RGO optimization, degradation performance and mechanism

Journal of Hazardous Materials 389 (2020) 121917

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Bio-photoelectrochemcial system constructed with BiVO4/RGO photocathode for 2,4-dichlorophenol degradation: BiVO4/RGO optimization, degradation performance and mechanism

T

Lingli Tua,1, Yanping Houa,b,1, Guiyun Yuana, Zebin Yua, Shanming Qina, Yimin Yana, Hongxiang Zhub,c,d, Hongfei Lind, Yongli Chend, Shuangfei Wangb,c,d,* a

School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, China College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China d Guangxi Bossco Environmental Protection Technology Co., Ltd, 12 Kexin Road, Nanning 530007, China b c

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Editor: Danmeng Shuai

A single-chamber bio-photoelectrochemical system (BPES) constructed with BiVO4/reduced graphene oxide (RGO) photocathode was proposed for 2,4-dichlorophenol (2,4-DCP) degradation under simulated solar irradiation. The BiVO4/RGO (B/G) composites were synthesized, optimized and characterized by various techniques to analyze their physico-chemical and photocatalytic properties. Results showed that B/G (5 wt% - 9 h - 150 °C) exhibited the best photocatalytic activity for 2,4-DCP degradation, which was 1.5 times of that of BiVO4, due to its better light absorption, faster electrons transfer, and more efficient photo-generated e− - h+ separation. Reactive species trapping experiments revealed that ·OH was the main radical leading to 2,4-DCP degradation, and h+ also influenced 2,4-DCP removal. The 2,4-DCP (20 mg/L) removal rate and current output from the illuminated BPES were much higher than those of the unilluminated reactor (68.5 % vs. 41.8 %, 60.31 A/m3 vs. 40.07 A/m3) in 24 h, and the cathode potential was more negative, indicating that photocathode catalytic process was favorable to pollutants degradation and energy generation. Intermediates of 2,4-DCP degradation in the BPES were identified, and accordingly, possible degradation pathway and mechanism were proposed. This research advanced the development of efficient photocathode and mechanism of recalcitrant wastewater treatment in the BPES.

Keywords: Bio-photoelectrochemcial system BiVO4/RGO photocathode 2, 4-Dichlorophenol Intermediates Degradation mechanism

Corresponding author at: Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, China. E-mail address: [email protected] (S. Wang). 1 These authors contributed equally to this study and share first authroship. ⁎

https://doi.org/10.1016/j.jhazmat.2019.121917 Received 28 September 2019; Received in revised form 24 November 2019; Accepted 15 December 2019 Available online 17 December 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 389 (2020) 121917

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1. Introduction

Zhang et al., 2018a). These suggested that the conduction band (CB) of BiVO4 may be lower enough to dechlorinate 2,4-DCP (Yang et al., 2018a). Meanwhile, the valence band (VB) of BiVO4 is close to the ·OH generation position, endowing it great potential for catalytic degradation of organic substances. Nevertheless, the photocatalytic performance of pristine BiVO4 would be relatively poor due to its low charge transfer efficiency (Shi et al., 2018; Wang et al., 2019b). Reduced graphene oxide (RGO) has been demonstrated to be a desired material to prepare photocatalyst composite for the following reasons (Hu et al., 2019; Iliev et al., 2018; Liu et al., 2018b; Lv et al., 2019; Wang et al., 2014b; Wu et al., 2018b): 1) RGO has unique 2-dimentional layered structure, great chemical stability, morphological diversity, and can be dissolved in numerous solvents to form a graphite oxide-based composite material; 2) RGO is an excellent electron acceptor and transporter that can enhance photocatalyst performance by facilitating the transfer of photo-induced electrons from the semiconductor and by acting as a scaffold that increase the surface area; 3) RGO can absorb more contaminants on the surface and strengthen the interaction between pollutants and photo-generated active species on semiconductor interface because of the existence of the π-π bond and the oxygencontaining functional groups. Therefore, preparing the BiVO4/RGO photocatalytic composites should increase electron transfer and surface area of the catalysts, as compared with pristine BiVO4. Herein, the objective of this study was to construct the BPES with the BiVO4/RGO photocathode for efficient 2,4-DCP degradation. More specifically, the BiVO4/RGO photocatalysts were synthesized and optimized, and the BiVO4/RGO photocathode was prepared and characterized; the 2,4-DCP degradation performances of the photocatalytic process and that in the BPES were evaluated; in addition, the intermediates were identified to deduce 2,4-DCP degradation pathway and mechanism.

