graphene catalytic cathodes

Accepted Manuscript Degradation mechanism, kinetics, and toxicity investigation of 4-bromophenol by electrochemical reduction and oxidation with Pd–Fe/graphene catalytic cathodes Dandan Xu, Xiaozhe Song, Wenzhi Qi, Hui Wang, Zhaoyong Bian PII: DOI: Reference:

S1385-8947(17)31677-7 https://doi.org/10.1016/j.cej.2017.09.173 CEJ 17758

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

3 May 2017 25 September 2017 26 September 2017

Please cite this article as: D. Xu, X. Song, W. Qi, H. Wang, Z. Bian, Degradation mechanism, kinetics, and toxicity investigation of 4-bromophenol by electrochemical reduction and oxidation with Pd–Fe/graphene catalytic cathodes, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej.2017.09.173

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Degradation mechanism, kinetics, and toxicity investigation of 4-bromophenol by electrochemical reduction and oxidation with Pd–Fe/graphene catalytic cathodes Dandan Xu a, Xiaozhe Songa, Wenzhi Qia, Hui Wanga,*, Zhaoyong Bianb,** a

College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, PR China

b

College of Water Sciences, Beijing Normal University, Beijing 100875, PR China

Abstract:

This study aimed to elucidate the electrodegradation intermediates, kinetics, and toxicity evolution of 4-bromophenol (4-BP), a compound abundant in water. In this study, 4-BP was subjected to electrochemical reduction and oxidation using a divided cell with two prepared Pd–Fe/graphene catalytic cathodes and a Ti/IrO2/RuO2 anode. In the cathodic compartment, reduction debromination in the presence of a Pd–Fe/graphene catalyst was processed with adsorbing hydrogen on the cathode. The cathode also generated hydroxyl radicals in aid of the feeding air to promote the oxidative degradation of 4-BP. At 60 min, the removal rate of 4-BP was 100% in the two catholytes and 99.5% in the anolyte. Most of the intermediate products formed during degradation were identified using liquid chromatography/mass spectrometry.

∗

Corresponding author. Tel.: +86 10 62336615; fax: +86 10 62336596 E-mail address: [email protected]

**

Corresponding author. Tel.: +86 10 58802736; fax: +86 10 58802739

E-mail address: [email protected]

1

The concentration evolution of three aromatic by-products and four carboxylic acids were detected by high-performance liquid chromatography and ion chromatography. Two possible pathways were proposed on the basis of the reduction and oxidation reaction. A kinetics experiment was carried out to evaluate significant degradation steps. A scheme for the complete mineralization of 4-BP was elucidated. Furthermore, the time course of 4-BP and intermediate concentrations satisfactorily correlated with the toxicity profiles determined from the inhibition of Photobacterium phosphoreum luminescence. Two toxicity evaluation methods demonstrated that a large amount of toxic substance (benzoquinone) was produced in the anolyte at 20 min. However, benzoquinone was not detected in the catholytes and the toxicity of the catholytes decreased with electrolysis time. The formation of the carboxylic acids induced a sharp toxicity decrease, thus ensuring overall detoxification. Keywords: 4-bromophenol; electrochemical reduction and oxidation; oxygen reduction; degradation pathway; kinetics; acute toxicity 1. Introduction Halogenated compounds constitute a large group of environmental pollutants. Bromophenols (BPs) widely exist in wastewater discharged from dyes, pulps, paints, polymer intermediates,

brominated flame retardants, herbicides and wood

preservatives [1, 2]. High-quality BPs in water and soil aggravate environmental pollution problems [3]. Most BPs are toxic and nonbiodegradable organic compounds. They are considered as priority pollutants due to their genotoxic, mutagenic, and

2

carcinogenic nature [4]. BPs exhibit strong toxicological effects, depending primarily on their pattern and degree of bromination [5]. The presence of bromine may influence not only the degradation efficiency of the targeted organic contaminants but also the potential generation of toxic brominated by-products [6]. The accumulation of BPs in the environment raises concerns because the compounds are suspected of having multiple adverse effects on living organisms [7]. Therefore, effective methods to eliminate 4-bromophenol (4-BP) in aqueous solution in contaminated environments are warranted. Several techniques and treatments for removal have been studied [8-10]. The removal of BPs from wastewater can be achieved using physico-chemical or biological processes. Biodegradation is an efficient and ecofriendly process for BPs treatment. However, the presence of brominated substitute groups increases the resistance of the aromatic ring of compounds to biodegradation by many microorganisms [11]. Arthrobacter chlorophenolicus A6T biodegrade 4-BP [12] but it requires a long reaction time. Moreover, photocatalytic degradation of BPs can be realized by nanoparticles under UV light irradiation [13]. The photocatalytic degradation rate of BPs by cobalt-doped nano-TiO2 reached 93.5%, but the phenol intermediates are not further degraded [14]. Electrochemical oxidation is a promising, versatile alternative to existing processes for bromophenol removal. However, the formation and accumulation of toxic intermediates of lower bromination degree during the treatment of brominated aromatics pose potential risks [15]. Efficient

