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Complexing agent and heavy metal removals from metal plating effluent by electrocoagulation with stainless steel electrodes

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In the present study, the treatability of a metal plating wastewater containing complexed metals originating from the nickel and zinc plating process by electrocoagulation using stainless steel electrodes was experimentally investigated. The study focused on the effect of important operation parameters on electrocoagulation process performance in terms of organic complex former, nickel and zinc removals as well as sludge production and specific energy consumption. The results indicated that increasing the applied current density from 2.25 to 9.0 mA/cm(2) appreciably enhanced TOC removal efficiency from 20% to 66%, but a further increase in the applied current density to 56.25 mA/cm(2) did not accelerate TOC removal rates. Electrolyte concentration did not affect the process performance significantly and the highest TOC reduction (66%) accompanied with complete heavy metal removals were achieved at the original chloride content ( approximately 1500 mg Cl/L) of the wastewater sample. Nickel removal performance was adversely affected by the decrease of initial pH from its original value of 6. Optimum working conditions for electrocoagulation of metal plating effluent were established as follows: an applied current density of 9 mA/cm(2), the effluent's original electrolyte concentration and pH of the composite sample. TOC removal rates obtained for all electrocoagulation runs fitted pseudo-first-order kinetics very well (R(2)>92-99).
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Journal of Hazardous Materials 165 (2009) 838–845
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Complexing agent and heavy metal removals from metal plating effluent by
electrocoagulation with stainless steel electrodes
Is¸ ık Kabdas¸lı, Tülin Arslan, Tu˘
gba Ölmez-Hancı, Idil Arslan-Alaton, Olcay Tünay
Istanbul Technical University, Civil Engineering Faculty, Environmental Engineering Department, 34469 Maslak, Istanbul, Turkey
article info
Article history:
Received 8 August 2008
Received in revised form 15 October 2008
Accepted 16 October 2008
Available online 1 November 2008
Keywords:
Metal plating effluent
Complexed metal treatment
Electrocoagulation with stainless steel
electrodes
Organic complex former removal
Nickel and zinc removal
abstract
In the present study, the treatability of a metal plating wastewater containing complexed metals origi-
nating from the nickel and zinc plating process by electrocoagulation using stainless steel electrodes was
experimentally investigated. The study focused on the effect of important operation parameters on elec-
trocoagulation process performance in terms of organic complex former, nickel and zinc removals as well
as sludge production and specific energy consumption. The results indicated that increasing the applied
current density from 2.25 to 9.0mA/cm2appreciably enhanced TOC removal efficiency from 20% to 66%,
but a further increase in the applied current density to 56.25mA/cm2did not accelerate TOC removal
rates. Electrolyte concentration did not affect the process performance significantly and the highest TOC
reduction (66%) accompanied with complete heavy metal removals were achieved at the original chloride
content (1500mg Cl/L) of the wastewater sample. Nickel removal performance was adversely affected
by the decrease of initial pH from its original value of 6. Optimum working conditions for electrocoagu-
lation of metal plating effluent were established as follows: an applied current density of 9mA/cm2, the
effluent’s original electrolyte concentration and pH of the composite sample. TOC removal rates obtained
for all electrocoagulation runs fitted pseudo-first-order kinetics very well (R2>92–99).
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The increasing demand for the control of toxic metals being
released from industrial activities has led to the search of more
effective treatment methods for heavy metal abatement. Conven-
tional treatment methods have proven to be satisfactory in meeting
the discharge consents. Among them, metal hydroxide precipita-
tion is the most common treatment method employed for the metal
finishing effluents [1]. On the other hand, complexed metals cannot
be efficiently removed by this method under conventional treat-
ment conditions. Complexed metals are the form of metals which
are bound to strong complexing agents (complex former) keeping
them in solution. In the metal finishing industry, electroless plat-
ing and immersion plating are two major sources of wastewater
bearing complexed metals [1]. Modified hydroxide precipitation or
pretreatment (such as oxidation and reduction) preceding hydrox-
ide precipitation are applicable treatment methods to complexed
metal effluents. Modified hydroxide precipitation is based on the
addition of calcium to which complexing agents preferentially bind,
allowing freed metal ions to precipitate effectively [1–4]. The main
Corresponding author. Tel.: +90 212 285 65 86; fax: +90 212 285 65 75.
