Electrochemical Pre-Treatment Combined with Biological Treatment

Article pubs.acs.org/IECR

Electrochemical Pre-Treatment Combined with Biological Treatment for the Degradation of Methylene Blue Dye: Pb/PbO2 Electrode and Modeling-Optimization through Central Composite Design Idris Yahiaoui,*,† Farida Aissani-Benissad,† Katia Madi,† Nassima Benmehdi,† Florence Fourcade,‡,§ and Abdeltif Amrane‡,§ †

Laboratoire de Génie de l’Environnement (LGE), Faculté de Technologie, Université A. MIRA, Route de Targa Ouzemour, 06000 Béjaïa, Algeria ‡ Ecole Nationale Supérieure de Chimie de Rennes, Université Rennes1, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France § Université européenne de Bretagne, Rennes, France ABSTRACT: Electrochemical pretreatment of wastewater containing methylene blue was studied on an Pb/PbO2 electrode and showed enhancement of the color removal efficiency for increasing current density and decreasing dye concentration. The effect of the temperature was more pronounced at a high initial dye concentration (134 mg L−1) compared to a low initial concentration (12 mg L−1), whereas it was the opposite for the agitation speed. The model obtained by central composite design led to the following optimal conditions for MB degradation: 41 ≤ T ≤ 60 °C, 10.66 ≤ i ≤ 25 mA cm−2, [MB]0 = 134 mg L−1, and ω = 720 rpm, which gave 95.61% degradation efficiency. Electrolysis improved biodegradability from 0.034 to 0.54 for the BOD5/COD ratio, indicating the feasibility of a biological treatment combined to an electrochemical pretreatment process. The degradation of the generated intermediate compounds was performed using activated sludge and led to 92.03% mineralization (overall DOC removal).

1. INTRODUCTION Nonbiodegradable organic compounds are common pollutants in waste effluents from many industrial sectors such as textiles, pharmaceutical, pesticide, and herbicide factories. Removing these nonbiodegradable organics is difficult and requires expensive processes.1−4 Both physicochemical and biological methods for the removal of dyes have been widely investigated. The physicochemical dye removal techniques such as flocculation−coagulation,5,6 electrochemical oxidation, photocatalytic oxidation, and electro-Fenton oxidation appear to have several technical benefits for the degradation of dyes, but the high cost, requirement for expensive equipment, and high reagent or energy requirement limits its use. On the other hand, biological methods such as activated sludge process and anaerobic treatment, the most cost-effective for wastewater treatment, which are destructive and have been extensively studied,7−13 do not always appear relevant for the removal of recalcitrant compounds, owing to their low biodegradability. For the degradation of persistent organic compounds contained in wastewater, several studies recommend integrated processes, especially the coupling of physicochemical processes and biological treatment.14−16 In addition, in recent years, the combination of electrochemical pretreatment with a biological treatment has attracted the attention of several researchers.17−19 This combination of processes seems to be a good alternative, because this method can lead to a total degradation of nonbiodegradable organic compounds and generated intermediate products, as shown for a pesticide, phosmet (Imidan, C11H12NO4PS2) by Alonso-Salles et al.18 Belkheiri et al.19 examined the removal of tetracycline on a graphite-felt © 2013 American Chemical Society

electrode in view of a subsequent biological treatment and showed that biodegradability increased with the oxidation potential. The degradation of procion blue by combined electrochemical, photocatalytic, and microbial methods and the tests of degradation capability of different microorganisms were studied by Ahmed Basha et al.17 The effectiveness of an electro-oxidation pretreatment depends on the nature of the anodes that are used.20 The choice of electrode material is of fundamental importance from an electrochemical point of view. Active electrodes mediate the oxidation of organic species by the formation of higher oxidation state oxides of the metal whenever such a higher oxidation state is reached by the metal oxide (e.g., RuO2 or IrO2), leading to selective oxidation, in which case biodegradability increases. Nonactive electrodes (e.g., SnO2 or PbO2) present no higher oxidation state available, and the organic species is directly oxidized by an adsorbed hydroxyl radical, generally resulting in complete combustion of the organic molecule. Effective and economic electrochemical treatment of organic pollutants requires the choice of appropriate catalytic electrode materials as well as appropriate electrolysis conditions. On traditional electrode materials, such as Au, Pt, and C, although the oxidation reaction by oxygen transfer is spontaneous, it is characterized by a low reaction rate constant. Oxygen transfer is usually favored on an anode material with Received: Revised: Accepted: Published: 14743

