Application of anodic oxidation, electro-Fenton and UVA photoelectro-Fenton to decolorize and mineralize acidic solutions of Reactive Yellow 160 azo dye

Article abstract of DOI:10.1016/j.electacta.2016.04.166

The degradation of 100 cm³ of a solution with 0.167 mmol dm^-3 Reactive Yellow 160 (RY160) azo dye in sulfate medium at pH 3.0 has been comparatively studied by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). Trials were carried out with a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and an air-diffusion cathode for H₂O₂ production, upon addition of 0.50 mmol dm^-3 Fe²⁺ as catalyst in EF and PEF. The solution was slowly decolorized by AO-H₂O₂ because of the low rate of reaction of the azo dye and its colored products with hydroxyl radicals generated at the BDD anode from water oxidation. The color loss was enhanced in EF by the larger oxidation ability of hydroxyl radicals produced in the bulk from Fenton's reaction between added Fe²⁺ and generated H₂O₂, whereas the solution was more rapidly decolorized by PEF owing to the additional generation of hydroxyl radicals from the photolysis of Fe(III)-hydroxy complexes by UVA light. The relative mineralization ability of the processes also increased in the sequence AO-H₂O₂ < EF < PEF. The PEF method was the most powerful due to the synergistic oxidation by hydroxyl radicals and UVA irradiation, yielding 94% mineralization after 360 min at 100 mA cm^-2. The influence of current density and RY160 concentration on the performance of all processes was assessed. Final carboxylic acids like maleic, fumaric, tartronic, acetic, oxalic, oxamic and formic were quantified by ion-exclusion HPLC. All these acids were totally removed by PEF, but the formation of small amounts of other highly recalcitrant products impeded the total mineralization. Chloride, sulfate, ammonium and, to a smaller extent, nitrate ions were released to the solution from the heteroatoms of the azo dye in all cases.

Full text of DOI:10.1016/j.electacta.2016.04.166

Electrochimica Acta 206 (2016) 307–316
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Application of anodic oxidation, electro-Fenton and UVA
photoelectro-Fenton to decolorize and mineralize acidic solutions of
Reactive Yellow 160 azo dye
Alejandro Bedolla-Guzmana, Ignasi Sirésb,1, Abdoulaye Thiamb,
a,1
1,b,
1,a,**
*
Juan Manuel Peralta-Hernández
, Silvia Gutiérrez-Granados , Enric Brillas
a
Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Cerro de la Venada s/n, Pueblito de Rocha, Guanajuato,
Mexico
b
Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física, Facultat de Química, Universitat de Barcelona, Martí i
Franquès 1-11, 08028 Barcelona, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
The degradation of 100 cm3 of a solution with 0.167 mmol dmꢀ3 Reactive Yellow 160 (RY160) azo dye in
sulfate medium at pH 3.0 has been comparatively studied by anodic oxidation with electrogenerated
H2O2 (AO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). Trials were carried out with a
stirred tank reactor equipped with a boron-doped diamond (BDD) anode and an air-diffusion cathode for
H2O2 production, upon addition of 0.50 mmol dmꢀ3 Fe2+ as catalyst in EF and PEF. The solution was slowly
decolorized by AO-H2O2 because of the low rate of reaction of the azo dye and its colored products with
hydroxyl radicals generated at the BDD anode from water oxidation. The color loss was enhanced in EF by
the larger oxidation ability of hydroxyl radicals produced in the bulk from Fenton’s reaction between
added Fe2+ and generated H2O2, whereas the solution was more rapidly decolorized by PEF owing to the
additional generation of hydroxyl radicals from the photolysis of Fe(III)-hydroxy complexes by UVA light.
The relative mineralization ability of the processes also increased in the sequence AO-H2O2 < EF < PEF.
The PEF method was the most powerful due to the synergistic oxidation by hydroxyl radicals and UVA
irradiation, yielding 94% mineralization after 360 min at 100 mA cmꢀ2. The influence of current density
and RY160 concentration on the performance of all processes was assessed. Final carboxylic acids like
maleic, fumaric, tartronic, acetic, oxalic, oxamic and formic were quantified by ion-exclusion HPLC. All
these acids were totally removed by PEF, but the formation of small amounts of other highly recalcitrant
products impeded the total mineralization. Chloride, sulfate, ammonium and, to a smaller extent, nitrate
ions were released to the solution from the heteroatoms of the azo dye in all cases.
Received 4 February 2016
Received in revised form 27 April 2016
Accepted 27 April 2016
Available online 28 April 2016
Keywords:
Anodic oxidation
Electro-Fenton
Photoelectro-Fenton
Reactive Yellow 160
Water treatment
ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction
The Guanajuato region is the largest National producer, generating
about 65% of tanning and leather finishing. In the city of Leon, for
The manufacture of leather products, along with the availability
of raw materials and labor, has favored the development of the
tanning industry. Currently, Mexico is among the ten world’s
largest producers of pelts, accounting for 4% of total production.
example, there are more than 500 tanneries, thus constituting its
main economic activity. Leather production is divided into four
stages: riviera, tanning, RTE (retanning, dyeing, greasing) and
finishing. At the end, the leather has acquired softness, and
especially color, among other features that are necessary to
produce commercial products. Dyeing is a chemical process that
imparts color to the leather in the drum. Coloring can be just
superficial or reach the entire thickness of leather and, for this
purpose, direct and basic anionic dyes are used to avoid adding
trills. In this context, the wastewater treatment containing
* Corresponding author. Tel.: +34 934021223; fax: +34 934021231.
** Corresponding author.
E-mail addresses: juan.peralta@ugto.mx (J.M. Peralta-Hernández),
brillas@ub.edu (E. Brillas).
1
ISE Active Member.
http://dx.doi.org/10.1016/j.electacta.2016.04.166
0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

308
A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
industrial dyes, produced as a result of the increasing activity of the
tannery industry in Guanajuato state, is a coordinated task
between the society and government, aiming to avoid their
noxious effects on the aquatic environment, animals and human
beings [1].
