Potential of Sludge Carbon as New Granular Electrodes for

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Potential of sludge carbon as new granular electrodes for degradation of AO 7 Hongwei Sun, Ting Chen, Lingjun Kong, Quan Cai, Ya Xiong, and Shuanghong Tian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00780 • Publication Date (Web): 08 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015

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Industrial & Engineering Chemistry Research

Potential of sludge carbon as new granular electrodes for degradation of AO 7 Hongwei Suna, c, Ting Chena, c, Lingjun Konga, b, Quan Caia, c, Ya Xionga, c, Shuanghong Tiana, c

a

School of Environmental Science and Engineering, Sun Yat-Sen University,

Guangzhou 510275, P.R. China; b

School of Environmental Science and Engineering, Guangzhou University,

Guangzhou 510006, P.R. China. c

Guangdong Provincial Key Laboratory of Environmental Pollution Control and

Remediation Technology, Guangzhou 510275, P.R. China



Corresponding author. Tel.: +86 20 84115556. Fax: +86 20 39332690. E-mail address: [email protected] (Ya Xiong); [email protected] (Shuanghong Tian).

1

ACS Paragon Plus Environment


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Abstract Powdered sludge carbons were prepared by pyrolysis at 800 oC and firstly used as the micro-particle electrodes of three-dimensional electrode reactor. It was found that the acid orange 7 (AO 7) could be efficiently removed by adsorption and electro-oxidation. The electrochemical oxidation could degrade both the AO 7 in aqueous solution and the AO 7 adsorbed on the micro-particle electrodes. The degradation efficiency of the former reached 80.2% and that of the latter is estimated to be 13.6% of the adsorption amount under the conditions of 20 V cell voltage, 300 mL min-1 airflow and 60 min reaction time. This degradation is contributed to not only

the

electro-catalytic

activity

of

carbon

component

but

also

the

electro-Fenton-like effect of Fe3O4 and the co-Fenton-like effect of SiO2 and Al2O3 in the sludge carbon. Key words: Sludge carbon, particle electrodes, three-dimensional electrode reactor.

2


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1. Introduction Sewage sludge is the main solid waste generated from municipal wastewater treatment process. In the past decades, it has dramatically increased due to the rapid urbanization, resulting in potential environmental problems.1 For example, its annual production exceeded 10 in the EU and 16 million tons in China (by dry mass).2 The traditional disposal methods of sewage sludge, such as ocean dumping, landfilling and incineration, frequently caused debate due to the potential of secondary pollution.3 Therefore, much great efforts have been made to convert them into valuable products for different applications in recent years.4 The products includes pyrolytic oil, H2-rich gas, sewage sludge carbon (SC) etc. Among them, SC has been extensively investigated as adsorbents and catalysts. 5-7 Up to now, the exploration for its new application has attracted an intensive interest, due to the prompt increasing of sewage sludge yield, coupling with economic and environmental concerns. As one of the advanced oxidation processes, the electrochemical oxidation has been attracting much attention with its unique features, such as simplicity and robustness in structure and operation, capability of degrading refractory organics.8,

9

In the

electrochemical oxidation processes, three-dimensional electrode has gained increased technical importance since it can provide a more extensive interfacial electrode surface, lower energy cost, and higher mass transfer efficiency comparing to traditional plate-type electrodes.10,

11

Recently, various types of three-dimensional

electrode reactors have been designed and developed, such as divided and undivided 3



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reactors, plug flowing and continuously stirred tank-type reactors, packed bed and fluidized reactors etc.12-16 No matter what type of three-dimensional electrode reactor is, they basically consist of two main electrodes (anode and cathode) and granular electrodes loaded between main electrodes. The performances of these reactors are intensively the electrochemical characteristics of the granular electrodes because they act as working electrodes. They can form lots of charged micro-electrodes under the influence of an additional electric field, then organic pollutants are degraded by direct and indirect electrochemical oxidation on the charged surfaces of these granular electrodes. To date, many kinds of particles or powdered materials have been used as the granular electrodes, such as activated carbon, carbon aerogel, graphite particles, quartz sand and catalytic metal oxide-coated Al2O3, titanium, ceramics and zeolite etc.12, 17-21 Among them, carbon-based particle electrodes, especially activated carbon are the most commonly used in the wastewater treatment and showed good performance.17 SC contains not only rich carbon element, but also considerable Al2O3, SiO2 and some other metal oxides. It is known from the previous investigations that these substances are all the ingredients of the investigated-well granular electrodes. 17, 20-21

However, there is no report about using SC as the three-dimensional electrodes.

