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Heavy metal ions removal from metal plating wastewater
using electrocoagulation: Kinetic study and process performance
Mohammad Al-Shannag
a
, Zakaria Al-Qodah
b,
⇑
, Khalid Bani-Melhem
c
, Mohammed Rasool Qtaishat
a
,
Malek Alkasrawi
d
a
Chemical Engineering Department, Faculty of Engineering and Technology, The University of Jordan, 11942 Amman, Jordan
b
Chemical Engineering Department, Taibah University, Saudi Arabia
c
Department of Water Management and Environment, Faculty of Natural Resources and Environment, Hashemite University, Al-Zarqa, Jordan
d
Department of Paper Science and Engineering, Faculty of Natural Resources, University of Wisconsin Stevens Point, Stevens Point, WI 54481, USA
highlights
High removal of heavy metal ions from metal plating wastewater using EC treatment.
Pseudo first-order kinetic model describes heavy metal ions removal adequately.
Electrocoagulation time and DC current density are the key parameters in EC process.
Metal plating wastewater treatment by electrocoagulation is economically rewarding.
article info
Article history:
Received 15 July 2014
Received in revised form 8 September 2014
Accepted 9 September 2014
Available online 21 September 2014
Keywords:
Electrocoagulation
Heavy metal ions
Metal plating wastewater
Iron electrodes
abstract
The main objective of the present study was the removal of heavy metal ions, namely Cu
2+
,Cr
3+
,Ni
2+
and
Zn
2+
, from metal plating wastewater using electrocoagulation technique. An electro-reactor was used
with six carbon steel electrodes of monopolar configurations. Three of the electrodes were designated
as cathodes meanwhile the other three as anodes. The results showed that the removal efficiency of
heavy metal ions increases with increasing both electrocoagulation (EC) residence time and direct cur-
rent (DC) density. Over 97% of heavy metal ions were removed efficiently by conducting the EC treatment
at current density (CD) of 4 mA/cm
2
, pH of 9.56 and EC time of 45 min. These operating conditions led to
specific energy consumption and specific amount of dissolved electrodes of around 6.25 kWh/m
3
and
1.31 kg/m
3
, respectively. The process of metal plating removal using EC consumes low amount of energy,
making the process economically feasible and possible to scale up. Moreover, the kinetic study
demonstrated that the removal of such heavy metal ions follows pseudo first-order model with cur-
rent-dependent parameters.
Ó2014 Elsevier B.V. All rights reserved.
1. Introduction
Metal plating industry is one of the major chemical processes
that discard large amounts of wastewaters. These industrial waste-
waters contain various types of harmful heavy metals and toxic
substances such as chromium, nickel, copper, zinc, cyanide and
degreasing solvents [1]. Numerous approaches such as physical,
chemical and biological processes including adsorption, biosorp-
tion, precipitation, ion-exchange, reverse osmosis, filtration and
other membrane separations are employed to treat wastewaters
[2]. Precipitation of heavy metals in an insoluble form of hydrox-
ides is the most effective and economical method to treat heavy
metals wastewater. The main idea of precipitation method is to
adjust the pH of wastewater and to add chemical coagulants like
aluminum or iron salts to remove pollutants as colloidal matter
[3]. The precipitation typically occurs according to the following
reaction:
M
þn
ðaqÞ
þnOH
ðaqÞ
$MðOHÞ
nðsÞ
ð1Þ
Although the chemical coagulation technique is considered to
be effective in treating industrial wastewater effluents, it has quite
high cost. On the other hand, the addition of chemical coagulants
to the wastewater may produce side-products that are considered
as secondary pollutants [4]. Alternatively, electrocoagulation (EC)
http://dx.doi.org/10.1016/j.cej.2014.09.035
1385-8947/Ó2014 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Tel.: +966 560948161.
E-mail addresses: z_alqodah@hotmail.com,zqudah@taibahu.edu.sa
(Z. Al-Qodah).
