An electrochemical mechanisms study on steel/IrO2-Sb2O3 electrodes for oxidation of phenol in water

Article abstract of DOI:10.1139/cjc-2014-0304

Phenol, a known water pollutant, was electrochemically oxidized on a steel/IrO₂-Sb₂O₃ novel anode. Since the oxidation mechanisms vary based on the anode material, a mechanisms study of electrooxidation of phenol on it was conducted. The phenol oxidation was carried out at 20 mA/cm² constant current density with a pH 11.00 Na₂SO₄ medium at room temperature. During 6 h of electrolysis, samples were tested for chemical oxygen demand removal efficiency of the anode. The steel/IrO₂-Sb₂O₃anode showed 76.3% chemical oxygen demand removal efficiency. Both 4-nitroso-N,N-dimethylaniline and the HCO₃^-/CO₃^2- radical scavenger tests confirmed the formation and presence of the hydroxyl radicals in the system. Therefore, it was concluded that the hydroxyl radicals that are generated on the anode surface are the main cause for the oxidation mechanism. Moreover, ICE, HPLC, and UV-vis absorbance and cyclic voltammetry results confirmed the presence of catechol and benzoquinone as intermediates and the reaction mechanism.

Full text of DOI:10.1139/cjc-2014-0304

Page 1 of 23
An electrochemical mechanisms study: On Steel/IrO ꢀSb O
2
2
3
electrodes for oxidation of phenol in water
1
, 4
1
2
Pavithra Bhakthi Jayathilaka , Gayani Chathurika Pathiraja , Athula Bandara , Nalaka
1
1, 3,4
Deepal Subasinghe , Nadeeshani Nanayakkara
*
1
Environmental Engineering/Electrochemistry Research Group, Institute of Fundamental
2
Studies, Kandy, Sri Lanka, Department of Chemistry, University of Peradeniya, Sri Lanka,
3
Department of Civil Engineering, Faculty of Engineering, University of Peradeniya, Sri
4
Lanka , Postgradate Institute of Science, University of Peradeniya, Sri Lanka
*
Corresponding author. E mail: kgnn@pdn.ac.lk, TP: +94 812 393574, Fax: +94ꢀ81ꢀ2388158

Page 2 of 23
Abstract
2 2 3
Phenol, a known water pollutant, was electrochemically oxidized on Steel/IrO ꢀSb O novel
anode. Since the oxidation mechanisms vary based on the anode material, mechanisms study
of electroꢀoxidation of phenol on it was conducted. The phenol oxidation was carried out at
2
2
0 mA/cm constant current density with pH 11.00 Na
2
SO
4
medium at room temperature.
During 6h of electrolysis, samples were tested for Chemical Oxygen Demand (COD) removal
efficiency of anode. The Steel/IrO ꢀSb anode showed 76.3% of COD removal efficiency.
Both4ꢀnitrosoꢀN, Nꢀdimethylaniline (RNO) and the HCO /CO radical scavenger tests
2
2 3
O
ꢁ
ꢂꢁ
ꢀ
ꢀ
confirmed the formation and presence of the hydroxyl radicals in the system. Therefore, it
was concluded that the hydroxyl radicals which are generated on the anode surface are the
main cause for the oxidation mechanism. Moreover, ICE, HPLC and UVꢀvis absorbance and
cyclic voltammetry results confirmed the presence of catechol and benzoquinone as
intermediates and the reaction mechanism.
Keywords:
2 2 3
Benzoquinone, Catechol, Oxidation mechanisms, Phenol, Steel/ IrO ꢀSb O anode
2

