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Heterogeneous Degradation of Organic Pollutants by Persulfate Activated by CuO-Fe3O4: Mechanism, Stability, and Effects of pH and Bicarbonate Ions Yang Lei,†,‡ Chuh-Shun Chen,‡ Yao-Jen Tu,§ Yao-Hui Huang,*,‡,∥ and Hui Zhang*,† †

Department of Environmental Engineering, Wuhan University, Wuhan 430079, China Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan § Institute of Urban Study, Shanghai Normal University, No.100 Guilin Rd. Shanghai 200234, China ∥ Sustainable Environment Research Center, National Cheng Kung University, Tainan 701, Taiwan ‡

S Supporting Information *

ABSTRACT: Magnetic CuO-Fe3O4 composite was fabricated by a simple hydrothermal method and characterized as a heterogeneous catalyst for phenol degradation. The effects of pH and bicarbonate ions on catalytic activity were extensively evaluated in view of the practical applications. The results indicated that an increase of solution pH and the presence of bicarbonate ions were beneficial for the removal of phenol in the CuO-Fe3O4 coupled with persulfate (PS) process. Almost 100% mineralization of 0.1 mM phenol can be achieved in 120 min by using 0.3 g/L CuO-Fe3O4 and 5.0 mM PS at pH 11.0 or in the presence of 3.0 mM bicarbonate. The positive effect of bicarbonate ion is probably due to the suppression of copper leaching as well as the formation of Cu(III). The reuse of catalyst at pH0 11.0 and 5.6 showed that the catalyst remains a high level of stability at alkaline condition (e.g., pH0 11.0). On the basis of the characterization of catalyst, the results of metal leaching and EPR studies, it is suggested that phenol is mainly destroyed by the surface-adsorbed radicals and Cu(III) resulting from the reaction between PS and Cu(II) on the catalyst. Taking into account the widespread presence of bicarbonate ions in waste streams, the CuO-Fe3O4/PS system may provide some new insights for contaminant removal from wastewater.

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INTRODUCTION With the decrease of freshwater resources, recovery of water from wastewater is increasingly important for the sustainable development of the world.1 Among the various water remediation technologies, advanced oxidation processes (AOPs), which are based on the generation of reactive oxygen species (ROS), are regarded as an effective technology for the degradation of hazardous organic pollutants in wastewater.2,3 Therefore, much effort has been invested to improve the treatment efficiency of AOPs as well as extending their working conditions and cutting down on secondary pollution. In contrast to homogeneous AOPs, heterogeneous AOPs have received extensive attention as they can usually work in mild conditions and produce almost no byproduct sludge after reaction.4,5 Catalysts have played a very important role in heterogeneous AOPs with iron oxides (e.g., Fe3O4 and γ-Fe2O3)6,7 and ironimmobilized silica4,8 being particularly studied due to the fact that they are highly accessible because iron is the second most abundant element in the earth’s crust.9 For example, our group previously fabricated shape-controlled nano Fe3O4 as an efficient Fenton-like catalyst for the mineralization of phenol.7 However, many of those catalysts are often confronted with weak catalytic © 2015 American Chemical Society

activity, which often needs the aid of electricity, ultrasonic sound and UV or visible light irradiation.10,11 However, a drawback is that the introduction of external energy increases the cost of the treatment process. Alternatively, an efficient way to improve the catalytic activity is to replace some activate sites of the iron oxides with other metal ions. Accordingly, the new formed catalysts, named (Co, Cu, Zn)Fe2O4, have been the focus of many studies in past decades.12−18 The bimetallic catalyst, CuFe2O4 has found the most favor, not only because of its good catalytic performance but also copper is not regarded as a potential carcinogen.19 Some researchers have reported that the CuFe2O4 activated peroxymonosulfate (PMS) process is quite effective for the destruction of organic contaminants in water.15−17 However, it was also concluded by Guan et al.17 that only PMS could be easily activated by CuFe2O4. Indeed, the asymmetric structure of PMS appeared to makes it more readily activated than persulfate (PS) Received: Revised: Accepted: Published: 6838

