Ligand-Assisted Sequential Redox Degradation of

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Kinetics, Catalysis, and Reaction Engineering

Ligand-Assisted Sequential Redox Degradation of Tetrabromobisphenol A using Bimetallic Zero-Valent Iron Nanoparticles Chung-Seop Lee, Da-Som Oh, Thao Thanh Le, Jianyu Gong, and Yoon-Seok Chang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03991 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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Industrial & Engineering Chemistry Research

Ligand-Assisted Sequential Redox Degradation of Tetrabromobisphenol A using Bimetallic Zero-Valent Iron Nanoparticles

Chung-Seop Lee a, Da-Som Oh a, Thao Thanh Le a, Jianyu Gong b, and Yoon-Seok Chang a,*

aDivision

of Environmental Science and Engineering,

Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea

bSchool

of Environmental Science and Engineering,

Huazhong University of Science and Technology, Wuhan 430074, PR China

*Corresponding author’s contact information: Phone: +82-54-279-2281 Fax: +82-54-279-8299 E-mail : [email protected]

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Abstract This study presents a sequential reduction–oxidation (redox) process composed of a reduction process using bismuth-modified zero-valent iron nanoparticles (Bi/Fe0 NPs) and

a

ligand-enhanced

oxidation

process

for

the

complete

removal

of

tetrabromobisphenol A (TBBPA). In the first reduction process, TBBPA was reductively degraded to bisphenol A (BPA) by Bi/Fe0 NPs. In the subsequent oxidation process, citric acid (CA) was used to enhance the oxidation capability for degradation of BPA over Bi/Fe0 NPs in the presence of oxygen, thereby completely degrading TBBPA. The addition of CA to oxygen-containing solution of Bi/Fe0 NPs significantly promoted the production of reactive oxygen species (ROS) by accelerating the rate of key reactions through its ligand function, which could form Fe(III)-citrate complex. The intermediates formed during the sequential redox treatment were identified and the plausible degradation pathway of TBBPA was proposed. These results provide a new strategy for degradation of various recalcitrant pollutants.

Keywords: Tetrabromobisphenol A; Bimetallic nanomaterials; Zero-valent iron; Bismuth; Citric acid; Sequential degradation;


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1. Introduction Tetrabromobisphenol A (TBBPA) is the most extensively used brominated flame retardant in industrial and consumer products.1 Large amounts of TBBPA have already been released into the environment due to the broad use of TBBPA and its incomplete removal from waste printed circuit boards (WPCBs).2-4 Various methods have been applied to remove TBBPA, including bioremediation,5 adsorption,6 ozonation,7 photocatalytic degradation,8 and reduction.9 Nanoscale zero-valent iron (nZVI) is well known to be an effective agent for reducing halogenated organic compounds (HOCs), and therefore many nZVI-based technologies have been implemented for reductive debromination of TBBPA in previous studies.9-14 However, it was reported that nZVI alone was not effective in reducing TBBPA which has many bromine atoms.9 Some of the less brominated compounds still pose a serious threat to the environment, which can be more toxic when compared with the mother compounds.15,16 Bisphenol A (BPA), the final reduction product of TBBPA, belongs to one of the numerous anthropogenic compounds considered as endocrine disruptors.17 To alleviate these drawbacks, several surface modification methods have been developed to enhance the reactivity of nZVI to effectively degrade TBBPA. nZVIbased bimetals (e.g., Cu/Fe, Pd/Fe, Ag/Fe, and Ni/Fe) showed better reactivity towards TBBPA than bare nZVI.10-13 Reactivity enhancement was also achieved through sulfidation. Li et al. reported that debromination of TBBPA could be enhanced by sulfidated nZVI.14 However, partially debrominated intermediates and/or BPA were still observed, indicating that such an approach failed to completely degrade TBBPA. Therefore, it is crucial not only to enhance the reductive debromination of TBBPA by nZVI but also to achieve further oxidative degradation and ring-opening of BPA. 3 ACS Paragon Plus Environment


