Proceedings e report - Firenze University Press

FUNGAL DECONTAMINATION BY COLD PLASMA: AN INNOVATING PROCESS FOR WOOD TREATMENT
order to limit the electrical current and thus allow the generation of electrical discharges in gases at
atmospheric pressure.
2.2. The flowing afterglow
The experimental device uses the flowing afterglow to process on samples. Many advantages of using
afterglows are recognised. First of all, afterglow is only composed of neutral species (atoms,
molecules, radicals and photons), and depending on the experimental conditions the gas temperature
can be made close to ambient temperature, an important factor to treat heat-sensitive materials.
Moreover, studies have demonstrated that neutral species [4], in large amounts in the afterglow, play
the major role in the sterilisation process. Operating in the discharge itself is thus not required and
anyway the little gas gap (1mm) does not allow to introduce large samples in the reactor. Then, the
afterglow can fill large chamber volumes to process on large pieces, and can be carried out via a tube
directly through a contaminated surface, to be one day directly supplied with a portable equipment.
Finally, interactions between the contaminated surface and neutral species from the afterglow can be
considered as a phase gas chemical process, thereby ensuring material integrity and protecting the
material intrinsic properties. Consequently, decontamination process by afterglows issued from DBD
answers to specifications required regarding the preservation of cultural heritage.
3. Experiment
3.1. Isolation and identification of fungal strains
To isolate fungi responsible for maritime pine biodeterioration, samples were taken from decayed
wood and deposed on malt agar plates. After 10 days of incubation, subcultures of single colonies
were carried out on fresh agar plates in order to isolate and purify the different fungal strains. Once
isolated, identification could be performed. Mains species identified are moulds: Penicillium sp. ,
Gliocladium sp. and Trichoderma sp. and blue-stain fungi: Ceratocystis sp. and Aureobasidium
pullulans.
3.2. Samples preparation
The fungus Aureobasidium pullulans is cultured on malt agar medium for ten days at 24°C. A fungal
suspension is prepared adding 20 mL of sterile water to the plate culture and then filtered through a
glass filter in order to recover only fungal spores (mycelium is retained in the filter). An aliquot of the
suspension is next spread on sterile nitrocellulose membrane filters stuck on a glass slide and let to dry
at ambient temperature before being exposed to the afterglow. Initial spore concentration of the fungal
suspension is evaluated. Dilutions in sterile water are then plated on malt agar plates and incubated 5
days at 24°C. The number of colonies formed are counted, and thus initial concentration is assessed.
3.3. Afterglow exposure
The DBD reactor (Fig. 1) is an industrial reactor (AXCYS Technologies) electrically supplied by a
generator which delivers chopped quasi sinusoidal voltage and current waveforms in a frequency of
125 kHz. Total delivered power was fixed to 900 Watts, and the duty cycle was adjusted to 10% in
order to keep the temperature inferior to 40°C so that thermal effect of the gas is not to be considered.
Outer
electrode
Inner
electrode
60
mm
Gas out
Fig. 1 - Schematic representation of the industrial reactor used for the process (AXCYS Technologies).
An adapter is added to the reactor in order to collect the gas exiting from the reactor and guide it in a
quartz tube of 1 cm of diameter. Glass slides are placed at 1 cm from the exit of the tube. The sample
moves thanks to a custom-made conveyor belt so that the treatment is dynamic and the entire surface
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Gas in
Dielectric
coating