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It is known that wastewater produced by the metal-plating industry contains several heavy metals, which are acidic in nature and therefore toxic for the environment and for living creatures. In particular, heavy metals enter the food chain and accumulate in vital organs and cause serious illness. The precipitation of these metals is mostly achieved by pH adjustment, but as an alternative to this method, the electrocoagulation process has investigated in this study using iron and aluminum electrodes. The effects of the pH adjustment on removal before and after the electrocoagulation process were investigated, and cost analyses were also compared. It was observed that a high proportion of removal was obtained during the first minutes of the electrocoagulation process; thus, the current density did not have a great effect. In addition, the pH adjustment after the electrocoagulation process using iron electrodes, which are 10% more effective than aluminum electrodes, was found to be much more efficient than before the electrocoagulation process. In the process where kinetic modeling was applied, it was observed that the heavy metal removal mechanism was not solely due to the collapse of heavy metals at high pH values, and with this modeling, it was seen that this mechanism involved adsorption by iron and aluminum hydroxides formed during the electrocoagulation process. When comparing the ability of heavy metals to be adsorbed, the sequence was observed to be Cr>Cu>Ni>Zn, respectively.
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RESEARCH ARTICLE
Electrocoagulation process for the treatment of metal-plating
wastewater: Kinetic modeling and energy consumption
Fatih Ilhan (), Kubra Ulucan-Altuntas, Yasar Avsar, Ugur Kurt, Arslan Saral
Environmental Engineering Department, Yildiz Technical University, Esenler/Istanbul 34220, Turkey
1 Introduction
Ensuring the treatability of wastewaters formed by rapidly
increasing industrialization carries vital signicance from
an environmental point of view. Among industries, metal-
plating industrial wastewaters in particular need to be
treated because of their highly toxic nature. Metal-plating
industrial wastewater and sludge are toxic for all living
creatures, because they contain cyanide and are acidic in
character, as well as containing a wide variety of heavy
metal types. Heavy metals can cause serious illnesses or
even death as a result of their bioaccumulation in the vital
organs of living bodies through the food chain (Choi and
Meier, 2001; Hashim and Chu, 2004; Martins et al., 2004;
Lee et al., 2016).
This branch of industry, which aims to prepare the metal
for use by processing its external surface, uses a wide
variety of processes. These processes are generally one of
Corresponding author
E-mail: lhan@yildiz.edu.tr
Front. Environ. Sci. Eng. 2019, 13(5): 73
https://doi.org/10.1007/s11783-019-1152-1
HIGHLIGHTS
The wastewater from industrial area was treated
by EC via Fe and Al electrodes.
Cu, Ni, Cr and Zn were highly removed at the
rst minutes, simultaneously.
Pseudo-2nd-order was found to be more suitable
for kinetics.
Adsorption capacities based on kinetic modeling
were observed as Cr>Cu>Ni>Zn.
The chemical cost in the case of pH adjustment
after EC was less as 3.83 $/m
3
.
ARTICLE INFO
Article history:
Received 15 April 2019
Revised 25 July 2019
Accepted 2 August 2019
Available online 20 September 2019
Keywords:
Electrochemical treatment
Heavy metals
Kinetic modeling
Pseudo rst order kinetic
Pseudo second order kinetic
GRAPHIC ABSTRACT
ABSTRACT
It is known that wastewater produced by the metal-plating industry contains several heavy metals,
which are acidic in nature and therefore toxic for the environment and for living creatures. In particular,
heavy metals enter the food chain and accumulate in vital organs and cause serious illness. The
precipitation of these metals is mostly achieved by pH adjustment, but as an alternative to this method,
the electrocoagulation process has investigated in this study using iron and aluminum electrodes. The
effects of the pH adjustment on removal before and after the electrocoagulation process were
investigated, and cost analyses were also compared. It was observed that a high proportion of removal
was obtained during the rst minutes of the electrocoagulation process; thus, the current density did
not have a great effect. In addition, the pH adjustment after the electrocoagulation process using iron
electrodes, which are 10%more effective than aluminum electrodes, was found to be much more
efcient than before the electrocoagulation process. In the process where kinetic modeling was
applied, it was observed that the heavy metal removal mechanism was not solely due to the collapse of
heavy metals at high pH values, and with this modeling, it was seen that this mechanism involved
adsorption by iron and aluminum hydroxides formed during the electrocoagulation process. When
comparing the ability of heavy metals to be adsorbed, the sequence was observed to be
Cr>Cu>Ni>Zn, respectively.
© Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
three main groups: cleaning, painting and coating. Organic
solvents, organic or inorganic acids, bases and metal salts
such as nickel and chromium are used in a plating bath to
coat the metal in these industries. Metal-coating industries
constitute an important source of pollution discharged into
the environment due to the use of toxic chemical materials
(Choi et al., 2000). The removal of heavy metals is mainly
achieved chemically. The most common method is to
enable removal at high pH values (Huang et al., 2011; Lee
et al., 2017). However, high costs are encountered due to
the buffering capacity of the metal-coating wastewaters.
Since different pHs are required for each type of metal, the
use of this process on its own has signicant disadvan-
tages. Consequently, it is thought that this necessary
process can be achieved through electrocoagulation, which
is a more highly developed model for chemical precipita-
tion.
The basic principle of electrocoagulation is to produce
coagulants by means of an electric current using iron
electrodes instead of iron salts and using aluminum
electrodes instead of aluminum salts (Ilhan et al., 2008;
Coskun et al., 2012; Qi et al., 2018). The metals dissolved
by the anodic reactions caused by the electric current can
function as coagulants. One of the most important
advantages of the electrocoagulation process is that, in
addition to the chemical coagulation as a result of the
electrolytic reactions during the process, the partial
adsorption, electro-oxidation and electro-otation pro-
cesses are carried out simultaneously by the same
mechanism (Avsar et al., 2007). Because of all these
advantages, electrocoagulation is used in a wide range of
areas (in domestic and industrial wastewater treatment)
(Vasudevan et al., 2008; Vasudevan et al., 2009; Kobya et
al., 2014; Ulucan and Kurt, 2015; Can et al., 2016; Deghles
and Kurt, 2016; Song et al., 2016; Hashim et al., 2018; Xu
et al., 2018; Maher et al., 2019). The pH is one of the most
important operating conditions in the electrocoagulation
process. Adjustment of pH before or after the treatment
should be investigated according to wastewater and the
optimum pH should be determined (Naje et al., 2017;
Ahangarnokolaei et al., 2018). For all these reasons, it can
be seen that the electrocoagulation process can be a very
efcient method to treat metal-plating industrial waste-
waters. Moreover, it is of great importance that not only the
basic cost but also the possible costs of chemicals are
minimized. When iron or aluminum electrodes are used in
the electrocoagulation process, electrolytic dissociation
formed as a result of the electrical voltage results in the
formation of metal hydroxides such as Fe(OH)
3
with the
dissolved iron electrode, and Al(OH)
3
with the aluminum
electrode. The main removal mechanism of pollutants from
the electrocoagulation is by adsorption via the metal
hydroxides produced, which have a very high adsorption
capacity. For this reason, electrocoagulation is currently
used in many places today. During this process, the
electrolytic reactions formed on the electrodes result in
very small gas bubbles from the water, especially in the
cathode area. Considering that these gas bubbles are the
basis of electro-otation, the pollutants can also be
removedfromthewaterbyelectro-otation during
electrocoagulation. In this way, the electrocoagulation
process can also achieve high removal efciencies for the
pollutant concentrations by electro-otation.
The aim of this study was to investigate the removal of
the parameters of COD, chromium (Cr), copper (Cu), zinc
(Zn) and nickel (Ni), which is one of the most important
parameters in treating wastewater from metal plating. The
current density, initial pH and time effects, which are
important parameters affecting the electrocoagulation (EC)
process, were investigated. The effects of pH adjustment
before and after EC were investigated and compared by
cost analysis. At the same time, the removal mechanism to
be introduced was investigated with kinetic model studies.
2 Experimental
2.1 Wastewater characterization
In the experimental studies, industrial wastewater from
metal plating was collected from the entrance channel of
the wastewater treatment plant in the Galvanotechnic
Industrial Site. Wastewaters at this entrance originated
from many industrial sites. Eight-hour composite samples
were collected on a daily basis in order for the samples to
be representative. The characterization of pollutant para-
meters from the collected wastewater is shown in Table 1.