To initiate the oxidation process, introduction of organic ligands to Fe0 has been considered as a good strategy to increase the yields of oxidants in the Fe0+O2 system.18,19 Our previous studies reported the bismuth-modified zero-valent iron nanoparticles (Bi/Fe0 NPs) with high reductive reactivity20 and the organic acidinduced reactivity enhancement of Bi/Fe0 NPs which could oxidatively degrade 4chlorophenol.21 The 4-chlorophenol removal was found to be accelerated by adding organic acids such as formic acid, tartaric acid, oxalic acid, and citric acid, in conjunction with Bi/Fe0 NPs under oxic conditions. Among the tested ligands, citric acid (CA) was the most efficient organic ligand and performed an important role in the generation of reactive oxygen species (ROS), including hydroxyl radical (•OH), superoxide radical (O2•−), and hydrogen peroxide (H2O2) in the Bi/Fe0-CA+O2 system. Therefore, considering the dual reduction–oxidation (redox) capability of Bi/Fe0 NPs and the ability of CA to increase the production of ROS, herein, a sequential redox process comprised of a reduction process using Bi/Fe0 NPs and ligand-enhanced oxidation process was demonstrated. Consequently, it could initiate further oxidation after the reduction process and thus overcoming the abovementioned problems associated with incompletely debrominated intermediates of TBBPA. The objectives of our study were to evaluate the performance of the sequential redox treatment to achieve complete degradation of TBBPA, and to examine various treatment parameters for optimizing the TBBPA debromination and BPA degradation efficiency. Furthermore, the major degradation intermediates were identified and a possible pathway of the sequential redox degradation of TBBPA was proposed. It is anticipated from our model study with TBBPA that this strategy could offer a promising method of achieving complete degradation of many recalcitrant pollutants in contaminated water. 4 ACS Paragon Plus Environment

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2. Materials and methods 2.1. Materials Bismuth nitrate (Bi(NO3)3·5H2O), ferrous sulfate (FeSO4·7H2O), sodium borohydride (NaBH4), citric acid (CA), 1,4-benzoquinone (BQ), 1,10-phenanthroline, isopropanol (IPA), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and potassium iodide were supplied by Sigma-Aldrich (USA). Tetrabromobisphenol A (TBBPA) standard (97%) and bisphenol A (BPA) standard (99%) were purchased from Alfa Aesar (USA). Deionized (DI) water obtained from a water purification system (Millipore, France) was used in all experiments.

2.2. Synthesis of Bi/Fe0 NPs The preparation of Bi/Fe0 NPs was performed according to our previous study.20 Briefly, 0.068 g of Bi(NO3)3·5H2O and 1.0 g of FeSO4·7H2O were mixed in degassed water (100 mL). Then, 5 mL of 0.85 M NaBH4 was slowly added into the mixture. The atomic ratio of Bi to Fe was 4%, as optimized in our previous study. The iron nanoparticles were then rinsed several times with degassed water and vacuum dried at 60°C. Pure Fe0 nanoparticles (Fe0 NPs) were synthesized by the same method without adding Bi(NO3)3∙5H2O.

2.3. Sequential redox process Batch experiments were performed in 100-mL serum bottles capped with Teflon septa and aluminum crimps. The TBBPA solution was prepared in N2-degassed water and the initial TBBPA concentration was fixed at 0.037 mM (20 mg L‒1). Acetonitrile (ACN), as a co-solvent, was used for enhancing the TBBPA solubility. The content of ACN in the reaction bottles (ACN/water mixture) was 0.2% (v/v). The reduction 5 ACS Paragon Plus Environment


process (anoxic treatment) was initiated by adding Bi/Fe0 NPs into the prepared solution, which was stirred at 150 rpm under anoxic condition. Samples were withdrawn at specific time intervals during the reaction and filtered through a 0.22-μm PTFE syringe filter for TBBPA analysis. The oxidation process (oxic treatment) was conducted by adding an appropriate amount of Bi/Fe0 NPs and CA in serum bottles. The BPA concentration used in this experiment was fixed at 0.037 mM, estimated by the hypothesis that TBBPA totally transformed to BPA in the prior reduction process. The solution was purged with O2 gas for the oxic condition. The sample was collected and quenched with methanol for BPA analysis. For the sequential redox process (anoxic followed by oxic treatment), the reduction process was initiated by adding a predetermined amount of Bi/Fe0 NPs into the reaction solution, which was continuously stirred during the reaction. After the reductive debromination process, to initiate the oxidation process, CA was added into the solution, and subsequently, oxygen gas was purged into the solution so that molecular oxygen was present in the test environment. The reduction and oxidation process proceeded for 60 h and 12 h, respectively, and samples were withdrawn at specific time intervals during the reaction for TBBPA and BPA analysis. A control test using only TBBPA was also conducted to evaluate the loss of TBBPA due to volatilization and sorption. All experiments were carried out in duplicate.

