A Review on Improvement and Application of Fenton Reagent

1Utilization of single-chamber microbial fuel cells as renewable power sources for

2electrochemical degradation of nitrogen-containing organic compounds

3Zhijun Wanga, Baogang Zhanga*, Alistair G.L. Borthwickb, Chuanping Fenga, Jinren

5a School of Water Resources and Environment, China University of Geosciences 6Beijing, Key Laboratory of Groundwater Circulation and Evolution (China University 7of Geosciences Beijing), Ministry of Education, Beijing 100083, China 8b School of Engineering, The University of Edinburgh, The King’s Buildings, 9Edinburgh EH9 3JL, UK 10c Department of Environmental Engineering, Peking University, The Key Laboratory 11of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China 12Abstract 13 By employing promising single-chamber microbial fuel cells (MFCs) as renewab

14le power sources, an aerated electrochemical system is proposed and for nitrogen-cont

15aining organic compounds (pyridine and methyl orange) removals. Carbon felt perfor

16med the best as electrode material while lower initial contaminant concentration and l

17ower initial pH value could improve the performance. A degradation efficiency of 82.

189% for pyridine was achieved after 360 min electrolysis with its initial concentration

19of 200 mg/L, initial pH of 3.0 and applied voltage of 700 mV. Mechanisms study impl

20ied that indirect electrochemical oxidation by generated hydrogen peroxide was respo

21nsible for their degradation. This study provides an alternative utilization form of low

22bioelectricity from MFCs and reveals that applying it to electrochemical process is hig

23hly-efficient as well as cost-effective for degradation of nitrogen-containing organic c

2,* Corresponding author. Tel.: +86 10 8232 2281; fax: +86 10 8232 1081. 3 E-mail: [email protected], [email protected] (B. Zhang)

1ompounds.

2Keywords: Microbial fuel cells; Nitrogen-containing organic compounds; Pyridine;

3Methyl orange

41. Introduction

5 Among various global environmental pollutions, recalcitrant pollutants have dra

6wn extensive attention in the last few decades, especially nitrogen-containing organic

7compounds discharged from coal coking, textile, petroleum refining, pharmaceutical f

8actories and other industries [1,2]. They usually contain complex structure compounds

9such as aromatic rings or azo bond that may cause harmful and irreversible environme

10ntal problems even at quite low concentrations [2,3]. A large portion of papers have be

11en devoted to the degradation of nitrogen-containing organic compounds based on ph

12ysical and biological methods [4-6]. The physical means realize nitrogen-containing o

13rganic compounds removals with only transferring them from water to another phase,

14which may cause secondary pollution [7] and unavoidably, the post-treatments includi

15ng regeneration of the adsorbent and reactivation of membrane fouling that will lead t

16o highly consumption of the energy and restriction of the actual application. As to biol

17ogical treatments, they often suffer from low efficiency due to microbial inhibition [8,

189]. Comparatively, chemical-oxidative processes have been employed for nitrogen-co

19ntaining organic compounds removals removal from aqueous solutions extensively as

20they can decompose molecules effectively. By omitting the generation of sludge from

21Fenton process and the utilization of photocatalyst, electrolysis technology exhibits pr

1omising future for nitrogen-containing organic compounds contamination controls am

2ong chemical-oxidative processes [10,11]. As higher applied voltage (almost up to 12

3V) or current (up to 6 A) are often required in these processes [12-14], thus other alter

4natives should be explored to strengthen their efficiencies with energy conservation.

5 Recently, microbial fuel cells (MFCs), devices using microorganisms as the catal

6ysts to oxidize organic and inorganic matters and converting chemical energy into elec

7tricity, draw researchers’ attentions extensively for their innovative features and envir

8onmental benefits [15-17]. Up to now, numerous wastewater treatment processes base

9d on MFCs have been developed to enhance pollutants removal efficiencies with bioel

10ectricity generation [18-20], including the efforts on nitrogen-containing organic com

11pounds removals [21,22]. Moreover, several studies have utilized bioelectricity produ

12ced from MFCs to generate active substances with added reactants to effectively treat

13wastewater containing pollutants, such as phenol [23,24], p-nitrophenol [25] and arse

14nic [26]. These bioelectro-Fenton systems enhance the efficiencies at the cost of large

15amount of reactants as well as sludge [27], while processes without reactants consump

16tion and sludge generation can be more applicable. Though some attempts to enhance

17nitrogen-containing organic compounds treatment by bioelectricity from MFCs directl

18y have been made [28], factors affecting the performances should be intensively inves

19tigated and mechanisms should be further clarified.

