Electrochemical incineration of dyes using a boron‐doped diamond anode

Article (PDF Available)inJournal of Chemical Technology & Biotechnology 82(6):575 - 581 · June 2007with56 Reads
DOI: 10.1002/jctb.1703
Abstract
The electrochemical oxidation of a synthetic wastewater containing the model dyes alizarin red (an anthraquinone) and Eriochrome black T (an azoic compound) has been studied on a boron-doped diamond electrode (BDD) by both cyclic voltammetry and bulk electrolysis. The influence of the current density and dye concentration were investigated. The results obtained show that complete chemical oxygen demand (COD) and colour removal was obtained for both wastewaters. However, the nature of the pollutant, and specially the presence of functional groups (such as the azoic group) seems to strongly influence the performance and efficiency of the electrochemical process. The electro-oxidation of alizarin red behaves as a mass-transfer-controlled process. In such a system, an increase in the current density leads to a decrease in the current efficiency. This can be explained by direct or hydroxyl radical mediated oxidation. The contrary tendency has been observed in Eriochrome black T electro-oxidation. In this case, higher efficiencies were obtained working at high current densities. This may indicate that the mediated oxidation by electrogenerated reagent (such as peroxodisulphate) is the main oxidation mechanism involved in Eriochrome black T treatment. These compounds have a longer average lifetime than hydroxyl radicals, and it allows the reaction to be extended to the whole wastewater volume. This study has shown the suitability of the electrochemical process for completely removing the COD and total organic carbon and effectively decolourising of wastewaters containing synthetic dyes. Copyright © 2007 Society of Chemical Industry
Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 82:575581 (2007)
Electrochemical incineration of dyes
using a boron-doped diamond anode
Cristina Saez,
1
Marco Panizza,
2
Manuel A Rodrigo
1
and Giacomo Cerisola
2
1
Universidad de Castilla La Mancha, Ciudad Real, Spain
2
University of Genoa, Genova, Italy
Abstract: The electrochemical oxidation of a synthetic wastewater containing the model dyes alizarin red (an
anthraquinone) and Eriochrome black T (an azoic compound) has been studied on a boron-doped diamond
electrode (BDD) by both cyclic voltammetry and bulk electrolysis. The influence of the current density and dye
concentration were investigated. The results obtained show that complete chemical oxygen demand (COD) and
colour removal was obtained for both wastewaters. However, the nature of the pollutant, and specially the presence
of functional groups (such as the azoic group) seems to strongly influence the performance and efficiency of the
electrochemical process. The electro-oxidation of alizarin red behaves as a mass-transfer-controlled pro cess. In
such a system, an increase in the current density leads to a decrease in the current efficiency. This can be explained
by direct or hydroxyl radical mediated oxidation. The contrary tendency has been observed in Eriochrome black
T electro-oxidation. In this case, higher efficiencies were obtained wo rking at high current densities. This may
indicate that the mediated oxidation by electrogenerated reagent (such as peroxodisulphate) is the main oxidatio n
mechanism involved in Eriochrome black T treatment. These compounds have a longer average lifetime than
hydroxyl radicals, and it allows the reaction to be extended to the whole wastewater volume. This study has shown
the suitability of the electrochemical process for completely removing the COD and total organic carbon and
effectively decolourising of wastewaters containing synthetic dyes.
2007 Society of Chemical I ndustry
Keywords: boron-doped diamond anode; electrochemical oxidation; decolourization, dyes
INTRODUCTION
In recent years, textile wastewaters have become the
focus of environmental remediation efforts because of
the presence of large quantities of reactive dyes (mainly
polyaromatic hydrocarbons) that cannot be oxidized
aerobically and tend to accumulate in sediments,
where they are prone to anaerobic degradation to
potentially carcinogenic aromatic amines. In fact, the
classical processes of physico-chemical methods such
as precipitation, coagulation, filtration, adsorption
1,2
and biological oxidation used in their treatment are
not always sufficient to completely remove all the
pollutants, and therefore, many studies have focused
on the search for novel methods.
In this framework, electrochemical oxidation, pro-
viding versatility, energy efficiency, amenability to
automation, environmental compatibility and cost
effectiveness, is a promising technique for the destruc-
tion of toxic or biorefractory organics in textile
wastewaters.
