WO2009143629A1

WO2009143629A1 - Apparatus and method for decontamination of wastes comprising toxic organic molecules

Info

Publication number: WO2009143629A1
Application number: PCT/CA2009/000752
Authority: WO
Inventors: Patrick Drogui; Lan Huong Tran; Jean-François BLAIS; Guy Mercier
Original assignee: Institut National De La Recherche Scientifique (Inrs)
Priority date: 2008-05-30
Filing date: 2009-05-29
Publication date: 2009-12-03
Also published as: CA2632788A1

Abstract

Disclosed is a method for decontaminating wastes containing toxic organic molecules and for degrading them. This method comprises the use of a surfactant for forming micelles entrapping the toxic organic molecules present in the wastes. The wastes are then decontaminated by extracting an aqueous solution comprising the micelles. An electrolytic cell is then filled with the aqueous solution, the electrolytic cell comprising at least one pair of electrodes; and applying to said at least one pair of electrodes a current for a period of time suitable to electro-oxidise said toxic organic molecules. Also disclosed is a cylindrical electrolytic cell that can be used for carrying out this method. The electrolytic cell comprises cylindrical electrodes and a tubular weir allowing a uniform distribution of the electrolyte to the electrodes. The method and electrolytic cell of the invention allows the decontamination of wastes stemming from aluminum industry containing creosotes, oil and grease.

Description

APPARATUS AND METHOD FOR DECONTAMINATION OF WASTES COMPRISING

TOXIC ORGANIC MOLECULES

Field of the invention

The present invention belongs to the field of decontamination of wastes comprising toxic organic molecules. More particularly, the invention is directed to a method for decontaminating wastes comprising different toxic organic molecules including one-type or a mixture of polycyclic aromatic hydrocarbons (PAHs), oils, greases, chlorinated compounds, pesticides, endocrine disruptors, petroleum hydrocarbons, PCBs, PCDD/F or other types of organic compounds. The invention is also directed to an electrolytic cell for electro-oxidizing organic molecules contained in an electrolyte.

Background of the invention

In recent years, numerous research works have focused on electro-oxidation (EO) process owing to the appearance of emerging pollutants such as chlorinated molecules, pesticides, endocrine disruptors (EDCs), polychlorinated biphenyl molecules (PCBs), polychlorinated dibenzodioxins (PCDDs), dyes 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, textile effluents, landfill leachate, olive oil mill wastewater, municipal sewage sludge, and tannery effluent 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 (HCIO, HBrO, H₂O₂, H₂S₂Og, and others) is electrochemically generated to carry out the oxidation. Two different ways can be followed in anodic oxidation: electrochemical conversion or electrochemical combustion. 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 is generally admitted that the direct anodic oxidation is carried out using two steps:

a first reaction (equation 1) that is the anodic oxidation of water molecule leading to the formation of hydroxyl radicals (HO°) adsorbed on active sites on the electrode "M":

H₁O + 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 + nH₂O + 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 favours the combustion reaction (equation 3). The hydroxyl radicals are species capable of oxidizing numerous complex organics, non-chemically oxidizable or difficulty oxidizable. 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):

H₂O + M[HO°] → M + O₂ + 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 sulphate and chloride ions, these ions can be respectively oxidized at the anode electrodes and formed in peroxodisulfuric acid (H₂S₂Oe) and hypochlorous acid (HCIO) solutions (see equations 5 and 6). Both HCIO and H₂S₂O₈ are powerful oxidants capable of oxidizing and modifying the structure of organic molecules and leading to more oxidized and less toxic compounds.

2SO]^' + 2H⁺ → H₂S₂O₈ + 2e - (5)

Cl^' + 2H₂O → HClO + H_}0⁺ + 2e - (6)

Likewise, during electrolysis, hydroxide peroxide (Η₂O₂) can be produced from dissolved oxygen by cathodic reduction (equation 7):

O_2UM + 2H⁺ + 2e- → H₁O₂ (7)

In the 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.

Electrochemical oxidation of PAHs present in sediment has been studied by (Stichnothe et al., J. Foils Sediments, 5, 21-29, 2005). A total of sixteen PAHs have been measured before and after the electrochemical treatment. A titanium anode electrode coated with iridium oxide (Ti/lrO₂) 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.

An oxidative electrochemical method has also been developed for conditioning and stabilizing sewage sludge from municipal and paper mill industries (Canadian laid-open patent application No. 2,472,879). The treatment of sludge is carried out using a cylindrical electrolytic cell having two concentric electrodes. The anode material is made of titanium coated with ruthenium (Ti/RuO₂) whereas titanium material (Ti) is used as cathode electrode This process comprises an 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, a treatment of the acidified sludge in an electrolytic cell able to generate in situ a bactericidal oxidant (HCIO or HsSzO₈) 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-PO₄ and N-NH₄) in dewatered-sludge

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 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 In particular, creosote is one of the important sources of PAHs release in the environment

Creosote is a distillate of coal tar and it is an excellent fungicide and insecticide Creosote can be toxic to animal, and direct contact with creosote can lead to skin irritation and disease The organic constituent of creosote includes PAHs (up to 85%), phenolic compounds (10%) and N-, S- and 0-heterocyclιc aromatic compounds (5%) 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. 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. 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, (Goel et al , Water Res , 37, 891-901 , 2003), electrochemical oxidation (Stichnothe et al , J. Soil Sediments, 5, 21-29, 2005); (Panizza et al , J. Chem. Techno Biotechnol , 81 , 225-232, 2006) or biological oxidation.

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 Quality criteria for water, EPA/440/5-86/001 , U. S Environmental Protection Agency, Washington, D C. (see Figure 5)) Main compounds in the creosote used in this study were naphthalene (NAP), phenanthrene (PHE), fluoreπe (FLU), pyrene (PYR) and fluoranthene (FLE). PAHs in creosote solution have the potential to contaminate both surface and ground waters

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 SnU₂, PbO₂ and Irθ₂ (Comninellis, C. Electrochimica Acta, 39(1 1-12), 1857-1862 (1994); Panizza et al., Water Res., 34(9), 2601-2605 (2000)).

The electro-oxidation of wastes is energy consuming, and in spite of the extensive bibliography which exists on aspects related to degradation process of organic compounds, no publication is known to date which considers a method for an exhaustive and direct decontamination of wastes comprising toxic organic molecules, said process including an electrolytic degradation of organic pollutants with a significant reduction of the energy consuming.