Chlorophenols have been widely used in productions of pesticides, dyes and phenolic resins, etc; thus, large amounts of wastewater containing chlorophenols are produced annually (Yang et al., 2018a; Yu et al., 2019). Chlorophenols are difficult to degrade (Li et al., 2018), and what’s worse, they have estrogenic, mutagenic and carcinogenic effects towards human beings (Kwean et al., 2018). Among various chlorophenols, 2,4-dichlorophenol (2,4-DCP) is a pivotal chemical raw material for synthesis of herbicide, 2,4-dichlorophenoxyacetic acid, preservative, etc. Since Cl is in the para position, the structure of 2,4DCP is more stable, and it can promote disturbances in the cell membrane bilayer and accumulate in living organisms; hence, it has been listed as a key pollutant by the US Environmental Protection Agency (EPA) (Chen et al., 2014; Wang et al., 2018c). Therefore, it is urgent to develop technologies to efficiently degrade chlorophenols. At present, a variety of methods, including physical methods, chemical methods, biological methods and their combinations, have been employed to degradade chlorophenols. For instance, Ghobashy et al. synthesized a cross-linked polystyrene organogel which exhibited higher absorption affinities for all chlorophenols (> 99 %) with good stability and reusability for the treatment of petroleum wastewater reclamation (Ghobashy et al., 2018). In spite of high efficiency of physical methods, the contaminants cannot be thoroughly degraded. Among various chemical methods, photocatalysis has been considered as one of most efficient methods to degrade chlorophenols, since many active species with strong redox ability including hydroxyl radicals (%OH), superoxide free radicals (%O2−) and photo-generated holes (h+) can be generated under enough light irradiation (Zhao et al., 2018). Wang et al. demonstrated that high efficient degradation of 4-CP (96.2 %) could be achieved owing to ·OH generated from Fe2O3/Polystyrene composite fibers (Wang et al., 2019a). Alafif et al. (Alafif et al., 2019) prepared an efficient visible light active CuO-GO/TiO2 for 2-chlorphenol and complex aromatics decomposition in real dairy wastewater, and their result showed that tCOD significantly decreased from 12,747 mg/L to 134.8 mg/L within 27 days. The above reports suggested that photocatalysis is effective for chlorophenol wastewater treatment. Recently, bioelectrochemical systems (BESs) related systems, including microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and their coupling systems, have been employed to degrade persistent organic pollutants, like hexachlorobenzene (Cao et al., 2015), fluoroaniline (Zhang et al., 2014), 4-CP (Kong et al., 2014), and so on. Our group also contributed to the advance of BESs coupled systems. Specifically, photocatalysis process was introduced to the BES to construct the bio-photoelectrochemcial system (BPES) for recalcitrant organic pollutants degradation and simultaneously clean energy recovery. Our results indicated that, with the assistance of photocatalysis occurred at the cathode, the BPES exhibited excellent performance in terms of azo dye degradation, antibiotics mineralization and energy generation (Hou et al., 2017a, 2019; Hou et al., 2017b). Hence, the BPES is a promising technology to treat wastewater and produce energy. As known, the cathode is one of the most important components of BESs, which would affect the performance and application potential of BESs (Kundu et al., 2013). Similarly, the photocathode with photocatalyst loaded on it also plays an essential role in the BPES, and generally, it has a significant influence on pollutants degradation efficiency, energy generation and microorganisms’ activity. Therefore, it is very important to develop desired photocatalytic material. Non-toxic Bi-based and Bi-containing materials have been used in many fields and attracted broad attention (Bai et al., 2019; Guan et al., 2018a). Among them, BiVO4 has been widely used in photocatalytic hydrogen evolution and PEC photo-anodes on account of its narrow direct bandgap (∼2.4 V) (which makes it visible light responsive) and unique monoclinic scheelite structure (Gao et al., 2019; Wang et al., 2018b), and superior photocatalytic properties (Li et al., 2019a; Wu et al., 2018a;

2. Materials and methods Graphene oxide (GO) was purchased from Shenzhen Turing Evolution Technology Co., Ltd. 2,4-DCP was obtained from China Pharmaceutical Group Chemical Reagents Co., Ltd. NaVO3 was purchased from Aladdin Reagent (Shanghai) Co., Ltd. Bi(NO3)3∙5H2O, NH4Cl, KCl, Na2HPO4·12H2O, NaH2PO4·2H2O and so on were obtained from Guangzhou Huada Chemical Reagent Co., Ltd. All reagents were of analytical grade and used without further purification. 2.1. Synthesis and characterization of the samples 2.1.1. Synthesis of BiVO4/RGO The BiVO4/RGO photocatalyst with visible light response was prepared via a simple one-step hydrothermal method at pH value of 6. The influences of GO amounts (0 wt%, 1 wt%, 5 wt% and 10 wt%), hydrothermal times (6 h, 9 h, 12 h and 15 h) and temperatures (120 °C, 150 °C, 170 °C and 190 °C) on the performance of the photocatalyst were determined. Details were shown in supplementary materials. For simplicity, the composites were named as B/G (amount of GO - hydrothermal time - hydrothermal temperature) below. 2.1.2. Preparation of photocathodes The photocathode was prepared as follows: 127.5 mg B/G (5 wt% 9 h - 150 °C) composite was added to the mixture of 200 μL nafion and 100 μL isopropanol. After 30 min of ultrasound, the mixed liquor was coated to a piece of nickel foam (5 cm × 3 cm) with a brush layer by layer and air-dried. 2.1.3. Characterizations of the samples The samples were analyzed by various techniques. The X-ray diffraction (XRD) was tested to characterize the crystal structure with high power (4 kW) polycrystalline X-ray diffractometer (DX2700A, Dandong Haoyuan Instrument Co., Ltd.), using Cu Kα radiation. Scanning 2

