Elsevier

Journal of Power Sources

Volume 371, 15 December 2017, Pages 26-34
Journal of Power Sources

Solar promoted azo dye degradation and energy production in the bio-photoelectrochemical system with a g-C3N4/BiOBr heterojunction photocathode

https://doi.org/10.1016/j.jpowsour.2017.10.033Get rights and content

Highlights

  • A bio-photoelectrochemical system was developed with a g-C3N4/BiOBr photocathode.

  • Azo dye degradation and H2 production were enhanced in the solar irradiated BPES.

  • Solar irradiation can lower photocathode resistance and increase current generation.

  • Contributions of various processes to MO degradation in the BPES were determined.

  • The g-C3N4/BiOBr photocathode was optimized and system stability was evaluated.

Abstract

In this study, a single-chamber bio-photoelectrochemical system (BPES), integrating advantages of bioelectrochemical system and photocatalysis process, is developed using a g-C3N4/BiOBr heterojunction photocathode for methyl orange (MO) degradation and simultaneous energy recovery. Photocatalytic activities of g-C3N4/BiOBr, g-C3N4 and BiOBr are characterized by UV-vis diffuse reflectance spectra (UV-vis DRS) and Photoluminescence (PL) spectra; and electrochemical activities of photocathodes are examined by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Results show that with an applied voltage of 0.8 V and under simulated solar irradiation, MO decolorization with g-C3N4/BiOBr photocathode reaches 97.8% within 4 h, higher than those with g-C3N4 (85.3%) and BiOBr (87.3%) photocathodes. Likewise, higher hydrogen production rate (143.8 L m−3d−1) is observed using g-C3N4/BiOBr photocathode; while values for g-C3N4 and BiOBr photocathodes are 124.3 L m−3d−1 and 117.1 L m−3d−1, respectively. PL and EIS reveal that superior performance of g-C3N4/BiOBr photocathode can be attributed to more efficient separation of photogenerated electron-hole pairs, lower resistance and better charge transfer. Synergistic effect occurs among biological, electrochemical and photocatalytic processes in illuminated BPES for MO removal. Photocathode optimization and system stability evaluation are conducted. This study demonstrates that the BPES holds great potential for efficient refractory organics degradation and energy production.

Introduction

Synthetic organic dyes are extensively used in the textile, paper, plastic leather, food, cosmetics and other industries. According to the statistics, about 70 million tons of synthetic dyes are produced and used in the above industries annually, and approximately 30–150 thousand tons of dyes are discharged into water bodies [1]. About 80% of these organic dyes are azo dyes, which contain chromophore (–N=N–) in their molecular structures, like methyl orange (MO), Congo red, and so on [2], [3]. Currently, dye wastewater discharge is a severe environmental issue, since it can cause aquatic environment problems owing to high colority, high COD, and biological toxicity, etc. [4]. Besides, dye wastewater can also threaten human health due to their carcinogenic, toxic and mutagenic properties [1]. Dye pollutants were found to be largely persistent in aquatic environment, because they are recalcitrant organics and conventional removal processes in wastewater treatment plants are poor [5]. Therefore, it is of great significant to develop more efficient treatment processes for removing dyes from wastewaters.

Up to now, various methods, including physical methods [6], physical-chemical methods [7], biological processes [8] and bioelectrochemical methods [9], [10] have been employed to remove azo dyes [6], [7], [8], [9], [10]. Physical methods, mainly based on coagulation, flocculation, filtration and adsorption processes, could be effective for dyes removal, but they may have drawbacks of producing large amount of sludge, high cost, subsequent disposal and secondary pollution problems [6], [7], [8], [11]. Physical-chemical methods include those processes using strong oxidizing agents (e.g. H2O2, O3 and Fenton's reagent) or photocatalysis [8]. Among these processes, photocatalysis has been demonstrated to be an efficient method for dye pollutants degradation since it can mineralize organic dyes into H2O, CO2, and mineral acids without bringing any secondary pollution [11], [12]. However, the recovery and reuse of the photocatalyst is a problem. Biological methods, which remove dyes by metabolic pathways (mainly refer to anaerobic biological treatment) or adsorption by living/dead biomass, are inexpensive and ecofriendly. Nevertheless, they suffer from issues of long processing time and incomplete degradation [8], [13]. Recently, the bioelectrochemical system (BES), combining advantages of physical-chemical and biological processes, has emerged as promising technology for simultaneous wastewater treatment and energy/resource recovery [14], [15], [16]. The BES mainly includes the microbial fuel cell (MFC) and microbial electrolysis cell (MEC) and other derivative system. It has been reported that simultaneous azo dye decolorization and electricity generation could be achieved in the MFC [4], and decolorization rates could be significantly enhanced in MEC mode [2], [3]. However, toxic intermediates (such as amines) generated from the reductive cleavage of azo bond can accumulate in the MEC owing to the incomplete anaerobic reduction, which could be harmful to microorganisms [17]. To further degrade the intermediates, coupled systems have been proposed, such as integrating BES with conventional biological reactors or constructed wetland [17], [18]. Considering highly efficient photocatalysis process for organic pollutants degradation, Du et al. [19] developed a dual-chamber biocathode coupled photoelectrochemical cell for azo dye degradation and electricity generation. Our group also proposed to couple the MEC with photocatalysis using the TiO2-coated nickel foam as photocathode for azo dye degradation and simultaneous hydrogen production [20], and our results showed that accelerated of azo dye degradation and concurrent hydrogen production could be achieved. This MEC-photocatalysis coupled system holds great advantages over the BES and photocatalysis. For one thing, enhanced azo dye degradation could be achieved as compared with the BES; for another, as compared with photocatalysis, by loading the photocatalyst on the electrode, the issue of difficult recovering photocatalyst could be avoided, and energy could be recovered simultaneously. Hence, this coupled system has great potential for efficient dyeing wastewater treatment and energy recovery.

