A Review on Improvement and Application of Fenton Reagent

Utilization of single-chamber microbial fuel cells as renewable power sources for electrochemical degradation of nitrogen-containing organic compounds

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

a School of Water Resources and Environment, China University of Geosciences Beijing, Key Laboratory of Groundwater Circulation and Evolution (China University of Geosciences Beijing), Ministry of Education, Beijing 100083, China

b School of Engineering, The University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JL, UK

c Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China

Abstract

By employing promising single-chamber microbial fuel cells (MFCs) as renewable power sources, an aerated electrochemical system is proposed and for nitrogen-containing organic compounds (pyridine and methyl orange) removals. Carbon felt performed the best as electrode material while lower initial contaminant concentration and lower initial pH value could improve the performance. A degradation efficiency of 82.9% for pyridine was achieved after 360 min electrolysis with its initial concentration of 200 mg/L, initial pH of 3.0 and applied voltage of 700 mV. Mechanisms study implied that indirect electrochemical oxidation by generated hydrogen peroxide was responsible for their degradation. This study provides an alternative utilization form of low bioelectricity from MFCs and reveals that applying it to electrochemical process is highly-efficient as well as cost-effective for degradation of nitrogen-containing organic compounds.

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

1. Introduction

Among various global environmental pollutions, recalcitrant pollutants have drawn extensive attention in the last few decades, especially nitrogen-containing organic compounds discharged from coal coking, textile, petroleum refining, pharmaceutical factories and other industries [1,2]. They usually contain complex structure compounds such as aromatic rings or azo bond that may cause harmful and irreversible environmental problems even at quite low concentrations [2,3]. A large portion of papers have been devoted to the degradation of nitrogen-containing organic compounds based on physical and biological methods [4-6]. The physical means realize nitrogen-containing organic compounds removals with only transferring them from water to another phase, which may cause secondary pollution [7] and unavoidably, the post-treatments including regeneration of the adsorbent and reactivation of membrane fouling that will lead to highly consumption of the energy and restriction of the actual application. As to biological treatments, they often suffer from low efficiency due to microbial inhibition [8,9]. Comparatively, chemical-oxidative processes have been employed for nitrogen-containing organic compounds removals removal from aqueous solutions extensively as they can decompose molecules effectively. By omitting the generation of sludge from Fenton process and the utilization of photocatalyst, electrolysis technology exhibits promising future for nitrogen-containing organic compounds contamination controls among chemical-oxidative processes [10,11]. As higher applied voltage (almost up to 12 V) or current (up to 6 A) are often required in these processes [12-14], thus other alternatives should be explored to strengthen their efficiencies with energy conservation.

Recently, microbial fuel cells (MFCs), devices using microorganisms as the catalysts to oxidize organic and inorganic matters and converting chemical energy into electricity, draw researchers’ attentions extensively for their innovative features and environmental benefits [15-17]. Up to now, numerous wastewater treatment processes based on MFCs have been developed to enhance pollutants removal efficiencies with bioelectricity generation [18-20], including the efforts on nitrogen-containing organic compounds removals [21,22]. Moreover, several studies have utilized bioelectricity produced from MFCs to generate active substances with added reactants to effectively treat wastewater containing pollutants, such as phenol [23,24], p-nitrophenol [25] and arsenic [26]. These bioelectro-Fenton systems enhance the efficiencies at the cost of large amount of reactants as well as sludge [27], while processes without reactants consumption and sludge generation can be more applicable. Though some attempts to enhance nitrogen-containing organic compounds treatment by bioelectricity from MFCs directly have been made [28], factors affecting the performances should be intensively investigated and mechanisms should be further clarified.

Herein a novel aerated electrochemical system with utilization of MFCs as renewable power sources and omission of reactants addition was proposed for effective degradation of two representative nitrogen-containing organic compounds, ie. pyridine and methyl orange (MO), belonging to nitrogen-heterocyclic compounds and azo dyes, respectively, with their wide use and strong toxic effect to aquatic species and human. Operating factors affecting the performance of above system were studied and the possible degradation mechanisms were also further examined. This MFC intensified electrochemical oxidation process was demonstrated efficient and cost-effective for nitrogen-containing organic compounds degradation.

