Environ Sci Pollut Res DOI 10.1007/s11356-014-2558-8

RESEARCH ARTICLE

Electrochemical treatment of domestic wastewater using boron-doped diamond and nanostructured amorphous carbon electrodes Rimeh Daghrir & Patrick Drogui & Joel Tshibangu & Nazar Delegan & My Ali El Khakani

Received: 1 October 2013 / Accepted: 13 January 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The performance of the electrochemical oxidation process for efficient treatment of domestic wastewater loaded with organic matter was studied. The process was firstly evaluated in terms of its capability of producing an oxidant agent (H2O2) using amorphous carbon (or carbon felt) as cathode, whereas Ti/BDD electrode was used as anode. Relatively high concentrations of H2O2 (0.064 mM) was produced after 90 min of electrolysis time, at 4.0 A of current intensity and using amorphous carbon at the cathode. Factorial design and central composite design methodologies were successively used to define the optimal operating conditions to reach maximum removal of chemical oxygen demand (COD) and color. Current intensity and electrolysis time were found to influence the removal of COD and color. The contribution of current intensity on the removal of COD and color Responsible editor: Angeles Blanco R. Daghrir : P. Drogui (*) : J. Tshibangu Institut National de la Recherche Scientifique, Centre Eau, Terre et Environnement, 490 rue de la Couronne, Québec, QC, Canada G1K 9A9 e-mail: [email protected] R. Daghrir e-mail: [email protected] J. Tshibangu e-mail: [email protected] N. Delegan : M. A. El Khakani Institut National de la Recherche Scientifique, INRS-Énergie, Matériaux et Télécommunications, 1650 Blvd. Lionel-Boulet, Varennes, QC, Canada J3X 1S2 N. Delegan e-mail: [email protected] M. A. El Khakani e-mail: [email protected]

was around 59.1 and 58.8 %, respectively, whereas the contribution of treatment time on the removal of COD and color was around 23.2 and 22.9 %, respectively. The electrochemical treatment applied under 3.0 A of current intensity, during 120 min of electrolysis time and using Ti/BDD as anode, was found to be the optimal operating condition in terms of cost/effectiveness. Under these optimal conditions, the average removal rates of COD and color were 78.9±2 and 85.5± 2 %, whereas 70 % of total organic carbon removal was achieved.

Keywords Electrochemical oxidation process . Domestic wastewater . Boron-doped diamond . Nanostructured amorphous carbon

Abbreviations a-C Amorphous carbon ANOVA Analysis of variance BDD Bored doped diamond CCD Central composite design CF Carbon felt COD Chemical oxygen demand DSA Dimensionally stable anodes DWW Domestic wastewater EO Electrochemical oxidation FD Factorial design Gr Graphite RSM Response surface methodology SEM Scanning electron microscope TOC Total organic carbon VC Vitreous carbon WTP Wastewater treatment plant

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Introduction Domestic wastewater (DWW) generated from individual homes, clusters of home, or isolated communities and industries contain high amounts of organic, inorganic, and microbial pollutants (Vlyssides et al. 2002). Their exposure and accumulation in the aquatic environment lead to adverse effect toward human life and nature and cause eutrophication of surface waters and transmission of waterborne diseases (Konnerup et al. 2009; Denny 1997). All over the world, DWW is conventionally treated by septic tank used as primary treatment. However, this process is not efficient for domestic wastewater treatment as only 40 % of biological oxygen demand is removed. The use of these waters for certain purposes might be excluded or at least hampered because most of the dissolved organic pollutants and pathogen bacteria remain in the effluent (Meuler et al. 2008; Schmalz et al. 2009). The stricter new legislation has caused extensive effort to focus on the development of alternative methods applied for achieving the complete removal of organic pollutants from decentralized wastewater. Recently, there has been an increasing interest in the use of electrochemical oxidation (EO) process for the treatment of DWW (Schmalz et al. 2009; Vlyssides et al. 2002; Daghrir et al. 2013a). EO has been found to be a promising environmental remediation technology because of its simplicity without requirement for special equipment, high efficiency in organic pollutants removal and environmentally benign (Zhang et al. 2008; MartinezHuitle and Ferro 2006; Wang et al. 2005). The interest of using EO is based on its capability of reacting on pollutants by using both direct and indirect effect of electrical current (Zaviska et al. 2011; Martinez-Huitle and Ferro 2006; Brillas et al. 2004). Direct oxidation may be achieved through mineralization with hydroxyl radical (OH°) produced at the electrode surface by dimensionally stable anodes having high oxygen overvoltage, such as iridium dioxide (IrO2), PbO2, and bored doped diamond (BDD) among others (Comninellis and Pulgarin 1991; Comninellis 1994; Panizza et al. 2000). In fact, OH° radicals are exclusively generated on the anode electrodes from the oxidation of water (Eq. (1)), and organic compounds can be completely transformed or degraded by reaction with adsorbed OH° radicals (Eq. (2)) (Oturan et al. 2013; Hamza et al. 2009; Dirany et al. 2010). Anodic oxidation using BDD has attracted a great attention due to its inert surface with low adsorption properties, its remarkable corrosion stability even in acidic media, and its high oxygen evolution over-potential (Panakoulias et al. 2010; Brillas et al. 2004). The majority of the studies (Brillas et al. 2004; Oturan et al. 2013; Sirés et al. 2007) have shown that

