Efficient degradation of tetrabromobisphenol A via electrochemical sequential reduction-oxidation: Degradation efficiency, intermediates, and pathway

Journal of Hazardous Materials 343 (2018) 376–385

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Efficient degradation of tetrabromobisphenol A via electrochemical sequential reduction-oxidation: Degradation efficiency, intermediates, and pathway Yanping Hou a,1 , Zhenbo Peng a,1 , Li Wang a , Zebin Yu a,∗ , Lirong Huang a , Lingfang Sun b , Jun Huang a a b

School of Environment, Guangxi University, Nanning 530004, PR China Guangxi Zhongxinhengtai Engineering Consulting Co. Ltd, Nanning 530022, PR China

h i g h l i g h t s • • • •

Electrochemical sequential reduction-oxidation of TBBPA was proposed. Synergism of Fe0 and electrochemical reduction occurred on Pd-Fe/Ni electrode. High conversion efficiency and debromination ratio were achieved. Intermediates were identified and possible degradation pathway was proposed.

a r t i c l e

i n f o

Article history: Received 23 May 2017 Received in revised form 6 September 2017 Accepted 2 October 2017 Available online 4 October 2017 Keywords: Electrochemical sequential reduction-oxidation Pd-Fe/Ni electrode Tetrabromobisphenol A Intermediates Degradation pathway

a b s t r a c t Tetrabromobisphenol A (TBBPA), a toxic persistent pollutant, should be effectively removed from the environment. In this study, an electrochemical sequential reduction-oxidation system was proposed by controlling reaction atmosphere with Pd-Fe nanoparticles modified Ni foam (Pd-Fe/Ni) electrode as cathode for TBBPA degradation. To obtain an efficient Pd-Fe/Ni electrode for TBBPA degradation, various factors, like Pd loading, Fe2+ adding amounts, were examined. The Pd-Fe/Ni electrode exhibited higher TBBPA conversion and debromination than the counterparts, due to the synergism of Fe0 and electrochemical reduction. Similar TBBPA conversions and debromination ratios were observed for the cases of sparging N2 only and sparging N2 followed by air, which were higher than those of aeration. Reductive debromination occurred while first bubbling N2 , forming tri-BBPA, di-BBPA, mono-BBPA and BPA; and these intermediates were likely to be further oxidized by • OH generated from H2 O2 together with PdFe/Ni electrode under aeration. Reductive and oxidative intermediates (including aromatic ring-opened product) were identified by HPLC and UPLC-QTOF-MS. Based on the intermediates, the possible TBBPA degradation mechanism and pathway were proposed. This study demonstrates that sequential reductionoxidation process tuned by N2 and air bubbling was favored for TBBPA degradation, thus, it should be a promising process for HOCs degradation. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Tetrabromobisphenol A (TBBPA), with good flame retardancy and stability, has been widely used in printed circuit boards, insulated wires and polycarbonated plastics [1]. The widespread application of TBBPA has caused its ubiquitous presence in various

∗ Corresponding author. E-mail address: [email protected] (Z. Yu). 1 These authors contributed equally to this study and share first authroship. https://doi.org/10.1016/j.jhazmat.2017.10.004 0304-3894/© 2017 Elsevier B.V. All rights reserved.

environmental media, including water, soil, sediment, air and even biological matrices, such as fish and human organs [2]. TBBPA has a structure similar to steroid hormones and acts as an endocrine disruptor, which could affect the metabolism and lead to disorder of hormone secretion, even cause damage to endocrine system, bone and brain [3,4]. Besides, TBBPA and its transformation products can also cause ecological destruction [5]. As a result, increasing concern has been raised by TBBPA pollution. Therefore, it is of great significance to develop efficient technology to eliminate TBBPA from environment.

