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CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants
Journal Pre-proof CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants Shan Chen, Xiudan Liu, Shiyuan Gao, Yanchao Chen, Longjun Rao, Yuyuan Yao, Zhiwei Wu PII:
S0013-9351(20)30137-7
DOI:
https://doi.org/10.1016/j.envres.2020.109245
Reference:
YENRS 109245
To appear in:
Environmental Research
Received Date: 21 August 2019 Revised Date:
9 January 2020
Accepted Date: 7 February 2020
Please cite this article as: Chen, S., Liu, X., Gao, S., Chen, Y., Rao, L., Yao, Y., Wu, Z., CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants, Environmental Research (2020), doi: https:// doi.org/10.1016/j.envres.2020.109245. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Inc.
CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants
Shan Chen1, Xiudan Liu1, Shiyuan Gao, Yanchao Chen, Longjun Rao, Yuyuan Yao∗, Zhiwei Wu
National Engineering Lab of Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, PR China
1 ∗
These authors contributed equally to this work. Corresponding author. Tel.: +86 571 86843810; fax: +86 571 86843255.
E-mail address: [email protected] (Y. Y. Yao). 1
Abstract
CuCo2O4 was synthesized via a relatively simple method, and innovatively supported onto the activated carbon (AC) by calcination to obtain a novel heterogeneous catalyst (AC-CuCo2O4). Brilliant red 3BF (3BF) was selected as the probe compound to investigate the catalytic activity of AC-CuCo2O4 in the presence of peroxymonosulfate (PMS). The results showed that 98% removal rate could be achieved and the reaction rate constant (0.476 min-1) was 5.2 times greater than that of CuCo2O4 alone (0.091min-1), suggesting that the introduction of AC could greatly enhance the catalytic activity of pure CuCo2O4. Typically, the 3BF removal was as high as 96% after five cycles, showing the good stability of catalyst reuse. Additionally, the effects of the initial pH, catalyst dosage, PMS concentration and reaction temperature on the 3BF removal were investigated, demonstrating that AC-CuCo2O4 effectively remove 3BF over a wide pH range (5.0–10.0) and possessed temperature-tolerant performance. To further explore the 3BF removal mechanism, electron paramagnetic resonance technology combining with trapping agents was employed to confirm the involvement of reactive oxygen species including SO4•−, •OH, O2•− and 1
O2, which distinctly differed from the reported CuCo2O4 for PMS activation. These findings
provided an addition promising strategy in environmental remediation.
Keywords: peroxymonosulfate (PMS), activated carbon, heterogeneous catalyst, organic contaminants
1. Introduction Recently, water pollution has become one of the significant issues with the rapid development 2
of industrialization, which has triggered the worldwide concerns. In the past years, considerable research efforts have been devoted to environment remediation, particularly, advanced oxidation processes (AOPs) has received a great deal of attention due to the promising capability in effective water treatment [1-3]. Among AOPs, methods based on peroxymonosulfate (PMS) activation to produce reactive radicals have been broadly applied owing to the unique characteristics including the higher oxidation potential, a wider pH range and easier storage and transportation than H2O2 as one of the most common oxidants [4, 5]. Various approaches for PMS activation such as UV irradiation [6], heat [7], alkaline [8] and transition metals [9], can effectively degrade the emerging organic contaminants. Among the diverse methods investigated, transition metals have been considered as a more feasible and efficient method for activating PMS. Typically, Co2+ is recognized as the best PMS activator, compared to ZVI, iron-based catalysts and other transition metal catalysts, yet the dissolution of stubborn toxic Co limits the long-term applications to great extent [10-12]. Therefore, the heterogeneous catalytic systems using supported cobalt [13, 14] or cobalt oxides and derived composites [15] have been developed to be the alternative pathways to avoid potential Co leaching. In the past decade, spinel with the general formula AB2X4, has been continuously used in several applications such as energy storage and environmental treatments, because of their easy magnetic collection, high catalytic activity and good chemical stability [16]. Among spinel type oxides, cobalt-based spinel oxides due to the promising activity, low cost, high stability and environmental benignity, have been actively pursued [17]. In particular, substituted Co3O4 with Cu, CuCo2O4, is a typical spinel and has gained much attention by virtue of their electrochemical properties in diverse applications such as sensors, anode materials and capacitors [18-20]. These 3
properties are significant in the interfacial charge transfer processes for redox reactions. A few studies have attempted to correlate CuCo2O4 to electrocatalysis [21, 22], nevertheless, the catalytic performance of CuCo2O4, especially the environment remediation, is rarely reported. In addition, compared with Co3O4, CuCo2O4 not only shows higher catalytic performance but the leaching of heavy metals from the solid phase can be significantly reduced owing to the relatively stable spinel structure. To be precise, it is not at all inevitable that the small molecular catalysts used directly in environment treatment will bring about the decrease in catalytic activity and inconvenient recovery. It is a common practice to introduce a support for addressing the above mentioned problems, for example, Ning et al. designed a hybrid electrocatalyst, CuCo2O4/N-rGO, which showed that the enhancement of electrocatalytic activity and stability could benefit from the graphene support introduction [23]. However, few studies on the CuCo2O4 supported catalyst were reported. Among the various materials, it was noticed that activated carbon (AC) has a widespread application in the field of catalysis either as supports or catalysts with the advantages of the huge specific surface area, high pore volume, cost-effective and environmental friendliness [24-26]. Accordingly, we conjecture that the introduction of AC enables CuCo2O4 to enhance the catalytic performance. On the one hand, AC not only can improve the adsorption capacity of CuCo2O4 for contaminants, but make CuCo2O4 uniform dispersion to provide more active sites. Additionally, AC may play a role in providing electrons to accelerate the generation of reactive oxygen species (ROS) [27]. To the best of our knowledge, the reports on the applications of CuCo2O4 activating PMS in the remediation of contaminated water are rarely focused, and there are no reviews in investigating the relationships between CuCo2O4 and AC. Therefore, we first proposed to construct a novel supported catalyst by coupling AC with CuCo2O4 (AC-CuCo2O4) to activate 4
PMS for organic contaminants removal. In this study, CuCo2O4 was prepared in a relatively facile manner, and then combined with AC by calcination to synthesize a superior catalyst (AC-CuCo2O4). 3BF is a typical azo dye and has gained much concern because of high toxicity, carcinogenicity and significant environmental risk, so 3BF was selected as the model pollutant to evaluate the catalytic performance of AC-CuCo2O4 towards PMS activation. The mechanism of the AC-CuCo2O4/PMS system was explored with the radical trapping experiments and EPR analysis, and the influencing operational parameters, such as catalyst dosage, PMS concentration, initial pH and reaction temperature, have been investigated in our work.
2. Experimental 2.1 Chemicals and reagents Activated carbon (AC), copper nitrate hydrate (Cu(NO3)2·xH2O, AR) and urea (CH4N2O, AR 99%) were obtained from Shanghai Macklin Biochemical Co., Ltd. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR 99%), L-histidine, ammonium fluoride (NH4F, AR 98%) and potassium peroxymonosulfate (PMS, 2KHSO5•KHSO4•K2SO4, AR) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). 5,5-dimethyl-pyrroline-oxide (DMPO, 97%) was provided by Beijing Bailingwei Chemical Technology Co., Ltd and 2,2,6,6-tetramethyl-4-piperidone (TEMP, 98%) was obtained from the Shanghai Saan Chemical Technology Co., Ltd. Other reagents applied in the experiments were obtained from the Hangzhou Mike Chemical Instrument Co., Ltd. (Hangzhou, China). All organic dyes, including Brilliant Red 3BF (3BF), Acid Red 1 (AR1), Reactive Red X-3B (X-3B), Reactive Brilliant (KN-R) and Reactive Yellow (M-3RE), were
5
commercial compounds without further purification. All solutions in the experiments were prepared with deionized water.
2.2 Synthesis of AC-CuCo2O4 In detail, AC was preliminarily impregnated into HNO3 (5 M) for 6 h at 80 oC. After that, the resultant AC was taken out and washed several times with deionized water, and dryed at 80 oC for 8 h. In terms of CuCo2O4 sample it was prepared by a solvothermal method according to a previous report with slight improvement [22]. Firstly, Cu(NO3)2•6H2O (0.5 mmol), Co(NO3)2•6H2O (1 mmol), NH4F (3 mmol) and CO(NH2)2 (6 mmol) were mixed and dissolved in 60 mL solution (20 mL aqueous ethanol and 40 mL deionized water). Subsequently, 1 g resultant AC was added into the above solution, and then treated with ultrasonic treatment for 2 min. Whereafter, the mixture solution was transferred to a 100 mL Teflon-lined autoclave and maintained at 95 °C for 12 h. The obtained sample was rinsed by more than 100 mL deionized water for 5 times until the pH of the filtrate kept stable under neutral condition and then dried at 80 °C overnight. The final product was then annealed in a pipe furnace and kept at 400 °C for 2 h with a heating speed of 5 °C/min under an argon atmosphere.
