# Electrochemical Degradation of Reactive Yellow 160 Dye in Real Wastewater Using C/PbO2-, Pb + Sn/PbO2 + SnO2-, and Pb/PbO2 Modified Electrodes

Authors: Mohamed Gaber; Nasser Abu Ghalwa; Abdalla M. Khedr; Munther F. Salem
Journal: Journal of Chemistry
 (2013)

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## Abstract

Three modified electrodes (C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2) were prepared by electrodeposition and used as anodes for electrochemical degradation of Reactive Yellow 160 (RY160) dye in aqueous solution. Different operating conditions and factors affecting the treatment process including current density, temperature, initial concentration of (RY160), pH, conductive electrolyte and time of electrolysis were studied and optimized. The best degradation occurred in the presence of NaCl (4 g L−1) as a conductive electrolyte. After 15 min, nearly complete degradation of RY160 was achieved (97.9%, 96.65 and 95.35% using C/PbO2, Pb+Sn/PbO2+SnO2, and Pb/PbO2 electrodes, resp.) at pH 7.13. Higher degradation efficiency was obtained at 25°C. The optimum current density for the degradation of RY160 on all electrodes was 50 mA cm−2. The prepared C/PbO2, Pb+Sn/PbO2+SnO2 and Pb/PbO2 electrodes were found to  be highly efficient in the treatment of effluents obtained from dyeing factory which contain RY160 dye with very slight matrix effect.


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## Body

## 1. Introduction

The electrochemical performance and stability of the PbO2 film are related to substrate preparation and electrodeposition conditions, as well as to the organic- and inorganic-doping species that might be used [1, 2]. New methodologies have led to improved adhesion of the PbO2 film onto the substrate [3], and also to an oxidation power of the PbO2 anode comparable to that of the boron-doped diamond (DDB) anode [2]; however the most used substrate is still the Ti-Pt [1–6]. Comninellis and Chen [7] point the possible release of Pb2+ ions, especially in basic solutions, as the main drawback of PbO2 anodes; in many instances, a short lifetime might be another important drawback [8]. The degradation of reactive dye was studied by wet-air oxidation (WAO) [9], wet-peroxide oxidation (WPO) [10], photooxidation [11–13], electro-fenton (EF) advanced oxidation [14, 15], ozonation [16, 17], H2O2/UV [18, 19], catalytic electro-oxidation [20] and electrocoagulation [21, 22]. The performances of the Ti-Pt/β-PbO2 and the boron-doped diamond (BDD) electrodes in the electro-oxidation of simulated wastewaters containing 85 mg L−1 of the Reactive Orange 16 and Blue Reactive 19 dye were investigated using a filter-press reactor [1, 4]. The electrochemical characterization of Ti/SnO2-Sb-Pt anodes prepared by thermal decomposition was researched using different techniques. These results were also compared to those obtained for the Ti/SnO2, Ti/SnO2-Sb, and deactivated Ti/SnO2-Sb-Pt anodes. COD, HPLC, UV-Vis, and potential control measurements were also used to verify the viability of these anodes to degrade and decolorize wastewater solutions containing a reactive dye: C.I. Reactive Orange 4 [23, 24]. The electrochemical degradation of C.I. Reactive Red 195 (RR195) and 4-chloro-3-methyl phenol (CMP) in aqueous solution on a Ti/SnO2-Sb/PbO2 electrode was investigated. The influence of operating variables on the mineralization efficiency was studied as a function of the current density, the initial pH, the initial concentration of the dye, and the addition of NaCl [25]. Basic Yellow 28 (SLY) and Reactive Black 5 (CBWB), which are, respectively, methane and sulfoazo textile dyes were individually exposed to electrochemical treatment using diamond-, aluminum-, copper- and iron-zinc alloy electrodes. Four different electrodic materials were tested, and presented 95% color removal and COD removal of up to 65–67% in the case of CBWB dye solution treated with the copper and iron electrodes [26]. Electrochemical degradation experiments were conducted to degrade a textile dye, namely, Reactive Blue 19 (RB-19). The oxidation of RB-19 using titanium-based dimensionally stable anode (DSA) takes place in the bulk solution with electrolytically generated chlorine/hypochlorite. Increasing the initial pH and increasing the reaction temperature decreases the de-colorization efficiency. At the same time, increasing the chloride concentration and increasing the current density showed an increase in the color removal. The complete removal of color was achieved within a short period of electrolysis for different concentrations of RB-19. However, the removal of COD and TOC was 55.8% and 15.6%, respectively, for 400 mg L−1 RB-19 with 1.5 g L−1 sodium chloride concentration [27]. The electrochemical oxidation of simulated textile wastewater was studied on iron electrodes in the presence of NaCl electrolyte in a batch electrochemical reactor. The simulated textile wastewater was prepared from industrial components based on the real mercerized and nonmercerized cotton and viscon process [28]. Electrochemical oxidation of O-Toluidine (OT) was studied by galvanostatic electrolysis using lead dioxide (PbO2) and (BDD) as anodes. The influence of operating parameters, such as current density, initial concentration of OT, and temperature were investigated [29].In this study, an electrodegradation method was applied on Reactive Yellow 160 (RY160) dye (Scheme1), by using three modified electrodes (C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2). Different factors including the pH, concentration of electrolyte, conductive electrolyte type, current density, time of electrolysis, initial concentration of RY160 solution, and temperature were studied and optimized for its removal from water. Two main parameters were measured to evaluate the electrochemical treatment efficiency, the remaining pollutant concentration and the chemical oxygen demand (COD).Scheme 1
The structure formula of RY160.


## 2. Experiments

### 2.1. Chemicals and Instrumentation

Sodium chloride, sodium fluoride, sodium carbonate, sodium sulphate, calcium chloride, potassium chloride, sodium hydroxide, sulphuric acid, and potassium dichromate, silver sulfate were of analytical grade and purchased from Merck. Distilled water was used for the preparation of solutions. Standard solutions of potassium dichromate, sulfuric acid reagent with silver sulfate, and potassium hydrogen phthalate (KHP) were prepared to measure the COD. Different standard solutions of RY160 with concentration from 20–200 mg L−1 were prepared to measure its degradation at different conditions. The double-beam UV-Vis spectrophotometer is from Shimadzu, the DC power supply is model GP4303D, LG Precision CO. Ltd. (Korea), a pH meter model AC28, TOA electronics Ltd., (Japan) to adjust pH of the solutions and a digital multimeter is kyoritsu model 1008, (Japan) for reading out the current and potential values. A closed reflux titrimetric unit was used for the COD determination [30].