Page 3 of 23
Introduction
Phenol (C
6
H
5
OH) is a known hazardous aromatic hydrocarbon which is produced through
1
both natural and anthropogenic processes. In recent, phenolic compounds have been detected
2
in ground and surface water as contaminants. Moreover, they have a potential to impact on
1
, 3
human health and ecology even at low concentration due to high toxicity, high oxygen
demand and low bio degradability. As such, it is important to degrade them in to harmless
species. Hence, a great deal of attention has been created on contamination of water by these
phenolic compounds.
Several treatment technologies have been researched for recovery or destruction of phenol in
4
5
6
water such as photoꢀcatalytic oxidation , biological treatment , chemical oxidation ,
7
8
electrochemical oxidation and adsorption. But there are drawbacks in these techniques such
chemical oxidation require large amount of reactive chemicals and it caused harmful effects
on environment. Even though biological treatment methods are environmental friendly they
are inefficient and slow. Also they are impractical for high toxicity conditions and limited in
6
the case of nonꢀbiodegradable compounds. More on the adsorption methods are efficient but
it not widely applicable due to higher reproducible cost of absorbents.
Recently, the electrochemical process (EC) has been popular among these treatment
9
,10
2
techniques. As one of the EC method, electrochemical oxidation is more attractive due to
its environmental compatibility, safety, high efficiency, cost effectiveness, inꢀsitu chemical
9
,11
generation and ease in operation.
The efficiency and the selectivity of electrochemical
2,7
oxidation is mainly depends on electrode materials. Therefore, anode material and its
10
properties are more important. The ideal anode material for the oxidation of organic
pollutant must be highly active towards organic oxidation while low activity towards
secondary reactions such as oxygen evolution reactions. In addition, both stability in
electrolyte medium and the inexpensiveness are needed. Considering above facts, several
3

Page 4 of 23
anode materials have been studied under various experimental conditions. Such investigated
1
2
anode materials for phenol/phenolic compounds oxidation include Ti/SnO
2
ꢀSb , Ti/RuO
, Ti/PdO–Co and
and graphite . Though, there are similar electrodes on different substrates
2
–
13
14
15
TiO
2
, boronꢀdoped diamond , Ti/IrO
2
, Pb/PbO
2
, Ti/Pt–Ir, Ti/PbO
2
3 4
O
16
17
Ti/RhO –TiO
x
2
18, 19
20
21
such as Ti/IrO2
, Ti/ IrO ꢀSnO ꢀSb O , Ti/SnO ꢀSb2O /PbO , all these electrodes
2 2 2 5 2 3 2
show different oxidation efficiencies and mechanisms which are specific to their surface.
Although the electrochemical oxidation of phenol has been studied and reported, literature on
2
2
mechanisms involved are limited. In addition, these mechanisms may vary based on the
anode material as well. More specifically, phenol oxidation mechanisms by different types of
anodes still remain to be investigated. Objectives of this paper are to develop and study a
2 2 3
novel Steel/IrO ꢀSb O anode material for phenol oxidation and investigating the mechanism
and reaction pathways of the possible reactions. Authors expect that this work will shed more
light on the field of wasteꢀwater treatment and understanding the reaction mechanisms of
phenol oxidation.
Materials & method
Materials
All chemicals including H
IrCl .3H O (Ir ꢀ 53ꢀ56 %, ACRDS Organics), Sb standard solution (Fisher Scientifics),
ethanol (99%, AR, Fisher Scientifics), isopropanol, K Cr (99%, Avondale Laboratories),
SO (98%, Fisher Scientifics), Ag SO (Reagent grade , Fisher Scientifics), phenol (99%,
SigmaꢀAldrich), Na SO anhydrous (AR, 99%, SDFCL), Na CO (99%, BDH) , NaOH
2 2 4 2
C O .2H O (99%, LOBAL Chemie), HCl (37%, BDH),
3
2
2
2 7
O
H
2
4
2
4
2
4
2
3
2 4
(97.5%, BDH), RNO (SigmaꢀAldrich), KH PO (AR, 99.5%, BDH)were of analytical reagent
grade. All aqueous solutions were prepared by using deionized (DI) water.
Methods
4

