Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic

Nov 14, 2018 - Biogas Institute of Ministry of Agriculture, Chengdu 610041 , P. R. China. # SCIEX Analytical Instrument Trading Co., Shanghai , 200335...
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Article Cite This: Environ. Sci. Technol. 2018, 52, 14302−14310

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Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic Sulfamethoxazole Jianfei Yan,† Jiali Peng,† Leiduo Lai,† Fangzhou Ji,† Yunhong Zhang,⊥ Bo Lai,*,†,‡,§,|| Qixuan Chen,† Gang Yao,‡,|| Xi Chen,# and Liping Song#

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†

Department of Environmental Science and Engineering, College of Architecture and Environment, Sichuan University, Chengdu 610065, P. R. China ‡ Sino-German Centre for Water and Health Research, Sichuan University, Chengdu 610065, P. R. China § National Engineering Research Center for Flue Gas Desulfurization, Sichuan University, Chengdu 610065, P. R. China || Institute of Environmental Engineering, RWTH Aachen University, Aachen 52056, Germany ⊥ Biogas Institute of Ministry of Agriculture, Chengdu 610041, P. R. China # SCIEX Analytical Instrument Trading Co., Shanghai, 200335, P. R. China S Supporting Information *

ABSTRACT: A new potential oxidation process is provided by CuFe2O4/hydroxylamine (HA) system for degradation of antibiotics in water. The CuFe2O4/HA system can generate reactive oxygen species (ROS) for the degradation of sulfamethoxazole (SMX). The addition of radical scavengers, including benzoquinone (BQ) and catalase (CAT), inhibited the oxidation of SMX in CuFe2O4/HA system. Electron transfer in the CuFe2O4/HA system played a key function for the generation of ROS and the degradation of SMX. The main ROS, was the superoxide radical (O2•−) mainly generated from adsorbed oxygen (O2(A)), which came from the oxidation of the lattice oxygen (O2−(L)) in CuFe2O4. The CuFe2O4/HA system was effectively applicable for a broad pH range (approximately 5−10). In addition, the activation mechanism for CuFe2O4/HA system was studied with the target contaminant SMX. Finally, the degradation pathways of SMX were proposed under the optimal conditions in CuFe2O4/HA system.

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INTRODUCTION Due to the overuse of antibiotics, sulfamethoxazole (SMX) is frequently detected in related aquatic environment.1−3 The concentration of SMX in surface water has been detected at the level from 0.01 μg L−1 to 2.0 μg L−1 in different countries, which do not directly show side effects or toxicity on human health.4,5 Residual SMX leads to faster emergence of antibioticresistant bacteria and antibiotic resistant genes, reducing its the potential for healing human and animal pathogens.6 When antibiotic resistant bacteria causing infections are no longer sensitive to antibiotics treatment, a serious threat is posed to human health. Thus, the increase of antibiotic resistant bacteria caused by the residual SMX has caused serious public concern.7,8 Therefore, it is urgent to exploit effective technologies to degrade or eliminate SMX in water. Cupric ion (Cu2+) can catalyze H2O2 to generate the reactive oxygen species (ROS).9 Ferrous iron (Fe2+), due to the superiority of high activity, environmental-friendliness, and low cost, has been widely used as a catalyst in Fenton and other Fenton-like processes.10,11 However, the homogeneous catalytic process has the problem of secondary pollution.3,12 The heterogeneous catalytic process is more efficient than the © 2018 American Chemical Society

homogeneous catalytic process, due to its mild operating conditions and little generated iron sludge.3,13,14 CuFe2O4, a type of magnetic spinel nanoparticle, has widely aroused people’s attention because of its good separation, remarkable catalysis, and desirable stability.14,15 The catalyst has been widely applied for the heterogeneous catalysis of ozone, peroxydisulfate (PDS), H2O2, and peroxymonosulfate (PMS) to degrade organic pollutants.14−17 In addition, the reduction of Fe3+ and Cu2+ to Fe2+ and Cu+ was difficult in advanced oxidation processes (AOPs), which greatly limits the catalytic ability of catalyst.14,18 Hydroxylamine (HA), an inorganic substance used to prepare oximes,19−21 has been used to active oxidants (H2O2, PDS and PMS), showing good activation performance.19,22 What is more, the addition of HA, has been proven to be an effective way for accelerating the cycle of Fe3+ to Fe2+ or Cu2+ to Cu+ in a homogeneous catalytic process.10,18,23 In addition, previous Received: Revised: Accepted: Published: 14302

July 5, 2018 November 4, 2018 November 14, 2018 November 14, 2018 DOI: 10.1021/acs.est.8b03340 Environ. Sci. Technol. 2018, 52, 14302−14310

