Peroxymonosulfate enhanced photoelectrocatalytic degradation of ofloxacin using an easily coated cathode

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Peroxymonosulfate enhanced photoelectrocatalytic degradation of ofloxacin using an easily coated cathode ⁎

Keyi Wanga,b, Gaozhou Liangb, Muhammad Waqasb, Bo Yanga,b, , Ke Xiaoa,b, Caizhen Zhub, Junmin Zhangb a b

Water Science and Environmental Engineering Research Center, College of Chemical and Environmental Engineering, Shenzhen University, Shenzhen 518060, China College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Photoelectrocatalysis Peroxymonosulfate Polydopamine Sulfate radicals Ofloxacin

This study described a promising photoelectrocatalytic (PEC) system enhanced by peroxymonosulfate (PMS) for the treatment of ofloxacin in wastewater. The PEC system, which employed a BiVO4 as photoanode and a polydopamine modified carbon felt (PDA/CF) as cathode, was constructed. The PDA/CF cathode was easily prepared by a dopamine self-polymerization process. The effect of PMS concentrations, applied potential and initial solution pH on the ofloxacin degradation was investigated in the enhanced PEC system. Ofloxacin can be completely removed after 120 min reaction under the optimal conditions of applied potential 1.5 V, pH 5.0 and 2 mM PMS addition. Based on the electron spin resonance measurement, sulfate radicals (SO4%−), hydroxyl radicals (%OH) and singlet oxygen (1O2) were mainly responsible for ofloxacin degradation. Analyses of the degradation intermediates indicated that the CeN bond, piperazine ring and the fluoride group of ofloxacin were attacked in its degradation process. The introduction of dopamine on the cathode significantly enhanced the PMS activation to produce active radicals, which further improved the ofloxacin degradation. The possible mechanism was proposed for ofloxacin degradation in the PEC system enhanced by the addition of PMS.

1. Introduction

[12]. The recent studies have demonstrated that the PEC system combined with SO4%− exhibited a great success in improving the photoelectrocatalytic activity of the PEC system [13]. SO4%− possess even stronger oxidation capacity (2.5–3.1 V vs SHE) than that of hydroxyl radical (%OH) [14]. In addition, SO4%− has higher selectivity, wider working range of pH (2–8) and longer half-life (30–40 s) than %OH in most cases [15,16]. Therefore, SO4%− could decompose the organic pollutants or even mineralize them into CO2 and H2O [16]. SO4%− can be generated by the activation of PMS through transition metal ions, metal oxide, and carbonaceous-based materials [17–19]. Carbonaceous materials have been widely used as catalysts for PMS activation due to its advantages such as large specific surface area, satisfactory conductance and high strength [20,21]. To improve the PMS activation ability, many methods such as nitrogen, sulfur modification and cobalt oxides doping have been applied to modify carbonaceous materials [22–24]. Sun et al [25] has reported that the Nmodified carbon nanotube exhibited superior PMS activation ability for the phenol removal, and Ouyang et al [26] has designed a nFe3O4/ biochar to improve photocatalytic activity in the presence of persulfate for 1,4-Dioxane degradation. It has been proved that the modification

Ofloxacin, as a fluoroquinolone antibiotic, has negative affects to the ecology and human being, leading to the selection of antibiotic resistant bacteria and chronic toxicity [2]. However, ofloxacin is frequently detected in various water bodies such as surface water, wastewater treatment plant effluents, and even drinking water [1]. For this reason, various strategies have been used to remove ofloxacin from waters, such as biological method [3], chemical oxidation [4], and sonochemical method [5]. The intrinsic drawbacks of low efficiency and high cost have limited their widespread application. In the past decades, photoelectrocatalysis (PEC) utilized semiconductor electrodes has exhibited a huge potential for organic pollutants elimination [6,7]. However, it is still a great challenge to develop a high-efficiency PEC system [8]. Recent works have focused on the improvement of photoelectrocatalytic activity and/or solar light response of the photoanode [9–11]. However, the cathode in the PEC system has been routinely underutilized for the organic degradation. It is worthwhile to mention that the electron transferred to cathode could activate peroxymonosulfate (PMS) to produce sulfate radical (SO4%−)

⁎

Corresponding author. E-mail address: [email protected] (B. Yang).

https://doi.org/10.1016/j.seppur.2019.116301 Received 20 August 2019; Received in revised form 5 November 2019; Accepted 6 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Keyi Wang, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116301

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Fig. 1. Structural characterization of the as-prepared cathodes. The morphology of (a) CF, (b) PDA/CF electrodes, (c) The binding energies of O, N, C of the asprepared cathodes, and (d) the high-resolution spectrum of C 1s of PDA/CF electrode.

