Vis system for degradation of bisphenol A: Environmental factors, degradation pathways, and toxicity evaluation

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M88/PS/Vis system for degradation of bisphenol A: Environmental factors, degradation pathways, and toxicity evaluation ⁎

Jiawei Lina,b, Yongyou Hua,b,c, , Luxiang Wanga,b, Donghui Lianga,b, Xian Ruana,b, Sicheng Shaoa,b a

School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou Higher Education Mega Centre, Guangzhou 510006, China c Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

effect in M88/PS/Vis • Synergistic system could effectively degrade BPA. M88/PS/Vis system could work • The over a wide pH range (3.0–9.0). activation by photocatalyst has a • PS great potential application. assessment in BPA/M88/PS/Vis • Risk system were firstly estimated via ECOSAR.

intermediate could be removed • Toxic and even mineralized by extending reaction time.

A R T I C LE I N FO

A B S T R A C T

Keywords: MIL-88B (Fe) Persulfate Photocatalytic Influence factors Toxicity analysis

In this study, we synthesized a typical iron-based metal-organic framework, MIL-88B (Fe), and constructed a MIL-88B (Fe)/persulfate/visible light (M88/PS/Vis) system for photocatalytic degradation of bisphenol A (BPA) in aqueous solution. The removal efficiency, environmental factors, and degradation mechanism and pathways were explored. The results showed that there were synergistic effects in the M88/PS/Vis system: BPA (10 mg/L) could be removed completely within 25 min with 0.6 g/L M88 and 2 mM PS under visible light, while the removal rates with individual M88/Vis or M88/PS was only 26.0% and 34.1%, respectively. In addition, the BPA degradation rate in the M88/PS/Vis system (0.107 min−1) was 4.7 times higher than that in the M88/PS system (0.023 min−1) and 8.9 times higher than that in the M88/Vis system (0.012 min−1), respectively. Additionally, environmental factors could strongly impact the degradation of BPA in the studied system in which O2%− and SO4%− played a dominant role. More importantly, risk assessment in the BPA/M88/PS/Vis system was, for the first time, estimated in our study via the “ecological structure activity relationships” program at three trophic levels and Cell Counting Kit-8 method. Although there were some intermediates with higher biological toxicity than BPA and increased the toxicity of solution, most intermediates were one or more level less toxic than BPA. In most cases, the toxicity of solutions were lower than the initial samples, which demonstrated the effectiveness of M88/PS/Vis system in the reduction of toxicity of BPA, and the toxicity hazard in general could be effectively avoided by prolonging the duration of light exposure. This research provides a

⁎ Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, China. E-mail address: [email protected] (Y. Hu).

https://doi.org/10.1016/j.cej.2019.122931 Received 4 June 2019; Received in revised form 20 September 2019; Accepted 21 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Jiawei Lin, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.122931

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profound understanding of the potential application of PS activation by photocatalyst for both BPA removal and toxicity shift under visible light.

1. Introduction

inhibition of the oxygen uptake rate by activated sludge increased from 40.8% to 55.1% during the first 5 min, which indicated that some toxic intermediates were generated during the initial stage of the reaction. Coincidentally, Wu et al. [23] evaluated the toxicity of the BPA reaction mixtures and found that the inhibition rate of Daphnia magna in the initial solution was 0.55, reaching a maximum of 0.80 after a reaction time of 60 min. It should be noted that, on one hand BPA could be converted into various by-products that might possess a much higher estrogen-binding and toxic activity than BPA during the degradation process. On the other hand, few studies have focused on removing emerging organic pollutants in an actual complex water environment. Both of the cases might encumber the development of actual applications and hinder technological improvement. Therefore, studies in related fields are necessary to clarify the environmental trends associated with BPA. Hence, to better understand the potential application of heterogeneous PS activation by a photocatalyst for both BPA removal and toxicity shift under visible light, we synthesized MIL-88B and constructed a MIL-88B/persulfate/visible light (M88/PS/Vis) system for photocatalytic degradation of BPA in aqueous solution. In the present study, a comprehensive investigation was performed for the degradation of BPA by the M88/PS/Vis system. The experiments focused on influence of various reaction parameters, including M88 concentration, PS concentration, pH, dissolved oxygen, co-existing inorganic ions (HCO3−, Cl−, SO42−, H2PO4−, NO3−), humid acid, and the effect of real background of water matrix (deionized water, tap water, river water and dyeing wastewater) on the reaction rate. The intermediates during the BPA degradation were determined, after which possible degradation mechanisms and pathways were proposed. Ultimately, risk assessment of BPA and its degradation intermediates were via the Ecological Structure Activity Relationships (ECOSAR) system [24] and the toxicity of the effluent samples was assessed by a commercial toxicity test kit (Cell Counting Kit-8 (CCK-8). The mineralization of BPA were studied based on the total organic carbon (TOC) parameter. This study highlights the potential application of novel photocatalysts for coupling with PS to address growing concerns with micropollutants in water and wastewater.

