OH• ) and sulfate radicals, for degrading organic pollutants. OH• is particularly attractive because it is a nonselective radical with a very high oxidation potential (2.80 eV versus NHE) [1,2], enabling it to degrade refractory pollutants effectively. OH• is conventionally generated from decomposition of hydrogen peroxide (H2 O2 ). While extensive studies have been conducted to facilitate the production of OH• from H2 O2 [3–6], such as “Fenton’s reaction”, several alterative reagents are proposed to substitute H2 O2 for production of OH• , including sodium percarbonate (SPC) (2 Na2 CO3 •3H2 O2 ) [7–13] and calcium peroxide (CaO2 ) [14,15]. These solid H2 O2 carriers can be more advantageous than the traditional H2 O2 solution as they can be handled much eas-
∗
Corresponding authors. E-mail addresses: [email protected] (K.-Y.A. Lin), yfl[email protected] (Y.-F. Lin).
ily and transported safely. In particular, SPC has received increasing attention as it has been successfully adopted for environmental remediation [7–13]. SPC (2Na2 CO3 •3H2 O2 ) is an adduct of 33 wt% of H2 O2 and 67 wt% Na2 CO3 . In comparison with the conventional H2 O2 solution, SPC is a solid reagent which can be considered as a dry carrier of H2 O2 because SPC in water can dissociate into Na2 CO3 and H2 O2 as follows [13] (Eq.(1)):
2Na2 CO3 ·3H2 O2 → 2Na2 CO3 +3H2 O2 .
(1)
The SPC reagent is usually in granulated-form, making it very convenient to use and transport. In addition, while Na2 CO3 would be added to water simultaneously, Na2+ and CO3 2− from Na2 CO3 inherently exist in natural water systems [13]. Thus, SPC is recognized as a non-toxic and environmentally friendly oxidant [13]. Especially, unlike Fenton reaction, which is suitable at acidic conditions [16], SPC can be implemented at a wide range of pH [7–13]. SPC also exhibits an alkaline property, and thus acidification of water environments, such as groundwater, can be prevented [17]. These features of SPC make it attractive for degradation of organic contaminants. For instance, Danish et al. proposed to degrade trichloroethene using SPC activated by zero-valent metals [12], whereas
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Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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Fig. 1. Ferrocene (Fc): (a) chemical structure, (b) SEM images of Fc; (c) SEM image of used Fc (The scale bar is 500 nm).
More importantly, Fc is readily available as a commercial and nonexpensive product, making it a practical catalyst for activating oxidants [20,22–24]. However, almost no studies have been conducted to investigate the catalytic activity of Fc for activating SPC. Herein, we aim to evaluate activation of SPC by Fc and select degradation of a toxic acid azo dye, amaranth, as a model reaction. Amaranth dye is extensively used in textiles, foods and cosmetics, and has been proven to be potentially carcinogenic [25,26]. Behaviors of Fc-activated SPC were studied by investigating different factors influencing degradation of amaranth, such as Fc and SPC doses, temperature, pH, and salts as well as inhibitors. The recyclability of Fc was also evaluated by employing used Fc for activating SPC to degrade amaranth. 2. Experimental Fig. 2. (a) XRD patterns and (b) FT-IR of pristine Fc and used Fc after activation of SPC.
