Removal of 2-MIB and geosmin by electrogenerated persulfate: Performance, mechanism and pathways

Chemosphere 168 (2017) 1309e1316

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Removal of 2-MIB and geosmin by electrogenerated persulfate: Performance, mechanism and pathways Lingjun Bu a, Shiqing Zhou a, *, Zhou Shi a, Lin Deng a, Naiyun Gao b a Key Laboratory of Building Safety and Energy Efficiency, Ministry of Education, Department of Water Engineering and Science, College of Civil Engineering, Hunan University, Changsha, Hunan, 410082, PR China b State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


In-situ formed and activated persulfate is responsible for 2-MIB and GSM removal.
Both anodic and cathodic reactions participate in the generation of persulfate.
High current density and low pH were favorable for 2-MIB and GSM removal.
Possible pathways of 2-MIB and GSM degradation were proposed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 September 2016 Received in revised form 25 November 2016 Accepted 26 November 2016 Available online 1 December 2016

In this study, the degradation of 2-methylisoborneol (2-MIB) and geosmin (GSM) was evaluated by electrochemical oxidation (EO) using boron-doped diamond (BDD) electrode. Both 2-MIB and GSM could be degraded efficiently in sulfate electrolyte compared to inert nitrate or perchlorate electrolytes, implying that in-situ generated persulfate may be responsible for contaminants degradation. The observed linear relationship between 2-MIB (GSM) degradation rates and persulfate generation rates further proved that the in-situ generated persulfate enhanced 2-MIB (GSM) degradation. Moreover, a divided electrolytic cell was employed to investigate the effect of cathodic reactions on contaminants degradation and persulfate generation, and results confirmed that both anodic and cathodic reactions participated in 2-MIB (GSM) degradation. High current density and low solution pH were found to be favorable for 2-MIB and GSM degradation. The degradation intermediates were identified and the possible pathways of 2-MIB and GSM degradation were proposed. This study indicated that the EO process with BDD anode could be considered as a potential alternative for the removal of 2-MIB and GSM. © 2016 Elsevier Ltd. All rights reserved.

Handling Editor: E. Brillas Keywords: Electrochemical oxidation In-situ generated persulfate Taste & odor compounds Pathway

1. Introduction

* Corresponding author. Department of Water Engineering and Science, College of Civil Engineering, Hunan University, Changsha, Hunan, 410082, PR China. E-mail address: [email protected] (S. Zhou). http://dx.doi.org/10.1016/j.chemosphere.2016.11.134 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

In recent years, the presence of taste and odor (T&O) compounds in lakes and reservoirs has been a hot topic of public concern and drawn increasing attention around the world (Izaguirre et al., 1982; Antonopoulou et al., 2014). For water utilities,

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the most common complaints from customers are due to T&O compounds in their drinking water, especially in summer (Khiari, 2004; Srinivasan and Sorial, 2011). Among various kinds of T&O compounds, 2-methylisoborneol (2-MIB) and geosmin (GSM), two tertiary non-toxic and semi-volatile alcohols, were identified to be the major T&O compounds (Pirbazari et al., 1993), which can be generated by cyanobacteria or blue-green algae. During the cyanobacteria blooms, the peak concentrations of 2-MIB and GSM can even reach up to 1 mg L1, while the guideline value are set at 10 ng L1 for drinking water in China (Mizuno et al., 2011; Xie et al., 2015). The conventional water treatment processes (such as coagulation, sedimentation and filtration) can poorly remove 2-MIB and GSM from water supply source, and new methods towards efficient abatement of these compounds need to be explored as the odor threshold concentrations have been reported to be as low as 15 ng L1 for 2-MIB and 4 ng L1 for GSM, respectively (Young et al., 1996; Antonopoulou et al., 2014). Advanced oxidation processes (AOPs) such as photocatalysis, ultraviolet (UV), persulfate, and ozone-based process have been investigated to removal T&O compounds by many researchers (Peter and Von Gunten, 2007; Kutschera et al., 2009; Li et al., 2010; Mizuno et al., 2011; Xie et al., 2015). Among various kinds of oxidation processes, electrochemical oxidation (EO) has attracted
s et al., increasing interest for treating contaminated water (Sire 2014; Brillas and Martínez-Huitle, 2015; Radjenovic and Sedlak, 2015). Compared to common AOPs, EO process requires no extra oxidants and appears to be more eco-friendly, and can easily combined with other technologies (Anglada et al., 2009). In EO process, the electrode material plays a significant role for the removal efficiency of contaminants. Several electrodes including mixed metal oxides (MMO), Pt and carbon electrodes have been investigated in the previous studies (Bagastyo et al., 2011; El-Ghenymy et al., 2014). In recent years, the boron-doped diamond (BDD) electrode has received increasing attention for electrochemical oxidation because of its superior electro-catalytic capability to degrade persistent organic contaminants (Antonin et al., 2015; Farhat et al., 2015), presumably due to the presence of weakly adsorbed
OH formed at the anode surface as follows (Comninellis, 1994; Thiam et al., 2016).