All of the conducted analyzes were performed according to
the methods described by the Standard Methods for the
Examination of Water and Wastewater of the American
Public Health Association (APHA). All heavy metal
analyses in the studies were performed by SM 3111
Table 1 Characteristics of industrial wastewater of raw metal coating
Parameter Wastewater characterization
COD (mg/L) 2100100
Sulfate (mg/L) 150050
Chloride (mg/L) 2500200
Conductivity (mS/cm) 155
Cr (Total) (mg/L) 25010
Cr
+6
(mg/L) 352
Ni (mg/L) 755
Cu (mg/L) 755
Zn (mg/L) 353
Pb (mg/L) <0.1
Fe (mg/L) <0.1
Cd (mg/L) <0.1
pH 1.80.1
2 Front. Environ. Sci. Eng. 2019, 13(5): 73
using a Perkin Elmer-branded Atomic Absorption Spectro-
photometer (Germany). A HACH-branded pH probe
(Germany) was used that could read and record pH and
conductivity values in the system (SM 4500 H
+
-B, 2520-
B), and a KERN PFB 300-branded precision scale
(Germany) was used for preparing solutions and weighing
operations for other processes.
2.2 Electrocoagulation assembly
For electrocoagulation (EC) studies, the reactor volume
was 500 mL and made of plexiglass. In separate studies
two different types of electrodes were used, aluminum and
iron. The space between the two electrodes (one anode and
one cathode) was 6.0 cm, and the tanks were stirred. The
effective area of electrodes in electrocoagulation was
10.5 cm 7.5 cm and 16.5 cm 10.5 cm. A GW Instek,
GPS 3030 DD-branded power supply that could stabilize
the current and voltage between 0 to 6 A and 0 to 20 V,
respectively, was used to power the system. A schematic
diagram based on the apparatus used in the study can be
seen in Fig. 1.
During the conducted studies, the pH value of the
wastewater was recorded by a WTW pH 720-branded pH
meter, before, during and after EC. After the EC, the
wastewaters were left for 1 hour by collecting them in
500 mL measuring cylinders for precipitation to occur.
After the precipitation process, samples were collected
from the supernatant and ltered through a coarse lter and
then placed in sample containers. To adjust the pH in the
conducted studies, 1 N and 6 N of NaOH and H
2
SO
4
solutions prepared from Merck-branded NaOH and H
2
SO
4
chemicals were used.
2.3 Kinetic modeling
The adsorption mechanism was considered to be effective
in the removal of pollutants with the metal hydroxides
formed during the EC process. In this sense, the suitability
of the models that characterize the adsorption process was
investigated. Various kinetic models were used, which
were proposed to determine what kind of mechanism
played a role in the adsorption of the adsorbent material
onto the adsorbent surface. Among these models, a Pseudo
First Order kinetic model and a Pseudo Second Order
model were used.
The Pseudo First Order kinetic model was developed by
Lagergren and expressed as follows (Eq. (1)) (Ho, 2004):
logðqeqtÞ¼logqe
k1
2:303t, (1)
q
e
= amount of adsorbed matter per gram of adsorbent at
equilibrium (mg/g);
q
t
= amount of adsorbed matter per gram of adsorbent at
any time (mg/g);
k
1
= speed constant (min
1
);
t= contact time (min).
The speed constant k
1
was calculated from the slope of
the graph drawn from log (q
e
q
t
) against tand the
theoretical q
e
value was calculated from the break point on
the graph.
The Pseudo Second Order kinetic model was developed
by Ho and expressed as follows (Eq. (2)) (Ho, 2004; Ho,
2006; Qiu et al., 2009):
t
qt
¼1
k2qe2þ1
qe
t, (2)
q
e
= amount of adsorbed matter per gram of adsorbent at
equilibrium (mg/g);
q
t
= amount of adsorbed matter per gram of adsorbent at
any time (mg/g);
k
2
= speed constant (g/mg/min);
k
2
q
e
2
= initial adsorption speed.
The speed constant k
2
and the theoretical q
e
values are
calculated respectively from the slope and breakpoint of
the graph drawn from t/q
t
against t.
3 Results and discussion
3.1 Studies conducted without changing the initial pH
Observing the COD parameter will properly represent the
organic pollution in terms of both reaching the limit values
and examining the efciency of the process applied to
other pollutant parameters. This study investigated the
effects of current density, initial pH and reaction time.
These are the primary parameters affecting the removal of
Cr, Ni, Cu and Zn metals during the electrocoagulation
process, especially in a high concentration of wastewater
and in COD removal. In this part of the study, the removal
efciencies of the pollutants were calculated without
changing the pH of the efuent samples.
Fig. 1 Schematic diagram of electrocoagulation process
(1: EC Cell, 2: Electrodes, 3: Power Supply, 4: Magnetic Stirrer,
5: pH meter).
