CA2632788A1 - Degradation of organic toxics by electro-oxidation - Google Patents

Abstract

This invention relates to an electrochemical process for the degradation of toxic organic molecules in solution. This process includes the treatment of organic toxics containing solutions in an electrolytic cell having dimensionally stable anodes (DSA) with high oxygen overvoltage. The anodes are made of titanium coated with iridium oxide (IrO2), ruthenium oxide (RuO2) or tin oxide (SnO2). The solution is treated with a current density ranging between 3.0 to 23 mA/cm2 and for a period of time ranging between 10 to 200 min. This process can be used for degradation of different organic molecules including one-type or a mixture of polycyclic aromatic hydrocarbons (PAHs), chlorinated compounds, pesticides, endocrine disruptors, oils and greases, petroleum hydrocarbons, PCBs, PCDD/F or other types of organic compounds.

Description

TITLE

DEGRADATION OF ORGANIC TOXICS BY ELECTRO-OXIDATION
INVENTORS

Patrick Drogui, Lan Huong Tran, Jean-Frangois Blais and Guy Mercier INVENTION FIELD

This invention relates to an electrochemical process for degradation of toxic organic molecules in solution. Particularly, this process includes the treatment of organic toxics containing solutions in an electrolytic cell having dimensionally stable anodes (DSA) with high oxygen overvoltage. The anodes are made of titanium coated with iridium oxide (IrO2), ruthenium oxide (Ru02) or tin oxide (Sn02), with different geometrical forms (plane, cylindrical or circular mesh anodes). This process can be used for degradation of different organic molecules including one-type or a mixture of polycyclic aromatic hydrocarbons (PAHs), chlorinated compounds, pesticides, endocrine disruptors, oils and greases, petroleum hydrocarbons, PCBs, PCDD/F or other types of organic compounds.

STATE-OF-THE-ART
In recent years, numerous research works have focused on electro-oxidation (EO) process owing to the appearance of emerging pollutants (PCBs, PAHs, EDCs, pesticides, and others), which are recalcitrant organic compounds and difficult to oxidize by traditional biological and chemical treatments. This type of technology has been widely applied for the treatment of different effluents: wastewater (Martinez-Huitle and Ferro 2006), textile effluents (Wang et al.
2004), landfill leachate (Moraes and Bertazzoli, 2005; Deng and Englehardt 2007), olive oil mill wastewater (Gotsi et al., 2005), municipal sewage sludge (Zheng et al. 2007), tannery effluent (Rao et al. 2001; Panizza and Cerisola 2004) using different electrode materials.

The interest of using electrochemical oxidation is based on its capability of reacting on the pollutants by using both direct and indirect effect of electrical current.
Direct anodic oxidation, where the organics can be destroyed at the electrode surface, and indirect oxidation where a mediator (HC1O, HBrO, H202, H2S208, and others) is electrochemically generated to carry out the oxidation (Grimm et al. 1998; Drogui et al. 2001). Two different ways can be followed in anodic oxidation: electrochemical conversion or electrochemical combustion (Comminellis and Pulgarin 1993). Electrochemical conversion only transforms the non-biodegradable organic pollutants into biodegradable compounds, whereas electrochemical combustion yields water and carbon dioxide and no further treatment is then required.

Direct anodic oxidation It has been admitted that the direct anodic oxidation is carried out using two steps (Comminellis 1994; Gandini et al. 1998). The first reaction (equation 1) is the anodic oxidation of water molecule leading to the formation of hydroxyl radicals (HO ) adsorbed on active sites on the electrode "M":

H2O + M -> M[HO ] + H+ + e - (1) Subsequently, the oxidation of organics "R" is mediated by adsorbed hydroxyl radicals (equation 2) and may result in fully oxidized reaction product as CO2 (equation 3).

R+M[HO ]-> M+RO+H++e- (2) R+M[HO ]-* M+mCO2+nHZO+H++e- (3) Where "RO" represents the oxidized organic molecule, which can be further oxidized by hydroxyl radical while it is continuously produced at anode electrodes. The accumulation of HO
radicals favors the combustion reaction (equation 3). The hydroxyl radicals are species capable of oxidizing numerous complex organics, non-chemically oxidizable or difficulty oxidizable (Pulgarin et al. 1994). They efficiently react with the double bonds -C=C- and attack the aromatic nucleus, which are the major component of refractory organic compounds.
However, during direct anodic oxidation of organic pollutant, competitive reactions (parasitic reaction) can take place and limit hydroxyl radical formation, such as molecular oxygen formation (equation 4):

HzO + M[HO ] --> M+ OZ + 3H+ + 3e - (4) Indirect electro-oxidation effect The indirect effect of electrolysis is also interesting to destroy recalcitrant organics. For instance, in the presence of sulfate and chloride ions, these ions can be respectively oxidized at the anode electrodes and formed in peroxodisulfuric acid (H2S208) and hypochlorous acid (HC1O) solutions (see equations 5 and 6). Both HC1O and H2S208 are powerful oxidants capable of oxidising and modifying the structure of organic molecules and leading to more oxidized and less toxic compounds (Canizares et al. 2002, 2005).

2SO4- + 2H+ -> H2S208 + 2e - (5) Cl -+ 2H20-> HCIO + H3O+ + 2e - (6) Likewise, during electrolysis, hydroxide peroxide (H202) can be produced from dissolved oxygen by cathodic reduction (Bernard and Rumeau 1998; Drogui et al. 2001) (equation 7):
02(d,ssoUs) + 2H+ + 2e- -> H202 (7) In such an electrolytic system, the required oxygen was supplied by oxidation of water and by transfer from the atmosphere or by pure oxygen injection. Hydroxide peroxide was produced by direct current electrolysis using only two electrodes, a carbon felt cathode and a Ru02 coated titanium anode. A high peroxide production rate was reached and a 15 mg/L
concentration was maintained. The dissolved organic carbon (DOC) removal in effluent of municipal sewage plant corresponded to a breakage of the double bonds (Drogui et al. 2001). A
remarkable remnant effect was ensured and induced non offensive by products contrary to chlorine or hypochlorous acid. It has also been reported that hydroxide peroxide can be produced in the electrolytic cell whose cathode is made of porous carbon polytetrafluorethylene (PTFE) with oxygen feeding. The degradation of 95% of aniline has been recorded in the presence of ultraviolet (UV) irradiation (Brillas et al. 1998).

Indirect EO can also contribute to generating two mediators such as ozone (03) and hydroxide peroxide (H202) by means of a compartmented electrolytic cell using an ionic exchange membrane separating anode and cathode chambers (Murphy and Duncan 1999). The cathode is coated with both a catalytic layer and a diffusion gas layer. The diffusion gas layer is comprised of hydrophobic (carbon fiber) and hydrophilic (PTFE) parts. The oxygen is transferred into the diffusion gas layer. The anodic oxidation of water induces ozone and proton (H) formation. The protons are transferred into cathodic chamber and H202 is produced from dissolved oxygen by cathodic reduction. The formation of hydroxyl radical at the surface of anode electrode does not take place on all the electrodes. These reactions depend on the experimental conditions and, above all, on the nature of the electrode materials.

Catalytic anodes for organic pollutants destruction In EO process, two types of insoluble electrode are often used dependently on the objectives of the treatment. When the objective is the simple electrolysis of water (oxygen formation), an electrode material having a low over-potential of oxygen evolution is required.
However, when the objective is the degradation of pollutants, a high over-potential of oxygen evolution is used. The latter parameter governs the choice of the electrodes for anodic combustion or conversion of organic pollutants. Table 1 gives a comparison of the most investigated anode materials. Numerous high over-potential electrodes have been investigated (Ti/Pt, Ti/Pt-Ir, Ti/IrO2, Ti/Ru02, Ti/Pb02, Ta/PbO2, BDD, Ti/BDD (boron-doped diamond), Ti/Sn02, Ti/Sn02-Sb2O5, Gr, and others) to treat effluents containing varied and relatively high amount of organic matter (PCBs, PAHs, EDCs, pesticides, phenolic compounds, surfactants) which are difficult to oxidize biologically or chemically (Rajeshwar and Ibanez 1997). Among these organics, phenolic compounds are the most investigated applications in EO studies. The influence of the nature of anodic electrode has been clearly put into evidence while oxidising phenol-containing synthetic effluent (Comminellis 1994). Different types of titanium anode electrodes coated with a catalytic layer of Pt, Ru02, IrO2, Pb02, and Sn02 have been studied for electrochemical treatment (i = 50 mA/cm2) of solutions containing phenol (10.6 mM). The best removal yield was obtained using Ti/Sn02 electrode with an average oxidation yield of 55% of current efficiency compared to Ti/Pt, Ti/Ru02, Ti/IrO2 and Ti/Pb02 electrodes whose values ranged between 10 and 15%. The relatively high oxidation yield recorded with Ti/Sn02 has been attributed to the highly crystalline nature of the electrode that catalyzed the electrochemical oxidation of phenol.

TABLE 1. Summary of the effectiveness of different anode materials used in electro-oxidation of water pollutants Pollutants Anode Pollutant Current References materials removal efficiency (%) (%) Aliphatic alcohol Ti/Ir02 90%, TOC 30-40 Simond and Comninellis (1997) Phenol Ti/Ir0z - 17 Comninellis (1992) Phenol Ti/Ru02 - 14 Comninellis (1992) Phenol Ti/Pt 13 Comninellis (1992) Carboxylic acids Ti/Pt-Ir 99% 80-100 Bock and MacDougall (2002) Bock et al. (2002) Dyes Ti/Pt-Ir 50% 80-100 Szpyrkowicz et al. (2000) 2-chlorophenol Ti/Pb02 80-95%, COD 35-40 Polcaro et al. (1999) Phenol Ti/Pb02 18 Comninellis (1992) Phenol Si/BDD 97%, TOC Beck et al. (2000) Phenol Ta/PbO2 57%, TOC Beck et al. (2000) Phenol Ti/Sn02 80%, TOC Beck et al. (2000) Phenol Pt 37%, TOC Beck et al. (2000) Phenol Ti/Sn0z-Sbz05 71% 58 Comninellis (1992) Panizza and Cerisola (2004) studied the electrochemical oxidation as a final treatment of synthetic tannery wastewater using lead dioxide (Ti/Pb02) and mixed titanium and ruthenium oxides as anodes (Ti/TiRuO2). Complete mineralization of the wastewater was recorded using either Ti/Pb02 or Ti/TiRuO2 electrode. In particular, the oxidation of organics took place on the Ti/Pb02 anode by direct electron transfer and indirect oxidation mediated by active chlorine, while it occurred on the Ti/TiRuO2 anode only by indirect oxidation. Likewise, Ti/Pb02 gave higher oxidation rate than that observed for the Ti/TiRuO2.

Electrochemical oxidation of PAHs present in sediment has been studied by Stichnothe et al. (2005). A total of sixteen PAHs have been measured before and after the electrochemical treatment. A titanium anode electrode coated with iridium oxide (Ti/Ir02) operated at a current density of 80 mA/cm2 during 120 min has been used. At the end of the treatment, the residual concentration was 0.53 mg PAH/kg, compared to 4.1 mg PAHs/kg recorded in the initial sediment, which corresponded to 90% of degradation. IrO2 electrode has been also studied for electrochemical elimination of aliphatic alcohols, allowing current efficiency of 30-40% and 90%
conversion to CO2 (Simond and Comminellis 1997). Overall, low current efficiencies and high removal efficiencies are obtained at longer times, resulting in the competition between the oxidation of organic and the oxygen evolution.

In particular, the boron-doped diamond (BDD) electrode represents an interesting and attractive anode material for the degradation of refractory pollutants such as phenol, chiorophenol, carboxylic acids, aniline, various dyes, surfactants and many others compounds (Kraft et al. 2003; Martinez-Huitle 2004a, 2004b, 2005). The use of BDD as anode for organic pollutant oxidation has been patented by Carey et al. (1995). The effectiveness of Si/BDD
electrode for the degradation of phenol has been compared to Ti/Sn02, Ta/PbO2 and Pt electrodes. At a charge loading of 20 A/h, the yield of total organic carbon (TOC) removal of 97% was recorded using Si/BDD. In comparison, 80, 57 and 37% of TOC removal were respectively recorded for Ti/Sn02, Ti/Pb02 and Pt electrodes. Another peculiarity of BDD
electrode results from its capacity to produce H2S208, a powerful oxidant capable of participating in the oxidation of the organic substrates, allowing higher efficiencies.
However, the fragility and the relatively low conductivity of Si-supported device, constitutes an obstacle for BDD
application at full scale. Titanium coated with BDD (Ti/BDD) would be more conducive, stable and effective for organic compound destruction.

Based on the background of the EO method, several processes have been developed for environmental applications. For instance, Kinder et al. (1998) have developed a procedure for the treatment of urban wastewater using an oxidative-electrochemical process. This process is effective to treat wastewater having high level of colloids and high-molecular organic residues.
The wastewater was passed multiple times over the stainless steel electrode, which functions as the anode, generates oxygen, and has a catalytic effect so that the colloid and partially dissolved oxidized dissolved materials are flocculated. In the second stage of the process, the flocculated substances are concentrated using membrane filtration. The concentrate volume (1/10 of the volume of wastewater) is then passing over a water-swellable polymer, an acrylamide-acrylic acid copolymer to remove the water. The water-swellable polymer can be air-dried and reused.

An oxidative electrochemical method has also been developed for conditioning and stabilizing sewage sludge from municipal and paper mill industries (Drogui et al. 2005). The treatment of sludge is carried out using a cylindrical electrolytic cell having two concentric electrodes are used. The anode material is made of titanium coated with ruthenium (Ti/Ru02) whereas titanium material (Ti) is used as cathode electrode. This process is characterized by the following steps: acidification of the sludge (4.0 < pH < 5.0) in such a manner so as to reach a sufficiently high pH to avoid corrosion and sufficiently low to significantly reduce the indicators of pathogens. Treatment of the acidified sludge in an electrolytic cell able to generate in situ a bactericidal oxidant (HC1O or H2S208) in a sufficiently high concentration to disinfect the sludge and a sufficiently low concentration to avoid the formation of organochlorinated compounds in the sludge; electrolysis of the sludge for a period of time sufficient for stabilization of the sludge and to improve their ability to be dewatered. Dryness gain of dewatered-sludge as high as 10 units are expected when the process is applied. The increasing of the total solids of treated sludge allowed reduction from 20 to 30% of the volumes of dewatered-sludge produced.
The process was also found to be effective in removing indicator microorganisms such as FC
(log-inactivation of FC was higher than 6 units), while at the same time preserving its fertilizing properties by maintaining the concentration of organic matter (chemical oxygen demand: COD) and inorganic nutrients (P-P04 and N-NH4) in dewatered-sludge.

Held and Chauhan (2002) describe a process using high electrical voltage (pulsed-electric filed, PEF) for dewatering and treating waste-activated sludge (WAS) from municipal wastewater treatment plant, from paper mill wastewater treatment plant, and effluents from agrofood industries. The method consists in subjecting waste-activated sludge to electroporation, which incorporate nonarcing, cyclical high voltages in the range of 15,000 and 100,000 V, which break down intercellular and intracellular molecular bonds. The release of intercellular and intracellular water allows reduction to 50% the amount of dewatered sludge.

An electrochemical process capable of oxidising and disinfecting water and wastewater by electroperoxidation has been developed by Bernard and Rumeau (1998). This process consists of oxidisation by direct anodic oxidation dissolved organic compound, viruses and deactivated bacteria, while at the cathode electrode, hydrogen peroxide is produced from dissolved oxygen reduction. This peroxide has a remarkable remnant effect and induces non offensive by products contrary to chlorine. Percolating electrodes of vitreous carbon (cathode) and platinated titanium or DSA (anode) are used in the electrolytic cell. Experiments have been carried out using synthetic effluent and effluent from municipal wastewater treatment plant. A
10 to 50% of DOC
removal yield can be reached depending on the nature of organic compouds (aliphatic or aromatic compounds). Bacterial (FC) removal is as high as 6 log units.