BDD provides more rapid destruction of pollutants and total mineralization of the organic loading with high current efficiencies. M þ H2 O→Mð OHÞ þ Hþ þ e−

ð1Þ

Mð OHÞ þ organics→M þ oxidation products

ð2Þ

Besides, indirect oxidation can be achieved through electrochemical generation of a mediator in solution such as H2O2 to convert toxic organics to a less harmful product (Drogui et al. 2001; Rajeshwar and Ibanez 1995). Hydrogen peroxide (H2O2) (1.77 V) is an environmentally metastable molecule with high disinfecting and oxidizing properties (Drogui et al. 2001; Khataee et al. 2011). H2O2 can be electrochemically produced by a two-electron reduction of oxygen (Eq. (3)) at appropriate cathodic potential on certain electrodes such as reticulated vitreous carbon, graphite, and carbon felt (CF) among others (Zhang et al. 2008; Guinea et al. 2008; Isarain-Chàvez et al. 2011). In undivided electrolytic cell, H2O2 could be oxidized to O2 at the anode leading to the formation of hydroperoxyl radical (HO2°) as intermediate (Eq. (4)), a much weaker oxidant than OH° (Brillas et al. 2000). O2 þ 2Hþ þ 2e− →H2 O2

ð3Þ

H2 O2 →HO2  þ Hþ þ e−

ð4Þ

In this context, the EO process has been explored at the laboratory pilot scale, to remove the chemical oxygen demand (COD) and color from DWW. The COD includes soluble and insoluble organic pollutants. The DWW was an effluent provided from a tank (septic tank) and collected after biofiltration treatment of wastewater from isolated residences. Therefore, the main purposes of this study are to evaluate the feasibility of the EO process in treating DWW and to determine the optimal operational conditions to efficiently remove organic pollutants. To do this, a statistical methodology was used for a rational analysis of the combination of operational factors that led to the best treatment process. Factorial design (FD) and central composite design (CCD) methodologies have been successively applied in order to point out the main and interaction effects of the factors and to optimize COD removal by the EO process.

Materials and methods Sampling and characterization of DWW The DWW used throughout this study was an effluent provided from the Riviere du Loup community domestic wastewater treatment plant (WTP; Riviere du Loup, QC, Canada). It is a

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conventional WTP having a primary treatment (septic tank) of wastewater providing from isolated residences followed by a bio-filtration process. The effluents were sampled at the outlet of the bio-filtration unit. Samples were collected and stored in polypropylene bottles and kept at 4 °C until use. The treated effluent had an initial content of COD of 79.17±4 mg/L, total organic carbon (TOC) of 19.9±0.8 mg/L, color of 100.2± 4 mg/L, turbidity of 11.4±0.3 NTU, pH of 7±0.03, and conductivity of 941.7±10 μs/cm. Preparation of amorphous carbon coatings The magnetron-sputtering technique was used to coat a 12cm-diameter deployed titanium grid substrates with nanostructured amorphous carbon (a-C) films. The a-C films were grown by means of 13.56 MHz radio frequency (RF) magnetron source sputtering up from a 3-in.-diameter graphite target (99.995 % purity) at a power density of 7.7 W/cm2. The target-substrate distance was set at 20 cm, and the substrate holder was heated to provide a substrate deposition temperature of 450 °C. The a-C film deposition was simultaneously achieved onto both quartz and deployed Ti grids (for chemical reactor applications). Prior to deposition, the sputterdeposition chamber was cryopumped to a background pressure of ∼2×10−8 Torr. Then, high purity (99.999 %) argon sputtering gas was introduced in the chamber. The gas flow rates were monitored to keep a constant total pressure of 1.4 mTorr in the chamber during the deposition process. Before the deposition, the target was cleaned by presputtering the target, for 10 min with the substrates isolated from the plasma by shutters. A post-acceleration bias of −90 V was applied to the substrate through the application of an RF power to the substrates. The typical thickness of the a-C films was of about 650 nm.