E-mail address: ikabdasli@ins.itu.edu.tr (I. Kabdas¸lı).
obstacle of this process is that complexing agents which may be
harmful in many ways are simultaneously released into the envi-
ronment. Via application of pretreatment the complexing agent
can be completely destroyed or converted into a form that does
not interfere with conventional precipitation. Reduction and oxi-
dation processes employed as pretreatment methods have several
disadvantages such as the high cost and toxicity of the chemi-
cals employed [1–5]. Hence, the development of new treatment
methods for effluents bearing complexed metals is an urgent issue.
Among the methods that have recently been studied, electroco-
agulation (EC) is deemed a promising one as it proved to be very
efficient in removing pollutants such as organic and inorganic mat-
ters from the industrial wastewaters [6–17]. In addition, a number
of scientific works have indicated that heavy metalsin the free form
can be successfully removed by EC using aluminum, iron and cast
iron electrodes [18–24].
Adhoum et al. [18] experimentally investigated the treatability
of synthetic samples for the electroplating wastewater contain-
ing copper, zinc and hexavalent chromium by EC with aluminum
electrodes. Their study demonstrated that the increase of current
density that ranged between 8 and 48 mA/cm2enhanced the treat-
ment rate and highest removal efficiencies could be achieved when
the pH was kept between 4 and 8. Lai and Lin [19] experimen-
tally explored the EC application to chemical mechanical polishing
0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2008.10.065
I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845 839
(CMP) wastewater from semiconductor fabrication using cast iron,
aluminum and titanium plates in different combinations as the
anode/cathode pairs. Their results revealed that aluminum/cast
iron (anode/cathode) was a good electrode pair and was able to
achieve 99% copper removal, 96.5% turbidity reduction and 75%
COD removal in less than 100 min. In other studies published by
Lai and Lin [20,21], sludge characteristics of CMP treated by EC
were investigated and it was emphasized that the EC process gen-
erates a significant amount of sludge that needs to be properly
disposed of. Golder et al. [22] searched the removal of chromium
(III) using a synthetic sample by EC with mild steel electrodes.
Almost complete chromium removal (99.9%) was obtained after
50 min of EC in the case of bipolar electrode arrangement at approx-
imately 33.52 mA/cm2against 81.5% removal for the monopolar
electrode configuration. In other study conducted by Golder et
al. [23] removal of trivalent chromium from synthetic solution
was investigated using EC with mild steel electrodes in order
to determine the effect of critical operating parameters such as
current density (16–26mA/cm2); initial pH (3–5) and NaCl con-
centration (1000 and 2000 mg/L) on process performance. In their
study, high chromium removal was observed at higher current
densities and solution pH. They found that increasing the chlo-
ride concentration from 1000 to 2000 mg/L reduced chromium
removal efficiency. Golder et al. [24] studied electrotreatment of
nickel and boron containing plating rinse effluent by using mild
steel and aluminum electrodes. Their results showed that nickel
could be removed with 86% efficiency at a current density of
48.78 mA/cm2.
The above-mentioned studies have been carried out in order
to remove free metal ions that can easily be treated by hydroxide
precipitation. Although the electrochemical processes have been
applied to some organic complex formers such as EDTA, NTA and
citric acid that are also widely employed in the metal finishing
industry, these studies were also conducted on synthetic solutions
and involved membrane separation to recover metals and/or com-
plex formers [25–28]. On the other hand, the EC process has the
potential of simultaneously removing both organic complex former
and heavy metals. Therefore, taking into account the high perfor-
mance of EC in removing a variety of pollutants, the performance of
the process for complexed metal treatmentand the ef fectof operat-
ing parameters on the process efficiency need to be experimentally
explored.
The target of the present study was to examine the treatabil-
ity of a metal plating wastewater containing complexed metals
originating from the nickel and zinc plating process, by EC using
stainless steel electrodes and to explore the effect of varying
operating parameters such as the applied current density, ini-
tial pH of solution, electrolyte concentration and electrical energy
consumption on heavy metal removal as well as organic matter
removal.
2. Materials and methods
2.1. Metal plating effluent
The source-based composite samples were taken from a metal
finishing factory located in Yalova, Turkey. These samples com-
prised of the exhausted nickel and zinc plating bath and its
subsequent rinses. The metal plating bath consisted of an organic
complexing agent, carrier, brightener, zinc chloride, nickel chloride
and potassium chloride. The contribution of the complexing agent
to the TOC content of the total effluent was approximately 90% [29].
The environmental characterization of the composite samples used
in our experimental study is delineated in Table 1 in terms of main
pollution parameters.