2.4. Analytical methods The concentrations of TBBPA and its brominated products were analyzed using a high-performance liquid chromatograph (HPLC, Agilent 1100, USA) with a C18 reversed phase column and diode-array detector. The mobile phase was an ACN/water 6 ACS Paragon Plus Environment

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(70:30, v/v) mixture at a flow rate of 1.0 mL min‒1. Concentration of Br− was measured by ion chromatography (IC, Dionex DX-120) equipped with a Dionex IonPac AS 14 column (4 mm × 250 mm). The eluent composition was 3.5 mM Na2CO3 and 1 mM NaHCO3 at a flow rate of 1.2 mL min‒1. The dissolved oxygen (DO) concentration was measured using a DO meter that included a 083005MD probe (Orion 3-star plus, Thermo Scientific, USA) during the course of the entire sequential redox process. Total dissolved iron concentration in the solution was determined by inductively coupled plasma-optical emission spectrometer (ICP-OES, iCAP 6300 Duo, Thermo Scientific). The dissolved Fe2+ and H2O2 were quantified using the 1,10-phenanthroline method and the iodometric method,22 respectively. Electron spin resonance spectrometer (ESR, A200 Bruker, Germany) was used to confirm the ROS generation. UV–vis absorption spectra of CA and iron solutions were characterized using UV–vis spectrophotometery (Varian, Palo Alto, USA).



3. Results and discussion 3.1. Reductive debromination of TBBPA by Bi/Fe0 NPs The reactivities of Fe0 NPs and Bi/Fe0 NPs for reductive TBBPA debromination were evaluated under anoxic conditions. As shown in Figure 1a, after 12 h of reaction, >99% degradation of the initial TBBPA was accomplished by Bi/Fe0 NPs, compared to 60% degradation by bare Fe0 NPs, implying the enhanced reactivity for TBBPA debromination by Bi/Fe0 NPs. In the control experiments, TBBPA alone showed negligible degradation for 24 h.

(a)

(b)

0.16

1.0

Br concentration (mM)

0.14 Control Fe 0 NPs Bi/Fe 0 NPs

0.8

C/C0

0.6

0.4

-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2

0.12 0.10 0.08 Measured Br- (Bi/Fe 0 NPs) Measured Br- (Fe 0 NPs)

0.06 0.04 0.02

0.0

0.00 0

5

10

15

20

0

25

10

20

30

40

50

60

Time (hour)

Time (hour)

Figure 1. (a) Comparison of TBBPA removal by Fe0 and Bi/Fe0 NPs. (b) The concentration of bromide ion released during the reduction of TBBPA. The symbols represent experimental data and the dashed lines are theoretical curves ([TBBPA]0 = 0.037 mM; [Fe NPs] = 4.0 g L‒1; pHi = 6.0).

With regard to the extent of debromination of TBBPA, bromide ion (Br−) released from the TBBPA by iron NPs was further measured (Figure 1b). As seen from Figure 8 ACS Paragon Plus Environment

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1b, the greater degradation of TBBPA by Bi/Fe0 NPs compared with Fe0 NPs was consistent with the determined Br− concentrations. The concentration of Br− released sharply increased within the 8-hour reaction and reached 0.1448 mM after 60 h, indicating that 98.5% of TBBPA was debrominated by Bi/Fe0 NPs. This result suggested that even though initial TBBPA was degraded by Bi/Fe0 NPs within 24 h, reductive debromination of partially debrominated intermediates of TBBPA further continued to occur during the reduction process. The gap between measured and theoretical concentrations of Br− was gradually narrowed over time, and finally it was negligible after 60 h, indicating that the initial TBBPA was fully transformed to BPA. However, in the degradation of TBBPA with Fe0 NPs, the amount of Br− released was stoichiometrically much less than the amount of TBBPA degraded by Fe0 NPs, implying that debromination of TBBPA was incomplete and partially debrominated intermediates were produced. Similar to previous studies where the optimal pH values for the highest debromination efficiency of TBBPA by bimetallic iron NPs ranged from 5.0 to 7.0,10,13 high removal kinetics of TBBPA by Bi/Fe0 NPs was observed at pH 6.0 (data not shown). This can be explained by the fact that H+ might compete with TBBPA for the active surface sites to produce H2 (Fe0 + 2H+ → Fe2+ + H2) under acidic conditions. The results of TBBPA debromination studies at various Bi/Fe0 NP concentrations are shown in Figure 2a. Figure 2a indicates that the degradation rate constants of TBBPA increase with the increase of Bi/Fe0 NPs addition. The highest observed rate constant of TBBPA was observed when Bi/Fe0 NPs addition is 4.0 g L‒1. Thus, 4.0 g L‒1 of Bi/Fe0 NPs was selected in the following experiments.



