Electrochemical degradation of linuron in aqueous solution using Pb/PbO2 and C/PbO2 electrodes

Nasser Abu Ghalwa; Mazen Hamada; Hazem M. Abu Shawish; Omar Shubair

doi:10.1016/j.arabjc.2011.08.006

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Original article

9 (

1_suppl

); S821-S828

doi:

10.1016/j.arabjc.2011.08.006

Electrochemical degradation of linuron in aqueous solution using Pb/PbO₂ and C/PbO₂ electrodes

Nasser Abu Ghalwa^{^a,⁎}, Mazen Hamada^{^a}, Hazem M. Abu Shawish^{^b}, Omar Shubair^{^a}

a Faculty of Science, Department of Chemistry, Al Azhar University Gaza, Gaza Strip, Palestine

b Chemistry Department, College of Sciences, Al-Aqsa University, Gaza, Palestine

⁎Corresponding author. Tel.: +970 82066119. dr.nasser.galwa@hotmail.com (Nasser Abu Ghalwa)

Received: 2011-5-14, Accepted: 2011-8-20, Published: 2011-09-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Two modified electrodes (Pb/PbO₂ and C/PbO₂) were prepared by electrodeposition and used as anodes for electrochemical degradation of linuron (phenylurea pesticide) in aqueous solution. Different operating conditions and factors affecting the treatment process including current density, temperature, initial concentration of linuron, pH, conductive electrolyte and time of electrolysis were studied and optimized. The best degradation occurred in the presence of NaCl (1 gL⁻¹) as conductive electrolyte. After 30 min, nearly complete degradation of linuron was achieved (92% and 84%) using C/PbO₂ and Pb/PO₂ electrodes at pH 7 and 1.5, respectively. Higher degradation efficiency was obtained at low temperature (5–10 °C). The optimum current density for the degradation of linuron on both electrodes was (150 mAcm⁻²).

Keywords

Electrochemical degradation

Linuron

Electrodes

Electrocatalytic oxidation

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1 Introduction

The environmental problem generated by phenylurea herbicides makes necessary their removal from waters matrices. In general, different technologies developed for the elimination of refractory organic micropollutants from drinking and wastewaters include chemical oxidation methods, which are successfully applied in drinking water purification plants. These chemical procedures are based on the application of oxidizing reagents such as UV radiation, ozone, hydrogen peroxide, etc. (Meunier et al., 2006) or combination of oxidants in the advanced oxidation processes (AOPs) (Benitez et al., 2009; Hoigne, 1998). Half-life in soil ranges from 38 to 67 days for linuron (Fig. 1) (Caux et al., 1998). Therefore, this compound has been found as contaminants in surface and ground waters (Garmouma et al., 1997) and microbial degradation is considered to be the primary mechanism for their dissipation from soil.

Few of treatment techniques for wastewater which contains linuron have been reported by using O₃/H₂O₂ (Tahmasseb et al., 2002), direct photolysis (Faure and Boule, 1997) and Fenton (Barlas, 2000). Recent reports indicate that a combination of H₂O₂ and UV irradiation with Fe(II), so-called the photo-Fenton process, can significantly enhance decomposition of many refractory organic compounds (Rao and Chu, 2010a). The degradation of linuron, one of phenylurea herbicides, was investigated for its reaction kinetics by different treatment processes including ultraviolet irradiation (UV), ozonation (O₃), and UV/O₃ (Rao and Cho, 2009a). The application of TiO₂/H₂O₂/Vis (visible light) process for the aqueous degradation of linuron has been investigated. The performance of TiO₂/H₂O₂/Vis process has been compared with other processes such as TiO₂/H₂O₂ in the dark, TiO₂/Vis, and H₂O₂/Vis in terms of linuron decay. The result showed that more than 70% linuron could be decomposed in the TiO₂/H₂O₂/Vis (Rao and Chu, 2009b, 2010a,b ). A comprehensive study of the degradation of linuron, was conducted by using different treatment processes including UV, ozonation and UV/O₃ (Rao and Chu, 2010b).