February 4, 2015 May 4, 2015 May 8, 2015 May 8, 2015 DOI: 10.1021/acs.est.5b00623 Environ. Sci. Technol. 2015, 49, 6838−6845

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Environmental Science & Technology and hydrogen peroxide (H2O2) which have symmetrical structures.20 However, as is well-known, PS is the most suitable of the three common used oxidants in AOPs because it is cheaper than PMS and more stable than H2O2 while have similar oxidation ability after activation.20,21 In order to effectively activate PS, it is therefore necessary to develop new catalysts. Recently, the activation of PS by copper oxide particles was reported whereby Liang et al.22 explored the degradation of pchloroaniline by copper oxidate activated PS and concluded that different radicals were involved at various solution pHs. However, Zhang et al.23 reported that the CuO/PS coupled system could selectively attack some organic pollutants but did not rely on the generation of radicals. Thus, apparently, there is some controversy about the mechanism of copper oxide activated PS process. On the other hand, although copper oxide showed good performance under laboratory conditions, the leaching of hazardous copper ions may produce separate problems in industrial applications. From the viewpoint of the possible application of copper based AOPs for wastewater treatment, it is therefore urgent to solve these problems, as well as determining the mechanism of the CuO activated PS process. A possible way to overcome these drawbacks is to combine CuO with Fe3O4, which simultaneously utilizes the high catalytic property of CuO and the magnetic property of Fe3O4. Therefore, in this current study, CuO-Fe3O4 was fabricated using a simple one-step method and its role as a heterogeneous catalyst for activating PS was studied. Specially, the effect of pH on the coupled process (CuO-Fe3O4/PS) was especially studied. The primary reason why solution pH greatly affected the performance and the stability of CuO-Fe3O4 may be mainly due to the leaching of copper ions, rather than the electrostatic interaction between the substrate and catalyst, which had been suggested previously.16,17 In addition, as bicarbonate is widely present in natural water and wastewater, the influence of NaHCO3 in CuOFe3O4/PS system was also studied. Interestingly, it was found that the coupled system performed well at higher pH and even better in the presence of bicarbonate. Finally, based on the characterization of the catalyst and the experimental results, the mechanism of CuO-Fe3O4 activated PS process is clearly elucidated.

III), scanning electron microscopy coupled to energy dispersive spectrometer (SEM-EDS, JEOL JSM-6700F), and Brunauer− Emmett−Teller (BET) sorptometer were used to characterize the catalyst. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was conducted for the determination of the chemical species of copper and iron. Experimental Procedure. Batch trials were performed in glass beakers with phenol solution (400 mL, 0.1 mM). The catalyst (0.12 g) was added, followed by the PS (5.0 mM) to start the reaction. Where required, the initial pH (pH0) was adjusted by addition of appropriate amounts of H2SO4 (0.1 M) or NaOH (0.1 M). During the investigation of the effect of NaHCO3 appropriate amounts of NaHCO3 were added before addition of PS. Samples were removed at predetermined time intervals and were filtered through 0.22 μm syringe filters prior to analysis. Unless specifically noted, all the experiments were carried out at an ambient temperature of 20 ± 2 °C and were exposed to air. EPR Studies. For Electron Paramagnetic Resonance (EPR) studies, experiments were conducted using DMPO as a spintrapping agent. A solution containing 10 mM DMPO, 5.0 mM PS was prepared at pH 5.6 ± 0.1, and then catalyst was added to initiated the reaction. After 5 min of reaction, samples were taken and analyzed on a JEOL FA200 spectrometer at room temperature. EPR measurements were conducted using a radiation of 9.147 GHz (X band) with a modulation frequency of 100 kHz, modulation width of 0.1 mT, a sweep width of 20 mT, center field of 326.0 mT, scan time of 60 s, time constant of 0.03 s, and microwave power of 5 mW. Analysis. The phenol concentration were determined with high performance liquid chromatograph (HPLC, Shimadzu 6A) equipped with a TSK-GEL ODS-100S column (4.6 mm × 250 mm) and a UV detector at wavelength of 270 nm. An eluent of methanol and water (V/V, 70/30) was used as the mobile phase with a constant flow rate (0.8 mL/min). The injection volume was 20 μL and the retention time was 5.2 min. The mineralization of phenol was measured with a TOC Analyzer (Sievers 900, GE). The zero point charge (pHzpc) of CuO-Fe3O4 was determined by mass titration.17 The metal ions were quantified by inductively coupled plasma optical emission spectrometer (ICP-OES, ULTIMA 2000, HORIDA).