2.3 Catalytic activity tests The experiments of 3BF removal was carried out in a 100 mL conical flask and vibrated in a water bath at 25°C or other temperatures were regulated by a constant temperature shaker water bath. A typical reaction mixture contained 3BF (0.1 mmol/L), AC-CuCo2O4 (0.2 g/L) and PMS (0.4 mmol/L). During each interval, the reaction solution was withdrawn using a syringe and filtered, and then analyzed with UV-vis spectrometer (Unico V-1800) at a wavelength of 542 nm. The adjustment of initial pH was dependent on NaOH or H2SO4 before catalyst and oxidant 6
addition. According to the same method mentioned above, the catalytic performance of CuCo2O4, CuCo2O4/PMS and AC/PMS were also performed. 2.4 Catalyst characterizations Scanning electron microscopy coupled to energy dispersive spectrometer (SEM-EDS, JSM-7001F) was used to characterize the catalyst. X-ray photoelectron spectroscopy (XPS, Thermo Fisher 250XI) was conducted with monochromatic Al Ka X-ray source to characterize AC-CuCo2O4. The binding energies were calibrated utilizing C1s peak at 284.7 eV as reference.
2.5 Analytical methods The pseudo-first-order kinetics was used to evaluate the catalytic reaction kinetics according to Eq. (1): k = −ln(C/C0)/t
(1)
where C was the concentration of pollutants at time t, C0 referred to the initial pollutants concentration, and the pseudo-first-order reaction rate constant (kobs) was calculated from the experimental data. EPR spectra were analyzed using a Bruker A300 spectrometer with DMPO and TEMP as the spin-trapping agents.
3. Results and discussion 3.1 Characterization of catalysts Scanning electron microscope (SEM) images of AC and AC-CuCo2O4 catalyst samples were presented in Fig. 1(A–D). As depicted in Fig. 1(A) and (B), the surface of AC was smooth and there was no apparent covering on it. After CuCo2O4 was anchored on AC support, AC became relatively rough and some particles were distributed well on the surface of AC, as shown in Fig. 7
1(C) and (D), indicating that AC-CuCo2O4 had been synthesized successfully. Additionally, the elemental mapping images were employed to analyze the chemical composition of AC-CuCo2O4 in Fig. 1(E–G), revealing that the amount of Cu and Co were present on the surface structure. Furthermore, pore volume and the Brunauer-Emmett-Teller (BET) specific surface area were obtained by measuring the N2 adsorption/desorption, which were exhibited in Fig S1. To gain more insight into the detailed elemental composition and valence states of prepared catalyst, X-ray photoelectron spectroscopy (XPS) was carried out. In the whole scanning spectrum (Fig. 2(A)), it could be observed that the tiny new peaks of copper and cobalt were detected in the AC-CuCo2O4 compared with the bare AC. Fig. 2(B) and (C) exhibited the high-resolution XPS spectra of Cu and Co 2p peaks in the AC-CuCo2O4, respectively. The detailed content of other elements detected were summarized (in Fig. S2). For the XPS spectrum of Cu in Fig. 2(B), the binding energy positions of 932.78 (Cu 2p3/2) and 953.38 eV (Cu 2p1/2). The main peaks at 932.78 eV for Cu 2p3/2 were attributable to Cu (II), and the peaks at 953.38 eV for Cu 2p1/2 existed in Cu (II) oxide species [28, 29]. The spectrum of Co was presented in Fig. 2(C), the peaks with binding energies of 780.98 and 795.98 eV corresponded to Co 2p3/2 and Co 2p1/2, respectively, which were in accordance with Co (II) and Co (III), respectively [30]. Thus, the results of XPS spectra further verified that CuCo2O4 was successfully supported onto AC. Furthermore, there was no obvious metal valence change after the reaction (in Fig. S3), indicating the high stability of the as-prepared AC-CuCo2O4 for PMS activation during the catalytic process.
3.2 Catalytic activity evaluation 3BF as the target pollutant was selected to evaluate the catalytic activity of AC-CuCo2O4 in the presence of PMS, as displayed in Fig. 3(A). There was no obvious change in 3BF removal by only 8
PMS or AC was present, while 12.7% of 3BF was obtained in 10 min when AC and PMS existed simultaneously, suggesting that the AC could activate PMS for the 3BF elimination to some extent. In the presence of CuCo2O4 alone, negligible 3BF was removed, while the removal of 65.1% occurred when CuCo2O4 was combined with PMS, indicating that CuCo2O4 could activate PMS to remove 3BF in a certain degree. Notably, the removal rate increased to 98% when AC-CuCo2O4 and PMS existed simultaneously, showing that the enhanced PMS activation by CuCo2O4 benefitted from the introduction of AC. To our surprise, the catalytic activity in the AC-CuCo2O4/PMS system was superior to that in the Co3O4/PMS system and AC-Co3O4/PMS system under the same condition, in which the Co3O4 as a commercial catalyst could effectively activate PMS. To quantitatively determine the catalytic activity for the above curves on kinetics, the kobs were estimated and the results was depicted in Fig. 3(B). The kobs of AC-CuCo2O4/PMS system was calculated 0.476 min-1, which was 35, 11, 5 and 3 times better than that in AC/PMS, Co3O4/PMS, CuCo2O4/PMS and AC-Co3O4/PMS system, respectively, indicating that AC-CuCo2O4 possessed an excellent catalytic performance in removing 3BF. To further investigate the catalytic activity of AC-CuCo2O4 and CuCo2O4 for PMS activation, the experiments were carried out at different pH conditions, as displayed in Fig. 3(C). It showed that 3BF removal enhanced with increasing pH in a range from 5 to 10 in both AC-CuCo2O4/PMS and CuCo2O4/PMS system, and the maximum 3BF removal occurred at pH 10, which might be attributed to the charged state of PMS species. Under acidic pH conditions, the possible attachment of the H+ to the more electronegative peroxide bond (O–O) of the PMS induced the interfacial repulsion between PMS and the protonated catalytic sites (e.g. hydroxyl species) to inhibit the catalytic performance. However, the catalytic performance was greatly improved on the 9
alkaline condition, which could be ascribed to several factors. On the one hand, more surface OH-groups had capable of enhancing the electron density of the transition metal and act as the donor ligand to accelerate the PMS activation reaction. On the other hand, the activation of PMS would produce more powerful reactive oxygen species by base. Moreover, the catalytic activity of the AC-CuCo2O4/PMS system was evidently better than that of the CuCo2O4/PMS system in the same situation, further indicating that AC introduction could remarkably enhance the catalytic performance and improved the pH adaptability of CuCo2O4 for PMS activation. The reusability of AC-CuCo2O4 in the presence of PMS is a significant aspect in the practical applications. The catalyst was taken out after each recycling experiment and rinsed with deionized water, and then dried at 80 °C. As presented in Fig. 3(D), 3BF removal rate remained almost the same over 5 cycles, which suggested that AC-CuCo2O4 was not only an efficient but also a durable catalyst for PMS activation. Furthermore, an atlas of time series for 3BF removal in the AC-CuCo2O4/PMS system by UV absorption spectra were displayed (in Fig. S4). In this investigation, the catalytic activity of the AC-CuCo2O4/PMS system for removing several other contaminants was also examined, as shown in Fig. 4. It illustrated that the removal rates for the AC-CuCo2O4/PMS system were almost 98%, 98% and 99% within 10 min for X-3B, AR1 and ciprofloxacin (CIP), respectively. Additionally, about 90% of KN-R, M-3RE and phenol was removed within 15 min in the AC-CuCo2O4/PMS system. Additionally, an atlas of time series for the different dyes removal in the system by UV absorption spectra were displayed (in Fig. S5). The above results indicated that the AC-CuCo2O4/PMS system could exhibit good catalytic performances for different organic pollutants and held promising application prospects in water treatment. 10
3.3 Effects of the reaction temperature The reaction temperature of a catalyst is vital for its performance assessment and practical application. Therefore, the experiments were undertaken to investigate the reaction temperature influence on 3BF removal, as described in Fig. 5(A) and (B). The role of reaction temperature on 3BF removal in the AC-CuCo2O4/PMS system was presented in Fig. 5(A), which showed that 3BF removal was improved slightly when the reaction temperature varied from 25 to 55 °C. Comparatively, an obvious enhancement of removing 3BF was observed with the increase of the reaction temperature in the CuCo2O4/PMS system, as depicted in Fig. 5(B). For example, 53.8% 3BF removal was reached in the CuCo2O4/PMS system in 10 min at 25 °C, while when the reaction temperature was increased to 55 °C, the removal efficiency of 3BF could achieve 97.1% only 4 min. To further discuss the relationship, kobs of the AC-CuCo2O4/PMS and the CuCo2O4/PMS system were displayed in Fig. 5(C). The results suggested that the higher removal efficiency was achieved upon increasing the temperature, which might be ascribed to higher temperatures resulting in molecules enhanced movement. In further experiments, the operating temperature of 25 °C was selected since higher temperatures gave rise to an increase in the energy requirement. As shown in Fig. 5(D), the activation energy (Ea) for removing 3BF was calculated according to the Arrhenius plot (lnkobs = lnA − Ea/RT) for further discussion. The values of Ea for 3BF removal were calculated to be 38.29 kJ·mol−1 and 69.57 kJ·mol−1 in the AC-CuCo2O4/PMS and the CuCo2O4/PMS system, respectively, indicating that Ea in the AC-CuCo2O4/PMS system was less than that in CuCo2O4/PMS system. The above results demonstrated that the AC-CuCo2O4/PMS system could proceed easily to remove the organic pollutants, which could potentially reduce the energy consumption owing to the lower energy activation and expanded the 11
practical application of the system to some extent.