0.04

(b) BPA concentration (mM)

(a) TBBPA concentration (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Control Bi/Fe 0 1.0 g/L Bi/Fe 0 2.0 g/L Bi/Fe 0 4.0 g/L

0.03

0.02

0.01

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0.04 Control Bi/Fe 0 1.0 g/L Bi/Fe 0 2.0 g/L Bi/Fe 0 4.0 g/L

0.03

0.02

0.01

0.00

0.00 0

10

20

30

40

50

0

60

Time (hour)

10

20

30

40

50

60

Time (hour)

Figure 2. (a) Debromination of TBBPA and (b) generation of BPA during the reduction process with the Bi/Fe0 NPs at various concentrations ([TBBPA]0 = 0.037 mM; pHi = 6.0).

When 4.0 g L‒1 Bi/Fe0 NP concentration was applied, the concentration of tri- and di-BBPA increased instantly at initial stage and decreased after 4 and 8 h, respectively, indicating that they underwent further debromination in the reduction process. The mono-BBPA concentration gradually increased and then slowly decreased after 24 h (data not shown). The BPA concentration during the initial period of reaction (8 h) was zero and gradually increased up to 0.0361 mM after 60 h, implying that 98.3% of the initial TBBPA was transformed to BPA (Figure 2b).

3.2. Oxidative degradation of BPA with Bi/Fe0-CA+O2 The effectiveness of using CA to improve the oxidation performance of Bi/Fe0 NPs towards BPA has been investigated.


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(a)

CA (5 mM)

1.0

Bi/Fe0 + CA (0 mM) Bi/Fe0 + CA (2 mM) Bi/Fe0 + CA (3 mM) Bi/Fe0 + CA (5 mM) Bi/Fe0 + CA (10 mM)

0.8 0.6

(b)

0.4

Bi/Fe 0 + O 2

1.0

Bi/Fe 0 + N 2 Bi/Fe 0 + CA + O 2

0.8

C/C0

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Bi/Fe 0 + CA + N 2

0.6

(c)

0.4

0.2

0.2

0.0

0.0

CA 5 mM

CA 0 mM

0

20

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80

100

120

0

20

40

60

80

Time (min)

Time (min)

100

120

332

334

336

338

Magnetic Field (mT)

Figure 3. (a) Degradation of BPA in the aqueous solution over Bi/Fe0 NPs in the presence of different concentrations of CA under O2 purging. (b) Effect of oxygen on the oxidative degradation of BPA. (c) ESR spectra of DMPO-•OH in the oxidation process ([BPA]0 = 0.037 mM; [Bi/Fe0 NPs] = 0.4 g L‒1).

The degradation efficiency of BPA significantly depended on the CA concentration (Figure 3a). CA could largely increase the degradation efficiency of BPA in the system of Bi/Fe0+O2 within the range of 2-10 mM. The degradation of TBBPA with CA alone was negligible, indicating that the activity of Bi/Fe0 NPs was improved due to the ligand reaction. However, BPA removal efficiency decreased when the CA concentration exceeded 5 mM, indicating that too high concentration of CA might block the reactive sites of Bi/Fe0 NPs or hinder the CA deprotonation. Thus, 5 mM CA was chosen for further studies of BPA removal. Since the oxidation of Fe0 and Fe(II) by oxygen could produce strong oxidants,18 the effect of DO was also considered as a crucial factor for the BPA degradation activity of the Bi/Fe0-CA system. Therefore, the BPA degradation activity of the Bi/Fe0-CA system was evaluated under anoxic conditions by nitrogen purge and under oxic conditions by oxygen purge. Figure 3b show that oxygen has a considerable effect on the oxidation reaction. The increase in DO concentration in solution enhanced not only 11 ACS Paragon Plus Environment