3

removal of the bromine group from the substrate has been recently considered as an alternative to the more popular and exhaustive electro-oxidative process of total degradation [16]. Reductive processes have attracted attention for the remedy of contaminated environments and have been proposed for pollutant abatement [17]. Moreover, electrochemical hydrodebromination has been used to treat brominated organic wastes [18]. Electrochemical reduction can effectively reduce the toxicity of bromine organic wastewater but cannot realize the mineralization of intermediate products [19]. Recent reductive debromination methods are still incapable of complete debromination and degradation. Therefore, electrochemical reduction and oxidation were combined in the present study to achieve the effective debromination and complete degradation of BPs. Identification of the major transformation products of debromination degradation and evaluation of their toxicity, which could be higher than that of the original molecule, are important [20]. In addition, establishment of a degradation kinetics model is crucial to improve the understanding of the degradation reaction and the in-situ application of remediation methods [21]. Although many studies have focused on enhancing the removal of toxic organic compounds [22], the kinetics and reaction mechanism of electrochemical degradation remain unclear to date. In addition, only few reports are available about the mixture toxicity of intermediates during BPs degradation. Therefore, 4-BP was chosen as a target compound for electrochemical reduction and oxidation degradation, product identification, and degradation pathway.

4

The kinetics and toxicity assessments of 4-BP degradation still warrant a systematic investigated. The present study investigated the electrochemical reduction and oxidation degradation of 4-BP using Pd–Fe/graphene catalytic cathodes in a three-electrode diaphragm system. In specific, the transformation pathways were elucidated by identifying the intermediates, and a kinetics experiment was carried out to evaluate significant electrocatalytic degradation steps. The toxicity of 4-BP and its intermediates was assessed during the treatment to identify potential environmental risks. 2. Materials and Methods 2.1 Procedures Graphene oxide was prepared using the modified Hummers’ method [23]. A Pd–Fe/graphene catalyst with a 0.5% weight ratio of Pd and a 0.5% weight ratio of Fe was fabricated by photo-induced reduction, and the optimal surface performance of this catalyst was evaluated by performing different characterization methods and electrochemical measurement on a CHI-660d workstation [24]. The Pd–Fe/graphene catalyst exhibited optimal surface performance, and the bimetallic Pd-Fe nanoparticles were approximately 6.05±0.05 nm in size. Pd 0 and Fe0 nanoparticles showed bright white spots uniformly distributed on the graphene sheets. The Pd–Fe/graphene catalyst demonstrated a high electrocatalytic activity for accelerating the reduction of O2 to H2O2. The cathode composed of a Pd–Fe/graphene catalyst

5

layer and a gas-diffusion layer stacked onto a stainless steel screen (200 mesh) was finally cold-pressed at 10 MPa for 1 min. All the degradation experiments for 4-BP were conducted in an electrochemical divided reactor of 150 mL capacity. The electrolytic cell consisted two self-prepared Pd–Fe/graphene cathodes, a Ti/IrO2/RuO2 anode (Wuhan Kaida Technology Engineering Co., Ltd, China), a polyester fabric diaphragm (Tangshan Fengrun Jinxiang Chemical Fiber Co., Ltd., Tangshan, China), an anodic compartment, cathodic compartment 1, cathodic compartment 2 and two gas compartments. The effective area of the electrode was 16 cm2. The schematic of the experimental setup was reported previously [25]. A laboratory direct current power supply with a currentvoltage monitor was employed to provide electric power. The trials were made by electrolyzing 100 mg L-1 of 4-BP in 0.03 mol L-1 Na2SO4 as supporting electrolyte. The initial solution pH was 7.0, and the electrolysis time was 60 min. The current density of each cathode was 25 mA cm-2. Before starting electrolysis, hydrogen gas was fed into gas compartments for 5 min at a rate of 25 mL s-1 to keep dissolved gas saturation. Hydrogen gas was then fed into the gas compartment at 0-30 min, following air untill the end of electrolysis process (30-60 min). 2.2 Analysis methods 2.2.1 Electrocatalytic measurements The total organic carbon (TOC) was measured with the TOC analyzer (TOC4000, Shimadzu, Japan). The H2O2 concentration accumulated during electrolysis was

6

determined spectrophotometrically with titanium oxalate using a SHIMADZU TU-1601 UV spectrometer. The hydroxyl radicals (•OH) were determined by electron spin resonance (ESR) measurements, which were recorded at room temperature on a JES-FE3AX ESR spectrometer (JEOL, Tokyo, Japan) operating at the X-band. Aqueous solutions, containing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, 0.08 mol L-1) as the spin-trapping reagent were performed. The samples were then quickly placed in a quartz tube (d=1 mm) and measured under the following conditions: microwave frequency, 9.86 GHz; microwave power, 20 mV; modulation frequency, 100 kHz; modulation amplitude, 0.5 G; center field, 3522 G; scan width, 100 G; receiver gain, 20; time constant, 0.3 s; and sweep time, 2 min. The

intermediate

products

of

4-BP

were

identified

using

a

liquid

chromatography/mass spectrometer (Q Exactive orbitrap, Thermo, CA, USA) equipped with a Cortecs C18 column (2.1 × 100 mm). The mobile phase consisted of solvent A (5 mM CH3COONH4 in H2O) and solvent B (100% methanol). The data with mass rang m/z 70–1050 were acquired in negative ion mode, and the sample injection volume was 1 µL. The MS parameters were optimized and set as follows: spray voltage, 3000 V; capillary temperature, 320 °C; heater temperature, 300 °C; sheath gas flow rate, 35 mL min-1; and auxiliary gas flow rate, 10 mL min-1. During electrochemical reduction and oxidation, the concentrations of 4-BP and its aromatic intermediate products were analyzed by high-performance liquid