Hence, in light of the aforementioned, there is still a need for new apparatus and method for the decontamination of polluted wastes and destruction of toxic organic molecules.

The present invention, by virtue of its design, components and steps, is able to overcome some and preferably all of the aforementioned prior art problems.

Summary of the invention

The invention is directed to a method for decontaminating wastes containing toxic organic molecules and for degrading the toxic organic molecules.

The method of the invention comprises the steps of:

a) obtaining an aqueous mixture by mixing the wastes with an effective amount of a surfactant to form micelles of surfactants, said micelles entrapping said toxic organic molecules;

b) extracting from the aqueous mixture obtained in step a), an aqueous solution comprising the micelles entrapping said toxic organic molecules; c) filling an electrolytic cell with said aqueous solution obtained in step b), the electrolytic cell comprising at least one pair of electrodes; and

d) applying to the at least one pair of electrodes a current for a period of time suitable to electro-oxidise the toxic organic molecules.

The invention is also directed to an electrolytic cell for electro-oxidizing organic molecules contained in an electrolyte, the cell comprising:

an electrolytic vessel comprising an inlet for filling the vessel with the electrolyte and an outlet for draining the electrolyte;

a tubular weir having a closed end, an open end and a tubular wall provided with a plurality of perforations, the tubular weir being installed into the vessel with its open end in connection with the inlet for receiving the electrolyte; and

a pair of cylindrical electrodes installed into the vessel for passing an electric current through the electrolyte, the pair of electrodes comprising a first perforated cylindrical electrode surrounding the tubular weir and a second cylindrical electrode surrounding the first perforated cylindrical electrode.

It is to be understood that in use, the tubular weir allows a uniform distribution of the electrolyte towards the electrodes, thereby enhancing the electro-oxidation of the organic molecules.

The invention is also directed to an electrolytic system for electro-oxidizing organic molecules comprising at least one electrolytic cell as defined above.

The invention actually lies in a simultaneous method of extraction of toxic pollutants wastes to decontaminate them by the use of a surfactant. The surfactant in aqueous solution form micelles that entrap the toxic pollutants. The solution containing micelles and the pollutants are then sent to an electrochemical cell to be destructed by electro-oxidation.

Thanks to the invention, the pollutants can be of different types and can be treated in a single-cell process. Compared to the prior art, the electro-oxidation is only applied to an aqueous solution containing the molecules to degrade and not to the whole waste containing inorganic material that is not necessary to electro-oxidize. Then, the invention may allow the use of a current of reduced voltage, saving energy. For example, the invention may allow the use of a current having a voltage lower than 4O VoItS (V)₁ preferably between 1 and 20 V.

For example, toxic polycyclic aromatic hydrocarbons (PAHs) can be treated simultaneously in the presence of oils and grease (O&G), petroleum hydrocarbons (C₁₀-C₅₀) which are not soluble in water. The micelles formed by the surfactant entrap the organic molecules and allow an electro-oxidation process of the organic molecules of all types, at the same time.

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 (H₂S₂O₈), ozone (O₃) and hydrogen peroxide (H₂O₂) in order to enhance organic pollutant degradation.

Furthermore, the potential applied may increase the temperature from about 20 to 25^CC 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 6O⁰C in pressurized system), although economic consideration make it advisable to work at moderate temperature (up to a maximum of about 40⁰C) in non pressurized system.

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 method breaks the double bonds of PAHs producing smaller molecules. For instance, pyreπe molecule having four aromatic rings is transform into furaπone 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.

The objects, advantages and other features of the present invention will be better understood upon reading of the following non-restrictive description of preferred embodiments thereof, made with reference to the accompanying drawings and following example.

Brief description of the drawings

FIGURE 1 is a schematic illustration of an electrolytic cell using a plurality of plate electrodes (anodes: TiZRuO₂; cathodes: stainless steel) (not drawn to scale).

FIGURE 2 is a schematic illustration of an electrolytic cell according to a preferred embodiment of the invention using cylindrical electrodes: Cell-1 (anode: Ti/lrCV, cathode: Ti) (not drawn to scale).

FIGURE 3 is a schematic illustration of an electrolytic cells using circular electrodes: Cell-2 (anode: Ti/lrO₂; cathode: Ti) and Cell-3 (anode: Ti/SnO_∑; cathode: Ti), (not drawn to scale).

FIGURE 4 is a schematic diagram of an electro-oxidation cell with a recirculation loop according to a preferred embodiment of the invention, (not drawn to scale).

FIGURE 5 identified as "Prior art", is a table comprising the molecular structures of PAHs identified in creosote solution.

FIGURE 6 is a diagram illustrating the effect of charge loading on the yields of PAHs degradation and on energy consumption (current density = 9.23 mA/cm², without initial pH adjustment (pH, = 6.0), [Na₂SO₄] = 0 mg/L, T = 21⁰C).

FIGURE 7 is a diagram illustrating the effect of temperature on the residual PAHs concentrations (current density = 9.23 mA/cm², treatment time = 90 min, without initial pH adjustment (pH, = 6.0), [Na₂SO₄] = 500 mg/L).

FIGURE 8 is a diagram illustrating the variation of cell potential and pH with the reaction time using the electrochemical Cell-3 or C₃ during the recycling batch tests (operating conditions: current density: 15 mA/cm², recycling rate: 3.6 L/min). FIGURE 9 is a diagram illustrating the 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).

FIGURE 10 is a diagram illustrating the 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).

FIGURE 11 is a diagram illustrating the variation of normalized concentration with the reaction time using the electrochemical C₃ during continuous mode operation at different high retention time (HRT) (operating conditions: current density: 15 mA/cm²). Values reported after a period of time equal to three HRT.

FIGURE 12 is a diagram illustrating the effect of initial chloride ion concentration on active chlorine production during electrolysis (current density: 15 mA. cm^"2; recycling flow rate: 2 Lmin^"1).

FIGURE 13 is a diagram illustrating the influence of the current density on active chlorine production (recycling flow rate: 2 Lmin^"1; chloride ion concentration: 17.1 mM).

FIGURE 14 is a diagram illustrating the influence of the current density on MV2B dye degradation during electrolysis (recycling flow rate: 2 l.min^"1; chloride ion concentration: 17.1 mM; initial MV2B concentration: 50 mg.l^"1).

