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Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: Implications for the removal of organic pollutants
Accepted Manuscript Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: Implications for the removal of organic pollutants Fengya Zhou, Chao Lu, Yuyuan Yao, Lijie Sun, Fei Gong, Daiwen Li, Kemei Pei, Wangyang Lu, Wenxing Chen PII: DOI: Reference:
S1385-8947(15)01000-1 http://dx.doi.org/10.1016/j.cej.2015.07.034 CEJ 13932
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 April 2015 5 July 2015 11 July 2015
Please cite this article as: F. Zhou, C. Lu, Y. Yao, L. Sun, F. Gong, D. Li, K. Pei, W. Lu, W. Chen, Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: Implications for the removal of organic pollutants, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.07.034
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Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: Implications for the removal of organic pollutants Fengya Zhou, Chao Lu, Yuyuan Yao∗, Lijie Sun, Fei Gong, Daiwen Li, Kemei Pei, Wangyang Lu, Wenxing Chen∗∗
National Engineering Laboratory of Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University,
Hangzhou 310018, PR China
Abstract: The development of an efficient and green catalytic system for recalcitrant pollutant removal is an attractive yet challenging research topic in the field of environmental catalysis. In the present work, the use of activated carbon fibers (ACFs) as a novel and excellent metal-free catalyst is proposed for peracetic acid (PAA) activation, constructing a pro-environmental and efficient catalytic system for the removal of organic pollutants. In this system, ACFs could effectively activate PAA to remove the dye Reactive Brilliant Red X-3B (RR X-3B) over a wide pH range (3−11), exhibiting a remarkable pH-tolerant performance. Moreover, the ACFs also displayed excellent sustained catalytic ability and regeneration capability, avoiding secondary contamination. A hybrid method that combines various radical scavengers with electron paramagnetic resonance (EPR) technology has been used to confirm that both hydroxyl radicals (HO•) and alkoxyl radicals (CH3C(O)O•) were generated in the ACFs/PAA catalytic system. Furthermore, a combination of EPR with density functional theory calculations has been employed to evaluate the role of ACFs in the ACFs/PAA system. The results suggested that the introduction of ACFs facilitated homolytic cleavage of the O−OH bond, resulting in the generation of HO• and CH3C(O)O• for the effective removal of organic dyes. Based on these results and analyses, a possible mechanism is *
Corresponding author. Tel.: +86 571 86843810; fax: +86 571 86843255.
∗∗
Corresponding author. Tel.: +86 571 86843005; fax: +86 571 86843255. E-mail addresses: [email protected](Y. Yao), [email protected](W. Chen). 1
proposed. Keywords: activated carbon fibers, peracetic acid, alkoxyl radicals, metal-free catalyst 1. Introduction In the past decades, serious environmental pollution has led to extensive efforts towards the development of suitable techniques for the removal of harmful organic compounds from industrial and municipal wastewater [1-3]. Among various treatment methods, advanced oxidation processes (AOPs) have attracted great interest and have proved to be among the most promising technical options for the removal of pollutants [4-7]. In AOPs, hydrogen peroxide (H2 O2), as the most commonly used oxidant [2, 8], is considered as a preferred source of highly reactive oxidizing species (e.g., hydroxyl radicals (HO•)), which are capable of non-selectively and completely destroying recalcitrant organic contaminants [9]. Compared with the traditional oxidant, H2O2, peracetic acid (PAA) has a weaker O−OH bond, which can potentially lead to more intense radical production to attack target pollutants [10]. PAA has other desirable features, such as ease of treatment implementation, less dependence on pH, and an absence of persistent toxic or mutagenic residues or by-products [11-13]. Therefore, PAA may be an attractive and ideal alternative to H2O2 as a novel oxidant in AOPs, and the development of efficient and environmentally friendly approaches to activate PAA is of great significance. Many researchers are dedicated to developing effective catalysts for the activation of PAA, and considerable progress has been made. Transition metal ions have been reported as efficient homogeneous catalysts for the activation of PAA to generate active species for recalcitrant pollutant removal [14, 15]. However, the poor reusability and toxicity of metal catalysts are the main hindrances to such homogeneous processes [16]. To tackle these problems, heterogeneous catalysts such as metal oxides were employed, but catalyst deactivation and secondary contamination caused by metal leaching persistently occurred [16], 2
thus limiting further application of metal oxides in wastewater treatment engineering. Hence, a novel non-toxic and environmentally benign catalyst for activation of PAA would be highly desirable in the context of green and sustainable development. The applications of green, metal-free catalysts are milestone achievements of modern chemistry, and their proliferation looks assured because they can completely prevent leaching of the potentially toxic metal and secondary contamination of the water body [17-19]. Among them, carbon materials exhibit significant advantages in terms of activity, stability, and regenerability over traditional metal or metal oxide catalysts, and are characterized by cost-effectiveness and environmental benignity [20], making them attractive alternatives to conventional catalysts to meet the requirements of sustainable chemistry [21]. Activated carbon fibers (ACFs), a common form of carbon material, have been widely used as adsorbents or supports for other active materials in the field of water treatment because of their unique structure and intrinsic properties, which include high specific surface area and ease of surface modification [22-24]. However, research on the direct use of ACFs as catalysts remains comparatively limited. We envisaged that ACFs might serve as ideal metal-free catalysts to activate PAA for the removal of organic pollution, based on the following considerations. (i) The abundance of nonbonding free electrons and the versatility of surface functional groups may endow ACFs with a large number of active sites for accessibility and activation of the oxidant, generating active species by electron transfer [25]. (ii) The excellent chemical stability and inertness of ACFs allow retention of their original structure and catalytic activity under the attack of active species during oxidation reactions, thus improving their sustained catalytic ability and reusability [22]. (iii) The various forms (such as cloth, felt, monoliths, etc.) of ACFs can make them more amenable to recovery from reaction media and increase the possibility of their application in reactors of complex construction, which may be advantageous for their large-scale practical 3
application. Herein, we report the first deployment of ACFs as a metal-free catalyst to accelerate the activation of PAA for the removal of dyes. The catalytic properties of ACFs were evaluated in terms of the removal rates of dyes in the presence of PAA. A central composite design (CCD) matrix was constructed to assess the effects of the main variables (pH, temperature, ACFs dosage, PAA concentration). Various radical scavengers were employed in combination with electron paramagnetic resonance (EPR) technology to identify the active species involved in the ACFs/PAA system. These experiments were supplemented by density functional theory (DFT) calculations. On the basis of the obtained results, a possible catalytic mechanism is proposed. This study not only offers a novel insight for the application of ACFs as a metal-free catalyst, but also provides a viable process for efficient remediation of wastewater. 2. Experimental 2.1. Materials and reagents ACFs (specific area: 1700 m2/g) were provided by Jiangsu Sutong Carbon Fiber Co., Ltd. (Jiangsu, China). Peracetic acid (36%−40%) was purchased from Aladdin Reagent Co. (Shanghai, China). The model dye for degradation was Reactive Brilliant Red X-3B (RR X-3B), the molecular structure of which is shown in Fig. S1. The spin trapping reagent 4-hydroxy-5,5-dimethyl-2-trifluoromethylpyrroline 1-oxide (FDMPO) was purchased from Enzo Life Sciences (USA). All chemicals were used as received without further purification. Doubly-distilled water was used throughout this study. 2.2. Catalyst preparation ACFs were immersed in distilled water and the mixture was heated under reflux at 100 °C for 1 h to remove inorganic salts and other impurities. They were then collected, washed with distilled water, and dried at 80 °C for 12 h. 4
