J. Chin. Inst. Chem. Engrs., Vol. 33, No. 4, 415-421, 2002
In Situ Oxidative Degradation of Methyl Red via
Electrogenerated Anodic Br-/Br2 and Cathodic
O2/H2O2 Redox Mediator
Ming-Liao Tsai[1] , Li-Fong Cheng[2]
Department of Chemical Engineering, National Chin-Yi Institute of Technology,
Taichung, Taiwan 411, R.O.C.
Tse-Chuan Chou[3]
Department of Chemical Engineering, National Cheng Kung University,
Tainan, Taiwan 701, R.O.C.
Abstract─In an undivided cell, the degradation of methyl red via indirect single and paired
electrolysis in the presence of bromide and hydrogen peroxide redox mediators was explored.
The factors that affected the degradation rate of methyl red are discussed, including the
concentration of methyl red, the kind of electrolyte, the concentration of sodium bromide, the
anodic current density, the material of electrode, the cathodic current density and the flow rate
of oxygen at the catholyte. The results revealed that the major factors were the concentration of
methyl red, the kind of electrolyte and the current density, and that the minor factors were the
concentration of NaBr, the type of electrolysis, the flow rate of oxygen and the cathode material.
The optimum operating conditions of this system were found to be 40 ppm methyl red, 2.0 M
NaBr, 3.38 mA·cm-2 anode current density, 2.96 mA·cm-2 cathode current density, 50 ml·min-1
O2, and graphite used in working and counter electrodes.
Key Words : Degradation, Undivided cell, Methyl red, Redox mediator
INTRODUCTION
The treatment of azo compounds is one of the
important problems in wastewaters due to their wide
application in the pigment and electronic industry.
Traditionally, aromatic compounds, which contain
multiple benzylic rings, are more stable and cannot
be destroyed even under long-term exposure to
sunlight or the presence of a stronger base or acid in
a higher temperature system (Kale, 1984; Demanin
and Uhrich, 1988). So far, many methods for treating
dye pollutants in wastewater have been reported,
such as physical methods, chemical methods and
biological methods (Gardiner and Borne, 1978; Chen
and Chou, 1993; Tsai and Chou, 1994; Tsai et al.,
2001; Unno and Tanji, 2001). The physical methods,
including adsorption via active carbon, active silica
gel, and ultra-filtration at room temperature or ambient temperature, are effective methods for decreasing
the BOD and COD of a solution. However, the cost
of a physical procedure is higher. Some accidents
may also happen due to the exothermic reaction, and
secondary pollution is created if excess oxidants are
used in the degradation of azo-compounds in the
[1]
蔡明瞭, To whom all correspondence should be addressed
陳儷方
[3]
周澤川
[2]
case of chemical methods, such as those that involve
adding oxidants like KmnO4, HClO,O3 , and H2O2.
The biological methods achieve powerful degradation in BOD, but in this case the system is very unstable and a long reaction time is required. In some
cases, biological methods are not applicable since
several chemicals, for example, phenol and formaldehyde kill bacteria. In previous papers, not only
higher current efficiency, but also higher reactivity
was achieved via electrochemical reactions in organic electrochemical processes (Tsai and Chou,
1996; 1997a; Tsai et al., 2001; Bringmann et al.,
1997; Comninellis and Pulgarin, 1991; Do, 1994; Do,
1993). Unfortunately, few reports have studied the
treatment of wastewater containing dyes using the
electrochemical method (Tsai and Chou, 1994; Tsai
et al., 2001). Up to now, no paper has examined the
treatment of methyl red in wastewaters via the electrochemical method. This study investigated indirect
anodic degradation of methyl red via single and
paired electrolysis. In general, the applied potential
was lower by using indirect anodic oxidation of
compounds than that of direct electrolysis for choosing a suitable redox mediator. The power consump-
J. Chin. Inst. Chem. Engrs., Vol. 33, No. 4, 2002
416
were purchased from the WAKO Chemical Company. The experimental setup included an electrolytic cell, an anode, an cathode, a condenser and so
on as shown in Fig. 1. The concentration of methyl
red was detected with a UV spectrograph (Shimadzu
UV-160A). The solution was prepared in a 150 ml
0.01 M sodium hydroxide solution containing methyl
red and electrolyte, and this solution was put into the
electrolytic cell. The solution was controlled at
25± 0.1o C via a Julabo F40 refrigerator and stirred at
a constant agitation rate by a magnetic stirrer,
respectively. At steady state, a constant current was
supplied from a potentiostate/galvonostate (Amel731).