2.2. Preparation of the electrodes

of carbonaceous materials can effectively activate PMS to generate SO4%−. However, most of the reported modification methods were complicated [22,23], which limited their industrial application. Dopamine with oxygen and nitrogen functional groups can easily self-polymerize under alkaline condition to produce polydopamine (PDA) coatings on various surfaces [27]. However, there is no research on the performance and mechanism of carbon felt (CF) modified by PDA in the activation of PMS to degrade organic pollutants in wastewater. In this study, a PDA modified CF electrode (PDA/CF) was prepared via the self-polymerization process. A PEC system was assembled with a PDA/CF cathode and a BiVO4 photoanode. PMS can be activated by the PDA/CF cathode in the PEC system and the oxidative radicals such as % OH, SO4%− and singlet oxygen (1O2) were generated, which could degrade ofloxacin efficiently. The effect of process parameters (PMS concentration, pH and applied potential) on ofloxacin degradation was studied. The degradation pathways of ofloxacin were explored and the activation mechanism was proposed.

The PDA/CF cathode was prepared via DPA self-polymerization under alkaline conditions. Before reaction, the CF was firstly conditioned using 1 M acetone. Then the resulting CF was dried at 60 °C after washing repeatedly with deionized water. For the synthesis of PDA/CF cathode, 0.1 g hydrochloride dopamine was dissolved in 50 mL phosphate buffered solution (pH of 8.4–8.6), and then a piece of CF (4 × 5 cm) was immerged into the solution and magnetically stirred for 12 h. The obtained cathode was thoroughly washed with deionized water and dried at 60 °C. BiVO4 photoanode was synthesized according to the previous study [28]. 2.3. Photoelectrocatalytic experiments The ofloxacin degradation was conducted in a glass reactor (effective volume of 160 mL) with a quartz tube by using a CHI 660E potentiostat (Chenhua Instruments Inc.). A three-electrode system was adopted with a working electrode (BiVO4 photoanode, 16 cm2), a counter electrode (PDA/CF cathode, 16 cm2) and a reference electrode (saturated Ag/AgCl/KCl electrode). A 300 W Xenon lamp (Perfect Inc., China) was used to simulate the solar light source. 50 mM NaClO4 was used as the electrolyte. H2SO4 (0.2 M) or NaOH (0.2 M) was used to adjust the initial solution pH.

2. Methods 2.1. Chemicals Hydrochloride dopamine and ofloxacin were obtained from Ark Pharm USA, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 2,2,6,6-tetramethyl-4-piperidone (TEMP), Methanol (MeOH), and Tert-butyl alcohol (TBA) were acquired from ACROS USA. CF was obtained from Tetan Co. Ltd., China. Peroxymonosulfate (2KHSO5·KHSO4·K2SO4), sodium azide (NaN3), H2SO4, HCl, NaOH, and phosphate buffered solution were acquired from Sinopharm chemical regent Co. Ltd., China.

2.4. Analytical methods The concentration of ofloxacin and its degradation intermediates were measured using a high performance liquid chromatography (HPLC, Shimadzu UV model with SPD-M20A) with a BEH-C18 column. 2

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The mobile phase consisted of methanol and 0.1% formic acid (v/v, 70/ 30). The mineralization of ofloxacin was studied using a TOC analyzer (Analytikjena, Germany). The electron spin resonance (ESR) signals were performed on a Bruker A300-10/12 spectrometer system (Germany). X-ray photoelectron spectroscopy (XPS) measurement was conducted on a Thermal theta 300xt spectrometer. The surface morphology of the cathode was recorded by scanning electron microscopy (SEM, HITACHI SU8020, Japan). The crystalline structures of the electrodes were recorded by an X-ray diffractometer (XRD, X’pert Pro MPD).