2. Experimental

2.1 Constructions of the reactors and chemicals

The configuration of the proposed system consisting of MFCs and an aeration electrolytic reactor (AER) was shown in Fig. 1. Two identical single-chamber air-cathode MFCs in cubic shape which exhibited higher performance were constructed with an effective volume of 125 mL (5 cm × 5 cm × 5 cm) as our previous study noted [29]. The anode of MFCs was carbon fiber felt (4 cm × 4 cm × 1 cm) and the cathode was made of plain carbon paper (with 0.5 mg/cm2 of Pt on one side) with a projected surface area of 16 cm2. They were connected together by copper wire with a 1000 Ω external resistor during start-up. Each MFC was inoculated with 25 mL anaerobic sludge obtained from an up-flow anaerobic sludge blanket reactor treating high sulfate wastewater [30]. The anolyte contained 0.75 g/L glucose in a phosphate buffer (0.31 g/L NH4Cl; 0.13 g/l KCl; 4.97 g/L NaH2PO4•H2O; 2.75 g/L Na2HPO4•H2O) with 1.25 ml/L vitamin solution and 12.5 ml/L trace mineral element solution [31].

The AER was built with a glass beaker and had a working volume of 200 mL. The anode and cathode with geometric dimension of 2.5 cm × 4 cm respectively were made of the same carbon materials and three kinds of carbon materials (carbon felt, carbon paper, carbon cloth) were tested. They were connected to the electrodes of MFCs by copper wire with electrode spacing of 1.0 cm during the formal experiments. Air was sparged near the cathode at a flow rate of 2.5 L/min by an air inflator. A variable resistor was also connected into the circuit in series to obtain the desired voltage across the AER. Two similar glass beakers were also employed and acted as control sets. Freshly prepared solution with pyridine concentration of 200 mg/L was filled into the reactors, while the MO (C14H14N3O3SNa) stock solution (1000 mg/L) was prepared before electrolytic experiments [32] and it was diluted to specific concentration with deionized water during experiments. All chemicals used in this study were analytical grade without further purification.

2.2 Experimental procedures

The MFCs had been well developed by refreshing electrolyte every 3 d before formal experiments. After start-up, the abilities of power outputs of single MFC and two MFCs connected in series were investigated respectively. Prior to electrolysis, the two electrodes of the AER were immersed in target solutions for 360 min to exclude the adsorption influence. After that, feasibilities of degradation of pyridine (initial concentration of 200 mg/L) and MO (initial concentration of 50 mg/L) were evaluated in the proposed system, respectively, with the applied voltage settled at 700 mV. Considering the volatility, the volatile pyridine was again dissolved in the aqueous solution by collecting the air and introducing it to the AER. The factors affecting the system performance were also studied, with MO as target contaminant, including electrode materials (carbon felt, carbon paper, carbon cloth), applied voltage (300, 500, 700, 900 mV), initial MO concentration (20, 50, 80, 100 mg/L) and initial pH (2.5, 3.0, 4.0, 5.0). The initial pH was adjusted by 0.1 M H2SO4 and NaOH. When the applied voltage across the AER settled at 300, 500, 700 mV, single MFC was employed in the system. When 900 mV was applied, two MFCs connected in series were used as power sources. Adjustment of a variable resistor (0 to 5000 Ω) in the circuit was companied under both two situations. After that, the degradation process and its mechanism were studied through monitoring active substances and degradation products synchronously, compared with control sets. All the experiments were conducted at room temperature (22 ± 2 ºC). Each test was repeated three times and their average results were reported.