Fatih Ilhan et al. Kinetic modelling of electrocoagulation process for metal plating wastewater 3
Firstly, the effect of current density without changing the
initial pH of the wastewater was investigated. COD
removal efciencies for both iron electrodes and aluminum
electrodes for current densities from 50 to 175 A/m
2
are
shown in Fig. 2. In this study, the reaction time was taken
to be 15 minutes.
As understood from Fig. 1, there is no major change in
the removal efciencies by increasing the current density.
Approximately 80%removal efciencies were achieved
for the iron electrodes, and 70%for the aluminum
electrodes. At 100 A/m
2
,efuent removal rates were
the highest for both electrodes, so a current density of
100 A/m
2
was selected as the optimum value to be used in
further studies. Changes in pH values were investigated
after the EC process without changing the raw wastewater
pH value. The results obtained are shown in Fig. 3. The
wastewater pH value of 1.8 before the EC process,
increased up to 4.08 by the end after 40 minutes, and the
efuent pH for the aluminum electrode was 5.8 when the
iron electrode was used. The relatively high pH values
obtained with the aluminum electrode reveal more
advantageous results for the precipitation of heavy metals
after EC.
The reaction time of the EC process between 1 and
40 minutes was investigated for both the iron and
aluminum electrodes in order to investigate the effect of
the reaction time, which is another important parameter
(Fig. 3). This study was conducted without changing the
initial pH. Figure 4 shows that removal efciencies with
the iron electrode are more efcient, with an average of
between 10%and 15%more than with aluminum
electrodes. Optimum results were obtained in the rst
minutes of the EC study. At the end of the rst minute,
80%removal efciency was achieved with iron electrodes
and 68.6%with aluminum electrodes. Although 84.8%
was reached with the iron electrodes after 30 minutes, it
can not be considered the optimum value in terms of
economy. It can be said that the high removal efciency
during the rst minutes is due to the continued adsorption
of pollutants by the metal hydroxides produced by EC
during the precipitation process. The effective process here
is considered to be the adsorption onto the metal
hydroxide.
The common removal method of heavy metals is carried
out by precipitation in the form of metal hydroxides and
high pH. The most important operating parameter for metal
removal is considered to be the pH. It is very difcult to
remove metals at low pH values. As a result, the study was
conducted to show both the effect of low pH and the effect
of time on the removal of metals, as shown in Fig. 5. The
removal of Cu, Ni, Zn and Cr was noted.
Considering the characterization of wastewater, one of
the most common heavy metals is copper. In this study, the
electrocoagulation process was performed at the original
pH (~1.8) using commonly used iron and aluminum
electrodes. When Fig. 4(a) is analyzed, it can observed that
although the same results were obtained for both electrodes
in the rst 15 minutes, removal rates at an 80%level were
achieved for iron electrodes for this process, whereas for
aluminum electrodes, the efciency is at a 50%level.
In the facility where the wastewater is generated, nickel-
plating processes are carried out intensively. In this case,
nickel forms part of the wastewater. In the studies
conducted, both the iron and aluminum electrodes were
used to analyze the ability to remove nickel simultaneously
with other components. The results obtained are given in
Fig. 5(b).
Fig. 2 The effect of current density on COD removal for iron and
aluminum electrode (Reaction time 15 min, pH 1.8, sample
volume 500 mL).
Fig. 3 pH efuent values at the end of EC applied with original
initial pH.
Fig. 4 The effect of electrode type on COD removal with
electrocoagulation (V: 500 mL, pH; ~1.8, Current density:
100 A/m
2
).
4 Front. Environ. Sci. Eng. 2019, 13(5): 73
When Fig. 5(b) is examined, it can be observed that a
removal of Ni at a 50%level could be achieved. However,
it should be noted that the original pH (pH ~1.8) value of
the plant was used in this study. As a matter of fact, the
metals are treated by using their ability to precipitate in the
form of metal hydroxides, especially when the pH value is
almost over 11. Therefore, studies were carried out at the
optimum pH values (partially neutral) required for
electrocoagulation and at the high pH values applied for
metal removal.