In the MEC and photocatalysis coupled system, the photocathode plays a key role. In our previous study, ultraviolet (UV) responsive TiO2 was adopted as the photocatalyst; however, UV light only accounts for about 4% of the solar radiation, which limits its practical application. To efficiently utilize solar irradiation, visible light responsive photocatalysts should be employed, as visible light accounts for ∼50% of the solar radiation. Up to now, various visible light responsive photocatalysts have been explored, and commonly reported ones include but not limited to: WO3 (2.7eV), CdS (2.4eV), Cu2O (2.2 eV), CuO (1.7eV), Fe2O3 (2.2 eV), BiVO4 (2.4eV), CuFeO2 (1.5 eV), graphite carbon nitride (g-C3N4, 2.7eV), and bismuth oxyhalides (BiOX, X = Cl, Br, I) [12], [21], [22], [23]. Among these visible light responsive semiconductors, g-C3N4, a metal-free photocatalyst, is one of the most promising materials for photocatalytic hydrogen evolution and pollutants degradation, because it is earth-abundant, easily prepared, environmentally friendly and chemically and thermally stable [24], [25]. Besides, it has been reported that the bismuth oxyhalides exhibited higher photocatalytic activities towards organic pollutants degradation compared to TiO2, and BiOBr has gained great interest due to its visible light response and high stability [12], [26]. Nevertheless, the pristine photocatalyst suffers from photogenerated electron and hole pairs recombination, which deteriorates the photocatalytic performance [26]. Constructing heterojunction has been demonstrated as an effective means to promote photogenerated carriers separation, which would result in enhanced photocatalytic performance [27]. Because of the excellent properties and the match energy band structures between g-C3N4 and BiOBr, the heterojunction of g-C3N4/BiOBr composite exhibited desired photocatalytic activity [28]. It is believed that while using g-C3N4/BiOBr composite as the photocatalyst on photocathode in the BES-photocatalysis coupled system, enhanced azo dye degradation should be achieved under solar irradiation. To our best knowledge, information on employing the visible light responsive g-C3N4/BiOBr heterojunction photocathode in the BES-photocatalysis coupled system is not available so far.

Therefore, the objective of this study was to develop an efficient integrated BES-photocatalysis system, named bio-photoelectrochemical system (BPES), using a g-C3N4/BiOBr heterojunction photocathode (using Ni foam as supporting material) for enhanced azo dye degradation and simultaneous hydrogen production. Methyl orange (MO) was chosen as the azo dye pollutant. For comparison, the g-C3N4 and BiOBr photocathodes were also prepared as the counterparts. Optical properties of the photocatalysts and electrochemical properties of the photocathodes would be characterized, and the BPES performance would be determined mainly in terms of MO decolorization, current generation, biogas production, and COD removals under solar irradiation. Besides, the effect of g-C3N4/BiOBr loading on the photocathode performance would be determined.

Section snippets

g-C3N4, BiOBr and g-C3N4/BiOBr synthesis and photocathodes preparation

All reagents for photocatalysts synthesis were commercially available and used without further purification. The g-C3N4 was obtained by directly annealing urea at 550 °C for 3 h with the heating rate of 15 °C/min in a muffle furnace. The resultant yellow g-C3N4 sample was milled into powder for further use. The g-C3N4/BiOBr heterojunction composite was synthesized as follows: 3.000  g g-C3N4 and 1.17 g KBr were dissolved in 100 mL deionized water (solution A); 4.77 g Bi(NO3)3·5H2O was dissolved

Properties of the g-C3N4/BiOBr composite, g-C3N4 and BiOBr photocatalysts

The XRD patterns of the g-C3N4/BiOBr composite, pure g-C3N4 and BiOBr samples were shown in Fig. 2a. Two diffraction peaks at 13.04° and 27.47° from the g-C3N4 sample can be indexed as the (100) and (002) peaks for graphitic materials (JCPDS 87-1526), corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments, respectively [12], [28]. A series of diffraction peaks could be observed from the pure BiOBr sample, assigned as (001), (002), (011), (012),

Conclusions

In summary, a single-chamber BPES constructing with solar responsive g-C3N4/BiOBr heterojunction photocathode was developed for efficient azo dye degradation and energy recovery. Various characterizations were performed to reveal photocatalytic properties of the as-synthesized photocatalysts and electrochemical activities of the photocathodes. Results showed that enhanced performance was observed for all the photocathodes with light illumination than those without light illumination. Moreover,

Acknowledgements

This work was supported by grants from National Science Foundation of China (No. 21707021), Ability Promotion Project of Education Department of Guangxi Province for the University Middle Age and Youth Teachers (No.2017KY0031) and Guangxi Colleges and Universities Key Laboratory of Environmental Protection (No. C3050097902).

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