Matsumoto (2005) has developed a process for electrolytic treatment of photographic wastewater for COD removal using a conductive diamond electrode as an anode.
The pH of the effluent is maintained at a pH 4Ø The efficiency of EO on photographic wastewater is improved while diluting the gas to be generated /emitted from the waste photographic processing solution with air or oxidising gas-mixed air. Ozone may be used as the oxidising gas.
COD components contained in photographic wastewater can be suppressed to satisfy ambient environmental quality standard.

A method and apparatus for treating wastewater containing organic matter and nitrogen compounds has been developed by Tabata et al. (2005). The wastewater treatment apparatus comprised of an electrolytic cell partitioned with an ion-exchange membrane into an anode zone and a cathode zone. Pollutants comprising organic matter and nitrogenous matter contained in raw wastewater are oxidatively decomposed by means of hypochlorous acid generated by chloride ion oxidation in the anode zone. Then, the treated-effluent is sent into the cathode zone and the residual oxidising substance is reduced due to a membrane module capable of separating chloride ions provided downstream of the cathode zone.

Coupling electro-oxidation and biological processes EO process can be advantageously combined with biological process while treating effluents containing refractory organic compounds. This system takes advantage of coupling a biodegradation (reduction of operating cost) and a physicochemical process (shorter retention time). While the effluent is previously subjected to EO process, the non-biodegradable organic pollutants are transformed into biodegradable compounds, which contribute to increasing the depurative efficiency of the subsequent biological process. When installed downstream of biological process, electrochemical combustion yields water and carbon dioxide and no further treatment is then required (Comminellis and Pulgarin 1991, 1993).

The effectiveness of such treatment has been put into evidence by Panizza et al. (2006) while treating naphthalenesulfonates in effluent from landfill sites using biological oxidation followed by electrochemical oxidation. This indicates that non biodegradable compounds have been mineralized by electrochemical oxidation. Likewise, the coupling allowed reducing energy consumption from 80 kWh/m3 (in the absence of biological treatment) to 60 kWh/m3 (in the presence of biological treatment).

Creosote as a source of PAHs Many industrial processes generate very toxic residual wastes or wastewaters that are hardly biodegradable and require a chemical or physicochemical treatment.
Among these organic pollutants, there are polycyclic aromatic hydrocarbons (PAHs), which are usually classified as priority pollutants of water due to their toxic xenobiotics and dangerous character for humans, plants, and animals (USEPA 1987; Beltran et al. 1998; Zheng et al. 2007). The presence of PAHs in water is due to different sources like pyrolysis of carbon, electrolysis with graphite electrodes (waste from aluminium industries), coke plant, creosote rubber or hydrocarbon synthesis from natural gas (Deschesne 1995; Beltran et al. 1998). In particular, creosote is one of the important sources of PAHs release in the environment.

Creosote is a distillate of coal tar (USEPA 1984) and it is an excellent fungicide and insecticide (Betts 1990). Creosote can be toxic to animal, and direct contact with creosote can lead to skin irritation and disease (Betts 1990; Becker et al. 2001). The organic constituent of creosote include PAHs (up to 85%), phenolic compounds (10%) and N-, S- and 0-heterocyclic aromatic compounds (5%) (Mueller et al. 1989; Engwall et al. 1999).

Creosote is commonly used as wood preservative. Creosote-treated wood is widely used for railway construction and poles for the transport of electricity or for telephone lines (Gouvernment of Canada 1993; Engwall et al. 1999; Becker et al. 2001; Ikarashi et al. 2006).
One concern involved in the use of creosote is the long-term release into the environment. In natural environment, creosoted wood is in contact with rainwater and moisture and water contained in the surrounding soil and may be responsible for severe pollution of ground water and surface water. Ikarashi et al. (2006) reported that creosote contaminated sites have been identified in Canada, United States, Greenland, Denmark, Sweden and the United Kingdom.
Creosote contains high quantities of polycyclic aromatic hydrocarbons (PAHs).
The removal of these compounds from water is a difficult task due to their low solubility and refractory character but it can be achieved through some treatment methods, such as chemical advanced oxidation (Trapido et al. 1995; Beltran et al. 1998; Goel et al. 2003; Flotron et al.
2005), electrochemical oxidation (Stichnothe et al. 2004, 2005; Panizza et al. 2006) or biological oxidation (Romero et al. 1998; Juhasz and Naidu 2000).

PAHs are usually classified as priority pollutants of water due to their dangerous or toxicity character for plants and animals. The United States Environmental Protection Agency (USEPA) has specified 16 main PAHs as priority pollutants because of their known toxicity, mutagenicity, and carcinogenicity to mammals and aquatic organisms (USEPA
1987; Wilson and Jones 1993; Mangas et al. 1998) (Figure 5). Main compounds in the creosote used in this study were naphthalene (NAP), phenanthrene (PHE), fluorene (FLU), pyrene (PYR) and fluoranthene (FLE). The present study focuses on PAHs in creosote solution because of their potential to contaminate both surface and ground waters.

The removal of PAHs from water is a difficult task due to their low concentration and refractory character but it can be achieved through some treatment methods, such as chemical advanced oxidation (Trapido et al. 1995; Beltran et al. 1998; Goel et al., 2003; Flotron et al., 2005), electrochemical oxidation (Stichnothe et al. 2005; Panizza et al. 2006) or biological oxidation using micro-organisms (Romero et al. 1998; Juhasz and Naidu 2000).
Advanced oxidation processes (AOPs) are often used for PAHs degradation (Trapido et al.
1995; Goel et al.
2003; Zheng et al. 2007). The aim of AOPs (including, 03/H2O2, UV/03, UV/H202, H2O2/Fe2+

etc.) is to produce the hydroxyl radical in water, a very powerful oxidant capable of oxidising a wide range of organic compounds with one or many double bonds. In spite of good oxidation of refractory organic compounds, the complexity of these methods (AOPs), high chemical consumption and relatively higher treatment cost constitutes major barriers in the field application (Panizza and Cerisola 2004; Martinez-Huitle and Ferro 2006).

Electrochemical oxidation treatment can be used as an alternate method for PAHs degradation. Electro-oxidation process opens new ways and can advantageously replace or complete already existing processes. There are two types of anodic oxidations that are indirect oxidation process and direct oxidation. The latter may be achieved through mineralization with hydroxyl radical (OH ) produced by dimensionally stable anodes (DSA) having high oxygen overvoltage, such as Sn02, Pb02 and IrO2 (Comninellis and Pulgarin 1991;
Comninellis 1994;
Panizza et al. 2000).

SUMMARY OF THE INVENTION

This invention relates to an electrochemical process for the degradation of toxic organic molecules in solution. This process includes the treatment of organic toxics containing solutions in an electrolytic cell having dimensionally stable anodes (DSA) with high oxygen overvoltage.
The anodes are made of titanium coated with iridium oxide (IrO2), ruthenium oxide (Ru02) or tin oxide (Sn02). The cathodes are made of stainless steel, titanium or another type of metal. The electrodes may have different geometrical forms (plane, cylindrical or circular mesh anodes).
This process can be used for degradation of different organic molecules including one-type or a mixture of polycyclic aromatic hydrocarbons (PAHs), chlorinated compounds, pesticides, endocrine disruptors, oils and greases, petroleum hydrocarbons, PCBs, PCDD/F
or other types of organic compounds. The solution is treated with a current density ranging between 3.0 to 23 mA/cm2 and for a period of time ranging between 10 to 200 min. The inter-electrode distance is adjusted between 0.5 to 2 cm. A surfactant can be added to keep the organic toxic molecules in solution, whereas a supporting electrolyte, like NaZSO4, can be added with a concentration ranging from 0.5 to 4.0 g/L to reduce the energy consumption. The temperature of the solution is preferentially maintained between 4 and 35 C. Finally, this process can be operated in batch, semi-continuous or continuous mode.

DETAILED DESCRIPTION OF THE INVENTION

In spite of the extensive bibliography which exists on aspects related to the process described, no publication is known to date which considers the method of electrolytic degradation described herein. The technique to be applied is therefore novel, and consists in a simultaneous method of electrochemical destruction of different types of organic pollutants in a single-cell process, since the oxidation of polycyclic aromatic hydrocarbons (PAHs), oils and grease (O&G), petroleum hydrocarbons (Clo-Cs0), take place at the same time on the anode electrode owing to hydroxyl radical generation on the electrode, whereas others oxidizing species can be simultaneously generated in solution, such as hypochlorous acid (HCIO), peroxodisulfuric acid (H2S208), ozone (03) and hydrogen peroxide (H202) in order to enhance organic pollutant degradation.

According to an embodiment of the invention, this electrochemical process can be used for degradation of one-type or a mixture of polycyclic aromatic hydrocarbons (PAHs) in synthetic solution or real effluent. The process described herein is effective in simultaneously oxidizing several PAHs having 2 to 6 of aromatic rings. More than 85% of PAHs degradation can be reached irrespective of the number of aromatic rings.

Another interesting characteristic of the process described herein results from its capacity of simultaneously reducing oils and greases (O&G) by direct anodic oxidation or by neutralization of charged droplets owing to the electric field induced by the potential difference, resulting in oils and grease destabilization.

Moreover, according to an embodiment of the invention, this electrochemical process is also effective in simultaneously oxidizing compounds in form of hydrocarbon chains from 10 to 50 units (Clo-Cso) contained in synthetic or real effluent. More than 80% of Clo-C5o reduction can be reached.

On the other hand, reduction in COD is about three times higher than TOC
reduction, indicating that only a small fraction of PAHs was completely oxidized into water and carbon dioxide, the majority of the pollutants being transformed into small molecules that reduce the oxygen demand in the treated-effluent.

In another configuration of the invention, this process can be used to degrade others types of toxic organic molecules, like chlorinated compounds, pesticides, endocrine disruptors, BPCs, PCDD/F or other types of organic compounds.

According to an embodiment of the invention, the surfactant cocamidopropyl hydroxysultaine (CAS) can be added to keep the organic toxic molecules in solution. In another configuration of the invention, other types of surfactants can be used in replacement of CAS.

In the cell it is advantageous to promote turbulence of the effluent.
Possibly, it should be provided using a system with recycling flow rate which would allow to overcoming the formation of organic substances on the electrode surface. Higher recycling flow rate decreases the thickness of the diffusion layer, which may results in a higher removal rate of organics. It is preferable that the raw water to be treated circulate in turbulent regime either by imposing conventional and mechanical agitation, or by forced circulation through turbulence promoters, in order to favour transportation of the electro-active species to the electrodes. Preferably, a recirculation flow rate, between 1.0 and 5.4 L/min is applied.

A direct current of voltage lower than 40 V, preferably between 1 and 20 V, is applied.
The potential applied must be contributed to increase temperature from about 20 to 25 C during electrolysis. The increase of the temperature accelerates the electrochemical decomposition of organics. However, work can be carried in the entire range of temperature in which the effluent to be treated is liquid (over 60 C in pressurized system), although economic consideration make it advisable to work at moderate temperature (up to a maximum of about 40 C) in non pressurized system.

According to an embodiment of the invention, the temperature of the solution is preferentially maintained between 4 and 35 C. Inside the cell, oxygen from the air or pure oxygen can be injected in a close loop in order to gradually saturate the liquid in oxygen and be able to further generate radical species (OH ) or oxidants (such as ozone, 03) capable of enhancing PAHs degradation. It has been already demonstrated by several authors that ozone could be formed during electrolysis of water using high oxygen overvoltage anodes (Wabner and Grambow, 1985; Tatapudi and Fenton, 1993; Foller and Tobias, 1982). A maximum for PAHs degradation efficiency was observed at 5.0 mL 02/min.

Amongst the many metals and alloys which can be used as anode, noble metallic oxides fixed on titanium metal are preferably used. According to an embodiment of the invention, the process includes the treatment of solution in an electrolytic cell having dimensionally stable anodes (DSA) with high oxygen overvoltage. The anodes are preferentially made of titanium coated with iridium oxide (Ir02), ruthenium oxide (Ru02) or tin oxide (Sn02).
The cathodes are made of stainless steel, titanium or another type of metal. The electrodes may have different geometrical forms (plane, cylindrical or circular mesh anodes). Mesh electrode or expanded electrodes are used in order to favor high transfer coefficient between electrode and effluent to be treated. The solution is treated with a current density ranging between 3.0 to 23 mA/cm2 and for a period of time ranging between 10 to 200 min. The inter-electrode distance is adjusted between 0.5 to 2 cm.

According to an embodiment of the invention, an electrolyte can be added to reduce the energy consumption. The electrolyte can be one or a mixture of Na2SO4, NaC1, KCI, MgC12, CaCl2, HCI, H2SO4, MgSO4, (NH4)2SO4, NH4C1. The concentration of the electrolyte is usually ranging between 0.5 to 4.0 g/L.

Alkaline media is not favourable for PAHs oxidation, whereas high performance of PAHs degradation can be recorded preferably between pH 4.0 and 7Ø

The method of electrolytic degradation described herein could be used as an alternative or complementary method to the conventional biological treatment used today in many sewage/wastewater treatment plants (STP). This is because the biological process suffers from a number of defects. For instance, the biological purification plant is essentially a culture of microorganisms, especially, bacteria, which feed on pollutants, oxidizing them. Since it is an ecosystem, it is not easy to maintain in a stationary state. Effectiveness of biological process depends to many environmental parameters, such as temperature, nutrients, oxygen transfer, but mainly depend of the quantity and type of pollutant contained in the input water. In order to avoid unsatisfying results of the conventional biological process in the presence of refractory organic pollutants, the method described herein can be advantageously used as pre-treatment or as tertiary treatment. While the effluent is previously subjected to the described process, the non-biodegradable organic pollutants are transformed into biodegradable compounds, which contribute to increasing the depurative efficiency of the subsequent biological process. When installed downstream of biological process, electrochemical combustion yields water and carbon dioxide and no further treatment is then required. The described process breaks the double bonds of PAHs producing smaller molecules. For instance, pyrene molecule having four aromatic rings is transform into furanone compounds which are less toxic than the initial pyrene compound.
Indeed, the described process is able to efficiently reduce more than 90% of the toxicity of PAH-containing effluent, based on a biotest battery using Microtox and Daphnia test.

Finally, this electrochemical process can be operated in batch, semi-continuous or continuous mode.

Methodology Creosote and PAHs solubilization Commercially-available creosote used in this study was provided by Stella-Jones Inc.
(Montreal, QC, Canada). It was comprised of 50% (v/v) of creosote and 50% of petroleum hydrocarbons. The creosote effluent was prepared in a 100 mL glass-tank containing 10 to 50 g of oily-creosote in which 10 to 50 g of an amphoteric surfactant, CAS
(Cocamidopropyl Hydroxysultame, Chemron, Ohio, USA) was added. Conditioning was carried out at a speed of 750 rpm for a period of time of 24 h. At the end of the conditioning stage, the suspension was transferred into a 20 L polypropylene tank containing 10 L of distilled water (final concentration = 1.0 to 5.0 g creosote/L). The resulting suspension constituted the synthetic creosote-oily solution (COS), which was then subjected to settling for at least 24 h in order to separate the insoluble and suspended solids before electrochemical treatment.

Electrochemical treatment using plate electrodes Electrochemical degradation of PAHs in COS was carried out in a batch square electrolytic cell made of acrylic material with a dimension of 12 cm (width) x 12 cm (length) x 19 cm (depth) (Figure 1). The electrode sets (anode and cathode) consisted of ten parallel pieces metal with an inter-electrode distance of 1 cm. Five anodes and five cathodes alternated in the electrode pack. The electrodes were placed in stable position and submerged in COS. The anodes are presented in the form of expanded titanium (Ti) covered with ruthenium oxide (RuO2), each one having a solid surface area of 65 cm2 and a void area of 45 cm2. The cathodes are plate stainless steel (SS, 316L) and having a surface area of 110 cm2 (10 cm width x 11 cm height).
The electrodes were placed 2 cm from the bottom of the cell. Mixing in the cell was achieved by a Teflon-covered stirring bar installed between the set of electrodes and the bottom of the cell.
For all tests, a working volume of 1.5 L of COS was used. The anodes and cathodes were connected respectively to the positive and negative outlets of a DC power supply Xantrex XFR
40-70 (Aca Tmetrix, Mississauga, Canada) with a maximum current rating of 70 A
at an open circuit potential of 40 V. Current was held constant for each run. Between two tests, electrolytic cell (including the electrodes) were rubbed with a sponge and rinsed with tap water, and then soaked with 5% (v/v) nitric acid solution for 15 min.

