Revista Facultad de Ingeniería, Universidad de Antioquia, No.110, pp. 65-76, Jan-Mar 2024
A comparative analysis of Cr(VI) reduction
with Cu2O, ZnO and F e2O3 coatings grown by
PEO
Análisis comparativo de la reducción de Cr(VI) con Cu2O, ZnO y Fe2O3 crecidos por OEP
Fernando Gordillo-Delgado 1, 2*John Alexander García-Giraldo 1
1Grupo de Investigación en Ciencia Aplicada para el Desarrollo de la Ecorregión (GICADE), Instituto Interdisciplinario de
las Ciencias, Universidad del Quindío. Carrera 15 # 12N. C. P 630004. Quindío, Colombia.
2Programa de Tecnología en Instrumentación Electrónica de la Facultad de Ciencias Básicas y Tecnologías, Universidad
del Quindío. Carrera 15 # 12N. C. P 630004. Quindío, Colombia.
CITE THIS ARTICLE AS:
F. Gordillo-Delgado and J.
Alexander García-Giraldo. ”A
comparative analysis of Cr(VI)
reduction with Cu2O, ZnO and
F e2O3 coatings grown by
PEO”, Revista Facultad de
Ingeniería Universidad de
Antioquia, no. 110, pp. 65-76,
Jan-Mar 2024. [Online].
Available: https:
//www.doi.org/10.17533/
udea.redin.20230418
ARTICLE INFO:
Received: February 04, 2022
Accepted: April 19, 2023
Available online: April 19, 2023
KEYWORDS:
Plasma electrolytic oxidation;
Cu2O; ZnO; Fe2O3;
photoreduction of Cr(VI)
Oxidación electrolítica con
plasma; Cu2O; ZnO; F e2O3;
fotoreducción de Cr(VI)
ABSTRACT: Coatings on copper, zinc and stainless-steel substrates were fabricated using
the plasma electrolytic oxidation (PEO) technique and their photocatalytic activity was
evaluated in the reduction of Cr(VI), a highly toxic agent present in wastewater from
industrial processes such as electroplating, manufacture of textile dyes, wood curing,
and leather tanning. The concentration of hexavalent chromium in drinking water has
been regulated to a maximum value established by national and international legislation
of 0.05 ppm. The strategy of reduction to less toxic species such as Cr(III), followed
by its precipitation in a basic medium, use several methods derived from chemistry,
physics and biology for the treatment of water contaminated with this material. In the
present work, some coatings of copper, zinc, and iron oxides were obtained over the
corresponding metal sheets exposed to PEO, which were tested in a heterogeneous
process of advanced oxidation with 1 ppm Cr(VI) solution under ultraviolet radiation.
Thus, a reduction rate to Cr(III) close to 100% in 60 min was obtained.
RESUMEN: Se fabricaron recubrimientos sobre sustratos de cobre, zinc y acero inoxidable
mediante la técnica de oxidación electrolítica con plasma (OEP) y se evaluó su actividad
fotocatalítica en la reducción del Cr(VI), un agente altamente tóxico presente en las
aguas residuales de procesos industriales como la galvanoplastia, la fabricación de
tintes textiles, el curtido de la madera y el curtido del cuero. La concentración
de cromo hexavalente en el agua potable se ha regulado hasta un valor máximo
establecido por la legislación nacional e internacional de 0.05 ppm. La estrategia de
reducción a especies menos tóxicas como el Cr(III), seguida de su precipitación en un
medio básico fundamenta varios métodos de la química, la física y la biología para
el tratamiento del agua contaminada con este material. En el presente trabajo, se
obtuvieron recubrimientos de óxidos de cobre, zinc y hierro sobre las correspondientes
láminas expuestas a OEP, que se ensayaron en un proceso heterogéneo de oxidación
avanzada con una solución de 1 ppm de Cr(VI) bajo radiación ultravioleta. Así, se obtuvo
una tasa de reducción a Cr(III) cercana al 100% en 60 min.
1. Introduction
Plasma Electrolytic Oxidation (PEO) has been reported in
the fabrication of ceramic-type coatings on metals such
as aluminum, titanium, magnesium, and alloys in order to
increase their resistance to exposure and wear, improving
their tribological properties [1, 2]. In the particular case
of titanium, this technique has been used for surface
modification with the aim of obtaining the oxide of this
metal, widely studied for its wide bandgap of forbidden
energy, which makes it a good candidate for photocatalyst
for pollutant charge reduction [3]. However, to guarantee
the viability of this process at an industrial scale, it is
important to explore the PEO with other metals, such as
Zn, Cu, and stainless steel, costing $3.78 [4], $9.59 [5]
and $5 [6] per kilogram, respectively, which are cheaper
than titanium ($34.75) [7]. This study presents the results
65
* Corresponding author: Fernando Gordillo-Delgado
E-mail: fotoacustica@uniquindio.edu.co
ISSN 0120-6230
e-ISSN 2422-2844
DOI: 10.17533/udea.redin.20230418 65
A comparative analysis of Cr(VI) reduction
with Cu2O, ZnO and F e2O3 coatings grown by
PEO
Análisis comparativo de la reducción de Cr(VI) con Cu2O, ZnO y Fe2O3 crecidos por OEP
Fernando Gordillo-Delgado 1, 2*John Alexander García-Giraldo 1
1Grupo de Investigación en Ciencia Aplicada para el Desarrollo de la Ecorregión (GICADE), Instituto Interdisciplinario de
las Ciencias, Universidad del Quindío. Carrera 15 # 12N. C. P 630004. Quindío, Colombia.
2Programa de Tecnología en Instrumentación Electrónica de la Facultad de Ciencias Básicas y Tecnologías, Universidad
del Quindío. Carrera 15 # 12N. C. P 630004. Quindío, Colombia.
CITE THIS ARTICLE AS:
F. Gordillo-Delgado and J.
Alexander García-Giraldo. ”A
comparative analysis of Cr(VI)
reduction with Cu2O, ZnO and
F e2O3 coatings grown by
PEO”, Revista Facultad de
Ingeniería Universidad de
Antioquia, no. 110, pp. 65-76,
Jan-Mar 2024. [Online].
Available: https:
//www.doi.org/10.17533/
udea.redin.20230418
ARTICLE INFO:
Received: February 04, 2022
Accepted: April 19, 2023
Available online: April 19, 2023
KEYWORDS:
Plasma electrolytic oxidation;
Cu2O; ZnO; Fe2O3;
photoreduction of Cr(VI)
Oxidación electrolítica con
plasma; Cu2O; ZnO; F e2O3;
fotoreducción de Cr(VI)
ABSTRACT: Coatings on copper, zinc and stainless-steel substrates were fabricated using
the plasma electrolytic oxidation (PEO) technique and their photocatalytic activity was
evaluated in the reduction of Cr(VI), a highly toxic agent present in wastewater from
industrial processes such as electroplating, manufacture of textile dyes, wood curing,
and leather tanning. The concentration of hexavalent chromium in drinking water has
been regulated to a maximum value established by national and international legislation
of 0.05 ppm. The strategy of reduction to less toxic species such as Cr(III), followed
by its precipitation in a basic medium, use several methods derived from chemistry,
physics and biology for the treatment of water contaminated with this material. In the
present work, some coatings of copper, zinc, and iron oxides were obtained over the
corresponding metal sheets exposed to PEO, which were tested in a heterogeneous
process of advanced oxidation with 1 ppm Cr(VI) solution under ultraviolet radiation.
Thus, a reduction rate to Cr(III) close to 100% in 60 min was obtained.
