Chemosphere 117 (2014) 309–315

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Application of electrochemical technology for removing petroleum hydrocarbons from produced water using lead dioxide and boron-doped diamond electrodes Boutheina Gargouri a, Olfa Dridi Gargouri a, Bochra Gargouri b, Souhel Kallel Trabelsi a, Ridha Abdelhedi a, Mohamed Bouaziz a,b,⇑ a b

Laboratoire d’Electrochimie et Environnement, Ecole Nationale d’Ingénieurs de Sfax, BP‘‘1173’’, 3038-Sfax, Université de Sfax, Tunisia Institut Supérieur de Biotechnologie de Sfax, 3038, BP‘‘1175’’, 3038-Sfax, Université de Sfax, Tunisia

h i g h l i g h t s  Produced Petroleum water (PW) is a serious environmental problem.  Remediation of industrial PW by electrochemical oxidation is performed.  Different current densities was carried out using PbO2 and BDD anodes.  The BDD anode showed a better performance than PbO2 to the treatment of PW.

a r t i c l e

i n f o

Article history: Received 18 April 2014 Received in revised form 16 July 2014 Accepted 20 July 2014

Handling Editor: E. Brillas Keywords: Produced water Electrochemical oxidation Petrochemical pollution Cytotoxicity

a b s t r a c t Although diverse methods exist for treating polluted water, the most promising and innovating technology is the electrochemical remediation process. This paper presents the anodic oxidation of real produced water (PW), generated by the petroleum exploration of the Petrobras plant-Tunisia. Experiments were conducted at different current densities (30, 50 and 100 mA cm2) using the lead dioxide supported on tantalum (Ta/PbO2) and boron-doped diamond (BDD) anodes in an electrolytic batch cell. The electrolytic process was monitored by the chemical oxygen demand (COD) and the residual total petroleum hydrocarbon [TPH] in order to know the feasibility of electrochemical treatment. The characterization and quantification of petroleum wastewater components were performed by gas chromatography mass spectrometry. The COD removal was approximately 85% and 96% using PbO2 and BDD reached after 11 and 7 h, respectively. Compared with PbO2, the BDD anode showed a better performance to remove petroleum hydrocarbons compounds from produced water. It provided a higher oxidation rate and it consumed lower energy. However, the energy consumption and process time make useless anodic oxidation for the complete elimination of pollutants from PW. Cytotoxicity has shown that electrochemical oxidation using BDD could be efficiently used to reduce more than 90% of hydrocarbons compounds. All results suggest that electrochemical oxidation could be an effective approach to treat highly concentrated organic pollutants present in the industrial petrochemical wastewater and significantly reduce the cost and time of treatment. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Petroleum effluents are hazardous compounds, whose discharge into the environment adversely affects the ecosystem. They are pollutants that are caused mainly by the increasing global ⇑ Corresponding author at: Institut Supérieur de Biotechnologie de Sfax, BP ‘‘1175’’, 3038, Université de Sfax, Tunisia. Tel.: +216 98 667 581; fax: +216 74 674 364. E-mail address: [email protected] (M. Bouaziz). http://dx.doi.org/10.1016/j.chemosphere.2014.07.067 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

energy demand required by the greater exploration and exploitation of the raw materials in crude oil. Produced water (PW) is considered as one of the largest waste streams in the petroleum, oil and gas industry. The drilling and extraction operations aiming to maximize the production of oil may be counterbalanced by the huge production of contaminated water with pollutants, such as heavy metals and organic compounds (Wake, 2005; Santos et al., 2006; Ahmaduna et al., 2009). In addition, PW is a serious environmental problem, whose discharge is directly poured into