BDD þ H2 O/BDDð· OHÞ þ Hþ þ e

(1)

Furthermore, BDD anodes may also form ozone, ferrate, chlorine and other peroxosalts in the presence of inorganic ions such as
s et al., 2014). chloride, sulfate and phosphate (Bergmann, 2010; Sire Sulfate solution is considered to be good supporting electrolyte because of its low cost, high efficiency, easy access, and nonhalogenated property. The relatively high removal efficiency of organic contaminants at BDD electrodes in the sulfate solution can be attributed to the formation of persulfate during the following ~ izares et al., 2009; Davis et al., 2014; Balaji et al., 2015; reaction (Can Zhu et al., 2016). 2 þ  2HSO 4  2e /S2 O8 þ 2H

(2)

2  2SO2 4  2e /2S2 O8

(3)

Unfortunately, the information regarding the removal of 2-MIB and GSM in electrochemical oxidation at BDD electrode is limited, and the role of cathodic reactions in EO was usually ignored in previous studies. In this study, the objectives are: (1) to investigate the effect of electrolyte, electrode material, current density and water quality parameters (solution pH and algal organic matters) on the 2-MIB and GSM degradation; (2) to compare the 2-MIB and GSM degradation in the divided and undivided cell; (3) to evaluate

the reaction mechanisms of 2-MIB and GSM degradation at BDD anode; (4) to identify the intermediates of 2-MIB and GSM in the EO process and propose possible degradation pathways. 2. Materials and methods 2.1. Materials All chemicals were at least of analytical grade except as noted and used as received without further purification. All solutions were prepared with ultrapure water, unless otherwise specified. Nhexane of HPLC grade, 2-MIB and geosmin were purchased from Sigma-Aldrich (St Louis, MO). Sodium sulfate, sodium nitrate, sodium perchlorate, sodium persulfate, potassium iodide, sodium bicarbonate, methanol (MeOH), and tertiary butanol (TBA) were obtained from Sinopharm Chemical Reagent Co. (Shanghai). Salt bridge used in divided system was laboratory-made. BDD/Nb electrodes (2500 ppm boron, 25  50  1 mm, bipolar, 5 mm coating thickness) were purchased from NeoCoat® SA, Switzerland. Mixed metal oxide electrodes (MMO, Ti/IrO2/Ta2O5) and Pt electrodes were supplied by Shanxi Kaida Chemical Ltd, China. 2.2. Experimental procedures Batch experiments were conducted open to the air and in a series of 100-mL borosilicate glass beakers. The anode and cathode were set in parallel at a distance of 2.0 cm and the total submerged area of electrodes was 6 cm2 in the electrolytic cell. A direct current source was used to supply power to the system. The experiments were performed with 4 mg L1 2-MIB and GSM, 30 mM electrolyte at different pH and current densities. To determine the concentrations of in-situ generated persulfate with time, we performed the experiments at different pH without added organic contaminants. To identify the primary reactive species formed in the electrochemical system, quenching experiments were conducted with the addition of radical scavengers (MeOH and TBA). To evaluate the effect of cathodic reactions on the degradation of contaminants, a divided electrolytic system was employed and compared with the undivided electrolytic system. For the divided system, two 100-mL borosilicate glass beakers were used to separate anodic and cathodic solutions. The two solutions were exactly the same as that used in undivided system, and a salt bridge was used to combine these two beakers. The distance and submerged area of two electrodes were also the same as undivided system. Algal organic matters (AOM) were extracted from Microcystis aeruginosa according to the method of Zhou et al. (2015a): M. aeruginosa cells in late exponential growth phase were harvested and centrifuged at 5000 r min1 for 10 min. The supernatants were subsequently filtered through 0.45 mm glassfiber membrane (Whatman), and the filtrates were referred as AOM. All electrochemical experiments were conducted at room temperature (25 ± 2  C). At each given time interval, samples were collected and mixed immediately with 0.1 mL Na2S2O3 (0.5 M) to quench the residual oxidants. All the experiments were conducted at least twice. The relative standard deviations (RSD) for different batches were normally less than 10%. 2.3. Analytical methods 2-MIB and GSM were quantified by liquid/liquid extraction with n-hexane followed by a GC-MS (7890A-5975C, Agilent, USA) using selective ion mode. The chromatograph was coupled with a HP-5 column (30 m  0.25 mm, ID  0.32 mm). The temperature program of oven began at 60  C for 3 min, ramped up to 145  C at 30  C