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electron microscopy (SEM) images were taken to display the morphology of samples using a Hitachi SU8220 (Hitachi Corporation of Japan) instrument, operated at acceleration voltage of 5 kV. Transmission electron microscope (TEM) and high resolution transmission electron microscope (HR-TEM) images which can also display the morphology were obtained by a FEI TECNAI G2 F30 (FEI) instrument. The X-ray photoelectron spectroscopy (XPS) was examined to confirm the chemical composition of the samples by a X-ray photoelectron spectrometer (ESCALAB 250XI+, Thermo Fisher Scientific) equipped with an Al (mono) X-ray source, and the binding energies were calibrated with respect to the signal for adventitious carbon (binging energy =284.8 eV). The fourier transform infrared (FT-IR) spectra of the samples were obtained to depict the chemical bonds by a Fourier Infrared Spectrometer (Nicolet iS 50, Somerfly, USA), using KBr as diluents. The UV–vis diffuse reflectance spectra (UV–vis DRS) of the samples were measured to characterize the property of light absorption by an UV-2501PC (SHIMADZU Japan) spectrophotometer, using BaSO4 as a reference. The photoluminescence (PL) spectra of the samples were conducted to test the recombination of photo-generated electrons and holes with a spectrofluoro-photometer (ZLX-PL-I, Beijing Zhuoli Hanguang Instrument Co., Ltd.) at excitation wavelength of 325 nm. Photoelectrochemical measurements were performed to obtain the properties of oxidation-reduction, impedance and transportation of carriers in a quartz cell on a conventional three-electrode cell with an electrochemical workstation (CHI 660EZ, China), using 50 mM PBS solution (pH = 7.0) as the electrolyte. The working electrodes (1 cm × 2 cm) were cut down from the as-prepared photocathodes, platinum gauze electrode and saturated calomel electrode were used as counter and reference electrodes, respectively. LSV tests were performed from 0 V to 1.2 V (vs. SCE) with the scan rate of 100 mV/s. Cyclic voltammetry (CV) tests were carried out between −1.2 V to 0.2 V (vs. SCE) with the scan rate of 100 mV/s. The electrochemical impedance spectroscopy (EIS) tests were performed at an open circuit potential (OCP) in the frequency range of 105 Hz to 0.1 Hz, and the charge transfer resistance (Rct) was obtained by ZSimpWin 3.10 software (Echem, US). Transient photocurrent (I-T) measurements were carried out with light-emitting diode (420 nm, 3 W) as the visible-light source. The Mott-Schottky (M-S) plot was recorded with an AC frequency of 1 kHz.

circuit. The bottle-type reactor was sealed with a rubber stopper. The bioanode of the BPES was firstly acclimated in the MEC, which consisted of graphite brush and nickel foam cathode, and was fed with the mixture of municipal wastewater (used as the inoculation, collected from Langdong wastewater treatment plant (Nanning, China)) and the medium. The medium of the startup period consisted of 1.0 g/L sodium acetate, 50 mM phosphate buffer solution (0.31 g/L NH4Cl, 0.13 g/L KCl, 103.5 g/L Na2HPO4·12H2O, 33.1 g/L NaH2PO4·2H2O), mineral solution and vitamin solution. Minerals and vitamins were necessary for the growth of microorganisms, and the ingredients of mineral solution and vitamin solution were provided in supplementary material (Tables S1–2). Applied voltage of 0.7 V was added to the circuit by a power source (LN I-T, UTP 3313 TF L-ll). Voltage across the resistor was recorded by a multimeter data acquisition system (model 2700, Keithley Instruments, Inc., Cleveland, OH). Tripartite reactors were operated in fed batch mode. The anode was considered to be fully acclimated when reproducible current generations were observed from the external circuit for at least 3 cycles. For each cycle, the medium was replaced when the current decreased to below 1.0 mA. After startup, 2,4-DCP (20 mg/ L) was added to the medium with sodium acetate (1.0 g/L) as a cosubstrate to cultivate the anodic respiring bacteria to adjust to the 2,4DCP wastewater. After several cycles of cultivation, the reactor was operated as the BPES with simulated solar irradiation, and 2,4-DCP degradation performance was examined. The current outputs from the BPESs with and without light illumination were also compared. Besides, the effect of initial 2,4-DCP concentration on degradation performance was determined. 2.3. Analysis and calculation All 2,4-DCP solution samples were filtered with 0.22 μm nylon filter before determination. The concentration of 2,4-DCP was measured by a high performance liquid chromatography (HPLC, LC-20AD, Shimadzu International Trade Co., Ltd.) equipped with an ultraviolet detector and a ZORBAX SB-C18 column (5 μm, 4.6 mm × 150 mm). The mobile phase was consisted of 80 % methanol and 20 % ultrapure water (v/v) with a flow rate of 0.8 mL/min. The wavelength was set at 286 nm. Intermediates were determined by a liquid chromatography tandem mass spectrometer (LC/MS, AGILENT 6460, USA) equipped with ACQUITY UPLCBEHC18 (1.7 μm, 2.1 mm × 50 mm) under negative electrospray ionization mode. The mobile phase consisted with ultrapure water that added 0.1 % formic acid and methanol with a flow rate of 0.3 mL/min. The ion source was heating electrospray at 300 °C. The removal rate (η) of 2,4-DCP was calculated by Eq. (1), where C0 is the initial concentration and Ct is the concentration at sampling time t.

2.1.4. Evaluation of photocatalytic activity The photocatalytic performances of as-prepared photocatalysts for 2,4-DCP decomposition were evaluated under simulated solar light using a 300 W Xe lamp (Beijing Zhongtuo Jinyuan Technology Co., Ltd.). In a typical test, 50 mg catalyst was placed in 50 mL 2,4-DCP aqueous solution (20 mg/L). Before irradiation, the suspension was vigorously stirred in dark for 30 min to ensure the establishment of the desorption-adsorption equilibrium between 2,4-DCP and photocatalyst. Afterward, the light was turned on. During the photocatalytic degradation process, 5 mL suspension was sampled at regular time intervals of 30, 30, 60, 120 and 120 min, respectively. Active species that may contribute to degradation of 2,4-DCP were investigated by radical trapping experiments. Three radical groups (%O2−, h+ and %OH) were captured by benzoquinone (BQ), ethylenediaminetetraacetic acid (EDTA) and isopropanol (IPA) in the reaction solution under light irradiation, respectively.

