Ti electrode

Environmental Technology & Innovation 15 (2019) 100382

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Environmental Technology & Innovation journal homepage: www.elsevier.com/locate/eti

Comparison on efficiency of electrochemical phenol oxidation in two different supporting electrolytes (NaCl and Na2 SO4 ) using Pt/Ti electrode Johanna Zambrano, Booki Min

∗

Department of Environmental Science and Engineering, Kyung Hee University, Seocheon-dong, Yongin-si, Gyeonggi-do 446-701, Republic of Korea

highlights • • • • •

Na2 SO4 electrolyte did not interfere with the electrochemical degradation of phenol. Chloride was an important parameter for the efficient oxidation of phenol. The presence of chloride ions seems to inhibit oxygen-evolution reaction. Basic pH conditions could reduce the formation of halogenated by products. Phenol oxidation with NaCl consumes less energy and has higher TCE than with Na2 SO4 .

article

info

Article history: Received 12 March 2019 Received in revised form 3 May 2019 Accepted 5 May 2019 Available online 9 May 2019 Keywords: Electrochemical oxidation Phenol Electrolytes Chloride Sulfate Pt/Ti electrode

a b s t r a c t The electrochemical oxidation of phenol was evaluated with two different electrolytes using a Pt/Ti anode electrode in a single-chambered reactor. The bulk electrolysis was conducted at a fixed current density of 9.6 mA cm−2 , and the average applied potential difference was 6 V (anode potential 4 V; cathode potential -2 V). The feasibility of electrochemical treatment of phenol contaminated water was determined in terms of phenol degradation rate, byproduct formation, pH stability, and current efficiency. Galvanostatic electrolysis with NaCl electrolyte removed phenol and chemical oxygen demand (COD) faster than with Na2 SO4 electrolyte, possibly due to the formation of chloride–oxychloride radicals, which are strong oxidation agents. The phenol and COD removal rates with NaCl were 2.26 × 10 −2 and 3.00 × 10−4 min−1 , which was 9.5 and 1.5 times higher respectively than with Na2 SO4 . Energy consumption and current efficiency during the electrolysis suggested that chloride ions hinder the oxygen-evolution reaction, causing a growth in current efficiency and a decrease in energy consumption. This result clearly indicated that the presence of chloride in the solution was an important parameter for the efficient electrochemical oxidation of phenol-contaminated water. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Phenolic pollutants are considered as one of the most toxic organic compounds, which widely exist in industrial wastewater. Industries, such as oil refineries, coal conversion plants, petrochemicals, polymeric resins, coal tar distillation, ∗ Corresponding author. E-mail address: [email protected] (B. Min). https://doi.org/10.1016/j.eti.2019.100382 2352-1864/© 2019 Elsevier B.V. All rights reserved.