12cm 12eM

~
19cm r~ 1 FIG. 1. Configuration of electrolytic cells using plate electrodes: (anodes:
Ti/RuO2;
cathodes: stainless steel) The first set of electrodegradation experiments consisted to test successively different operating parameters such as, current densities (3.08 to 12.3 mA/cm2), retention times (20 to 180 min), initial pH (2.0 to 9.0), initial PAHs concentration (240 to 540 mg/L), concentration of electrolyte (500 to 4,000 mg Na2SO4/L) and temperature (4 to 35 C) in view of determining the optimal conditions (reduce cost and increase effectiveness) for treating COS.
The pH was adjusted using sulphuric acid (H2SO4, 5 mol/L) or sodium hydroxide (NaOH, 2 mol/L). Sodium sulphate, sodium hydroxide and sulphuric acid were analytical grade reagent and supplied by Fisher Scientific. During these assays, only the residual PAHs concentrations were measured to evaluate the performance of the experimental unit in oxidizing these refractory organic compounds. Once the appropriate values of these parameters were determined, the optimal conditions were repeated in triplicate to verify the effectiveness and the reproducibility of the electro-oxidation process. In addition to residual PAHs analyzed during the second set of experiments, dissolved organic carbon (DOC), total organic carbon (TOC), oil and grease (O&G) and petroleum hydrocarbons (Cio-C50) were simultaneously measured. Likewise, biotests (Microtox and Daphnia tests) were carried out to have information about the toxicity of the initial and treated solution under optimum experimental conditions.

Electrochemical treatment using concentrical and circular electrodes Three cylindrical electrolysis cells (C1, C2 and C3), each having 2 L of capacity, were operated. The cells were made of PVC material with a dimension of 15 cm (height) x 14 cm (diameter) and all electrodes were presented in the form of expanded metal.
The electrolytic Cl was comprised of two concentrical electrodes (Figure 2). The cylindrical anode electrode (14 cm height x 10 cm diameter x 0.1 cm thick) was titanium coated with iridium oxide (Ti/IrO2) having a solid surface area of 270 em2 and a void area of 170 cm2. The cylindrical cathode electrode (14 cm height x 12 cm diameter x 0.1 cm thick) was made of titanium (Ti) having a solid surface area of 325 cm2 and a void area of 202 cm2. A perforated cylindrical weir (2 mm diameter of holes) made of PVC material, was placed in the centre of the C1 cell and allowed uniformly distributing the effluent toward the concentrical electrodes. Likewise, the cylindrical weir allowed wastewater to remain in the cell for a certain period. The weir had a diameter of 4.0 cm and a height of 14 cm. By comparison, inside both electrolytic C2 and C3, the electrodes were circular disks (12 cm diameter) and titanium (Ti) was used as cathode with a solid surface area of 65 cm2 and a void surface area of 45 em2 (Figure 3). Circular Ti/Ir02 and Ti/Sn02 electrodes were respectively used as anode electrode in C2 and C3 cells with a solid surface area of 65 cm2 and a void surface area of 45 cm2. The inter-electrode gap was 10 mm in the three electrolytic cells. The circular electrodes were supplied by Electrolytica Inc (Amherst, NY), whereas the cylindrical electrodes were provided by Electech (Chardon, Ohio).

The assays were carried out in a closed loop, depicted schematically in Figure 4. A one litre of PVC tank (4), a centrifugal gear pump (6) and the electrolytic cell (1) (fully detailed in Figures 2 and 3) constitute the loop. The first assays were conducted in batch recirculation mode with a flow of wastewater entering the bottom of the cell. The recycle flow rate (varying from 1.8 to 7.3 L/min) was measured using a flow-meter (13). It worth noting that, the recycle flow (QR), induced by the centrifugal pump maintained the liquid phase in sufficient mixing. A needle valve (2) installed before a manometer (3) allowed controlling the hydrostatic pressure inside the cell.
The apparatus included oxygen injection (8) in the closed loop in order to favour the hydroxyl radical at anode electrode. The rate of oxygen injected was measured using a flow-meter (7). An oxygen probe (9) was connected to an oxymeter and installed in the pipe to measure dissolved oxygen concentration (8.0 to 20 mg/L) during electrolysis. The excess of oxygen was rejected out of the system by means of a venting pipe (10) fixed on the 1-L PVC reservoir.
At the start of each assay, the raw effluent was injected in the experimental unit by means of a funnel (14) installed in the pipe and connected to a peristaltic pump, which allowed adding a working volume of 4.5 L. An addition of sulphate sodium (0.5 g/L of Na2SO4) was necessary to increase the electrical conductivity. The electrochemical cells were operated under galvanostatic conditions, with current densities varying from 4.0 to 23 mA/cm2 imposed during a period of treatment ranging from 10 to 180 min. Current densities were imposed by means of a DC
power source, Xantrex XFR40-70. During the experiments the pH was monitored but not controlled. While the experimental unit operated in continue mode, valve (15) was closed, whereas valves (11) and (12) were opened. However, in the batch mode, valves (11), (12) and (15) were closed. Prior to continue mode operation, the apparatus was initially operated in batch recirculation mode until the steady sate for PAHs degradation was reached, followed by feeding the electrolytic cell with untreated and freshly creosote effluent by means of peristaltic pump. The inlet (QE) and outlet (Qs) flow rates were quite the same and ranged between 50 and 100 mL/min. It worth noting that during the continue mode operation, the centrifugal pump was closed and the recycling flow (QR) was nil.

Treated effluent Current connector 2,5 cm Perforated PVC plate --~~
anode (8 cm diameter) 14cm 0.5 m Cathode (12cm Scm ~ ~
9cm diameter) 1cm Perforated f cylindrical weir ` ~'' ~_---~r----r 2,5 cm Inlet zone = Effluent T distributor Untreated effluent .F
13 cm FIG. 2. Configuration of electrolytic cells using cylindrical electrodes: Cell-1 (anode:
Ti/IrO2; cathode: Ti) Treated effluent = =
~ ~.. -+.
Outlet zone - 6 cm ,.~
=
---Cathode 1cm . .. . _._... . ..`
Anode = (' ~ ', ~
~ 1cm =---- __ - 15cm Cathode ---- - ~ -- -1cm Anode Inlet zone - -6 cm, ~ =
Untreated effluent ~ -_ - --_ - -13 cm U05~
`n!i~~;~tt~i~
$~-FIG. 3. Configuration of electrolytic cells using circular electrodes: Cell-2 (anode:
Ti/Ir02; cathode: Ti) and Cell-3 (anode: Ti/Sn02; cathode: Ti) (14) (3) (2) (15) (9) outlet ~_ (11) Inlet _ (12) (10) I
I
(4) (~) ~ ----- ----------- (5) (7) (8) (6) (13) (1) : Electro-oxydation cell (11) : Valve (Outlet of water in continuous operation mode) (2) : Needle valve (0-5 bars) (12) : Valve (Inlet of water in continuous operation mode) (3) : Manometer (13) : Water flowmeter (4) : PVC tank (1 L) (14) : Funnel for filling in effluent the experimental unit (5) : Exhaust pipe (15) : Valve (6) : Centrifugal Pump (7) : Air and 0Z flowmeter (8) : Oxygen bottle (9) : Oxygen probe (10) : Venting pipe FIG. 4. Schematic view of the electro-oxidation cell with a recirculation loop Analytical techniques Operating parameters The pH was determined using a pH-meter (Fisher Acumet model 915) equipped with a double-junction Cole-Palmer electrode with Ag/AgCI reference cell. A
conductivity meter (Oakton Model 510) was used to determine the ionic conductivity of the solution. The temperature of treated-solution was monitored using a thermo-meter (Cole-Parmer, model Thermo Scientific Ertco).

Extraction and analysis of PAHs Analyses of PAHs were carried out after extraction and purification on a solid phase using polypropylene-cartridges (Enviro-Clean sorbents, United Chemical Technology Inc.). The Enviro-Clean polypropylene-cartridge was successively conditioned by rinsing with 10 mL of dichloromethane (99.9% ACS reagent, EMD chemicals Inc., USA), 10 mL of methanol (99.8%
reagent, Fisher Scientific, Canada) and 10 mL of distilled water.
Subsequently, 500 mL of sample (creosote-oily solution) containing 5 mL of methanol was loaded onto the cartridge where it is entirely filtered drip. PAHs retained on the polypropylene-cartridge were then eluted with 10 mL of dichloromethane. After elution, the sample was transferred into a filter containing anhydrous MgSO4 (EMD chemicals Inc., USA) in order to eliminate all traces of water, followed by evaporation of dichloromethane using a rotary evaporator (Buchi Rotavapor-R, Rico Instrument Co.). The extraction solution was diluted with dichloromethane, and a series of diluted solution (1 x 10 x 100) was prepared and analyzed. PAHs were quantified using a Perkin Elmer 500 gas chromatography coupled Mass Spectrometry (GC-MS) on a VF-5MS-FS
column (0.25 mm diameter, 30 m long and 0.25 m film thickness). A polycyclic aromatic hydrocarbons (PAHs) mixture containing 44 PAHs at a concentration of 1,000 mg/L in dichloromethane-benzene (3:1) (Supelco, Canada) was used as a standard for PAHs. The PAHs standard solution was commercially-available from Sigma Aldrich Canada Ltd (Oakville, ON, Canada). The 16 major PAHs identified in the creosote solution with the relatively largest peak area in the chromatogram are presented in Figure 5. Likewise, Table 2 indicates some physicochemical properties of these compounds.

\ \ \ \ H3 / / / / \ \ \ \

Naphthalene 2-Methyl Naphtalene Acenaphtylene Acenaphtene CioHs C12H8 C12HIo cCc9ccCct Fluorene Phenanthrene Anthracene Fluoranthene C13H,o C1aHto CiaHio Ci6Hio \ \ \ \ \ \

\ \ \ \ \ \ \ / \ \ \
Pyrene Benzo(a) anthracene Chrysene Benzo(b)fluoranthene C16H,o CaH12 C18H12 C2oH12 co Benzo(j)fluoranthene Benzo(k)fluoranthene Benzo(a) pyrene Indeno (1,2,3-c,d) pyrene C2oH12 C2oHl2 C20H12 C22Ht2 \ \ \ \ \

Dibenzo (a,h) anthracene Benzo(ghi)perylene FIG. 5. Molecular structures of PAHs identified in creosote solution Organic measurements Chemical oxygen demand (COD) determination was made by Hach COD method (HACH 1995) and a reading spectrophotometer Carry UV 50 (Varian Canada Inc.).
TOC was measured using a Shimadzu TOC 5000A analyzer (Shimadzu Scientific Instruments Inc.) equipped with an autosampler. Samples BOD determinations with required controls were made by Standard Methods (APHA 1999). The quality of the treated-solution was also measured in terms total oil and grease (O&G) and CI o-C5o petroleum hydrocarbons. O&G were determined by gravimetric method which consisted in extracting fat and grease from sample with hexane at pH
below 2.0 followed by the evaporation of the organic solvent. The concentration of petroleum hydrocarbons present in the samples was determined by comparing the total area of group of peaks of n-C10 to n-C50 with area of the standard curves obtained under similar reaction conditions.

Toxicity tests The quality of treated-solution (versus untreated solution) has been evaluated using a biotest battery to have information about its toxic effect. Microtox and Daphnia bioassay tests were applied. Microtox analysis is a standardized toxicity test using the luminescent marine, Vibrio fisheri (Software MTX6, version 6.0, Microbics Corporation) (Environnement Canada 1992; USEPA 1993). This test consisted of one control and six serial dilutions of each sample (1.5, 3.0, 6.25, 12.5, 25.0, and 50% v/v). The endpoint of Microtox test is the measurement of bioluminescence reduction. The bioluminescence emitted by V. fisheri was first measured after min of incubation (without adding any sample, control assay), after which the creosote-solution (treated or untreated-solution) was added to the bacterial suspension. The bioluminescence reduction was determined after a 5, 15 and 30 min exposure to the contaminant.
The toxicity effects were monitored as the average percentage of light emission inhibition compared to the control assay. By comparison, Daphnia bioassay test consisted in determining the lethal concentration for which at least 50% of mortality of crustacean Daphania magna is observed after 48 h exposure to the contaminant. This test consisted of one control and five serial dilutions of each sample (6.25, 12.5, 25.0, 50.0, and 100% v/v). After 48 h exposure, the survival and death organisms was counted and the toxicity effect was evaluated using a statistic calculation software (Computer Basic, Spearman Karber tests, version 2.01, Microsoft) (Environnement Canada 2000; Ministere de 1'environnement du Quebec 2000).

Economic aspect The economic study included chemical and energy consumption. The electric cost was estimated about of 0.06 US$/kWh. A unit cost of 0.30 US$/kg was used of electrolyte (Na2SO4 industrial grade). The acid used to adjust the pH of the solution before and a long the treatment was a H2SO4 solution (5 mol/L) which have a cost of 80 US$/t of concentrated acid (H2SO4 93%). The base pH was adjusted using a NaOH (2 mol/L) solution and it was about 600 US$/t.
The total cost was evaluated in terms of U.S. dollars spent par cubic meter of treated solution (US$/m3).

a~ p ~x o o N N N m 7 7 'n ~ r ~ ~ r ~
O C O C C O O O O O O O p p - ti -Q" X X X X X X X X X X X X X X ~ X
V l~ 00 O~ v'i N 00 O o0 N M O ~O p p O~ N o0 "O ('4 ,T ,--..-CC

> N
L er:

oo 00 00 M ~ ~O ~ 00 O
O O 3 M 00 O O~ =--d: d: O~ 00 ~O ~ O O o0 ~n v'~
Vl kn aL74 00 v) ~O O N O
y^~ ~ ~ p~ ~ oM0 pN N O O O O O ~ O
a, _7 (, M N M O p p p p p O O p O
N
..i G y V1 r.+
di 00 N N d' oo 00 N N 00 00 N N o0 ~O ~O
CE cJ tn I'O l- t- O O N N ~ v~ [~ l~ l~
.~

.~ .C w N N M M M M M kn V) CC
x zo Q~ L ~ O O O O O O O ~ N_ ~ N_ ^ ~ ~
++ ~~ ~" x x x x x:T x:~ x x~; x x x x rN, rN` rN\
.(,~. r00 r, r, V V V V
O ~~ v V ~ V V V V V V V V V

V O
~ ~ a z z d~ w H w r~ d a a z a a a~ > d w U U a x z~~ d x~ d d z a ~ i: z~" d d w a d w a CG U~ A A'-' A
A d ct ~ _a c Y u N a~ c3 ^
a z Z % -0 cz 0 p ~A
C 6 C ~ v NN NN N
o E-~ a z N d d w w¾ u o. aa v cn m Q~ aa Example 1: PAHs solubilization from creosote The electro-oxidation has been explored at the laboratory pilot scale, to oxidize refractory organic compounds from creosote-oily solution (COS). The COS was a synthetic solution prepared from a commercial creosote solution in the presence of an amphoteric surfactant. The main objective of the present study was to examine the feasibility of electro-oxidation process in treating COS and to determine the optimal operating conditions to efficiently oxidize PAHs.

