Accepted Manuscript Treatment of reverse-osmosis concentrate of print and dyeing wastewater by electrooxidation process with controlled oxidation-reduction potential (ORP) Jiade Wang, Tian Zhang, Yu Mei, Bingjun Pan PII:
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Received Date: 22 January 2018 Revised Date:
6 March 2018
Accepted Date: 7 March 2018
Please cite this article as: Wang, J., Zhang, T., Mei, Y., Pan, B., Treatment of reverse-osmosis concentrate of print and dyeing wastewater by electro-oxidation process with controlled oxidationreduction potential (ORP), Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.03.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Treatment of reverse-osmosis concentrate of print and dyeing wastewater by
electro-oxidation process with controlled oxidation-reduction potential (ORP)
Jiade Wang1, Tian Zhang1, Yu Mei2, Bingjun Pan1*
College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China
College of Biological and Environmental Engineering, Zhejiang Shuren University, Hangzhou,
Zhejiang 310005, China
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E-mail: [email protected]
Tel: +86-571-88320915; Fax: +86-571-88320882
1. Removal efficiencies of COD, TN and chroma are positively correlated with Qsp.
2. The optimal current efficiency can be obtained at a certain current density.
3. A good correlation between ORP and the COD/Cl- removal efficiencies, Qsp is found.
4. The developed constant-ORP system substantially increases energy-efficiency.
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ABSTRACT Reverse osmosis concentrate (ROC) of printing and dyeing wastewater remains as a
daunting environmental issue, which is characterized by high salinity, chemical oxygen
demand (COD), chroma and low biodegradability. In this study electro-oxidation process
(PbO2/Ti electrode) coupled with oxidation-reduction potential (ORP) online monitor was
applied to treat such a ROC effluent. The results show that with the increase of specific
electrical charge (Qsp), the removal efficiencies of COD, TN and chroma increased significantly
at the incipience and then reached a gentle stage; the optimal total current efficiency (12.04
kWh m-3) was obtained with the current density of 10 mA cm-2 (Qsp, 2.45 Ah L-1). Meanwhile,
some inorganic ions can be simultaneously removed to varying degrees. FTIR analyses
indicated that the macromolecular organics were decomposed into smaller molecules. A
multi-parameter linear relationship between ORP and Qsp, COD and Cl- concentration was
established, which can quantitatively reflect the effect of current density, chloride ion
concentration, pollutants and reaction time on the performance of the electro-oxidation
system. As compared to a traditional constant-current system, the constant-ORP system
developed in this study (through the back-propagation neural network [BPN] model with
ORP monitoring) approximately reduced the energy cost by 24-29%. The present work is
expected to provide a potential alternative in optimizing the electro-oxidation process.
Keywords: reverse osmosis concentrate (ROC); printing and dyeing wastewater;
electro-oxidation process; oxidation-reduction potential (ORP); constant-ORP system
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1. Introduction Reverse osmosis process has been widely applied in printing and dyeing wastewater
treatment as it can effectively barrier various organic/inorganic contaminants and also
biological constituents (Šostar-Turk et al., 2005). After being treated, a large proportion of
feed stream can be reused directly in printing and dyeing process (Bagastyo et al., 2013),
while the residual reverse osmosis concentrate (ROC) of printing and dyeing wastewater,
which is characterized by recalcitrant organics, high hardness and high salinity, cannot be
directly discharged and still remains serious environmental risks (Combernoux et al., 2015).
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Lots of conventional techniques have been proposed to further treat stubborn ROC
effluents (Pérez-González et al., 2012). However, the physical and chemical treatments such
as coagulation (Chen et al., 2017), membrane distillation (Qu et al., 2009) and adsorption
(Zhao et al., 2012) will eventually cause secondary pollution and most of them come with a
high waste disposal cost. The biochemical method is also not a good choice for terminal
treatment because of the very low biodegradability and high salinity in ROC effluent (Ersever
et al., 2014). Advanced oxidation processes such as ozonation (Benner et al., 2008),
heterogeneous photocatalysis (Lu et al., 2013) and Fenton (Zhou et al., 2012), are not
economically feasible and need extra reagents. Among the existing treatment options,
electro-oxidation has been widely applied in ROC treatment (Pérez et al., 2010, Radjenovic et
al., 2013). The high content of salts in ROC effluent ensures the good conductivity and can
reduce the cell voltage during electrolysis (Yao et al., 2016). Moreover, massive chloride, a
main component in ROC effluent can serve as active chlorine resource and thus enhance the
indirect oxidation in electro-oxidation process (Pérez et al., 2010). Weng and Pei (2016)
ACCEPTED MANUSCRIPT found that the removal efficiency of COD can reach 59.35% by using Co doped PbO2 anode at
a current density of 80 mA cm-2. Zhou et al. (2011) investigated the COD degradation in
high-salinity ROC effluent and found that it could almost be completely removed by
electro-oxidation at 3.0 Ah L-1 with the electrode of IrO2-RuO2/Ti. Hege et al. (2002) reported
that 70% of COD and 75% of ammonia nitrogen in ROC of printing and dyeing wastewater
was removed by using boron-doped diamond (BDD) as working electrode at a specific
electrical charge of 1.5 Ah L-1.