1.0 0.8

C/C0

0.6 CF-PMS PDA/CF-PMS BiVO4-CF

0.4 0.2

BiVO4-PDA/CF BiVO4-CF-PMS

0.0

3. Results and discussion

BiVO4-PDA/CF-PMS

0

3.1. Characterizations of electrodes

20

40

60

80

100

120

Time (min)

The surface morphology of the pristine CF and PDA/CF samples was characterized by SEM (Fig. 1a). The pristine CF was consisted of uniform fibers with a diameter of about 15 µm, and exhibited a smooth surface. After modification by PDA, as shown in Fig. 1b, it was observed that the original smooth surface of CF was covered by the cylindricallike PDA and the modification of PDA did not change the structure of CF. The chemical bonds of pristine CF and PDA/CF samples were further characterized by XPS. As shown in Fig. 1c, on pristine CF, only carbon and oxygen were detected; and oxygen existed at a very low ratio of 5.2%, partially due to the surface adsorption of oxygen. After PDA modification, the oxygen and nitrogen content increased to 36.6 and 6.8%. The atomic molar percent of N 1s to C 1s was 0.120, which was comparable to the theoretical value of PDA (0.125). The C1s peak of the PDA/CF sample could be deconvoluted into three peaks at 284.8 (CeC), 285.8 (CeO) and 286.7 eV (CeN) (Fig. 1d), indicating the existence of oxygen-containing and nitrogen-containing groups on the PDA/CF electrode. The microstructure of BiVO4 photoanode were also characterized (Fig. S1). From XRD patterns of the BiVO4 photoanode, characteristic peaks at 18.7°, 28.6°, 30.5°, corresponding to the monoclinic BiVO4 and FTO, could be observed. These indicated that the monoclinic scheelite structure of BiVO4, which confirmed good photocatalytic performance of the photoanode, was successfully deposited on the FTO substrate. The magnified SEM in Fig. S1 further confirmed a well-structured film which included a top layer with large nanoparticles (> 200 nm) and a bottom layer with smaller nanoparticles (< 100 nm). The band gap energy was measured as 2.48 eV (Fig. S1), the conduction and valance bands at the point of zero charge for BiVO4 crystallites were calculated as 0.31 and 2.79 eV, respectively (Table. S1). These indicated that the prepared BiVO4 crystallites had well photocatalytic activity.

Fig. 2. Ofloxacin removal using various electrodes and in the absence/presence applied of PMS. Experimental conditions: [ofloxacin]0 = 8 mg/L; potential = 1.5 V; pH = 5.0; [PMS]0 = 2 mM.

removal efficiency increased to 70.3% with addition of 2.0 mM PMS using CF cathode (BiVO4-CF-PMS). This enhancement may be attributed to the fact that PMS was effectively activated and produced high active SO4%− to degrade ofloxacin. At the same conditions, the prepared PDA/CF cathode was used with the addition of PMS (BiVO4-PDA/ CF-PMS) and the ofloxacin can be completely removed within 120 min. The outstanding catalytic performance of BiVO4-PDA/CF-PMS system can be attributed to the excellent PMS activation capability of PDA, which enhanced the production of active radicals in the ofloxacin degradation.