Table 1
The environmental characterization of the composite samples.
Parameter Unit Composite
Sample I
Composite
Sample II
Composite
Sample III
TO C mg / L 173 170 170
Zinc mg/L 232 217 236
Nickel mg/L 248 270 282
Chloride mg/L 1480 1515 1725a
aAfter acidification to pH 3 with HCl.
2.2. The electrocoagulation unit
The electrocoagulation unit is a 1800-mL-effectivecapacity rect-
angular reactor (with a height of 28.3cm, a length of 34.3 cm and
a width of 12.5cm) which is made of polyethylene (PE 1000). It
is equipped with six monopolar, parallel connected stainless steel
(SS 304) electrodes used as both anodes and cathodes with effective
surface areas of 38.5 cm2(L: 11.9cm and d: 1.02cm). The distance
between the electrodes was fixed at 3 mm. The applied currentden-
sity (Jc) was kept constant by means of a high precision DC power
supply (Maksimel Proffesional Systems UPS 023 model; Emax:20V;
Jc: 2.25–88.00 mA/cm2). After each experimental run, the EC reac-
tor and electrodes were carefully rinsed twice with 50% (v/v) nitric
acid solution for 2–4 min and several times with distilled water
to remove impurities and metal hydroxide precipitates from the
electrocoagulation unit. The electrodes were renewed each time
whenever more than 10% of electrode material was consumed. The
electrode consumption was followed by precise measuring of the
diameter.
2.3. Experimental procedure
The electrocoagulation experiments were run at room tempera-
ture. The initial pH values of the composite samples were adjusted
with 6N KOH or 6N HCl in order to keep the original composition
of the effluent and to avoid the addition of foreign ions into the
solution for Runs 15–20. Samples were withdrawn from the reac-
tor at proper time intervals during the course of EC. Thereafter, the
samples were allowed to settle for 30min prior to vacuum filtration
through Millipore membrane filters with a pore size of 0.45m.
2.4. Analytical procedure
In the composite (raw) and EC-treated samples, TOC, nickel, zinc
and pH were analyzed in order to follow the process performance.
All analyses were performed as defined in Standard Methods [30]
except the COD measurements that was determined by the Open
Reflux Titrimetric Method according to ISO 6060 (1986). TOC was
monitored on a Tekmahr-Dorhmann Apollo 9000 model carbon
analyzer. Unicam 929 model atomic absorption spectrophotometer
was used for nickel and zinc determination, while pH measure-
ments were made with Orion 920 model pH meter. The amount
of produced sludge consisting of Fe(OH)3together with removed
nickel, as Ni(OH)2, zinc, as Zn(OH)2, and adsorbable organic mat-
ter was determined as the total suspended solids (TSS) parameter
according to Standard Methods after EC process [30].
3. Results
3.1. Effect of applied current density
Applied current density is one of the main operating param-
eters directly affecting process performance and operating cost.
Therefore, as a beginning, a series of EC experiments was car-
840 I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845
Fig. 1. The effect of applied current density on EC process performance (initial conditions: TOC0: 173 mg/L; Ni0: 248 mg/L; Zn0: 232 mg/L; pH0:6;Cl
0: 1480mg/L)
(:2.25 mA/cm2;:4.5 mA/cm2;:6.75 mA/cm2;×:9 mA/cm2; *:11 mA/cm2;:22.5 mA/cm2; +:33.75 mA/cm2;:45 mA/cm2;:56.25 mA/cm2).
ried out on the Composite Sample I at a wide range of applied
current densities varying between 2.25 and 56.25mA/cm2at
an initial pH 6.0 and a chloride concentration of 1480 mg/L
(the original composition of Composite Sample I) to assess the
effect of applied current density on process performance and
to determine the optimum current density in terms of removal
efficiencies as well as electrical energy requirements and sludge
production.
3.1.1. Effect on the organic complex former removal
Fig. 1 displays changes in TOC (a), nickel (b), zinc (c) and pH (d)
during the EC process run under different applied current densities.
As can be seen from Fig. 1(a), increasing the applied current density
from 2.25 to 9.0mA/cm2appreciably improved the TOC removal
efficiency from 20% to 66%. On the other hand, in the range of
22.5–56.25 mA/cm2practically the same TOC removal efficiencies
(60%) were obtained, however, within relatively shorter reaction
times. For instance, 60% TOC was removed after 150min at the
applied current density of 9.0 mA/cm2and the same removal effi-
ciency was obtained in only 45 min at 45 mA/cm2[29]. Hence, it can
be easily deduced that an increase in the applied current density
accelerated TOC removal kinetics. This observation can also be sup-
Table 2
TOC removal rate coefficients for EC applied under different applied current density
(pH0: 6.0 and chloride: 1450mg Cl/L).
