Treatment of reverse-osmosis concentrate of printing and dyeing wastewater by electro-oxidation process with controlled oxidation-reduction potential (ORP)

Treatment of reverse-osmosis concentrate of printing and dyeing wastewater by electro-oxidation process with controlled oxidation-reduction potential (ORP)

Accepted Manuscript Treatment of reverse-osmosis concentrate of print and dyeing wastewater by electrooxidation process with controlled oxidation-redu...

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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:

S0045-6535(18)30465-X

DOI:

10.1016/j.chemosphere.2018.03.051

Reference:

CHEM 20995

To appear in:

ECSN

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

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electro-oxidation process with controlled oxidation-reduction potential (ORP)

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Jiade Wang1, Tian Zhang1, Yu Mei2, Bingjun Pan1*

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College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China

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College of Biological and Environmental Engineering, Zhejiang Shuren University, Hangzhou,

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Zhejiang 310005, China

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E-mail: [email protected]

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Tel: +86-571-88320915; Fax: +86-571-88320882

Corresponding author

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Highlights:

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1. Removal efficiencies of COD, TN and chroma are positively correlated with Qsp.

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2. The optimal current efficiency can be obtained at a certain current density.

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3. A good correlation between ORP and the COD/Cl- removal efficiencies, Qsp is found.

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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

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daunting environmental issue, which is characterized by high salinity, chemical oxygen

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demand (COD), chroma and low biodegradability. In this study electro-oxidation process

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(PbO2/Ti electrode) coupled with oxidation-reduction potential (ORP) online monitor was

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applied to treat such a ROC effluent. The results show that with the increase of specific

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electrical charge (Qsp), the removal efficiencies of COD, TN and chroma increased significantly

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at the incipience and then reached a gentle stage; the optimal total current efficiency (12.04

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kWh m-3) was obtained with the current density of 10 mA cm-2 (Qsp, 2.45 Ah L-1). Meanwhile,

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some inorganic ions can be simultaneously removed to varying degrees. FTIR analyses

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indicated that the macromolecular organics were decomposed into smaller molecules. A

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multi-parameter linear relationship between ORP and Qsp, COD and Cl- concentration was

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established, which can quantitatively reflect the effect of current density, chloride ion

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concentration, pollutants and reaction time on the performance of the electro-oxidation

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system. As compared to a traditional constant-current system, the constant-ORP system

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developed in this study (through the back-propagation neural network [BPN] model with

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ORP monitoring) approximately reduced the energy cost by 24-29%. The present work is

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expected to provide a potential alternative in optimizing the electro-oxidation process.

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Keywords: reverse osmosis concentrate (ROC); printing and dyeing wastewater;

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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

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treatment as it can effectively barrier various organic/inorganic contaminants and also

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biological constituents (Šostar-Turk et al., 2005). After being treated, a large proportion of

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feed stream can be reused directly in printing and dyeing process (Bagastyo et al., 2013),

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while the residual reverse osmosis concentrate (ROC) of printing and dyeing wastewater,

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which is characterized by recalcitrant organics, high hardness and high salinity, cannot be

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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

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effluents (Pérez-González et al., 2012). However, the physical and chemical treatments such

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as coagulation (Chen et al., 2017), membrane distillation (Qu et al., 2009) and adsorption

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(Zhao et al., 2012) will eventually cause secondary pollution and most of them come with a

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high waste disposal cost. The biochemical method is also not a good choice for terminal

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treatment because of the very low biodegradability and high salinity in ROC effluent (Ersever

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et al., 2014). Advanced oxidation processes such as ozonation (Benner et al., 2008),

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heterogeneous photocatalysis (Lu et al., 2013) and Fenton (Zhou et al., 2012), are not

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economically feasible and need extra reagents. Among the existing treatment options,

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electro-oxidation has been widely applied in ROC treatment (Pérez et al., 2010, Radjenovic et

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al., 2013). The high content of salts in ROC effluent ensures the good conductivity and can