7

chromatography (HPLC, Shimadzu, Japan). The HPLC instrument was equipped with an ODS-SP C18 separation column using a mobile phase of methanol: water (80:20, v/v) at a flow rate of 1.0 mL min-1. The column temperature was 35 °C, the injection volume was 20 µL, and the UV wavelengths were 245 nm and 280 nm. An ion chromatography system (IC-3000, Dionex, USA) was used to measure significant low-carbon aliphatic carboxylic acid intermediates. The system was equipped with an AS11-HC separation column using a mobile phase of water and NaOH solution (250 mmol L-1) as a flow rate of 1.2 mL min-1. 2.2.2 Toxicity measurements Acute toxicity was tested using the luminescent freshwater bacterium, Photobacterium phosphoreum. Freeze-dried powder of Photobacterium phosphoreum was purchased from the Institute of Soil Science, Chinese Academy of Sciences (Nanjing, China). The freeze-dried powder was hydrated using 2.5 mg 100 mL-1 of cold sodium chloride solution. The temperature during the test was maintained at 20 °C. Exposure of luminescent bacteria to toxic substances inhibits the bacterial luciferase and rapidly decreases light intensity [26]. The effects of toxicity are expressed as EC50, which is the concentration of toxicant corresponding to the inhibition value of 50% at 15 min of exposure time [27]. The toxicity variation can be expressed as I
%=

LB -LS LB

×100%,

8

(1)

where I represents the inhibition of the concentrated sample to luminescent bacteria, LB is the luminescent intensity of the bank, and LS is the luminescent intensity of the samples. However, several factors affect the results of the luminescent bacteria method in the actual degradation through direct toxicity determination. For instance, the concentration of the intermediate product is nonlinear. Therefore, the two methods included the toxicity factor value and toxicity inhibition effect, which were the evaluated and analyzed toxicity change of the electrochemical reduction and oxidation degradation of BPs. The feasibility of the two methods was compared. The values of mixture toxicity are presented either as EC50 or as toxicity factor (f) [28]: f=1/EC50 (L·mg-1 ).

(2)

Then, the toxicity of mixture solution during degradation was determined using the following equation: fmix =

∑ fi Ci ∑ Ci

,

(3)

where Ci is the concentration of intermediates and fmix is the mixture toxicity factor. 3. Results and discussion 3.1 Hydrogen peroxide and hydroxyl radical electrogenerated on the Pd-Fe/graphene catalytic cathode For electrochemical reduction and oxidation, hydrogen peroxide formed by the two-electron reduction of dissolved O2 at the catalytic cathode was determined and quantified by spectrophotometry in the divided electrolysis device [29]. The

9

production of hydrogen peroxide (H2O2) and hydroxyl radicals (•OH) by electrochemical reduction and oxidation under the optimized current condition was investigated. Fig. 1a shows the global production of H2O2 versus time by applying 25 mA cm-2 in the absence of 4-BP in the catholytes of the two cathodic compartments. The amount of H2O2 increased in the system during electrolysis. After 60 min, the concentration of H2O2 increased to 13.9 and 14.7 mg L-1 in the Pd–Fe/graphene double-cathode system, which was higher than the concentration of 6.2 mg L-1 in the C/PTFE gas-diffusion electrode system and 7.5 mg L-1 in the Pd/C gas-diffusion electrode system [30]. This result indicated that the Pd–Fe/graphene catalyst accelerated the two-electron reduction of O2 to H2O2 on the cathode when feeding air. 15

0.15

a

0.10

12

0.05

-1

H2O2(mg·L )

b

Intensity

9 6

-0.05

Catholyte 1 Catholyte 2

3 0

0.00

0

10

20

30

40

50

-0.10 -0.15 3470 3480 3490 3500 3510 3520 3530 3540 3550

60

Time (min)

Magnetic field (G)

Fig. 1 (a) Variation of accumulated H2O2 concentration with electrolysis time and (b) ESR spectrum of the •OH-DMPO adduct detected in 0.08 mol L-1 DMPO solution in 0.03 mol L-1 Na2 SO4 in the cathodic compartments. Current density: 25 mA cm-2. A Ti/IrO2 /RuO2 anode and a Pd–Fe/graphene catalytic cathode were used. Air was passed into the gas compartment during the whole experiment.