3.4 Effects of the catalyst dosage and PMS concentration Catalyst dosage is a significant factor during the catalytic process, therefore, the experiments were performed to estimate the influence of the catalytic activity in AC-CuCo2O4/PMS system, as illustrated in Fig. 6(A). It was generally noticed that the removal rate of 3BF distinctly increased as the AC-CuCo2O4 dosage ranging from 0.1 to 0.3 g/L, while the further increase of the AC-CuCo2O4 dosage could not obviously promote the removal rate. Moreover, the kobs values enhanced significantly from 0.325 to 0.538 min-1 with the dosage of AC-CuCo2O4 ranging from 0.1 to 0.3 g/L, while a slight increase was observed from 0.538 to 0.566 min-1 as AC-CuCo2O4 dosage increased to 0.5 g/L in Fig. 6(B). The improvement of the removal rate might be attributable to the increasing amount of active sites. Additionally, the influence of oxidant concentration varying from 0.1 to 2.0 mmol/L on 3BF removal was investigated in the AC-CuCo2O4/PMS system, as shown in Fig. 6(C). A remarkable improvement was observed when the PMS concentration increased from 0.1 to 0.4 mmol/L, while a decrease in the system with the further increase in PMS concentration. Obviously, similar tendencies of the kobs values for 3BF removal were presented in Fig. 6(D). In consideration of the cost and catalytic efficiency, 0.2 g/L AC-CuCo2O4 and 0.4 mmol/L PMS were chosen for the removal of 3BF in this study.
3.5 Analysis of reaction mechanism It is generally recognized that the free radicals are the dominant ROS for the organic contaminants degradation towards PMS activation [31, 32], so the radical quenching experiments were conducted to investigate the catalytic mechanism. Ascorbic acid (AA), a typical scavenger, can capture ROS such as superoxide radical (O2•−), sulfate radical (SO4•−) and hydroxyl radical 12
(•OH) during the catalytic process [33]. There was an obvious inhibition when AA was added into the AC-CuCo2O4/PMS system with the concentration ranging from 0 to 1.0 mmol/L, suggesting that the free radicals were involved in the catalytic process (in Fig. S6). In general, SO4•−and •OH were regarded as the primary types of ROS for PMS activation. Therefore, methanol (MA) was applied to quench both SO4•− and •OH, and tert-Butyl alcohol (TBA) was employed as a unique scavenger for •OH [34, 35]. As illustrated in Fig. 7(A), the removal rate achieved 30.5% at a MA concentration of 5 M, and 49.2% removal was reached when TBA was introduced into the AC-CuCo2O4/PMS system, indicating that both SO4•− and •OH might be generated during the catalytic process. EPR spectroscopy with DMPO as the spin trapping agent was further applied in the subsequent experiments. As displayed in Fig. 7(B), the signals of DMPO-OH ( ) and DMPOSO4•− ( ) adducts were detected from the EPR spectra, which further confirmed that SO4•− and •
OH participated in the 3BF removal during the catalytic process. It was noticed that the 3BF removal was not entirely inhibited when SO4•− and •OH were
scavenged by adding MA, so we speculated that there were other ROS in the AC-CuCo2O4/PMS system. To best of our knowledge, singlet oxygen (1O2) was a selective ROS to display negligible reactivity towards alcohols. Therefore, the trapping experiments were conducted using L-histidine as the scavenger for 1O2. As described in Fig. 7(C), a completely inhibitory effect could be observed with the addition of L-histidine, suggesting that 1O2 might be involved in the 3BF removal during the catalytic process. EPR spectroscopy was employed to further confirm our hypothesis, as presented in Fig. 7(D). Obviously, it was found that a typical 1:1:1 triplet signal of TEMP-1O2 adducts in the presence of TEMP, which further confirmed that 1O2 was indeed generated in this system. Furthermore, it has been reported that superoxide radical (O2•−) acting as 13
the precursor involved the formation of 1O2 [36]. Herein, benzoquinone (BQ) as the scavenger for O2•− was adopted, and the obvious inhibition could be observed when BQ was added in the system (in Fig. S7), suggesting that O2•− was produced in the removal process. The above results indicated that AC introduction improved the catalytic activity of CuCo2O4 for PMS activation, and the mechanism of AC-CuCo2O4 for activating PMS differed from the reported CuCo2O4. Hence, it is of great significance to further investigate the function of AC in the ROS production process. EPR spectroscopy was performed to ascertain whether electrons were involved in the AC-CuCo2O4/PMS system. The signals of the persistent free radicals (PFRs) were detected and the intensity decreased after the catalytic oxidation reaction (in Fig. S8), showing that the PFRs in the AC might be responsible for accelerating the key reaction step of Cu3+ to Cu2+ and Co3+ to Co2+ in the AC-CuCo2O4/PMS system[37]. On the basis of the above-mentioned results and detailed discussion, a possible catalytic mechanism for PMS activation by AC-CuCo2O4 was proposed in Scheme 1. First of all, Co3+ preliminarily reacted with PMS to form Co2+ and SO5•−, and then Co2+ reacted with HSO5− to produce SO4•− with the production of SO4•− and •OH (Eqs. (2) – (4)) [38, 39]. Co3+ + HSO5− →Co2+ + SO5•− + H+
(2)
Co2+ + HSO5− →Co3+ + SO4•− + H2O
(3)
SO4•− +OH− → OH + SO42−
(4)
Meanwhile, Cu2+ activated PMS for 3BF removal based on a series of reactions as following (Eqs. (5) – (8)) [40, 41] and Cu3+ interacted with Co2+ to be reduced to Cu2+ at the same time. Cu2+-OH− + HSO5− → Cu2+-(HO)OSO3− + OH−
(5)
Cu2+-(OH)OSO3− → Cu3+-OH− + SO4•−
(6)
Cu3+ -OH− +HSO5− → Cu2+-•OOSO3− + H2O
(7)
Cu2+ -•OOSO3 + 2H2O → Cu2+-OH− + O2 + 2SO4•−+ 2H+ 14
(8)
Consequently, O2 was further involved in the formation of 1O2 (Eqs. (9) and (10)) [42] through electron transfer process. The PFRs in the AC might be responsible for accelerating the electron transfer of Cu3+ to Cu2+ and Co3+ to Co2+. Via a series of cyclic reactions for PMS activation by going through Co2+-Co3+ -Co2+ and Cu2+-Cu3+ -Cu2+ states, SO4•−, •OH and
1
O2 were
simultaneously generated to remove the organic pollutants in the AC-CuCo2O4/PMS system. O2 + e−→ O2•−
(9)
O2•−+ e−→ 1O2
(10)
Conclusion In this work, AC-CuCo2O4 was successfully prepared by a solvothermal method. The catalytic activity investigations confirmed that the combination of CuCo2O4 with AC for PMS activation resulted in the superior catalytic activity for 98 % of 3BF removal than that of pure CuCo2O4 (65 %). In addition, the 3BF removal of AC-CuCo2O4 was about 2.4 times more than that of the commercial Co3O4 catalyst for PMS activation alone (41%). Furthermore, the AC-CuCo2O4/PMS system exhibited low value of Ea (38.29 kJ·mol-1) for 3BF removal, which was significantly lower than that in CuCo2O4/PMS system. Combined with the EPR technology and the radical quenching experiments, the mechanism of the catalytic system was explored that the ROS such as SO4•−, •OH, O2•− and 1O2 contributed to the 3BF removal in the AC-CuCo2O4/PMS system, which significantly differed from the CuCo2O4 for activating PMS in previous studies. The reusability of AC-CuCo2O4 was also investigated and it was found that 96% removal of 3BF still remained during the PMS activation process after five successive runs, showing the good recyclability of AC-CuCo2O4 in the presence of PMS. Taking into account the superior temperature tolerance, highly catalytic activity and good recycling ability, AC-CuCo2O4 can be considered as a promising 15
catalyst to activate PMS for organic contaminants removal in practical water treatment.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51772274).