Page 5 of 23
Electrode preparation
In the pre treatment process, substrate was mechanically treated by using a sand paper with
grit of 120 to increase the adhesiveness of the surface followed by a chemical pretreatment
process. Chemical pre treatment was done with 5% (w/w) oxalic acid solution and 37%
2
(
w/w) HCl acid prior to the coating process (geometrical area of one electrode was ~1 cm ).
ᴏ
23
Finally, the substrate was sonicated in distilled water and dried at 100 C . The anode was
developed on pretreated steel support with rare metal oxide coatings using the dip coating
process followed by calcinations steps. The coating solution was prepared by mixing 0.56 g
of IrCl .3H O in 4.6 ml ethanol and 0.33 ml Sb (1000 ppm) standard solution in 1.65 ml
3
2
ᴏ
isopropanol. Then the wet coating surface was air dried in 80 C airflow to evaporate solvents
ᴏ
and calcinated in an oven at 450 C for 10 min. The coating process was repeated until
2
ᴏ
achieving the final coating load of 1mg/cm . Finally, the electrode was post backed at 500 C
24
in muffle furnace (OTꢀHTMFꢀ05, Optics technology, Delhi India) for 1 h.
Electrochemical oxidation of phenol
Electrochemical oxidation of phenol was conducted for 50ml batch contains 40ml of 5.00mM
phenol solution and 10 ml of 10g/L Na
2
SO
4
supporting electrolyte. The electrochemical cell
2
2
was formed with 1 cm Steel/IrO
2
ꢀSb
2
O
3
anode and 1 cm stainless steel cathode. During the
6
h of electrolysis, the continuous mixing was carried out by using magnetic stirrer and the
distance between two electrodes was set at 1.0 cm. The initial conductivity of solution was
recorded as 2760 µS/cm. The initial pH was adjusted to 11 by adding aqueous NaOH drops,
because majority of industrial wastewater samples are having pH values close to 11. Further,
2 2 3
based on the preliminary studies, Steel/IrO ꢀSb O anode gave the highest performance when
the pH of the medium is between 9 and 12. The electrochemical measurements were carried
out in room temperature, 25°C. Electrolytic analysis apparatus ANAꢀ2 (Tokyo photoelectric
Co. Ltd., Japan) was employed as the DC power supply. All the voltage variations were
5

Page 6 of 23
2
recorded under 20 mA/cm of consistent current density and initial voltage of the system was
set to 4 V. During the electrolysis, 0.5 ml samples were withdrawn at one hour intervals for
the determination of COD and the analysis of intermediates. The effect of variation of batch
volume to the concentration was negligible.
Analysis and detection of intermediates
25
The COD test was conducted according to the standard potassium dichromate method. The
pH and conductivity of each sample was recorded by using a pH meter (Model: Thermo
Scientific, Orion 5 star).The intermediates were recognized by high performance liquid
4
chromatograph (HPLC) and UVꢀvis absorbance spectra. In HPLC (Shimadzu, Japan) a C18
reversed phase was used to determine intermediates. The samples were injected (8 µL) and
intermediates were monitored at 245 nm wavelength. 1ml/min was the flow rate of the
mobile phase (35% methanol and 65% distilled water). In UV analysis, different natures of
intermediates were identified by their specific wave lengths.190ꢀ1100 nm wide scan spectra
of the solutions were acquired by UVꢀ2450(Shimadzu, Japan) spectrophotometer and a quartz
cell with 1 cm light path length.
Qualitative analysis of radicals
Hydroxyl radical generation at electrodes was qualitatively investigated with 4ꢀnitrosoꢀN, Nꢀ
dimethylaniline (RNO).RNO is a good candidate to use in identification of hydroxyl radical
ꢃ
due to its selective reaction with OH radicals, electrochemical inertness and the easy color
22,26
ꢀ5
determination.
2 × 10 M RNO in 10 g/L Na
2
SO
4
solution was electrochemically treated
2
at a constant current density of 20 mA/cm . The color bleaching was monitored through
3
absorbance readings at 440 nm wave length.
Cyclic voltammetry (CV)
6