FIGURE 15 is a diagram illustrating the influence of anode material and supporting electrolyte on the degradation rate of MV2B (current density: 15 mA.cm^"2, recycling flow rate: 2.0 L.min^"1; chloride concentration: 17.1 mM; initial MV2B concentration: 50 mg.L^"1). Detailed description of the invention

As aforesaid, the invention is directed a method for decontaminating wastes containing toxic organic molecules and for degrading the toxic organic molecules

Preferably, the wastes may be hazardous or polluted municipal or industrial wastewaters, soils, sands, sediments or sludge, and may be stemmed from aluminium industry

By "hazardous waste", it is to be understood a waste that poses substantial or potential threats to public health or the environment and generally exhibits one or more of these characteristics: carcinogenic, ignitable (ι e , flammable), oxidizing, corrosive, toxic, radioactive, explosive U S environmental laws (see Resource Conservation and Recovery Act, Public Law at http.//epw senate.gov/rcra.pdf) additionally describe a "hazardous waste" as a waste (usually a solid waste) that has the potential to cause, or significantly contribute to an increase in mortality (death) or an increase in serious irreversible, or incapacitating reversible illness, or pose a substantial present or potential hazard to human health or the environment when improperly treated, stored, transported, or disposed of, or otherwise managed.

Preferably, the toxic organic molecules may be creosotes, polycyclic aromatic hydrocarbons (PAHs), hydrocarbons from oils, hydrocarbons from greases, hydrocarbons from petroleum, chlorinated molecules, pesticides, endocrine disruptors (EDCs), polychlorinated biphenyl molecules (PCBs), polychloriπated dibenzodioxins (PCDDs), dyes or mixtures thereof

More preferably, the toxic organic molecules may be polycyclic aromatic hydrocarbons (PAHs), such as the ones illustrated in Figure 5, and hydrocarbons from oils and/or hydrocarbons from grease These hydrocarbons may be those having from 10 to 50 carbon atoms (Cio - C₅₀), saturated or unsaturated, linear or branched Polluted wasted may also comprise petroleum wastes which comprise the above mentioned Cio - C₅₀ hydrocarbons The method of the invention comprises a first step a) wherein an aqueous mixture is obtained by mixing the wastes with an effective amount of a surfactant to form micelles of surfactants. The micelles entrap the toxic organic molecules.

Preferably, water may be added to the mixture in case the wastes are dry wastes that do not contain water or the original amount of water present in the wastes is insufficient to forms micelles

It is to be understood that the amount and the nature of the chosen surfactant will depend on the amount and nature of the organic molecules to treat.

Preferably, the surfactant may be an amphoteric or non-ionic surfactant. More preferably, the surfactant is cocamidopropyl hydroxysultaine (CAS), polyoxoethylene(20)sorbitan monooleate (Tween 80™), polyoxyethylene(10)isooctylphenyl ether (Triton X-100 ™), polyoxyethylene-(12)isooctylphenyl ether (Igepal CA 720 ™), or polyethylenglycol dodecyl ether (Brij 35 ™). The invention is not limited to these specific.

The method of the invention comprises a second step b) in which an aqueous solution is extracted from the aqueous mixture obtained in step a). The aqueous then comprise the micelles entrapping the toxic organic molecules

Extraction of the aqueous solution can be made by different methods well known in the art, such as decantation, flotation, centrifugation, pumping, filtering, or the like. Preferably, the aqueous solution is extracted from the wastes by decantation or flottation.

Preferably, the toxic aqueous solution to treat may comprise polycyclic aromatic hydrocarbons (PAHs) at a concentration up to about 30 g by kg of the toxic aqueous solution. The term "about" as used in the present description means that the values or measures have a precision which cannot be inferior to the precision of the apparatus used to get this measure. It is commonly accepted that a 10% precision measure is acceptable and encompassed by the term "about"

The method of the invention comprises a third step c) in which an electrolytic cell is filled with the aqueous solution containing the micelles entrapping the toxic molecules to degrade

The electrolytic cell comprises at least one pair of electrodes. Electrolytic cell with a plurality of electrodes can be used, such as the one illustrated on Figure 1

Preferably, each pair of electrodes comprises a dimensionally stable anode with high oxygen overvoltage and a cathode Dimensionally stable anodes (DSA) were developed in the mid-1960's for the chlor-alkali industry. They are called dimensionally stable because they utilize precious metal containing electro-catalysts, like IrO₂ or RuO₂, coated on titanium The titanium substrate is corrosion resistant in the chlorine generating environment which allows for the structure to maintain its dimensional tolerance during its life unlike the graphite anodes they replaced.

For the present invention, the dimensionally stable anodes are preferably made of titanium coated with iridium oxide, ruthenium oxide or tin oxide, and the cathode may be made of titanium or stainless steel.

The electrodes used may be plain or meshed. They also can be plate, circular or cylindrical shape More preferably, the electrodes are cylindrical meshed electrodes, such as the ones of the electrolytic cell defined herein after Mesh electrode or expanded electrodes may be preferably used in order to favour high transfer coefficient between electrode and effluent to be treated Preferably, the electrolytic cell has an inter-electrode distance ranging from about 0.5 to about 2.0 cm. The invention is however not limited to these range.

The method of the invention further comprises a fourth step d) in which a current is applied to the electrodes in order to electrolyse the electrolytic solution.

Preferably, a current having a voltage lower than about 40 Volts, preferably from 1 to 20 Volts, and a density ranging from about 3.0 to 30 mA/cm² is applied for a period of time suitable to electro-oxidise the toxic organic molecules that are then degraded into non or less toxic molecules, such as CO₂ and H₂O. The voltage may be higher in that the electrolytic cells used for the method are industrial cells.

Preferably, in step d), the period of time suitable for electro-oxidizing organic molecules is ranging from about 10 to 200 minutes.

Preferably, the method of the invention further comprise adding a given amount of air or oxygen to the toxic aqueous solution before or during steps c) or d). 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, O₃) capable of enhancing PAHs degradation. Preferably, a maximum for PAHs degradation efficiency may be observed at an injection rate of about 5.0 ml_ 0₂/min.

The method of the invention may further comprise the addition of a given amount of at least one supporting electrolyte to the toxic aqueous solution before or during steps c) or d).