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Environmental Science & Technology

were synthesized by using a sol−gel combustion method.14 Details pertaining method is provided in SI Text S2. Experimental Setup and Procedure. H2SO4 and NaOH solutions were used to adjust the initial solution pH. The batch experiments were conducted in 500 mL glass beaker at 30 °C, 300 mL stock SMX solution was added into the glass beaker and mixed by using a mechanical stirrer (300 rpm). The addition of CuFe2O4 and stock solution of HA would initiate reactions. One mL samples were taken out and quenched with 50 μL sodium thiosulfate immediately at fixed time intervals, then which were filtered with 0.45 μm filter membrane and stored at 4 °C. Quenching experiments with tert-butyl alcohol (TBA), methanol (MeOH), catalase (CAT), and 1,4benzoquinone (BQ) were performed by adding the desired quenchers into the reaction solution before the addition of CuFe2O4 and HA. In order to investigate source of oxygen in O2•− of CuFe2O4/HA system, the air was pumped into the CuFe2O4/HA system during the reaction. What’s more, the no dissolved oxygen environment of CuFe2O4/HA system was obtained by pumping N2 in a sealed three-necked flask. All experiments were done three times, and the error bars of standard deviations and the average values are provided. Analytical Methods. The SMX concentration was analyzed by high-performance liquid chromatograph (HPLC) (Agilent U.S.A.) with an Eclipse XDB C-18 column. Acetonitrile and oxalic acid solution (0.01 M) mixture was used as the mobile phase at a flow rate of 1 mL min−1, and the detection wavelength was 264 nm. The samples were analyzed by HPLC within 6 h, and the quenched results of 50 μL sodium thiosulfate were provided in SI Figure S1. Total nitrogen (TN) and residual HA concentrations were determined by UV−vis spectrometer and the same HPLC system respectively, the detailed procedures were shown in SI Texts S3 and S4.19,27 The concentration of NO3− and NO2− were analyzed by ion chromatograph (ICS-90). Dissolved N2O was measured using a microsensor (UNISENSE). Total organic carbon (TOC) of degraded samples were analyzed using TOC analyzer (Shimadzu, Japan). Dissolved oxygen was measured with a DO Meter (2FD354, Germany). The method of potassium titanium(IV) oxalate was used to determine the H2O2 concentration at 400 nm with a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan).28 Electron paramagnetic resonance (EPR) was used to detect O2•−, and detailed experiments can be seen in SI Text S5. The SCIEX ExionLC AC -SCIEX X500R QTOF-MS/MS (LC-QTOF-MS/MS) system was used to identify the intermediates of SMX, and detailed information can be seen in SI Text S6. The characterization analytical methods of CuFe2O4 can be seen in SI Text S7.

reports suggest the combination of HA with Cu2+ could produce O2•− and H2O2 by reducing dissolved oxygen, and Fe2+ could also generate O2•− by oxygen reduction in homogeneous catalytic process.18,24 The following reactions describe the generation of ROS (reactions 1−16).9,13,18,24−26 Recently, superoxide radical (O2•−) catches attention from the public for its potential to destroy highly toxic organic chemicals such as pesticides, dioxins, chlorinated solvents and other carcinogenic chemicals.11,13 O2•−, as a reactive oxygen species, was found in photocatalytic reactions under visible light irradiation, magnetite nanoparticles activation persulfate process, reaction process of Fe2+ and O2, and Cu2+/ HA system.4,13,18,24 Thus, the CuFe2O4/HA system might also be an effective way to degrade SMX by heterogeneous generation of O2•− or other ROS. Cu 2 + + 1 2 NH 2OH → 1 4 N2O + 1 4 H 2O + Cu+ + H+ (1)

Cu 2 + + NH 2OH → 1 2 N2 + H 2O + Cu+ + H+

(2)

2+ Cu+ + O2 → O•− 2 + Cu

(3)

2+ + Cu+ + O•− + H 2O2 2 + 2H → Cu +

Cu + H 2O2 → Cu

2+

•

(4) −

+ HO + OH or Cu

3+

+ 2OH

−

(5)

Cu

2+

+

O•− 2

+

→ Cu + O2

(6)

+ 2O•− 2 + 2H → O2 + H 2O2

Cu

3+

+

O•− 2

→ Cu

2+

(7)

+ O2

(8)

+ Cu 2 + + H 2O2 → Cu+ + O•− 2 + 2H

(9)

+ Cu 3 + + H 2O2 → Cu 2 + + O•− 2 + 2H

(10)

3+ Fe 2 + + O2 → O•− 2 + Fe

(11)

Fe

2+

+

O•− 2

+

+ 2H → Fe

3+

+ H 2O2

(12)

+ • O•− 2 + H ↔ HO2

(13)

Fe2 + + H 2O2 → Fe3 + + HO•

(14)

Fe3 + + H 2O2 → Fe2 + + HO•2

(15)

HO•2

(16)

→

O•− 2

+H

+

In this study, CuFe2O4 magnetic spinel nanoparticles were synthesized with sol−gel combustion method and coupled with HA to degrade SMX in aqueous solution. This study has the following purposes: (i) evaluating the degradation potential of the CuFe2O4/HA system for SMX; (ii) identifying the reactive oxidants generated from the CuFe2O4/HA system under the optimal conditions; (iii) revealing the activation mechanism of the CuFe2O4/HA system; and (iv) deducing the SMX degradation pathways.