Experimental setup The tests were carried out in a closed loop as depicted schematically in Fig. 1. A 2.0 L of PVC tank (1), a peristaltic recycling pump (2), and the electrolytic cell (3) constitute the loop. The electrolysis cell had a working volume of 1 L. The experiment unit included a stirred 0.5-L tank in a closed loop with the electrolysis cell in which a recycling pump (120 mL/ min) induced perfect mixing of the liquid phase. For all experiments, a total working volume of 1.5 L was used. The recycle flow rate of 120 mL/min was maintained using a peristaltic recycling pump (Master flex, Model 77200-50, USA). The electrochemical cell was operated under galvanostatic conditions, with a current intensity (from 0.59 to 3.41 A) imposed during treatment period ranging from 47.57 to 132.43 min. Current intensity was imposed by means of DC power supply Xantrex XFR 40-70 (Aca Tmetrix, Mississauga, ON, Canada) with a maximum current rating of 70 A at an open circuit potential of 40 V. All experiments were carried out at room temperature (25±0.1 °C). Owing to the high conductivity, the treatment of DWW by electrochemical process was performed without adding of Na2SO4. The effectiveness of the electrochemical process was evaluated by simultaneously measuring the residual concentration of COD, TOC, and color. Energy consumption and pH were also determined. Experimental procedure The first set of experiments was carried out to test successively different operating parameters, such as cathode materials (carbon felt or a-C) and current intensity (0.5 to 4 A) in order to evaluate the efficacy of the electrolytic cell to produce H2O2 as oxidant agent. The capacity for H2O2 production in the

+

Electrolytic cell The electrolytic cell used was 1 L of capacity and was made of polyvinyl chloride (PVC) material with a dimension of 17 cm (depth)×5 cm (width)×15 cm (length). The electrochemical cell was comprised of one anode and one cathode electrodes in the form of expanded metal, each having a solid surface area of 65 cm2 and a void surface area of 45 cm2. The circular anode electrode (12 cm of diameter×0.1 thick) was either made of titanium coated with boron doped diamond (Ti/ BDD) or titanium coated with iridium–ruthenium dioxide (Ti/IrO2–RuO2). The circular cathode electrode was either made of CF (11 cm of diameter×0.6 cm thick) or titanium coated with a-C (12 cm of diameter×0.1 thick). The interelectrode gap was 1.0 cm in the electrolytic cell. The electrodes were vertically installed on a perforated Plexiglas plate at 2 cm from the bottom of the cell.

-

(3) (6)

(1) (7)

(4)

(5)

(2)

Fig. 1 Schematic diagram of the experimental setup: (1) PVC tank, (2) peristaltic recycling pump, (3) electrolytic reactor, (4) anode, (5) cathode, (6) inlet, (7) outlet

