“Self-degradation” of 2-chlorophenol in a sequential cathode-anode cascade mode bioelectrochemical system
Introduction
Chlorinated organic compounds (COCs) are refractory organic compounds that are commonly used in many fields and widely exist in our environment (Li et al., 2021; Liang et al., 2013; Scheutz et al., 2011). Because COCs are toxic, harmful and difficult to degrade, most of them have been listed as priority pollutants by the United States Environmental Protection Agency (Igbinosa et al., 2013). Among COCs, chlorophenols (CPs) constitute an important category due to their strong odor emission, high toxicity and persistence in the environment (Miran et al., 2017). Therefore, the treatment of CPs wastewater has become a major environmental concern, and more and more current researches focused on achieving the efficient degradation of CPs.
Since the existence of chlorine group increases the toxicity and persistence of CPs, dechlorination is the most critical step in CPs degradation process (Kong et al., 2014b). Several technologies achieved the dechlorination of CPs, but limitations still exist. As for anaerobic biotechnology, because of the high toxicity of CPs and the deficiency of sustainable electron donors (e.g. H2 and short-chain organics), the anaerobic dechlorination rate was low (Lin et al., 2019). Electrochemical technology could provide sufficient electron donors for the dechlorination reaction, but the energy consumption and cost were relatively high (Wen et al., 2013).
Bioelectrochemical system (BES) has great potential in wastewater treatment field for supplying electrons (Lin et al., 2021; Liu et al., 2021; Zeng et al., 2021; Zhai et al., 2021). Previous researches have shown that several types of CPs (e.g. 2-chlorophenol (2-CP), 4-chlorophenol (4-CP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP)) could be efficiently and selectively reduced into phenol in BES cathode (Lin et al., 2019; Strycharz et al., 2010; Wang et al., 2012b; Wen et al., 2013). The mechanism of BES cathode dechlorination is that the carbon source in the anode chamber was firstly converted into electrons by the electroactive bacteria in the bioanode, then the electrons would flow to the cathode through the external circuit and be served as electron donors to complete the dechlorination of CPs in the cathode chamber. Compared with conventional anaerobic biotechnology, BES has lower carbon consumption and higher dechlorination rate (Jiang et al., 2016; Kong et al., 2014a). Compared with conventional electrochemical technology, BES has lower energy consumption (Cui et al., 2016). However, as an electron donor system, BES cathode could not efficiently achieve the further degradation of the dechlorinated product phenol, resulting in the incomplete degradation of CPs. Many strategies have been proposed to solve this problem. For instance, BES cathode was combined with denitrification bioreactor to achieve the further ring-cleavage of 2-CP (Arellano-Gonzalez et al., 2016). Micro-aeration was also applied in BES cathode, coupled with facultative anaerobic degradative bacteria, to promote the complete degradation of 2,4,6-TCP (Khan et al., 2019). However, the combined application of reactors made the whole system complicated, and the proper micro-aeration condition was difficult to control in BES cathode.
Therefore, the objective of this study is to achieve both dechlorination and ring-cleavage degradation of 2-CP in BES. Since previous studies have shown that phenol, the dechlorinated product of 2-CP, could be used as carbon source in the bioanode to perform transformation and degradation (Friman et al., 2013; Hassan et al., 2018; Hedbavna et al., 2016; Kong et al., 2014b), the combination of cathode and anode activities seems like a promising way to degrade 2-CP. Accordingly, a sequential cathode-anode cascade mode BES was developed and investigated to achieve the “self-degradation” of 2-CP, which means that the electrons responsible for the cathode 2-CP dechlorination come from its own dechlorinated product in the anode, phenol. This operating mode can not only achieve the further degradation of the intermediate phenol, but also convert the waste phenol into useful carbon source in the bioanode, thus reducing the exogenous carbon consumption of the 2-CP degradation process.