3–5
The overall performance of electrochemical pro-
cesses is determined by the complex interplay of
parameters that may be optimized to obtain effective
and economical degradation of pollutants. The prin-
cipal factors determining the electrolysis performance
are electrode potential and current density, mass trans-
port regime, cell design, electrolyte composition and
temperature. Above all, electrode materials should be
totally stable and exhibit high activity towards organic
oxidation and low activity towards secondary reactions
(e.g. oxygen evolution).
Many studies have demonstrated that the complete
mineralisation of organics can be obtained with
high efficiency by direct electro-oxidation using only
high oxygen overvoltage anodes such as SnO
2
,
6–10
PbO
2
11 16
and boron-doped diamond (BDD).
17 26
With the use of these anodes, organics are oxidised to
CO
2
by hydroxyl radicals electrogenerated from water
discharge:
2H
2
O −− 2OH
+ 2H
+
+ 2e
(1)
Of these, SnO
2
and PbO
2
have the common draw-
backs of a short service-life and the release of toxic
ions,
27
while BDD electrodes exhibit good chemi-
cal and electrochemical stability, a long life and a
wide potential window for water discharge, and are
thus promising anodes for industrial-scale wastewater
treatment.
24
In fact, it has been demonstrated that
many biorefractory compounds such as phenols,
28,29
chlorophenols
15,17,30
nitrophenol,
31,32
pesticides,
33
synthetic dyes
34 36
and industrial wastes
37 40
can be
completely mineralised with high current efficiency,
even close to 100%, using BDD anodes.
The aim of this work was to study the electro-
chemical oxidation of a model wastewater containing
Correspondence to: Marco Panizza, Department of Chemical and Process Engineering. University of Genoa, P.le J.F. Kennedy 1, 16129 Genova, Italy
E-mail: marco.panizza@unige.it
(Received 5 March 2007; revised version received 26 March 2007; accepted 26 March 2007)
Published online 21 May 2007
; DOI: 10.1002/jctb.1703
2007 Society of Chemical Industry. J Chem Technol Biotechnol 02682575/2007/$30.00
CSaezet al
synthetic dye using a boron-doped diamond anode.
Alizarin red, an anthraquinone dye, and Eriochrome
black T, an azoic dye, were chosen as model com-
pounds because they have been used in textile dyeing
for many years early antiquity, and they contain aro-
matic rings that make them difficult to treat with
traditional processes, and although they are large
molecules the functional groups are quite different.
The influence of the main operating parameters, such
as current density and dye concentration, affecting
COD and colour removal was investigated, and some
insights into the oxidant mechanism have been pro-
posed.
EXPERIMENTAL
A BDD thin-film electrode was supplied by CSEM
(Centre Swiss d’Electronique et de Microtechnique
of Neuch
ˆ
atel). It was prepared by the hot filament
chemical vapour deposition technique (HF-CVD)
on single crystal p-type Si < 100 > wafers. The
diamond film thickness obtained was about 1
µm with
a resistivity of 1030 m
˙
cm and the doping level
of the boron in the diamond layer, expressed as B/C
ratio, was about 3500 mg dm
3
.
Cyclic voltammetry was carried out in a conven-
tional three-electrode cell using a computer-controlled
EG&G potentiostat model M 273 (Princeton, NJ,
USA). A BDD was used as the working electrode,
Hg/Hg
2
SO
4
.K
2
SO
4
(sat) as the reference and Pt
as the counter electrode. Bulk oxidations were per-
formed in a one-compartment electrolytic flow cell
under galvanostatic conditions using an AMEL 2055
potentiostat/galvanostat (AMEL, Rome, Italy). BDD
was used as the anode and stainless steel as the cath-
ode. Both electrodes were in the form of disks with
a geometrical area of 50 cm
2
each, with an inter-
electrode gap of 1 cm. The solution was stored in a
250 mL thermo-regulated glass tank and circulated
through an electrochemical reactor by a centrifugal
pump with a flow-rate of 180 dm
3
h
1
, corresponding
to a mass-transfer coefficient in the cell, measured with
a ferri/ferrocyanide couple, of about 2.0 × 10
5
ms
1
.
The dyestuff solution was prepared by dissolving
different amounts of alizarin red (C
14
H
7
O
7
SNa) or
Eriochrome black T (C
20
H
12
O
7
N
3
SNa) in distilled
water in 5000 mg dm
3
Na
2
SO
4
. Their molecular
structures are illustrated in Fig. 1.