### 2.2. Electrodeposition of Doped Lead Dioxide at Different Substrates

#### 2.2.1. Preparation of Pb/PbO2-Modified Electrode

Lead Surface Treatment.Pretreatments of the lead substrate were carried out before anodization to ensure good-adhesion lead dioxide film. Lead was first roughened to increase the adhesion of PbO2 deposit via subjecting its surface to mechanical abrasion by sand papers of different grades, down to 40/0. Then, it was cleaned by acetone to remove sand particles or any other particles lodged in the metal surface. This process has a great application and good penetrating power. Then it was treated with an alkali solution, a mixture of sodium hydroxide (50 g L−1) and sodium carbonate (20 g L−1), to remove any organic materials in the surface, and tri-sodium orthophosphate (20 g L−1) and sulphuric acid (2 g L−1) to remove any oxides. Uniform and well-adhesive deposit necessitates a smooth surface with no oxide or scales. To confirm our preparation, the lead substrate was soaked for 2 min in a pickling solution consisting of nitric acid (400 g L−1) and hydrofluoric acid (5 g L−1), and then chemically polished in boiled oxalic acid solution (100 g L−1) for 5 min [31].Electrochemical Deposition of
P
b
O
2
.PbO2 was deposited galvanostatically on the pretreated lead substrate by electrochemical anodization of lead in oxalic acid solution (100 g L−1). This acid solution was electrolyzed galvanostatically for 30 min. at ambient temperature using an anodic current density of 100 mA cm−2. The cathode was stainless steel (austenitic type), and the two electrodes were concentric with the lead electrode axially. This arrangement gave the formation of a regular and uniform deposit [31].


#### 2.2.2. Preparation of Pb + Sn/PbO2+ SnO2-Modified Electrode

Preparation and Fabrication of Pb-Sn Alloy Electrodes.Binary Pb-Sn alloy with concentration (1 : 1 w/w) were prepared according to the standard following procedure and the fabrication of the electrodes as discussed in detail elsewhere. Anodic oxidation of alloy electrodes was carried out, and the film was characterized for its structure [32].Electrochemical Deposition of
P
b
O
2
+
S
n
O
2
.Three electrodes assembly was used for making thin films, in which the working alloy electrodes was of 1 cm2 area with Pt (4 cm) as the counter-electrode and saturated calomel electrode (SCE) as the reference. Prior to oxidation, the working electrode surface was successively polished on 1000~ grit paper on roughing stone using water as lubricant and finally with methanol-acetic acid mixture. The alloy substrate was cleaned by acetone to remove greases or oils lodged in the metal surface, treated with an alkali solution, a mixture of sodium hydroxide (50 g L−1), and sodium carbonate (20 g L−1), to remove any organic materials in the surface, and tri-sodium orthophosphate (20 g L−1), sulphuric acid (2 g L−1) to remove any oxides. To confirm our preparation, the alloy substrate was soaked for 2 min in a pickling solution consisting of nitric acid (400 g L−1) and hydrofluoric acid (5 g L−1), and then chemically polished in boiled oxalic acid solution (100 g L−1) for 5 min. Potentiodynamic anodization of Pb-Sn alloy was carried out at 80°C in the potential range from −1.25  V to +2.35  V with a sweep rate 200 mV s−1. After 20 min of continuous anodization [32], the electrode was taken out of the electrolysis bath and washed thoroughly in doubly distilled water followed by drying in air at 120°C for 2 h.


#### 2.2.3. Preparation of Modified C/PbO2 Electrode

Carbon Surface Treatment.Pretreatment of carbon rod (8 mm × 25 cm) was carried out following the procedure applied by Narasimham and Udupa [33]. The carbon rod was soaked in 5% NaOH solution, washed with distilled water, dried in furnace at 105°C, and cooked with linseed oil to reduce the porosity of rod. After that, the electrode was ready to receive doped PbO2.Electrochemical Deposition of
P
b
O
2. The electrodeposition of PbO2 was performed at constant anodic current of 20 mA cm−2 in 12% (w/v) Pb(NO3)2 solution containing 5% (w/v) CuSO4·5H2O and 3% surfactant. The role of the surfactant is to minimize the surface tension of the solution. Electrodeposition was carried out for 60 min. at 80°C with continuous stirring [33].


### 2.3. Electrolysis of Reactive Yellow 160 Degradation

Galvanostatic electrolyses were carried out at C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes, with current density ranging from 0 to 400 mA cm−2 and electrical potential ranging from 1–12 volts. Runs were performed at 10–40°C. Solutions of 100 mg L−1 of pure RY160 solution were used. The investigations of this study were carried out in the presence of sodium chloride (0.5–20 g L−1) and 4 g L−1 of different conductive electrolytes such as; NaCl, CaCl2, KCl, Na2CO3, NaF, NaPO4, and Na2SO4 with pH between 1.5 and 12. The electrolysis duration ranges from 0–30 min. The electrochemical degradation of the RY160 solutions was carried out in a 100 mL Pyrex glass cell where the prepared electrodes C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 work as anode and austenitic stainless steel as cathode. The electrodes were connected to a DC power supply, while the current and potential measurements were read out using the digital multimeter.


### 2.4. Analysis

Two main parameters were measured to evaluate the electrochemical treatment efficiency, remaining pollutants (RY160) concentration was measured with the double-beam UV-visible spectrophotometer from Shimadzu atλmax=415 nm using calibration curve with standard error ±0.5%, and the COD was determined using a closed reflux titrimetric method [30].


### 2.5. Cost Calculation of RY160 Degradation

The cost of electrochemical degradation of RY160 per liter was calculated as follows:(1)Cost=Electricalenergyconsumption×priceindollarsElectricalenergyconsumption(kws-1)=Itv(1000×3600),
where I: applied current density (A),  t: duration (h), and v: applied volt (V) Priceindollar=1.02×10-4$.


## 2.1. Chemicals and Instrumentation

Sodium chloride, sodium fluoride, sodium carbonate, sodium sulphate, calcium chloride, potassium chloride, sodium hydroxide, sulphuric acid, and potassium dichromate, silver sulfate were of analytical grade and purchased from Merck. Distilled water was used for the preparation of solutions. Standard solutions of potassium dichromate, sulfuric acid reagent with silver sulfate, and potassium hydrogen phthalate (KHP) were prepared to measure the COD. Different standard solutions of RY160 with concentration from 20–200 mg L−1 were prepared to measure its degradation at different conditions. The double-beam UV-Vis spectrophotometer is from Shimadzu, the DC power supply is model GP4303D, LG Precision CO. Ltd. (Korea), a pH meter model AC28, TOA electronics Ltd., (Japan) to adjust pH of the solutions and a digital multimeter is kyoritsu model 1008, (Japan) for reading out the current and potential values. A closed reflux titrimetric unit was used for the COD determination [30].


