Journal of Hazardous Materials 145 (2007) 100–108

Electrochemical degradation and toxicity reduction of C.I. Basic Red 29

solution and textile wastewater by using diamond anode
A. Savaş Koparal ∗ , Yusuf Yavuz, Canan Gürel, Ülker Bakır Öğütveren
Anadolu Üniversitesi, Çevre Sor.Uyg. ve Araş, Merkezi, Eskişehir, Turkey
Received 17 August 2006; received in revised form 31 October 2006; accepted 31 October 2006
Available online 7 November 2006

Abstract
Electrochemical oxidation of Basic Red 29 (BR29) was studied in a bipolar trickle tower (BTT) reactor by using Raschig ring shaped boron-
doped diamond (BDD) electrodes, which were originally employed by the present researchers, in a recirculated batch mode. The model solution
was prepared with BR29 using distilled water. The effects of initial dye concentration, Na2 SO4 concentration as supporting electrolyte, current
density, flow rate and initial pH on the removal efficiency were investigated, and practically, complete BR29 removal (over 99%) was obtained in
all the studies. After optimum experimental conditions were determined, textile wastewater has also studied by monitoring the destruction of color
and COD. With the textile wastewater, 97.2% of color and 91% of COD removal were, respectively, achieved at the current density of 1 mA/cm2 .
Microtox toxicity tests were performed in both BR29 solution and textile wastewater under optimum experimental conditions, and relatively good
toxicity reductions were obtained with respect to the initial values. According to the results, BDD anode was seen to be a unique material for the
degradation of BR29 and COD and also the reduction of toxicity simultaneously.
© 2006 Elsevier B.V. All rights reserved.

Keywords: Electrochemical oxidation; Dye; Textile wastewater; Boron-doped diamond; Toxicity; Bipolar trickle tower reactor

1. Introduction process) in comparison with most industrial wastewaters [3].

For all these reasons, textile wastewater needs to be treated to
Textile wastewater is characterized by strong color, large satisfy discharging standards.
amount of suspended solids, broadly fluctuating pH, high chem- Textile wastewater is typically treated by conventional meth-
ical oxygen demand (COD) and biotoxicity and causes coloring ods. During chemical precipitation, although additives increase
of the receiving water environment [1]. Different dyes result treatment efficiency, a sludge disposal problem is created [3].
in wastewater with different colors and the variations in color Ozone and hypochlorite oxidation are efficient decolorization
induce variation in chemical oxygen demand (COD) of the methods, but they are not desirable due to the high investment
wastewater [2]. The pH change is primarily caused by differ- and operational cost, and the secondary pollution arising from
ent kinds of dye stuffs used in the dyeing process. The pH the residual chlorine [1]. The wide range of pH and elevated tem-
value of the wastewater can range from 2 to more than 12. peratures are also additional problems encountered when textile
Textile wastewater, depending on dye used, can have a toxic wastewaters are treated by conventional methods [3]. Azo dyes
effect on the living organisms in the receiving water, affects like BR29 (contain –N N– bonds) are resistant to biodegrada-
the ecosystem adversely, and reduces the assimilative capacity tion under aerobic conditions whereas anaerobic treatment is
of the environment. The temperature of textile wastewaters is applied successfully. However, textile wastewater is not proper
unusually high (typically 40 ◦ C because of hot rinse waters and to use anaerobic process because the breakdown of azo dye leads
the temperatures up to 90 ◦ C used in various steps in the dyeing to the formation of aromatic amines, which may be more toxic
than the dye molecules themselves [4]. Electrochemical meth-
ods are also used successfully for the degradation of dyes and
Abbreviations: BDD, boron-doped diamond; BR29, basic red 29; BTT, treatment of textile wastewaters.
bipolar trickle tower; COD, chemical oxygen demand; HF CVD, hot filament
In recent years there has been an increasing interest in the
chemical vapor deposition
∗ Corresponding author. Tel. +90 222 321 35 50; fax: +90 222 323 95 01. use of electrochemical methods for the destruction of toxic and
E-mail address: askopara@anadolu.edu.tr (A.S. Koparal). biorefractory organic pollutants. These methods use the electron