20 Herein a novel aerated electrochemical system with utilization of MFCs as renew

21able power sources and omission of reactants addition was proposed for effective degr

22adation of two representative nitrogen-containing organic compounds, ie. pyridine and

1methyl orange (MO), belonging to nitrogen-heterocyclic compounds and azo dyes, res

2pectively, with their wide use and strong toxic effect to aquatic species and human. Op

3erating factors affecting the performance of above system were studied and the possib

4le degradation mechanisms were also further examined. This MFC intensified electroc

5hemical oxidation process was demonstrated efficient and cost-effective for nitrogen-c

6ontaining organic compounds degradation.

72. Experimental

82.1 Constructions of the reactors and chemicals

9 The configuration of the proposed system consisting of MFCs and an aeration ele

10ctrolytic reactor (AER) was shown in Fig. 1. Two identical single-chamber air-cathode

11MFCs in cubic shape which exhibited higher performance were constructed with an ef

12fective volume of 125 mL (5 cm × 5 cm × 5 cm) as our previous study noted [29]. The

13anode of MFCs was carbon fiber felt (4 cm × 4 cm × 1 cm) and the cathode was made

14of plain carbon paper (with 0.5 mg/cm2 of Pt on one side) with a projected surface are

15a of 16 cm2. They were connected together by copper wire with a 1000 Ω external resi

16stor during start-up. Each MFC was inoculated with 25 mL anaerobic sludge obtained

17from an up-flow anaerobic sludge blanket reactor treating high sulfate wastewater [3

180]. The anolyte contained 0.75 g/L glucose in a phosphate buffer (0.31 g/L NH4Cl; 0.1

193 g/l KCl; 4.97 g/L NaH2PO4•H2O; 2.75 g/L Na2HPO4•H2O) with 1.25 ml/L vitamin s

20olution and 12.5 ml/L trace mineral element solution [31].

21 The AER was built with a glass beaker and had a working volume of 200 mL. Th

1e anode and cathode with geometric dimension of 2.5 cm × 4 cm respectively were m

2ade of the same carbon materials and three kinds of carbon materials (carbon felt, carb

3on paper, carbon cloth) were tested. They were connected to the electrodes of MFCs b

4y copper wire with electrode spacing of 1.0 cm during the formal experiments. Air wa

5s sparged near the cathode at a flow rate of 2.5 L/min by an air inflator. A variable resi

6stor was also connected into the circuit in series to obtain the desired voltage across th

7e AER. Two similar glass beakers were also employed and acted as control sets. Fresh

8ly prepared solution with pyridine concentration of 200 mg/L was filled into the react

9ors, while the MO (C14H14N3O3SNa) stock solution (1000 mg/L) was prepared before

10electrolytic experiments [32] and it was diluted to specific concentration with deionize

11d water during experiments. All chemicals used in this study were analytical grade wit

12hout further purification.

132.2 Experimental procedures

14 The MFCs had been well developed by refreshing electrolyte every 3 d before fo

15rmal experiments. After start-up, the abilities of power outputs of single MFC and two

16MFCs connected in series were investigated respectively. Prior to electrolysis, the two

17electrodes of the AER were immersed in target solutions for 360 min to exclude the ad

18sorption influence. After that, feasibilities of degradation of pyridine (initial concentra

19tion of 200 mg/L) and MO (initial concentration of 50 mg/L) were evaluated in the pr

20oposed system, respectively, with the applied voltage settled at 700 mV. Considering t

21he volatility, the volatile pyridine was again dissolved in the aqueous solution by colle

22cting the air and introducing it to the AER. The factors affecting the system performan

1ce were also studied, with MO as target contaminant, including electrode materials (ca

2rbon felt, carbon paper, carbon cloth), applied voltage (300, 500, 700, 900 mV), initial