## 2.2. Electrodeposition of Doped Lead Dioxide at Different Substrates

### 2.2.1. Preparation of Pb/PbO2-Modified Electrode

Lead Surface Treatment.Pretreatments of the lead substrate were carried out before anodization to ensure good-adhesion lead dioxide film. Lead was first roughened to increase the adhesion of PbO2 deposit via subjecting its surface to mechanical abrasion by sand papers of different grades, down to 40/0. Then, it was cleaned by acetone to remove sand particles or any other particles lodged in the metal surface. This process has a great application and good penetrating power. Then it was treated with an alkali solution, a mixture of sodium hydroxide (50 g L−1) and sodium carbonate (20 g L−1), to remove any organic materials in the surface, and tri-sodium orthophosphate (20 g L−1) and sulphuric acid (2 g L−1) to remove any oxides. Uniform and well-adhesive deposit necessitates a smooth surface with no oxide or scales. To confirm our preparation, the lead substrate was soaked for 2 min in a pickling solution consisting of nitric acid (400 g L−1) and hydrofluoric acid (5 g L−1), and then chemically polished in boiled oxalic acid solution (100 g L−1) for 5 min [31].Electrochemical Deposition of
P
b
O
2
.PbO2 was deposited galvanostatically on the pretreated lead substrate by electrochemical anodization of lead in oxalic acid solution (100 g L−1). This acid solution was electrolyzed galvanostatically for 30 min. at ambient temperature using an anodic current density of 100 mA cm−2. The cathode was stainless steel (austenitic type), and the two electrodes were concentric with the lead electrode axially. This arrangement gave the formation of a regular and uniform deposit [31].


### 2.2.2. Preparation of Pb + Sn/PbO2+ SnO2-Modified Electrode

Preparation and Fabrication of Pb-Sn Alloy Electrodes.Binary Pb-Sn alloy with concentration (1 : 1 w/w) were prepared according to the standard following procedure and the fabrication of the electrodes as discussed in detail elsewhere. Anodic oxidation of alloy electrodes was carried out, and the film was characterized for its structure [32].Electrochemical Deposition of
P
b
O
2
+
S
n
O
2
.Three electrodes assembly was used for making thin films, in which the working alloy electrodes was of 1 cm2 area with Pt (4 cm) as the counter-electrode and saturated calomel electrode (SCE) as the reference. Prior to oxidation, the working electrode surface was successively polished on 1000~ grit paper on roughing stone using water as lubricant and finally with methanol-acetic acid mixture. The alloy substrate was cleaned by acetone to remove greases or oils lodged in the metal surface, treated with an alkali solution, a mixture of sodium hydroxide (50 g L−1), and sodium carbonate (20 g L−1), to remove any organic materials in the surface, and tri-sodium orthophosphate (20 g L−1), sulphuric acid (2 g L−1) to remove any oxides. To confirm our preparation, the alloy substrate was soaked for 2 min in a pickling solution consisting of nitric acid (400 g L−1) and hydrofluoric acid (5 g L−1), and then chemically polished in boiled oxalic acid solution (100 g L−1) for 5 min. Potentiodynamic anodization of Pb-Sn alloy was carried out at 80°C in the potential range from −1.25  V to +2.35  V with a sweep rate 200 mV s−1. After 20 min of continuous anodization [32], the electrode was taken out of the electrolysis bath and washed thoroughly in doubly distilled water followed by drying in air at 120°C for 2 h.


### 2.2.3. Preparation of Modified C/PbO2 Electrode

Carbon Surface Treatment.Pretreatment of carbon rod (8 mm × 25 cm) was carried out following the procedure applied by Narasimham and Udupa [33]. The carbon rod was soaked in 5% NaOH solution, washed with distilled water, dried in furnace at 105°C, and cooked with linseed oil to reduce the porosity of rod. After that, the electrode was ready to receive doped PbO2.Electrochemical Deposition of
P
b
O
2. The electrodeposition of PbO2 was performed at constant anodic current of 20 mA cm−2 in 12% (w/v) Pb(NO3)2 solution containing 5% (w/v) CuSO4·5H2O and 3% surfactant. The role of the surfactant is to minimize the surface tension of the solution. Electrodeposition was carried out for 60 min. at 80°C with continuous stirring [33].


## 2.2.1. Preparation of Pb/PbO2-Modified Electrode

Lead Surface Treatment.Pretreatments of the lead substrate were carried out before anodization to ensure good-adhesion lead dioxide film. Lead was first roughened to increase the adhesion of PbO2 deposit via subjecting its surface to mechanical abrasion by sand papers of different grades, down to 40/0. Then, it was cleaned by acetone to remove sand particles or any other particles lodged in the metal surface. This process has a great application and good penetrating power. Then it was treated with an alkali solution, a mixture of sodium hydroxide (50 g L−1) and sodium carbonate (20 g L−1), to remove any organic materials in the surface, and tri-sodium orthophosphate (20 g L−1) and sulphuric acid (2 g L−1) to remove any oxides. Uniform and well-adhesive deposit necessitates a smooth surface with no oxide or scales. To confirm our preparation, the lead substrate was soaked for 2 min in a pickling solution consisting of nitric acid (400 g L−1) and hydrofluoric acid (5 g L−1), and then chemically polished in boiled oxalic acid solution (100 g L−1) for 5 min [31].Electrochemical Deposition of
P
b
O
2
.PbO2 was deposited galvanostatically on the pretreated lead substrate by electrochemical anodization of lead in oxalic acid solution (100 g L−1). This acid solution was electrolyzed galvanostatically for 30 min. at ambient temperature using an anodic current density of 100 mA cm−2. The cathode was stainless steel (austenitic type), and the two electrodes were concentric with the lead electrode axially. This arrangement gave the formation of a regular and uniform deposit [31].


## 2.2.2. Preparation of Pb + Sn/PbO2+ SnO2-Modified Electrode

Preparation and Fabrication of Pb-Sn Alloy Electrodes.Binary Pb-Sn alloy with concentration (1 : 1 w/w) were prepared according to the standard following procedure and the fabrication of the electrodes as discussed in detail elsewhere. Anodic oxidation of alloy electrodes was carried out, and the film was characterized for its structure [32].Electrochemical Deposition of
P
b
O
2
+
S
n
O
2
.Three electrodes assembly was used for making thin films, in which the working alloy electrodes was of 1 cm2 area with Pt (4 cm) as the counter-electrode and saturated calomel electrode (SCE) as the reference. Prior to oxidation, the working electrode surface was successively polished on 1000~ grit paper on roughing stone using water as lubricant and finally with methanol-acetic acid mixture. The alloy substrate was cleaned by acetone to remove greases or oils lodged in the metal surface, treated with an alkali solution, a mixture of sodium hydroxide (50 g L−1), and sodium carbonate (20 g L−1), to remove any organic materials in the surface, and tri-sodium orthophosphate (20 g L−1), sulphuric acid (2 g L−1) to remove any oxides. To confirm our preparation, the alloy substrate was soaked for 2 min in a pickling solution consisting of nitric acid (400 g L−1) and hydrofluoric acid (5 g L−1), and then chemically polished in boiled oxalic acid solution (100 g L−1) for 5 min. Potentiodynamic anodization of Pb-Sn alloy was carried out at 80°C in the potential range from −1.25  V to +2.35  V with a sweep rate 200 mV s−1. After 20 min of continuous anodization [32], the electrode was taken out of the electrolysis bath and washed thoroughly in doubly distilled water followed by drying in air at 120°C for 2 h.