Bisphenol A (BPA, 2,2-bis (4-hydroxyphenyl) propane) is a typical endocrine disruption substance (EDS) [1]. Despite a relatively short environmental half-life of 2.5–5 days [2], BPA is frequently detected in the environment, including in sewage treatment plant tail water, natural water, and tap water [3]. Many ecological toxicity assessments have pointed out that BPA exhibits neuro-, immuno-, developmental, and reproductive toxicities against fish, amphibians, and invertebrates, which could result in dysfunction [4]. According to the latest report, BPA levels in wastewater, rivers/lakes/ponds, and surface seawater were 370.00, 3.92, and 0.19 μg·L−1, respectively [5], which is well above the predicted no effect concentration (PNEC) of BPA, updated to 0.06 μg·L−1 in 2011 [6]. This indicates that natural water systems have adverse health effects because of the ecological risk of BPA. Therefore, it is urgent to remove BPA from water systems to reduce harm to people and the environment. Recently, persulfate (PS) has received increasing attention as an emerging oxidizing agent. It could be activated to produce sulfate radicals (SO4−%), which behave like hydroxyl radicals with an unselective oxidation pattern and are very reactive with a wide range of contaminants over a wide pH range. There are many methods to activate PS, including ultraviolet light irradiation and heat [7] or using lowvalence metals such as Fe2+, Co2+, and Mn2+ [8]. Among numerous advanced oxidation processes (AOPs), photocatalysis is a convenient and environment-friendly approach to convert solar energy into chemical energy and been increasingly regarded as a promising strategy to effectively remove BPA from aquatic environments. The earliest study was based on modified TiO2 semiconductor materials and it’s composites [9]. Up to now, many photocatalysts have emerged, including MoS2 [10], g-C3N4 [11], BiOCOOH [12] and their composites [13]. However, the large band gap and the non-adjustable structure of conventional semiconductor materials have limited their further development and many efforts have been made to overcome this problem [14,15]. Because of its large surface area, well-ordered porous structure, and tunable organic linkers or metal clusters, metal organic frameworks (MOFs), a class of 3D crystalline micro-mesoporous hybrid materials, have emerged as novel heterogeneous catalysts with promising photocatalytic activity [16]. MIL-88B (Fe), a water-stable Febased MOF, was constructed from 1,4-benzenedicarboxylic acid (BDC) and trimeric Fe octahedral (Fe3-μ3-oxo) cluster [17]. It was considered to be a potential photocatalyst primarily since the size of the Fe3-μ3-oxo clusters limits electron-hole recombination, leading to a sensitive optical response to visible light. Furthermore, a photocatalysis system coupled with persulfate could accelerate the degradation of contaminants by forming SO4−% under visible light and scavenging conduction band electrons [18]. Many novel photocatalysts have been designed and coupled with persulfate to achieve high degradation rate or mineralization of BPA, including Ti3+ self-doped TiO2/graphene nanocomposite [19], Fe3O4-α-MnO2 nanoflower-like catalyst [20], and Fe(II)-immobilized chitosan/alginate composite [21]. Based on these studies, it was possible that this technology could be widely used in the treatment and detoxification of sanitary sewage and industrial wastewater. Despite the fast degradation rate, these processes might suffer from the generation of highly toxic intermediates. It has been confirmed that some intermediates with higher biological toxicity than BPA itself could be generated during the degradation process. To evaluate the biological toxicity of BPA-degraded intermediates, activated sludge inhibition tests were conducted by Du et al. [22], and the results showed that

2. Materials and methods 2.1. Materials Bisphenol A, iron (III) chloride hexahydrate (FeCl3·6H2O) and 1,4Benzenedicarboxylic (H2BDC) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). N,N-dimethylformamide (DMF) was purchased from Guangzhou Wenrui Scientific Instrument Co., Ltd. (Guangzhou, China). Dichloromethane (chromatographically pure) and sodium persulfate were obtained from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Unless specified, chemicals used in this study were analytical-reagent grade and used as received. All the solutions were prepared using deionized water. The solution pH was adjusted using dilute H2SO4 and NaOH solutions. 2.2. Characterization MIL-88B (Fe) nanooctahedra was prepared following the reported method with some key improvement and specific method was shown in Text S1. The phase of MIL-88B (Fe) was analyzed using X-ray diffraction (XRD) performed on a Bruker D8 Advance diffractometer with Cu Kα radiation operated at 40 kV and 40 mA. The morphology of MIL-88B 2

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2.4. Detection of reaction species (RSs)

(Fe) was investigated with scanning electron microscopy (SEM), using a MERLIN compact instrument with an acceleration voltage of 20 kV. UV–vis diffuse reflectance spectra (DRS) were recorded on a UV–visNIR spectrophotometer (Varian Cary 500). A conventional three-electrode system consisting of a Pt wire as the counter electrode, a SCE electrode as the reference electrode and MIL-88B electrodes as working electrodes was used to analyze the photo-electrochemical properties of MIL-88B on an electrochemical workstation which was connected to a computer (CHI 660B, China). A Na2SO4 solution (0.2 mol·L−1) was used as the electrolyte. The visible light source was a 300 W Xe lamp (BoPhilae Technology Co., LTD, Beijing, China) with a 420 nm cut off filter. The photocurrent test was performed under chopped light irradiation with Ampermetrio i-t curve. The Mott-Schottky measurements were carried out with impedance-potential model to evaluate the band position of the MIL-88B.