3MO concentration (20, 50, 80, 100 mg/L) and initial pH (2.5, 3.0, 4.0, 5.0). The initial

4pH was adjusted by 0.1 M H2SO4 and NaOH. When the applied voltage across the AE

5R settled at 300, 500, 700 mV, single MFC was employed in the system. When 900 m

6V was applied, two MFCs connected in series were used as power sources. Adjustmen

7t of a variable resistor (0 to 5000 Ω) in the circuit was companied under both two situa

8tions. After that, the degradation process and its mechanism were studied through mon

9itoring active substances and degradation products synchronously, compared with con

10trol sets. All the experiments were conducted at room temperature (22 ± 2 ºC). Each te

11st was repeated three times and their average results were reported.

122.3 Analytical methods

13 Pyridine concentration was analyzed by a High Performance Liquid Chromatogr

14aphy (HPLC) system (Shimadzu LC10ADVP, SPD10AVP UV–vis Detector; Rheodyne

157725i manual injector; Diamonsil C18 reverse-phase column, 250 mm × 4.6 mm, 5 μ

16m) as reported previously [6]. A methanol and water solution (4:1 or 1:1) was used as

17mobile phase at a ﬂow rate of 1.0 mL/min and pyridine was detected at 254 nm. MO c

18oncentration was monitored by a UV-vis spectrophotometer (DR 5000, HACH, USA)

19at 464 nm [32]. pH was measured by a pH-201 meter (Hanna, Italy). Total organic car

20bon (TOC) was monitored by Multi N/C 3000 TOC analyzer (Analytik Jena AG, Ger

21many). Ammonia-N was determined by ultraviolet spectrophotometer (DR 5000, HA

22CH, USA) according to standard methods. The concentration of hydrogen peroxide w

1as determined by the spectrophotometer at 350 nm after the sample mixed with 0.1 M

2potassium iodide and 0.01 M ammonium heptamolybdate tetrahydrate [33]. To further

3confirm the degradation products, gas chromatography/mass spectrometry (GC/MS) a

4nalysis was performed. During this part, the samples were extracted by dichlorometha

5ne and the extraction solutions were dehydrated with nitrogen. Hereafter, the organic

6phases were enriched and subjected to GC/MS (Trace GC-DSQ, Thermo Fisher, US

7A) analysis as reported previously [34]. The GC was equipped with a TR-35MS capill

8ary column (30 m × 0.25 mm × 0.25 μm). Ultrapure helium was used as the carrier ga

9s with a constant flow rate of 1.0 ml/min. An autosampler was used, and split injection

10was performed at a split ratio of 50. The oven temperature was programmed from 50 º

11C for 4 min, then increased at a ramp of 15 ºC/min to 280 ºC and hold for 3 min. MS

12was operated under the following conditions: transfer line, 220 ºC; ion source, 220 ºC,

13and electron energy, 70 eV [34]. The voltages were recorded by a data acquisition syst

14em (PMD1208LS, Measurement Computing Corp., Norton, MA, USA) at an interval

15of 5 min. Polarization curves were drawn with external resistances ranging from 5000

16to 10  using a resistor box to evaluate the performance of the MFCs and obtain the m

17aximum power density.

183. Results and Discussion

193.1 Evaluation of power outputs of the MFCs and pollutants removals

20 After start-up, polarization curves of single MFC and two MFCs connected in ser

21ies were investigated respectively and results were shown in Fig. 2a. Maximum power

1densities of 502.5 ± 17 mW/m2 and 401.6 ± 23 mW/m2 were exhibited for single MFC

2and two MFCs connected in series, respectively. When the AER was connected to the

3single MFC directly, the voltage outputs of the MFC monitored during a 10 d operatio

4n with fresh anolyte were 100-800 mV, while voltage outputs of 200-1100 mV were o

5bserved when the AER was connected to two serially linked MFCs (Fig. 2b), due to th

6e small current level in the closed circuit. As to the power output of two serially linked

7MFCs was smaller than that of single MFC, this could be due to voltage reversal caus

8ed by slight substrate concentrations differences between two MFCs which altered bac

9terial activity [35]. One of the stacked MFC units had relatively poor performance real

10ized in total lower power densities in the stacked MFCs [36]. Even so, these results sti

11ll demonstrated that the present MFCs could function as renewable power sources sep

12arately or in series for electrolysis experiments.