The first set of experiments consisted to determine the best way of solubilizing PAHs from creosote using an amphoteric surfactant (Cocamidopropyl Hydroxysultaine, CAS).
Different creosote/surfactant mass ratios (1.0, 2.0, 3.0 and 5.0) have been tested by imposing either a creosote (CR) concentration of 0.5 g/L or by holding constant the surfactant concentration to 1.0 g/L during the assays. The results are summarized in Table 3. 16 PAHs were investigated in the creosote and were comprised of different number of aromatic rings (2-, 3-, 4-, 5- and 6-rings PAHs). The highest total concentration of PAHs in solution were obtained at a fixed concentration of surfactant of 1.0 g/L with solubilization of 274, 404, 471 and 538 mg/L
recorded while imposing creosote/surfactant ratios of 1.0, 2.0, 3.0, and 5.0, respectively. The total PAHs concentrations increased while increasing CR/CAS for CAS
concentration imposed of 1.0 g/L, whereas the PAHs concentration decreased with CR/CAS ratio while imposing a creosote concentration of 0.5 g/L. It can also be seen that, the total PAHs measured in solution were greatly linked to the amount of creosote concentration utilized rather than surfactant (CAS) concentration. For instance, for the lowest (1.0 w/w) and the highest (5.0 w/w) CR/CAS ratios imposed, 123 and 53.3 mg/L of total PAHs were respectively recorded using 0.5 g/L of creosote concentration. By comparison, while using a fixed concentration of 1.0 g/L of CAS, 274 and 538 mg/L of PAHs were solubilized for the same ratios of 1.0 and 5.0 imposed, respectively. The latter concentrations of PAHs were 2.2 and 10.0 times higher than the first ones. Indeed, 1.0 and 5.0 g/L of creosote were respectively required to impose the ratios 1.0 and 5.0 in the presence of 1.0 g/L of CAS. Consequently, the best performance of PAHs solubilization results more importantly from the amount of creosote concentration in the mixture creosote-surfactant.

TABLE 3. PAHs solubilization (mg/L) from creosote PAHs Creosote (0.5 g/L) Surfactant (1.0 g/L) Creosote/surfactant ratio (w/w) Creosote/surfactant ratio (w/w) 1.0 2.0 3.0 5.0 5.0 3.0 2.0 1.0 2-ring PAHs NAP 16.8 12.9 9.27 6.75 69.2 66.4 52.1 35.3 MEN 15.0 11.9 8.63 6.06 61.1 64.2 49.2 33.7 Sum 31.8 24.8 17.9 12.8 130 131 101 69.0 3-ring PAHs ACN 0.80 0.88 0.70 0.37 3.88 3.50 2.60 1.67 ACA 12.2 10.3 9.78 7.54 72.9 59.1 43.2 40.3 FLU 10.8 8.50 7.43 6.24 61.9 43.5 39.5 33.2 PHE 25.8 18.1 17.6 11.1 127 103 97.5 52.6 ANT 5.67 4.00 2.69 1.16 12.4 10.5 10.4 9.37 Sum 55.3 41.8 38.2 26.4 278 220 193 137 4-ring PAHs FLE 13.9 12.5 7.36 4.93 53.3 43.9 41.7 24.5 PYR 12.2 10.2 5.72 5.16 44.4 41.5 37.3 25.6 BAA 2.83 3.07 1.93 1.06 10.1 9.71 8.60 5.95 CHR 3.94 2.94 1.88 1.42 11.7 14.1 12.0 6.18 Sum 32.9 28.7 16.9 12.6 120 109 99.6 62.2 5-ring PAHs BJK 1.82 1.69 0.90 0.91 6.53 6.44 5.93 3.82 BAP 0.74 0.70 0.57 0.38 2.50 3.83 2.85 1.55 DAN 0.13 0.13 0.06 0.07 0.66 0.29 0.53 0.28 Sum 2.69 2.52 1.53 1.36 9.69 10.6 9.31 5.65 6-ring PAHs INP 0.03 0.03 0.10 0.07 0.26 0.52 0.24 0.16 BPR 0.09 0.04 0.04 0.05 0.51 0.38 0.38 0.18 Sum 0.12 0.07 0.14 0.12 0.77 0.90 0.62 0.34 E PAHs 123 97.9 74.7 53.3 538 471 404 274 From the Table 3, it can also be seen that 3-ring PAHs (FLU, PHE, ANT, CAN and ACA) were present in the highest concentration with the percentage of solubilization ranging from 42.7 to 51.7%, followed by 2-rings-PAHs (NAP, MEN) with the yields of solubilization varying between 24.0 to 27.7% and 4-rings PAHs (FLE, PYR, BAA and CHR) with the yields of solubilization ranging from 22.2 to 29.3%. The lowest yields of PAHs solubilization from creosote were recorded for 5-rings PAHs (BJK, BAP and DAN) and for 6-rings PAHs (INP and BPR) with the percentage ranging from 0.10 to 2.6%. Despite the maximal PAHs solubilization recorded using the ratio 5/1 (creosote/surfactant) (538 mg/L of total PAHs recorded), the ratio of 3/1 leading to 471 mg/L of PAHs was selected as an optimal ratio to reduce as much as possible the concentration of creosote while preparing COS. The COS was then subjected to electrochemical oxidation.

Several batch electrolytic tests were performed in order to determine economical and optimal conditions for PAHs degradation in COS. Majors operating conditions investigated included: (i) current density; (ii) retention time; (iii) initial pH; (iv) electrolyte concentration; and (iv) temperature.

Example 2: Effect of current density on electrochemical oxidation of PAHs Current density is one of the most important parameters that can affect organic removal.
Table 4 indicates the initial untreated COS and residual PAHs concentrations after treatment while imposing different current densities (3.08, 4.62, 6.15, 9.23 and 12.3 mA/cm2) for 180 min.
The control assay consisted only in agitating the COS in the electrolytic cell without imposing any current density. The yields of PAHs degradation were obtained by subtracting the residual PAHs concentration from the initial value recorded in COS and the resulting operation was divided by the same initial concentration of PAHs. A total PAHs concentration of 476 mg/L was measured in the initial solution, compared to 418 mg/L recorded in the control assay, which corresponded to an abatement of 13.2% of PAHs. The decrease in PAHs concentration during the control assay was probably attributed to the volatilization of the fraction of the molecular organic while agitating the solution. For instance, some compounds such as, PYR, FLE, MEN were more sensitive to the volatilization than CAN, NAP and CHR. While the current density was imposed the degradation of PAHs increased from 72 to 82%. Considering the possible volatilization of some organic compounds, the real contribution of electro-oxidation for PAHs degradation can be obtained by subtracting the yields of PAHs removal (while imposing current density) from the yields recorded without current density. Thus, in our experimental conditions, the real yields of PAHs degradation varied from 59 and 69%. The yields of PAHs degradation increased with current density unti19.23 mA/cm2 and then remained quite stable at 12.3 mA/cm2. Similar results have been recorded by Yavuz and Kaporal (2006) while studying electrochemical oxidation of phenol by using ruthenium mixed metal oxide electrode. Phenol removal of 47, 67 and 78% were obtained with current densities of 10, 15 and 20 mA/cm2, respectively for a charge loading of 269 F/m3. Using a current density of 9.23 mA/cm2, the rates of PAHs degradation (around 81 to 84%) were quite similar regardless of the number of aromatic rings (2-, 3-, 4-, 5- and 6-rings PAHs) of the compounds. Finally, the current density of 9.23 mA/cm2 was retained for the next step of the study. The power consumption was 78 kWh/m3 while the current density of 9.23 mA/cm2 was held constant for a period of treatment of 180 min.

O v'~ O 110 O~ N N m Q~ M 110 ~ O~ ^-~ v1 00 d; ~O
06 lO O~ v'i [~ O~ N O O O ,-.
N O~ M o0 kn O O O c0 o0 U
o_ M ~O M o0 O ^~ W1 N \O 00 v1 (~ O ~ 00 O~
N M o0 ~O V ~n O~ ~t N N ~ N O O N N II
p~ .-00 O O~ O~ ^ N 00 W) O O O 00 00 i-. O

~ II
a~ ^ v~ ~ o 00 - o ~ ~ t~ o0 0o r v~ ~n ~ ~o oo r-:
.~ 'r ^ O ~ _ N O O N M in N O O v Q
0 O O l- C/) V O
r.+ O
V
~"' N [~ O N N l- O~ [- 'r1 C~ \O m N 00 00 r-e~y ~p = M ~ N 00 O 6 W~ N N ',I: ~O N O ^" ~ M Cl.
4~ ~ N ~ --~ ^ O~ N ~ ~ ~ N N ~ O O O O l~ ~=.i .r ct 00 ~1 '^ 00 M O l~ ~ 00 ~ l~ 00 O M ~ 00 O~ O '~" M
N ^O O O O
y =+:+
w O
--oo M kn W) M pp =ct 3 p p ~ v Mi M vi M o40 .-~~ M ~O o0 ~ N O O O d= ~ L~~.
U
4. ~
^ I I
~ =õRy .~ O1 --~ - 00 M m N M O 00 O~ ~ --~ 02 M (0~ N ~n O ~C c+~ O ~ N kn kn ~ 00 O~ ~O N '-=O O N

O
~ cz O 'C
Ci ~
~y U

~ o N
W ~n ~3 d' ¾ W U U~'~ z W-1 J ¾ x x¾¾ p" a a aEi ~
[-~ w Z~¾¾ w ¾ w a. a U a~ w Q~ W w 04 Example 3: Effect of treatment time on electrochemical oxidation of PAHs In view of reducing the power consumption and further optimizing the electrochemical oxidation of COS, additional experiments were conducted by testing different retention times.
During these assays, the current density of 9.23 mA/cm2 was imposed. Two sets of experiments were carried out: the first one consisted to test relatively short retention times (0, 10 and 20 min), whereas the second one allowed testing long retention time periods (30, 60, 90, 150 and 180 min). The results are summarized in Table 5. During the first set of experiments a total PAHs concentration of 513 mg/L was measured in the initial solution. By comparison, 474, 364 and 299 mg/L were recorded while imposing 0, 10 and 20 min, respectively. The PAHs degradation yield increased with the retention time. However, it is surprising to see that, the initial concentration of PAHs recorded in the untreated solution was different to that measured at t= 0 min (i = 0 mA/cm2) in the electrolytic cell. In fact, before each assay, 10 L of COS was prepared in a 20 L cylindrical tank from which 1.5 L was withdrawn and transferred into the electrolytic cell. PAHs concentrations in the initial solution was measured using a sample obtained from the 20 L cylindrical tank, whereas the initial values measured at t = 0 min was obtained from a sample withdrawn in the electrolytic cell. Thus, this discrepancy can be attributed to two main factors. Firstly, the initial solution was not very homogenous, and secondly a fraction of PAHs could be deposited on the wall on the tank or on the electrode material, so that PAHs concentrations initially measured in both tanks (cylindrical tank and electrolytic cell) were different. It was the reason why, at the start of each set of experiment (before imposing the current density), a sample of COS (untreated sample) was withdraw from the cell and analyzed. During the 2"d set of experiment, a total concentration of 525 mg PAHs/L
was measured in the untreated solution. The application of electro-oxidation process allowed reducing PAHs content and the residual concentration varied from 88.5 to 143 mg/L and contributed to removal of about 74 to 83% of PAHs depending on the retention time imposed.

O\ Ql V'1 M M N N M O~ N O t l~ M
oo O d' v~ N N
CO N N ~ O\ ^ N O O O O O 01 oo p t7 ~ ~ ~ ~ "t ~ ~ l~ OM p00p M M O Vl V'1 1~0 a [- 0 ~ ON N -~ 00 O N O O O O O o0 00 N
cM tn O M v~ QNi O ~ M r-+ ~O ~ l\ _M ~
~~, ~"~ ~ ~O O ~ ~ N M =+ oo .--N -~ O O O O O~ 00 ~
K O
'~,~,~ ~0 N M O d ~O O ~D O cl ~n C~ N --~ ~n d N ~D O
~= ~ ~ M ~O O~ O O vi O ~ O T O d r, O O p C/~
-- O~ N ^ O O O O o0 N
O l~ O N M
tr, Cd ~ (~ G
y O
~+ a ~ N 0 O~ l- 00 l~ M l- ~ M ~t M .O M .p ^ ~O
O 4 4 ~
N
M M 6 N N ^~ O O O O "~ ~
l- O o0 IC 0 O', O', N N O~ 0 O', tr .-1+ O~O 6 M - V ~ N N ~ O O O
. ~.

p N N O ~n vt ~ O N O ~ O ~n O~ =--~O N Ll.
ON ~G N O O 0 n cd 1.4 .~ M
~,a +r i. [~ ~D =-= O, O, l l- t'- v) 00 O_ 00 O M ~D =-~ Q~ <y (D6 C.6 cn 00 00 M N M 0 O O O

N s~ y y kn O [~ M tn Ul oo O O 00 N ',O l- O'~ l'~ ~
Ni --~ O O 0 M r o d N N ~ ~ ~ ~t N d tn O
r~~y 0 F ~G N O ~'1 M l~ ~O l~ N ~O ~D ~ M 00 II
-+ I~ N M a O C Vl M 00 4 (-i 01 M M I.O
~y d M M [~ ~Y ~ M l~ 00 M O O
==
vz "" N
oc d' kn tti vi O~ W~ M 'O
00 O~ M ~ O O
U
6~ y U
O

x a z Z¾ a w H w rx d c~ x a z a u; d~
d Q w U U..a 'x Z...a ~ d x=-, d¾ a a ~
z a1 w n: ~
E-+ a. Z~ ¾¾ w a¾ cL. a am U aa W a) Figure 6 shows the changes in PAHs degradation yield as a function of charge loading.
Two different regions could be distinguished. When the charge loading is below 1 A.h/L, the yield of PAHs degradation increased linearly with charge loading. Beyond 1 A.h/L, the rate of PAHs degradation remained quite stable. These results are consistent with those obtained by Chen and Chen (2006) while oxidizing orange II dye molecular on titanium (Ti) recovered with boron-doped diamond (BBD) electrode. The anodic oxidation of pollutant occurs heterogeneously. First, organic pollutants must be transported toward the anode electrode surface, and then be oxidized there. The organic pollutant degradation may be subjected either to current control or mass transfer control. In fact, at the start of the electrolysis, the PAHs concentration was relatively high, and accordingly the PAHs reduction rate was subjected to current control. As the PAHs concentration was lowered to a certain level, the PAHs reduction rate was subjected to the mass transfer control (Panizza et al. 2001). In that case, only a fraction of current intensity (or charge loading) supplied was used to oxidize pollutants, while the remaining charge loading was wasted for generation of oxygen. It was the reason for which the yields of PAHs degradation remained stable in spite of high charge loading applied. Figure 6 presents also the change in energy consumption as a function of charge loading. The energy consumption varied in a linear fashion between 0.0 and 6.0 A.h/L, from 0.0 to 78 kWh/m3. Since the maximum increase in PAHs reduction rate was reached between 1.0 and 3.0 A.h/L, the energy consumption could be reduced by curtailing the charge loading at 3.0 A.h/L. Indeed, a charge loading of 3.0 A.h/L was selected (rather than 1.0 A.h/L) to further oxidize by-products resulting from PAHs oxidation and, render the treated-solution less toxic. A
charge loaded of 3.0 A.h/L corresponded to a period of treatment time of 90 min and the energy consumption was reduced to 41 kWh/m3 (rather than 78 kWh/m3) as expected.

O m a G

20 -C~ PAH degradation 20 --*-Energy consumption l - 10 Charge loading (A.h/L) FIG. 6. Effect of charge loading on the yields of PAHs degradation and on energy consumption (current density = 9.23 mA/cm2, without initial pH adjustment (pHi = 6.0), [Na2SO4] = 0 mg/L, T = 21 C) Example 4: Effect of initial pH on electrochemical oxidation of PAHs In order to know if the electrolysis cell could work well in oxidizing PAHs in a wide pH
range, the removal efficiency at four different initial pH values (2.0, 4.0, 7.0, and 9.0) was investigated. Initial pH of the solution was adjusted using sulfuric acid (H2SO4, 5 mol/L) and sodium hydroxide (NaOH, 2 mol/L). In addition, a control assay was carried out without pH
adjustment (original pH was around 6.0). During these assays, the current density was maintained at 9.23 mA/cm2 and a retention time of 90 min was imposed. The results are shown in Table 6. It was found that COS having an initial pH closed to the neutral value (pH 6.0 and 7.0) was more favorable for PAHs reduction (PAHs removal of 81 and 84% were recorded, respectively). This is consistent with the results of Yavuz and Kaporal (2006) while oxidizing phenol using ruthenium mixed metal oxide electrode. They reported that electro-oxidation without initial pH
adjustment (initial pH around 7.0) was more effective in removing phenol, compared to pH 3.0 and pH 11Ø However, one study showed that the pH effect is not significant while oxidizing orange II dye on Ti/BDD anode at a current density imposed of 200 mA/rn2 (Chen and Chen 2006). The COD reduction (nearly 100%) at pH 1.2 was comparable with that at pH 12Ø
Another study showed that the current efficiency increased from 3.0 to 13% as pH increased from 2.0 to 11.0 while oxidizing aniline on Pb02 anode (Kirk et al. 1985).
PAHs electro-oxidation rate variations with pH recorded in the present study are contradictory with those obtained by the authors mentioned above. This may be associated with the differences in properties of chemical compounds tested and the anodic electrode used.
Finally, as the highest PAHs removal yield (84%) recorded at pH 7.0 was quite similar to that measured (81 %) without pH adjustment (original pH 6.0), it was not necessary to modify the initial pH
before treatment.