RESUMEN: Se fabricaron recubrimientos sobre sustratos de cobre, zinc y acero inoxidable
mediante la técnica de oxidación electrolítica con plasma (OEP) y se evaluó su actividad
fotocatalítica en la reducción del Cr(VI), un agente altamente tóxico presente en las
aguas residuales de procesos industriales como la galvanoplastia, la fabricación de
tintes textiles, el curtido de la madera y el curtido del cuero. La concentración
de cromo hexavalente en el agua potable se ha regulado hasta un valor máximo
establecido por la legislación nacional e internacional de 0.05 ppm. La estrategia de
reducción a especies menos tóxicas como el Cr(III), seguida de su precipitación en un
medio básico fundamenta varios métodos de la química, la física y la biología para
el tratamiento del agua contaminada con este material. En el presente trabajo, se
obtuvieron recubrimientos de óxidos de cobre, zinc y hierro sobre las correspondientes
láminas expuestas a OEP, que se ensayaron en un proceso heterogéneo de oxidación
avanzada con una solución de 1 ppm de Cr(VI) bajo radiación ultravioleta. Así, se obtuvo
una tasa de reducción a Cr(III) cercana al 100% en 60 min.
1. Introduction
Plasma Electrolytic Oxidation (PEO) has been reported in
the fabrication of ceramic-type coatings on metals such
as aluminum, titanium, magnesium, and alloys in order to
increase their resistance to exposure and wear, improving
their tribological properties [1, 2]. In the particular case
of titanium, this technique has been used for surface
modification with the aim of obtaining the oxide of this
metal, widely studied for its wide bandgap of forbidden
energy, which makes it a good candidate for photocatalyst
for pollutant charge reduction [3]. However, to guarantee
the viability of this process at an industrial scale, it is
important to explore the PEO with other metals, such as
Zn, Cu, and stainless steel, costing $3.78 [4], $9.59 [5]
and $5 [6] per kilogram, respectively, which are cheaper
than titanium ($34.75) [7]. This study presents the results
65
* Corresponding author: Fernando Gordillo-Delgado
E-mail: fotoacustica@uniquindio.edu.co
ISSN 0120-6230
e-ISSN 2422-2844
DOI: 10.17533/udea.redin.20230418 65
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
of Cr(VI) photoreduction using the coatings over these
metallic substrates.
In some variations of conventional PEO, the
substrate-electrode acts as the anode (anodic PEO),
and the electrolyte container vessel or a platinum
electrode is the cathode [8]; these methods include power
supplies with variable or switchable output voltage at
a given frequency and duty cycle [9]. This methodology
allows control of the grain size of the oxide, directly formed
on the substrate surface. Unlike conventional anodizing,
in which low electrical voltages are used for exposure
times longer than 20 min, the PEO involves the use of high
magnitude voltages for less time.
The PEO consists of the surface modification of a metal
that is immersed in an electrolyte solution contained in a
metallic container. The concentration of the electrolyte
determines the current intensity that flows through the
substrate while it is subjected to an electrical voltage
higher than the breakdown voltage of the material. When
the breakdown voltage is exceeded, microdischarges are
generated on the surface of the sample; thereby, the local
pressure and temperature abruptly increase, producing
microcavities. The current density can be controlled by
considering the substrate shape and the applied electric
potential [10].
Several processes use hexavalent chromium (Cr(VI))
to manufacture some products; in the automotive
industry, one of them is electroplating or deposition of a
metallic layer of chromium on objects to add reflective
brightness and provide corrosion protection [11]; in the
manufacture of paint and pigments, Cr(VI) is added to the
product to improve its stability [12], and in the tannery, it is
used to transform the collagen in the leather into a stable
material to prevent its degradation and to increase its
useful life [13, 14]. The Cr quantity that is not deposited,
impregnated or absorbed in these techniques is returned
to the environment through wastewater. This is harmful to
health [15–17], so it is necessary to reduce Cr(VI) to Cr(III),
which is less toxic and more environmentally friendly.
Although other reduction processes of hexavalent
to trivalent chromium are currently used, such as
bioremediation and chemical reactors [18–20], the fixation
of materials for advanced oxidation presents advantages,
such as the reuse of the catalyst, without complex recovery
or addition of toxic agents.
The use of the cathodic configuration has been reported
for PEO on certain metals and alloys; in this configuration,
the substrate is placed as the cathode and the container
vessel, or another electrode, as the anode [21]. Thus,
several authors have modified stainless steel surfaces
decreasing the electric potential and low concentrations
of the electrolyte [22]. In this work, the PEO anodic
configuration was used with copper and zinc substrates,
and the cathodic mode with the steel substrates. Based on
the results, the existence of the corresponding oxides on
each substrate and their efficiency in Cr(VI) photoreduction
was probed; which do not have been reported before, in
the best of our knowledge; most of the reports are related
to oxide samples in powder for this application.
2. Materials and methods
Sheets of dimensions 20 mm x 20 mm x 1 mm of stainless
steel (304), copper (Cu-DHP 1/2 hard CW024A) and zinc
(shiny sheet EN 988) were previously polished with an
abrasive paper and used as substrates. Before the PEO,
these metal sheets were subjected to an ultrasonic bath
for 20 min with distilled water and isopropanol. In all
cases, the exposure time was 10 min, and a mixture of 0.2
g/L sodium metasilicate nonahydrate (N a2SiO3 − 9H2O)
and 0.05 g/L potassium hydroxide (KOH) was utilized as
electrolyte.
The PEO of copper and zinc substrates was carried out by
conventional mode with potential differences between the
electrodes of 450, 550, 650 and 750 V, while the surface of
stainless-steel substrates was modified with PEO cathodic
configuration, applying 350, 450, and 550 V. In all cases,
an INSTEK PSW 800-4.32 GW source was used by constant
voltage mode. A schematic of the experimental setup for
the PEO in anodic configuration is shown in the schema of
Figure 1, in which the positive of the source was connected
to the substrate-electrode and the negative to the metal
vessel. In the cathodic arrangement, the polarity of the
source leads was reversed.
Figure 1 Experimental setup of the anodic PEO system
The X-ray diffractograms (XRD) of the samples were
obtained with the grazing angle method, using an XBruker
66
of Cr(VI) photoreduction using the coatings over these
metallic substrates.
In some variations of conventional PEO, the
substrate-electrode acts as the anode (anodic PEO),
and the electrolyte container vessel or a platinum
electrode is the cathode [8]; these methods include power
supplies with variable or switchable output voltage at
a given frequency and duty cycle [9]. This methodology
allows control of the grain size of the oxide, directly formed
on the substrate surface. Unlike conventional anodizing,
in which low electrical voltages are used for exposure
times longer than 20 min, the PEO involves the use of high
magnitude voltages for less time.
The PEO consists of the surface modification of a metal
that is immersed in an electrolyte solution contained in a
metallic container. The concentration of the electrolyte
determines the current intensity that flows through the
substrate while it is subjected to an electrical voltage
higher than the breakdown voltage of the material. When
the breakdown voltage is exceeded, microdischarges are
generated on the surface of the sample; thereby, the local
pressure and temperature abruptly increase, producing
microcavities. The current density can be controlled by
considering the substrate shape and the applied electric
potential [10].
Several processes use hexavalent chromium (Cr(VI))
to manufacture some products; in the automotive
industry, one of them is electroplating or deposition of a
metallic layer of chromium on objects to add reflective
brightness and provide corrosion protection [11]; in the
manufacture of paint and pigments, Cr(VI) is added to the
product to improve its stability [12], and in the tannery, it is
used to transform the collagen in the leather into a stable
material to prevent its degradation and to increase its
useful life [13, 14]. The Cr quantity that is not deposited,
impregnated or absorbed in these techniques is returned
to the environment through wastewater. This is harmful to
health [15–17], so it is necessary to reduce Cr(VI) to Cr(III),
which is less toxic and more environmentally friendly.