2.3. Experimental procedures and analysis The removal of dyes was carried out in a 150 mL glass beaker with the temperature controlled at 25 °C by means of a constant temperature shaker water bath (DSHZ-300A, Taicang, Jiangsu). A typical reaction mixture contained the following concentrations or initial amounts: dye (50 µM), ACFs (2 g/L), and PAA (5 mM). The aqueous solution was adjusted to different pH values with dilute aqueous solutions of sodium hydroxide (NaOH) or perchloric acid (HClO4). We assumed that the removal of RR X-3B followed pseudo-first-order kinetics. The values of apparent rate constant kobs for the catalytic reactions were obtained from the slopes of plots of ln(Ct/C0) versus time. At appropriate time intervals, samples were withdrawn and immediately analyzed on a UV/Vis spectrophotometer (Hitachi U-3010). EPR spectra of radicals trapped by FDMPO were recorded on a Bruker A300 spectrometer. EPR signals of liquid samples were recorded at ambient temperature. The settings for the EPR spectrometer were as follows: center field, 3520 G; sweep width, 100 G; microwave frequency, 9.77 GHz; modulation frequency, 100 kHz; power, 20.00 mW. EPR signals of ACF samples were recorded at 77 K. The settings were as follows: center field, 3300 G; sweep width, 500 G; microwave frequency, 9.43 GHz; modulation frequency, 100 kHz; power, 20.13 mW. 2.4. Experimental response surface methodology (RSM) RSM is an efficient technique for the optimization of a multivariable system [26-28]. In this study, optimal removal of RR X-3B was obtained by RSM using Design Expert 8.0. The four independent factors chosen were pH, temperature, ACFs dosage, and PAA concentration, with the removal rate of RR X-3B as the response. The ranges and levels of the independent variables are given in Table 1. CCD, which is the most frequently used form of RSM, was employed to evaluate the influence of the four independent variables in 30 sets of experiments [26]. 5
A second-order (quadratic) polynomial equation was used to fit the experimental results of CCD as follows: Y = +b0 + b1X1 + b2X2 + b3X3 + b4X4 + b12X1 X2 + b13X1X3 + b14X1X4 + b23X2X3 + b24X2X4 + b34X3X4 + b11X12 + b22X22 + b33X32 + b44X42 where Y represents the response variable (removal rate), bi, b ii, and bij are the regression coefficients for linear and quadratic effects and the coefficients of the interaction parameters, respectively, and Xi are the independent variables studied. 2.5. Computational methods Density functional theory (DFT) calculations were performed using the Gaussian-03 program. The B3LYP hybrid functional and the 3-21G basis set were combined to optimize the structures in this work. All of the DFT calculations were based on a simplified model and the adsorption effect was neglected. 3. Results and discussion 3.1. Oxidative removal of RR X-3B Control experiments were carried out under various conditions to compare the removal rates of RR X-3B. As shown in Fig. 1A (C0 and C are the initial concentration and the reaction concentration of RR X-3B, respectively), RR X-3B was scarcely removed in the presence of either H2O2 or PAA alone. With ACFs alone, 47.6% of RR X-3B was removed, which can be ascribed to the adsorption capacity of the ACFs. When both H2O2 and ACFs were present in an aqueous solution, only 54.4% of RR X-3B was removed in 45 min. However, to our surprise, the simultaneous presence of PAA and ACFs was much more efficient, with almost 97% removal of RR X-3B being observed after 45 min. The removal rate of RR X-3B was greatly enhanced when PAA was used in place of the traditional oxidant H2O2 , hence it was deemed of great interest to investigate the broader dye removal ability of the ACFs/PAA system. Indeed, 6
we observed that the ACFs/PAA system was also highly effective for removing other organic dyes of different colors and structures under the same conditions (Fig. 1B). Therefore, the non-selective catalytic removal ability of the ACFs/PAA system makes it ideally suited for practical organic wastewater treatment. 3.2. Sustaining catalytic ability and reusability of ACFs The performance of a catalyst in sustainable catalytic experiments is important for its application in environmental remediation. Therefore, a set of oxidation processes to remove RR X-3B were carried out sequentially by adding the same amount of RR X-3B with or without PAA to investigate the sustainable catalytic ability of ACFs. As shown in Fig. 2, the concentration of residual RR X-3B in the aqueous solution increased markedly by adding RR X-3B without PAA, and finally reached 199.4 µM. However, when the above experiments were repeated with PAA, the remaining amount of RR X-3B was only 4.0 µM. According to these experiments, we preliminarily proposed that the RR X-3B adsorbed in the ACFs was destroyed and then more RR X-3B could sequentially enter the ACFs from the solution, resulting in a remarkable decrease in the dye concentration in solution. Our results indicated that ACFs in the presence of PAA showed sustained catalytic activity for the oxidation of RR X-3B. On the basis of the above experiments, we conjectured that the removal of RR X-3B in the ACFs/PAA system involves two processes, namely adsorption of RR X-3B onto the ACFs and rapid in situ catalytic oxidation of the adsorbed dye. Therefore, it was essential to investigate whether catalytic oxidation could still take place effectively after different amounts of dye had been adsorbed on the ACFs. As can be seen in Fig. 3, in the presence of ACFs alone, the removal rate of RR X-3B decreased because the adsorption of ACFs was close to the saturation level. However, on adding PAA after adsorption for 15, 30, and 45 min, respectively, the concentrations of RR X-3B in each solution decreased dramatically, which further proved that the RR X-3B adsorbed on the ACFs was rapidly destroyed in situ and was promptly replaced by more 7
dye from the solution. These results implied that RR X-3B was adsorbed on ACFs in the vicinity of active sites, allowing rapid and effective catalytic oxidation. According to the above analysis, it was considered that ACFs retain the ability to regenerate their active sites, and that this is not affected by different amounts of adsorbed dye. In addition, the reusability of the catalyst is another major concern for practical application. Hence, the removal of RR X-3B from successive batches of the same solution under the same conditions was investigated to assess the reusability of the ACFs. The ACFs were recovered after each such experiment and rinsed with deionized water in order to remove the residual PAA. This process was repeated ten times. As can be seen in Fig. 4, the removal rate of RR X-3B showed no obvious decrease, remaining as high as 94% after the tenth cycle, suggesting that the ACFs retained stable catalytic activity and showed remarkable reuse performance. These experiments indicated that not only can ACFs be used for sustainable catalytic oxidation, but also that they have excellent reusability, making them suitable as a stable and environmentally friendly catalyst for PAA activation. 3.3. Analysis of response surface design The experimental matrix design and the responses based on the experiments proposed by the CCD for the removal of RR X-3B were provided by Design Expert 8.0 [26]. In this study, the four independent factors chosen were PAA concentration, ACFs dosage, pH, and temperature, with the removal rate of RR X-3B as the response. According to the experimental design, a second-order polynomial equation in terms of actual factors was established, which demonstrates the empirical relationships between the independent variables and the response: Removal rate = −1.665 + 0.13704X1 + 0.056911X2 + 0.11534X3 + 0.45804X4 − 0.00100235X1X2 + 0.00113593X1X3 − 0.00456936X1X4 − 0.00202296X2X3 + 0.000512477X2X4 + 0.00212755X3X4 − 8
0.00740956X12 − 0.000463107X2 2 − 0.0045482 X32 − 0.065873 X42 The analysis of variance (ANOVA) required to evaluate the adequacy of the model is elaborated in Table 1. In accordance with the ANOVA, the values of F and p for the model are 16.67 and <0.0001, respectively, suggesting that the prediction model is significant. The lack-of-fit F-value of 4.15 implies that the lack-of-fit is not significant relative to the pure error, which indicates good predictability of the model. In addition, the R2 value of 0.9396 is in reasonable agreement with the adjusted R2 value of 0.8832, further confirming good predictability of the model. The accuracy of the model is illustrated in Fig. S2, in which the measured values and the predicted responses of the model are compared. Based on the monomial coefficients of the regression model, p(X1) = 0.5761 (pH), p(X2) < 0.0001 (temperature), p(X3) = 0.0003 (PAA concentration), and p(X4) < 0.0001 (ACFs dosage), values of less than 0.05 indicate that model terms are significant, suggesting that temperature (X2) and ACFs dosage (X4) exert the greatest influences on the removal rate of RR X-3B, whereas pH has little impact. The combined effect of temperature and ACFs dosage is depicted in Fig. 5 as a response surface plot. It is clear that the removal rate could be enhanced by increasing either the ACFs dosage or the temperature. On the basis of the desirability function, optimization was performed to determine the optimal conditions for the removal of RR X-3B. Numerical optimization software was used to identify the specific point that maximizes the desirability function. To confirm the adequacy of the model for predicting the maximum removal percentage of RR X-3B, a verification experiment was carried out under the optimum conditions (pH 5.92, temperature 28.94 °C, PAA concentration 6.04 mM, and ACFs dosage 2.81 g/L). An average maximum removal of 92.5% obtained from three replicate experiments was close to the predicted value of 96.0%; the good agreement between the predicted and experimental values confirmed the validity of the model. Hence, further research was carried out according to the RSM optimization conditions, and 9