Run Jc(mA/cm2)kTOC (min1) Time interval (min) R2
1 2.25 0.0014 0–180 0.92
2 4.50 0.0021 0–180 0.96
3 6.75 0.0037 0–180 0.97
4 9.00 0.0042 0–105 0.99
5 11.00 0.0046 0–90 0.99
6 22.50 0.0083 0–120 0.98
7 33.75 0.0121 0–90 0.97
8 45.00 0.0173 0–60 0.92
9 56.25 0.0247 0–30 0.99
ported by regression analyses applied to fit the TOC concentrations
as a function of EC operation time obtained for different applied cur-
rent densities to the pseudo-first-order kinetic expression as given
below:
ln (TOC)t
(TOC)0
=−kTOC ×t(1)
where tis EC treatment time (in min) and kTOC is the pseudo-
first-order reaction rate constant (in min1). The calculated
pseudo-first-order TOC rate constants are tabulated in Table 2.As
can be seen from the table and Fig. 2, the fitness to the data was
very satisfactory (R2> 0.92) and increasing the applied current den-
sity significantly enhanced TOC removal rates.This interpretation is
consistent with the results given in the related literature [9,10,16].
In addition, a linear increase has also been obtained for the pseudo-
first-order TOC rate constants versus the applied current densities
Fig. 2. Application of the pseudo-first-order reaction kinetic for EC run at dif-
ferent applied current densities (:2.25 mA/cm2;:4.5 mA/cm2;:6.75 mA/cm2;
×:9 mA/cm2; * 11mA/cm2;:22.5 mA/cm2; +:33.75 mA/cm2;:45 mA/cm2;
:56.25 mA/cm2).
I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845 841
(kTOC =0.0004×Jc,R2= 0.98) which may facilitate decision mak-
ing for the operation of the EC process.
3.1.2. Effect on nickel and zinc removals
Three mechanisms being involved in the abatement of pollu-
tants via EC process are of importance for heavy metal removal. One
of these mechanisms is the removal of organic matter, which con-
sists of the organic complex former for the present case, by indirect
oxidation apparently through chlorine species formed from chlo-
ride ions (according to Eqs. (4)–(7))[6]. TOC abatements obtained
through the EC process implied the degree of mineralization e.g.
complete oxidation of the organic matter. However, complete oxi-
dation of the organic matter is not required to free the heavy metal
bound by organic complex former. Organic matter (organic complex
former) undergoes the structural changes by which it loses its com-
plexing ability. This may take place at a certain extent of oxidation
and the complexed metals convert into free metal forms which are
removed by the hydroxide precipitation. In our experimental study,
for the case of nickel this conversion was realized after achieving
40–50% TOC removal. However, zinc removal was observed to be
independent of TOC abatement indicating that the organic complex
former used in the investigated metal plating bath does not form
complex species with zinc and it is only able to bind the nickel as
a complexed metal. This observation was justified with hydroxide
precipitation run at the pH range of 7–12 as control experiments
(data not shown). The results showed that only zinc can completely
be removed by hydroxide precipitation at its optimum pH [29].
The second reaction mechanism having an integral role in the
heavy metal removal with EC is hydroxide precipitation of heavy
metals by increasing the solution pH. The increase in pH is a conse-
quence of the following cathodic reaction taking place during the
EC process [17,23]:
2H2O(aq) +2eH2(g) +2OH(2)
In hydroxide precipitation, the optimum pH of metal hydroxide
solubility is the key parameter of the heavy metal removal perfor-
mance. The optimum pH values for zinc and nickel removal are 9.2
and 10.2, respectively [1].
The third mechanism is the removal of colloidal material
through the formation of Fe(OH)3flocs. This mechanism enhances
the removal of metal hydroxide solids which have a colloidal form.
Since the amount of coagulation material namely Fe(OH)3was
always higher than the amount required for sweep floc destabiliza-
tion of colloids, the effect of this mechanism could be considered
equivalent in all experimental studies carried out in the present
work.