RESULTS AND DISCUSSION
Indirect electrolysis of the methyl red
1. Potentiostate/Galvonostate
2. Amperes
3. Amperes
4. Volt meter
5. Coolers
6. Cell
7. Coolers
8. Stirrer
9. Sampling
Fig. 1. Experimental set-up for the degradation of
methyl red.
tion and the concentration of the redox mediator used
were lower because the mediators played a role in
catalysis and could be regenerated (Tsai et al., 1997a;
1997b; Tsai and Chou, 1996; Tsai et al., 2001;
Bringmann et al., 1997; Comninellis and Pulgarin,
1991; Do, 1994; Do, 1993). Furthermore, side reactions effectively decreased when a redox mediator
was used compared with the direct electrochemical
system. The overall indirect degradation reaction of
methyl red is shown by the following equation (Chen
and Chou, 1993; Do, 1994; Tsai et al., 2001):
oxidation

→ xCO 2 + yH 2 O
methyl red indirect
+ other species
EXPERIMENTAL
Methyl red was bought from KOCH-LIGHT
LABORATORIES LTD. The extra pure chemicals,
such as sodium chloride, sodium hydroxide, sodium
acetate, acetic acid and potassium bromide were
supplied from the OSAKA Chemical Company. Sodium bromide and sodium chloride aqueous solution
Based on the experimental results, several
small molecules and colorlessness products, such as
CO2, H2O and other species, were found in the in situ
degradation of methyl red via electrogenerated
Br − /BrO − (anodic chamber) and O2/H2O2 (cathodic
chamber) redox mediator producing. The reaction
mechanism of this system has been proposed as follows (Tsai and Chou, 1997; Bringmann et al., 1997;
Do, 1993; Tsai et al., 2001):
Anode:
1
Br − →
Brad + e − ,
(1)
2
Brad + Br − →
Br2 + e − ,
k
(2)
Br2 + H 2 O ⇔ HBr + HBrO,
(3)
k
k3
k −3
4
methyl red + HBrO →
xCO 2 + yH 2 O
k
+ species ,
(4)
Cathode:
5
O 2 + 2H + + 2e − →
H 2O 2
(5)
6
xCO 2 + yH 2 O
methyl red + H 2 O 2 →
+ species .
(4)
k
k
At the beginning of a run, the bromide anion
was adsorbed at the surface of the working electrode
and then transfered an electron to an anode, as shown
in Eq. (1). In the next step, Br − reacts with Brad to
produce bromine and transfered another electron to
an anode as shown in Eq. (2). The oxidant, HBrO,
was generated as shown in Eq. (3), and a molecule of
methyl orange was oxidized by HBrO to produce
CO2, H2O and other species. In the cathodic chamber,
hydrogen peroxide was simultaneously generated in
Ming-Liao Tsai, Li-Fong Cheng and Tse-Chuan Chou : In Situ Oxidative Degradation of Methyl Red via Electrogenerated
–
Anodic Br / Br2 and Cathodic O2/H2O2 Redox Mediator
Fig. 2. Concentration profile of methyl red over time.
Electrolyte: 1 M NaCl; Anode: graphite; Cathode: graphite; Current density: 6.76 mA·cm-2;
Temperature: 273K; pH: 11.18; Cell voltage:
3.44 V.
417
Fig. 3. Effect of the initial concentration of methyl red
on the degradation fraction.