2.3 Analytical methods

Pyridine concentration was analyzed by a High Performance Liquid Chromatography (HPLC) system (Shimadzu LC10ADVP, SPD10AVP UV–vis Detector; Rheodyne 7725i manual injector; Diamonsil C18 reverse-phase column, 250 mm × 4.6 mm, 5 μm) as reported previously [6]. A methanol and water solution (4:1 or 1:1) was used as mobile phase at a flow rate of 1.0 mL/min and pyridine was detected at 254 nm. MO concentration was monitored by a UV-vis spectrophotometer (DR 5000, HACH, USA) at 464 nm [32]. pH was measured by a pH-201 meter (Hanna, Italy). Total organic carbon (TOC) was monitored by Multi N/C 3000 TOC analyzer (Analytik Jena AG, Germany). Ammonia-N was determined by ultraviolet spectrophotometer (DR 5000, HACH, USA) according to standard methods. The concentration of hydrogen peroxide was determined by the spectrophotometer at 350 nm after the sample mixed with 0.1 M potassium iodide and 0.01 M ammonium heptamolybdate tetrahydrate [33]. To further confirm the degradation products, gas chromatography/mass spectrometry (GC/MS) analysis was performed. During this part, the samples were extracted by dichloromethane and the extraction solutions were dehydrated with nitrogen. Hereafter, the organic phases were enriched and subjected to GC/MS (Trace GC-DSQ, Thermo Fisher, USA) analysis as reported previously [34]. The GC was equipped with a TR-35MS capillary column (30 m × 0.25 mm × 0.25 μm). Ultrapure helium was used as the carrier gas with a constant flow rate of 1.0 ml/min. An autosampler was used, and split injection was performed at a split ratio of 50. The oven temperature was programmed from 50 ºC for 4 min, then increased at a ramp of 15 ºC/min to 280 ºC and hold for 3 min. MS was operated under the following conditions: transfer line, 220 ºC; ion source, 220 ºC, and electron energy, 70 eV [34]. The voltages were recorded by a data acquisition system (PMD1208LS, Measurement Computing Corp., Norton, MA, USA) at an interval of 5 min. Polarization curves were drawn with external resistances ranging from 5000 to 10 ( using a resistor box to evaluate the performance of the MFCs and obtain the maximum power density.

3. Results and Discussion

3.1 Evaluation of power outputs of the MFCs and pollutants removals

After start-up, polarization curves of single MFC and two MFCs connected in series were investigated respectively and results were shown in Fig. 2a. Maximum power densities of 502.5 ± 17 mW/m2 and 401.6 ± 23 mW/m2 were exhibited for single MFC and two MFCs connected in series, respectively. When the AER was connected to the single MFC directly, the voltage outputs of the MFC monitored during a 10 d operation with fresh anolyte were 100-800 mV, while voltage outputs of 200-1100 mV were observed when the AER was connected to two serially linked MFCs (Fig. 2b), due to the small current level in the closed circuit. As to the power output of two serially linked MFCs was smaller than that of single MFC, this could be due to voltage reversal caused by slight substrate concentrations differences between two MFCs which altered bacterial activity [35]. One of the stacked MFC units had relatively poor performance realized in total lower power densities in the stacked MFCs [36]. Even so, these results still demonstrated that the present MFCs could function as renewable power sources separately or in series for electrolysis experiments.

With initial pyridine concentration of 200 mg/L and the voltage across the AER was fixed at 700 mV by adjusting the variable resistor in the circuit, pyridine was gradually removed and the degradation efficiency reached as high as 82.9% after 360 min operation (Fig. 3), showing advantage to foregoing pyridine degradation experiments. For example, only about half pyridine with initial concentration of 200 mg/L was degraded by Rhodococcus strain after 10 h [37], while 14 h was required to remove 90% of pyridine with initial concentration of 98.27 mg/L and addition of 146.72 mM H2O2 [38]. Promising results were also obtained from MO degradation in the proposed system as 90.4% of MO was removed after 360 min operation with initial concentration of 50 mg/L, which was comparable to previous study [34] and showed advantage to existing decolorization studies conducted in MFCs [39-41]. These results implied that nitrogen-containing organic compounds could be electrochemically degraded effectively with MFCs as renewable power sources.

Though the capital costs of full scale MFCs might be several times higher than those of conventional wastewater treatment systems and those of electricity from common DC power supplies, MFCs could also offer other interesting opportunities for improving their economic feasibility such as simultaneous treatment of domestic sewage which may also exist in factories, apart from bioelectricity production, confirming the promising application future of the proposed system [42].

3.2 Studies of operating factors

For better evaluation of the proposed system, operating factors were taken into account, with MO as representative nitrogen-containing organic compound.

The influences of three kinds of electrode materials on MO decolorization were conducted with MO initial concentration of 50 mg/L, applied voltage of 700 mV and initial pH of 3.0. A significant difference was noted when using different carbon materials as electrodes with eliminating the materials’ adsorption effects (Fig. S1a in Supporting Information). Only 23.0% decolorization efficiency was achieved after 360 min electrolysis with carbon cloth as electrodes, in contrast, significant decolorization performances of 76.5% and 90.4% were exhibited using carbon paper and carbon felt as electrodes, respectively. The excellent capability of carbon felt to color removal could be attributed to its larger surface area, which can provide more reactive sites for decolorization as previous studies indicated [29]. Thus carbon felt was selected as efficient electrode material for the AER in the following experiments.