Page 7 of 23
Cyclic voltammograms were obtained by using a potentiostatgalvanostat equipment (Autolab
PGSTAT128N). In this study, Steel/IrO ꢀSb , stainless steel, and Ag/AgCl electrodes
2
2 3
O
were used as working electrode (WE), counter electrode (CE) and reference electrode (RE),
respectively. 40 ml of 5.00 mM phenol solution and pH 11.00, 10 ml of 0.1 M phosphate
buffer were involved in CV. Each reading was taken in nitrogen environment to avoid
27
atmospheric oxygen diffusion into the solution.
Results and discussion
2 2 3
Performance of phenol oxidation by Steel/IrO -Sb O anode
Chemical oxygen demand (COD)
2 2 3
The mineralization capacity of the Steel/IrO ꢀSb O anode was tested during the
electrochemical oxidation process of phenol. The initial COD concentration, 282.86 O mg/L,
2
was decreased during 6.0 h of electrolysis time as shown in Fig. 1. A COD concentration of
2
27.90 O
of 19.6%. After 2 h, 26.8% COD removal (i.e. 208.19 O
observed. ACOD removal efficiency of 26.34% (i.e. 188.38 O
was achieved at the end of three hours of electrolysis. COD concentrations at the end of four
and five hours of electrolysis were 122.63 O mg/L and 82.26 O mg/L, respectively. At the
2
mg/L was observed after 1 hour of electrolysis, showing a COD removal efficiency
mg/L of COD concentration) was
mg/L of COD concentration)
2
2
2
2
end of the 6.0 h of electrolysis period, COD concentration dropped up to 66.60 O
Removal rate at the end of the electrolysis time (6.0 h) was 76.3%.
2
mg/L.
The resultant COD removal efficiency of 76.3% can be due to the formation of oxidizing
agents on the anode surface. Under the experimental conditions used, i.e. in chloride free
2 4
electrolyte (Na SO solution), it was first assumed that the major oxidant contributing to the
oxidation would be the hydroxyl radical. In order to investigate if this assumption is valid,
further experimental investigations were carried out.
7

Page 8 of 23
RNO test to validate the assumption
2 2 3
In order to confirm above assumption on generating hydroxyl radicals on Steel/ IrO ꢀSb O
anode surface, a RNO test was carried out. RNO solution is yellow in color in absence of
hydroxyl radicals. Therefore, initially the highest absorbance reading was recorded at 440 nm
2
wave length. While applying 20 mA/cm of constant current density to the cell hydroxyl
2 2 3
radicals are supposed to generate on Steel/IrO ꢀSb O anode surface. When these radicals
entered into the system RNO color bleach was initiated. Due to the bleach absorbance values
were decreased with time. During the first 10 minutes, rapid color bleach was obtained, and a
noticeable rapid decrease of absorbance was observed (Fig. 2). The rapid decrease of
absorbance during first 10 minutes confirmed the generation of hydroxyl radicals. Therefore,
it can be said that the active sites of the Steel/ IrO
hydroxyl radical.
2 2 3
ꢀSb O anode surface generates the
ꢃ
ꢄꢅꢆradicals scavenger test to confirmation of presence hydroxyl radicals
ꢃ
After confirming the generation of OH radical in the system, investigations were extended to
ꢃ
identify the effect of OH radical in more quantitative manner. In order to accomplish this,
28
ꢃ
Na
2
CO
3
, a OH radical scavenger, was used as the electrolyte. Due to the scavenging effect,
increase of Na
2
CO concentration decreases the hydroxyl radical up to a considerable level.
3
As a result, COD removal efficiency of the system was low after adding the scavenger. Fig. 3
shows the COD removal efficiency after adding different amount of radical scavengers to the
system. When the concentration of Na
2
CO
3
was 10 g/L, the COD removal rate was 35.14%
CO concentration of 20
after 6 h of reaction time. It was further lowered to 15.29% at a Na
2
3
g/L. These removal efficiencies were considerably low comparing to nonꢀradical scavenger
system.
Phenol oxidation pathway and intermediate products identification
8