However, the energy cost of electro-oxidation process is still a major factor that restricts
its wide application in wastewater advanced treatment. Various studies have accordingly
carried out and found that optimizing electrode materials and developing new reactors can
improve the current efficiency and thus reduce the energy consumption (Raghu et al., 2009;
Zhu et al., 2015; Cao et al., 2016). Zou et al. (2016) also found that increasing electrolyte
concentration in the wastewater can improve the current efficiency and save about 32% of
energy consumption. Fortunately, the ROC of printing and dyeing wastewater usually comes
along with high electrolyte concentration. Additionally, since the current efficiency depends
on the current density and is affected by the composition of wastewater, the optimal current
density often fluctuate during operation process. Also, the electro-generated active hydroxyl
radicals (serving as the functional oxidizing species) in electro-oxidation process are difficult
to be online monitored.
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Since 1980s, oxidation-reduction potential (ORP) has been reported as an important
indicator in some wastewater advanced treatment such as Fenton dosage control (Wu and
Wang, 2012), simultaneous nitrification-denitrification in oxidation ditch process (Hou et al., 4
ACCEPTED MANUSCRIPT 2006), and the nanoscale zero-valent iron process for trichloroethylene degradation (Yu et al.,
2015). Some quantitative relationships have been established among ORP, operating
parameters and treatment efficiency to optimize the dosage of chemical agents, guide the
operation of wastewater plant, and save the operation cost and energy consumption. As
compared to current density, ORP is considered to be a more suitable indicator for the
optimal operation of the electro Fenton process (Kishimoto et al., 2015). In electro-oxidation
process, ORP is also the key factor to explore the redox effect of substances at the electrode
surfaces or in the solution. However, until now no literature reports the relationship between
ORP and treatment efficiencies in electro-oxidation processes.
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In this study, the removal efficiencies of COD, TN and chroma for ROC effluent of a real
printing and dyeing wastewater by electro-oxidation process was investigated with different
operation parameters. Fourier transform infrared spectroscopy (FTIR) was used to analyze
the degradation of large organic molecules by electro-oxidation. Meanwhile, the removal of
the inorganic ions in ROC effluent was also inspected during electro-oxidation. Finally, the
optimum response interval and the quantitative relationship among ORP, Qsp, COD and
chloride ion concentration were established to verify the possibility of reducing energy
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2. Materials and methods
2.1. Characteristics of ROC effluent of printing and dyeing wastewater The ROC effluent was collected from a printing and dyeing factory (Zhejiang, China),
which built a wastewater treatment plant with the capacity of 12000 T d-1. With being
treated by microfiltration (MF) and reverse osmosis (RO), 65% of the total wastewater was
reused as productive water, and 35% of the total wastewater (ROC) was discharged into the
public wastewater treatment plant for further purification. The main characteristics of the
ROC effluent are listed in Table 1. Briefly, the COD value is 272±10 mg L-1, and the total
dissolved solids (TDS) is 8490 mg L-1, which is beyond the suitable value of 5000 mg L-1 for
biodegradation, although some microorganism was reported to maintain metabolic activity
in higher TDS concentration (He et al., 2016). Additionally, Due to nitrification in aerobic
reactor, the ROC effluent also contains ≈40 mg L-1 total nitrogen (TN, most of them are
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The dissolved solids in ROC effluent were further analyzed by ion chromatography. As
shown in Table 1, the main anions in ROC effluent are Cl- (1471.8 mg L-1) and SO42- (4739.9
mg L-1), and the major cations are Na+ (4330.5 mg L-1) and K+ (40.6 mg L-1). High
concentration of TDS and chloride would inhibit the biological activity in bioprocess, but are
in favor of the electro-oxidation process. High salinity can provide excellent conductivity to
reduce energy consumption, and high chlorine content promotes indirect oxidation of the
organic matter by increasing active chlorine (Cl2, HOCl) and consequently improves the
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2.2. Electrode selection The working electrode is a key part in electro-oxidation process. Table SM-1 compares
the oxygen evolution potentials (OEP), the removal efficiency of pollutants and the electrode
life of several commercial electrodes. That is, BDD electrode has the highest OEP of 2.3 V, but
it is more expensive. IrO2/Ti electrode and RuO2/Ti electrode have low OEP values (1.6~1.65
V) with poor performance on COD removal. SnO2-Sb2O5/Ti and PbO2/Ti have the same OEP
value of 1.9 V. However, the working life and COD removal efficiency of SnO2-Sb2O5/Ti were
worse than that of PbO2/Ti. Moreover, the price of PbO2/Ti is cheaper than BDD,
SnO2-Sb2O5/Ti, and RuO2/Ti. Therefore, PbO2/Ti is selected as the experimental electrode.