Miao et al. also employed SPC to degrade perchloroethylene using Fe2+ as a catalyst [7]. In addition, Lu et al. also developed several methods to activate SPC in order to degrade a number of contaminants in groundwater [7,9–13,18], demonstrating the promising potential of SPC for environmental applications. Nevertheless, as revealed in these previous studies, proper activation of SPC is still required for accelerating production of OH• as for H2 O2 , in order to improve degradation efficiency. As iron appears as the most powerful catalyst for Fenton’s reaction, many iron species have been proposed to activate SPC, such as Fe2+ and Fe3+ [7,10,13]. However, Fe3+ tends to precipitate out under neutral conditions, thereby constraining the effectiveness of SPC. Thus, several studies have proposed to chelate iron species with chelating agents, such as citric acid, ascorbic acid, ethylenediamine tetraacetic acid, glutamate, and ethylenediamineN,N-disuccinic acid [8,9,11]. While activation of SPC by these chelated iron catalysts are successfully demonstrated, a few studies aim to develop heterogeneous iron-based catalysts for activating SPC. These heterogeneous catalysts can be even more advantageous as they can be easily recovered and separated from solutions [12,18]. Nevertheless, the reported heterogeneous catalysts for activating SPC are very limited and preparation of these catalysts usually involves complex procedures and reagents [12,18]. Therefore, there is still a need for developing simple but effective heterogeneous catalysts for activating SPC. To this end, we propose a Fe2+ -containing reagent, Ferrocene (Fc), as a heterogeneous catalyst for activating SPC. Fc is an organometallic compound, which consists of a central Fe2+ atom bound to two cyclopentadienyl (Cp) rings (Fig. 1(a)). As Fc is comprised of an electron donor–acceptor conjugated structure and it exhibits superior redox reversible properties [19–21]. Fc is also non-toxic as well as highly stable in aqueous environments [20,22].
2.1. Materials Chemicals employed in this study were all commercially available and used directly without purification. Sodium percarbonate (SPC), amaranth dye and ferrocene were received from SigmaAldrich (USA). tert-butyl alcohol (TBA), and chloroform were obtained from Alfa Aesar (USA), whereas NaCl was purchased from Showa Chemicals (Japan). Deionized (DI) (<18 MOhm-cm) was used for preparing aqueous solutions. 2.2. Characterization of Fc Although Fc is a readily available chemical reagent, it was essential to characterize Fc to determine its physical and chemical characteristics. The morphology of Fc was revealed by a scanning electron microscopy (SEM) (JEOL JSM-6700, Japan), whereas the crystalline structure of Fc was obtained using an X-ray diffractometer (Bruker D8 Discover, USA). IR spectrum of Fc was measured by a FT-IR spectrophotometer (Perkin-Elmer Spectrum Two, U.S.A.). Surface chemistry of Fc was determined using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, Inc., Japan). 2.3. Degradation of amaranth dye using SPC activated by Fc To evaluate catalytic activity of Fc for activating SPC, degradation of the toxic acidic azo dye, amaranth, by activated SPC was adopted as a model reaction. In a typical degradation experiment, 50 mg of Fc was added to 0.2 L of amaranth solution with an initial concentration (C0 ) of 50 mg/L. The resulting mixture was stirred to disperse Fc and maintained at a desired temperature; a certain dose of SPC (e.g., 300 mg) was then added to the mixture to start degradation experiments. At pre-set times, sample aliquots were taken from the mixture and filtered through syringe disk membranes to separate Fc from amaranth solutions, which were then
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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3.2. Degradation of amaranth dye using Fc-activated SPC
Fig. 3. XPS spectra of Fc: (a) Fe2p core-level and (b) C1s core-level spectra.
analyzed using a UV–vis spectrophotometer (ChromTech CT-2200, Taiwan) for determining the remaining concentration of amaranth at a given time t, (Ct ). The effect of co-existing ions on amaranth degradation by Fc-SPC was investigated by adding NaCl to amaranth solutions. The concentrations of NaCl adopted in this study were selected based on a few previous studies in which the concentration of NaCl ranged from a few hundreds to a few thousands mg/L [27,28]. A hot filtration test was also conducted by removing Fc from the reaction solution at the half of the reaction time (i.e., 60 min) and the concentration of amaranth was continuously monitored until the end of the reaction. In addition, the recyclability of Fc for activating SPC to degrade amaranth was evaluated by re-using Fc for activating SPC over multiple times. The used Fc was recovered via centrifugation and added to a subsequent batch experiment of amaranth degradation without regeneration treatments.