the test conditions were adjusted to pH 7, temperature 25 °C, PAA concentration 5 mM, and ACFs dosage 2 g/L under cost considerations. 3.4. Effect of initial pH Wastewater from the textile industry usually falls within a certain pH range, while some treatment methods are pH dependent [29, 30]. Accordingly, in order to investigate the effect of initial pH on the catalytic oxidation of RR X-3B, a series of experiments was performed at pH values in the range 3−11. As shown in Fig. 6, in the presence of PAA alone, less than 3% of RR X-3B was removed in 45 min, indicating that the dye was stable and hardly decomposed over a wide pH range. With ACFs alone, the removal rates were 53.85%, 53.54%, 53.31%, 51.56%, and 48.47% at pH values of 3, 5, 7, 9, and 11, respectively, which suggests that there were no remarkable changes in the adsorption capability of the ACFs on going from acidic to alkaline conditions. In addition, at the same pH values, the ACFs/PAA system achieved removal rates of 97.6%, 98.7%, 98.9%, 97.7%, and 98.4%, respectively. These results revealed that the ACFs/PAA system exhibited excellent pH tolerance over a wide range, which further demonstrated that pH plays a minor role in this catalytic reaction. Compared with other systems using metal-free carbon materials as catalysts, the ACFs/PAA system displays remarkable catalytic removal ability under neutral and alkaline conditions, thereby avoiding the need for additional acidic or alkaline reactants, and reducing costs as well as environmental and operational risks. 3.5. Effect of NaCl Aquatic environments are rich in various dissolved salts, and sodium chloride (NaCl) is the most commonly found salt in industrial wastewater [31, 32]. Therefore, it was of crucial importance to investigate the effect of NaCl on the ACFs/PAA system. Fig. 7 shows that in the presence of both PAA and ACFs, the removal rate of RR X-3B decreased remarkably with increasing concentration of NaCl from 0 to 10
0.1 M. To our surprise, however, with further addition of NaCl, the removal of RR X-3B was significantly enhanced. When the concentration of NaCl exceeded 0.3 M, the removal rate was even larger than that without NaCl. A similar dual effect of chloride on dye removal has previously been observed in other AOP systems, such as Co/PMS [31]. This interesting phenomenon may be rationalized as follows. At low NaCl concentration, the active species involved in this system reacts with Cl− to generate the less reactive Cl2−• radical, resulting in inhibition of the dye removal process, whereas at high concentration (0.3 M) the reaction of Cl− with PAA proceeds to form HClO, accelerating the dye removal rate. In contrast to the common phenomenon reported in most AOP technologies [33, 34], whereby NaCl usually exerts a negative effect on the catalytic oxidation process, in the ACFs/PAA system it has a positive effect at high concentrations, resulting in a higher dye removal rate. Thus, the present system would seem to be particularly well suited for treating chloride-rich effluent. 3.6. Catalytic oxidation mechanism 3.6.1. Identification of active species Ascorbic acid (AA), a typical radical scavenger, was used to preliminarily test whether radical species were generated in the ACFs/PAA system [35, 36]. As shown in Fig. 8, the removal rate of RR X-3B in the presence of PAA decreased from 98% to 66.01% in 45 min, and even the corresponding kobs decreased sharply as the amount of ascorbic acid was increased, indicating that radical species might be involved in this system. Moreover, when excess ascorbic acid was present, the removal rate of RR X-3B in the ACFs/PAA/AA catalytic system was almost consistent with that in the absence of PAA, suggesting that the removal rate was mainly attributed to adsorption rather than the oxidation reaction in the presence of ascorbic acid (Fig. 1). These results demonstrated that the addition of ascorbic acid led to an obvious inhibition of the dye removal, indicating that radical species played a major role in the removal process of 11
RR X-3B in the ACFs/PAA system. Besides, with the aim of investigating the possible radical species involved in the ACFs/PAA and ACFs/H2O2 systems, tert-butyl alcohol (TBA), a common hydroxyl radical scavenger [37, 38], was used to investigate whether hydroxyl radical was generated in these catalytic systems. As shown in Fig. 9, 41.37% of RR X-3B was eliminated in the ACFs/TBA system after 45 min. In addition, about 54.4% and 39.3% of RR X-3B were removed in the ACFs/H2O2 and ACFs/H2 O2/TBA systems, respectively, suggesting that the addition of tert-butyl alcohol to the ACFs/H2O2 system conspicuously inhibited the removal of RR X-3B. Hence, we speculate that hydroxyl radical may play a dominant role in the ACFs/H2O2 system. However, when the same amount of tert-butyl alcohol was added to the ACFs/PAA system, only a slight decrease in the removal rate of RR X-3B was observed. Moreover, the removal rate of RR X-3B remained almost unchanged on further increasing the TBA concentration (Fig. S5), implying that hydroxyl radicals are not the sole active species in the ACFs/PAA system. On the basis of the aforementioned detection of hydroxyl radicals in the ACFs/PAA system, we hypothesized that the initiation reaction involves cleavage of the peroxy bond of PAA into two primary radicals: alkoxyl (CH3C(O)O•) and hydroxyl (HO•). To verify this hypothesis, electron paramagnetic resonance (EPR) spin-trapping was employed to investigate whether CH3C(O)O• radicals were generated in this catalytic system. Fig. 10 shows the EPR spectrum of the FDMPO adduct recorded for the ACFs/PAA system. The spectral shape corresponds to the superposition of two spectra originating from two different radical species. From the spectrum, it can be observed that the ACFs/PAA system could form a typical triplet of 1:3:3:1 quartet signals of hydroxyl adducts and carbon-centered adducts on the basis of their hyperfine coupling constants (FDMPO/•OH adduct: αF = 2.60 G, αN = 13.7 G; FDMPO/•CH3 C(O)O adduct: αF = 2.46 G, αN = 13.8 G), which is in agreement with a previous report [39]. This result confirms 12
our above inference and indicates that both alkoxyl radicals and hydroxyl radicals were generated during the catalytic reaction. 3.6.2. The role of ACFs during the catalytic reaction The above findings suggested that ACFs play a pivotal role in removing dyes during the catalytic reaction. Hence, it was deemed necessary to investigate the role of ACFs in the catalytic oxidation reaction. To the best of our knowledge, activated carbon fibers have a turbostratic disordered graphite-like structure with an abundance of nonbonding free electrons [23, 25]. On the basis of previous structural analysis of ACFs, we speculated that the free electrons might play an important role during the catalytic reaction. To support this, electron paramagnetic resonance (EPR) spin-trapping was employed to investigate changes in the free electron density on the ACFs before and after the reaction. As depicted in Fig. 11, a narrow singlet signal located at 3365 G with a g-factor of 2.0014−2.0015 was observed in the samples before and after the reaction, typical of a carbon-centered radical [40]. This indicated the presence of unpaired free electrons in the ACFs. Moreover, it is noteworthy that the signal intensity observed for the ACFs clearly decreased after 40 min, indicating that the amount of unpaired free electrons decreased markedly over this reaction time. As any oxidation process involves the transfer of electrons, we speculated that unpaired free electrons in the ACFs might be transferred to PAA to promote the generation of radical species in the oxidation reaction, thus accounting for the remarkable catalytic activity of the ACFs/PAA system. To further explore the effect of ACFs in the ACFs/PAA system from the perspective of PAA, we modeled the reaction process by DFT using the B3LYP hybrid functional with the 3-21G basis set. In homolytic cleavage, a chemical bond of a neutral molecule is broken with the generation of two radicals [39], which can be assessed by the bond length. As shown in Fig. 12, the length of the CH3C(=O)O−OH bond (between two oxygen atoms) is 1.52141 Å, implying that spontaneous homolysis of PAA is highly 13
unlikely. However, for PAA in the presence of ACFs, the bond distance is distinctly elongated to 1.5516 Å, favoring the homolysis of PAA. Therefore, it was clearly demonstrated that ACFs can exert a significant influence on the homolytic cleavage of PAA for the generation of the radical species. Based on the above analysis, a mechanism for the catalytic activation of PAA is proposed as follows. The unique structure of ACFs offers a convenient and feasible pathway for transfer of their nonbonding electrons to PAA. Electron transfer may induce elongation of the CH3C(=O)O−OH bond (between two oxygen atoms) in PAA, accelerating its homolytic cleavage to generate a greater amount of alkoxyl and hydroxyl radicals. These can then attack dye molecules in the ACFs/PAA system, leading to their rapid destruction. 4. Conclusion In summary, this study has demonstrated the feasibility of using ACFs as a metal-free catalyst for PAA activation as a novel strategy for wastewater treatment. Emphasis has been placed on examining the dye removal in the ACFs/PAA system, the effects of different variables, and the reaction mechanism. The collected experimental data indicate the following. (i) PAA can be effectively activated by ACFs to remove RR X-3B, which generates no secondary pollution due to the absence of a metal catalyst. (ii) ACFs exhibit excellent sustained catalytic ability and reuse capability, indicating that the described catalytic system is green and environmentally benign. (iii) The ACFs/PAA system displays remarkable pH-tolerant performance without an obvious decrease in catalytic ability over a wide pH range (3−11). (iv) The introduction of ACFs facilitates homolytic cleavage of the O−O bond in CH3C(=O)O−OH to generate hydroxyl and alkoxyl radicals. These valuable findings provide an insight into ACFs-based metal-free catalysts, which should contribute to the promising research field of green, metal-free catalysts for use in environmental remediation. 14
Acknowledgments This work was supported by the State Key Program of National Natural Science of China (No. 51133006), the National Natural Science Foundation of China (No. 51103133, 51302246) and Zhejiang Provincial Natural Science Foundation of China (No. LY14E030013, LY14E030015).