TABLE 6. PAHs concentrations (mg/L) before and after treatment by imposing different initial pH values*

PAHs Initial Final solution solution without pH with pH adjustment adjustment pH 6.0 pH 2.0 pH 4.0 pH 7.0 pH 9.0 NAP 72.4 11.7 14.1 13.0 10.9 16.7 MEN 69.0 10.2 11.3 9.70 7.88 13.7 ACN 3.25 0.75 0.78 0.76 0.59 0.83 ACA 75.3 11.2 11.9 11.9 9.19 14.32 FLU 50.1 10.8 11.7 10.5 9.48 12.0 PHE 113 25.8 22.4 24.6 17.3 22.7 ANT 13.3 2.37 2.36 2.25 2.06 2.91 FLE 55.1 10.6 13.4 10.9 8.34 12.5 PYR 35.4 7.15 9.49 7.65 6.50 8.81 BAA 7.94 1.54 1.26 1.29 1.41 2.24 CHR 9.70 2.21 2.35 2.09 2.34 2.92 BJK 4.94 1.44 1.24 1.41 0.58 1.55 BAP 1.56 0.24 0.26 0.26 0.19 0.27 DAN 1.00 0.21 0.25 0.24 0.23 0.34 INP 0.30 0.04 0.06 0.05 0.04 0.07 BPR 0.30 0.06 0.08 0.05 0.06 0.06 E PAHs (mg/L) 513 96.3 103 96.7 77.1 112 Removal (%) - 80.5 78.4 80.2 83.6 75.7 * Operating conditions: current density = 9.23 mA/cmZ, treatment time = 90 min, [NaZSO4] _ O mg/L, T = 21 C.

Example 5: Effect of supporting electrolyte on electrochemical oxidation of PAHs The addition of an electrolyte in solution during electrolysis can influence the treatment since it modifies the conductivity of the solution and facilitates the passage of the electrical current. Thus, various concentrations of sodium sulfate (NaZSO4 used as electrolyte) were added to the system and changes in PAHs reduction rate were noted. The current density of 9.23 mA/cm 2 was held constant over the retention time of 90 min without initial pH adjustment.
Table 7 represents the PAHs reduction yields with increasing concentration of Na2SO4. The PAHs degradation yields (80 to 83%) were quite similar regardless of supporting electrolyte concentration imposed. There was not a significant effect of electrolyte concentration on the oxidation efficiency in the investigated range of 500 to 4,000 mg Na2SO4/L.
This is consistent with the results of Chen and Chen (2006) while oxidizing orange II dye synthetic solution. The same trend has also been recorded by Fernandes et al. (2004). However, it can be interesting to add a certain quantity of electrolyte in order to reduce the power consumption and consequently, to reduce the cost related to the electrochemical treatment. For instance, for the same oxidation efficiency of 80% recorded, the treatment cost (including only, energy and electrolyte cost) was estimated to 1.35 US$/m3 while adding 500 mg Na2SO4/L in COS, compared to 2.52 US$/m3 recorded without any addition of supporting electrolyte.

TABLE 7. PAHs concentrations (mg/L) before and after treatment using different concentrations of supporting electrolyte (Na2SO4)*

PAHs Initial Na2SO4 concentration (mg/L) 0 500 1,000 2,000 3,000 4,000 NAP 75.0 9.57 9.89 9.88 9.87 10.8 10.7 MEN 72.8 9.21 10.9 10.8 10.1 10.8 9.85 ACN 3.31 0.56 0.64 0.75 0.52 0.60 0.58 ACA 70.2 11.7 11.8 11.1 11.7 11.2 10.9 FLU 59.0 10.1 10.8 10.9 9.7 9.7 8.86 PHE 133 26.3 31.0 29.8 27.1 26.1 25.6 ANT 12.3 2.11 2.28 2.50 2.23 2.04 2.17 FLE 60.2 12.4 12.5 12.4 12.0 11.8 10.3 PYR 41.8 8.17 8.14 8.44 8.08 8.18 7.96 BAA 8.48 1.79 1.59 1.83 1.94 1.82 1.46 CHR 9.42 2.07 2.01 2.11 1.93 1.88 1.52 BJK 5.04 1.15 1.25 1.26 1.16 1.10 0.96 BAP 1.83 0.41 0.36 0.43 0.47 0.43 0.33 DAN 0.20 0.08 0.05 0.04 0.04 0.04 0.05 INP 0.99 0.21 0.29 0.19 0.25 0.28 0.15 BPR 0.61 0.08 0.09 0.08 0.07 0.06 0.07 F. PAHs (mg/L) 554 95.9 104 102 97.1 96.7 91.5 Removal (%) 80.3 80.0 80.3 81.3 81.3 83.1 Cost ($/m3) 2.52 1.32 1.34 1.39 1.66 1.90 * Operating conditions: current density = 9.23 mA/cm2, treatment time = 90 min, without initial pH
adjustment (pHi = 6.0), T = 21 C.

Example 6: Effect of initial PAHs concentration on electrochemical oxidation of PAHs The effect of initial PAHs concentration was investigated while preparing different synthetic COS by using creosote/surfactant ratios of 1.0, 2.0, 3.0, and 5.0 (w/w). The surfactant concentration was held constant at 1.0 g/L, whereas the concentration of creosote varied from 1.0 to 5.0 g/L (Table 8). Initial PAHs concentrations varied from 274 to 538 mg/L.
At the end of the treatment, residual PAHs concentrations recorded ranged between 65 to 108 mg/L. Irrespective of the initial PAHs concentration, the PAHs removal yield was quite similar with 78 to 80% of PAHs degradation. There was no effect of initial PAHs concentration on the oxidation efficiency in the investigated range 274 to 538 mg PAHs/L.

TABLE 8. PAHs concentrations (mg/L) before and after treatment at different initial PAHs concentrations*

PAHs Initial solution Final solution Creosote/surfactant ratio (w/w) Creosote/surfactant ratio (w/w) 5.0 3.0 2.0 1.0 5.0 3.0 2.0 1.0 NAP 69.2 66.4 52.1 35.3 12.7 12.0 9.10 7.90 MEN 61.1 64.2 49.2 33.7 11.7 11.0 8.38 6.10 ACN 3.88 3.50 2.60 1.67 0.83 0.76 0.72 0.41 ACA 72.9 59.1 43.2 40.3 13.1 14.1 12.4 5.92 FLU 61.9 43.5 39.5 33.2 14.2 11.0 9.51 7.83 PHE 127 103 97.5 52.6 23.4 20.6 17.5 15.6 ANT 12.4 10.5 10.4 9.37 2.92 2.45 2.04 2.16 FLE 53.3 43.9 41.7 24.5 12.6 11.7 9.69 7.93 PYR 44.4 41.5 37.3 25.6 9.71 8.74 7.54 6.33 BAA 10.1 9.71 8.60 5.95 1.96 1.86 1.73 1.71 CHR 11.7 14.1 12.0 6.18 2.43 2.63 2.08 1.51 BJK 6.53 6.44 5.93 3.82 1.67 1.22 1.55 0.93 BAP 2.50 3.83 2.85 1.55 0.58 0.73 0.66 0.30 DAN 0.26 0.29 0.24 0.16 0.05 0.06 0.04 0.03 INP 0.66 0.52 0.53 0.28 0.07 0.09 0.06 0.03 BPR 0.51 0.38 0.38 0.18 0.06 0.04 0.04 0.03 F, PAHs (mg/L) 538 471 404 274 108 98.7 83.0 64.0 Removal (%) - - - - 80.1 80.1 80.0 78.0 * Operating conditions: current density = 9.23 mA/cm2, treatment time = 90 min, without initial pH
adjustment (pHi = 6.0), [Na2SO4] = 500 mg/L, T = 21 C.

Example 7: Effect of temperature on electrochemical oxidation of PAHs The effect of the temperature on PAHs degradation was examined by controlling the temperature of the solution in a water bath. Figure 7 shows residual PAHs concentration of different number of aromatic rings (2-, 3- and 4-ring PAHs) at different temperatures (4, 21 and 35 C). These results compare the untreated-solution (initial solution maintained at the desired temperature without current imposition) with electro-oxidation of solution (treated-solution).
Firstly, considering the untreated-solution subjected but maintained only, at different temperatures, it can be seen that residual 2-ring PAHs concentrations increase slightly while increasing the temperature from 4 to 21 C. The same trend could be observed for 4-ring and 3-ring PAHs. The temperature of 21 C enhanced PAHs solubilization. However, while maintaining the temperature at 35 C, residual PAHs (2-, 3- and 4-rings) concentrations decreased compared to that recorded at 4 C or at 21 C. For instance, at 21 C, the 2-rings PAHs concentration measured was 26.0 mg/L. When the temperature increased to 35 C, the residual 2-rings PAHs concentration was lowered to 14.9 (42.9% 2-rings PAHs reduction). It is believed that, from a certain level of temperature, the heat induced a loss of a fraction of PAHs either by volatilization or by PAHs deposition on the wall of the electrolytic cell so that PAHs concentrations in solution were reduced. Considering now the effectiveness of electro-oxidation process at different temperatures imposed, it can be seen that about 50% of PAHs was oxidized at 4 C. However, the yields of PAHs removal increased to around 80% while increasing the temperature either at 21 or 35 C. The increase of the temperature accelerates the electrochemical decomposition of PAHs.

These results were consistent with several works mentioned in the literature (Sharifian and Kirk 1986; Tahar and Savall 1998; Chen and Chen 2006; Yavuz and Kaporal 2006). Since the temperature of the solution naturally (without temperature control) increased from about 20 to 25 C during electrolysis, it was not necessary to adjust the temperature to have its beneficial effect on PAHs degradation.

^ 4oC (initial solution) ^ 21 oC (initial solution) 35oC (initial solution) 4oC (final solution) B 21oC (final solution) 150 o islut'on 35 C(f a o~ ) L
=+
O
d V

2-ring PAHs 3-ring PAHs 4-ring PAHs FIG. 7. Effect of temperature on the residual PAHs concentrations (current density =
9.23 mA/cm2, treatment time = 90 min, without initial pH adjustment (pHi =
6.0), (Na2SO4] = 500 mg/L) Example 8: Effectiveness and reproducibility of electro-oxidation performance in treating COS

According to the results mentioned above, the electrolytic cell operated at current density of 9.23 mA/cm2 through 90 min of treatment in the presence of 500 mg/L but without pH and temperature adjustment gave the best performance of electro-oxidation of COS.
It was now important to determine whether the results of these tests are reproducible or not. For that, the optimal assay (determined in terms of effectiveness and cost) was repeated in triplicate to verify the effectiveness and reproducibility of electro-oxidation performance in treating COS.

Degradation of PAHs Table 9 compares the untreated and treated-solutions by electro-oxidation. An average value of total PAHs concentration of 462 5 mg/L were measured in the initial solution. It was found that PHE (77.7 0.5 mg/L), ACA (66.5 0.1 mg/L), NAP (65.3 0.3 mg/L) and MEN
(62.2 0.7 mg/L) were present in the highest concentrations (2 to 3-rings PAHs). In contrast, the compounds having 5 and 6-rings PAHs were represented in the lowest concentrations: INP
(0.79 0.00 mg/L), DAN (0.15 0.04 mg/L) and BPR (0.48 0.01 mg/L). By comparison, the application of electrochemical treatment reduced the total concentration of PAHs to an average value of 105 2 mg/L. The PAHs removal yield had a mean value of 80.1 % with a standard deviation of only 0.2, which means that it can be considered as constant with 0.3% accuracy. The compounds initially represented in the highest concentrations in untreated-solution were effectively oxidized. The residual concentrations of these PAHs were as follows: PHE (17.4 0.4 mg/L), ACA (16.9 0.5 mg/L), NAP (14.4 0.2 mg/L) and MEN (11.5 0.4 mg/L). It worth noting that these residual concentrations were obtained with a percentage of accuracy inferior to 4.0%, consequently, they can be considered as constant. It corresponded to PAHs degradation rates of 78, 75, 78 and 81 %, respectively.

TABLE 9. PAHs concentrations (mg/L) before and after treatment in optimal conditions*

PAHs Solution Removal Initial Final (%) NAP 65.3 0.3 14.4 0.2 77.9 MEN 62.2 0.7 11.5 0.4 81.4 ACN 3.18 0.02 0.81 0.04 74.6 ACA 66.5 0.1 16.9 0.5 74.6 FLU 49.9 1.0 12.8 0.3 74.4 PHE 77.7 0.5 17.4 0.4 77.6 ANT 16.5 0.1 3.28 0.05 80.1 FLE 50.8 1.0 11.4 0.1 77.5 PYR 39.4 0.6 10.4 0.2 73.5 BAA 11.4 0.4 2.20 0.07 80.8 CHR 10.2 0.4 2.38 0.05 76.6 BJK 5.12 0.13 1.01 0.00 80.2 BAP 1.94 0.05 0.40 0.01 79.2 DAN 0.15 0.04 0.01 0.00 93.3 INP 0.79 0.00 0.07 0.00 91.1 BPR 0.48 0.01 0.05 0.00 90.2 E PAHs 462 5 105 2 80.1 0.2 * Operating conditions: current density = 9.23 mA/cm2, treatment time = 90 min, without initial pH
adjustment (pHi = 6.0), [Na2SO4] = 500 mg/L, T = 21 C.

Organics removal In addition to PAHs measurements, other parameters such as O&G, Clo-C50, COD
and TOC related to the organics were also measured in the initial and treated-solution. The results are summarized in Table 10. The residual O&G and CIo-Cso concentrations recorded at the end of the treatment were 290 mg/L and 27 mg/L, respectively, compared to 940 mg O&G/L and 170 mg O&G/L measured in the initial solution. A yield of 69% of O&G removal was recorded, whereas 84% of C10-C50 was removed.

On the other hand, reduction in COD and TOC were 62% and 27%, respectively.
The residual concentration COD and TOC recorded at the end of electro-oxidation were 809 mg DCO/L and 174 mg TOC/L. By comparison, 2,102 mg/L and 237 mg/L were measured respectively in the initial solution. The relatively low yield of TOC removal (27%) compared to 62% of COD removal, indicated that only a small fraction of PAHs was completely oxidized into water and carbon dioxide, the majority of the pollutants being transformed into small molecules that reduce the oxygen demand in the treated-solution. In fact, the electrolytic cell broke the double bonds producing smaller molecules. It is worth noting that, during electrolysis, organic pollutants can be subjected to two different paths in anodic oxidation:
electrochemical conversion or electrochemical combustion (Comninellis 1992; Grimm et al. 1998;
Drogui et al.
2001; Drogui et al. 2007). Electrochemical conversion only transforms the non-biodegradable organic pollutants into biodegradable compounds, whereas electrochemical combustion yields water and carbon dioxide and no further treatment is then required. In the present study, electrochemical conversion was the predominant reaction.