Y. Hou et al. / Journal of Hazardous Materials 343 (2018) 376–385

Many methods have been employed to remove TBBPA, such as adsorption [6], biological method [7], ozonation [8], photocatalytic technology [3,9], reduction technology based on nanoscale zerovalent iron [10], electrochemical method [11] and so on. Among the reported methods, TBBPA could not be degraded with adsorption, which may lead to secondary pollution and remain potential environmental risk. It has been found that some microorganisms can decompose TBBPA; yet, the low water solubility of TBBPA limits the removal efficiency through aerobic biodegradation in aquatic environment; while the anaerobic biodegradation products (mainly bisphenol A, BPA) of TBBPA are harmful to the environment and human beings [12]. Though rapid TBBPA removal could be achieved using ozonation, the mineralization of TBBPA and its intermediates is poor [8]. Photocatalysis has been considered as an efficient method for TBBPA degradation; nevertheless, the photocatalyst usually suffers from far recombination of photo-generated electrons and holes, and it is difficult to recover the powder photocatalyst from reaction system [3]. Electrochemical reduction is an effective way to treat halogenated organics (HOCs) like TBBPA due to its high efficiency, versatility and environmental compatibility [13,14]. However, for the reduction of HOCs, halogen atoms are always removed from backbone of HOCs, possibly forming toxic residues [15]. To completely degrade TBBPA, the sequential reductionoxidation process has been proposed recently. For instance, Luo et al. [16] used Fe-Ag bimetallic nanoparticle under ultrasound radiation for TBBPA reduction; then, the following oxidation process for removing TBBPA debromination products was realized by Fenton-like oxidation reaction caused by adding H2 O2 into the system. Nevertheless, it costs a lot of reagents and should be assisted by ultrasound radiation. Guo et al. [17] reported sequential reductionoxidation of TBBPA in a photocatalytic system by controlling the atmosphere. Under N2 ambient, photo-induced electrons dominated the reduction of TBBPA; when changing the atmosphere to O2 , photo-excited holes or hydroxyl radicals (• OH) played a leading role on oxidation of debromination products. However, as aforementioned, the photocatalyst suffers from photo-generated electrons and holes recombination and catalyst recovery issues, and its efficiency should be further improved. Considering the high efficiency of electrochemical technology, and complete degradation process of sequential reductionoxidation, it is believed that combining these two procedures, named electrochemical sequential reduction-oxidation process, could not only improve TBBPA degradation efficiency, but also avoid the problem of recovery suspended powder catalyst. For electrochemical system, Pd-based electrode is one of the most popular cathodes [13,14]. A noteworthy characteristic of Pd nanoparticles in electrochemical system is that it can in situ catalyze generation of H2 O2 and • OH with the presence of H2 and O2 [18], which could be expressed by Eqs. (1)-(2) [19,10]. The • OH is a strong oxidative species (oxidation potential 2.8 V) and can almost react with organics nonselectively [21]. Pd

H2 + O2 →H2 O2 Pd

H2 O2 →2 · OH

(1) (2)

However, an obvious drawback of Pd-based electrode is the high cost [22]. To reduce the cost, inducing Fe nanoparticles to the Pd-modified electrode has been developed, and it was reported that high reduction efficiency could be achieved even with a low Pd loading amount owing to the in situ formation of Fe0 [23,24]. Herein, we intended to prepare Pd-Fe nanoparticles modified Ni foam (Pd-Fe/Ni) electrodes as the cathode for sequential reductionoxidation of TBBPA in an electrochemical system via controlling the atmosphere. The graphite, as one of the commonly used carbon

377

anode materials, with high conductivity, electrochemical stability as well as robustness [25], was adopted as the anode. Compared with other metal anodic materials such as nickel, platinum, stainless steel, and molybdenum etc., no anodic dissolution would occur since graphite is highly stable [25], which could avoid the side effect of the anodic metal dissolution on electrochemical sequential reduction-oxidation of TBBPA. The performance of TBBPA degradation in the system was determined detailedly. More specifically, effects of electrode preparation parameters such as Fe addition amount, Pd loading amount, Cetyltrimethylammonium Bromide (CTAB, which could affect the process of electrodeposition) addition on the as-prepared electrode performance for TBBPA degradation, kinetics of TBBPA degradation, as well as the effect of atmospheric conditions on the performance of sequential reduction-oxidation of TBBPA were examined. More importantly, intermediates were identified and possible degradation mechanism and pathway were proposed. 2. Materials and methods 2.1. Synthesis and characterization of Pd-Fe/Ni electrodes The reagents used in this study and the Pd-Fe/Ni electrode synthesis procedure were provided in Supporting Information. To optimize the performance of Pd-Fe/Ni electrode for TBBPA degradation, the effects of Fe2+ capacity (5, 10, 15 and 20 mmol L−1 ), Pd loading (0, 0.32, 0.61, and 1.18 mg/cm2 ) and CTAB addition amount (0, 1, 5, and 10 mmol L−1 ) were investigated. The Pd/Ni electrode and Ni electrode were used as the counterparts. Electrodes characterization, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and the Brunauer-Emmett-Teller (BET) specific surface area were also described in Supporting Information. 2.2. Electrochemical reduction of TBBPA Electrochemical reduction experiments were carried out in a two-compartment cell (the working volume of each compartment was 150 mL) separated by Nafion117 membrane, the schematic diagram of the experimental device was shown in Fig. S1. The as-prepared Pd-Fe/Ni electrode was applied as cathode and graphite electrode was served as anode. The electrochemical degradation of TBBPA was conducted in the cathode compartment with continuously nitrogen gas (N2 ) sparging. The solution in cathode compartment contained Na2 SO4 (0.1 mol L−1 ) and TBBPA (20 mg L−1 ). 2.3. Electrochemical degradation of TBBPA with atmosphere control According to the experimental results of Section 2.2 (as presented in Section 3.2), the initial conditions for electrochemical degradation of TBBPA with different electrodes and under different atmospheres were as follows except as noted: the optimized PdFe/Ni electrode prepared with Fe2+ capacity of 15 mmol L−1 and Pd loading of 1.18 mg cm−2 , and CTAB addition amount of 1 mmol L−1 , initial pH of 3 and applied current density of 0.83 mA cm−2 . In the cathode compartment, different reaction atmospheres would result in different reactions. With N2 sparging, electrochemical reduction would occur; with air sparging, oxidation process would take place; while altering the atmospheric condition, the reactions could be tuned. To determine the effect of atmospheric conditions on TBBPA degradation, three atmospheric conditions were examined: i.e. sparging with N2 for 60 min, N2 for 30 min + Air for 30 min, and Air for 60 min. For all cases, the sparging rate was 1.5 L/min.