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76 (2014) 435 - 445. [26] M. Vaez, A.Z. Moghaddam, S. Alijani, Optimization and modeling of photocatalytic degradation of azo dye using a response surface methodology (RSM) based on the central composite design with immobilized titania nanoparticles, Ind. Eng. Chem. Res. 51 (2012) 4199 - 4207. [27] S. Chatterjee, A. Kumar, S. Basu, S. Dutta, Application of response surface methodology for methylene blue dye removal from aqueous solution using low cost adsorbent, Chem. Eng. J. 181-182 (2012) 289 - 299. [28] F. Torrades. S. Saiz. J.A. García-Hortal, Using central composite experimental design to optimize the degradation of black liquor by Fenton reagent, Desalination. 268 (2011) 97 - 102. [29] Y.H. Guan, J. Ma, X.C. Li, J.Y. Fang, L.W. Chen, Influence of pH on the formation of sulfate and hydroxyl radicals in the UV/peroxymonosulfate system, Environ. Sci. Technol. 45 (2011) 9308 - 9314. [30] F. Ji, C.L. Li, X.Y. Wei, J. Efficient performance of porous Fe2O3 in heterogeneous activation of peroxymonosulfate for decolorization of Rhodamine B, Chem. Eng. J. 231 (2013) 434 - 440. [31] R.X. Yuan, S.N. Ramjaun, Z.H. Wang, J.S. Liu, Effects of chloride ion on degradation of Acid Orange 7 by sulfate radical-based advanced oxidation process: Implications for formation of chlorinated aromatic compounds, J. Hazard. Mater. 196 (2011) 173 - 179. [32] Y. Miao, R.H. Liao, X.X. Zhang, B. Liu, Y. Li, B. Wu, A.M. Li, Metagenomic insights into salinity effect on diversity and abundance of denitrifying bacteria and genes in an expanded granular sludge bed reactor treating high-nitrate wastewater, Chem. Eng. J. 277 (2015) 116 - 123. [33] W.X. Chen, W.Y. Lu, Y.Y. Yuan, M.H. Xu, Highly Efficient Decomposition of organic dyes by aqueous-fiber phase transfer and in Situ catalytic oxidation using Fiber-Supported cobalt phthalocyanine, Environ. Sci. Technol. 41 (2007) 6240 - 6245. 18
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Tables and figures Table 1. ANOVA for analysis of variance and adequacy of the quadratic model. Fig. 1. (A) Concentration changes of RR X-3B (50 µM) under different conditions: (a) H2 O2 (5 mM); (b) 19
PAA (5 mM); (c) ACFs (2 g/L); (d) ACFs (2 g/L) with H2 O2 (5 mM); (e) ACFs (2 g/L) with PAA (5 mM); (B) digital photographs of three primary color dyes (AR1, AO7, and BG1) before (A) and after (B) oxidation in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 2. Concentration changes of RR X-3B (initial concentration: 50 µM) with continuous addition of RR X-3B with PAA (initial concentration: 5 mM) or without PAA (pH 7.0, T = 25 °C). The four subsequent identical additions of RR X-3B finally resulted in 250 µM RR X-3B and 25 mM PAA. Fig. 3. Concentration changes of RR X-3B (50 µM) without PAA or adding PAA (5 mM) after adsorption for 5, 15, 30, and 45 min, respectively (pH 7.0, T = 25 °C). Fig. 4. Repeated recycling of ACFs (2 g/L) with PAA (5 mM) for the removal of RR X-3B (50 µM) (pH 7.0, T = 25 °C). Fig. 5. Response surface plot for removal of RR X-3B showing interaction between temperature (°C) and ACFs dosage (g/L) (pH 7.0, initial PAA concentration 5 mM). Fig. 6. Effect of initial pH on the removal of RR X-3B (50 µM) under different conditions: (a) ACFs (2 g/L); (b) ACFs (2 g/L) and PAA (5 mM); and (c) PAA (5 mM) (T = 25 °C). Fig. 7. Effects of different dosages of NaCl on the removal rate and the kobs of RR X-3B removal in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 8. Influence of ascorbic acid (AA) concentration on the apparent rate constant kobs for the removal of RR X-3B in the ACFs/PAA system. The inset shows concentration changes of RR X-3B under different conditions: (a) ACFs (2 g/L), ascorbic acid (5 mM); (b) ACFs (2 g/L), PAA (5 mM), ascorbic acid (5 mM); (c) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C) Fig. 9. Influence of tert-butyl alcohol (TBA) on the removal of RR X-3B (50 µM) in the ACFs/H2O2 and ACFs/PAA systems: (a) ACFs (2 g/L), H2O2 (5 mM), TBA (500 mM); (b) ACFs (2 g/L), TBA (500 mM), 20
(c) ACFs (2 g/L), H2O2 (5 mM); (d) ACFs (2 g/L), PAA (5 mM), TBA (500 mM); (e) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 10. Experimental EPR spectrum of FDMPO adduct for the removal of RR X-3B (50 µM) in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 11. EPR spectra of ACFs at 0 and 40 min. Conditions: [RR X-3B] = 50 µM, ACFs = 2 g/L and PAA = 5 mM, pH 7, T = 25 °C. Fig. 12. Structures of PAA in the simplified model of DFT calculations: (A) in the model of PAA alone; (B) in the model of PAA with ACFs.
21
Fig. 1. (A) Concentration changes of RR X-3B (50 µM) under different conditions: (a) H2O2 (5 mM); (b) PAA (5 mM); (c) ACFs (2 g/L); (d) ACFs (2 g/L) with H2 O2 (5 mM); (e) ACFs (2 g/L) with PAA (5 mM); (B) the digital picture of three primary color dyes (AR1, AO7 and BG1) before (Ⅰ) and after (Ⅱ) oxidation in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C).
22
Fig. 2. Concentration changes of RR X-3B (initial concentration: 50 µM) with continuous addition of RR X-3B with PAA (initial concentration: 5 mM) or without PAA (pH 7.0, T = 25 °C). The four subsequent identical additions of RR X-3B resulted finally in 250 µM RR X-3B and 25 mM PAA.
23
Fig. 3. Concentration changes of RR X-3B (50 µM) without PAA or adding PAA (5 mM) after adsorption for 5, 15, 30, 45 min, respectively (pH 7.0, T = 25 °C).
24
Fig. 4. Repeated recycling of ACFs (2 g/L) with PAA (5 mM) for the removal of RR X-3B (50 µM) (pH 7.0, T = 25 °C).
25
Fig. 5. Response surface plot for removal of RR X-3B shows interaction between temperature (°C) and ACFs dosage (g/L) (pH 7.0, initial PAA concentration 5mM).
26
Fig. 6. Effect of initial pH on the removal of RR X-3B (50 µM) under different conditions: (a) ACFs (2 g/L); (b) ACFs (2 g/L) and PAA (5 mM); and (c) PAA (5 mM) (T = 25 °C).
27
Fig. 7. Effects of different dosages of NaCl on the removal rate and the kobs of RR X-3B removal in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C).
28
Fig. 8. Influence of ascorbic acid (AA) concentration on the apparent rate constant kobs for the removal of RR X-3B in ACFs/PAA system. The inset shows concentration changes of RR X-3B under different conditions : (a) ACFs (2 g/L), ascorbic acid (5 mM); (b) ACFs (2 g/L), PAA (5 mM), ascorbic acid (5 mM); (c) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C).
29
Fig. 9. Influence of tert-butyl alcohol (TBA) on the removal of RR X-3B (50 µM) in ACFs/H2O2 system and ACFs/PAA system: (a) ACFs (2 g/L), H2O2 (5 mM), TBA(500 mM); (b) ACFs (2 g/L), TBA(500 mM), (c) ACFs (2 g/L), H2O2 (5 mM); (d) ACFs (2 g/L), PAA (5 mM), TBA(500 mM); (e) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C).
30
Fig. 10. Experimental EPR spectrum of FDMPO adduct for the removal of RR X-3B (50 µM) in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C).
31
Fig. 11. EPR spectra of ACFs at 0 and 40 min. Conditions: [RR X-3B] = 50 µM, ACFs = 2 g/L and PAA = 5 mM, pH 7, T = 25 °C.