Chromium is different from conventional heavy metals
and the treatment is a two-stage process. According to the
conventional chromium removal process, Cr
6+
s, which is
in a dangerous form in the water must be converted into
Cr
3+
by a controlled process. While this rst stage is
carried out at low pH values, Cr
3+
s must be precipitated in
the second stage. The need for high pH values in this case
makes heavy metal removal extra difcult. In electro-
coagulation, it is aimed to perform these two operations in
one stage. The results obtained are demonstrated in
Fig. 5(c). Total chromium removal efciencies for
aluminum and iron electrodes used for the different types
are shown in Fig. 5(c). Over 90%removal efciency can
be achieved especially with the use of iron electrodes. The
main reason for this can be explained by the fact that the
raw pH of the wastewater is about 2 as a result of the
electrolytic reactions, and therefore Cr
6+
is converted to
Cr
3+
in a very short time due to the ferric ions given to the
water by the iron electrode. In this way, it is possible to
remove Cr
3+
only by increasing the pH or with the
electrocoagulation process.
As in other metal types, zinc is also present in high
concentration in the raw wastewater emerging from the
plating workshops and arriving at the treatment plant due
to the intensive zinc-coating operations. The study stages
for the removal of zinc were carried out for the other metal
parameters. Figure 5(d) shows the effects of different
electrode types on the removal of zinc. Iron and aluminum
electrodes show signicant similarity with respect to
efciency. With both types of electrodes, a removal rate
of 50%could be achieved. It should be noted, however,
that this study was carried out at the original pH and
without any pH adjustment. It is thought that the efciency
can be increased with pH adjustment.
3.2 The effect of pH adjustment on COD removal efciency
The electrocoagulation process is based on the chemical
precipitation mechanism. Simultaneously, the adsorption,
oxidation and otation mechanisms are partially effective,
resulting in effective removal of different pollutants. Each
of these mechanisms alone is particularly closely related to
the pH parameter. In addition to the previous studies
performed with the original pH, the initial pH values were
Fig. 5 The effect of electrode type on (a) Copper, (b) Nickel, (c) Total Chromium and (d) Zinc removal with electrocoagulation
(V: 500 mL, pH; ~1.8, Current density: 100 A/m
2
).
Fatih Ilhan et al. Kinetic modelling of electrocoagulation process for metal plating wastewater 5
adjusted and their effects on the efciency were investi-
gated at this stage of the study.
In the previous section, the removal efciencies after the
EC process were given and no pH adjustment was
performed after the EC process. In this part of the study,
the pH adjustment before the EC process and the pH
adjustment after the EC process were compared. Iron
electrodes were selected for the study as they showed more
effective results; these results are given in Table 2. The
initial and efuent pH values were adjusted to a pH value
from 6 to 12 at which metals could be precipitated.
Although there was a partial increase in the efuent pH
values due to the electrocoagulation mechanism itself, an
additional pH adjustment was implemented. When Table 2
is examined, it is observed that higher removal rates are
achieved with the adjustment of pH after EC. When the
results obtained for pH 8 in the table are examined, it is
found that the removal of COD, Zn, Cr, Ni and Cu were
achieved at levels of 45.6%, 98.3%,95%,56%and 26.6%,
respectively, with the adjustment of pH before the EC
process, and levels of 67%, 93.9%, 96.9%,72%and 91.7%
were achieved when the pH was adjusted after EC. In
general, the removal efciencies are higher with the pH
adjustment after EC. The major factor here is that the metal
hydroxides formed in the EC process adsorb the metals.
3.3 Kinetic modeling
The studies conducted show that removal can be achieved
by the adsorption of metals on the metal hydroxides
produced by the EC process as well as the precipitation of
metals by pH adjustment in the removal of COD. Kinetic
modeling was applied from this perspective in order to
prove this mechanism. Table 3 shows the results of Pseudo
rst order and Pseudo second order kinetic models.
Considering the regression coefcients, the R
2
s obtained
for both kinetic models are observed to be quite high. The
Pseudo second order kinetic model can be said to be
compatible for the removal of both COD and metals as the
R
2
values of the Pseudo second order kinetic model were
higher. This case shows that the metals are removed by
chemisorption (Ho and McKay, 1998; Kumar et al., 2010).
In addition, the adsorption capacities of iron hydroxides
produced in the EC process are given in Table 3. When
adsorption capacities are compared, it is observed that the
highest adsorbed pollutant parameter is COD. When heavy
metals are compared, Cr had the highest capacity to be
adsorbed and was followed by Cu>Ni>Zn.
Al-Shannag et al. applied kinetic modeling in their study
on electrocoagulation treatment of metal-plating water
treatment (Al-Shannag et al., 2015). According to the
regression coefcient neither the rst order nor the second
order kinetic models were applicable for Cr, Ni and Zn,
whereas copper ions were suitable with rst order kinetics.
