Anode biofilm influence on the toxic response of microbial fuel cells under different operating conditions
Graphical abstract
Introduction
Recently, water safety issues have been occurring with increasing frequency. Qualified and reliable water quality is not only related to the stable operation of water plants but also has a significant impact on human health (Jiang et al., 2018). Biological treatment units in water plants are often affected or even stalled as a result of a sudden shock of contaminants. Therefore, it is necessary to test the water quality online in a timely manner. Microbial fuel cells (MFCs) have promising potential as biosensors for water quality detection (Wang et al., 2013). Kim used MFCs detected Pb, Hg, and mixture of 1 mg/L Cd and 1 mg/L Pb (Kim et al., 2007). Di Lorenzo developed a small-scale single MFC which can detect 1 μg/L of cadmium in water (Di Lorenzo et al., 2014). Jiang developed a biocathode MFC more sensitive than bioanode which can detect formaldehyde of 0.0005% (Jiang et al., 2017). Xing used an indicator of average current inhibition rate to evaluate the inhibitory effect of toxic substances such as 2,4-dichlorophenol (Xing et al., 2020). Moreover, MFCs are self-sufficient (Logan et al., 2006) as they use electrical signals to indicate microorganism activity changes. Electroactive microorganisms, which play an important role in MFCs, have been of increasing interest in recent years. Further, a higher electroactivity was recorded for a mixed bacterial community than that of a single strain (Sun et al., 2015). Through the gravity settling of planktonic bacteria, a cost-effective method was proposed to improve the electrochemical activity of biofilms over a short period (Li et al., 2017). For dual-chamber MFCs, the anode, cathode, and cation exchange membrane are the main components. When exposed to toxic substances, the activity of the electroactive microbes is inhibited. Thus, the electrical signal decreases intuitively with a reduction in the number of electrons and protons. Traditionally, electroactive microorganisms in the anode were the primary observable component for testing. Compared with the anaerobic environment of an anode, the low cost and long-term stability of a biocathode is attractive (Xia et al., 2013). Jiang et al. used a biocathode as the sensing element of an MFC for toxicity monitoring (Jiang et al., 2017). Meanwhile, it has been proposed that bioanodes and biocathodes working simultaneously as biosensors would offer better toxic sensitivity than that of a single biosensor (Zhao et al., 2019). Qi et al. found that luminescent bacteria (Vibrio fischeri) bioanodes are more sensitive than biocathodes, and proposed an optical and electrical signal biosensor for an improved visual understanding of the toxicity process (Qi et al., 2019). MFC performance is closely associated with microorganisms (Yuan et al., 2013). Chu discussed in detail about the characteristics of anode and cathode sensitive units (Chu et al., 2021). However, few studies focus on the effects of flow rate and culture time conditions on MFC toxicity testing.
Previous studies have shown that the characteristics of biofilms, such as biofilm density, can be affected by flow rates and porosity. Di Lorenzo et al. found the coulombic efficiency was best at the lowest flow rate (Di Lorenzo et al., 2010). Further, the high porosity biomass density was low when the flow rate was reduced to 1.3 mL/min, improving the sensitivity to heavy metals (Shen et al., 2013). Some studies have shown that a higher flow rate is conducive for matrix diffusing into the biofilm, wherein the biofilm density increases and the diffusivity of the matrix within the biofilm reduces (Liu and Tay, 2002). The effect of flow rate on material diffusion and biomass density has been intermittently studied while the effect of microbial community structure, microbial redox capacity, and extracellular polymer on the toxicity response has not been reported. One advantage of MFC is that it can operate for a long time. However, 2 months operation will inevitably lead to an increased thickness of the biofilm on the electrode surface (Shen et al., 2013). Some MFC types can sustainably produce energy (Tommasi and Lombardelli, 2017). After 21 days of operation, the anode performance deteriorates with the increasingly thick biofilm (Islam et al., 2016). Further, the conductivity and matrix diffusion rates are affected by thick biofilms because of the long distance of extracellular electron transfer resistances (Bond et al., 2012; Islam et al., 2016). Regardless, researchers are unsure how the long-term operation of MFCs affects the redox capacity and microbial community structure. As a biosensor, it is necessary to conduct research on the microorganisms and electrochemical performance of MFCs under different conditions, especially for toxicity testing. However, there is limited research regarding the culture time and flow rates of bioanodes as associated with MFC performance and toxic response, especially the redox capacity among biofilms and the conformation of intact cells.
To evaluate the effect of the structure and properties of the anode biofilm under different operating conditions on the toxic response of MFCs, electrochemical methods were quantified for individual conditions to evaluate the performance of MFCs. The microbial community structure was also studied to further investigate the impact of operating conditions on MFC toxicity response. From a micro-perspective, operating conditions have a great impact on the anode oxidation reduction potential (ORP) among biofilms and conformation of intact cells. Extracellular polymeric substances (EPS), which can characterize the performance of microorganisms under different conditions, were also investigated. Petrochemical wastewater is a kind of complex composition and large discharge industrial wastewater. This study chose 2,4-DCP as a toxic substance because it is a typical chlorophenol substance present in petrochemical wastewater. The goal of this study is to determine how to improve both MFC toxic response sensitivity and accuracy in actual application.
Section snippets
Construction and operation of MFCs
In an MFC, a cation exchange membrane (Membranes N117, USA) separates the anode and cathode chambers (Fig. S1). With a diameter of 3 cm and a 1 cm long cylindrical interior chamber, the volumes of the anode and cathode are the same. A diameter of 3.8 cm and thickness of 0.3 cm graphite felt (Sanye Carbon, China) on a titanium sheet are used as the electrodes. Then, the electrodes are connected to the current and voltage signal collection system (Xinwei 4008, China). Next, anolyte is dissolved
Effect of flow rate
The flow rates of 0.5 mL/min, 2 mL/min, and 5 mL/min with and without 2,4-DCP tests are presented in Fig. 1 (a). At a flow rate of 0.5 mL/min, a voltage of 621 ± 5 mV (fluctuating less than 5%) lasted for 26 h. Meanwhile, at 2 mL/min and 5 mL/min, the voltage lasted for 21 h and 26 h, respectively at 616 ± 6 mV and 612 ± 14 mV. Further, the matrix length varied at different flow rates. Notably, the substrate took more time to run out at a fast (5 mL/min) flow rate than a slow (0.5 mL/min) one,
Conclusions
As a biosensor, the toxicity response of MFCs was significantly affected by the structure and properties of the anode biofilm under different operating conditions, such as flow rate and culture time. The flow rate and culture time not only affect the microbial community structure of the MFC, especially its Geobacter sp. content, but also affect the composition of the EPS, the ORP among biofilms, and the conformation of intact cells. The flow rate mainly affected the ORP between biofilms, while
CRediT authorship contribution statement
No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
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.
Acknowledgments
The work is financially supported by National Water Pollution Control and Treatment Science and Technology Major Project of China (2017ZX07402002).
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