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the reaction rate but also the oxidation efficiency of BPA regardless of the existence of CA. In the absence of CA, 43% removal of BPA was observed in the Bi/Fe0+O2 system, indicating that small amounts of ROS were generated. In contrast, no degradation occurred in the Bi/Fe0+N2 system. However, in the presence of CA, 100% removal of BPA was observed with oxygen purging, while 20% removal of BPA was observed in the Bi/Fe0-CA+N2 system. The BPA degradation rate and efficiency under the O2 purged condition were much higher than those under the N2 purged condition, providing indirect evidence that oxygen was involved in the oxidation of BPA. The enhanced BPA degradation in the presence of oxygen was attributed to the in situ generation of H2O2 by the reaction between Fe0 and O2.21 With a supply of oxygen, H2O2 could be generated by the reaction of dissolved oxygen and Fe0 (Eq. 1), providing an opportunity for conversion into hydroxyl radicals by the Fenton reaction (Eq. 2).

Fe0 + O2 + 2H+ → Fe2+ + H2O2

(1)

Fe2+ + H2O2 → Fe3+ + •OH + OH‒

(2)

The hydroxyl radical produced by the Fenton reaction is a strong oxidant capable of rapidly oxidizing many organic compounds. To identify the hydroxyl radicals generated during the oxidation process in the Bi/Fe0-CA+O2 system, an ESR spin-trapping technique was adopted. As shown in Figure 3c, four-line spectra with an intensity ratio of 1:2:2:1were observed, indicating that •OH was involved in the oxidation process and the generation of •OH was enhanced in the presence of CA. Therefore, it was concluded that CA played a crucial role in the increase of ROS production with the assistance of 12 ACS Paragon Plus Environment

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oxygen, resulting in the synergistic effect on BPA removal.

3.3. Sequential redox degradation of TBBPA On the basis of the abovementioned results, the sequential redox process of TBBPA was examined (Figure 4). Figure 4a shows the time profiles for the disappearance of TBBPA and the appearance of BPA by Bi/Fe0 NPs. In the reduction process, reductive debromination of TBBPA by Bi/Fe0 NPs occurred with concomitant increase of BPA concentration over 60 h. When the reaction time reached 60 h, 99.1% of TBBPA conversion was achieved with the rise of Br− and BPA as major products. The rises of Br− and BPA were consistent with the reduction of TBBPA, and BPA and Br− were detected as the final products of the reduction process.

Adding CA

0.04 TBBPA BPA O2 purging

0.03

Air exposing N2 purging

0.02

0.04

BPA C on c. (m M)

Cocentration (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01

0.03 0.02 0.01 0.00

(a)

60.00

60.25

(b)

60.50

Tim e (h ou r)

0.00 0

10

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30

40

Time (hour) 13 ACS Paragon Plus Environment

50

60

70


Figure 4. Sequential redox degradation of TBBPA and BPA. (a) Anoxic reductive debromination of TBBPA by Bi/Fe0 NPs with corresponding generation of BPA. (b) Oxidative degradation of BPA with Bi/Fe0-CA under N2 purging (×), air exposure (▽), and O2 purging (△). The inset shows the magnified decay profile of BPA during 30 min after O2 purging ([TBBPA]0 = 0.037 mM; [Bi/Fe0 NPs] = 4.0 g L‒1; [CA] = 5 mM; pHi = 6.0).

After the first reduction process for 60 h, three atmospheric conditions (N2 purging, O2 purging, and air exposure) were examined for the subsequent oxidation process (Figure 4b). As shown in Figure 4b, BPA degradation varied under different atmospheres. When the solution was kept anoxic (N2 atmosphere) after adding CA, the BPA concentration decreased slightly. However, when the solution was exposed to atmospheric air after adding CA, the BPA concentration gradually decreased, while BPA removal was much accelerated when the solution was purged with oxygen. Rapid and complete degradation of BPA was only achieved under the O2 purged condition. The zoomed region (inset in Figure 4) shows the decay profile of BPA during 30 min after O2 purging, indicating that BPA was completely degraded within 30 min. The results presented in Figure 4a illustrate that the reductive debromination of TBBPA was a dominant reaction under anoxic condition. However, if there existed an oxygen supply after the reduction process, or if air was saturated continuously, the CAassisted oxidative degradation of BPA played a leading role. The Fenton reaction was induced by the produced Fe2+ during the reduction process as well as by residual Bi/Fe0 NPs upon the addition of CA with O2 purging, leading to the generation of •OH in the oxidation process. Furthermore, the atmospheric condition was found to be important 14 ACS Paragon Plus Environment

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for the BPA degradation activity of the Bi/Fe0-CA system in the oxidation process, suggesting that the oxidative reactivity of this system can be controlled by purging the solution with oxygen gas.