This case arouses our interest in extending the use of SC as particle electrodes in a three-dimensional electrode reactor. A soluble acid azo-dye, Acid Orange 7 (C16H11N2NaO4S, AO 7), was selected as a model dyeing pollutant in this investigation because azo-dyes are among the largest-group of colorants used in a 4


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variety of industries ranging from textile to paper and stable with respect to biochemical oxidation.23 As an initial work, this paper will be devoted to investigating the performances of SC powder-based granular electrodes in the three-dimensional electrode reactor for eliminating organic pollutants in wastewater. The performances mainly include their degradation efficiencies under varied dosage, cell voltage and airflow. A special interest was to compare the efficiency of SC with common carbon particle electrodes, further to approach the origin of their efficiency discrepancy. The aim of these investigations is to probe the potential of SC as new granular electrodes in the three-dimensional electrode reactor for wastewater treatment. 2. Materials and Experiment 2.1. Materials and Chemicals The dewatered sewage sludge was obtained from Datansha municipal wastewater treatment plant in Guangzhou, China. The sludge was dried at 105 oC to a constant weight, subsequently being ground and sieved through a 100 mesh sieve, and finally stored in a desiccator at room temperature. Pine wood sawdust was washed thoroughly with deionized water before use. The AO 7 is a commercial dye (Hengrun Dyestuff Chemical Co., Ltd., Guangzhou) and used without further purification. Other regents are all analytical grade. 2.2. Preparation of carbon materials

5



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SC was prepared by a simple pyrolytic process as shown in previous study. 24 The pretreated sludge was directly put into ceramic ark, heated in programmable tube electric furnace (SKF-210, Hangzhou Lantian Instrument Co., China) at a rate of 10 o

C min-1 to 800 oC and hold for 3 h in the presence of N2. After cooling to room

temperature, the resulted SC was ground, rinsed with deionized water until the pH reached 7, dried at 105 oC over night. The dried SC was sieved with 100 to 200 meshes, that is, the fraction in the range of 0.075 to 0.147 mm diameter was selected as particle electrodes. Wood carbons-containing various metal oxides were prepared by the following process. Given amounts of ethanol-TEOS, FeSO4, Al2(SO4)3 and their mixed solution was stirred with wood sawdust, respectively, at 80 oC for 24 h. These mixtures were dried at 105 oC for 12 h, and then pyrolyzed as described above. The resulted products were labeled as SiWC, AlWC, FeWC and AlSiFeWC, respectively. The mass content of metal oxide (SiO2, Al2O3 and Fe3O4) in the former three products was 22.1%, 11.7% and 5.4%, respectively. The mass content of metal oxides in the AlSiFeWC is 11.7% for Al2O3, 22.1% for SiO2 and 5.4% for Fe3O4. These contents are similar to those of SC. 2.3. SC-based three-dimensional electrode reactor The experimental apparatus is a batch rectangular undivided three-phase three-dimensional electrode reactor, as shown in Figure 1, similar to our previous study.23 The reactor support was made from plastic. The main anode and cathode 6