According to a preferred embodiment of the invention, a supporting electrolyte can be added to reduce the energy consumption. The electrolyte can be one or a mixture of Na₂SO₄, NaCI, KCI, MgCI₂, CaCI₂, HCI, H₂SO₄, MgSO₄, (NH₄)₂SO₄, or NH₄CI. The concentration of the electrolyte may be preferably ranging between 0.5 to 4.0 g/L. The method of the invention may further comprise the step, an initial pH of the toxic aqueous solution of measuring before steps c) or d). In the case this pH is lower than 4, it is preferable to adjust it to a new pH ranging from 4 to 7 by adding to the solution a given amount of a base. In the case the measured initial pH is greater than 7, it is preferable to adjust it the new pH ranging from 4 to 7 by adding to the solution a given amount of an acid. Indeed, it is known that alkaline media may be not favourable for PAHs oxidation, whereas high performance of PAHs degradation can be recorded preferably between pH 4.0 and 7.0.

The method of the invention may further comprise analyzing the solution after step d) for detecting remaining toxic organic molecules. In the case remaining toxic organic molecules are detected, it may be possible to apply again steps c) and d) of the method until clearing the toxic organic molecules from the toxic aqueous solution.

The method of the invention may further comprise providing turbulences to the solution during step d) in order to enhance the electro-oxidation of the toxic organic molecules. Preferably, the turbulence of the effluent is provided in the cell. 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. In another hand, an agitator, mechanic or magnetic, may be added into the cell.

Preferably, the method of the method of the invention may be carried out in a batch, semi- continuous or continuous mode. More preferably, the invention may be carried out in semi- continuous or continuous mode.

Preferably, the temperature of the solution may be maintained between about 4 and 35⁰C. 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 method 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 present method 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 (C₁₀-C₅₀) contained in synthetic or real effluent. More than 80% of C_1O-C₅₀ 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.

That is why the method of the invention is particularly adapted for decontaminating wastes polluted with toxic organic molecules. The wastes may be polluted municipal or industrial wastewaters, soils, sands, sludge or mixtures thereof. The toxic organic molecules may be creosotes, polycyclic aromatic hydrocarbons (PAHs), hydrocarbons from oils, hydrocarbons from greases, hydrocarbons from petroleum, chlorinated molecules, pesticides, endocrine disruptors (EDCs), polychlorinated biphenyl molecules (PCBs), polychlorinated dibenzodioxins (PCDDs), dyes or mixtures thereof.

The method of the invention is particularly adapted for decontaminating wastes stemmed from aluminium industry.

The invention is also directed to an electrolytic cell for electro-oxidizing organic molecules contained in an electrolyte, like the one illustrated on Figure 2.

The cell comprises:

Preferably, the electrolytic cell of the invention may further comprise a wall installed into the vessel above the inlet for supporting the weir and electrodes, the wall and vessel forming as such an inlet zone. The wall is preferably provided with at least one orifice allowing the electrolyte to flow through the wall from the inlet zone toward the weir. The inlet zone may be useful for containing an agitator installed for providing turbulences to the electrolyte.

Preferably also, the vessel and/or the tubular weir are made of an inert material such as a plastic material. More preferably, the polymer material may be made of polyvinylchloride (PVC). Other inert and resistant plastic materials may used.

As illustrated on Figure 2, the first perforated electrode is an anode and the second electrode is a cathode. Of course, the first perforated electrode may be alternatively a cathode whereas the second electrode may an anode.

Preferably also, the anode is a dimensionally stable anode as the one defined above. Preferably, the anode may be made of titanium coated with iridium oxide, ruthenium oxide or tin oxide, whereas the cathode may be made of titanium or stainless steel.

Each of the tubular weir and the first cylindrical electrodes, the first cylindrical electrodes and the second electrode, and the second electrode and the vessel, are preferably separated by a gap ranging from 0.5 to 4 cm, independently.

To enhance the circulation of the electrolyte into the vessel, the second electrode may be also perforated.

This electrolytic cell is particularly useful for the application of the method according to the present invention. One or several identical cells can be used in parallel and/or in continuity inside a closed loop circuit.

The electrolytic cell according the to the invention is particularly useful for electro-oxidizing organic molecules such as creosotes, polycyclic aromatic hydrocarbons (PAHs), hydrocarbons from oils, hydrocarbons from greases, hydrocarbons from petroleum, chlorinated molecules, pesticides, endocrine disruptors (EDCs), polychloπnated biphenyl molecules (PCBs), polychloπnated dibenzodioxins (PCDDs), dyes or mixtures thereof

EXAMPLES

Methodology

Creosote and PAHs solubilisation

Commercially-available creosote used in this study is provided by Stella-Jones lnc (Montreal, QC, Canada), and comprises 50% (v/v) of creosote and 50% of petroleum hydrocarbons. The creosote effluent is 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 hydroxysultaine, Chemron, Ohio, USA) is added. Conditioning is 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 is 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 is 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

In this example, electrochemical degradation of PAHs in COS is 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 cells (anodes and cathodes) consist of ten parallel pieces metal with an inter-electrode distance of 1 about cm Five anodes and five cathodes alternate in the electrode cell The electrodes are 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 1 10 cm² (10 cm width x 11 cm height) The electrodes are placed about 2 cm from the bottom of the cell Mixing in the cell is 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 is used. The anodes and cathodes are 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 is held constant for each run. Between two tests, electrolytic cell (including the electrodes) are preferably rubbed with a sponge and rinsed with tap water, and then soaked with 5% (v/v) nitric acid solution for 15 min.

The first set of electrodegradation experiments consists 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 Na₂SO₄ZL) and temperature (4 to 35⁰C) in view of determining the optimal conditions (reduce cost and increase effectiveness) for treating COS.