MATERIALS AND METHODS Chemicals. Phenol and methanol were obtained from SigmaAldrich, Taiwan. Sodium persulfate and sodium bicarbonate were bought from Merck, Taiwan. The 5,5-dimethyl-1pyrrolidine N-oxide (DMPO) was purchased from Aladdin, China. All reagents used were at least analytical grade and prepared in Milli-Q water. Preparation and Characterization of Catalyst. The CuO-Fe3O4 catalyst was fabricated using a simple and one-step hydrothermal method. CuSO4·5H2O (500 mL, 0.1 M) and FeSO4·7H2O (500 mL, 0.5 M) were mixed under continuous air purging (flow rate = 3 L/min). During the reaction, the pH and temperature were kept constant at 8.0 and 80 °C, respectively. The fabrication process can be simply expressed as follows:

RESULTS AND DISCUSSION Characterization of the CuO-Fe3O4 Catalyst. Figure 1(A) shows the XRD pattern of CuO-Fe3O4 at 2θ from 10 to 70°. The sharpness of XRD reflections clearly illustrates that the synthesized catalyst is highly crystalline. The well matched peaks with standard Fe3O4 (JCPDS: 65-3107) and CuO (JCPDS: 45-0937) demonstrate that the fabricated catalyst is CuO-Fe3O4. The EDS results (see Figure 1(B)) show that the catalyst is mainly formed of the three elements, O, Cu and Fe. In addition, the atomic% ratio of Fe to Cu is 4.7, which is very close to the designed value (5.0). Furthermore, the SEM photo (Figure 1(B)) shows that the size of the fabricated CuO-Fe3O4 was at nano level and this was further confirmed by BET analysis (Supporting Information Table S1). These results clearly show that nano sized CuO-Fe3O4 was successfully prepared by the simple one-step method. Phenol Degradation. As can be seen from Figure 2, while it was not removed by PS or CuO-Fe3O4 alone, phenol was effectively degraded when they were used together. The removal and mineralization percentages of phenol were 80% (Figure 2) and 78% (Supporting Information Figure S1) in 120 min, respectively. All these evidence suggest that CuO-Fe3O4 coupled

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3Fe2 + + 6HO− + 0.5O2 → Fe3O4 + 3H 2O

(1)

Cu 2 + + 2HO− → CuO + H 2O

(2)

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The desired solids were separated with a magnet and then washed 3 times with 1.5 L distilled water and dried at 105 °C. Various techniques such as Fourier transform infrared (FTIR, Bruker Tensor 27), X-ray powder diffraction (XRD, Rigaku, RX 6839

DOI: 10.1021/acs.est.5b00623 Environ. Sci. Technol. 2015, 49, 6838−6845

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Environmental Science & Technology

the experiment. To check the effect of leached Cu2+, control experiments with Cu2+ activated PS were performed. Figure 3

Figure 3. Degradation of phenol with Cu2+ /PS process. Conditions: [PS] = 5.0 mM, [phenol] = 0.1 mM. [Cu2+] = 5.6−64 mg/L, pH0 5.6 ± 0.1, H2A means ascorbic acid.

