A new type of continuous-flow heterogeneous electro-Fenton reactor for Tartrazine degradation

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A new type of continuous-flow heterogeneous electro-Fenton reactor for Tartrazine degradation Chao Zhang, Gengbo Ren, Wei Wang, Xinmin Yu, Fangke Yu, Qizhan Zhang, Minghua Zhou

⁎

Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China Tianjin Key Laboratory of Urban Ecology Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China Tianjin Advanced Water Treatment Technology International Joint Research Center, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Continuous-flow reactor Electro-Fenton Fenton Fe-C micro-electrolysis Heterogeneous Tartrazine

A new type of continuous-flow heterogeneous electro-Fenton (EF) reactor, in which electrochemically formed H2O2 was externally injected to the iron-carbon granules packed bed, was designed to degrade a model dye, Tartrazine. The gas diffusion electrode (GDE) can generate H2O2 efficiently, and the iron-carbon granules were used as the heterogeneous catalyst for the Fenton oxidation of the pollutants. The important operating parameters such as initial concentration of Tartrazine, pH, modified Fe-C dosage, wastewater flow rate and current were optimized. The continuous-flow EF reactor exhibited good performance even after 12 h continuous operations with the Tartrazine removal of 80% and TOC removal of nearly 30%. Compared with the conventional EF reactor, this novel continuous EF reactor can efficiently degrade the target pollutants from low concentration electrolyte. The electric energy consumption in this new type continuous-flow reactor was 0.15 kWh/(g TOC), whereas it was 0.69 kWh/(g TOC) in the batch reactor under the same experimental conditions. All results demonstrated that this novel EF system was energy-efficient and potential for treatment of wastewater without or low concentration electrolyte.

1. Introduction Wastewaters, especially those generated in the chemical, papermaking, printing and dyeing, and pharmaceutical industries, contain large amounts of toxic, hazardous and non-biodegradable substances, which are difficult to be degraded by conventional methods. Electrochemical advanced oxidation processes (EAOPs) have attracted significant application on the refractory industrial wastewater treatment because of strong oxidation ability and high efficiency [1]. In addition, these EAOPs are environmental friendly by the utilization of electrochemically generated free radical to degrade the contaminants, which can avoid secondary pollution caused by adding other chemical reagents. In the last decade, the interest in electro-Fenton (EF), a new EAOPs, has grown considerably. EF is one of the most important indirect oxidation processes, in which H2O2 can be electro-generated in-situ by the reduction of oxygen (Eq. (1)), and then catalyzed to a highly powerful % OH in the presence of Fe2+ (Eq. (2)) [2]. Therefore, an appropriate

cathode material which can produce enough H2O2 in-situ is essential. Carbonaceous materials are the most familiar materials used as cathode, such as activated carbon fiber [3], graphite [4], carbon sponge [5], carbon or graphite felt [6] and carbon–polytetrafluoroethylene (PTFE) gas diffusion electrode (GDE) [7]. Among these cathodes, GDE is regarded as the most effective one to produce H2O2 [7,8]. O2 + 2H+ + 2e− → H2O2

(1)

H2O2 + Fe2+ → Fe3+ + •OH + OH−

(2)

EF system can be divided into homogeneous EF and heterogeneous EF. In a common homogeneous EF system, aqueous iron source (e.g., Fe2+) is usually used as catalyst to reaction with the electrochemically produced H2O2. Homogeneous reaction is efficient, fast and simple, but there are some disadvantages difficult to overcome, such as the narrow pH, iron ions inactivation and iron sludge generation [9,10]. These drawbacks limit the practical application of EF, so researches increasingly focus on the study of heterogeneous systems.

⁎ Corresponding author at: Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China. E-mail address: [email protected] (M. Zhou).

https://doi.org/10.1016/j.seppur.2018.05.016 Received 5 February 2018; Received in revised form 4 May 2018; Accepted 8 May 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Zhang, C., Separation and Purification Technology (2018), https://doi.org/10.1016/j.seppur.2018.05.016

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anode, sodium sulfate (Na2SO4), ferrous sulfate heptahydrate (FeSO4·7H2O), sulfuric acid (H2SO4) and sodium hydroxide (NaOH) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received without further purification.