32
Fig. 12. Structures of PAA in the simplified model of DFT calculations: (A) in the model of sole PAA; (B) in the model of PAA with ACFs.
33
Table 1 ANOVA for analysis of variance and adequacy of the quadratic model.
Source Model
A-pH B-Temp C-PAA D-ACF Lack of Fit Pure Error
Sum of Squares
Degree of freedom
Mean Square
1.57 0.0022 0.42 0.15 0.83 0.090 0.011
14 1 1 1 1 10 5
0.11 0.0022 0.42 0.15 0.83 0.00903 0.00217
34
F-Value
Prob > F
16.67 0.33 62.36 22.09 123.13 4.15
< 0.0001 0.5761 < 0.0001 0.0003 < 0.0001 0.0649
Graphical abstract
35
Highlights: 1. An efficient and green metal-free catalytic system of ACFs/PAA was developed for pollutants removal 2. ACFs/PAA presented a remarkable pH-tolerant performance with enough activity. 3. ACFs/PAA exhibited the excellent sustained catalytic ability and recycling regeneration capability. 4. Both alkoxyl radicals and hydroxyl radicals were generated in the catalytic reaction.
36
S1385-8947(15)01000-1 http://dx.doi.org/10.1016/j.cej.2015.07.034 CEJ 13932
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 April 2015 5 July 2015 11 July 2015
Please cite this article as: F. Zhou, C. Lu, Y. Yao, L. Sun, F. Gong, D. Li, K. Pei, W. Lu, W. Chen, Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: Implications for the removal of organic pollutants, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.07.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: Implications for the removal of organic pollutants Fengya Zhou, Chao Lu, Yuyuan Yao∗, Lijie Sun, Fei Gong, Daiwen Li, Kemei Pei, Wangyang Lu, Wenxing Chen∗∗
National Engineering Laboratory of Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University,
Hangzhou 310018, PR China
Abstract: The development of an efficient and green catalytic system for recalcitrant pollutant removal is an attractive yet challenging research topic in the field of environmental catalysis. In the present work, the use of activated carbon fibers (ACFs) as a novel and excellent metal-free catalyst is proposed for peracetic acid (PAA) activation, constructing a pro-environmental and efficient catalytic system for the removal of organic pollutants. In this system, ACFs could effectively activate PAA to remove the dye Reactive Brilliant Red X-3B (RR X-3B) over a wide pH range (3−11), exhibiting a remarkable pH-tolerant performance. Moreover, the ACFs also displayed excellent sustained catalytic ability and regeneration capability, avoiding secondary contamination. A hybrid method that combines various radical scavengers with electron paramagnetic resonance (EPR) technology has been used to confirm that both hydroxyl radicals (HO•) and alkoxyl radicals (CH3C(O)O•) were generated in the ACFs/PAA catalytic system. Furthermore, a combination of EPR with density functional theory calculations has been employed to evaluate the role of ACFs in the ACFs/PAA system. The results suggested that the introduction of ACFs facilitated homolytic cleavage of the O−OH bond, resulting in the generation of HO• and CH3C(O)O• for the effective removal of organic dyes. Based on these results and analyses, a possible mechanism is *
Corresponding author. Tel.: +86 571 86843810; fax: +86 571 86843255.
∗∗
Corresponding author. Tel.: +86 571 86843005; fax: +86 571 86843255. E-mail addresses: [email protected](Y. Yao), [email protected](W. Chen). 1
proposed. Keywords: activated carbon fibers, peracetic acid, alkoxyl radicals, metal-free catalyst 1. Introduction In the past decades, serious environmental pollution has led to extensive efforts towards the development of suitable techniques for the removal of harmful organic compounds from industrial and municipal wastewater [1-3]. Among various treatment methods, advanced oxidation processes (AOPs) have attracted great interest and have proved to be among the most promising technical options for the removal of pollutants [4-7]. In AOPs, hydrogen peroxide (H2 O2), as the most commonly used oxidant [2, 8], is considered as a preferred source of highly reactive oxidizing species (e.g., hydroxyl radicals (HO•)), which are capable of non-selectively and completely destroying recalcitrant organic contaminants [9]. Compared with the traditional oxidant, H2O2, peracetic acid (PAA) has a weaker O−OH bond, which can potentially lead to more intense radical production to attack target pollutants [10]. PAA has other desirable features, such as ease of treatment implementation, less dependence on pH, and an absence of persistent toxic or mutagenic residues or by-products [11-13]. Therefore, PAA may be an attractive and ideal alternative to H2O2 as a novel oxidant in AOPs, and the development of efficient and environmentally friendly approaches to activate PAA is of great significance. Many researchers are dedicated to developing effective catalysts for the activation of PAA, and considerable progress has been made. Transition metal ions have been reported as efficient homogeneous catalysts for the activation of PAA to generate active species for recalcitrant pollutant removal [14, 15]. However, the poor reusability and toxicity of metal catalysts are the main hindrances to such homogeneous processes [16]. To tackle these problems, heterogeneous catalysts such as metal oxides were employed, but catalyst deactivation and secondary contamination caused by metal leaching persistently occurred [16], 2
thus limiting further application of metal oxides in wastewater treatment engineering. Hence, a novel non-toxic and environmentally benign catalyst for activation of PAA would be highly desirable in the context of green and sustainable development. The applications of green, metal-free catalysts are milestone achievements of modern chemistry, and their proliferation looks assured because they can completely prevent leaching of the potentially toxic metal and secondary contamination of the water body [17-19]. Among them, carbon materials exhibit significant advantages in terms of activity, stability, and regenerability over traditional metal or metal oxide catalysts, and are characterized by cost-effectiveness and environmental benignity [20], making them attractive alternatives to conventional catalysts to meet the requirements of sustainable chemistry [21]. Activated carbon fibers (ACFs), a common form of carbon material, have been widely used as adsorbents or supports for other active materials in the field of water treatment because of their unique structure and intrinsic properties, which include high specific surface area and ease of surface modification [22-24]. However, research on the direct use of ACFs as catalysts remains comparatively limited. We envisaged that ACFs might serve as ideal metal-free catalysts to activate PAA for the removal of organic pollution, based on the following considerations. (i) The abundance of nonbonding free electrons and the versatility of surface functional groups may endow ACFs with a large number of active sites for accessibility and activation of the oxidant, generating active species by electron transfer [25]. (ii) The excellent chemical stability and inertness of ACFs allow retention of their original structure and catalytic activity under the attack of active species during oxidation reactions, thus improving their sustained catalytic ability and reusability [22]. (iii) The various forms (such as cloth, felt, monoliths, etc.) of ACFs can make them more amenable to recovery from reaction media and increase the possibility of their application in reactors of complex construction, which may be advantageous for their large-scale practical 3
application. Herein, we report the first deployment of ACFs as a metal-free catalyst to accelerate the activation of PAA for the removal of dyes. The catalytic properties of ACFs were evaluated in terms of the removal rates of dyes in the presence of PAA. A central composite design (CCD) matrix was constructed to assess the effects of the main variables (pH, temperature, ACFs dosage, PAA concentration). Various radical scavengers were employed in combination with electron paramagnetic resonance (EPR) technology to identify the active species involved in the ACFs/PAA system. These experiments were supplemented by density functional theory (DFT) calculations. On the basis of the obtained results, a possible catalytic mechanism is proposed. This study not only offers a novel insight for the application of ACFs as a metal-free catalyst, but also provides a viable process for efficient remediation of wastewater. 2. Experimental 2.1. Materials and reagents ACFs (specific area: 1700 m2/g) were provided by Jiangsu Sutong Carbon Fiber Co., Ltd. (Jiangsu, China). Peracetic acid (36%−40%) was purchased from Aladdin Reagent Co. (Shanghai, China). The model dye for degradation was Reactive Brilliant Red X-3B (RR X-3B), the molecular structure of which is shown in Fig. S1. The spin trapping reagent 4-hydroxy-5,5-dimethyl-2-trifluoromethylpyrroline 1-oxide (FDMPO) was purchased from Enzo Life Sciences (USA). All chemicals were used as received without further purification. Doubly-distilled water was used throughout this study. 2.2. Catalyst preparation ACFs were immersed in distilled water and the mixture was heated under reflux at 100 °C for 1 h to remove inorganic salts and other impurities. They were then collected, washed with distilled water, and dried at 80 °C for 12 h. 4