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(main electrodes), situated 6 cm apart from each other, were made from graphite plates. SC particle electrodes (working electrodes) were packed between the two main electrodes to form sludge carbon slurry. The length, width and height of the three-dimensional electrodes is 6310 cm3. Compressed air was sparged into three-dimensional electrode reactor by micro-pore pipes from the bottom of the reactor to form a kind of fluidized-type three-dimensional electrode reactor. The airflow rate changed from 0 to 400 mL/min when its influence on the removal of AO7 was investigated. Otherwise, the airflow rate was fixed at 300 mL/min. When electric potential is applied across the sludge carbon slurry, every SC particle is polarized, charged and behaved as an anode on its one side and a cathode on the other side. Thus, Faradic reactions will take place on both sides of the SC particles, as a micro-cell.25, 26 The polarized SC can provide a more extensive interfacial electrode surface area compared with the two-dimensional electrode. Then the working efficiency could be highly improved. The working voltage was supplied with regulated DC power supply, TPR640, China. The cell potential ranged from 0 to 25 V when its influence on the removal of AO7 was investigated. Otherwise, the cell potential was fixed at 20 V. 2.4. Electrochemical degradation of AO 7 The test solution is the synthetic wastewater containing 5,000 mg L-1 AO 7 and 0.015 mol/l Na2SO4 as supporting electrolyte which was frequently found in many wastewaters. After SC was immersed in the test solution for 1 h for pre-adsorption, basically reaching the adsorption/desorption equilibrium, the suspension (50 ml) was 7



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added in the three-dimensional electrode reactor and started electrochemical degradation. Samples were withdrawn at certain time intervals during the degradation and immediately analyzed after filtration with 0.45 m Millipore membrane filter. 2.5. Equipments and analytical methods The morphology of sludge carbon was observed using a field emission scanning electron microscope (SEM) (JEOL JSM-6330F). The specific surface area (SBET) was measured by physical adsorption of N2 at 77 K on an auto-adsorption system (Auto-sorb-6, Quantachrome). X-ray diffraction (XRD) analysis was conducted using D/max 2200 vpc Diffratometer (Rigaku Corporation, Japan) with a Cu Kα radiation at 40 kV and 30 mA. The measurement of metal content was conducted by ICP-OES spectrometry (ICPS 7500, Shimadzu Corp., Kyoto, Japan). Concentration of AO 7 was determined with a visual spectrophotometer (Hach company, DR 2800, USA) at 484 nm. The concentration of H2O2 was determined spectrophotometrically.27 Hydroxyl

radicals (·OH) are determined with

trapping and quantified

dimethyl

sulfoxide

(DMSO)

by HPLC. 28

3. Results and discussion 3.1. Characterization of SC granular electrodes The basic physical parameters and chemical composition of SC are listed in Table 1. It can be seen that its surface area is only 57.65 m2/g, although it is formed by assembling of many micro-particles, as shown in the SEM image (insert of Figure 2). The less surface area is possible dependent on their composition. The ash content 8


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reached as high as 77.6 % while its C content was only 17.8 %, being much lower than that of the commercial coal-based activated carbon. Therefore, SC is actually a hybrid material with carbon and a lot of non-carbon inorganic ash although it is accustomed to be called as “sludge carbon”. ICP analysis results shown that the ash mainly contained aluminum, silicon and iron elements with a content of 10.3%, 6.2% and 3.9%, respectively. They occurred in the form of SiO2, Al2O3 and Fe3O4, correspondingly, as shown in their XRD pattern (Figure 2), while the carbons existed mainly in the type of graphite. It is well known that graphite possess a favorable conduct for charge transfer. The high graphitized degree is a base for electrode materials. 3.2. Catalytic performance of SC granular electrodes Figure 3 presents the treatment efficiency of three-dimensional electrode reactor with various dosages of SC granular electrodes. It can be seen from the figure that the removal efficiency is not persistently direct proportion to the dosage of SC. The two maximum treatment efficiencies of decoloration and COD removal appear at a dosage of 100 g L-1 although the change of the former is not more apparent than that of the latter. The initial increase can be simply attributed to the increase of the electrochemical reaction sites with the increase of the dosage. However, the increase of SC dosage also leads to the increase in the collision frequency of their opposite charges, moreover these collisions possible breakdown liquid film resistance on the surfaces of SC particles, resulting in the quenching of their positive and negative 9



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charges each other. Thus, the decrease in degradation efficiency can be ascribed to the charge quenching. The result is similar to Hong et al’s observation in the piezo-degradation of AO 7.29 The bottleneck effect of the dosage is possible a special characteristics of fluidized-type electrochemical reactors with micro-particle electrodes. In general, there are two main pathways to degrade organic pollutants in an electrochemical reactor. One is direct oxidation of anode, and the other is indirect oxidation of electro-generated H2O2 from the two-electron reduction of oxygen on cathode, as described by equation (1). 30-33 O2 + 2H+ + 2e = H2O2