April 29, 2013 September 22, 2013 October 1, 2013 October 14, 2013 dx.doi.org/10.1021/ie401367q | Ind. Eng. Chem. Res. 2013, 52, 14743−14751

Industrial & Engineering Chemistry Research

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high oxygen evolution overpotential.1 Many studies have demonstrated that the degradation of dyes can be obtained with SnO2,21 PbO2,1,22−24 Ta/PbO2,25,26 and Boron-Doped Diamond electrodes (BDD).3,27 In this study, a Pb/PbO2 electrode was used; this type of electrode has received a great deal of attention because of its easy and rapid preparation as well as its low cost. In addition, the PbO2 electrode is quite stable at high applied potentials in solution, and lead dioxide (PbO2) is characterized by high oxygen overpotential; it is therefore one of the most commonly used anodes for electrochemical degradation of many pollutants.1,28 Investigating an electrochemical pretreatment for the degradation of methylene blue on Pb/PbO2 electrode combined with a biological treatment was therefore the purpose of this study, whose objectives were (i) to examine the individual effects of four operating parameters, namely the temperature, the current density, the initial dye concentration, and the agitation speed, (ii) to optimize these parameters through central composite design (CCD), and finally, (iii) to assess the feasibility of the combined process.

Na2HPO4, 6.80; KH2PO4, 2.80. The BOD5 value was initially estimated by BOD5 = COD/1.46, where the COD value is measured experimentally by means of a Nanocolor test CSB 160 (Macherey-Nagel, Hoerd, France). The range of expected BOD5 values was then deduced and, hence, led to the volumes of sample of activated sludge solution and of nitrification inhibitor (10 mg L−l solution of N-allylthiourea), which had to be added in the shake flask of the Oxitop apparatus. A similar protocol was applied for the control flask except that it was replaced by a solution of easily biodegradable compounds, namely glutamic acid (150 mg L−1) and glucose (150 mg L−1). Before use, KOH was added to achieve a neutral pH (7.0 ± 0.2). The same protocol was also used for the blank solution, for which the sample was replaced by water to deduce the biological oxygen demand corresponding to the endogenous respiration. 2.4. Response Surface Methodology (RSM). RSM is a collection of statistical and mathematical methods useful for the modeling and the analysis of engineering problems, which consist of the optimization of the response surface that is influenced by various process parameters. RSM also quantifies the relationship between the controllable input parameters and the obtained response surfaces.29,30 Its greatest applications concern industrial research, when a lot of variables affect the system feature. The runs were designed according to a central composite design (CCD). This type of design comprises two levels of factorial design (+1, −1) superimposed by the center points (coded 0) and the star points (+α, -α); α is the distance of the axial point from the center domain.24,30−32 The precise value of α depends on the desired properties for the design and on the number of factors involved. In the present study, to respect the rotatability of the design, α = (2k)1/4 (k was the number of studied parameters). This characteristic provides good predictions at points equally distant from the center.24,30−32 The response is expressed as the percent of color removal efficiency of methylene bleu as follows:

2. MATERIALS AND METHODS 2.1. Lead Surface Treatment and Formation of PbO2. Pretreatment of the lead substrate (50 mm × 40 mm × 1 mm) was carried out before anodization to ensure good adhesion of lead dioxide. Lead was first roughened to increase the adhesion of PbO2 via subjecting its surface to mechanical abrasion by sandpapers of different grades. It was then cleaned to remove sand particles or any other particles lodged in the metal surface by immersing the lead substrate in H2SO4 solution heated at 65 °C over the course of 30 min. The formation of PbO2 was carried out for 150 min by electrochemical anodization of lead in an H2SO4 solution (1M) at 65 °C using an anodic current density of 25 mA cm−2.24 2.2. Electrolysis and Chemicals. Electrolysis of the synthetic dye solution was carried out in a one-compartment Pyrex glass cell of 500 mL volume with the prepared Pb/PbO2 electrode as anode, grid platinum electrode as cathode, and the saturated calomel electrode was used as reference. All reagents and materials used in this study were of analytical grade (Biochem Chemopharma, Montréal, Québec, Canada), and distilled water was used to prepare synthetic dye solution of methylene blue (C16H18CIN3S; 100% purity). The pH of the solutions was adjusted to 2 by the addition of sulphuric acid solution (H2SO4; 96% purity), and experiments lasted 180 min. The agitation of the solution was assured by smooth, Tefloncoated magnetic bar (35 × 8 mm). 2.3. Analysis. Dye concentration was determined from its absorbance characteristic in the UV−vis. A SAFAS SP2000 (Monaco, France) spectrophotometer connected to a PC was used. The chemical oxygen demand (COD) was carried out by Nanocolor 500D photometer type (Macherey-Nagel, Hoerd, France). Dissolved organic carbon (DOC) was measured by TOC-VCPH/CPN Total Organic Analyzer Schimadzu (Marne la Vallée, France). Samples were taken and filtered through a 0.45 μm membrane syringe filter for measurement of dissolved organic carbon (DOC). Biodegradability was deduced from BOD5 measurements carried out in Oxitop IS6 (WTW, Alès, France). Activated sludge from a wastewater treatment plant was used to inoculate the flasks; the initial microbial concentration was 0.5 g L−1. The following mineral basis was used for all experiments (g L−1): MgSO4·7H2O, 22.5; CaCl2, 27.5; FeCl3, 0.15; NH4Cl, 2.0;

y (%) =

[MB]0 − [MB]t × 100% [MB]0

(1)

[MB]0 and [MB]t are the initial dye concentration and its concentration at a given time t (mg L−1); The original values of each factor and their corresponding levels are collected in Table 1. Table 1. Values and Levels of the Operating Parameters levels operating factors Z1: Z2: Z3: Z4:

T (°C) i (mA cm−2) [MB]0 (mg L−1) ω (rpm)

−2

−1

0

1

2

20 2.5 12 240

30 8.125 42.5 360

40 13.75 73 480

50 19.375 103.5 600

60 25 134 720

The central composite design was composed of 2 4 experiments of factorial design, 12 experiments realized at the center work domain and eight-star points (Table 2). The experiments were performed according to the central composite design. The stars points and the replicate runs are added to the factorial design to provide an estimation of the curvature of the model and to allow an estimation of the experimental error. 14744

dx.doi.org/10.1021/ie401367q | Ind. Eng. Chem. Res. 2013, 52, 14743−14751


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Table 2. Results of MB Dye Degradation for 24 Full Factorial Design (Centre Points and the Star Points) run no.

natural values of parameters coded values of parameters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Z1

Z2

Z3

Z4

x0

x1

x2

x3

x4

y (%)

ŷ (%)

30 30 30 30 30 30 30 30 50 50 50 50 50 50 50 50 40 40 40 40 40 40 40 40 40 40 40 40 20 60 40 40 40 40 40 40

8.125 8.125 8.125 8.125 19.375 19.375 19.375 19.375 8.125 8.125 8.125 8.125 19.375 19.375 19.375 19.375 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 13.75 2.5 25 13.75 13.75 13.75 13.75

42.5 42.5 103.5 103.5 42.5 42.5 103.5 103.5 42.5 42.5 103.5 103.5 42.5 42.5 103.5 103.5 73 73 73 73 73 73 73 73 73 73 73 73 73 73 73 73 12 134 73 73

360 600 360 600 360 600 360 600 360 600 360 600 360 600 360 600 480 480 480 480 480 480 480 480 480 480 480 480 480 480 480 480 480 480 240 720

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

−1 −1 −1 −1 −1 −1 −1 −1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 −2 +2 0 0 0 0 0 0

−1 −1 −1 −1 1 1 1 1 −1 −1 −1 −1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 −2 +2 0 0 0 0

−1 −1 1 1 −1 −1 1 1 −1 −1 1 1 −1 −1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 −2 +2 0 0

−1 1 −1 1 −1 1 −1 1 −1 1 −1 1 −1 1 −1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 −2 +2