## 2.2.3. Preparation of Modified C/PbO2 Electrode

Carbon Surface Treatment.Pretreatment of carbon rod (8 mm × 25 cm) was carried out following the procedure applied by Narasimham and Udupa [33]. The carbon rod was soaked in 5% NaOH solution, washed with distilled water, dried in furnace at 105°C, and cooked with linseed oil to reduce the porosity of rod. After that, the electrode was ready to receive doped PbO2.Electrochemical Deposition of
P
b
O
2. The electrodeposition of PbO2 was performed at constant anodic current of 20 mA cm−2 in 12% (w/v) Pb(NO3)2 solution containing 5% (w/v) CuSO4·5H2O and 3% surfactant. The role of the surfactant is to minimize the surface tension of the solution. Electrodeposition was carried out for 60 min. at 80°C with continuous stirring [33].


## 2.3. Electrolysis of Reactive Yellow 160 Degradation

Galvanostatic electrolyses were carried out at C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes, with current density ranging from 0 to 400 mA cm−2 and electrical potential ranging from 1–12 volts. Runs were performed at 10–40°C. Solutions of 100 mg L−1 of pure RY160 solution were used. The investigations of this study were carried out in the presence of sodium chloride (0.5–20 g L−1) and 4 g L−1 of different conductive electrolytes such as; NaCl, CaCl2, KCl, Na2CO3, NaF, NaPO4, and Na2SO4 with pH between 1.5 and 12. The electrolysis duration ranges from 0–30 min. The electrochemical degradation of the RY160 solutions was carried out in a 100 mL Pyrex glass cell where the prepared electrodes C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 work as anode and austenitic stainless steel as cathode. The electrodes were connected to a DC power supply, while the current and potential measurements were read out using the digital multimeter.


## 2.4. Analysis

Two main parameters were measured to evaluate the electrochemical treatment efficiency, remaining pollutants (RY160) concentration was measured with the double-beam UV-visible spectrophotometer from Shimadzu atλmax=415 nm using calibration curve with standard error ±0.5%, and the COD was determined using a closed reflux titrimetric method [30].


## 2.5. Cost Calculation of RY160 Degradation

The cost of electrochemical degradation of RY160 per liter was calculated as follows:(1)Cost=Electricalenergyconsumption×priceindollarsElectricalenergyconsumption(kws-1)=Itv(1000×3600),
where I: applied current density (A),  t: duration (h), and v: applied volt (V) Priceindollar=1.02×10-4$.


## 3. Results and Discussion

### 3.1. Mechanism of Electrochemical Oxidation of Organic Pollutants

The electrochemical oxidation of many organic pollutants in aqueous solutions on anode could take place by direct electron transfer or oxygen-atom transfer. In addition to direct oxidation, organic pollutants can also be treated by an indirect electrolysis generating chemical reactant to convert them into less deleterious products. Oxidation of these pollutants might go further to carbon dioxide and water via successive reactions. Each of them could proceed through several steps such as mass transport, adsorption, and direct or indirect reaction at the anode surface [31]. The direct electrochemical oxidation of organic compounds could generally occur through the following mechanism in which the first step is the oxidation of water molecules on the electrode surface (MOx). This process may give rise to formation of hydroxyl radicals according to the following equation
(2)MOx+H2O⟶k1MOx[OH∙]+H++e-
The produced hydroxyl radicals can be oxidized to a higher state forming the so-called higher oxide as follows
(3)MOx[OH∙]⟶k2MOx[O]+H++e-
The role of the formed higher oxide is the participation in the formation of selective oxidation of the organic pollutants (R) without complete incineration (4):
(4)MOx[O]+R⟶k3RO+MOx
The above route can take place only if the transition of the underlying oxide to a higher oxidation state occurred. The electrodes of this class are called “active electrodes” [34]. However, if the product of (4) is not obtained, the electrogenerated hydroxyl radicals could directly oxidize the organic compound to carbon dioxide and water, predominantly causing the combustion of the organic compound through hydroxylation of these compounds as follows
(5)MOx[OH∙]+R⟶k4Ox+mCO2+nH2O+H++e-
and this class of electrodes are called “nonactive electrodes” [35]. On the basis of the abovementioned mechanism, the lead dioxide anode employed in this investigation is characterized by high oxygen overvoltage on which (OH∙) is generated from the oxidation of water. Hydroxyl radicals (OH∙) are electrosynthesized in aqueous solutions and can react rapidly with aromatic pesticides, leading to a polyhydroxylation reaction, followed by complete mineralization of the initial pollutants [35]. However, PbO2 does not have a higher oxidation state; consequently it is classified as a “nonactive electrode”. It was reported that lead dioxide electrode is hydrated one, and the electrogenerated hydroxyl radicals are expected to be more strongly adsorbed on its surface. This behavior makes lead dioxide anode very reactive towards organic oxidation. The degradation of the organic pollutants is completed by reaction with adsorbed hydroxyl radicals forming carbon dioxide and water. Indirect electrochemical oxidation of organic pollutants occurs through the “in situ” electrogeneration of catalytic species with powerful oxidizing property. This process is capable of eliminating the detrimental pollutants from their solutions by converting them into harmless compound.Although a large number of electrogenerated oxidants can be used such as Fenton’s reagent and ozone, the hypochlorite ion is the most widely employed oxidant in wastewater treatment [36]. The mechanism of electrogeneration, from a solution, containing chloride ions involves two steps. The first one is primary oxidation of chloride ions to chlorine at the anode surface according to [36] the following equation:
(6)2Cl-⟶k5Cl2+2e-
The second step is formation of hypochlorous acid as follows
(7)Cl2+H2O⟶k6HClO+Cl-+H+
The HClO undergoes dissociation into hypochlorite and hydrogen ions as follows
(8)HClO⟶k7ClO-+H+


### 3.2. Effect of Various Factors on the Rate of Degradation

The effect of different operating conditions such as: type of conductive electrolyte, current density, pH of simulated solution, temperature, time interval of treatment, initial concentration, and NaCl concentration were studied. The remaining concentration (mg L−1) and COD removal (%) were illustrated in Figures 1–7.Figure 1
The effect of pH on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2 and Pb/PbO2 electrodes.Figure 2
The effect of NaCl concentration on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 3
The effect of current density on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 4
The effect of the conductive electrolyte type on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 5
The effect of temperature on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 6
The effect of initial concentration on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 7
The effect of distance between the cathode and anode on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.