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the sea in offshore areas. Actually, being loaded with hazardous compounds that remain soluble in water, PW is regarded as the most harmful pollutant that affects life directly. So, there is an urgent need to develop efficient and economical methods to simultaneously remove organic and inorganic pollutants from such effluents. Due to their toxicity and potential carcinogenic effect, they may cause irreparable damage not only to human health but also to the environment (Diya’uddeen et al., 2011; Rocha et al., 2012; Da Silva et al., 2013). Several alternative treatments of PW have been studied by several research groups around the world (Ahmaduna et al., 2009; Ramalho et al., 2010; Rocha et al., 2012). Yet, the physico-chemical and bioremediation methods utilized for the degradation of these compounds have shown various operational problems, such as generation of toxic gases, phase transfer of pollutants, residual sludge production, demand for large territorial areas and the impossibility of destroying refractory compounds (Cañizares et al., 2011). During recent years, electrochemical methods have been proved as both an efficient and viable alternative for the treatment of wastewater practicability in various industrial wastewaters treatment, like refinery wastewater (Dos Santos et al., 2014). This is due to its unique ability to oxidize or reduce contaminants in the water near the well-controlled electrode (Hamza et al., 2009; Martínez-Huitle and Brillas, 2009; Yavuz et al., 2010; Méndez et al., 2012) and its attractive characteristics, such as versatility, energy efficiency, amenability of automation and environmental compatibility (free-chemical reagents). The electrochemical treatment has various advantages, among which wide application, simple equipment, easy operation, no consumption of chemical, lower temperature requirements and lack of sludge formation can be mentioned (Chen, 2004; Martínez-Huitle and Ferro, 2006). Among these electrochemical methods, the anodic oxidation is useful to directly destroy organic pollutants by reaction with hydroxyl radical (HO) formed at the anode surface from water oxidation (Belhadj-Tahar and Savall, 1998):

H2 O ! HOads þ Hþ þ e

ð1Þ

Over the past years, the application of anodic oxidation to water remediation has received great attention owing to the use of special electrodes. Moreover, the total mineralization to CO2 and H2O as well as the optimal faradic efficiency was strictly obtained by using high oxygen over-potential anodes, such as Tin dioxide SnO2 (Ramalho et al., 2010), lead dioxide PbO2 (Belhadj-Tahar and Savall, 1998; Weiss et al., 2008) and boron-doped diamond BDD, which brought about different removal organic matter efficiencies (Michaud et al., 2003; Zhao et al., 2010; Da Silva et al., 2013). Non-active anodes, as BDD, is a new anode material and possesses technologically important characteristics such as an inert surface with low adsorption properties, remarkable corrosion stability and an extremely wide potential window in aqueous medium (Panizza and Cerisola, 2009; Da Silva et al., 2013; Dos Santos et al., 2014). In this context, BDD anodes have a great potential for the electrochemical applications in the treatment of wastewater. This is due to their extraordinary chemical inertness and their ability to mineralize a wide range of recalcitrant organic compounds (Dos Santos et al., 2014; Urtiaga et al., 2014). This is explained by the very high over-voltage of oxygen production and the generation of hydroxyl radicals, a very powerful oxidant (Gomes de Lima et al., 2009; Panizza and Cerisola, 2009, 2010b; Sales et al., 2013). The BDD electrode is often used as anode material for petroleum wastewater treatment (Dos Santos et al., 2014). Nevertheless, none of the previously done works reports the electrochemical treatment of the petroleum wastewater on the PbO2 anodes. The main objective of the present work is to study