3.3. Effect of various parameters on the ofloxacin degradation The ofloxacin degradation at different conditions was studied in the BiVO4-PDA/CF-PMS system. Fig. 3a illustrated the effect of PMS concentration on the ofloxacin degradation. An enhancement of ofloxacin degradation was observed by increasing the concentration of PMS. When the concentration of PMS was 0.0 mM, the ofloxacin degradation efficiency was 55.2% after 120 min. When the PMS concentration increased to be 2.0 mM, the ofloxacin was nearly completely degraded. Further increasing the PMS concentration would not improve the ofloxacin degradation. However, TOC removal increased from 13.1% to 44.0% with increasing the PMS concentration from 1 to 3 mM (Fig. 3c), indicating that the mineralization of ofloxacin can be significantly improved with the increase of PMS concentration. In the BiVO4-PDA/CFPMS system, as the PMS concentration increased from 0 to 2.0 mM, the rate constants of ofloxacin degradation increased from 0.0238 to 0.0766 min−1 (Fig. 3b). These results demonstrated that ofloxacin can be degraded more efficiently in the BiVO4-PDA/CF-PMS system with an optimal amount of PMS. The effect of different applied potential on the ofloxacin removal was depicted in Fig. 4a. The ofloxacin removal increased from 7.2 to 99.3% with increasing the applied potential from 0.0 to 1.5 V. There was no significant difference in ofloxacin degradation when the applied potentials varied from 1.5 to 2.0 V. At a high applied potential, a decrease in ofloxacin degradation was observed, due to the cathodic evolution of H2 [33]. Furthermore, ofloxacin degradation was adjusted in agreement to a pseudo first-order rate constant. The rate constants increased linearly from 0.008 to 0.0766 min−1 with increasing the applied potential from 0.0 to 1.5 V (Fig. 4b). As well known, the increase of applied potential can facilitate the separation of photoinduced holes and electrons on the BiVO4 photoanode in the PEC system [34], which makes more holes to participate in the oxidation of ofloxacin; therefore the ofloxacin degradation was improved. In addition, PMS activation ability of PDA could be enhanced by the increase of applied potential, which improved the SO4%− production and thus increased

3.2. Degradation of ofloxacin in different systems The ofloxacin degradation in the BiVO4-PDA/CF system with or without PMS was evaluated in a three-electrode system by using BiVO4 as photoanode and PDA/CF as cathode. For comparison, the ofloxacin degradation in the BiVO4-CF system was also studied. Before this, the PMS activation abilities of CF and PDA /CF electrodes for ofloxacin degradation were firstly studied. As shown in Fig. 2a, no ofloxacin was removed by single CF and PMS within 120 min, indicating that the CF, as a kind of carbonaceous-based material, could not activate PMS. In the previous studies, it was reported that carbonaceous-based materials, such as activated carbon and graphene, could act as a catalyst for activating PMS [29–31]. Chen et al. proved that carbon nanotube can effectively activate PMS to generate active radicals and decolorize azo dye [32]. However, the ofloxacin degradation efficiency was improved to 29.2% using the PDA/CF electrode in the presence of PMS. In the individual PEC process, under visible-light illumination with the applied potential of 1.5 V, the ofloxacin removal achieved 48.1% and 55.2% by the BiVO4-CF and BiVO4-PDA/CF system, respectively. The 3

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1.0

a

0.8

C/C0

0.6 0.4

0.0 V 0.5 V 1.0 V 1.5 V 2.0 V

0.2 0.0 0

20

40

60

80

100

120

2.0

2.5

3.0

Time (min)

0.08

-1

Rate constant (min )

b 0.06

0.04

0.02

0.00

0.0

0.5

1.0

1.5

Applied bias (V) Fig. 4. Influence of applied potentials on (a) the ofloxacin removal and (b) the pseudo-first-order kinetic modeling in the BiVO4-PDA/CF-PMS system. Reaction conditions: [ofloxacin]0 = 8 mg/L; pH = 5.0; [PMS]0 = 2 mM.

a

1.0 0.8

C/C0

0.6 0.4

3 5 7 9 11

0.2 0.0

Fig. 3. Influence of PMS concentration on (a) the ofloxacin removal, (b) the pseudo-first-order kinetic modeling and (c) the TOC removal in the BiVO4-PDA/ CF-PMS system. Reaction conditions: [ofloxacin]0 = 8 mg/L; applied potential = 1.5 V; pH = 5.0.