Different applied voltages on MO decolorization efficiency were also studied with MO initial concentration of 50 mg/L and initial pH of 3.0. The decolorization efficiency of MO increased with the increase of applied voltages across the AER (Fig. S1b in Supporting Information). As the variable resistor was adjusted manually, there was a deviation of 20 mV under each specific applied voltage. A notable improvement of decolorization efficiency was realized with applied voltage increasing from 300 mV to 500 mV. The increased production of oxidant such as hydrogen peroxide at higher applied voltage might be responsible for this improvement as indicated by other studies [11,43]. As electrolysis was performed under the relatively low voltage without water decomposition in present study, further increase of applied voltage did not result in the more generation of above active substance, thus only a slight enhancement was obtained when the applied voltage further increased from 700 mV to 900 mV and the following experiments were conducted at 700 mV unless otherwise stated.

Four different initial MO concentration (20, 50, 80, 100 mg/L) were used to evaluate the effect of initial concentration on decolorization efficiency with applied voltage of 700 mV and initial pH of 3.0. A higher initial dye concentration resulted in a relatively lower decolorization efficiency from Fig. S1c in Supporting Information. After 360 min electrolysis, the degradation efficiency of MO reached 93.5% (18.7 mg/L MO was removed) when initial MO concentration was 20 mg/L and a decolorization efficiency of 90.4% (45.2 mg/L MO was removed) was achieved with initial MO concentration of 50 mg/L. Further increasing initial dye concentration to 80 mg/L and 100 mg/L, the decolorization efficiencies decreased to 88.6% (70.9 mg/L MO was removed) and 85.5% (85.5 mg/L MO was removed), respectively. Fig. 3c also revealed that decolorization efficiencies of MO had reached relatively high levels after first 180 min and further electrolysis had almost no significant effect. This might be ascribed to the limited generation of active substances while they could also react with MO degradation intermediates instead of MO itself at the same time [44].

Solution pH is always an important factor in electrochemical tests. Notably, the color of MO solution varied with pH value. However, this phenomenon does not affect the MO determination at 464 nm as the corresponding relationship between the absorbance and concentration is constant. Fig. S1d in Supporting Information showed the effect of various initial pH on decolorization efficiency with initial MO concentration of 50 mg/L and applied voltage of 700 mV. It was found that decolorization efficiency decreased with the increase of initial pH and color removal favored at acidic conditions (Fig. 3d). A significant enhancement on color removal was noted when initial pH decreased from 5.0 to 4.0 and the decolorization efficiency increased from 68.5% to 87.9% at the end of operating cycle (360 min) correspondingly. Obviously, higher hydrogen peroxide production under lower pH was in charge of higher decolorization efficiency. However, further decreasing of initial pH from 4.0 to 2.5 did not lead to great increase in color removal efficiency due to the limitation of active substances generation at relatively lower applied voltage of 700 mV.

3.3 Investigation of degradation mechanisms

Electrolysis system functions mainly in two aspects, ie. direct and indirect electrochemical effects [34]. In the pyridine degradation experiment, slight pyridine removal was observed (17.1% within 360 min) with applied voltage of 700 mV from the same MFCs, while the air aeration was replaced by mechanical stirring under anaerobic conditions. In this case, pyridine was found to be degraded almost linearly as a function of reaction time, which fully illustrated the direct electrochemical effect worked a little on degradation due to the relatively lower applied voltage in present study. While there was rare pyridine removal with only air aeration without applied voltage, by dissolving the volatile pyridine in the aqueous solution again. Compared with the significant enhancement of pyridine removal in the AER (82.9% after 360 min operation), it implied that indirect electrochemical oxidation was mainly responsible for pyridine removal.

In fact, hydrogen peroxide concentration was also monitored in the AER and it was found that the accumulation of hydrogen peroxide increased almost linearly in first 120 min electrolysis, after which the increment of hydrogen peroxide became weaker (Fig. 3). Generally, O2 reduction will lead to the generation of hydrogen peroxide which was consumed directly for pyridine degradation in present study according to Eq. (1):

O2 + 2H+ + 2e- → H2O2 (1)

Actually, hydrogen peroxide could also be generated through water electrolysis [34]. However, this pathway was negligible as rare hydrogen peroxide could be detected in Control 1, due to no electron and proton sources in the water (except protonation by H2O) under relatively lower voltage. Though the current flowing from the single MFC to the AER is relatively lower, it can support the hydrogen peroxide production. Moreover, hydrogen peroxide production in proposed system is a spontaneous reaction while its generation with water electrolysis is non-spontaneous electrochemical reaction, further exhibiting promising future in actual applications.