L. Bu et al. / Chemosphere 168 (2017) 1309e1316

min1, then ramped up to 180  C at 10  C min1 and held for 1 min, and finally reached up to 250  C at 40  C min1. The degradation products of 2-MIB and GSM were also determined using the GC-MS analyzer. The temperature program of oven was the same as that shown above. The concentrations of in-situ generated persulfate were determined via a spectrophotometric method using a UV-Vis spectrophotometer (Hitachi, U-3900, Japan) based on iodometric titration, as described by Liang et al. (2008). Standard solutions with different concentrations of persulfate were prepared in advance. 1 mL standard solution together with 0.125 g NaHCO3 (to avoid airoxidation of iodide) and 2.5 g KI were added into 25 mL pure water in a 25 mL-colorimetric tube. Then the solutions were hand shaken and allowed to equilibrate for 15 min. The analytical wavelength was fixed at 352 nm and the calibration concentrations ranged from 0.0 mM to 1.5 mM in this study. The total organic carbon (TOC) of AOM was measured using a TOC-V analyzer (Shimadzu, Japan). 3. Results and discussion 3.1. Effect of electrolytes and electrode materials Sulfate, nitrate, perchlorate, and chloride solutions were often used as supporting electrolytes in electrochemical oxidation of organic contaminants (Mascia et al., 2010; Farhat et al., 2015; JalifeJacobo et al., 2016). The formation of halogenated by-products may limit the application of electrochemical oxidation in the presence of chloride ions due to their high toxicity (Bagastyo et al., 2011; Zhou et al., 2014). In this study, sulfate, nitrate and perchlorate ions were chosen as the target electrolytes. The degradation of 2-MIB and GSM fit pseudo-first order kinetics well. As shown in Fig. 1a and Fig. SM-1a, GSM was found to be oxidized faster than 2-MIB with higher rate constants, which may be attributed to that GSM is more hydrophobic and volatile than 2-MIB (Song and O'Shea, 2007). Moreover, the degradation rate constants of 2-MIB and GSM in the sulfate electrolyte were 0.045 min1 and 0.058 min1 respectively, and showed a better efficiency than the other two electrolytes. It was probably because of the formation of persulfate and other reactive species and the detailed mechanisms would be discussed in the next section. Besides, different types of anodes (Pt, MMO electrodes) were frequently used in electrochemical oxidation (Jeong et al., 2009; Du et al., 2011; Isarain-Ch
avez et al., 2011). To make sure that the