2.3. Experimental procedures and analysis The removal of dyes was carried out in a 150 mL glass beaker with the temperature controlled at 25 °C by means of a constant temperature shaker water bath (DSHZ-300A, Taicang, Jiangsu). A typical reaction mixture contained the following concentrations or initial amounts: dye (50 µM), ACFs (2 g/L), and PAA (5 mM). The aqueous solution was adjusted to different pH values with dilute aqueous solutions of sodium hydroxide (NaOH) or perchloric acid (HClO4). We assumed that the removal of RR X-3B followed pseudo-first-order kinetics. The values of apparent rate constant kobs for the catalytic reactions were obtained from the slopes of plots of ln(Ct/C0) versus time. At appropriate time intervals, samples were withdrawn and immediately analyzed on a UV/Vis spectrophotometer (Hitachi U-3010). EPR spectra of radicals trapped by FDMPO were recorded on a Bruker A300 spectrometer. EPR signals of liquid samples were recorded at ambient temperature. The settings for the EPR spectrometer were as follows: center field, 3520 G; sweep width, 100 G; microwave frequency, 9.77 GHz; modulation frequency, 100 kHz; power, 20.00 mW. EPR signals of ACF samples were recorded at 77 K. The settings were as follows: center field, 3300 G; sweep width, 500 G; microwave frequency, 9.43 GHz; modulation frequency, 100 kHz; power, 20.13 mW. 2.4. Experimental response surface methodology (RSM) RSM is an efficient technique for the optimization of a multivariable system [26-28]. In this study, optimal removal of RR X-3B was obtained by RSM using Design Expert 8.0. The four independent factors chosen were pH, temperature, ACFs dosage, and PAA concentration, with the removal rate of RR X-3B as the response. The ranges and levels of the independent variables are given in Table 1. CCD, which is the most frequently used form of RSM, was employed to evaluate the influence of the four independent variables in 30 sets of experiments [26]. 5
A second-order (quadratic) polynomial equation was used to fit the experimental results of CCD as follows: Y = +b0 + b1X1 + b2X2 + b3X3 + b4X4 + b12X1 X2 + b13X1X3 + b14X1X4 + b23X2X3 + b24X2X4 + b34X3X4 + b11X12 + b22X22 + b33X32 + b44X42 where Y represents the response variable (removal rate), bi, b ii, and bij are the regression coefficients for linear and quadratic effects and the coefficients of the interaction parameters, respectively, and Xi are the independent variables studied. 2.5. Computational methods Density functional theory (DFT) calculations were performed using the Gaussian-03 program. The B3LYP hybrid functional and the 3-21G basis set were combined to optimize the structures in this work. All of the DFT calculations were based on a simplified model and the adsorption effect was neglected. 3. Results and discussion 3.1. Oxidative removal of RR X-3B Control experiments were carried out under various conditions to compare the removal rates of RR X-3B. As shown in Fig. 1A (C0 and C are the initial concentration and the reaction concentration of RR X-3B, respectively), RR X-3B was scarcely removed in the presence of either H2O2 or PAA alone. With ACFs alone, 47.6% of RR X-3B was removed, which can be ascribed to the adsorption capacity of the ACFs. When both H2O2 and ACFs were present in an aqueous solution, only 54.4% of RR X-3B was removed in 45 min. However, to our surprise, the simultaneous presence of PAA and ACFs was much more efficient, with almost 97% removal of RR X-3B being observed after 45 min. The removal rate of RR X-3B was greatly enhanced when PAA was used in place of the traditional oxidant H2O2 , hence it was deemed of great interest to investigate the broader dye removal ability of the ACFs/PAA system. Indeed, 6
we observed that the ACFs/PAA system was also highly effective for removing other organic dyes of different colors and structures under the same conditions (Fig. 1B). Therefore, the non-selective catalytic removal ability of the ACFs/PAA system makes it ideally suited for practical organic wastewater treatment. 3.2. Sustaining catalytic ability and reusability of ACFs The performance of a catalyst in sustainable catalytic experiments is important for its application in environmental remediation. Therefore, a set of oxidation processes to remove RR X-3B were carried out sequentially by adding the same amount of RR X-3B with or without PAA to investigate the sustainable catalytic ability of ACFs. As shown in Fig. 2, the concentration of residual RR X-3B in the aqueous solution increased markedly by adding RR X-3B without PAA, and finally reached 199.4 µM. However, when the above experiments were repeated with PAA, the remaining amount of RR X-3B was only 4.0 µM. According to these experiments, we preliminarily proposed that the RR X-3B adsorbed in the ACFs was destroyed and then more RR X-3B could sequentially enter the ACFs from the solution, resulting in a remarkable decrease in the dye concentration in solution. Our results indicated that ACFs in the presence of PAA showed sustained catalytic activity for the oxidation of RR X-3B. On the basis of the above experiments, we conjectured that the removal of RR X-3B in the ACFs/PAA system involves two processes, namely adsorption of RR X-3B onto the ACFs and rapid in situ catalytic oxidation of the adsorbed dye. Therefore, it was essential to investigate whether catalytic oxidation could still take place effectively after different amounts of dye had been adsorbed on the ACFs. As can be seen in Fig. 3, in the presence of ACFs alone, the removal rate of RR X-3B decreased because the adsorption of ACFs was close to the saturation level. However, on adding PAA after adsorption for 15, 30, and 45 min, respectively, the concentrations of RR X-3B in each solution decreased dramatically, which further proved that the RR X-3B adsorbed on the ACFs was rapidly destroyed in situ and was promptly replaced by more 7
dye from the solution. These results implied that RR X-3B was adsorbed on ACFs in the vicinity of active sites, allowing rapid and effective catalytic oxidation. According to the above analysis, it was considered that ACFs retain the ability to regenerate their active sites, and that this is not affected by different amounts of adsorbed dye. In addition, the reusability of the catalyst is another major concern for practical application. Hence, the removal of RR X-3B from successive batches of the same solution under the same conditions was investigated to assess the reusability of the ACFs. The ACFs were recovered after each such experiment and rinsed with deionized water in order to remove the residual PAA. This process was repeated ten times. As can be seen in Fig. 4, the removal rate of RR X-3B showed no obvious decrease, remaining as high as 94% after the tenth cycle, suggesting that the ACFs retained stable catalytic activity and showed remarkable reuse performance. These experiments indicated that not only can ACFs be used for sustainable catalytic oxidation, but also that they have excellent reusability, making them suitable as a stable and environmentally friendly catalyst for PAA activation. 3.3. Analysis of response surface design The experimental matrix design and the responses based on the experiments proposed by the CCD for the removal of RR X-3B were provided by Design Expert 8.0 [26]. In this study, the four independent factors chosen were PAA concentration, ACFs dosage, pH, and temperature, with the removal rate of RR X-3B as the response. According to the experimental design, a second-order polynomial equation in terms of actual factors was established, which demonstrates the empirical relationships between the independent variables and the response: Removal rate = −1.665 + 0.13704X1 + 0.056911X2 + 0.11534X3 + 0.45804X4 − 0.00100235X1X2 + 0.00113593X1X3 − 0.00456936X1X4 − 0.00202296X2X3 + 0.000512477X2X4 + 0.00212755X3X4 − 8
0.00740956X12 − 0.000463107X2 2 − 0.0045482 X32 − 0.065873 X42 The analysis of variance (ANOVA) required to evaluate the adequacy of the model is elaborated in Table 1. In accordance with the ANOVA, the values of F and p for the model are 16.67 and <0.0001, respectively, suggesting that the prediction model is significant. The lack-of-fit F-value of 4.15 implies that the lack-of-fit is not significant relative to the pure error, which indicates good predictability of the model. In addition, the R2 value of 0.9396 is in reasonable agreement with the adjusted R2 value of 0.8832, further confirming good predictability of the model. The accuracy of the model is illustrated in Fig. S2, in which the measured values and the predicted responses of the model are compared. Based on the monomial coefficients of the regression model, p(X1) = 0.5761 (pH), p(X2) < 0.0001 (temperature), p(X3) = 0.0003 (PAA concentration), and p(X4) < 0.0001 (ACFs dosage), values of less than 0.05 indicate that model terms are significant, suggesting that temperature (X2) and ACFs dosage (X4) exert the greatest influences on the removal rate of RR X-3B, whereas pH has little impact. The combined effect of temperature and ACFs dosage is depicted in Fig. 5 as a response surface plot. It is clear that the removal rate could be enhanced by increasing either the ACFs dosage or the temperature. On the basis of the desirability function, optimization was performed to determine the optimal conditions for the removal of RR X-3B. Numerical optimization software was used to identify the specific point that maximizes the desirability function. To confirm the adequacy of the model for predicting the maximum removal percentage of RR X-3B, a verification experiment was carried out under the optimum conditions (pH 5.92, temperature 28.94 °C, PAA concentration 6.04 mM, and ACFs dosage 2.81 g/L). An average maximum removal of 92.5% obtained from three replicate experiments was close to the predicted value of 96.0%; the good agreement between the predicted and experimental values confirmed the validity of the model. Hence, further research was carried out according to the RSM optimization conditions, and 9