indicates that no phenol was degraded at the leached Cu2+ concentration (5.6 mg/L), and no phenol was removed even when the dose of Cu2+ was increased to 64 mg/L. These results showed that the potential effect of leached ions was negligible and also demonstrated that the degradation process only occurred at the surface of catalyst. Moreover, the heterogeneous reaction mechanism was supported by a 44% inhibition of phenol removal when a magnet was used to attract CuO-Fe3O4 (Supporting Information Figure S2). Effect of pH. It is well-known that the performance of homogeneous AOPs is strongly correlated to the solution pH.24 Recently, some studies that focused on copper oxide activated PS and CuFe2O4/PMS systems also found that the processes were influenced by pH.16,17,22 Therefore, the degradation of phenol at different solution pH was explored and Figure 4(A) shows that the degradation of phenol is obviously pH dependent. There was almost no phenol removed at pH0 2.5. The amount removed increased dramatically, reaching 68%, at pH0 4.0 and then smoothly increased with the increase of pH0 before it reached 8.5 (80%). Thereafter, there was a second apparent jump when the pH0 was increased to 10.0. The phenol removed was further increased if the pH0 rose to 12.0 (98%). On the other hand, when the reaction rate constant in the first stage (10 min) of removal was calculated (Supporting Information Figure S3), it was found that the reaction rate constant increased by 5 times when the pH0 was increased from 4.0 to 12.0, as seen by rates of 0.001 and 0.005 mM/min, respectively. Thus, the effect of pH0 on the degradation of phenol is clear, the higher the solution pH, the higher the removal rate. Nevertheless, as stated, the electrical interaction between catalyst and pollutants may not fully explain the results in our system. For instance, the electrical interaction between CuO-Fe3O4 and phenol (pKa = 10) should be ignored at the pH0 range (2.5−7.0), as the pHzpc of the CuO-Fe3O4 is about 7.3. Furthermore, at pH0 above 10.0, the phenol is present in its deprotonated form and the catalyst is also negatively charged, which means electrical repellent is dominant between phenol and CuO-Fe3O4. Thus, it is speculated that there must be other factors rather than electrical interaction that largely affect the degradation process. In order to better understand the impact

Figure 1. XRD pattern (A) and SEM-EDS data (B) of the fabricated catalyst.

Figure 2. Degradation efficiency of phenol and concentrations of leached metal ions. Conditions: [PS] = 5.0 mM, [phenol] = 0.1 mM, [CuO-Fe3O4] = 0.3 g/L, pH0 5.6 ± 0.1.

with PS is an effective system for the degradation of organic pollutants from water. Concentrations of leached metals were also monitored in this process. Considering the solution at pH = 5.6 ± 0.1, Figure 2 shows that there was almost no iron leached while the amount of copper ions in solution gradually increased over the 120 min of 6840