η = (1-Ct/C0)/100 %

(1)

The band edge position and optical absorption are related to Eq. (2), where α is absorption coefficient (which can be replace by absorbance (A), hv and Eg are respectively photon energy and band gap, A is a constant, and n is 1/2 for BiVO4 with direct band gap (Wu et al., 2018a). αhv = A(hv-Eg)n

2.2. Configuration and operation of the BPES

3. Results and discussion

The bottle-type reactor was made from quartz glass (with UV transmittance > 80 %), with the outer diameter of 5.5 cm, height of 7.5 cm, and a working volume of 125 mL. Graphite brush (made of graphite fiber, 2.5 cm diameter × 4.0 cm length) was used as the anode, which was heated in a muffle furnace at 450 °C prior to use. The anode and cathode were placed paralleled in the reactor with electrode distance of ∼1.5 cm. A resistor of 10 Ω was connected to the external

3.1. Structure characterization of photocatalysts

(2)

The surface morphological features of pristine GO, BiVO4 and B/G (5 wt% - 9 h - 150 °C) hybrid sample were shown in Fig. 1a–e. Fig. 1a depicts the SEM image of GO, and it is obviously that GO showed a thin yarn morphology, and wrinkles were present at the edge of GO sheet layer, as the sp3 hybridized oxygen-containing functional groups on GO 3

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destroyed the C]C double bond. As shown in Fig. 1b, the irregular (mostly square) BiVO4 particles with the size of ∼500 nm were agglomerated and there was a staggered connection between the crystals. Fig. 1c–e shows the SEM image of B/G (5 wt% - 9 h - 150 °C) hybrid sample, BiVO4 particles were well combined with RGO, which reduced the agglomeration of BiVO4 crystal particles. This would result in exposing more active sites, improving light absorption and increasing the

transfer and separation of photogenic charges, thereby enhancing the photocatalytic performance of the hybrid sample. And Fig. 1f (the inset in Fig. 1d) showed the section SEM of B/G (5 wt% - 9 h - 150 °C), and it was obvious that BiVO4 particles contacted closely with RGO and presented a regular monoclinic crystal, which consistent with the result of XRD. The TEM and HR-TEM images of GO and B/G (5 wt% - 9 h - 150 °C)

Fig. 1. SEM images of GO (a), BiVO4 (b) and B/G (5 wt % - 9 h - 150 °C) (c–e); TEM images of B/G (5 wt% - 9 h - 150 °C) (f–g) and HR-TEM images of B/G (5 wt% - 9 h - 150 °C) (h–i). 4

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were shown in Fig. 1f–i. Fig. 1f–g indicates that GO with thin yarn morphology was embedded into the surface of the block BiVO4 structures, which was favorable for rapid transfer of photogenic charges. As shown in the HR-TEM of B/G (5 wt% - 9 h - 150 °C) hybrid sample (Fig. 1h–i), the lattice space of 0.583 nm corresponding to the (020) planes of the BiVO4 monoclinic phase, which confirmed the crystallinity of BiVO4 in the composite material. Fig. 2 depicts the XRD pattern of GO, BiVO4 and hybrid samples. In Fig. 2a, the GO shows diffraction peaks at 9.3° and 42.2°, corresponding to the (002) and (100) reflection planes, respectively (Hafeez et al., 2018). It is confirmed the formation of monoclinic BiVO4 (space group 12/a) with lattice constant values of a =0.5195 nm, b =1.1701 nm, c =0.4092 nm, γ = 89.9°, which was consistent with the standard diffraction pattern of BiVO4 (JCPDS card No.014-0688). Previous study has demonstrated that BiVO4 with only monoclinic scheelite structure has photocatalytic activity (Wang et al., 2014a). The main peaks (2 theta = 18.6°, 18.9°, 28.9°, 30.5°, 34.5°, 35.2°, 39.8°, 47.3°, 53.3° and 59.2°) of BiVO4/RGO composites, corresponded to the monoclinic scheelite phase (110), (011), (121), (040), (200), (002), (211), (042), (161) and (123) crystal planes (Guan et al., 2018b). The (040) crystal plane of B/G (5 wt% - 9 h - 150 °C) had the strongest peak among all samples, suggesting that the hybrid sample had the best photocatalytic activity, which was further proved by the photoelectrochemical characterizations and experiments of degradation of 2,4-DCP (Wang et al., 2014b). The diffraction peaks of the composites were similar to those of BiVO4, and the reduction of GO could account for the disappearance of characteristic diffraction peaks of GO. Fig. 2b depicts the XRD pattern of the BiVO4/RGO hybrid samples that synthesized at different hydrothermal times. As shown, the BiVO4 of the composite obtained with the hydrothermal time of 6 h was mainly a tetragonal phase without