10

In basic electrolyte, H2O2 may be transformed into HO2-, and then H2O2 and HO2are converted to •OH, O2-• under warm alkaline conditions as follows [31, 32]: H2O2+OH- → H2O+HO2-

(4)

H2O2+HO2- → •OH+O2-•+H2O

(5)

Detecting the ESR spectrum of •OH by the normal method is difficult because of the high activity and short life of this radical. Hence, DMPO was added as a trapping agent. The ESR spectra of DMPO-OH adducts were detected using the spin trapping method in the oxygen reduction cathodic system. The pH in the catholyte rapidly become to alkaline (pH>12) due to the presence of diaphragm and the temperature increased with the electrolysis time and stabilized at 75-85 °C after 15 min. Fig. 1b shows the typical ESR spectrum obtained after 15 min of electrolysis in the DMPO solution. The observed quartet signal was attributed to the formed DMPO-OH adduct with a peak high ratio of 1:2:2:1, g-value of 2.0065, and hyperfine constants of aN=aH=14.0 G [33]. This finding indicated that •OH was produced in the Pd–Fe/graphene catalytic electrode system in the transformation of the formed H2O2. The heterogeneous electro-Fenton reaction may also contribute the •OH formation on the Pd–Fe/graphene catalyst [34]. The concentration change of H2O2 and •OH reflected that the Pd–Fe/graphene catalyst accelerated the two-electron reduction of O2 to H2O2 when feeding air, which promoted the production of •OH. Then, the Pd–Fe/graphene catalytic cathode was investigated to degrade 4-BP solution in a divided electrolysis system.

11

3.2 Electrocatalytic debromination and oxidation of 4-BP The prepared Pd–Fe/graphene catalytic cathode was used to degrade 4-BP in aid of electrochemical reduction and oxidation under optimal conditions. 4-BP was completely removed in different electrode compartments within 60 min when the initial concentration of 4-BP was 100 mg L-1 (Fig. 2). The concentration of 4-BP decreased with the electrolysis time, and the conversion rates of 4-BP in catholyte 1, catholyte 2, and anolyte were 100%, 100%, and 99.5%, respectively (Fig. 2). The Pd–Fe/graphene catalytic cathode could enhance the electrocatalytic degradation rate of 4-BP compared with other traditional processes [35]. In the present system, the cathodic compartments favored 4-BP degradation. This result could be attributed to the Pd–Fe/graphene catalytic cathode that adsorbed hydrogen, reductively produced highly active chemisorbed hydrogen atom (Hads) within 30 min, and then reacted with the bromine of 4-BP [36]. This finding indicated that the electrochemical hydrodebromination of 4-BP was processed, and Hads was the main reducing agent for reductive debromination. The break of C–Br bond is energetically favorable than that of C–OH bond [37]. After 30 min of electrolysis, oxidation reaction occurred in the cathodic compartments due to •OH generation. During anodic oxidation, H2O was discharged on the Ti/IrO2/RuO2 anode surface and adsorbed MOx(•OH) formed under acidic conditions. The MOx(•OH) was involved in the mineralization of 4-BP on the anode [38]. The removal rate of 4-BP in the anodic compartment showed a similar trend to those in the cathodic

12

compartments. The halogen substituent in organic compounds represented the toxicity of aromatic pollutants, and reduction dehalogenation contributed to detoxification. Therefore, the concentration of bromide ions released from 4-BP with electrolysis time in the catholytes and anolyte was measured (Fig. 2). The bromide ion concentration of 4-BP was 41.2 mg L-1 in the anodic compartment and 39.3 and 37.0 mg L-1 in the two cathodic compartments after 30 min of electrolysis. The brominated organic compounds were not presented in the cathodic and anodic compartments. The removal rate of bromide ions could exceed 90%. The concentration of bromide ions in the anolyte was higher than those in the catholytes. The phenomenon occurred because the electrostatic repulsion from the negatively charged cathode and bromide ions in the cathodic compartments could diffuse to the anodic compartment through the diaphragm. The concentration of bromide ions decreased in the three compartments after 30 min. Possibly, the bromide ions released during oxidation were transformed into molecular bromine in the anodic compartment, and then molecular bromine was separated from the electrolysis system due to volatilization.

13

Anolyte (4-BP) Catholyte 1 (4-BP) Catholyte2 (4-BP)

80

50 Anolyte (Br-) Catholyte 1 (Br-) Catholyte 2 (Br-) 40

60

30

40

20

20

10

0

0

10

20

30 Time (min)

40

50

Br- Concentration (mg L-1)

4-BP Concentration (mg L-1)

100

0 60

Fig. 2 Variation of 4-bromophenol and Br- concentrations with electrolysis for electrochemical reduction and oxidation treatment in different electrode compartments. Concentration of supporting electrolyte: 0.03 mol L-1 Na2 SO4.; current density: 25 mA cm-2. Initial pH: 7.0.