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RESULTS AND DISCUSSION Characterization of CuFe2O4. The specific surface areas of the new CuFe2O4 and used CuFe2O4 were 22 and 24 m2/g, respectively. The saturation moments of the new CuFe2O4 and used CuFe2O4 were about 27.96 and 28.16 emu/g (Figure S2), respectively. These results suggest that the specific surface area of used CuFe2O4 was increased, and the saturation moment of CuFe2O4 had almost no change. Compared to the fresh CuFe2O4, no morphology change was found in the used CuFe2O4 by SEM (Figure S3) and TEM (Figure S4) images results. What’s more, the SEM-EDS results show that surface oxygen content of the reunion CuFe2O4 was increased and the atomic ratio of Cu and Fe (closed to 1:2) had almost no

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MATERIALS AND METHODS Reagents. Sources of chemicals are provided in the Supporting Information (SI) Text S1. Synthesis of CuFe2O4 Magnetic Nanoparticles. According to previously reported protocol, CuFe2O4 particles 14303

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Environmental Science & Technology change after reaction (Figure S5). The increased oxygen content of the reunion CuFe2O4 and the increased specific surface area were presumed to be the conversion of lattice oxygen to adsorbed oxygen according to the experimental results of X-ray photoelectron spectroscopy (XPS) (Figure S11(b)).29 XRD patterns of the fresh and used CuFe2O4 in CuFe2O4/HA system were illustrated in SI Figure S6. The characteristic peaks of the catalyst were observed at 2θ = 29.9°, 35.1°, 43.8°, 57.8°, and 62.2°, which could be indexed to the (112), (221), (220), (321), and (224), respectively.14 The diffraction patterns were matched well with the standard of the CuFe2O4 particles (JCPDS File No. 34-0425), suggesting the formation of CuFe2O4 with a considerable degree of crystallization. Similarly, according to the standard JCPDS File No. 45-0937, the peak at about 2θ = 39° was CuO. Little change and no additional peaks were observed in the used CuFe2O4, suggesting that the component and structure of CuFe2O4 were not destroyed. The result suggests that the structure of CuFe2O4 was stable in the CuFe2O4/HA system. In addition, the average crystallite size of CuFe2O4 was 13.9 nm, calculating by Scherrer equation.14 When the solid particles size was smaller than 20 nm especially, which could cause quantum confinement effects,13,30 the O2•− solvation shell and the surface free energy might be altered by quantum confinement, which might enhance the reactivity of reactants.13,30,31 Although the reactivity of O2•− was lower than that of •OH and SO4•− in aqueous solution,13,30 which would be enhanced when the O2•− presented on the surface of CuFe2O4.13,28,29 Parameters Optimization. To obtain optimal experimental conditions, the effects of different CuFe2O4 dosages (0−7 g/L), HA dosages (0−1.2 mM), and initial pH (3−11) on oxidation SMX were examined in CuFe2O4/HA system (Figure S7). The optimized CuFe2O4 dosage and HA dosage were 6 g/L and 1.0 mM, and the CuFe2O4/HA system showed a wide range of pH applications (approximately 5−10). Detail results and discussion can be seen in SI Texts S8−S10. Effects of Inorganic Anion. The presence of Cl− and Br− had a little adverse effect on CuFe2O4/HA system (Figure S8(a, b)), while the NO3−, HCO3−, and H2PO4− anions obviously inhibited the degradation of SMX in CuFe2O4/HA system (Figure S8(c−e)). Detailed results and discussion can be seen in SI Text S11. Stability of the Spinel CuFe2O4. he CuFe2O4/HA system still exhibited favorable degradation ability for SMX after using five times of CuFe2O4 (Figure S9). Detailed results and discussion can be seen in SI Text S12. SMX Degradation in Different System. Six control systems (i.e., CuFe2O4 alone, HA alone, CuFe2O4/PMS, CuFe2O4/HA, CuFe2O4/PDS, and CuFe2O4/H2O2) were used to degrade SMX. The SMX removal rates were 0%, 0%, 4%, 65%, 86%, and 100% within 300 s in CuFe2O4 alone, HA alone, CuFe2O4/H2O2, CuFe2O4/HA, and CuFe2O4/PMS systems, respectively (Figure 1). Neither CuFe2O4 or HA alone could not degrade SMX, which might be the absence of reactive oxidants. When CuFe2O4 and HA were simultaneously added to the SMX solution, the CuFe2O4/HA system exhibited a greater oxidation capacity for SMX. All these evidence suggest that CuFe2O4 coupled with HA was an effective system for the degradation of SMX from water. In addition, due to the excellent activation ability of CuFe2O4 for PMS, the degradation of SMX in CuFe2O4/PMS system was faster than other systems.