2.3. Electro-oxidation experiment
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A laboratory-scale reactor with an effective volume of 1 L was built as shown in Fig. 1.
The anode is titanium-based lead dioxide (PbO2/Ti) and the cathode is titanium mesh-plate
with the same size of 10 cm × 15 cm. The effective area of the anode is 300 cm2 and the
electrode gap between the anode and cathode is 1.0 cm. A direct current converter (MPS
601, Tradex, United States) was used to deliver power to the electrodes. An ORP probe
(Sanxin, China) and a pH probe (Sanxin, China) were installed in the Electrolysis bath for
online monitoring ORP/pH during the Electro-oxidation. Both probes and direct current
converter were connected to a computer, and the software of Matlab was utilized for
acquiring data and controlling specific electrical charge input. In order to provide a good
mass transfer in electrolyte, a circulation system of wastewater was set up with silicone
rubber tube and peristaltic pump. Five current densities (5, 10, 15, 20 and 25 mA cm-2) were
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The removal efficiencies of COD, TN, chroma and inorganic ions were investigated for
evaluating the efficacy of electro-oxidation. 1000 mL of ROC effluent was continuously
recirculated and a 6.0 mL aliquot of supernate was sampled each time. All electrolysis
processes were carried out without adding any chemical reagent.
2.4. Analysis and calculation methods
The COD values of samples were determined according to the standard of China EPA.
Specifically, sample was mixed with suitable amount of DI water, K2Cr2O7, Ag2SO4/H2SO4 and
HgSO4 (to eliminate the interference of chloride ion), and the mixed solution was digested at
spectrophotometer (DR6000, HACH). Chroma was measured by dilution method, and it
means the dilution times of sample by pure water when its color of the diluted sample is
barely visible by visual inspection. The values of conductivity and total dissolved solids (TDS)
were determined by Conductivity meter (Five Easy Plus, Mettler Toledo, Switzerland). The
concentrations of inorganic ions were measured by ion chromatography (Dionex ICS-2000,
USA). The molecular structures of the organics before and after electrolysis were determined
by Fourier transform infrared spectroscopy (FTIR, Nicole 6700, thermo, USA).
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for 2 h and then measured at a wavelength of 440 nm by UV–visible
The specific removal of pollutants in unit water (R, mg L-1) and the specific electrical
charge (Qsp, Ah L-1) were calculated by the equations (1, 2), respectively:
R = -
Qsp = j ∙ A ∙
Where C0 and Ct are the concentrations of samples at electrolytic time 0 and t (mg L-1), 8
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respectively; j is current density (A cm-2); A is the effective area of electrode (cm2); V is the
effective volume of the electrolysis bank (L); t is reaction time during electrolytic process (h).
The total energy consumption (TEC, kWh m-3) and the total current efficiency (TCE, %) were calculated using the following equations (3, 4): TEC = Qsp ∙ U
TCE = ∑ C ∙ Ei =(
∑[( − ) ∙
Where U (V) is the cell voltage; i is pollutant (COD, NO3- and NH4+, respectively); Ci0 and
Cit are pollutants values measured at electrolytic time 0 and t, respectively; F (96485.3 C
mol-1) is the Faraday constant; ni is the number of electrons transferred; Mi (g mol-1) is the
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The energy consumption (EC, kWh [kg COD]-1) and the instantaneous current efficiency (ICE, %) were calculated by equations (5, 6):
EC=1000 ∙ Qsp ∙
= ∙ ∙ ∙
Where RCOD (mg L-1) is the specific removal of COD, which can be obtained according to Eq.