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electrolytic cell was evaluated during 90 min of electrolysis time and under the typical synthetic conditions: distilled water+0.5 g/L of Na2SO4. The addition of a supporting electrolyte such as Na2SO4 or NaCl in the solution can facilitate the passage of the electrical current and improve the conductivity of the effluent. The major disadvantage of using NaCl as supporting electrolyte is the possible formation of organochlorinated compounds during the electrolysis, which by-products can be more toxic than the initial pollutant (Aquino Neto and de Andrade 2009). To this end, Na2SO4 was selected as a supporting electrolyte to improve the conductivity of the solution. Subsequently, response surface methodology (RSM) was selected to evaluate and determine the optimum operating conditions for electrochemical treatment of DWW. RSM is a collection of mathematical and statistical methods for modeling, optimizing, and analyzing a process in which the response can be influenced by several variables (Zodi et al. 2010; Zaroual et al. 2009). RSM provides a simple and efficient modeling of treatment process (Bhatti et al. 2009). FD and CCD are widely used in RSM. In this study, FD was used to determine the interaction effects of the factors on the response, whereas CCD was employed to optimize EO process in treatment of DWW. A three-factorial and two-level central composite design, with five replicate at the center point for each categorical factor, led to a total number of 26 experiments employed for response surface modeling. The variables investigated in our study were current intensity (X1), electrolysis time (X2), and type of anode (Ti/ BDD or Ti/IrO2–RuO2) (X3). The removal efficiency of COD and color was considered as responses (Y). The values of different variables were selected based on the preliminary assays carried out to determine the concentration of H2O2 produced during electrochemical oxidation process. The experiment region investigated for DWW treatment and the code values are shown in Table 1. Experimental data were analyzed using Design Expert 7.1 program software (Design Expert 7, 2007, Stat-Ease Inc., Minneapolis, MN, USA). Analysis of variance (ANOVA) was used to analyze graphical data and to obtain the interactions between the variables and the response.

Analytical technique The pH was determined using a pH meter (Fisher Acumet model 915) equipped with a double junction Cole–Palmer electrode with Ag/AgCl reference cell. A conductivity meter (Oakton Model 510) was used to determine the ionic conductivity of the effluent. Turbidity (in nephelometric turbidity units) was measured using a turbidimeter Hach 2100 AN. COD determination was measured based on APHA (1999) and a reading spectrophotometer UV 0811 M136 (Varian, Australia). TOC measurements were performed using a Shimadzu TOC 5000A analyzer (Shimadzu Scientific Instruments Inc) equipped with an autosampler. The initial and the residual concentrations of colors were established by spectrophotometric method. A calibration curve of known color concentration versus absorbance (400 nm) was used to calculate the residual concentrations of color. The H2O2 concentration was measured using volumetric dosage (Sigler and Masters 1957). The cerium ion solution (Ce (SO4) 2 , 2(NH4)2 SO4, 2 H2O) (5.88×10−3 M) was used under acidic conditions (H2SO4, 9 N) in the presence of three drops of Fe(o-phen)32+ as an indicator. To quantitatively determine the concentration of H2O2, the calibration curve was obtained by plotting the cerium volume (in milliliters) as a function of H2O2 concentration (from 0 to 2.94×10−3 mol/L). The gradual change of the solution color from red to blue indicates a total oxidation of H2O2 using the cerium solution. Economic aspect Evaluation of operation costs related to the electrochemical oxidation treatment process included energy consumption. The energy consumed was estimated at a cost of US $0.06/ kWh, which correspond to the cost in the province of Quebec (Canada). The total cost was evaluated in terms of US dollars spent per cubic meter of treated solution.

Results and discussions Characterization of the amorphous carbon coatings

Table 1 Data for optimization operation: experimental range and levels of independent process variables Factor (Ui) Coded variables (Xi)

Experimental field

Ui,0 ΔUi

Min Max value (−1) value (+1) X1 X2 X3

U1: current 1 intensity (A) U2: electrolysis 60 time (min) U3: type of Ti/BDD anode

3

2

1

120

90

30

Ti/IrO2–RuO2 –



Figure 2 shows a typical cross-section scanning electron microscope (SEM) image of the sputter-deposited a-C films. The a-C films are seen to exhibit a relatively dense columnar-like nanostructure with more or less irregular nodular features and an apparent nanoporosity. The bonding states of the nanostructured a-C films were also characterized by Raman spectroscopy. Figure 3 shows a typical Raman spectrum of the sputter-deposited a-C films, where the characteristic D and G bands are clearly distinguishable. The D and G peaks (located around 1,360 and 1,590 cm−1) are found to be quite intense ad particularly sharp (with a FWHM of only about 284 and

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Fig. 2 Typical cross-section image of the magnetron sputter-deposited aC films grown at a deposition temperature of 450 °C

120 cm−1,respectively), suggesting a more diamond-like character of the films (higher Sp3/Sp2 hybridization ratio and a certain degree of structural ordering). Finally, the broad band centered around 2,900 cm−1 is attributed to the second order D and G overlapping Raman peaks (Tabbal et al. 2005). The behavior of a-C cathode on the electrochemical generation of H2O2 will be investigated hereafter in the following section.