3.2 Exploration of reaction mechanism 3.2.1 Identification of intermediate products To confirm the predicted mechanism and degradation of 4-BP, the degradation products were identified on the basis of their molecular ion and mass spectrometric fragmentation peaks compared with those of chemical standards in the presence of electrochemical reduction and oxidation system (Table S1). In the cathodic compartments, the debromination of 4-BP induced the formation of phenol. The signal at m/z=93 belonged to phenol. In the anodic compartment, the hydroxylation of 4-BP triggered the formation of hydroquinone by •OH. The dominant signals in the MS spectrum were located at m/z=109. The signals at m/z=115 and 117 were from fumaric acid and succinic acid, respectively. The removal of aqueous 4-BP was primarily initiated by •OH attacking, and benzoquinone and hydroquinone were the 14

major products [39]. Benzoquinone and hydroquinone are known as an active redox couple in equilibrium in aqueous solution, but benzoquinone is unstable [40, 41]. The concentration changes of 4-BP and the main intermediates were evaluated to analyze the degradation mechanism using HPLC and IC. Therefore, the concentration–time courses of major aromatics and carboxyl acid intermediates are shown in Fig. 3. 10

10

fumaric acid phenol acetic acid

a

8

6 4 2 0

0

10

20

fumaric acid phenol acetic acid

b Concentration (mg/L)

Concentration (mg/L)

8

formic acid succinic acid

30 40 Time (min)

50

60

succinic acid formic acid

6 4 2 0

0

10

20

30 Time (min)

40

50

60

20

Concentration (mg/L)

c 15

fumaric acid hydroquinone benzoquinone phenol

acetic acid succinic acid formic acid

10

5

0

0

10

20

30 40 Time (min)

50

60

Fig. 3 Time course of the concentration of the main aromatic intermediates and carboxylic acids products formed during the electrochemical reduction-oxidation treatment with Pd–Fe/graphene catalytic cathode and Ti/IrO2 /RuO2 anode in catholyte 1 (a), catholyte 2 (b) and anolyte (c). Concentration of supporting electrolyte: 0.03 mol L-1; current density: 25 mA cm-2.

In the present study, three aromatic intermediate products (i.e., phenol,

15

benzoquinone, and hydroquinone) were generated in the different electrode compartments during electrolysis. In the cathodic compartments, phenol was initially produced as the primary intermediate at almost the same time of 4-BP debromination. However, the oxidation degradation of 4-BP to hydroquinone and benzoquinone by •OH addition was identified in the anodic compartment. Then, the opening of the hydroxylated ring could result in the formation of low-molecular-weight carboxylic acids [42]. The debromination products of 4-BP in the catholytes of the two cathodic compartments increased before 30 min and then decreased with reaction time. The phenol concentrations during debromination were 0.65 and 0.85 mg L-1 in the catholytes of the two cathodic compartments, which increased to 1.51 and 1.72 mg L-1, respectively. Meanwhile, the concentration of 4-BP decreased rapidly at 30 min. The phenol concentrations decreased after 30 min, indicating that electrochemical oxidation occurred. Reduction debromination occurred in the catholytes. Hence, only phenol and not hydroquinone and benzoquinone were detected. This result can be attributed to the fact that phenol is a less toxic, inhibitory, and recalcitrant compound than hydroquinone and benzoquinone. After 30 min, the catholytes produced H2O2 through the reduction of pumped O2 in air [43]. H2O2 was transformed into HO2- and then converted to •OH and O2-. The oxidizing power of •OH and O2- was stronger than that of H2O2. Then, phenol was degraded into organic acid by the strong oxidant •OH. However, benzoquinone reached the maximum value of 8.91 mg L-1 at 20 min in the anolyte, and the concentration of hydroquinone (0.12 mg L-1) was lower than that

16

of benzoquinone (1.55 mg L-1) at 60 min. These findings indicated that the C–Br bond cracked, and the subsequent •OH attack at the para-position corresponded to the formation of hydroquinone. Subsequently, hydroquinone was transformed into benzoquinone. The results indicated that the Pd–Fe/graphene catalytic cathodes promoted hydrogenolysis to replace the bromide atom in organic compounds with activated hydrogen atom. Four carboxylic acids were determined and monitored as final transformation products from the opening of the benzene rings of aromatic intermediates, including formic, acetic, succinic, and fumaric acids. All detected organic acids were produced at the first minute of reaction. As shown in Fig. 3, succinic and fumaric acids gradually accumulated up to the highest concentration (18.12 and 2.15 mg L-1) at 30 min, followed by a slow decrease in the anolyte. The amount of succinic acids may be derived from the decomposition of benzoquinone and hydroquinone and the conversion of fumaric acid (maleic acid) in the anolyte. The concentration of formic acid (acetic acid) increased from 0 to 1.14 mg L-1 (0.27 mg L-1) in the first 30 min, which revealed that generation rate was higher than the decomposition rate. This result showed that 4-BP was oxidized to macromolecular fumaric acid, decomposed into succinic acid, and then degraded to small-molecule acids (formic acid and acetic acid). Maleic acid and oxalic acid were undetected in the solution probably because they were initially degraded by their strong oxidation capability in a short time. However, the cathodic compartments mainly underwent the reduction reaction in the