The principal reason for combining on-site H2O2 generation and
Fenton’s reaction (2) is the enhancement of the oxidation abilities
of the two individual treatments, creating a synergistic EF system.
When this process is performed in an undivided cell with a large
O2-overpotential anode M, organics are destroyed by both, ꢁOH
formed in the bulk from Fenton’s reaction (2) and physisorbed
hydroxyl radicals originated at the anode surface from water
oxidation (M(ꢁOH)) [9,12,32,33]. Wastewater decontamination
based on the removal of pollutants upon the sole action of the
anode (without Fenton’s reagent) gives rise to the well-known and
popular anodic oxidation (AO) method, whereas if H2O2 is
produced at the cathode, it is so-called AO with electrogenerated
H2O2 (AO-H2O2) [11,32]. Currently, BDD thin-layer electrodes are
considered the best anodes for AO and AO-H2O2 since they can
efficiently mineralize a large number of aromatics and aliphatic
carboxylic acids [34–37]. The generation of the physisorbed
oxidant BDD(ꢁOH) at the BDD anode proceeds as follows:
For the remediation of dyeing wastewater, different traditional
physicochemical techniques including adsorption, coagulation and
filtration have been tested [2–4]. Although these methods can yield
fast decolorization, they generate large volumes of sludge to be
disposed and/or need the regular regeneration of adsorbent
materials. Recently, advanced oxidation processes (AOPs) have
appeared as powerful alternatives for the treatment of such
wastewater since they are able to transform the organic pollutants
into innocuous substances [5–8]. AOPs are based on the in situ
generation of very oxidizing reactive oxygen species (ROS) such as
the free hydroxyl radical (ꢁOH). When it is generated, ꢁOH can react
quickly and non-selectively with most organics, being added to a
double bond or abstracting a hydrogen atom, reaching their
mineralization in many cases, i.e., their conversion into CO2 and
H2O.
Lately, the electrochemical advanced oxidation processes
(EAOPs) based on Fenton’s reaction chemistry have received
increasing attention for the treatment of acidic wastewater [9–12].
The significance of these methods is due to their environmental
compatibility, amenability of automation, high energy efficiency,
versatility and safe operation under mild conditions. In these
EAOPs, hydrogen peroxide is electrochemically supplied to the
treated solution by the two-electron reduction of O2 gas,
preferentially at a carbonaceous cathode as follows [9,11]:
BDD + H2O ! BDD(ꢁOH) + H+ + eꢀ
(4)
An important limitation of the EF treatment of azo dyes is the
production of final Fe(III)-carboxylate complexes that are very
hardly attacked by generated hydroxyl radicals [38–42]. This
drawback can be solved by illuminating the solution with UVA
radiation supplied by a lamp in the so-called UVA photoelectro-
Fenton (PEF) process [9,11,39–42], which favors: (i) the enhance-
ment of Fe2+ regeneration and ꢁOH production by photoreduction
of Fe(OH)2+, the predominant Fe(III) species at pH ꢂ3.0, from
reaction (5) and (ii) the photodecarboxylation of Fe(III)–
O
2ðgÞ þ 2Hþ þ 2eꢀ ! H2O2
ð1Þ
a
120
100
80
60
40
20
0
Carbon nanotubes [13,14], activated carbon fiber [15,16], carbon
sponge [17], graphite [18], graphite felt [19], carbon felt [19–23],
carbon-polytetrafluoroethylene (PTFE) gas (O2 or air) diffusion
[24–27] and boron-doped diamond (BDD) [28–30] have been used
to electrogenerate H2O2 from reaction (1). Since H2O2 is a weak
oxidant, its oxidation power is greatly improved by the addition of
Fe2+ to the acidic solution to form ꢁOH and Fe3+ according to
Fenton’s reaction (2), with optimum pH of 2.8 [9,11,12,31,32]:
ꢁ
Fe2þ þ H2O2 ! Fe3þ þ OH þ OHꢀ
ð2Þ
This EAOP is commonly referred to as electro-Fenton (EF),
whose main advantage compared to chemical Fenton’s reagent is
the propagation of reaction (2) from the reduction of Fe3+ to Fe2+ at
the cathode through reaction (3) [9,12]:
b
50
40
30
20
10
0
Fe3þ þ eꢀ ! Fe2þ
ð3Þ
0
60
120
180
240
300
360
420
time / min
Fig. 2. (a) Decolorization efficiency and (b) dissolved organic carbon (DOC) removal
with electrolysis time for the treatment of 100 cm3 of a solution of 0.167 mmol dmꢀ3
RY160 with 0.050 mol dmꢀ3 Na2SO4 at pH 3.0 using a stirred tank reactor equipped
with a 3 cm2 BDD anode and a 3 cm2 air-diffusion cathode at 100 mA cmꢀ2 and 25 ꢃC.
(ꢄ) Anodic oxidation with electrogenerated H2O2 (AO-H O ), ( ) electro-Fenton
4
2
2
(EF) with 0.50 mmol dmꢀ3 Fe2+ and (~) photoelectro-Fenton (PEF) with 0.50 mmol
Fig. 1. Chemical structure of Reactive Yellow 160.
dmꢀ3 Fe2+ and a 6 W UVA light.

A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
309
carboxylate intermediates from the general reaction (6) [43]:
different PbO2-based and SnO2-based anodes has been previously
reported in the literature [44].
FeðOHÞ þ hv ! Fe2þ þ ꢁ OH
ð5Þ
ð6Þ
2þ
This paper presents the results obtained for the comparative
color removal and mineralization of RY160 solutions by applying
AO-H2O2, EF and PEF with a BDD anode and a carbon-PTFE air-
diffusion cathode. The effect of the applied current density (j) and
azo dye content on the degradation by each EAOP was examined to
better clarify the role of generated hydroxyl radicals and/or
UVA irradiation. Final short-linear aliphatic carboxylic acids
and released inorganic ions were quantified by high-performance
liquid chromatography (HPLC) and ion chromatography,
respectively.