Chemical Engineering Journal 260 (2015) 749–756
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
was found to be an effective technique for precipitating industrial
wastewater pollutants [5,6]. The simplicity of EC operation, low
energy consumption, high quality effluent, low sludge formation
and low dissolved solids made electrocoagulation a desirable treat-
ment method [5,7,8].
In electrocoagulation process, no chemicals are added to form
coagulant agents. Basically, wastewater solution is subjected to a
direct electrical (DC) current field through sacrificial electrodes
(cathodes and anodes) that are generally made of iron or alumi-
num [1,5,6]. Though it is traditional to use solid flat electrodes,
cylindrical perforated ones are adopted in some previous studies
to have better distribution of the applied DC field onto the
wastewater treated [9,10]. Due to electrical potential difference
between cathodic and anodic electrodes in electrocoagulation,
water is oxidized to produce hydrogen ions (H
+
) and oxygen
gas and the metal oxidation will generate its cations. Simulta-
neously, water reduction occurs at the cathode to generate
hydroxyl ions (OH
) and hydrogen gas. For iron-iron electrodes,
as in the present study, two ferric hydroxides, Fe(OH)
2
and
Fe(OH)
3
are produced according to the following electrolytic
reactions [11,12]:
Fe
ðsÞ
$Fe
2þ
ðaqÞ
þ2e
ð2Þ
2H
2
O
ð1Þ
þ2e
$H
2ðgÞ
þ2OH
ðaqÞ
ð3Þ
Fe
2þ
ðaqÞ
þ2HO
ðaqÞ
$FeðOHÞ
2ðsÞ
ð4Þ
Overall : Fe
ðs
Þþ2H
2
O
ð1Þ
$FeðOHÞ
2ðsÞ
þH
2ðgÞ
ð5Þ
4Fe
ðsÞ
$4Fe
2þ
ðaqÞ
þ8e
ð6Þ
8H
þ
ðaqÞ
þ8e
$4H
2ðsÞ
ð7Þ
4Fe
2þ
ðaqÞ
þ10H
2
O
ð1Þ
þO
2ðsÞ
$4FeðOHÞ
3ðsÞ
þ8H
þ
ðaqÞ
ð8Þ
Overall : 4Fe
ðsÞ
þ10H
2
O
ð1Þ
þO
2ðsÞ
$4FeðOHÞ
3ðgÞ
þ4H
2ðsÞ
ð9Þ
The generated ferric hydroxide flocs serve as coagulant agents
that can precipitate various wastewater pollutants. It is reported
that Fe(III) hydroxide coagulants are more effective than Fe(II)
hydroxide due to the higher stability of Fe(OH)
3
[13]. There are
many physiochemical phenomena involved in electrocoagulation
that can be summarized as [6]: (i) anodic oxidation and cathodic
reduction, (ii) generation and migration of flocculating agents in
the aqueous phase (iii) coagulation and adsorption of pollutants
on flocculating agents and (iv) electroflotation or sedimentation
of coagulated aggregates. In order to achieve optimal treatment
effectiveness, the chemical/physical properties of wastewater must
be monitored during the EC operation.
Electrocoagulation has been successfully applied for the treat-
ment of different types of wastewater generated from municipal
wastewater [4,10], pulp and paper mill industries [12,14], olive
mills [15], textile processing [16], potato chips manufacturing
[17], baker’s yeast production [18] and pigments industries
[13,19]. Several studies have proved the high efficiency of
electrocoagulation in the removal of heavy metal ions from
industrial/synthesis wastewater [1,20–22]. Unlike these studies,
the present work investigated simultaneous removal of chro-
mium (Cr
3+
), copper (Cu
2+
), nickel (Ni
2+
) and zinc (Zn
2+
) ions
from metal plating wastewater using electrocoagulation (EC)
technique. In addition, a kinetic study was conducted for the
first time to describe the removal rates of heavy metal ions.
The impact of EC time, direct current density, pH and electrical
conductivity (
r
) on the heavy metal ions removal by electroco-
agulation was investigated. Finally, the consumption levels of
both electrical energy and electrode material were assessed at
different operating conditions to demonstrate qualitatively the
cost-effective features.