Figure 1. SMX degradation in different processes: HA alone, CuFe2O4 alone, CuFe2O4/HA, CuFe2O4/PMS, CuFe2O4/PDS, and CuFe2O4/H2O2 systems. Experimental conditions: [SMX]0 = 39.5 μM, [CuFe2O4]0 = 6 g/L, [HA]0 = 1 mM, [PDS] = 1 mM, [PMS] = 1 mM, [H2O2] = 1 mM, initial pH = 6.2, and stirring speed = 300 rpm.

The TOC removal rate of SMX solution was measured to investigate the mineralization capacity of the CuFe2O4/HA system. TOC removal of 0%, 0% and 37% were observed in CuFe2O4 alone, HA alone, and CuFe2O4/HA systems in 300 s, respectively (Figure not shown). The result suggests that the CuFe2O4/HA system possessed certain mineralization capacity for organic pollutants and some organic intermediates were successfully mineralized to CO2. The major transformation product of HA was N2, which suggests that the final fate of HA was friendly to environment in the CuFe2O4/HA system (Figure S10). Detailed results and discussion of HA transformation products can be seen in SI Text S13. Identification of Reactive Oxidants Species in CuFe2O4/HA System. Previous literatures reported that the degradation of organic pollutants was generally free radical mechanism or metal oxo.18,32 The H2O2 and O2•− could be generated from Cu2+/HA system,18 and the •OH could be generated from CuFe 2 O 4 /H 2 O 2 and Fe 2+ /H 2 O 2 systems.16,18,33 Thus, O2•−, H2O2, and •OH probably presented in the CuFe2O4/HA system. It is widely accepted in the literature that tert-butyl alcohol (TBA), 1,4-benzoquinone (BQ) and catalase (CAT) are quenchers for •OH, O2•−, and H2O2, respectively.10,34−36 The addition of BQ and CAT obviously inhibited the SMX degradation in the CuFe2O4/HA system, while the presence TBA weakly suppressed the degradation of SMX (Figure 2(a)). With 500 mM TBA, the SMX degradation rate only decreased from 86% to 83% in 300 s (Figure 2(a)), which suggests that there was a small amount of produced •OH radicals, but they were not the major oxidant species for the degradation of SMX. However, in the presence of 2 mM BQ and exceed 500 U/mL CAT, the SMX degradation efficiencies were notably decreased from 86% to 24% and 64% in 300 s, respectively. The strong inhibitory effects of BQ and CAT evidenced that the O2•− and H2O2 generated in the CuFe2O4/HA system, and the degradation of SMX was mostly induced by the O2•−. To further identify the production of O2•− in the CuFe2O4/HA system, the EPR technique with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) as a spin-trapping agent was used (Figure 2(b)). The hyperfine splitting constant of DMPO radical adducts were aN = 13.59 G and aH = 7.74 G in the CuFe2O4/HA system. These were 14304

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Figure 2. Effects of scavengers on SMX degradation by CuFe2O4/HA system (a); EPR spectra in CuFe2O4/HA system (b); and effects of K2Cr2O7 in CuFe2O4/HA system (c). Experimental conditions: [SMX]0 = 39.5 μM, [CuFe2O4]0= 6 g/L, [HA]0 = 1 mM, initial pH = 6.2, stirring speed = 300 rpm, [BQ] = 2 mM, [TBA] = 500 mM, [CAT] ≥ 500 U/mL, and [K2Cr2O7] = 5 mM.

representative of O2•− radicals added to DMPO (DMPO-O2), suggesting that the O2•− radicals presented in the CuFe2O4/ HA system. Quenching experiments results show that H2O2 presented in the CuFe2O4/HA system but the produced •OH radicals were very limited, which suggests only a small part of H2O2 were translated to •OH radicals. However, the concentration of the produced H2O2 in solution was not measured in the CuFe2O4/HA system by the potassium titanium(IV) oxalate method. Thus, it was deduced that the produced H2O2 presented on the surface of CuFe2O4, and the filtration affected the concentration detection of H2O2. Due to the lower direction oxidation capacity of H2O2 for SMX, the degradation of SMX by H2O2 was achieved by converting to superoxide radicals (reactions 26, 27, 31, and 32). The reactive oxidants species in the CuFe2O4/HA system contained •OH, H2O2, and O2•−, in which the O2•− radicals were the major reactive oxidants species. Electron transfer in the CuFe2O4/HA system might be the reason for O2 •− formation. As potassium dichromate (K2Cr2O7) was able to remove electrons, it was determined to be used to investigate the electron transfer in the CuFe2O4/ HA system.35,37 The degradation efficiencies of SMX had an obvious decrease in the CuFe2O4/HA system after the addition of 5 mM K2Cr2O7 (Figure 2(c)). According to previous data, the SMX degradation rate in 300 s was 86% in the CuFe2O4/ HA system. However, the removal rates in 300 s were only 3% and 4% in the K2Cr2O7/CuFe2O4/HA system and the