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buffering ability of the catalyst.17 However, if the solution pH0 is very high or very low then the buffering ability of CuO-Fe3O4 becomes negligible. For example, in the case of 12.0 and 2.5, the solution pH did not change much during the reaction process. The loss of catalytic ability of CuO-Fe3O4 at pH0 2.5 is mainly due to the high leaching of the active component in the strong acid conditions. This can be seen from Figure 4(C) where values as high as 32.8 mg/L of copper ion and 0.6 mg/L of iron ion can be detected by ICP-OES after 120 min reaction. Another reason can be attributed to the adverse effect of H+ on PS activation.23,25 In the pH0 range of 4.0−10.0, the leaching of copper ions gradually declined from 7.7 mg/L (pH0 4.0) to 1.3 mg/L (pH0 10.0). By contrast, almost no leaching of iron ions can be detected at all these pH0 values. The XRD characterization of the catalyst that was produced at different values of pH0 provides more powerful evidence, as shown in Supporting Information Figure S4. The typical peak of CuO at 2θ = 38.7° and 48.7° has totally disappeared at pH0 2.5 which indicates that this is responsible for the loss of catalytic ability of CuO-Fe3O4. This evidence also indicates that copper is the active component in the catalyst, whereas iron may not directly involve in the activation of PS. On the other hand, the rise of pH0 is also beneficial for the formation of surface hydroxyl groups on the catalyst, as illustrated by the increased intensity of the band at 3454 cm−1 (Supporting Information Figure S5). The generated surface hydroxyl groups can behave as an active site for electron transfer and thus improve the performance of the catalyst.16,26 However, the enhanced removal of phenol in highly basic solution cannot be simply attributed to metal leaching and the formation of hydroxyl groups. The work investigated by Ahmad et al.27 could provide some insight into the good performance of CuO-Fe3O4/ PS for the degradation of phenol at pH0 11.0 and 12.0. Based on their research, PS can be activated by the phenoxide ion which will be formed from phenol (pKa = 10.0) when pH0 is 11.0 and 12.0. Additionally, alkaline conditions will increase the reactively of PS and this could account for the increased removal of phenol at higher pH0.28 This is supported by control experiments, as shown in Supporting Information Figure S6, where 14% and 18% of phenol were removed at pH0 11.0 and 12.0 respectively, in the absence of any catalyst. Effect of NaHCO3. Bicarbonate is usually present in the aquatic environment at the concentration of 1.0 to 5.0 mM.29 Generally, bicarbonate was previously thought to have a negative effect on AOPs as it was a radical quencher.30 However, some recent studies suggest that in both homogeneous and heterogeneous AOPs, NaHCO3 can accelerate the degradation of pollutants.30,31 As a preliminary step in investigating the potential application of CuO-Fe3O4/PS system in real water, the effect of bicarbonate on CuO-Fe3O4/PS system at its natural concentration was studied. As can be noticed from Figure 5(A), the removal of phenol increased with the addition of NaHCO3 and reached a plateau when 3.0 or 5.0 mM NaHCO3 were added. Interestingly, a further rise in NaHCO3 concentration to 10 mM slightly inhibited the degradation process but the amount removed (98%) is still about 18% higher than the control group (without NaHCO3). Analysis of the TOC results, as illustrated in Supporting Information Figure S1, showed a decrease in TOC of over 95% either at pH0 11.0 or in the presence of 3.0 mM NaHCO3. This almost 100% removal of TOC means that the final degradation products of phenol were CO2 and H2O. This evidence once again suggests that the CuO-Fe3O4/PS system is

Figure 4. (A). Influence of pH0 on the degradation of phenol in CuOFe3O4/PS system. (B) pH variation and (C) metal leaching during the reaction in the CuO-Fe3O4/PS system. Conditions: [PS] = 5.0 mM, [phenol] = 0.1 mM, [CuO-Fe3O4] = 0.3 g/L, Time = 120 min.

of pH, the change of solution pH with time elapse was monitored and the concentration of leached free metal ions was measured for all pHs studied. Interestingly, it was found that the removal trend of phenol at various pH0 corresponds to the change of solution pH during the reaction, as well as the leaching of Cu2+. Figure 4(B) shows that the pH values remain unchanged after 40 min at 5.6 ± 0.1 when the pH0 of the solutions were 4.0, 5.6, and 8.5 probably due to the 6841


