st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Effect of Micro- and Nano-particles on
Submerged Arc Decomposition of Methylene
N. Parkansky1, E. Faktorovich-Simon1, V. Yacubov1, B. Alterkop1, O. Berkh2,
R.L. Boxman1, Z. Barkay3, Yu. Rosenberg3, L. Burstein3, A. Khatchtouriants4
1
3
Electrical Discharge and Plasma Laboratory, 2 School of Electrical Engineering,
Wolfson Applied Materials Research Center, 4 The Center for Nanoscience and Nanotechnology,
Tel Aviv University, POB 39040, Tel Aviv 69978, Israel
Abstract: The pulsed submerged arc (SA) can decompose contaminant molecules in water.
Recently, SA decomposition of Methylene Blue (MB) contamination in aqueous solutions was
demonstrated. The particles eroded from the electrodes were shown to influence the decomposition efficiency. However, no comparative studies of effects of the particles eroded from
Fe and Ti electrodes were conducted. The objective of this work was to determine the effects
of Fe and Ti electrodes on the efficiency of MB decomposition in aqueous solutions. Electrode pairs of the same material (Fe or Ti) and combinations of these materials were used.
Solutions were treated using a SA with Fe and Ti electrodes in the presence of H2O2 and
without it, and then aged. The treated solutions were examined by Raman and absorption
spectroscopy. The produced particles were studied by SEM, XPS and XRD.
In the absence of H2O2 in the solutions, the decontamination ratio after SA treatment was
higher with Fe/Fe than with Ti/Ti electrodes. In the presence of H2O2 in the solutions, SA
treatment with Ti/Ti electrodes was more effective than with Fe/Fe electrodes. One week aging of the solutions treated in the presence of H2O2 with all electrode pairs very effectively
decontaminated the solutions, reaching a decontamination ratio close to 99 %). According to
the XPS and Raman spectroscopy analysis, titanium peroxide was formed by SA treatment
with Ti electrodes on the surface of particles eroded from the electrodes. We believe that
Ti-peroxide forms on the surface of eroded particles during SA treatment and gradually oxidizes MB during aging. A similar processes can proceed on the surface of the particles eroded
from Fe electrodes owing to Fe-peroxide compounds formed in Fenton chemistry during discharge.
Keywords: Submerged pulsed arc, water treatment, micro- and nano-particles plasma, decontamination.
1. Introduction
respectively) were used.
Plasma processes to treat water proceed by several 2. Experimental Details
mechanisms such as radical reactions, shock waves, ulPulsed arcs were applied between low carbon (0.2%)
tra-violet radiation, ionic reactions, electron processes and steel (designated herein as Fe) and 99.5% Ti electrodes.
thermal dissociation [1-4]. The submerged pulsed Electrode pairs of the same material and combinations of
high-current and high voltage electrical discharge, i.e. a these materials were used in a setup which was previously
discharge between two electrodes in a liquid, referred to presented [2]. 40 ml samples of 10mg/l MB solutions,
as an electro-hydraulic discharge [1], has been shown to most containing 0.5% H2O2, were SA treated with the
oxidize many organic compounds [5-9]. The pulsed low above electrodes. All solutions were prepared using devoltage submerged arc (SA) can decompose contaminant ionized water. The SA comprised discharging a 15 F
molecules in water. Recently, SA decomposition of Meth- capacitor charged to 80 V, and hence storing an energy of
ylene Blue (MB) contamination was demonstrated [2,9]. 48 mJ, my momentarily contacting and separating the
However, similarities and differences of the effects of electrodes, which were submerged in the solution. This
particles eroded from different electrodes on decontami- was accomplished by mounting one of the electrodes on a
nation efficiency have not been studied. The objectives of vibrator, which had a vibration amplitude of zz mm, and
this research were determining and comparing these ef- vibration frequency of 100 Hz. This produced arc pulses
fects for the particles eroded from Fe and Ti electrodes. with a 100 Hz repetition frequency, which was applied to
Electrode pairs Fea/Fec, Tia/Tic, Tia/Fec and Fea/Tic (where the solution for 2 min.
the subscripts a and c designate the anode and cathode,
The SA treatment was followed by aging of the treated
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
liquid, i.e. storing it in the dark. The decontamination was
monitored by absorption spectroscopy, specifically of the
664 nm MB absorption peak [10]. The treated solutions
were examined by Raman spectroscopy, and the eroded
particles by SEM, XPS and XRD.
3. Results
The SA treatment without H2O2 using Fe/Fe and Ti/Ti
electrodes removed only a ~50 % and ~10 % of the MB,
respectively. During 1 week aging, the removal ration of
the Fe/Fe treated solution decreased by up to 40%,
whereas it increased to 20% in Ti/Ti treated solutions
The presence of H2O2 during SA treatment decreased the
removal ratio observed immediately after SA treatment
using Fe/Fe electrodes and increased it for Ti/Ti electrodes.
Fig.1 Ti concentration in eroded material and removal ratio of
MB for different electrode pairs.
trode pair. The removal ratio after 2 hour aging time increased with the increase of Ti concentration in eroded
material. After one week aging time, the removal ratio of
at least 97% was achieved for all electrode pairs.