2. Materials and methods
2.1. Experimental setup
Fig. 1 shows the schematic diagram of the electrocoagulation
(EC) laboratory scale setup. The EC reactor was constructed from
Pyrex glass with dimensions of 120 mm 112 mm 89 mm. Iron
(carbon steel) plates were used as sacrificial electrodes, arranged in
monopolar configurations. Six electrodes were positioned verti-
cally with spaces of 15 mm. Three plates were connected as cath-
odes and the other three as anodes. The plates have rectangular
geometry with the dimensions of 45 mm 53 mm 3 mm. The
total effective surface area of electrodes immersed in wastewater
solution was around 247.5 cm
2
. The electrodes were connected
to a direct current (DC) power supply providing voltage in the
range of 0–30 V and electrical current in the range of 0–6 A. During
electrocoagulation experiments, the solution was agitated continu-
ously using mechanical mixer (Stuart Scientific, UK) with rota-
tional speed of about 1000 rpm.
2.2. Experimental procedure
The metal plating wastewater samples were collected from the
Union Locks Company/Sayegh Group located in the region of Abu-
Alanda, Amman, Jordan. The physical and chemical characteristics
of the metal plating wastewater used in this study are listed in
Table 1. The EC reactor shown in Fig. 1 was filled with nearly
600 ml of the wastewater solution to run out the electrocoagula-
tion experiments. The DC was adjusted to give the desired current
density (CD) which is defined as the ratio of the applied direct cur-
rent to the total effective surface area of electrodes. After each
experiment, the EC reactor was rinsed with diluted HCl, followed
by frequent distilled water washes. Before analyzing the
concentrations of the heavy metal ions, the original and treated
Fig. 1. Schematic diagram of experimental setup: (1) DC power supply; (2)
cathode; (3) anode; (4) mechanical stirrer; (5) carbon steel electrodes; (6) EC
reactor.
750 M. Al-Shannag et al. / Chemical Engineering Journal 260 (2015) 749–756
wastewater was filtrated using filtration papers (0.45
l
m, Milli-
pore, USA). Samples of the filtrate were taken to measure Cr
3+
,
Cu
2+
,Ni
2+
and Zn
2+
ions concentrations using atomic absorption
spectrophotometer. Lamotte (CON 6) conductivity meter (Model
4071) was employed to determine the electrical conductivity (
r
)
of wastewater. In addition, the pH of wastewater was adjusted to
the desirable value using either 0.1 M NaOH or 0.1 M H
2
SO
4
. All
experimental runs were carried out at the ambient temperature
of around 26 ± 1 °C. Each experiment was performed in triplicate
to rule out the uncertainty in the measurements.
3. Results and discussion
The heavy metal ions removal was measured in terms of
percent removal efficiency defined as:
g
ð%Þ¼100 C
0
C
C
0
%ð10Þ
where C
0
and Care the concentrations of Cr
3+
,Cu
2+
,Zn
2+
or Ni
2+
in
the original wastewater sample and in the treated one at the given
EC time (t), respectively.
3.1. Effect of electrical conductivity on the removal of heavy metal ions
It is well known that electrical conductivity is a key parameter
that significantly affects heavy metal ions removal from wastewa-
ter using electrocoagulation process. This was supported by the
study of Akbal et al. [1] for the removal of copper, chromium and
nickel heavy metal ions from metal plating wastewater using EC
process. They found that the removal efficiency was strongly
increased with increasing electrical conductivity from 2 (original
wastewater) to 6 mS/cm. However in the present study, the effect
of electrical conductivity on the removal of Cr
3+
,Cu
2+
,Zn
2+
and Ni
2+
was not noticeable when the electrical conductivity increased from
8.9 mS/cm (original wastewater) to 12.0 mS/cm using sodium
chloride salt as shown in Table 2. This leads to the conclusion that
there is no need to adjust the electrical conductivity of metal plat-
ing wastewater above 8.9 mS/cm in order to enhance the EC per-
formance. This result confirms that the contribution of electrical
conductivity on EC performance seems to be negligible at high lev-
els which explains the disagreement between the trend of this
study and the corresponding one reported by Akbal et al. [1].