Scavenging experiments were conducted as an indirect method for the detection of RSs. Formate (sodium formate), p-benzoquinone (BQ), t-butanol (TBA), and methanol (MeOH) were used to quench active oxides including photo-induced holes (h+), superoxide anion radicals (O2−%), hydroxyl radicals (%OH), and sulfate radicals (SO4−%) in the reaction system.

2.5. Identification of intermediates The products and intermediates generated during the degradation process were examined with both a high-performance liquid chromatography mass spectrometry (HPLC/MS/MS, Agilent Technologies) instrument, and a gas chromatography-mass spectrometry (GC/MS/MS Agilent 6890-GC/5973i-MS) analyzer (Text S2).

2.3. Photoctalytic degradation test BPA solutions (10 mg·L−1, pH 6.5–7.2) were prepared to investigate the photocatalytic performances of MIL-88B under visible light irradiation. A certain amount of MIL-88B was placed in a 100 mL cylindrical Pyrex vessel with 100 mL BPA solution as-prepared. Prior to turnon of the light and addition of PS, the mixed solution was undergoing an adsorption-desorption equilibrium with magnetic stirring for 30 min in dark. Constant stirring was maintained during the experiment, the temperature was steady at 25 ± 2 °C, and the pH was adjusted with NaOH and H2SO4 if necessary. At predetermined time intervals, 0.8 mL of the sample was extracted and mixed with methanol for quenching of residual free radicals, and then filtered through a 0.22 μm filter for testing the concentration of BPA. The concentration of BPA was determined using a High Performance Liquid Chromatography (Waters Corp., Milford, MA, USA) at the maximum absorbance of BPA (276 nm). HPLC-grade methanol and ultrapure water were employed as the mobile phase at 70/30 (v/v) with a flow rate of 1 mL·min−1 and the sample injection volume was 10 μL. The total organic carbon (TOC) was determined with an Elementar TOC II analyzer (DKSH, Shanghai, China).

2.6. Risk assessment The risk assessment calculations of BPA and it’s degradation intermediates were carried out using the ECOSAR program, and the acute and chronic toxicities of BPA and its transformation by-products on fish, daphnia and green algae were predicted. The acute toxicity was expressed as EC50 for the algae and LC50 for the fish and daphnia. It is noted that EC50 is the concentration at which 50% of green algae are adversely impaired after a 96-h exposure and LC50 is the concentration at which 50% of fish and daphnia die after a 96-h and 48-h exposure, respectively. Specific data could be directly required through a calculation process when corresponding chemical formulas were drew. The toxicity of the effluent samples was assessed by a commercial toxicity test kit (Cell Counting Kit-8 (CCK-8), Sangon Biotech (Shanghai), China), which could allow sensitive assay for the determination of cell viability in cell proliferation and cytotoxicity assays. The toxicity of samples against Escherichia coli (E. coli) bacteria was assessed in the form of relative absorption intensity according to the manufacturer's protocol.

Fig. 1. XRD pattern (a); SEM images ((b) and (c)); Elemental mapping of the as-synthesized MIL-88B ((d), (e) and (f)). 3

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3. Results and discussion

performance of various systems, while control experiments were conducted to compare the removal efficiency of BPA by various processes with an initial BPA concentration of 10 mg·L−1. As shown in Fig. 3(a), in the absence of the catalyst and PS, photolysis of BPA was not observed under visible light irradiation, indicating that BPA was quite stable. Additionally, the photo-decomposition of BPA was extremely low during illumination in the absence of the materials; the BPA concentration showed no significant changes in either the dark or during the illumination process, demonstrating that non-activated PS had little effect on BPA and suggesting that oxidizing radicals could not be generated from PS without a catalyst. The dark adsorption experiments with MIL-88B indicated that the adsorption of BPA attained equilibrium after 30 min. The adsorption of BPA concentration was 10%. The M88/ PS/Vis system had the highest BPA degradation rate, compared with M88/Vis and M88/PS after 30 min of irradiation, the values of which were 100.0%, 26.0%, and 34.1%, respectively. This result also revealed that M88 coupled with PS had a synergistic effect under visible light irradiation. Fig. S2 shows the corresponding rate constants of BPA under various photocatalytic conditions. The data showed that the BPA degradation rate in the M88/PS/Vis system (0.107 min−1) was 4.7 times higher than that in the M88/PS system (0.023 min−1) and 8.9 times higher than that in the M88/Vis system (0.012 min−1), revealing that M88/PS/Vis could effectively degrade BPA and both the light source and PS played a remarkable role in the system. To elucidate the synergistic effect between PS and M88 under visible light, photo-current experiments were carried out. As showed in Fig. 3(b), both systems were prompt in generating a photo-current with a reproducible response to on–off cycles. M88/PS/Vis presented a lower transient current density than M88/Vis, which indicated that photogenerated electrons reacted with PS to generate SO4−% (Eq. (1)) [30]. The observed change in the transient photocurrent directly revealed the efficient charge carrier separation ability in the presence of persulfate, which could scavenge conduction band electrons and prevent the recombination of electrons and photoholes.