Page 9 of 23
2 2 3
Since Steel/IrO ꢀSb O anode achieved 76.3% mineralization efficiency, it may yield
ꢃ
intermediates due to incomplete oxidation of phenols by OH radicals. The formation and the
presence of such intermediates were determined by using the instantaneous current efficiency
(ICE) calculations, HPLC, UVꢀ vis spectral analysis and cyclic voltammetry studies.
3
Based on COD values, the instantaneous current efficiency (ICE) was calculated as follows ,
ꢇ(COD) − (COD)ꢈꢉ∆ꢈꢊFV
ꢈ
ICE =ꢆ
ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(Eq. 01)ꢆ
8I∆t
3
Where (COD) and (COD)ꢈꢆꢉ∆ꢈ are the initial chemical oxygen demand (gO
2
/m ) at time t
ꢈꢆꢆ
and t + ∆t (s) respectively. I is the applied current (A), F is the Faraday constant (C/mol) and
3
V is the volume of the electrolyte (m ).
This ICE variation indicates the actual amount of current applied for the necessary reaction
compared to the amount of current applied. The change of ICE in each hour of electrolysis
2
for Steel/IrO ꢀSb O anode is shown in Fig. 4. It can be seen that, at 20 mA/cm of consistent
2
2
3
current density, ICE is relatively higher at 0ꢀ1 h and 3ꢀ4 h stages. ICE increment at initial
stage of 0ꢀ1 h is due to the oxidation of phenol in the system. During this 0ꢀ1 h period phenol
oxidation with resulting series of intermediates is the principal reaction of the system.
Initially at high phenol concentration, hydroxyl radicals react rapidly before starting the
oxygen evolution reactions. It might be the formation of catechol or benzoquinone from
phenol oxidation. Therefore, ICE shows relatively high value in first hour comparing second
and third hours. There is a rapid decrease of ICE values between first and second hours due to
oxidation of the intermediates which are formed by phenol oxidation. There can be further
oxidation reactions such as aromatic ring opening. Then there is a noticeable increment in
ICE during 3ꢀ4 h period indicating the oxidation of easily oxidizable products.
9

Page 10 of 23
Moreover, in HPLC analysis, the primary intermediates; catechol (i.e. at 3.20 min retention
time) & benzoquinone (i.e. at 1.65 min retention time) were detected. It was found that the
phenol (i.e. at 4.00 min retention time) concentration was decreased with time.
In addition, the intermediates were further monitored by using the UVꢀvis scan for 200 nm to
400 nm wavelength range. Results are shown in Fig. 5 (a) and (b). The characteristic
absorbance wave length for phenol is 270 nm and the peak belongs to the 270 nm was
decreased with time. The spectra which were obtained during regular time intervals (1h) were
different from the initial one (results not shown here).
The new peaks were appeared after 6 h at 276 nm and 234 nm. Those peaks are associated
with catechol and benzoquinone, respectively (Fig. 5 (b)). Hence, the UVꢀvis spectral
findings confirmed that phenol was oxidized into catechol and subsequently catechol was
2 2 3
oxidized into benzoquinone on Steel/IrO ꢀSb O anode (Eq. 02)
ꢋꢌꢍꢎꢏꢐꢌꢑꢒꢓꢍꢔꢕꢓꢑ
ꢋꢌꢍꢎꢏꢐꢌꢑꢒꢓꢍꢔꢕꢓꢑ
Phenol ꢖꢗꢗꢗꢗꢗꢗꢗꢗꢗꢗꢗꢘ Catecol ꢖꢗꢗꢗꢗꢗꢗꢗꢗꢗꢗꢗꢘ Benzoquinoneꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(Eq. 02)
Further the above reaction sequence, the oxidation mechanism and the intermediates were
confirmed by a cyclic voltammetric behavior as shown in Fig. 6(a).
In Fig. 6 (a) CV, the batch initially contained only phenol molecules. Then the anode was
initially at a potential of ꢀ2.5 V Vs. Ag/AgCl reference electrode. At this negative potential,
only the reduced form of the molecules are present in the solution and further reduction
products cannot be expected. Therefore, either no or very little current was observed at ꢀ2.5
V. This potential was not enough to cause oxidation of phenol. While electrode potential was
slowly swept to anodic direction (or positive direction), single peak was appeared at +0.26 V.
If the system contains only phenol molecules, that peak must be related to phenol oxidation.
a
It can be seen from the Fig. 6 (a), as the phenol oxidation peak 1 (at Epla = +0.26 V).
Therefore, appearance of above peak confirmed phenol oxidation into catechol according to
following reaction,
1
0