The oxidation of different model compounds was studied at Pb/PbO₂ anodes in undivided cells employing various current densities. Considerable differences in oxidizability of organic substances were found by continuous measuring of CO₂ during anodic oxidation (Mann and Ndler, 1998).

The electrochemical removal of tramadol hydrochloride, herbicidal 2,4-D (albar super) and pure 2,4-dichlorophenoxy acetic acid from aqueous solutions was investigated under several operating conditions using a Pb/PbO₂ electrode (Abu Ghalwa et al., 2014; Zourab et al., 2009).

In this study, an electrodegradation method was applied on linuron pesticide by using modified electrodes (Pb/PbO₂ and C/PbO₂). Different factors including the pH, concentration of electrolyte, conductive electrolyte type, current density, time of electrolysis, initial concentration of linuron 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).

2 Experiments

2.1

2.1 Chemicals and instrumentation

Sodium chloride, sodium fluoride, sodium carbonate, sodium sulfate, calcium chloride, potassium chloride, sodium hydroxide, sulfuric acid, potassium dichromate, silver sulfate, and sodium thiosulfate were of analytical grade and purchased from Merck. Linuron pesticide was purchased from Dr. Ehrenstorfer GmbH (Germany) with purity of 99.9%. Other reagents were of the analytical grade. Distilled water was used for the preparation of solutions. Standard solutions of potassium dichromate (K₂Cr₂O₇), sulfuric acid (H₂SO₄) reagent with silver sulfate (Ag₂SO₄) were prepared to measure the COD. Different standard solutions of linuron with concentration from 10 to 70 mg/L were prepared to measure its degradation at different conditions. The double-beam UV-visible 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 multi-meter is kyoritsu model 1008, (Japan) for reading out the current and potential values. A closed reflux titrimetric unit was used for the COD determination.

2.2

2.2 Electrodeposition of doped lead dioxide at different substrates

2.2.1

2.2.1 Preparation of Pb/PbO₂ modified electrode

2.2.1.1

2.2.1.1 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, acetone was used for degreasing. This process has a great application and good penetrating power. Then it was treated with an alkali solution [a mixture of sodium hydroxide (50 gL⁻¹) and sodium carbonate (20 gL⁻¹), tri-sodium orthophosphate (20 gL⁻¹) and sulfuric acid (2 gL⁻¹)]. 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 gL⁻¹) and hydrofluoric acid (5 gL⁻¹) and then chemically polished in boiled oxalic acid solution (100 gL⁻¹) for 5 min (Awad and Abu Galwa, 2005).

2.2.1.2

2.2.1.2 Electrochemical deposition of PbO₂

PbO₂ was deposited galvanostatically on the pretreated lead substrate by electrochemical anodization of lead in oxalic acid solution (100 gL⁻¹) at 25 °C. This acid solution was electrolyzed galvanostatically for 30 min at ambient temperature using an anodic current density of 100 mAcm⁻². The cathode was stainless steel (austenitic type), the two electrodes were concentric with the lead electrode as axial. This arrangement gave the formation of a regular and uniform deposit (Awad and Abu Galwa, 2005).

2.2.2

2.2.2 Preparation of modified C/PbO₂ electrode

2.2.2.1

2.2.2.1 Carbon surface treatment

Pretreatment of carbon rod (8 mm × 25 cm) was carried out following the procedure applied by Narasimham and Udupa (Narasimham and Udupa, 1976). The carbon rod was soaked in 5% NaOH solution, washed with distilled water, dried in furnace at 105 °C, cooked with linseed oil to reduce the porosity of rod. The electrode after the previous treatment is ready to receive doped PbO₂.

2.2.2.2

2.2.2.2 Electrochemical deposition of PbO₂

The electrodeposition of PbO₂ was performed at constant anodic current of 20 mAcm⁻² from 12% w/v Pb(NO₃)₂ solution at 25 °C. Containing 5% w/v CuSO₄·5H₂O and 3% surfactant, cetyl trimethyl ammonium bromide (CTAB). The role of the surfactant is minimizing the surface tension of the solution. Electrodeposition was carried out for 60 min at 80 °C with continuous stirring (Narasimham and Udupa, 1976).