activated carbon fiber and CF can be used for electrochemical production of H2O2. Two types of cathode materials CF and a-C were tested to evaluate their capacity for H2O2 production using distilled water in which Na2SO4 (3.52×10−3 M) was added. Experiments using Ti/BDD at the anode were carried out over 90 min of treatment time and at different current intensities (from 0.5 to 4.0 A). Hydrogen peroxide production concentrations generated under the different experimental conditions are shown in Fig. 4. The rate of H2O2 production was proportional to the current intensity imposed. The maximum concentration of H2O2 (0.064 mM) was recorded at 4.0 A of current intensity and using a-C. Under the same conditions, only 0.031 mM of H2O2 was generated using CF electrode. The H2O2 concentration electrogenerated after 90 min of treatment time using nanostructured a-C was nearly two times higher than that recorded using CF at the cathode. As shown in SEM view (Fig. 2), the nanostructured a-C electrode has large specific area and a great number of apparent nanoporosity allowing fast diffusion of O2 molecule and consequently high production of H2O2. According to these results, it can be concluded that the nanostructured a-C is a favorable material for the electrochemical generation of H2O2. Hence, a-C cathode electrode was selected for the next step of this study dealing with FD methodology.

Electrochemically hydrogen peroxide production The primary objective of the preliminary tests was to evaluate the efficacy of the electrolytic cell to produce hydrogen peroxide (H2O2). Such an oxidant can be electrochemically generated at the cathode (Khataee et al. 2011; Isarain-Chàvez et al. 2011; Zhang et al. 2008). According to previous studies (Oturan and Oturan 2005; Daghrir et al. 2013a, b), several cathode materials including graphite, vitreous carbon,

Intensity (A.U.)

800

D

G

600

400

200

500

1000

1500

2000

Raman Shift

2500

3000

(cm-1)

Fig. 3 Typical Raman spectrum of the magnetron sputter-deposited a-C films grown at a deposition temperature of 450 °C, where the characteristic D and G bands are identified

Effect of the experiment parameters on the DWW treatment using the experimental factorial design methodology The results presented above allowed us to clearly define the experimental region for RSM to study the treatment of DWW using electrochemical oxidation process. The influence of different variables: current intensity (U1), electrolysis time (U2), and type of anode (U3) on COD and color removal was investigated using factorial matrix (2k, k being the number of factors; k=3). In this type of design, variables (k) are studied at two different levels normalized as (−1) and (+1). The experimental design and result are shown in Table 2. From this table, it can be seen that the best removal efficiency of COD and color was recorded using Ti/BDD electrode. Using Ti/ BDD electrode, COD concentration in solution could be optimally diminished up to 79.3 % by imposing a current intensity of 3.0 A and after 120 min of treatment time. Under the same conditions, 93.0 % of color has been also removed. It is well-known that the decolorization processes are mainly based on redox reactions on the electrodes surface. These processes can be enhanced by indirect redox reactions by producing a powerful oxidant such as HClO, H2O2, O3, and among others (Rajkumar and Muthukumar 2012). This high removal rate of color from DWW was attributed to the hydroxyl radical (OH°) formation at the surface of Ti/BDD anode and to H2O2 electrochemically generated at the a-C cathode electrode. By using such a design methodology, it is

Environ Sci Pollut Res Fig. 4 Hydrogen peroxide production using carbon felt or aC at the cathode during 90 min of treatment time

For COD removal

possible to determine the principal effects of each factor and their interaction 2 to 2 and 3 to 3 (Myers and Montgomery 2002). The experimental response associated with a 23 factorial design (three variables) is represented by a linear polynomial model with interaction (Eq. (5)).