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During these experiments, TBBPA degradation performance was determined. 2.4. Analytical methods and calculations The concentration of TBBPA was monitored by the highperformance liquid chromatograph (HPLC, Agilent 1260 series, USA) with an Agilent Eclipse XDB-C18 column (5 ␮m, 4.6 mm × 250 mm) and column temperature of 30 ◦ C. The mobile phase was composed of a mixture of methanol and 0.1% acetic acid aqueous solution (v:v = 80:20). The flow rate was 1.0 mL/min and the detection wavelength was 230 nm. The volume of the injected sample was 10 ␮L. Degradation intermediates were identified by ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) (XEVO G2-SQTOF, Waters Corporation, Milford, MA, USA) equipped with a Waters Acquity UPLC BEH C18 column (100 mm × 2.1 mm, 1.7 ␮m) at 35 ◦ C. The flow rate of mobile phase (consisting eluent A (water + 0.1% formic acid) and eluent B (acetonitrile) was 0.4 mL/min. The B linear solvent gradient was as follows: 0–3 min, 20%–40%; 3–7 min, 40%–80%; 7–7.5 min, 80%–20%; 7.5–10 min, 20%–80%. The injection volume was 1 ␮L. The mass spectrometry conditions were as follows: MS detection was performed using a XEVO G2-S QTOF MS system, and the ionization mode was negative electrospray (ESI− ). The source temperature was set at 100 ◦ C, and the desolvation temperature was kept at 300 ◦ C. The capillary was 2.7 kV. The desolvation gas flow was 700 L h−1 . The concentration of bromide ion was determined by ion meter (PXS 270, Shanghai INESA Scientific Instrument Co., Ltd., P.R. China).The conversion of TBBPA was calculated by Eq. (3) [26]: Conversion (%) =



1−

Ct C0



× 100%

(3)

Where C0 is the initial concentration; Ct is the reaction concentration at time t (min). Debromination ratio of TBBPA was calculated by Eq. (4) [27]: Debrominationratio (%) =

CBr × 100% C0 × n

(4)

Where CBr is the concentration of Br− (mg L−1 ); C0 is the initial concentration of TBBPA (mg L−1 ); and n refers to the number of Br atoms contained in TBBPA molecule. Electrochemical reduction of HOCs always follows a pseudofirst order kinetic model (Eq. (12)) [20], thus, in this study, the rate constant was calculated as: ln(

Ct ) = kobs t+b C0

(5)

The kobs is a constant of apparent kinetics. The concentration of H2 O2 in the system was measured by spectrophotometric determination using potassium titanium (IV) oxalate as previously described [28]. 3. Results and discussion 3.1. Electrodes characteristics As shown by SEM images, (Fig. S2(a)–(d)), Pd and Fe nanoparticles were successfully deposited on the surface of Ni foam, forming the Pd-Fe/Ni electrode; while with CTAB addition during the preparing procedure, more uniform nanoparticles distribution could be observed, as compared with the counterpart without CTAB. The uniform distribution of particles on Ni foam was likely due to the fact that adding CTAB to the electrolyte during electrodeposition process could reduce the agglomeration of Pd and