Electrolyte: 1 M NaCl; Anode: graphite; Cathode: graphite; Current density: 6.76 mA·cm-2;
Temperature: 273K; pH: 11.18; Cell voltage:
3.44 V.
the presence of oxygen at the surface of the cathode
and was used to degrade methyl red as shown in Eqs.
(5) and (6). The solution of methyl red quickly
changed in color from red to clear during electrolysis.
Effect of the concentration of methyl red
During 80 minutes of electrolysis under a constant current as shown in Fig. 2, the concentration of
methyl red decreased from 50 to 18.4 ppm, from 40
to 4.6 ppm, from 30 to 4.2 ppm, and from 20 to 5.0
ppm under different initial concentrations of methyl
red, respectively. The degradation percentage of
methyl red was found to be 75.4, 86.5, 88.7, and
64.4% when the initial concentration of methyl red
was 50, 40, 30, and 20, respectively, as shown in Fig.
3. According to Eqs. (4) and (6), the degradation rate
of methyl red was proportional to the initial concentration of methyl red. In addition, the degradation
rate decreased because the precipitation occurred
when the initial concentration of methyl red was up
to 50 ppm.
Effect of electrolyte
The concentration and degradation fraction of
methyl red were severely affected by the use of different kinds of electrolyte, as shown in Figs. 4 and 5,
respectively. For a 100-minute run, the concentration
of methyl red decreased from 40 to 2.9 ppm ,and the
degradation fraction of methyl red was 88% in the
presence of 1.0 M NaCl solution. In 1.0 M NaBr so-
Fig. 4. Concentration profile of methyl red over time
via different kinds of electrolyte.
MR: 40 ppm; Anode: graphite; Cathode:
graphite; Current density: 6.76 mA·cm-2;
Temperature: 273K; pH: 11.36; Cell voltage:
3.17 V.
lution, the residue concentration was 2.6 ppm, and
the highest degradation fraction of methyl red was
found to be 92.8% as shown in Fig. 5. The degradation fraction of methyl red was 61 and 45% using 1.0
M KBr and CH3COONa solution, respectively. Furthermore, the degradation rate of methyl red was
faster in the presence of NaBr than NaCl because the
oxidized potential of Cl − was higher than that of the
Br − as shown in the Eqs. (1) and (2) (Sawyer et al.,
1995). Therefore, at the same cell potential, the side
418
J. Chin. Inst. Chem. Engrs., Vol. 33, No. 4, 2002
Fig. 5. Effect of the type of electrolyte on the
degradation fraction.
MR: 40 ppm; Anode: graphite; Cathode:
graphite; Current density: 6.76 mA·cm-2;
Temperature: 273K; pH: 11.36; Cell voltage:
3.17 V.
Fig. 7. Effect of the anode current density on the
degradation fraction.
MR: 40 ppm; Anode: graphite; Cathode:
graphite; Concentration of NaBr: 2.0 M;
Temperature: 273K; pH: 11.75; Cell voltage:
4.14 V.
The degradation fraction of methyl red increased
from 84 to 90% in the presence of 0.5 M NaBr when
the run time increased from 60 to 120 minutes. As
shown in Fig. 6, the maximum degradation fraction
of methyl red was found to be 92% using 2.0 M
NaBr as the electrolyte. Increasing the concentration
of Br − resulted in an increase in the current efficiency of HBrO（Eqs. (1) to (3)）and then the degradation fraction of methyl red also increased （Eq.
(4)）.
Effect of the anodic current density
Fig. 6. Effect of the concentration of NaBr on the
degradation fraction.
MR: 40 ppm; Anode: graphite; Cathode:
graphite; Current density: 6.76 mA·cm-2;
Temperature: 273K; pH: 11.38; Cell voltage:
3.71 V.
reaction on the anode in the presence of Cl − was
greater than that of Br − .
Effect of the NaBr concentration
The effect of the NaBr concentration on the
degradation fraction of methyl red is shown in Fig. 6.
During 27 minutes of electrolysis, the concentration
of methyl red rapidly decreased from 40 to close to 5
ppm, and the color quickly changed from red to clear.