#### 3.2.1. Effect of pH Value

The pH of the solution was varied while the other conditions where kept constant. As shown in Figure1, maximum removal of RY160 and COD was achieved at pH 7.13 for C/PbO2, Pb+Sn/PbO2+SnO2, and Pb/PbO2, respectively. The pH values of the solutions were adjusted by adding drops of H2SO4 and NaOH. The reactions were carried out for 15 min for three electrodes under the following conditions: the initial concentration of 100 mg L−1, a current density of 50 mA cm−2, a temperature of 25°C and NaCl concentration of 4 g L−1. The distance between the two electrodes was adjusted to 1 cm. It was found that the maximum rate of degradation using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes was achieved in neutral medium as the optimal medium.


#### 3.2.2. Effect of the NaCl Concentration

Different concentrations of NaCl were applied to study their effect on the removal of RY160 and the corresponding COD elimination as indicated in Figure2. The results indicate that an increase of the electrolyte concentration up to 4 g L−1 leads to increase in the RY160 degradation rate and COD removal for three C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. The NaCl solution liberates Cl2 gas which is considered as the active species for the degradation of organic compound. Further increase of the NaCl concentration has slight effect on the degradation rate of RY160 and COD removal.


#### 3.2.3. Effect of Current Density

As shown in Figure3, RY160 degradation and COD removal increase with increasing the applied current density up to 50 mA cm−2 by using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. Further increase of the current density was followed by gradual decrease in RY160 degradation and COD removal due to increase in temperature. Above a temperature 35°C, sodium hypochlorite tends to chemically decompose to sodium chlorate as follows:
(9)3NaClO⟶NaClO3+2NaCl
So when temperature rises higher than 35°C, production of NaClO falls. But at higher current densities the rate of hypochlorite decomposition increases with increase in current density.


#### 3.2.4. Effect of Type of Electrolyte

Electrolytes of 4 g L−1 of the following salts; NaCl, CaCl2, KCl, Na2CO3, NaF, Na3PO4, and Na2SO4 were studied by three electrodes. As appears in Figure 4, The NaCl, KCl, and CaCl2 were the most effective conductive electrolytes for the electrocatalytic degradation of the investigated RY160 and COD removal. The Cl− anion is a powerful oxidizing agent. It enhances the degradation of pollutants. Therefore, addition of NaCl, KCl, and CaCl2 provides the effective Cl− ion. This behavior may be due to the small ionic size of K+ and Na+ which increases the ion mobilities and the loss ability of Cl− ion. Na2SO4 and NaF electrolytes showed the least efficiency in the degradation of pollutant. This may be attributed to the formation of an adherent film on the anode surface which poisons the electrode surface. Also, these electrolytes do not contain chloride ions (Cl−) in their structures and may form stable intermediate species that could not be oxidized by direct electrolysis. These observations were also confirmed in other studies [31].


#### 3.2.5. Effect of the Electrolysis Time

To assess the effect of electrolysis time, experiments were conducted with operating treatment conditions that were consistent with those described for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. The maximum removal of RY160 was achieved using C/PbO2, Pb + Sn/PbO2 + SnO2 and Pb/PbO2electrodes after at least 15 min. Therefore, this was taken as optimal degradation time for the removal of RY160. The optimal time for COD removal for three electrodes was 270, 340, and 360 min, respectively.


#### 3.2.6. Effect of Temperature

It is well known that the rate of diffusion of ions increases with increasing temperature. Figure5 represents the correlation between the concentration of the remaining RY160 dye and COD residual as a function of the solution temperature. The rate of the RY160 degradation and COD removal increase significantly with increasing the solution temperature until 25°C. Therefore, 25°C was fixed as optimal electrolysis temperature for the next experiments.


#### 3.2.7. Effect of Initial RY160 Concentration

Figure6 shows the effect of different initial RY160 concentrations on the rate of RY160 degradation and corresponding COD removal. Total removal of the RY160 and COD can be achieved in the presence of initial RY160 load up to 100 mg L−1. However, increasing the RY160 concentration above this level results in a decrease in the electrocatalytic rate of degradation. The removal efficiency of the RY160 by using C/PbO2, Pb + Sn/PbO2, + SnO2 and Pb/PbO2 electrodes at 100 mg L−1 was the optimum concentration for the initial load concentration of RY160. As the initial RY160 concentration increase, the degradation efficiency decrease. This evidence that the generation of the powerful oxidizing agent Cl− ions on electrode surface was not increased in constant current density. The optimum operating conditions for degradation RY160 dye for each electrode were determined and summarized in Table 1. At optimized conditions, the percentages of RY160 degradation and COD removal for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 are 97.9%, 96.65, and 95.35%, respectively. The results indicate that the C/PbO2 electrode is more adequate than Pb + Sn/PbO2 + SnO2- and than Pb/PbO2-modified electrode for the degradation of RY160. Their behavior may be attributed to the color and structure of tested electrodes. C/PbO2 modified electrodes have a black color, while Pb + Sn/PbO2 + SnO2 and Pb/PbO2 modified electrodes have a brown color. It was reported that PbO2film has two structures, α-structure (brown color) and β-structure (black color) [31]. The black one has a tetrahedral crystal structure which is close-packed and more disordered in comparison with the close-packed structure of the brown α-form (orthorhombic). Therefore, the surface area in case of tetrahedral structure is more than that of the orthorhombic one, and hence the β-PbO2 form will be more effective than α-PbO2 form. Because the overpotential for oxygen evolution of β-PbO2 is higher than that of α-PbO2, it is expected that the electrocatalytic properties for C/PbO2-modified electrodes are more efficient than that of Pb/PbO2-modified electrode [34]. In this work, the degradation rate of RY160 was nearly completed and reached 97.9, 96.65, and 95.35 percentage using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes, respectively, after 15 min.Table 1
Percentage of COD and concentration removal of RY160 on C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes in the optimum conditions.
Type of electrode
Removal percent of RY160 at 15 min
Removal percent of COD
Removal percent of COD for real samples
C/PbO2
97.9
100 at 270 min
100 at 300 min
Pb + Sn/PbO2 + SnO2
96.65
100 at 340 min
100 at 380 min
Pb/PbO2
95.35
100 at 360 min
100 at 400 min


#### 3.2.8. Effect of Distance between the Cathode and Anode

The effect of distance between the two electrodes of the cell was studied. It was found from Figure7 that there was an increase of hypochlorite generation by decreasing the distance between the two electrodes up to 1 cm for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. Therefore 1 cm was chosen as optimum distance between electrodes for sodium hypochlorite generation. The experiments were carried out under the following conditions: current density 50 mA cm−2, pH of 7.13, temperature of 25°C, and the concentration of NaCl 4 g L−1. The time of electrolysis was 15 min. It is clear that the sodium hypochlorite production increase with decreasing distance down to 1 cm. This is due to drop of electrolyte ohmic potential, and hence the cell voltage [37]. The highest hypochlorite production was achieved with narrow distance between the cell electrodes of 1 cm.