photocatalytic activity, which demonstrated that the hydrothermal time had a great influence on crystal phase of BiVO4. As the hydrothermal time increased, BiVO4 transformed into monoclinic phase with photocatalytic activity, but still had tiny hetero peaks, which were attributed to the tetragonal phase. As shown in Fig. 2c, photocatalysts obtained at 120 °C and 170 °C were mainly tetragonal phase with poor photocatalytic activity. When the hydrothermal conditions were 9 h and 150 °C, the crystal diffraction intensity of the obtained sample was larger, indicating that the crystallinity was better and the composite had more outstanding photocatalytic performance (Wang et al., 2014b). The significant reduction of GO into RGO was also confirmed by FTIR spectroscopy. The FT-IR spectra of GO, BiVO4 and composites are presented in Fig. 2d. As shown, GO showed characteristic vibrational peaks of C]O, in-plane vibrations of sp2-hybridized CeC bonding, alkoxy CeO stretches and the CeOeC stretching vibration absorption peaks at ca. 1730 cm−1, 1621 cm−1, 1051 cm−1 and 1225 cm−1, respectively. The characteristic absorption peak at approximately 3420 cm−1 corresponded to the deformation vibration and stretching vibration of OeH (Guo et al., 2012). The peaks around 433 cm−1 and 741 cm−1 corresponded to the stretching vibration absorption peaks of BieO and VeO in BiVO4, respectively. However, in the case of composites, the absorption peaks of GO at 1730 cm−1, 1621 cm−1, 1051 cm−1 and 1225 cm−1 were disappeared. This strongly suggested the significant reduction of GO, indicating that the composite had been successfully prepared. For the sample of B/G (5 wt% - 9 h - 150 °C), the absorption peaks of GO in the hybrid sample were the weakest, which manifested that the GO had the highest degree of reduction, and BiVO4 could be well combined with RGO to improve the photocatalytic performance. The surface composition and elemental states of BiVO4 and B/G

Fig. 2. XRD patterns of the photocatalysts: different GO doping amounts (a), different hydrothermal times (b) and different hydrothermal temperatures (c) and FT-IR spectra of GO, BiVO4, and various composites (d). 5

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(5 wt% - 9 h - 150 °C) hybrid sample were characterized by XPS (Fig. S1, Fig. 3), which confirmed that the hybrid sample was consisted of Bi, V, O and C elements (Fig. 3a). As shown in Fig. 3b, the XPS peak assigned to Bi 4f in the composite showed a positive peak shift of about 0.4 eV compared with Bi 4f in bare BiVO4, suggesting that BiVO4 might be chemically bonded to RGO via Bi-C bond, resulting in a strong electron transfer from BiVO4 to RGO. XPS spectra in V 2p region presented a similar trend as those in Bi 4f region (Fig. 3c) (Han et al., 2018; He et al., 2019; Lakhera et al., 2019). The presence of RGO was demonstrated by the carbon peaks in the XPS spectrum of B/G (5 wt% - 9 h 150 °C). Specially, the characteristic peaks in the C1 s spectrum (Fig. 3d) can be attributed to the graphitic sp2 carbon atoms of CeC species (284.7 eV) (Patil et al., 2017) and the non-oxygenated ring C (285.4 eV) (Wang et al., 2013), the other the two carbon species (CeOH (286.5 eV) and C]OeH (288.5 eV)) in the composite were ascribed to the residual oxygenated groups on the surface of RGO (Patil et al., 2017). Besides, the weak peak intensity indicated that some oxygencontaining functional groups on the GO surface were reduced, and GO had been reduced. The peak at 284.8 eV in BiVO4 was in correspondence with C 1s, which might be the carbon contamination during the test (Wang et al., 2018a). More details were provided in the supplementary materials.

showing benign visible light response, which was consistent with literature (Guan et al., 2019). The light absorption ability in the range of 520∼800 nm of hybrid samples was significantly increased with the increasing of GO doping amount. This could be ascribed to the alleviation of BiVO4 crystal particles agglomeration with addition of GO and black color of GO (Xu et al., 2018), which was consistent with SEM and XRD results. The optical absorption edges of hybrid samples (except B/G (5 wt% - 6 h - 150 °C)) appeared red shift that might be owing to GO doping, which should generate an impurity band and narrow the band gap of BiVO4 (Yang et al., 2018b). As indicated by XRD, the B/G (5 wt% - 6 h - 150 °C) was mainly a tetragonal phase, therefore slight blue shift of the optical absorption edge appeared compared to bare BiVO4 (Liu et al., 2017). The calculated band gap of BiVO4 was around 2.38 V according to Eq. (2). The band gap position of BiVO4 was determined directly by MottSchottky plot, and the result was shown in Fig. S2. The flat band potential of BiVO4 can be determined at -0.25 V (vs. SCE) according to the intersection points of potential and linear C−2 potential curves, thus the conduction band of BiVO4 was -0.11 V (vs. NHE). According to the UV–vis DRS, the band gap of BiVO4 was 2.38 V, thus the valance band of BiVO4 was 2.27 V. Furthermore, the MS plot of B/G (5 wt% - 9 h 150 °C) showed a smaller slope than that of BiVO4, which confirmed the charge carrier density of the composite was higher (Han et al., 2018). Fig. 4b displays PL spectra of the as-prepared samples. It is generally believed that the higher PL intensity reveals the faster the recombination rate of e− - h+ pairs, and a lower intensity means more are transferred or trapped (Ni et al., 2018). The BiVO4 displayed an emission peak at approximately 425 nm (λex=325 nm), while the PL

3.2. Photoelectrochemical properties of BiVO4/RGO photocatalysts and photocathodes The UV–vis-DRS results of various photocatalysts were shown in Fig. 4a. Pure BiVO4 exhibited an absorption edge at around 520 nm,

Fig. 3. XPS spectra of pure BiVO4 and B/G (5 wt% - 9 h - 150 °C): survey spectrum (a) Bi4f (b), V2p and O1 s (c) and the C1 s spectra of B/G (5 wt% - 9 h - 150 °C) (d). 6

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Fig. 4. The photoelectrochemical properties of the as-prepared samples: UV–vis-DRS of different photocatalysts (a); PL spectra of the as-obtained photocatalysts (b); I-T curves (c); EIS of samples (d); LSV (e) and CV (f) curves of BiVO4 and B/G (5 wt% - 9 h - 150 °C) with and without illumination.