Y 1 ¼ 46:25 þ 14:57X 1 þ 9:13X 2 −5:75X 3 −5:48X 1 X 3 þ 1:07X 2 X 3

ð6Þ For color removal Y 1 ¼ 61:84 þ 11:92 X 1 þ 7:44 X 2 −5:59 X 3 −3:55X 1 X 3 −0:53X 2 X 3

Y ¼ b0 þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b12 X 1 X 2 þ b13 X 1 X 3 þ b23 X 2 X 3 ð5Þ

where Y represents the experimental response (for COD and color removal), b0 represents the average value of the responses of the eight assays, Xi the coded variable (−1 or +1), bi represents the principal effect of each factor i on the response, and bij represents the interaction effect between factor i and factor j on the response. The coefficients of the model were calculated using the half-difference between the arithmetic average of the response values when the associated coded variable is at a level (+1) and the arithmetic average of the response values when the associated coded variable is at level (−1). Design-Expert ® Program Software (Design Expert 7, Stat-Ease Inc., Minneapolis, MN, USA) was used to calculate the experimental data:

ð7Þ The value of the regression coefficients, R2, for removal of COD and color was 0.997 and 0.999, respectively. The coefficient b0 (b0 =46.25 for COD removal and b0 =61.84 for color removal) represents the average value of the response of eight assays. From Eqs. (6) and (7), it can be seen that the current intensity influenced the removal efficiency of COD (b1 = 14.57) and color (b1 =11.92). The percentage of COD removal increases on average 29.14 % (2×14.57) when the current intensity goes from 1.0 to 3.0 A. The acceleration in the removal of COD with increasing the applied current intensity could be explained by the higher production of OH° at the surface of the anode, as well as the greater electrogeneration of H2O2 at the O2-diffusion cathode (a-C) (Guinea et al. 2008;

Table 2 Experimental factorial matrix in the 23 design and experimental results Test

Experimental design

Experimental plan

X1

X2

X3

1

−1

−1

−1

1

60

Ti/BDD

23.1

46.1

2 3 4 5 6 7 8

−1 +1 +1 −1 −1 +1 +1

+1 −1 +1 −1 +1 −1 +1

−1 −1 −1 +1 +1 +1 +1

1 3 3 1 1 3 3

120 60 120 60 120 60 120

Ti/BDD Ti/BDD Ti/BDD Ti/IrO2–RuO2 Ti/IrO2–RuO2 Ti/IrO2–RuO2 Ti/IrO2–RuO2

38.8 62.8 79.3 21.5 37.1 47.3 60.1

57.8 72.8 93.0 38.0 57.8 60.7 68.6

U1 (A)

Experimental responses U2 (min)

U3

COD removal (%)

Color removal (%)

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Isarain-Chàvez et al. 2011). The second most important factor on the removal of COD (b2 =9.13) and color (b2 =7.44) is the electrolysis time with a positive effect. The increase of electrolysis duration contributes to increase the removal rate of the COD and color. However, the effects of the electrode anode on the removal of COD (−5.75) and color (−5.59) are relatively weak, the electrode effect being negative. The removal efficiency of COD and color decreases when the electrode Ti/ BDD is replaced by Ti/IrO2–RuO2. As reported in previous research (Oturan et al. 2013; Murugananthan et al. 2010), the Ti/BDD anode has a higher oxidation power and a great ability to produce active radicals BDD(OH°) from the EO of water than the Ti/IrO2–RuO2. The interaction effects were globally weaker than the main effects. With the exception of the interaction X1X3 (current intensity and type of anode), the other interactions have a negligible effect. The importance of the factors and interactions on the removal of COD and color from DWW has been put into evidence (Eq. (8)). Indeed, it is possible to give more significant information by calculating the contribution of each factor on the response. Pi ¼

! b2i X 100 b2i

ði ≠ 0 Þ

ð8Þ

where bi represents the estimation of the principal effect of the factor i. Thus, it is found that the contribution of current intensity and electrolysis time on the removal of COD is around 59.1 and 23.2 %, respectively, whereas that of type of anode accounts only for 9.2 % (Fig. 5a). Moreover, the contribution of current intensity and electrolysis time on the removal of color is for 58.8 and 22.9 %, respectively, while the type of anode accounts only for 12.9 % (Fig. 5b). As discussed above, the production rate of reactive oxygen species (e.g.`, OH° and H2O2) was proportional to the current intensity. Generally, the higher the concentration of oxidant species in the bulk solution, the more effective the oxidation is for organic pollutant degradation. However, the current intensity influences the removal of COD, but it directly depends of the type of anode used (Fig. 6a). It can be seen that when the Ti/BDD is used at the anode, current intensity has a significant influence on the removal of COD. The degradation rate of COD passed from 30.9 to 71 % (a reduction gain of 40.1 U). Nevertheless, when the Ti/IrO2–RuO2 is used at the anode, the degradation rate of COD passed from 29.3 to 53.7 % (a reduction gain of 24.4 %). From Fig. 6b, it can be also seen that current intensity and the type of anode influence the removal efficiency of color from DWW. When the current intensity is fixed at the highest level (3.0 A), the type of anode (Ti/BDD or Ti/IrO2–RuO2) has a significant influence on the removal of color. The degradation rate