1.20

3.2. Mechanisms of 2-MIB and GSM degradation by BDD anode 3.2.1. Relationship between 2-MIB (GSM) degradation and persulfate generation As discussed in subsection 3.1, efficient removal of 2-MIB and GSM was observed in the sulfate electrolyte. To investigate the role of the in-situ formed persulfate, the relationship between contaminants degradation and persulfate generation was evaluated under different solution pH. The rate constants of persulfate generation and 2-MIB (GSM) degradation in the electrochemical oxidation system were evaluated as shown in Table S1, Fig. 2 and Fig. SM-2. It was found that persulfate generation rates were linearly correlated with the contaminants degradation rates (R2 ¼ 0.999 and 0.988 for 2-MIB and GSM, respectively). Moreover, the kinetics of contaminants degradation and persulfate generation could be expressed as the following Eqs. (4) and (5).

ln

½T&Ot ¼ kc t ½T&O0

(4)

½PSt ¼ kPS t

(5)

The contaminants degradation rates increased in accordance with the persulfate generation rate, confirming that contaminants reacted with the oxidants in-situ formed on the anode. The results demonstrated that the in-situ formed oxidants were responsible for 2-MIB and GSM degradation and the persulfate generation was the rate-limiting step in the oxidation process. Furthermore, as presented in Fig. SM-3, the degradation rate of 2-MIB and GSM decreased with the increase of solution pH from 2.0 to 5.0. Meanwhile, an interesting phenomenon was observed during the experiments: the generation of bubble on the electrodes

1.20

sulfate

(b)

1.00

nitrate perchlorate

0.80

MIB (Ct/C0)

0.80

MIB (Ct/C0)

reactive species were generated via BDD anode, experiments were performed with different anodes and the results were illustrated in Fig. 1b and Fig. SM-1b. The removal rate of 2-MIB and GSM with BDD anode was much higher than that with Pt and MMO anode. The degradation rate was in the order of BDD > MMO > Pt, proving that BDD anode was responsible for 2-MIB and GSM removal. This phenomenon could be attributed to that MMO and Pt electrodes belong to active electrode (Comninellis, 1994), which have a low potential for O2 evolution, and exhibit low ability for organics degradation (Radjenovic and Sedlak, 2015).

(a)

1.00

1311

0.60

0.60

0.40

0.40

0.20

0.20

0.00

BDD anode

MMO anode Pt anode

0.00

0

5

10

15 20 Time (min)

25

30

35

0

5

10

15 20 Time (min)

25

30

35

Fig. 1. (a) 2-MIB degradation with different electrolytes; (b) 2-MIB degradation with different anode materials. Experimental conditions: Current density ¼ 5.0 mA cm2, [electrolyte]0 ¼ 30 mM, [2-MIB]0 ¼ 4 mg L1, pH ¼ 3.0, reaction time ¼ 30 min.

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0.06

0.00 R² = 0.971

MIB ln(Ct /C0)

0.04

-1.00

R² = 0.980 R² = 0.991

-2.00

pH=2 pH=3

R² = 0.996

pH=4 pH=5

-3.00 0

0.03

0.02

10

20

30 Time (min)

40

50

0.8 Persulfate (mM)

MIB Degradation rate (min -1 )

0.05

y = 0.302x - 0.002 R² = 0.999

0.01

0.6

0.4 0.2 0

0

10

0

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

20 Time (min)

0.16

30

0.18

0.2

Persulfate generation rate (mM·min -1 ) Fig. 2. Relationship between persulfate generation rates and 2-MIB degradation rates. Experimental conditions: Current density ¼ 5.0 mA cm2, [Na2SO4]0 ¼ 30 mM, [2MIB]0 ¼ 4 mg L1, pH ¼ 2.0e5.0, reaction time ¼ 15e45 min.

increased along with the solution pH. This was because that the potential for oxygen evolution at anode is relatively high in acidic medium, leading to a better degradation efficiency than that in neutral medium (Zhang et al., 2013).