73.95 72.52 56.92 58.04 82.49 85.68 78.78 69.26 79.73 76.90 66.28 74.04 85.49 90.84 78.32 81.63 75.77 72.16 76.43 72.42 71.89 76.13 73.66 72.28 72.33 71.92 72.00 72.41 64.33 81.60 58.17 88.17 91.68 76.48 75.67 83.04

73.05 72.32 61.11 60.39 85.94 85.21 74.00 73.28 75.65 79.99 68.46 72.80 88.54 92.88 81.35 85.69 73.28 73.28 73.28 73.28 73.28 73.28 73.28 73.28 73.28 73.28 73.28 73.28 65.77 80.79 60.39 86.17 92.47 73.35 76.38 80.00

a shake-flask system. The activated sludge was collected from a local wastewater treatment system (Beaurade Rennes, France). The activated sludge was first washed thoroughly at least five times with distilled water and centrifuged at 2500 rpm for 5 min to remove any residual carbon and mineral sources. 2.6. Biological Treatment. The biological treatment was carried out in a shake-flask system (250 mL) at 25 °C containing 150 mL of MB dye solution electrolyzed under the optimal conditions obtained by means of the central composite design (Figure 6), 0.5 g L−1 of activated sludge and the following mineral supplementation to allow microorganisms’ growth: 0.5 mL KH2PO4, 43.8 mg L−1; 0.5 mL Na2HPO4, 33.4 mg L−1; 0.150 mL CaCl2, 27.5 g L−1; 0.150 mL MgSO4·7H2O, 22.5 g L−1; 0.150 mL(NH4NO3, 3 g L−1) and 0.5 mL FeSO4· 7H2O, 1.36 g L−1; CuSO4·2H2O, 0.24 g L−1; ZnSO4·5H2O, 0.25 g L−1; NiSO4·6H2O, 0.11 g L−1; MnSO4·H2O, 1.01 g L−1; H3BO3, 0.1 g L−1. The pH was then adjusted to 7.0 with 1 mol L−1 NaOH solution.

The correlation of the independent variables and the response were estimated by a second-order polynomial eq 2 using the least-squares method as follows: 4

y ̂ = b0 +

4

4

∑ bixi +

∑ biixi 2+ ∑

i=1

i=1

4

∑

i=1 j=i+1

bijxixj (2)

When the response data are obtained from the test work, a regression analysis is carried out to determine the coefficients of the response model (b1, b2, ..., bn) as well as their standard errors and their significance. In addition to the constant b0, the response model incorporates:24,30,32 · the predicted color removal efficiency of MB dye (ŷ) · linear terms corresponding to the variables (x1, x2, ..., xn) · squared terms corresponding to the variables (x12, x22, ..., xn2) · first-order interaction terms for each paired combination (x1x2, x1x3, ..., xn−1xn) 2.5. Adsorption Tests. The adsorption of intermediate compounds formed after electro-oxidation pretreatment was performed with 0.5 g L−1 of activated sludge at 25 °C and pH 7 with an agitation speed of 300 rpm and agitation time of 3 h in

3. RESULTS AND DISCUSSION 3.1. Electrode Surface Characterization after Electrochemical Anodization. Figure 1 presents the X-ray diffracto14745



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Table 3. Apparent Rate Constant (Kapp), Apparent MassTransfer Coefficients (Km), and R2 Values [MB]0 (mg L−1)

T (°C)

12

20 45 65 20 45 65 i (mA cm−2)

0.0145 0.181 0.0249 0.311 0.0315 0.394 0.0086 0.107 0.0133 0.166 0.0164 0.205 Kapp (min−1)

R2

1 10 25 1 10 25

0.0075 0.0116 0.0284 0.0051 0.0092 0.0130

0.978 0.991 0.940 0.935 0.971 0.976

134

12

134

Figure 1. X-ray diffractograms of the electrode surface before and after electrochemical anodization.

Kapp (min−1)

Km (cm min−1)

R2 0.99 0.98 0.97 0.99 0.98 0.99

Figure 3. Influence of the temperature on color removal efficiency of MB dye, where i = 25 mA cm−2, ω = 360 rpm, pH = 2, and initial dye concentrations are 12 mg L−1 (a) and 134 mg L−1 (b).