0

20

40

60

80

100

120

Time (min)

0.08

-1

Rate constant (min )

the ofloxacin degradation. Similar results were reported in the previous study for the diclofenac degradation in the PEC process [35]. The influence of initial pH on ofloxacin degradation was conducted by varying the initial pH from 3 to 11 (Fig. 5a). A slight decrease of ofloxacin removal was observed at strong acidic and weak alkaline conditions. At the optimal pH of 5, nearly complete removal of ofloxacin was observed at 120 min, and the rate constant was 0.0766 min−1 (Fig. 5b). At pH 9, the ofloxacin removal was decreased to 49.7%, and the rate constant was 0.0371 min−1. While a significant improvement on ofloxacin degradation was observed at the initial pH of 11, the removal efficiency increased to 99.6% and the reaction rate significantly increased to 0.0798 min−1 (Fig. 5). In the acid solution, SO4%− was the primary reactive species, which had a higher redox potential (2.5–3.1 V) than that of %OH (1.8–2.7 V in acid and neutral conditions). These resulted in a relatively high degradation efficiency. However, when the initial pH was 9, the ofloxacin removal was only 49.7%. Low

b

0.07 0.06 0.05 0.04 0.03

2

4

6

8

10

12

pH Fig. 5. Influence of initial solution pH on (a) the ofloxacin removal and (b) the pseudo-first-order kinetic modeling in the BiVO4-PDA/CF-PMS system. Conditions: [ofloxacin]0 = 8 mg/L; applied potential = 1.5 V; [PMS]0 = 2 mM. 4

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% OH for pollutant degradation [16]. To identify the reactive species involved in the ofloxacin degradation in the BiVO4-PDA/CF-PMS system, radicals quenching experiments were conducted (Fig. 6a). In this work, 0.25 M tert-butyl alcohol (TBA) was used as the %OH scavenger; while 0.25 Methanol (EtOH) scavenged both SO4%− and %OH with quite similar rate constants. The addition of 0.25 M TBA slightly decreased the ofloxacin degradation efficiency, implying that %OH showed certain contribution to the ofloxacin degradation in the BiVO4PDA/CF-PMS system. With the addition of 0.25 M EtOH, the ofloxacin removal decreased significantly from 100% to 37% after 120 min. However, sodium azide (SA, 0.1 M) was used as 1O2 scavenger; as a result, the ofloxacin removal was almost completely suppressed in the system. These indicated that both of SO4%− and 1O2 played a key role in the ofloxacin removal in the BiVO4-PDA/CF-PMS system. To confirm the active species that was responsible for the degradation of ofloxacin, ESR measurement was also employed using DMPO or TEMP as spin trap agents (Fig. 6b). Generally, DMPO was used to trap %OH and SO4%− with the formation of DMPO-%OH and DMPO-SO4%−; and TEMP was employed to identify 1O2 by formatting TEMPO. It was found that all signals of DMPO-SO4%−, DMPO-%OH and TEMPO can be detected in the BiVO4-PDA/CF-PMS system. Moreover, only DMPO-%OH was detected nearby the BiVO4 photoanode and both TEMPO and DMPO-SO4%− could be identified nearby the PDA/CF cathode (Fig. 6b and c). These further confirmed the excellent PMS activation ability of PDA/CF for the SO4%− and 1O2 production. It also could be found that the peak intensity of DMPO-SO4%− and DMPO-%OH around the PDA/CF cathode was stronger than those produced around the BiVO4 photoanode. The %OH radicals produced around the BiVO4 photoanode can be attributed to the chemical reaction between photoinduced holes and H2O. The ESR results indicated that the PMS activation was mainly occurred on the surface of PDA/CF cathode. It has been reported that photoinduced electrons can activate PMS for SO4%− and 1O2 production [38]. For instance, BiVO4 was used in PMS activation under visible light and this couple could degrade Rhodamine B efficiently [39]. But in the present study, the ofloxacin degradation was limited in the BiVO4-CF-PMS system, which indicated a low PMS activation ability of photogenerated electrons. By contrast, the ofloxacin degradation was significantly improved by the PDA/CF cathode in the BiVO4-PDA/CF-PMS system. From the above results, it can be concluded that the introduction of PDA on the cathode strengthened PMS activation and further resulted in an enhancement of ofloxacin degradation.

3.5. Degradation intermediates of ofloxacin Fig. 6. (a) Effect of different scavengers on the ofloxacin removal; (b), and (c) ESR spectra at the BiVO4 photoanode and PDA/CF cathode in the BiVO4-PDA/ CF-PMS system. Reaction conditions: [ofloxacin]0 = 8 mg/L; applied potential = 1.5 V; pH = 5.0; [PMS]0 = 2 mM.