3. Results and discussion 3.1. Characterization of Fc Fig. 1(b) reveals the morphology of commercially available Fc, which exhibited very fine configurations, including filamentlike and ribbon-like nanoscale structures. This indicates that even though Fc was used as received, Fc was inherently nanoscale, facilitating its contact with other reactants. The crystalline structure of Fc is shown in Fig. 2(a), which agrees with the reported pattern of Fc (JC-PDS # 29-1711) [29]. On the other hand, chemical functionalities of Fc were determined using IR and are shown in Fig. 2(b). Several significant peaks were observed: the peak at 475 cm−1 attributed to the Fe-Cp stretching mode; the peak at 815 cm−1 derived from the C–H bending mode; the peak at 10 0 0 cm−1 owing to the C–H in-plane bending mode; the peaks at 1103 and 1407 cm−1 ascribed to the C=C stretching mode; and the peak at 3094 cm−1 attributed to the C–H stretching mode. Furthermore, the surface chemistry of Fc was analyzed using XPS and is shown in Fig. 3. According to the reported studies [23,30], the peak at 709 eV can be attributed to Fe2+ , whereas the peak at 712.1 eV is derived from Fe3+ . The peak at 715.1 eV is the satellite peak, and the peaks at 721.8 and 724.8 eV can be assigned to Fe2+ and Fe3+ , respectively. The previous studies have indicated that a small fraction of Fe2+ in Fc can be oxidized to Fe3+ which thus can be still detected in Fc [31]. However, the XPS analysis reveals that Fe2+ is still the dominant species of Fc. In addition, Fig. 3(b) displays the core-level spectrum of C1s, in which a tall peak was observed at 284.3 eV, attributed to C–C bonds of Cp rings in Fc.
As degradation of amaranth was selected as a model reaction for examining activation capability of Fc for SPC, amaranth was possibly decolorized via adsorption to Fc. To elucidate this possibility, Fc alone was present in an amaranth solution and no amaranth was removed even after 120 min (Fig. 4(a)). This demonstrates that Fc itself could not decolorize or degrade amaranth. Fig. 4(a) also reveals that when SPC alone was present, very negligible amount of amaranth was removed (Ct /C0 ∼ 0.98), validating that SPC, without activation, was ineffective. However, once Fc was combined with SPC, amaranth was rapidly degraded and Ct /C0 approached zero within 80 min. The spectral variation of amaranth during degradation was determined and is displayed in Fig. S1. When the spectral pattern of amaranth remained consistent, the intensity of maximum absorbance at 520 nm was reduced with the reaction time, demonstrating that amaranth was degraded. Since SPC and Fc were incapable of degrading amaranth individually, this result indicates that SPC was activated in the presence of Fc. Furthermore, a hot filtration test was conducted to examine whether the activation of SPC by Fc was attributed the leaching of iron from Fc. In the hot filtration test, Fc was separated at the half of the reaction time (i.e., 60 min). Fig. 1(a) shows that amaranth degradation in the hot filtration was slowed down and almost stopped as a small fraction of amaranth still remained in the end of the reaction. This suggests that no significant amount of iron leached out from Fc and left in the solution for further activation of SPC. To further investigate respective roles of Fc and SPC during amaranth degradation, effects of Fc and SPC doses were further examined. Fig. 4(b) shows amaranth degradation by the combination of Fc and SPC at different Fc doses. As the Fc dose decreased significantly from 500 mg/L to 125 and 50 mg/L, Ct /C0 still reached below 0.2 after 120 min, showing that the degradation extents remained quite similar with much less Fc doses. However, the degradation kinetics was noticeably influenced. In order to distinguish the effect of Fc dose on degradation kinetics, the pseudo first order equation was adopted to interpret the kinetic data via the following equation (Eq. (2)):
Ct = C0 exp (−kobs t )
(2)
where kobs is the observed rate constant and calculated kobs values are summarized in Table S1. When Fc was 50 mg/L, kobs was 0.014 min−1 , which increased to 0.018 and 0035 min−1 at Fc = 125 and 500 mg/L, respectively. Even though Fc decreased significantly from 500 to 50 mg/L (i.e., 90% reduction), kobs merely changed from 0.035 to 0.014 min−1 . This suggests that Fc played a catalytic role during amaranth degradation as only small doses of Fc are needed to facilitate amaranth degradation. We also examined the effect of SPC does and Fig. 5(a) shows amaranth degradation by the combination of Fc and SPC at various doses of SPC. When SPC decreased from 1500 to 750 mg/L, the degradation extent and kinetics were both adversely influenced as Ct /C0 merely approached 0.4 and kobs reduced from 0.035 to 0.007 min−1 . When SPC was even further reduced to 150 mg/L, the corresponding Ct /C0 only reached 0.75. This indicates that degradation extent of amaranth was highly dependent on SPC dose because oxidation of amaranth was attributed to SPC instead of Fc. On the other hand, when SPC dose was doubled (i.e., 30 0 0 mg/L), the degradation extent remained the same but kobs increased to 0.040 min−1 , demonstrating that higher doses of SPC also facilitated the degradation kinetics. As SPC is a solid carrier of H2 O2 , adding SPC to water leads to generation of H2 O2 . It is well known that H2 O2 can be decomposed to form radicals (i.e., OH• ) in the presence of iron species, including Fe2+ . Since Fc consists of Fe2+ , Fe2+ is expected to react with
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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Fig. 4. Activation of SPC by Fc for degradation of amaranth: (a) a comparison between adsorption to Fc, SPC, Fc-activated SPC and hot filtration test (i.e., Fc was removed from the reaction solution at half of the reaction time); (b) effect of Fc dosage (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, T = 30 °C, Fc = 250 mg/L).