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(2015) 76862-76874. [15] L. Chen, S. Yang, X. Zuo, Y. Huang, T. Cai, D. Ding, Biochar modification significantly promotes the activity of Co3O4 towards heterogeneous activation of peroxymonosulfate, Chem. Eng. J. 354 (2018) 856-865. [16] Y Pang, L Kong, H Lei, D Chen, G Yuvaraja. Combined microwave-induced and photocatalytic oxidation using zinc ferrite catalyst for efficient degradation of tetracycline hydrochloride in aqueous solution[J]. J Taiwan Inst Chem E, 93(2018) 397-404. [17] M.H. Habibi, Z. Rezvani, Photocatalytic degradation of an azo textile dye (C.I. Reactive Red 195 (3BF)) in aqueous solution over copper cobaltite nanocomposite coated on glass by Doctor Blade method, Spectrochim Acta A, 147 (2015) 173-177. [18] P. Liang, F. Wang, Z.-A. Hu, Controlled synthesis of ordered sandwich CuCo2O4/reduced graphene oxide composites via layer-by-layer heteroassembly for high-performance supercapacitors, Chem. Eng. J. 350 (2018) 627-636. [19] A. Pendashteh, S.E. Moosavifard, M.S. Rahmanifar, Y. Wang, M.F. El-Kady, R.B. Kaner, M.F. Mousavi, Highly Ordered Mesoporous CuCo2O4 Nanowires, a Promising Solution for High-Performance Supercapacitors, Chem. Mater. 27 (2015) 3919-3926. [20] E. Alizadeh-Gheshlaghi, B. Shaabani, A. Khodayari, Y. Azizian-Kalandaragh, R. Rahimi, Investigation of the catalytic activity of nano-sized CuO, Co3O4 and CuCo2O4 powders on thermal decomposition of ammonium perchlorate, Powder Technol. 217 (2012) 330-339. [21] X. Wang, Y. Li, T. Jin, J. Meng, L. Jiao, M. Zhu, J. Chen, Electrospun Thin-Walled CuCo2O4@C Nanotubes as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries, Nano Lett. 17 (2017) 7989-7994. 18
[22] F. Niu, N. Wang, J. Yue, L. Chen, J. Yang, Y. Qian, Hierarchically Porous CuCo2O4 Microflowers: a Superior Anode Material for Li-ion Batteries and a Stable Cathode Electrocatalyst for Li-O2 Batteries, Electrochim. Acta, 208 (2016) 148-155. [23] R. Ning, J. Tian, A.M. Asiri, A.H. Qusti, A.O. Al-Youbi, X. Sun, Spinel CuCo2O4 nanoparticles supported on N-doped reduced graphene oxide: a highly active and stable hybrid electrocatalyst for the oxygen reduction reaction, Langmuir, 29 (2013) 13146-13151. [24] E. Saputra, S. Muhammad, H. Sun, S. Wang, Activated carbons as green and effective catalysts for generation of reactive radicals in degradation of aqueous phenol, RSC Adv. 3 (2013) 21905-21910. [25] E.M. Iannicelli-Zubiani, P. Gallo Stampino, C. Cristiani, G. Dotelli, Enhanced lanthanum adsorption by amine modified activated carbon, Chem. Eng. J. 341 (2018) 75-82. [26] J. Zhu, J. Yang, B. Deng, Enhanced mercury ion adsorption by amine-modified activated carbon, J. Hazard. Mater. 166 (2009) 866-872. [27] F. Ghanbari, M. Moradi, F. Gohari, Degradation of 2,4,6-trichlorophenol in aqueous solutions using peroxymonosulfate/activated carbon/UV process via sulfate and hydroxyl radicals, J. Water Pro. Eng. 9 (2016) 22-28. [28] J. Li, Y. Ren, F. Ji, B. Lai, Heterogeneous catalytic oxidation for the degradation of p -nitrophenol in aqueous solution by persulfate activated with CuFe2O4 magnetic nano-particles, Chem. Eng. J. 324 (2017) 63-73. [29] W.D. Oh, S.K. Lua, Z. Dong, T.T. Lim, A novel three-dimensional spherical CuBi2O4 consisting of nanocolumn arrays with persulfate and peroxymonosulfate activation functionalities for 1H-benzotriazole removal, Nanoscale, 7 (2015) 8149-8158. 19
[30] A. Khan, Z. Liao, Y. Liu, A. Jawad, J. Ifthikar, Z. Chen, Synergistic degradation of phenols using peroxymonosulfate activated by CuO-Co3O4@MnO2 nanocatalyst, J. Hazard. Mater. 329 (2017) 262-271. [31] C. Li, J. Wu, W. Peng, Z. Fang, J. Liu, Peroxymonosulfate activation for efficient sulfamethoxazole degradation by Fe3O4/β-FeOOH nanocomposites: Coexistence of radical and non-radical reactions, Chem. Eng. J. 356 (2019) 904-914. [32] G.P. Anipsitakis, D.D. Dionysiou, Radical generation by the interaction of transition metals with common oxidants, Environ. sci. tech. 38 (2004) 3705-3712. [33] X. Liu, Y. Chen, Y. Yao, Q. Bai, Z. Wu, Iodine-doped carbon fibers as an efficient metal-free catalyst to activate peroxymonosulfate for the removal of organic pollutants, Catal. Sci. Tech. 8 (2018) 5482-5489. [34] Y. Zhou, J. Jiang, Y. Gao, J. Ma, S.Y. Pang, J. Li, X.T. Lu, L.P. Yuan, Activation of Peroxymonosulfate by Benzoquinone: A Novel Nonradical Oxidation Process, Environ Sci Technol, 49 (2015) 12941-12950. [35] P. Wang, S. Yang, L. Shan, R. Niu, X. Shao, Involvements of chloride ion in decolorization of Acid Orange 7 by activated peroxydisulfate or peroxymonosulfate oxidation, J. Environ. Sci. 23 (2011) 1799-1807. [36] D. Dai, Z. Yang, Y. Yao, L. Chen, G. Jia, L. Luo, Highly efficient removal of organic contaminants based on peroxymonosulfate activation by iron phthalocyanine: mechanism and the bicarbonate ion enhancement effect, Catal. Sci. Tech. 7 (2017) 934-942. [37] Z Yang, D Dai, Y Yao, L Chen, Q Liu, L Luo. Extremely enhanced generation of reactive oxygen species for oxidation of pollutants from peroxymonosulfate induced by a supported 20
copper oxide catalyst[J]. Chem Eng J, 322(2017) 546-555. [38] W. Guo, S. Su, C. Yi, Z. Ma, Degradation of antibiotics amoxicillin by Co3O4-catalyzed peroxymonosulfate system, Environ. Prog. Sustain. Energy. 32 (2013) 193-197. [39] K.-Y. Andrew Lin, F.-K. Hsu, W.-D. Lee, Magnetic cobalt–graphene nanocomposite derived from self-assembly of MOFs with graphene oxide as an activator for peroxymonosulfate, J. Mater. Chem. A 3 (2015) 9480-9490. [40] Y. Lei, C.S. Chen, Y.J. Tu, Y.H. Huang, H. Zhang, Heterogeneous Degradation of Organic Pollutants by Persulfate Activated by CuO-Fe3O4: Mechanism, Stability, and Effects of pH and Bicarbonate Ions, Environ. Sci. Technol. 49 (2015) 6838-6845. [41] T. Zhang, H. Zhu, J.P. Croue, Production of sulfate radical from peroxymonosulfate induced by a magnetically separable CuFe2O4 spinel in water: efficiency, stability, and mechanism, Environ. Sci. Technol. 47 (2013) 2784-2791. [42] T. Daimon, T. Hirakawa, M. Kitazawa, J. Suetake, Y. Nosaka, Formation of singlet molecular oxygen associated with the formation of superoxide radicals in aqueous suspensions of TiO2 photocatalysts, Appl. Catal. A-Gen. 340 (2008) 169-175.
Figure and Table Captions Fig. 1 SEM images of (A) and (B) AC, (C) and (D) AC-CuCo2O4. Elemental mapping images of AC-CuCo2O4 for (E) Cu and Co, (F) Cu and (G) Co.
Fig. 2 (A) XPS survey spectra of AC and AC-CuCo2O4. (B) Cu 2p and (C) Co 2p XPS spectra of AC-CuCo2O4, respectively. 21
Fig. 3 (A) 3BF removal versus time in different systems. (B) Rate constants of 3BF removal in different systems. (C) Influence of initial pH on 3BF removal in AC-CuCo2O4/PMS and CuCo2O4/PMS system, respectively. (D) Repeated recycling of AC-CuCo2O4 for the 3BF removal. Reaction conditions: [AC] = [AC-Co3O4] = [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, [3BF]: 0.1 mmol/L, [CuCo2O4]: 15 mg/L, pH 10, T=25 oC.
Fig. 4 Removal of different contaminants in the AC-CuCo2O4/PMS system. Reaction Conditions: [AR1] = [X-3B] = [Phenol]: 0.1 mmol/L, [M-3RE] = [KN-R] = [CIP]: 0.05 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 oC.
Fig. 5 Removal of 3BF in the (A) AC-CuCo2O4/PMS and (B) CuCo2O4/PMS system at different reaction temperatures; (C) kobs of 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system; (D) Arrhenius plots for 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [CuCo2O4]: 15 mg/L, [PMS]: 0.4 mmol/L, pH 10.
Fig. 6 (A) Effects of catalyst dosage on the 3BF removal in the AC-CuCo2O4/PMS system and (B) kinetic fitting. Conditions: [3BF]: 0.1 mmol/L, [PMS]: 0.4 mmol/L pH 10, T=25 °C. (C) Effects of PMS concentration on 3BF removal in the AC-CuCo2O4/PMS system; (D) the corresponding kobs removal of 3BF. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, pH 10, T=25 °C.
