Methylene blue can be decomposed by OH radicals. Decoloration
characteristics of organic dye in an aqueous solution by a
pulsed-discharge plasma Shin Ikoma 1, Yasushi Miyazaki 1, Kohki
Satoh 1,2, and Hidenori Itoh 1 1 Department of Electrical and
Electronic Engineering, Muroran Institute of Technology, 27-1
Mizumoto, Muroran 050-8585, Japan 2 Center of Environmental Science
and Disaster Mitigation for Advanced Research, Muroran Institute of
Technology, 27-1 Mizumoto, Muroran 050-8585, Japan E-mail :
[email protected] INTRODUCTION APPARATUS &
CONDITIONS Background Various methods and techniques to reduce
environmental pollutants like dioxin, toluene, trichloroethylene,
etc. have been developed. Advanced Oxidation Processes (AOPs) [1],
such as ozone-UV oxidation, photo catalytic oxidation,
pulsed-discharge etc., have been used as effective methods for
purifying polluted water. Electrode : A multi-needle electrode and
a water bath electrode A multi-needle electrode: 1, 4, 37 and 55
stainless-steel needles are arranged on a stainless-steel plate. A
water bath electrode: A water bath made of stainless-steel, which
has the dimensions of 119mm inner diameter and 12mm depth, is
placed under the multi-needle electrode. The water bath electrode
is earthed. Gap length: A distance between the tip of multi-needle
electrode and the surface of the test liquid is fixed at 4mm.
Discharge chamber : A cylindical discharge chamber is made of
acrylic resin (140mm inner diameter, 100mm height and 1.5L volume).
H.V. pulse generator: A high-voltage pulse, generated by a blumlein
pulse-generator, is applied to the multi-needle electrode. The
coaxial transmission lines are charged at +14.14kV. Pulse
repetition rate is 20pps (pulses per second). Concentration
measurements of methylene blue : The test liquid is exposed to the
pulsed discharge for 60min, and the temporal variation in
concentration of methylene blue, O 3 and NO x are measured.
Concentration of methylene blue mesured by absorption photometry
using 650nm laser and Photonic Multi-Channel Analyzer (PMA). The
hazardous substances can be directly decomposed by the collision
with high energy electrons [1]. The species which have high
oxidation potential, such as OH, O, O 3 etc., are produced by a
pulsed-discharge plasma generated above a water surface [2], and
that the species can dissolve into the water, reacting with
substances in the water. UV radiation may contribute to the
decomposition of the hazardous substances [1][2]. Objective We
generated a pulsed-discharge plasma above test liquid (containing
low concentration of methylene blue) and investigated the
decomposition characteristics of methylene blue by the plasma
exposure. RESULTS & DISCUSSION (2) Influence of electrode
configuration on methylene blue decomposition We investigated the
decomposition characteristics of aqueous methylene blue solution,
by pulsed discharge, with the different electrode configuration and
composition of the background gas. 55needles 1needle Filamental
discharges from the needle tip reach the water surface, and the
discharges split into several branches, then some of them reach the
water bath electrode. Methylene blue can be decomposed by OH
radicals generated by the reaction between electrons and H 2 O
vapour. 55 and 37 needles 1 and 4 needles A mixture of effects
caus- ing decomposition of the hazardous substances is expected by
using pulsed- discharge plasma. Pulsed discharge [5] C.Mavroyannis
et al.:Can. J. Chem., 39 (1961) 1601. [6] O.Eichwald et al.:J.
Appl. Phys. 82 (1997) 4781. Pawlat et al. [3] investigated the
effects of the pulse repetition rate and gas flow rate on the
decomposition characteristics of methylene blue. Georgescu et al.
[4] reported that the O 3 contributes to the decomposition of
methylene blue. The number of needles (1, 4, 37 and 55) Influence
of electrode configuration on the decomposition characteristics
Influences of background-gas composition on the decomposition
characteristics 55 3714 The decomposition rate Decomposition rate
of methylene blue increases monotonously with the increase of O 3
concentration except for 1needle. The O 3 contributes to the
decomposition of methylene blue. O 2 + e 2O + e O 2 + O O 3
O3O3O3O3 OH O3O3O3O3 OH OH OH O3O3O3O3 O3O3O3O3 O 3 produced with
the high concentration, O 3 contributes to the decomposition of
methylene blue. Mesurement of voltage and current Applied voltage
and discharge current are measured by high voltage probe
(Tektronix, P6015A) and digital oscilloscope (Yokogawa, DL1620).