13 With initial pyridine concentration of 200 mg/L and the voltage across the AER

14was fixed at 700 mV by adjusting the variable resistor in the circuit, pyridine was grad

15ually removed and the degradation efficiency reached as high as 82.9% after 360 min

16operation (Fig. 3), showing advantage to foregoing pyridine degradation experiments.

17For example, only about half pyridine with initial concentration of 200 mg/L was degr

18aded by Rhodococcus strain after 10 h [37], while 14 h was required to remove 90% o

19f pyridine with initial concentration of 98.27 mg/L and addition of 146.72 mM H2O2

20[38]. Promising results were also obtained from MO degradation in the proposed syste

21m as 90.4% of MO was removed after 360 min operation with initial concentration of

2250 mg/L, which was comparable to previous study [34] and showed advantage to exist

1ing decolorization studies conducted in MFCs [39-41]. These results implied that nitro

2gen-containing organic compounds could be electrochemically degraded effectively w

3ith MFCs as renewable power sources.

4 Though the capital costs of full scale MFCs might be several times higher than th

5ose of conventional wastewater treatment systems and those of electricity from

6common DC power supplies, MFCs could also offer other interesting opportunities

7for improving their economic feasibility such as simultaneous treatment of domestic

8sewage which may also exist in factories, apart from bioelectricity production,

9confirming the promising application future of the proposed system [42].

103.2 Studies of operating factors

11 For better evaluation of the proposed system, operating factors were taken into ac

12count, with MO as representative nitrogen-containing organic compound.

13 The influences of three kinds of electrode materials on MO decolorization were

14conducted with MO initial concentration of 50 mg/L, applied voltage of 700 mV and i

15nitial pH of 3.0. A significant difference was noted when using different carbon materi

16als as electrodes with eliminating the materials’ adsorption effects (Fig. S1a in Suppor

17ting Information). Only 23.0% decolorization efficiency was achieved after 360 min e

18lectrolysis with carbon cloth as electrodes, in contrast, significant decolorization perfo

19rmances of 76.5% and 90.4% were exhibited using carbon paper and carbon felt as ele

20ctrodes, respectively. The excellent capability of carbon felt to color removal could be

21attributed to its larger surface area, which can provide more reactive sites for decolori

1zation as previous studies indicated [29]. Thus carbon felt was selected as efficient ele

2ctrode material for the AER in the following experiments.

3 Different applied voltages on MO decolorization efficiency were also studied wit

4h MO initial concentration of 50 mg/L and initial pH of 3.0. The decolorization efficie

5ncy of MO increased with the increase of applied voltages across the AER (Fig. S1b i

6n Supporting Information). As the variable resistor was adjusted manually, there was a

7deviation of 20 mV under each specific applied voltage. A notable improvement of de

8colorization efficiency was realized with applied voltage increasing from 300 mV to 5

900 mV. The increased production of oxidant such as hydrogen peroxide at higher appli

10ed voltage might be responsible for this improvement as indicated by other studies [11,

1143]. As electrolysis was performed under the relatively low voltage without water dec

12omposition in present study, further increase of applied voltage did not result in the m

13ore generation of above active substance, thus only a slight enhancement was obtained

14when the applied voltage further increased from 700 mV to 900 mV and the following

15experiments were conducted at 700 mV unless otherwise stated.

16 Four different initial MO concentration (20, 50, 80, 100 mg/L) were used to eval

17uate the effect of initial concentration on decolorization efficiency with applied voltag

18e of 700 mV and initial pH of 3.0. A higher initial dye concentration resulted in a relat

19ively lower decolorization efficiency from Fig. S1c in Supporting Information. After 3

2060 min electrolysis, the degradation efficiency of MO reached 93.5% (18.7 mg/L MO

21was removed) when initial MO concentration was 20 mg/L and a decolorization effici

22ency of 90.4% (45.2 mg/L MO was removed) was achieved with initial MO concentra

2 10 1

1tion of 50 mg/L. Further increasing initial dye concentration to 80 mg/L and 100 mg/L,

2 the decolorization efficiencies decreased to 88.6% (70.9 mg/L MO was removed) and

385.5% (85.5 mg/L MO was removed), respectively. Fig. 3c also revealed that decolori

4zation efficiencies of MO had reached relatively high levels after first 180 min and fur

5ther electrolysis had almost no significant effect. This might be ascribed to the limited

6generation of active substances while they could also react with MO degradation inter

7mediates instead of MO itself at the same time [44].

