INVESTIGADORES
FLEXER Victoria
congresos y reuniones científicas
Título:
Plasma treatment of electrodes significantly enhances the development of electrochemically active biofilms
Autor/es:
VICTORIA FLEXER; MARQUE, M.; DONOSE, B.C.; LEFEVBRE, C.; VIRDIS, B.; KELLER, J.
Lugar:
Cairns
Reunión:
Conferencia; 4th International Microbial Fuel Cell Conference; 2013
Institución organizadora:
ISMET-International Society for Microbial Electrochemistry and Technologies
Resumen:
Summary of key findings
Here
we studied the effect of pre-treating carbonaceous electrodes with either an
oxygen or a nitrogen plasma before reactor inoculation with a mixed microbial
consortia. The plasma produces chemical modifications at the electrode surface
level. X-ray photoelectron spectroscopy and water contact angle analysis showed
that the plasma removes surface contamination, produces ion implantation and
renders the hydrophobic surfaces highly hydrophilic. Both in the case of
cathodic and anodic inoculae, plasma pre-treatment considerably accelerated the
generation of a bioelectrochemical current after inoculation. Nitrogen plasma
yielded the best performance, followed closely by oxygen plasma. Plasma
pre-treated electrodes reached a plateau of maximum current density twice as
fast as untreated electrodes. Analysis of the current development profiles
suggests that the plasma pre-treatment is neither producing a preferential
attachment of certain types of bacteria over others, nor accelerating the
extracellular electron transfer rate. The results indicate that the plasma
treatment considerably enhances the initial cell adhesion, which results in
subsequently faster biofilm development. Plasma pre-treatment of electrodes is
an inexpensive, fast, safe and straightforward technique to achieve more rapid
start-up of bio-electrochemical processes.
Background and relevance
The viability of
prospective applications of microbial bioelectrochemical systems is highly
dependent on performance improvement, i.e.
in current increase. Current production is dependent, among other factors, on
the microbial consortia, bioelectrochemical reactor design, and electrode
materials. One strategy to improve
bacteria-electrode interaction is to apply some type of pre-treatment to the
electrodes before exposing them to biofilm development. Although some
pre-activation techniques proposed in the past have shown improvements in
performance, they employ dangerous chemicals or extreme conditions, and
sometimes even long, cumbersome, and multistep techniques. We propose the use
of plasma treatment as a new pre-activation technique for carbonaceous
electrode materials. Plasma treatment can
be used to modify a wide variety of material surfaces in a nonspecific manner
by changing the wettability, or in a more specific manner by introducing a
variety of functional groups depending on the processing gas. We aim at changing
the inherited hydrophobicity of carbonaceous materials, in order to increase
microorganisms adhesion.
Results
Experiments were performed on glassy carbon (GC),
graphite felt (GF), and graphite plates (GP). Water contact angle measurement
showed that after 15 minutes of either O2 or N2 plasma
treatment, the water contact angles were always below 5o. XPS
analysis revealed important changes in the atomic compositions at surface level
after plasma treatment, attributed both to the removal of surface contamination
and to effective ion implantation.
We chose to study the effect of the plasma
pre-treatment on current production by electroactive biofilm developing from
mixed culture inocula. This is a more relevant and broadly applicable case for
real applications. We studied the current development on classical
three-electrode electrochemical cells.
As a model anodic system, we used the effluent of an
acclimatized BES reactor (acetate fed) in operation. Figure 1 (left) shows the
catalytic current development from time of inoculation for 3 GC electrodes,
oxygen plasma treated electrode (GC-O), nitrogen plasma treated electrode
(GC-N), and the blank electrode (no plasma treatment, GC-blank). We observed
that the current development was fastest for the nitrogen plasma treated electrode,
with the oxygen plasma treated electrode following shortly after. The current
for GC-O and GC-N arrives to an initial maximum plateau value at day 18 from
inoculation. The current increase for the blank electrode is much slower. By
the time the GC-O and GC-N electrodes have arrived at their respective maxima,
the current value for the blank electrode is only 20% of the values reached by
the treated electrodes. Approximately at day 32, the currents for the three
electrodes became very similar. The current values were very similar for the
rest of the experiment. Similar results were also observed on graphite felt
electrodes. At the end of the chronoamperometry experiments, cyclic
voltammograms were measured both under turnover and non-turnover conditions.
As a model cathodic system, we inoculated the reactor with
a broad consortia of denitrifier organisms (feeding NaNO3 and NaHCO3,
with no organic carbon source). Figure 1 (right) shows preliminary results for
3 graphite plates electrodes. Catalytic current development from time of
inoculation is shown for oxygen plasma treated electrode (GP-O), nitrogen
plasma treated electrode (GP-N), and the blank electrode (no plasma treatment,
GP-blank). Once again the current development was faster for the GP-N electrode,
followed by the GP-O, with the blank electrode lagging behind. These
experiments are still undergoing.
Discussion
For the anodic biofilms, during the first weeks after
reactor inoculation, plasma treated electrodes showed considerably higher
currents than non-treated electrodes. After a certain period of rapid current
increase, the treated electrodes show constant current values, indicating that
a mature biofilm has developed. It takes much longer for the blank electrode to
reach this mature biofilm stage, but eventually, the same current densities
are. Therefore, we do not believe that plasma treatment is increasing the
extracellular electron transfer rate of the attached microorganisms; otherwise,
we would expect a higher current density throughout the duration of the
experiment. Very similar cyclic voltammetry curves both in turnover and
non-turnover conditions, suggest that the plasma treatment does not seem to
favour the preferential attachment of certain bacterial species over others, i.e. similar bacterial communities seem
to be responsible for current generation in the three electrode set. This
hypothesis is reinforced by a careful analysis of the logarithmic current
plots. These show parallel profiles that suggest equal bacterial developing
rates, suggesting once again the similarities of the bacterial communities.
Therefore our results suggest that plasma treatment considerably enhances the
initial cell adhesion, which results in subsequently faster biofilm
development.
Preliminary results on the cathodic biofilms indicate
that the effect of plasma treatment would be longer lasting that in the anodic
biofilms, i.e. the current response
would still be higher for the N2 plasma pre-treated electrode even
after development of a mature biofilm. The origin of this longer lasting effect
is currently under investigation. These results are yet to be confirmed.