the electro-oxidation process in treating petroleum PW using Ta/ PbO2 and BDD anodes to remove petroleum hydrocarbons. Effectiveness was measured in terms of the reduction in total petroleum hydrocarbon [TPH], COD and petroleum hydrocarbons (C10–C40) by GC/MS analysis. Besides, the toxicity of the water after treatment was tested using cell culture. 2. Experimental 2.1. Produced water samples and chemicals The waste analyzed in this study was continuously produced as a by-product during crude oil extraction in secondary extraction wells. Since there was no actual way for the deposition of this waste, it was disposed in closed reservoirs at a rate of 10–15 m3 per day per well. PW samples were supplied by Petrobras plant ‘‘SEREPT’’ an industry for research and exploitation of petroleum located in the southwest of Tunisia–Sfax. The oil was separated from the produced water in that terminal. Its pH was around 6.9 and its electric conductivity around 1540 lS cm1. It contained 4.38 g L1 of total dissolved solids, 59 mg L1 of phenol and 56.7 mg L1 of oils and greases. Chemical oxygen demand (COD) was about 19 842 ppm. Table 1 lists the average composition of the produced water as received in the terminal, according to the physicochemical parameters. The chosen chemicals were of the highest-quality commercially-available ones, and they were used without further purification. NaCl was purchased from Chemi-Pharma and used as a supporting electrolyte for increase effluent conductivity. Besides, H2SO4 and NaOH (Merck, Germany) were used for the pH adjustment. All the solutions were freshly prepared with ultra pure water. Cytotoxicity using cell lines HeLa was carried out to get information about the toxicity of the initial and treated produced water under optimum experimental conditions. 2.2. Extraction procedure of hydrocarbon The analyses of hydrocarbons were carried out after dichloromethane extraction. The aqueous phase sample was removed and put in a sealed flask for sub-sequent analysis. Then, it was concentrated to approximately 3 mL using a rotary evaporator under reduced pressure in a water bath. Afterwards, it was dissolved in equal volume of dichloromethane and further cleaned through a column filled with florisil (SUPELCLEAN LC-FLORISIL, USA) and then analyzed by gas chromatography–mass spectrometry apparatus. After the evaporation of the solvent, the amount of residual TPH was determined by gravimetric methods after dichloromethane evaporation by simple distillation at 60 °C (Mishra et al., 2001). 2.3. Cytotoxicity experiments 2.3.1. HeLa cell culture The continuous human cell lines HeLa (epithelial cervical cancer cell line) was investigated for cytotoxicity of petrochemical wasteTable 1 Main physical and chemical characteristics of produced wastewater. Characteristics

Values

pH COD (mg O2 L1) BOD5 (mg L1) Phenol (mg L1) Oil and greases (g L1) Electrical conductivity (lS cm1) Total petroleum hydrocarbons (g L1)

6.9 19 842 475 59 56.7 1540 11.22

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water. This adherent cell line was grown in RPMI 1640 medium (Gibco) supplemented with 10% foetal calf serum (FCS) and 2 mM L1 glutamin in tissue culture flasks (Nunc). It was passed twice a week and kept at 37 °C in a humidified atmosphere of 95% air and 5% CO2. 2.3.2. MTT cell proliferation assay The MTT (3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide) cell proliferation assay measures the cell proliferation rate and, conversely, the reduction in cell viability when metabolic events lead to apoptosis or necrosis. The yellow compound MTT (Sigma) is reduced by mitochondrial dehydrogenases to the water insoluble blue formazan compound, depending on the viability of the cells. Cells (6  104 cells mL1) were grown on microtiter plates (200 lL of cell suspension/well) in 96 well microplates with serial dilutions of different extracts for 48 h before the addition of 20 lL of an MTT solution (5 mg mL1 in Phosphate-Buffered Saline (PBS)). The plate was incubated for 4 h at 37 °C in a CO2 incubator. After incubation, 180 lL of medium were removed from each well and 180lL of DMSO/methanol (50:50) were added to each sample. The preparations were thoroughly mixed on a plate shaker with the cells containing formazan crystals. When all the crystals were dissolved, absorbance was measured at 570 nm with a microplate reader (Elx 800 microplate reader). 2.4. Electrochemical experiment 2.4.1. The preparation of the PbO2 electrode The deposit of lead dioxide was prepared by the electrochemical oxidation of an aqueous solution of lead nitrate on a rectangular tantalum plate (70 mm  10 mm  1 mm). Tantalum was chosen for its chemical and electrochemical stability. From the potential–pH equilibrium data, tantalum undergoes passivation once it is in contact with an aqueous solution, by the formation of a non-conducting oxide layer Ta2O5 (Pourbaix, 1963). The tantalum surface is chemically stripped for 30 s in 40% hydrofluoric acid at room temperature. Such a chemical treatment dissolves the layer of tantalum oxide. The lead dioxide was galvanostatically deposited by using a double compartment cell (V = 200 mL) thermo-regulated at 65 °C. The analyte is an aqueous solution containing lead nitrate at a concentration of 1 M. The cathode is a cylindrical polycrystalline platinum grid (Ø = 5 cm, l = 2.8 cm). The PbO2 deposition was carried out for 2.5 h using an anodic current density of 20 mA cm2. The geometric surface area of PbO2 film deposited on the two faces of tantalum plate was of 7 cm2 (3 cm2 for each face). The obtained film was mat grey, adherent, regular and uniform. 2.4.2. The boron-doped diamond electrodes The BDD thin-film electrode was supplied by CSEM (Centre Swiss d’Electronique et de Microtechnique, Neuchatel, Switzerland). It was synthesized by the hot filament chemical vapor deposition technique (HF-CVD) on single-crystal p-type Si h1 0 0i wafers (1–3 mW cm, Siltronix) (Perret et al., 1999). The doping level of boron in the diamond layer, expressed as B/C ratio, was about 3500 ppm. The resulting diamond film thickness was about 1 mm with a resistivity of 10–30 mX cm. The electrode area was 7 cm2. Before using it in galvanostatic electrolysis assays, it was polarized during 30 min with a 0.5 M H2SO4 solution at 50 mA cm2 in order to remove any kind of impurity from its surface. This treatment made the surface hydrophilic, as previously indicated by Panizza and Cerisola (2010a). 2.4.3. Electrolysis The electrolysis of the aqueous solutions containing PW was carried out in an isothermal reactor using a single thermo