Figure 2. Influence of the current density on color removal efficiency of MB dye, where T = 45 °C, ω = 360 rpm, pH = 2, and the initial dye concentrations are 12 mg L−1 (a) and 134 mg L−1 (b).

an electrode. Indeed, the rate of diffusion of pollutants from the bulk liquid to the liquid boundary layer surrounding the electrode becomes higher owing to an enhancement of turbulence and a decrease of the thickness of the liquid boundary layer, leading to an increase of the mass transfer coefficient.33−35

grams of the lead electrode before and after electrochemical anodization; the figure shows the formation of PbO2 at the surface of the electrode. Surface analysis by profilometer type Veeco Dektak 150 indicated 0.12 mm and 6.63 μm of thickness and roughness, respectively. An increase of surface roughness leads to an increase in the rate of mass transfer at the surface of 14746



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Figure 4. (a) Influence of the initial dye concentration on color removal efficiency. (b) Plots of ln([MB]0/[MB]t) versus time for dye degradation at various initial dye concentrations, where i = 25 mA cm−2, T = 45 °C, pH = 2, and ω = 360 rpm.

Figure 5. Influence of the agitation speed on color removal efficiency of MB dye, where T = 45 °C, i = 25 mA cm−2, and pH = 2, 12 mg L−1 (a) and 134 mg L−1 (b) initial dye concentrations.

3.2. Parameters’ Effect. 3.2.1. Current Density Effect. According to Figure 2 and Table 3, increasing the current density from 1 to 25 mA cm−2 improved color removal efficiency of MB dye from 62 to 97% and from 48 to 79% for 12 and 134 mg L−1 initial concentrations, respectively (Figure 2a,b). This effect, which is in agreement with those reported by Awad and Galwa;1 Bahadir;2 Song et al.,36 can be explained by an increase of production of oxidizing OH• radicals originated from water oxidation on the electrode surface (PbO2) according to the following eq 3:1

mg L−1. It can be seen from Figure 3 that the color removal efficiency of MB dye was temperature- and concentrationdependent. The color removal efficiency increased with raising temperature, and the effect of this parameter was especially pronounced at higher initial dye concentration (134 mg L−1). In order to compare the course of the color removal efficiency of methylene blue at different temperatures, the apparent rate constants (Kapp) were calculated by plotting ln([MB]0/[MB]t) versus t. The obtained plots with high correlation coefficients R2 (Table 3) suggest that a first-order model can describe the kinetic behavior. Apparent rate constants for the degradation of MB increased with temperatures and decreased with increasing initial concentrations. The observed increase on Kapp and the apparent mass-transfer coefficient (Km) in eq 5 (Table 3) when the temperature increased can be explained in terms of the reduction of the viscosity. The same results are reported by Panizza and Cerisola39 and Awad and Galwa.1

PbO2 (h+) + H 2Oads → PbO2 [OH•] + H+

(3)

•

A physisorbed hydroxyl radical (OH ) react with the organic compound (MB) according to the following eq 4: PO2 [OH•]z + MB → CO2 + z H+ + z e− + PbO2

(4)

For the degradation of organic compounds, a high concentration of hydroxyl radicals is necessary at the anode. This case occurs when the rate of the reaction given by the eq 3 is greater than the transfer of oxygen in the oxide lattice. 3.2.2. Temperature Effect. The effect of temperature was obtained by studying the color removal efficiency of MB at different temperatures (i.e., 20, 45, and 65 °C) with an initial solution concentration of 12 (Figure 3a) and 134 (Figure 3b)

Km =

K appV S

(5)

where V is the volume of the solution (mL) and S is the anode surface (cm2). 14747



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Figure 6. Model validation. (a) Comparison of experimental and predicted responses. (b) Residual analysis for the estimated model. (c) Contour plots showing the effect of current density and temperature on the yield of color removal efficiency of MB dye.