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reduce the cell voltage during electrolysis (Yao et al., 2016). Moreover, massive chloride, a

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main component in ROC effluent can serve as active chlorine resource and thus enhance the

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indirect oxidation in electro-oxidation process (Pérez et al., 2010). Weng and Pei (2016)

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ACCEPTED MANUSCRIPT found that the removal efficiency of COD can reach 59.35% by using Co doped PbO2 anode at

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a current density of 80 mA cm-2. Zhou et al. (2011) investigated the COD degradation in

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high-salinity ROC effluent and found that it could almost be completely removed by

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electro-oxidation at 3.0 Ah L-1 with the electrode of IrO2-RuO2/Ti. Hege et al. (2002) reported

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that 70% of COD and 75% of ammonia nitrogen in ROC of printing and dyeing wastewater

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was removed by using boron-doped diamond (BDD) as working electrode at a specific

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electrical charge of 1.5 Ah L-1.

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However, the energy cost of electro-oxidation process is still a major factor that restricts

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its wide application in wastewater advanced treatment. Various studies have accordingly

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carried out and found that optimizing electrode materials and developing new reactors can

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improve the current efficiency and thus reduce the energy consumption (Raghu et al., 2009;

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Zhu et al., 2015; Cao et al., 2016). Zou et al. (2016) also found that increasing electrolyte

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concentration in the wastewater can improve the current efficiency and save about 32% of

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energy consumption. Fortunately, the ROC of printing and dyeing wastewater usually comes

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along with high electrolyte concentration. Additionally, since the current efficiency depends

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on the current density and is affected by the composition of wastewater, the optimal current

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density often fluctuate during operation process. Also, the electro-generated active hydroxyl

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radicals (serving as the functional oxidizing species) in electro-oxidation process are difficult

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to be online monitored.

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Since 1980s, oxidation-reduction potential (ORP) has been reported as an important

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indicator in some wastewater advanced treatment such as Fenton dosage control (Wu and

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Wang, 2012), simultaneous nitrification-denitrification in oxidation ditch process (Hou et al., 4

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2015). Some quantitative relationships have been established among ORP, operating

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parameters and treatment efficiency to optimize the dosage of chemical agents, guide the

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operation of wastewater plant, and save the operation cost and energy consumption. As

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compared to current density, ORP is considered to be a more suitable indicator for the

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optimal operation of the electro Fenton process (Kishimoto et al., 2015). In electro-oxidation

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process, ORP is also the key factor to explore the redox effect of substances at the electrode

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surfaces or in the solution. However, until now no literature reports the relationship between

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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

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printing and dyeing wastewater by electro-oxidation process was investigated with different

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operation parameters. Fourier transform infrared spectroscopy (FTIR) was used to analyze

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the degradation of large organic molecules by electro-oxidation. Meanwhile, the removal of

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the inorganic ions in ROC effluent was also inspected during electro-oxidation. Finally, the

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optimum response interval and the quantitative relationship among ORP, Qsp, COD and

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chloride ion concentration were established to verify the possibility of reducing energy

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consumption.

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2. Materials and methods

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2.1. Characteristics of ROC effluent of printing and dyeing wastewater The ROC effluent was collected from a printing and dyeing factory (Zhejiang, China),

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which built a wastewater treatment plant with the capacity of 12000 T d-1. With being

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treated by microfiltration (MF) and reverse osmosis (RO), 65% of the total wastewater was

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reused as productive water, and 35% of the total wastewater (ROC) was discharged into the

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public wastewater treatment plant for further purification. The main characteristics of the

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ROC effluent are listed in Table 1. Briefly, the COD value is 272±10 mg L-1, and the total

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dissolved solids (TDS) is 8490 mg L-1, which is beyond the suitable value of 5000 mg L-1 for

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biodegradation, although some microorganism was reported to maintain metabolic activity

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in higher TDS concentration (He et al., 2016). Additionally, Due to nitrification in aerobic

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reactor, the ROC effluent also contains ≈40 mg L-1 total nitrogen (TN, most of them are

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nitrate).