regulated compartment Pyrex glass cell of 200 mL volume with the BDD or the PbO2 anodes and a cylindrical platinum grid (Ø = 5 cm, l = 2.8 cm) as cathode. The PW solutions were electrolyzed in galvanostatic mode by using a DC power supply (model ATTEN TPR 3003-3C-LG Precision Co. Ltd, Korea). The total surface area of the working electrode was 7 cm2. 2.5. Analytical methods During the electrolysis, samples were withdrawn from the reactor at selected intervals. The treatment of PW was followed by the COD, [TPH] and GC/MS analysis. During the processing, the COD measurement allows the evaluation of the kinetics of organic matter decay. The COD values were determined by the dichromate method (Knechtel, 1978). All experiments were performed in triplicate with appropriate controls. The analytical determination of phenol was carried out according to the Folin–Ciocalteu method as described by Gargouri et al.(2014). GC/MS (Agilent 5975B inert MSD) was used to determine the component of hydrocarbons. The gaseous phase separation of the intermediates was performed in the capillary column (Agilent HP-5MS, with the filler of 5% phenyl-polysiloxane, 30 m  0.25 mm  0.25 lm), with a mass range between m/z 50 and 450. The GC column temperature was programmed as follows: it was firstly maintained at 70 °C for 2 min, and then heated at a rate of 20 °C min1 up to 230 °C. After that, it was heated at a rate of 40 °C min1 up to 300 °C, and then held at this temperature for 10 min. The injection temperature was maintained at 250 °C. The carrier gas was helium and the column flow was maintained at 1.0 mL min1. The energy consumption per volume of treated effluent was estimated and expressed in kW h m3. During the electrolysis, the average cell voltage was taken for the calculation of the energy consumption as follows:

Energy consumption ¼

U cell It V 3600

where Ucell is the average cell voltage (V), I is the current (A), t is the electrolysis time (s) and V is the volume (L). 3. Results and discussion 3.1. Electrochemical oxidation using PbO2 electrode To achieve a high level of efficiency in electrochemical treatment, several factors must be taken into consideration, such as the current density and the anode material for the anodic treatment of the petrochemical wastewater. In this study a set of experiments were performed for studying the role of anode material (Ta/PbO2 and BDD anodes). Preliminary electrochemical experiments were performed using Ta/PbO2 anode at different applied current densities (30, 50 and 100 mA cm2). Fig. 1 shows the influence of applied current density on COD removal, as a function of electrolysis time, during the electro-oxidation of PW. As can be seen in Fig. 1, the increase in japp causes a quicker degradation of the PW solution, as expected from the greater generation of OH from reaction (Eq. (1)). At 7 h, for example, COD is reduced by about 65%, 83% and 98% at 30, 50 and 100 mA cm2, respectively. At Ta/PbO2 electrode, organic pollutants in PW are directly destroyed by reaction with hydroxyl radicals formed at the anode surface from anodic discharge of water (Eq. (1)). These radicals react very rapidly organic pollutants in PW and its intermediates (Eqs. (2) and (3)).