As mentioned above, nickel removal was closely related not
only to the degree of TOC abatement but also pH increases being
occurred during the EC process. As is evident from Fig. 1(d), pH
gradually increased through the EC processes and very limited pH
changes were obtained at the end of reaction at lower applied cur-
rent densities than 9 mA/cm2. Hence nickel removal efficiencies
were limited to 80%. Significant nickel removal began after TOC
concentrations fell below 100 mg/L in our EC applications. Together
with these TOC abatements and when the solution pH was attained
to or exceeded the optimum pH of nickel, complete removals were
achieved (cf. Fig. 1(b) and (d)). These two prerequisites were pro-
vided for the applied current densities of 9 and 22.5 mA/cm2in 180
and 90 min, respectively. On the other hand, as zinc removal was
only related to the final solution pH, complete zinc removals were
obtained in all EC applications (Runs 1–9).
3.1.3. Effect on specific energy consumption and sludge
production
The major operating cost of the process is considered to be
directly related to the specific energy consumption during EC [16].
The specific energy consumption (SEC) is defined as the amount
of electrical energy consumed per unit mass of pollutant (TOC and
nickel in the present case) removed [31].Table 3 summarizes the
specific electrical energy consumptions obtained for EC processes
run with a wide range of the applied current densities. The specific
energy consumptions were calculated using the equation given as
follows;
SEC =
U×I×t
g removed pollutant (3)
where Uis the voltage measured during the reaction (in V), Ithe
applied electrical current (in A) and tthe reaction time (in hour).
As clearly seen from Table 3, the calculated specific energy con-
sumptions for TOC and nickel increased with increasing applied
current densities as expected. When the applied current density
was elevated from 2.3 to 9.0mA/cm2, the SEC values increased in
parallel from 0.08 to 0.16kWh/g TOC removed for 3 h operation
time. As also delineated in Table 3, the calculated SEC value for
22.5 mA/cm2(corresponding to 0.44 kWh/g TOC removed) was sig-
nificantly higher than for the calculated value for 9.0mA/cm2. The
same trend was also observed for nickelremoval. The calculated SEC
values for TOC removal were relatively higher than the calculated
values for nickel removal. However, the main target of the process
is to remove nickel almost completely to comply with the national
discharge standards (2 mg/L according to[32]) and only the SEC val-
ues satisfying this prerequisiteare meaningful from this standpoint.
Table 3
Specific electrical consumptions and sludge characteristics.
Run TOCrem (%) SECTOC (kWh/g TOCrem )Ni
rem (%) SECNi (kWh/g Nirem) Sludge (mg TSS/L) SVI (mL/g TSS)
1 15 0.08 80 0.01 2,290 185
2 36 0.10 80 0.03 4,075 108
3 50 0.14 80 0.06 5,900 68
4 66 0.16 100 0.07 8,085 60
5 48 0.20 96 0.07 11,100 45
6 63 0.44 100 0.20 13,900 36
10 66 0.16 100 0.05 7,490 75
11 62 0.15 100 0.06 7,615 74
12 63 0.15 100 0.06 7,250 59
13 62 0.14 100 0.05 7,500 47
14 56 0.14 100 0.05 7,270 53
15 45 0.14 78 0.049 3,000 90
16 40 0.15 79 0.046 3,200 82
17 44 0.14 78 0.048 4,130 74
18 50 0.14 77 0.054 4,000 69
19 46 0.15 83 0.049 4,200 55
20 40 0.14 96 0.037 3,800 60
842 I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845
Fig. 3. Effect of electrolyte (chloride) concentration on EC process performance (at 9.0mA/cm2; initial conditions: TOC0: 170 mg/L; Ni0: 270mg/L; Zn0: 217mg/L; pH0:6;Cl
0:
1515mg/L) (:1510 mg Cl/L; :1875mg Cl/L; :2250 mgCl/L; ×:2635 mg Cl/L; *:3000mg Cl/L).
Consequently,among the applied current densities employed in the
present study, 9.0mA/cm2gave the optimum result on the basis of
both organic matter (66% TOC) and nickel (100%) removals as well
as the calculated SEC values.
The amount of sludge produced at the end of the EC processes
run at varying applied current densities are also given in Table 3.
It is not surprising that the sludge production during EC process
increased upon increasing the applied current density. The results
indicated that an increase in the applied current density from 2.25
to 9.0 mA/cm2brought about a threefold increase in the sludge
formation rate. On the other hand, the sludge volume index (SVI)
values revealed that the sludge settling characteristics significantly
improved with increasing the applied current density. Neverthe-
less, SVI values measured for all EC runs represented a good settling
character except the value obtained for lower applied current den-
sity employed e.g. Runs 1 and 2.