Heterogeneous EF process uses heterogeneous catalysts, which are generally very slightly soluble or insoluble in water. Therefore the solid catalysts play the core role in heterogeneous EF, determining the degradation efficiency and subsequent post-treatment. The basic requirements of the catalyst are of highly catalytic activity, long life span, stability and low price. Iron-carbon (Fe-C) particles can be good alternatives as catalysts for Fenton reaction due to their inexpensive price, high catalytic property, long life span and easy practical utilization. Once Fe-C particles are in contact with the electrolyte, numerous microscopic galvanic cells will be formed [11,12]. Iron as anode will lose two electrons and the carbon as cathode can accelerate the reduction by accepting electrons and transferring the electrons to the pollutants or oxygen. The reactions within one electrode and no potential applied can be represented as follows [13,14]: Anodic oxidation: Fe-2e− → Fe2+

2.2. Preparation of modified Fe-C catalyst and GDE Fe-C catalysts with particle size of 0.5–1.0 cm were dipped into pure ethanol and ultrasonically treated for 30 min, then filtered and washed two times with ethanol. 20 g Fe-C were dipped into 20% PTFE solution. After 1 h dipping, Fe-C particles were separated from the solution by vacuum filtration, then they were added into a three-neck flask and dried 2 h at 100 °C with N2 protection. The GDE was prepared using the following procedure. A mixture of 1.5 mL distilled water, 1.5 mL PTFE (mass fraction 60%) and 0.27 g carbon black, stirred for 20 s. Then the mixture was pasted uniformly by banister brush onto one side of multi-dimensional carbon cloth of 8 × 5 cm2. After dried naturally, the coated carbon cloth was heated at 350 °C for 30 min. After stirred for 20 s, the mixture of 5.82 mL distilled water, 0.09 mL PTFE (mass fraction 60%), 0.18 g carbon black and 0.5 mL isopropanol was coated onto same side of the carbon cloth by the same procedure. The first carbon black layer was for electric conduction and waterproof, the second layer was for H2O2 production.

(3)

Cathodic reduction: Acidic: 2H+ + 2e− → 2[H] → H2

(4) −

Acidic with oxygen sparged: O2 + 4H + 4e → 2H2O +

−

−

Neutral to alkaline: O2 + 2H2O + 4e → 4OH

(5) (6)

2+

by Fe-C micro-electrolysis can be iron source for The formed Fe Fenton reaction. To date, most of the heterogeneous EF reactors reported in literature are integrated the electrochemical production of hydrogen peroxide with wastewater treatment in one reactor, and have been widely studied for many wastewaters treatment [15,16]. However, this kind of heterogeneous EF reactor has some common drawbacks [8]: (1) the catalyst loading cannot be too high, because the distance between anode and cathode cannot be too large, (2) the EF system need sufficient electrolyte to maintain conductivity and reduce treatment cost. In some case, parts of industrial wastewaters have no or low concentration of electrolyte, which is unfavorable for such an EF reactor due to the high energy consumption. In addition, the batch reactor for wastewater treatment is not suitable for practical utilization. To overcome the aforementioned drawbacks of traditional EF process, in this work, a new type of continuous-flow heterogeneous Fenton reactor was developed using the modified iron-carbon as catalyst, while electrochemically formed H2O2 was externally injected to the wastewater. Tartrazine, one of azo dyes, is selected as the target pollutant in this investigation. Wastewater containing azo dyes usually has the following characteristics: high COD concentration, poor biochemical properties, chemical stability, so it is widely recognized as one of the non-biodegradable organic wastewater [17]. It will accumulate in the environment and pose a threat to all kinds of creatures and even human health if the Tartrazine discharged into the water environment without any treatment [18–20]. In this work, GDE was employed as the cathode for H2O2 production and modified Fe-C particles were used as heterogeneous Fenton catalysts as prepared in our previous work [8]. The effects of operating parameters on the Tartrazine and TOC removal efficiencies in the new continuous-flow heterogeneous Fenton reactor were explored. And the comparison between this reactor and traditional reactor was investigated to explore its advantages.