3.2.3. Initial Dye Concentration Effect. In order to determine the effect of the initial dye concentration, the electrochemical oxidation of MB on Pb/PbO2 at i = 25 mA cm−2, T = 45 °C, pH = 2, and ω = 360 rpm was carried out by varying this parameter from 12 to 200 mg L−1. As the concentration of the dye was increased, the color removal efficiency decreased. Figure 4a depicts the time-dependent graphs of color removal efficiency (%) at different concentrations of dye solutions. For dye solution of 12 mg L−1, almost 97% degradation occurred within 100 min; in the case of 200 mg L−1, only 60% and 74% degradations of dye were observed within 100 and 180 min, respectively. The relationship between ln([MB]0/[MB]t) and time t for the four investigated dye concentrations was plotted. As shown in Figure 4b, a good linear relationship between ln([MB]0/[MB]t) and time t is obtained, indicating that the electrochemical oxidation on Pb/ PbO2 electrode of MB dye followed pseudo-first-order kinetics.

The apparent rate constant (Kapp) decreased when the initial dye concentration was increased. It was observed that the increase in MB concentration induced a decrease of the apparent rate constant (Kapp) and the apparent mass transfer coefficient (Km) (Figure 4b and Table 3), which could be attributed to the competitive consumption of oxidizing OH• radicals between MB dye and the generated intermediates formed, in agreement with other findings.25,26,36−38,40 A second source of the decrease of the apparent mass transfer coefficient can be a decrease of the oxygen formation rate when the initial concentration of MB increased, as reported by Belhadj et al.25 3.2.4. Agitation Speed Effect. In order to determine the effect of the agitation speed on the color removal efficiency of MB dye, experiments have been carried out at 240, 360, and 600 rpm for 12 mg L−1 (Figure 5a) and 134 mg L−1 (Figure 5b) initial dye concentrations at 45 °C, i = 25 mA/cm2, and pH = 2. 14748



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test.24,29,30,32 The second-order model obtained according to central composite design (CCD) for color removal efficiency of methylene blue after performing 36 experiments and discarding the insignificant effects was as follows: y ̂ = 73.28 + 3.75x1 + 6.45x 2 − 4.78x3 + 0.90x4 + 1.19x1x3 + 1.26x1x4 + 2.41x32 + 1.23x4 2

(6)

A good adjustment of eq 6 to the experimental data was checked through the high correlation coefficient value obtained R2 = 0.92 (Figure 6a). Furthermore, the random distribution of the residuals (Figure 6b) shows the absence of a trend. These results clearly indicate that the second-order model accurately describes experimental data. The quadratic model (eq 6) obtained by central composite design (CCD) was used to find the optimal values of the operating parameters giving the highest color removal efficiency. The contour plots (Figure 6c) were drawn using STATISTICA software, which clearly indicates that the optimal conditions found for the highest color removal efficiency were 41 ≤ T ≤ 60 °C, 10.66 ≤ i ≤ 25 mA cm−2, [MB]0 = 134 mg L−1, and ω = 720 rpm. These conditions led to 95.6% dye removal efficiency and 81% reduction of the chemical oxygen demand (COD). 3.4. Biodegradation. Biological methods that are destructive do not always appear relevant for the removal of recalcitrant compounds owing to their low biodegradability. For the degradation of persistent organic compounds, the coupling of physicochemical processes such as electrochemical process and biological treatment is recommended. The electrochemical processes modify the structure of the pollutants leading to byproducts which are expected to be more biodegradable and less toxic, allowing a subsequent biological treatment. 3.4.1. Biodegradability Tests. Biodegradability tests were realized on the solution electrolyzed in the above optimal conditions (Figure 6c). The (BOD5/COD) ratio increased from 0.034 initially to 0.54 after electrolysis, namely above the limit of biodegradability, 0.4, showing the feasibility of a combined process, namely a biological treatment after the electrochemical pretreatment. 3.4.2. Biological Treatment. Biodegradation (made in duplicates) of pure methylene blue solution (134 mg L−1) were carried out with an activated sludge culture. The results show that [DOC]t/[DOC]0 values remained constant throughout the culture time (30 day), indicating the refractory character of methylene blue (data not shown). Sixty-seven percent of the dissolved organic carbon (DOC) remained after the electrochemical pretreatment. To try to complete the mineralization of the solution, a biological treatment was carried out in a shake-flask system with activated sludge in an aerobic system for 30 days. According to Figure 7a, the biodegradability of the electrolyzed solution by activated sludge was confirmed. Indeed, the significant decrease of the DOC within the first 2 days was only partially related to biosorption on the activated sludge of intermediate compounds formed (Figure 7b) because biosorption accounted for only approximately 9% of the DOC decrease and, hence, about 37% DOC decrease corresponded to the biodegradation of the intermediate compounds. From 2 to 14 days, a low degradation rate was observed, leading to 13.03% DOC removal. From 14 to 30 days, the [DOC]t/[DOC]0 remained constant (Figure 7a); this behavior can be attributed to acclimation of the activated sludge to the intermediates compound, which are only

