2 Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang 330022, China;
Abstract
Generally, high bioelectroactivity of anodophilic biofilm favors high power generation of microbial fuel cell (MFC), however, it is not clear whether it can promote denitrification of MFC synchronously. In this study, the impact of anodophilic biofilms bioelectroactivity on denitrification behavior of single-chamber air-cathode MFC (SAMFC) in steady state was studied for the first time. Anodophilic biofilms of various bioelectroactivity were acclimated at conditions of open circuit (OC), Rext of 1000Ω and 20Ω (denoted as SAMFC-OC, SAMFC-1000Ω and SAMFC-20Ω, respectively) and run for 100 days in the presence of nitrate. Electrochemical tests and microbial analysis results showed that the anode of the SAMFC-20Ω delivered higher oxidation and denitrification current response and had a higher abundance of electroactive bacteria, likeGeobacter , Pseudomonas and Comamonas, which possessed bidirectional electron transfer function, demonstrating a higher bioelectroactivity of the anodophilic biofilm. Moreover, these electroactive bacteria favored the accumulation of denitrifers, likeThauera and Alicycliphilus , probably by consuming trace oxygen through catalyzing oxygen reduction. The SAMFC-20Ω not only delivered a 61.7% higher power than the SAMFC-1000Ω, but also achieved a stable and high denitrification rate constant(kDN ) of 1.9, which was 50% and 40% higher than that of the SAMFC-OC and SAMFC-1000Ω, respectively. It could be concluded that the high bioelectroactivity of the anodophilic biofilms not only favored high power generation of the SAMFC, but also promote the growth of denitrifers at the anodes and strengthened denitrification. This study provided an effective method and important theoretical basis for enhancing power generation and denitrification performance of the SAMFC synchronously.
Keywords: Denitrification, single-chamber air-cathode microbial fuel cell, bioelectroactivity, anodophilic biofilms, steady state
INTRODUCTION
As one of newly developed microbial electrochemical technologies, microbial fuel cell (MFC) attracts special attentions due to its double functions of waste removal and electrical energy recovery from wastewater. MFC utilizes the electrode respiration of exoelectrogens (also referred to as electroactive microorganisms and anode respiring bacteria) (Borole et al., 2011; Logan, 2009; Lovley, 2012) to convert the pollutants from water to available electrical energy or resources (Falk Harnisch & Schroder, 2010; Rabaey & Verstraete, 2005). Based on the electrode respiration of the exoelectrogens, multiple pollutants in the wastewater can be synergistically removed. As one of important pollutants in wastewater that usually cause serious water eutrophication, nitrate can serve as electron acceptor and is (bio)electrochemical reduced at the cathode of MFC (Falk Harnisch & Schroder, 2010; Li, Yu, & He, 2014), this provide a new idea for nitrate removal. The simultaneous organic oxidation at bioanode and bioelectrochemical denitrification (BEDN) at biocathode was firstly performed in a dual-chamber MFC (Clauwaert et al., 2007), in which autotrophic denitrifers at cathode conducted BEDN by uptaking electrons from the organic oxidation of exoelectrogens at the anode. However, the autotrophic denitrifers usually reproduced slowly and thus led to a poor denitrification performance (H. Huang et al., 2019; Patureau, Zumstein, Delgenes, & Moletta, 2000). Recently, it had been found that the exoelectrogens enriched at the anode of MFC could not only conduct organics oxidation, but also be able to catalyze cathodic oxygen reduction (Ka Yu Cheng, Ho, & Cord-Ruwisch, 2010) and bioelectrochemical denitrification (BEDN) (K. Y. Cheng, Ginige, & Kaksonen, 2012). It meant that the BEDN could also be carried out at the anode of MFC. It had been reported that exoelectrogens can transfer electrons directly to denitrifers by direct attachment or through electrode (Liang et al., 2019). The heterotrophic environment of bioanode could facilitate the reproduction of the exoelectrogens thus would substantially increase the denitrification performance of MFC. Anodic BEDN was firstly conducted in dual-chamber MFC (Zhang et al., 2013), however, the use of ion-exchange membrane decreased the performance of MFCs on nitrate removal. Moreover, the high operating cost sourced from the use of ion-exchange membrane also limits the application of dual-chamber MFC in practical wastewater treatment(Haobin Huang et al., 2018). In comparison, single-chambered air-cathode (SAMFC) would be more favorable for practical wastewater treatment due to the merits of membrane-less configuration, low internal resistance, low cost. In SAMFC, the BEDN mainly occurs at bioanode (Sukkasem, Xu, Park, Boonsawang, & Liu, 2008), might be due to the presence of dissolved oxygen which restrict the enrichment of denitrifers on the air-cathode.
In recent years, many investigations on BEDN of the SAMFC have been advanced in recent years. (H. Huang et al., 2019; Haobin Huang et al., 2018; Sukkasem et al., 2008). In one study, Liu et al compared the denitrification performance under electricity generation (EG) (with loads of 1000 and 270 Ω) and open circuit (OC) conditions and found that the denitrification activity of the SAMFC under current generation conditions was slightly lower than that under OC state. They thought that the possible reason was attributed to the electron competition between the electrode and nitrate under the EG condition (Sukkasem et al., 2008). However, it should be noted that this study was based on transient state, in which the operation of SAMFC under specific condition was in a relatively short time, e.g. several hours, and difficult to reflect the real denitrification performance of SAMFC. In another study, Huang et al also compared the denitrification performance of SAMFC under the EG (with load of 1000 Ω) and OC conditions in steady state and achieved the opposite result that the denitrification performance under the EG was higher than that under OC state (H. Huang et al., 2019). Generally, MFC with lower external resistance favored the acclimation of anodophilic biofilms with higher bioelectroactivity, thus favors higher power generation, however, it is not clear whether it can promote denitrification of MFC synchronously? How does the bioelectroactivity of anodophilic biofilms affect the denitrification behavior of the SAMFC in steady state?
In this study, we studied the impact of anodophilic biofilms bioelectroactivity on the denitrification behavior of the SAMFC in steady state for the first time, trying to achieve high power generation and denitrification performance of SAMFC synchronously. Anodophilic biofilms of the SAMFC with various bioelectroactivity was acclimated at conditions of open circuit, Rext of 1000Ω and 20Ω (denoted as SAMFC-OC, SAMFC-1000Ω and SAMFC-20Ω, respectively) and run for 100 days in the presence of nitrate. The anode potentials, voltage of the SAMFCs were monitored in the whole operation process, then the bioelectroactivity of the anodophilic biofilms, microbial community and denitrification performance of the SAMFCs were analyzed and compared.