(1); I (A) is the current, t (s) is reaction time during electrolytic process.
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3. Results and discussion
3.1. Effect of specific electrical charge on removal efficiencies Fig. 2 shows the removal of COD, TN and chroma in ROC effluent of printing and dyeing
wastewater under different specific electrical charges. The specific removal of COD, TN and
chroma increased with the increase of Qsp. In order to analyze the effect of specific electrical
charge on pollutant removal and obtain the appropriate specific electrical charge, the curve
fittings were conducted and the regression equations, regression coefficients and rate
constants are listed in Table 2. The degradations of COD, TN and chroma follow the
first-order kinetics, the reaction constants for COD, TN and chroma are 0.217, 0.021 and
0.258, respectively. As shown, 99 % of chroma can be degraded with only 2.7 Ah L-1 of Qsp,
and the removal efficiency of COD could reach 95.5 % (where the residual COD concentration
is 12.42 mg L-1) under the specific electrical charge of 13.0 Ah L-1.
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In China, the limitation of COD was set as 100 mg L-1 for printing and dyeing factory
effluent discharge (China EPA, 2013, Discharge standards of water pollutants for dyeing and
finishing of textile industry). Therefore, the residual COD value of 80 mg L-1 is set as the end
point of electro-oxidation reaction in this study. Fig. 3 shows the total energy consumptions
(TEC) and the total current efficiencies (TCE) at five current densities when the reaction end
point was reached. With the increase of the current density, TCE increased at first stage and
then decreased, while the values of TEC increased monotonously. The inflection point of TCE
was at the current density of 10 mA cm-2, where the TEC and TCE were 12.04 kWh m-3 and
27.86 %, respectively. Therefore, the suitable current density was 10mA cm-2 and the
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electrolysis time was 40 min, with the specific electrical charge of 2.45 Ah L-1.
3.2. Evaluation of removal efficiencies of inorganic ions Fig. SM-1 shows the removal of inorganic ions in ROC effluent by electro-oxidation
process under the suitable operating parameters (current density: 10 mA cm-2; reaction time:
40 min). Comparatively, higher removal efficiencies of chloride ion (42.0 %), ammonia
nitrogen (54.3 %) and calcium ion (62.7 %) were observed. The removal efficiencies of nitrate
and magnesium were 19.1 % and 21.3 %, respectively, while only 8.2 % of potassium ion and
almost no sulfate and sodium removal were found.
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The chloride in the ROC effluent was oxidized to chlorine (Cl2) (Eq. 7) by the anodic
electron transfer reaction. Subsequently, the chlorine was hydrolyzed to hypochlorous acid
(HClO) and chloric acid (ClO3-) (Eqs. 8-9), which could greatly enhance the indirect oxidation.
2Cl- → Cl2 + 2e-
Cl2 + H2 O → HClO + HCl
12HClO + 6H2 O → 4HClO3 + 8HClO + 3O2 + 12H+ +12e-
During the electro-oxidation process, the calcium ions and magnesium ions in the ROC
effluent moved to the cathode and deposited on the cathode due to the reaction with CO32-
in wastewater (Rocha et al., 2012). Standard electrode potential of Na and K are -2.7 and -2.9
V, respectively, which result in their insignificant removal. The sulfate might not react due to
its low reduction potential (S2-/SO42-, E0= 0.82 V) at cathode in alkaline environment and its
higher oxidation potential (SO42−/S2O82−, E0= 2.10 V) at anode (Espinoza et al., 2016). So the
concentration of sulfate ion almost kept stable in the whole electro-oxidation process.
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3.3. Variation of organic molecular structure The components of organics in the ROC effluent are usually complex. According to the
previous work (Xu, 2010), the main components of organic compounds in the ROC effluent of
printing and dyeing wastewater are ketone ethers (RC-O-R/R-O-R, 40.6 %), amines
(RNH2/RNHR'/RN[R']R'', 29.1 %); and nitriles, alkanes, esters, aromatic hydrocarbons,
carboxylic acids and olefin were also detected.