3.2.2. Comparison of 2-MIB (GSM) degradation in divided and undivided cells Electrochemical oxidation of organics is ascribed to the anodic reactions in most cases and cathodic reactions are usually ignored (Martínez-Huitle et al., 2015). However, some researchers found that undivided cells with a BDD anode and an active cathode can enhance the removal rate of contaminants because of the in-situ formed H2O2 on the cathode via the following equation (Peralta
ndez et al., 2006; Dirany et al., 2012): Herna

O2 þ 2Hþ þ 2e /H2 O2

(6)

Similarly, it was assumed that the following reaction may occur on the cathode during oxidation in the sulfate electrolyte: 2 þ  O2 þ 2SO2 4 þ 4H þ 2e /S2 O8 þ 2H2 O

(7)

Therefore, 2-MIB and GSM degradation in divided cell were conducted as reference experiments to investigate the effect of cathodic reactions. As depicted in Fig. 3a and Fig. SM-4, the removal rate of 2-MIB and GSM was 63% and 71% in the anodic compartment, decreased by 31% and 28% compared to that in undivided cell, and the contaminant removal in the cathodic compartment was negligible. Since in-situ formed persulfate was responsible for 2MIB and GSM degradation, the persulfate concentrations in the anodic and cathodic compartments were also evaluated. As expected, concentrations of in-situ formed persulfate in undivided cell and anodic compartment were 0.77 and 0.54 mM, respectively, and no persulfate was detected in the cathodic compartment. Therefore, persulfate in the undivided cell should be generated from two pathways as shown in Eqs. (3) and (7). The proposed mechanism was presented in Fig. 4.

2  Anode: 2SO2 4 /S2 O8 þ 2e (3) 2 þ Cathode: O2 þ 2SO4 þ 4H þ 2e /S2 O2 8 þ 2H2 O (7)

3.2.3. Identification of reactive species in the EO process Since persulfate was in-situ formed during the oxidation process, many researchers proposed that sulfate radicals were generated and played significant roles in degrading organic contaminants. In this study, quenching experiments were conducted to confirm reactive species. As illustrated in Fig. 5a and Fig. SM-5a, when 10 mM MeOH or TBA were added into the solution, two curves of 2-MIB degradation almost coincided (GSM degradation appeared the same trend), suggesting that sulfate radicals were not generated. Similar phenomenon was also observed by Farhat and his colleagues (Farhat et al., 2015). As a reference experiment, 0.5 mM persulfate was added to the EO system with nitrate electrolyte (10 mM). 68% 2-MIB and 73% GSM were removed in 30 min, which was significantly enhanced compared to that with nitrate electrolyte (30 mM). Therefore, persulfate could be activated to produce reactive species at BDD anode. Quenching experiments with 10 mM MeOH or TBA were also performed and similar results were obtained as shown in Fig. 5b and Fig. SM-5b, further refuting the existence of sulfate radicals. Based on the aforementioned results, other reactive species (e.g., superoxide and singlet oxygen) may generate during the EO process as the following Eqs. (8) and (9) (Zhang et al., 2014; Zhou et al., 2015b).

2· OH þ S2 O2 8

BDD þ 1 2SO2 4 þ O2 þ 2H /

(8)

4· OH þ S2 O2 8

BDD $ þ 2SO2 4 þ 2O2 þ 4H /

(9)

3.3. Effect of some other parameters 3.3.1. Effect of current density Since current density is a key influence factor in the EO process, the effect of current density on 2-MIB and GSM degradation at BDD

L. Bu et al. / Chemosphere 168 (2017) 1309e1316

1.20

(a)

(b)

0.8

1.00

1313

undivided cell anodic compartment

MIB (Ct/C0)

Persulfate (mM)

undivided cell

0.80

anodic compartment 0.60

cathodic compartment

0.40

cathodic compartment

0.6

0.4

0.2

0.20 0.00

0

0

5

10 Time (min)

15

0

5

10 Time (min)

15

20

Fig. 3. (a) Comparison of 2-MIB degradation; (b) persulfate generation in undivided cell, anodic compartment and cathodic compartment. Experimental conditions: Current density ¼ 5.0 mA cm2, [Na2SO4]0 ¼ 30 mM, [2-MIB]0 ¼ 4 mg L1, pH ¼ 2.0, reaction time ¼ 15 min.

