A Review on Improvement and Application of Fenton Reagent

11Utilization of single-chamber microbial fuel cells as renewable power sources for2electrochemical degradation of nitrogen-containing organic compounds3Zhijun Wanga, Baogang Zhanga*, Alistair G.L. Borthwickb, Chuanping Fenga, Jinren4Nic5a 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 renewab14le power sources, an aerated electrochemical system is proposed and for nitrogen-cont15aining organic compounds (pyridine and methyl orange) removals. Carbon felt perfor16med the best as electrode material while lower initial contaminant concentration and l17ower initial pH value could improve the performance. A degradation efficiency of 82.189% for pyridine was achieved after 360 min electrolysis with its initial concentration19of 200 mg/L, initial pH of 3.0 and applied voltage of 700 mV. Mechanisms study impl20ied that indirect electrochemical oxidation by generated hydrogen peroxide was respo21nsible for their degradation. This study provides an alternative utilization form of low22bioelectricity from MFCs and reveals that applying it to electrochemical process is hig23hly-efficient as well as cost-effective for degradation of nitrogen-containing organic c2,* Corresponding author. Tel.: +86 10 8232 2281; fax: +86 10 8232 1081. 3 E-mail: [email protected], [email protected] (B. Zhang)4 1 11ompounds.2Keywords: Microbial fuel cells; Nitrogen-containing organic compounds; Pyridine;3Methyl orange41. Introduction5 Among various global environmental pollutions, recalcitrant pollutants have dra6wn extensive attention in the last few decades, especially nitrogen-containing organic7compounds discharged from coal coking, textile, petroleum refining, pharmaceutical f8actories and other industries [1,2]. They usually contain complex structure compounds9such as aromatic rings or azo bond that may cause harmful and irreversible environme10ntal problems even at quite low concentrations [2,3]. A large portion of papers have be11en devoted to the degradation of nitrogen-containing organic compounds based on ph12ysical and biological methods [4-6]. The physical means realize nitrogen-containing o13rganic compounds removals with only transferring them from water to another phase,14which may cause secondary pollution [7] and unavoidably, the post-treatments includi15ng regeneration of the adsorbent and reactivation of membrane fouling that will lead t16o highly consumption of the energy and restriction of the actual application. As to biol17ogical treatments, they often suffer from low efficiency due to microbial inhibition [8,189]. Comparatively, chemical-oxidative processes have been employed for nitrogen-co19ntaining organic compounds removals removal from aqueous solutions extensively as20they can decompose molecules effectively. By omitting the generation of sludge from21Fenton process and the utilization of photocatalyst, electrolysis technology exhibits pr2 2 11omising future for nitrogen-containing organic compounds contamination controls am2ong chemical-oxidative processes [10,11]. As higher applied voltage (almost up to 123V) or current (up to 6 A) are often required in these processes [12-14], thus other alter4natives should be explored to strengthen their efficiencies with energy conservation.5 Recently, microbial fuel cells (MFCs), devices using microorganisms as the catal6ysts to oxidize organic and inorganic matters and converting chemical energy into elec7tricity, draw researchers’ attentions extensively for their innovative features and envir8onmental benefits [15-17]. Up to now, numerous wastewater treatment processes base9d on MFCs have been developed to enhance pollutants removal efficiencies with bioel10ectricity generation [18-20], including the efforts on nitrogen-containing organic com11pounds removals [21,22]. Moreover, several studies have utilized bioelectricity produ12ced from MFCs to generate active substances with added reactants to effectively treat13wastewater containing pollutants, such as phenol [23,24], p-nitrophenol [25] and arse14nic [26]. These bioelectro-Fenton systems enhance the efficiencies at the cost of large15amount of reactants as well as sludge [27], while processes without reactants consump16tion and sludge generation can be more applicable. Though some attempts to enhance17nitrogen-containing organic compounds treatment by bioelectricity from MFCs directl18y have been made [28], factors affecting the performances should be intensively inves19tigated and mechanisms should be further clarified. 