3.2. Effect of pH on the removal of heavy metal ions
Previous studies have reported strong dependency between the
performance of electrocoagulation and the pH of wastewater
[1,10,12,20–23]. Therefore, the influence of the pH upon the heavy
metal ions removal from metal plating wastewater was investi-
gated. The initial pH of the original metal plating wastewater
was 9.56; see Table 1. Three pH values: 6.56, 7.89 and 10.68 were
considered. Fig. 2 shows the impact of pH variations on the
removal efficiencies of heavy metal ions with and without electro-
coagulation. Fig. 2(a) illustrates the effect of pH on the removal
efficiency of Cr
3+
,Ni
2+
,Cu
2+
and Zn
2+
ions from wastewater sam-
ples without electrocoagulation. For such samples, Fig. 2(a) shows
no significant effect of pH on the removal of copper and nickel ions.
However, the removal efficiencies of chromium ions were 46% and
10% at pH = 6.56 and 10.68, respectively. At pH = 6.56, the removal
efficiency of zinc ions was 38% and it was negligible at pH = 10.68.
Hence, it can be concluded that the precipitate formed in metal
plating wastewater of nearly neutral pH has the potential to
remove Cr
3+
and Zn
2+
heavy metal ions effectively.
In fact, the dependency of heavy metals ions removal on pH will
differ when EC technique is applied. In order to understand the dif-
ferent reaction mechanisms that may occur when iron is used as
electrodes in the designed EC process, the theory of electrocoagu-
lation by iron anode needs to be highlighted. Many researchers
reported the reaction mechanisms that occur in electrocoagulation
Table 1
Some chemical/physical characteristics of metal plating wastewater obtained from
Union Locks Company/Sayegh Group located in Abu-Alanda zone, Amman, Jordan.
Parameter Unit Value
pH – 9.6 ± 0.3
Cr
3+
ppm 93.2 ± 2.2
Cu
2+
ppm 33.3 ± 1.1
Ni
2+
ppm 57.6 ± 1.2
Zn
2+
ppm 20.4 ± 0.8
Electrical conductivity (
r
) ms/cm 8.9 ± 0.2
Color – Yellow
Table 2
Heavy metal ions removal efficiencies at different electrical conductivities (
r
) after
one-hour of EC treatment of metal plating wastewater with CD = 4 mA/cm
2
, solution
volume = 600 ml and pH = 9.6.
r
(mS/cm)
g
exp
(%)
Cr
3+
Cu
2+
Ni
2+
Zn
2+
8.9 (original wastewater) 100 99 98 99
10.3 100 100 99 100
11.1 99 99 98 99
12.0 100 99 98 99
(a)
η (%)
0
20
40
60
80
100
Cr3+
Cu2+
Ni2+
Zn2+
6.56 7.89 9.56 10.68
pH
(b)
η (%)
0
20
40
60
80
100
Cr3+
Cu2+
Ni2+
Zn2+
6.56 7.89 9.56 10.68
pH
Fig. 2. Variations of removal efficiencies of heavy metal ions with pH: (a) without
electrocoagulation; (b) with one-hour EC treatment under applied current density
of 4 mA/cm
2
. The metal plating wastewater volume is 600 ml and its
r
= 8.9 mS/cm.
M. Al-Shannag et al. / Chemical Engineering Journal 260 (2015) 749–756 751
with iron [5,23–24]. It is recommended to refer to the study
reported by Moreno-Casillas et al. [23] in which they described
in details the mechanisms of different reactions at different pH
together with a description of the solution’s color changes.