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and pharmaceuticals, are found to be the sources of phenolic pollutants (Iniesta et al., 2002; Ma et al., 2014; Wu et al., 2001). Nowadays, the treatment of phenolic wastewater has become considerably important due to their carcinogenicity, toxicity, mutagenicity and teratogenicity. Furthermore, low concentration (2 mg l−1 phenol) are known to be toxic for aquatic-life, resulting in death within 96 h when concentrations reach between 10 and 100 mg l−1 (Li et al., 2005; Ma et al., 2014; Rajkumar and Palanivelu, 2004; Wu et al., 2001). Treatment technologies available for phenolic contaminants are biological, chemical, physical, and electrochemical processes. Coagulation, adsorption, and chemical oxidation are found to be ineffective due to low efficiency, high cost, and secondary pollution issues. Although biodegradation would be highly cost-efficient, phenol-containing wastewater has been found to be refractory in many cases since phenolic pollutants inhibit the regular function of microbial populations, thus affecting the performance of biological treatment process (Jin et al., 2014; Li et al., 2005; Ma et al., 2014; Rajkumar and Palanivelu, 2004). On the other hand, the electrochemical process has attracted considerable research attention due to its easy control, environmental compatibility, robust oxidation performance and simplicity (Cabeza et al., 2007; Flox et al., 2009; Hastie et al., 2006; Wang et al., 2009; Yavuz et al., 2010). Moreover, the coexisted ions present in the industrial wastewaters and its higher conductivity are beneficial to enhance the removal of organic contaminants and reduce the cost of the electrochemical oxidation process (Duan et al., 2013; Ma et al., 2014). Chloride and sulfate ions are often present in a wide range of natural waters and industrial wastewater, making possible the participation of active chlorine and sulfate compounds during electrochemical treatment, which is used as powerful oxidants for degrading biorefractory organic compounds effectively (De Paiva Barreto et al., 2014; Shiying et al., 2009). Zhou et al. (2011) compared the electrochemical degradation of methyl orange by using a mixed metal oxide (MMO) and a boron-doped diamond (BDD) electrode with sodium sulfate (Na2 SO4 ) and sodium chloride (NaCl) as supporting electrolytes. In both cases, the presence of NaCl increased significantly the decolonization and mineralization. However, the direct comparison between two electrolytes for electrochemical phenol treatment was not conducted at identical operational conditions. Therefore, in this study, we evaluated comparatively the treatment efficiency of phenol with two different electrolytes of Na2 SO4 and NaCl in synthetic wastewater. Ti anodes coated with Pt were used as a working electrode to degrade phenol pollutants. Dimensionally stable anodes (DSAs), constituted of a Ti coated by noble metals improve catalytic activity towards oxygen and chlorine evolution reactions (De Paiva Barreto et al., 2014). Chemical oxygen demand (COD) changes as a function of time and gas chromatography coupled with mass spectrometry (GC/MS) were investigated to determine byproduct transformation, and electrochemical analysis were used to evaluate phenol oxidation on the electrode in the presence of two electrolytes. 2. Materials and methods 2.1. Electrochemical reactor setup The single-chambered reactors were operated for the electrochemical degradation of phenol. Reactors consisted of a 250 ml glass bottle sealed by rubber caps with two open slots for the electrodes assembled. A platinized titanium (Ti) mesh electrode of 6.25 cm2 , was used as the anode and a Ti mesh with the same area as the cathode. Electrodes were separated 2 cm from each other (Fig. S1). Electrodes were supplied by William Gregor Ltd (London, United Kingdom). A DC potentiostat with a cell potential (difference) range of 0–36 V or a current of 0–3 A was employed as a power supply for system. 2.2. Reactor operations For the electrolysis test, phenol-containing solution (200 ml) each reactor was added with an initial concentration of 100 mg l−1 . Different concentrations of Na2 SO4 (10 g l−1 ) or NaCl (10 g l−1 ) were used as the supporting electrolyte. Additionally, the solutions were adjusted to different pH using buffer solution; pH 3 H3 PO4 / KH2 PO4 , pH 5 and pH 7 phosphate buffer, pH 9 Na2 HPO4 •7H2 O/NaH2 PO4 •H2 O. A DC current of 0.12 A was applied to the electrodes, which resulted in a current intensity of 9.6 mA cm−2 . For continuous mixing the EC cell was placed on a magnetic stirrer, and the temperature stayed at 30 ◦ C. Samples were taken from the electrolytic cell at various intervals for chemical analysis of pH, phenol, chemical oxygen demand (COD), and intermediate products. Experiments were replicated three times to maintain consistency in the obtained results. In the figures, error bars represent one standard deviation of the three replicate measurements. 2.3. Chemical analysis and calculations Phenol concentrations, pH variation, COD degradation and current profiles were defined as main parameters to assess the present study. Phenol and COD concentration were measured according to the standard methods (APHA, 2005) using COD and Phenol kits (HUMAS Co., Korea). pH was measured using a pH/conductivity meter (Thermo scientific, USA). Output potential difference and cathode potential were monitored with a data acquisition system (Keithley 2700, Keithley Instruments, US). The current was loaded by an external resistor of 10 Ω .