TABLE 10. Concentrations of parameters related to the organics and toxicity measurements before and after treatment in optimal conditions*

Parameters Solution Removal Initial Final (%) Organics O&G (mg/L) 940 290 69.2 (Cio--Cso) (mg/L) 170 27 84.1 COD (mg/L) 2,102 809 61.5 TOC (mg/L) 237 174 26.6 Toxicity Daphnia magna test (TU) 4,762 453 90.5 Vibriofischeri test (Microtox) (TU) 1,000 200 80.0 * Operating conditions: current density = 9.23 mA/cm2, treatment time = 90 min, without initial pH
adjustment (pH; = 6.0), [NazSO4] = 500 mg/L, T= 21 C.

Toxicity reduction Microtox and Daphnia bioassay tests were carried out to estimate the toxic effect of the initial and treated solutions under optimum experimental conditions. The Microtox test used the luminescent marine bacterium (Vibrio fisheri) and the toxicity results effects were monitored as the average percentage of light emission inhibition. The Daphnia test consisted in determining the lethal concentration for which at least 50% of mortality of crustacean Daphnia magna is observed after 48 h exposure to the contaminant. The results are given in toxicity unit (TU) and are summarized in Table 10. The comparison of the results shows a reduction of the toxicity while applying electro-oxidation treatment. Thus, relatively high toxicity of 4,762 TU was measured for crustacean Daphnia and 1,000 TU was recorded for luminescent bacterium Vibrio fisheri in the initial solution. By comparison, only 453 TU and 200 TU were recorded after treatment, respectively. It corresponded to 91 % of toxicity reduction on crustacean Daphnia, whereas 80% of toxicity reduction was accomplished on luminescent bacterium V.
fisheri. In fact, the electro-oxidation process breaks the double bonds of PAHs producing smaller molecules which are less toxics. For instance, the electrolysis of pyrene-containing synthetic solution is transform into furanone compounds which are probably less toxic than the initial pyrene compound.

Example 9: Selection of electrolytic cell configuration and anode material Initial characteristics of the COE are given in Table 11. 16 PAHs were investigated in the COE and were comprised of different number of aromatic rings (2-, 3-, 4-, 5-and 6-rings PAHs).
From the Table 11, it can be seen that 3-ring PAHs (ACA, PHE, FLU, ANT, and ACN) were present in the highest concentration with a sum of 104 mg/L, followed by 4-rings PAHs (FLE, PYR, BAA and CHR) with a total concentration of 91 mg/L, and 2-rings-PAHs (NAP
and MEN) with a sum of 41 mg/L. The lowest concentration of PAHs in COE were recorded for 5-rings PAHs (BJK, BAP and DAN) and for 6-rings PAHs (INP and BPR) with total concentrations of 10.1 and 0.9 mg/L, respectively. Effectiveness of electro-oxidation process in treating COE was evaluated by measuring the residual 16 PAHs concentrations.

The primary objective of the preliminary screening tests was to verify the efficacy of PAHs degradation in COE. The assays were carried out using electrolytic cells made up of either Ti/Ir02 or Ti/Sn02 anode electrodes at current densities of 9.0 mA/cm2 and 12 mA/cm2 for 90 min. Table 12 presents initial and final conditions of each test as well as PAHs degradation rates obtained during treatment using different electrolytic reactors, C1, C2 and C3. The initial pH was around 6.0, whereas at the end of the treatment the values varied from 6.9 to 7.8. The power consumption has been evaluated between 3.09 and 9.50 kWh/m3, and the highest consumption was obtained for CI (6.14 and 9.50 kWh/m3) comprising of cylindrical electrodes. This was mainly due to higher current intensities imposed to reach the desired current densities with regard to high surface area of cylindrical anode in the C1. For instance, for the same current density of 9.0 mA/cm2 and the same nature of electrode of Ti/Ir02 imposed (comparison between C1 and C2), the current intensities required were 2.4 A and 1.2 A, respectively, whereas the average voltage was around 7.1 or 7.4 using either the C, or the C2. However, considering the energy consumption, it can be seen that the electrical energy (6.14 kWh/m3) using C, was approximately two times higher than that (3.09 kWh/m3) recorded with C2. This confirms that the parameter that influenced the energy consumption during assays using C1 and C2 is the current intensity.

TABLE 11. Characterization of the creosote oily effluent Parameters Means values and standard deviation pH 6.0 0.1 Conductivity ( S/cm) 322 8 POR (mV) 213 8 Temperature 20 I
PAHs (mg/L) Aromatic rings Naphthalene (NAP) 2 21.8 3.8 2-Methyl-naphtalene (MEN) 2 18.8 3.4 Acenaphtylene (ACN) 3 2.10 0.5 Acenaphtene (ACA) 3 43.6 17.5 Fluorene (FLU) 3 18.4 2.0 Phenanthrene (PHE) 3 28.3 5.8 Anthracene (ANT) 3 11.4 3.9 Fluoranthene (FLE) 4 42.7 19.8 Pyrene (PYR) 4 30.8 10.9 Benzo(a)anthracene (BAA) 4 9.1 3.8 Chrysene (CHR) 4 8.7 3.2 Benzo(b,j,k)fluoranthene (BJK) 5 5.8 1.9 Benzo(a)pyrene (BAP) 5 3.5 1.5 Dibenzo(a,h)anthracene (DAN) 5 0.8 0.3 Indeno(1,2,3-c,d)pyrene (INP) 6 0.3 0.2 Benzo(ghi)perylene (BPR) 6 0.6 0.3 E PAHs 247 65 TABLE 12. Treatment of creosote oily effluent using different electrolytic cells Parameters Electrolytic cells Ci C2 C3 Anodic current density (mA/cm2) 9 12 9 12 9 12 Current intensity (A) 2.4 3.2 1.2 1.6 1.2 1.6 Anode electrode Ti/IrOz Ti/IrOz Ti/IrOz Ti/IrO2 Ti/Sn02 Ti/Sn02 Cathode electrode Ti Ti Ti Ti Ti Ti Geometric form concentric concentric circular circular circular circular Recycling rate (L/min) 3.6 3.6 3.6 3.6 3.6 3.6 Treatment time (min) 90 90 90 90 90 90 Average voltage (V) 7.4 9.5 7.1 9.7 9.8 10.5 Initial pH 6.0 6.0 6.0 6.0 6.0 6.0 Final pH 6.9 7.1 7.8 7.3 7.3 7.5 Energy cons. (kWh/m3) 6.14 9.50 3.09 5.54 4.33 6.00 Energy cost ($/m3) 0.37 0.57 0.19 0.33 0.26 0.36 Electrolyte cost ($/m3) 0.15 0.15 0.14 0.14 0.14 0.14 Total cost ($/m3) 0.52 0.72 0.33 0.48 0.40 0.50 E PAHs (before treatment) 146 140 146 146 155 155 E PAHs (after treatment) 43.0 33.4 27.9 28.0 44.5 32.2 Removal (%) 67.3 73.5 79.8 78.0 74.8 82.4 The efficacy of the electro-oxidation process in terms of PAHs removal from COE using different electrolytic cells was in the following order: C3 (75 to 82%) > C2 (78 and 80%) > C1 (67 to 74%). In fact, the electrolytic cells (C2 and C3) including circular electrodes were more effective than the other one comprised of cylindrical electrodes. Considering both electrolytic cells (C1 and C2) for which the same material anode electrode (i.e., Ti/IrO2) was used, it can be seen that the PAHs removal yields (80 and 78%, respectively) using the C2 were better than those recorded (67 and 74%, respectively) using the C, while imposing respectively 9.0 and 12 mA/cm2 of current densities. This can be attributed to the different hydrodynamic conditions (or mass transfer) imposed inside the cells. It is well-known that, hydrodynamic conditions insides the reactors are greatly linked to the cell configuration or cell design. Indeed, in the direct anodic oxidation, the oxidation of pollutants occurs heterogeneously.
Pollutant must be transported to the electrode surface first, and then be oxidized there owing to hydroxyl radical formation (OH ) (Grimm et al. 1998; Drogui et al. 2001; Martinez-Huitle and Ferro 2006). In the C2, the liquid arrived rapidly and directly on the anode material and passed through the cathode material, followed by the circulation through a second anode and cathode electrodes. By comparison, in the C1 comprising of cylindrical electrodes, the liquid firstly arrived in the centre of the cell inside a perforated cylindrical weir, before being distributing gradually and successively toward the anode and cathode electrodes. From the hydrodynamic descriptions (mentioned above), it believed that, the mass transfer between electrode and electrolyte was better inside the C2, resulting in an increase in PAHs oxidation rates by comparison to the CI. On the other hand, in view of putting into evidence the influence of anode electrode material on PAHs removal from COE, additional experiments were conducted by using Ti/SnO2 circular electrodes (C3). The hydrodynamic conditions and the configuration of the C2 and C3 were the same; all parameters were kept constant with the exception of the anode material. For the relatively high current density of 12 mA/cm2 imposed, the highest yield of PAHs degradation (82.4%) was recorded using Ti/Sn02 anode electrode installed in the C3 in comparison to 78%
PAHs removal obtained with Ti/IrO2 anode using the C2 for the same current density imposed.
As reported by Comninellis and Nerini (1995), Comninellis (1992) and Feng et al. (2003), tin oxide is one of the noble metal oxides having a better performance for organic compounds degradation in comparison to traditional electrodes (Pt, IrOZ and Ru02). This is attributed to the highly crystalline nature of tin oxide, which catalyzes the reaction of electrochemical oxidation (Comninellis 1992). Finally, the C3 including circular Ti/Sn02 anode was selected for the next experiments.

Example 10: Influence of applied current density on PAHs degradation using Ti/Sn02 circular mesh anode In order to determine economical and better conditions for PAHs degradation in COE, several batch electro-oxidation assays were performed using the C3 containing circular Ti/SnO2 anode electrode. Majors operating conditions such as current density, retention time, recycling flow rate and oxygen flow rate in the close loop were investigated.

One of the main factors affecting the electrochemical oxidation efficiency is the current density. Current densities were obtained by dividing each current by the corresponding total anode area. The effect of current density on PAHs degradation is shown in Table 13. This table indicates the initial untreated COE and residual PAHs concentrations after treatment while imposing different current densities (4.0, 9.0, 12, 15 and 23 mA/cm2) for 90 min at a recycling flow rate of 3.6 L/min. The residual PAHs concentrations recorded at the end of the treatment varied from 52 to 26 mg/L compared to 155 mg/L measured in untreated COE. PAHs degradation increased with current density in the range of 4.0 to 15 mA/cm2.
The largest PAHs oxidation was observed at 15.0 mA/cm2. However, when a current intensity of 23 mA/cm2 was imposed, the PAHs removal slightly decreased. Indeed, the increase of current intensity above 15.0 mA/cm2 further induced parasitic reactions such as water reduction, leading to high amount of oxygen bubbles (02) formation, which disturbed PAHs oxidation on anode electrodes.

TABLE 13. PAHs concentration (mg/L) before and after treatment using experimental C3 (Ti/SnO2) operated at different current densities*

PAHs Control Current density (mA/cmz) 4.0 9.0 12 15 23 NAP 17.7 3.65 3.99 2.35 1.73 2.34 MEN 14.3 3.18 2.05 2.02 1.73 1.62 CAN 1.46 0.38 0.25 0.21 0.17 0.17 ACA 19.8 7.72 5.92 4.91 3.91 4.12 FLU 16.6 6.34 4.80 4.14 3.34 3.35 PHE 35.6 13.5 10.1 8.28 6.74 7.63 ANT 6.76 1.71 1.36 1.05 0.78 0.78 FLE 14.9 6.13 5.48 3.85 3.59 3.33 PYR 15.8 5.02 4.53 3.01 2.43 2.43 BAA 4.16 1.33 1.25 0.80 0.46 0.60 CHR 4.35 1.31 1.24 0.77 0.43 0.59 BJK 3.27 0.88 0.92 0.48 0.28 0.35 BAP 1.78 0.36 0.33 0.22 0.11 0.14 DAN 0.13 0.03 0.03 0.02 0.01 0.01 INP 0.50 0.14 0.12 0.07 0.06 0.05 BPR 0.31 0.09 0.08 0.05 0.04 0.03 E PAHs (mg/L) 155 51.7 42.4 32.2 25.8 27.5 Removal (%) - 70.5 74.8 82.4 86.9 86.2 * Operating conditions: treatment time = 90 min, recycling rate = 3.6 L/min.

Example 11: Influence of reaction time on PAHs degradation using Ti/SnOZ
circular mesh anode Figures 8, 9 and 10 show the results of electrolysis of COE for various retention times (10 to 180 min). It can be observed that the pH of COE first increased and then remained quite stable around pH 6.8 (compared to the original value of 5.8) until the end of experiment. These changes can be justified in terms of anodic and cathodic processes that develop in the cell. On the cathode electrode, the main reaction is the water reduction which generates hydroxyl ions and induces an increase of the pH. On the anode electrode several reactions take place simultaneously. The main reaction is the oxidation of organic matter. Generally, the first stages in electro-oxidation processes are the formation of carboxylic acid in addition to proton formation owing to water oxidation. These acidic compounds compensate the cathodic hydroxyl ion generation rate. The cell potential decreases slightly during the electrolysis and then remains constant (around 10.5 V). This fact could be explained in terms of the increase of the ionic conductivity due to water oxidation and reduction reactions that generate ions in solution. The behavior of electrochemical oxidation of PAHs is presented in Figure 9. PAHs removal increase to 92% with the reaction time elapsed 180 min. From Figure 10, it can be seen that the decomposition of PAHs followed first order kinetics. Therefore, the reaction rate constant "k"
could be calculated from the slope value of the plot (t) versus -Ln(C/Co) of equation (8).

- Ln ~ = k.t (8) Where "Co" is the initial concentration of PAHs, "C" the concentration of PAHs at time t, "t" the reaction time, and "k" is the first order reaction rate constants (f'). As shows in Figure 10, the first order decomposition reaction rate constant of PAHs by the electrochemical oxidation was 0.015 miri l. It is interesting to compare the constant rate of PAHs degradation in COE with those obtained under different experimental conditions. The constant rate of organic degradation has been determined by Kim et al (2003) while studying electrochemical oxidation of polyvinyl alcohol (PVA) using titanium coated with ruthenium oxide (Ti/Ru02). The constant rate decreased from 0.0162 miri I to 0.0021 miri 1 while increasing initial PVA
concentration from 33 to 2,400 mg/L. The smaller the initial PVA concentration, the faster it could be removed from solution by anodic oxidation. It can be seen that, the kinetic rate constant determined in the present study (0.015 miri ) was quite similar to that measured (0.0162 min") by Kim et al.
(2003) while imposing the lowest concentration of 33 mg/L of PVA, although the experimental conditions were different. For instance, in the present study a current density of 15 mA/cm2 was imposed, whereas Kim et al. (2003) imposed a current density of 1.34 mA/cm2, which was times lower. For the same kinetic constant rate, high current density was required in treating CEO probably due to the fact that PAHs in COE was more difficult to oxidize than PVA.

i.+

"C'-Cetl potential -4HpH

Time (min) FIG. 8. Variation of cell potential and pH with the reaction time using the electrochemical C3 during the recycling batch tests (operating conditions:
current density: 15 mA/cm2, recycling rate: 3.6 L/min) 1.0 ._........._..____..... _.... _...... ...._.... 300 0.8 o ..
~ 200 0.6 u ~ C
0 Normalized concentration 150 -O-Residual PAHs concentration d 0.4 I - w L y z 0.2 0.0 0 Time (min) FIG. 9. Variation of residual PAHs and yields of PAHs degradation with the reaction time using the electrochemical C3 during the recycling batch tests (operating conditions: current density: 15 mA/cm2, recycling rate: 3.6 L/min) 3.0 ~ ~ _.. _._....._._..____.
2.5 = 3 2.0 y = 0.0151x V = R2 = 0.9614 V 1.5 =
1.0 0.5 .=0.0 Time (min) FIG. 10. First-order relationship of PAHs degradation by electrochemical oxidation using the C3 during the recycling batch tests (operating conditions: current density: 15 mA/cm2; recycling rate: 3.6 L/min) Example 12: Influence of recycling flow rate on PAHs degradation using Ti/Sn02 circular mesh anode Due to Joule effect, the temperature of the liquid can increase dramatically due to low flow rates in the cell and excessive electricity consumption. Recirculating waste could be absolutely necessary for efficient treatment. Experiments were conducted at constant current density (15 mA/cm2) for different recycling flow rates (1.8, 2.7, 3.6, 5.4 and 7.3 L/min) during a period of treatment of 90 min. Degradation efficiency increased slightly (from 81 to 85%) as recycling flow rate increased from 1.8 to 5.4 L/min, as shown in Table 14. A
maximum for PAHs degradation of 85% was observed at 5.4 L/min. Higher recycling flow rate decreases the thickness of the diffusion layer, which may results in a higher removal rate.
These results can be compared to those obtained by Nagata et al. (2006) while treating different effluents containing endocrine disrupting chemicals (17(3-estradiol, biphenol, pentachlorophenol, dichlorophenol, etc.) using electro-oxidation process with a three-dimensional electrode system. They observed that removal efficiency increased from 60 to 90% as the recycling flow rate increased from 0.1 to 0.8 L/min. However, in our experiment conditions, while increasing the recirculation rate to 7.3 L/min, degradation efficiency decreased to 81%. It is worth noting that an increase in the recirculation rate is accompanied by higher velocity in the cell and shorter residence times. For instance, a linear velocity of 0.71 cm/s was imposed for 7.4 L/min compared to 0.55 cm/s measured for 5.4 L/min. It is believed that from a certain level of the linear velocity imposed, the fluid did not sufficiently remain inside the cell, so that the degradation efficiently decreased.
Thus, a recycling flow rate of 3.6 L/min was selected for the next step of the experiments, as PAHs degradation efficiency was quite similar to that at 5.4 L/min.