ꢁ
2þ
FeðOOCRÞ þ hv ! Fe2þ þ CO2 þ R
To gain a better understanding on the viability of EAOPs to
decolorize and mineralize azo dyes, we have undertaken a study on
the degradation of a complex azo dye, Reactive Yellow 160 (RY160,
Na2C25H22ClN9O12S3,M = 818.13 g molꢀ1
, lmax = 440 nm, see formu-
la in Fig. 1), widely used in many tannery factories in the Guanajato
region. Only one paper dealing with the AO of this azo dye using
a
120
100
80
60
40
20
0
60
50
40
30
20
10
0
a
0
60
120
180
240
300
360
420
time / min
120
100
80
60
40
20
0
b
b
50
40
30
20
10
0
100
80
60
40
20
0
c
c
50
40
30
20
10
0
0
30
60
90
120
150
180
210
time / min
0
60
120
180
240
300
360
420
time / min
Fig. 3. Effect of current density on the percentage of color removal vs electrolysis
time for the degradation of 100 cm3 of a 0.167 mmol dmꢀ3 RY160 and 0.050 mol
dmꢀ3 Na2SO4 solution at pH 3.0 and 25 ꢃC using a stirred BDD/air-diffusion cell. (a)
AO-H2O2, (b) EF and (c) PEF. Applied current density: (^) 33.3 mA cmꢀ2, (&)
Fig. 4. Change of DOC with electrolysis time for the assays of Fig. 3. (a) AO-H2O2, (b)
EF and (c) PEF. Applied current density: (^) 33.3 mA cmꢀ2, (&) 66.7 mA cmꢀ2 and
ꢀ2
ꢀ2
66.7 mA cmꢀ2 and ( ) 100 mA cm
.
.
5
5
( ) 100 mA cm

310
A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
2. Experimental
2.1. Chemicals
2.2. Electrochemical system
The electrolytic experiments were performed in batch mode at
laboratory scale using an open, cylindrical and undivided two-
electrode cell containing a 100 cm3 RY160 solution. The cell was
surrounded by a jacket where thermostated water was recirculated
to keep the solution at 25 ꢃC. In all trials, the solution was vigorously
stirredwith a magnetic barat800 rpm forthe mixingoforganicsand
their mass transport toward/from the electrodes. The anode was a
3 cm2 BDD thin film purchased from NeoCoat and the cathode was a
carbon-PTFE air-diffusion electrode supplied by E-TEK. The inter-
electrode gap was about 1 cm. The cathode was mounted as
described elsewhere [45] and was fed with pumped air at a flow
rate of 1 dm3 minꢀ1 for H2O2 generation by reaction (1). All the trials
were made at constant j provided by an EG&G PAR 273A
potentiostat-galvanostat. For the PEF runs, the solution was
illuminated with a 6 W Philips black light blue tube lamp of
Commercial RY160 (66% content, plus inorganic salts for
stabilization) was supplied by PCL S.A. de C.V (Mexico). The
solutions were prepared with ultrapure water from a Millipore
Milli-Q system (resistivity > 18 M
V
cm, 25 ꢃC). The catalyst and
background electrolyte were introduced as FeSO4.7H2O and
anhydrous Na2SO4, both of analytical grade purchased from Fluka
and Panreac, respectively. The solution pH was adjusted to 3.0 with
analytical grade H2SO4 supplied by Merck. Carboxylic acids,
solvents and other chemicals were either of analytical or HPLC
grade purchased from Panreac and Sigma-Aldrich.
9
8
7
6
5
4
3
2
1
0
a
12
10
8
a
6
4
2
0
b 7
b 35
6
5
4
3
2
1
0
30
25
20
15
10
5
0
c
c 45
6
40
35
30
25
20
15
10
5
5
4
3
2
1
0
0
20
40
60
80
100
0
0
60
120
180
time / min
240
300
360
420
% DOC removal
Fig. 6. Variation of the specific energy consumption per unit DOC mass obtained
Fig. 5. Mineralization current efficiency calculated from Eq. (10) vs electrolysis time
from Eq. (11) with percentage of DOC removal for the experiments shown in Fig. 3
for the trials of Fig. 3 and 4. (a) AO-H2O2, (b) EF and (c) PEF. ꢀA2pplied current density:
and 4. (a) AO-H2O2, (b) EF and (c) PEF. Applied current density: (^) 33.3mA cmꢀ2
,
(^) 33.3 mA cmꢀ2, (&) 66.7 mAcmꢀ2 and ( ) 100 mA cm
.
(&) 66.7 mA cmꢀ2 and ( ) 100 mA cm
.
ꢀ2
5
5

A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
311
lmax = 360 nm, placed 7 cm above it and supplying an UV irradiance
of 5 W mꢀ2, as detected with a Kipp & Zonen CUV 5 global UV
radiometer. Cleaning of the BDD surface and activation of the air-
diffusion cathode were made before the treatments by polarizing
100 cm3 of 0.050 mol dmꢀ3 Na2SO4 at 100 mA cmꢀ2 for 180 min.
Solutions with 0.167 and 0.333 mmol dmꢀ3 of RY160, equivalent
to 50 and 100 mg dmꢀ3 of dissolved organic carbon (DOC),
respectively, in 0.050 mol dmꢀ3 Na2SO4 were degraded at pH
3.0 by means of AO-H2O2, EF and PEF. The influence of j between
33.3 and 100 mA cmꢀ2 on the performance of each EAOP was
assessed. In all the EF and PEF assays, 0.50 mmol dmꢀ3 Fe2+ was
initially added to the solution to act as catalyst of Fenton’s reaction
(2).
where Ecell is the average cell voltage (in V) and the rest of
parameters have been defined above. This equation is valid for AO-
H2O2 and EF, but it is an approach for PEF because the energy
required by the UVA lamp, which can be replaced by free sunlight
[26], is not considered.