3.1. Characterization of MIL-88B The powder XRD patterns of the as-synthesized MIL-88B were shown in Fig. 1(a); they were well matched with the [0 0 2], [1 0 1], [1 0 3], [2 0 2], and [2 1 1] diffraction planes of MIL-88B (Fe) as well as with those in previous report [17,25]. The sharp and high-intensity diffraction peaks confirmed that the MIL-88 (Fe) was synthesized successfully. Meanwhile, no other peaks could be observed, indicating that a pure phase of MIL-88 (Fe) was obtained. Fig. 1(b) and (c) illustrate that the morphology of MIL-88 (Fe) was needle-like with a length of approximately 1.1 μm and an average width of 0.26 μm. A compositional analysis carried out with energy-dispersive X-ray spectroscopy measurements (Fig. S1) showed the presence of Fe, C, and O elements in the as-prepared MIL-88B (Fe) sample. Fig. 1(d–f) was the elemental mapping, which further confirmed that these elements were homogeneously distributed in the MIL-88B (Fe). To further analyze the molecular structure and identify the functional groups of the MIL-88 (Fe) sample, FTIR spectroscopy was performed, and the results were shown in Fig. 2(a). The characteristic absorption peaks were identical to those reported in the literature [26], which were observed at 552, 750, 1394, 1548, and 1660 cm−1. The intense bands at 552 cm−1 were assigned to Fe-O vibrations [27]. The peak at 750 cm−1 corresponded to C-H bending vibrations of benzene. The two sharp peaks at 1548 and 1394 cm−1 were assigned to asymmetric and symmetric vibrations of carboxyl groups, which confirms the presence of the dicarboxylate linker within the MIL-88B (Fe) material. The FTIR band at 1660 cm−1 was characteristic of dimethylformamide (DMF) [28]. To sum up, the results of XRD analysis and the FTIR spectra clearly confirm the formation of the MIL-88B (Fe) structure. Additionally, UV–vis diffuse reflectance spectra (DRS) were acquired for the obtained samples. Fig. 2(b) display the optical absorption spectra of the sample. The as-prepared MIL-88B exhibited fundamental absorption edges at 200–600 nm, demonstrating visiblelight responsive characteristics and a wide light absorption region. Clearly, the onset of the main absorption edge of MIL-88 (Fe) was located at 518 nm. Based on the relationship Eg = 1240/λ [29], the calculated bandgap from the absorption onset was approximately 2.39 eV, which implied an efficient response to visible light. Mott-Schottky plots were determined for the obtained samples to provide a post-hoc explanation of the reaction mechanism. The measurement (Fig. 2c) gave a conduction-band potential of approximately −0.61 eV. As a result of the DRS results, the valence band (VB) was located at 1.78 eV (EVB = Eg + ECB).

S2 O82 − + HO2− → SO42 − + SO4−· + H+ + O2−·

(3)

2S2 O82 −

(4)

3.2. Enhanced degradation rate by synergistic effect

SO4- · + H2 O → SO42 − + OH ·+H+

(5)

SO4·− + OH− → HO·+SO42 −

(6)

S2 O82 − + e− → SO4−· + SO42 −

Based on these results, it was proposed that the high degradation rate of the synergistic M88/PS/Vis system was ascribed to the abundant production of SO4−%. In addition, the excitation of PS could also produce other active species, such as O2−% and OH% (Eqs. (2)–(7)) [31].

S2 O82 − + H2 O → 2SO42 − + HO2− + H+

The degradation of BPA was used to evaluate the catalytic

(a)

(1)

(b)

+ 2H2 O →

SO4−·

+

3SO42 −

+ H+ +

(c)

Fig. 2. (a) FTIR spectrum; (b) UV–vis spectra and (c) Mott-Schottky plots of MIL-88B (Fe). 4

(2)

O2−·

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(a)

(b)

Fig. 3. (a) Degradation rate of BPA under different conditions ([PS] = 2 mM, [M88] = 0.6 g/L); (b) Transient photocurrent responses of MIL-88B ([PS] = 2 mM, [M88] = 0.6 g/L, [Na2SO4] = 0.2 M).

O2−· + H2 O → OH ·+HO2−·

implying that SO4−% was responsible for BPA degradation. A moderate decrease occurred in the presence of formate, while the degradation of BPA decreased markedly with the addition of BQ, implying the dominant role of O2−% species. In order to further confirm the role of radicals, both methanol (250 mM) and BQ (1 mM) were added in the same reaction system, the result showed that degradation of BPA could be inhibited completely (Fig. S3b), which confirmed that O2−% and SO4−% played the key roles in M88/PS/Vis system. The reaction rate in the presence of different scavengers and the removal rate of BPA in different quenching experiments were shown in Fig. 4(b), which demonstrated the same trend with degradation rate.