Fig. 4. Proposed mechanism for persulfate generation in the system.

1.20

1.20

(a) control

1.00

TBA

MIB (Ct/C0)

0.60

0.40

0.40

0.20

0.20

0.00

0.00

5

10

15 20 Time (min)

25

30

MeOH TBA

0.80

0.60

0

with persulfate

1.00

MeOH

0.80

(b)

35

0

5

10

15

20

25

30

35

Time (min)

Fig. 5. (a) 2-MIB degradation using BDD anode with the presence of MeOH and TBA; (b) 2-MIB degradation in BDD/persulfate process with the presence of MeOH and TBA. Experimental conditions: Current density ¼ 5.0 mA cm2, [2-MIB]0 ¼ 4 mg L1, [MeOH] ¼ [TBA] ¼ 10 mM, pH ¼ 3.0, reaction time ¼ 30 min (a) [Na2SO4]0 ¼ 30 mM; (b) [NaNO3]0 ¼ 10 mM, [Na2S2O8]0 ¼ 0.5 mM.

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1.20

1.20

(a)

1.00

control 2 mg-C L-1

1.00

5.0 mA cm -2

4 mg-C L-1

7.5 mA cm -2

0.80

0.80

10.0 mA cm -2

MIB (Ct/C0)

MIB (Ct/C0)

(b)

2.5 mA cm -2

0.60

0.60

0.40

0.40

0.20

0.20

0.00

6 mg-C L-1

0.00

0

5

10

15 20 Time (min)

25

30

35

0

5

10

15 20 Time (min)

25

30

35

Fig. 6. (a) 2-MIB degradation with different current densities; (b) 2-MIB degradation with different AOM concentrations. Experimental conditions: Current density ¼ 2.5e10.0 mA cm2, [electrolyte]0 ¼ 30 mM, [2-MIB]0 ¼ 4 mg L1, [AOM]0 ¼ 0e6 mg-C L1, pH ¼ 3.0, reaction time ¼ 30 min.

anode was investigated with current density ranging from 2.5 mA cm2 to 10 mA cm2 and the results were presented in Fig. 6a and Fig. SM-6a. As expected, 2-MIB and GSM degradation

were current-dependant and the increase of current density resulted in higher removal rate of organic contaminants. Specifically, 2-MIB (GSM) degradation rate was 53% (56%), 66% (82%), 81%

Fig. 7. (a) Proposed pathway of 2-MIB after electrochemical oxidation; (b) proposed pathway of GSM after electrochemical oxidation.