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In order to estimate the total Current efficiency (TCE), the following relationship was used:

( %TCE = FV

CODo − CODf 8I ∆t

) x100

(1)

where F the Faraday constant (96,487 C mol−1 ), V the volume (l), COD0 and CODf are chemical oxygen demands at initial times t = 0 and final time f in g O2 l−1 , respectively; 8 is the oxygen equivalent mass (g eq.−1 ), I the current (A), and ∆t is the total time of the electrochemical process (Rocha et al., 2012). Additionally, the energy consumption per volume of treated effluent was estimated as follows: Energy consumption =

∆Ec x I x t 3600 x V

(2)

where ∆Ec (V) and I (A) are the averages applied potential difference and the electrolysis current, respectively; t is the time of electrolysis (s); and V is the sample volume (dm3 ) (Rocha et al., 2012). Gas chromatography coupled with mass spectrometry (GC/MS) and gas chromatography (GC) were used for the measurement of the organic acids that were produced by the phenol electrolysis. To allow the GC measurement of the organic acids, the samples were extracted following the procedure described by Jones et al. (1993). Extraction was performed by adding 2 ml of ethyl ether to 1 ml of sample in a centrifuge tube which was then vortexed. 1.5 g for NaC1 was added to the samples resulting in a small amount of solid remaining after vortexing. 60 µL of 2.4 N HCl was added prior to the addition of solvent, to adjust the pH to approximately 1. The mixture was vortexed in order to separate the supernatant liquid using a Pasteur pipet. Another 1 ml of the extracting solvent was added, the mixture vortexed, and the supernatant removed. Repeating this 1 ml extraction one more time giving a total extraction solvent volume of approximately 4 ml. The GC (HP6890, Agilent) used a capillary column (HP-624, 25.0 m × 200 µm × 1.12 µm, Agilent) followed by detection with an MS (Netwaik 5973, Agilent). The temperature program began at 70 ◦ C and increased at a rate of 20 ◦ C min−1 up to 230 ◦ C, with a holding time of 2 min for each increment. The carrier gas was Helium at a constant flow rate of 53.6 ml min−1 . 1 ml of samples were injected in the splitless mode. Intermediate organic acids, such as maleic acid, malonic acid, acetic acid, succinic acid, and oxalic acid, were quantified by another GC (HP6890, Agilent) equipped with a capillary column (HP-4, 30.0 m x 320 µm x 0.25 µm, Agilent) and a flame ionization detector (FID) was used. The temperature was increased from 70 ◦ C at 10 ◦ C min−1 up to 180 ◦ C and then at 40 ◦ C min−1 up to 260 ◦ C with a holding time of 2 min for each increment. The helium carrier gas had a flow rate of 13.7 ml min−1 . The samples were injected in the splitless mode, and each injection was 1 ml (Li et al., 2005). Chemicals were of reagent grade, and water solutions were prepared by DI water. 2.4. Cyclic voltammetry A three-electrode cell was used to conduct the cyclic voltammetry experiments, with Ti chosen as the counter electrode, and Ag/AgCl (197 mV vs. SHE) reference electrode (Bio-analytical Systems Inc., USA). CV curves of the testing anodes were performed under galvanostatic conditions using a VERSTAT3 (Princeton Applied Research). The CV analyses were carried out in a potential range of −1.5 V to 1.5 V with a scan rate of 100 mV s−1 . This potential range (−1.5 V to 1.5 V) was successfully used in other studies carried out by Feng and Li (2003). 3. Results and discussion 3.1. Electrolyte influence on electrochemical oxidation of phenol Electrochemical oxidation of phenol was conducted in two different reactors with NaCl and Na2SO4 respectively as electrolytes and 0.12 A as applied current to comparatively evaluate their effect on phenol removal in wastewater (Fig. 1A). In solutions containing NaCl, active chloro-species such as chlorine, hypochlorous acid or hypochlorite ion might be generated on anode surface and consequently oxidize organic matter (De Paiva Barreto et al., 2014; Rajkumar et al., 2005; Ramalho et al., 2010). While electrolysis in aqueous media containing sulfate ions generates peroxodisulfate (S2 O8 2− ), under thermally enhanced conditions (temperature 30–100 ◦ C), S2 O8 2− can be converted into activate persulfate and thus to produce sulfate free radicals, which are powerful oxidants to degrade biorefractory organic compounds such as phenol (Dahmani et al., 2006; Liang et al., 2008; Shiying et al., 2009). Using NaCl as supporting electrolyte 98.21 ± 3.10% of phenol removal was reached within 3-h treatment meanwhile, by using Na2 SO4 as supporting electrolyte, complete phenol removal was achieved at 24-h (Fig. 1B). Complete and faster removal of phenol was observed when NaCl was present as electrolyte compared to operation with Na2 SO4 as electrolyte, probably due to a successful generation of active chlorines. In this study, temperature inside the reactors reached around 30 ◦ C, however this condition could not be enough to produce the theoretical sulfate free radicals when Na2SO4 is used as electrolyte. Sulfate was measured along the first 24-h treatment and no change in its initial concentration (Ci ) was observed (Ci ≈ 6700 mg l−1 ); therefore, in solutions containing Na2 SO4 , the electrolyte did not interfere in the oxidation process of phenol. Hence, it is possible to conclude that oxidation is mainly caused by hydroxyl radicals electrogenerated from water discharge at the anode (Panizza et al., 2007). On the other hand, a decrease in chloride concentration was observed during phenol degradation (Ci ≈ 5800 mg