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The dissolved solids in ROC effluent were further analyzed by ion chromatography. As

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shown in Table 1, the main anions in ROC effluent are Cl- (1471.8 mg L-1) and SO42- (4739.9

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mg L-1), and the major cations are Na+ (4330.5 mg L-1) and K+ (40.6 mg L-1). High

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concentration of TDS and chloride would inhibit the biological activity in bioprocess, but are

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in favor of the electro-oxidation process. High salinity can provide excellent conductivity to

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reduce energy consumption, and high chlorine content promotes indirect oxidation of the

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organic matter by increasing active chlorine (Cl2, HOCl) and consequently improves the

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current efficiency.

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2.2. Electrode selection The working electrode is a key part in electro-oxidation process. Table SM-1 compares

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the oxygen evolution potentials (OEP), the removal efficiency of pollutants and the electrode

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life of several commercial electrodes. That is, BDD electrode has the highest OEP of 2.3 V, but

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it is more expensive. IrO2/Ti electrode and RuO2/Ti electrode have low OEP values (1.6~1.65

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V) with poor performance on COD removal. SnO2-Sb2O5/Ti and PbO2/Ti have the same OEP

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value of 1.9 V. However, the working life and COD removal efficiency of SnO2-Sb2O5/Ti were

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worse than that of PbO2/Ti. Moreover, the price of PbO2/Ti is cheaper than BDD,

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SnO2-Sb2O5/Ti, and RuO2/Ti. Therefore, PbO2/Ti is selected as the experimental electrode.

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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.

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The anode is titanium-based lead dioxide (PbO2/Ti) and the cathode is titanium mesh-plate

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with the same size of 10 cm × 15 cm. The effective area of the anode is 300 cm2 and the

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electrode gap between the anode and cathode is 1.0 cm. A direct current converter (MPS

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601, Tradex, United States) was used to deliver power to the electrodes. An ORP probe

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(Sanxin, China) and a pH probe (Sanxin, China) were installed in the Electrolysis bath for

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online monitoring ORP/pH during the Electro-oxidation. Both probes and direct current

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converter were connected to a computer, and the software of Matlab was utilized for

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acquiring data and controlling specific electrical charge input. In order to provide a good

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mass transfer in electrolyte, a circulation system of wastewater was set up with silicone

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rubber tube and peristaltic pump. Five current densities (5, 10, 15, 20 and 25 mA cm-2) were

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ACCEPTED MANUSCRIPT designed to observe the effect of the specific electrical charge on the electrolysis efficiency.

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The removal efficiencies of COD, TN, chroma and inorganic ions were investigated for

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evaluating the efficacy of electro-oxidation. 1000 mL of ROC effluent was continuously

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recirculated and a 6.0 mL aliquot of supernate was sampled each time. All electrolysis

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processes were carried out without adding any chemical reagent.

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2.4. Analysis and calculation methods

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The COD values of samples were determined according to the standard of China EPA.

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Specifically, sample was mixed with suitable amount of DI water, K2Cr2O7, Ag2SO4/H2SO4 and

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HgSO4 (to eliminate the interference of chloride ion), and the mixed solution was digested at

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spectrophotometer (DR6000, HACH). Chroma was measured by dilution method, and it

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means the dilution times of sample by pure water when its color of the diluted sample is

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barely visible by visual inspection. The values of conductivity and total dissolved solids (TDS)

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were determined by Conductivity meter (Five Easy Plus, Mettler Toledo, Switzerland). The

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concentrations of inorganic ions were measured by ion chromatography (Dionex ICS-2000,

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USA). The molecular structures of the organics before and after electrolysis were determined

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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

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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:

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R =  -

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Qsp = j ∙ A ∙ 

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(1) 

(2)

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

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effective volume of the electrolysis bank (L); t is reaction time during electrolytic process (h).