MO degradation experiment also exhibited the similar regularity and confirmed indirect electrochemical oxidation of MO by the generated hydrogen peroxide were the main effects. It should be noted that the amount of hydrogen peroxide accumulation of about 5.4 mg/L within 360 min was relatively less than other electro-generation of hydrogen peroxide processes [45] as lower applied voltage without pure oxygen was applied in present study. Moreover, the generation and consumption of hydrogen peroxide could occur simultaneously. More efforts should be made to maximize hydrogen peroxide yields in this proposed system.

In the pyridine degradation experiment, TOC was monitored and it gradually decreased with 57.4% of TOC removal after 360 min (Fig. 3), suggesting a successful partial mineralization of pyridine with a fraction of degradation of the intermediates. Ammonia-N was also accumulated during the operation (Fig. 3), consistent with previous study [21]. To further ascertain degradation fractions in the solution after reaction, GC/MS was performed to identify the degradation fractions and results indicated are ring opening and desamination happened with generation of less toxic small molecule aldehyde and carboxylic acid, similar with the degradation intermediates reported by Zalat et al. [46], suggesting the proposed system could also reduce pyridine toxicity efficiently.

In the MO degradation experiment, there were also 62.4% TOC removal was achieved after 360 min and UV-vis spectra in the range of 200-800 nm also suggested the changes of the molecule and structural characteristics of MO with electrochemical degradation (Fig. S2 in Supporting Information), including some derivatives of benzene, such as phenol and phthalic acid, assistant with previous research [34], indicating the destruction of MO molecule (Table S1 in Supporting Information). However, species of intermediates detected here were more complex than results obtained by Li et al. [34], which might be caused by effective destroy of MO as well as the bonding between the intermediates.

In comparison with foregoing studies on degradation of nitrogen-containing organic compounds [47,48], this study exhibited a promising performance. Moreover, the present study realized the in situ generation and utilization of hydrogen peroxide for degradation with bioelectricity from MFC instead of extra addition, reducing the cost of advanced oxidation processes and providing a promising utilization form of low bioelectricity from MFC not only in nitrogen-containing organic compounds removals, but also in other environmental contaminants controls. Other factors affecting the performance of the proposed system besides what had been considered in present study, such as flow regime, would be investigated individually afterwards for better evaluation of the proposed system for actual application.

4. Conclusions

An effective electrochemical system was constructed with MFCs as renewable power sources and successful degradation of nitrogen-containing organic compounds (pyridine and MO) was achieved in present study. Carbon felt performed the best as electrode material and higher applied voltage, while lower initial contaminant concentration and lower initial pH value could improve the performance. Comparative studies implied that indirect electrochemical oxidation by generated hydrogen peroxide was the main effect. Partial mineralization of nitrogen-containing organic compounds were also observed. This work constituted a step ahead in developing strategy for enhancing electrochemical degradation of nitrogen-containing organic compounds with bioelectricity from MFCs.

Acknowledgements

This research work was supported by the National Natural Science Foundation of China (NSFC) (No. 21307117, No. 41440025), the Research Fund for the Doctoral Program of Higher Education of China (No. 20120022120005), the Beijing Excellent Talent Training Project (No. 2013D009015000003), the Beijing Higher Education Young Elite Teacher Project (No. YETP0657) and the Fundamental Research Funds for the Central Universities (No.2652015226, No. 2652015131).

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Figure Captions

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

Fig. 2. Polarization curves (a) and voltage outputs (b) of individual MFC and two MFCs in series. The voltage outputs were monitored when the AER was connected to MFCs.

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

Figure 1

0

200

400

600

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050010001500200025003000

Current density (mA/m

2

)

Voltage (mV)

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Power density (mW/m

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)

MFC-1

MFC-2

Two MFCs in series

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0246810

Time (d)

Voltage (mV)

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Figure 2

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050100150200250300350400

Time (min)

Concentration (mg/L)

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Pyridine

TOC

Hydrogen peroxide

Ammonia-N

Figure 3

b

a

* Corresponding author. Tel.: +86 10 8232 2281; fax: +86 10 8232 1081.

E-mail: � HYPERLINK "mailto:zbgcugb@gmail.com" ��zbgcugb@gmail.com�, baogangzhang@cugb.edu.cn (B. Zhang)

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