2.3. New heterogeneous Fenton reactor As shown in Fig. 1, this novel reactor consists of three parts: solution tank (5.0 L), the H2O2 electrochemical generation system (0.3 L) using GDE cathode and the heterogeneous Fenton reaction cell (0.65 L) using suitable dosage of modified Fe-C particles as catalysts packing at the bottom. The heterogeneous Fenton reaction cell was operated in a continuous-flow mode with continuous H2O2 inflow from the cell bottom, supplied by the H2O2 electrochemical generation system. In the H2O2 electrochemical generation system, GDE (3.14 cm2) was used as cathode, Ti/IrO2-RuO2 (4 cm × 2 cm) as anode, and 0.05 M Na2SO4 as the supporting electrolyte (300 mL). The H2O2 was generated at current of 200 mA, and started to flow into the Fenton reactor at a flow rate of 3 mL/min after 20 min, and the target pollutants also flowed into the heterogeneous Fenton reaction reactor at the same time. The solution pH was adjusted by H2SO4 or NaOH, and the flow rate was adjusted by a flow meter. At certain time intervals, samples were taken from the outlet of the heterogeneous Fenton reaction cell for analysis. For performance comparison, the removal of Tartrazine was carried out in a bath heterogeneous electro-Fenton reaction cell with the same solution volume (0.65 L), the dosage of modified Fe-C particles and the same electrodes under similar conditions. 2.4. Analytical methods The Tartrazine concentration was determined by spectrophotometric method at λmax = 428 nm, and the removal efficiency was calculated by Eq. (7) [21],

A Removal = ⎛1− t ⎞ × 100% ⎝ A0 ⎠ ⎜

⎟

(7)

where A0 and At denote the absorbance at initial time and after an electrolysis time t, respectively. The electric energy consumption (EEC) of the electrochemical process was calculated by Eq. (8) [22],

2. Experimental 2.1. Chemicals and materials Fe-C catalyst with particle size of 0.5–1.0 cm was purchased from Weifang Hua Yun Environmental Protection Technology Co., Ltd, and the iron content was about 39%. Detailed characteristics of Fe-C could be found in our previous work [8]. Carbon black was purchased from Jilin Carbon Plant. Tartrazine was used as target pollutant. Tartrazine,

EEC(kWh/kg) =

1000UIt CVs

(8)

where t is the electrolysis time (h), I is the applied current (A), U is the voltage (V), C is the concentration of the TOC removed and VS is the 2

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Fig. 1. Schematic drawing of the continuous-flow reactor. 70

volume (L). The residence time (T) was calculated by the following formula (9) [23]:

1000 × (Vr−Vc) j

50

(9)

H2O2 (mg/L)

T=

160 g Fe-C no Fe-C

60

where Vr is the volume of the heterogeneous Fenton reaction cell (L), Vc is the volume of the iron-carbon (L), j is the flow rate (mL/min). The concentration of H2O2 was monitored by spectrophotometer (VI-1501, Tianjin Dong Gang Sci & Tech Development Co., Ltd) at λmax = 400 nm, using the potassium titanium (IV) oxalate method [24]. The TOC of the initial and final samples was determined by a TOC analyzer (Analytikjena multi N/C 3100, Germany). The total iron concentration was analyzed by spectrophotometer at λmax = 510 nm, using 1,10phenanthroline method [25].

40 30 20 10 0 5

3. Results and discussion

10

15

20

25

30

Time (min) 3.1. Generation of H2O2 in the system Fig. 2. The accumulation of H2O2 in different systems. Conditions: 0.05 M Na2SO4, pH 7, O2 0.05 L/min, 200 mA, H2O2 3 mL/min, H2O 20 mL/min.

The variation of H2O2 accumulation in the continuous-flow reactor was investigated. The impact of the presence and absence of ironcarbon in the heterogeneous reactor on the concentration of H2O2 was studied. As shown in Fig. 2, in the absence of iron-carbon, the concentration of H2O2 gradually increased to 60 mg/L after 30 min continuous operating due to the accumulation of H2O2 with time in the H2O2 generation device. When 160 g/L of iron-carbon was packed into the reactor, the concentration of H2O2 decreased firstly and then remained between 10 and 20 mg/L. This showed that the leaching Fe2+ from the iron-carbon played an important role in the catalytic decomposition of H2O2.

rates were 10, 15 and 20 mL/min, the Tartrazine removal efficiency was stable at about 80%. At the flow rate of 30 mL/min, the initial Tartrazine removal efficiency was only 30–40%, and the maximum was no more than 70%. In this case, Fenton reaction was relatively weak, which can be explained by the fact that the residence time was too short and the iron ions in solution were too little. Fig. 3b shows the TOC removal and iron leaching under different flow rates. When the flow rate was 15 mL/min, the obtained maximum TOC removal was 29%. Further increased the flow rate to 20 mL/min, the TOC removal decreased to 25%, which was well in agreement of the leaching of iron. At a flow rate of 10 mL/min, the leaching of iron exceeded 20 mg/L, and a large amount of iron floc formed due to the low flow rate and long residence time, which raised the solution pH (Eq. (6) and thus the Fenton reaction weakened and in turn led to the decrease of TOC removal.