0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2006.10.090
 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108 101

as the main reagent, but also require the presence of supporting Table 1
electrolytes. In general, the supporting electrolytes exist in the Properties of Basic Red 29 (BR29)
wastewaters to be treated, but not always in sufficient concen-
trations. These processes can operate at ambient temperature
without a need of temperature control [5].
Decolorization can be achieved either by electrooxidation Structural formulae (chemical structure)
with insoluble anodes [1,5–12] or by electrocoagulation using
consumable materials [13–18]. The degradation products in the
oxidation of azo dyes are typically carbon dioxide, nitrate and Synonym Basacryl Red GL
sulfate, with the possible formation of aromatic esters, phenols, Molecular formula C19 H17 ClN4 S
aromatic carboxylic acids, cyclic and aliphatic hydrocarbons, MW (g/mol) 368.88
etc., as intermediates. Usually, the oxidation of azo group occurs, λmax (nm) 511
followed by the oxidation of the decomposition products [5]. Source/purity Aldrich/19%
Electrode material can influence the mechanism and conse-
quently the products of anodic reaction. Electrode material is
conductive (<0.1 (cm) BDD films, deposited on niobium sub-
the most important parameter in the electrochemical oxidation
strates via the hot filament chemical vapor deposition technique
of organics. BDD has a great potential for electrochemical appli-
(HF CVD) from a gaseous feed of methane and a boron doping
cations, especially for the treatment of wastewater and drinking
agent in dihydrogen.
water because of the extraordinary chemical inertness offering
Experiments were carried out in a recirculated batch mode
the opportunity to use such electrodes (anodes as well as cath-
at ambient temperature and wastewater was fed to the reactor
odes) in very aggressive media. The electrochemical properties
by means of a peristaltic pump. Model solution was recirculated
of diamond provide a wide range of applications due to the
through the electrochemical reactor with the flow rates of 24.83,
extreme electrochemical window (>3 V) for almost any reac-
36.3 and 47.8 mL/min (1.5, 2.2 and 2.9 L/h). Current densities
tion at the surface, before hydrogen forms at the cathode and
of 0.25, 0.5, 0.75 and 1 mA/cm2 were studied to observe their
oxygen at the anode [19,20].
effects. pH values of 3, 5.8 (original pH of the BR29 solution)
BDD anodes allow the direct production of hydroxyl radicals
and 11 were studied to investigate the effect of initial pH of the
(OH• ) from aqueous electrolysis with very high current effi-
solution. In addition, supporting electrolyte concentration was
ciencies as dominant degradation mechanism in dye oxidation
also studied using the solutions of 0.01, 0.03, 0.04 and 0.05 M
[21,22] according to Eq. (1).
Na2 SO4 . pH of the working solution was monitored throughout
H2 O → OH• + e− + H+ (1) the studies.
Wastewater characteristics of textile wastewater which was
Dissimilarity of this study from the others in the literature provided from a local plant are given in Table 2.
is the shape (Raschig ring) of BDD electrodes and BTT reac-
tor used. Furthermore, toxicity studies performed in both model 2.1. Analysis
solution and textile wastewater could be a remarkable contribu-
tion to the literature. BR29 (ALDRICH) was preferred to prepare the model solu-
tion and, Na2 SO4 (MERCK) for the formation of the supporting
electrolyte solution. pH of the solutions were adjusted with
2. Materials and methods
H2 SO4 (MERCK) and NaOH (MERCK). All chemicals used
were analytical grade.
Aqueous solution of BR29 was chosen as the model wastew-
ater. BR29 has been reported by the manufacturer as the
mutagenic to the microorganisms and mammalian somatic cells
at the chronic exposure [23]. The chemical structure and other
characteristics of BR29 are shown in Table 1.
Experimental setup is illustrated in Fig. 1. BTT reactor had a
volume of 125 mL and consisted of two concentric glass pipes
with the inner diameters of 4 and 2.5 cm. The distance between
the feeding electrodes was about 22.5 cm. BDD Raschig rings
were used as working electrode and placed in the inner glass pipe
at the bipolar electrode configuration. Raschig rings employed in
the reactor had a height of 8 mm, and inner and outer diameters of
6 and 8 mm, respectively. Total surface area of BDD electrodes
was 352 cm2 . BDD electrodes were provided from Magneto
Special Anodes B.V. (Schiedam, The Netherlands) shaped as
Raschig rings had an outer diameter of 0.8 cm and a height of
0.8 cm. The BDD electrodes consisted of thin (2–7 (m), highly Fig. 1. Experimental setup used in the studies.
102 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108