Fig. 5 Graphical Pareto analysis of the effect of current intensity, electrolysis time, and type of anode on the removal of a COD and b color from DWW

passed from 64.6 to 82.9 % (a reduction gain of 18.3 %). It is well-known that the type of anode material can greatly influence EO processes (Zaviska et al. 2009; Comninellis and Nerini 1995). This can be attributed to the different crystalline natures of electrodes that catalyzes the reaction of the electrochemical oxidation (Comninellis 1992; Feng et al. 2003). The influence of the anode material (Ti/BDD and Ti/IrO2–RuO2) was also put into evidence by Panakoulias et al. (2010) while studying the electrochemical degradation of reactive red 120 (RR120). The rate of the discoloration of RR120 was much higher in the case of BDD (100 % removal of TOC). The low oxidation power of Ti/IrO2–RuO2 anode results in partial oxidation of organic compounds (40 % removal of TOC). Finally, the factorial plan design was used to determine the interactions affecting the response and indicates if the lowest or the highest levels of the factors are favorable or not. The results show that the response is greatly influenced by the factors having a significant effect. However, the factorial plan design cannot be used to determine the optimal conditions for removal both COD and color. For this reason, a RSM should be used in a second step to determine the optimal conditions for DWW treatment using EO process.

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gion and the usefulness of interpolating the response. The CCD matrix allows the description of a region around an optimal response. It is comprised of factorial matrix (described here) and nine additional experiments carried out for each categorical factor (Ti/BDD and Ti/IrO2–RuO2 electrode, respectively). The nine additional assays consisted of five runs required at the center of the experimental region investigated, plus four other axial runs. For the axial runs matrix, α has been chosen in order to have iso-variance property by using rotation, with α=(Nf)1/4 = 1.41, Nf being the number of points required for the factorial matrix. For the evaluation of data, the experimental response associated to CCD matrix was described by a second-order model in the form of quadratic polynomial equation presented below (Daghrir et al. 2013b; Mohajeri et al. 2010): Y ¼ b0 þ

Xk

bi X i þ

i ¼ 1

Fig. 6 Interaction X13 between current intensity and type of anode on the removal of a COD and b color from DWW

In this section, the optimal values of the factors: current intensity (X1), electrolysis time (X2) and type of anode (X3) for the removal of COD and color were studied using CCD (Table 3). One of the advantages of CCD is the possibility to explore the whole of the experimental re-

i ¼ 1

bii X 2i þ

X Xk j

i ¼ 2

bij X i X j

þ ei

ð9Þ

where Y is the experimental response, Xi and Xj are the independent variables, b0 is the average of the experimental response, bi is the estimation of the principal effect of the factor i on the response Y, bii is the estimation of the second effect of the factor i on the response Y, bij is the estimation of the interaction effect between i and j on the response Y, and ei represents the error on the response Y. The experimental values of Ui can be calculated from the coded variables Xi using the following equation: Xi ¼

Optimization conditions for DWW treatment using CCD methodology

Xk

U i − U i;0 ΔU i

ð10Þ

where Ui,0 = (Ui,max + Ui,min)/2 represents the value of Ui at the center of the experimental field and ΔUi = (Ui,max − Ui,min)/2 represents the step of the variation, with Ui,max and Ui,min which are the maximum and minimum values of the effective variable Ui, respectively. Thus, the regression model in terms of coded variables has been expressed by the following second-order polynomial equation:

For Ti/BDD Y 1 ¼ 8:61 þ 6:08X 1 þ 0:21X 2 þ 0:024X 1 X 2

þ 3:45 X 21 þ 2:63E − 0:03X 22

Y 2 ¼ 13:78 þ 30:62X 1 þ 0:0224X 2 − 0:014X 1 X 2

− 2:30X 21 þ 1:05208 E − 0:03X 22

ð11Þ ð12Þ

For Ti/IrO2–RuO2 Y 1 ¼ 15:80 þ 1:65X 1 þ 0:24X 2 þ 0:024 X 1 X 2 þ 3:45 X 21 þ 2:63E − 0:03X 22