Accordingly, Fig. 2(b) presents the pH effect on metal ion con-
centrations and their removal efficiencies after one-hour of EC
treatment with DC = 4 mA/cm
2
. It is clear in the figure that the
maximum removal efficiencies for all heavy metal ions occur at
pH = 7.89 and 9.56. Lower removal efficiencies were obtained at
pH = 6.56 and 10.68. This indicates that the heavy metal ions
removal decreases in lower acidic and higher basic mediums. In
alkaline medium, the oxidation of hydroxyl ions at the anode and
the formation of Fe(OH)
4
and Fe(OH)
6
3
anions lowered the
removing capacity [5]. In strongly acidic medium, the protons in
the solution were reduced to hydrogen gas at the cathode and
the same proportion of hydroxyl ions could not be produced [5].
In addition, the pH affects the EC performance through varying
the solution physiochemical properties, such as the solubility of
metal hydroxides and the electrical conductivity, and the size of
colloidal particles of iron (III) complexes that are strongly reactive
agents with the heavy metal ions [19,24].
3.3. Effect of current density on the removal of heavy metal ions
The effect of CD variation on heavy metal removal was investi-
gated by running the EC experiments at different direct current
fields with CD of 1, 2, 3 and 4 mA/cm
2
. It was clearly observed that
increasing the current density led to a significant removal of heavy
metal ions concentrations. Fig. 3 shows the heavy metal ions
removal efficiencies after one-hour of EC treatment at different
current densities. It is clear that the removal efficiency has values
in the ranges of
g
= 23–29%, 52–62%, 75–83% and 98–100% for
applied CD of 1, 2, 3 and 4 mA/cm
2
, respectively. In other words,
the current density represents a key parameter in enhancing heavy
metal ions removal. This can be attributed to the direct proportion-
ality between direct current field and potential electrolysis which
implies more release of ferric ions, and thereby more generation
of iron hydroxides necessary to form coagulants [16,17].
3.4. Effect of EC time and kinetic study
In this work, the kinetic study for the removal of heavy metal
ions was considered for various current densities at the ambient
temperature and constant wastewater volume (600 ml). For such
EC batch process, the mass conservation of heavy metal ion is:
dC
dt¼ðr
D
Þð11Þ
where (r
D
) is the removal rate of heavy metal ion in ppm/min and
tis the electrocoagulation (EC) time in min. First-order, second-
order and pseudo first-order models were tested to describe
the removal rate equations [25,26]. With the first-order model
(r
D
=k
1
C), the integration of Eq. (11) at the initial concentration
C(0) = C
0
, gives:
CðtÞ¼C
0
e
k
1
t
ð12Þ
where k
1
is the first-order rate constant in min
1
. For the second-
order model (r
D
=k
2
C
2
), the time-dependent concentration is
obtained as:
1
CðtÞ¼1
C
0
þk
2
tð13Þ
where k
2
is the second-order rate constant in ppm
1
min
1
. In addi-
tion, when pseudo first-order model, r
D
=k
app
(CC
e
), is prevailed,
the integration of Eq. (11), gives:
CðtÞ¼C
e
þðC
0
C
e
Þe
k
app
t
ð14Þ
CD (mA/cm2)
01234
η (%)
0
20
40
60
80
100
Cr3+
Cu2+
Ni2+
Zn2+
Fig. 3. Variations of the removal efficiencies of heavy metal ions with direct current
density after one-hour EC treatment of 600 ml metal plating wastewater (pH = 9.6
and
r
= 8.9 mS/cm).
Table 3
Predicted parameters of first- and second-order removal rates of heavy metal ions at different current densities with solution volume = 600 ml,
pH = 9.6 and
r
= 8.9 mS/cm.