(7)

Therefore, methanol was chosen to scavenge both OH% and SO4−% radicals and tert-butyl alcohol (TBA) was chosen to scavenge SO4−% radicals [32]. Besides, sodium formate and p-benzoquinone (BQ) were used to quench hv+ and O2−% in this study [33]. At first, reactions without scavengers was set as the control group; different amounts of methanol and TBA were added to the reaction solution at scavenger concentrations of 100 mM, 250 mM, and 500 mM to explain the probable contribution of OH% and SO4−% because methanol could quench SO4−%, whereas both methanol and TBA could quench OH%. As depicted in Fig. S3(a), the presence of both methanol and TBA inhibited the degradation of BPA; as the scavenger concentration increased, the inhibitory effect became more obvious. Additionally, the inhibiting ability of methanol was greater than that of TBA, which might be due to the contribution of SO4−%. The negative effects on the removal rate indicate the presence of both OH% and SO4−% in the M88/Vis/PS system. Furthermore, methanol (250 mM) and TBA (250 mM) were used to compare with formate (1 mM) and BQ (1 mM) as scavengers to quench hv+ and O2−%. Fig. 4(a) showed the influence of different scavengers on the BPA degradation efficiency. Obviously, the inhibition of performance with the addition of methanol was much higher than that with TBA, and the BPA removal rate decreased from 97.1% to 76.8%,

3.3. Photocatalytic performance of M88/Vis/PS system MIL-88 (Fe) alone under visible light irradiation was used as a control group to investigate the effect of PS concentration. Approximately 26.0% of BPA was removed within 30 min, which might be the result of reactive species generated from the excited photoactive MIL-88 (Fe). The BPA removal efficiency was observed to increase as the concentration of PS increased from 0.5 mM to 2.0 mM (corresponding to PS/BPA molar ratios of 114:1 to 456:1), and it subsequently attained a plateau when the concentration reached 3.0 mM (Fig. 5a). One possible reason for this result was that the photon output

Fig. 4. Free radical quenching studies: (a) Effects of different quenching scavenger; (b) degradation constant rate and removal rate of BPA accordingly [[MIL88B] = 0.6 g/L, [PS] = 2.0 mM]. 5

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Fig. 5. (a) Degradation of BPA under different PS concentration; (b) Constants rate of BPA under different PS concentration; (c) Degradation of BPA under different M88 dosage; (d) Constants rate of BPA under different M88 dosage.

3.4. Effects of environmental factors in natural water systems

and electron-hole yield was steady for a certain amount of catalyst. An increase in PS electron acceptors might drive the reaction to attain a stable plateau according to Eq. (1). Minimal additional degradation occurred after 20 min when the PS initial concentration was 2.0 mM, indicating that PS was in excess when the dosage reached more than 2.0 mM, such as 3.0 mM. Therefore, 2.0 mM PS was considered to be the optimal dosage during the rest of the experiments. The photocatalytic degradation rate constant of BPA under different PS dosages were shown in Fig. 5(b); when the PS dosages were 0.5, 1.0, 2.0, and 3.0 mM, the rate constants were 0.069, 0.089, 0.107, and 0.114 min−1, i.e., the constant rates were 5.75, 7.42, 8.92, and 9.50 times of that of the control group, respectively. Fig. 5(c) shows the BPA degradation curve over the reaction time when various MIL-88B dosages were used with a 2.0 mM PS concentration. The BPA removal efficiency increased with the increase of the catalyst dosage, and BPA could be completely degraded in 30 min when the dosage of MIL-88B was greater than 0.4 g·L−1. The apparent rate constants of BPA degradation (k·min−1) were 0.076, 0.090, 0.107, 0.126, and 0.137 min−1 at initial catalyst dosages of 0.2, 0.4, 0.6, 0.8, and 1.0 g·L−1, respectively (Fig. 5d). With a higher dosage of catalyst, the number of active sites increased, which resulted in a higher yield of the oxidizing radicals and higher removal rate of BPA. Therefore, we chose a median catalyst dosage of 0.6 g·L−1 for all of the following experiments.

The effect of initial pH on the degradation efficiency of BPA was determined (Fig. 6a). Different from other catalyst/PS or PMS/light studies in which the solution pH was an important factor that could remarkably affect the degradation process, our results showed that initial pH had a negligible influence on the degradation of BPA in the M88/PS/Vis system. Normally, SO4−% could be generated more easily in an acidic environment according to Eqs. (8) and (9) while in alkaline environment, OH%, whose redox potential (1.8–2.7 V) was lower than SO4−% (2.5–3.1 V) [34], could be generated and consume SO4−% (Eq. (10)) to cause a decrease of BPA removal. Besides, surface charge of MIL-88B remained negative and zeta potential increased with the increase of pH value [35], and pKa value of BPA remained as 10.2 (Fig. S4a), which indicated that inhibiting effect would occur in high pH range due to electrostatic repulsion between M88 and BPA. Hence, variation of pH at a function of time under different pH solution values was measured in order to further understand the effect of pH (Fig. S4b), it was interesting to find that pH would quickly reach a similar value of 3.5 in the first 5 min and remained stable with the proceeding of the reaction at all studied initial reaction pH values, which might the result of PS acidification. This observation was in agreement with previous studies regarding the degradation of organic contaminants by activated PS process [31].

S2 O82 − + H+ → HS2 O8− 6

(8)

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(a)

(b)

(c)

(d)

Fig. 6. (a) Photocatalytic degradation rate of BPA with different pH; (b) Photocatalytic degradation rate of BPA with different DO concentration; (c) Photocatalytic degradation rate constants and removal rate of BPA with existence of different inorganic anions (100 mM); (d) Photocatalytic degradation rate of BPA in different kinds of actual water body. ([MIL-88B] = 0.6 g/L,[PS] = 2.0 mM).