Short-linear aliphatic carboxylic acids were quantified by HPLC
using a Waters 600 LC fitted with a Bio-Rad Aminex HPX 87H,
300 mm
996 photodiode array detector set at
aliquots of 20
ꢅ
7.8 mm, column at 35 ꢃC, coupled to
a
Waters
= 210 nm. In these analyses,
L were injected into the LC and a 4 mmol dmꢀ3
l
m
H2SO4 solution at 0.6 cm3 minꢀ1 was used as mobile phase. The
chromatograms exhibited peaks at retention times of 7.0 min for
oxalic acid, 7.9 min for tartronic acid, 8.2 min for maleic acid,
9.4 min for oxamic acid,13.7 min for formic acid,14.8 min for acetic
acid and 15.7 min for fumaric acid. NH4+, Clꢀ, NO3 and SO4
ꢀ
2ꢀ
2.3. Instruments and analytical procedures
contents were analyzed by ion chromatography upon injection of
25 L aliquots into a Shimadzu 10Avp LC coupled with a Shimadzu
CDD 10Avp, following the procedure reported elsewhere [41].
The solution pH was determined with a Crison GLP 22 pH-
meter. Samples withdrawn from electrolyzed solutions were
m
microfiltered with 0.45 mm PTFE filters from Whatman prior to
immediate analysis. Hydrogen peroxide concentration was mea-
sured from the light absorption of the titanic-hydrogen peroxide
3. Results and discussion
3.1. Characterization of the electrochemical systems
complex at
l= 408 nm using a Shimadzu Unicam 1800 UV–vis
spectrophotometer thermostated at 25 ꢃC [46]. The loss of solution
color was followed from their absorbance (A) decay at the
maximum visible wavelength of lmax = 440 nm of RY160, deter-
mined with the above spectrophotometer. The decolorization
efficiency or percentage of color removal was determined by
measuring the change from the starting absorbance A0 to the value
At at time t, as follows [11,12]:
Prior to the degradation of RY160 solutions, the supporting
electrolyte was treated under AO-H2O2, EF and PEF conditions to
clarify the ability of these EAOPs to produce and accumulate H2O2.
For each electrochemical system, 100 cm3 of a 0.050 mol dmꢀ3
Na2SO4 solution at pH 3.0 and 25 ꢃC were electrolyzed at j values of
33.3, 66.7 and 100 mA cmꢀ2 for 360 min. Treatments under EF and
A0 ꢀ At
%Color removal ¼
ꢅ 100
ð7Þ
A0
a
120
100
80
60
40
20
0
The mineralization of electrolyzed solutions was assessed from
their DOC decay, measured on a Shimadzu VCSN TOC analyzer. This
analysis was made by injecting 50 mL aliquots into this system,
obtaining reproducible DOC values with ꢆ1% accuracy. The
percentage of DOC removal for a given experiment at time t was
then calculated from Eq. (8) [12]:
D
ðDOCÞ
%DOC removal ¼
where
exp ꢅ 100
ð8Þ
DOC0
D
(DOC)exp is the experimental DOC decay (in mg C dmꢀ3
and DOC0 is the starting DOC.
)
The theoretical total mineralization of the anionic form of
RY160 can be assumed to yield carbon dioxide and chloride, sulfate
and ammonium as pre-eminent ions, as discussed below:
b
C25H22ClN9O12S32ꢀ + 50H2O ! 25CO2 + Clꢀ + 3SO42ꢀ + 9NH4+ + 122
100
80
60
40
20
0
H+ + 126 eꢀ
(9)
Then, the mineralization current efficiency (MCE, in %) for each
trial was estimated as [38]:
nFV
4:32 ꢅ 107mIt
D
ðDOCÞ
%MCE ¼
exp ꢅ 100
ð10Þ
where n = 126 is the number of electrons for the total combustion
of the azo according to reaction (9), F is the Faraday constant
(96,487C molꢀ1), V is the solution volume (in dm3), 4.32 ꢅ 107 is a
conversion factor (3,600 s hꢀ1 ꢅ12,000 mg C molꢀ1), m = 25 is the
number of carbon atoms of RY160, I is the current (in A) and t is the
electrolysis time (in h).
0
60
120
180
240
300
360
420
time / min
The specific energy consumption per unit DOC mass (ECDOC
)
was estimated by Eq. (11) [12,38]:
Fig. 7. (a) Percentage of color removal and (b) DOC vs electrolysis time for the: (ꢄ)
AO-H O , ( ) EF and (~) PEF treatments of 100 cm3 of a solution of 0.334 mmol
4
2
2
À
Á
dmꢀ3 RY160 in 0.050 mol dmꢀ3 Na2SO4 at pH 3.0 using a stirred BDD/air-diffusion
tank reactor at 100 mAcmꢀ2 and 25 ꢃC. In EF and PEF, 0.50 mmol dmꢀ3 Fe2+ was
added to the initial solution.
EcellIt
ðDOCÞexp
ECDOC kWh gꢀ1DOC ¼
ð11Þ
V
D

312
A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
PEF conditions were carried out with 0.50 mmol dmꢀ3 Fe2+ as
catalyst. No change of solution pH was found during these runs.
In all cases, the H2O2 content in solution rose rapidly at short
electrolysis time, tending to a plateau at prolonged electrolysis
time. The gradual decrease in the accumulation rate of H2O2 at long
time can be related to the progressive increase in its decomposition
rate as the oxidant concentration became higher. This phenome-
was even much more rapid by using PEF, where the solution
became colorless in only 60 min, as can be seen in Fig. 2a. This is
due to the production of much larger quantities of ꢁOH in the bulk
from the photolytic reaction (5), also yielding faster Fe2+
regeneration that enhances Fenton’s reaction (2).