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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

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TCE = ∑ C ∙ Ei =(

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(3)

∑[( −  ) ∙

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Where U (V) is the cell voltage; i is pollutant (COD, NO3- and NH4+, respectively); Ci0 and

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Cit are pollutants values measured at electrolytic time 0 and t, respectively; F (96485.3 C

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mol-1) is the Faraday constant; ni is the number of electrons transferred; Mi (g mol-1) is the

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molar mass.

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The energy consumption (EC, kWh [kg COD]-1) and the instantaneous current efficiency (ICE, %) were calculated by equations (5, 6):

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EC=1000 ∙ Qsp ∙ 

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Where RCOD (mg L-1) is the specific removal of COD, which can be obtained according to Eq.

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(1); I (A) is the current, t (s) is reaction time during electrolytic process.

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3. Results and discussion

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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

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wastewater under different specific electrical charges. The specific removal of COD, TN and

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chroma increased with the increase of Qsp. In order to analyze the effect of specific electrical

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charge on pollutant removal and obtain the appropriate specific electrical charge, the curve

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fittings were conducted and the regression equations, regression coefficients and rate

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constants are listed in Table 2. The degradations of COD, TN and chroma follow the

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first-order kinetics, the reaction constants for COD, TN and chroma are 0.217, 0.021 and

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0.258, respectively. As shown, 99 % of chroma can be degraded with only 2.7 Ah L-1 of Qsp,

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and the removal efficiency of COD could reach 95.5 % (where the residual COD concentration

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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

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effluent discharge (China EPA, 2013, Discharge standards of water pollutants for dyeing and

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finishing of textile industry). Therefore, the residual COD value of 80 mg L-1 is set as the end

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point of electro-oxidation reaction in this study. Fig. 3 shows the total energy consumptions

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(TEC) and the total current efficiencies (TCE) at five current densities when the reaction end

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point was reached. With the increase of the current density, TCE increased at first stage and

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then decreased, while the values of TEC increased monotonously. The inflection point of TCE

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was at the current density of 10 mA cm-2, where the TEC and TCE were 12.04 kWh m-3 and

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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.

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3.2. Evaluation of removal efficiencies of inorganic ions Fig. SM-1 shows the removal of inorganic ions in ROC effluent by electro-oxidation

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process under the suitable operating parameters (current density: 10 mA cm-2; reaction time:

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40 min). Comparatively, higher removal efficiencies of chloride ion (42.0 %), ammonia

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nitrogen (54.3 %) and calcium ion (62.7 %) were observed. The removal efficiencies of nitrate

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and magnesium were 19.1 % and 21.3 %, respectively, while only 8.2 % of potassium ion and

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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

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electron transfer reaction. Subsequently, the chlorine was hydrolyzed to hypochlorous acid

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(HClO) and chloric acid (ClO3-) (Eqs. 8-9), which could greatly enhance the indirect oxidation.

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2Cl- → Cl2 + 2e-

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Cl2 + H2 O → HClO + HCl

(8)

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12HClO + 6H2 O → 4HClO3 + 8HClO + 3O2 + 12H+ +12e-

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(7)

During the electro-oxidation process, the calcium ions and magnesium ions in the ROC

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effluent moved to the cathode and deposited on the cathode due to the reaction with CO32-

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in wastewater (Rocha et al., 2012). Standard electrode potential of Na and K are -2.7 and -2.9

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V, respectively, which result in their insignificant removal. The sulfate might not react due to

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its low reduction potential (S2-/SO42-, E0= 0.82 V) at cathode in alkaline environment and its

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higher oxidation potential (SO42−/S2O82−, E0= 2.10 V) at anode (Espinoza et al., 2016). So the

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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

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previous work (Xu, 2010), the main components of organic compounds in the ROC effluent of

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printing and dyeing wastewater are ketone ethers (RC-O-R/R-O-R, 40.6 %), amines

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(RNH2/RNHR'/RN[R']R'', 29.1 %); and nitriles, alkanes, esters, aromatic hydrocarbons,

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carboxylic acids and olefin were also detected.