Figure 7. (a) Time courses of [DOC]t/[DOC]0 values during the activated sludge culture on electrolyzed MB dye solution (3 h). (b) Biosorption on activated sludge of intermediate compounds formed after the electrochemical oxidation of MB dye solution (3 h).

It can be seen that the maximum degradation efficiency of MB dye is obtained at 360 rpm for 12 mg L−1 of initial dye concentration and the decolorization efficiency increased when the agitation speed was increased from 240 to 360 rpm. This effect may be attributed to the existence of a resistance to the mass transfer; the diffusion rate of MB dye from the bulk liquid to the liquid boundary layer surrounding electrode became higher owing to an enhancement of the turbulence and a decrease of the thickness of the liquid boundary layer, which is caused by an increase of the mass transfer coefficient.24 At 600 rpm, a decrease of the degradation efficiency was also observed. The possible reason for this behavior may be attributed to the short contact time between the MB dye molecules and the surface of Pb/PbO2 electrode. In case of 134 mg L−1 initial dye concentration (Figure 5b), the degradation efficiencies were similar (≅90%) after 3h of electrochemical treatment for the various agitation speeds investigated. 3.3. Modeling and Optimization by Central Composite Design (CCD). The model coefficients were estimated by standard least-squares regression method using Microsoft Excel software. Three tests were required to evaluate the adequacy of the model: the student’s t test, the R2 test, and the Fisher 14749



