Journal of Hazardous Materials 343 (2018) 376–385

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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

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

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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

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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%)