ð13Þ

Y 2 ¼ 15:72 þ 21:87X 1 þ 0:0298X 2 − 0:014 X 1 X 2 − 2:3081 X 21 þ 1:05208 E − 0:03X 22

ð14Þ

Environ Sci Pollut Res Table 3 Central composite matrix and experimental results

Tests

Experimental design

Experimental plan

Experimental responses

X1

U1 (A)

U2 (min)

Removal COD (%)

Color removal (%)

0.59 3.41 2.0 2.0 2.0 2.0 2.0 2.0 2.0

90 90 47.57 132.43 90 90 90 90 90

4.84 60.69 22.98 42.34 30.92 32.5 31.2 30.90 32.0

30.86 71.47 56.62 69.43 60.26 62.4 61.2 60.9 59.3

0.59 3.41 2.0 2.0 2.0 2.0 2.0 2.0 2.0

90 90 47.57 132.43 90 90 90 90 90

11.36 79.95 26.48 56.39 40.79 41.82 43.20 41.20 42.10

21.83 91.85 53.71 62.29 61.57 59.20 61.80 64.95 60.54

X2

For Ti/IrO2–RuO2 electrode (U3) 9 −1.41 0 10 1.41 0 11 0 −1.41 12 0 1.41 13 0 0 14 0 0 15 0 0 16 0 0 17 0 0 For Ti/BDD electrode (U3) 18 −1.41 0 19 1.41 0 20 0 −1.41 21 0 1.41 22 0 0 23 0 0 24 0 0 25 0 0 26 0 0

Fig. 7 The effect of electrolysis time (in minutes) and current intensity (in ampere) on the removal of a COD and b color (anode electrode: Ti/BDD): results obtained from central composite matrix

Environ Sci Pollut Res

where Xi varying from −1.41 to +1.41 and Y1 and Y2 represent the percentage of COD and color removal, respectively. From these equations (Eqs. (11)–(14)), it can be seen that current intensity and electrolysis time are very meaningful for the removal of COD and color. When the Ti/BDD was used at the anode, the removal efficiency of color and COD increased with increasing current intensity at all electrolysis time studied. As seen from the three-dimensional presentation (Fig. 7a, b), more than 60 % of COD and 80 % of color could be reported at 2.5 A of current intensity while the treatment time was higher than 105 min. As observed from the analysis of variance (Table 4), the model F value of 32.36 (or 31.93) and the low probability value (Pr>F=0.0001) imply that the model is significant for efficient removal of COD and color from DWW. According to Cescut et al. (2011), the model terms are significant when the values of “Prob>F” are less than 0.0500. The value of the correlation coefficient for COD (R2 =0.94) and color (R2 =0.88) indicates that only 6 and 12 % of the total variation could not be explained by the empirical model. An agreement between actual and predicted values of COD and color removal is satisfactory and consistent with the quadratic model (Fig. 8). According to Joglekar and May (1987), R2 should be at least 0.80 for a good fit of model. The R2 value reported in the present study for the removal of COD and color was higher than 0.80, indicating that the regression model fitted well the EO process. The main objective of the optimization is to determine the optimum values for DWW treatment using the EO process. To rigorously determine the optimal conditions for the removal of COD and color in terms of cost/effectiveness, the energy consumption during electrolysis process has to be taken into account. In fact, the choice of optimal condition must take into consideration not only the removal efficiency of color and COD but also the cost of energy consumed. Table 4 ANOVA results for removal of CODT and color Source

Analysis of variance d.f. Sum of square Mean of square F value Pr>F

ANOVA results for CODT removal Model 8 7,824.89 Residual 17 513.86 Lack of fit 9 513.86 Pure error 8 0.000 ANOVA results for color removal Model 3 4,572.55 Residual 22 1,050.29 Lack of fit 14 1,050.29 Pure error 8 0.000

978.11 30.23 57.10 0.000

32.36 – – –

Electrochemical treatment of domestic wastewater using boron-doped diamond and nanostructured amorphous carbon electrodes.

The performance of the electrochemical oxidation process for efficient treatment of domestic wastewater loaded with organic matter was studied. The pr...
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