The pH is adjusted using sulphuric acid (H₂SO₄, 5 mol/L) or sodium hydroxide (NaOH, 2 mol/L). Sodium sulphate, sodium hydroxide and sulphuric acid all analytical grade reagent and supplied by Fisher Scientific. During these assays, only the residual PAHs concentrations are measured to evaluate the performance of the experimental unit in oxidizing these refractory organic compounds. Once the appropriate values of these parameters are determined, the optimal conditions are 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 (C₁0-C₅₀) are simultaneously measured. Likewise, biotests (Microtox and Daphnia tests) are 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 new cylindrical electrolysis cells (C₁, C₂ and C3) have been constructed. Each of them has 2 L of capacity. The cells are made of PVC material with a dimension of 15 cm (height) x 14 cm (diameter) and all electrodes are presented in the form of expanded metal. The electrolytic Ci comprises two concentrical electrodes (Figure 2). The cylindrical anode electrode (14 cm height x 10 cm diameter x 0.1 cm thick) is made of titanium coated with iridium oxide (Ti/lrθ₂) has a solid surface area of 270 cm² and a void area of 170 cm². The cylindrical cathode electrode (14 cm height x 12 cm diameter x 0.1 cm thick) is 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, is 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 given period. The weir had a diameter of about 4.0 cm and a height of about 14 cm.

By comparison, inside both electrolytic C₂ and C₃, the electrodes are circular disks (12 cm diameter) and titanium (Ti) is used as cathode with a solid surface area of 65 cm² and a void surface area of 45 cm² (Figure 3). Circular Ti/lrO₂ and Ti/SnO₂ electrodes are 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 is about 10 mm in the three electrolytic cells. The circular electrodes were supplied by Electrolytica lnc (Amherst, NY), whereas the cylindrical electrodes were provided by Electech (Chardon, Ohio).

The assays were carried out in a closed loop as the one depicted schematically in Figure 4.

A one liter PVC tank (4), a centrifugal gear pump (6) and the electrolytic cell (1) (as the ones illustrated on Figures 2 and 3) constitute the loop. The first assays are 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 is worth noting that, the recycle flow (QR), induced by the centrifugal pump maintains the liquid phase in sufficient mixing. A needle valve (2) installed before a manometer (3) allows controlling the hydrostatic pressure inside the cell. The apparatus includes oxygen injection inlet (8) in the closed loop in order to favor the hydroxyl radical at anode electrode. The rate of oxygen injected is measured using a flow-meter (7). An oxygen probe (9) is 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 is 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 is 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 sodium sulfate (0.5 g/L of Na_∑SO-i) is added to increase the electrical conductivity.

The electrochemical cells are 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 are imposed by means of a DC power source, Xantrex™ XFR40-70. During the experiments the pH is monitored but not controlled. While the experimental unit operated in continue mode, valve (15) is closed, whereas valves (1 1) and (12) are opened. However, in the batch mode, valves (1 1), (12) and (15) are closed.

Prior to continue mode operation, the apparatus is initially operated in batch recirculation mode until the steady sate for PAHs degradation is 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 are quite the same and ranged between 50 and 100 mL/min. It worth noting that during the continue mode operation, the centrifugal pump is closed and the recycling flow (QR) is nil.

Analytical techniques

Operating parameters

The pH is 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) is used to determine the ionic conductivity of the solution. The temperature of treated-solution is monitored using a thermo-meter (Cole-Parmer, model Thermo Scientific Ertco™).

Extraction and analysis of PAHs

Analyses of PAHs are carried out after extraction and purification on a solid phase using polypropylene-cartridges (provided by Enviro-Clean sorbents, United Chemical Technology Inc.). The Enviro-Clean™ polypropylene-cartridge is 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 is loaded onto the cartridge where it is entirely filtered drip. PAHs retained on the polypropylene-cartridge are then eluted with 10 mL of dichloromethane. After elution, the sample is 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 (Bϋchi Rotavapor-R, Rico Instrument Co.). The extraction solution is diluted with dichloromethane, and a series of diluted solution (1 x 10 x 100) is prepared and analyzed. PAHs are 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 μm 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) is used as a standard for PAHs. The PAHs standard solution is 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 1 indicates some physico- chemical properties of these compounds. All the Tables mentioned herein are located at the end of the present description.

Organic measurements Chemical oxygen demand (COD) determination is made by Hach COD method (HACH (1995) Hach water analysis handbook: Chemical oxygen demand, reactor digestion method 467. Hach Company, Loveland, Colorado) 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 are made by Standard Methods (American Public Health Association (APHA) (1999) Standards Methods for Examination of Water and Wastewaters, 20^th Edition, American Public Health Association, Washington, D. C)).

The quality of the treated-solution is also measured in terms total oil and grease (O&G) and CiO-C₅₀ petroleum hydrocarbons. O&G are determined by gravimetric method which consists 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 is determined by comparing the total area of group of peaks of n- C₁₀ to n-C_δo with area of the standard curves obtained under similar reaction conditions.

Toxicity tests

The quality of treated-solution (versus untreated solution) is evaluated using a biotest battery to have information about its toxic effect. Microtox and Daphnia bioassay tests are applied. Microtox analysis is a standardized toxicity test using the luminescent marine, Vibrio fisheri (Software MTX6™, version 6.0, Microbics Corporation) (Environnement Canada (1992) Essai de toxicite sur Ia bacterie luminescente Photobacterium phospherum, Report SPE 1/RM/24 SPE 1/RM/24, Ottawa, ON, Canada (in French) 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 is first measured after 10 min of incubation (without adding any sample, control assay), after which the creosote-solution (treated or untreated-solution) is added to the bacterial suspension. The bioluminescence reduction is determined after a 5, 15 and 30 min exposure to the contaminant. The toxicity effects are monitored as the average percentage of light emission inhibition compared to the control assay. By comparison, Daphnia bioassay test consists 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 consists 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 are counted and the toxicity effect is evaluated using a statistic calculation software (Computer Basic, Spearman Karber™ tests, version 2.01 , Microsoft™) (Environnement Canada (2000) Methode d'essai biologique: methode de reference pour Ia determination de Ia lέtalite aigϋe d'effluents chez Daphnia magna., Report SPE1/RM/14, Ottawa, ON, Canada (in French)); Ministere de I'environnement du Quebec (2000) Determination de Ia toxicite letale CL50-48h Daphnia magna, Method MA 500-D mag 1.0, Ministere de I'environnement du Quebec, Qc, Canada (in French)).

Economic aspect

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

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 is a synthetic solution prepared from a commercial creosote solution in the presence of an amphoteric surfactant.

The main objective of the present study is 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 Hydroxysultaϊne, 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 2. 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 solubilisation 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.