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The samples before and after electro-oxidation treatment were measured by FTIR

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instrument and the results are shown in Fig. SM-2. The peak at 1661.1 cm-1 for the untreated

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ORC effluent (Fig. SM-2a) represents the structural vibration of C=O, and peaks at 990.7 and

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694.1 cm-1 represent the C=C stretching. The peaks at 1362.4 and 830.7 cm-1 indicate the

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existence of aromaticity or benzene ring, while the other peaks at 875.8 and 616.4 cm-1

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indicate the presence of S-O stretching.

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After electro-oxidation, the components of organics in ROC effluent changed

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significantly. The new peak in Fig. SM-2b appears at 1419.9 cm-1, indicating the production of

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carboxylic acids (-COO-). while the peaks of 1362.4, 990.7, 830.7 and 694.1 cm-1 completely

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disappear and the peaks of 1661.1, 875.8 and 616.4 cm-1 shift to 1637.1, 876.6 and 614.6

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cm-1, respectively. After being treated by eletro-oxidation, the macromolecular components

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in ROC was degraded to small molecular matters such as organic acids. The above results

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show that electro-oxidation process has a destructive function for complex structures of

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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

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surfaces. The relative oxidizing or reducing strength of solution can be expressed using ORP

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(oxidation-reduction potential) to explore the redox interaction effect of substances at a

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macroscopic level. Qsp and the concentration of Cl- are two major factors which influence the

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removal of pollutants in wastewater during the electro-oxidation process. Fig. 4 shows the

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dependences of the values of ORP, pH, COD and Cl- on Qsp during the electro-oxidation

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process. The ORP value decreased from 260 ± 25 mV to -75 mV in 2min of electrolyzing and

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then increased rapidly to reach a steady stage at 720 ± 10 mV. In contrast, the pH value kept

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relatively stable throughout the reaction process (increasing from 8.3 to 8.9). Meanwhile,

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the values of COD and Cl- decreased obviously with the increase of specific electrical charge;

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and their values can be reduced to 77 mg L-1 and 866 mg L-1, respectively, at the specific

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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

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concentration and the specific electrical charge during the electro-oxidation process, and can

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affect the COD removal efficiency. In order to find out the relationship among ORP, Qsp, COD

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and chloride ion concentration under the suitable operating parameters, multiple regression

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was conducted using Origin 8.0 software (Wu and Wang, 2012), their relationships can be

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expressed as following:

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ORP = -699.35 + 112.01 Qsp - 5.70 COD + 1.75 Cl-

(10)

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The regression coefficient (R2) is 0.976, reflecting a good correlation among ORP, Qsp,

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COD and Cl-. The ORP value is in a positive correlation with Cl- content and Qsp, which 13

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reduction power. The good correlation shows that the ORP value gives a comprehensive

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indicator to the effect of current density, chloride ion concentration, pollutants and reaction

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time on the performance of the electro-oxidation system. Therefore, the ORP value can be

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used as an effective controlling factor to optimize the electro-oxidation process.

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3.5. The optimization of specific electrical charge (Qsp) control

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Fig. 5 shows the relationship between energy consumption (EC), instantaneous current

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efficiency (ICE) and ORP during the electro-oxidation. As shown, ORP has significant effect on

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the values of EC and ICE, and their relation curves look like two opposite waves along with

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the increase of ORP value. It can be found that, in the ORP ranger of 690 to 740 mV, the

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maximum value of ICE (≈33 %) and the minimum value of EC (≈50 kWh [kg COD]-1) are

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simultaneously achieved at 721±3mV, which can be considered as the optimum response

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interval for electro-oxidation. Note that when ORP ≤ 690 mV the residual COD value of the

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treated ROC was still larger than the target concentration (80 mg L-1).