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Fig. 1. Influence of chloride on the electrochemical degradation of phenol on Ti/Pt anode. (A) Phenol and COD degradation (B) Phenol and COD removal.

l−1 , Cf ≈ 3900 mg l−1 ), which portrays that in NaCl solutions chloride ions oxidation occurred (Mascia et al., 2010). The oxidation of chloride ions gave a bulk contribution to the electrochemical treatment with formations of HOCl or OCl− making the phenol degradation reaction faster. The first order constant from the galvanostatic electrolysis of phenol in presence of 10 g l−1 of NaCl were 2.26 × 10−2 ± 5.49 × 10−3 min−1 which is 9.5 times higher than those obtained with 10 g l−1 of Na2 SO4 (2.37 × 10−3 ± 1.15 × 10−4 min−1 ) (Fig. 2A). Present results are in concordance with the findings of Panizza et al. (2007) and Zhou et al. (2011), they found that the oxidation of organic contaminants in the presence of NaCl is faster due to the formation of chloride ions. After 168-h operation, 82.81 ± 2.00% of COD removal was reached using Na2 SO4 as supporting electrolyte, while 96.15 ± 0.26% COD removal was achieved with the presence of NaCl electrolyte (Fig. 1B). The COD removal rate was divided into two phases (Fig. 2B): removal rate while phenol was present in the solution and removal rate without phenol present in the solution. The trend of phenol and COD removal followed a pseudo-first-order kinetics in both Na2 SO4 and NaCl electrolytes. For both phases, a faster removal rate was observed in solutions containing NaCl. As mentioned previously oxidation mediated by chloride ions might be the reason for a faster COD removal when NaCl is used as the electrolyte, pH variation during the electrochemical oxidation of phenol support this statement. The initial pH in both cases was 5.8; in the solution with Na2 SO4 pH decreased until 4.4 after 7-h treatment and remained around this pH during the whole operation. The drop of pH might be caused by the production of hydrogen cations generated from water electrolysis as well as the gradual consumption of hydroxyl anions (Li et al., 2009). These results portray that phenol and COD removal is owed to the oxidation of organic compounds by hydroxyl radicals. Meanwhile, in the solution containing NaCl pH raised until 7.8 after 24-h treatment and remained around this pH until complete COD removal, suggesting that HClO had a role in the oxidation of organic compounds. It is found that in an electrochemical process mediated by oxidation with active chlorine species, the predominant species in the pH range of 3–8 is HClO, which is the strongest oxidizing agent of the

J. Zambrano and B. Min / Environmental Technology & Innovation 15 (2019) 100382

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Fig. 2. Kinetic analysis for pseudo-first-order reaction for (A) phenol and (B) COD removal during electrochemical oxidation. Two linear regressions were separately conducted for each data set.