For the 120 min degradation experiments, > 99% of ofloxacin was degraded but the TOC removal was only 44% (Fig. 3c). The striking difference between the ofloxacin removal and TOC removal indicated a relatively low degree of mineralization in the BiVO4-PDA/CF-PMS system. Instead, ofloxacin degradation in the BiVO4-PDA/CF-PMS system produced a variety of organic intermediates which were more resistant towards radical oxidation. To better elucidate the degradation mechanisms of ofloxacin, the degradation intermediates were analyzed by mixing samples collected at 30, 60, 90, and 120 min of reaction for HPLC-MS analysis, which confirmed 12 major degradation intermediates. The details of the degradation intermediates such as the retention time, molecular ion, elemental formula, and proposed structure were obtained and shown in Table 1. Based on the above results, three main pathways of ofloxacin degradation was proposed as depicted in Fig. 7. In pathway I, the CeN bond in ofloxacin was firstly attacked by the oxidative radicals to form acetaldehyde groups (P1). The acetaldehyde groups in P1 were tended to hydrogenate and subsequent hydrolysis to form the P2 and P3 isomers. Then, the isomers were further decomposed to form P4. In pathway II, the oxidative radicals attacked the methyl of piperazinyl substituent in ofloxacin to generate P5. P6 was formed in the following

concentration of OH− could scavenge the SO4%− to %OH (Eq. (1)) which had a weaker oxidation ability, leading to a decrease of the ofloxacin removal. In contrast, both of the ofloxacin removal and reaction rate increased significantly as the initial pH increased to 11.0 (Fig. 5b). It had been reported that base-activated PMS was an effective way to degrade organic pollutants [36,37]. Therefore, at pH 11 in the present system, PMS was activated by base, resulting in the efficient production of active radicals for ofloxacin removal. SO4%− + OH− → %OH + SO42−

(1)

3.4. Involved reactive oxidizing species Many oxidative radicals, such as SO4%−, %OH, and peroxymonosulfate radical (SO5%−), are generated in the activation of PMS. But the activity of SO5%− is negligible in comparison with SO4%− and 5

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Table 1 Transformation products generated during the ofloxacin degradation. Compound

Retention time (min)

Molecular ion

Elemental formula

P1

16.9

391

C18H17FN3O6

P2

21.7

363

C17H17FN3O5

P3

22.1

363

C17H17FN3O5

P4

14.0

334

C16H17FN3O4

P5

8.6

347

C17H18FN3O4

P6

9.7

304

C16H18FN3O2

P7

10.7

279

C14H18FN3O2

P8

6.3

220

C11H9FN2O2

P9

12.8

343

C18H19N3O4

P10

23.1

369

C18H15N3O6

P11

24.5

313

C15H11N3O5

P12

26.9

327

C15H9N3O6

Molecular formula

was only a slight decrease in the ofloxacin degradation efficiency after 5 consecutive operations. The morphology of PDA/CF cathode before and after five consecutive operations was investigated by SEM (Fig. S3). No obvious change in the morphology of PDA/CF cathode was observed after 5 consecutive operations. The above results indicated the robust stability of the PDA/CF cathode used in the BiVO4-PDA/CF-PMS system.

hydroxylation process. Then the piperazinyl substituent, the oxazinyl substituent and the carboxyl group were attacked to form P7 and P8. In pathway III, ofloxacin was firstly defluorinated to form P9. Piperazinyl substituent in P9 can be attacked by the oxidative radicals and formed P10. Besides, oxazinyl substituent could be attacked to form P11. The ring of piperazinyl substituent and oxazinyl substituent was degraded to some small molecules (P12). Eventually, the identified degradation intermediates from all the pathways were oxidized into smaller organic compounds before being mineralized into H2O andCO2.