Although other reduction processes of hexavalent
to trivalent chromium are currently used, such as
bioremediation and chemical reactors [18–20], the fixation
of materials for advanced oxidation presents advantages,
such as the reuse of the catalyst, without complex recovery
or addition of toxic agents.
The use of the cathodic configuration has been reported
for PEO on certain metals and alloys; in this configuration,
the substrate is placed as the cathode and the container
vessel, or another electrode, as the anode [21]. Thus,
several authors have modified stainless steel surfaces
decreasing the electric potential and low concentrations
of the electrolyte [22]. In this work, the PEO anodic
configuration was used with copper and zinc substrates,
and the cathodic mode with the steel substrates. Based on
the results, the existence of the corresponding oxides on
each substrate and their efficiency in Cr(VI) photoreduction
was probed; which do not have been reported before, in
the best of our knowledge; most of the reports are related
to oxide samples in powder for this application.
2. Materials and methods
Sheets of dimensions 20 mm x 20 mm x 1 mm of stainless
steel (304), copper (Cu-DHP 1/2 hard CW024A) and zinc
(shiny sheet EN 988) were previously polished with an
abrasive paper and used as substrates. Before the PEO,
these metal sheets were subjected to an ultrasonic bath
for 20 min with distilled water and isopropanol. In all
cases, the exposure time was 10 min, and a mixture of 0.2
g/L sodium metasilicate nonahydrate (N a2SiO3 − 9H2O)
and 0.05 g/L potassium hydroxide (KOH) was utilized as
electrolyte.
The PEO of copper and zinc substrates was carried out by
conventional mode with potential differences between the
electrodes of 450, 550, 650 and 750 V, while the surface of
stainless-steel substrates was modified with PEO cathodic
configuration, applying 350, 450, and 550 V. In all cases,
an INSTEK PSW 800-4.32 GW source was used by constant
voltage mode. A schematic of the experimental setup for
the PEO in anodic configuration is shown in the schema of
Figure 1, in which the positive of the source was connected
to the substrate-electrode and the negative to the metal
vessel. In the cathodic arrangement, the polarity of the
source leads was reversed.
Figure 1 Experimental setup of the anodic PEO system
The X-ray diffractograms (XRD) of the samples were
obtained with the grazing angle method, using an XBruker
66
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
Figure 2 Schema of photocatalytic method (top) and experimental setup (bottom)
D8-Advance equipment, with a scan between 20° and
90°, at 5 s per degree, an operating voltage of 40 kV and
current of 8 mA. The average crystalline size (S) of the
coatings was estimated from XRD patterns, according to
the Scherrer equation (Equation (1)) [23]:
S = Kλ
B cos(θ) (1)
Where β is the full width at half maximum (FWHM) in
radian, corresponding to the most intense diffraction peak;
K = 0.9 is the shape factor or Scherrer constant; λ is
the X-ray wavelength of CuKα radiation (1.54056 Å) and
θ is the Bragg’s diffraction angle. The contributions of
the crystallite size and lattice strain to diffraction peak
broadening were calculated by the Williamson–Hall (WH)
method, using Equation (2) [24]:
β cos(θ) = 0.9 K
S + 4σ sin(θ) (2)
Where β is the measured FWHM of XRD, corresponding to
different crystal planes, and σ is the effective strain.
The porous formation, cracks, and agglomerations
during the growing of oxide layers could increase the
surface-volume ratio, which has a direct influence on the
photocatalytic activity. For this reason, in the present
work, images of the coatings were taken with a JEOL
JSM-5900LV Scanning Electron Microscope (SEM) to
observe the sample surface morphology.
The physicochemical process of photoreduction involves
the in situ generation of active radical species, especially
hydroxyls, which lead to the destruction of pollutant
targets. Therefore, a photocatalytic agent is used under
the action of light in the presence of substances containing
OH groups such as EDTA (ethylenediaminetetraacetic
acid). A schema of the photocatalysis method is shown
in Figure 2. The photocatalytic activity was evaluated
by irradiating the films, immersed in a solution of
Cr(VI)-EDTA, with ultraviolet light of wavelength 255 nm
from a 15 W bactericidal lamp. The complete reduction
efficiency was obtained after 60 minutes. The test
was performed in a climatic chamber, controlling the
temperature to around 23 °C.
The standard method ASTM-3500 B-Cr [25] was applied to
calculate the removal rate (R), using a spectrophotometer
(EU-2800DS) to measure the absorbance of the sample
solutions under the maximum absorbance wavelength
(352 nm). Considering the direct relationship between
concentration and absorbance, the R of Cr(VI) in the
solution was calculated through Equation (3):
R(%) = C0 − Cf
C0
100% (3)
Where C0 and Cf are the initial and final Cr(VI)
concentration, expressed in mg/L.
The kinetic model of the photocatalytic reaction, using
the simplification to first-order that assumes isothermal
conditions, can be expressed as follows (Equation (4))
<[26]:
ln
( C0
C
)
= kr t (4)
Where, C is the concentration at the time t and kr is the
rate constant of degradation.
In the experiments, three concentrations during the
treatment of Cr(VI) aqueous solution were measured, so
the analysis of experimental data was carried out with
three points for the fit-linear process, using Equation (4).
Although this number of points is low, the obtained values
of the rate constant of degradation were only considered
67
Figure 2 Schema of photocatalytic method (top) and experimental setup (bottom)
D8-Advance equipment, with a scan between 20° and
90°, at 5 s per degree, an operating voltage of 40 kV and
current of 8 mA. The average crystalline size (S) of the
coatings was estimated from XRD patterns, according to
the Scherrer equation (Equation (1)) [23]:
S = Kλ
B cos(θ) (1)
Where β is the full width at half maximum (FWHM) in
radian, corresponding to the most intense diffraction peak;
K = 0.9 is the shape factor or Scherrer constant; λ is
the X-ray wavelength of CuKα radiation (1.54056 Å) and
θ is the Bragg’s diffraction angle. The contributions of
the crystallite size and lattice strain to diffraction peak
broadening were calculated by the Williamson–Hall (WH)
method, using Equation (2) [24]:
β cos(θ) = 0.9 K
S + 4σ sin(θ) (2)
Where β is the measured FWHM of XRD, corresponding to
different crystal planes, and σ is the effective strain.
The porous formation, cracks, and agglomerations
during the growing of oxide layers could increase the
surface-volume ratio, which has a direct influence on the
photocatalytic activity. For this reason, in the present
work, images of the coatings were taken with a JEOL
JSM-5900LV Scanning Electron Microscope (SEM) to
observe the sample surface morphology.
The physicochemical process of photoreduction involves
the in situ generation of active radical species, especially
hydroxyls, which lead to the destruction of pollutant
targets. Therefore, a photocatalytic agent is used under
the action of light in the presence of substances containing
OH groups such as EDTA (ethylenediaminetetraacetic
acid). A schema of the photocatalysis method is shown
in Figure 2. The photocatalytic activity was evaluated
by irradiating the films, immersed in a solution of
Cr(VI)-EDTA, with ultraviolet light of wavelength 255 nm
from a 15 W bactericidal lamp. The complete reduction
efficiency was obtained after 60 minutes. The test
was performed in a climatic chamber, controlling the
temperature to around 23 °C.
The standard method ASTM-3500 B-Cr [25] was applied to
calculate the removal rate (R), using a spectrophotometer
(EU-2800DS) to measure the absorbance of the sample
solutions under the maximum absorbance wavelength
(352 nm). Considering the direct relationship between
concentration and absorbance, the R of Cr(VI) in the
solution was calculated through Equation (3):
R(%) = C0 − Cf
C0
100% (3)
Where C0 and Cf are the initial and final Cr(VI)
concentration, expressed in mg/L.