(1)

In order to simultaneously utilize the functions of anode and cathode or direct and indirect oxidation, the compressed air was uniformly sparged into the electrochemical reactor by an microspore aeration pipe during electrolyses. The purpose of the sparged-air is to supply the essential oxygen for electro-generated H2O2 in addition to stirring up and suspending sludge carbon-based particle electrodes. However, the removal efficiency is not always positively dependent on airflow. As show in Figure 4, the decoloration and COD removal efficiencies first increased and the maximum removal efficiency appears at 300 mL min-1 airflow. The increase can be simply attributed to the increase in mass transfer and reactant O2 of electro-generating H2O2, while the decrease seems difficult to be understood to some degree. A possible reason for the decrease is the increase in the collision frequency and intensity between these 10


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charged particle electrodes with increase of airflow, leading to the increase of quenching due to their positive and negative charges each other. This is also a special characteristic of fluidized-type electrochemical reactors with micro-particle electrodes. This phenomenon was also observed by Lv et. al. 18 Figure 5 shows the decoloration efficiency of AO 7 under various cell voltages. It can be seen from the figure that the removal efficiency is considerably dependent on cell voltage. The removal efficiency for a 60-min electrolysis first increases and then decreases with the increase of cell voltages. The highest decoloration and COD removal efficiencies appeared at 20 V. The reason is easily to be understood because the cell voltage is the driving force of electrochemical reaction. The decrease is possible due to many side reactions, such as generation of oxygen and hydrogen etc.34 Considering that the sludge carbon-based particle electrodes have adsorbed considerable AO 7 during immersing within 1 h before electrolysis, the electro-oxidation process degraded not only the unabsorbed AO 7 in the solution, but also the adsorbed AO 7 in these sludge carbon particle electrodes. In order to assess the electrochemical degradation amounts of the adsorbed AO 7, the used sludge carbon-based particle electrodes in the electro-oxidation process were re-immersed in the AO 7 solution with a concentration of 1423.2 mg L-1 since the equilibrium concentration was 1423.2 mg L-1 after pre-adsorption before electrolysis, and the adsorbed amounts could be approximately regarded as the electrochemical degradation amounts of the adsorbed AO 7. Thus, the total amount of the 11



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electrochemical degradation was equal to the sum of the degradation amounts in solution and in these sludge carbon particles. These experimental results showed that the three dimensional electrode reactor not only degraded AO 7 in solution but also decreased the adsorbed AO 7 in the sludge carbon-based particle electrodes, as expectedly, but the change of their degradation amounts with electrolysis time was rather different. As shown in Figure 6, the degradation amount of AO 7 in the solution increased rapidly in the initial 1 h and then increased slowly, while the degradation amount of AO 7 in the sludge carbon increased fast after 2 h and reached as high as 100.3 mg at 3 h. It excesses the degradation amount in the solution, being 60% of the total degradation amount. After 1 h reaction, in the solution, the degradation amount divided the total amount in the solution is the degradation efficiency, which was calculated to be 80.2%. In the sludge carbon, the electrochemical degradation amounts of the adsorbed AO 7 divided the total adsorbed amounts is the degradation efficiency, which was calculated to be 13.6%.

The degradation amount and

adsorption amount can be calculated according the AO7 concentration change and the reaction volume. The result also indicates that the three-dimensional electrode process is possible developed as an electrochemical regeneration technology for the exhausted sludge carbon in adsorption. 3.3. Comparison of various particle electrodes As previous mentioned, SC is actually a hybrid material of carbon and a lot of non-carbon inorganic ash. The non-carbon inorganic substance are mainly SiO2, 12