20 Herein a novel aerated electrochemical system with utilization of MFCs as renew21able power sources and omission of reactants addition was proposed for effective degr22adation of two representative nitrogen-containing organic compounds, ie. pyridine and2 3 11methyl orange (MO), belonging to nitrogen-heterocyclic compounds and azo dyes, res2pectively, with their wide use and strong toxic effect to aquatic species and human. Op3erating factors affecting the performance of above system were studied and the possib4le degradation mechanisms were also further examined. This MFC intensified electroc5hemical oxidation process was demonstrated efficient and cost-effective for nitrogen-c6ontaining organic compounds degradation.72. Experimental82.1 Constructions of the reactors and chemicals9 The configuration of the proposed system consisting of MFCs and an aeration ele10ctrolytic reactor (AER) was shown in Fig. 1. Two identical single-chamber air-cathode11MFCs in cubic shape which exhibited higher performance were constructed with an ef12fective volume of 125 mL (5 cm × 5 cm × 5 cm) as our previous study noted [29]. The13anode of MFCs was carbon fiber felt (4 cm × 4 cm × 1 cm) and the cathode was made14of plain carbon paper (with 0.5 mg/cm2 of Pt on one side) with a projected surface are15a of 16 cm2. They were connected together by copper wire with a 1000 Ω external resi16stor during start-up. Each MFC was inoculated with 25 mL anaerobic sludge obtained17from an up-flow anaerobic sludge blanket reactor treating high sulfate wastewater [3180]. The anolyte contained 0.75 g/L glucose in a phosphate buffer (0.31 g/L NH4Cl; 0.1193 g/l KCl; 4.97 g/L NaH2PO4•H2O; 2.75 g/L Na2HPO4•H2O) with 1.25 ml/L vitamin s20olution 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. Th2 4 11e anode and cathode with geometric dimension of 2.5 cm × 4 cm respectively were m2ade of the same carbon materials and three kinds of carbon materials (carbon felt, carb3on paper, carbon cloth) were tested. They were connected to the electrodes of MFCs b4y copper wire with electrode spacing of 1.0 cm during the formal experiments. Air wa5s sparged near the cathode at a flow rate of 2.5 L/min by an air inflator. A variable resi6stor was also connected into the circuit in series to obtain the desired voltage across th7e AER. Two similar glass beakers were also employed and acted as control sets. Fresh8ly prepared solution with pyridine concentration of 200 mg/L was filled into the react9ors, while the MO (C14H14N3O3SNa) stock solution (1000 mg/L) was prepared before10electrolytic experiments [32] and it was diluted to specific concentration with deionize11d water during experiments. All chemicals used in this study were analytical grade wit12hout further purification.132.2 Experimental procedures14 The MFCs had been well developed by refreshing electrolyte every 3 d before fo15rmal experiments. After start-up, the abilities of power outputs of single MFC and two16MFCs connected in series were investigated respectively. Prior to electrolysis, the two17electrodes of the AER were immersed in target solutions for 360 min to exclude the ad18sorption influence. After that, feasibilities of degradation of pyridine (initial concentra19tion of 200 mg/L) and MO (initial concentration of 50 mg/L) were evaluated in the pr20oposed system, respectively, with the applied voltage settled at 700 mV. Considering t21he volatility, the volatile pyridine was again dissolved in the aqueous solution by colle22cting the air and introducing it to the AER. The factors affecting the system performan2 5 11ce were also studied, with MO as target contaminant, including electrode materials (ca2rbon felt, carbon paper, carbon cloth), applied voltage (300, 500, 700, 900 mV), initial3MO concentration (20, 50, 80, 100 mg/L) and initial pH (2.5, 3.0, 4.0, 5.0). The initial4pH was adjusted by 0.1 M H2SO4 and NaOH. When the applied voltage across the AE5R settled at 300, 500, 700 mV, single MFC was employed in the system. When 900 m6V was applied, two MFCs connected in series were used as power sources. Adjustmen7t of a variable resistor (0 to 5000 Ω) in the circuit was companied under both two situa8tions. After that, the degradation process and its mechanism were studied through mon9itoring active substances and degradation products synchronously, compared with con10trol sets. All the experiments were conducted at room temperature (22 ± 2 ºC). Each te11st was repeated three times and their average results were reported. 122.3 Analytical methods13 Pyridine concentration was analyzed by a High Performance Liquid Chromatogr14aphy (HPLC) system (Shimadzu LC10ADVP, SPD10AVP UV–vis Detector; Rheodyne 157725i manual injector; Diamonsil C18 reverse-phase column, 250 mm × 4.6 mm, 5 μ16m) 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 c18oncentration 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 car20bon (TOC) was monitored by Multi N/C 3000 TOC analyzer (Analytik Jena AG, Ger21many). Ammonia-N was determined by ultraviolet spectrophotometer (DR 5000, HA22CH, USA) according to standard methods. The concentration of hydrogen peroxide w2 6 11as 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 further3confirm the degradation products, gas chromatography/mass spectrometry (GC/MS) a4nalysis was performed. During this part, the