chlorine oxyanions (Boucher, 1989; Körbahti and Artut, 2010; Sirés et al., 2014). This behavior has been reported by other authors, phenol degradation in the presence of Na2 SO4 show a drop of pH around 3 (Li et al., 2005; Rajkumar et al., 2005). While, regardless its initial pH, phenol degradation in the presence of chlorine found a final pH around 8 (Rajkumar et al., 2005; Scialdone et al., 2009). To determine if the removal rate between different electrolytes is statistically significant, ANOVA analysis was used. Relying on statistical analysis, for the conditions used in this study, the electrolyte has a significant influence on the reaction rate for phenol or COD removal. A comparative analysis of specific reaction rates previously reported in the literature for electrochemical treatment studies showed that the results obtained in the present study had the same order of magnitude with those of literature (Table 1). Ma et al. (2014) using a Ti/IrO2 -RuO2 electrode and Na2 SO4 as electrolyte reported a rate constant of 1.29 × 10−4 min−1 , for 90% of COD removal. Still, its operation time was three times higher than the one used in this study. Similarly, Mascia et al. (2010) using BDD electrode and NaCl as electrolyte obtained a rate constant of 1.23 × 10−4 min−1 , for 99% of COD removal. Despite using only 7-h treatment, BDD energy consumption was 27 times higher than the one used by Ti-Pt electrode under the present study conditions. Duan et al. (2013) with a Ti/PbO2 electrode achieved complete phenol removal after 3-h treatment using Na2 SO4 as supporting electrolyte. However, the current density used was 5 times higher than the one applied in the present study. As well as, for phenol removal using 30 mA cm−2 and NaCl as supporting electrolyte De Paiva Barreto et al. (2014) reported a similar rate constant of 2.00 × 10−2 and 2.70 × 10−2 min−1 using Ti/IrO2 and Ti-Pt-SnO2 -SbO5 , respectively. Belhadj Tahar and Savall (2009) using NaOH as electrolyte obtained complete phenol removal with similar reaction rates as the present study. However, the current density was 20 times higher (200 mA cm−2 ). In addition, the use of electrodes coated with Pb oxides causes

6

Electrode

Phenol (mg l−1 )

Supporting electrolyte

pH

Ti/IrO2 Pt

900–1000

NaOH

13

5

Na2 SO4

–

TiO2 /ACF graphite

160

Na2 SO4

6.2

Ti/IrO2 -RuO2

5000

Na2 SO4

–

100

NaCl

7

3

COD

Phenol

100

–

7.00 × 10−3 1.10 × 10−2

24

–

3.67 × 10−2 1.15 × 10−2

16

100

NaCl

5.5–6.2

Na2 SO4 NaCl

6

36

55

73

75

90

80

98

3.00 × –

7.00 × 1.10 × 1.16 × 2.50 ×

–

100

47.03

83.26

–

3.06 × 10−2

120

22

25

336

65

90

3.10 × 10−5 6.85 × 10−5

–

504

90

100

1.29 × 10−4

6.7

99

–

1.23 × 10−4

2

22.5 46.2 58.6 34 86.7 71.59

–

COD 168

82.81

58,31

Phenol 7

96.15

100

6.5–6.8

Current density

Energy consumption (kWh dm−3 )

Reference

(mA cm−2 )

200

–

Serpone et al. (1993)

2.71 2.42

–

Flox et al. (2009)

10 7.6

2.15

20 30 40

1.77

50 –

7.14

–

Dahmani et al. (2006)

3

–

–

Boucher (1989)

4.5

5

0.002

Radjenovic and Sedlak (2015)

3.5

10 20 30 10 20 30

36 42 69 28 40 54

6

9.6

35.7 47.1 1.8 – 2.00 × 10−2

– 2.70 × 10−2 1.05 × 10−4 2.80 × 10−4

Cell potential (V)

36 3

90

Ti-Pt-SnO2 -SbO5

100

30

22

10−3 10−3 10−2 10−2 10−2

1

Ti/IrO2

Ti-Pt

Phenol

4

50

Current efficiency (%)

COD

2

Nb/β -PbO2

BDD

Rate Constant (min−1 )

Removal (%)

7

Ta/β -PbO2

Ti/ PbO2

Time (h)

2.35 × 10−3 2.26 × 10−2

– 59 39 26 72 56 49 0.65 0.74

Iniesta et al. (2002)

COD 0.168

Present

Phenol 0.007

study

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Table 1 Comparative analysis of Phenol and COD removal on Ti/Pt anode with studies previously reported in the literature.