The kinetic model of the photocatalytic reaction, using
the simplification to first-order that assumes isothermal
conditions, can be expressed as follows (Equation (4))
<[26]:
ln
( C0
C
)
= kr t (4)
Where, C is the concentration at the time t and kr is the
rate constant of degradation.
In the experiments, three concentrations during the
treatment of Cr(VI) aqueous solution were measured, so
the analysis of experimental data was carried out with
three points for the fit-linear process, using Equation (4).
Although this number of points is low, the obtained values
of the rate constant of degradation were only considered
67
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
Figure 3 Diffractograms of copper substrates, modified by PEO
with voltages between 450 and 750 V. In each case, the inserted
plot corresponds to a magnification between 36 and 44°
Table 1 Grain sizes (S) of PEO coatings on copper substrates
Sample Scherrer method WH method σ
S (nm) S (nm)
450 V 47.78 128.97 0.0024
550 V 53.62 127.86 0.0018
650 V 48.23 83.8 0.0042
750 V 40.89 60.98 0.0052
as estimations for comparison purposes; although the
behavior of these experimental points clearly showed
a decrease of C/C0 in the function of the exposition
time, more data could be necessary to guarantee this
information.
3. Results
3.1 Coatings of Cu2O over Cu substrates
The diffractograms of the coatings of Cu
substrate-electrode obtained with each PEO potential
difference are shown in Figure 3. In the case of the pure
copper substrate, two peaks were presented around
43.5° and 50.5°, which is similar to that reported by
Theivasanthi and Alagar [27]. On the other hand, four
low- intensity peaks around 29.66°, 36.53°, 42.44° and
61.57° are observed, related to the (110), (111), (200) and
(220) crystallographic directions of cuprous oxide (Cu2O)
(JCPDS: 05-0667), after surface modification by PEO
[28, 29]. In any case, the formation of cupric oxide (CuO)
was observed.
In Figure 4, the WH plots were made for all the observed
diffraction peaks; the sin(θ) values along the x-axis
and β cos(θ) values along the y-axis were drawn. The
intercept with the y-axis of the best linear-fitting is related
to the nanocrystalline size (S), and the slope gives the
lattice strain multiplicated by four (4 σ). In Table 1, the
corresponding values were registered. The S values
obtained by Scherrer and WH methods are different due
to the distribution of particle sizes in the sample [30]. It
is possible to note that the size grain decreases while
PEO voltage increases. On the other hand, the strain has
a proportional behavior to this growth parameter, which
suggests an increase in the dislocation density caused by
this rising voltage.
The SEM micrograph of the 550 V-treated substrate,
shown in Figure 5, indicates a surface modification with
irregular topography of a flak-like shape.
The use of copper (I) and (II) oxides in the degradation of
organic pollutants has been reported by other authors
[31, 32]; these p-type semiconductors have a narrow
bandgap of 1.2–1.8 eV and 2.0–2.2 eV, respectively, which
allows their photoactivity with UV and visible light.
The photocatalytic activity of the Cu2O coatings in
the photoreduction of Cr(VI)-EDTA was evaluated through
the data plotted in Figure 6. It can be seen a reduction rate
close to 100% within 60 min with the samples, although
with the coating grown at 450 V, the highest efficiency was
achieved at 30 min of the process.
The kr values were obtained from the slope of the
linearly fitted plot in Figure 6 c) by using Equation (4), and
the data were registered in Table 2. It is possible to see
that the highest kr value was obtained with the coating
grown at 650 V PEO.
3.2 Coatings of ZnO over Zn substrates
The diffractograms in Figure 7 correspond to the
PEO-modified zinc substrates. It can be observed the
formation of zinc oxide related to the broad bands around
31.80, 34.47, 36.29, 47.60, 56.65, 62.94, 66.45, 68.03,
and 69.16° (JCPDS: 036-1451), by which the intensity
augmented as a function of increasing voltage. This is in
agreement with those reported by other authors [33, 34].
In the experiments, once the plasma is obtained on
68
Figure 3 Diffractograms of copper substrates, modified by PEO
with voltages between 450 and 750 V. In each case, the inserted
plot corresponds to a magnification between 36 and 44°
Table 1 Grain sizes (S) of PEO coatings on copper substrates
Sample Scherrer method WH method σ
S (nm) S (nm)
450 V 47.78 128.97 0.0024
550 V 53.62 127.86 0.0018
650 V 48.23 83.8 0.0042
750 V 40.89 60.98 0.0052
as estimations for comparison purposes; although the
behavior of these experimental points clearly showed
a decrease of C/C0 in the function of the exposition
time, more data could be necessary to guarantee this
information.
3. Results
3.1 Coatings of Cu2O over Cu substrates
The diffractograms of the coatings of Cu
substrate-electrode obtained with each PEO potential
difference are shown in Figure 3. In the case of the pure
copper substrate, two peaks were presented around
43.5° and 50.5°, which is similar to that reported by
Theivasanthi and Alagar [27]. On the other hand, four
low- intensity peaks around 29.66°, 36.53°, 42.44° and
61.57° are observed, related to the (110), (111), (200) and
(220) crystallographic directions of cuprous oxide (Cu2O)
(JCPDS: 05-0667), after surface modification by PEO
[28, 29]. In any case, the formation of cupric oxide (CuO)
was observed.
In Figure 4, the WH plots were made for all the observed
diffraction peaks; the sin(θ) values along the x-axis
and β cos(θ) values along the y-axis were drawn. The
intercept with the y-axis of the best linear-fitting is related
to the nanocrystalline size (S), and the slope gives the
lattice strain multiplicated by four (4 σ). In Table 1, the
corresponding values were registered. The S values
obtained by Scherrer and WH methods are different due
to the distribution of particle sizes in the sample [30]. It
is possible to note that the size grain decreases while
PEO voltage increases. On the other hand, the strain has
a proportional behavior to this growth parameter, which
suggests an increase in the dislocation density caused by
this rising voltage.
The SEM micrograph of the 550 V-treated substrate,
shown in Figure 5, indicates a surface modification with
irregular topography of a flak-like shape.
The use of copper (I) and (II) oxides in the degradation of
organic pollutants has been reported by other authors
[31, 32]; these p-type semiconductors have a narrow
bandgap of 1.2–1.8 eV and 2.0–2.2 eV, respectively, which
allows their photoactivity with UV and visible light.
The photocatalytic activity of the Cu2O coatings in
the photoreduction of Cr(VI)-EDTA was evaluated through
the data plotted in Figure 6. It can be seen a reduction rate
close to 100% within 60 min with the samples, although
with the coating grown at 450 V, the highest efficiency was
achieved at 30 min of the process.
The kr values were obtained from the slope of the
linearly fitted plot in Figure 6 c) by using Equation (4), and
the data were registered in Table 2. It is possible to see
that the highest kr value was obtained with the coating
grown at 650 V PEO.
3.2 Coatings of ZnO over Zn substrates
The diffractograms in Figure 7 correspond to the
PEO-modified zinc substrates. It can be observed the
formation of zinc oxide related to the broad bands around
31.80, 34.47, 36.29, 47.60, 56.65, 62.94, 66.45, 68.03,
and 69.16° (JCPDS: 036-1451), by which the intensity
augmented as a function of increasing voltage. This is in
agreement with those reported by other authors [33, 34].
In the experiments, once the plasma is obtained on
68
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
(a) (b)
(c) (d)
Figure 4 WH analysis corresponding to diffractograms in Figure 3 of Cu2O coatings grown by PEO with voltages between a) 450, b)
550, c) 650, and d) 750 V. The continuous line represents the best linear-fitting for calculus of grain size of nanocrystals and strain
effect
Figure 5 SEM micrograph of PEO-modified copper substrate at
an electrical potential 550 V. The behavior was similar in the
other samples
the substrate surface, the current tends to be a constant
value, but the electrolyte evaporates rapidly due to the high
temperature reached, leading to the partial uncovering
of the substrate-electrode. The band gap energy of this
semiconductor material has been reported to be between
3.2 and 3.5 eV at room temperature [35, 36].