17

presence of H2, which led to the unclear concentration variety of carboxylic acids. This event could explain that the degradation rate of macromolecular acids was similar to that of small-molecule acids. The Pd–Fe/graphene catalytic electrode catalyzed the two-electron reduction of O2 to H2O2, which was converted to a large amount of •OH in the catholytes. The result promoted the rapid oxidation of the aromatic compounds formed by 4-BP debromination. 3.2.2 Proposed reaction pathways of 4-BP degradation The concentration ratio of total carbon (∑CBP+Int, BP represents 4-BP, and Int represents intermediates) and the TOC were compared to monitor the organic carbon balance with the electrochemical reduction and oxidation of 4-BP in the three-electrode system (Fig. 4). When the TOC relative values were the same as the ∑CBP+Int relative values, then all the reaction intermediates were detected. The initial TOC of 4-BP was 55.1 mg L-1. The TOC decreased sharply during electrolysis, and the removal efficiencies of TOC were 94.7% and 94.9% in the two catholytes and 93.4% in the anolyte at 60 min (Fig. S1). The mineralization degree of bromide organic compounds and the TOC removal showed stronger effects in the catholytes than in the anolyte. Correlated with the 4-BP degradation rate above, the TOC values decreased slowly because organic intermediates were deeply oxidized to CO2. Furthermore, the concentration ratio of ∑CBP+Int and the TOC gradually decreased with reaction time, and ∑CBP+Int was slightly lower than TOC. These findings indicated that the intermediate products were mostly already detected and could

18

accurately infer reaction process. 1.0 ∑CBP+Int (anolyte) ∑CBP+Int (catholyte 1)

0.8

CCi/CC0

∑CBP+Int (catholyte 2) TOC (anolyte) TOC (catholyte 1) TOC (catholyte 2)

0.6 0.4 0.2 0.0

0

10

20 30 40 Time (min)

50

60

Fig. 4 TOC decay versus electrolysis time for the mineralization of 4-bromophenol solution by electrochemical reduction and oxidation with two Pd–Fe/graphene cathodes and a Ti/IrO2/RuO2 anode in three electrode compartments. Electrolyte: 0.03 mol L-1 Na2SO4, current density: 25 mA cm-2.

On the basis of the structure elucidation of the degradation products, two possible electrodegradation pathways of 4-BP under the Pd–Fe/graphene catalytic cathodes and the Ti/IrO2/RuO2 anode were proposed to evaluate reaction steps (Fig. 5). One pathway in the cathodic compartments was the hydrodebromination of 4-BP, and no other aromatic compounds aside from phenol were detected in the catholytes. Hydrogen gas was adsorbed on the surface of the Pd–Fe/graphene catalytic cathodes, formed a powerful activated hydrogen atom, and then was exchanged with bromide atoms. Hads provided reduction driving force and made a C–Br cleavage in the cathodic compartments. This active hydrogen atom was responsible for the hydrodechlorination of 4-CP [44]. The phenol was formed by the debromination 19

degradation of 4-BP and oxidized into other organic acids rapidly resulting from the amount of •OH produced in the cathodic compartment. Another pathway was the electrochemical oxidation by •OH produced on the anode surface. Given that •OH was added at the para-position of the aromatic ring, the C–Br bond cracked and subsequent •OH attack at the para-position corresponded to the formation of hydroquinone. Then, the hydroquinone was oxidized to benzoquinone by •OH in anolyte. Under the strong oxidation of the anode, •OH broke the aromatic ring structure of benzoquinone and generated simple acids, which were later oxidized further to CO2 and H2O.

Fig. 5 Scheme of the degradation mechanism and possible degradation pathways of 4-bromophenol by electrochemical reduction and oxidation in anodic compartment and cathodic compartments.

The electrochemical degradation of 4-BP to organic acids through and not through hydroquinone and benzoquinone compounds possibly occurred between the anodic compartment and cathodic compartments. The Pd–Fe/graphene catalyst played a 20

significant

role

in

the

production

of

adsorbing

hydrogen

and

in

the

hydrodebromination. The reductive debromination system with the Pd–Fe/graphene catalytic cathodes accelerated the degradation process to form less toxic organic acids and prevent the accumulation of highly toxic compounds in the catholytes. 3.3 Kinetics analysis of 4-BP degradation A kinetic model was proposed to understand the key mechanism underlying the electrochemical reduction and oxidation of 4-BP. This model was used for providing a design reference for actual industrial wastewater treatments. The reaction in any path is not a single step, and the main reaction steps for relatively stable and detectable intermediates must be identified [28]. Three assumptions were made to simplify the model. First, the solution was incompressible. Second, the reaction took place on the electrode surface, and the degradation of 4-BP was mainly attributed to •OH. The mass transport of organic compounds from the bulk of the solution toward the electrode surface was very quick, and the adsorption of 4-BP onto the catalytic sites of the electrode triggered a reaction. In the present experiment, mass transports should not be a rate-determining stage due to the fluctuation of the solution by gas feeding. The adsorption of organic compounds onto the electrode would also be rather fast. Third, given its strong oxidation ability, •OH did not accumulate in the solution. Consequently, all reactions were simplified to be pseudo-first order kinetics. According to the main reaction pathways in the anodic compartment and cathodic compartments, degradation reaction rate constants were obtained.