### 3.3. Application of the Treatment Process in Real Wastewater Samples

The treatment of RY160 effluents obtained from dyeing factory was carried out by using the prepared C/PbO2-, Pb+Sn/PbO2-, + SnO2, and Pb/PbO2-modified electrodes. The treatment was performed, first by collecting actual waste samples from the wastewater effluents of the RY160 dyeing bath. The initial dye-load concentration of these samples was 170 mg/L taken from Hubbub dyeing factory located in the industrial area at Biet Hanon, Gaza Strip, PNA. The dyestuff solutions were treated by the electrocatalytic oxidation technique using the same method as applied to the treatment of RY160 in aqueous solution to investigate the optimum condition for real wastewater containing the dye. After the treatment process, the removal percentages of RY160 dye at 15 min using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes were 97.9%, 96.65, and 95.35%, respectively. The removal percentages of COD were 100% at 300, 380 and 400 min for the above electrodes, respectively. These results indicated that the suggested modified electrodes are highly efficient in the treatment of effluents containing RY160 dye with very slight effect of matrix.


## 3.1. Mechanism of Electrochemical Oxidation of Organic Pollutants

The electrochemical oxidation of many organic pollutants in aqueous solutions on anode could take place by direct electron transfer or oxygen-atom transfer. In addition to direct oxidation, organic pollutants can also be treated by an indirect electrolysis generating chemical reactant to convert them into less deleterious products. Oxidation of these pollutants might go further to carbon dioxide and water via successive reactions. Each of them could proceed through several steps such as mass transport, adsorption, and direct or indirect reaction at the anode surface [31]. The direct electrochemical oxidation of organic compounds could generally occur through the following mechanism in which the first step is the oxidation of water molecules on the electrode surface (MOx). This process may give rise to formation of hydroxyl radicals according to the following equation
(2)MOx+H2O⟶k1MOx[OH∙]+H++e-
The produced hydroxyl radicals can be oxidized to a higher state forming the so-called higher oxide as follows
(3)MOx[OH∙]⟶k2MOx[O]+H++e-
The role of the formed higher oxide is the participation in the formation of selective oxidation of the organic pollutants (R) without complete incineration (4):
(4)MOx[O]+R⟶k3RO+MOx
The above route can take place only if the transition of the underlying oxide to a higher oxidation state occurred. The electrodes of this class are called “active electrodes” [34]. However, if the product of (4) is not obtained, the electrogenerated hydroxyl radicals could directly oxidize the organic compound to carbon dioxide and water, predominantly causing the combustion of the organic compound through hydroxylation of these compounds as follows
(5)MOx[OH∙]+R⟶k4Ox+mCO2+nH2O+H++e-
and this class of electrodes are called “nonactive electrodes” [35]. On the basis of the abovementioned mechanism, the lead dioxide anode employed in this investigation is characterized by high oxygen overvoltage on which (OH∙) is generated from the oxidation of water. Hydroxyl radicals (OH∙) are electrosynthesized in aqueous solutions and can react rapidly with aromatic pesticides, leading to a polyhydroxylation reaction, followed by complete mineralization of the initial pollutants [35]. However, PbO2 does not have a higher oxidation state; consequently it is classified as a “nonactive electrode”. It was reported that lead dioxide electrode is hydrated one, and the electrogenerated hydroxyl radicals are expected to be more strongly adsorbed on its surface. This behavior makes lead dioxide anode very reactive towards organic oxidation. The degradation of the organic pollutants is completed by reaction with adsorbed hydroxyl radicals forming carbon dioxide and water. Indirect electrochemical oxidation of organic pollutants occurs through the “in situ” electrogeneration of catalytic species with powerful oxidizing property. This process is capable of eliminating the detrimental pollutants from their solutions by converting them into harmless compound.Although a large number of electrogenerated oxidants can be used such as Fenton’s reagent and ozone, the hypochlorite ion is the most widely employed oxidant in wastewater treatment [36]. The mechanism of electrogeneration, from a solution, containing chloride ions involves two steps. The first one is primary oxidation of chloride ions to chlorine at the anode surface according to [36] the following equation:
(6)2Cl-⟶k5Cl2+2e-
The second step is formation of hypochlorous acid as follows
(7)Cl2+H2O⟶k6HClO+Cl-+H+
The HClO undergoes dissociation into hypochlorite and hydrogen ions as follows
(8)HClO⟶k7ClO-+H+


## 3.2. Effect of Various Factors on the Rate of Degradation

The effect of different operating conditions such as: type of conductive electrolyte, current density, pH of simulated solution, temperature, time interval of treatment, initial concentration, and NaCl concentration were studied. The remaining concentration (mg L−1) and COD removal (%) were illustrated in Figures 1–7.Figure 1
The effect of pH on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2 and Pb/PbO2 electrodes.Figure 2
The effect of NaCl concentration on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 3
The effect of current density on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 4
The effect of the conductive electrolyte type on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 5
The effect of temperature on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 6
The effect of initial concentration on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.Figure 7
The effect of distance between the cathode and anode on RY160 and COD removal using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes.


### 3.2.1. Effect of pH Value

The pH of the solution was varied while the other conditions where kept constant. As shown in Figure1, maximum removal of RY160 and COD was achieved at pH 7.13 for C/PbO2, Pb+Sn/PbO2+SnO2, and Pb/PbO2, respectively. The pH values of the solutions were adjusted by adding drops of H2SO4 and NaOH. The reactions were carried out for 15 min for three electrodes under the following conditions: the initial concentration of 100 mg L−1, a current density of 50 mA cm−2, a temperature of 25°C and NaCl concentration of 4 g L−1. The distance between the two electrodes was adjusted to 1 cm. It was found that the maximum rate of degradation using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes was achieved in neutral medium as the optimal medium.


### 3.2.2. Effect of the NaCl Concentration

Different concentrations of NaCl were applied to study their effect on the removal of RY160 and the corresponding COD elimination as indicated in Figure2. The results indicate that an increase of the electrolyte concentration up to 4 g L−1 leads to increase in the RY160 degradation rate and COD removal for three C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. The NaCl solution liberates Cl2 gas which is considered as the active species for the degradation of organic compound. Further increase of the NaCl concentration has slight effect on the degradation rate of RY160 and COD removal.


### 3.2.3. Effect of Current Density

As shown in Figure3, RY160 degradation and COD removal increase with increasing the applied current density up to 50 mA cm−2 by using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. Further increase of the current density was followed by gradual decrease in RY160 degradation and COD removal due to increase in temperature. Above a temperature 35°C, sodium hypochlorite tends to chemically decompose to sodium chlorate as follows:
(9)3NaClO⟶NaClO3+2NaCl
So when temperature rises higher than 35°C, production of NaClO falls. But at higher current densities the rate of hypochlorite decomposition increases with increase in current density.