A SEM micrograph of the zinc substrate modified by PEO at
550 V is shown in Figure 9. In this case, the accumulation
of irregular grains of different sizes and shapes distributed
over the entire surface of the coating can be observed,
which shows internal porosity and a sponge-like structure.
Table 2 The rate constant of degradation corresponding to
analysis in Figure 6c) of degradation of Cr(VI)-EDTA with the
Cu2O coating samples. R2 is the R-square statistic of the
linear fitting
Sample kr (min−1) R2
450V 0.02 0.988
550V 0.02 0.999
650V 0.03 0.988
750V 0.02 0.995
Cu 0.001 0.999
Table 3 Grain sizes (S) of the ZnO coatings
Sample Scherrer method WH method σ
S (nm) S (nm)
450V 27.16 20.12 0.0094
550V 10.75 7.92 -0.0053
650V 16.57 31.08 0.0044
750V 9 6.09 -0.0060
69
(a) (b)
(c) (d)
Figure 4 WH analysis corresponding to diffractograms in Figure 3 of Cu2O coatings grown by PEO with voltages between a) 450, b)
550, c) 650, and d) 750 V. The continuous line represents the best linear-fitting for calculus of grain size of nanocrystals and strain
effect
Figure 5 SEM micrograph of PEO-modified copper substrate at
an electrical potential 550 V. The behavior was similar in the
other samples
the substrate surface, the current tends to be a constant
value, but the electrolyte evaporates rapidly due to the high
temperature reached, leading to the partial uncovering
of the substrate-electrode. The band gap energy of this
semiconductor material has been reported to be between
3.2 and 3.5 eV at room temperature [35, 36].
A SEM micrograph of the zinc substrate modified by PEO at
550 V is shown in Figure 9. In this case, the accumulation
of irregular grains of different sizes and shapes distributed
over the entire surface of the coating can be observed,
which shows internal porosity and a sponge-like structure.
Table 2 The rate constant of degradation corresponding to
analysis in Figure 6c) of degradation of Cr(VI)-EDTA with the
Cu2O coating samples. R2 is the R-square statistic of the
linear fitting
Sample kr (min−1) R2
450V 0.02 0.988
550V 0.02 0.999
650V 0.03 0.988
750V 0.02 0.995
Cu 0.001 0.999
Table 3 Grain sizes (S) of the ZnO coatings
Sample Scherrer method WH method σ
S (nm) S (nm)
450V 27.16 20.12 0.0094
550V 10.75 7.92 -0.0053
650V 16.57 31.08 0.0044
750V 9 6.09 -0.0060
69
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
(a) (b)
(c)
Figure 6 a) R (%), b) C/C0 and c) log (C0/C) of the photocatalytic process of the Cu2O samples in the reduction of Cr(VI)-EDTA as
a function of the exposition time. In this last plot, the solid line represents the best linear fitting
Figure 7 Diffractograms of PEO-modified zinc substrates at
voltages between 450 and 650 V
The photocatalytic performance of the ZnO coatings
was tested in the reduction of Cr(VI)-EDTA with an initial
Table 4 The rate constant of degradation corresponding to the
analysis in Figure 10c of degradation of Cr(VI)-EDTA with the ZnO
coating samples
Sample kr (min-1) R2
450V 0.05 0.92
550V 0.05 0.93
650V 0.05 0.95
750V 0.05 0.96
Zn 0.01 0.99
concentration of 1 ppm. The reduction rates as a function
of time are presented in Figure 10, showing a behavior
similar to that described above for the Cu2O coatings,
reaching a reduction of around 100% within 60 min. The
highest efficiency was achieved at 30 min with the samples
obtained by PEO, using 650 and 750 V.
The kr values, obtained from the slope of the linearly
fitted plot in Figure 10c) by using Equation (4), were
registered in Table 4. It is possible to see that kr was the
same for all ZnO coatings.
3.3 Coatings of Fe2O3and Fe3O4 over
stainless steel substrates
Substrate-electrodes of stainless steel 304 were treated
by PEO, and its modified surface was analyzed to
70
(a) (b)
(c)
Figure 6 a) R (%), b) C/C0 and c) log (C0/C) of the photocatalytic process of the Cu2O samples in the reduction of Cr(VI)-EDTA as
a function of the exposition time. In this last plot, the solid line represents the best linear fitting
Figure 7 Diffractograms of PEO-modified zinc substrates at
voltages between 450 and 650 V
The photocatalytic performance of the ZnO coatings
was tested in the reduction of Cr(VI)-EDTA with an initial
Table 4 The rate constant of degradation corresponding to the
analysis in Figure 10c of degradation of Cr(VI)-EDTA with the ZnO
coating samples
Sample kr (min-1) R2
450V 0.05 0.92
550V 0.05 0.93
650V 0.05 0.95
750V 0.05 0.96
Zn 0.01 0.99
concentration of 1 ppm. The reduction rates as a function
of time are presented in Figure 10, showing a behavior
similar to that described above for the Cu2O coatings,
reaching a reduction of around 100% within 60 min. The
highest efficiency was achieved at 30 min with the samples
obtained by PEO, using 650 and 750 V.
The kr values, obtained from the slope of the linearly
fitted plot in Figure 10c) by using Equation (4), were
registered in Table 4. It is possible to see that kr was the
same for all ZnO coatings.
3.3 Coatings of Fe2O3and Fe3O4 over
stainless steel substrates
Substrate-electrodes of stainless steel 304 were treated
by PEO, and its modified surface was analyzed to
70
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
(a) (b)
(c) (d)
Figure 8 WH analysis corresponding to the diffractograms of PEO-modified zinc substrates using a) 450, b) 550 V, c) 650 V, and d)
750 V
Figure 9 SEM surface morphology of the ZnO sample, obtained
by PEO with an electrical potential of 550 V. The behavior of the
surface modification was similar using the other voltages
search for the growth of iron oxides, known as Hematite
(F e2O3) and Magnetite (F e3O4). These compounds
have been reported in advanced oxidation process
applications for textile industry wastewater treatment
and as antibacterial for general purposes [37–39]. In
Figure 11, the plotted diffractograms correspond to the
coatings that were prepared by cathodic PEO, using 350,
450 and 550 V between the electrodes; the diffraction
peaks were associated with the (2 0 2), (3 0 0) and (0 2 10)
crystallographic planes of F e2O3 (JCPDS: 330664); (6 2
2) of F e3O4 (JCPDS 190629) and (2 0 0) of γ − F e (ICCD
98-004-1506). This intensity changed for the samples
according to the surface modification of the steel sheet by
cathodic PEO [40, 41].
Table 5 Grain sizes (S) of F e2O3 and F e3O4 coatings on
stainless steel 304 substrates
Sample Scherrer (F e3O4) Scherrer (F e2O3) W-H σ
S (nm) S (nm) S (nm)
350V 17.28 21.45 10.49 -0.0034
450V 26.04 38.98 18.85 0.0070
550V 21.94 39.43 25.96 0.0054
In Figure 12, the WH plots were made for all the
observed diffraction peaks corresponding to F e2O3. The
intercept with the y-axis of the best linear-fitting is related
to S and from the slope was estimated σ. In Table 5,
the corresponding values were registered. The S values
obtained by Scherrer and WH methods for both F e2O3
and F e3O4 are different, due to the distribution of particle
sizes in the sample [26]. It is possible to note that the
grain size increases while PEO voltage rises. On the other
hand, the strain was negative for the lowest voltage and
the strain minimum was obtained with the highest voltage.