L. Bu et al. / Chemosphere 168 (2017) 1309e1316

(89%), 88% (99%) when the current density was fixed at 2.5, 5.0, 7.5, 10.0 mA cm2, respectively. This could be explained from two aspects: One is that persulfate and other reactive species increased along with current density, and the other is that a higher current density results in the increase of the oxygen evolution potential (Czarnetzki and Janssen, 1992). 3.3.2. Effect of AOM As 2-MIB and GSM are derivative byproducts of cyanobacteria, different dosages of AOM (0e6 mg-C L1) were added in the solution to simulate the actual conditions and elucidate their effects on the degradation of 2-MIB and GSM. As shown in Fig. 6b and Fig. SM-6b, 2-MIB (GSM) degradation rate exhibited a decreased trend and was 79% (82%), 69% (74%), 68% (75%), 66% (72%) when AOM concentration was fixed at 0, 2, 4 and 6 mg-C L1, respectively. This is because that AOM acted as a scavenger and played a competitive role in the oxidation process. AOM can compete with 2-MIB and GSM and reduce the steady-state concentration of reactive species. However, removal of 2-MIB and GSM using BDD anode could still be deemed as a promising technology since AOM of actual water sources were usually around 4 mg-C L1. 3.4. Proposed pathways of 2-MIB and GSM degradation The degradation intermediates of 2-MIB and GSM during the EO process were investigated in this study. The extracted chromatograms were shown in the supporting information (Fig. SM-7 and Fig. SM-8) and the possible pathways were illustrated in Fig. 7. As presented in Fig. 7a, part of 2-MIB were directly transformed to P1 and P2 by elimination reaction via dehydration, while others were degraded to ketone-derivatives (P3) by b-scission. Then, P2 was further oxidized to alcohol-derivatives (P4) by addition reaction. These products could be subsequently oxidized to other intermediates with smaller molecular weight (Fig. 7a) (Fotiou et al., 2014). Moreover, with the ring opening, all the organics would be converted to inorganics, such as CO2 and H2O. Furthermore, two preliminary pathways of GSM degradation were also proposed (Fig. 7b). The formation of ketone-derivatives (P5) was owing to ahydrogen abstraction and b-scission on GSM. P5 was further oxidized to P6 via an elimination reaction. Besides, GSM could also convert to P7 through dehydration and addition reaction. Then, P8 and P9 were formed with the destruction of the hexatomic ring. Finally, complete mineralization to CO2 and H2O occurred in the EO process. 4. Conclusions In this study, the performance and mechanisms of 2-MIB and GSM degradation by the EO process using BDD anode were investigated and the following conclusions can be drawn. (a) BDD anode showed better degradation efficiency of 2-MIB and GSM than other active anodes such as MMO and Pt, and the contaminants degradation in the sulfate electrolyte was faster than that in the nitrate and perchlorate electrolytes. (b) In-situ formed persulfate in the EO process was found to be responsible for 2-MIB and GSM degradation. Both anodic and cathodic reactions participate in the persulfate generation. Hydroxyl radical and some other reactive species (not sulfate radical) are assumed to be generated during the EO process. (c) High current density, low solution pH and AOM concentration were favorable for 2-MIB and GSM degradation at BDD anode.

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(d) Some degradation intermediates were identified and possible pathways of 2-MIB and GSM degradation in the EO process were proposed. Acknowledgement This work was financially supported by the National Natural Science Foundation (51508174). Appendix A. Supplementary data Supplementary data related to this article can be found at 10. 1016/j.chemosphere.2016.11.134. References Anglada, A., Urtiaga, A., Ortiz, I., 2009. Contributions of electrochemical oxidation to waste-water treatment: fundamentals and review of applications. J. Chem. Technol. Biotechnol. 84, 1747e1755. Antonin, V.S., Santos, M.C., Garcia-Segura, S., Brillas, E., 2015. Electrochemical incineration of the antibiotic ciprofloxacin in sulfate medium and synthetic urine matrix. Water Res. 83, 31e41. Antonopoulou, M., Evgenidou, E., Lambropoulou, D., Konstantinou, I., 2014. A review on advanced oxidation processes for the removal of taste and odor compounds from aqueous media. Water Res. 53, 215e234. Bagastyo, A.Y., Radjenovic, J., Mu, Y., Rozendal, R.A., Batstone, D.J., Rabaey, K., 2011. Electrochemical oxidation of reverse osmosis concentrate on mixed metal oxide (MMO) titanium coated electrodes. Water Res. 45, 4951e4959. Balaji, S., Muthuraman, G., Moon, I.S., 2015. Influence of cathode on the electrogeneration of peroxydisulfuric acid oxidant and its application for effective removal of SO2 by room temperature electro-scrubbing process. J. Hazard. Mater 299, 437e443. Bergmann, M.E.H., 2010. Drinking Water Disinfection by In-line Electrolysis: Product and Inorganic By-product Formation. Electrochemistry for the Environment. Springer, pp. 163e204. Brillas, E., Martínez-Huitle, C.A., 2015. Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. an updated review. Appl. Catal. B Environ 166, 603e643. ~ izares, P., Sa
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