### 3.2.4. Effect of Type of Electrolyte

Electrolytes of 4 g L−1 of the following salts; NaCl, CaCl2, KCl, Na2CO3, NaF, Na3PO4, and Na2SO4 were studied by three electrodes. As appears in Figure 4, The NaCl, KCl, and CaCl2 were the most effective conductive electrolytes for the electrocatalytic degradation of the investigated RY160 and COD removal. The Cl− anion is a powerful oxidizing agent. It enhances the degradation of pollutants. Therefore, addition of NaCl, KCl, and CaCl2 provides the effective Cl− ion. This behavior may be due to the small ionic size of K+ and Na+ which increases the ion mobilities and the loss ability of Cl− ion. Na2SO4 and NaF electrolytes showed the least efficiency in the degradation of pollutant. This may be attributed to the formation of an adherent film on the anode surface which poisons the electrode surface. Also, these electrolytes do not contain chloride ions (Cl−) in their structures and may form stable intermediate species that could not be oxidized by direct electrolysis. These observations were also confirmed in other studies [31].


### 3.2.5. Effect of the Electrolysis Time

To assess the effect of electrolysis time, experiments were conducted with operating treatment conditions that were consistent with those described for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. The maximum removal of RY160 was achieved using C/PbO2, Pb + Sn/PbO2 + SnO2 and Pb/PbO2electrodes after at least 15 min. Therefore, this was taken as optimal degradation time for the removal of RY160. The optimal time for COD removal for three electrodes was 270, 340, and 360 min, respectively.


### 3.2.6. Effect of Temperature

It is well known that the rate of diffusion of ions increases with increasing temperature. Figure5 represents the correlation between the concentration of the remaining RY160 dye and COD residual as a function of the solution temperature. The rate of the RY160 degradation and COD removal increase significantly with increasing the solution temperature until 25°C. Therefore, 25°C was fixed as optimal electrolysis temperature for the next experiments.


### 3.2.7. Effect of Initial RY160 Concentration

Figure6 shows the effect of different initial RY160 concentrations on the rate of RY160 degradation and corresponding COD removal. Total removal of the RY160 and COD can be achieved in the presence of initial RY160 load up to 100 mg L−1. However, increasing the RY160 concentration above this level results in a decrease in the electrocatalytic rate of degradation. The removal efficiency of the RY160 by using C/PbO2, Pb + Sn/PbO2, + SnO2 and Pb/PbO2 electrodes at 100 mg L−1 was the optimum concentration for the initial load concentration of RY160. As the initial RY160 concentration increase, the degradation efficiency decrease. This evidence that the generation of the powerful oxidizing agent Cl− ions on electrode surface was not increased in constant current density. The optimum operating conditions for degradation RY160 dye for each electrode were determined and summarized in Table 1. At optimized conditions, the percentages of RY160 degradation and COD removal for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 are 97.9%, 96.65, and 95.35%, respectively. The results indicate that the C/PbO2 electrode is more adequate than Pb + Sn/PbO2 + SnO2- and than Pb/PbO2-modified electrode for the degradation of RY160. Their behavior may be attributed to the color and structure of tested electrodes. C/PbO2 modified electrodes have a black color, while Pb + Sn/PbO2 + SnO2 and Pb/PbO2 modified electrodes have a brown color. It was reported that PbO2film has two structures, α-structure (brown color) and β-structure (black color) [31]. The black one has a tetrahedral crystal structure which is close-packed and more disordered in comparison with the close-packed structure of the brown α-form (orthorhombic). Therefore, the surface area in case of tetrahedral structure is more than that of the orthorhombic one, and hence the β-PbO2 form will be more effective than α-PbO2 form. Because the overpotential for oxygen evolution of β-PbO2 is higher than that of α-PbO2, it is expected that the electrocatalytic properties for C/PbO2-modified electrodes are more efficient than that of Pb/PbO2-modified electrode [34]. In this work, the degradation rate of RY160 was nearly completed and reached 97.9, 96.65, and 95.35 percentage using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes, respectively, after 15 min.Table 1
Percentage of COD and concentration removal of RY160 on C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes in the optimum conditions.
Type of electrode
Removal percent of RY160 at 15 min
Removal percent of COD
Removal percent of COD for real samples
C/PbO2
97.9
100 at 270 min
100 at 300 min
Pb + Sn/PbO2 + SnO2
96.65
100 at 340 min
100 at 380 min
Pb/PbO2
95.35
100 at 360 min
100 at 400 min


### 3.2.8. Effect of Distance between the Cathode and Anode

The effect of distance between the two electrodes of the cell was studied. It was found from Figure7 that there was an increase of hypochlorite generation by decreasing the distance between the two electrodes up to 1 cm for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. Therefore 1 cm was chosen as optimum distance between electrodes for sodium hypochlorite generation. The experiments were carried out under the following conditions: current density 50 mA cm−2, pH of 7.13, temperature of 25°C, and the concentration of NaCl 4 g L−1. The time of electrolysis was 15 min. It is clear that the sodium hypochlorite production increase with decreasing distance down to 1 cm. This is due to drop of electrolyte ohmic potential, and hence the cell voltage [37]. The highest hypochlorite production was achieved with narrow distance between the cell electrodes of 1 cm.


## 3.2.1. Effect of pH Value

The pH of the solution was varied while the other conditions where kept constant. As shown in Figure1, maximum removal of RY160 and COD was achieved at pH 7.13 for C/PbO2, Pb+Sn/PbO2+SnO2, and Pb/PbO2, respectively. The pH values of the solutions were adjusted by adding drops of H2SO4 and NaOH. The reactions were carried out for 15 min for three electrodes under the following conditions: the initial concentration of 100 mg L−1, a current density of 50 mA cm−2, a temperature of 25°C and NaCl concentration of 4 g L−1. The distance between the two electrodes was adjusted to 1 cm. It was found that the maximum rate of degradation using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes was achieved in neutral medium as the optimal medium.


## 3.2.2. Effect of the NaCl Concentration

Different concentrations of NaCl were applied to study their effect on the removal of RY160 and the corresponding COD elimination as indicated in Figure2. The results indicate that an increase of the electrolyte concentration up to 4 g L−1 leads to increase in the RY160 degradation rate and COD removal for three C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. The NaCl solution liberates Cl2 gas which is considered as the active species for the degradation of organic compound. Further increase of the NaCl concentration has slight effect on the degradation rate of RY160 and COD removal.


## 3.2.3. Effect of Current Density

As shown in Figure3, RY160 degradation and COD removal increase with increasing the applied current density up to 50 mA cm−2 by using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. Further increase of the current density was followed by gradual decrease in RY160 degradation and COD removal due to increase in temperature. Above a temperature 35°C, sodium hypochlorite tends to chemically decompose to sodium chlorate as follows:
(9)3NaClO⟶NaClO3+2NaCl
So when temperature rises higher than 35°C, production of NaClO falls. But at higher current densities the rate of hypochlorite decomposition increases with increase in current density.