3.4. Structural transformation of Bi/Fe0 NPs The structural transformation of Bi/Fe0 NPs after the sequential redox process was investigated by using transmission electron microscopy (TEM, JEM-2200FS, JEOL, Japan).

(a)

(b)

(c)

(d)



Figure 5. TEM images of (a) Pristine Bi/Fe0 NPs, (b) Bi/Fe0 NPs after the reduction process, (c) Bi/Fe0 NPs after the redox process in the absence of CA, and (d) Bi/Fe0 NPs after the redox process in the presence of CA.

Figure 5 shows the TEM images of the Bi/Fe0 NPs before and after use in the reduction/oxidation processes. The freshly prepared Bi/Fe0 NPs were spherical and showed chain-like aggregates of spherical nanoparticles (Figure 5a). As shown in Figure 5b, after the reduction process, over an extended period of time (60 h), the spherical structure was well maintained, similar to that of fresh Bi/Fe0 NPs. However, thin layers of sheets and needle-shaped crystals on the surface of Bi/Fe0 NPs were observed because of the partial oxidation of Bi/Fe0 NPs resulting from the reaction with water. In the absence of CA, the complete loss of the Fe0 core and the emergence of an acicular structure were observed on the Bi/Fe0 NPs after the sequential redox process, which was probably due to the long-term exposure to the oxic environment and the resulting reaction with oxygen (Figure 5c). By contrast, Figure 5d displays that the Bi/Fe0 NPs were covered by smaller-sized precipitates in the presence of CA, indicating that CA adsorbed on the surface of Bi/Fe0 NPs slightly changed the surface structure through complexation and reductive dissolution. It was reported that polycarboxylic acids could chelate the dissolved Fe2+ and remove the iron (hydr)oxides coating.23 Thus, it was proposed that the presence of CA prevented the accumulation of iron (hydr)oxides on the Bi/Fe0 NP surface, facilitating electron transfer from Bi/Fe0 NPs to the contaminant or oxidant.

3.5. Mechanism for the enhanced BPA degradation by CA It is well documented that the corrosion of Fe0 to Fe2+ accompanied by two-electron 16 ACS Paragon Plus Environment

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transfer to the DO produces H2O2 (Eq. 1).18,19 Subsequently, the reaction of Fe2+ with H2O2 produces •OH (Eq. 2). In this study, strong radical species, mainly hydroxyl radicals, were generated, which could facilitate the oxidative degradation of BPA (Figure 3a). However, only 34.8% of BPA was degraded in the Bi/Fe0+O2 system, whereas complete oxidative degradation of BPA occurred in the Bi/Fe0-CA+O2 system after 60 min of treatment (Figure 3b). Therefore, through the comparison of results with and without CA under the same experimental condition, the enhanced BPA degradation mechanism was attributed to the formation of a ligand complex of Fe(III)-citrate and its role in promoting the higher generation of ROS. The Fe(III)-citrate complex has a higher stability constant (log K = 14.29) than that of Fe(II)-citrate complex (log K = 5.52).24 Spectrophotometric measurements of the solution of Fe(III) and CA indicated that CA formed a complex with Fe(III) (Figure S1). An absorption shoulder at around 365 nm was observed, indicating that Fe(III) was complexed by carboxyl groups in CA according to ligand exchange.25 Therefore, the reactivity of Fe(III) to decompose H2O2 (Eq. 3) decreased, separating Fe(III) from the Fenton’s catalytic cycle.

