Submerged Arc Breakdown of Methylene Blue
in Aqueous Solutions
R.L. Boxman, N. Parkansky, A. Vegerhof, B. Alterkop,
Electrical Discharge and Plasma Laboratory, Tel Aviv University, Tel Aviv 69978, Israel
O. Berkh
School of Electrical Engineering, Tel Aviv University, POB 39040, Tel Aviv 69978, Israel
Abstract: Low voltage, low energy submerged pulsed arcs between a pair of carbon or iron electrodes with a
pulse repetition rate of 100 Hz, energies of 2.6-192 mJ and durations of 20, 50 and 100 s were used to remove
methylene blue (MB) contamination from 30 ml aqueous solutions.
The MB concentration decreased
-1
exponentially with rates of 0.0006-0.0143 s during processing with the carbon electrode pair. With the iron
electrodes, the MB concentration initially decreased faster than with the carbon electrodes, but later saturated. The
effects of the treatment on the pH of the solution andon the Zeta–potential of the particles formed as a result of
electrode erosion were studied in the presence and in the absence of MB. .
Keywords: Submerged pulsed arc, water treatment, plasma, decontamination.
applications is rather recent and remains to be
1. Introduction
optimized for various types of contaminants and
It has been shown that plasma technologies have the
capability to treat water using several mechanisms
such as radical reactions, shock waves, ultra-violet
radiation, ionic reactions, electron processes and
thermal dissociation [1-4]. It is suspected that these
factors, singularly or synergistically, may be
responsible
for
concurrently
oxidizing
trace
microorganisms. A low voltage arc applied between
two submerged carbon electrodes was used to
breakdown sulfadimethoxine and to decontaminate
aqueous solutions from other biological and
chemical agents [2,9]. However low voltage
submerged arc removal of MB or other dyes from
aqueous solutions has not been reported previously.
contaminates and disinfecting microorganisms in
The objectives of this research were to:
water. In particular, the submerged pulsed highcurrent and high voltage electrical discharge, i.e. a
discharge between two electrodes in a liquid,
sometimes referred to as an electro-hydraulic
discharge [1], has been shown to oxidize many
organic compounds [5-8]. The use of electrohydraulic discharge systems use in water treatment
(1) remove MB from aqueous solutions using low
voltage,
low energy submerged
pulsed arcs,
(2) determine the removal efficiency with iron and
carbon electrode pairs, and (3) determine the
influence of the discharge on the solution pH,
mobility and Zeta–potential of particles produced
from electrode erosion.
2.5
2. Experimental Details
a
Absorbance, a.u.
2
Pulsed arcs were applied between two
99.5% carbon or low carbon (0.2%) steel electrodes
(referred to as C/C and Fe/Fe respectively) using a
0 min
1.5
0.5 min
1
1 min
3 min
0.5
setup which was previously presented [2]. 30 ml of
5 min
deionized water and 10mg/l MB solution in
0
200
deionized water were submerged arc treated. Treated
processing times between 0 and 5 min. The MB
2
solution
was
monitored
Absorbance, a.u.
2.5
in
400
500
600
700
800
Wavelength, nm
liquid was collected after four or more selected
concentration
300
by
measuring the solution absorbance at the absorption
maximum (664 nm). Electrokinetic mobility and the
Zeta-potential were measured with a Zetasizer Nano
0 min
b
1.5
0.5 min
1
1 min
3 min
0.5
5 min
ZS. The pH of the solution before and after arc
0
200
treatment was recorded at room temperature.
3. Results
The absorption spectra after various
Fe/Fe electrodes are shown in Figs. 1a and 1b,
4
ln(Co/Ct)
5
concentration. Similar behavior was observed for
400
500
600
Wavelenght, nm
700
800
Figure 1. Absorption spectra of the aqueous MB solution
after various treatment times with W=192 mJ, using (a) C/C and
(b) Fe/Fe electrode pairs.
treatment times (W=192 mJ) with C/C and
respectively. The arc treatment decreased the MB
300
C/C
3
2
treatments using discharge pulses with other
1
energies. The impact of the submerged pulsed arc
0
Fe/Fe
0
treatment on the MB degradation ln(C0/Ct) was
50
100
150
200
250
300
Treatment time, s
considered as a function of processing time t, where
C0 and Ct are the MB concentrations (proportional to
Figure 2. ln(Co/Ct) vs. treatment time for Fe/Fe and C/C
electrode pairs and W=192 mJ.
the height of the absorbance peak at 664 nm)
Dissolved MB shifted the charge of the
initially and after processing time t. Fig. 2 presents
carbon and iron particles to more positive values.
typical dependences of the degradation. With the
The
C/C electrodes, the degradation increased linearly
1.796 mm-cm/V-s
with t up to ln(C0/Ct)=4.2 at t=300 s. With the Fe/Fe
2.237 mm-cm/V-s
electrodes, the degradation proceeded more rapidly
respectively. The shift was more pronounced for
than with C/C during the first 60 s, and then
carbon particles, changing the sign of the C particle
saturated at ln(C0/Ct)=1.5.
mobility.
mobility
changed
and
for
from
from
Fe
and
1.064
to
-0.9245
to
C
particles
Fig. 3 shows typical dependences of pH on
(PS). This increases the positive charge on the PS. In
treatment time of deionized water and 10 mg/l MB
the opposite case, the increase of pH decreases the
solution for Fe/Fe and C/C electrode pairs. During
positive charge down to zero and can even
arcing in water with Fe electrodes, the pH increased
negatively charge the PS.
by ~15% during the first 30 s of treatment, and than
saturated. The maximum pH increase was 30%.
During arcing with carbon electrodes, the pH
decreased by 22-28% for the same pulse energies.
The addition of MB solution did not significantly
change the pH with the Fe electrodes, while with the
C electrodes the MB decreased it by a factor of 30%.
The stronger effect of MB on the mobility of
carbon particles (including a sign change) than on
iron particles is consistent with the higher adsorption
activity of the carbon particles. The enhanced
adsorption activity of carbon particles can be
associated first of all with their more negative
charge, which attracts positively charged cations
such as MB+.
Iron
oxides
and
hydroxides
with
iron
electrodes and carbonic acid with carbon electrodes
are produced during arcing. Electrolytic dissociation
of iron oxides and hydroxides increases the pH while
carbonic acid decreases the pH. The decrease of pH
after arcing, in the presence of MB, might be
augmented by the acidic character of the products of
Figure 3. pH vs treatment time for arcing with 192 mJ pulses
with Fe/Fe and C/C electrodes submerged in deionized water
and 10 mg/l MB solution.
MB degradation.
4. Discussion
above water produce chemically active species such
It is widely accepted that discharges in or
MB degradation demonstrated in the present
as high-energy electrons, H•, OH•, O•, O3, H2O2,
work seems to be mainly a result of oxidation by
excited neutral molecules and ionic species. OH•
active species formed during arcing in a high
radicals have the highest oxidation potential.
concentration and assisted by adsorption on the
Additional OH• radicals may be produced by
produced particles. Surfaces of activated carbon or
Fenton's reaction
iron oxide particles suspended in water contain
hydroxyl groups which behave amphoterically as
shown below.
Fe 2+ + H2O2 = Fe 3+ + OH− + OH•
Additional production of OH• radicals probably
provided the higher degradation rate in the
-H