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from 5.6 mg/L to less than 0.1 mg/L when 1.0 mM NaHCO3 was added (Figure 5(C)). Furthermore, when more than 5.0 mM NaHCO3 was added, almost no copper ions could be determined in solution. Consequently, it is safe to say that the increased solution pH and the decreased leaching of copper ions are mainly responsible for the better performance of CuO-Fe3O4/PS/ NaHCO3 system. Another role that NaHCO3 plays may be its complexing ability with Cu2+. Very recently, Chen et al.31 calculated the Cu(II) species under different bicarbonate concentration and found that Cu2+ (0.03 mM) exists mostly as CuCO3 when the concentration of bicarbonate was between 1.0 and 10 mM. They also suggested that only CuCO3 was the main species responsible for the formation of the reactive species, Cu(III), which further led to the decolorization of acid orange 7 in their Cu2+-HCO3−-H2O2 system. The formation of high oxidation state metals in bicarbonate/H2O2 coupled process is not new. In fact, one intermediate in the mechanism that Lane et al.32 speculated for Mn(II)−catalyzed epoxidations of alkenes by bicarbonate/H2O2 is the formation of Mn(IV). As PS has a similar structure to H2O2, it was believed that PS could react with bicarbonate ions associated with the formation of peroxymonocarbonate via eq 3, which may further accelerate the conversion of Cu(II) to Cu(III)31 but probably does not rely on the direct reaction between Cu(II) and PS. HCO3− + S2 O82 − → HCO4 − + SO4 − + SO3−

(3)

Mechanistic Study. Two major alternative mechanisms have been proposed on copper activated persulfate: the free radical process15−17,22 versus a nonradical process.23 In general, the free radical is formed as a result of the electron transfer from copper to persulfate, as shown in eqs 4-6. By contrast, the nonradical process assumed that the outer-sphere interaction between copper and PS makes PS more active to some organic pollutants. Cu 2 + + PMS/H 2O2 → Cu+ + SO5•− /O2•−

(4)

Cu+ + PMS/H 2O2 → Cu 2 + + SO4•− /HO•

(5)

Cu 2 + + PMS/PS/H 2O2 → Cu 3 + + SO4•− /HO•

(6)

As stated above, it seems that alternative mechanisms are involved in different situations. First, the mechanism based on eqs 4 and 5 is ruled out in our system as PS has limited reduction ability. This can be further confirmed by our control studies as shown in Figure 3 where it can be seen that only when Cu2+/PS was kinetically activated by ascorbic acid, can phenol be degraded to any extent. This result indicates that Cu+ (generated from the reduction of Cu2+ by ascorbic acid) but not Cu2+ was effective for the activation of PS and this was in line with previous reports. For example, Liang et al.22 found that Cu2+ reacts much less with PS than with copper oxidate. Similarly, Liu et al.33 explored the destruction of propachlor with a Cu2+/PS system and although 70% of propachlor could be removed, the process took 72 h. The higher activity of Cu(II) on the catalyst surface than soluble Cu2+ in aqueous solution is likely due to the higher E0Cu(III)/Cu(II) in solid phase than that in aqueous solution.16 It was reported that the reduction potential of Cu(III)/Cu(II) is to be 2.3 V in solid form and 1.57 V in the ionized phase.16,34 Thus, much published work as well as our own results indicate that the formation of Cu(I) and hence a Cu(I) based mechanism is not possible in our system. The second aspect that is worthy of discussion is whether radicals are produced or not. In order to check this, radical

Figure 5. (A). Influence of NaHCO3 concentration on the degradation of phenol in the CuO-Fe3O4/PS system. (B) pH variation and (C) metal leaching during the reaction in the CuO-Fe3O4 /PS/NaHCO3 system. Conditions: [PS] = 5.0 mM, [phenol] = 0.1 mM, [CuO-Fe3O4] = 0.3 g/ L, Time = 120 min.

highly effective for the treatment of organic wastewater, even in the presence of bicarbonate. To fully understand the role of NaHCO3, pH variation and the leached free metal ions were also monitored. In general, due to the buffering ability of NaHCO3, the pH stayed constant at 8.0 ± 0.5 (Figure 5(B)) when 1.0 to 10 mM NaHCO3 was added although the more NaHCO3 added the higher the solution pH achieved. Correspondingly, the amount of Cu2+ leached dropped 6842