intensity of the composites decreased dramatically. This indicated that the recombination of photo-generated electron–hole pairs was effectively inhibited in the composites, as compared with the pristine BiVO4. Among various BiVO4/RGO composites, the B/G (5 wt% - 9 h - 170 °C) exhibited the lowest PL intensity, suggesting that the photo-generated charges separation and transfer on it was the fastest. Transient photocurrent results were shown Fig. 4c. The pure BiVO4 exhibited the weakest photocurrent response, which could be owing to relatively high e−-h+ recombination rate; while the composites showed enhanced photocurrent-response ability, which was likely due to the fact that RGO has intimately contacted with BiVO4, resulting in better charge separation and transfer performances. B/G 10 wt% - 9 h - 150 °C) exhibited the best photocurrent generation, which might because B/G (10 wt% - 9 h - 150 °C) could absorb more light than other samples (as shown in the UV–vis-DRS), and produce more photo-generated carriers.

As an electron collecting fluid, RGO could enhance the conductivity of composites, and thus improve carriers transferring; however, excessive RGO could serve as the center of electron hole recombination. Therefore, the PL intensity was higher for B/G (10 wt% - 9 h - 150 °C). For B/ G (5 wt% - 6 h - 150 °C), although it had low PL intensity, the photocurrent was relatively low, which may be ascribed to the tetragonal phase of BiVO4 in the composites. B/G (5 wt% - 9 h - 150 °C) also showed great photocurrent generation. Notably, although high PL intensity was observed from the B/G (5 wt% - 9 h - 150 °C), its much higher transient photocurrent and much lower and interfacial layer resistance (discussed below) could lead to better photocatalytic performance. These results were basically consistent with the results of structure characterizations. The EIS was conducted to evaluate the electrical properties of all the samples. As can be seen in Fig. 4d, the Nyquist arc radius of B/G (5 wt% 7

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- 9 h - 150 °C) decreased dramatically compared to other samples, indicating that the charge transfer on this sample is faster (Lv et al., 2019). Accordingly, more efficient interfacial charge transfer of B/G (5 wt% - 9 h - 150 °C) would be achieved, as compared with other samples, which was probably attributed to the excellent electrical productivity of RGO and the intimate contact interface between BiVO4 and RGO (Ni et al., 2018; Yu et al., 2018), as demonstrated by XPS, TEM and FT-IR. In addition, B/G (5 wt% - 9 h - 150 °C) showed the largest radius of the arc, indicating that the transfer of carriers was low. Fitting circuit diagram was provided in supplementary materials (Fig. S3). Fig. 4e–f showed the LSV and CV curves of the BiVO4 and B/G (5 wt % - 9 h - 150 °C) photocathodes with and without illumination. It was obvious that higher currents were generated under illumination for both BiVO4 and B/G (5 wt% - 9 h - 150 °C), which could be due to that photocurrent was produced under light irradiation (Hou et al., 2017a). In addition, the composite exhibited higher current than the bare BiVO4 as the introduction of RGO could enhance the transfer of electrons. As reported in the literature, CV curves with redox peaks for electrodes indicated pseudo-capacitor behavior (Patil et al., 2016). The overall integral area (current) of B/G (5 wt% - 9 h - 150 °C) electrode was much greater than the BiVO4 electrode, indicating a higher specific capacitance (Sengottaiyan et al., 2019). Stronger oxidation peaks were observed when the photocathodes were illuminated, which might be ascribed to the photo-induced electrons gathered as the good capacitance.

the highest 2,4-DCP removal efficiency of 55 % was achieved within 6 h using B/G (5 wt% - 9 h - 150 °C), which was about 1.5 times of that of pristine BiVO4 (37 %), followed by that of the B/G (10 wt% - 9 h 150 °C) (46 %). Yet, the absorption of the hybrid samples increased gradually with the amount of GO increasing. This could be ascribed to the fact that the oxygen-containing functional groups on the surface and edge of RGO (as proved by XPS) are capable of forming hydrogen bonds, allowing RGO to absorb more contaminants on the surface and strengthening the interaction between pollutants and photo-generated active species (Iliev et al., 2018). Fig. 5b shows that similar 2,4-DCP removals were achieved after 6 h for the composites obtained from hydrothermal times of 9 h, 12 h, and 15 h, indicating that the influence of hydrothermal time on the photocatalytic activity towards 2,4-DCP was insignificant within the range of 9 h–15 h. It was worth noting that B/G (5 wt% - 9 h - 190 °C) removed only 47 % of 2,4-DCP (Fig. 5c), which might be ascribed to the largest resistance as EIS shown and the damage of the structure of BiVO4 caused by the high temperature (Yang et al., 2009) Fig. S4 shows that the degradation process followed pseudo first-order kinetics, and the kinetic constant of B/G (5 wt% - 9 h - 150 °C) was 0.00184 min−1, higher than other samples. This was likely due to faster transfer of photo-induced carriers, less recombination of electrons and holes and more active sites. To explore the photocatalytic degradation mechanism of 2,4-DCP, reactive species trapping experiments were performed. The results showed that the removal rate of 2,4-DCP decreased from 55 % to 50 %, 43 % and 21 % with addition of BQ, EDTA and IPA, respectively (Fig. 5d), indicating that the ·OH was the main radicals leading to 2,4DCP degradation mediated by BiVO4/RGO, h+ also influenced the removal of 2,4-DCP and ·O2− hardly worked. The possible mechanism of photocatalytic degradation could be as follows:

3.3. Evaluation of photocatalytic activity As shown in Fig. 5a, the composites exhibited higher photocatalytic activities towards 2,4-DCP degradation than pure BiVO4. Specifically,

Fig. 5. Removal rate of the as-prepared samples with different GO doping amounts (a), different hydrothermal times (b) and different hydrothermal temperatures (c) and effects of scavengers on the degradation of 2,4-DCP (d). 8

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Fig. 6. Removal rate of 2,4-DCP with and without illumination in the BPES (a), removal rate of 2,4-DCP with different initial 2,4-DCP concentration in the BPES (b).