From Figure 13, two SEM micrographs of the surface
of the steel substrate modified by PEO at 550 V can
be observed. The formation of crater-like pores and
spheroidal nodules over the coating is observed, which
increases the surface area, enhancing its photocatalytic
activity. This morphology is different from that obtained
by other authors, using cathodic PEO with a pulsed DC
power supply; for instance, Jin et al. reported a surface in
some hill-shaped embossment, employing voltages low
280 V [42], while Wu et al., found a rough surface with
some granular bulges under 400 V [43]. This behavior is as
71
(a) (b)
(c) (d)
Figure 8 WH analysis corresponding to the diffractograms of PEO-modified zinc substrates using a) 450, b) 550 V, c) 650 V, and d)
750 V
Figure 9 SEM surface morphology of the ZnO sample, obtained
by PEO with an electrical potential of 550 V. The behavior of the
surface modification was similar using the other voltages
search for the growth of iron oxides, known as Hematite
(F e2O3) and Magnetite (F e3O4). These compounds
have been reported in advanced oxidation process
applications for textile industry wastewater treatment
and as antibacterial for general purposes [37–39]. In
Figure 11, the plotted diffractograms correspond to the
coatings that were prepared by cathodic PEO, using 350,
450 and 550 V between the electrodes; the diffraction
peaks were associated with the (2 0 2), (3 0 0) and (0 2 10)
crystallographic planes of F e2O3 (JCPDS: 330664); (6 2
2) of F e3O4 (JCPDS 190629) and (2 0 0) of γ − F e (ICCD
98-004-1506). This intensity changed for the samples
according to the surface modification of the steel sheet by
cathodic PEO [40, 41].
Table 5 Grain sizes (S) of F e2O3 and F e3O4 coatings on
stainless steel 304 substrates
Sample Scherrer (F e3O4) Scherrer (F e2O3) W-H σ
S (nm) S (nm) S (nm)
350V 17.28 21.45 10.49 -0.0034
450V 26.04 38.98 18.85 0.0070
550V 21.94 39.43 25.96 0.0054
In Figure 12, the WH plots were made for all the
observed diffraction peaks corresponding to F e2O3. The
intercept with the y-axis of the best linear-fitting is related
to S and from the slope was estimated σ. In Table 5,
the corresponding values were registered. The S values
obtained by Scherrer and WH methods for both F e2O3
and F e3O4 are different, due to the distribution of particle
sizes in the sample [26]. It is possible to note that the
grain size increases while PEO voltage rises. On the other
hand, the strain was negative for the lowest voltage and
the strain minimum was obtained with the highest voltage.
From Figure 13, two SEM micrographs of the surface
of the steel substrate modified by PEO at 550 V can
be observed. The formation of crater-like pores and
spheroidal nodules over the coating is observed, which
increases the surface area, enhancing its photocatalytic
activity. This morphology is different from that obtained
by other authors, using cathodic PEO with a pulsed DC
power supply; for instance, Jin et al. reported a surface in
some hill-shaped embossment, employing voltages low
280 V [42], while Wu et al., found a rough surface with
some granular bulges under 400 V [43]. This behavior is as
71
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
(a) (b)
(c)
Figure 10 Photocatalytic efficiency of Cr(VI)-EDTA reduction with PEO-modified zinc sheets at electric potentials of 450, 550, 650,
and 750 V
Figure 11 Diffractograms of PEO-modified 304 stainless steel
substrates with inter-electrode voltages of 350, 450, and 550 V
expected, because the effects of spark discharges can be
mitigated by the application of pulsed current, providing
control over the impact of plasma discharge on the surface
[44].
Table 6 The rate constant of degradation corresponding to
analysis in Figure 14 c) of degradation of Cr(VI)-EDTA with
F e2O3 and F e3O4 coating samples
Sample kr (min−1) R2
350V 0.02 0.92
450V 0.02 0.94
550V 0.03 0.88
steel 0 0.995
In Figure 14, the reduction behavior of Cr(VI)-EDTA
using the PEO-modified stainless steel sheets is shown.
A complete reduction was found within 60 min, using the
samples obtained at 550 and 650 V. The kr values obtained
from the slope of the linearly fitted plot in Figure 14c) by
using Equation (4) were registered in Table 6. It is possible
to see that the highest kr value was achieved with the
coating grown at 550 V during the PEO.
The obtained coatings had a morphology that depended
on the type of substrate. For the copper, the flak-like
shape structures over the surface were grown, while for
zinc substrate-electrodes, it can be seen agglomerations
of the zinc oxide have a sponge-like appearance. In
contrast, stainless steel crater-like microcavities, and
spheroidal nodules were generated over its surface. This
may be related to the fact that this material contains other
elements such as chromium, manganese, and nickel,
apart from iron and carbon; thus, other types of bonds
can be produced on the surface of the substrate, and the
topography of the substrate changes [45].
72
(a) (b)
(c)
Figure 10 Photocatalytic efficiency of Cr(VI)-EDTA reduction with PEO-modified zinc sheets at electric potentials of 450, 550, 650,
and 750 V
Figure 11 Diffractograms of PEO-modified 304 stainless steel
substrates with inter-electrode voltages of 350, 450, and 550 V
expected, because the effects of spark discharges can be
mitigated by the application of pulsed current, providing
control over the impact of plasma discharge on the surface
[44].
Table 6 The rate constant of degradation corresponding to
analysis in Figure 14 c) of degradation of Cr(VI)-EDTA with
F e2O3 and F e3O4 coating samples
Sample kr (min−1) R2
350V 0.02 0.92
450V 0.02 0.94
550V 0.03 0.88
steel 0 0.995
In Figure 14, the reduction behavior of Cr(VI)-EDTA
using the PEO-modified stainless steel sheets is shown.
A complete reduction was found within 60 min, using the
samples obtained at 550 and 650 V. The kr values obtained
from the slope of the linearly fitted plot in Figure 14c) by
using Equation (4) were registered in Table 6. It is possible
to see that the highest kr value was achieved with the
coating grown at 550 V during the PEO.
The obtained coatings had a morphology that depended
on the type of substrate. For the copper, the flak-like
shape structures over the surface were grown, while for
zinc substrate-electrodes, it can be seen agglomerations
of the zinc oxide have a sponge-like appearance. In
contrast, stainless steel crater-like microcavities, and
spheroidal nodules were generated over its surface. This
may be related to the fact that this material contains other
elements such as chromium, manganese, and nickel,
apart from iron and carbon; thus, other types of bonds
can be produced on the surface of the substrate, and the
topography of the substrate changes [45].
72
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
(a) (b)
(c)
Figure 12 WH analysis corresponding to the diffractograms of PEO-modified stainless steel substrates using a) 350, b) 450 V, and c)
550 V
(a) (b)
Figure 13 SEM micrographs of the surface of stainless-steel substrate modified by PEO with 550 V, using a magnification of a)
X1000 and b) X5000
The high-energy processes, such as the PEO, promote
the formation of new compounds that cannot be obtained
by conventional chemical or physical methods [46].
Particularly for soft metals, the melting point plays an
important role; this temperature for copper is 1357.77 K,
while for zinc is 692.68 K, so the modification of surfaces
by PEO with different characteristics in both such cases is
expected.
According to the estimated values of kr , the highest
photocatalytic efficiency of Cr(VI) reduction was achieved
using the ZnO coatings, possibly due to the wide bandgap
of this semiconductor. However, the size grain did not
have an influence in these results.