Results of XPS analysis of Ti particles produced by erosion of Ti/Ti electrodes during SA with H2O2 are presented in Figs. 3a, b. The O1s XPS spectrum (Fig.3a) was
fitted with the sum of three Lorentzian curves with maxima located at 530eV, 532eV and 533eV. The components
with peaks near 530eV and at 532eV were attributed to
Ti-O and C-O bonds, respectively. The component with
peak near 533eV was attributed to titanium peroxide
Ti-O-O bonds (the same peak was observed by Ohno et.
al [11] in the XPS spectrum of titanium peroxide). Fig. 3b
indicated the presence of TiO2.
a
Removal ratio, %
The effect of the electrode material and aging time on the
removal ratio and on the Ti fraction in the eroded partices
is shown in Fig.1. It was obtained that the Ti fraction in
the eroded material increased in the sequence of electrode
pairs which were used: Fea/Fec, Tia/Fec, Fea/Tic and
Tia/Tic. Change in the removal ratio as a function of Ti%
in eroded particles is presented in Fig.2. This graph was
derived from the graph in Fig.1.
100
1 week aging
80
60
b
40
Figs. 3a,b. High resolution O1s (a) and Ti2p (b) XPS spectra
from the surface of the particles produced during the discharge
Ti/Ti electrodes in solution with 0.5% H2O2.
2 hours aging
20
0
0
20
40
60
80
100
Ti in eroded material,%
Fig.2 Removal ratio vs. Ti component in eroded material
Fig.2. Removal ratio vs. Ti% in the eroded material
Increase of Ti component in electrode erosion leads to the
acceleration of MB decomposition by arcing Ti-Fe elec-
Raman spectrum of SA treated solutions indicated the
presence of Ti-O-O bonds (Fig. 4).Thus, according to the
XPS and Raman spectroscopy analysis, titanium peroxide
was formed as a result of discharge with Ti electrodes and
presented in the treated solutions and on the particles surface.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fe-peroxide species formed during discharge and accumulated on the surface of the particles eroded from Fe
electrodes. They, likely, provided MB oxidation during
aging and very high values of removal ratio.
Although the exact composition of Ti-peroxide and its
formation mechanism on the surface of Ti oxide particles
are unknown, oxidative radicals such as HO2•and OH•
were found during degradation of Ti-peroxide. We believe that Ti-peroxide decomposed during aging with
formation of these radicals and in such way caused oxidation of MB with removal ratio of about 99 %.
5. Conclusions
1.
The particles eroded from electrodes defined the
type and concentration of oxidative species and the level
Fig. 4. Raman spectrum of Ti/Ti SA treated solution with
0.5% H2O2.
.
4. Discussion
It is widely accepted that discharges in or above water
produce chemically active species such as high-energy
electrons, H•, OH•, O•, O3, H2O2, excited neutral molecules and ionic species. OH• radicals have the highest
oxidation potential [3,12,13]. Decontamination rate is
presumably a function of OH• concentration. The concentration of OH • radicals is determined by the balance between their generation and consumption.
The low removal ratios (50 and 10 % for Fe/Fe and
Ti/Ti electrodes, respectively) obtained by SA treatment
without H2O2 may be associated with the decrease of the
oxidative species concentration with the SA time. When
the SA with Fe/Fe treatment was interrupted by periodic
filtration of the solutions under treatment, the removal
ratio increased up to 99 % [10]. We assume that by using
of Ti/Ti electrodes a similar phenomenon may be observed. Really, aggregation (coagulation) of eroded particles with SA treatment time decreased a specific area of
the particles suspended in the solution. In the case of Ti
electrodes, their high activity towards the oxidative species was the essential factor leading to the decrease of
their concentration and to the limitation of removal ratio.
The effect of H2O2 on the removal ratio is associated
with Fenton reaction in the case of Fe/Fe electrodes [10]
and with formation of Ti oxide and peroxide on the surface of the eroded particles in the case of Ti/Ti electrodes. .
Fenton reaction provides formation of OH• radicals
(Fe 2+
is generated by SA from electrodes):
Fe 2+ + H2O2 = Fe 3+ + OH− + OH•
(1)
Fenton chemistry is very complex. Along with OH•,
other oxidative species can be produced in treated solution and on the particle surface. Evidence for the
iron-peroxide species, outer-sphere complexes Fe3+HO2and ferryl ion FeO2+ in Fenton chemistry was provided
[14]. These species are the active oxidants.
of removal ratio after SA treatment. Decrease of the oxidative species concentration with SA treatment time leads
to the low limited removal ratio for the both Fe/Fe and
Ti/Ti electrodes.
2.
The presence of H2O2 in the solution led to the in-
crease of removal ratio after SA treatment, likely, owing
to increase of concentration of oxidative species mainly
on the surface of the particles.
3.
The aging of the solutions SA treated in the presence
of H2O2 with all combinations of electrodes provided the
very effective decontamination of solutions (removal ratio
was close to 99 %).
The effect of aging may be associ-
ated with accumulation of oxidative species, particularly
peroxides on the surface of eroded particles and with
gradual oxidation of MB.
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21 International Symposium on Plasma Chemistry (ISPC 21)
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Cairns Convention Centre, Queensland, Australia
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