Fig. 5. Effects of (a) SPC dosage (T = 30 °C) and (b) temperature on degradation of amaranth by Fc-activated SPC (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, Fc = 250 mg/L).
H2 O2 to produce OH• , and Fe2+ is oxidized to Fe3+ simultaneously as follows (Eq.(3)) [4,7–13,32,33]:
Fe2+ @Fc+H2 O2 → Fe3+ @Fc + OH• +OH− .
(3)
The resulting Fe3+ then reacts with H2 O2 to return to Fe2+ via the following reaction (Eq. (4)):
Fe3+ @Fc+H2 O2 → Fe2+ @Fc+H+ +HO2 • . •
H+
(4) •−
HO2 possibly evolves into and O2 , which can react with Fe3+ to form O2 , while Fe3+ transforms back to Fe2+ [34]. Therefore, the transformation between Fe2+ and Fe3+ leads to the catalytic decomposition of H2 O2 and production of high oxidation potential radicals, such as OH• , for attacking amaranth molecules. This potential mechanism for activation of SPC by Fc can be schematically illustrated in Fig. 6. 3.3. Effects of temperature and pH In addition to the effects of Fc and SPC doses, we further investigated the effect of temperature. Fig. 5(b) shows amaranth degradation by Fc-activated SPC at different temperatures. While amaranth was fully degraded at all tested temperatures, degradation kinetics was substantially accelerated at elevated temperatures. As kobs was 0.035 min−1 at 30 °C, it increased to 0.078 min−1 at 40 °C and even further 0.198 min−1 at 50 °C, validating the enhancing effect of higher temperatures on degradation kinetics. This is possibly because higher temperatures facilitated mass transfer of reactants. Since kobs increased with temperature, the relationship between kobs and temperature was further correlated through the Arrhenius equation as follows (Eq. (5)):
In kobs = ln A − −Ea /RT
(5) (min–1 );
where A is the pre-exponential factor R is the universal gas constant; and T is the solution temperature in Kelvin (K). Based
Fig. 6. A proposed mechanism for activation of SPC by Fc.
on the Arrhenius equation, a plot of ln kobs versus 1/T is shown in Fig. S2, in which the data points are well fitted by linear regression with R2 = 0.996. This suggests that the relationship between kobs and temperature can be properly interpreted by Arrhenius equation and the corresponding activation energy was thus calculated as 70.4 kJ/mol. Furthermore, the effect of pH was also examined as pH has been considered as the most important factor for Fenton-like reactions. Fig. 7(a) reveals amaranth degradation by Fc-activated SPC at pH = 3, 6, 9 and 11. While degradation extents remained similar at various pH values, the corresponding kinetics were noticeably influenced. As kobs was 0.035 min−1 at pH = 6 (i.e., the pH-unadjusted case), it increased to 0.041 min−1 at pH = 9 and 0.061 min−1 at
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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Fig. 7. Effects of (a) pH and (b) NaCl on degradation of amaranth by Fc-activated SPC (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, Fc = 250 mg/L, T = 30 °C).