To compare the obtained results about reduction of
Cr(VI) to Cr(III) with the reports of other authors, it
was necessary to consider nanoparticle photocatalysts
because corresponding studies with pure oxide coatings
were not found. In this way, for example, with ZnO
nanoparticles prepared by co-precipitation method with
an average size of 20–50 nm, the photocatalytic ability of
Cr(VI) reduction was 90% under UV-light irradiation for 17
h with and kr = 0.025 min−1 [47], while in the present
work, this value was doubled.
On the other hand, with Cu2O, the corresponding
reduction rate of Cr(VI) was kr = 0.00294 min−1, under
visible light irradiation, according to Xincheng-Dou et al.
[48], which is a value 90% less than that reported in this
work, but using UV-light radiation.
73
(a) (b)
(c)
Figure 12 WH analysis corresponding to the diffractograms of PEO-modified stainless steel substrates using a) 350, b) 450 V, and c)
550 V
(a) (b)
Figure 13 SEM micrographs of the surface of stainless-steel substrate modified by PEO with 550 V, using a magnification of a)
X1000 and b) X5000
The high-energy processes, such as the PEO, promote
the formation of new compounds that cannot be obtained
by conventional chemical or physical methods [46].
Particularly for soft metals, the melting point plays an
important role; this temperature for copper is 1357.77 K,
while for zinc is 692.68 K, so the modification of surfaces
by PEO with different characteristics in both such cases is
expected.
According to the estimated values of kr , the highest
photocatalytic efficiency of Cr(VI) reduction was achieved
using the ZnO coatings, possibly due to the wide bandgap
of this semiconductor. However, the size grain did not
have an influence in these results.
To compare the obtained results about reduction of
Cr(VI) to Cr(III) with the reports of other authors, it
was necessary to consider nanoparticle photocatalysts
because corresponding studies with pure oxide coatings
were not found. In this way, for example, with ZnO
nanoparticles prepared by co-precipitation method with
an average size of 20–50 nm, the photocatalytic ability of
Cr(VI) reduction was 90% under UV-light irradiation for 17
h with and kr = 0.025 min−1 [47], while in the present
work, this value was doubled.
On the other hand, with Cu2O, the corresponding
reduction rate of Cr(VI) was kr = 0.00294 min−1, under
visible light irradiation, according to Xincheng-Dou et al.
[48], which is a value 90% less than that reported in this
work, but using UV-light radiation.
73
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
(a) (b)
(c)
Figure 14 Cr(VI)-EDTA reduction rate with PEO-modified stainless steel sheets at voltages of 350, 450, and 550 V
Liu TY et al. reported that kr ranged from 0.0072 min−1
to 0.0304 min−1, using dosages between 0.5 and 3
g/L of F e2O3 nanoparticles [49]; while pure F e3O4
nanoparticles hardly reduced Cr(VI) within 90 min under
visible light irradiation with a value of kr = 0.008cm−1,
as reported by Ting-Ge et al. [50]. In contrast, in this
work, with coatings containing both F e3O4 and F e2O3,
a kr = 0.03min−1 was found, a similar value to that
maximum reported by Liu TY et al.
4. Conclusions
The PEO modification of the surfaces of the copper, zinc,
and stainless-steel substrates was obtained by varying
the voltage between the electrodes. The formation of
layers of Cu2O, ZnO and F e2O3 and F e3O4, over each
corresponding material, was verified by X-ray diffraction.
As a growth parameter, this differential of potential
affected the crystalline structure of the coatings since
the intensity and width of the peaks changed in the
related diffractograms. On the other hand, according
to the micrographs of the surface of the samples, the
morphology of the coating depends on the current demand
and the characteristics of the power supply to support the
output voltage.
The lattice strain factor had an important role in the
formation of the nanocrystalline Cu2O coatings; this was
evidenced in the differences between estimated sizes with
Scherrer and WH methods. Using the sample grown to
650 V, the rate of degradation of Cr(VI) was slightly higher
than those obtained with the others coatings on the Cu
substrates. On the other hand, the strain factor had a low
influence on the generation of ZnO, F e2O3, and F e3O4
nanostructures, which can be inferred from the minor
alteration of the grain size, calculated with both models,
Scherrer and WH. While the rate of degradation with the
coatings containing F e2O3 and F e3O4 was similar to that
obtained with Cu2O, the highest value of this parameter
was achieved using the ZnO coatings.
Using ultraviolet radiation over the samples, the total
photoreduction of Cr(VI)-EDTA was obtained within
60 minutes. For this reason, these coatings have the
potential for use in the treatment of chromium-containing
wastewater. In particular, PEO-treated stainless steel
can be the best candidate, due to its high porosity
corresponding to uniformly distributed microcavities.
5. Declaration of competing interest
We declare that we have no significant competing interests,
including financial or non-financial, professional, or
personal interests interfering with the full and objective
presentation of the work described in this manuscript.
74
(a) (b)
(c)
Figure 14 Cr(VI)-EDTA reduction rate with PEO-modified stainless steel sheets at voltages of 350, 450, and 550 V
Liu TY et al. reported that kr ranged from 0.0072 min−1
to 0.0304 min−1, using dosages between 0.5 and 3
g/L of F e2O3 nanoparticles [49]; while pure F e3O4
nanoparticles hardly reduced Cr(VI) within 90 min under
visible light irradiation with a value of kr = 0.008cm−1,
as reported by Ting-Ge et al. [50]. In contrast, in this
work, with coatings containing both F e3O4 and F e2O3,
a kr = 0.03min−1 was found, a similar value to that
maximum reported by Liu TY et al.
4. Conclusions
The PEO modification of the surfaces of the copper, zinc,
and stainless-steel substrates was obtained by varying
the voltage between the electrodes. The formation of
layers of Cu2O, ZnO and F e2O3 and F e3O4, over each
corresponding material, was verified by X-ray diffraction.
As a growth parameter, this differential of potential
affected the crystalline structure of the coatings since
the intensity and width of the peaks changed in the
related diffractograms. On the other hand, according
to the micrographs of the surface of the samples, the
morphology of the coating depends on the current demand
and the characteristics of the power supply to support the
output voltage.
The lattice strain factor had an important role in the
formation of the nanocrystalline Cu2O coatings; this was
evidenced in the differences between estimated sizes with
Scherrer and WH methods. Using the sample grown to
650 V, the rate of degradation of Cr(VI) was slightly higher
than those obtained with the others coatings on the Cu
substrates. On the other hand, the strain factor had a low
influence on the generation of ZnO, F e2O3, and F e3O4
nanostructures, which can be inferred from the minor
alteration of the grain size, calculated with both models,
Scherrer and WH. While the rate of degradation with the
coatings containing F e2O3 and F e3O4 was similar to that
obtained with Cu2O, the highest value of this parameter
was achieved using the ZnO coatings.
Using ultraviolet radiation over the samples, the total
photoreduction of Cr(VI)-EDTA was obtained within
60 minutes. For this reason, these coatings have the
potential for use in the treatment of chromium-containing
wastewater. In particular, PEO-treated stainless steel
can be the best candidate, due to its high porosity
corresponding to uniformly distributed microcavities.
5. Declaration of competing interest
We declare that we have no significant competing interests,
including financial or non-financial, professional, or
personal interests interfering with the full and objective
presentation of the work described in this manuscript.
74
F. Gordillo-Delgado et al., Revista Facultad de Ingeniería, Universidad de Antioquia, No. 110, pp. 65-76, 2024
6. Acknowledgements
The authors thank the Universidad del Quindío for partial
funding of this work through projects 982 and 1071 and
the Instituto Interdisciplinario de las Ciencias for the XRD
measurements. García-Giraldo expresses his gratitude
to Minciencias and Universidad del Quindío for the Young
Researcher grant.
7. Funding
This work was supported by Universidad del Quindío and
Minciencias.
8. Author contributions
F. G. D conceived and designed the analysis. J. A. G.
collected the data and contributed to the analysis tools. F.