8 Solution pH is always an important factor in electrochemical tests. Notably, the c

9olor of MO solution varied with pH value. However, this phenomenon does not affect

10the MO determination at 464 nm as the corresponding relationship between the absorb

11ance and concentration is constant. Fig. S1d in Supporting Information showed the eff

12ect of various initial pH on decolorization efficiency with initial MO concentration of

1350 mg/L and applied voltage of 700 mV. It was found that decolorization efficiency de

14creased with the increase of initial pH and color removal favored at acidic conditions

15(Fig. 3d). A significant enhancement on color removal was noted when initial pH decr

16eased from 5.0 to 4.0 and the decolorization efficiency increased from 68.5% to 87.9

17% at the end of operating cycle (360 min) correspondingly. Obviously, higher hydroge

18n peroxide production under lower pH was in charge of higher decolorization efficien

19cy. However, further decreasing of initial pH from 4.0 to 2.5 did not lead to great incre

20ase in color removal efficiency due to the limitation of active substances generation at

21relatively lower applied voltage of 700 mV.

223.3 Investigation of degradation mechanisms

2 11 1

1 Electrolysis system functions mainly in two aspects, ie. direct and indirect electro

2chemical effects [34]. In the pyridine degradation experiment, slight pyridine removal

3was observed (17.1% within 360 min) with applied voltage of 700 mV from the same

4MFCs, while the air aeration was replaced by mechanical stirring under anaerobic con

5ditions. In this case, pyridine was found to be degraded almost linearly as a function o

6f reaction time, which fully illustrated the direct electrochemical effect worked a little

7on degradation due to the relatively lower applied voltage in present study. While ther

8e was rare pyridine removal with only air aeration without applied voltage, by dissolvi

9ng the volatile pyridine in the aqueous solution again. Compared with the significant e

10nhancement of pyridine removal in the AER (82.9% after 360 min operation), it impli

11ed that indirect electrochemical oxidation was mainly responsible for pyridine remova

13 In fact, hydrogen peroxide concentration was also monitored in the AER and it w

14as found that the accumulation of hydrogen peroxide increased almost linearly in first

15120 min electrolysis, after which the increment of hydrogen peroxide became weaker

16(Fig. 3). Generally, O2 reduction will lead to the generation of hydrogen peroxide whic

17h was consumed directly for pyridine degradation in present study according to Eq.

18(1):

+ - 19 O2 + 2H + 2e → H2O2 (1)

20 Actually, hydrogen peroxide could also be generated through water electrolysis

21[34]. However, this pathway was negligible as rare hydrogen peroxide could be detect

2 12 1

1ed in Control 1, due to no electron and proton sources in the water (except protonation

2by H2O) under relatively lower voltage. Though the current flowing from the single M

3FC to the AER is relatively lower, it can support the hydrogen peroxide production. M

4oreover, hydrogen peroxide production in proposed system is a spontaneous reaction

5while its generation with water electrolysis is non-spontaneous electrochemical reacti

6on, further exhibiting promising future in actual applications.

7 MO degradation experiment also exhibited the similar regularity and confirmed i

8ndirect electrochemical oxidation of MO by the generated hydrogen peroxide were the

9main effects. It should be noted that the amount of hydrogen peroxide accumulation o

10f about 5.4 mg/L within 360 min was relatively less than other electro-generation of h

11ydrogen peroxide processes [45] as lower applied voltage without pure oxygen was ap

12plied in present study. Moreover, the generation and consumption of hydrogen peroxi

13de could occur simultaneously. More efforts should be made to maximize hydrogen pe

14roxide yields in this proposed system.