The concentration of methyl red was decreased
significantly during 30 minutes of electrolysis by increasing the current density from 0.68 to 13.51
mA·cm-2. At a lower anodic current density, 0.68
mA·cm-2, the current efficiency of methyl red was
higher than that under other current densities, and the
degradation fraction of methyl red was 88% for 62.5
coulombs of electricity. The degradation fraction of
methyl red was 91% for 500 coulombs of electricity
passed at a 3.38 mA·cm-2 current density. In the
other runs, where the anodic current density was 6.76,
10.14 and 13.51 mA·cm-2, the degradation fraction of
methyl red was reduced as shown in Fig. 7. In the
changed range of the anodic current density, i.e.,
from 0.68 to 13.5 mA·cm-2, the degradation fraction
of methyl red increased when electricity was supplied at 600 coulombs. Furthermore, the degradation
fraction of methyl red decreased slightly when the
electrochemical reaction lasted. Of course, the side
reactions had been happened such as the electrolysis
Ming-Liao Tsai, Li-Fong Cheng and Tse-Chuan Chou : In Situ Oxidative Degradation of Methyl Red via Electrogenerated
–
Anodic Br / Br2 and Cathodic O2/H2O2 Redox Mediator
419
Fig. 8. Effect of the flow rate of O2 on the degradation
fraction via paired electrolysis.
MR: 40 ppm; Anode: graphite; Cathode:
graphite; Concentration of NaBr: 2.0 M;
Temperature: 273K; pH: 11.84; Cell voltage:
3.35 V; Current density: 3.38 mA·cm-2.
Fig. 9. Effect of the cathode materials on the
degradation fraction via paired electrolysis.
MR: 40 ppm; Anode: graphite; Cell voltage:
3.38 V; Concentration of NaBr: 2.0 M;
Temperature: 273K; pH: 11.90; Anodic current
density: 3.38 mA·cm-2; Flow rate of O2: 50
ml·min-1.
of the water or the impurity of solution in this case.
In addition, this may have been caused by the production of some substances that could absorb UV
light for analysis of methyl red and caused the concentration of methyl red to increase. The degradation
fraction, hence, decreased when the amount of electricity passed was greater than 600 coulombs.
Effect of the cathode material via paired
electrolysis
Types of indirect electrolysis
The degradation of methyl red was completed
via single and paired electrolysis using an indirect
redox mediator. The results revealed that the degradation rate was slightly affected by the type of indirect electrolysis. The maximum degradation fraction
of methyl red was found to be 93% for a 50 ml·min-1
O2 flow rate, and the degradation fraction of methyl
red increased from 83 to 93% with an increase in the
run time from 20 to 120 minutes as shown in Fig. 8.
In electrolysis without bubbling of O2, i.e., a 0
ml·min-1 O2 flow rate, the mechanism of degradation
of methyl red mainly followed Eqs. (1) to (4). The
paired electrolysis, i.e., in situ indirect oxidation of
methyl red via bromine and hydrogen peroxide redox
mediators, was described by Eqs. (1) to (6) for passing O2 into the electrolyte. In general, current efficiency was higher in paired electrolysis than in the
single working electrode electrolysis (Do and Yeh,
1996; Do and Chao 1999; Tsai and Chou, 1993).
The concentration variation of methyl red was
slightly affected by the use of different cathodic materials. During 40 minutes of electrolysis, the concentration of methyl red fast decreased from 40 to 8
and 7 ppm when Pt and graphite were used in cathodes. The residue concentrations of methyl red were
4.1 and 2.9 ppm using both Pt and graphite cathodes
during 120-minute reactions. The degradation fraction of methyl red was, 93%, better for a graphite
cathode than that, 89%, for a Pt cathode by paired
electrolysis as shown in Fig. 9. These results show
that cathodic reduction of O2 on graphite predominantly occurred through H2O2 formation. On the
other hand, cathodic reduction of O2 on Pt occurred
through both 4e − and peroxide pathways (Yeager et
al., 1984).