22
Fig. 7 (A) Radical trapping tests using MA and TBA 3BF removal in AC-CuCo2O4/PMS system; (B) EPR spectra of DMPO in AC-CuCo2O4/PMS system; (C) Radical trapping tests using L-histidine on the 3BF removal in AC-CuCo2O4/PMS system; (D) EPR spectra of TEMP in AC-CuCo2O4/PMS system. Conditions: [MA] = [TBA] = 5 mol/L, [L-histidine]:10 mmol/L, [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 °C.
Scheme 1 Proposed mechanism of PMS activation on AC-CuCo2O4 for 3BF removal.
23
Fig. 1 SEM images of (A) and (B) AC, (C) and (D) AC-CuCo2O4. Elemental mapping images of AC-CuCo2O4 for (E) Cu and Co, (F) Cu and (G) Co.
Fig. 2 (A) XPS survey spectra of AC and AC-CuCo2O4. (B) Cu 2p and (C) Co 2p XPS spectra of AC-CuCo2O4, respectively.
Fig. 3 (A) 3BF removal versus time in different systems. (B) Rate constants of 3BF removal in different systems. (C) Influence of initial pH on 3BF removal in AC-CuCo2O4/PMS and CuCo2O4/PMS system, respectively. (D) Repeated recycling of AC-CuCo2O4 for the 3BF removal. Reaction conditions: [AC]=[AC-Co3O4]= [AC-CuCo2O4]: 0.2 g/L, [Co3O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, [3BF]: 0.1 mmol/L, [CuCo2O4]: 15 mg/L, pH 10, T=25 oC.
Fig. 4 Removal of different contaminants in the AC-CuCo2O4/PMS system. Reaction Conditions: [AR1] = [X-3B] = [Phenol]: 0.1 mmol/L, [M-3RE] = [KN-R] = [CIP]: 0.05 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 oC.
Fig. 5 Influence of different reaction temperature on the 3BF removal in the (A) AC-CuCo2O4/PMS and (B) CuCo2O4/PMS system; (C) kobs of 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system; (D) Arrhenius plots for 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [CuCo2O4]: 15 mg/L, [PMS]: 0.4 mmol/L, pH 10.
Fig. 6 (A) Effects of catalyst dosage on the 3BF removal in the AC-CuCo2O4/PMS system and (B) kinetic fitting. Conditions: [3BF]: 0.1 mmol/L, [PMS]: 0.4 mmol/L，pH 10, T=25 °C. (C) Effects of PMS concentration on 3BF removal in the AC-CuCo2O4/PMS system; (D) the corresponding kobs removal of 3BF. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, pH 10, T=25 °C.
Fig. 7 (A) Radical trapping tests using MA and TBA on the removal of 3BF in AC-CuCo2O4/PMS system; (B) DMPO spin-trapping EPR spectra in AC-CuCo2O4/PMS system; (C) Radical trapping tests using L-histidine on the 3BF removal in AC-CuCo2O4/PMS system; (D) TEMP spin-trapping EPR spectra in AC-CuCo2O4/PMS system. Reaction conditions: [MA] = [TBA] = 5 mol/L, [L-histidine]:10 mmol/L, [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 °C.
Scheme 1 Proposed mechanism of PMS activation on AC-CuCo2O4 for 3BF removal.
Highlights
AC-CuCo2O4 exhibited excellent catalytic activity and reusability for PMS activation.
AC introduction enhanced the catalytic activity of CuCo2O4 for PMS activation.
The mechanism of AC-CuCo2O4 for activating PMS differed from the reported CuCo2O4.
1
Novelty Statement In this work, AC-CuCo2O4 was successfully prepared by a facile method. The catalytic activity investigations indicated that combining CuCo2O4 with AC for PMS activation resulted in the superior catalytic activity for 3BF removal than that of pure CuCo2O4. In addition, AC-CuCo2O4/PMS exhibited outstanding catalytic activity and good reusability for PMS activation. Importantly, AC-CuCo2O4 could effectively remove 3BF over a wide pH range and possessed temperature-tolerant performance. Interestingly, the mechanism of AC-CuCo2O4 for activating PMS differed from the reported CuCo2O4.
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants”
S0013-9351(20)30137-7
DOI:
https://doi.org/10.1016/j.envres.2020.109245
Reference:
YENRS 109245
To appear in:
Environmental Research
Received Date: 21 August 2019 Revised Date:
9 January 2020
Accepted Date: 7 February 2020
Please cite this article as: Chen, S., Liu, X., Gao, S., Chen, Y., Rao, L., Yao, Y., Wu, Z., CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants, Environmental Research (2020), doi: https:// doi.org/10.1016/j.envres.2020.109245. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Inc.
CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants
Shan Chen1, Xiudan Liu1, Shiyuan Gao, Yanchao Chen, Longjun Rao, Yuyuan Yao∗, Zhiwei Wu
National Engineering Lab of Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, PR China
1 ∗
These authors contributed equally to this work. Corresponding author. Tel.: +86 571 86843810; fax: +86 571 86843255.
E-mail address: [email protected] (Y. Y. Yao). 1
Abstract
CuCo2O4 was synthesized via a relatively simple method, and innovatively supported onto the activated carbon (AC) by calcination to obtain a novel heterogeneous catalyst (AC-CuCo2O4). Brilliant red 3BF (3BF) was selected as the probe compound to investigate the catalytic activity of AC-CuCo2O4 in the presence of peroxymonosulfate (PMS). The results showed that 98% removal rate could be achieved and the reaction rate constant (0.476 min-1) was 5.2 times greater than that of CuCo2O4 alone (0.091min-1), suggesting that the introduction of AC could greatly enhance the catalytic activity of pure CuCo2O4. Typically, the 3BF removal was as high as 96% after five cycles, showing the good stability of catalyst reuse. Additionally, the effects of the initial pH, catalyst dosage, PMS concentration and reaction temperature on the 3BF removal were investigated, demonstrating that AC-CuCo2O4 effectively remove 3BF over a wide pH range (5.0–10.0) and possessed temperature-tolerant performance. To further explore the 3BF removal mechanism, electron paramagnetic resonance technology combining with trapping agents was employed to confirm the involvement of reactive oxygen species including SO4•−, •OH, O2•− and 1
O2, which distinctly differed from the reported CuCo2O4 for PMS activation. These findings
provided an addition promising strategy in environmental remediation.
Keywords: peroxymonosulfate (PMS), activated carbon, heterogeneous catalyst, organic contaminants
1. Introduction Recently, water pollution has become one of the significant issues with the rapid development 2
of industrialization, which has triggered the worldwide concerns. In the past years, considerable research efforts have been devoted to environment remediation, particularly, advanced oxidation processes (AOPs) has received a great deal of attention due to the promising capability in effective water treatment [1-3]. Among AOPs, methods based on peroxymonosulfate (PMS) activation to produce reactive radicals have been broadly applied owing to the unique characteristics including the higher oxidation potential, a wider pH range and easier storage and transportation than H2O2 as one of the most common oxidants [4, 5]. Various approaches for PMS activation such as UV irradiation [6], heat [7], alkaline [8] and transition metals [9], can effectively degrade the emerging organic contaminants. Among the diverse methods investigated, transition metals have been considered as a more feasible and efficient method for activating PMS. Typically, Co2+ is recognized as the best PMS activator, compared to ZVI, iron-based catalysts and other transition metal catalysts, yet the dissolution of stubborn toxic Co limits the long-term applications to great extent [10-12]. Therefore, the heterogeneous catalytic systems using supported cobalt [13, 14] or cobalt oxides and derived composites [15] have been developed to be the alternative pathways to avoid potential Co leaching. In the past decade, spinel with the general formula AB2X4, has been continuously used in several applications such as energy storage and environmental treatments, because of their easy magnetic collection, high catalytic activity and good chemical stability [16]. Among spinel type oxides, cobalt-based spinel oxides due to the promising activity, low cost, high stability and environmental benignity, have been actively pursued [17]. In particular, substituted Co3O4 with Cu, CuCo2O4, is a typical spinel and has gained much attention by virtue of their electrochemical properties in diverse applications such as sensors, anode materials and capacitors [18-20]. These 3
properties are significant in the interfacial charge transfer processes for redox reactions. A few studies have attempted to correlate CuCo2O4 to electrocatalysis [21, 22], nevertheless, the catalytic performance of CuCo2O4, especially the environment remediation, is rarely reported. In addition, compared with Co3O4, CuCo2O4 not only shows higher catalytic performance but the leaching of heavy metals from the solid phase can be significantly reduced owing to the relatively stable spinel structure. To be precise, it is not at all inevitable that the small molecular catalysts used directly in environment treatment will bring about the decrease in catalytic activity and inconvenient recovery. It is a common practice to introduce a support for addressing the above mentioned problems, for example, Ning et al. designed a hybrid electrocatalyst, CuCo2O4/N-rGO, which showed that the enhancement of electrocatalytic activity and stability could benefit from the graphene support introduction [23]. However, few studies on the CuCo2O4 supported catalyst were reported. Among the various materials, it was noticed that activated carbon (AC) has a widespread application in the field of catalysis either as supports or catalysts with the advantages of the huge specific surface area, high pore volume, cost-effective and environmental friendliness [24-26]. Accordingly, we conjecture that the introduction of AC enables CuCo2O4 to enhance the catalytic performance. On the one hand, AC not only can improve the adsorption capacity of CuCo2O4 for contaminants, but make CuCo2O4 uniform dispersion to provide more active sites. Additionally, AC may play a role in providing electrons to accelerate the generation of reactive oxygen species (ROS) [27]. To the best of our knowledge, the reports on the applications of CuCo2O4 activating PMS in the remediation of contaminated water are rarely focused, and there are no reviews in investigating the relationships between CuCo2O4 and AC. Therefore, we first proposed to construct a novel supported catalyst by coupling AC with CuCo2O4 (AC-CuCo2O4) to activate 4
PMS for organic contaminants removal. In this study, CuCo2O4 was prepared in a relatively facile manner, and then combined with AC by calcination to synthesize a superior catalyst (AC-CuCo2O4). 3BF is a typical azo dye and has gained much concern because of high toxicity, carcinogenicity and significant environmental risk, so 3BF was selected as the model pollutant to evaluate the catalytic performance of AC-CuCo2O4 towards PMS activation. The mechanism of the AC-CuCo2O4/PMS system was explored with the radical trapping experiments and EPR analysis, and the influencing operational parameters, such as catalyst dosage, PMS concentration, initial pH and reaction temperature, have been investigated in our work.