Colour removal was monitored by measuring the
decrease in absorbance using a spectrophotometer
(Perkin-Elmer Lambda 2; Waltham, MA, USA). The
chemical oxygen demand (COD) of the solution
was measured during electrolysis using a Dr. Lange
LASA50 system (Italy), the total organic carbon
(TOC) was determined using a TOC-5050 Shimadzu
(Tokyo, Japan) apparatus. The instantaneous current
efficiency (ICE) for anodic oxidation is the ratio
between the charge consumed for the oxidation of the
dyes and the total charge passed, and was calculated
Alizarin Red
CAS: 130-22-3
SO
3
Na
O
OH
OH
O
N
N
SO
3
Na
OH
OH
NO
2
Eriochrome Black T
CAS: 1787-61-7
Figure 1. Molecular structure of alizarin red and Eriochrome black T.
from values of the COD using the relationship:
41
ICE =
(COD
t
COD
t+t
)
8It
FV (2)
where (COD)
t
and (COD)
t+t
are the chemical
oxygen demands at times t and t + t (in g
O2
L
1
)
respectively, I is the current (A), F is the Faraday
constant (96487 C mol
1
), V is the volume of the
electrolyte (L) and 8 is the oxygen equivalent mass
(g eq). The formation of peroxodisulfuric acid during
the anodic oxidation was determined by iodometric
titration.
RESULTS AND DISCUSSIONS
Figure 2 shows the changes in COD as a function of
TOC during the electrolysis of synthetic waste polluted
with the anthraquinone dye Alizarin red (AR). As can
be observed, the electrochemical treatment leads to
almost complete mineralization of the waste. Likewise,
simultaneous changes in COD and TOC are obtained.
This fact means that total oxidation dominates over
partial oxidation, indicating a low generation of
intermediates, and it could be interpreted in terms of
the almost direct oxidation of the pollutant to carbon
dioxide.
42
To obtain additional qualitative information about
intermediate generation and consequently about the
0
500
1000
1500
2000
0 100 200 300 400 500 600 700
TOC / mg dm
-3
COD / mg dm
-3
Figure 2. Changes of COD as a function of TOC during electrolysis of
AR polluted synthetic wastes. Operating conditions: current density:
600 A m
2
; T:25
C. COD
0
: 1800 mg dm
3
; electrolyte: Na
2
SO
4
5000
mg dm
3
;pH:2.
576 J Chem Technol Biotechnol 82:575581 (2007)
DOI: 10.1002/jctb
Incineration of dyes using boron-doped diamond
0
0.5
1
1.5
2
2.5
270 370 470
Wavelength / nm
Absorbance
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200
Time / min
A/ A°
1
2
3
4
5
6
a
b
Figure 3. (a) Changes in the UV-vis spectra with time during
electrolysis of an AR polluted synthetic waste. Operating conditions.
Current density: 600 A m
2
; T:25
C. COD
0
: 1800 mg dm
3
;
electrolyte: Na
2
SO
4
5000 mg dm
3
; pH:2. (b) Changes in the ratio
A/A
0
measured at 280 (
) and 420 (
)nm.
oxidation pathways involved in the electro-oxidation
process of AR, the samples obtained were measured
by UV-visible techniques. Figure 3 shows the changes
in the UV-Visible spectra obtained during the same
electrolysis assay. As AR is a coloured compound, its
UV-Visible spectrum shows an absorption maximum
within the range of visible range (wavelength 420 nm).
Likewise, it also gives a strong sharp peak in the ultra-
violet region for benzenoid and/or quinoid absorption
(wavelength = 280 nm). Figure 3 inset shows the vari-
ation of the ratio absorbance/initial absorbance (A/A
0
)
at these two wavelengths. As can be observed, the
intensity of both bands decreases continuously and in
the same ratios during treatment. This fact seems to
indicate that AR is the main species present in the
solution, and that discoloration of the waste occurs
simultaneously with oxidation of aromatic rings.
As can be observed in Fig. 4, the experimen-
tal results obtained for the treatment of this
anthraquinone dye are in contrast with those obtained
in the electrochemical oxidation of another kind of
dye, Eriochrome black T (EBT) that was selected as
a model azoic compound.