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Al2O3 and a little of Fe3O4. In order to probe the dependence of the removal efficiency on non-carbon inorganic substance, the carbon materials containing similar amount of SiO2, Al2O3 and Fe3O4 to those of the sludge carbon were prepared with wood powders and used as particle electrodes. Figure 7a presents the total decoloration efficiencies of various electrodes, the contribution of adsorption and electro-oxidation to it. To discriminate the adsorption and electro-oxidation of AO7, pre-adsorption to equilibrium was evolved without apply the voltage in the first 60 min. Interestingly, the decoloration efficiency was kept increasing after the application of voltage although the adsorption was equilibrium for the SC and other tested electrodes. More interestingly, the FeWC and SiWC had low adsorption ability to AO7 but performed high decoloration efficiency after applying the voltage, meaning the high electro-oxidation to AO7. It can be seen from the figure that these non-carbon inorganic substances show a complicated effect on the performance and total decoloration of wood carbon, however, they all can increase its electro-oxidation efficiency. The electro-oxidation contributions of AlWC, SiWC and FeWC reach 40.0%, 62.9% and 79.5%, respectively. It is noted that the total decoloration efficiency of SC 3D electrode reached as high as 94.4%. However, it is difficult to clarify the contribution of electro-oxidation since 71.5% AO7 was removed by adsorption. Therefore, degradation of AOII in different batch runs were repeated by using the recycled the SC particle electrodes. In each run, fresh AOII solution with the concentration of 1423 mg L-1 was applied. As shown in Figure 13



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7b, the 3D electrode can almost completely remove AOII due to the high adsorption rate by SC and WC. After four runs, the degradation efficiency kept a constant value of about 85% for SC and about 71% for WC because the adsorption reached a steady state. In other words, the contribution of electro-oxidation can be regarded as about 85% for SC and about 71% for WC. As is well known, Fe3O4 is a kind of typical Fenton-like catalyst. If the reactor with sludge carbon-based particle electrodes can electrochemically generate H2O2, the increased effect of Fe3O4 can be simply contributed to the electro-Fenton-like reaction (1). Hence, the production of H2O2 and OH in 2D electrode and SC 3D electrode reactors were detected, as shown in Figure 7c and d. The H2O2 concentration of 2D electrode reached a steady value of 6.26 mg L-1 after 90 min while that of SC 3D electrode is only 2.60 mg L-1, much lower than 2D electrode. In contrast, much more hydroxyl radicals formed in 3D electrode than 2D. It is deduced that Fe3O4 contained in SC, acted as a Fenton-like catalyst to decompose H2O2 into OH, as described by equation (2).35 2H2O2 + Fe3O4

= OH + OOH + H2O + Fe3O4

(2)

Generally, SiO2 and Al2O3 both have less electrochemical activity, not directly participate in electrochemical oxidation or reduction. However, it was reported that SiO2 and Al2O3 could promote the adsorption of H2O2, in addition to many organic pollutants, on the sludge carbon by the formation of hydrogen bonds between hydrogen in H2O2 in SiO2 or Al2O3.24,

36

The adsorption will lead to a higher

concentration of H2O2 on surface of the sludge carbon compared with the bulk 14


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solution

and

facilitate

indirect

electrochemical

oxidation,

such

as

the

electro-Fenton-like reaction. SiO2 or Al2O3 possess a co-Fenton-like effect. It can be inferred from these results that SiO2, Al2O3 and Fe3O4 in SC possess a positive effect on the electro-oxidation of carbon materials, although the role of other inorganic substances need be further investigated. Finally, the products in the solution were meaused by capillary electrophoresis according to our previous work after the electrochemical reactions.37 Six compounds, including sulfamic acid, oxalic acid, benzenesulfonic

acid,

4-hydroxybenzene

sulfonic

acid,

phthalic

acid,

and

4-aminobenzene sulfonic acid were detected. 3.4 Stability of SC electrodes The durability of the electrode, as an important property in practical applications, is measured by successive batch experiments. After six runs in Figure 7b, the SC 3D electrodes were further reused six times. The results are shown in Figure 8, the degradation efficiency of AOII treated by SC 3D electrode reactor does not obviously change after six recycle times. It indicates that SC electrodes are durable.