21

On the basis of the above assumption and degradation mechanism, the principal pathways of 4-BP degradation are represented in Reaction (6)-(9). Anodic compartment 4-BP
A+MOx (•OH) → aromatic intermediates (B1 )

(6)

Cathodic compartment 4-BP
A+H → phenol (B1 )

(7)

Aromatic intermediates
B+•OH → organic acids (C)

(8)

Organic acids
C+[•OH] → CO2 +H2 O

(9)

k1

k1

k2

k3

In the above reaction pathway, ki (i=1, 2, 3,) is the apparent rate constant. The following set of differential equations could be obtained according to the reaction diagram in the anodic compartment and cathodic compartments: dA dt dB dt dC ct

=-k1 A

(10)

=-k2 B+k1 A

(11)

=k2 B-k3 C

(12)

The integration constants were evaluated from initial conditions: at time t=0, [A]=[A]0, and [B]=[C]=0. The generalized kinetics model was given by using the Laplace transform: A=A0 e-k1t B=A0

(13)

k1

k2 -k

C= A0  k

e-k1 t -e-k2t k1 k2 e-kt

3 -k1 k2-k1 

(14) k1 k2 e-k2 t k k e-k3 t  - k -1k 2k -k  k  k k  2 1 2 3 3 1 3 2

 + k

(15)

For 4-BP, aromatic intermediates (phenol), and organic acids, conversions are defined by the Equations (16) – (18), respectively. These conversion expressions were 22

used to normalize the value of the different compound concentrations ranging from 0 to 1. The conversion values provide relevant information related to the state of 4-BP electrochemical reduction and oxidation processes. [A]

X = 

(16)



[B]

X =  X =

(17)



[C]

(18)

 

The proposed model was evaluated by testing against 100 mg L-1 4-BP data sets obtained at the different electrode compartments used in the present work. Fig. 6 and Table S2 shows a high correlation coefficient for the kinetics model with the 4-BP degradation using electrochemical reduction–oxidation. The symbols in Fig. 6 represent the experimental data, and the continuous lines represent the values calculated using the proposed model. A good kinetics fitting effect was obtained between the experimental and calculated curves fitted by the model for all the experimental series. The model was essentially suitable for the electrochemical reduction and oxidation of 4-BP at 100 mg L-1 of initial concentration. The kinetics parameters calculated are listed in Table 1.

23

1.0 a

Catholyte 1 model Catholyte 2 model Anolyte model Catholyte 1 Catholyte 2 Anolyte

0.8

XA

0.6 0.4 0.2 0.0 0

10

20

30 40 Time (min)

50

60

0.25

Catholyte 1 Catholyte 2 Anolyte

0.20

b

XB

0.15 0.10

Catholyte 1 model Catholyte 2 model Anolyte model

0.05 0.00

0

10

20

30

40

50

60

Time (min) 0.30

Catholyte 1 Catholyte 2 Anolyte

0.25

Catholyte 1 model Catholyte 2 model Anolyte model

XC

0.20 0.15 0.10 0.05 c 0.00

0

10

20

30 40 Time (min)

50

60

Fig. 6 Experimental data and model-fitting results of 4-bromophenol (a), aromatics/phenol (b), and carboxyl acids (c) with electrolysis time during electrochemical reduction and oxidation processes in catholyte 1, catholyte 2 and anolyte. Electrolyte: 0.03 mol L-1 Na2SO4, current density: 25 mA cm-2, Initial pH: 7.0.

24

As shown in Table 1, the kinetic constants (k1, k2, and k3) in the different electrode compartments were obtained. The apparent rate constant k2 was larger in the cathodic compartment 1 and cathodic compartment 2 than in the anodic compartment. Table 1 also indicates that the ring opening of aromatic intermediates was faster in the cathodic compartments than in the anodic compartment. The finding proved that the Pd–Fe/graphene catalytic cathodes were more effective in degrading 4-BP completely and rapidly than anode. Furthermore, the apparent kinetics constant k2 both in the cathodic compartment and anodic compartments was larger than k1 and k3. The result suggested that the second step that the aromatic ring was opened to form organic acids was faster than the first step that 4-BP was degraded to aromatics and the third step that organic acids were converted into carbon dioxide and water. The electrochemical reduction and oxidation systems showed better properties due to more rapid removal and higher reaction kinetic constants of the generated intermediates than those of other methods [28, 45]. Table 1 Estimated apparent kinetics constant (ki) obtained in the fitting of 4-bromophenol degradation experiments in the anodic compartment, cathodic compartment 1 and cathodic compartment 2 with an initial concentration of 100 mg L-1. Cathodic

Cathodic

Anodic

compartment1

compartment 2

compartment

k1

0.0848

0.0934

0.1424

k2

0.1764

0.1750

0.1502

k3

0.1026

0.0967

0.1171

25

3.4 Toxicity evolution Given the lack of data on the toxicity of 4-BP, which are essential to assess the potential risk derived from their discharge into the environment, the EC50 of 4-BP and toxicity variation during electrochemical reduction and oxidation were determined. The toxicity of the identified compounds is presented in Table S3 according to the literature [28]. The EC50 of 4-BP was 0.69 mg L-1 (Fig. S2). 3.4.1 Toxicity factor evaluation According to the measured EC50 of 4-BP and its intermediate products, toxicity variation was calculated using Formula (3) every 10 min in the three-electrode system (Fig. 7). Meanwhile, 4-BP degradation was rapidly completed with TOC removal almost achieved, and toxicity exhibited a difference effect. The value of f increased at the start of the reaction and peaked within the first 20 min. Subsequently, it decreased gradually at 60 min in the anolyte. The f value reached the maximum (11.8 mg L-1) at 20 min because of the formation of new intermediates with different toxicities, such as benzoquinone, hydroquinone and phenol. The toxicity of benzoquinone was higher than that of 4-BP, and the maximum concentration of benzoquinone was reached in the anolyte at 20 min. Further degradation and mineralization of these intermediates reduced toxicity and even generated nontoxic products. Toxicity decreased gradually in the catholytes in the two cathodic compartments during degradation. The phenomenon indicated that 4-BP initially generated phenol by hydrogenation debromination using the Pd–Fe/graphene catalytic cathodes when