samples were extracted by dichlorometha5ne and the extraction solutions were dehydrated with nitrogen. Hereafter, the organic 6phases were enriched and subjected to GC/MS (Trace GC-DSQ, Thermo Fisher, US7A) analysis as reported previously [34]. The GC was equipped with a TR-35MS capill8ary column (30 m × 0.25 mm × 0.25 μm). Ultrapure helium was used as the carrier ga9s with a constant flow rate of 1.0 ml/min. An autosampler was used, and split injection10was performed at a split ratio of 50. The oven temperature was programmed from 50 º11C 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 syst14em (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 m17aximum power density.183. Results and Discussion193.1 Evaluation of power outputs of the MFCs and pollutants removals20 After start-up, polarization curves of single MFC and two MFCs connected in ser21ies were investigated respectively and results were shown in Fig. 2a. Maximum power2 7 11densities of 502.5 ± 17 mW/m2 and 401.6 ± 23 mW/m2 were exhibited for single MFC2and two MFCs connected in series, respectively. When the AER was connected to the3single MFC directly, the voltage outputs of the MFC monitored during a 10 d operatio4n with fresh anolyte were 100-800 mV, while voltage outputs of 200-1100 mV were o5bserved when the AER was connected to two serially linked MFCs (Fig. 2b), due to th6e small current level in the closed circuit. As to the power output of two serially linked7MFCs was smaller than that of single MFC, this could be due to voltage reversal caus8ed by slight substrate concentrations differences between two MFCs which altered bac9terial activity [35]. One of the stacked MFC units had relatively poor performance real10ized in total lower power densities in the stacked MFCs [36]. Even so, these results sti11ll demonstrated that the present MFCs could function as renewable power sources sep12arately or in series for electrolysis experiments. 13 With initial pyridine concentration of 200 mg/L and the voltage across the AER14was fixed at 700 mV by adjusting the variable resistor in the circuit, pyridine was grad15ually removed and the degradation efficiency reached as high as 82.9% after 360 min16operation (Fig. 3), showing advantage to foregoing pyridine degradation experiments.17For example, only about half pyridine with initial concentration of 200 mg/L was degr18aded by Rhodococcus strain after 10 h [37], while 14 h was required to remove 90% o19f pyridine with initial concentration of 98.27 mg/L and addition of 146.72 mM H2O220[38]. Promising results were also obtained from MO degradation in the proposed syste21m as 90.4% of MO was removed after 360 min operation with initial concentration of2250 mg/L, which was comparable to previous study [34] and showed advantage to exist2 8 11ing decolorization studies conducted in MFCs [39-41]. These results implied that nitro2gen-containing organic compounds could be electrochemically degraded effectively w3ith MFCs as renewable power sources.4 Though the capital costs of full scale MFCs might be several times higher than th5ose of conventional wastewater treatment systems and those of electricity from6common DC power supplies, MFCs could also offer other interesting opportunities7for improving their economic feasibility such as simultaneous treatment of domestic8sewage 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 factors11 For better evaluation of the proposed system, operating factors were taken into ac12count, 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 i15nitial pH of 3.0. A significant difference was noted when using different carbon materi16als as electrodes with eliminating the materials’ adsorption effects (Fig. S1a in Suppor17ting Information). Only 23.0% decolorization efficiency was achieved after 360 min e18lectrolysis with carbon cloth as electrodes, in contrast, significant decolorization perfo19rmances of 76.5% and 90.4% were exhibited using carbon paper and carbon felt as ele20ctrodes, respectively. The excellent capability of carbon felt to color removal could be21attributed to its larger surface area, which can provide more reactive sites for decolori2 9 11zation as previous studies indicated [29]. Thus carbon felt was selected as efficient ele2ctrode material for the AER in the following experiments.3 Different applied voltages on MO decolorization efficiency were also studied wit4h MO initial concentration of 50 mg/L and initial pH of 3.0. The decolorization efficie5ncy of MO increased with the increase of applied voltages across the AER (Fig. S1b i6n Supporting Information). As the variable resistor was adjusted manually, there was a7deviation of 20 mV under each specific applied voltage. A notable improvement of de8colorization efficiency was realized with applied voltage increasing from 300 mV to 5900 mV. The increased production of oxidant such as hydrogen peroxide at higher appli10ed 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 