The samples before and after electro-oxidation treatment were measured by FTIR
instrument and the results are shown in Fig. SM-2. The peak at 1661.1 cm-1 for the untreated
ORC effluent (Fig. SM-2a) represents the structural vibration of C=O, and peaks at 990.7 and
694.1 cm-1 represent the C=C stretching. The peaks at 1362.4 and 830.7 cm-1 indicate the
existence of aromaticity or benzene ring, while the other peaks at 875.8 and 616.4 cm-1
indicate the presence of S-O stretching.
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After electro-oxidation, the components of organics in ROC effluent changed
significantly. The new peak in Fig. SM-2b appears at 1419.9 cm-1, indicating the production of
carboxylic acids (-COO-). while the peaks of 1362.4, 990.7, 830.7 and 694.1 cm-1 completely
disappear and the peaks of 1661.1, 875.8 and 616.4 cm-1 shift to 1637.1, 876.6 and 614.6
cm-1, respectively. After being treated by eletro-oxidation, the macromolecular components
in ROC was degraded to small molecular matters such as organic acids. The above results
show that electro-oxidation process has a destructive function for complex structures of
organics and thus is an effective way to treat ROC of printing and dyeing wastewater.
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3.4. Relationship among ORP, Qsp, COD and chloride ion concentration Electro-oxidation/reduction is essentially the process of electronic gain/loss at electrode
surfaces. The relative oxidizing or reducing strength of solution can be expressed using ORP
(oxidation-reduction potential) to explore the redox interaction effect of substances at a
macroscopic level. Qsp and the concentration of Cl- are two major factors which influence the
removal of pollutants in wastewater during the electro-oxidation process. Fig. 4 shows the
dependences of the values of ORP, pH, COD and Cl- on Qsp during the electro-oxidation
process. The ORP value decreased from 260 ± 25 mV to -75 mV in 2min of electrolyzing and
then increased rapidly to reach a steady stage at 720 ± 10 mV. In contrast, the pH value kept
relatively stable throughout the reaction process (increasing from 8.3 to 8.9). Meanwhile,
the values of COD and Cl- decreased obviously with the increase of specific electrical charge;
and their values can be reduced to 77 mg L-1 and 866 mg L-1, respectively, at the specific
electrical charge of 3.0 Ah L-1.
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According to the previous analysis, ORP has a close correlation with chloride ion
concentration and the specific electrical charge during the electro-oxidation process, and can
affect the COD removal efficiency. In order to find out the relationship among ORP, Qsp, COD
and chloride ion concentration under the suitable operating parameters, multiple regression
was conducted using Origin 8.0 software (Wu and Wang, 2012), their relationships can be
expressed as following:
ORP = -699.35 + 112.01 Qsp - 5.70 COD + 1.75 Cl-
The regression coefficient (R2) is 0.976, reflecting a good correlation among ORP, Qsp,
COD and Cl-. The ORP value is in a positive correlation with Cl- content and Qsp, which 13
ACCEPTED MANUSCRIPT presents oxidizing power; and in a negative correlation with COD value, which presents
reduction power. The good correlation shows that the ORP value gives a comprehensive
indicator to the effect of current density, chloride ion concentration, pollutants and reaction
time on the performance of the electro-oxidation system. Therefore, the ORP value can be
used as an effective controlling factor to optimize the electro-oxidation process.
3.5. The optimization of specific electrical charge (Qsp) control
Fig. 5 shows the relationship between energy consumption (EC), instantaneous current
efficiency (ICE) and ORP during the electro-oxidation. As shown, ORP has significant effect on
the values of EC and ICE, and their relation curves look like two opposite waves along with
the increase of ORP value. It can be found that, in the ORP ranger of 690 to 740 mV, the
maximum value of ICE (≈33 %) and the minimum value of EC (≈50 kWh [kg COD]-1) are
simultaneously achieved at 721±3mV, which can be considered as the optimum response
interval for electro-oxidation. Note that when ORP ≤ 690 mV the residual COD value of the
treated ROC was still larger than the target concentration (80 mg L-1).