26

passed into hydrogen. Fe plays a significant role in the production of hydrogen, which might be collected by Pd to form adsorbing hydrogen, and performs direct reductive dechlorination [46]. Benzoquinone was not detected in the catholytes, indicating that the intermediate products decomposed quickly after debromination. The hydroxyl radicals generated by the transformation of H2O2 broke the benzene ring structure of phenol rapidly for further degradation. At 60 min, the toxicity values were 0.12 L mg-1 in the anodic compartment and 0.06 and 0.04 L mg-1 in the cathodic compartments. The 4-BP solution was almost nontoxic after 60 min. Nevertheless, the toxicity in anolyte was higher than that in the catholytes.

12

Catholyte 1 Catholyte 2

10

Anolyte

f(L·mg-1)

8 6

1

0

0

10

20

30 40 Time (min)

50

60

Fig. 7 Evolution of solution toxicity during the electrocatalytic degradation of 100 mg L-1 4-bromophenol solution using Pd-Fe/graphene catalytic cathodes in different compartments. Electrolyte: 0.03 mol L-1 Na2SO4, Current density: 25 mA cm-2, Initial pH: 7.0.

3.4.2 Inhibition effect evaluation The toxicity inhibition effect was evaluated (Table S4–S6) during the degradation of 4-BP in the catholytes in the two cathodic compartments and in the anolyte. In the 27

anloyte, the value of EC50 decreased from 47.6% to 46.2% in 20 min. This behavior could be correlated with the intermediates (benzoquinones) that were generated in extremely high concentrations during the degradation and that presented higher toxicity levels than the parent compound (4-BP). After 20 min, EC50 increased gradually and reached 79.2%. Compared with that of anolyte, the EC50 of the catholytes increased constantly and reached the maximum (100%) at 40 min. Toxicity was removed because benzoquinone and hydroquinone were absent in the catholytes. Correlation analysis between two methods was performed using SPSS statistics 17.0. Toxicity inhibition and toxicity factor methods were significantly correlated, (r=1) at the P=0.01 level. This correlation proved that the experimental results of toxicity inhibition and toxicity factor were reliable and that the two toxicity tests were feasible.

Electrochemical

reduction

and

oxidation

was

an

effective

and

environment-friendly technology that can be widely used to remove 4-BP in aqueous solutions. 4. Conclusion The electrochemical reduction and oxidation of 4-BP in a divided compartment reactor with two Pd-Fe/graphene catalytic cathodes and a Ti/IrO2/RuO2 anode was presented. The cathode generated H2O2 through the two-electron reduction of feeding air, and •OH was determined in the electrochemical systems by ESR. The Pd–Fe/graphene catalysts accelerated the two-electron reduction of O2 to H2O2, which promoted the production of •OH. Furthermore, the Pd–Fe/graphene catalytic cathodes

28

played an important role in debromination, particularly in the formation of absorbed hydrogen atom. The application of electrochemical reduction and oxidation resulted in high degradation efficiency in catholyte 1 (100%), catholyte 2 (100%), and anolyte (99.5%). Meanwhile, 94.7%, 94.9% and 93.4% of TOC removal were obtained, respectively. The Pd–Fe/graphene catalytic cathodes led to greater mineralization efficiency than the anode during electrolysis. The degradation of 4-BP was accompanied by the formation of phenol, benzoquinone, and hydroquinone, as well as aliphatic hydrocarbons such as formic, acetic, succinic, and fumaric acids. The kinetics model indicated that the ring opening of the aromatic intermediates was faster in the cathodic compartments than in the anodic compartment. Moreover, two toxicity test methods simultaneously evidenced that 4-BP achieved complete detoxification through electrochemical reduction and oxidation. However, a relatively high toxicity was observed in the anolyte at 20 min due to the formation of benzoquinone. Electrochemical degradation by the Pd–Fe/graphene catalytic cathodes is an effective and fast method of removing 4-BP and decreasing the toxicity of aqueous 4-BP solutions. Acknowledgments This work was supported by Beijing Natural Science Foundation (grant numbers 8172035), and the National Natural Science Foundation of China (grant numbers 21373032). References

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36




4-BP was reductively debrominated and degraded using Pd-Fe/graphene cathode.




Pd-Fe/graphene accelerated the reduction of O2 to form H2O2 and •OH.




Kinetics of 4-BP and intermediates were investigated to evaluate reaction steps.




Relationship between intermediates and toxicity of the solution was established.

37

38