G. D and J.A.G wrote the paper.
9. Data availability statement
The authors confirm that the data supporting the findings
of this study are available within the article [and/or] its
supplementary materials.
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J. García-Giraldo, and D. Torres-cerón, “S-incorporated tio2
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of cr(vi)-edta with sunlight,” Environmental Science and Pollution
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//doi.org/10.1007/s11356-018-2695-6
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aluminium alloys using electroless nickel as underlayer,” Journal of
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[Online]. Available: https://doi.org/10.4236/msce.2014.27007
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jclepro.2019.03.335
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583646
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removal of trivalent chromium from leather wastewater using
engineered bacteria immobilized on magnetic pellets,” Science of
The Total Environment, vol. 775, no. 145647, Jun. 25 2021. [Online].
Available: https://doi.org/10.1016/j.scitotenv.2021.145647
[19] S. Mishra, S. Chen, G. D. Saratale, R. G. Saratale, L. F. R.
Ferreira, and /textitet al., “Reduction of hexavalent chromium by
microbacterium paraoxydans isolated from tannery wastewater and
characterization of its reduced products,” Journal of Water Process
Engineering, vol. 39, no. 101748, Feb. 2021. [Online]. Available:
https://doi.org/10.1016/j.jwpe.2020.101748
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G. M. Ratnamala, and /textitet al., “Removal of hexavalent
chromium-industry treated water and wastewater: A review,”
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Available: https://doi.org/10.1016/j.matpr.2020.12.492
[21] M. Aliofkhazraei, A. S. Rouhaghdam, and P. Gupta, “Nano-fabrication
by cathodic plasma electrolysis,” Critical Reviews in Solid State and
Materials Sciences, vol. 36, no. 3, Sep. 2, 2011. [Online]. Available:
https://doi.org/10.1080/10408436.2011.593269
[22] W. Gui, J. Lin, G. Hao, Y. Qu, Y. Liang, and et al., “Electrolytic plasma
processing-an innovative treatment for surface modification of 304
stainless steel,” Scientific Reports, vol. 7, no. 308, Mar. 22, 2017.
[Online]. Available: https://doi.org/10.1038/s41598-017-00204-w
[23] H. G. Jiang, M. Rühle, and E. J. Lavernia, “On the applicability of
the x-ray diffraction line profile analysis in extracting grain size
and microstrain in nanocrystalline materials,” Journal of Materials
Research, vol. 14, no. 22, pp. 549–559, 1999.
[24] M. Birkholz, Thin film analysis by X-ray scattering. John Wiley& Sons,
2006.
[25] A. Apha, Standard methods for the examination of water and
wastewater, 23rd ed. Washington, DC: American Public Health
Association and American Water Works Association and Water
75
6. Acknowledgements
The authors thank the Universidad del Quindío for partial
funding of this work through projects 982 and 1071 and
the Instituto Interdisciplinario de las Ciencias for the XRD
measurements. García-Giraldo expresses his gratitude
to Minciencias and Universidad del Quindío for the Young
Researcher grant.
7. Funding
This work was supported by Universidad del Quindío and
Minciencias.
8. Author contributions
F. G. D conceived and designed the analysis. J. A. G.
collected the data and contributed to the analysis tools. F.
G. D and J.A.G wrote the paper.
9. Data availability statement
The authors confirm that the data supporting the findings
of this study are available within the article [and/or] its
supplementary materials.
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oxidation of titanium in a phosphate/ silicate electrolyte and
tribological performance of the coatings,” Applied Surface Science,
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apsusc.2014.08.037
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plasma electrolytic oxide coatings on magnesium alloys,” Tribology
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Available: https://doi.org/10.1179/1751584X12Y.0000000006.
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J. García-Giraldo, and D. Torres-cerón, “S-incorporated tio2
coatings grown by plasma electrolytic oxidation for reduction
of cr(vi)-edta with sunlight,” Environmental Science and Pollution
Research, vol. 26, Feb. 20, 2019. [Online]. Available: https:
//doi.org/10.1007/s11356-018-2695-6
[4] (n.d.) Zinc price. Markets Insider. [Online]. Available: https:
//markets.businessinsider.com/commodities/zinc-price
[5] (n.d.) Cooper price. Markets Insider. [Online]. Available: https:
//markets.businessinsider.com/commodities/copper-price
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savoypipinginc.com/stainless-steel-plates-sheet-type/
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html
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businessinsider.com/commodities/tin-price
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electrolytic oxidation (peo) coating growth behavior,” Surface and
Coatings Technology, vol. 226, Jul. 15, 2013. [Online]. Available:
https://doi.org/10.1016/j.surfcoat.2013.03.041
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progress in surface modification of metals coated by plasma
electrolytic oxidation: Principle, structure, and performance,”
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https://doi.org/10.1016/j.pmatsci.2020.100735
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aluminium alloys using electroless nickel as underlayer,” Journal of
Materials Science and Chemical Engineering, vol. 7, no. 2, Jul. 2014.
[Online]. Available: https://doi.org/10.4236/msce.2014.27007
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degradation: some case studies,” in Issues in contemporary oil paint,
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of retanning, fatliquoring and dyeing,” Journal of Cleaner Production,
vol. 226, Jul.20, 2019. [Online]. Available: https://doi.org/10.1016/j.
jclepro.2019.03.335
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of water, chemicals and effluents in leather post-tanning process: A
review,” Environmental Impact Assessment Review, vol. 89, Jul. 2021.
[Online]. Available: https://doi.org/10.1016/j.eiar.2021.106597
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Elements in Medicine and Biology, vol. 65, no. 126729, May. 2021.
[Online]. Available: https://doi.org/10.1016/j.jtemb.2021.126729
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toxicity,” Current Opinion in Toxicology, vol. 14, Apr. 2019. [Online].
Available: https://doi.org/10.1016/j.cotox.2019.05.003
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chromium,” Journal of Coordination Chemistry, vol. 64, no. 10, May.
2011. [Online]. Available: https://doi.org/10.1080/00958972.2011.
583646
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removal of trivalent chromium from leather wastewater using
engineered bacteria immobilized on magnetic pellets,” Science of
The Total Environment, vol. 775, no. 145647, Jun. 25 2021. [Online].
Available: https://doi.org/10.1016/j.scitotenv.2021.145647
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Ferreira, and /textitet al., “Reduction of hexavalent chromium by
microbacterium paraoxydans isolated from tannery wastewater and
characterization of its reduced products,” Journal of Water Process
Engineering, vol. 39, no. 101748, Feb. 2021. [Online]. Available:
https://doi.org/10.1016/j.jwpe.2020.101748
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G. M. Ratnamala, and /textitet al., “Removal of hexavalent
chromium-industry treated water and wastewater: A review,”
Materials Today: Proceedings, vol. 42, no. 2, Mar. 2021. [Online].
Available: https://doi.org/10.1016/j.matpr.2020.12.492
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by cathodic plasma electrolysis,” Critical Reviews in Solid State and
Materials Sciences, vol. 36, no. 3, Sep. 2, 2011. [Online]. Available:
https://doi.org/10.1080/10408436.2011.593269
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processing-an innovative treatment for surface modification of 304
stainless steel,” Scientific Reports, vol. 7, no. 308, Mar. 22, 2017.
[Online]. Available: https://doi.org/10.1038/s41598-017-00204-w
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the x-ray diffraction line profile analysis in extracting grain size
and microstrain in nanocrystalline materials,” Journal of Materials
Research, vol. 14, no. 22, pp. 549–559, 1999.
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2006.
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wastewater, 23rd ed. Washington, DC: American Public Health
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5402/2012/372505
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characterization of zno films on zinc substrate by plasma electrolytic
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water: Alteration of finite size effects from ferrihydrite to hematite,”
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