Heavy metal CD (mA/cm
2
) First-order model
dC/dt=k
1
C
R
2
(–) Second-order model
dC/dt=k
2
C
2
R
2
(–)
k
1
(min
1
)k
2
(ppm
1
min
1
)
Cr
3+
2 0.0114 0.7476 0.0002 0.8283
3 0.0218 0.7686 0.0006 0.8904
4 0.0856 0.6619 0.0616 0.8228
Cu
2+
2 0.0123 0.9338 0.0006 0.9627
3 0.0246 0.9487 0.0017 0.9720
4 0.0787 0.9772 0.0465 0.7043
Ni
2+
2 0.0133 0.6696 0.0004 0.7573
3 0.0281 0.8219 0.0015 0.9248
4 0.0671 0.9565 0.0168 0.9177
Zn
2+
2 0.0116 0.6855 0.001 0.7669
3 0.0263 0.8150 0.0036 0.8783
4 0.0763 0.9294 0.0723 0.7753
752 M. Al-Shannag et al. / Chemical Engineering Journal 260 (2015) 749–756
where k
app
is the apparent pseudo first-order rate constant in min
1
and C
e
is the equilibrium concentration. The pseudo first-order
model was first proposed by Legergen [26] in which the adsorption
rate is directly proportional to the concentration difference at time t
and at equilibrium. Obviously, if the equilibrium concentration has
zero value, the pseudo first-order model gets back to the first-order
model.
Least-square method was used in order to determine the best
values of the kinetic parameters [27]. The sum of squared errors
(SSE) was minimized for each heavy metal ion. Concentrations of
seven EC experiments (EC time = 5, 10, 15, 20, 30, 45 and 60 min)
were incorporated to build the SSE objective function. The
squared-correlation coefficient, R
2
, was used to measure the good-
ness of the kinetic model. Furthermore, graphical comparison
between the experimental and the corresponding predicted con-
centrations was depicted.
The kinetic parameters of both first- and second-order models
together with the R
2
values are given in Table 3. At current density
of 2 mA/cm
2
, the R
2
values that correspond to both kinetic models
were found to be far from unity for Cr
3+
,Ni
2+
and Zn
2+
. Hence, nei-
ther first-order nor second-order kinetic model can describe the
removal rate of these heavy metal ions (Cr
3+
,Ni
2+
and Zn
2+
). The
copper ions were the only heavy metal ions that their removal rate
can be modeled by first-order kinetics since the corresponding R
2
values at all current densities were close to unity.
Table 4 presents the kinetic parameters and the correlation
coefficients for pseudo first-order model at different current densi-
ties. As can be observed, all R
2
values are too close to unity. Thus,
pseudo first-order kinetics can model the removal rate of all heavy
metal ions adequately. On the other hand, when current density
increased from 2 to 4 mA/cm
2
, the apparent constant increased
from k
app
= 0.0939 to 0.274 min
1
for Cr
3+
ions. For other heavy
metal ions, the influence of current density, up to 4 mA/cm
2
,on
k
app
was marginal. The apparent constant had an average value
of around k
app
= 0.053, 0.113, 0.099 min
1
for Cu
2+
,Ni
2+
and Zn
2+
ions, respectively. The values of equilibrium concentrations (C
e
),
shown in Table 3, strongly matched the corresponding experimen-
tal ones after one-hour of electrocoagulation treatment.
Fig. 4 shows the variations of both experimental and pseudo
first-order concentrations versus time at CD = 2, 3 and 4 mA/cm
2
.
Table 4
Predicted parameters of pseudo first-order removal rates of heavy metal ions at
different current densities with solution volume = 600 ml, pH = 9.6 and
r
= 8.9 mS/
cm.
Heavy metal CD (mA/cm
2
) Pseudo first-order model
-dC/dt=k
app
(CC
e
)
R
2
(–)
k
app
(min
1
)C
e
(ppm)
Cr
3+
2 0.0939 43.89 0.9859
3 0.1165 22.52 0.9933
4 0.2740 00.33 0.9991
Cu
2+
2 0.0504 16.00 0.9823
3 0.0539 08.10 0.9866
4 0.0560 00.31 0.9921
Ni
2+
2 0.1224 22.53 0.9965
3 0.0983 09.82 0.9998
4 0.1180 01.73 0.9939
Zn
2+
2 0.1168 09.02 0.9972
3 0.0878 03.81 0.9980
4 0.0928 00.16 0.9949
t(min)
0 10203040506070
C(ppm)
0
20
40
60
80
100
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
t(min)
0 10203040506070
C(ppm)
0
10
20
30
40
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
t(min)
0 10203040506070
C(ppm)
0
10
20
30
40
50
60
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
t(min)
0 10203040506070
C(ppm)
0
5
10
15
20
25
30
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
(a)
(b)
(c)
(d)
Fig. 4. Variations of heavy metal ions concentrations with EC time during electrocoagulation of 600 ml metal plating wastewater (pH = 9.6 and
r
= 8.9 mS/cm) at different
applied current densities: (a) Cr
3+
; (b) Cu
2+
; (c) Ni
2+
; (d) Zn
2+
.