Within the context of the above evaluations and from the stand-
points of both removal efficiencies and process operation, it was
decided that 9 mA/cm2is the optimum applied current density for
the particular effluent under study. This optimum current density
is markedly lower than that found by Golder et al. as 32.82mA/cm2
[22,23] for chromium removal. Hence, the forthcoming experi-
ments were all run at an applied current density of 9.0mA/cm2.
3.2. Effect of electrolyte concentration
As mentioned in the literature, increasing the electrolyte
(NaCl) concentration accelerates inorganic and organic pollutant
removals, decreases the power consumption and shortens the
reaction time [33,34]. In addition, chloride may generate chlo-
rine/hypochlorite serving as an oxidizing agent during EC process
at a proper pH range (pH< 11) and for an appropriate electrode
material e.g. stainless steel [6,33].
2Cl(aq) Cl2(g) +2e(4)
2H2O+2eH2(g) +2OH(5)
Cl2(g) +H2OHOCl(aq) +H++Cl(6)
HOCl H++OCl(7)
The chlorine/hypochlorite couple oxidizes the pollutants
thereby getting reduced to chloride ions [6]. Therefore, in order
to determine the effect of electrolyte concentration on the pro-
cess performance, another set of EC experiments was carried out
at varying chloride concentrations at the optimum applied cur-
rent density of 9 mA/cm2and original pH (=6) of the effluent. As
Composite Sample II already bore 1515 mg/L chloride which was an
advantage of the wastewater sample thus providing a suitable elec-
trolyte concentration that is essential for EC, the experiments were
initiated with the original sample composition and the chloride
concentration was then increased up to 3000 mg/L.
Fig. 4. Application of the pseudo-first-order reaction kinetic for EC run at
various electrolyte (chloride) concentration (:1510 mg Cl/L; :1875mg Cl/L;
:2250 mgCl/L; ×:2635 mg Cl/L; *:3000 mg Cl/L).
I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845 843
Table 4
TOC removal rate coefficients for EC applied at varying chloride concentrations (Jc:
9 mA/cm2and pH0: 6).
Run Chloride (mg/L) kTOC (min1) Time interval (min) R2
10 1512 0.0054 0–90 0.99
11 1875 0.0 053 0–90 0.99
12 2250 0.0 052 0–90 0.99
13 2635 0.0050 0–90 0.99
14 3000 0.0050 0–90 0.99
3.2.1. Effect on the organic complex former
Fig. 3(a) illustrates time-dependent changes in TOC values as a
function of chloride concentration during the EC process. From the
figure it is clear that the originally present chloride concentration of
1515mg/L in the composite sample was sufficient for the removal
of organic complex former and provided a high degree of mineral-
ization yielding the lowest remaining TOC of 57 mg/L. On the other
hand, further increase in chloride concentration did not enhance
TOC removal efficiencies even resulting in a slight decrease in the
process performance. The highest TOC removalefficiency (6 6%) was
achieved at the lowest chloride concentration (1515mg/L). How-
ever, as can be seen from Fig. 4 the pseudo-first-order kinetics
(Eq. (1)) was obeyed for all tested chloride concentrations and
the TOC removal rate coefficients were all close to each another
(Table 4). Nevertheless, a slight decrease in TOC removal rates with
increasing chloride concentration was evident. These findings were
all in accordance with the literature data [9] where no signifi-
cant effect of chloride concentrations was observed over 1500 mg/L
while increasing chloride concentrations up to 1500 mg/L improved
organic matter removal.
3.2.2. Effect on nickel and zinc removals
Although gradually increasing within the first 120 min of reac-
tion, the pH shifted to highly alkaline pH values at the end of
EC. The final effluent pH values were around 11 for all experi-
mental runs except the EC run with a chloride concentration of
3000 mg/L (Fig. 3(d)) yielding a final pH of 12. From Fig. 3(c) it is
evident that more or less the same zinc removal efficiencies were
attained at any time of EC application for all experimental runs.
Though the solution pH values were in the range of 6.5–7.5 being
much lower than optimum pH of zinc after 60min EC, almost com-
plete zinc removals were obtained. This zinc removal other than
hydroxide precipitation may be attributable to binding onto freshly
produced Fe(OH)3flocs described in Section 3.1.2 as the third
mechanism.