3.2. Optimization of important parameters 3.2.1. Effect of flow rate Wastewater flow rate directly affects the residence time in the reactor, thus affecting the quality of effluent. The Tartrazine and TOC removal was studied under the flow rate from the solution tank of 10, 15, 20 and 30 mL/min. The corresponding residence time was 65, 43.3, 32.5 and 21.7 min, respectively. As shown in Fig. 3a, when the flow

3.2.2. Effect of iron-carbon dosage As shown in Fig. 4a, the Tartrazine removal efficiency increased 3

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(a)

90

(a)

80

80

Removal (%)

Removal (%)

70

60

50

10 ml/min 15 ml/min 20 ml/min 30 ml/min

40

70

Fe-C 86 g Fe-C 120 g Fe-C 160 g Fe-C 195 g

60

50

30

40 10

15

20

25

30

Time (min) 30

5

(b)

15

20

25

30

Time(min)

TOC

20

(b) 25

25 15

10

15

10 5

16

20

TOC removal (%)

20

18

TOC

Iron leaching (mg/L)

TOC removal (%)

10

20

5

14

15

12 10 10 5

0

0

10mL/min

15mL/min

20mL/min

8

0

30mL/min

Iron leaching (mg/L)

5

6

86g

120g

160g

195g

Fig. 3. (a) Tartrazine removal efficiency, (b) TOC removal and the amount of iron ions leaching under different flow rates. Condition: Tartrazine 60 mg/L, pH 3, Fe-C catalyst 160 g, current for H2O2 generation 200 mA.

Fig. 4. (a) Tartrazine removal efficiency, (b) TOC removal and the amount of iron ions leaching under different iron-carbon dosage. Condition: Tartrazine 60 mg/L, pH 3, flow rate 20 mL/min, current for H2O2 generation 200 mA.

with the increase of the iron-carbon dosage. When the iron-carbon dosage was 160 g and 195 g, the Tartrazine removal efficiency remained above 80% during investigated time. At an iron-carbon dosage of 86 g, the Tartrazine removal efficiency raised from initially 40% to 70% within 15 min and then remained above 70%. When the dosage was 120 g, the removal trend also remained stable. Therefore the larger amount of iron-carbon did increase the depletion of Tartrazine, presumably more hydroxyl radicals were produced with more iron available. Fig. 4b shows the TOC removal and iron leaching under different dosages of iron-carbon. The TOC removal increased as the iron-carbon dosage increased. When the dosage of iron-carbon were 86, 120, 160 and 195 g, the TOC removal were 18.5%, 22.9%, 25% and 26.2%, respectively, and the amount of leached iron ions were 7.2, 9.65, 8.95 and 14.32 mg/L, respectively. When the iron-carbon dosage was more than 120 g, the increase in the TOC removal efficiency was not obvious. Taking the removal efficiency of Tartrazine and TOC as well as the amount of leached iron ions into consideration, 160 g of iron-carbon was used as the optimal catalyst dosage.

was only about 60%, and it was below 40% at the initial concentration of 100 mg/L. When the initial concentration was 40, 60, 80 and 100 mg/L, the removed Tartrazine was about 32.4, 59.17, 49.12 and 38.5 mg/L. Therefore the removed Tartrazine and removal efficiency performed the best at the initial concentration of 60 mg/L and decreased when the initial concentration further increased. It can be seen from Fig. 5b that TOC removal trend was consistent with the Tartrazine removal efficiency. When the initial concentration was 40 and 60 mg/L, the TOC removal were 38% and 29.8%, respectively. However, when the initial concentration was higher than 80 mg/ L, TOC removal was below 20%. When the initial concentration was 40, 60, 80 and 100 mg/L, the TOC removal were 3.55, 4.17, 3.42 and 3.17 mg, respectively. This trend was in agreement with the removed Tartrazine, indicating that under optimum conditions, the maximum treatment capacity of this system could not exceed 60 mg/L. The initial concentration has little effect on the amount of leached iron ions, which remained at about 9 mg/L. 3.2.4. Effect of initial pH value pH value is a very critical factor in Fenton reaction. Under neutral or alkaline conditions, leached iron and HO− quickly generate floc precipitation, thus losing the catalytic activity of Fenton process. Actually, the organic waste treated by Fenton reaction was adjusted to alkaline, a portion of TOC could be removed by iron flocculation. The experiments investigate the effect of initial pH on the wastewater treatment efficiency. Fig. 6a shows the effects of different initial pH on the Tartrazine removal efficiency. When the initial pH was 3, the Tartrazine removal efficiency was maintained at 80%, which was due to the reaction of