2.3

2.3 Electrolysis for linuron degradation

Galvanostatic electrolyses were carried out at Pb/PbO₂ and C/PbO₂ electrodes, with current density ranging from 0 to 400 mAcm⁻² and electrical potential ranging from 1 to 12 volts. Runs were performed at 5–40 °C. Solutions of 50 mgL⁻¹ of pure linuron solution were used. Electrolysis was done with 1 gL⁻¹ of different types of electrolytes NaCl, CaCl₂, KCl, Na₂CO₃, NaF, and Na₂SO₄, at sodium chloride concentration from 0.5 to 10 gL⁻¹ with pH around 1.5–12. The electrolysis of time ranges from 0-180 min. The electrolysis of the aqueous solution containing the linuron to be treated electrochemically was carried out in one compartment Pyrex glass cell of 50 ml volume with the prepared Pb/PbO₂ and C/PbO₂ as anode and austenitic stainless steel as cathode. DC power supply was used for the degradation of linuron pesticide. The current and potential measurements were carried out using digital multi-meter.

2.4

2.4 Analysis

Two main parameters were measured to evaluate the electrochemical treatment efficiency, the remaining pollutant concentration and the chemical oxygen demand (COD). Remaining pollutants (linuron) concentration was measured using a double-beam UV-visible spectrophotometer from Shimadzu at λ_max = 242.5 nm using calibration curve with standard error ± 0.2%. While the COD was determined using a closed reflux titrimetric method.

3 Results

3.1

3.1 The effect of different operating factors on degradation of linuron and COD removal using Pb/PbO₂ and C/PbO₂ electrodes

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 (mgO₂ L⁻¹) were illustrated in Figs. 2–8 .

3.1.1

3.1.1 Effect of pH value

The pH of the solution was varied while the other conditions were kept constant. As shown in Fig. 2, maximum removal of linuron and COD were achieved at pH 1.5 and 7 for Pb/PbO₂ and C/PbO₂, respectively. The pH values of the solutions were adjusted by adding drops of H₂SO₄ and NaOH. The reactions were carried out for 30 min under the following conditions: the initial concentration of 50 mgL⁻¹, a current density of 150 mA cm⁻², a temperature of 10 °C and NaCl concentration of 1 gL⁻¹. The distance between the two electrodes was adjusted to 1 cm.

The effect of pH on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

3.1.2

3.1.2 Effect of the NaCl concentration

Different concentrations of NaCl were applied to study their effect on the removal of linuron and the corresponding COD elimination as indicated in Fig. 3. The results indicate that an increase of the electrolyte concentration up to 1 gL⁻¹ lead to increase in the linuron degradation rate and COD removal for both Pb/PbO₂ and C/PbO₂ electrodes. Further increase of the NaCl concentration reflected negatively on the degradation rate of linuron and COD removal. The operating conditions of the treatment process were: current density of 150 mA cm⁻², pH 1.5 and 7 using Pb/PbO₂ and C/PbO₂, respectively, temperature of 10 °C, initial concentration 50 mg L⁻¹, and the distance between the two electrodes was 1 cm. The reaction was allowed to proceed for 30 min.

The effect of NaCl concentration on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

3.1.3

3.1.3 Effect of current density

As shown in Fig. 4 linuron degradation and COD removal increase with increasing the applied current density up to 150 mA cm⁻² by using Pb/PbO₂ and C/PbO₂ electrodes. Further increase of the current density was followed by gradual decrease in linuron degradation and COD removal due to increase in temperature. These experiments were carried out under the following operating conditions: initial concentration 50 mgL⁻¹, pH 1.5 and 7 using Pb/PbO₂ and C/PbO₂, respectively, temperature 10 °C, NaCl 1 gL⁻¹, and the distance between the two electrodes of 1 cm. The time of electrolysis was 30 min using Pb/PbO₂ and C/PbO₂ electrodes.