K2Cr2O7 alone system, respectively, and the little SMX degradation caused by the oxidation of K2Cr2O7. These results suggest that the electron transfer presented in the CuFe2O4/ HA system played an irreplaceable function in the generation of O2•− for the degradation of SMX. The XPS experiments were conducted to detect chemical states of the fresh and used CuFe2O4, which would further verify the electron transfer behavior of CuFe2O4 in the CuFe2O4/HA system (Figure S11). Figure S11(a) shows that Cu (2p), Fe (2p), and O (1s) presented in CuFe2O4 particles. The acquired spectra were calibrated by the carbon 1s signal at 284.8 eV. All XPS core level spectra were fitted using linear or Shirley background. Figure S11(c) shows that before catalytic oxidation process, the peak at binding energies of 934.1 eV was attributed to Cu(II), the Cu 2p peaks at 941.3 and 943.7 eV were attributed to Cu(II) oxide species.14 For the used CuFe2O4, a new peak at 933.2 eV was found on the catalyst, which was attributed to Cu(I).14 In addition, the effects of alcohols (Figure S12) and Br− (Figure S8(b)) on CuFe2O4/ HA system provided evidence of the existence of Cu(III), detail results and discussion can be seen in SI Text S14. The three peaks located at 710.9 and 713.0 eV for Fe 2p 3/2 and 724.4 eV for Fe 2p 1/2, which were attributed of Fe(III) in the new CuFe2O4 (Figure S11(d)).14 A new Fe 2p peak at 713.9 eV was found in the used CuFe2O4, which was attributed to Fe(II).38 These results indicate that reduction reactions of Fe(III)/Fe(II) and Cu(II)/Cu(I) were involved in the 14305

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Figure 3. Effects of air and N2 on CuFe2O4/HA system (a); effect of BQ in CuFe2O4/HA/N2 system (b). Experimental conditions: [SMX]0 = 39.5 μM, [CuFe2O4]0 = 6 g/L, [HA]0 = 1 mM, initial pH = 6.2, and stirring speed = 300 rpm.

no dissolved oxygen was found in the CuFe2O4/HA/N2 system (data not shown), which suggests that the disappeared surface lattice oxygen completely converted to surface hydroxyl group and adsorbed oxygen. These results suggest that the oxygen in superoxide radicals came from the surface lattice oxygen of CuFe2O4 in the CuFe2O4/HA/N2 system. Previous literature reported that the reactive oxidants could not be produced in the Cu(II)/HA/N2 system.18 Thus, it could be indicated that the oxygen in superoxide radicals mainly came from the O2(A) in CuFe2O4/HA system. In the PDS/MNPs (magnetite nanoparticles) system and photocatalyst process, the increasing concentration of dissolved oxygen greatly accelerated the degradation of pollutants, which was due to the fact that the increasing concentration of dissolved oxygen led to the generation of more O 2 •− radicals.4,13,41 Furthermore, due to the lack of electronic acceptor (dissolved oxygen) in the anoxic environment, the degradation of pollutant was markedly inhibited in the photocatalysis process.41 However, in the CuFe2O4/HA system, the increasing concentration of dissolved oxygen reduced the degradation of SMX (Figure 3(a)). The reaction (reaction 17) of HA with the increase dissolved oxygen reduced the HA amounts reacted with Cu(II) and Fe(III), which might cause the decrease of SMX removal rate in the CuFe2O4/HA/air system.9

CuFe2O4/HA system. As shown in Figure S11(b), for the fresh CuFe2O4, the main O 1s peak at 530.0 eV was attributed to the surface lattice oxygen of metal oxides (O2−, denoted as O2−(L)), and the peak at 531.2 eV was interpreted as oxygen in a single bond of the hydroxyl group adsorbed at the sample surface.14,39 For the used CuFe2O4, the O 1s peaks at 531.8 and 533.0 eV were interpreted as adsorbed oxygen (denoted as O2(A)) or surface hydroxyl species and oxygen in the adsorbed H2O, in which the O 1s peak with the observed position around 530.0 eV was attributed to the surface lattice oxygen of metal oxides.37,38 Previous research reported that lattice oxygen played a critical role in oxidation reaction and was active oxygen species.29 The appearance of adsorbed oxygen on the used CuFe2O4 was ascribed to lattice oxygen, which was initially oxidized to O2 and then adsorbed on the catalyst surface.29 The decrease of lattice oxygen and the appearance of adsorbed oxygen suggest that the lattice oxygen participated the reduction of Fe(III)/Fe(II) and Cu(II)/Cu(I), and the electrons transfer presented in the CuFe2O4/HA system.29 Effects of Air and N2 on CuFe2O4/HA System. ROS, including O2•− and hydrogen peroxide (H2O2), could generated from O2 by accepted electron from Fe(II) or Cu(I).9,13,18,24,40 To verify the source of oxygen in superoxide radicals, the effects of air and N2 on the CuFe2O4/HA system were done (Figure 3(a)). The initial dissolved oxygen concentration ([O2]0) of the CuFe2O4/HA system was 6.9 mg/L, without aeration during the reaction. When air was continuously pumped into SMX solution during the reaction, the initial dissolved oxygen concentration was 7.1 mg/L, and the SMX degradation rate decreased from 86% to 81% in 300 s. While the N2 presented in the CuFe2O4/HA system (the dissolved oxygen concentration was 0 mg/L), the SMX degradation rate increased from 86% to 94% in 300 s. The presence of air inhibited the SMX degradation while without dissolved oxygen presented in CuFe2O4/HA system accelerated the SMX degradation. To identify the reactive oxidants in the CuFe2O4/HA/N2 system, 2 mM BQ was used (Figure 3(b)). After adding BQ, the SMX degradation rate decreased from 94% to 11%. The result suggests that O2•− radicals were still produced when there was no dissolved oxygen in the CuFe2O4/HA system. In addition, the O 1s peaks of the used CuFe2O4 in CuFe2O4/HA/N2 were presented in Figure S13, only surface hydroxyl group (530.6 eV) and adsorbed oxygen (531.8 eV) were found on the used CuFe2O4.10 What’s more,