Fig. 7. Current curves (a) and cathodes’ potential (b) of the BPES with and without illumination.

B/G (5 wt% - 9 h - 150 °C) + hv → e- +h+ -

e + O2 +2H

+

microorganisms (68.5 % vs. 41.8 % vs. 36.6 %) in 24 h (Fig. 6a). The enhancement of 2,4-DCP removal under illumination could not only be ascribed to the faster electrons transfer of the photocatalysts, but also more ·OH generation at the VB of the photocatalysts which could oxide 2,4-DCP effectively. It should be noted that the removal rate of 2,4-DCP at the fourth hour declined as compared with that at the second hour, probably because microorganisms on the anode would quickly absorbed 2,4-DCP at the beginning of the reaction, resulting in decreasing its concentration; as time went on, microorganisms desorbed the 2,4DCP, leading to increasing 2,4-DCP concentration. The removal rate of 2,4-DCP in the abiotic BPES only with light irradiation was about 21 %, as shown in Fig. S6, which was not as high as expected compared with photocatalysis experiments above. The reason could be summarized as follows: in the photocatalytic experiments, the reaction solutions were homogeneous under magnetic stirring, allowing photocatalyst and contaminant evenly mixed and fully contacted, which was favorable for 2,4-DCP degradation; but in systems, the photocatalyst was loaded onto a nickel foam cathode, which limited the contact between catalyst and 2,4-DCP, thereby retarding its efficient degradation. In addition, 2,4-DCP removal of 28 % was achieved from the abiotic BPES with applied voltage of 0.7 V and light irradiation, indicating that the electrolysis could assist the degradation of 2,4-DCP during the photocatalytic process. This might be due to band-bending could be controlled and a more efficient separation of the charge carriers can be achieved when added applied bias (Zlamal et al., 2007). It was noting that synergistic effect might exist in BPES, as the removal efficiency in BPES was higher than the sum of photocatalytic and electrolytic process and microbial process (68.5 % > 28 % + 36.6 % = 64.6 %), which was consistent with our previous findings (Hou et al., 2017a, b).

→ H2O2

-

e + H2O2 → ·OH + OHH2O → H+ + OHH2O + h+ → ·OH + H+ 2,4-DCP + ·OH /h+→ Products Furthermore, as shown in Fig. S5, the CB edge potential of BiVO4 was -0.11 V (vs. NHE), which was more positive than the standard redox potential Eθ (O2/·O2−) (-0.33 V vs. NHE), indicating that O2 could not be reduced to ·O2− by the electrons at CB of BiVO4. However, the CB edge potential of BiVO4 was more negative than the standard redox potential Eθ (O2/H2O2) (0.685 V vs. NHE), indicating O2 absorbed on the surface of photocatalyst could react with electrons to generated H2O2, subsequently generated ·OH. On the other hand, the VB edge potential of BiVO4 (2.27 V vs. NHE) was more positive than the standard redox potential Eθ (·OH /OH-) (1.99 V vs. NHE), thus holes at the VB of BiVO4 could oxide OH- to ·OH (Ren et al., 2019; Wu et al., 2019). Besides, 2,4-DCP could be removed by ·OH and h+ according to the above reactive species trapping experiments. 3.4. 2,4-DCP degradation and current generation in the BPES The degradation efficiencies of 2,4-DCP were evaluated in the BPES with and without illumination. With lower 2,4-DCP concentration of 20 mg/L, the removal rate from the illuminated BPES was much higher than that of the unilluminated reactor and the system with only 9

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Table 1 Parameters for the identified intermediates of 2,4-DCP degradation. Compound

Structural formula

m/z

Retention time

2,4-diclorophenol

162.84

8.94

4-chlorophenol 2-chlorophenol

128.03

5.27

4-chlorocatechol

144.87

6.27

2-chloro-hydroquinone phenol

94.92

16.54

catechol 1,3-benzenediol hydroquinone

111.01

0.92

benzene-1,2,4-triol

126.90

11.76

2-hydroxy-benzoquinone

123.90

9.02

quinone

108.90

10.32

maleic acid

116.93

14.97

dimethyl malonic acid

132.87

9.15

acetic acid

59.01

2.83

ethylene glycol

61.99

12.78

malonic acid

103.04

8.16

The effect of initial 2,4-DCP concentration on BPES performance was also determined. As it was shown in Fig. 6b, the degradation efficiency gradually declined as the initial 2,4-DCP concentration increased; and the removal rate was only 14 % when the initial concentration went up to 50 mg/L. This was likely due to the toxicity of 2,4-DCP that strongly restrained microbial activity, which decreased the efficiency of biodegradation process.