Fig. 8. (a) Effect of inhibitors on degradation of amaranth by Fc-activated SPC; and (b) recyclability of Fc for activating SPC (C0 of amaranth = 50 mg/L, SPC = 1500 mg/L, Fc = 250 mg/L, T = 30 °C, TBA = chloroform = DMSO = 1 M).
pH = 3 but decreased to 0.024 min−1 at pH = 11. This demonstrates that amaranth degradation by Fc-activated SPC was more favorable under relatively acidic conditions. It has been reported that OH• generated from Fenton-like reaction exhibits the highest oxidizing potential at pH = 3 [35]. Thus, the amaranth degradation by Fc-activated SPC at the acidic condition was the most effective. On the contrary, the oxidizing potential of OH• is greatly diminished under alkaline conditions [36]; therefore amaranth degradation became even less effective at pH = 11. 3.4. Effects of salts and inhibitors Since salts, especially NaCl, are usually added to enhance dyefixation performance [37–39], the effect of NaCl was further investigated. While amaranth concentration in this study was merely 50 mg/L, we particularly added much higher concentrations of NaCl to amaranth solutions to reveal its effect. Fig. 7(b) shows the effect of NaCl on amaranth degradation by Fc-activated SPC. When the concentration of NaCl was relatively low (i.e., 500 and 10 0 0 mg/L), no obvious effect was observed and the degradation extent remained the same. However, once the concentration of NaCl increased to 50 0 0 mg/L, the degradation extent and kinetics both substantially hindered. A previous study reveals that when the concentration of NaCl was lower than 10 mM (i.e., 580 mg/L), the adverse effect of NaCl on degradation using SPC is insignificant. Nevertheless, once the concentration of NaCl is higher than 10 0 mM (i.e., 580 0 mg/L), the degradation kinetics is remarkably slowed because the as-generated OH• was consumed by reaction with the excessive amount of Cl− [10]. Additionally, radical scavengers were also evaluated for their effects on amaranth degradation by Fc-activated SPC in order to elucidate degradation mechanism. As discussed earlier, OH• should be generated from SPC, and therefore a particular OH• scavenger, TBA,
was evaluated. Fig. 8(a) shows that amaranth degradation was substantially affected in the presence of TBA as kobs decreased from 0.035 to 0.014 min−1 , validating that OH• certainly involved in amaranth degradation. To validate the existence of OH• , another common OH• scavenger, DMSO, was also evaluated for its scavenging effect. The presence of DMSO indeed inhibited amaranth degradation (Fig. 8(a)) as kobs also lowered to 0.019 min−1 , confirming that the amaranth degradation by Fc-SPC involved with OH• . On the other hand, as discussed earlier, HO2 • can be also produced when H2 O2 reacts with Fe3+ @Fc. Since HO2 • can be further decomposed to H+ and O2 •− , chloroform, a radical scavenger specifically for O2 •− was also evaluated. Fig. 8(a) shows that the presence of chloroform slightly influenced the degradation kinetics as kobs decreased from 0.035 to 0.023 min−1 , suggesting the existence of O2 •− . Based on the effects of these radical scavengers, the mechanism of amaranth degradation can be primarily attributed to OH• and other reactive oxygen species to a lesser extent. 3.5. Recyclability of Fc for activating SPC As a heterogeneous catalyst, Fc should be more conveniently recovered after reactions. However, it is essential to investigate whether the recovered Fc can be reused for activating SPC. Thus, the long-term recyclability of Fc for SPC activation was evaluated by reusing Fc for activating SPC to degrade amaranth. Fig. 8(b) shows that amaranth was still fully degraded using SPC activated by used Fc and the degradation kinetics remained similar without significant delay. This demonstrates that Fc can be reused for activation of SPC even without regeneration treatments. On the other hand, reused Fc was characterized using XRD and FT-IR to examine whether the multi-cycle activation of SPC changed characteristics of Fc. Fig. 2(a) and (b) show that the reused Fc exhibited the consistent XRD pattern and IR spectrum to those
Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017