2. Experimental 2.1 Chemicals and reagents Activated carbon (AC), copper nitrate hydrate (Cu(NO3)2·xH2O, AR) and urea (CH4N2O, AR 99%) were obtained from Shanghai Macklin Biochemical Co., Ltd. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR 99%), L-histidine, ammonium fluoride (NH4F, AR 98%) and potassium peroxymonosulfate (PMS, 2KHSO5•KHSO4•K2SO4, AR) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). 5,5-dimethyl-pyrroline-oxide (DMPO, 97%) was provided by Beijing Bailingwei Chemical Technology Co., Ltd and 2,2,6,6-tetramethyl-4-piperidone (TEMP, 98%) was obtained from the Shanghai Saan Chemical Technology Co., Ltd. Other reagents applied in the experiments were obtained from the Hangzhou Mike Chemical Instrument Co., Ltd. (Hangzhou, China). All organic dyes, including Brilliant Red 3BF (3BF), Acid Red 1 (AR1), Reactive Red X-3B (X-3B), Reactive Brilliant (KN-R) and Reactive Yellow (M-3RE), were
5
commercial compounds without further purification. All solutions in the experiments were prepared with deionized water.
2.2 Synthesis of AC-CuCo2O4 In detail, AC was preliminarily impregnated into HNO3 (5 M) for 6 h at 80 oC. After that, the resultant AC was taken out and washed several times with deionized water, and dryed at 80 oC for 8 h. In terms of CuCo2O4 sample it was prepared by a solvothermal method according to a previous report with slight improvement [22]. Firstly, Cu(NO3)2•6H2O (0.5 mmol), Co(NO3)2•6H2O (1 mmol), NH4F (3 mmol) and CO(NH2)2 (6 mmol) were mixed and dissolved in 60 mL solution (20 mL aqueous ethanol and 40 mL deionized water). Subsequently, 1 g resultant AC was added into the above solution, and then treated with ultrasonic treatment for 2 min. Whereafter, the mixture solution was transferred to a 100 mL Teflon-lined autoclave and maintained at 95 °C for 12 h. The obtained sample was rinsed by more than 100 mL deionized water for 5 times until the pH of the filtrate kept stable under neutral condition and then dried at 80 °C overnight. The final product was then annealed in a pipe furnace and kept at 400 °C for 2 h with a heating speed of 5 °C/min under an argon atmosphere.
2.3 Catalytic activity tests The experiments of 3BF removal was carried out in a 100 mL conical flask and vibrated in a water bath at 25°C or other temperatures were regulated by a constant temperature shaker water bath. A typical reaction mixture contained 3BF (0.1 mmol/L), AC-CuCo2O4 (0.2 g/L) and PMS (0.4 mmol/L). During each interval, the reaction solution was withdrawn using a syringe and filtered, and then analyzed with UV-vis spectrometer (Unico V-1800) at a wavelength of 542 nm. The adjustment of initial pH was dependent on NaOH or H2SO4 before catalyst and oxidant 6
addition. According to the same method mentioned above, the catalytic performance of CuCo2O4, CuCo2O4/PMS and AC/PMS were also performed. 2.4 Catalyst characterizations Scanning electron microscopy coupled to energy dispersive spectrometer (SEM-EDS, JSM-7001F) was used to characterize the catalyst. X-ray photoelectron spectroscopy (XPS, Thermo Fisher 250XI) was conducted with monochromatic Al Ka X-ray source to characterize AC-CuCo2O4. The binding energies were calibrated utilizing C1s peak at 284.7 eV as reference.
2.5 Analytical methods The pseudo-first-order kinetics was used to evaluate the catalytic reaction kinetics according to Eq. (1): k = −ln(C/C0)/t
(1)
where C was the concentration of pollutants at time t, C0 referred to the initial pollutants concentration, and the pseudo-first-order reaction rate constant (kobs) was calculated from the experimental data. EPR spectra were analyzed using a Bruker A300 spectrometer with DMPO and TEMP as the spin-trapping agents.
3. Results and discussion 3.1 Characterization of catalysts Scanning electron microscope (SEM) images of AC and AC-CuCo2O4 catalyst samples were presented in Fig. 1(A–D). As depicted in Fig. 1(A) and (B), the surface of AC was smooth and there was no apparent covering on it. After CuCo2O4 was anchored on AC support, AC became relatively rough and some particles were distributed well on the surface of AC, as shown in Fig. 7
1(C) and (D), indicating that AC-CuCo2O4 had been synthesized successfully. Additionally, the elemental mapping images were employed to analyze the chemical composition of AC-CuCo2O4 in Fig. 1(E–G), revealing that the amount of Cu and Co were present on the surface structure. Furthermore, pore volume and the Brunauer-Emmett-Teller (BET) specific surface area were obtained by measuring the N2 adsorption/desorption, which were exhibited in Fig S1. To gain more insight into the detailed elemental composition and valence states of prepared catalyst, X-ray photoelectron spectroscopy (XPS) was carried out. In the whole scanning spectrum (Fig. 2(A)), it could be observed that the tiny new peaks of copper and cobalt were detected in the AC-CuCo2O4 compared with the bare AC. Fig. 2(B) and (C) exhibited the high-resolution XPS spectra of Cu and Co 2p peaks in the AC-CuCo2O4, respectively. The detailed content of other elements detected were summarized (in Fig. S2). For the XPS spectrum of Cu in Fig. 2(B), the binding energy positions of 932.78 (Cu 2p3/2) and 953.38 eV (Cu 2p1/2). The main peaks at 932.78 eV for Cu 2p3/2 were attributable to Cu (II), and the peaks at 953.38 eV for Cu 2p1/2 existed in Cu (II) oxide species [28, 29]. The spectrum of Co was presented in Fig. 2(C), the peaks with binding energies of 780.98 and 795.98 eV corresponded to Co 2p3/2 and Co 2p1/2, respectively, which were in accordance with Co (II) and Co (III), respectively [30]. Thus, the results of XPS spectra further verified that CuCo2O4 was successfully supported onto AC. Furthermore, there was no obvious metal valence change after the reaction (in Fig. S3), indicating the high stability of the as-prepared AC-CuCo2O4 for PMS activation during the catalytic process.