However, the study showed that the rst order pseudo
kinetics matched all metal ions. For Cu, Ni and Zn, the
obtained k
1
coefcients were 0.053, 0.113, 0.099 min
1
,
respectively. In the study by Kim et al., the removal of
metal-plating wastewater with PAC and modied PAC was
examined (Kim et al., 2018). According to the kinetic
results, both adsorbents showed that the Pseudo second
order kinetic modeling was suitable and also claimed that
the adsorption process was performed by chemisorption.
3.4 Energy consumption of electrocoagulation process
The total electrical and chemical costs of the process were
calculated and the pH adjustments before and after EC
were also compared. As the iron electrodes were more
efcient, calculations were made for the iron electrodes. A
current of 1.13 A and a constant voltage of 17 V were used
for the optimum current density 100 A/m
2
. The optimum
reaction time was chosen to be 15 minutes and the
calculations were carried out in a way to adjust the pH
value of the wastewater to be 8.
The operational cost of the electrocoagulation, accord-
ing to the steps of this study, can be given as follows:
EC operation cost: ðIVtÞ=V¼1:13 17 ð15=
=60Þ=0:5L¼9.61 W/L of wastewater = 9.61 kW/m
3
of
wastewater.
Considering that the prescribed unit electricity con-
sumption price for industrial or commercial facilities is
0.066 $/kWh, the cost for 1 m
3
of wastewater treatment is
as follows:
Table 2 The effect of pH adjustment before EC and pH adjustment after EC on the pollutant removal (V: 500 mL, Reaction time: 15 min, Iron
electrode, Current Density: 100 A/m
2
)
pH
COD (%)Cu(%)Ni(%) Total Cr (%)Zn(%)
Before EC After EC Before EC After EC Before EC After EC Before EC After EC Before EC After EC
6 55.3 65 30.4 90.5 49.6 47.8 97.7 97.6 94 91.1
7 50 65 26.5 90.9 46.2 65.7 97.3 97.3 98.1 93.6
8 45.5 67 26.6 91.7 56 72 95 96.9 98.3 93.9
9 38 69 19.8 92.5 28.4 72.3 94 96.4 97.7 94.2
10 30.1 89 17.9 93 27.8 79.2 93 96.5 98.9 94.4
12 32.9 89 66.4 93 61.6 79 93.9 96.4 96 95.1
6 Front. Environ. Sci. Eng. 2019, 13(5): 73
EC operational cost = 9.61 kW/m
3
0.066 $/kW =
0.634 $/ m
3
.
The pH value for the raw water used for EC was
conrmed to be 1.8. A 5.3 mL amount of 6 N NaOH
solution was used to change the pH before EC to a pH of 8.
To change the pH after EC to a pH of 8, 4.8 mL of 6 N
NaOH solution was used. According to this, while the
amount of caustic wastewater that is used pre-EC and to
increase the pH value to 8, is 1.27 kg/m
3
, the amount of
caustic wastewater used post-EC was 1.15 kg/m
3
.
As the unit price of experimental quality NaOH is
1 $/kg, the cost required to increase the pH value of 1 m
3
of
wastewater to 8 is 1.27 $/m
3
for pre-EC and 1.15 $/m
3
for
post-EC. Accordingly, the total cost for the process
conducted with the pH adjustment before and after EC is
1.90 $/m
3
and 1.78 $/m
3
. These values are approximately 4
to 5 times lower than published literature (Rodriguez et al.,
2007; Oden and Sari-Erkan, 2018).
4 Conclusions
As the wastewater exhibits oxidation mechanisms, ota-
tion mechanisms and the formation of ocs, their presence
provides an important environment for other processes to
be carried out after the EC process. The most important of
these is the chemical consumption required for pH
adjustment to meet discharge standards. The current
density, pH and the reaction time, which are important
parameters for EC, were studied and it was observed that
current density had no signicant effect on metal removal.
One of the expected efuents increases the initial pH of the
EC process. When the iron and aluminum electrodes were
compared, iron was excellent in removing chromium and
copper. Furthermore, while aluminum and iron electrodes
had a similar effect in the removal of Zn with a 50%
removal efciency, both electrodes were weak in eliminat-
ing Ni with less than a 40%removal efciency. In addition,
they were found to be very effective in removing both
COD and heavy metals even over very short periods (1
minute). When kinetic modeling was applied, pseudo
second order kinetics was found to be more suitable. This
proves that the metal hydroxides formed during the EC
process remove heavy metals by adsorption. The relative
rates of adsorption of the heavy metals was seen to be
Cr>Cu>Ni>Zn. According to the cost analysis, it was
found that the pH adjustment after the EC process
decreased the chemical use and was 1.78 $/m
3
.