As aforementioned, in order to ensure complete nickel removal,
two prerequisites have to be fulfilled: (i) conversion into the free
metal state through the structural change of the organic complex
former and (ii) reaching its optimum pH condition. In the present
study, 40–50% TOC abatement spoke for the loss of the agent’s
complexing ability. It can be inferred that these prerequisites were
practically satisfied at the end of 105min EC (Fig. 3(a)). Although
the remaining TOC content was in the range of 90–100 mg/L, unsat-
isfactory nickel removal efficiencies (80%) were obtained at the end
of this time interval due to low pH and resulting effluent nickel con-
centrations varying between 25 and 50mg/L and thus exceeding
the national discharge standards (Fig. 3(b)). Under all EC condi-
tions, as the solution pH values reached the optimum pH, nickel
was completely precipitated after 150min EC. It is interesting that
similar to the zinc removal pattern, more or less the same remain-
ing nickel concentrations weremeasured at any time for all chloride
concentrations tested.
3.2.3. Effect on the specific energy consumption and sludge
production
The calculated SEC values for both TOC and nickel removals
obtained during EC at varying chloride concentrations were prac-
tically identical as obvious from Table 3. The amount of sludge
produced during EC remained unchanged with increasing chlo-
ride concentration as well. Thus it can be concluded that increasing
the electrolyte (chloride) concentration had no positive influence
on treatment performance. Therefore, as long as the targeted TOC
Fig. 5. Effect of initial pH on EC process performance (at 9.0 mA/cm2; initial conditions: TOC0: 170mg/L; Ni0: 282 mg/L; Zn0: 236 mg/L; pH0:6;Cl
0: 1725mg/L) (:pH 3; :pH
4; :pH 5; ×:pH6;*pH7;:pH 8).
844 I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845
Fig. 6. Application of the pseudo-first-order reaction kinetic for EC initiated at dif-
ferent initial pH values (:pH 3; :pH 4; :pH 5; ×:pH 6; *:pH 7; :pH 8).
reductions and complete heavy metal removals are achieved, there
is no need to increase the conductivity by chloride addition.
3.3. Effect of initial pH
It has already been established that the initial as well as final
pH achieved after through EC are important operation parameters
dramatically affecting the EC process performance [6–8,16,17].In
order to examine the effect of initial pH on TOC and heavy metal
abatements, another set of EC process was performed using acid-
ified Composite Sample III (see Table 1) at the optimum applied
current density (9 mA/cm2). The reason of primary acidification
was to work with the same initial chloride concentration; hence,
first the initial pH of the composite sample was adjusted to 3 with
1N HCl and then it was increased to the desired pH with 1N KOH.
A wide range of initial pH varying between 3 and 8 was tested.
3.3.1. Effect on the organic complex former removal
The results illustrated in Fig. 5(a) indicated that TOC reduced
with increasing treatment time and TOC abatements were not very
different from each other for all initial pH values for 120 min of EC
operation. At the end of EC operation, the highest TOC removal effi-
ciency (50%) together with the highest reaction rate were observed
for EC started at an initial pH of 6 and resulting in a remaining
TOC of 85 mg/L. Regression analyses were applied to fit the data
obtained for EC initiated at different pH values to the pseudo-first-
order kinetic (Fig. 6). The fitness to the data was very satisfactory
and all the TOC removal rate coefficients found to be close to one
another (see Table 5). TOC removal rates were also very close to
those run with varying chloride concentrations.
3.3.2. Effect on nickel and zinc removal
Fig. 5(d) depicts time-dependent pH evolution as a function of
EC treatment time. pH slowly shifted from slightly acidic to around
neutral pH values (7.2–7.9) except the EC initiated at pH 8. After
90 min of operation time, reaction solution pH values remained
Table 5
TOC removal rate coefficients for EC started at varying initial pH (Jc: 9 mA/cm2and
chloride: 1725 mg Cl/L).
Run pH0kTOC (min1) Time interval (min) R2
15 3.0 0.0049 0–120 0.99
16 4.0 0.0035 0–105 0.99
17 5.0 0.0048 0–120 0.99
18 6.0 0.0056 0–120 0.99
19 7.0 0.0049 0–120 0.99
20 8.0 0.0042 0–120 0.99
practically at the same level with increasing time. In spite of the
lower pH values than the optimum of zinc, complete zinc removals
were achieved at the end of 60 min for these runs (Fig. 5(c)). During
EC starting at an initial pH of 8, the pH gradually increased upon
increasing EC time and the final pH was obtained as 10.34. In this
case, the discharge standard (3mg/L [32]) for zinc was met even
after 10min of operation time.