Table 2 Table 3
Physical and chemical characteristics of textile wastewater Typical results of the study with BR29 solution (40 mg/L BR29 + 0.03 M
Na2 SO4 , i = 1 mA/cm2 , Q = 36.3 mL/min, pH = original)
Parameter Value
Time (min) pH Remaining dye Dye removal
pH 12.51 concentration (mg/L) efficiency (%)
Conductivity (mS/cm) 5.74
COD (mg/L) 566.45 0 6.07 40 –
TOC (mg/L) 380 1 7.40 0.71 98.2
SS (mg/L) 155 2 9.57 0.53 98.7
Cl−1 (mg/L) 350 3 9.95 0.47 98.8
4 10.09 – –
5 10.25 – –
During the model solution studies, BR29 concentration was
determined spectrophotometrically at λmax = 508 nm according
to Lambert–Beer law by using a Shimadzu UV-1700 model monitored as a percent decrease of the light emission of V. fis-
spectrophotometer. cheri after 5 min of incubation at 15 ◦ C with 95% confidence
The rate of the electrochemical degradation of BR29 solu- intervals by using the Microtox calculation software (version
tion and textile wastewater was monitored by COD tests besides 1.18).
being followed by UV–visible spectrophotometry. COD tests In the experimental studies, Statron 3234.9 model power sup-
were carried out using a COD reactor (HACH) according to the ply, OGSM 3900 digital multimeter, Multifix MC 1000 PEC
standard methods. model peristaltic pump, Polyscience 9605 model water bath,
Toxicities of the model solutions and textile wastewater were Orion 420 A model pH meter were employed as auxiliary equip-
established with Microtox Model 500 Analyzer. The Microtox® ments.
system was supplied by AZUR Environmental (Carlsbad, CA),
and consisted of lyophilized bacterial reagent Vibrio fischeri,
reconstitution reagents and the Model 500 Toxicity Analyzer 3. Results and discussion
(AZUR Environmental), which utilizes freeze-dried lumines-
cent bacteria (V. fischeri, previously named Photobacterium In this section, electrochemical oxidation of BR29 was
phosphoreum) as test organisms. The short-term luminescent searched in preliminary experiments. After the best experimen-
bacteria assay was done according to the supplier’s protocol for tal conditions had been determined, electrochemical oxidation
the duplicate basic test [24]. Each test consisted of one con- of textile wastewater was studied. COD kinetics and toxicity
trol and four serial dilutions of each sample. Toxic effects were results were also given in separate subtitles.

Fig. 2. Remaining amount of dye for different current densities (40 mg/L BR29 + 0.03 M Na2 SO4 , Q, 36.3 mL/min, pH = original) (a) 1 min of electrolysis and (b)
5 min of electrolysis.
 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108 103

Fig. 3. Variation of removal efficiency vs. time for different current densities Fig. 5. Variation of pH with time in BR29 solution for different support-
(40 mg/L BR29 + 0.03 M Na2 SO4 , Q = 36.3 mL/min, pH = original). ing electrolyte concentrations (40 mg/L BR29, i = 1 mA/cm2 , Q = 36.3 mL/min,
pH = original).
3.1. Electrochemical oxidation of BR29
for an initial dye concentration of 40 mg/L at a current density
The effects of current density, initial pH, supporting elec- of 1 mA/cm2 .
trolyte concentration, initial BR29 concentration and solution Current density was an important parameter for dye removal
flow rate on the removal efficiency were examined to obtain best as presented in Fig. 2. In this figure, UV–vis spectra of BR29
values. Current density of 1 mA/cm2 , flow rate of 36.3 mL/min, can be seen for different current densities both after 1 and
Na2 SO4 concentration of 0.03 M and original pH of the solution 5 min of electrolysis. Increase in the current density resulted
were found as the best values. Since BR29 solution has a lower in a decrease in the BR29 concentration. About 1 mA/cm2 was
electrical conductivity, sodium sulfate was used as supporting obtained as the most appropriate current density value. As it
electrolyte in the studies. can also be seen from Fig. 2 that almost same UV–vis spec-
It is clear that the method employed is very effective for trums were obtained at the current density of 0.25 mA/cm2 after
dye removal as can be seen from Table 3. After only 1 min of 1 and 5 min of electrolysis. Variation of dye removal efficiency
electrolysis, final dye concentration of 0.71 mg/L was reached versus time for different current densities is depicted in Fig. 3.