Acknowledgements The authors received research grants from the
Research Fund of the Republic of Turkey Ministry of Industry and
Technology with a Project Number 0274.STZ-2013-2.
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Table 3 Pseudo 1st order and Pseudo 2nd order kinetic modeling for EC prosess
Parameter Pseudo 1st order Pseudo 2nd order
R
2
q
e1
(mg/g) k
1
(min
1
)R
2
q
e2
(mg/g) k
2
(g/mg/min)
COD 0.936 9418.84 0.993 0.978 282.3 0.00002
Cu 0.936 666.04 0.993 0.976 19.36 0.0003
Ni 0.968 578.94 0.993 0.972 18.21 0.0003
Cr 0.956 2259.01 0.993 0.957 58.96 0.0001
Zn 0.950 419.00 0.992 0.967 11.59 0.0004
Fatih Ilhan et al. Kinetic modelling of electrocoagulation process for metal plating wastewater 7
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8 Front. Environ. Sci. Eng. 2019, 13(5): 73
... To determine the energy required for removing Congo red dye from simulated water by using a combined system (EC-EO), by using the composite electrodes (C and A) as anode for the EO process besides Al as anode for the EC process, Table 6 would represent the energy consumption for the cathode (C) and Table 7 would represent the energy consumption for the anode (A), Eq. 12 was used to calculate energy consumption [54]. ...
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The combined system of electrocoagulation (EC) and electro-oxidation (EO) is one of the most promising methods in dye removal. In this work, a solution of 200 mg/l of Congo red was used to examine the removal of anionic dye using an EC-EO system with three stainless steel electrodes as the auxiliary electrodes and an aluminum electrode as anode for the EC process, Cu-Mn-Ni Nanocomposite as anode for the EO process. This composite oxide was simultaneously synthesized by anodic and cathodic deposition of Cu (NO3)2, MnCl2, and Ni (NO3)2 salts with 0.075 M as concentrations of each salt with a fixed molar ratio (1:1:1) at a constant current density of 25 mA/cm 2. The characteristics structure and surface morphology of the deposited nano oxides onto the graphite substrates were determined by (XRD), (FE-SEM), (AFM), and (EDX). The results shown that nano Cu-Mn-Ni oxides were successfully deposited onto the anode and cathode. The crystal size and root mean square for the cathode were 30.79 nm and 79.36 nm, respectively, while for the anode, they were 24.19 nm and 41.88 nm, respectively. Furthermore, the combined system was examined for C.D, NaCl concentration, and time. In the EC-EO combined system, the cathode and anode were efficient when used as anodes for the EO process, besides aluminum. The cathode was more effective in the removal process than the anode due to its larger crystal size and the rough, granular shape of its surface. When current density (C.D) increased from 3 to 6 mA/cm², the removal efficiency shifted from 95% to 98%. However, excellent removal of 98% and 96.5% was attained with 1.665 and 2.0859 kWh/kg of dye as energy consumption in the presence and absence of NaCl salt, respectively by applying 6 mA/cm 2 within 20 min of electrolysis.
... Hydrogen gas is liberated from the water in the cathode zone in the form of tiny gas bubbles, which are the basis of the electroflotation (EF) process, allowing the pollutants to be removed from water during the EC process. Accordingly, the EF process makes the EC process perform better to remove the contaminants [26]. ...
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Combining different treatment strategies successively or simultaneously has become recommended to achieve high purification standards for the treated discharged water. The current work focused on combining electrocoagulation, ion-exchange, and ultrasonication treatment approaches for the simultaneous removal of copper, nickel, and zinc ions from water. The removal of the three studied ions was significantly enhanced by increasing the power density (4–10 mA/cm2) and NaCl salt concentration (0.5–1.5 g/L) at a natural solution pH. The simultaneous removal of these metal ions at 4 mA/cm2 and 1 g NaCl/L was highly improved by introducing 1 g/L of mordenite zeolite as an ion-exchanger. A remarkable removal of heavy metals was reported, as the initial concentration of each metal decreased from approximately 50 ppm to 1.19 for nickel, 3.06 for zinc, and less than 1 ppm for copper. In contrast, ultrasonication did not show any improvement in the treatment process. The extended Langmuir isotherm model convincingly described the experimental data; the Temkin and Dubinin-Radushkevich isotherm models have proven that the removal processes were physical and exothermic. Finally, the pseudo-second-order kinetics model appropriately explained the kinetics of the process with correlation coefficients of 0.9337 and 0.9016, respectively.