NH 2OH + O2 → N2 + H 2O2 + 2H 2O

(17)

However, reaction 17 was very slow, which was not the major reason for the removal rate decline in the CuFe2O4/ HA/air system.18 Previous literature reported that, in Cu(II)/ HA/O2 system, increasing concentration of O2 increased concentration of H2O2, but reduced the concentration of Cu(I).9,18 Thus, the concentration Cu(I) and Fe(II) might be reduced by the increased dissolved oxygen (reactions 3,11, 20, and 28), which would reduce the amount of Cu(I) and Fe(II) in heterogeneous reactions. Heterogeneous reactions were mainly responsible for the degradation of SMX (Figure S14(b)), thus the reduction of Cu(I) and Fe(II) in heterogeneous reactions might cause the decline of SMX removal rate. However, the Cu(I) might be oxidized to Cu(III) (reactions 3, 4, 5, 20, 21, and 22) and a large amount of H2O2 presented in the CuFe2O4/HA system according to the previous analysis, thus the generated Cu(III) could further produce radicals (reactions 10 and 27), which means that the 14306

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Environmental Science & Technology reduction of Cu(I) might not affect the SMX removal rate and the reduction of Fe(II) was the major reason for the decrease in SMX removal rate. In addition, the reactivity of O2•− would be enhanced when it presented in the solid surface.13 According to the above analysis, the major amount of O2•− radicals generated from the O2(A) and presented on the surface of CuFe2O4, and the increase of dissolved oxygen would affect the amount of O2•−(A) (the O2•− adsorbed on the CuFe2O4 surface), which would also cause the degradation of SMX removal rate. Activation Mechanism of CuFe2O4/HA System. Fe2O3 and CuO were used to test the activity of Cu(II) and Fe(III) in CuFe2O4 (Figure S14(a)). The oxidative degradations of SMX in the CuO/HA, Fe2O3/HA, CuO/Fe2O3/HA, and CuFe2O4/ HA systems were examined, respectively. Less than 12% SMX was degraded by HA coupling with CuO or Fe2O3 or CuO and Fe2O3 in 300 s (Figure S14(a)). A rapid decrease of the SMX concentration (86%) was observed for the HA/CuFe2O4 system. The results suggest that Cu(II) or Fe(III) in a structure other than CuFe2O4 could not couple HA to degrade effectively SMX. Due to the stable spinel structure, the leaching of copper ions from CuFe2O4 were extremely few in CuFe2O4/ PMS, CuFe2O4/PDS, and CuFe2O4/O3 systems.14,15,17 The leaching of copper and iron were 12.5 mg/L and 0.068 mg/L in the CuFe2O4/HA system, occupying the 0.8% and 0.002% of Cu and Fe quality in CuFe2O4 respectively. Previous studies have also found that the dissolved copper concentration was higher than that of iron.14 Due to the fact that the formation of Fe(OH)3 was easier than Cu(OH)2 around pH 5.2, which reduced the more concentration of Fe3+ comparing to Cu2+. The leaching of copper and iron coupled with 1 mM HA made 11% contribution for the SMX degradation (Figure S14(b)). The heterogeneous activation reactions made 75% contribution in the CuFe2O4/HA system. It can be concluded that the heterogeneous activation reactions were the major reason for SMX degradation in the CuFe2O4/HA system. In addition, the dissolved copper ions concentration was decreased to 0.46 mg/L by adding 3 g/L Fe0 to the effluent solution and stirring for 5 min. Then, the Fe/Cu bimetallic particles in the effluent solution were magnetically separated from solution, which could be used to activate PDS and O3 for the degradation of contaminant.42,43 Thus, the dissolved copper ions of final effluent solution were below maximum contaminant level (1.3 mg/L) set by the U.S. Environmental Protection Agency.18 According to above analysis and results, a likely catalytic mechanism in the CuFe2O4/HA system was proposed (Figure 4). In heterogeneous activation process, ≡Cu(II) and ≡Fe(III) in CuFe2O4 were first reduced to ≡Cu(I) and ≡Fe(II) by HA (reactions 18, 19, and 33−36), and the O2−(L) was oxidized to O2(A).9,29,33 HA and O2− (L) provided electrons to CuFe2O4, which could speed up the transformation of ≡Cu(II) to ≡Cu(I) and ≡Fe(III) to ≡Fe(II). Second, reactive oxidants (O2•− and O2•−(A)) and H2O2(A) (the H2O2 adsorbing on the surface of CuFe2O4) were produced in CuFe2O4/HA system by electrons transfer processes. The ≡Cu(I) and ≡Fe(II), generated by a series of single-electron transfer reactions (reactions 18, 19, and 33−36), reacted with the dissolved oxygen and O2 (A) to produce O2•− and O2•−(A) (reactions 20 and 28).9,13,18,24 Previous literature also reported the adsorbed O2 on the Fe0 surface could accept an electron and form O2•−.44 The concentration of H2O2 was not detected in solution, thus the H2O2 came from the reactions of O2•−(A) with ≡Cu(I) and ≡Fe(II) (reactions 21 and 29) and adsorbed