HS2 O82 − + e− → SO4−· + SO42 − + H+ SO4−· + OH− → OH ·+SO42 −

could act as a chelating agent by attaching to the surface of photocatalysts, resulting in a spatial barrier with respect to stereochemistry and a quenching effect for radicals [19]. Meanwhile, SO42− had a moderate inhibition effect on BPA removal; this was due to the fact that the excess sulfate could act as a scavenger for S2O82− and SO4−% [40,41]. Additionally, the presence of electrolytes might change the electrochemical characteristics of the M88/PS/Vis system. If depletion of SO4−% could be simplified into Eq. (17), the corresponding Nernst equation could be formulated as Eq. (18). It was clear that a lower redox potential of SO4−·/ SO42 − was achieved with a higher SO42− ion intensity, which directly depressed the degradation of BPA. Usually, the presence of Cl− in the water matrix results in a lower degradation rate, as Cl− reacts with SO4−% to produce less-reactive chlorine radicals (Cl%, Cl2−%) (Eqs. (19)–(21)). Interestingly, the BPA degradation process in M88/PS/Vis system could be facilitated by Cl−. This might be attributed to the following reasons. First, the enhanced formation of OH% was induced by Cl− [42]. Second, Cl− could not compete with the photocatalyst to absorb visible light [43]. Third, the reactive chlorine species generated from chloride oxidation by PS should be responsible for BPA removal by a non-radical mechanism (Eqs. (22) and (23)), and the contribution of HClO and Cl2 produced from the oxidation of Cl− was convinced to exhibit a higher reaction rate toward BPA [44,45].

(9) (10)

Fig. 6(b) shows the effect of DO on the degradation of BPA. Experiments were conducted before reaction solutions were purged with oxygen, air, and nitrogen vigorously for 30 min at a gas flow of 15 mL·min−1 during the whole catalytic process. The removal rate of BPA was oxygen-dependent. This might be due to the following two reasons. First, oxygen could accept electrons to produce reactive species, including HO2% and O2−% [36]. Second, oxygen could act as an electron acceptor to reduce the recombination of holes and electrons, which could enhance the photocatalytic ability [37]. Inorganic anions are commonly present in aqueous media and may significantly affect the photocatalytic degradation of pollutants. Therefore, considering the practical applications of this system, the influences of various water matrix species (HCO3−, Cl−, SO42−, H2PO4−, and NO3−) on BPA degradation was investigated, and the results were shown in Fig. S5. Most inorganic anions have an inhibitory effect on the degradation, and the reasons for this were given below. HCO3− could greatly inhibit BPA degradation, which could be attributed to the fact that HCO3− and CO32− compete for hydroxyl (OH%) and sulfate radicals (SO4−%) to yield weak oxidants (CO3−% and HCO3%) (Eqs. (11)–(15)) [38]. Similarly, NO3− could react with SO4−% to generate NO3%% (Eq. (16)) [39]. It should be noted that the BPA removal efficiency in the M88/PS/Vis process after 30 min were reduced to 70.8% for CO32− and 78.8% for NO3− (Fig. 6c), which could be due to the higher reactive ability of CO32− with OH% and SO4−% compared with that of NO3−. Regarding H2PO4−, it was reported that H2PO4−

HCO3− + SO4−· → SO42 − + HCO3·

OH ·+HCO3− → CO3−·+H2 O HCO3− ⇔ H+ + CO32 − 7

k = 6.1 × 106

k = 1.6 × 106

kpa = 10.3

(11) (12) (13)

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CO32 − + SO4−· → SO42 − + CO3−·

k = 6.1 × 106

CO32 − + OH ·→OH− + CO3−·

k = 3.9 × 108

(15)

SO4−· + NO3− → SO42 − + NO3·

k = (5.6 ± 0.5) × 10 4

(16)

SO4−· + e− → SO42 − E (SO4−·/ SO42 −) = E 0 (SO4−·/ SO42 −) +

(17)

RT zF

SO−· ln ⎜⎛ 42 − ⎞⎟ ⎝ SO4 ⎠

(18)

3.5. Degradation reaction mechanism

SO4%− + Cl− → Cl% + SO42− k = 3.0 × 108

(19)

Cl% + Cl− → Cl2%− k = 8.5 × 109

(20)

Cl2%− + SO42− → SO4%− + 2Cl− k = 2.1 × 108

(21)

2C l−

(22)

→ Cl2 (aq) +

can be another reason for the direct decrease in the removal rate of BPA in the M88/PS/Vis system. It should be noted that adsorption for the complex water matrix was enhanced, which might be due to colloids in the water, which could interact strongly with many organic pollutants [48]. In short, all of the above results reveal that environmental factors could strongly impact the degradation of BPA in the M88/PS/Vis system, demonstrating the necessity of study focused on how to promote the degradation of BPA and other pollutants in complex water systems before practical application could be achieved.

(14)

2e−

Cl2 (aq) + H2 O → HClO + H+ + Cl−

Based on the above discussion, the results of the present study could be explained by taking the electronic characteristics of MIL-88B into account. Three plausible pathways for the photodegradation of BPA by the M88/PS/Vis system could be expounded, and they were depicted in Fig. 7.