Page 11 of 23
ꢁ
ꢉ
ꢃ
C H OHꢆ +ꢆOH → C H (OH) ꢆ+ 1e ꢆ+ 1H ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(Eq. 03)
ꢙ
ꢚ
ꢙ
ꢛ
ꢂ
Then, anodic current was reached up to +0.007 A at 2.5 V potential relative to Ag/AgCl
reference electrode. At this point, any phenol molecule which reaches the anode will
immediately be converted into its oxidized form, catechol.
Then the potential was swept to the cathodic direction, cathodic current was observed. While
c
scanning, there was a cathodic peak 2 at Ep2c= ꢀ0.70 V and it corresponds to reduction of
catechol, which is the oxidation product of phenol. This peak position was tally with 99%
catechol oxidation peak at ꢀ0.36 V which was obtained by cyclic voltammetry analysis of
standard 30 µM catechol (see Fig. 6(b)) under similar experimental conditions.
2 2 3
However, formation of polymeric layer or change of Steel/IrO ꢀSb O anode performance
was not observed during the process. Specially, the anode layer was stable and actively
involved in phenol oxidation even at very negative (i.e.ꢀ2.5 V) and very positive (+ 2.5 V)
potentials. Therefore, during the cyclic voltammetry measurements, electrode passivation was
not observed at high negative and positive potentials.
The second scan in positive direction was done for the same solution without cleaning the
anode surface. Then, one anodic peak was appeared at E p2a, at ꢀ0.32 V. It was related to
oxidation of catechol into benzoquinone. Catechol reduction into benzoquinone must be a
reversible reaction involving twoꢀ electrons. Theoretical potential difference between anodic
and cathodic peaks can be calculated as |E pa – E pc|. If the difference is close to 30 mV, that
29
reaction is theoretically defined as a twoꢀelectron reversible reaction. In this study, the
difference between the anodic and cathodic peak potentials, |Ep2aꢀ Ep2c| was 38 mV and it is
close to the theoretical value of 30 mV. Therefore, it confirms the reversibility of the peak 2
a
and 2 and it might be due to the presence of catechol in the system. This catechol reduced
c
into benzoquinone through twoꢀelectron reversible reaction as following,
ꢁ
ꢉ
C H (OH) ↔ C H O ꢆ+ ꢆ2e ꢆ+ 2H ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(Eq. 04)
ꢙ
ꢛ
ꢂ
ꢙ
ꢛ ꢂ
1
1

Page 12 of 23
Moreover, phenol oxidation products were adsorbed to the anode surface and resulting a
decrease in the peak 1 current.
a
According to the above investigations on intermediates products, the following reaction
pathway was resulted for the electrochemical oxidation of phenol on Steel/IrO ꢀSb O novel
2
2
3
anode. No polymer formation was observed on the anode surface and no significant amount
of metal was dissolved into the solution (Fig. 7.).
Conclusions
ꢃ
Investigations on bleaching of RNO proved the generation of OH radical on Steel/IrO ꢀSb O
2
2
3
ꢃ
anode surface. Studies based on OHradical scavengers showed a sharp decrease in
ꢃ
mineralization efficiency, concluding the involvement of OH radical in the oxidation process
compared to the other possible mechanisms.
ICE, UVꢀvis and HPLC studies showed that the oxidation of phenol into catechol as the
major intermediate of the reaction. Further, catechol was found to be oxidized into
benzoquinone. In conclusion, reaction mechanism pathway of oxidation of phenol on
Steel/IrO ꢀSb O anode was followed below oxidation sequence,
2
2
3
ꢁ
ꢉ
ꢁ
ꢉ
ꢃ
C H OHꢆ +ꢆOH → C H (OH) ꢆ+ 1e ꢆ+ 1H ↔ C H O ꢆ+ ꢆ2e ꢆ+ 2H ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(Eq. (05)
ꢙ
ꢚ
ꢙ
ꢛ
ꢂ
ꢙ
ꢛ ꢂ
This work enhances the overall understanding on the mechanisms of oxidation of phenol in
water on Steel/IrO ꢀSb anode.
2
2 3
O
Acknowledgement
Authors acknowledge the International Foundation for Scienceꢀ Sweden for providing funds
W 5336/1) and the National Research Councilꢀ Sri Lanka (NRCꢀ11ꢀ054).
(
1
2