Products formed during the RY160 degradation were hardly
transformed into CO2, as deduced from the larger time required for
gradual mineralization compared with the time for total decolori-
zation. Fig. 2b shows that the 0.167 mmol dmꢀ3 azo dye solution
underwent a continuous DOC reduction with electrolysis time for
all EAOPs, without attaining overall mineralization in any case.
From these DOC-time plots, one can infer that the relative
oxidation power increases in the sequence AO-H2O2 < EF < PEF,
similarly to their decolorization ability. The slow degradation of
organics by BDD(ꢁOH) at the BDD surface in AO-H2O2 only allowed
attaining 79% DOC abatement in 360 min, whereas their additional
oxidation by ꢁOH in the bulk upgraded the mineralization in EF
reaching 91% DOC decay. PEF was the most powerful EAOP for both,
decolorization and mineralization processes. In the latter case,
Fig. 2b depicts a very quick DOC abatement up to 78% in only
60 min, owing to the large synergistic action of UVA light on initial
photoactive by-products that favors their fast conversion into CO2.
At longer time, however, DOC abatement rate was strongly
reduced, attaining a final value of 94%. This suggests the formation
of highly recalcitrant products at the end of PEF that are hardly
removed by BDD(ꢁOH), ꢁOH and UVA radiation, thereby preventing
the total mineralization of the azo dye solution.
non can be ascribed to the acceleration of H2O2 oxidation to O2 at
ꢁ
the BDD anode, with the weak oxidant hydroperoxyl radical (HO2
formed as intermediate as follows [11]:
)
H2O2 ! HOꢁ2 þ Hþ þ eꢀ
ð12Þ
HOꢁ2 ! O2ðgasÞ þ Hþ þ eꢀ
ð13Þ
The plateau in each trial is just attained when the rate for H2O2
generation from reaction (1) and its destruction from reactions
(12) and (13) became equal. In AO-H2O2, final concentrations of
33.4, 59.7 and 75.5 mmol dmꢀ3 were found for this species at 33.3,
66.7 and 100 mA cmꢀ2, respectively. The greater H2O2 accumula-
tion at higher j can be explained by the concomitant upgrading of
reaction (1), which also entails an acceleration of reactions (12)
and (13). When the EF conditions were tested, the H2O2 content
decayed up to 30.6, 45.8 and 57.8 mmol dmꢀ3 at the end of the
electrolyses at the above j values, respectively. This decrease is due
to the action of Fenton’s reaction (2) promoting the removal of
H2O2 content, to a larger extent when greater j was applied. The
H2O2 concentration was even more sharply reduced under PEF
conditions, attaining final values of 16.3, 23.8 and 28.1 mmol dmꢀ3
for the same j values, as a result of the production of more ꢁOH and
quicker Fe2+ regeneration from photolytic reaction (5), which
induces a greater acceleration of Fenton’s reaction (2). These
findings confirm the large ability of all the electrochemical systems
to accumulate high amounts of H2O2 in the medium, which allow
the production of enough ꢁOH in the bulk to effectively remove
RY160 and its oxidation products.
30
a
25
20
15
10
5
An additional blank experiment was made by adding
0.167 mmol dmꢀ3 RY160 to the above supporting electrolyte and
irradiating the resulting solution with a 6 W UVA lamp for 120 min
without current supply. Neither decolorization nor DOC abatement
were observed under these conditions, corroborating the high
photostability of the azo dye upon UVA illumination and
suggesting that its destruction in EAOPs can be basically caused
by generated hydroxyl radicals.
0
0
60
120
180
240
300
360
420
time / min
3.2. Comparative decolorization and mineralization of RY160 solutions
by EAOPs
5
b
A first series of comparative degradation assays was made by
treating 100 cm3 of a 0.167 mmol dmꢀ3 RY160 solution in 0.050
mol dmꢀ3 Na2SO4 at pH 3.0 and 25 ꢃC by AO-H2O2, as well as by EF
and PEF upon addition of 0.50 mmol dmꢀ3 Fe2+, using a stirred
BDD/air-diffusion cell at j = 100 mA cmꢀ2 for 360 min. In these
trials, the solution pH became slightly acid to reach a final value
near 2.6-2.7, probably due to the production of acidic products like
final carboxylic acids [38–42], as will be discussed below.
Fig. 2a depicts a very slow loss in solution color by applying AO-
H2O2, attaining 98% decolorization efficiency at the end of the
treatment. This means that physisorbed BDD(ꢁOH) radicals
originated from reaction (4) at the BDD anode attack very slowly
the azo dye and its colored products, thus needing long time for
removing the color. In contrast, Fig. 2a also highlights a very quick
and complete decolorization in 180 min by EF, as a result of the
additional oxidation of RY160 and colored products by ꢁOH formed
in the bulk via Fenton’s reaction (2). The decolorization process
4
3
2
1
0
0
20
40
60
80
100
% DOC removal
Fig. 8. Change of (a) mineralization current efficiency with electrolysis time and (b)
specific energy consumption per unit DOC mass with percentage of DOC removal for
the degradation trials of Fig. 7. (ꢄ) AO-H O , ( ) EF and (~) PEF.
4
2
2

A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
313
3.3. Effect of current density on the degradation of the azo dye
solutions
radicals at higher j did not improve significantly the mineralization
rate of the RY160 solutions.
Fig. 5a-c presents the MCE values calculated from Eq. (10) for
the experiments given in Fig. 4a-c, respectively. While higher j
accelerated all the mineralization processes due to the generation
of more oxidizing species and/or an effective photolysis of
photoactive intermediates, the opposite trend can be observed
for current efficiency since it gradually dropped. This phenomenon
can be associated with the progressive increase in rate of non-
oxidizing reactions of hydroxyl radicals, thereby promoting a
smaller number of reactive events with organic molecules.
Consequently, a clear decrease in current efficiency was observed.