Fe nanoparticles and consequently lead to better dispersion. Both EDS data and XRD pattern conformed that Pd and Fe on the Ni foam after electrodeposition process (Fig. S2(e), (f)). The BET surface area of Pd-Fe/Ni with CTAB addition was higher than that without CTAB (3.993 vs. 2.515 m2 g−1 , Table S1), which accords with the results of previous studies [29]. 3.2. Performance of TBBPA electrochemical reduction 3.2.1. Effect of Pd-Fe/Ni electrode preparation parameters on TBBPA electrochemical reduction Electrodeposition of Fe0 would be affected by Fe2+ capacity, which would further affect the electrode performance. Result showed that after 60 min reaction, TBBPA debromination ratios with electrodes prepared with various Fe2+ concentrations followed the order of 5 mM (66.6%) <10 mM (71.5%) <20 mM (73.8%) <15 mM (76.7%) (Fig. 1a). That is, when increasing Fe2+ capacity from 5 to 15 mmol L−1 , performance of the as-prepared PdFe/Ni electrode for TBBPA removal was enhanced. This could be attributed to the in situ formation of Fe0 , which has a positive effect on Pd dispersion and activity, thus enhancing electrons transport on the cathode [20]. Besides, as discussed in Section 3.2.2 below, Fe0 nanoparticles could also be involved in debromination process. Though the effect of Fe2+ capacity ranging from 10 to 20 mM on TBBPA removal and debromination was not as significant as expected, it was supposed that Fe2+ capacity of 15 mM would be more appropriate. This is because with this Fe2+ concentration, suitable electrodeposition amount of uniform distribution Fe0 would be involved in both TBBPA reduction and its intermediates oxidation (discussed in Section 3.3); when using a Fe2+ capacity of 10 mM, less Fe0 would be deposited on the electrode, causing less Fe0 and Fe2+ involved in intermediates oxidation, which would slow down TBBPA degradation; while increasing the Fe2+ capacity to 20 mM, Fe0 particles might cause conglomeration on the electrode surface [30], which may reduce its involvement in TBBPA degradation. According to electrocatalytic hydrogenolysis (ECH) mechanism [24], for Pd-Fe/Ni electrode, Pd nanoparticles, as the main catalytic metal, play the decisive role on generation of active hydrogen atom (Hads ). Hence, Pd loading amount is a significant variable for electrochemical reduction of TBBPA on the surface of Pd-Fe/Ni electrode. As illustrated in Fig. 1b, TBBPA conversions and debromination ratios increased with increase of Pd loadings, especially debromination ratios. To be more concrete, TBBPA conversions and debromination ratios nearly doubled when increasing the Pd loading amount from 0.32 to 0.61 mg cm−2 . With the Pd loading amount of 1.18 mg cm−2 , 100% of conversion and 76.7% of debromination ratio were achieved after 60 min of electrochemical reduction. The explanation for this could be as follows: high Pd loading amount would be favorable for the production of Hads [14,22,24], and Hads on Pd nanoparticles surface (Hads Pd) would react with the C-Br bond of TBBPA adsorbed on Pd nanoparticle surface (C-Brads Pd), and then a rapid exchange of C-Brads Pd bond of Hads Pd would occur; thus leading to enhanced TBBPA degradation. The addition of CTAB is considered as an important factor that affects particles distribution on the electrode and further affects electrode performance on TBBPA debromination. As shown in Fig. 1c, when 1 mmol L−1 CTAB was added into the electrolyte during electrodeposition process, debromination ratio increased. While, the amount of CTAB exceeded 1 mmol L−1 , debromination ratio declined. The reasons for this might be as follows: first, the addition of CTAB would enlarge the BET surface area of Pd-Fe/Ni electrode (3.993 m2 /g with CTAB vs. 2.515 m2 /g without CTAB, Table S1), making it easier for TBBPA molecule to absorb on the electrode surface; second, adding CTAB during electrodeposition process could result in more uniform nanoparticles (as shown in Fig. S2), thereby reduce the chance of nanoparticle agglomeration [29];

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379

Fig. 1. Effects of Fe2+ capacity (a) and Pd loading capacity (b) and CTAB addition amount (c) on Pd-Fe/Ni electrode performance for TBBPA reduction. (C/C0 is the ratio of TBBPA concentration at time t to initial concentration).

both of which favor the catalytic debromination. On the contrary, excessive amount of CTAB could form a CTAB film on Pd-Fe/Ni elec-

trode, which would result in less active sites available on Pd-Fe/Ni electrode surface for electrochemical reduction [31].