TABLE 14. PAHs concentration (mg/L) before and after treatment using experimental C3 (Ti/SnO2) operated at different recycling flow rates*

PAHs Control Recycling rates (L/min) 1.8 2.7 3.6 5.4 7.3 NAP 26.7 5.36 5.27 4.28 4.02 3.89 MEN 22.8 4.36 3.82 3.65 3.54 4.43 CAN 2.30 0.36 0.34 0.32 0.30 0.39 ACA 63.6 14.3 12.9 12.3 11.5 14.9 FLU 18.7 4.21 4.17 4.02 3.73 3.75 PHE 20.4 2.30 1.88 1.75 1.65 2.18 ANT 10.4 2.30 1.88 1.87 1.65 2.18 FLE 59.5 10.7 8.63 8.29 8.03 10.0 PYR 35.9 8.25 6.75 6.33 6.25 7.82 BAA 9.69 1.73 1.41 1.38 1.36 1.64 CHR 9.59 1.65 1.36 1.31 1.29 1.57 BJK 5.89 1.02 0.87 0.84 0.80 1.00 BAP 3.10 0.50 0.44 0.42 0.41 0.51 DAN 0.25 0.03 0.03 0.03 0.02 0.03 INP 0.70 0.15 0.14 0.14 0.15 0.23 BPR 0.50 0.12 0.11 0.10 0.09 0.12 E PAHs (mg/L) 290 57.3 50.0 47.0 44.8 54.7 Removal (%) 81.2 83.5 84.3 85.0 81.2 * Operating conditions: current density = 15 mA/cmZ, treatment time = 90 min.

Example 13: Influence of injection of oxygen in a close loop on PAHs degradation using Ti/Sn02 circular mesh anode It is worth underling that, the results discussed above were obtained without any oxygen injection in the close loop. Then, some experiments have been carried out for different oxygen flow rates (5, 10 and 20 mL 02/min) injected in the close loop and compared with a control assay without 02 injection. The interest of continuously injecting oxygen in the system was to gradually saturate the liquid in oxygen and be able to further generate radical species (OH ) or oxidants (such as ozone, 03) capable of enhancing PAHs degradation. It has been already demonstrated by several authors that ozone could be formed during electrolysis of water using high oxygen overvoltage anodes (Foller and Tobias 1982; Wabner and Grambow 1985; Tatapudi and Fenton 1993). The results are presented in Table 15. The initial PAHs concentration measured in the untreated COE was 264 mg/L. While injecting oxygen in the close loop, residual PAHs concentrations varied from 31.2 to 52.9 mg/L. By comparison, a residual PAHs concentration of 40.5 mg/L was recorded during the assay without Oz injection (control assay). A
maximum for PAHs degradation efficiency (88%) was observed at 5 mL 02/min.
While the oxygen flow rate increased to 10 mL/min, no significant effect was observed by comparison with the assay carried out without oxygen injection (83% of PAHs was removed).
However, for 20 mL 02/min imposed, a negative effect was recorded, PAHs degradation efficiency decreased to 79%. This can be due to the fact that, high oxygen flow rates may favor hydrophobic conditions inside the cell, so that the reaction at the electrodes were hampered or disturbed. As this operating parameter had moderately significant effect, oxygen injection in the close loop was not pursued.

Finally, the best operating conditions retained for PAHs degradation in COE
were as followed: the utilization of the C3 containing circular electrode comprised of Ti/Sn02 anode operated at a current density of 15 mA/cm2 through 90 min of treatment with a recycling rate of 3.6 L/min in the presence of 500 mg Na2SO4/L (used as electrolyte support) but without 02 injection the close loop.

TABLE 15. PAHs concentration (mg/L) before and after treatment using experimental C3 (Ti/SnO2) operated at different oxygen flow rates*

PAHs Raw Control Oxygen flow rates (mL/min) effluent (without 02 5 10 20 injection) NAP 18.9 4.59 5.87 7.57 8.53 MEN 18.0 2.41 2.35 3.14 3.83 CAN 2.41 0.36 0.36 0.45 0.54 ACA 48.8 6.39 5.07 7.43 8.71 FLU 16.7 3.34 3.24 3.01 3.79 PHE 23.8 2.14 1.87 2.12 2.67 ANT 13.8 2.14 1.17 2.12 2.67 FLE 53.0 7.65 4.77 7.11 8.47 PYR 35.4 5.37 3.38 5.09 6.20 BAA 10.3 1.88 1.00 1.75 2.25 CHR 10.2 1.82 0.96 1.67 2.14 BJK 6.78 1.36 0.63 1.24 1.88 BAP 3.45 0.71 0.32 0.65 0.90 DAN 0.60 0.06 0.06 0.02 0.06 INP 0.84 0.17 0.07 0.18 0.21 BPR 0.63 0.11 0.05 0.12 0.10 E PAHs (mg/L) 264 40.5 31.2 43.7 52.9 Removal (%) 83.5 88.2 82.8 78.7 * Operating conditions: current density = 15 mA/cm2, treatment time = 90 min, recycling rate =
3.6 L/min.

Example 14: Efficacy and reproducibility of batch electro-oxidation treatment for PAHs degradation using Ti/Sn02 circular mesh anode It was now important to determine whether the results of these tests are reproducible or not. For that, the optimal assay (determined in terms of effectiveness and cost) was repeated in triplicate to verify the effectiveness and reproducibility of electro-oxidation performance in treating COE.

The Table 16 compares the untreated and treated-effluents by electro-oxidation. An average value of total PAHs concentration of 292 24 mg/L was measured in untreated effluent.
It was found that ACA (59.5 5.1 mg/L), FLE (55.0 3.1 mg/L), PYR (38.3 2.2 mg/L) and PHE (24.5 3.6 mg/L) were present in the highest concentrations (3 to 4-rings PAHs). In contrast, the compounds having 5 and 6-rings PAHs were represented in the lowest concentrations: INP (0.42 0.21 mg/L), DAN (0.96 0.28 mg/L) and BPR (0.71 0.22 mg/L).
By comparison, the application of electrochemical oxidation reduced the total concentration of PAHs to an average value of 50.5 4.3 mg/L. The yield of PAHs removal had a mean value of 81.6% with a standard deviation of 2.2, which means that it can be considered as constant with 4.3% accuracy. The compounds initially represented in the highest concentrations in untreated-effluent were effectively oxidized. The residual concentrations of these PAHs were as following:
ACA (9.10 0.39 mg/L), FLE (8.94 0.70 mg/L), PYR (6.78 0.52 mg/L) and PHE
(3.78 0.42 mg/L). It worth noting that these residual concentrations were obtained with a percentage of accuracy inferior to 4.0%, consequently, they can be considered as constant. It corresponded to PAHs degradation rates of 85, 84, 82 and 84%, respectively.

TABLE 16. PAHs concentration before and after treatment using experimental C3 (Ti/SnO2) and the optimal conditions*

PAHs Effluent Degradation (%) Untreated Treated NAP 23.5 0.8 4.86 0.20 79.3 0.4 MEN 20.5:L 0.5 3.26t0.12 84.0f0.8 CAN 2.46 f 0.18 0.39 t 0.03 84.3f 0.5 ACA 59.5 5.1 9.10f0.39 84.7t0.9 FLU 19.1 t 1.1 3.90 0.51 79.6 3.4 PHE 24.5 f 3.6 3.78 0.42 84.5 0.6 ANT 14.3t 1.7 2.89t0.74 79.8f4.5 FLE 55.0 f 3.1 8.94 0.70 83.7 t 1.4 PYR 38.3t2.2 6.78f0.52 82.2 2.3 BAA 11.4f 1.7 2.12 0.14 81.1t3.3 CHR 10.8f1.1 2.11f0.31 80.5 1.9 BJK 6.99 0.95 1.28f0.10 81.4 3.8 BAP 4.13 f 1.23 0.83 t 0.11 79.2 3.2 DAN 0.96f0.28 0.18t0.03 80.5f2.7 INP 0.42f0.21 0.07t0.02 83.0t2.9 BPR 0.71f0.22 0.16t0.05 77.7f0.7 E PAHs (mg/L) 293 t 24 50.5 f 4.3 -Removal (%) - 81.6 2.2 -* Operating conditions: current density = 15 mA/cm2, treatment time = 90 min, recycling rate =
3.6 L/min, without oxygen injection.

Example 15: Combining successively batch and continuous electro-oxidation treatment for PAHs degradation using Ti/SnO2 circular mesh anode Three sets of experiments were performed to evaluate the performance of the electro-oxidation process while combining successively batch and continuous mode operations. During these assays, a constant current density of 15 mA/cm2 was imposed for various inlet flow rates (50, 75 and 100 mL/min). The experimental conditions are summarized in Table 17. For the first set of experiments, the electrochemical system was previously maintained in the recirculating batch test (run A, 3.6 L/min of recycling flow rate) for 90 min, followed by the continuous mode operation (runs B to F) by imposing a constant inlet flow rate at 50 mL/min, which corresponded to 90 min of HRT. By comparison, during the second set of experiment (runs H
to K) 60 min of HRT was imposed in continuous mode operation by imposing a constant inlet flow of 75 mL/min, whereas the system was previously maintained in the recirculating batch test (run G, 3.6 L/min of recycling flow rate) for 90 min. Similarly to the 1 St and 2nd set of experiments, a recirculating batch test (run L) was carried out prior to continuous mode operation (runs M to 0) during the third set of experiment where 45 min of retention time (100 mL/min of inlet flow rate). The interest of imposing recirculating batch tests (Runs A, G and L) was to maintain initially a steady state inside the cell prior to start the continuous run tests. The results are presented in Table 17. This table compares sum of PAHs concentration measured in the inlet solution versus those recorded in the outlet solution. As expected, the best performance of the electrolytic C3 operated in continuous mode was obtained while a HRT of 90 min was imposed.
Residual PAHs concentration varied from 19.1 to 34.4 mg/L compared to 150 mg/L
of PAHs continuously injected inside the electrochemical system. By comparison, while decreasing HRT
(60 or 45 min), residual PAHs concentration increased rapidly and residual concentrations up to 80 and 90 mg/L could be recorded in the outlet solution (compared to 176 mg/L
injected in the system). Figure 11 represents the change in PAHs degradation with reaction time for various HRT. The values reported correspond to the values obtained after a period of time equal at least to three HRT (i.e. when the initial effluent electrolyzed in the recirculating batch test was completely replace by freshly effluent). The percentage of PAHs oxidized remained in a steady state (around 85%) for a long period of time (from 300 to 1,200 min), then slightly decreased to 79% of total PAHs removal. The slight decrease of degradation efficiency cans probably due the formation of organic substances on the electrode surface that reduce its electrode active surface.
Nagata et al. (2006) analyzed the electrode surface (Ti/Pt anode electrode) before and after the continuous electrochemical by using X-ray photoelectron spectrometry (XPS).
Before treatment, a big Pt peak, a small oxygen d and carbon were observed, translating to the fact that electrode surface was comprised a clean Pt. However, after treatment a big carbon peak was observed instead of the Pt peak, meaning that the electrode surface was covered with organic substances that were formed during the treatment of organic-containing effluent. From Figure 11, it can be seen that PAHs degradation efficiency decrease rapidly using 60 min of HRT
(with a relatively high slope). Degradation efficiency passed from 77% to 54% between 360 min and 1,080 min of treatment period. In fact, the formation of organic substances on the electrode surface increased while HRT decreased to 60 min. Otherwise, while further decreasing HRT, the percentage of PAHs degradation was low, but it remained quite stable around 50%, meaning that the process of the formation of organic substances on the electrode surface decreased owing to a relatively high linear velocity of liquid. In all case, during continuous treatment, the electrode surface can be easily recovered with organics dependently on HRT imposed. This situation may affect the treatment performance in a long term experiment. To overcome this process, the polarity inversion during the treatment could be one of the easier and feasible regeneration methods of the electrode surface.

~
~.+ u G ~ M ~t l~ 00 O o G 10 Vt V' O
=~ a ~ ~ N ~ N N M N M 00 N ~O 00 O~
bQ

a a - - - - - - - - - - r -W C v .., E o O o p 00 oV O o O O 00 o O C) O
~ M N
ZC ~ M O 'Y O~ M
Cd O O~ ~ r O + E .- .-~
y ..i ~ F
ei O

~ i : o p p p o o p o p o O o ~., n n o, o, c, rn c, ~o ~o ~ ~c o, v rr ~
4u õ ~.

~ O O a p i/'1 i!1 i!1 vl ~ p l~ l~ [~ [~ o ~i O
~
i, ++ G
p o O O
O O
O
~r ++ ^
L ~O O O O O O ~O O O O O ~O O O O

O

L H ` ~/'~ ~fl Ul ~1 V~ V1 V'1 v"~ V1 Vl ~1 v~ ~!1 Vl ~
.--.--~ ,~
- - - - - - - - .-r - r., ~ O ~ ~ tn rn N V~ V~ N V) tn v1 V1 V1 ~ ~ ~ ~ ~ ~ =~ '~' ~ ~ ~ ~
=0000 ~~.y F ~ ~ o ~ c .~ ~ ~ .~ ~ .~ ~ ~ ^ ro ~ ~ ~
~.y `~ ~ ~+ Y Y Y Y Y Y Y ~-+ Y ~+ Y
h=l d 0 0 0 0 0 0 0 0 0 0 0 0 u U
O D U U U U U U U U U U U
0 ro) k a F~ W s . Q m U ~1 W w C7 x .~ ti ~C . a ~ Z O

1.00 0.80 U
U
0.60 a 'a 0.40 -~

-4- HRT= 90 min 0.20 fHRT=60 min j f HRT= 45 min 0.00 Time (min) FIG. 11. Variation of normalized concentration with the reaction time using the electrochemical C3 during continuous mode operation at different HRT
(operating conditions: current density: 15 mA/cm2). Values reported after a period of time equal to three HRT

REFERENCES

American Public Health Association (APHA) (1999) Standards Methods for Examination of Water and Wastewaters. 20`fi Edition. American Public Health Association, Washington, D.C.

Beck, F., Kaiser, W., and Krohn, H. (2000) Boron doped diamond (BDD)-layers on titanium substrates as electrodes in applied electrochemistry. Electrochim. Acta, 45, 4691-4695.
Becker, L., Matuschek, G., Lenoir, D., and Kettrup, A. (2001) Leaching behaviour of wood treated with creosote. Chemosphere, 42, 301-308.

Beltran, F., Gonzalez, M., Rivas, F.J., and Alvarez, P. (1998) Fenton reagent advanced oxidation of polynuclear aromatic hydrocarbon in water. Water Air Soil Pollut., 105, 685-700.
Bernard, C., and Rumeau, M. (1998) Patent FR2784979.