From the Table 2, 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

Table 3 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 consists 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 is 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 is 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. The real yields of PAHs degradation is measured to vary from 59 and 69%. The yields of PAHs degradation increases with current density until 9.23 mA/cm² and then remains quite stable at 12.3 mA/cm². Using a current density of 9.23 mA/cm², the rates of PAHs degradation (around 81 to 84%) are 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 is retained for the next step of the study. The power consumption was 78 kWh/m³ while the current density of 9.23 mA/cm² is held constant for a period of treatment of 180 min.

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 are conducted by testing different retention times. During these assays, the current density of 9.23 mA/cm² is imposed. Two sets of experiments are carried out: the first one consists to test relatively short retention times (0, 10 and 20 min), whereas the second one allows testing long retention time periods (30, 60, 90, 150 and 180 min). The results are summarized in Table 4. During the first set of experiments a total PAHs concentration of 513 mg/L is measured in the initial solution. By comparison, 474, 364 and 299 mg/L are recorded while imposing 0, 10 and 20 min, respectively. The PAHs degradation yield increases with the retention time. However, it is surprising to see that, the initial concentration of PAHs recorded in the untreated solution is 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 are 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 are obtained from a sample withdrawn in the electrolytic cell.

This discrepancy may 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 is the reason why, at the start of each set of experiment (before imposing the current density), a sample of COS (untreated sample) is withdraw from the cell and analyzed. During the 2^nd set of experiment, a total concentration of 525 mg PAHs/L is measured in the untreated solution. The application of electro-oxidation process allows reducing PAHs content and the residual concentration varied from 88.5 to 143 mg/L and contributes to removal of about 74 to 83% of PAHs depending on the retention time imposed.

Figure 6 shows the changes in PAHs degradation yield as a function of charge loading. Two different regions can be distinguished. When the charge loading is below 1 A.h/L, the yield of PAHs degradation increases linearly with charge loading. Beyond 1 A.h/L, the rate of PAHs degradation remained quite stable. The anodic oxidation of pollutant occurs heterogeneously. First, organic pollutants may 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 is relatively high, and accordingly the PAHs reduction rate is subjected to current control. As the PAHs concentration was lowered to a given level, the PAHs reduction rate is subjected to the mass transfer control. 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 is the reason for which the yields of PAHs degradation remains 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 varies 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 is reached between 1.0 and 3.0 A.h/L, the energy consumption can be reduced by curtailing the charge loading at 3.0 A.h/L. Indeed, a charge loading of 3.0 A.h/L is 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 is reduced to 41 kWh/m³ (rather than 78 kWh/m³) as expected. Example 4: Effect of initial pH on electrochemical oxidation of PAHs

In order to know if the electrolysis cell may 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) are investigated. Initial pH of the solution is adjusted using sulfuric acid (H₂SO₄, 5 mol/L) and sodium hydroxide (NaOH, 2 mol/L). In addition, a control assay is carried out without pH adjustment (original pH was around 6.0). During these assays, the current density is maintained at 9.23 mA/cm² and a retention time of 90 min is imposed.

The results are shown in Table 5. It is shown that COS having an initial pH closed to the neutral value (pH 6.0 and 7.0) is more favourable for PAHs reduction (PAHs removal of 81 and 84% are recorded, respectively). As the highest PAHs removal yield (84%) recorded at pH 7.0 is quite similar to that measured (81%) without pH adjustment (original pH 6.0), it is not necessary in that case to modify the initial pH before treatment.

Example 5: Effect of supporting electrolyte on electrochemical oxidation of PAHs

The addition of an electrolyte in solution during electrolysis may influence the treatment since it modifies the conductivity of the solution and facilitates the passage of the electrical current.

Various concentrations of sodium sulfate (Na₂SO₄ used as electrolyte) are added to the system and changes in PAHs reduction rate were noted. The current density of 9.23 mA/cm² is held constant over the retention time of 90 min without initial pH adjustment.

Table 6 represents the PAHs reduction yields with increasing concentration of Na₂SO₄. The PAHs degradation yields (80 to 83%) are quite similar regardless of supporting electrolyte concentration imposed. There is not a significant effect of electrolyte concentration on the oxidation efficiency in the investigated range of 500 to 4,000 mg Na₂SO₄/!.. However, it may be interesting to add a given 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) is estimated to 1.35 US$/m³ while adding 500 mg Na₂SO₄/.. in COS, compared to 2.52 US$/m³ recorded without any addition of supporting electrolyte.

Example 6: Effect of initial PAHs concentration on electrochemical oxidation of PAHs

The effect of initial PAHs concentration is 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 is held constant at 1.0 g/L, whereas the concentration of creosote varies from 1.0 to 5.0 g/L (Table 7). Initial PAHs concentrations vary from 274 to 538 mg/L. At the end of the treatment, residual PAHs concentrations are recorded to range between 65 to

108 mg/L. Irrespective of the initial PAHs concentration, the PAHs removal yield is quite similar with 78 to 80% of PAHs degradation. There is no effect of initial PAHs concentration on the oxidation efficiency in the investigated range 274 to 538 mg PAHs/L.

Example 7: Effect of temperature on electrochemical oxidation of PAHs

The effect of the temperature on PAHs degradation is 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 35⁰C). 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 21⁰C. The same trend can be observed for A- ring and 3-ring PAHs. The temperature of 21⁰C enhanced PAHs solubilization. However, while maintaining the temperature at 35⁰C₁ residual PAHs (2-, 3- and 4-rings) concentrations decreased compared to that recorded at 4⁰C or at 21⁰C. For instance, at 21⁰C, the 2-rings PAHs concentration measured is 26.0 mg/L. When the temperature increased to 35°C, the residual 2-rings PAHs concentration is lowered to 14.9 (42.9% 2- rings PAHs reduction). It is believed that, from a given 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 are 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 increase to around 80% while increasing the temperature either at 21 or 35°C. The increase of the temperature accelerates the electrochemical decomposition of PAHs.

Since the temperature of the solution naturally (without temperature control) increased from about 20 to 25°C during electrolysis, it is not necessary in that case to adjust the temperature to have its beneficial effect on PAHs degradation.