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Fig. 2. TBBPA conversions (a) and debromination ratios (b) based on Ni, Pd/Ni and Pd-Fe/Ni electrodes during the electrochemical reduction process.

Fig. 3. Electrochemical degradations of TBBPA under different atmospheres (N2 for 60 min, N2 for 30 min +Air for 30 min, and Air for 60 min).

Among the electrode preparation parameters, it seems that Pd loading is the most significant factor. With loadings of 0, 0.31, 0.61 and 1.18 mg/cm2 , TBBPA debromination efficiencies within 60 min were 9.8%, 41.5%, 59.5% and 76.7%, respectively. The differences are significant, with the largest difference of 66.9% (76.7%–9.8%). While TBBPA debromination efficiencies varied from 66.7% to 76.7% with Fe2+ capacity ranging from 5 to 20 mM; and the debromination values were within the range of 75.3% to 85.4% when the CTAB amount ranged from 0 to 10 mM. 3.2.2. Effect of different electrodes on TBBPA reduction and Pd-Fe/Ni electrode stability Electrochemical reduction performance comparison of Ni, Pd/Ni and Pd-Fe/Ni electrode was performed. According to Fig. 2, Pd-Fe/Ni electrode exhibited an obvious advantage over Pd/Ni electrode and Ni electrode in terms of TBBPA conversion and debromination. As expected, Pd-Fe/Ni electrode exhibited the best performance for TBBPA degradation (100% of conversion and 76.7% of debromination ratio after 60 min reaction), followed by Pd/Ni electrode, and Ni electrode showed the poorest efficiency for TBBPA degradation–conversion efficiency of 16% and a debromination ratio of 10%. The superior performance of Pd-Fe/Ni electrode was likely because: on the one hand, ECH reactions occurred on the surface of Pd-Fe/Ni electrode, as listed as Eqs. (6)–(9) [13,14,32]; on the other hand, the Fe0 nanoparticles mediated degradation of TBBPA was also involved, that is, reaction of C-Br bond occurred on the surface of Fe0 nanoparticles (Eq. (10)) [10,33,34]. Besides, TBBPA could also be debrominated through electrochemical reduction process via H substitution and accepting electrons on the cathode (Eq. (11)). As a result, rapid debromination was achieved with Pd-Fe/Ni electrode. To obtain this desirable performance, the Pd loading on the nickel foam was 1.18 mg/cm2 , which was much lower than that used for electrochemical hydrodechlorination with the Pd-loaded nickel foam cathode (Pd loading of 3.5 mg/cm2 ) [35]. In contrast, for

Fig. 4. H2 O2 accumulations in the electrochemical system with and without TBBPA under atmospheric conditions of N2 (60 min) and N2 (30 min) + Air (30 min).

Pd/Ni electrode, only ECH was involved, while barely ECH would occur for the case of Ni electrode. 2H2 O + 2e− + M → 2(H)ads M + 2OH−

(6)

R − Br + M  (R − Br)ads M

(7)

(R − Br)ads M + 2(H)ads M → (R − H)ads M + HBr

(8)

(R − H)ads M  R − H + M

(9)

0

+

2+

−

−

Fe + RBr + H → RH + Fe +

RBr + H + 2e → RH + Br

+ Br

−

(10) (11)

The stability of the Pd-Fe/Ni electrode was evaluated in the electrochemical reduction system under optimized conditions, and the result showed that Pd-Fe/Ni cathode remained stable after 4 cycles

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381

Fig. 5. The proposed mechanism of electrochemical sequential reduction-oxidation degradation of TBBPA on Pd-Fe/Ni electrode surface.