291 292 293
By converting Eq. (10), the relationship between Qsp and ORP, COD and chloride ion
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concentration can be obtained: !"# = 6.24 + 0.0009 ORP + 0.05 COD - 0.015 Cl-
Thus, to maintain a relatively constant ORP value, we can accordingly adjust the input of
Qsp by Eq. (11) when providing different concentrations of COD and chloride ion. A computer
control procedure based on back-propagation neural network (BPN) model was developed
with Matlab software to optimize the input of Qsp according to the content of residual 14
ACCEPTED MANUSCRIPT pollutants (Yu et al., 2013). The desired residual value of COD and the constant ORP value are
the two objective parameters to control. The initial content of pollutants in wastewater,
temperature, pH value and Chroma are the original parameters. The Qsp value was optimized
by the computer control procedure to control the ORP at a stable value, and the energy
consumption could be calculated at this constant ORP system during electro-oxidation
process. With the target residual COD value (80 mg L-1), the total energy consumption (TEC)
in constant-ORP system was 9.06 kWh m-3, while that for constant-current system was
12.04kWh m-3. Thus, the developed constant-ORP system can save the energy consumption
by 24.75% as compared to the constant-current system. Consequently, monitoring of ORP
has high potential to effectively optimize the electro-oxidation process.
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The electro-oxidation technique can be considered as an effective, feasible and robust
treatment method for ROC effluent in printing and dyeing factory by using PbO2/Ti electrode.
With a suitable specific electrical charge of 2.45 Ah L-1, high removal efficiencies of COD
(72 %), TN (18 %) and chroma (99 %) were achieved. FTIR analyses show that the
electro-oxidation technique has a significant degradation effects to the organic matters of
complex structures in ROC effluent. Additionally, the electro-oxidation process can also
simultaneously remove some inorganic ions. The quantitative relationship among ORP, Qsp,
COD and chloride ion concentration was established. This good correlation shows that the
ORP value gives a comprehensive indicator to the effect of current density, chloride ions,
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pollutants and reaction time on the performance of electro-oxidation system. The developed
constant-ORP system showed the success of online monitoring of ORP in optimizing the
The authors are grateful for the financial support provided by the National Natural
Science Foundation of China (Grant No.51278465) and the Transformation Projects in the
Science & Technology Pillar Program of Zhejiang Province (Grant No.20130176).
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Tables and Figures
Table 1. General characteristics of the ROC effluent. Physical-chemical properties COD
272±10 mg L-1 -1
Inorganic ions Na+
40.6 mg L-1
40.8 mg L
1.8 mg L-1
26.9 mg L-1
7.5 mg L-1
4.0 mg L-1
17.0 ms cm-1
4739.9 mg L-1
8.49 g L-1
1471.8 mg L-1
0.6 mg L-1
RCOD = -203.6 exp(-Qsp/1.993) + 241.6
RTN = -9.78 exp(-Qsp /5.913) + 11.6
RCOD (mg L-1), RTN (mg L-1) and RChroma (times) are the specific removal of COD, TN and chroma, respectively; Qsp is the specific electrical charge (Ah L-1).
Chroma RChroma = -85.9 exp(-Qsp /0.879) + 156.5
1.3 mg L-1
Table 2. Regression equations, coefficients and rate constants for the dependences of the special removals of COD, TN and chroma on the specific electrical charge. rate constants regression equationa regression coefficient (R2) (k,s-1)
24.9 mg L-1
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Fig.1. Diagram of experimental setup of eletro-oxidation.
RTN (mg L
45 30 15
Qsp (Ah L
442 443 444
RCOD (mg L
COD (mg L
225 COD RCOD
TN (mg L
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Fig.2. Effect of specific electric charge on COD, TN and chroma removal efficiencies; the initial values of COD, TN and chroma were 272 mg L-1, 40.82 mg L-1, and 160 Times, respectively.
445 446 21
ACCEPTED MANUSCRIPT 447 29 TCE
TEC (kWh m )
current density (mA cm )
Fig. 3. The influence of current density on the total current efficiency (TCE) and the total energy consumption (TEC); the residual COD value was set as 80 mg L-1.
400 200 0 -200
453 454 455
200 150 100 50 1400
Cl- (mg L-1)
COD (mg L-1)
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448 449 450
Qsp (Ah L )
Fig. 4. The effect of Qsp on the values of ORP, pH, COD and chloride ion concentration; current density: 10 mA cm-2, reaction time: 40 min.
456 457 458 459 22
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461 462 463
Fig. 5. The relationship between energy consumption (EC), instantaneous current efficiency (ICE) and ORP during the electro-oxidation process.