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of pristine Fc. Fig. 1(c) also reveals that used Fc retained its morphology as the filament-like and ribbon-line nanostructures were still maintained. This suggests that Fc remained intact and no obvious damage on Fc occurred even after the multi-cycle activation of SPC, validating that Fc can be a recyclable and stable heterogeneous catalyst for SPC activation. 4. Conclusion In this study, Fc was employed for the first time for activating the solid H2 O2 carrier, SPC, for degrading the toxic acidic azo dye, amaranth. While SPC and Fc could not degrade amaranth individually, the combination of SPC and Fc rapidly and effectively degrade amaranth, indicating that Fc can activate SPC. The activation of SPC by Fc can be attributed to Fe2+ contained in Fc; Fe2+ @Fc reacted with H2 O2 to generate OH• , which decolorize amaranth. The amaranth degradation by Fc-activated SPC can be further improved by elevating reaction temperature and lowering solution pH. Through investigating effects of radical scavengers, the mechanism of amaranth degradation was primarily attributed to OH• and other reactive oxygen species to a lesser extent. Fc was also reusable and remained highly effective for activating SPC to degrade amaranth even without regeneration treatments. These features validate that Fc, a readily available Fe2+ -containing compound, can be a promising heterogeneous catalyst for activating SPC in AOPs. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.05.017. References [1] Andreozzi R, Insola A, Caprio V, Marotta R, Tufano V. The use of manganese dioxide as a heterogeneous catalyst for oxalic acid ozonation in aqueous solution. Appl Catal A 1996;138:75–81. [2] Legube B, Karpel Vel Leitner N. Catalytic ozonation: a promising advanced oxidation technology for water treatment. Catal Today 1999;53:61–72. [3] Khaksar M, Amini M, Boghaei DM, Chae KH, Gautam S. Mn-doped ZrO2 nanoparticles as an efficient catalyst for green oxidative degradation of methylene blue. Catal Commun 2015;72:1–5. [4] Ali MEM, Gad-Allah TA. Badawy MI heterogeneous Fenton process using steel industry wastes for methyl orange degradation. Appl Water Sci 2013;3:263–70. [5] Velichkova F, Julcour-Lebigue C, Koumanova B, Delmas H. Heterogeneous Fenton oxidation of paracetamol using iron oxide (nano)particles. J Environ Chem Eng 2013;1:1214–22. [6] Amini M, Khaksar M, Ellern A, Keith WL. A new nanocluster polyoxomolybdate [Mo36O110(NO)4(H2O)14]•52H2O: synthesis, characterization and application in oxidative degradation of common organic dyes. Chin J Chem Eng 2017. doi:10.1016/j.cjche.2017.03.031. [7] Miao Z, Gu X, Lu S, Zang X, Wu X, Xu M, et al. Perchloroethylene (PCE) oxidation by percarbonate in Fe2+ -catalyzed aqueous solution: PCE performance and its removal mechanism. Chemosphere 2015;119:1120–5. [8] Fu X, Gu X, Lu S, Xu M, Miao Z, Zhang X, et al. Enhanced degradation of benzene in aqueous solution by sodium percarbonate activated with chelated-Fe(II). Chem Eng J 2016;285:180–8. [9] Cui H, Gu X, Lu S, Fu X, Zhang X, Fu GY, et al. Degradation of ethylbenzene in aqueous solution by sodium percarbonate activated with EDDS–Fe(III) complex. Chem Eng J 2017;309:80–8. [10] Fu X, Gu X, Lu S, Sharma VK, Brusseau ML, Xue Y, et al. Benzene oxidation by Fe(III)-activated percarbonate: matrix-constituent effects and degradation pathways. Chem Eng J 2017;309:22–9. [11] Fu X, Gu X, Lu S, Miao Z, Xu M, Zhang X, et al. Enhanced degradation of benzene by percarbonate activated with Fe(II)-glutamate complex. Environ Sci Pollut Res 2016;23:6758–66. [12] Danish M, Gu X, Lu S, Brusseau ML, Ahmad A, Naqvi M, et al. An efficient catalytic degradation of trichloroethene in a percarbonate system catalyzed by ultra-fine heterogeneous zeolite supported zero valent iron-nickel bimetallic composite. Appl Catal A: Gen 2017;531:177–86.
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Please cite this article as: K.-Y.A. Lin et al., Heterogeneous catalytic activation of percarbonate by ferrocene for degradation of toxic amaranth dye in water, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.05.017