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concern about toxicity issues due to possible Pb leaching. Therefore, present findings show that Pt/Ti electrode can show a better performance than titanium coated electrodes and use less energy than BDD, making them a feasible option for industrial use as it can achieve similar rate constants using lower current densities. 3.2. Phenol degradation and byproduct formation It was observed that, byproducts formation caused a difference between phenol and COD removal rates under same electrolyte condition. When NaCl was used as supporting electrolyte, phenol removal rate reported two orders of magnitude which is higher than COD removal rate. On the contrary, for Na2 SO4 solutions, phenol removal rate was only one order of magnitude higher than the COD removal rate. To understand this difference GC analysis for byproducts detection was performed during first-hour treatment. GC analysis showed that in presence of NaCl only hydroquinone was formed as a first step of phenol degradation. Hydroquinone is a hydrophilic species that will react with OH radicals generated at the anode surface and in a large extent with the OH radicals and the HClO present in the solution bulk (Li et al., 2005; Mascia et al., 2010; Serpone et al., 1993). On the other hand, when Na2 SO4 is used as supporting electrolyte, the first step of phenol degradation shows the presence of hydroquinone and benzoquinone. Benzoquinone is and hydrophobic species that would react at the hydrophobic gas bubble/liquid interface only with OH and H radicals making the phenol degradation rate slower in the presence of Na2 SO4 (Serpone et al., 1993). GC analysis showed the formation of hydroquinone at first-hour treatment for both conditions studied in present research, which is an intermediate in phenol oxidation when OH is the responsible for the degradation, as reported by several authors (Duan et al., 2013; Enache and Oliveira-Brett, 2011; Li et al., 2005). However, solutions containing NaCl have an additional oxidizing agent thanks to the action of active chlorine species. Which gives a higher degradation performance to solutions containing NaCl. Additionally, GC/MS analysis was performed to detect the byproducts formed during the complete electrochemical degradation of phenol in order to determine whereas there is a difference in the oxidation pathway when different electrolytes are used. Aromatic ring opening is a limiting step to complete electrochemical phenol oxidation, which was successfully achieved by solutions with NaCl after one-hour treatment (Li et al., 2005). At three-hour treatment, solutions containing NaCl did not show more presence of hydroquinone (Fig. 3A), while when Na2 SO4 is used as electrolyte hydroquinone was present until 24-h treatment (Fig. 3B). In both conditions, results showed the formation of polymers. Li et al. (2005) proposed a pathway of phenol oxidation in presence of Na2 SO4 as supporting electrolyte where without the strong oxidizing power of free radicals, hydroquinone and benzoquinone were accumulated, leading to the formation of polymeric products that are more refractory than phenol to the electrochemical oxidation process. However, the present study found the formation of polymers produced from simple alkenes as monomers, polyethylene, and polypropylene. These polymers showed to be degraded by the electrochemical process as they decreased their concentration as operational time increased. On the other hand, the electrochemical degradation of phenol, when chloride is present in water, could produce halogenated byproducts that are often more toxic and persistent than the parent compound (Radjenovic and Sedlak, 2015). GC/MS analysis showed the formation of chloroform and 1,2dichloroethane when NaCl was used as supporting electrolyte (Fig. 3B). However, under the conditions used in the present study, electrochemical treatment using Pt/Ti electrode succeed to degrade both halogenated byproducts. 3.3. Cyclic voltammetry between two electrolytes for phenol degradation The cyclic voltammograms with 100 mg l−1 phenol were obtained in the presence of NaCl and Na2 SO4 electrolytes. Solutions containing phenol and electrolyte showed an anodic peak current about 0.9 (vs. Ag/AgCl)/V corresponding to the oxidation of phenol (Fig. 4). This finding pointed out that phenol could be directly oxidized on the Pt/Ti electrode (Duan et al., 2013). The oxygen evolution potential for NaCl was higher than the one shown for Na2 SO4 , 1.1 and 0.8 (vs. Ag/AgCl)/V respectively. The increase of the potential at which oxygen evolution occurs may be due to the presence of chloride; chloride ions seems to inhibit the oxygen-evolution reaction. Higher oxygen evolution potential would extend the lifetime of the hydroxyl radicals and favor the production of active chlorine species (De Paiva Barreto et al., 2014; Li et al., 2005; Ramalho et al., 2010; Rocha et al., 2012), which as mentioned on the previous sections are responsible for the faster oxidation of organic compounds in the electrolytic degradation of phenol using NaCl. In addition, the maximum anodic current of 110 mA cm−2 was obtained at 1.48 V for solutions containing NaCl. While for Na2 SO4 electrolyte the maximum anodic current was 30 mA cm−2 at 1.5 V. This suggests that the electrocatalytic performance of Pt/Ti electrode is enhanced when NaCl is used as the electrolyte. Several authors have studied Ti-based multilayer metal-oxide electrodes for electrochemical treatment, RuO2 and IrO2 have been the preferred coatings since these materials exhibit the lowest overpotentials for the oxygen evolution reaction. Nevertheless, the high cost and low natural abundance of these materials reduce their widespread commercial utilization (Doyle and Lyons, 2016). The present study by using Pt/Ti electrode, which has industrial availability, obtains the same oxygen evolution potential of 1.1 V like the one shown by Li et al. (2005) by using Ti/RuO2 electrode.