Examples of such parasitic reactions are the oxidation of BDD(ꢁOH)
to O2 at the BDD anode by reaction (14) and the consumption of
ꢁOH in the bulk in the presence of H2O2 and Fe2+ through reactions
(15) and (16), respectively [12,33]. The quicker generation of
The applied j is an important operation parameter in EAOPs
because it determines the quantity of oxidizing hydroxyl radicals
that are formed to destroy organic pollutants. The influence of this
parameter on the degradation of
a
0.167 mmol dmꢀ3
RY160 solution of pH 3.0 was then checked for the AO-H2O2, EF
and PEF processes between 33.3 and 100 mA cmꢀ2 for 360 min. The
solution pH in all these trials did not vary significantly and attained
final values near 2.6-2.7 due to the formation of acidic products, as
pointed out above.
The changes in decolorization efficiency with electrolysis time
for the EAOPs at the three j values tested are presented in Fig. 3a-c.
The percentage of color removal in each process always grew when
j increased, at least at the beginning of the runs. This can be
accounted for by the progressive production of larger amounts of
BDD(ꢁOH) from reaction (4) in all cases, as well as of ꢁOH from
Fenton’s reaction (2) in EF and PEF because of the larger generation
and accumulation of H2O2 in the bulk, as pointed out in section 3.1.
For each j value, moreover, the decolorization efficiency decreased
in the order PEF > EF > AO-H2O2, in agreement with their relative
oxidation power. Fig. 3a shows that for the less potent process
(AO-H2O2), higher j caused a larger loss of color until 240 min of
electrolysis, whereupon it was decelerated up to attain quite
similar decolorization efficiencies of 93%, 94% and 98% for 33.3,
66.7 and 100 mA cmꢀ2, respectively. This is indicative of the
generation of very recalcitrant colored products that need long
time to be removed by BDD(ꢁOH). The production of great amounts
2ꢀ
2ꢀ
weaker oxidants at the anode such as S2O8 from SO4 of the
electrolyte by reaction (17) and O3 by reaction (18) plays also their
role, both of them decreasing BDD(ꢁOH) concentration that is
available to oxidize organics [34].
2BDDðꢁOHÞ ! 2BDD þ O2 þ 2Hþ þ 2eꢀ
ð14Þ
H2O2þꢁ OH ! HOꢁ þ H2O
ð15Þ
2
Fe3þþꢁ OH ! Fd3þ þ OHꢀ
2SO42ꢀ ! S2O82ꢀ þ 2eꢀ
3H2O ! O3 þ 6Hþ þ 6eꢀ
ð16Þ
ð17Þ
ð18Þ
ꢁ
of OH with much higher oxidation ability to remove the azo dye
and its colored products in EF gave rise to total decolorization in
about 180 min for all j values, as can be observed in Fig. 3b.
However, faster decolorization with increasing j from 33.3 to
100 mA cmꢀ2 was found until 90 min, whereupon it was quite
similar for all j values because of the slower removal of small
amounts of very recalcitrant colored products. This phenomenon
was less apparent in PEF as a result of the greater generation of ꢁOH
induced by photolytic reaction (5). For this treatment, Fig. 3c
highlights that the azo dye solution became colorless after 180,
Fig. 5a-c also shows a decay of all MCE values with prolonging
electrolyses owing to the gradual loss of organic load and the
formation of more recalcitrant products [33]. Low current
efficiencies (< 10%) were obtained in AO-H2O2 treatments, which
dropped very slowly attaining final values from 5.6% to 2.5% by
varying j between 33.3 and 100 mA cmꢀ2 (see Fig. 5a). Much
greater MCE values were found in the EF process at times < 180
min, but they further decayed very rapidly up to 8.1%, 3.7% and
2.9% for 33.3, 66.7 and 100 mA cmꢀ2, respectively (see Fig. 5b).
Although the current efficiencies for PEF were clearly superior to
those of EF until 120 min, they decayed so quickly at longer time in
the former process that only slightly greater MCE values compared
with the latter one were finally obtained (see Fig. 5c). These
findings suggest a conversion of organics into CO2 at similar rate by
BDD(ꢁOH) in the AO-H2O2 runs. In contrast, the higher MCE values
in EF at short times are indicative of a much quicker destruction of
organics by ꢁOH, which is enhanced in PEF by the synergistic action
of UVA light on photoactive products.
The change of ECDOC determined from Eq. (11) with the
percentage of DOC removal for the above trials is depicted in
Fig. 6a-c. As expected, the specific energy consumption increased
when j was raised and diminished as the relative oxidation power
of processes was enhanced. The lowest ECDOC values were then
obtained at j = 33.3 mA cmꢀ2, being 1.73,1.21 and 1.11 kWh gꢀ1 DOC
at the end of the AO-H2O2, EF and PEF runs, respectively, all of them
representing a volumetric consumption of about 52 kWh mꢀ3. This
value is similar to that obtained for the treatment of other azo dyes
under analogous conditions [39,40]. It is also noticeable from
Fig. 6c that at high DOC removal of PEF treatments (77-79%),
0.44 kWh gꢀ1 DOC were consumed at 33.3 mA cmꢀ2, being much
lower than 1.3-1.4 kWh gꢀ1 DOC at 66.7 and 100 mA cmꢀ2. These
120 and 60 min of electrolysis at 33.3, 66.7 and 100 mA cmꢀ2
,
respectively. That means that the formation of oxidant ꢁOH was
significantly enhanced at higher j, which can be explained by the
greater generation of H2O2 yielding larger quantities not only of
ꢁOH from Fenton’s reaction (2), but also of Fe3+ that is more quickly
photoreduced byꢁ reaction (5), eventually upgrading the Fe2+
regeneration and OH formation.