Betts, W.D. (1990) Information about coal-tar creosote for wood. Proceedings of the International Tar Conference, Paris, France.

Bock C, and MacDougall, B. (2002) The electrochemical oxidation of organics using tungsten oxide based electrodes. Electrochim. Acta, 47(20), 3361- 3373.

Bock, C, Smith, A, and MacDougall, B. (2002) Anodic oxidation of oxalic acid using WOx based anodes. Electrochim. Acta, 48(1), 57-67.

Brillas, E, Mur, E, Sauleda, R, Sanchez, L, Peral, F, Domenech, X, and Casado, J. (1998) Aniline mineralisation by AOP's : anodic oxidation, photocatalysis, electron-Fenton and photoelectro-process. Appl. Catal. B Environ., 16, 31-42.

Canizares, P., Lobato, J., Paz, R., Rodrigo, M.A., Saez, C.J., and Rodrigo, M.A. (2005) Electrochemical oxidation of phenolic wastes with boron-doped diamond anodes.
Water Res., 39, 2687-2703.

Canizares, P., Martinez, C., Diaz, M., Garcia-Gomez, J., and Rodrigo, M.A.
(2002) Electrochemical oxidation of aqueous phenol wastes using active and nonactive electrodes. J. Electrochem. Soc., 149(8), 118-124.

Carey, J.J., Christ, J.C.S., and Lowery, S.N. (1995) Patent US5399247.

Chen, X., and Chen, G. (2006) Anodic oxidation of orange II on Ti/BDD
electrode: variable effects. Sep. Purif. Technol., 48, 45-49.

Comninellis, C. (1992) Electrochemical treatment of wastewater containing phenol. Trans. Inst.
Chem. Eng., 70(4), 219-224.

Comninellis, C. (1994) Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for wastewater treatment. Electrochimica Acta, 39(11-12), 1857-1862.
Comninellis, C., and Nerini, A. (1995) Anodic oxidation of phenol in the presence of NaC1 for wastewater treatment. J. Appl. Electrochem., 25, 23-28.

Comninellis, C., and Pulgarin, C. (1991) Anodic oxidation of phenol for wastewater treatment using Sn02 anodes. J. Appl. Electrochem., 21, 703-708.

Comninellis, C., and Pulgarin, C. (1993) Electrochemical oxidation of phenol for wastewater treatment using Sn02. J. Appl. Electrochem., 23, 108-112.

Deng, Y, and Englehardt, J.D. (2007) Electrochemical oxidation for landfill leachate treatment.
Waste Manag., 27, 380-388.

Deschenes, L. (1995) Impact de surfactants biologiques et du SDS sur la mobilisation et la biodegradation des PAHs contenus dans un sol contamine a la creosote. Ph.D.
thesis, INRS-ETE, Universite du Quebec, Sainte-Foy, QC, Canada, 184 p.

Drogui, P., Blais, J.F., and Mercier, G. (2007) Review of electrochemical technologies for environmental application. Recent Patent Eng., 1, 257-272.

Drogui, P., Bureau, M.A., Blais, J.F., and Mercier, G. (2005) Patent CA2472879.

Drogui, P., Elmaleh, S, Rumeau, M, Bernard, C, and Rambaud, A. (2001) Hydroxide peroxide production by water electrolysis: Application to disinfection. J. Appl.
Electrochem., 31, 877-882.

Engwall, M.A, Pignatello, J.J., and Grasso, D. (1999) Degradation and detoxification of the wood preservatives creosote and pentachlorophenol in water by the photo-Fenton reaction. Water Res., 33(5), 1151-1158.

Environnement Canada (1992) Essai de toxicite sur la bacterie luminescente Photobacterium phosphoreum. Report SPE 1/RM/24 SPE 1/RM/24, Ottawa, ON, Canada (in French).
Environnement Canada (2000) Methode d'essai biologique: methode de reference pour la determination de la letalite aigue d'effluents chez Daphnia magna. Report SPE1/RM/14, Ottawa, ON, Canada (in French).

Feng, C., Sugiura, N., Shimada, S., and Maekawa, T. (2003) Development of a high performance electrochemical wastewater treatment system. J. Hazard. Mater., B 103, 65-78.

Fernandes, A., Morao, M., Magrinho, M., Lopes, A., and Concalves, I. (2004) Electrochemical oxidation of C.I. acid orange 7. Dyes Pigments, 61, 287-296.

Flotron, V., Delteil, C., Pedellec, Y., and Camel, V. (2005) Removal of sorbed polycyclic aromatic hydrocarbon from soil, sludge and sediment samples using the Fenton's reagent process. Chemosphere, 59, 1427-1437.

Foller, P.C., and Tobias, C.W. (1982) The anodic evolution of ozone. J.
Electrochem. Soc., 129, 506-515.

Gandini, D, Comninellis, C., Tahar, N.B., and Savall, A. (1998) Electroddpollution: traitement electrochimique des eaux residuaires chargees en matieres organiques toxiques.
Actualite Chimique, 10, 68-73 (in French).

Goel, R.K., Flora, J.R.V., and Ferry, J. (2003) Mechanisms for naphthalene removal during electrolytic aeration. Water Res., 37, 891-901.

Gotsi, M., Kalogerakis, N., Psillakis, E., Samaras, P., and Mantzavinos, D.
(2005) Electrochemical oxidation of olive oil mill wastewaters. Water Res., 39(17), 4177-4187.
Gouvernment of Canada. 1993. Matieres residuaires impregnees de creosote. Loi Canadienne sur la protection de l'environnement, liste des substances d'interet prioritaire.
Report No. En 40-215/13-F, Ottawa, ON, Canada.

Grimm, J., Bessarabov, D., and Sanderson, R. (1998) Review of electro-assisted methods for water purification. Desalination, 115, 285-294.

HACH (1995) Hach water analysis handbook: Chemical oxygen demand, reactor digestion method 467. Hach Company, Loveland, Colorado.

Held, D., and Chauhan, S.P. (2002) Patent CA2382357.

Ikarashi, Y., Kaniwa, M., and Tsuchiya, T. (2006). Monitoring of polycyclic aromatic hydrocarbons and water-extractable phenols in creosotes and creosote-treated woods made and procurable in Japan. Chemosphere, 60(9), 1279-1287.

Juhasz, A.L., and Naidu, R. (2000) Bioremediation of high molecular weight polycyclic aromatic hydrocarbons: a review of the microbial degradation of benzo[a]pyrene. Int.
Biodeterior.
Biodegrad., 45(1-2), 57-88.

Kim, S., Kim, T., Park, C., and Shin, E.B. (2003). Electrochemical oxidation of polyvinyl alcohol using a Ru02/Ti anode. Desalination, 155, 49-57.

Kinder, R., Kuehn, R., and Richter, W. (1998) Patent DE19634208A1.

Kirk, D.W., Sharifian, H., and Foulkes, F.R. (1985) Anodic oxidation of aniline for waste water treatment. J. Appl. Electrochem., 15(2), 285-292.

Kraft, A., Stadelmann, M., and Blaschke, M.J. (2003) Anodic oxidation with doped diamond electrodes: a new advanced oxidation process. J. Hazard. Mater., 103, 247-261.

Mangas, E., Vaquero, M.T., Comellas, L., and Broto-Puig, F. (1998) Analysis and fate of aliphatic hydrocarbons, linear alkylbenzene, polychlorinated biphenyls and polycyclic aromatic hydrocarbon in sewage-amended soil. Chemosphere, 36, 61-72.

Martinez-Huitle, C.A., Ferro, S., and De Battisti, A. (2004a) Electrochemical incineration of oxalic acid: role of electrode material. Electrochem. Acta, 49, 4027-4034.
Martinez-Huitle, C.A., Ferro, S., and De Battisti, A. (2005) Electrochemical incineration of oxalic acid: reactivity and engineering parameters. J. Appl. Electrochem., 35, 1087-1093.
Martinez-Huitle, C.A., Quiroz, M.A., Comninellis, C., Ferro, S., and De Battisti, A. (2004b) Electrochemical incineration of chloranilic acid using Ti/IrO2, Pb/Pb02 and Si/BDD
electrodes. Electrochem. Acta, 50, 949-956.

Martinez-Huitle, CA, and Ferro, S. (2006) Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes. Chem. Soc. Rev., 35, 1324-1340.
Matsumoto, K. (2005) Patent JP2005185872.

Ministere de 1'environnement du Quebec (2000) Determination de la toxicite letale CL50-48h Daphnia magna. Method MA 500-D.mag 1.0, Minist6re de 1'environnement du Qu6bec, Quebec, QC, Canada (in French).

Moraes, P.B., and Bertazzoli, R. (2005) Electrodegradation of landfill leachate in a flow electrochemical reactor. Chemosphere 58(1), 41-46.

Mueller, J.G., Chapman, P.J., and Pritchard, P.H. (1989) Creosote contaminated sites-their potential for bioremediation. Environ. Sci. Technol., 23(10), 1197-1201.

Murphy, O.J., and Duncan, H.G. (1999) Patent US5972196.

Nagata, R.., Prosnansky, M., and Sakakibara, Y. (2006). Electrochemical treatment of trace endocrine disrupting chemicals with a three-dimensional electrode system. J.
Adv. Oxid.
Technol., 9(1), 97-102.

Panizza, M, and Cerisola, G. (2004) Electrochemical oxidation as a final treatment of synthetic tannery wastewater. Environ. Sci. Technol., 38, 5470-5475.

Panizza, M., Cristina, C., and Cerisola, G. (2000) Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res., 34(9), 2601-2605.

Panizza, M., Michaud, P.A., Cerisola, G., and Comninellis, C. (2001) Anodic oxidation of 2-naphthol at boron-doped diamond electrodes. J. Electroanal. Chem., 507(1-2), 206-214.
Panizza, M., Zolezzi, M., and Nicolella, C. (2006) Biological and electrochemical oxidation of naphthalenesulfonates. J. Chem. Technol. Biotechnol., 81, 225-232.

Polcaro, A.M., Palmas, S., Renoldi, F., and Mascia, M. (1999) On the performance of Ti/Sn02 and Ti/Pb02 anodes in electrochemical degradation of 2-chlorophenol for wastewater treatment. J. Appl. Electrochem., 29, 147-151.

Pulgarin, C., Adler, N., Peringer. P., and Comninellis, C. (1994) Electrochemical detoxification of a 1,4-benzoquinone solution in wastewater treatment. Water Res., 28(4), 887-893.
Rajeshwar, K., and Ibanez, J. (1997) Environmental electrochemistry -Fundamentals and applications in pollution abatement. Academic Press, San Diego, CA.

Rao, N.N., Somasekhar, K.M., Kaul, S.N., and Szpyrkowicz, L. (2001) Electrochemical oxidation of tannery wastewater. J. Chem. Technol. Biotechnol., 76, 1124-1131.

Romero, M.C., Cazau, M.C., Giorgieri, S., and Arambarri, A.M. (1998) Phenanthrene degradation by microorganisms isolated from a contaminated stream. Environ.
Pollut., 101(3), 355-359.

Sharifian, H., and Kirk, D.W. (1986) Electrochemical oxidation of phenol. J.
Electrochem. Soc., 133(5), 921-924.

Simond, 0., and Comninellis, C. (1997) Anodic oxidation of organics on Ti/IrO2 anodes using Nafiong as electrolyte. Electrochim. Acta, 42, 2013-2018.

Szpyrkowicz, C,, Juzzolino, S, Kaul, S.N,, Daniele S., and De Faveri, M.D.
(2000) Electrochemical oxidation of dying baths bearing disperse dyes. Ind. Eng.
Chem., 39, 3241-3248.

Stichnothe, H., Calmano, W., Arevalo, E., Keller, A., and Thiming, J. (2005) TBT-contaminated sediments. Treatment in a pilot scale. J. Soils Sediments, 5, 21-29.

Tabata, M., Asano, M., Nagai, M., and Sakimura, M. (2005) Patent JP2005218983.

Tahar, N.B., and Savall, A. (1998) Mechanistic aspects of phenol electrochemical degradation by oxidation on a Ti/Pb02 anode. J. Electrochem. Soc., 145(10), 3427-3434.

Tatapudi, P., and Fenton, J. (1993) Synthesis of ozone in a proton exchange membrane electrochemical reactor. J. Electrochem. Soc., 140, 3527-3530.

Trapido, M., Veressinina, Y., and Munter, R. (1995) Ozonation and advanced oxidation processes of polycyclic aromatic hydrocarbons in aqueous solutions - a kinetic study.
Environ. Technol., 16(8), 729-740.

USEPA (1984) Creosote - special review position. Document 2/3. U.S.
Environmental Protection Agency, Washington, D.C.

USEPA (1987) Quality criteria for water. EPA/440/5-86/001, U.S. Environmental Protection Agency, Washington, D.C.

USEPA (1993) Methods for measuring the acute toxicity of effluent and receiving waters to freshwater and marine organisms. EPA/600/4-90/027F, U.S. Environmental Protection Agency, Cincinnati, Ohio.

Wabner, D., and Grambow, C. (1985) Reactive intermediates during oxidation of water at lead dioxide and platinum electrodes. J. Electroanal. Chem., 195, 95-108.

Wang, A, Qu, J, Liu, H, and Ge, J. (2004) Degradation of azo dye Acid Red 14 in aqueous solution by electrokinetic and electrooxidation process. Chemosphere, 55(9), 1189-1196.

Wilson, S.C., and Jones, K.C. (1993) Bioremediation of soil contaminated with polynuclear aromatic hydrocarbon (PAH): a review. Environ. Pollut., 81, 229-249.

Yavuz, Y., and Kaporal, A.S. (2006) Electrochemical oxidation of phenol in a parallel plate reactor using ruthenium metal oxide electrode. J. Hazard. Mater., B136, 296-302.

Zheng, X.J., Blais, J.F., Mercier, G., Bergeron, M., and Drogui, P. (2007) PAHs removal from spiked municipal wastewater sewage sludge using biological, chemical and electrochemical treatments. Chemosphere, 68(6), 1143-1152.

Claims (15)

1) An electrochemical process for organic toxics degradation in solution, the process comprising the treatment of the said solution in an electrolytic cell having dimensionally stable anodes (DSA) with high oxygen overvoltage.

2) The process according to claim 1, characterized in that the anodes are made of titanium coated with iridium oxide (IrO2), ruthenium oxide (RuO2) or tin oxide (SnO2).

3) The process according to claim 1, characterized in that a surfactant is added to maintain the organic toxic molecules in solution.

4) The process according to claim 1, wherein a current density is ranging between 3.0 to 23 mA/cm2.

5) The process of claim 1, characterized in that the reaction time in the electrolytic cell is ranging between 10 to 200 min.

6) The process of claim 1, characterized in that the pH of the solution is ranging between 2.0 and 9Ø

7) The process of claim 1, characterized in that an electrolyte is added to reduce the energy consumption.

8) The process of claim 1, characterized in that the temperature of the solution is ranging between 4 and 35°C.

9) The process of claim 1, characterized in that the inter-electrode distance is ranging between 0.5 to 2 cm.

10) The process according to any one of claims 1 to 9, characterized in that the process is operated in batch, semi-continuous or continuous mode.

11) The process according to any one of claims 1 to 10, characterized in that the cathodes are made of stainless steel, titane or another type of metal.

12) The process according to any one of claims 1 to 11, characterized in that the electrodes are plane, cylindrical, circular or other geometrical forms.

13) The process according to any one of claims 1 to 12, characterized in that the toxic organic molecules includes one-type or a mixture of polycyclic aromatic hydrocarbons, organochlorides, pesticides, endocrine disruptors, BPCs, PCDD/F or other types of organic compounds.

14) The process according to any one of claims 1 to 13, characterized in that the electrolyte is one or a mixture of Na2SO4, NaCl, KCl, MgCl2, CaCl2, HCl, H2SO4, MgSO4, (NH4)2SO4, NH4Cl.

15) The process of claim 14, characterized in that the electrolyte is added in a concentration ranging between 0.5 to 4.0 g/L.