Effect of the cathode current density via paired
electrolysis
As the cathode current density increased from
2.96 to 5.92 mA·cm-2, the degradation fraction of
methyl red decreased from 93 to 88% for 400 coulombs of electricity. When the cathode current density was increased further, the degradation fraction of
methyl red was affected slightly. In this case, the
degradation reactions of methyl red were controlled
J. Chin. Inst. Chem. Engrs., Vol. 33, No. 4, 2002
420
ACKNOWLEDGEMENT
Financial support provided by the National Science Council, R.O.C. (NSC 90-2214-E-167-001), the
National Chin-Yi Institute of Technology and Cheng
Kung University is gratefully acknowledged.
NOMENCLATURE
Br ad
k1
k2
Fig.10 Effect of the cathode current density on the
degradation fraction.
MR: 40 ppm; Anode: graphite; Cell voltage:
3.64 V; Concentration of NaBr: 2.0 M;
Temperature: 273K; pH: 11.92; Anodic current
density: 3.38 mA·cm-2; Flow rate of O 2 : 50
ml·min-1.
k3
k-3
k4
k5
k6
by an electrode reaction and diffusion reaction. The
effect of the cathodic current density on the degradation of methyl red was similar to that of the anodic
current density. The maximum degradation fraction
of methyl red was found to be 93% when a 2.96
mA·cm-2 cathodic current density was used (Fig. 10).
The side reaction increased with the increase in the
current density, hence, the degradation fraction decreased.
CONCLUSION
Degradation of methyl red was carried out easily in the presence of a redox mediator via indirect
single and paired electrolysis. Not only did the red
color of methyl red disappear quickly, but also the
degradation products of methyl red were no poisonous such as CO2, H2O…etc. This is one of the most
powerful treatment methods for methyl red because
the residue concentration of methyl red was less than
2.9 ppm, and the degradation fraction was up to 93%
under 400 coulombs electricity. The best treatment
conditions of methyl red via paired electrolysis for
this system were as follows:
(1) an initial concentration of methyl red of 40 ppm,
(2) NaBr used as electrolyte,
(3) an NaBr concentration of 2.0 M,
(4) an anode current density of 3.38 mA·cm-2,
(5) a cathode current density of 2.96 mA·cm-2,
(6) a flow rate of O2 at the catholyte of 50 mA·cm-2,
(7) graphite used in both the anode and cathode.
bromine atom adsorbed at the surface of
the anode
forward reaction rate constant defined in
Eq. (2), min-1
forward reaction rate constant defined in
Eq. (3), M-1·min-1
forward reaction rate constant defined in
Eq. (4), min-1
backward reaction rate constant defined
in Eq. (4), M-1·min-1
forward reaction rate constant defined in
Eq. (5), M-1·min-1
forward reaction rate constant defined in
Eq. (6), M-2·min-1
forward reaction rate constant defined in
Eq. (7), M-1·min-1
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Anodic Br / Br2 and Cathodic O2/H2O2 Redox Mediator
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(Manuscript Reveived January 11, 2002)
經由 Br − /Br2 與 O 2 /H 2 O 2 氧化還原媒子同時間接電分解甲基紅
蔡明瞭 陳儷方
國立勤益技術學院化學工程學系
周澤川
國立成功大學化學工程學系
摘
−
要
本研究是於一未隔離式電解槽中, 藉由 Br /Br2 與 O 2 /H 2 O 2 氧化還原媒子之間接催化分解甲基紅之單一與組對電化學
反應。探討甲基紅濃度、電解質種類、電解質濃度、陽極電流密度、電極材料、陰極電流密度與陰極室氧氣流量等因素
對甲基紅電分解的影響。實驗結果証實影響間接電分解甲基紅速率之主要因素有甲基紅濃度、電解質種類、電流密度。
電極材料、溴化鈉濃度及氧氣流量影響較小。此系統之最佳操作條件為：40 ppm 甲基紅， 2.0 M NaBr，陽極電流密度
3.38 mA·cm-2，陰極電流密度 2.96 mA·cm-2，氧氣流量為 50 ml·min-1，使用石墨為陰陽電極。