Fig. 4. Remaining amount of dye for different initial pH’s (40 mg/L Fig. 6. Remaining amount of dye for different supporting electrolyte concentra-
BR29 + 0.03 M Na2 SO4 , i = 1 mA/cm2 , t = 1 min, Q = 36.3 mL/min). tions (40 mg/L BR29, i = 1 mA/cm2 , t = 1 min, Q = 36.3 mL/min, pH = original).
104 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108

Over 98% dye removal was achieved for all current densities, fore, 0.03 M was preferred because lower amount of additional
except 0.25 mA/cm2 , relating with the duration of electroly- chemical usage was necessary. As well known, supporting elec-
sis. Dye removals of over 98% were obtained after 1, 20 and trolyte concentration is inversely proportional to the energy
45 min of electrolysis with the current densities of 1, 0.75 and consumption. Therefore, an increase in its concentration results
0.5 mA/cm2 , respectively, whereas it was 88.9% at the current in increase in the electrical conductivity and decrease in energy
density of 0.25 mA/cm2 after 90 min of electrolysis. Higher the consumed. Electrical conductivities of dye solutions contain-
current density, shorter the time to reach desired effluent dye ing 0.01, 0.03, 0.04 and 0.05 M sodium sulfate were 2.82, 5.54,
concentration. 7.38 and 8.92 mS/cm and energy consumptions for these solu-
Studies were performed in three different initial pH values to tions after 1 min of electrolysis were 0.466, 0.342, 0.311 and
investigate their effects as depicted in Fig. 4. It was found that 0.303 kWh/g, respectively.
acidic initial pH caused the reduction in the efficiency of the BR29 concentrations of 20, 40 and 60 mg/L were studied
study, and the performances in the initial pH of 11 and original and the results were shown in Fig. 7. According to the UV–vis
pH studies were similar. Consequently, experiments were carried spectra results, not only the dye removal but also the organics
out at the original pH of ∼5.8 without any pH adjustment. Thus, degradations were seen in UV region for different initial dye con-
additional chemical usage was not required and this makes the centrations. Over 95% of dye removal efficiencies were achieved
process more cost effective. When the studies were performed in in 1 min of electrolysis for all dye concentrations. Variation of
the different pH values, pH of the solutions generally increased initial dye concentration did not affect the performance of pro-
as shown in Fig. 5. This is probably due to the degradation of cess. Actually, BR29 concentration of 60 mg/L can be thought
dye and the electrode reactions. As the studies were performed as quite high when the characteristics of a textile wastewater are
under uncontrolled pH conditions, initial pH of ∼6 increased up considered. However, it can be concluded that electrochemical
to ∼10 as illustrated in Fig. 5. degradation by using BDD electrode in a BTT reactor is very
The results obtained from the studies on the investiga- efficient method for dye removal even if in relatively higher dye
tion supporting electrolyte effect can be seen in Fig. 6. Dye concentrations.
removal efficiencies were higher in the 0.03 and 0.04 M sodium Remaining amount of dye and degradation of organics for
sulfate concentrations than that in the 0.01 and 0.05 M. There- different flow rates were given in Fig. 8. In general, insignificant
differences have been observed between the results of different

Fig. 7. Remaining amount of dye for different initial dye concentrations

(40 mg/L BR29 + 0.03 M Na2 SO4 , i = 1 mA/cm2 , t = 1 min, Q = 36.3 mL/min, Fig. 8. Remaining amount of dye for different flow rates (40 mg/L
pH = original). BR29 + 0.03 M Na2 SO4 , i = 1 mA/cm2 , t = 1 min, pH = original).
 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108 105

flow rates. However, 36.3 mL/min was chosen as optimum flow density of 0.25 mA/cm2 was applied, initial BR29 concen-
rate since the results of organics spectrums in the UV region tration of 40 mg/L decreased to lower concentrations. Faster
indicated this flow rate. Thus, the other studies were performed BR29 degradation rates were observed at the current density
at this flow rate. of 0.5 mA/cm2 than that seen at 0.25 mA/cm2 . However, dye
Time course changes of UV–vis spectra of BR29 for differ- degradations were also realized gradually at the current den-
ent current densities were given in Fig. 9. When the current sity of 0.5 mA/cm2 . On the other hand, sharp decrease in the