The effect of current density on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

3.1.4

3.1.4 Effect of type of electrolyte

Electrolytes of 1 gL⁻¹ of the following salts: NaCl, CaCl₂, KCl, Na₂CO₃, NaF, and Na₂SO₄ were studied by both electrodes. As appears in Fig. 5, the KCl and NaCl were the most effective conductive electrolytes for the electrocatalytic degradation of the investigated linuron and COD removal while Na₂SO₄ and Na₂CO₃ electrolytes show poor results. The operating conditions of the treatment process were: current density of 150 mA cm⁻², pH 1.5 and 7 using Pb/PbO₂ and C/PbO₂, respectively, temperature of 10 °C, initial concentration 50 mgL⁻¹, and the distance between the two electrodes was 1 cm. The reaction was allowed to proceed for 30 min.

The effect of the conductive electrolyte type on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

3.1.5

3.1.5 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 both Pb/PbO₂ and C/PbO₂ electrodes. As shown in Fig. 6, the maximum removal of linuron was achieved using Pb/PbO₂ and C/PbO₂ electrodes after at least 30 min. Therefore, this was taken as optimal degradation time for the removal of linuron. The optimal time for COD removal for both electrodes was 4 h. These experiments were carried out under the following operating conditions: current density of 150 mAcm⁻², pH 1.5 and 7 using Pb/PbO₂ and C/PbO₂, respectively, temperature 10 °C, NaCl 1 gL⁻¹, initial concentration 50 mgL⁻¹, and the distance between the two electrodes of 1 cm.

The effect of time on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

3.1.6

3.1.6 Effect of temperature

Fig. 7 represents the correlation between the concentration of the remaining linuron and COD residual as a function of the solution temperature. The rate of the linuron degradation and COD removal decrease significantly with increasing the solution temperature above 40 °C. Further decrease of the temperature below 10 °C did not bring any significant effect. Therefore, 10 °C was fixed as optimal electrolysis temperature under the same conditions mentioned previously. The reactions were carried out for 30 min under the following conditions: the initial concentration of 50 mgL⁻¹, a current density of 150 mAcm⁻², NaCl and pH 1.5 and 7 using Pb/PbO₂ and C/PbO₂, respectively, and concentration of 1 gL⁻¹. The distance between the two electrodes was adjusted to 1 cm.

The effect of temperature on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

3.1.7

3.1.7 Effect of initial linuron concentration

Fig. 8 shows the effect of different initial linuron concentrations on the rate of linuron degradation and corresponding COD removal. Total removal of the linuron and COD can be achieved in the presence of initial linuron load up to 50 mgL⁻¹. However, increasing the linuron concentration above this level results in a decrease in the electrocatalytic rate of degradation. The removal efficiency of the linuron by using Pb/PbO₂ and C/PbO₂ electrodes at 50 mgL⁻¹ was the optimum concentration for the initial load concentration of linuron. These experiments were carried out under the following operating conditions: current density of 150 mAcm⁻², pH 1.5 and 7 using Pb/PbO₂ and C/PbO₂, respectively, temperature 10 °C, NaCl 1 gL⁻¹, and the distance between the two electrodes of 1 cm. The time of electrolysis was 30 min using Pb/PbO₂ and C/PbO₂ electrodes.

The effect of initial concentration on linuron and COD removal using PbO2/Pb and PbO2/C electrodes.

4 Discussions

4.1

4.1 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 (Awad and Abu Galwa, 2005).

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 (MO_x). This process may give rise to formation of hydroxyl radicals according to:

(1)

{MO}_{x} + H_{2} O \overset{k1}{⟶} {MO}_{x} [{OH}^{\cdot}] + H^{+} + e^{-}

The produced hydroxyl radicals can, be oxidized to a higher state forming the so-called higher oxide:

(2)

{MO}_{x} [{OH}^{\cdot}] \overset{k2}{⟶} {MO}_{x} [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:

(3)

{MO}_{x} [O] + R \overset{k3}{⟶} RO + {MO}_{x}

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”. However, if the product of Eq. (3) 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:

(4)

{MO}_{x} [{OH}^{\cdot}] + R \overset{h 4}{⟶} O_{x} + {mCO}_{2} + {nH}_{2} O + H^{+} + e^{-}

And this class of electrodes are called “non-active electrodes”.