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Fig. 3. Byproduct formation with time for (A) Na2 SO4 and (B) NaCl as electrolytes.

Fig. 4. Cyclic voltammograms of Ti/Pt anode obtained at a scan rate of 100 mV s−1 .

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Fig. 5. (A) Total current efficiency and (B) energy requirements for electrochemical oxidation of phenol.

3.4. Total current efficiency The total current efficiency (TCE) during the electrochemical degradation of COD using Na2 SO4 or NaCl as supporting electrolyte decreased dramatically with the increase of electrolysis time (Fig. 5A). Comninellis and Pulgarin (1993) have pointed that the current efficiency at greater initial organic concentration was relatively high as it is related closely to the initial organic concentration. During this study, TCE at the beginning of operation kept high due to the presence of several organic compounds such as partial oxidized phenol, hydroquinone and series of ring-opened intermediates. Once, electrochemical process started the side reactions of oxygen evolution, the TCE reduced gradually. The highest values of TCE were achieved at the first-hour treatment. Furthermore, there is a considerable difference in TCE when chlorine is present in the solution. Solution with Na2 SO4 obtained a TCE around 8% while solution with NaCl obtained a TCE around 14%. These phenomena could be interpreted as SO4 2− may be adsorbed onto the electrode surface and hinder the •OH generation. Meanwhile, SO4 2− anion could react with •OH, as shown in the equation below: SO4 2− + •OH + H+ → •SO4 − + H2 O

(3)

reducing the •OH concentration which yields in a less phenol removal when Na2 SO4 is present in the solution (Jin et al., 2014). Duan et al. (2013) showed lower TCE values for phenol degradation on Ti/PbO2 electrode using Na2 SO4 as supporting electrolyte and 10 mA cm−2 current density. After 3-h treatment TCE was 3% in contrast to 7% TCE achieved in the present study using a similar current density. On the other hand, De Paiva Barreto et al. (2014) showed higher TCE values for phenol degradation using NaCl as supporting electrolyte and 10 mA cm−2 current density. After 2-h treatment TCE was 59% and 72% for Ti/IrO2 and Ti-Pt-SnO2 -SbO5 , respectively; while in the present study TCE was around 11% with an energy

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Fig. 6. Electrochemical degradation of phenol in presence of chloride at different pH conditions on Ti/Pt anode (A) removal and (B) kinetics.