the test conditions were adjusted to pH 7, temperature 25 °C, PAA concentration 5 mM, and ACFs dosage 2 g/L under cost considerations. 3.4. Effect of initial pH Wastewater from the textile industry usually falls within a certain pH range, while some treatment methods are pH dependent [29, 30]. Accordingly, in order to investigate the effect of initial pH on the catalytic oxidation of RR X-3B, a series of experiments was performed at pH values in the range 3−11. As shown in Fig. 6, in the presence of PAA alone, less than 3% of RR X-3B was removed in 45 min, indicating that the dye was stable and hardly decomposed over a wide pH range. With ACFs alone, the removal rates were 53.85%, 53.54%, 53.31%, 51.56%, and 48.47% at pH values of 3, 5, 7, 9, and 11, respectively, which suggests that there were no remarkable changes in the adsorption capability of the ACFs on going from acidic to alkaline conditions. In addition, at the same pH values, the ACFs/PAA system achieved removal rates of 97.6%, 98.7%, 98.9%, 97.7%, and 98.4%, respectively. These results revealed that the ACFs/PAA system exhibited excellent pH tolerance over a wide range, which further demonstrated that pH plays a minor role in this catalytic reaction. Compared with other systems using metal-free carbon materials as catalysts, the ACFs/PAA system displays remarkable catalytic removal ability under neutral and alkaline conditions, thereby avoiding the need for additional acidic or alkaline reactants, and reducing costs as well as environmental and operational risks. 3.5. Effect of NaCl Aquatic environments are rich in various dissolved salts, and sodium chloride (NaCl) is the most commonly found salt in industrial wastewater [31, 32]. Therefore, it was of crucial importance to investigate the effect of NaCl on the ACFs/PAA system. Fig. 7 shows that in the presence of both PAA and ACFs, the removal rate of RR X-3B decreased remarkably with increasing concentration of NaCl from 0 to 10
0.1 M. To our surprise, however, with further addition of NaCl, the removal of RR X-3B was significantly enhanced. When the concentration of NaCl exceeded 0.3 M, the removal rate was even larger than that without NaCl. A similar dual effect of chloride on dye removal has previously been observed in other AOP systems, such as Co/PMS [31]. This interesting phenomenon may be rationalized as follows. At low NaCl concentration, the active species involved in this system reacts with Cl− to generate the less reactive Cl2−• radical, resulting in inhibition of the dye removal process, whereas at high concentration (0.3 M) the reaction of Cl− with PAA proceeds to form HClO, accelerating the dye removal rate. In contrast to the common phenomenon reported in most AOP technologies [33, 34], whereby NaCl usually exerts a negative effect on the catalytic oxidation process, in the ACFs/PAA system it has a positive effect at high concentrations, resulting in a higher dye removal rate. Thus, the present system would seem to be particularly well suited for treating chloride-rich effluent. 3.6. Catalytic oxidation mechanism 3.6.1. Identification of active species Ascorbic acid (AA), a typical radical scavenger, was used to preliminarily test whether radical species were generated in the ACFs/PAA system [35, 36]. As shown in Fig. 8, the removal rate of RR X-3B in the presence of PAA decreased from 98% to 66.01% in 45 min, and even the corresponding kobs decreased sharply as the amount of ascorbic acid was increased, indicating that radical species might be involved in this system. Moreover, when excess ascorbic acid was present, the removal rate of RR X-3B in the ACFs/PAA/AA catalytic system was almost consistent with that in the absence of PAA, suggesting that the removal rate was mainly attributed to adsorption rather than the oxidation reaction in the presence of ascorbic acid (Fig. 1). These results demonstrated that the addition of ascorbic acid led to an obvious inhibition of the dye removal, indicating that radical species played a major role in the removal process of 11
RR X-3B in the ACFs/PAA system. Besides, with the aim of investigating the possible radical species involved in the ACFs/PAA and ACFs/H2O2 systems, tert-butyl alcohol (TBA), a common hydroxyl radical scavenger [37, 38], was used to investigate whether hydroxyl radical was generated in these catalytic systems. As shown in Fig. 9, 41.37% of RR X-3B was eliminated in the ACFs/TBA system after 45 min. In addition, about 54.4% and 39.3% of RR X-3B were removed in the ACFs/H2O2 and ACFs/H2 O2/TBA systems, respectively, suggesting that the addition of tert-butyl alcohol to the ACFs/H2O2 system conspicuously inhibited the removal of RR X-3B. Hence, we speculate that hydroxyl radical may play a dominant role in the ACFs/H2O2 system. However, when the same amount of tert-butyl alcohol was added to the ACFs/PAA system, only a slight decrease in the removal rate of RR X-3B was observed. Moreover, the removal rate of RR X-3B remained almost unchanged on further increasing the TBA concentration (Fig. S5), implying that hydroxyl radicals are not the sole active species in the ACFs/PAA system. On the basis of the aforementioned detection of hydroxyl radicals in the ACFs/PAA system, we hypothesized that the initiation reaction involves cleavage of the peroxy bond of PAA into two primary radicals: alkoxyl (CH3C(O)O•) and hydroxyl (HO•). To verify this hypothesis, electron paramagnetic resonance (EPR) spin-trapping was employed to investigate whether CH3C(O)O• radicals were generated in this catalytic system. Fig. 10 shows the EPR spectrum of the FDMPO adduct recorded for the ACFs/PAA system. The spectral shape corresponds to the superposition of two spectra originating from two different radical species. From the spectrum, it can be observed that the ACFs/PAA system could form a typical triplet of 1:3:3:1 quartet signals of hydroxyl adducts and carbon-centered adducts on the basis of their hyperfine coupling constants (FDMPO/•OH adduct: αF = 2.60 G, αN = 13.7 G; FDMPO/•CH3 C(O)O adduct: αF = 2.46 G, αN = 13.8 G), which is in agreement with a previous report [39]. This result confirms 12
our above inference and indicates that both alkoxyl radicals and hydroxyl radicals were generated during the catalytic reaction. 3.6.2. The role of ACFs during the catalytic reaction The above findings suggested that ACFs play a pivotal role in removing dyes during the catalytic reaction. Hence, it was deemed necessary to investigate the role of ACFs in the catalytic oxidation reaction. To the best of our knowledge, activated carbon fibers have a turbostratic disordered graphite-like structure with an abundance of nonbonding free electrons [23, 25]. On the basis of previous structural analysis of ACFs, we speculated that the free electrons might play an important role during the catalytic reaction. To support this, electron paramagnetic resonance (EPR) spin-trapping was employed to investigate changes in the free electron density on the ACFs before and after the reaction. As depicted in Fig. 11, a narrow singlet signal located at 3365 G with a g-factor of 2.0014−2.0015 was observed in the samples before and after the reaction, typical of a carbon-centered radical [40]. This indicated the presence of unpaired free electrons in the ACFs. Moreover, it is noteworthy that the signal intensity observed for the ACFs clearly decreased after 40 min, indicating that the amount of unpaired free electrons decreased markedly over this reaction time. As any oxidation process involves the transfer of electrons, we speculated that unpaired free electrons in the ACFs might be transferred to PAA to promote the generation of radical species in the oxidation reaction, thus accounting for the remarkable catalytic activity of the ACFs/PAA system. To further explore the effect of ACFs in the ACFs/PAA system from the perspective of PAA, we modeled the reaction process by DFT using the B3LYP hybrid functional with the 3-21G basis set. In homolytic cleavage, a chemical bond of a neutral molecule is broken with the generation of two radicals [39], which can be assessed by the bond length. As shown in Fig. 12, the length of the CH3C(=O)O−OH bond (between two oxygen atoms) is 1.52141 Å, implying that spontaneous homolysis of PAA is highly 13
unlikely. However, for PAA in the presence of ACFs, the bond distance is distinctly elongated to 1.5516 Å, favoring the homolysis of PAA. Therefore, it was clearly demonstrated that ACFs can exert a significant influence on the homolytic cleavage of PAA for the generation of the radical species. Based on the above analysis, a mechanism for the catalytic activation of PAA is proposed as follows. The unique structure of ACFs offers a convenient and feasible pathway for transfer of their nonbonding electrons to PAA. Electron transfer may induce elongation of the CH3C(=O)O−OH bond (between two oxygen atoms) in PAA, accelerating its homolytic cleavage to generate a greater amount of alkoxyl and hydroxyl radicals. These can then attack dye molecules in the ACFs/PAA system, leading to their rapid destruction. 4. Conclusion In summary, this study has demonstrated the feasibility of using ACFs as a metal-free catalyst for PAA activation as a novel strategy for wastewater treatment. Emphasis has been placed on examining the dye removal in the ACFs/PAA system, the effects of different variables, and the reaction mechanism. The collected experimental data indicate the following. (i) PAA can be effectively activated by ACFs to remove RR X-3B, which generates no secondary pollution due to the absence of a metal catalyst. (ii) ACFs exhibit excellent sustained catalytic ability and reuse capability, indicating that the described catalytic system is green and environmentally benign. (iii) The ACFs/PAA system displays remarkable pH-tolerant performance without an obvious decrease in catalytic ability over a wide pH range (3−11). (iv) The introduction of ACFs facilitates homolytic cleavage of the O−O bond in CH3C(=O)O−OH to generate hydroxyl and alkoxyl radicals. These valuable findings provide an insight into ACFs-based metal-free catalysts, which should contribute to the promising research field of green, metal-free catalysts for use in environmental remediation. 14
Acknowledgments This work was supported by the State Key Program of National Natural Science of China (No. 51133006), the National Natural Science Foundation of China (No. 51103133, 51302246) and Zhejiang Provincial Natural Science Foundation of China (No. LY14E030013, LY14E030015).