An enhancement of DOC decay was always found when j rose
from 33.3 to 100 mA cmꢀ2 as a result of the greater generation of
hydroxyl radicals, as stated above. Comparison of DOC-t plots of
Fig. 4a-c corroborates again a decay in the relative oxidation power
of EAOPs in the sequence PEF > EF > AO-H2O2, regardless of the
applied j. For AO-H2O2, Fig. 4a clearly shows that the oxidative
action of increasing amounts of BDD(ꢁOH) led to higher DOC
reduction, reaching 60%, 65% and 79% at the end of the treatment at
33.3, 66.7 and 100 mA cmꢀ2, respectively. The additional produc-
tion of ꢁOH in EF from Fenton’s reaction (2) improved DOC
abatement up to a final mineralization of 86%, 90% and 91% for the
above j values, as shown in Fig. 4b. In contrast, Fig, 4c reveals a
slightly higher mineralization rate with raising j using PEF, being
only noticeable until 120–180 min, when 82-86% mineralization
was attained, whereupon a similar DOC decay occurred up to
attaining 90%, 92% and 94% of mineralization at 360 min of 33.3,
66.7 and 100 mA cmꢀ2, respectively. This behavior suggests that
UVA irradiation photolyzes so rapidly the photoactive intermedi-
ates while they are formed that the production of more hydroxyl

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A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
values corresponded to 17, 48 and 52 kWh mꢀ3, respectively. This
means that decreasing j allows obtaining similar mineralization up
to about 80% DOC removal, but with much lower energy
consumption, therefore being advantageous the use of low j
(e.g., 33.3 mA cmꢀ2) in terms of cost for the application of EAOPs to
the remediation of wastewater containing RY160 azo dye.
produced. These chromatograms displayed well-defined peaks
related to maleic, fumaric, tartronic, acetic, oxalic, oxamic and
formic acids. The former four acids can proceed from the cleavage
of the benzene rings of the azo dye (see Fig. 1), and they are
subsequently oxidized to oxalic and formic acids [11,12,38–42]. In
contrast, oxamic acid can be formed from the degradation of N-
derivatives. Oxalic, oxamic and formic acids are final products that
can be directly mineralized to CO2 [9,12,42]. Note that all the acids
form Fe(III)-carboxylate complexes to a large extent in the EF and
PEF treatments [38–42]. The evolution of the most significant acids
is illustrated in Fig. 9a-c for AO-H2O2, EF and PEF, respectively.
Maleic, fumaric and acetic acids were always accumulated in
very low concentrations (< 0.1 mg dmꢀ3), disappearing from the
medium within short times (< 300 min) in all cases. Tartronic acid
was only detected in EF and PEF, with maximum contents
3.4. Influence of azo dye concentration on the degradation processes
The study of the degradation of acidic RY160 solutions was
extended to a greater concentration of 0.334 mmol dmꢀ3 of the azo
dye to check the effect of the initial organic load on the
performance of all the EAOPs. Fig. 7 exemplifies the results
obtained for j = 100 mA cmꢀ2 during 360 min of electrolysis. As can
be seen in Fig. 7a, the loss of the solution color was accelerated in
the sequence AO-H2O2 < EF < PEF, in agreement with the larger
production of hydroxyl radicals, as pointed out above. A
decolorization efficiency of 96% was attained at the end of AO-
H2O2, whereas the solution became colorless after more than
300 min in EF and after 120 min in PEF, which were much longer
times than those found for the less concentrated solution with
0.167 mmol dmꢀ3 RY160, as shown in Fig. 2a. The decolorization
rate then decreases at higher azo dye content, which can be
simply associated with the slower destruction of more organic
load by similar quantities of hydroxyl radicals originated at the
same j value. A similar conclusion can be deduced by comparing
the DOC abatements of Fig. 2b and 7b, since lower percentage of
DOC removal at a given time was found upon AO-H2O2 and EF
treatment of the 0.334 mmol dmꢀ3 azo dye solution. For
example, after 360 min of electrolysis, 75% and 82% of
mineralization was obtained for these EAOPs with the latter
solution, respectively (see Fig. 7b), whereas 79% and 81% of DOC
abatement were found for the less concentrated solution (see
Fig. 2b). In contrast, similar DOC reductions of 96% and 94%
were determined at the end of PEF. This points to consider a
very positive action of UVA irradiation regarding the minerali-
zation of organics in this powerful procedure, which is
maintained and even improved in the presence of greater
amounts of pollutants.
The significant mineralization rate showed for the
0.334 mmol dmꢀ3 azo dye solution by all the EAOPs was also
reflected in higher MCE values (see Fig. 8a) and lower ECDOC
values (see Fig. 8b) compared to those of the less concentrated
solution (see Fig. 5 and 6). At the end of the treatment, current
efficiencies of 4.7%, 5.1% and 6.0% were obtained for AO-H2O2,
EF and PEF, respectively, whereas the corresponding specific
energy consumptions were of 4.7, 3.8 and 3.2 kWh gꢀ1 DOC.
These results indicate that the EAOPs, and especially the
powerful PEF, are more efficient and require a lower electrical
consumption as azo dye content increases, suggesting that
organics are more quickly removed by larger amounts of
hydroxyl radicals available due to the deceleration of their
parasitic reactions (14)-(18). This is advantageous for the
application of these procedures to real wastewater treatment,
since they usually contain high dye loads, usually between
100 and 250 mg dmꢀ3 [38]. The alternative use of solar PEF
could drastically reduce the excessive energy requirements of
artificial UVA light utilized in PEF [26].
6
5
a
4
3
2
1
0
b
25
20
15
10
5
0
c30
25
20
15
10
5
0
0
60
120
180
240
300
360
420
3.5. Identification of short-linear aliphatic carboxylic acids and
released inorganic ions
time / min
The 0.167 mmol dmꢀ3 RY160 solutions were analyzed at
regular times by ion-exclusion HPLC during their electrolysis at
33.3 mA cmꢀ2 by the different EAOPs for 360 min to identify and
quantify the final short-linear aliphatic carboxylic acids
Fig. 9. Time-course of the concentration of (^) tartronic, (*) oxalic, (&) oxamic
and (!) formic acids detected during the (a) AO-H2O2, (b) EF and (c) PEF
degradation of 100 cm3 of a solution of 0.167 mmol dmꢀ3 RY160 in 0.050 mol dmꢀ3
Na2SO4 at pH 3.0 using a stirred BDD/air-diffusion tank reactor at 33.3 mA cmꢀ2 and
25 ꢃC.