42
Fig. 4 shows the varia-
tion of the ratios COD/COD
0
, TOC /TOC
0
and A/A
0
(corresponding to the main absorption peak in the
visible region) obtained during the electrochemical
treatment of AR and EBT synthetic polluted wastes.
It can be observed that the electrochemical oxida-
tion with conductive-diamond electrodes can treat
satisfactorily both synthetic wastewaters, but the per-
formance of the process seems to strongly depend
on the nature and structure of the dye. Thus, in the
electro-oxidation on BDD anodes of AR solutions
the simultaneous removal of COD, TOC and discol-
oration are obtained, but not in the treatment of EBT
wastes. In the former case, the changes in COD are
more abrupt than those in TOC, and the discoloration
of the waste occurs in the initial stages of the pro-
cess (the total discoloration of the solution is obtained
for electrical charge passed below 20 Ah dm
3
). This
could be explained by taking into account the fact
0
0.2
0.4
0.6
0.8
1
0 1020304050607080
Q / Ah dm
-3
COD/COD
0
, TOC/TOC
0
, A/A
0
b
a
0
0.2
0.4
0.6
0.8
1
0 1020304050
Q / Ah dm
-3
COD/COD
0
, TOC/TOC
0
, A/A
0
Figure 4. Changes in the ratio COD/COD
0
(
), TOC/TOC
0
(
)and
A/A
0
() with charge current charge during electrolysis of AR (a) and
EBT (b) polluted synthetic wastes. Operating conditions. AR (600 A
m
2
; T:25
C, pH 2), EBT (300 A m
2
; T:25
C, pH 2).
that both the large molecular weight of the EBT
molecule and its complex molecule (with many func-
tional groups) could favour the formation of a great
variety of intermediates (changes in the COD) with-
out carbon dioxide formation (changes in the TOC).
Therefore, the EBT oxidation process could start with
the breakage of the azoic group and continue with
the oxidation of the intermediates generated. Figure 5
shows the changes in the UV-Visible spectra obtained
during this electrolysis assay. As can be observed, the
spectra show four main bands at 225, 285, 340 and
540 nm. The intensity of the last band (in the visi-
ble spectrum region) decreases continuously down to
zero (discoloration of the waste), but the intensities
of the other peaks increase in the initial stages and
later decrease (generation and later oxidation of inter-
mediates). Both observations, the quick discoloration
of the waste and the increment in the intensity of
the absorption peak in the ultraviolet region, seem to
support the assumptions of the rapid oxidation of the
azo group of the EBT molecule (colour removal) and
the generation of significant amounts of intermediates
(with lower COD) during the process. On the con-
trary, and according to Fig. 3, in the electro-oxidation
of AR the accumulation of large amounts of interme-
diates should not take place (absence of increments in
the intensity of UV-absorption bands). Hence, these
results confirm that the presence of functional group
that can be easily oxidized could strongly influence the
J Chem Technol Biotechnol 82:575 581 (2007) 577
DOI: 10.1002/jctb
CSaezet al
1
8
7
5
6
0
0.5
1
1.5
2
2.5
3
3.5
4
200 300 400 500 600 700
Wavelenght / nm
Absorbance
0
0 50 100 150 200
0.2
0.4
0.6
0.8
1
1.2
1.4
Time / min
A/ A°
a
b
2,3,4
Figure 5. (a) Changes in the UV-vis spectra with time during
electrolysis of an EBT polluted synthetic waste. Operating conditions.
Current density: 300 A m
2
; T:25
C. COD
0
: 1800 mg dm
3
;
electrolyte: Na
2
SO
4
5000 mg dm
3
; pH:2. (b) Changes in the ratio
A/A
0
measured at () 225, (
) 280, 340 (
) and 540 ()nm.
oxidation pathways, and also support the importance
of the nature of the pollutant, especially in those cases
in which the complexity of the pollutant is high.
The results obtained are compared with those
predicted by a model proposed in the literature
43,44
that assumes direct electrochemical reaction in the
anode surface, with efficiencies only limited by
mass transport. This model has been validated
with the results obtained in the treatment of many
compounds such as phenol, chlorophenols, naphthol,
nitrophenols, etc.