15 In the pyridine degradation experiment, TOC was monitored and it gradually dec

16reased with 57.4% of TOC removal after 360 min (Fig. 3), suggesting a successful par

17tial mineralization of pyridine with a fraction of degradation of the intermediates. Am

18monia-N was also accumulated during the operation (Fig. 3), consistent with previous

19study [21]. To further ascertain degradation fractions in the solution after reaction, GC

20/MS was performed to identify the degradation fractions and results indicated are ring

21opening and desamination happened with generation of less toxic small molecule alde

22hyde and carboxylic acid, similar with the degradation intermediates reported by Zalat

2 13 1

1et al. [46], suggesting the proposed system could also reduce pyridine toxicity efficien

3 In the MO degradation experiment, there were also 62.4% TOC removal was ach

4ieved after 360 min and UV-vis spectra in the range of 200-800 nm also suggested the

5changes of the molecule and structural characteristics of MO with electrochemical deg

6radation (Fig. S2 in Supporting Information), including some derivatives of benzene, s

7uch as phenol and phthalic acid, assistant with previous research [34], indicating the d

8estruction of MO molecule (Table S1 in Supporting Information). However, species of

9intermediates detected here were more complex than results obtained by Li et al. [34],

10which might be caused by effective destroy of MO as well as the bonding between the

11intermediates.

12 In comparison with foregoing studies on degradation of nitrogen-containing orga

13nic compounds [47,48], this study exhibited a promising performance. Moreover, the

14present study realized the in situ generation and utilization of hydrogen peroxide for d

15egradation with bioelectricity from MFC instead of extra addition, reducing the cost o

16f advanced oxidation processes and providing a promising utilization form of low bioe

17lectricity from MFC not only in nitrogen-containing organic compounds removals, but

18also in other environmental contaminants controls. Other factors affecting the perform

19ance of the proposed system besides what had been considered in present study, such

20as flow regime, would be investigated individually afterwards for better evaluation of

21the proposed system for actual application.

2 14 1

14. Conclusions

2 An effective electrochemical system was constructed with MFCs as renewable p

3ower sources and successful degradation of nitrogen-containing organic compounds

4(pyridine and MO) was achieved in present study. Carbon felt performed the best as el

5ectrode material and higher applied voltage, while lower initial contaminant concentra

6tion and lower initial pH value could improve the performance. Comparative studies i

7mplied that indirect electrochemical oxidation by generated hydrogen peroxide was th

8e main effect. Partial mineralization of nitrogen-containing organic compounds were a

9lso observed. This work constituted a step ahead in developing strategy for enhancing

10electrochemical degradation of nitrogen-containing organic compounds with bioelectr

11icity from MFCs.

12Acknowledgements

13 This research work was supported by the National Natural Science Foundation of

14China (NSFC) (No. 21307117, No. 41440025), the Research Fund for the Doctoral

15Program of Higher Education of China (No. 20120022120005), the Beijing Excellent

16Talent Training Project (No. 2013D009015000003), the Beijing Higher Education

17Young Elite Teacher Project (No. YETP0657) and the Fundamental Research Funds

18for the Central Universities (No.2652015226, No. 2652015131).

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1Figure Captions

2Fig. 1. Experimental apparatus consisting of MFCs and AER.

3Fig. 2. Polarization curves (a) and voltage outputs (b) of individual MFC and two

4MFCs in series. The voltage outputs were monitored when the AER was connected to

5MFCs.

6Fig. 3. Pyridine degradation and TOC removal with generations of hydrogen peroxide 7and Ammonia-N in the AER.

2 23 1

1 2Figure 1

2 24 1

1200 600 MFC-1 a 1000 MFC-2 500 Two MFCs in series ) 2 m 800 400 / ) W V m m ( (

y e t i

g 600 300 s a n t l e o d

V r 400 200 e w o P

200 100

0 0 0 500 1000 1500 2000 2500 3000

2 1 Current density (mA/m ) b 1200

MFC-1 1000 MFC-2 Two MFCs in series 800 ) V m (

e 600 g a t l o V 400

0 0 2 4 6 8 10 2 T im e (d)

3 Figure 2

2 25 1

250 10

Pyridine 200 TOC 8 Hydrogen peroxide ) ) L L / /

g Ammonia-N g m m

( 150 6 (

n n o o i i t t a a r r t t

e 100 4 e c c n n o o C C

0 0 0 50 100 150 200 250 300 350 400 1 Time (min)

2Figure 3