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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-

(11)

Thus, to maintain a relatively constant ORP value, we can accordingly adjust the input of

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Qsp by Eq. (11) when providing different concentrations of COD and chloride ion. A computer

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control procedure based on back-propagation neural network (BPN) model was developed

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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

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the two objective parameters to control. The initial content of pollutants in wastewater,

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temperature, pH value and Chroma are the original parameters. The Qsp value was optimized

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by the computer control procedure to control the ORP at a stable value, and the energy

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consumption could be calculated at this constant ORP system during electro-oxidation

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process. With the target residual COD value (80 mg L-1), the total energy consumption (TEC)

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in constant-ORP system was 9.06 kWh m-3, while that for constant-current system was

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12.04kWh m-3. Thus, the developed constant-ORP system can save the energy consumption

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by 24.75% as compared to the constant-current system. Consequently, monitoring of ORP

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has high potential to effectively optimize the electro-oxidation process.

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4. .Conclusions

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The electro-oxidation technique can be considered as an effective, feasible and robust

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treatment method for ROC effluent in printing and dyeing factory by using PbO2/Ti electrode.

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With a suitable specific electrical charge of 2.45 Ah L-1, high removal efficiencies of COD

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(72 %), TN (18 %) and chroma (99 %) were achieved. FTIR analyses show that the

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electro-oxidation technique has a significant degradation effects to the organic matters of

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complex structures in ROC effluent. Additionally, the electro-oxidation process can also

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simultaneously remove some inorganic ions. The quantitative relationship among ORP, Qsp,

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COD and chloride ion concentration was established. This good correlation shows that the

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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

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constant-ORP system showed the success of online monitoring of ORP in optimizing the

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electro-oxidation process.

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Acknowledgements

The authors are grateful for the financial support provided by the National Natural

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Science Foundation of China (Grant No.51278465) and the Transformation Projects in the

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Science & Technology Pillar Program of Zhejiang Province (Grant No.20130176).

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Tables and Figures

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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

TP

1.8 mg L-1

Ca2+

26.9 mg L-1

pH

8.3

Mg2+

7.5 mg L-1

Chroma

160 times

NH4+

4.0 mg L-1

Conductivity

17.0 ms cm-1

SO42-

4739.9 mg L-1

TDS

8.49 g L-1

Cl-

1471.8 mg L-1

NO3F-

430 431

TN

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COD

0.6 mg L-1

RCOD = -203.6 exp(-Qsp/1.993) + 241.6

0.948

0.217

RTN = -9.78 exp(-Qsp /5.913) + 11.6

0.931

0.021

0.898

0.258

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).

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Chroma RChroma = -85.9 exp(-Qsp /0.879) + 156.5

436 437

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)

432 433

434 435

24.9 mg L-1

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PO43-

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TN

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K

4330.5 mg/L

+

20

438

Fig.1. Diagram of experimental setup of eletro-oxidation.

440

RTN (mg L

-1

)

12 9

TN RTN

6

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Chroma RChroma

135

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RChroma (Times)

150

120

45 30 15

105

0 0

3

6

9

-1

Qsp (Ah L

12

Chroma (Times)

0 60

)

75

90 45

442 443 444

150

-1

RCOD (mg L

135

COD (mg L

225 COD RCOD

180

441

30 300

225

90

33

)

270

36

-1

3

39

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

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18

TEC

28

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TCE (%)

27

TEC (kWh m )

15

12

-3

26

25

5

10

15

20

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9

25

-2

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.

451 800

400 200 0 -200

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8

453 454 455

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0

200 150 100 50 1400

pH

1200

-

Cl

7 6

250

1000 1

2

3

Cl- (mg L-1)

9

pH

ORP COD

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ORP (mV)

600

300

COD (mg L-1)

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448 449 450

800

-1

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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Fig. 5. The relationship between energy consumption (EC), instantaneous current efficiency (ICE) and ORP during the electro-oxidation process.

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