17,25
Fig. 6 shows the current
efficiencies obtained versus the ratio COD/COD
lim
,
as a function of the current density applied in the
electrolyses of AR and EBT. The parameter COD
lim
depends on the current density applied and also on
the fluidodynamics characteristic of the system (k
m
),
as shown below:
COD
lim
=
j
appl
4·F·k
m
(3)
where j
appl
is the current density (A m
2
), F is the
Faraday constant (96487 C mol
1
), and k
m
is the
mass-transfer coefficient (m s
1
). It indicates the COD
value at which the system begins to be mass-transfer
controlled.
As can be seen, the efficiencies obtained in the
electrolysis of AR are lower than that predicted by
the model, but higher than those obtained in the
electrochemical oxidation of EBT, mainly at 300
Am
2
. Another important observation is that the
influence of the current density on the efficiency of
the process also depends on the characteristics of
the pollutant. In AR removal, no marked influences
are observed. In this case, mass transport seems to
be the controlling mechanism (an increase in the
current density cannot increase the rate of oxidation
of organics at the electrode, and only favours anodic
side reactions). On the contrary, in the EBT removal
process, the higher the current density, the higher the
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2
COD/COD
lim
ICE
Theoretical model
Figure 6. Variation of the ICE with the ratio COD/COD
lim
during
electrolyses of AR and EBT synthetic wastewaters with BDD anodes.
Solid line: trend predicted by model proposed in the literature.
Operating conditions. AR (T:25
C, pH 2): (
) 300 A m
2
;() 600 A
m
2
.EBT(T:25
C, pH 2): (
) 300 A m
2
;() 600 A m
2
.
current efficiencies. In this former case, the higher
efficiencies observed in the process carried out at 600
Am
2
can be explained by mediated oxidants activity
over the whole reaction volume and not limited to
a zone close to the anode surface. This indicates
the important role of the mediated oxidation (by
electrogenerated reagent) in the oxidation process of
azoic compounds. In fact during the electrolysis it
was observed that peroxidisulphuric acid was formed
and its concentration reached a maximum value
of about 20 mmol L
1
. In the literature
25,45 47
the
generation of powerful oxidants in the electrolysis
of sulphate solution on BDD electrodes is also
proposed. In addition, it has been verified that
these reagents can contribute abruptly to the global
oxidation process.
46
Since the oxidation carried out
by these electroreagents is a chemical oxidation
reaction, the oxidation rate depends on the nature
of the pollutants. Thus, although they are generated
in both cases (AR and EBT electro-oxidation), the
effect is more significant in the treatment of the
azoic dye.
Figure 7 plots the COD removal percentage
against specific electrical charge passed during
the electrolyses of synthetic wastes polluted with
different initial concentrations of AR and EBT.
The trends obtained in the electrolysis of AR are
coincident, and similar values of specific electrical
charge passed are required to achieve complete
mineralization of the waste. This indicates that both
oxidation rate and efficiency of the process are
directly proportional to the concentration of organic
matter, and it also confirms the presence of mass
transfer limitations. According to the literature,
12,17
this behaviour can be explained assuming that the
main mechanism involved in the electrochemical
oxidation on conductive diamond anodes is a direct or
hydroxyl-radical mediated electrochemical oxidation
process. In contrast, the COD removal obtained
in EBT essays changes abruptly with the initial
578 J Chem Technol Biotechnol 82:575581 (2007)
DOI: 10.1002/jctb
Incineration of dyes using boron-doped diamond
0
20
40
60
80
100
0 20 40 60 80 100
Q / Ah dm
-3
COD Removal / %
Figure 7. Variation of COD removal percentage during electrolysis of
AR and EBT polluted synthetic wastes. Operating conditions: current
density: 300 A m
2
; T:25
C. Electrolyte: Na
2
SO
4
5000 mg dm
3
.
COD
0
:AR:(
) 300 mg dm
3
,(
°
) 700 mg dm
3
and () 1800 mg
dm
3
.EBT:(
) 100 mg dm
3
,(
ž
) 1200 mg dm
3
and () 1800 mg
dm
3
.
concentration of pollutant. As stated before, this
could be explained by assuming an important role
for mediated peroxodisulphate oxidation in the overall
oxidation process.