## 3.2.4. Effect of Type of Electrolyte

Electrolytes of 4 g L−1 of the following salts; NaCl, CaCl2, KCl, Na2CO3, NaF, Na3PO4, and Na2SO4 were studied by three electrodes. As appears in Figure 4, The NaCl, KCl, and CaCl2 were the most effective conductive electrolytes for the electrocatalytic degradation of the investigated RY160 and COD removal. The Cl− anion is a powerful oxidizing agent. It enhances the degradation of pollutants. Therefore, addition of NaCl, KCl, and CaCl2 provides the effective Cl− ion. This behavior may be due to the small ionic size of K+ and Na+ which increases the ion mobilities and the loss ability of Cl− ion. Na2SO4 and NaF electrolytes showed the least efficiency in the degradation of pollutant. This may be attributed to the formation of an adherent film on the anode surface which poisons the electrode surface. Also, these electrolytes do not contain chloride ions (Cl−) in their structures and may form stable intermediate species that could not be oxidized by direct electrolysis. These observations were also confirmed in other studies [31].


## 3.2.5. Effect of the Electrolysis Time

To assess the effect of electrolysis time, experiments were conducted with operating treatment conditions that were consistent with those described for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. The maximum removal of RY160 was achieved using C/PbO2, Pb + Sn/PbO2 + SnO2 and Pb/PbO2electrodes after at least 15 min. Therefore, this was taken as optimal degradation time for the removal of RY160. The optimal time for COD removal for three electrodes was 270, 340, and 360 min, respectively.


## 3.2.6. Effect of Temperature

It is well known that the rate of diffusion of ions increases with increasing temperature. Figure5 represents the correlation between the concentration of the remaining RY160 dye and COD residual as a function of the solution temperature. The rate of the RY160 degradation and COD removal increase significantly with increasing the solution temperature until 25°C. Therefore, 25°C was fixed as optimal electrolysis temperature for the next experiments.


## 3.2.7. Effect of Initial RY160 Concentration

Figure6 shows the effect of different initial RY160 concentrations on the rate of RY160 degradation and corresponding COD removal. Total removal of the RY160 and COD can be achieved in the presence of initial RY160 load up to 100 mg L−1. However, increasing the RY160 concentration above this level results in a decrease in the electrocatalytic rate of degradation. The removal efficiency of the RY160 by using C/PbO2, Pb + Sn/PbO2, + SnO2 and Pb/PbO2 electrodes at 100 mg L−1 was the optimum concentration for the initial load concentration of RY160. As the initial RY160 concentration increase, the degradation efficiency decrease. This evidence that the generation of the powerful oxidizing agent Cl− ions on electrode surface was not increased in constant current density. The optimum operating conditions for degradation RY160 dye for each electrode were determined and summarized in Table 1. At optimized conditions, the percentages of RY160 degradation and COD removal for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 are 97.9%, 96.65, and 95.35%, respectively. The results indicate that the C/PbO2 electrode is more adequate than Pb + Sn/PbO2 + SnO2- and than Pb/PbO2-modified electrode for the degradation of RY160. Their behavior may be attributed to the color and structure of tested electrodes. C/PbO2 modified electrodes have a black color, while Pb + Sn/PbO2 + SnO2 and Pb/PbO2 modified electrodes have a brown color. It was reported that PbO2film has two structures, α-structure (brown color) and β-structure (black color) [31]. The black one has a tetrahedral crystal structure which is close-packed and more disordered in comparison with the close-packed structure of the brown α-form (orthorhombic). Therefore, the surface area in case of tetrahedral structure is more than that of the orthorhombic one, and hence the β-PbO2 form will be more effective than α-PbO2 form. Because the overpotential for oxygen evolution of β-PbO2 is higher than that of α-PbO2, it is expected that the electrocatalytic properties for C/PbO2-modified electrodes are more efficient than that of Pb/PbO2-modified electrode [34]. In this work, the degradation rate of RY160 was nearly completed and reached 97.9, 96.65, and 95.35 percentage using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes, respectively, after 15 min.Table 1
Percentage of COD and concentration removal of RY160 on C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes in the optimum conditions.
Type of electrode
Removal percent of RY160 at 15 min
Removal percent of COD
Removal percent of COD for real samples
C/PbO2
97.9
100 at 270 min
100 at 300 min
Pb + Sn/PbO2 + SnO2
96.65
100 at 340 min
100 at 380 min
Pb/PbO2
95.35
100 at 360 min
100 at 400 min


## 3.2.8. Effect of Distance between the Cathode and Anode

The effect of distance between the two electrodes of the cell was studied. It was found from Figure7 that there was an increase of hypochlorite generation by decreasing the distance between the two electrodes up to 1 cm for C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes. Therefore 1 cm was chosen as optimum distance between electrodes for sodium hypochlorite generation. The experiments were carried out under the following conditions: current density 50 mA cm−2, pH of 7.13, temperature of 25°C, and the concentration of NaCl 4 g L−1. The time of electrolysis was 15 min. It is clear that the sodium hypochlorite production increase with decreasing distance down to 1 cm. This is due to drop of electrolyte ohmic potential, and hence the cell voltage [37]. The highest hypochlorite production was achieved with narrow distance between the cell electrodes of 1 cm.


## 3.3. Application of the Treatment Process in Real Wastewater Samples

The treatment of RY160 effluents obtained from dyeing factory was carried out by using the prepared C/PbO2-, Pb+Sn/PbO2-, + SnO2, and Pb/PbO2-modified electrodes. The treatment was performed, first by collecting actual waste samples from the wastewater effluents of the RY160 dyeing bath. The initial dye-load concentration of these samples was 170 mg/L taken from Hubbub dyeing factory located in the industrial area at Biet Hanon, Gaza Strip, PNA. The dyestuff solutions were treated by the electrocatalytic oxidation technique using the same method as applied to the treatment of RY160 in aqueous solution to investigate the optimum condition for real wastewater containing the dye. After the treatment process, the removal percentages of RY160 dye at 15 min using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes were 97.9%, 96.65, and 95.35%, respectively. The removal percentages of COD were 100% at 300, 380 and 400 min for the above electrodes, respectively. These results indicated that the suggested modified electrodes are highly efficient in the treatment of effluents containing RY160 dye with very slight effect of matrix.


## 4. Conclusion

In this work, three modified electrodes (C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2) were prepared by elecrodeposition and used as anodes for electrodegradation of RY160 in aqueous solution at different parameters including conductive electrolyte, current density, temperature, initial concentration of RY160, pH, and time. The optimum conditions for three electrodes are: NaCl (4 g L−1), temperature at 25°C, degradation time of 15 min, initial concentration of 100 mg L−1, current density equals 50 mA cm−2 and 1 cm distance between the three electrodes of the cell. The degradation of RY160 was nearly completed (97.9%, 96.65, and 95.35%) using C/PbO2, Pb + Sn/PbO2 + SnO2, and Pb/PbO2 electrodes at pH 7.13, respectively. The obtained results indicated high efficiency of the suggested modified electrodes in the treatment of effluents containing RY160 dye with very slight matrix effect.



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*Source: 691763-2012-09-05.xml*