(Fig. S3). Debromination ratios exhibited a slight fluctuation over the four cycles of operation, and the average value was 76%, suggesting the Pd-Fe/Ni electrode was stable and reusable. 3.2.3. Kinetics for the electrochemical reduction of TBBPA As summarized in Table S2, electrochemical reduction of TBBPA on Pd-Fe/Ni electrode fits well with pseudo-first order kinetic model. The kobs reached 0.0784 min−1 (0.5223 min−1 L−1 , normalized to reaction working volume) with the Pd-Fe/Ni electrode prepared under the conditions of Fe2+ capacity of 15 mM, Pd loading of 1.18 mg/cm2 and CTAB addition amount of 1 mM, which is 3.5 times of that from UV/base/persulfate system 0.1480 min−1 L−1 (0.0074 min−1 , 50 mL solution) [36], and higher than that obtained in a TiO2 photocatalytic system 0.4220 min−1 L−1 (0.0211 min−1 , 50 mL solution) [17], indicating that electrochemical reduction of TBBPA with Pd-Fe/Ni electrode was highly efficient. 3.3. Electrochemical sequential reduction-oxidation degradation of TBBPA To evaluate the degradation of TBBPA and its intermediates, electrochemical reduction-oxidation was conducted. As shown in Fig. 3a, TBBPA conversions varied under different atmospheres. More specifically, for the cases of sparging N2 for 60 min and N2 (30 min) + Air (30 min), nearly complete conversions of TBBPA were achieved within the first 30 min; while for the case of aeration for 60 min, the conversion was only 60% within 30 min, and it increased to 83% at 60 min. During the first 30 min, similar debromination ratios (∼85%) were observed for the cases of N2 for 60 min and N2 (30 min) + Air (30 min) (Fig. 3b), which was consistent with the conversion result. Prolonged N2 sparging time to 60 min, the debromination ratio increased slightly to 89%; yet, when sparging with air until the solution was saturated in the later 30 min, debromination ratio decreased a little. Possible reason for this could be that • OH was produced when bubbling with air (as described in Eqs. (1), (2), (12) and (13)), and • OH can oxidize the aqueous Br− to Br2 [37], resulting in decrease of Br− concentration in the solution, and thereby declining the observed debromination ratio. For comparison, with aeration for 60 min, debromination ratios were the lowest, only 30% at 30 min and 43% at 60 min. The low TBBPA debromination ratio with air bubbling was possibly due to the competitive reactions between oxidation process (O2 as oxidant) and debromination process [38]. In general, the results in Fig. 3 were

in line with those from the HPLC spectrograms of TBBPA electrochemical degradation (Fig. S4–S6). Notably, as shown in Fig. S5, no reductive intermediates were observed in the first 10 min of aeration, while Fig. 3 showed that ∼17% of conversion and ∼10% of debromination was observed in the first 10 min, suggesting that other process instead of electrochemical reduction would contribute to this. Apart from the possible process related to Eqs. (10), (11), • OH substitution may be another involved process, since the • OH can directly substitute one bromine in the C-Br bond of TBBPA, yielding Br− ions [9]. In this system, • OH could be mainly generated from H2 O2 under air bubbling. In addition to the reaction shown in Eq. (2), Fenton reaction may also take place; since Fe0 on the Pd-Fe/Ni electrode surface would react with H2 O2 to form Fe2+ , inducing Fenton reaction together with H2 O2 (Eqs. (12), (13)) [38]. Fe0 + H2 O2 + 2H+ → Fe2+ + 2H2 O

(12)

Fe2+ + H2 O2 → Fe3+ + · OH+OH−

(13)

To demonstrate the generation of H2 O2 in the electrochemical system with aeration, as well as the involvement of H2 O2 (transforming to • OH in the presence of Pd and Fe2+ ) for TBBPA oxidative degradation, H2 O2 accumulations were determined with and without TBBPA under different atmospheres, as displayed in Fig. 4. Under the same atmosphere, H2 O2 accumulations with the presence of TBBPA were lower than those with the absence of TBBPA throughout the whole process, suggesting that oxidative degradation of TBBPA intermediates occurred in the system, consuming H2 O2 directly or indirectly (by consuming • OH which generated from H2 O2 , as discussed below). For the case of N2 for 30 min + Air for 30 min with TBBPA, obviously, no H2 O2 accumulation was observed during the first 30 min with N2 sparging, and when aeration started, H2 O2 accumulation increased rapidly, and the maximum H2 O2 accumulation occurred at 45 min. This maximum value was lower than that from the process with air sparging for 60 min (Fig. 4), which was likely owing to the fact that more H2 O2 would be consumed by more reductive intermediates in the process of N2 for 30 min + Air for 30 min with TBBPA as compared with that of Air for 60 min. Besides, there was a general trend that with aeration, the accumulation of H2 O2 first increased and then decreased. Four possible reasons could be supposed. Firstly, at the beginning, the amount of H+ is sufficient and thus the production rate of H2 O2 was high, resulting in accumulation of H2 O2 in the system. Secondary, accompanied with the debromination

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Fig. 6. Possible pathway for electrochemical sequential reduction-oxidation degradation of TBBPA on Pd-Fe/Ni electrode.

reaction, consumption of H+ occurred continuously, which led to a fast decline of H+ , and thus, slowed down the production of H2 O2 . Thirdly, both H2 O2 and • OH would involve in the degradation of TBBPA and its intermediates, leading to decrease of H2 O2 accumulation. Fourthly, decomposition of H2 O2 (Eq. (14)) would occur during the reaction [39]; when the decomposition rate is greater than the generation rate, the concentration of H2 O2 would decline (after 30 min).