3.7. A proposed mechanism In the BiVO4-PDA/CF system, the electrons and holes were photogenerated on the surface of BiVO4 photoanode by the solar-light radiation (Eq. (2)). The photogenerated holes could oxidize H2O or OH− into %OH (Eqs. 3–4) to degrade ofloxacin (Eq. (5)). But the results in Fig. 2 demonstrated that the ofloxacin degradation was limited by the

3.6. Stability of the prepared PDA/CF cathode To investigate the stability of PDA/CF cathode, recycle experiments were performed on the ofloxacin degradation under the same conditions in the BiVO4-PDA/CF-PMS system. As illustrated in Fig. S2, there 6

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Fig. 7. Possible ofloxacin degradation pathways in the BiVO4-PDA/CF-PMS system.

photogenerated holes in the BiVO4-PDA/CF system without the addition of PMS. BiVO4 + hv → e− + hvb+ −

(2)

%

+ OH → OH

(3)

hvb+ + H2O → %OH + H+

(4)

hvb+

%

OH + Ofloxacin → OH− + product

(5)

In the BiVO4-PDA/CF-PMS system, the linear sweep voltammetry (LSV) curves of the PDA/CF cathode with different PMS concentrations were investigated (Fig. S4). The current of PDA/CF cathode increased with the increase of PMS concentration, implying a strong relativity of PMS activation and PDA/CF cathode. Moreover, the changes in functional groups of the PDA/CF electrode before and after 120 min reaction were studied by XPS (Fig. 8). From the high-resolution spectrum of O 1s, the peaks centered at 531.2 ± 0.2 eV (CeO) and 533.1 ± 0.1 eV (C]O) were clearly observed [40]. The absorbance peak of CeO was 71.5% for the pristine PDA/CF cathode (Fig. 8a) and reduced to 23.2% after 120 min reaction (Fig. 8b), indicating that some CeO groups were oxidized to C]O. The above results further demonstrated the existence of redox reaction on the PDA/CF cathode in the activation of PMS. Based on these results, the possible mechanism of ofloxacin degradation in the BiVO4-PDA/CF-PMS system was proposed (Fig. 9). The CeO groups in the PDA/CF cathode could active PMS to generate C]O and SO4%− (Eq. (6)). C]O groups can be regenerated to C–O through the reduction of photogenerated electrons on BiVO4 photoanode (Eq. (7)). This redox cycle of CeO and C]O mainly improved the catalytic activity of PMS. The generated SO4%− with strong oxidation ability also oxidized H2O or OH− into %OH (Eqs. (1) and (8)). At a higher pH, SO4%− can react with OH− to generate 1O2 (Eq. (9)). These free radicals oxidized ofloxacin efficiently (Eq. (10)). eCeO + PMS → eC]O +

SO4%−

eC]O + e− → eCeO SO4%− + H2O → %OH + H+ + SO42−

Fig. 8. XPS analysis of O 1s on the PDA/CF electrode before (a) and after 120 min reaction (b).

(6) 4SO4%− + 4OH− → 1O2 + 4SO42− + 2H2O

(7)

%

(8) 7

OH/SO4%−/1O2 + Ofloxacin

→ Intermediates → CO2 + H2O

(9) (10)

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[5]

[6]

[7]

[8]

[9]

[10]

Fig. 9. Possible catalytic mechanism of ofloxacin degradation in the BiVO4PDA/CF-PMS system.

[11]

4. Conclusions

[12]

In this study, PDA/CF was prepared by a self-polymerization approach and employed as a cathode in the BiVO4-PDA/CF-PMS system. It was found that the ofloxacin degradation could be efficiently improved in the presence of PMS. With 2 mM PMS addition, ofloxacin can be completely degraded after 120 min reaction, and the corresponding rate constant increased to 0.0766 min−1. In the presence of PMS, PDA/CF cathode in the PEC system can efficiently active PMS and produce %OH, SO4%−, and 1O2 radicals, which showed the main responsible for ofloxacin degradation in the BiVO4-PDA/CF-PMS system. Moreover, the combination of applied potential and photoinduced electrons on BiVO4 photoanode can further improve the PMS activation, resulting in the enhancement of ofloxacin degradation. This study may expand a new perspective on construction of other novel PEC systems by utilization of different advanced oxidation techniques for organic pollutants degradation.

[13]

[14]

[15]

[16]

[17]

[18]

[19]

Declaration of Competing Interest [20]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[21]

Acknowledgements [22]

This work financially supported by the National Natural Science Foundation of China (21777106, 51708356), the National Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07202).

[23]

[24]

Appendix A. Supplementary material [25]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116301.

[26]

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