3.2 Bioelectroactivity of anodophilic biofilms
After acclimation of the SAMFCs in medium containing acetate for 30
days, the bioelectroactivity of the biofilms enriched at the bioanodes
are measured by CVs and the results are shown in Figure 3. Figure 3A1
shows that all the CVs of anodophilic biofilms acclimated in the
presence of acetate (turnover condition) present the typical positive
sigmoidal shape (Liang et al., 2019; Pous et al., 2016), indicating that
exoelectrogens are established successfully in all the SAMFCs. Obvious
differences are observed from the CVs of different SAMFC bioanodes. The
CVs of the SAMFC-20Ω bioanodes display a high bioelectrocatalytic
oxidation current of 7.8 mA, which is much higher than that of the
SAMFC-1000Ω (5.5 mA) and SAMFC-OC (0.3 mA), respectively. These results
indicate that the biofilms enrich at the bioanodes of the SAMFC-20Ω
possesses higher bioelectrocatalytic activity than that of the
SAMFC-1000Ω and SAMFC-OC. It has been reported that the
bioelectrocatalytic activity of exoelectrogens at the anode is highly
associated with the anode potential and current density (Aelterman,
Freguia, Keller, Verstraete, & Rabaey, 2008; Zhao & Chen, 2018).
Higher bioelectrocatalytic activity could be obtained at comparatively
more positive anode potential and higher current density (Aelterman et
al., 2008; Hong, Call, Werner, & Logan, 2011; Jiang et al., 2018),
because electron transfer from bacteria to anode is accelerated
substantially when SAMFC operated under lower external resistances (Zhu,
Tokash, Hong, & Logan, 2013). In this study, the SAMFC-20Ω could
deliver comparatively more positive anode potential and higher current
density than the SAMFC-1000Ω and SAMFC-OC, thereby a much higher anodic
electrocatalytic activity of exoelectrogen biofilms are enriched the
bioanodes. Furthermore, the high current density in the SAMFC-20Ω enable
anode-respiring bacteria to outcompete aerobic heterotrophs for
substrate to release much more electrons (Ren, Zhang, He, & Logan,
2014), thus SAMFC operated under 20Ω is an efficient means to enrich
exoelectrogens. The CVs in Figure 3A2 show that the exoelectrogens
enriched in medium containing only acetate substrate could also catalyze
BEDN, which is in accordance to that of previous reports (K. Y. Cheng et
al., 2012) .
When the nitrate is supplied to the SAMFCs accompanying with the
acetate, denitrifers are enriched at the bioanodes after acclimating for
a period time. The CVs of the anodophilic biofilms at the
20th day are performed in the PBS medium containing
only nitrate and shown in Figure 3B2. They all display a negative
sigmoidal shape which is extremely accordance with that in ref. (Pous et
al., 2016) , proving that anodophilic biofilms could successfully
catalyze nitrate reduction (K. Y. Cheng et al., 2012). Notably, the CVs
of the SAMFC-20Ω anode display a more negative bioelectrocatalytic
reduction current response of-1.15 mA, which is four and eight times as
that of the SAMFC-1000Ω (-0.28 mA) and SAMFC-OC (-0.13 mA) bioanodes,
respectively, representing a higher BEDN performance. The CVs of the
anodophilic biofilms at the 20th day shown in Figure
2B1 display typical positive sigmoidal shape but with increased
bioelectrocatalytic oxidation peak current of 11.5, 6.6 and 1.3 mA for
the SAMFC-20Ω, SAMFC-1000Ω and SAMFC-OC, respectively, indicating that
the bioelectrocatalytic activity of exoelectrogens at the anodes is
enhanced even in the presence of denitrifers, indicating the good
coexistence of exoelectrogens and denitrifers. Moreover, a higher
negative denitrification current is observed for all the SAMFCs at the
100th day (Figure 3C2), demonstrating an increase of
BEDN performance with increase of acclimation time. In addition, the
enrichment of denitrifers at the anode result in a distinctive “duck
head” shape (Figure 3B1 and 3C1) in the CVs of the SAMFC-20Ω anodes
measured in the presence of acetate. The CVs measured under non-turnover
conditions (absence of substrate) can further characterize the
electrochemical features and extracellular electron transfer (EET) modes
of the biofilm. The non-turnover CVs of the anodophilic biofilm
acclimated by feeding media containing acetate shown in Figure 3A3
present two major clear redox systems with the formal potential ranged
from -0.260 to -0.350 V, which is reported to be related to membrane
bound c-type cytochromes and also highly consistent with the CV
characteristic that described in the literature(Fricke, Harnisch, &
Schroder, 2008; F. Harnisch & Freguia, 2012). The two major redox
systems at the formal potentials of -0.256 V and -0.360 V in SAMFC-20Ω
are basically equal to that of -0.260 V and -0.350 V in SAMFC-1000Ω as
well as -0.263 V and -0.347 V in SAMFC-OC. The similar two major clear
redox systems with the formal potential ranged from 0.235 to 0.370 V can
also be observed in the non-turnover CVs of anodophilic biofilm
acclimated by feeding media containing both acetate and nitrate (Figure
3B3 and 3C3), hinting that the bioelectrochemical features and EET modes
of the exoelectrogen biofilms is preserved even in the presence of
denitrifers.