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 rnA/cm² through 90 min of treatment in the presence of 500 mg/L but without pH and temperature adjustment give in that case the best performance of electro-oxidation of COS. It is 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) is repeated in triplicate to verify the effectiveness and reproducibility of electro-oxidation performance in treating COS. Degradation of PAHs

Table 8 compares the untreated and treated-solutions by electro-oxidation. An average value of total PAHs concentration of 462 ± 5 mg/L is measured in the initial solution. It is 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) are present in the highest concentrations (2 to 3-rings PAHs). In contrast, the compounds having 5 and 6-rings PAHs are 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 reduces the total concentration of PAHs to an average value of 105 ± 2 mg/L. The PAHs removal yield has 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 are effectively oxidized.

The residual concentrations of these PAHs are 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 are 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.

Organics removal

In addition to PAHs measurements, other parameters such as oil and grease (O&G), Cio- C50, COD and TOC related to the organics are also measured in the initial and treated- solution. The results are summarized in Table 9. The residual O&G and Ci₀-C₅₀ concentrations recorded at the end of the treatment are 290 mg/L and 27 mg/L, respectively, compared to 940 mg/L and 170 mg/L of O&G measured in the initial solution. A yield of 69% of O&G removal is recorded, whereas 84% of Ci_O-C₅₀ are removed.

On the other hand, reduction in COD and TOC are 62% and 27%, respectively. The residual concentration COD and TOC recorded at the end of electro-oxidation are

809 mg/L pf DCO and 174 mg/L of TOC. By comparison, 2,102 mg/L and 237 mg/L are 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 is 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 breaks the double bonds producing smaller molecules.

As already mentioned, it is well known that during electrolysis, organic pollutants can be subjected to two different paths in anodic oxidation: electrochemical conversion or electrochemical combustion. 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, it is believed that electrochemical conversion may be the predominant reaction.

Toxicity reduction

Microtox and Daphnia bioassay tests are 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 are 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 9. The comparison of the results shows a reduction of the toxicity while applying electro-oxidation treatment. Thus, relatively high toxicity of 4,762 TU is measured for crustacean Daphnia and 1 ,000 TU is recorded for luminescent bacterium Vibrio fisheri in the initial solution. By comparison, only 453 TU and 200 TU are recorded after treatment, respectively. It corresponded to 91 % of toxicity reduction on crustacean Daphnia, whereas 80% of toxicity reduction is accomplished on luminescent bacterium V. fisheri. In fact, the electro-oxidation process breaks the double bonds of PAHs producing smaller molecules which are less toxic. 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 creosote oily effluent (COE) are given in Table 10. Sixteen PAHs were investigated in the COE and were comprised of different number of aromatic rings (2-, 3-, A-, 5- and 6-rings PAHs). From the Table 10, it can be seen that 3-ring PAHs (ACA, PHE, FLU, ANT, and ACN) are 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 are 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 is evaluated by measuring the residual 16 PAHs concentrations.

The primary objective of the preliminary screening tests is to verify the efficacy of PAHs degradation in COE. The assays are carried out using electrolytic cells made up of either Ti/lrθ₂ or Ti/Snθ₂ anode electrodes at current densities of 9.0 mA/cm² and 12 mA/cm² for 90 min. Table 11 presents initial and final conditions of each test as well as PAHs degradation rates obtained during treatment using different electrolytic reactors, Ci, C₂ and C₃. The initial pH is around 6.0, whereas at the end of the treatment the values vary from 6.9 to 7.8. The power consumption is evaluated between 3.09 and 9.50 kWh/m³, and the highest consumption is obtained for Ci (6.14 and 9.50 kWh/m³) comprising of cylindrical electrodes. This is 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/lrθ₂ imposed (comparison between Ci and C₂), the current intensities required were 2.4 A and 1.2 A, respectively, whereas the average voltage is around 7.1 or 7.4 using either the Ci or the C₂. However, considering the energy consumption, it can be seen that the electrical energy (6.14 kWh/m³) using Ci is 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.

The efficacy of the electro-oxidation process in terms of PAHs removal from COE using different electrolytic cells is 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 are 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/lrO₂) is used, it can be seen that the PAHs removal yields (80 and 78%, respectively) using the C₂ are better than those recorded (67 and 74%, respectively) using the Ci 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 (OH⁰). In the C₂, the liquid arrives 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 Ci 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 may be inside the C₂, resulting in an increase in PAHs oxidation rates by comparison to the d. 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/Snθ₂ 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%) is recorded using Ti/Snθ₂ anode electrode installed in the C₃ in comparison to 78% PAHs removal obtained with Ti/lrO₂ anode using the C₂ for the same current density imposed. Tin oxide is one of the noble metal oxides having a better performance for organic compounds degradation in comparison to traditional electrodes (Pt, IrO₂ and RuO₂). This is attributed to the highly crystalline nature of tin oxide, which catalyzes the reaction of electrochemical oxidation. Finally, the C₃ including circular Ti/SnO₂ anode was selected for the next experiments.

Example 10: Influence of applied current density on PAHs degradation using Ti/SnO? circular mesh anode

In order to determine economical and better conditions for PAHs degradation in COE, several batch electro-oxidation assays are 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 are investigated. As already mentioned, one of the main factors affecting the electrochemical oxidation efficiency is the current density. Current densities are obtained by dividing each current by the corresponding total anode area. The effect of current density on PAHs degradation is shown in Table 12. 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 vary from 52 to 26 mg/L compared to 155 mg/L measured in untreated COE. PAHs degradation increases with current density in the range of 4.0 to 15 mA/cm². The largest PAHs oxidation is observed at 15.0 mA/cm². However, when a current intensity of 23 mA/cm² is imposed, the PAHs removal slightly decreased. Indeed, the increase of current intensity above 15.0 mA/cm² further induces parasitic reactions such as water reduction, leading to high amount of oxygen bubbles (O₂) formation, which disturbs PAHs oxidation on anode electrodes.

Example 11 : Influence of reaction time on PAHs degradation using Ti/SnOg 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 increases and then remains 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 follows first order kinetics. Therefore, the reaction rate constant "k" could be calculated from the slope value of the plot (t) versus -Ln(C/C₀) of equation (8):

where C₀ is the initial concentration of PAHs, C the concentration of PAHs at tine t, t the reaction time, and k is the first order reaction rate constants (f¹).