H2 O2 + 2H+ + 2e− → 2H2 O

(14)

TOC change before and after oxidation process was not significant (data not shown), this was possibly because the initial TBBPA concentration in the system was low. Yet, oxidation of TBBPA reductive/debrominated products could be confirmed by UPLCQTOF-MS results, as oxidative intermediates were identified in the system with aeration (discussed below).

3.4. TBBPA degradation mechanism and possible pathways for electrochemical sequential reduction-oxidation degradation To reveal the degradation mechanism of TBBPA during the electrochemical sequential reduction-oxidation process, intermediates were identified using HPLC and UPLC-QTOF-MS. From the HPLC patterns (Figs. S4–S6), the retention time of TBBPA and intermediates production could be observed. With the qualitative analysis by the UPLC-QTOF-MS, the degradation intermediates peaks at retention time of 7.0 min, 5.1 min, 4.0 min and 3.3 min corresponded to tri-BBPA (m/z = 465), di-BBPA (m/z = 386), BBPA (m/z = 307) and BPA (m/z = 228) (Table 1), respectively. As clearly shown in Fig. S4, with N2 sparging throughout the process, TBBPA was quickly converted to a series of reductive products (tri-BBPA, di-BBPA, BBPA and BPA), and then tri-BBPA, di-BBPA, BBPA were further reduced to BPA; these transformations revealed that during electrochemical reduction process, debromination occurred stepwise, causing accumulation of BPA in the system. At the atmosphere of N2 (30 min) + Air (30 min), TBBPA was almost completely

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383

Table 1 The major identified intermediates of TBBPA in electrochemical system. Products

Compounds

Retention time/min

m/z

TBBPA

7.24

544

A

tri-BBPA

6.61

465

B1

di-BBPA

6.03

386

B2

di-BBPA

5.95

386

C

mono-BBPA

5.31

307

D

BPA

4.88

228

E

2,6-dibromo-4-methylphenol

6.82

266

F

5-(hydroxymethyl)benzene-3-bromo-1,2-diol

4.43

222

G

2,6-dibromo-4-isopropenyl-phenol

2.94

291

H

2,4-dihydroxybutanoic acid

7.72

118

disappeared within 30 min of electrochemical reduction, accompanied with the peaks of tri-BBPA, di-BBPA, BBPA and BPA appeared; and notably, the concentration of BPA was low after a follow up 30 min of oxidation reaction under aeration (Fig. S6), indicating that BPA was oxidized gradually. Apart from the above reduction intermediates (shown from HPLC patterns), some other products were also detected, including 2,6-dibromo-4-methylphenol (m/z = 266), 5-(hydroxymethyl) benzene-3-bromo-1, 2-diol (m/z = 222), 2,6-dibromo-4-isopropenyl-phenol (m/z = 291), and 2,4-dihydroxybutanoic acid (m/z = 118), as listed in Table 1 and Fig. S7. The other intermediates were not detected in the present study, probably due to their low concentration or fast degradation right after formation. These results confirmed that debromination process occurred stepwise during the electrochemical reduction process at N2 ambient, and then the reductive intermediates were further oxidized under air sparging, and eventually, the TBBPA and its intermediates would be mineralized to carbon dioxide and water. In summary, the reaction pathway can be selectively tuned via controlling the reaction ambient, and the process of electrochemical reduction-oxidation for TBBPA degradation is feasible.