consumption of 0.002 kWh dm−3 . Meanwhile, Ti/IrO2 and Ti-Pt-SnO2 -SbO5 consumed 36 and 28 kWh dm−3 , respectively. Energy consumption of Pt/Ti electrodes is lower than BDD or MMO electrodes energy consumption reported by other authors (Table 1). The present study showed an energy consumption of 0.003 and 0.024 kWh dm−3 for total phenol removal and of 0.168 and 0.252 kWh dm−3 for total COD removal with NaCl and Na2 SO4 as electrolytes, respectively (Fig. 5B). Results showed that energy consumption of Pt/Ti electrode was lower when chloride was present in the solution. Possibly due to the higher conductivity observed in solutions containing NaCl, 20.88 ± 0.18 ms cm−1 , in contrast to 10.06 ± 0.10 ms cm−1 in solutions with Na2 SO4 . NaCl dissociated faster in water, due to the type of ionic bond this salt has, which led to a better electric conductivity hence it permitted a lower energy consumption (Perry et al., 1997). Moreover, as mentioned in Section 3.3, anodic oxygen evolution in aqueous electrolyte represents an non-desirable power loss but the higher oxygen evolution potential detected in solutions with NaCl have suggested that the radical reaction forming oxygen was probably restrained thus the energy consumption was less (Feng and Li, 2003; Wang et al., 2009) 3.5. pH influence on the electrochemical degradation of phenol in presence of chloride The generation of hypochlorite/chlorine in a solution is pH dependent for this reason phenol degradation was evaluated at four different pH conditions (pH 3, 5, 7 and 9) to obtain optimum pH for maximum phenol removal. The electrochemical oxidation of phenol in presence of chloride proved to enhance the organic compounds oxidation as described in the previous sections because of the presence of chlorine oxyanions. During the first hour treatment, the highest removal percentage of 84% was achieved by pH 5 followed by pH 3, 9 and 7 with phenol removals of 81%, 74% and 64%, respectively

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(Fig. 6A). Within 3 h of electrolysis, phenol removal over 90% was reached in all pH conditions. Even though, acidic pH still showed a better phenol degradation, pH 3 and pH 5 reached a phenol removal of 97 and 96% respectively in contrast to 94 and 91% removal for pH 7 and pH 9 respectively. Results suggest that chlorine oxyanions are stronger oxidizers in acidic conditions. Lower pH conditions enhanced phenol removal as chlorine is more stable towards disproportionation in solutions with acidic pH than in alkaline ones (Boucher, 1989; Körbahti and Artut, 2010). For phenol removal from industrial wastewaters Iniesta et al. (2002), using PbO2 /Ti electrode, also showed that acidic conditions allowed a higher phenol removal. Fisher individual tests for differences of means were used to identify statistically significant differences between the reactions rates (Fig. 6B) of the four pH conditions evaluated in this study and to determine whether the differences were practically significant. Relying on statistical analysis pH did not have a significant influence on the reaction rate for phenol removal in the presence of chlorine. Given the above using basic pH conditions for electrochemical treatment in the presence of chlorine is recommended to transform the chlorine formed to ClO− which is a powerful oxidant too but less active chlorinating agent than HClO so it could be possible to reduce the formation of halogenated byproducts. 4. Conclusions The present study has shown that electrochemical oxidation can be successfully used to remove completely organic pollutants from wastewater contaminated by phenol. In solutions containing sulfates, the oxidation of organic compounds was mainly due to hydroxyl radical generated from water discharge at the anode. While in solutions containing chloride, oxidation occurred by chloride-oxychloride radicals. The presence of chloride ions seemed to inhibit the oxygen-evolution reaction, producing an increase in current efficiency and a decrease in energy consumption. The Pt/Ti electrodes used in this study are a suitable option for wastewater treatment on a large scale due to their industrial availability and lack of toxicity. Moreover, the presence of chloride in the solution was an important parameter to enhance the efficiency of the electrochemical oxidation of phenol. However, there is concern about the formation of halogenated byproducts, and the use of basic pH conditions could help to transform the chlorine formed to ClO− to reduce the formation of halogenated byproducts. 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