A. Bedolla-Guzman et al. / Electrochimica Acta 206 (2016) 307–316
315
< 4.4 mg dmꢀ3 at 20 min and total removal in about 90–120 min
4. Conclusions
(see Fig. 9b and c). These findings are indicative of a quick removal
of Fe(III)-maleate, Fe(III)-fumarate, Fe(III)-acetate and Fe(III)-
tartronate complexes by hydroxyl radicals, mainly by BDD(ꢁOH)
[9,11]. A different behavior can be observed in Fig. 9a-c for the
shorter carboxylic acids that were usually accumulated to a larger
extent and for longer time. Oxamic acid only appeared with 0.17,
2.5 and 3.8 mg dmꢀ3 as maximal in AO-H2O2, EF and PEF,
A 0.167 mmol dmꢀ3 RY160 solution of pH 3.0 was slowly
decolorized by AO-H2O2 using a BDD/air-diffusion tank reactor at
100 mA cmꢀ2 due to the slow destruction of the azo dye and its
colored products by BDD(ꢁOH). In contrast, a rapid decolorization
was observed in EF with 0.50 mmol dmꢀ3 Fe2+ because of the
quicker attack of ꢁOH formed in the bulk. The solution became
colorless even more rapidly using PEF owing to the production of
larger amounts of ꢁOH induced by the photolysis of Fe(III)-hydroxy
species. From DOC removal, it was established an increasing
relative oxidation power of the EAOPs in the order AO-H2O2 < EF
< PEF, in agreement with their decolorization trend. PEF was the
most powerful EAOP, since the synergistic action of BDD(ꢁOH), ꢁOH
and UVA light yielded 94% mineralization after 360 min. Higher j
always accelerated the decolorization and mineralization, but it
was detrimental regarding the current efficiency and the specific
energy consumption. The treatment of more concentrated
solutions was feasible by PEF, reaching 96% of mineralization
after 360 min at 100 mA cmꢀ2. Oxalic, oxamic and formic acids
were largely produced as final short-linear carboxylic acids. A large
amount of other persistent products was accumulated in AO-H2O2,
respectively, reaching final contents of 0.15, 1.0 and 0.10 mg dmꢀ3
.
The more pronounced accumulation of oxamic acid and the faster
destruction of Fe(III)-oxamate complexes in PEF can be ascribed to
the action of UVA light to produce higher amounts of oxidant ꢁOH
induced by reaction (5), to photo-oxidize N-derivatives and/or to
destroy the Fe(III)-oxamate species via reaction (6). In turn, formic
acid was not detected in AO-H2O2, but it was largely and
continuously accumulated up to 8.3 mg dmꢀ3 in EF (see Fig. 9b),
suggesting that it was pre-eminently formed when intermediates
were oxidized by ꢁOH in the bulk, whereas Fe(III)-formate
complexes were hardly destroyed by BDD(ꢁOH) and ꢁOH. In PEF,
however, formic acid was totally removed at 360 min after
achieving a maximal concentration of 21 mg dmꢀ3 at 120 min.
This means that UVA light contributed to the production of more
formic acid, probably by the generation of more ꢁOH, and
photodecomposed very quickly the resulting Fe(III)-formate
complexes up to their total disappearance. The same trend can
be deduced from Fig. 9a-c for oxalic acid. It was progressively
accumulated up to 5.0 mg dmꢀ3 in AO-H2O2, because of its slow
reaction with BDD(ꢁOH), much largely accumulated up to 26 mg
dmꢀ3 with further slow decay to 17 mg dmꢀ3 in EF, due to the slow
oxidation of Fe(III)-oxalate species by hydroxyl radicals, and almost
completely removed at the end of PEF, as a result of the effective
photolysis of these complexes by UVA irradiation through reaction
(6). A mass balance at the end of the treatments revealed that the
remaining solutions in AO-H2O2, EF and PEF contained 1.4, 6.9 and
0.2 mg dmꢀ3 of DOC related to all detected acids, respectively,
which represented a 7.0%, 100% and 4.2% of their corresponding
DOC (see Fig. 4). These findings allow inferring that the AO-H2O2
process led to large amounts of undetected products, even more
recalcitrant than short-linear carboxylic acids to the attack by BDD
(ꢁOH), whereas the final solution of EF was composed pre-
eminently by carboxylic acids thanks to the most effective
oxidation of ꢁOH in the bulk. In contrast, small quantities of
highly recalcitrant products remained at the end of PEF, suggesting
that they are the result of the photolytic action of UVA light on
several intermediates.
whereas
a smaller number of highly recalcitrant products
originated by UVA light was accumulated in PEF, thereby
preventing the total mineralization. Chloride, sulfate, ammonium
and nitrate were formed as inorganic ions from the heteroatoms of
the azo dye. Our results prove the viability of PEF to treat
wastewater containing azo dyes like RY160.
Acknowledgments
Financial support from MINECO (Spain) under the project
CTQ2013-48897-C2-1-R, co-financed with FEDER funds, as well as
from PRODEP-UGTO-PTC-472 and UGTO, Convocatoria Institucio-
nal para Fortalecer la Excelencia Académica 2015 (Mexico), is
acknowledged.
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Inorganic ions formed during the mineralization of the
0.167 mmol dmꢀ3 RY160 solution in 0.050 mol dmꢀ3 LiClO4 as
background electrolyte (to avoid the sulfate medium), regulated at
pH 3.0 with HClO4, at 33.3 mA cmꢀ2 were determined at the end of
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