In order to obtain more information about the
mechanisms (direct or mediated oxidation) of the oxi-
dation process, several voltammetric measurements
were carried out. Thus, Fig. 8 shows cyclic voltam-
mograms of aqueous solutions containing different
concentrations of AR or EBT and Na
2
SO
4
as sup-
porting electrolyte. The presence of AR leads to the
appearance of an anodic oxidation peak at approxi-
mately 1 V vs. SCE. This peak indicates the existence
of direct electrochemical reactions in the potential
region of stability of the electrolyte. Likewise, AR
and EBT voltammograms show an anodic oxidation
shoulder at values of potential close to that of water
decomposition. This shoulder can be explained in
terms of a peak very close to the oxygen evolution
potential region and partially overlapped with this
process. Moreover, in both cases the peaks decrease
in size in the following scans and the voltammograms
0
1
2
3
4
5
6
7
0 0.5 1 1.5 2 2.5 3
E / V vs. SCE
j/ mA cm
-2
AR: scan 1
AR: scan 2
EBT: scan 1
EBT: scan 2
Figure 8. Cyclic voltammograms on BDD anodes of AR or EBT and
Na
2
SO
4
(5000 mg dm
3
) solutions. Scan rate: 100 mV s
1
. Counter
electrode: Pt. Reference electrode: SCE.
are superposed in later cycles, suggesting the forma-
tion of polymeric materials that decrease electrode
activity. According to the literature,
43,48
the diamond
surface regains its initial activity by anodic polariza-
tion at a potential greater than the water oxidation
potential. It is known that water discharge on BDD
anodes involves the production of hydroxyl radicals
thatareabletooxidizethepolymericfilmonthesur-
face. This confirms the important role of the hydroxyl
radicals in electrochemical oxidation with conductive-
diamond electrodes.
49
Thus, although both direct
(mainly in AR electro-oxidation essays) and mediated
oxidation processes contribute to the global oxida-
tion rate, the mechanisms involved in them being
more severe than those occurring in the supporting
electrolyte instability region (mediated oxidation pro-
cesses).
CONCLUSIONS
The electrochemical treatment of a synthetic solution
containing alizarin red and Eriochrome black T was
investigated using a BDD electrode under different
experimental conditions. The experimental results
show that complete COD and colour removal was
obtained within the range studied regardless of
the current density, temperature and initial dye
concentration in the electrochemical oxidation of AR
and EBT. However, the oxidation mechanism seems
to depend on the nature of the dye. Hence, the
electrochemical oxidation of AR leads to almost direct
generation of carbon dioxide, without accumulation
in the system of large amounts of intermediates.
In contrast, the EBT oxidation process starts with
breakage of the azoic group and continues with
oxidation of the intermediates generated. On the
other hand, both oxidation pathway and efficiency
of the process seem to depend on the nature of
the pollutant. In the electrochemical oxidation of
AR, low current density favoured COD removal,
current efficiency and colour fading, meaning that
oxidation was under mass-transport control. In EBT
treatment, the higher the current density, the higher
the current efficiency, indicating the important role
of electrogenerated reagent in the global oxidation
process.
ACKNOWLEDGEMENT
The Erasmus agreement between the University
of Genoa (Italy) and the University of Castilla-La
Mancha (Spain) is gratefully acknowledged.
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    • "Over the years, electrochemical oxidation of refractory effluents has received a great deal of attention thanks to its attractive characteristics of energy efficiency, possibility of automation, versatility and environmental compatibility, because the main reagent, the electron, is a clean reagent [9]. Previous studies have demonstrated that the complete mineralization of organics can be obtained with high current efficiency by direct electrooxidation using only high-oxygen-overvoltage anodes such as SnO 2 [10][11][12][13], PbO 2 [13][14][15][16]and boron-doped diamond (BDD) [17][18][19][20][21]anodes. With the use of these anodes, organics are oxidized to CO 2 by highly reactive hydroxyl radicals electrogenerated from water discharge. "
    [Show abstract] [Hide abstract] ABSTRACT: The electrocatalytic properties of the Ti–Ru–Sn ternary oxide anode (TiRuSnO2), lead dioxide (PbO2) and boron-doped diamond (BDD) anodes were compared for the electrochemical incineration of Methyl Orange (MO), an azo dye, using an electrolytic flow cell with parallel-plate electrodes. The effects of several operating parameters such as current density, hydrodynamic conditions and initial dye concentration on the degradation rate, mineralization and current efficiency were determined. The experimental data indicate that, on PbO2 and BDD anodes, MO was completely oxidized by reaction with hydroxyl radicals electrogenerated from water discharge and the removal rate was favored by high flow rates, meaning that the oxidation was a diffusion-controlled process. It was also observed that the Methyl Orange decay followed a pseudo-first-order kinetics. After 3 h the BDD and PbO2 anodes lead to almost complete MO degradation with comparable removal rate, but BDD enables higher COD removal than PbO2, with 96% and 79% of mineralization, respectively. On the contrary, on DSA only a partial oxidation of MO was obtained corresponding to 75% degradation of the initial substrate with 60% mineralization.