dec12omposition in present study, further increase of applied voltage did not result in the m13ore generation of above active substance, thus only a slight enhancement was obtained14when the applied voltage further increased from 700 mV to 900 mV and the following15experiments were conducted at 700 mV unless otherwise stated.16 Four different initial MO concentration (20, 50, 80, 100 mg/L) were used to eval17uate the effect of initial concentration on decolorization efficiency with applied voltag18e of 700 mV and initial pH of 3.0. A higher initial dye concentration resulted in a relat19ively lower decolorization efficiency from Fig. S1c in Supporting Information. After 32060 min electrolysis, the degradation efficiency of MO reached 93.5% (18.7 mg/L MO21was removed) when initial MO concentration was 20 mg/L and a decolorization effici22ency of 90.4% (45.2 mg/L MO was removed) was achieved with initial MO concentra2 10 11tion 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) and385.5% (85.5 mg/L MO was removed), respectively. Fig. 3c also revealed that decolori4zation efficiencies of MO had reached relatively high levels after first 180 min and fur5ther electrolysis had almost no significant effect. This might be ascribed to the limited6generation of active substances while they could also react with MO degradation inter7mediates instead of MO itself at the same time [44]. 8 Solution pH is always an important factor in electrochemical tests. Notably, the c9olor of MO solution varied with pH value. However, this phenomenon does not affect10the MO determination at 464 nm as the corresponding relationship between the absorb11ance and concentration is constant. Fig. S1d in Supporting Information showed the eff12ect of various initial pH on decolorization efficiency with initial MO concentration of1350 mg/L and applied voltage of 700 mV. It was found that decolorization efficiency de14creased with the increase of initial pH and color removal favored at acidic conditions15(Fig. 3d). A significant enhancement on color removal was noted when initial pH decr16eased from 5.0 to 4.0 and the decolorization efficiency increased from 68.5% to 87.917% at the end of operating cycle (360 min) correspondingly. Obviously, higher hydroge18n peroxide production under lower pH was in charge of higher decolorization efficien19cy. However, further decreasing of initial pH from 4.0 to 2.5 did not lead to great incre20ase in color removal efficiency due to the limitation of active substances generation at21relatively lower applied voltage of 700 mV. 223.3 Investigation of degradation mechanisms2 11 11 Electrolysis system functions mainly in two aspects, ie. direct and indirect electro2chemical effects [34]. In the pyridine degradation experiment, slight pyridine removal3was observed (17.1% within 360 min) with applied voltage of 700 mV from the same4MFCs, while the air aeration was replaced by mechanical stirring under anaerobic con5ditions. In this case, pyridine was found to be degraded almost linearly as a function o6f reaction time, which fully illustrated the direct electrochemical effect worked a little7on degradation due to the relatively lower applied voltage in present study. While ther8e was rare pyridine removal with only air aeration without applied voltage, by dissolvi9ng the volatile pyridine in the aqueous solution again. Compared with the significant e10nhancement of pyridine removal in the AER (82.9% after 360 min operation), it impli11ed that indirect electrochemical oxidation was mainly responsible for pyridine remova12l.13 In fact, hydrogen peroxide concentration was also monitored in the AER and it w14as found that the accumulation of hydrogen peroxide increased almost linearly in first15120 min electrolysis, after which the increment of hydrogen peroxide became weaker16(Fig. 3). Generally, O2 reduction will lead to the generation of hydrogen peroxide whic17h 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 electrolysis21[34]. However, this pathway was negligible as rare hydrogen peroxide could be detect2 12 11ed in Control 1, due to no electron and proton sources in the water (except protonation2by H2O) under relatively lower voltage. Though the current flowing from the single M3FC to the AER is relatively lower, it can support the hydrogen peroxide production. M4oreover, hydrogen peroxide production in proposed system is a spontaneous reaction5while its generation with water electrolysis is non-spontaneous electrochemical reacti6on, further exhibiting promising future in actual applications. 