Fe3+ + H2O2 → Fe2+ + H+ + HO2•

(3)

In addition, CA greatly accelerated the release of iron from Bi/Fe0 NPs (Figures S2). Higher concentrations of total dissolved iron were observed during the reaction time of 90 min in the Bi/Fe0-CA+O2 system, indicating that the presence of CA increased the solubility of iron. Thus, the presence of CA contributes to the enhancement of BPA degradation through the CA-promoted dissolution and then CA-induced homogeneous 17 ACS Paragon Plus Environment


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Fenton reaction. The addition of CA may also lower the Fe3+/Fe2+ redox potential (Eo(Fe3+‒citrate/Fe2+‒citrate) = 0.372 VNHE vs. Eo(Fe3+/Fe2+) = 0.77 VNHE),26 making the Fenton reaction more thermodynamically favorable. To evaluate the contribution of ROS to the oxidative BPA degradation, the scavenging effect was examined by using IPA and BQ. The degradation efficiencies were decreased from 100% to 69% and 35% after BQ and IPA were added, respectively (Figure S3). Therefore, O2•− and •OH were considered as the predominant radical species for the oxidative degradation of BPA, and the relevant reactions in the Bi/Fe0CA+O2 system are proposed (Eqs. 4-8).

•OH + H2O2 → HO2• + H2O

(4)

HO2• → O2•− + H+

(5)

Fe2+ + O2•− → Fe3+ + H2O2 + OH−

(6)

O2•− + H2O2 → O2 + OH− + •OH

(7)

O2•− + Fe3+ → Fe2+ + O2

(8)

In the proposed reaction mechanism, the •OH and O2•− are generated by the Fenton reaction (Eqs. 1-2), and the reaction between the hydroxyl radicals and the hydrogen peroxide (Eqs. 4-5). At higher pH, the dominant species is O2•− (Eq. 5), which favors the formation of H2O2 (Eq. 6) and consequently •OH (Eq. 7). However, the ESR peaks of DMPO-O2•− were not observed because of the rapid reactions between O2•− and Fe2+ (Eq. 6) or Fe3+ (Eq. 8).27 18 ACS Paragon Plus Environment

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3.6. Identification of intermediates and the degradation pathway of TBBPA To understand the degradation pathway of BPA in the oxidation process, possible intermediates were identified by liquid chromatography–mass spectrometry and HPLC. Various degradation products, including monohydroxylated bisphenol A (MHBPA), 4,5-bisphenol-o-quinone, 4-hydroxyacetophenone, 1-(2,4-dihydroxyphenyl)ethanone, and ethylene glycol monoformate (EGMF), were identified and listed in Table S1. In addition to aromatic intermediates, some small aliphatic compounds, including acetaldehyde and formic acid, were also identified as the benzene ring opening products (Figure S4). Based on our results, the possible degradation pathway of TBBPA in the sequential redox system is proposed in Figure 6.



(a)

(b)

Figure 6. Proposed TBBPA degradation pathway by the sequential redox process. (a) Anoxic debromination of TBBPA and (b) oxidation of BPA.


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Based on the degradation products identified in this study, the reductive degradation pathway of TBBPA by Bi/Fe0 NPs was proposed in Figure 6a. The results indicate that TBBPA was transformed into tri- and di-BBPA by Bi/Fe0 NPs. Then, tri- and di-BBPA are debrominated to mono-BBPA, and finally, BPA is generated. A similar debrominaion pathway of TBBPA by nZVI has also been reported by Lin et al.9 The oxidative degradation of BPA during the oxidation process was mainly initiated by the attack of •OH resulting from the Fenton reaction (Figure 6b). The •OH could attack the aromatic ring and produce phenoxyl BPA, which could further form pisopropyl phenol (IPP) and/or phenol (PhO) radicals through beta-scission. The PhO radicals were converted into phenol and hydroquinone (HQ), and IPP radicals were converted into 4-hydroxyphenyl-2-propanol or 4-isopropenylphenol. Then, 4isopropenylphenol was subsequently converted to 4-hydroxyacetophenone (4-HAP) by means of oxidation.28-30 Meanwhile, •OH could also be added on the aromatic ring of BPA, forming the MHBPA. Then, the MHBPA was oxidized to give 4,5-bisphenol-o-quinone which was further oxidized to a muconic acid-based intermediate via the rupture of the aromatic ring.31-33 Finally, these aromatic compounds were further oxidized via ring opening reactions into aliphatic compounds.

