Co-metabolism for enhanced phenol degradation and bioelectricity generation in microbial fuel cell

Highlights

• Acetate-MFC has the highest phenol degradation (78.8%) and voltage output (389.0 mV).

• Extracellular electron transfer enzyme acitivity increased noteably for acetate.

• Intemediates were similar in all MFCs but acetate co-substrate had the lowest level.

• Electrochemically active and phenolic degradation bacteria were both enriched.

Abstract

Co-metabolism is one of the effective approaches to increase the removal of refractory pollutants in microbial fuel cells (MFCs), but studies on the links between the co-substrates and biodegradation remain limited. In this study, four external carbon resources were used as co-substrates for phenol removal and power generation in MFC. The result demonstrated that acetate was the most efficient co-substrate with an initial phenol degradation of 78.8% and the voltage output of 389.0 mV. Polarization curves and cyclic voltammogram analysis indicated that acetate significantly increased the activity of extracellular electron transfer (EET) enzyme of the anodic microorganism, such as cytochrome c OmcA. GC-MS and LC-MS results suggested that phenol was biodegraded via catechol, 2-hydroxymuconic semialdehyde, and pyruvic acid, and these intermediates were reduced apparently in acetate feeding MFC. The microbial community analysis by high-throughput sequencing showed that Acidovorax, Geobacter, and Thauera were predominant species when using acetate as co-substrate. It can be concluded that the efficient removal of phenol was contributed to the positive interactions between electrochemically active bacteria and phenolic degradation bacteria. This study might provide new insight into the positive role of the co-substrate during the treatment of phenolic wastewater by MFC.

Introduction

Phenol is a pollutant that is widely produced worldwide, especially in the petroleum, coke production and coal gasification industries [1], [2]. Many wastewater plants suffer from the shock loading of high phenols. As a result, it usually brings severe challenges and constraints to the conventional biological wastewater treatment process [3]. In addition, due to the severe toxicity, carcinogenicity and mutagenicity of phenol, it has been listed as a priority organic pollutant and is severely restricted in the environment [1]. The phenol contaminates wastewater, if not properly treated, may pose a serious threat to human health and the global ecosystem. Given such a situation, there is an urgent need to develop an effective biological process for the restoration of phenolic wastewater.

Microbial fuel cell (MFC) is a promising technology that recovers electrical energy from pollutants and could be used for bioremediation of the phenolic wastewater [4]. Previous research studies have shown that a wide range of organic substrates can be degraded in an MFC, ranging from various kinds of artificial wastewaters to real wastewaters [5], [6]. It is demonstrated that the substrate is critical for the MFC because the anodic electrogenic in MFC tend to utilize the biodegradable organics and transfer electrons to the anode. While the recalcitrant contaminants could damage the biofilm and inhibit bacterial activity, due to their biological toxicity [7]. In the study by Hedbavna and colleagues, the removal rate of the wastewater containing phenol alone was only 52.6% after 25 days in a two-chamber MFC and the corresponding power density was only 1.8 mW/m² [8]_. In another work, the 2,4-dichlorophenol were used as the substrate in MFC, only 62% of 2,4-dichlorophenol degraded after 100 h and a weak current production of 123 mA/m² [9]. In general, when MFC is fed with recalcitrant contaminants only, the performance of MFC is severely affected and even collapsed completely [10], [11].

Co-metabolism is considered as one of the preferred routes for degradation of recalcitrant pollutants in MFC [12], [13], [14]. The reinforcement of co-substrates could be attributed to the oxidizing enzyme induction and bacteria proliferation supported by the biodegradable carbon resource [15]. It has been reported that in the presence of the acetate, the removal rate of 4-chlorophenol (CP) was increased by 43% and the power generation was increased for about 4 times compared to MFC without co-substrate [10]. Besides, the application of biodegradable organics as co-substrate can also accelerate 2, 4, 6-trichlorophenol degradation (TCP) by enhanced bacterial metabolism [16], [17]. With co-substrate, the treatment of p-nitrophenol (PNP) can also be improved by the anode functional bacteria of the genera Corynebacterium, Comamonas, Chryseobacterium and Rhodococcus [18]. Buitrón et al discovered that the use of acetate as co-substrate resulted in simultaneous electricity generation and phenol degradation by Pseudomonas, Geobacter and Shewanella in MFC [19]. In another study, MFC can enhance the biodegradation of phenol with glucose as co-substrate [20]. In general, extensive studies have demonstrated promising outcomes in pollutant treatment using co-substrates. Therefore, it is a practical approach to incorporate co-substrates in MFC for phenolic wastewater treatment. In addition, it is also discovered that the system performance (electrochemical behavior, pollutant degradation and other factors) varies when different co-substrates used to promote the degradation of refractory organics [21]. There was a close link between the enrichment of bacterial community structure and substrates composition in MFC [22]. The different co-substrates may trigger a specific microbial metabolism mechanism which also affects the metabolism of organic and electronic transfer process consequently [23]. However, to the best of our knowledge, the effect of co-substrate on enhanced phenol degradation in MFC has not been well studied. How the easily degradable substrates promote the phenol degradation in MFC? And what are the connections between the electrochemical properties, phenol degradation pathways and the microbial community? These questions have not been answered yet.

In this study, four kinds of co-substrates sodium acetate, ethanol, glucose and sucrose were selected. These external carbon resources with different atoms and functional groups have different metabolic processes. Among them, acetate is the intermediate product of the tricarboxylic acid cycle that can be utilized directly by microorganisms. Ethanol is used by converting into acetate in the first place. While the conversion of glucose and sucrose is more complex since they have more carbon atoms [12], [14]. These external carbon resources as co-substrate may affect the phenol degradation in MFC. Furthermore, the investigation into the electrochemical behavior, intermediates and microbial community would benefit from revealing the underlying mechanism of co-substrates on phenolic degradation and bioelectricity generation in MFC. This study might help to further understand the role of co-substrate for refractory organics removal and provide practical support for the application of MFCs in phenolic industrial wastewater treatment.