3.2 Catalytic activity evaluation 3BF as the target pollutant was selected to evaluate the catalytic activity of AC-CuCo2O4 in the presence of PMS, as displayed in Fig. 3(A). There was no obvious change in 3BF removal by only 8
PMS or AC was present, while 12.7% of 3BF was obtained in 10 min when AC and PMS existed simultaneously, suggesting that the AC could activate PMS for the 3BF elimination to some extent. In the presence of CuCo2O4 alone, negligible 3BF was removed, while the removal of 65.1% occurred when CuCo2O4 was combined with PMS, indicating that CuCo2O4 could activate PMS to remove 3BF in a certain degree. Notably, the removal rate increased to 98% when AC-CuCo2O4 and PMS existed simultaneously, showing that the enhanced PMS activation by CuCo2O4 benefitted from the introduction of AC. To our surprise, the catalytic activity in the AC-CuCo2O4/PMS system was superior to that in the Co3O4/PMS system and AC-Co3O4/PMS system under the same condition, in which the Co3O4 as a commercial catalyst could effectively activate PMS. To quantitatively determine the catalytic activity for the above curves on kinetics, the kobs were estimated and the results was depicted in Fig. 3(B). The kobs of AC-CuCo2O4/PMS system was calculated 0.476 min-1, which was 35, 11, 5 and 3 times better than that in AC/PMS, Co3O4/PMS, CuCo2O4/PMS and AC-Co3O4/PMS system, respectively, indicating that AC-CuCo2O4 possessed an excellent catalytic performance in removing 3BF. To further investigate the catalytic activity of AC-CuCo2O4 and CuCo2O4 for PMS activation, the experiments were carried out at different pH conditions, as displayed in Fig. 3(C). It showed that 3BF removal enhanced with increasing pH in a range from 5 to 10 in both AC-CuCo2O4/PMS and CuCo2O4/PMS system, and the maximum 3BF removal occurred at pH 10, which might be attributed to the charged state of PMS species. Under acidic pH conditions, the possible attachment of the H+ to the more electronegative peroxide bond (O–O) of the PMS induced the interfacial repulsion between PMS and the protonated catalytic sites (e.g. hydroxyl species) to inhibit the catalytic performance. However, the catalytic performance was greatly improved on the 9
alkaline condition, which could be ascribed to several factors. On the one hand, more surface OH-groups had capable of enhancing the electron density of the transition metal and act as the donor ligand to accelerate the PMS activation reaction. On the other hand, the activation of PMS would produce more powerful reactive oxygen species by base. Moreover, the catalytic activity of the AC-CuCo2O4/PMS system was evidently better than that of the CuCo2O4/PMS system in the same situation, further indicating that AC introduction could remarkably enhance the catalytic performance and improved the pH adaptability of CuCo2O4 for PMS activation. The reusability of AC-CuCo2O4 in the presence of PMS is a significant aspect in the practical applications. The catalyst was taken out after each recycling experiment and rinsed with deionized water, and then dried at 80 °C. As presented in Fig. 3(D), 3BF removal rate remained almost the same over 5 cycles, which suggested that AC-CuCo2O4 was not only an efficient but also a durable catalyst for PMS activation. Furthermore, an atlas of time series for 3BF removal in the AC-CuCo2O4/PMS system by UV absorption spectra were displayed (in Fig. S4). In this investigation, the catalytic activity of the AC-CuCo2O4/PMS system for removing several other contaminants was also examined, as shown in Fig. 4. It illustrated that the removal rates for the AC-CuCo2O4/PMS system were almost 98%, 98% and 99% within 10 min for X-3B, AR1 and ciprofloxacin (CIP), respectively. Additionally, about 90% of KN-R, M-3RE and phenol was removed within 15 min in the AC-CuCo2O4/PMS system. Additionally, an atlas of time series for the different dyes removal in the system by UV absorption spectra were displayed (in Fig. S5). The above results indicated that the AC-CuCo2O4/PMS system could exhibit good catalytic performances for different organic pollutants and held promising application prospects in water treatment. 10
3.3 Effects of the reaction temperature The reaction temperature of a catalyst is vital for its performance assessment and practical application. Therefore, the experiments were undertaken to investigate the reaction temperature influence on 3BF removal, as described in Fig. 5(A) and (B). The role of reaction temperature on 3BF removal in the AC-CuCo2O4/PMS system was presented in Fig. 5(A), which showed that 3BF removal was improved slightly when the reaction temperature varied from 25 to 55 °C. Comparatively, an obvious enhancement of removing 3BF was observed with the increase of the reaction temperature in the CuCo2O4/PMS system, as depicted in Fig. 5(B). For example, 53.8% 3BF removal was reached in the CuCo2O4/PMS system in 10 min at 25 °C, while when the reaction temperature was increased to 55 °C, the removal efficiency of 3BF could achieve 97.1% only 4 min. To further discuss the relationship, kobs of the AC-CuCo2O4/PMS and the CuCo2O4/PMS system were displayed in Fig. 5(C). The results suggested that the higher removal efficiency was achieved upon increasing the temperature, which might be ascribed to higher temperatures resulting in molecules enhanced movement. In further experiments, the operating temperature of 25 °C was selected since higher temperatures gave rise to an increase in the energy requirement. As shown in Fig. 5(D), the activation energy (Ea) for removing 3BF was calculated according to the Arrhenius plot (lnkobs = lnA − Ea/RT) for further discussion. The values of Ea for 3BF removal were calculated to be 38.29 kJ·mol−1 and 69.57 kJ·mol−1 in the AC-CuCo2O4/PMS and the CuCo2O4/PMS system, respectively, indicating that Ea in the AC-CuCo2O4/PMS system was less than that in CuCo2O4/PMS system. The above results demonstrated that the AC-CuCo2O4/PMS system could proceed easily to remove the organic pollutants, which could potentially reduce the energy consumption owing to the lower energy activation and expanded the 11
practical application of the system to some extent.
3.4 Effects of the catalyst dosage and PMS concentration Catalyst dosage is a significant factor during the catalytic process, therefore, the experiments were performed to estimate the influence of the catalytic activity in AC-CuCo2O4/PMS system, as illustrated in Fig. 6(A). It was generally noticed that the removal rate of 3BF distinctly increased as the AC-CuCo2O4 dosage ranging from 0.1 to 0.3 g/L, while the further increase of the AC-CuCo2O4 dosage could not obviously promote the removal rate. Moreover, the kobs values enhanced significantly from 0.325 to 0.538 min-1 with the dosage of AC-CuCo2O4 ranging from 0.1 to 0.3 g/L, while a slight increase was observed from 0.538 to 0.566 min-1 as AC-CuCo2O4 dosage increased to 0.5 g/L in Fig. 6(B). The improvement of the removal rate might be attributable to the increasing amount of active sites. Additionally, the influence of oxidant concentration varying from 0.1 to 2.0 mmol/L on 3BF removal was investigated in the AC-CuCo2O4/PMS system, as shown in Fig. 6(C). A remarkable improvement was observed when the PMS concentration increased from 0.1 to 0.4 mmol/L, while a decrease in the system with the further increase in PMS concentration. Obviously, similar tendencies of the kobs values for 3BF removal were presented in Fig. 6(D). In consideration of the cost and catalytic efficiency, 0.2 g/L AC-CuCo2O4 and 0.4 mmol/L PMS were chosen for the removal of 3BF in this study.
3.5 Analysis of reaction mechanism It is generally recognized that the free radicals are the dominant ROS for the organic contaminants degradation towards PMS activation [31, 32], so the radical quenching experiments were conducted to investigate the catalytic mechanism. Ascorbic acid (AA), a typical scavenger, can capture ROS such as superoxide radical (O2•−), sulfate radical (SO4•−) and hydroxyl radical 12
(•OH) during the catalytic process [33]. There was an obvious inhibition when AA was added into the AC-CuCo2O4/PMS system with the concentration ranging from 0 to 1.0 mmol/L, suggesting that the free radicals were involved in the catalytic process (in Fig. S6). In general, SO4•−and •OH were regarded as the primary types of ROS for PMS activation. Therefore, methanol (MA) was applied to quench both SO4•− and •OH, and tert-Butyl alcohol (TBA) was employed as a unique scavenger for •OH [34, 35]. As illustrated in Fig. 7(A), the removal rate achieved 30.5% at a MA concentration of 5 M, and 49.2% removal was reached when TBA was introduced into the AC-CuCo2O4/PMS system, indicating that both SO4•− and •OH might be generated during the catalytic process. EPR spectroscopy with DMPO as the spin trapping agent was further applied in the subsequent experiments. As displayed in Fig. 7(B), the signals of DMPO-OH ( ) and DMPOSO4•− ( ) adducts were detected from the EPR spectra, which further confirmed that SO4•− and •
OH participated in the 3BF removal during the catalytic process. It was noticed that the 3BF removal was not entirely inhibited when SO4•− and •OH were
scavenged by adding MA, so we speculated that there were other ROS in the AC-CuCo2O4/PMS system. To best of our knowledge, singlet oxygen (1O2) was a selective ROS to display negligible reactivity towards alcohols. Therefore, the trapping experiments were conducted using L-histidine as the scavenger for 1O2. As described in Fig. 7(C), a completely inhibitory effect could be observed with the addition of L-histidine, suggesting that 1O2 might be involved in the 3BF removal during the catalytic process. EPR spectroscopy was employed to further confirm our hypothesis, as presented in Fig. 7(D). Obviously, it was found that a typical 1:1:1 triplet signal of TEMP-1O2 adducts in the presence of TEMP, which further confirmed that 1O2 was indeed generated in this system. Furthermore, it has been reported that superoxide radical (O2•−) acting as 13
the precursor involved the formation of 1O2 [36]. Herein, benzoquinone (BQ) as the scavenger for O2•− was adopted, and the obvious inhibition could be observed when BQ was added in the system (in Fig. S7), suggesting that O2•− was produced in the removal process. The above results indicated that AC introduction improved the catalytic activity of CuCo2O4 for PMS activation, and the mechanism of AC-CuCo2O4 for activating PMS differed from the reported CuCo2O4. Hence, it is of great significance to further investigate the function of AC in the ROS production process. EPR spectroscopy was performed to ascertain whether electrons were involved in the AC-CuCo2O4/PMS system. The signals of the persistent free radicals (PFRs) were detected and the intensity decreased after the catalytic oxidation reaction (in Fig. S8), showing that the PFRs in the AC might be responsible for accelerating the key reaction step of Cu3+ to Cu2+ and Co3+ to Co2+ in the AC-CuCo2O4/PMS system[37]. On the basis of the above-mentioned results and detailed discussion, a possible catalytic mechanism for PMS activation by AC-CuCo2O4 was proposed in Scheme 1. First of all, Co3+ preliminarily reacted with PMS to form Co2+ and SO5•−, and then Co2+ reacted with HSO5− to produce SO4•− with the production of SO4•− and •OH (Eqs. (2) – (4)) [38, 39]. Co3+ + HSO5− →Co2+ + SO5•− + H+
(2)
Co2+ + HSO5− →Co3+ + SO4•− + H2O
(3)
SO4•− +OH− → OH + SO42−
(4)
Meanwhile, Cu2+ activated PMS for 3BF removal based on a series of reactions as following (Eqs. (5) – (8)) [40, 41] and Cu3+ interacted with Co2+ to be reduced to Cu2+ at the same time. Cu2+-OH− + HSO5− → Cu2+-(HO)OSO3− + OH−
(5)
Cu2+-(OH)OSO3− → Cu3+-OH− + SO4•−
(6)
Cu3+ -OH− +HSO5− → Cu2+-•OOSO3− + H2O
(7)
Cu2+ -•OOSO3 + 2H2O → Cu2+-OH− + O2 + 2SO4•−+ 2H+ 14
(8)
Consequently, O2 was further involved in the formation of 1O2 (Eqs. (9) and (10)) [42] through electron transfer process. The PFRs in the AC might be responsible for accelerating the electron transfer of Cu3+ to Cu2+ and Co3+ to Co2+. Via a series of cyclic reactions for PMS activation by going through Co2+-Co3+ -Co2+ and Cu2+-Cu3+ -Cu2+ states, SO4•−, •OH and
1
O2 were
simultaneously generated to remove the organic pollutants in the AC-CuCo2O4/PMS system. O2 + e−→ O2•−
(9)
O2•−+ e−→ 1O2
(10)
Conclusion In this work, AC-CuCo2O4 was successfully prepared by a solvothermal method. The catalytic activity investigations confirmed that the combination of CuCo2O4 with AC for PMS activation resulted in the superior catalytic activity for 98 % of 3BF removal than that of pure CuCo2O4 (65 %). In addition, the 3BF removal of AC-CuCo2O4 was about 2.4 times more than that of the commercial Co3O4 catalyst for PMS activation alone (41%). Furthermore, the AC-CuCo2O4/PMS system exhibited low value of Ea (38.29 kJ·mol-1) for 3BF removal, which was significantly lower than that in CuCo2O4/PMS system. Combined with the EPR technology and the radical quenching experiments, the mechanism of the catalytic system was explored that the ROS such as SO4•−, •OH, O2•− and 1O2 contributed to the 3BF removal in the AC-CuCo2O4/PMS system, which significantly differed from the CuCo2O4 for activating PMS in previous studies. The reusability of AC-CuCo2O4 was also investigated and it was found that 96% removal of 3BF still remained during the PMS activation process after five successive runs, showing the good recyclability of AC-CuCo2O4 in the presence of PMS. Taking into account the superior temperature tolerance, highly catalytic activity and good recycling ability, AC-CuCo2O4 can be considered as a promising 15
catalyst to activate PMS for organic contaminants removal in practical water treatment.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51772274).
