Simultaneous high-concentration pyridine removal and denitrification in an electricity assisted bio-photodegradation system
Graphical abstract
Introduction
Pyridine (C5H5N), a nitrogen-containing heterocyclic compound, has been widely used as the solvent for the synthesis of pharmaceuticals, insecticides, paints, dyes, explosives and petrochemicals [1], [2], [3]. In real industrial wastewaters, pyridine concentration varied greatly from several milligrams per liter to thousands of milligrams per liter [4]. Pyridine would be inevitably discharged into the natural environment, causing carcinogenic, teratogenic and mutagenic risks to humans and animals, if the wastewater containing pyridine was not treated effectively [5], [6]. Various methods have been employed for the removal of pyridine from wastewater, such as advanced oxidation [2], [7], [8], [9], adsorption [10], extraction [11] and membrane separation [12]. However, high cost, high energy consumption and serious secondary pollution has become the main limitation of these physical and chemical methods.
Biological treatment is an efficient and economical method for the treatment of pyridine containing wastewater [13], [14]. However, because of the high toxicity and rebellious nature of pyridine, high-concentration pyridine posed a challenge to traditional biological treatment process [15]. Aerobic biodegradation involves mono-oxygenation reactions, in which molecular oxygen is served as the electron acceptor [16]. Unfortunately, conventional aerobic treatment would not only cause secondary air pollution due to the volatility of pyridine but also be unstable in a long-time treatment of high-loading organic pollution [17]. Other than molecular oxygen, electron acceptors such as nitrate could enhance pyridine biodegradation in the anoxic biodegradation systems [18]. Pervious study has proved that the solid electrode could act as continuous electron donors or acceptors, resulting remarkably enhanced biological metabolism during biodegradation [19], [20]. Xu et al. [21] indicated that the initial mono-oxygenation reaction during pyridine biodegradation required readily biodegradable electron donors to yield intracellular electron carriers (NADH or e−), which could accelerate the initial reactions for pyridine biodegradation. Zhang et al. [2] found that in a biodegradation system coupled with UV photolysis, pyridine biodegradation could be accelerated by both electron acceptor namely molecular oxygen and electron donor namely succinic acid. From the above analysis, it could be inferred that pyridine biodegradation could be improved through the multiple oxidation and reduction reactions.
According to our previous study [20], the solid electrode could act as a continuous electron sink for pyridine oxidation in an electrochemical-biodegradation system. However, the biofilm on the anode might be destroyed, when a higher current was applied in the anaerobic system in order to further increase the capacity of the anode to accept electrons. In our recent study, a bio-photodegradation system based on BiVO4/FeOOH semiconductor-microbe interface was developed, where pyridine was firstly reduced by photoelectrons and then oxidized by light-excited holes, resulting into the substantial enhanced pyridine mineralization [22]. However, the recombination of photoelectrons and holes is likely to occur at the light-excited semiconductor-microbe interface due to the transfer limitation of electron and hole pairs [23], which would lead to the failure of system operation when influent pyridine concentration was higher than 450 mg L-1. As indicated by Xia et al. [24], an appropriate bias potential was beneficial for the efficient separation and transfer of the photoelectrons and holes at photoanode. Zang et al. [25] has reported that the recombination of the photoelectron-hole pairs at the photocathode was retarded by introducing bioanode. Moreover, previous studies have reported that enhanced organic pollutant degradation was achieved in bio-photodegradation system with assistance of electrochemistry [26], [27]. Therefore, assistance of electrochemical technology for bio-photodegradation system would be promising in order to promote pyridine removal efficiency, especially for high-concentration pyridine. However, the feasibility of enhancing pyridine degradation by photoelectrical stimulation in biosystems has not been well documented. In addition, NH4+ released from pyridine ring during pyridine degradation could be nitrified to NO2- and NO3- in biosystems, which could be utilized as the electron acceptor for the degradation of pyridine and its intermediates. However, the mechanism involved in simultaneous pyridine degradation and nitrogen removal in biosystems under photoelectrical stimulation have not been clarified.
Therefore, an Electricity assisted bio-photodegradation system (EBPS) with a photoanode zone, a biocathode zone and an aerobic zone was constructed to enhance the performance of high-concentration pyridine degradation and simultaneous denitrification. In our previous study [22], enhanced bio-photodgradation of pyridine was achieved with BiVO4/FeOOH semiconductor-microbe interface, but the performance of pyridine photodegradation with BiVO4/FeOOH semiconductor was quiet poor. >65% pyridine was residual in photodegradation system after 48 h reaction. Besides, quite a number of N-containing organic intermediates were detected in the photodegradation process by using BiVO4/FeOOH semiconductor alone, revealing unsatisfactory pyridine mineralization. This result could be attribute to the relatively low catalytic activity of BiVO4/FeOOH semiconductor. In this study, the main function of photoanode zone was to destroy pyridine structure, decrease toxicity of pyridine and provide photoelectrons for biodegradation in biocathode zone. Thus, TiO2 with high catalytic activity was chosen as the semiconductor to construct photoanode, where the nontoxicity and stability of TiO2 were beneficial to operation of EBPS for a long time. The key role of photoelectrocatalysis for pyridine degradation in EBPS was investigated. The effects of various key factors on the reactor performance were evaluated, including pyridine concentration, recirculation ratio and photoanode potential. Based on the reactive species trapping experiments and microbial function analysis, possible mechanism for enhanced pyridine degradation in EBPS was proposed.
Section snippets
Materials
Carbon paper (CP) and graphite felts were purchased from Toray Co. (Japan) and Chemshine Carbon Co. (Sichuan, China), respectively. Prior to use, both CP and graphite felts were sonicated and then dried to remove impurities [19]. The commercial TiO2 nanoparticles (P25) were purchased from Degussa Co. (Germany). All reagents used were of analytical grade (purity > 99.9%) and used without further purification.
Synthesis of TiO2@CP
TiO2@CP was prepared by a simple coating method according to He et al. [28]. Briefly,
Characterization of TiO2@CP
As shown in Fig. 2a, TiO2 nanoparticles were evenly distributed on the surface of CP, which was beneficial to maintain a relatively large surface area. In the XRD pattern of TiO2@CP shown in Fig. 2b, two distinct diffraction peaks appeared at the 2θ values of 26.53° and 54.55°, corresponding to the (002) and (004) reflections of graphite (JCPDS No. 41–1487) [37]. The diffraction peaks observed at 2θ values of 25.54°, 37.93°, 48.23°, 69.15°, 70.43° and 75.32°, could be ascribed to (101), (004),
Conclusions
In this study, simultaneous pyridine degradation and denitrification was successfully achieved in EBPS and efficient separation of photoelectron-hole pairs accelerated both photodegradation and biodegradation. 1250 mg L-1 pyridine was completely degraded in EBPS with 60 h HRT and no additional electron donors to involve for microbe metabolism during degradation. The possible pyridine degradation pathway was provided in this study, including hydroxylation, carbonylation and pyridine ring
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financed by National Natural Science Foundation of China (No. 52170084, 51922050 and 51708293) and Natural Science Foundation of Jiangsu Province (BK20211574).
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These authors contributed to the paper equally.