+H


 PS-OH 

 PS- OH2
PS-O 


Decrease
of
the
pH
(i.e.
increase
of
initial period of the treatment process than
H+
concentration) increases the density of protonated
groups (i.e. OH and OH2+) on the particle surface
observed with the C/C electrodes. However, in
the Fe/Fe case, MB degradation ceased at ~78%.
Possibly the cessation was caused by consumption
of OH• radicals by Fe micro-and nano-particles
3.
which accumulated with time in the solution during
electrodes was 78 % and required 11.5 kW-hr/m3.
electrode erosion. 98.8% removal of MB by arcing
For 75% MB removal with J = 7.4 kW-hr/m3
with C/C electrodes was achieved in 5 min (Fig. 2).
Apparently OH• radicals were not consumed by the
C particles.
The maximum MB removal using Fe
4.
The particle surface charge and mobility
shifted in the positive direction with the addition of
MB into the treated solution. This indicates the
The minimum energy per unit volume J for
reduction of two orders of magnitude (99% removal)
of 10 mg/l MB contamination in water was
adsorption of MB+ and other positively charged
species produced during the discharge.
References
extrapolated to be J = 43920 kJ/m3 =12.2 kW-hr/m3
using C-C electrodes with a pulse energy of 7.5 mJ
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905
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3.
5. Conclusions
1.
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Low voltage, low energy submerged pulsed
arcs between pairs of carbon or iron electrodes
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concentration decreased with a maximum rate of
0.06 s-1 for arcing with a pulse energy of 192 mJ,
-1
i.e., faster than that with C electrodes (0.0143 s for
the same pulse energy), but later the MB
concentration saturated.
2.
The minimum energy density for reducing
MB concentration by two orders of magnitude in 10
mg/l water solution is extrapolated to be 12.2 kWhr/m3 using a pair of C electrodes, pulse energy of
7.5 mJ and pulse duration of 20 µs.
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