In this study, the ability of the double-chamber BES to separately degrade cathode 2-CP and anode phenol was firstly investigated to gain operating experience. The optimal concentration ratio of anode acetate to phenolic compound and the optimum concentration of phenolic compounds were measured. Based on the separate degradation experiments, the sequential cathode-anode cascade mode BES was further developed and applied to sequentially degrade 2-CP wastewater. The critical operating parameters including applied voltage and cathode influent pH were optimized to improve the 2-CP degradation efficiency in this novel operating mode. This study also analyzed the electrochemical activity and microbial community structure of the bioanodes, and clarified the decisive role of the bioanodes on the electrochemical and degradation performance of BES.
Section snippets
BES setup
The double-chamber BES was constructed using polymethyl methacrylate, and each chamber had a working volume of 75 cm3 (5 cm length × 3 cm width × 5 cm height). The anode and cathode chambers were separated by a proton exchange membrane (N117, Dupont, USA). Graphite felts (3 cm length × 3 cm height × 3 mm thickness) were used as anode and cathode, and the cathode was coated with Pt catalyst (1 mg Pt cm−2). The graphite felts were pretreated by heating at 450 °C for 30 mins before use (
BES acclimation and start-up
Acetate was used as the substrate to acclimate BES, during the acclimation period, the system current continuously increased and the electrode potential gradually stabilized (Fig. S1). In 250 h, the maximum output current reached around 2.93 mA and remained same in three consecutive cycles, showing the success of BES start-up. The anodic biofilm was finally formed successfully (Fig. S2). The anode and cathode potentials were also sustained at that time, at about −450 mV and −1050 mV,
Perspectives
Although this study demonstrated the feasibility and advantage of the sequential cathode-anode cascade mode BES to degrade 2-CP, further development of this system must appropriately address several key challenges. The first challenge is the requirement of the addition of co-substrate acetate. In this study, acetate had to be added every time before transferring the cathode effluent into the anode in case the BES had lower electrochemical and degradation performance due to the high toxicity of
Conclusion
This study designed and developed a sequential cathode-anode cascade mode BES. Through the combination and cooperation of the cathode and anode, 2-CP could achieve “self-degradation”. The optimum concentration ratio of anode acetate to phenol and the phenolic compound degradation ability of the BES were investigated and several critical operating parameters were optimized. In this operating mode, bioanodes played the most decisive role. The electrochemical activity and the microbial community
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 was supported by the National Key R&D Program of China (Grant No. 2018YFE0106400) and the Young Elite Scientists Sponsorship Program of Tianjin (Grant No. TJSQNTJ-2020–16).
References (58)
- et al.
Mineralization of 2-chlorophenol by sequential electrochemical reductive dechlorination and biological processes
J. Hazard. Mater.
(2016) - et al.
Fabrication of polypyrrole/β-MnO2 modified graphite felt anode for enhancing recalcitrant phenol degradation in a bioelectrochemical system
Electrochim. Acta
(2017) - et al.
Azo dye decolorization in an up-flow bioelectrochemical reactor with domestic wastewater as a cost-effective yet highly efficient electron donor source
Water Res
(2016) - et al.
Bioelectrochemical BTEX removal at different voltages: assessment of the degradation and characterization of the microbial communities
J. Hazard. Mater.
(2018) - et al.
Influence of applied voltage on the performance of bioelectrochemical anaerobic digestion of sewage sludge and planktonic microbial communities at ambient temperature
Bioresour. Technol.
(2016) - et al.
Phenol degradation in bio-electrochemical cells
Int. Biodeterior. Biodegrad.
(2013) - et al.
Enrichment of anodic biofilm inoculated with anaerobic or aerobic sludge in single chambered air-cathode microbial fuel cells
Bioresour. Technol.
(2014) - et al.
Mixed carbon sources for nitrate reduction in activated sludge-identification of bacteria and process activity studies
Water Res
(2008) - et al.
Microbial community and bioelectrochemical activities in MFC for degrading phenol and producing electricity: microbial consortia could make differences
Chem. Eng. J.
(2018) - et al.
Biodegradation of phenolic compounds and their metabolites in contaminated groundwater using microbial fuel cells
Bioresour. Technol.
(2016)