Figure 4. Schematic illustration of activation mechanism in CuFe2O4/ HA system. O2−(L) is surface lattice oxygen, O2(A) is the O2 adsorbed on the CuFe2O4 surface, H2O2(A) is the H2O2 adsorbed on the CuFe2O4 surface.

on the catalyst surface.18 In addition, the reactions of between H2O2 (A) and ≡Fe(III), ≡Cu(II), and ≡Cu(III) would also provide superoxide radicals (reactions 26, 27, 31, and 32).9,11 These reactions were responsible for the O2•− and O2•−(A) generation in the CuFe2O4/HA system, where the O2•−(A) was the main factor in the degradation and mineralization of SMX during heterogeneous activation process. In homogeneous activation process, the dissolved Fe3+ and Cu2+ were reduced to Fe2+ and Cu+, which then reacted with dissolved oxygen or O2 (A) to produce O2•− and O2•−(A).9,13,18,24 In addition, the Cu(III) presented in the CuFe2O4/HA system (reactions 5 and 22),18,33 which was also capable of oxidizing organic compounds.18 Meanwhile, the reaction between Fe(III) and Cu(I) was thermodynamically favorable (reaction 37),45 where the product Fe(II) (reaction 37) was beneficial to the generation O2•−(A) (reaction 11). In addition, the circulation of iron and copper might be accelerated in CuFe2O4/HA system, as shown in reaction 38.34 All in all, the O2•−(A) produced in the CuFe2O4/HA system played a particularly essential role in the SMX degradation. ≡Cu(II) + 1 2 NH 2OH → 1 4 N2O + 1 4 H 2O + ≡Cu(I) + H+

(18)

≡Cu(II) + NH 2OH → 1 2 N2 + H 2O + ≡ Cu(I) + H+ (19)

≡Cu(I) + O2 /O2(A) →

•− O•− 2 /O2(A)

+ ≡Cu(II)

(20)

+ ≡Cu(I) + O•− 2(A) + 2H → ≡Cu(II) + H 2O2(A) •

−

≡Cu(I) + H 2O2(A) → ≡Cu(II) + HO + OH or Cu(III) + 2OH

(21) −

(22)

≡Cu(II) +

•− O•− 2 /O2(A)

→ Cu(I) + O2 /O2(A)

+ 2O•− 2(A) + 2H → O2 + H 2O2(A)

(24)

•− ≡Cu(III) + O•− 2 /O2(A) → ≡Cu(II) + O2 /O2(A)

(25)

+ ≡Cu(II) + H 2O2(A) → ≡Cu(I) + O•− 2(A) + 2H

(26)

+ ≡Cu(III) + H 2O2(A) → ≡Cu(II) + O•− 2(A) + 2H

(27)

≡Fe(II) + O2 /O2(A) → 14307

(23)

•− O•− 2 /O2(A)

+ ≡Fe(III)

(28)

DOI: 10.1021/acs.est.8b03340 Environ. Sci. Technol. 2018, 52, 14302−14310

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Environmental Science & Technology

Figure 5. Pathways of SMX oxidation by the CuFe2O4/HA system. + ≡Fe(II) + O•− 2(A) + 2H → ≡Fe(III) + H 2O2(A) •

≡Fe(II) + H 2O2(A) → ≡Fe(III) + HO ≡Fe(III) + H 2O2(A) → ≡Fe(II) +

HO•2

≡Cu(III)/Cu 3 + + Fe(II)/Fe2 + → ≡Cu(II)/Cu 2 + + ≡Fe(III)/Fe3 +

(29)

(38)

(30)