Structure formula

Based on the above results, possible mechanisms of electrochemical sequential reduction-oxidation degradation of TBBPA were proposed, as interpreted in Fig. 5: at the stage of reduction, ECH and Fe0 catalytic reduction were main reactions related to TBBPA debromination; for oxidation stage, • OH generated from H2 O2 could be mainly responsible for the oxidation of TBBPA debromination products. According to the identified intermediates and information available from literature [9,38,40], possible pathways for electrochemical sequential reduction-oxidation degradation of TBBPA on Pd-Fe/Ni electrode surface were proposed. As shown in Fig. 6, the degradation pathways include: (I) TBBPA reduction route (stepwise debromination): TBBPA → tri-BBPA (A) → di-BBPA (B1, B2) → mono-BBPA (C) → BPA (D); (II) TBBPA intermediates oxidation process (as the TBBPA was almost converted into intermediates after 30 min of N2 sparging, the oxidation process began with TBBPA reduction intermediates): • OH could attack the bond between benzene ring and isopropyl of tri-BBPA (A), forming products G or E; and E could be further transformed into F through the reaction of • OH substitution; these intermediates would be further degraded by several steps with the presence of • OH to CO2 and H2 O; opening

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ring reaction could be occurred to BPA with the involvement of • OH to form product H, and H would be eventually mineralized to CO2 and H2 O. 4. Conclusions Electrochemical sequential reduction-oxidation process was proposed by controlling the reaction atmosphere (firstly bubbling N2 and afterwards sparging with air), aiming at achieving thorough degradation of TBBPA in the electrochemical system with an efficient Pd-Fe/Ni electrode as the cathode. TBBPA could be degraded faster and more completely in the sequential reductionoxidation process as compared with bubbling N2 only or aeration only. HPLC and UPLC–MS results demonstrated that mainly reductive debromination products were detected during the process with N2 sparging, while oxidative intermediates were identified during the followed up aeration process, indicating the feasibility of electrochemical sequential reduction-oxidation degradation of TBBPA. The oxidation of TBBPA debromination intermediates was likely to be ascribed to the reaction with strong oxidative species • OH originating from H2 O2 under aeration. H2 O2 accumulation was observed when air bubbling, which confirmed the involvement of • OH indirectly. The possible mechanisms and degradation pathways of TBBPA degradation through electrochemical sequential reduction-oxidation process were proposed. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21367002) and Ability Promotion Project of Education Department of Guangxi Province for the University Middle Age and Youth Teachers (No. 2017KY0031). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jhazmat.2017.10. 004. References [1] X. Zhou, J. Guo, W. Zhang, P. Zhou, J. Deng, K. Lin, Tetrabromobisphenol A contamination and emission in printed circuit board production and implications for human exposure, J. Hazard. Mater. 273 (2014) 27–35. [2] M. Fan, N. Zhou, P. Li, L. Chen, Y. Chen, S. Shen, S. Zhu, Anaerobic co-metabolic biodegradation of tetrabromobisphenol A using a bioelectrochemical system, J. Hazard. Mater. 321 (2017) 791–800. [3] Q. Zhou, A. Xing, D. Zhao, K. Zhao, Tetrabromobisphenol A photoelectrocatalytic degradation using reduced graphene oxide and cerium dioxide co-modified TiO2 nanotube arrays as electrode under visible light, Chemosphere 165 (2016) 268–276. [4] S. Pang, J. Jiang, Y. Gao, Y. Zhou, X. Huang, Y. Liu, J. Ma, Oxidation of flame retardant t Tetrabromobisphenol A by aqueous permanganate: reaction kinetics, brominated products, and pathways, Environ. Sci. Technol. 48 (2014) 615–623. [5] S. Yang, S. Wang, F. Sun, M. Zhang, F. Wu, F. Xu, Z. Ding, Protective effects of puerarin against tetrabromobisphenol a-induced apoptosis and cardiac developmental toxicity in zebrafish embryo-larvae, Environ. Toxicol. 9 (2015) 1014–1023. [6] Y. Zhang, L. Jing, X. He, Y. Li, X. Ma, Sorption enhancement of TBBPA from water by fly ash-supported nanostructured ␥-MnO2 , J. Ind. Eng. Chem. 21 (2015) 610–619. [7] X. Peng, Z. Zhang, W. Luo, X. Jia, Biodegradation of tetrabromobisphenol A by a novel Comamonas sp. strain JXS-2-02, isolated from anaerobic sludge, Bioresour. Technol. 128 (2013) 173–179. [8] R. Qu, M. Feng, X. Wang, Q. Huang, J. Lu, L. Wang, Z. Wang, Rapid removal of tetrabromobisphenol A by ozonation in water: oxidation products, reaction pathways and toxicity assessment, PLoS One 10 (2015) e0139580. [9] M. Cao, P. Wang, Y. Ao, C. Wang, J. Hou, J. Qian, Photocatalytic degradation of tetrabromobisphenol A by a magnetically separable graphene-TiO2 composite photocatalyst: mechanism and intermediates analysis, Chem. Eng. J. 264 (2015) 113–124.

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