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Tables and figures Table 1. ANOVA for analysis of variance and adequacy of the quadratic model. Fig. 1. (A) Concentration changes of RR X-3B (50 µM) under different conditions: (a) H2 O2 (5 mM); (b) 19
PAA (5 mM); (c) ACFs (2 g/L); (d) ACFs (2 g/L) with H2 O2 (5 mM); (e) ACFs (2 g/L) with PAA (5 mM); (B) digital photographs of three primary color dyes (AR1, AO7, and BG1) before (A) and after (B) oxidation in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 2. Concentration changes of RR X-3B (initial concentration: 50 µM) with continuous addition of RR X-3B with PAA (initial concentration: 5 mM) or without PAA (pH 7.0, T = 25 °C). The four subsequent identical additions of RR X-3B finally resulted in 250 µM RR X-3B and 25 mM PAA. Fig. 3. Concentration changes of RR X-3B (50 µM) without PAA or adding PAA (5 mM) after adsorption for 5, 15, 30, and 45 min, respectively (pH 7.0, T = 25 °C). Fig. 4. Repeated recycling of ACFs (2 g/L) with PAA (5 mM) for the removal of RR X-3B (50 µM) (pH 7.0, T = 25 °C). Fig. 5. Response surface plot for removal of RR X-3B showing interaction between temperature (°C) and ACFs dosage (g/L) (pH 7.0, initial PAA concentration 5 mM). Fig. 6. Effect of initial pH on the removal of RR X-3B (50 µM) under different conditions: (a) ACFs (2 g/L); (b) ACFs (2 g/L) and PAA (5 mM); and (c) PAA (5 mM) (T = 25 °C). Fig. 7. Effects of different dosages of NaCl on the removal rate and the kobs of RR X-3B removal in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 8. Influence of ascorbic acid (AA) concentration on the apparent rate constant kobs for the removal of RR X-3B in the ACFs/PAA system. The inset shows concentration changes of RR X-3B under different conditions: (a) ACFs (2 g/L), ascorbic acid (5 mM); (b) ACFs (2 g/L), PAA (5 mM), ascorbic acid (5 mM); (c) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C) Fig. 9. Influence of tert-butyl alcohol (TBA) on the removal of RR X-3B (50 µM) in the ACFs/H2O2 and ACFs/PAA systems: (a) ACFs (2 g/L), H2O2 (5 mM), TBA (500 mM); (b) ACFs (2 g/L), TBA (500 mM), 20
(c) ACFs (2 g/L), H2O2 (5 mM); (d) ACFs (2 g/L), PAA (5 mM), TBA (500 mM); (e) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 10. Experimental EPR spectrum of FDMPO adduct for the removal of RR X-3B (50 µM) in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C). Fig. 11. EPR spectra of ACFs at 0 and 40 min. Conditions: [RR X-3B] = 50 µM, ACFs = 2 g/L and PAA = 5 mM, pH 7, T = 25 °C. Fig. 12. Structures of PAA in the simplified model of DFT calculations: (A) in the model of PAA alone; (B) in the model of PAA with ACFs.
21
Fig. 1. (A) Concentration changes of RR X-3B (50 µM) under different conditions: (a) H2O2 (5 mM); (b) PAA (5 mM); (c) ACFs (2 g/L); (d) ACFs (2 g/L) with H2 O2 (5 mM); (e) ACFs (2 g/L) with PAA (5 mM); (B) the digital picture of three primary color dyes (AR1, AO7 and BG1) before (Ⅰ) and after (Ⅱ) oxidation in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C).
22
Fig. 2. Concentration changes of RR X-3B (initial concentration: 50 µM) with continuous addition of RR X-3B with PAA (initial concentration: 5 mM) or without PAA (pH 7.0, T = 25 °C). The four subsequent identical additions of RR X-3B resulted finally in 250 µM RR X-3B and 25 mM PAA.
23
Fig. 3. Concentration changes of RR X-3B (50 µM) without PAA or adding PAA (5 mM) after adsorption for 5, 15, 30, 45 min, respectively (pH 7.0, T = 25 °C).
24
Fig. 4. Repeated recycling of ACFs (2 g/L) with PAA (5 mM) for the removal of RR X-3B (50 µM) (pH 7.0, T = 25 °C).
25
Fig. 5. Response surface plot for removal of RR X-3B shows interaction between temperature (°C) and ACFs dosage (g/L) (pH 7.0, initial PAA concentration 5mM).
26
Fig. 6. Effect of initial pH on the removal of RR X-3B (50 µM) under different conditions: (a) ACFs (2 g/L); (b) ACFs (2 g/L) and PAA (5 mM); and (c) PAA (5 mM) (T = 25 °C).
27
Fig. 7. Effects of different dosages of NaCl on the removal rate and the kobs of RR X-3B removal in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C).
28
Fig. 8. Influence of ascorbic acid (AA) concentration on the apparent rate constant kobs for the removal of RR X-3B in ACFs/PAA system. The inset shows concentration changes of RR X-3B under different conditions : (a) ACFs (2 g/L), ascorbic acid (5 mM); (b) ACFs (2 g/L), PAA (5 mM), ascorbic acid (5 mM); (c) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C).
29
Fig. 9. Influence of tert-butyl alcohol (TBA) on the removal of RR X-3B (50 µM) in ACFs/H2O2 system and ACFs/PAA system: (a) ACFs (2 g/L), H2O2 (5 mM), TBA(500 mM); (b) ACFs (2 g/L), TBA(500 mM), (c) ACFs (2 g/L), H2O2 (5 mM); (d) ACFs (2 g/L), PAA (5 mM), TBA(500 mM); (e) ACFs (2 g/L), PAA (5 mM); (pH 7.0, T = 25 °C).
30
Fig. 10. Experimental EPR spectrum of FDMPO adduct for the removal of RR X-3B (50 µM) in the presence of ACFs (2 g/L) and PAA (5 mM); (pH 7.0, T = 25 °C).
31
Fig. 11. EPR spectra of ACFs at 0 and 40 min. Conditions: [RR X-3B] = 50 µM, ACFs = 2 g/L and PAA = 5 mM, pH 7, T = 25 °C.
32
Fig. 12. Structures of PAA in the simplified model of DFT calculations: (A) in the model of sole PAA; (B) in the model of PAA with ACFs.
33
Table 1 ANOVA for analysis of variance and adequacy of the quadratic model.
Source Model
A-pH B-Temp C-PAA D-ACF Lack of Fit Pure Error
Sum of Squares
Degree of freedom
Mean Square
1.57 0.0022 0.42 0.15 0.83 0.090 0.011
14 1 1 1 1 10 5
0.11 0.0022 0.42 0.15 0.83 0.00903 0.00217
34
F-Value
Prob > F
16.67 0.33 62.36 22.09 123.13 4.15
< 0.0001 0.5761 < 0.0001 0.0003 < 0.0001 0.0649
Graphical abstract
35
Highlights: 1. An efficient and green metal-free catalytic system of ACFs/PAA was developed for pollutants removal 2. ACFs/PAA presented a remarkable pH-tolerant performance with enough activity. 3. ACFs/PAA exhibited the excellent sustained catalytic ability and recycling regeneration capability. 4. Both alkoxyl radicals and hydroxyl radicals were generated in the catalytic reaction.
36