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... g/L of NaCl, pH = 7 after 20 min from the operation start. Equation (15) was used to calculate energy consumption depending on CR removal [75]. ...
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Removing Congo red (CR) is critical in wastewater treatment. We introduce a combination of electrocoagulation (EC) and electro-oxidation (EO) to address the elimination of CR. We also discuss the deposition of triple oxides (Cu–Mn–Ni) simultaneously on both anodic and cathodic graphite electrodes at constant current density. These electrodes efficiently worked as anodes in the EC-EO system. The EC-CO combination eliminated around 98 % of the CR dye and about 95 % of the Chemical Oxygen demand (COD), and similar results were obtained with the absence of NaCl. Thus, EC-EO is a promising technique to remove CR in an environmentally friendly pathway.
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Water pollution by metals is increasing. An electrode material, copper hexacyanoferrate (CuHCF), for capacitive deionization (CDI) to remove Co2+ from wastewater was developed. The asymmetric electrode structure of the CDI overcomes the limitations of traditional activated carbon electrodes (low ion removal capacity, ion removal rate, and charge efficiency). The specific capacitance of the CuHCF electrode (216.7 F g−1) at 1 mV s−1 was higher than that of a traditional electrode (92.0 F g−1), and its deionization capacity was 156.85 mg g−1 in 50 mg L−1 aqueous Co2+ (charge efficiency = 68.4%). CuHCF is a high-performance electrode material for wastewater remediation.
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A new batch, flow column electrocoagulation reactor (FCER) that utilises a perforated plate flow column as a mixer has been used to remove fluoride from drinking water. A comprehensive study has been carried out to assess its performance. The efficiency of fluoride removal (R%) as a function of key operational parameters such as initial pH, detention time (t), current density (CD), inter-electrode distance (ID) and initial concentration (C0) has been examined and an empirical model has been developed. A scanning electron microscopy (SEM) investigation of the influence of the EC process on morphology of the surface of the aluminium electrodes, showed the erosion caused by aluminium loss. A preliminary estimation of the reactor's operating cost is suggested, allowing for the energy from recycling of hydrogen gas hydrogen gas produced amount.
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A number of recent studies have demonstrated that electrochemical technologies, including electroreduction (ER), electrocoagulation (EC), and electrodialysis (ED), are effective in nitrate elimination in wastewater due to their high reactivity. To obtain the maximal elimination efficiency and current efficiency, many researchers have conducted experiments to investigate the optimal conditions (i.e., potential, current density, pH value, plate distance, initial nitrate concentration, electrolyte, and other factors) for nitrate elimination. The mechanism of ER, EC and ED for nitrate removal has been fully elucidated. The ER mechanism of nitrate undergoes electron transfer and hydrogenation reduction. The EC pathways of nitrate removal include reduction, coagulation and flotation. The ED pathways of nitrate include redox reaction and dialysis. Although the electrochemical technology can remove nitrate from wastewater efficiently, many problems (such as relatively low selectivity toward nitrogen, sludge production and brine generation) still hinder electrochemical treatment implementation. This paper critically presents an overview of the current state-of-the-art of electrochemical denitrification to enhance the removal efficiency and overcome the shortages, and will significantly improve the understanding of the detailed processes and mechanisms of nitrate removal by electrochemical treatment and provide useful information to scientific research and actual practice. Open image in new window
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In this study, the characterizations and adsorption efficiencies for chromium, copper and nickel were evaluated using manufacture-grade Fe2O3-carbon foam. SEM, XRD, XRF and BET analyses were performed to determine the characteristics of the material. Various pore sizes (12–420 μm) and iron contents (3.62%) were found on the surface of the Fe2O3-carbon foam. Fe2O3-carbon foam was found to have excellent adsorption efficiency compared to carbon foam for mixed solutions of cationic and anionic heavy metals. The adsorption capacities for chromium, copper and nickel were 6.7, 3.8 and 6.4 mg/g, respectively, which were obtained using a dual exponential adsorption model. In experiments with varying dosages of the Fe2O3 powder, no notable differences were observed in the removal efficiency. In a fixed-bed column test, Fe2O3-carbon foam achieved adsorption capacities for chromium, copper and nickel of 33.0, 12.0 and 9.5 mg/g, respectively, after 104 h. Based on these results, Fe2O3-carbon foam was observed to be a promising material for treatment of plating wastewater.