(23) 3.5.1. Reaction pathway I It was noted that the CB potential of MIL-88B was more negative than the redox potential of O2/·O2− (−0.33 V) [49], indicating that O2−% could be produced by photogenerated electrons (Eq. (24)). Meanwhile, part of the O2−% species might further form OH% radicals [50].

The inhibiting effect of various inorganic anions was shown in Fig. 6(c). The corresponding rate constants of HCO3−, H2PO4−, SO42−, and NO3− were 0.037, 0.038, 0.042, and 0.047 min−1, respectively. Additionally, the BPA degradation rate in Cl− (0.265 min−1) was 2.48 times of that of the control group (0.107 min−1). To explore the photocatalytic performance of the M88/Vis/PS system for degradation of BPA in natural water bodies, the degradation of BPA in deionized water, tap water, river water, and dye wastewater was investigated. The quality of the river water and dye wastewater was provided in Table S1. In all cases, BPA was added to the aquatic environment to obtain an initial concentration of 10 mM. As shown in Fig. 6(d), the removal rate in those water bodies mentioned above within 30 min were 100.0%, 100.0%, 52.8%, and 37.6%, respectively, which indicated that tap water has a very slight influence on the degradation of BPA, while the degradation process in both river water and dye wastewater was inefficient. NOM, a complex mixture of acidic organic molecules that originates from a variety of natural sources (e.g., soil, sediment, water, etc.), is ubiquitous in natural water [46]. The decrease in the removal rate of BPA in river water might attribute to competition for OH% and SO4−% between BPA and NOM [47]. In addition, as discussed above, several inorganic anions which were detected in both water have a negative impact on the degradation process, and the lower concentration of DO in river water and wastewater than that in deionized water and tap water should be taken into account. Beyond that, the presence of colloids increases the turbidity of water, decreases light transmittance, and causes a light screening effect, which

O2 + e− → O2−·

(24)

3.5.2. Reaction pathway II In the case of photocatalysis, MIL-88B (Fe) was excited to generate electrons and holes under visible light illumination. (Eq. (25)). Then, the photoinduced electrons in the CB could be trapped by PS to form SO4−%. In addition, the redox potential of BPA was higher than the EVB of MIL-88B (Fe). Therefore, BPA could be oxidized on the surface of MIL-88B (Fe) by donating an electron to the catalyst (Eq. (26)), then the electron could be transferred from VB to PS to produce SO4−% radicals (Eq. (27)). Finally, SO4−% radicals initiate a chain of reaction to generate hydroxyl radicals. The photo-generated holes could also damage the BPA which was adsorbed on the surface of the photocatalyst as a reactive oxygen species.

M 88v is h+ + e− → M 88 + BPA →

(25)

M 88(e−)⋯[BPA]+

M 88(e−) + S2 O82 − → M 88 + SO42 − + SO4−·

(26) (27)

Finally, generated radicals could combine with photoholes to

Fig. 7. Possible reaction mechanism for the activation of PS by M88 under visible light. 8

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was further decomposed to phenol and 4-t-butyl-2-(α,αdimethylbenzyl) phenol. Subsequently, the generated phenol was gradually oxidized to hydroquinone and p-benzoquinone, while 4-t-butyl-2-(α,αdimethylbenzyl) phenol was converted to phenol, 2,4-bis(1,1-dimethyl ethyl) (red arrow). The third pathway was led by O2−% hydroxylation, where a series of multihydroxylations occurred when the aromatic ring was attacked by water molecules via the electrophilic effect (black arrow) [51]. The hydroxylated BPA then underwent dehydration, forming quinone and carboxylic compounds. Finally, aromatic compounds were transformed into ring-opened intermediates and ultimately mineralized into CO2 and H2O. A similar degradation process for aromatic compounds was well supported in many degradation systems. The generated intermediates of BPA found in this study were also detected in previous studies [19,22,52].

directly oxidize BPA to some extent and completely mineralize it to CO2 and H2O. 3.6. BPA degradation pathways To elucidate the BPA degradation pathways in the M88/PS/Vis system, the reaction intermediates were identified by GC–MS (Fig. S6) and LC–MS analyses (Fig. S7). Table S2 presents the identified oxidation products obtained in the present study, including monohydroxylated BPA, quinone of dihydroxylated BPA, 1-methylbenzene, 2-glutaric acid dimethymethane, phenol, 2,4-bis(1,1-dimethyl ethyl), phenol, p-isopropenyl phenol, and p-benzoquinone. According to the identified intermediates, several reaction processes took place during BPA degradation, as shown in Fig. 8 (chemical formulae in black were detected by GC/MS; chemical formulae in purple were detected by LC/MS). At first, the C-C bond of BPA between benzene rings and isopropyl was ruptured by radicals to generate isopropenyl phenol radicals [51]. On one hand, it was attacked by photogenerated h+ with the formation of 4-isopropenylphenol through the desaturation and unsaturation progress (green arrow). On the other hand, in the combined oxidation by OH% and SO4−%, isopropenyl phenol radicals coupled with BPA to generate 4,4′-((4-hydroxy-1,3-phenylene)bis (propane-2,2-diyl)) diphenol, which had a higher molecular weight than BPA; the compound