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japp (mA cm-2)

25000

80 60

20000

COD (mg O2 L-1)

The energy consumption tendency with COD removal can be related to the gradual formation of oxidation intermediates, such as carboxylic acid, which are more difficult to destroy by hydroxyl radical on the one hand, and to the development of the secondary reaction Eq. (2), on the other hand.

100

40 20

15000

0

30

50

3.3. Effect of anode materials

100

10000

5000 0 0

2

4

6

8

10

12

Times (h) Fig. 1. Influence of applied current density on the COD trends, as a function of time, during the electrochemical treatment of PW using PbO2 anode. insert: % COD removal after 7 h of electrolysis as a function of current density. Electrolyte: pH = 6.9, T = 25 °C, 0.05 M of NaCl, agitation rate: 400 rpm.

Electrochemical oxidation experiments were carried out at 25 °C and at the minor current density (japp = 30 mA cm2) in order to favors the use of lower energy requirements and to assess the role of anode materials to investigate the treatment of PW. Indeed, in a recent research study, it has been reported that the highest efficient COD removal rate of petrochemical wastewater was attained by applying 30 mA cm2 using BDD anode (Rocha et al., 2012). The variation of COD with time, depending on the electrode materials, is plotted in Fig. 3a. In this condition, the anode material

a

25000

m OHads þ organic pollutants ! intermediates 

n OHads þ intermediates ! xH2 O þ yCO2

ð3Þ

However, hydroxyl radicals can be lost by the parasitic reaction (Eq. (4)):

2 OHads ! 1=2O2 þ H2 O

ð4Þ

COD (mg O2 L-1)

ð2Þ

♦ PbO2 BDD

20000 15000 10000 5000

Under the experimental conditions used in this study. The influence of applied current density was further considered to assess more clearly the mineralization power of electrogenerated oxidants PbO2 (OH).

0

0

100

6

8

10

12

b

COD removal %

80 60 40

♦ PbO2 BDD

20 0 0

10

20

30

40

50

Ec (KWh m-3) Fig. 3. (a) Comparison of the COD decay, as a function of time, during the electrochemical treatment of PW at PbO2 and BDD anodes. (b) Variation of energy consumption and electrode consumption for COD removal from petroleum wastewater at PbO2 and BDD anodes. Electrolyte: pH = 6.9, T = 25 °C, 0.05 M of NaCl, japp = 30 mA cm2, agitation rate: 400 rpm.

120 100 80

12

60

japp (mA

cm-2)

10

30 50 100

40

TPH (g L-1)

COD removal %

4

Times (h)

3.2. The electrical energy consumption using Ta/PbO2 anode The adoption of an electrochemical technology as an effluent treatment must consider some aspects in order to make its implementation feasible (performance of anode material, energy consumption and operating cost). The electrical energy consumption (Ec) required to the treatment of PW at different current densities was calculated by referring to data from Fig. 1 and the values shown in Fig. 2. As it is indicated, the minimum electrical energy’s consumption was 46.6 kW h m3 at 30 mA cm2 current density After 7 h of electrolysis. This energy increased slightly when japp went up from 30 to 100 mA cm2 (it increased from 46.6 to 182 kW h m3, respectively).

2

20 0 0

50

100

150

200

Ec (KWh m-3) Fig. 2. The evolution of the specific energy consumption against % of COD removal during the electrochemical treatment of petrochemical wastewater on the PbO2 anode at different applied current densities. Electrolyte: pH = 6.9, T = 25 °C, 0.05 M of NaCl, agitation rate: 400 rpm.