M. Al-Shannag et al. / Chemical Engineering Journal 260 (2015) 749–756 753
It is clear that the pseudo first-order curves strongly fit the exper-
imental concentrations. This also demonstrates the validity of
pseudo first-order model in analyzing the removal rates of heavy
metal ions. It is depicted in Fig. 4 that there is dramatic reduction
in the heavy metal ion concentrations within the first 45 min. For
example, after 20 min of EC treatment, the Cr
3+
ion concentration
decreased from an initial concentration of 93.2 to 52.4, 28.7 and
0.6 ppm at CD = 2, 3 and 4 mA/cm
2
, respectively. However, the
concentration reduction was moderately enhanced by increasing
the EC time above 45 min, especially at low current densities. It
is worth mentioning that at short EC time, the amount of ferric ions
released from anode will not be adequate to generate iron hydrox-
ide complexes necessary for destabilization and aggregation mech-
anisms involved in the electrocoagulation process [17]. Increasing
the current density increased the removal rate of heavy metal ions.
For example, at EC treatment of 45 min, when the current density
was doubled from 2 to 4 mA/cm
2
, the heavy metal ion concentra-
tions dropped down from 45.1 to 0.3, 16.9 to 1.2, 23.0 to 1.5 and
9.0 to 0.8 ppm for Cr
3+
,Cu
2+
,Ni
2+
and Zn
2+
ions, respectively; See
Fig. 4 (a)–(d).
By using pseudo first-order model, Eqs. (14) and (10), time-
dependent removal efficiency can be expressed as:
g
ð%Þ¼100
g
e
ð1e
k
app
t
Þð15Þ
where
g
e
is the equilibrium removal efficiency which can be calcu-
lated from Eq. (10) at equilibrium concentration.
The experimental and predicted removal efficiencies are illus-
trated in Fig. 5. As can be seen in Figs. 4 and 5, the decrease in
the heavy metal ions concentration with EC time is accompanied
by the increase in the removal efficiency. On the other hand, the
treated metal plating wastewater was visually clear with around
complete removal of heavy metal ions at CD = 4 mA/cm
2
after
one-hour of EC treatment. This result agrees strongly with the find-
ings of Akbal et al. [1] who reported a maximum removal efficiency
of
g
max
= 100%, 100% and 99% for Cr
+3
,Cu
2+
and Ni
3+
ions,
respectively.
3.5. Energy and electrodes consumptions
In the electrocoagulation process, electrical energy consump-
tion and the amount of electrode dissolved in solution exhibit sig-
nificant economical factor. The electrical energy consumption per
unit volume of treated wastewater was calculated using [13]:
E¼ðPÞðIÞðtÞ
Vð16Þ
where Eis the specific energy consumption in kWh/m
3
,Pis the
voltage in V, Iis the DC current in A, tis EC time in hour and Vis
the volume of the treated wastewater in liters. The amount of elec-
trodes dissolved per unit volume of treated metal plating wastewa-
ter, was estimated theoretically using Faraday’s law:
m
Fe
¼1000 ðIÞðtÞMwt
Fe
ðZ
Fe
ÞðFÞðVÞð17Þ
where m
Fe
is the specific amount of dissolved electrode in kg/m
3
,Iis
the direct electrical current in A, tis the EC time in seconds, Mwt
Fe
is the molecular weight of iron (56 g/gmol), Z
Fe
is the chemical
equivalence of iron (Z
Fe
= 2), Fis the Faraday constant
t(min)
0 10203040506070
η (%)
0
20
40
60
80
100
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
t(min)
0 10203040506070
η (%)
0
20
40
60
80
100
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
t(min)
0 10203040506070
η (%)
0
20
40
60
80
100
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
t(min)
0 10203040506070
η (%)
0
20
40
60
80
100
Measurements at CD = 2.0 mA/cm2
Measurements at CD = 3.0 mA/cm2
Measurements at CD = 4.0 mA/cm2
Based on pseudo first-order kinetics
(a) (c)
(b) (d)
Fig. 5. Variations of removal efficiencies of heavy metal ions with EC time during electrocoagulation of 600 ml metal plating wastewater (pH = 9.6 and
r
= 8.9 mS/cm) at
different applied current densities: (a) Cr
3+
; (b) Cu
2+
; (c) Ni
2+
; (d) Zn
2+
.