7 MO degradation experiment also exhibited the similar regularity and confirmed i8ndirect electrochemical oxidation of MO by the generated hydrogen peroxide were the9main effects. It should be noted that the amount of hydrogen peroxide accumulation o10f about 5.4 mg/L within 360 min was relatively less than other electro-generation of h11ydrogen peroxide processes [45] as lower applied voltage without pure oxygen was ap12plied in present study. Moreover, the generation and consumption of hydrogen peroxi13de could occur simultaneously. More efforts should be made to maximize hydrogen pe14roxide yields in this proposed system.15 In the pyridine degradation experiment, TOC was monitored and it gradually dec16reased with 57.4% of TOC removal after 360 min (Fig. 3), suggesting a successful par17tial mineralization of pyridine with a fraction of degradation of the intermediates. Am18monia-N was also accumulated during the operation (Fig. 3), consistent with previous19study [21]. To further ascertain degradation fractions in the solution after reaction, GC20/MS was performed to identify the degradation fractions and results indicated are ring21opening and desamination happened with generation of less toxic small molecule alde22hyde and carboxylic acid, similar with the degradation intermediates reported by Zalat2 13 11et al. [46], suggesting the proposed system could also reduce pyridine toxicity efficien2tly.3 In the MO degradation experiment, there were also 62.4% TOC removal was ach4ieved after 360 min and UV-vis spectra in the range of 200-800 nm also suggested the5changes of the molecule and structural characteristics of MO with electrochemical deg6radation (Fig. S2 in Supporting Information), including some derivatives of benzene, s7uch as phenol and phthalic acid, assistant with previous research [34], indicating the d8estruction of MO molecule (Table S1 in Supporting Information). However, species of9intermediates 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 the11intermediates. 12 In comparison with foregoing studies on degradation of nitrogen-containing orga13nic compounds [47,48], this study exhibited a promising performance. Moreover, the14present study realized the in situ generation and utilization of hydrogen peroxide for d15egradation with bioelectricity from MFC instead of extra addition, reducing the cost o16f advanced oxidation processes and providing a promising utilization form of low bioe17lectricity from MFC not only in nitrogen-containing organic compounds removals, but18also in other environmental contaminants controls. Other factors affecting the perform19ance of the proposed system besides what had been considered in present study, such20as flow regime, would be investigated individually afterwards for better evaluation of21the proposed system for actual application.2 14 114. Conclusions2 An effective electrochemical system was constructed with MFCs as renewable p3ower sources and successful degradation of nitrogen-containing organic compounds4(pyridine and MO) was achieved in present study. Carbon felt performed the best as el5ectrode material and higher applied voltage, while lower initial contaminant concentra6tion and lower initial pH value could improve the performance. Comparative studies i7mplied that indirect electrochemical oxidation by generated hydrogen peroxide was th8e main effect. Partial mineralization of nitrogen-containing organic compounds were a9lso observed. This work constituted a step ahead in developing strategy for enhancing10electrochemical degradation of nitrogen-containing organic compounds with bioelectr11icity from MFCs.12Acknowledgements13 This research work was supported by the National Natural Science Foundation of14China (NSFC) (No. 21307117, No. 41440025), the Research Fund for the Doctoral15Program of Higher Education of China (No. 20120022120005), the Beijing Excellent16Talent Training Project (No. 2013D009015000003), the Beijing Higher Education17Young Elite Teacher Project (No. YETP0657) and the Fundamental Research Funds18for the Central Universities (No.2652015226, No. 2652015131).19References20[1] Rosana M. Alberici, Maria C. Canela, Marcos N. Eberlin, Wilson F. Jardim,21 Catalyst deactivation in the gas phase destruction of nitrogen-containing organic2 15 11 compounds using TiO2/UV-VIS, Appl. Catal. B-Environ. 30 (2001) 389–397.2[2] L.G. Devi, S.G. Kumar, K.M. Reddy, C. 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Polarization curves (a) and voltage outputs (b) of individual MFC and two4MFCs in series. The voltage outputs were monitored when the AER was connected to5MFCs. 6Fig. 3. Pyridine degradation and TOC removal with generations of hydrogen peroxide 7and Ammonia-N in the AER.2 23 11 2Figure 12 24 11200 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 dV r 400 200 e w o P200 1000 0 0 500 1000 1500 2000 2500 30002 1 Current density (mA/m ) b 1200MFC-1 1000 MFC-2 Two MFCs in series 800 ) V m ( e 600 g a t l o V 4002000 0 2 4 6 8 10 2 T im e (d)3 Figure 22 25 1250 10Pyridine 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 n n e 100 4 e c c n n o o C C50 20 0 0 50 100 150 200 250 300 350 400 1 Time (min)2Figure 32 26

A Review on Improvement and Application of Fenton Reagent

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