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(7) Fu, Y.; Viraraghavan, T. Fungal decolorization of dye wastewaters: a review. Bioresour. Technol. 2001, 79, 251. (8) McMullan, G.; Meehan, C.; Conneely, A.; Kirby, N.; Robinson, T.; Nigam, P.; Banat, I. M.; Marchant, R.; Emyth, W. F. Microbial decolorization and degradation of the textile dyes. Appl. Microbiol. Biotechnol. 2001, 56, 81. (9) Robinson, T.; McMullan, G.; Marchant, R.; Nigam, P. Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative. Bioresour. Technol. 2001, 77, 247. (10) Stolz, A. Basic and applied aspects in the microbial degradation of azo dyes. Appl. Microbiol. Biotechnol. 2001, 56, 69. (11) Dafale, N.; Nageswara Rao, N.; Meshram, S. U.; Wate, S. R. Decolorization of azo dyes and simulated dye bath wastewater using acclimatized microbial consortium-Biostimulation and halo tolerance. Bioresour. Technol. 2008, 99, 2552. (12) Junghanns, C.; Krauss, G.; Schlosser, D. Potential of aquatic fungi derived from diverse freshwater environments to decolourise synthetic azo and anthraquinone dyes. Bioresour. Technol. 2008, 99, 1225. (13) Rodríguez Couto, S. Dye removal by immobilised fungi: a review. Biotechnol. Adv. 2009, 27, 227. (14) Scott, J. P.; Ollis, D. F. Integration of chemical and biological processes for water treatment: Review and recommendations. Environ. Prog. Sustainable Energy 1995, 14, 88. (15) Pulgarin, C.; Invernizzi, M.; Parra, S.; Sarria, V.; Polania, R.; Peringer, P. Strategy for the coupling of photochemical and biological flow reactors useful in mineralization of biorecalcitrant industrial pollutants. Catalysis Today 1999, 54, 341. (16) Farré, M. J.; Brosillon, S.; Domènech, X.; Peral, J. Evaluation of the intermediates generated during the degradation of Diuron and Linuron herbicides by the photo-Fenton reaction. J. Photochem. Photobiol., A: Chemistry 2007, 189, 364. (17) Basha, C. A.; Chithra, E.; Sripriyalakshmi, N. K. Electrodegradation and biological oxidation of non-biodegradable organic contaminants. Chem. Eng. J. 2009, 149, 25. (18) Salles, N. A.; Fourcade, F.; Geneste, F.; Floner, D.; Amrane, A. Relevance of an electrochemical process prior to a biological treatment for the removal of an organophosphorous pesticide, phosmet. J. Hazard. Mater. 2010, 181, 617. (19) Belkheiri, D.; Fourcade, F.; Geneste, F.; Floner, D.; Ait-Amar, H.; Amrane, A. Feasibility of an electrochemical pre-treatment prior to a biological treatment for tetracycline removal. Sep. Purif. Technol. 2011, 83, 151. (20) Li, X. Y.; Cui, Y. J.; Feng, Y. J.; Xie, Z. M.; Gu, J. D. Reaction pathways and mechanisms of the electrochemical degradation of phenol on different electrodes. Water Res. 2005, 39, 1972. (21) Kötz, R.; Stuki, S.; Carcer, B. Electrochemical waste-water treatment using high overvoltage anode I: physical and electrochemical properties of SnO2. J. Appl. Electrochem. 1991, 21, 14. (22) Schumann, U.; Grundler, P. Electrochemical degradation of organic substrates at PbO2 anode: monitoring by continuous CO2 measurements. Water Res. 1998, 32, 2835. (23) Iniesta, J.; Gonzalez-Garcia, J.; Exposito, E.; Montiel, V.; Aldaz, A. Influence of chloride ion on electrochemical degradation of phenol in alkaline medium using bismuth doped and pure PbO2 anodes. Water Res. 2001, 35, 3291. (24) Yahiaoui, I.; Aissani-Benissad, F.; Fourcade, F.; Amrane, A. Response surface methodology for the optimization of the electrochemical degradation of phenol on Pb/PbO2 electrode. Environ. Prog. Sustainable Energy 2012, 31, 515. (25) Belhadj Tahar, N.; Savall, A. Mechanistic aspects of phenol electrochemical degradation by oxidation on a Ta/PbO2 anode. J. Electrochem. Soc. 1998, 145, 3427. (26) Belhadj Tahar, N.; Abdelhédi, R.; Savall, A. Electrochemical polymerization of phenol in aqueous solution on a Ta/ PbO2 anode. J. Appl. Electrochem. 2009, 39, 663.

partially biodegradable. The biological treatment led to a removal of 59.03% of the dissolved organic carbon (DOC), and the overall DOC removal by means of the combined process was close to 92.03%, showing its efficiency for the treatment of methylene blue.

4. CONCLUSION In the first part of this study, the effect of the operating parameters on the color removal efficiency of methylene blue by an electrochemical pretreatment on Pb/PbO2 electrode was examined. The degradation of methylene blue follows pseudo-firstorder kinetics, and the rate constant increases with the applied current density and the temperature of the solution. The effect of the temperature was more pronounced at high initial dye concentration. As the concentration of the dye was increased, the color removal efficiency decreased. The model obtained by using central composite design (CCD) led to 95.6% electrochemical degradation of MB in the following optimal conditions: 41 ≤ T ≤ 60 C°, 10.66 ≤ i ≤ 25 mA cm−2, [MB]0 = 134 mg L−1, and ω = 720 rpm. The combination of an electrochemical pretreatment on Pb/ PbO2 electrode with biological treatment for the degradation of methylene blue dye was then performed, showing an overall dissolved organic carbon (DOC) removal close to 92.03%, confirming the relevance of the combined process, which therefore appears promising for the treatment of real wastewater. The corresponding validation is in progress in the laboratory

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AUTHOR INFORMATION

Corresponding Author

*I. Yahiaoui. E-mail: [email protected]. Phone: 00 213 34 21 51 05. Fax: 00 213 34 21 51 05. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received partial financial support from the TASSILI program between Algeria and France (CMEP Project No. 11 MDU 843-2011-2014), and the authors express their gratitude to the AUF (Grant fellowship for I.Y.).

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REFERENCES

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Electrochemical Pre-Treatment Combined with Biological Treatment

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