Background gas : N 2 (99.99%) O 2 (99.5%) and Ar(99.99%) Mixture
ratios are N 2 (or Ar) : O 2 =100:0, 90:10, 80:20, 60:40, 40:60,
20:80, 10:90 and 0:100 (3) Influence of the background-gas
composition (N 2 -O 2 ) on methylene blue decomposition O 2 + e 2O
+ e O 2 + O O 3 H 2 O + e OH + H N 2 (A) +H 2 O OH + N 2 +H H 2 O +
e OH + H O 2 + e 2O + e O 2 + O O 3 N 2 (A) +H 2 O OH + N 2 +H H 2
O + e OH + H N + O 3 NO + O 2 (6) N + O 3 NO + O 2 (6) NO + O 3 NO
2 + O 2 (9) N + O NO (5) NO + O NO 2 (8) Methylene blue
decomposition rate tends to increase when the number of
needle-electrodes increases, except for the case of 1needle. In
pure O 2, O 3 produced with the highest concentration (120ppm), and
the O 3 contributes to the decomposition of methylene blue. In pure
N 2, slight increase in the decomposition rate can be made by the
contribution of OH radicals, produced by the reaction between N 2
molecules exited in metastable state and H 2 O vapour. In N 2 -O 2
mixture, the decomposition rate of methylene blue decreases due to
the decomposition of O 3 by NO and N and the inhibition of O 3
generation by NO x production. (4) Influence of the background-gas
composition (Ar- O 2 ) on methylene blue decomposition In N 2 -O 2
mixture, NO x production inhibits O 3 generation. OH radicals
produced by the reaction between Ar exited in metastable state and
H 2 O vapour. H 2 O + Ar* OH + H + Ar Exited molecules of Ar (Ar*)
Exited molecules of Ar can decompose the benzen ringsExited
molecules of Ar can decompose the benzen rings [7]. in pure N 2 68%
in N 2 -O 2 mixture 40 65% in pure O 2 98% OH radicals produced by
the reaction between N 2 exited in metastable state and H 2 O. The
decomposition rate decreases due to the production of NO x. Excited
molecules of Ar may contribute Methylene blue decomposition.
Substitution of polluted water : A aqueous methylene blue solution
Weight : 70g Initial concentration : 10ppm Input energy per pulse
is 0.7J. Maximum values of discharge-voltage, discharge-current and
electrical power are respectively -20kV, -70A and 1.5MW. (1) The
characteristics of an applied HV pulse O and O 3 contribute to the
decomposition of methylene blue. NO x production processes In the
case of 1needle Methylene blue decomposition rate after 15min
plasma exposure Following reactions can be induced by the
pulsed-discharge plasma with the background gas consisting of N 2
and O 2 [1,2,5,6]. N 2 + e N + N + e (4) N 2 (A) + O NO + N (7) O 2
+ e 2O + e (1) O 2 + O O 3 (2) N 2 (A) +H 2 O OH + N 2 +H (3) Low
oxygenHigh oxygen Low oxygen concentration NO production High
oxygen concentration (NO 2 production) N produced by the reaction
(4), leads NO production reactions in (5) and (6). NO produced by
the reaction between N 2 molecules exited in metastable state and
O, as shown in (7). NO production reactions shown in (5), (6) and
(7) are activated, and they are consumed for NO 2 productions shown
in (8) and (9). The decomposition rate decreases due to the
consumption of O, O 3 and N 2 (A) by NO x production. N 2 (or Ar):O
2 =100:0 90:10 80:20 60:40 40:60 20:80 10:90 and 0:100 [3] J.Pawlat
et al.:Acta Physica Slovaca 55 (2005) 479. [4] N.Georgescu et
al.:Proceedings of GD 2006 F05 (2006) 497. The decomposition rate
of methylene blue is nearly-constant with O 3 concentration
decreases at the same time. O 3 concentration increases in
proportional to the increase of the O 2 concentration. C 16 H 18
ClN 3 S 3H 2 O Recent works (Decomposition of methylene blue by
using pulsed-discharge plasma) [1] M.A.Malik et al.:Plasma Sourse
Sci. Technol. 10 (2001) 82. [2] P.Lukes et al.:J. Phys. D: Appl.
Phys. 38 (2005) 409. [7] N.Goto et al.: IEEJ Trans., FM, 126 (2006)
321. H 2 O + e OH + H [needles] Pulse width is 500ns.
CONCLUSIONS