Page 13 of 23
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Page 16 of 23
Figure captions
Fig. 1. Variations of COD removal percentage with electrolysis time for Steel/IrO ꢀ Sb O
3
2
2
anode
ꢀ5
Fig. 2.Electrochemical bleaching of 2× 10 M RNO in 10 g/L Na
2
4
SO solution as a function
2
2
of treatment time (Current density = 20 mA/cm , anode area = 1 cm , batch volume=50 ml)
2
Fig. 3.COD removal percentage comparison with radical scavenger conditions ꢀSteel/IrO ꢀ
Sb O anode
2
3
Fig. 4 Variation of ICE with every 1 h of electrolysis – on Steel/IrO
Fig. 5. (a)UVꢀ vis absorption spectra of 5.00 mM phenol solution before electrolysis and (b)
after 6 h of electrolysis on Steel/IrO ꢀSb
Fig. 6.(a)CVs in pH 11.0 0.1 M phosphate buffer 30 µM phenol: (ꢀ) first scan and (…) fifth
2 2 3
ꢀ Sb O anode
2
2 3
O
ꢀ1
scan, ʋ = 50 mVs (b) Standards CV of 30 µM catechol in pH 11.0 0.1 M phosphate buffer,
ꢀ1
scan, ʋ = 50 mVs
Fig. 7 Proposed reaction pathway of electrochemical oxidation of phenol on Steel/IrO
2
ꢀSb
2
O
3
anode
1
6

Page 17 of 23
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
100
200
300
400
Electrolysis time (min)
Fig.1.
1
7

Page 18 of 23
0
0
.14
.12
0
.1
0
0
0
0
.08
.06
.04
.02
0
0
25
50
75
100
Electrolysis time (min)
Fig.2.
1
8

Page 19 of 23
1
00
Na2SO4 ꢀ 10g/L
Na2CO3 ꢀ 10g/L
Na2CO3 ꢀ 20g/L
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
100
200
300
400
Electrolysis time (min)
Fig.3.
1
9

Page 20 of 23
1
0
0
0
0
0
0
0
0
0
0
.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
.0
0
ꢀ1
1ꢀ2
2ꢀ3
3ꢀ4
4ꢀ5
5ꢀ6
Electrolysis Time (h)
Fig.4.
2
0

Page 21 of 23
3
.5
2
2
00
Initial
2
1
0
.5
1
2
70
.5
0
1
90
230
270
310
350
390
Wave length (nm)
3
2
00
Sixth hour
2
1
0
.5
2
.5
1
2
34
2
76
.5
0
1
90
230
270
310
350
390
Wave length (nm)
Fig. 5.
2
1

Page 22 of 23
0
.01
(
a)
First scan
Sixth scan
0
0
0
0
.008
.006
.004
.002
0
1a
2a
ꢀ0.002
ꢀ0.004
ꢀ0.006
ꢀ0.008
2
c
Fig.7.
ꢀ0.01
ꢀ4
ꢀ2
0
2
4
Potential/V vs. Ag/AgCl
0
.01
(
b)
0
0
0
0
.008
.006
.004
.002
0
Catechol Standard sample
ꢀ0.002
ꢀ0.004
ꢀ0.006
ꢀ0.008
ꢀ0.01
ꢀ3
ꢀ2
ꢀ1
0
1
2
3
Potential /V vs. Ag/AgCl
Fig. 6.
2
2

Page 23 of 23
Fig. 7.
2
3