3.7. Toxicity assessment and TOC removal evaluation The reaction products included many intermediates. To determine the potential environmental impact of BPA and the degradation products, the acute and chronic toxicities of BPA and the degraded products were first calculated in this study using the ECOSAR program, which was a reliable and cost-effective substitute for time-consuming and costly experimental risk assessment. First, we compared the

Fig. 8. Proposed transformation pathways for BPA degradation in M88/Vis/PS system. 9

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Guidelines for New Chemical Substances (HJ/T 1954-2004), which pointed out that BPA and the degradation products could be classified to different toxicity levels. The predicted values of some products produced via coupling between two compounds were higher than that of BPA, indicating the generation of intermediates with higher biological toxicity than BPA itself. Taking toxicity into account, the mineralization rate was more significant than the removal rate. CCK-8 method was based on the color change of solution since [2-(2-methoxy4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WST-8) included in CCK-8 could bioreduce by dehydrogenases in cells to give an yellow-color product (formazan) [56]. The amount of the formazan dye, generated by the activities of dehydrogenases in cells, is directly proportional to the number of living cells. According to Fig. 9(b), the toxicity was changing over time, the results obtained after 10 min of reaction also indicated that the degradation of BPA led to the generation of intermediates with higher toxicity than the initial BPA. In most cases, the toxicity of solutions were lower than the initial samples, which demonstrated the effectiveness of M88/PS/Vis system in the reduction of toxicity of BPA. TOC data were acquired to investigate the

predicted toxicity values and the experimental value of BPA to evaluate the validity of the ECOSAR system. The predicted half lethal concentration after 96 h (LC50-96 h) of BPA to fish was 6.27 mg L−1, while the experimental values were 4.70, 4.60, 8.04, and 9.06 mg·L−1 in previous studies [53,54]. Second, based on current U.S. Environmental Protection Agency standard evaluation procedures, bisphenol A was moderately to slightly toxic to the fish and invertebrates tested, with LC50 or EC50 values ranging from 1.1 to 10.0 mg·L−1 [55]. To summarize, the calculated toxicities agreed well with the experimental values, which indicates that ECOSAR was suitable for predicting the toxicity of BPA and the degradation products. A schematic representation of the evolution of the acute and chronic toxicities was shown in Fig. 9(a), and the corresponding toxic values were given in Table S3. The chronic toxicity was similar to the acute toxicity regarding the predicted toxicity values at all three levels. The acute toxicity of most intermediates was one level less toxic than BPA and not harmful, while the chronic toxicity was much more complex and distributed across four spans. The results were in accordance with the EU Directive No.67/548/EEC and the Chinese Hazard Evaluation

(a)

(c)

(b)

Fig. 9. (a) Risk assessment of BPA and it’s by-products via ECOSAR in M88/PS/Vis system; (b) The relative absorption intensity variation of M88/PS/Vis system to CCK-8; (c) The efficiency of BPA mineralization by the M88/PS/Vis system. 10

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References

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4. Recycling performance of M88/PS/Vis system The stability of M88 was evaluated through three consecutive operations. Fig. S8(a) shows that no noticeable degradation of activity was observed, implying that M88 was stable during the photocatalytic reactions. Furthermore, no obvious structural variation was observed from the XRD patterns, which further demonstrated the robust stability of M88. In addition, the concentration of leached-out Fe was measured using an ICP-MS to further evaluate the stability of M88 (Fig. S8b). The results demonstrated that as the number of iterations increased, the dissolved quantity of iron increased slightly, but the concentration throughout the entire recycling process was approximately 4 mg/L, which further showed the stability of M88. 5. Conclusion The major findings in this study were summarized as follows: (1) Because of the synergistic effects in the M88/PS/Vis system, M88/ Vis/PS could effectively degrade BPA, which might attribute to the presence of persulfate that could scavenge conduction band electrons and prevent the recombination of electrons and photoholes. (2) M88/Vis/PS could function effectively over a wide initial pH range (3.0–9.0). Various natural environmental reaction parameters, such as dissolved oxygen concentration, co-existing inorganic ions (Cl−, HCO3−, SO42−, H2PO4−, and NO3−), HA and real background of water matrices (deionized water, tap water, river water, and dye wastewater) could influence the reaction rate. (3) Seven intermediates were identified with GC/MS and LC/MS, and a possible transformation pathway was proposed, in which three mechanisms took place during BPA decomposition. These mechanisms include the scission of C-C between the isopropyl and benzene rings, the multihydroxylation of the aromatic ring via electrophilic attack, and coupling between two compounds. (4) The ECOSAR program was used to determine the potential environmental impact of BPA and its by-products. BPA and its transformation by-products were classified to different toxic levels at three trophic levels (fish, daphnid, and green algae). Although the toxicity of most intermediates was one or more level less toxic than that of BPA, there were intermediates with higher biological toxicity than original BPA molecules. TOC data indicated that toxicity hazards in general could be effectively avoided by prolonging the duration of light exposure. 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. Acknowledgment This work was financed by the National Natural Science Fund of China (Foundation of Guangdong Province of China; No. U1401235). We would also like to thank Editage (www.editage.cn) for English language editing. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122931. 11

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