8 6 4 2 0

PW

PbO2

BDD

Fig. 4. Hydrocarbon removal of petroleum wastewater before and after an electrochemical oxidation of PW at PbO2 and BDD anodes after 7 h, applying 30 mA cm2.

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effect on COD abatement was significant using BDD and Ta/PbO2 anodes under the experimental conditions (pH = 6.9, japp = 30 mA cm2, and T = 25 °C). Otherwise, the oxidation rate of the PW on the BDD anode is higher than that on the PbO2. Moreover, the relevant COD removal of about 96% and 84% were achieved after 8 h electrolysis by using BDD anode and after 11 h using

PbO2 anode (Fig. 3a). In this context, a recent research work has demonstrated that electrochemical oxidation using BDD anode is an efficient method for the treatment of petroleum wastewater (Yan et al., 2011). More recently, Rocha et al. (2012) have studied the electrochemical oxidation of brine PW in galvanostatic conditions using platinum supported on titanium (Ti/Pt) and BDD

Produced water inital Day

TIC: SSP_02102012.D\data.ms

a 2400000

Relative Abundance

2200000 2000000 1800000 1600000 1400000 1200000 1000000 800000 600000 400000 200000 0

5.00

6.00

7.00

8.00

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11.00

12.00

13.00

14.00

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16.00

Time (min)

b 2400000

Produced water treated with PbO2

Relative Abundance

2200000 2000000 1800000 1600000 1400000 1200000 1000000 800000 600000 400000 200000 5.00

6.00

7.00

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c

Produced water treated with BDD

Relative Abundance

2200000 2000000 1800000 1600000 1400000 1200000 1000000 800000 600000 400000 200000 5.00

6.00

7.00

8.00

9.00

10.00

11.00

12.00

13.00

14.00

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16.00

Time (min) Fig. 5. GC/MS analysis of petroleum wastewater before and after electrochemical treatment using PbO2 and BDD anode. (a) Produced water initial Day; (b) Produced water treated with PbO2 ; (c) Produced water treated with BDD. Electrolyte: pH = 6.9, T = 25 °C and japp = 30 mA cm2.

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anodes. Their results have shown that complete COD removal was achieved using BDD electrode due to the production of high amounts of hydroxyl radicals (OH) and oxidizing species (Cl2, HClO, ClO). The large difference in COD removal rate can be explained by the different nature of the physisorbed HO generated on both anodes. These radicals have a lower enthalpy of adsorption on the BDD surface (Kapalka et al., 2008; Panizza and Cerisola, 2008; Weiss et al., 2008), which suggests that they are more active and effective as they react very rapidly with all organics arriving at the surface and even in the vicinity of the anode. However, hydroxyl radicals are strongly adsorbed on the surface of PbO2 electrode and led to the secondary reaction of oxygen formation (Eq. (2)) which is competing with its counterpart, the pollutant mineralization, as was previously reported in many studies (Santos et al., 2006; Panizza and Cerisola, 2008; Sires et al., 2008). Energy consumption is one of the most important parameters in the electrochemical studies as well as in other wastewater treatment. Along with high removal efficiency, low energy requirement is a necessity for a treatment system. Fig. 3b reports the energy consumption (kW h m3) as a function of COD removal during the PW electrolysis at PbO2 and BDD anodes, performed under the studied condition (30 mA cm2). The average energy consumption values at the end of electrochemical treatment were 38 and 46 kW h m3 by using BDD and PbO2 electrodes, respectively. As it can be observed, BDD consumed less energy than PbO2 anode but achieved higher COD removal than PbO2. For example, to reach 85% of the COD removal, BDD and PbO2 consumed 24 and 46.2 kW h m3, respectively. These results show the high performance of BDD anodic oxidation in treating petrochemical wastewaters. Indeed, electrochemical oxidation using BDD anode could be a feasible process for produced waters generated by petrochemical industry in order to significantly reduce the cost and time of treatment (De Oliveira et al., 2011; Dos Santos et al., 2014). 3.4. Petroleum hydrocarbons degradation at BDD and PbO2 anodes The effect of electrochemical process on the degradation of hydrocarbons was investigated through conducting degradation in crude produced water and produced water treated by PbO2 and BDD anode under the best experimental conditions (pH = 6.9, japp = 30 mA cm2 and T = 25 °C) (Fig. 4). The obtained results have shown that the amount of hydrocarbons compounds (TPH) in PW treated with BDD anode is lower than that with PbO2 anodes. In fact, 97% of TPH was nearly completely removed within 7 h in produced water treated with BDD and it reach only 84% in produced water treated with PbO2 anode within 11 h. Thus, we come to the conclusion that the BDD anode has a better performance to remove the petroleum hydrocarbons [TPH]. 3.5. Change in GC/MS profile during electrochemical remediation process GC/MS has been known for its higher separation of complex organic compounds, greater sensitivity and shorter measuring time. Hence, it is suitable for the detection and identification of volatile organic compounds of the produced wastewater. Fig. 5 shows the GC results of the produced wastewater before and after electrochemical treatment process at PbO2 and BDD anodes during 11 and 7 h, respectively under the used conditions (pH = 6,5, japp = 30 mA cm2 and T = 25 °C). The corresponding organic compounds distribution in the PW is shown in Fig. 5a. According to GC/MS analysis, the main organic compounds in the PW are aliphatic hydrocarbons, phenols as well as little of phthalate, which contribute to the COD value and pollute the environment. Pro-