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Figure and Table Captions Fig. 1 SEM images of (A) and (B) AC, (C) and (D) AC-CuCo2O4. Elemental mapping images of AC-CuCo2O4 for (E) Cu and Co, (F) Cu and (G) Co.
Fig. 2 (A) XPS survey spectra of AC and AC-CuCo2O4. (B) Cu 2p and (C) Co 2p XPS spectra of AC-CuCo2O4, respectively. 21
Fig. 3 (A) 3BF removal versus time in different systems. (B) Rate constants of 3BF removal in different systems. (C) Influence of initial pH on 3BF removal in AC-CuCo2O4/PMS and CuCo2O4/PMS system, respectively. (D) Repeated recycling of AC-CuCo2O4 for the 3BF removal. Reaction conditions: [AC] = [AC-Co3O4] = [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, [3BF]: 0.1 mmol/L, [CuCo2O4]: 15 mg/L, pH 10, T=25 oC.
Fig. 4 Removal of different contaminants in the AC-CuCo2O4/PMS system. Reaction Conditions: [AR1] = [X-3B] = [Phenol]: 0.1 mmol/L, [M-3RE] = [KN-R] = [CIP]: 0.05 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 oC.
Fig. 5 Removal of 3BF in the (A) AC-CuCo2O4/PMS and (B) CuCo2O4/PMS system at different reaction temperatures; (C) kobs of 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system; (D) Arrhenius plots for 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [CuCo2O4]: 15 mg/L, [PMS]: 0.4 mmol/L, pH 10.
Fig. 6 (A) Effects of catalyst dosage on the 3BF removal in the AC-CuCo2O4/PMS system and (B) kinetic fitting. Conditions: [3BF]: 0.1 mmol/L, [PMS]: 0.4 mmol/L pH 10, T=25 °C. (C) Effects of PMS concentration on 3BF removal in the AC-CuCo2O4/PMS system; (D) the corresponding kobs removal of 3BF. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, pH 10, T=25 °C.
22
Fig. 7 (A) Radical trapping tests using MA and TBA 3BF removal in AC-CuCo2O4/PMS system; (B) EPR spectra of DMPO in AC-CuCo2O4/PMS system; (C) Radical trapping tests using L-histidine on the 3BF removal in AC-CuCo2O4/PMS system; (D) EPR spectra of TEMP in AC-CuCo2O4/PMS system. Conditions: [MA] = [TBA] = 5 mol/L, [L-histidine]:10 mmol/L, [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 °C.
Scheme 1 Proposed mechanism of PMS activation on AC-CuCo2O4 for 3BF removal.
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Fig. 1 SEM images of (A) and (B) AC, (C) and (D) AC-CuCo2O4. Elemental mapping images of AC-CuCo2O4 for (E) Cu and Co, (F) Cu and (G) Co.
Fig. 2 (A) XPS survey spectra of AC and AC-CuCo2O4. (B) Cu 2p and (C) Co 2p XPS spectra of AC-CuCo2O4, respectively.
Fig. 3 (A) 3BF removal versus time in different systems. (B) Rate constants of 3BF removal in different systems. (C) Influence of initial pH on 3BF removal in AC-CuCo2O4/PMS and CuCo2O4/PMS system, respectively. (D) Repeated recycling of AC-CuCo2O4 for the 3BF removal. Reaction conditions: [AC]=[AC-Co3O4]= [AC-CuCo2O4]: 0.2 g/L, [Co3O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, [3BF]: 0.1 mmol/L, [CuCo2O4]: 15 mg/L, pH 10, T=25 oC.
Fig. 4 Removal of different contaminants in the AC-CuCo2O4/PMS system. Reaction Conditions: [AR1] = [X-3B] = [Phenol]: 0.1 mmol/L, [M-3RE] = [KN-R] = [CIP]: 0.05 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 oC.
Fig. 5 Influence of different reaction temperature on the 3BF removal in the (A) AC-CuCo2O4/PMS and (B) CuCo2O4/PMS system; (C) kobs of 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system; (D) Arrhenius plots for 3BF removal in the AC-CuCo2O4/PMS and CuCo2O4/PMS system. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [CuCo2O4]: 15 mg/L, [PMS]: 0.4 mmol/L, pH 10.
Fig. 6 (A) Effects of catalyst dosage on the 3BF removal in the AC-CuCo2O4/PMS system and (B) kinetic fitting. Conditions: [3BF]: 0.1 mmol/L, [PMS]: 0.4 mmol/L，pH 10, T=25 °C. (C) Effects of PMS concentration on 3BF removal in the AC-CuCo2O4/PMS system; (D) the corresponding kobs removal of 3BF. Conditions: [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, pH 10, T=25 °C.
Fig. 7 (A) Radical trapping tests using MA and TBA on the removal of 3BF in AC-CuCo2O4/PMS system; (B) DMPO spin-trapping EPR spectra in AC-CuCo2O4/PMS system; (C) Radical trapping tests using L-histidine on the 3BF removal in AC-CuCo2O4/PMS system; (D) TEMP spin-trapping EPR spectra in AC-CuCo2O4/PMS system. Reaction conditions: [MA] = [TBA] = 5 mol/L, [L-histidine]:10 mmol/L, [3BF]: 0.1 mmol/L, [AC-CuCo2O4]: 0.2 g/L, [PMS]: 0.4 mmol/L, pH 10, T=25 °C.
Scheme 1 Proposed mechanism of PMS activation on AC-CuCo2O4 for 3BF removal.
Highlights
AC-CuCo2O4 exhibited excellent catalytic activity and reusability for PMS activation.
AC introduction enhanced the catalytic activity of CuCo2O4 for PMS activation.
The mechanism of AC-CuCo2O4 for activating PMS differed from the reported CuCo2O4.
1
Novelty Statement In this work, AC-CuCo2O4 was successfully prepared by a facile method. The catalytic activity investigations indicated that combining CuCo2O4 with AC for PMS activation resulted in the superior catalytic activity for 3BF removal than that of pure CuCo2O4. In addition, AC-CuCo2O4/PMS exhibited outstanding catalytic activity and good reusability for PMS activation. Importantly, AC-CuCo2O4 could effectively remove 3BF over a wide pH range and possessed temperature-tolerant performance. Interestingly, the mechanism of AC-CuCo2O4 for activating PMS differed from the reported CuCo2O4.
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “CuCo2O4 supported on activated carbon as a novel heterogeneous catalyst with enhanced peroxymonosulfate activity for efficient removal of organic pollutants”