Degradation Pathways of SMX. The products produced from the SMX degradation by the CuFe2O4/HA system were analyzed. The C2, C4, C6, S7, N8, O9, O10, and N11 of SMX were the most favorable sites for free radicals attack by the results of fukui function calculations (Figure 5).7 These products and their fragment ions could be seen in SI Table S1 and Figure S15−S29. Fourteen products including P-284, P300, P-273, P-289, P-255, P-271, P-109, P-99, P-114, P-270 (RT: 4.8 min), P-286, P-270 (RT: 2.7 min), P-288, and P-272 were identified by LC-QTOF-MS/MS, and six possible pathways of SMX oxidation were presented (Figure 5). Detailed results and discussion can be seen in SI Text S15. Degradation of Different Pollutants in CuFe2O4/HA System. The CuFe2O4/HA system showed good degradation abilities for four different contaminants (i.e., tetracycline,

(31)

+ HO•2 → O•− 2(A) + H

(32)

≡Fe(III)/Fe3 + + NH 2OH → NH 2O• + ≡Fe(II)/Fe2 + + H+

(33)

2NH 2O• → N2 + H 2O

(34)

≡Fe(III)/Fe3 + + NH 2O• → NHO + ≡Fe(II)/Fe2 + + H+

(35)

2NHO → N2O + H 2O

(36)

≡Cu(I)/Cu+ + ≡Fe(III)/Fe3 + → ≡Cu(II)/Cu 2 + + ≡Fe(II)/Fe2 +

(37) 14308

DOI: 10.1021/acs.est.8b03340 Environ. Sci. Technol. 2018, 52, 14302−14310

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Environmental Science & Technology ORCID

bisphenol A, SMX and sulfathiazole) (Figure S30(a)). Detailed results and discussion can be seen in SI Text S16. Environmental Implications. This study demonstrated the CuFe2O4 activation by HA for efficient degradation of SMX in water. Besides, the optimal operations (i.e., CuFe2O4 dosage of 6 g/L, HA dosage of 1.0 mM and initial pH of 6.2) were obtained. In addition, the CuFe2O4/HA system could effectively oxidize SMX in water at pH between 5 and 10. The •OH, O2•− and H2O2(A) simultaneously presented in the CuFe2O4/HA system, but the O2•− radicals presented on the surface of CuFe2 O 4 were mainly responsible for the degradation of SMX. Meanwhile, electron capture experiments using K2Cr2O7 proved the electron transfer presented in the CuFe2O4/HA system. What’s more, the comparative XPS analysis of the fresh and used CuFe2O4 further evidenced the electrons transfer and the change of oxygen species of CuFe2O4. Also, the degradation of SMX was accelerated when without dissolved oxygen in the CuFe2O4/HA system. SMX was degraded to 14 products through six degradation pathways in the CuFe2O4/HA system. CuFe2O4/HA showed stable excellent degradation performance and the CuFe2O4 showed a good stability, and the major transformation product of HA was N2 in the CuFe2O4/HA system.

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Bo Lai: 0000-0002-7105-1345 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge financial support from the National Natural Science Foundation of China (No. 51878423) and Fundamental Research Funds for the Central Universities (No. 2015SCU04A09).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b03340.

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REFERENCES

Reagents (Text S1), synthesis of CuFe2O4 magnetic nanoparticles (Text S2), total nitrogen (Text S3), residual HA (Text S4), EPR studies (Text S5), LCQTOF-MS/MS studies (Text S6), characterization analytical methods of catalysts (Text S7), effects of key factors (Text S8−S10), effects of inorganic anion (Text S11), stability of the spinel CuFe2O4 (Text S12), transformation products of HA (Text S13), effect of different alcohols (Text S14), degradation pathways of SMX (Text S15) degradation of different pollutants in CuFe2O4/HA system (Text S16), SMX products detected by LC-QTOF-MS during CuFe2O4/HA system (Table S1), effect of different SMX concentration detection time on experimental results (Figure S1), cyclic voltammograms (Figure S2), SEM (Figure S3), TEM (Figure S4), SEM-EDS (Figure S5), XRD patterns (Figure S6), effects of key factors (Figure S7), effects of inorganic anion (Figure S8), operational life of CuFe2O4 (Figure S9), residual HA (a) and residual TN (b) (Figure S10), XPS spectra (Figure S11), effect of different alcohols (Figure S12), O 1s peaks in CuFe2O4/HA/N2 system (Figure S13), SMX degradation in different catalyst/HA processes (Figure S14(a)), homogeneous activation and heterogeneous activation in the CuFe2O4/HA system (Figure S14(b)), SMX and its products (Figures S15−29), degradation of different pollutants in the CuFe2O4/HA system (a), and apparent pseudo-second-order reaction kinetics for different pollutants in CuFe2O4/HA system (b) (Figure S30) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel/fax: +86 15308172530; e-mail: [email protected] (B.L.). 14309

DOI: 10.1021/acs.est.8b03340 Environ. Sci. Technol. 2018, 52, 14302−14310

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