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Environmental Science & Technology quenching experiments were first explored. It can be noticed from Figure 6(A) that the removal of phenol is only slightly

the surface of CuO-Fe3O4.16 It is thus inferred the interaction between Cu(II) and PS leads to the formation of a weak bond at the surface of the CuO-Fe3O4 (eqs 7 and 8), accompanied by the generation of Cu(III) and SO4•− radicals. The formed Cu(III) is unstable in the absence of strong chealting anions33,35 and could result in the formation of HO• via eq 9.36 Cu(II) + S2O82 − → Cu(II) ···O3SO2 SO32 −

(7)

Cu(II)··· O3SO2 SO32 − → Cu(III) + SO4•− + SO4 − (8)

Cu(III) + H 2O → Cu(II) + HO + H •

+

(9)

Meanwhile, as the fabricated catalyst is a homogeneous mixture of CuO and Fe3O4, it is important to exam the role of each metal oxide. The control experiments using CuO or Fe3O4 as catalyst were investigated. Supporting Information Figure S8 shows only 11% of phenol was removed when PS was activated by 0.3 g/L Fe3O4, while the phenol removal was 18% when Fe3O4 was replaced by 0.05 g/L CuO. It should be noted a 0.05 g/L dosage of CuO was selected based on the equivalent metal content of 0.3 g/L CuO-Fe3O4. A comparable removal of phenol could be achieved only when at a higher dosage of CuO, i.e., 0.3 g/L. Furthermore, EPR studies clearly show the intensity of DMPO adducts signals in CuO-Fe3O4/PS system was much stronger than the signals in CuO or Fe3O4 activated PS system (see Figure 6(B)). The synergistic effect of the combined CuOFe3O4 catalyst may result from the reduction of Cu(III) by Fe(II) as shown in eq 10, considering E0Cu(III)/Cu(II) = 2.3 V16,34 and E0Fe(III)/Fe(II) = 0.77 V.15 Cu(III) + Fe(II) → Fe(III) + Cu(II)

(10)

To further verify this speculation, XPS spectra of fresh and used CuO-Fe3O4 were recorded and the results are shown in Figure 7. Regarding the fresh catalyst, the high-resolution spectra of the peaks at 710.6 ev and 713.0 ev are indicative of the presence of Fe(II) and Fe(III).11 For the XPS spectra of Cu 2p3/2 region, the main peak at binding energy of 933.6 ev is assigned to Cu(II)37 and the satelite peaks at 940.9 ev and 943.2 ev are basically similar to that of dominated Cu(II) oxide species.38,39 After reaction, while copper species remains in Cu(II) status, the proportion of Fe(II) species declined from 53.0% to 44.1%, indicating the oxidation of Fe(II) into Fe(III) species. This is probabaly attributed to the reaction between Fe(II) and PS as well as the redox reaction between Cu(III) and Fe(II) (see eq 10). On the basis of all the results obatained above, a possible mechanism for PS activation by CuO-Fe3O4 is proposed. First of all, a Fenton-like reaction occurred between Cu(II) and S2O82− at the surface of CuO-Fe3O4 associated with the formation of Cu(III) and SO4•−, which may lead to the formation of HO• radicals via eqs 9 and 11, respectively. In particular, the regeneration of Cu(II) by Fe(II) favors the continuous decomposition of PS as well as the production of radicals at the surface of catalyst. In the meanwhile, the suface-adsorbed radicals may diffuse into the aqueous solution. All these radicals as well as Cu(III) account for the destruction of phenol.

Figure 6. Effects of methanol on the degradation of phenol and in CuOFe3O4/PS system (A) and EPR spectra in CuO-Fe3O4/PS, CuO/PS and Fe3O4/PS systems (B). Conditions (A): [PS] = 5.0 mM, [phenol] = 0.1 mM, [CuO-Fe3O4] = 0.3 g/L, Conditions (B): [PS] = 5.0 mM, [CuO] = 0.05 g/L, [Fe3O4] = 0.3 g/L, [DMPO] = 10 mM, pH0 5.6.