On the basis of the above-mentioned mechanism, the lead dioxide anode employed in this investigation is characterized by high oxygen overvoltage on which (OH^•) are 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 (Panizza et al., 2000). However, PbO₂ does not have a higher oxidation state; consequently it is classified as a “non-active electrode”. It was reported that lead dioxide electrode is a 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 toward 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. 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 node surface according to (Cristina and Cerisola, 2000)

(5)

2 {Cl}^{-} \overset{k5}{⟶} {Cl}_{2} + 2 e^{-}

The second step is formation of hypochlorous acid:

(6)

{Cl}_{2} + H_{2} O \overset{k6}{⟶} HClO + {Cl}^{-} + H^{+}

The HClO undergoes dissociation into hypochlorite and hydrogen ions:

(7)

HClO \overset{k7}{⟶} {ClO}^{-} + H^{+}

4.2

4.2 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 and optimized. The remaining concentration (mg L⁻¹) and COD removal (mgO₂ L⁻¹) were illustrated in Figs. 2–8 . The percentages removal for each electrode was represented in the Table 1.

Table 1 Percentage of COD and concentration removal of linuron on Pb/PbO₂ and C/PbO₂ electrodes.

Type of electrode	Removal percent of linuron at 30 min	Removal percent of COD at 4 h
Pb/PbO₂	84	84.04
C/PbO₂	92	94.70

From Figs. 2–8 and Table 1, it was found that the C/PbO₂ electrode was more effective than Pb/PbO₂ modified electrode in the degradation of linuron. This behavior may be attributed to the color and structure of electrodes. C/PbO₂ modified electrodes have a black color, while Pb/PbO₂ modified electrode has a brown color. It was reported that PbO₂ film has two structures, α-structure (brown color) and β-one (black color) (Awad and Abu Galwa, 2005). The black one has a tetrahedral crystal structure which is a close-packed structure 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 the orthorhombic one, and then the β-PbO₂ form will be more effective than α-PbO₂ form. That because the over potential for oxygen evolution of β-PbO₂ is more than that of α-PbO₂ we can expect that the electrocatalytic properties for C/PbO₂ modified electrodes are more efficient than Pb/PbO2 modified electrode (Pelegrino et al., 2002).

The degradation of linuron was nearly completed and reached 92% and 84% using C/PbO₂ and Pb/PO₂ electrodes, respectively, on 30 min, while degradation using photo-Fentons reaction after 25 h was 90% (Katsumata et al., 2005).

In this study, it was found that the electrocatalytic oxidation of linuron was depending on pH value of solution as shown in Fig. 2. It was shown that the maximum rate of degradation using Pb/PbO₂ electrode was in the acidic medium. In this medium, the NaCl solution liberates Cl₂ gas which is considered as the active species for the degradation of organic compound. While by using C/PbO₂ electrode the neutral medium is the optimum condition.

As shown in Fig. 4, linuron degradation and COD removal increase with increasing the applied current density up to 150 mAcm⁻² by using Pb/PbO₂ and C/PbO₂ electrodes. Further increase of the current density was followed by gradual decrease in linuron degradation and COD removal due to increase in temperature. Above a temperature of 35 °C, sodium hypochlorite tends to chemically decompose to sodium chlorate.

(8)

3NaClO \to {NaClO}_{3} + 2 NaCl

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.

Fig. 5 shows that the effect of conductive electrolyte on degradation of linuron using Pb/PbO₂ and C/PbO₂ electrodes. From Fig. 5, it is clear that the less value of remaining concentration and COD were obtained in presence of KCl and NaCl. This indicates that KCl and NaCl are the most effective electrolytes in electrodegradation of linuron. The Cl⁻ anion is a powerful oxidizing agent. It enhances the degradation of pollutants. Therefore, addition of KCl or NaCl provides the effective Cl⁻ ion. In addition, NaCl is the cheapest electrolyte containing chloride ions. From Fig. 5, KCl and NaCl are considered as the most preferential electrolytes. This observed behavior may be due to the small ion size of K⁺ and Na⁺ which increases the ability of loss of Cl⁻ ion. From Fig. 5, it is clear that the less effective electrolytes in the degradation of pollutant are Na₂SO₄ and Na₃CO₃. This behavior may be attributed for the formation of an adherent film (PbO₂[O]) on the anode surface which poisoned the electrode process surface. Also these electrolytes do not contain chloride ion (Cl⁻). Also those electrolytes may form stable intermediate species that could not be oxidized by direct electrolysis. These observations were confirmed in other studies (Awad and Abu Galwa, 2005). Also, it was shown that CaCl₂ and NaF have an intermediate effect on the rate of degradation of pollutants because CaCl₂ contains effective ion (Cl⁻), while the electrodegradation of pollutants may be occur through electrocatalytic oxidation in the presence of NaF electrolyte.