Current generations were shown in Fig. 7a. The maximum current density from the illuminated BPES reached ∼60.31 A/m3, which was 19.61 A/m3 higher than that without illumination (∼40.70 A/m3), which indicated that the photocathode could enhance the transfer of electrons from anode to cathode. This was consistent with our previous findings (Hou et al., 2017a, b). Higher current density indicated that microorganisms on the anode were more energetic and generated more 10

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Fig. 8. Proposed pathway for 2,4-DCP degradation in BPES.

removed by h+, ·OH attacking or by accepting extracellular electrons (ebio−) which generated from the microorganisms to yield 4-CP and 2CP, or replaced by hydroxyl to generate 4-chlorocatechol and 2-chlorohydroquinone (Humayun et al., 2019a; Zada et al., 2018; Zhou et al., 2018). Then, 4-CP and 2-CP were further dechlorinated to produce phenol, afterwards one hydrogen atom could be replaced in the case of the ·OH, thereby, catechol, 1,3-benzenediol and hydroquinone were generated (Li et al., 2019b; Zhang et al., 2015; Zhou et al., 2018). The 4-Chlorocatechol and 2-chloro-hydroquinone could also be transformed into catechol and 1,3-benzenediol by the ·OH, respectively (Chen et al., 2016; Zhang et al., 2018b). Then the ·OH attacked the benzene ring of these dihydric phenols to yield maleic acid (Li et al., 2019b). On the other hand, the last chlorine atom of 4-chlorocatechol and 2-chlorohydroquinone could be replaced by hydroxyl to form benzene-1,2,4triol, too (Gong et al., 2019). And benzene-1,2,4-triol transferred to 2-

electrons because of the rapid removal of 2,4-DCP. The cathodes’ potentials were recorded with and without illumination in the reactors. As shown in Fig. 7b, at steady stage, the photocathode potential was lower under illumination than that without illumination; and reports has shown that lower cathode potential was beneficial for organic pollutants reduction (Kong et al., 2015). 3.5. Degradation pathway of 2,4-DCP To further explain the degradation mechanism, the intermediates during 2,4-DCP degradation by BPES were identified by LC–MS. Mass spectra of the intermediates were shown in Fig. S7. And Table 1 displays their detailed information. According to the detected intermediates, a reasonable pathway for the degradation of 2,4-DCP by BPES was proposed in Fig. 8. Firstly, one chlorine of 2,4-DCP could be 11

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Scheme 1. Possible reaction mechanism for degradation of 2,4-DCP in BPES.

hydroxy-benzoquinone, then quinone by the %OH, and maleic acid was produced (Li et al., 2017b). Afterwards, maleic acid could be decomposed to dimethyl malonic acid, then malonic acid could be degraded to some small molecule substances including acetic acid and ethylene glycol. Meanwhile, maleic acid could also be directly decomposed to acetic acid and ethylene glycol. Last, these small molecule substances could be mineralized to CO2 and H2O (Humayun et al., 2019b; Li et al., 2017a). Base on the above results, possible mechanism of 2,4-DCP degradation in the BPES could be summarized as follows (Scheme 1): anode respiring bacteria would generate extracellular electrons during the process of oxidizing the co-substrate. These electrons could be transmitted to the photocathode through the external circuit and current generated in this process (Liu et al., 2018a; Ortiz-Medina et al., 2019); also, they could be transferred directly to 2,4-DCP molecules, resulting in dechlorination of 2,4-DCP (Kong et al., 2014). Under the irradiation of simulated solar light, photo-generated electrons (e−) and holes (h+) could be produced by B/G (5 wt% - 9 h - 150 °C) on the photocathode, then the h+ can oxide H2O to generate %OH. Hence, the dechlorination of 2,4-DCP could be achieved by the bio-electrons, h+ and ·OH, and the intermediates would be decomposed to some small molecular substances such as acetic acid, ethylene glycol and malonic acid. And these small molecular substances could be further oxidized by microorganisms or by h+ or %OH to eventually form to CO2 and H2O (Hou et al., 2017a, b).

Author contribution statement Yanping Hou, Lingli Tu and Shuangfei Wang contributed to the conception of the study. Lingli Tu conducted the experiment. Lingli Tu, Yanping Hou and Guiyun Yuan performed the data analyses and wrote the manuscript. Zebin Yu, Shanming Qin, Yimin Yanm, Hongxiang Zhu, Hongfei Lin, Yongli Chen and Shuangfei Wang helped performed the analysis with constructive discussions. Declaration of Competing Interest 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. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21707021, 51668006), Natural Science Foundation of Guangxi (No. 2017GXNSFBA198186), China Postdoctoral Science Foundation Grant (No. 2018M633295), Open Fund of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (No. KF201723), Guangxi Science and Technology Research Program (No. AA17202032), and Young Teachers Innovation Cultivation Program (No. BRP180261) from Guangxi Bossco Environmental Protection Technology Co., Ltd.

4. Conclusions In this study, the single-chamber BPES with a BiVO4/RGO photocathode was constructed for 2,4-DCP degradation under simulated solar irradiation. The BiVO4/RGO photocatalysts was synthesized, optimized, and characterized. Results showed that RGO could significantly improve the performance of BiVO4, and BiVO4/RGO composite with GO doping of 5 wt%, hydrothermal time of 9 h and hydrothermal temperature of 150 °C exhibited the best photocatalytic activity towards 2,4-DCP degradation. In the BPES with the optimized BiVO4/RGO (5 wt % - 9 h - 150 °C) as catalyst of the cathode, both of 2,4-DCP removal rate of and current generation were much higher under solar irradiation as compared with that without illumination. During the 2,4-DCP degradation in the BPES, intermediates were identified, and possible degradation pathway and mechanism were proposed accordingly. This study indicated that BiVO4/RGO was a promising photocathode material in the BPES for refractory organics degradation and energy recovery.

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