(10%) decreased when 2.5 M (500 times the molar concentration of PS) methanol is added to the reaction. The first speculation is that a nonradical process may be involved and this is consistent with the findings of Zhang et al.,23 who reported a CuO activated PS system is effective for the removal of 2,4dichlorophenol and does not rely on the formation of sulfate radicals. To verify this speculation, the EPR experiments was performed to detect any radical generated. The result clearly shows the exsitance of both SO4•− and HO• radicals, as noticed from Figure 6(B). Furthermore, when 2.5 M methanol was added to the solution, the intensity of DMPO adducts in CuOFe3O4/PS system decreased but the EPR signals were still observed (Supporting Information Figure S7). This indicates alcohols fail to capture surface-adsorbed radicals completely and consequently the addition of 2.5 M mathanol only scavenged about 10% of phenol degradation. As identified above, surface radicals are probably responsible for the destruction of phenol. Supporting Information Figure S5 shows the ATR-FTIR spectral change of CuO-Fe3O4 at pH0 11.0 and in prescence of 3.0 mM NaHCO3. As can be seen, there is a new small band formed at 1125 cm−1 as well as the blue shift of the band at 1180 cm−1, indicating the fomation of a complex at

H 2O + SO4•− → H+ + SO4 2 − + HO•

(11)

Stability of CuO-Fe3O4. For the stability evaluation of catalyst, CuO-Fe3O4 was magnetically separated and then washed three times with 1.5 L distilled water and dried at 105 °C. The stability of catalyst in two differnt reaction 6843


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Figure 7. .XPS spectra for Fe 2p regions (A) and Cu 2p regions (B) of fresh and used CuO-Fe3O4.

conditions (pH0 5.6 and pH0 11.0) was evaluated by their reuse performance. As can be seen from Figure 8 (A), when the catalyst was reused at pH0 5.6, phenol removal decreased greatly from 80% to less than 33% over 3 runs. This corresponds to the leaching of copper ions (Figure 2 and Figure 3(C)) as loss of Cu2+ will result in the decrease of active sites (Cu(II)) on the catalyst surface. On the other hand, as phenol was not 100% mineralized at pH0 5.6 (Supporting Information Figure S1), any intermediate products may attach to catalyst surface so both the leached Cu2+ and intermediates may result in the decrease in catalyst activity and thus poor repeated catalyst performance. However, when the solution pH0 is 11.0, over 70% of phenol could still be removed in the third run (Figure 8(B)). And it should also be noted that phenol could be further removed if the reaction time is extended beyond 120 min (data not shown). In order to identify the change in surface area, the BET properties of the CuO-Fe3O4 after reaction were also measured (Supporting Information Table S1). After three runs, the BET surface area increased slightly to 31.63 and 34.02 m2/g at pH 5.6 and pH 11.0, respectively and similar trend is observed for the pore size. The greater surface area and pore size may due to the leaching of metals. However, the CuO-Fe3O4 still remained in the spinel crystalline form in the cubic phase at pH 11.0 (Supporting Information Figure S4). All these information suggests that the CuO-Fe3O4 catalyst is relatively stable. As very limited leached Cu2+ was detected by ICP-OES, the reason for the gradually decline of the catalytic activity at pH 11.0 is not obvious and will be the focus of future work.

Figure 8. Catalytic property of CuO-Fe3O4 for repeated use at pH0 5.6 (A) and pH0 11.0 (B). Conditions: [PS] = 5.0 mM, [phenol] = 0.1 mM, [CuO-Fe3O4] = 0.3 g/L.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information associated with this article includes Table S1 and Figures S1−S8. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b00623.

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AUTHOR INFORMATION

Corresponding Authors

*(Y.H. Huang) Phone: 886-2757575 ×62636; fax: 886-62344496; e-mail: [email protected] *(H. Zhang) Phone: 86-27-68775837; fax: 86-27-68778893; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grant No. 20977069). The generous help of Professor David H. Bremner in polishing this manuscript is also greatly appreciated. 6844


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