3.7. Implication for performance under environmentally relevant conditions TBBPA can be released into the environment during the manufacture, usage, and disposal of TBBPA-containing products.34 This is of great concern since TBBPA has been indeed found in stream sediments and sewage sludge, which also makes it a potential groundwater contaminant.35 The leakage of landfill leachate could also contribute to the contamination of both surface and groundwater sources.36,37 Therefore, 21 ACS Paragon Plus Environment


although the performance of sequential redox treatment for TBBPA has been demonstrated, further examination is highly required to evaluate the feasibility of the sequential redox treatment system under environmentally relevant conditions, especially groundwater conditions. Herein, to evaluate the effects of groundwater matrix on the TBBPA degradation performance, natural groundwater collected from rural area in Korea was used to mimic the environmental conditions. The chemical

0.04 DI water Groundwater

(b)

0.04

BPA conc. (mM)

(a)

0.03

0.03

0.02

0.01

0.00

DI water Groundwater

0.02

0.01

10

20

30

40

50

60

0

Time (hour)

0.04

0.03

DI water Groundwater

0.02

0.01

0.00

0.00

0

(c) BPA conc. (mM)

components of groundwater are given in Table S2.

TBBPA conc. (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

10

20

30

40

50

60

Time (hour)

0

5

10

15

20

25

30

Time (min)

Figure 7. (a) TBBPA debromination and (b) BPA generation by Bi/Fe0 NPs in the reduction process. (c) BPA degradation by CA-assisted Fenton reaction in the oxidation process. Control (DI water) results are shown with empty symbols, whereas real groundwater results are presented with full symbols ([TBBPA]0 = 0.037 mM; [Bi/Fe0 NPs] = 4.0 g L‒1; [CA] = 5 mM).

The TBBPA degradation rate was faster in DI water than that in groundwater (Figure 7a), presumably due to solutes in the groundwater. Moreover, it also took a longer time to achieve complete conversion of TBBPA to BPA, indicating that debromination was slightly inhibited by groundwater solutes (Figure 7b). It is well known that dissolved organic or inorganic species could reduce the oxidation efficiency 22 ACS Paragon Plus Environment

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by scavenging ROS or affecting the behavior of ligands, but these inhibitory effects were not observed in the oxidation process (Figure 7c). The values of kobs obtained in DI water and groundwater were similar. Therefore, this system exhibited a remarkable redox activity for TBBPA degradation, which showed a significant potential for groundwater remediation. To apply this sequential treatment strategy in groundwater remediation, other technologies for effectively supplying oxygen may be needed to initiate the oxidation process. We anticipate that the introduction of in situ air sparging (IAS) into the in situ chemical reduction (ISCR) can be considered not only to achieve sequential treatment strategy but also to completely degrade many recalcitrant pollutants including TBBPA in a contaminated environment matrix.

4. Conclusions In this study, a sequential redox process using Bi/Fe0 NPs coupled with CA has been developed and evaluated for the degradation of TBBPA. Experimental results showed that TBBPA was reductively degraded through the stepwise debromination into lowly brominated intermediates by the reduction process, which were then oxidized by the oxidation process. The addition of CA in the Bi/Fe0+O2 system appeared to greatly enhance the efficiency of BPA oxidation due to the integrated mechanisms including (i) chelating Fe(III) to form complexes and decreasing H2O2 dissociation, (ii) accelerating the release of Fe(II), and (iii) prevention of Fe(III) from precipitating into iron hydroxide and/or iron oxide in the oxidation process. Our sequential treatment system has the advantage of performing reduction and oxidation stages in the same reactor, making it possible to initiate each stage by tuning the experimental conditions. Therefore, this sequential redox treatment strategy is 23 ACS Paragon Plus Environment


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expected to be feasible for practical applications in the degradation of various organic brominated

compounds

such

as

polybrominated

dibenzofurans

(PBDFs),

polybrominated dibenzo-p-dioxins (PBDDs), and polybrominated diphenyl ethers (PBDEs), which are toxic and persistent in the aquatic environment.

ASSOCIATED CONTENT Supporting Information Supporting Information is available: Intermediates of BPA degradation in the Bi/Fe0-CA+O2 system (Table S1), physicochemical properties of the groundwater sample (Table S2), UV-Vis spectra of Fe(III)-citrate complexes (Figure S1), iron dissolution (Figure S2), inhibition kinetics of BPA degradation treated with the Bi/Fe0-CA+O2 system in the presence of various scavengers (Figure S3), and HPLC diagrams (Figure S4) (PDF).

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government.

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Abstract Graphics

O2

TBBPA

Oxidation Reduction

Citric acid BPA


Ligand-Assisted Sequential Redox Degradation of

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