3.2.3. Initial Tartrazine concentration As shown in Fig. 5a, in order to explore the treatment capability of this system, Tartrazine removal efficiency, TOC removal and the amount of iron ions leaching under different initial concentrations of Tartrazine were investigated. The Tartrazine removal efficiency showed big differences when the initial concentration increased from 40 to 100 mg/L. At initial concentrations of 40 mg/L and 60 mg/L, the Tartrazine removal efficiency were higher than 80%. However, when the initial concentration was 80 mg/L, the Tartrazine removal efficiency 4

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90

(a)

(a) 80

80

pH=3 pH=5 pH=7 pH=9

60

Removal (%)

60 50 40

40 mg/L 60 mg/L 80 mg/L 100 mg/L

30 20

40

20

0

10 5

10

15

20

25

5

30

10

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Time (min) 40

16

(b)

30

25

30

12

(b) TOC

TOC

10

25

12

20

10

4

0

0

Removal (%)

8

8

Iron leaching (mg/L)

30

TOC removal (%)

20

Time (min)

20

6

15

4

10

2

5

0

0

40 mg/L

60mg/L

80mg/L

pH=3

100mg/L

Iron leaching (mg/L)

Removal (%)

70

pH=5

pH=7

pH=9

Fig. 6. (a) Tartrazine removal efficiency, (b) TOC removal and the amount of iron ions leaching under different pH. Condition: Tartrazine 60 mg/L, modified Fe-C catalyst 160 g, flow rate 20 mL/min, current for H2O2 generation 200 mA.

Fig. 5. (a) Tartrazine removal efficiency, (b) TOC removal and the amount of iron ions leaching under different initial concentration. Condition: pH 3, modified Fe-C catalyst 160 g, flow rate 20 mL/min, current for H2O2 generation 200 mA.

Table 1 Performance and energy consumption comparison.

Fe2+ and H2O2, producing a large number of •OH under acidic conditions. At pH 5, the Tartrazine removal efficiency was only about 40%. Furthermore, when the pH were 7 and 9, the Tartrazine removal efficiency were less than 30%. The TOC removal continuously decreased as the initial pH increased and maintain nearly 30% under acidic conditions. Fenton reaction would not work effectively since the iron ion would form Fe(OH)2, Fe(OH)3 and m Fe(OH)2 · n Fe(OH)3. Thus, the majority of TOC removal was due to flocculation of iron ions under alkaline conditions. Meanwhile, the pH of the solution also affects the iron-carbon micro-electrolysis process, the leaching of iron ions under acidic conditions was much higher than that under alkaline condition. Considering these results, acidic condition was more favorable to this heterogeneous Fenton process because it both benefited for the ironcarbon micro-electrolysis and Fenton reaction.

Type

Electrolyte concentration (g/L)

TOC removal (mg)

Energy consumption (kWh/gTOC)

Iron leaching (mg/L)

Batch-flow Batch-flow Batch-flow Batch-flow Continuous-flow

0.93 2.79 5.58 7.44 0.93

5.88 6.02 6.12 6.17 4.17

0.69 0.37 0.14 0.10 0.15

11.3 8.8 13.4 9.7 8.9

electrolyte, and was even comparable with the batch-flow system with much higher electrolyte (5.58 g/L). The lower energy consumption could be explained by the voltage in this new reactor, which was observed to be much lower than batch system. 4.17 mg of TOC was removed in the continuous-flow, while the batch-flow system removed 5.88 mg of TOC within 30 min, which was due to the contribution of anodic oxidation in the removal of TOC in the batch flow system. Thus, it can be concluded that the continuous-flow process is recommended when the electrolyte was lower than 5.58 g/L from point of view of the energy consumption. In other words, the continuous-flow system is suitable for low electrolyte dyeing wastewater. The energy consumption of this new type EF reactor (150 kWh/(kg TOC)) was notably lower than the reported peer works (290–1280 kWh/(kg TOC)) as shown in Table 2. It was worth noting that the concentration of electrolyte in this EF reactor was very low compared with the peer works. Therefore, this continuous-flow EF reactor has great potential for the practical application in the wastewater treatment.