Fig. 9. Time course changes of spectrums for different current densities (40 mg/L BR29 + 0.03 M Na2 SO4 , Q = 36.3 mL/min, pH = original) (a) 0.25 mA/cm2 (b)
0.5 mA/cm2 (c) 1 mA/cm2 .
106 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108

BR29 concentration was experienced at 1 mA/cm2 . In addition

to the BR29 spectrums in the VIS region, organics degradations
were also investigated in the UV region between 250–300 nm.
It can be said that high removal efficiencies can be achieved
for BR29 at all current densities in the VIS region without
a time concern. However, while the organics degradation was
investigated in the UV region, only the current density of
1 mA/cm2 was found to be efficient whereas the other cur-
rent densities were insufficient. Decomposition of organics at
1 mA/cm2 in the region of 250–300 nm was shown in Fig. 9.
Although BR29 removal rate is higher at 0.5 mA/cm2 than
that of 0.25 mA/cm2 , organics removal rates were almost equal Fig. 11. Variation of COD and pH with time in textile wastewater study
(Fig. 9). (i = 1 mA/cm2 , Q = 36.3 mL/min, pH = original).

3.2. Electrochemical oxidation of textile wastewater tants decrease regularly as depicted between 250 and 300 nm.
As it can be seen from Fig. 11, initial COD of 566.45 mg/L
Under the optimum experimental conditions, a textile decreased gradually to final COD of 52.38 mg/L in a period of
wastewater was treated by electrochemical oxidation using BDD 8 h. COD removal of 91.0% was achieved with an energy con-
Raschig ring anodes and BTT reactor. Wastewater was obtained sumption of 1.4 kWh/g COD removed. Change of wastewater
from a local plant and had a COD of 566.45 mg/L. Electrical pH between 10 and 12 with time was also given in the same
conductivity of the wastewater was 5.74 mS/cm and pH 12.51. figure
In a textile wastewater, total organics removal is more
important than dye removal, because organics removal is more 3.3. COD kinetics
difficult. In this study variation of the concentrations of organics
and dye was also monitored by UV–vis spectroscopy (Fig. 10). In direct electrochemical oxidation process, the COD
According to the Fig. 10, concentrations of total organic pollu- removal rate is proportional to the concentration of the organic
pollutant. Therefore, the kinetics for COD removal is written as:
d
− [COD] = k[COD] (2)
dt
Rearranging and integrating the Eq. (2) gives:
[CODt ] Ct
ln = −k.t or ln = −k.t (3)
[CODt ] Co

where, Co is the initial COD of the solution in mg L−1 , and Ct

is the COD in mg L−1 at time t.
Plotting Ct /Co on the y-axis versus t on the x-axis on semilog
paper will result in straight line with the slope of k. According to
the Fig. 12, the rate constant (k) and r2 values for COD removal
were 5.5 × 10−3 min−1 and 0.9326, respectively.

Fig. 10. UV–visible spectra of textile wastewater study (i = 1 mA/cm2 , Fig. 12. Variation of ln(Ct /Co ) with time for the electrochemical degradation of
Q = 36.3 mL/min, pH = original). textile wastewater.
 A.S. Koparal et al. / Journal of Hazardous Materials 145 (2007) 100–108 107

Fig. 13. Variation of toxicity with time in: (a) 40 mg/L BR29 (+0.03 M Na2 SO4 ) and (b) the textile wastewater (i = 1 mA/cm2 , Q = 36.3 mL/min).

3.4. Toxicity studies Acknowledgement

Microtox® bioassay tests were performed to measure the tox- The authors are grateful to the Anadolu University Research
icity of model solution and textile wastewater treated in the Fund for supporting this research (Project no: 03.02.34).
electrochemical reactor in a given time intervals including the
time zero. Toxicity results were given as relative toxicity index
(RTI) [25]: References
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