The degradation of linuron pesticide in aqueous solution was characterized by a significant decrease in pesticide concentration with electrolysis time until about 30 min as shown in Fig. 6. This can be explained by the overall oxidizing agent in solution which was generated in the first half hour. However, the degradation of most linuron pesticide in all processes was reached after 30 min.

It is clear from Fig. 7 that the optimum temperature of sodium hypochlorite production was 10 °C for Pb/PbO₂ and C/PbO₂ electrodes. At low temperature, the losses of chlorine gas decrease, which lead to increase the sodium hypochlorite formation. Asokan and Kraft found that at 35 °C, the sodium hypochlorite tends to chemically decompose to sodium chlorate (Asokan and Subramanian, 2009; Kraft et al., 1999). In addition, increasing of remaining concentration and COD above 40 °C may be attributed to decomposition of adsorbed film on anodic side. The electrodes are unstable at high temperature above 40 °C.

Fig. 8 shows the effect of different initial linuron concentrations on the rate of linuron degradation and corresponding COD removal. As the initial linuron concentration increases, the degradation efficiency decreases. This is 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 linuron pesticide for each electrode were determined and summarized in Table 1.

It is clear that the sodium hypochlorite production increases with decreasing distance down to 1 cm. This is due to drop of electrolyte ohmic potential, and hence the cell voltage (Kelsall, 1984). The highest hypochlorite production was achieved with narrow distance between the cell electrodes of 1 cm.

5 Conclusion

In this work two modified electrodes (Pb/PbO₂ and C/PbO₂) were prepared by elecrodeposition and used as anodes for electrodegredation of linuron (phenyl urea pesticide) in aqueous solution at different parameters including conductive electrolyte, current density, temperature, initial concentration of linuron, pH and time. The optimum condition for both electrodes are: NaCl (1 gL⁻¹), temperature at (5–10 °C), degradation time of 30 min, initial concentration of 50 mgL⁻¹, and current density (150 mA cm⁻²). The degradation of linuron was nearly completed (92% and 84%) using C/PbO₂ and Pb/PO₂ electrodes at pH 7 and 1.5, respectively.

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Rao Y.F., Chu W., . Degradation of linuron by UV, ozonation, and UV/O₃ processes – effect of anions and reaction mechanism. J. Hazard. Mater.. 2010;180:514.
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Rao Y.F., Chu W., . Linuron decomposition in aqueous semiconductor suspension under visible light irradiation with and without H₂O₂. Chem. Eng. J.. 2010;158:181.
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Tahmasseb L.A., Ńelieu S., Kerhoas L., Einhorn J., . Ozonation of chlorophenylurea pesticides in water: reaction monitoring and degradation pathways. Sci. Total Environ.. 2002;291:33.
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Zourab S., Abu Ghalwa N., Zaggout F.R., Al-Asqalany M.Y., Khdear N., . Electrochemical degradation of herbicidal and pure 2,4-dichlorophenoxy acetic acid on Pb/PbO₂ modified electrodes. J. Dispersion Sci. Technol.. 2009;5:712.
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Introduction

Experiments

Chemicals and instrumentation
Electrodeposition of doped lead dioxide at different substrates
Electrolysis for linuron degradation
Analysis

Results

The effect of different operating factors on degradation of linuron and COD removal using Pb/PbO2 and C/PbO2 electrodes

Discussions

Mechanism of electrochemical oxidation of organic pollutants
Effect of various factors on the rate of degradation

Conclusion

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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Electrochemical degradation of linuron in aqueous solution using Pb/PbO₂ and C/PbO₂ electrodes