3.3. The comparison between the new reactor and batch flow reactor Table 1 summarizes the effect of electrolyte concentration on the TOC removal, energy consumption and iron leaching in the batch-flow and comparison with continuous-flow system. In the batch-flow system, electrolyte concentration poorly affected the TOC removal and the Tartrazine removal that was basically more than 90% after 30 min. However, the energy consumption decreased with the increase of electrolyte concentration. In the continuous-flow system, the energy consumption was only 0.15 kWh/(g TOC), which was much lower than that in the batch-flow system (0.69 kWh/g TOC) with the 0.93 g/L 5

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Table 2 The comparison of energy consumption of TOC removal with literature. Cell configuration

Pollutant

SPEF Cyclic liquid flow EF Cyclic liquid flow EF Batch EF Batch EF Continuous-flow

Operating conditions 130 mL, pH 3.0, 0.05 M Na2SO4, 0.5 mM Fe

Azo dye Carmoisine

2.5 L, 209.3 mg/L, flow rate 200 L/h, 0.05 M Na2SO4, 0.5 mM Fe

Antibiotic Amoxicillin Tartrazine

250 mL, pH 3.0, I 120 mA, 240 min, 0.1 mM AMX (19.6 mg/L TOC), 0.05 mol/L Na2SO4, 0.1 mmol/L Fe2+ 100 mg/L, voltage 4.0 V, pH 3, aeration rate 80 mL/min, Fe2+ 0.4 mmol/L, 0.05 mol/L Na2SO4, flow rate 20 mL/min

50

80

30

Ref.

290 kWh/kgTOC

[26]

1280 kWh/kgTOC

[27]

25

20 60

30

20

10

0

0

4

5

6

7

8

9

10

11

[29]

350 kWh/kgTOC

[30]

References

15

40 20

[28] 370 kWh/kgTOC 500 kWh/kgTOC

of China (Nos. 21773129 and 21273120), China National Water Project (Nos. 2017ZX07107002 and 2015ZX07203-11), Key Project of Natural Science Foundation of Tianjin (No. 16JCZDJC39300), National Key Research and Development Program (2016YFC0400706), and Fundamental Research Funds for the Central Universities.

Iron leaching (mg/L)

40

Removal (%)

TOC removal (%)

EEC

400 mL, 100 mg/L, 0.15 mM Fe2+, H2SO4 (0.1 M) and K2SO4 (0.1 M)

100

3

2+

Azo dye Amaranth

TOC removal

2

, flow rate 200 L/h

Food color additives

60

1

2+

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10

5

0

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Time (h) Fig. 7. Continuous-flow reactor in continuous operation (■) Tartrazine removal efficiency (▴) iron leaching. Condition: Tartrazine 60 mg/L, pH 3, modified Fe-C catalyst 160 g, flow rate 20 m L/min, current for H2O2 generation 200 mA.

3.4. Performance stability in continuous operations As shown in Fig. 7, this novel continuous-flow reactor was continuously operated for 12 h under the same conditions. After 12 h of continuous operation, the Tartrazine removal efficiency was all above 80%, and almost all TOC removal was more than 30%, indicating the continuous operation system showed good stability. The leached iron ions was relatively low (about 15–20 mg/L) after 12 h running, which was only 0.4% of the total iron in the iron-carbon catalyst, indicating the stable and long life of catalyst. 4. Conclusions In this study, a new type of continuous-flow heterogeneous Fenton reactor was designed to degrade Tartrazine, showing more cost-effective than the traditional batch reactor. With the pollutants flow rate of 20 mL/min, H2O2 flow rate of 3 mL/min, the Tartrazine removal was over 80% and TOC removal was near 30%. The optimum operating parameters were as follows: initial Tartrazine concentration was 60 mg/ L, pH was 3, modified Fe-C was 160 g, wastewater flow rate was 20 mL/ min, the current for H2O2 production was 200 mA, flow rate was 3 mL/ min. This new reactor exhibited good performance after 12 h continuous operations. This continuous-flow reactor is more suitable for practical utilization in low concentrated electrolyte wastewater, overcoming the drawbacks of conventional electrochemical process which need enough electrolyte. Acknowledgements This work was financially supported by Natural Science Foundation 6

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