4. Conclusions

As an initial work, sludge carbon was confirmed to be a kind of efficient particle electrode, and interestedly its potential is slightly greater that that of common wood carbon. Its electro-oxidation efficiency is contributed to not only the electro-catalytic activity of its carbon component but also the electro-Fenton-like effect of Fe3O4 and 15



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co-Fenton-like effect of SiO2 and Al2O3. This finding not only provides a new use of sludge or sludge carbon, but also offers an enlightenment to design and preparation of particle electrodes in the future. Acknowledgements This research was supported by Nature Science Foundations of China (20977117, 21107146), Nature Foundations of Guangdong Province (92510027501000005, 2014A030313218), Science and Technology Research Programs of Guangzhou City (2012J4300118) and Project of Education Bureau of Guangdong Province (cgzhzd1001), the Fundamental

Research Funds for the Central Universities

(121pgy20).

16


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Figure 1. Schematic diagram of sludge carbon-based 3D electrodes reactor (1: main anode, 2: main cathode, 3: sludge carbon particle electrodes, 4: air bubble, 5: compressed air, 6: support).

Figure 2. XRD pattern and SEM (insert) of sludge carbon.

17


330 300 270 240 210 0

20

40

60

80

-1

85 83 81 79 77 75 73 71 69 67 65 63 61 59 57 55

Page 18 of 28

Removal amounts of COD (mg L )

100 120 140

Dosage of SC (g/L)

Figure 3. Treatment efficiency of three-dimensional electrode reactor with various dosages of SC particle electrodes (cell voltage: 20 V, airflow: 300 mL min-1,

84 82 80 78 76 74 72 70 68 66 64 62

330 320 310 300 290 280 270 260

0

100

200

300

-1

Decoloration efficiency of AO7 (%)

electrolysis time: 60 min).

Removal amount of COD (mg L )

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



400

Airflow (mL/min)

Figure 4. Removal efficiencies of AO 7 under various airflows (cell voltage: 20 V, sludge carbon: 100 g L-1, electrolysis time: 60 min).

18


70 60 50 40 30 20 10 0

0

5

10

15

20

25

360 330 300 270 240 210 180 150 120 90 60 30 0

-1


80

Removal amount of COD (mg L )

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


Cell voltage (V)

Figure 5. Removal efficiencies of AO 7 in aqueous solution under various cell voltages (sludge carbon: 100 g L-1, airflow: 300 mL min-1, electrolysis time: 60 min).

Amount of degraded AO 7 (mg)

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100 80

(in sludge carbon) (in solution)

60 40 20 0 0.0

0.5

1.0 1.5 2.0 Electrolysis time (h)

2.5

3.0

Figure 6. Removal efficiencies of AO 7 in aqueous solution as a function of electrolysis time (sludge carbon: cell voltage: 20 V, 100 g L-1, airflow: 300 mL min-1, electrolysis time: 60 min).

19



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Figure 7. (a)Decoloration efficiency of various particle electrodes (sludge carbon: 100 g/l, cell voltage: 20 V, airflow: 300 mL min-1, electrolysis time: 60 min; the decoloration efficiency is calculated according to the total concentration of AO 7, e.g. 5000 mg L-1); (b) decoloration efficiency of various particle electrodes in different run numbers with initial AO 7 concentration of 1423 mg L-1 in each run; (c) H2O2 concentration detected of

2D electrode and SC 3D electrode; and (d) hydroxyl

radical concentrations of 2D electrode and SC 3D electrode in 60 min.

20


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1.0

Decoloration efficiency of AO 7

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0.8 0.6 0.4 1st 2nd 3rd 4th 5th 6th

0.2 0.0 0

60

120

180

240

300

360

Time (min)

Figure 8. Degradation efficiency of AO 7 within 60 min with reused SD 3D electrode sludge carbon: 100 g L-1, cell voltage: 20 V, airflow: 300 mL min-1, electrolysis time: 60 min; Initial AO 7 concentration: 1423 mg L-1) cc Table 1. Physic-chemical characteristics of SC particle electrodes. Physic characteristics

Chemical composition

Yield (%)

62.5

Ash (%)

77.6

Apparent density (g/l)

767.6

Si (%)

10.3

Particle size range (mm)

0.075-0.147

Al (%)

6.2

SBET (m2/g)

57.65

Fe (%)

3.9

11.25

C (%)

17.8

Methylene blue adsorption (mg/g)

21



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