    Full-text · Article · Feb 2016
    • "Over the years, electrochemical oxidation of refractory effluents has received a great deal of attention thanks to its attractive characteristics of energy efficiency, possibility of automation, versatility and environmental compatibility, because the main reagent, the electron, is a clean reagent [9]. Previous studies have demonstrated that the complete mineralization of organics can be obtained with high current efficiency by direct electrooxidation using only high-oxygen-overvoltage anodes such as SnO 210111213, PbO 213141516 and boron-doped diamond (BDD)1718192021 anodes. With the use of these anodes, organics are oxidized to CO 2 by highly reactive hydroxyl radicals electrogenerated from water discharge. "
    [Show abstract] [Hide abstract] ABSTRACT: The electrocatalytic properties of the Ti–Ru–Sn ternary oxide anode (TiRuSnO2), lead dioxide (PbO2) and boron-doped diamond (BDD) anodes were compared for the electrochemical incineration of Methyl Orange (MO), an azo dye, using an electrolytic flow cell with parallel-plate electrodes. The effects of several operating parameters such as current density, hydrodynamic conditions and initial dye concentration on the degradation rate, mineralization and current efficiency were determined. The experimental data indicate that, on PbO2 and BDD anodes, MO was completely oxidized by reaction with hydroxyl radicals electrogenerated from water discharge and the removal rate was favored by high flow rates, meaning that the oxidation was a diffusion-controlled process. It was also observed that the Methyl Orange decay followed a pseudo-first-order kinetics. After 3 h the BDD and PbO2 anodes lead to almost complete MO degradation with comparable removal rate, but BDD enables higher COD removal than PbO2, with 96% and 79% of mineralization, respectively. On the contrary, on DSA only a partial oxidation of MO was obtained corresponding to 75% degradation of the initial substrate with 60% mineralization.
    Full-text · Article · Feb 2016
    • "The increase in current may accelerate reaction (2) also enhancing the production of these active chlorine species which are able to oxidize more rapidly the organic matter in competition with M(@BULLETOH) [25]. The use of EAOPs by direct or indirect approaches has shown promising results on the complete abatement of POPs in synthetic and real effluents such dyes [20,262728, drugs293031, pesticides323334, petrochemical wastes35363738394041 and tannery wastewaters [42] . In contrast , no attempts have been published about the use of electrochemical technologies for decontamination of polluted effluents generated by fuel stations. "
    [Show abstract] [Hide abstract] ABSTRACT: In this work, a real effluent from oil–water separator of fuel station has been treated for the first time by electrochemical oxidation (EO) process. Electrochemical experiments were carried out under real discharged effluent conditions using Ti/Pt and Ti/IrO2–Ta2O5 anodes at different supporting electrolytesin order to study the influence of the different oxidants electrogenerated. The effect of applied current densities (j = 10, 20 and 30 mA cm− 2) was also evaluated. Results showed that good performances were achieved using Ti/Pt anode adding K2SO4 as supporting electrolyte to improve the solution conductivity. Dissolved organic carbon (DOC) and chemical oxygen demand (COD) abatements of 55.2% and 61.5% were achieved, reducing significant concentration of organic compounds (in terms of benzene–toluene–ethylbenzene–xylene (BTEX) and total petroleum hydrocarbons (TPHs)) at j = 30 mA cm− 2 after only 4 h of electrochemical treatment. Affordable costs of process expenditure of US$ 3.79 m− 3 were also achieved using Ti/Pt anodes; however, these costs could be reduced increasing the solution conductivity. The figures obtained in this investigation provide valuable information for developing of electrochemical technologies to their real application in order to propose a pre or post treatment alternative.
    Article · Dec 2015
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