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.