754 M. Al-Shannag et al. / Chemical Engineering Journal 260 (2015) 749–756
(F= 96500 C/mol) and Vis the volume of the treated wastewater in
m
3
.
Table 5 summarizes the amount of dissolved electrodes and
electrical energy consumption per one cubic meter of treated
wastewater and the corresponding removal efficiencies at different
current densities and EC treatment times. As expected, it is clear
that increasing current density and/or EC time increases the
removal of heavy metal ions, which is associated with increasing
both the specific electrical energy consumption and the specific
dissolution of electrodes. In order to maximize the removal effi-
ciency at the operating conditions of this study, the current density
must not be less than 4 mA/cm
2
and EC treatment time should be
in the range of 45 to 60 min. These operating conditions minimized
the specific energy consumption to the level of 6.25–8.33 kWh/m
3
and the specific amount of dissolved electrodes to the level of
1.31–1.74 kg/m
3
. These consumption levels were in very good
agreement with the results reported in the study of Akbal et al.
[1] in which 20 min EC treatment with current density of
10 mA/cm
2
at pH = 3.0 was able to achieve removal efficiency of
100%, 100% and 99% for Cr
3+
,Cu
2+
and Ni
2+
, respectively. The corre-
sponding energy and electrode consumptions were 10.07 kWh/m
3
and 1.08 kg/m
3
, respectively.
Furthermore, Table 6 gives a comparison of heavy metal ions
removal efficiencies from wastewater using various treatment
methods. It is obvious from Table 6 that all treatment processes
achieved high removal efficiencies for all heavy metal ions consid-
ered. However, a detailed cost analysis is necessary to get a real
conclusion about the feasibility of the most effective method for
the heavy metal ions removal which is not the scope of the current
study.
Finally, it is worth mentioning that the sludge generated in EC
reactor as a byproduct might contain a wide range of components
which will harm the environment if no proper management is con-
sidered. Therefore, sustainable end-use of the final sludge gener-
ated from EC reactor is an essential issue in order to minimize its
negative impact on the environment. Generally, the landfill is the
common used method for sludge disposal. However, sludge man-
agement and reuse became an interesting area for many research-
ers in the last few years, especially when the sludge contains
economic compounds like metallic hydroxides as in the present
study.
4. Conclusions
The present study investigated the removal of heavy metal ions
from metal plating wastewater, by a batch electrocoagulation pro-
cess. Electrocoagulation for long residence time with high current
density significantly improves the removal of heavy metal ions.
The results confirmed that the EC process is independent from
electrical conductivity at high levels. In order to further minimize
the energy consumption while maintaining higher removal effi-
ciency, the current density must not be more than 4 mA/cm
2
with
electrocoagulation time in the range of 45 to 60 min. Moreover, for
optimal removal of heavy metal ions, the pH value of the metal
plating wastewater must be adjusted to a level of slightly basic
conditions.
In conclusion, EC process is an efficient treatment method for
the removal of heavy metal ions from metal plating wastewater.
Indeed, a continuous EC process on a pilot scale level together with
a proper approach for sludge management should be first designed
and characterized. In this context, a detailed careful assessment of
both environmental and economic issues should be considered.
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