120

PW

PbO2

BDD

100

Cytotoxicity (%)

314

80 60 40 20 0

1

0.5

0.25

0.125

0.0625 0.03125

Concentrations (mg L-1) Fig. 6. Cytotoxic effect of produced water on HeLa cell line. The inhibitory effect of different doses on cell growth. Error bars indicate the SD of the mean of three replicates.

duced water consists mainly of n-alkanes C10 through C39, with intermediate branched chain hydrocarbons and other petroleumbased compounds. The PW electrochemical treatment allowed the degradation of the most organic substances. Indeed, hydrocarbons compounds were almost undetected by GC/MS for the treated produced water using BDD anodes under the studied conditions, which indicates that the removal of organic pollutants is satisfied in this reaction system (Fig. 5c). This result is in agreement with the reported reference (Liu et al., 2007; Yan et al., 2011). 3.6. Cytotoxicity analysis To investigate the cytotoxic effect of untreated and treated produced water, HeLa human cell line was used. The cells were incubated with various water samples, and then submitted to the MTT test (Fig. 6). The chemical analyses have shown that the produced water samples from the petrochemical industry plant contained the highest concentration of hydrocarbons. As for the results of MTT assay, they indicated that the untreated produced water also had the greatest cytotoxicity on Hella cells. However, the treated PW with PbO2 and BDD displayed some inhibition effects on human cells growth in a dose-dependent manner. The comparison of the results shows a reduction of the toxicity while applying electro-oxidation treatment (Fig. 6). Thus, relatively high toxicity of PW was measured in the untreated samples. 4. Conclusions The present research study has shown that anodic oxidation can be successfully used to remove completely organic pollutants from petrochemical wastewaters. In the case of PW, the efficiency decontamination and time process depend on the operating conditions, such as current density and anode materials. It was reported that PbO2 anode displayed noticeable oxidation abilities for treating PW, due to the production of a large amount of hydroxyl radicals on the electrode surface during electrolysis. However, the degradation of the PW at the BDD anode provided higher oxidation rate than the PbO2 anode for the same operating conditions. To reach 85% of COD removal, the BDD anode consumes less energy than PbO2. GC/MS analysis was found to be very useful tools in performing preliminary tests in order to predict remediation performance so as to select an appropriate approach for clean-up technologies. The results clearly indicate that the electrochemical treatment of PW under the operating conditions can lead to an efficient degradation of its organic compounds. As an effective approach, electrochemical technology may be applied in treating petrochemical wastewater in petroleum industry.

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