The experiments clearly indicated that as long as a TOC reduc-
tion of 40–50% was reached, nickel could be totally removed when
the solution pH reached a proper value of around 10 correspond-
ing to its optimum pH. Upon closer inspection of Fig. 5(a) and (d)
it can be seen that these two prerequisites were satisfied during
only one run, e.g. the EC initiated at pH of 8 resulted in almost
complete nickel removal. In case of the other EC runs, while the
TOC removal condition was ensured, the final pH values were
far from the optimum pH condition. Therefore, nickel concen-
trations were reduced to around 50mg/L accompanied with 80%
removal efficiency exceeding the discharge standards for these EC
runs (Fig. 5(b)). Considering these EC runs for which 40–50% TOC
removals were obtained, to cope with this pH constraint further pH
adjustment can be made using a proper alkali agent such as lime, or
NaOH. In this case, a significant cost reduction would be possible in
terms of sludge handling, treatment duration and electrical energy
consumption. Further pH adjustment of the effluents obtained at
the end of 120 min EC in the present studyjustifie d the applicability
of this alternative. An experiment that was conducted to check this
issue yielded complete nickel removal. Another alternative would
be the prolongation of EC operation just for the purpose of increas-
ing the pH at the expense of increasing the amount of operating
(electrical energy and sludge handling) costs (cf. Runs 10 and 18).
3.3.3. Effect on the specific energy consumption and sludge
production
Although the calculated SEC values for TOC removalswere found
to be similar, the lowest value for nickel was obtained for the EC run
initiated at a pH of 8 (Run 20) as can be seen from Table 3.
From Table 3 it is evident that shorter treatment time brought
about appreciably lower sludgeproduction. For instance, in the case
of EC run at a current density of 9 mA/cm2and an initial pH of 6,
the decrease in treatment time from 180 to 120min resulted in
a significant decrease in sludge formation from 7490 to 4000 mg
TSS/L, as would be expected.
4. Conclusions
In the present study, electrocoagulation using stainless steel
electrodes proved to be a promising treatment method for com-
plexed metal removal as well as organic matter reduction from
complexed metal wastewater originating from metal plating indus-
try. The following conclusions could be drawn from the present
study:
The experimental results revealed that the organic complex for-
mer used in the metal plating bath formulation could be oxidized
through the EC process to free nickelion corresponding to 40–50%
TOC removal. Therefore, the effective mechanism of the process
can be postulated as breaking of the complexing agent structure
via oxidation, followed by hydroxide precipitation and adsorp-
tion/entrapment on freshly produced ferric hydroxide flocs.
The experimental results indicated that the highest TOC abate-
ment (66%) as well as nickel and zinc removals (100%) were
achieved with an applied current density of 9 mA/cm2at the orig-
inal electrolyte (chloride) concentration and original pH of the
composite sample used. Therefore, it can be concluded that the
I. Kabdas¸ lı et al. / Journal of Hazardous Materials 165 (2009) 838–845 845
national discharge standards could be complied with by apply-
ing 9 mA/cm2current density without any electrolyte addition
and/or pH correction.
The results obtained at varying operation conditions in order to
elucidate their effects on process performance implied that TOC
and heavy metal removals as well as sludge production and spe-
cific energy consumption rates were significantly affected by the
applied current density. On the other hand, the process perfor-
mance was not significantly affected by increasing the chloride
concentration. Reduction of the initial pH of the composite sam-
ple did not enhance the process performance in terms of TOC
and zinc removals, but reduced nickel removal due to low efflu-
ent pH values achieved unless the reaction time was extended to
180min.
Results of regression analysesapplie d tothe obtaine d experimen-
tal data revealed that in all cases TOC removal perfectly obeyed
the pseudo-first-order reaction kinetics. While TOC removal rates
were significantly accelerated with increasing the applied cur-
rent density, increasing the chloride concentration and varying
the initial reaction pH did not significantly affect the TOC removal
efficiency.
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... Nickel removal was studied at an initial pH = 7.6, initial temperature = 21°C, and current densities of 5, 15, and 25 mA/cm 2 . This initial pH value is within the pH range = 7-8 for which similar results have been reported in the literature [36]. As shown in Fig. 2, the initial pH first dropped to 6.8 and then increased with time to 8.11 at CD = 5 mA/cm 2 , whereas it increased steadily to 8.78 at CD = 15 mA/ cm 2 . ...
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