As shown in Figure 10, the first order decomposition reaction rate constant of PAHs by the electrochemical oxidation is 0.015 min^"1. 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 S. et al. (Desalination, 155, 49-57 (2003)) while studying electrochemical oxidation of polyvinyl alcohol (PVA) using titanium coated with ruthenium oxide (Ti/Ruθ₂). The constant rate decreased from 0.0162 min^"1 to 0.0021 min^'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 min^"1) is quite similar to that measured (0.0162 min^"1) by Kim et al. (2003) while imposing the lowest concentration of 33 mg/L of PVA, although the experimental conditions are 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 10 times lower. For the same kinetic constant rate, high current density is required in treating CEO probably due to the fact that PAHs in COE are more difficult to oxidize than PVA.

Example 12: Influence of recycling flow rate on PAHs degradation using Ti/SnO? 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 Recirculation waste can be necessary for efficient treatment

Experiments are 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 mm Degradation efficiency increased slightly (from 81 to 85%) as recycling flow rate increases from 1 8 to 5.4 L/min, as shown in Table 13 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

While increasing the recirculation rate to 7 3 L/min, it is shown that degradation efficiency decreases 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 is imposed for 7 4 L/min compared to 0 55 cm/s measured for 5 4 L/min It is believed that from a given level of the linear velocity imposed, the fluid does not sufficiently remain inside the cell, so that the degradation efficiently decreased Thus, a recycling flow rate of 3.6 L/min is selected for the next step of the experiments, as PAHs degradation efficiency as quite similar to that at 5 4 L/min.

Example 13: Influence of injection of oxygen in a close loop on PAHs degradation using Ti/SnOg circular mesh anode

The results discussed above are obtained without any oxygen injection in the close loop. The following experiments are carried out for different oxygen flow rates (5, 10 and 20 mL CVmin) injected in the close loop and compared with a control assay without O₂ injection The interest of continuously injecting oxygen in the system is to gradually saturate the liquid in oxygen and be able to further generate radical species (OH⁰) or oxidants (such as ozone, O₃) capable of enhancing PAHs degradation. It is known that ozone can be formed during electrolysis of water using high oxygen overvoltage anodes.

The results are presented in Table 14. The initial PAHs concentration measured in the untreated COE is 264 mg/L. While injecting oxygen in the close loop, residual PAHs concentrations vary from 31.2 to 52.9 mg/L. By comparison, a residual PAHs concentration of 40.5 mg/L is recorded during the assay without O₂ injection (control assay). A maximum for PAHs degradation efficiency (88%) is observed at 5 mL 0₂/min. While the oxygen flow rate increases to 10 mL/min, no significant effect is observed by comparison with the assay carried out without oxygen injection (83% of PAHs was removed). However, for 20 mL O₂/min imposed, a negative effect is 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 are hampered or disturbed. As this operating parameter had moderately significant effect, oxygen injection in the close loop is 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/Snθ₂ 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 Na₂SO₄ZL (used as electrolyte support).

Example 14: Efficacy and reproducibility of batch electro-oxidation treatment for PAHs degradation using Ti/SnOg circular mesh anode

It is 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) is repeated in triplicate to verify the effectiveness and reproducibility of electro-oxidation performance in treating COE. Table 15 compares the untreated and treated-effluents by electro-oxidation. An average value of total PAHs concentration of 292 ± 24 mg/L is measured in untreated effluent. It is 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) are present in the highest concentrations (3 to 4-rings PAHs). In contrast, the compounds having 5 and 6-rings PAHs are 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 are 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). These residual concentrations are 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.

Example 15: Combining successively batch and continuous electro-oxidation treatment for PAHs degradation using Ti/SnOg circular mesh anode

Three sets of experiments are 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 is imposed for various inlet flow rates (50, 75 and 100 mL/min). The experimental conditions are summarized in Table 16.

For the first set of experiments, the electrochemical system is 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 is imposed in continuous mode operation by imposing a constant inlet flow of 75 mL/min, whereas the system is 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 2^nd set of experiments, a recirculating batch test (run L) was carried out prior to continuous mode operation (runs M to O) 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) is to maintain initially a steady state inside the cell prior to start the continuous run tests.

Table 16 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 is obtained while a HRT of 90 min is 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 can 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 remains 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.

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 passes 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 increases while HRT decreases to 60 min. Otherwise, while further decreasing HRT, the percentage of PAHs degradation is low, but it remains quite stable around 50%, meaning that the process of the formation of organic substances on the electrode surface decreases 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.

Example 16: Coupling extraction-flotation with surfactant and electrochemical degradation for the treatment of PAH contaminated hazardous wastes

The performance of a two-stage process combining extraction of polycyclic aromatic hydrocarbons (PAHs) with an amphoteric surfactant (CAS) followed by electro-oxidation of PAH-foam concentrate is studied for the decontamination of aluminum industry wastes (AIW) and polluted soils. The PAH extraction from AIW and soils is performed in a Denver™ flotation cell (Joy Manufacturing Company, Denver, Colorado, USA), containing 400 g of polluted wastes mixed with 4.0 L of tap water in the presence of 0.20% (w.w^~1) of surfactant. Air flow rate within pulp is adjusted to 1 mL.min^"1. The foam is formed and moved up to the surface of the pulp. After each flotation step, the flotation concentrate (comprised of foam and fine particles) named as FCO, was recovered by overflow in a container. FCO-AWI represents foam concentrate from aluminum waste industry; whereas FCO-soil represents foam concentrate from soil.

Both FCO-AIW and FCO-soil are respectively dried for 48 h at 55⁰C, and weighed in order to determine the TS of the hazardous wastes. The FCO-AWI and FCO-soil suspensions are respectively prepared in 500 mL Erlenmeyer flasks with stainless cap containing 1.5 to 6.0 g of dry FCO, in which 300 mL of tap water is added. The resulting slurries are transferred into an electrolytic cell containing 1.2 L of tap water (process water, PW2). The mixtures are then agitated for a period of 30 min before the current intensity is imposed for electrochemical treatment of the foam concentrates.

The application of electrochemical treatment of FCO-AWI reduces the total PAH

5 concentration to an average value of 12 091 ±302 mg kg^"1 compared to 22 380 ± 171

(Table 17). The PAH removal from FCO-AWI has a mean value of 49±1%. The effectiveness of the electrolytic cell in treating FCO-Soil suspensions are evaluated (Table

18). The application of electrochemical treatment of FCO-soil reduces the total PAH concentration to average values of 14 675±90 mg kg^"1 compared to 22 886±395 measured

I O in untreated FCO-soil. The PAH removal from FCO-soil has a mean value of 44±2%.

Example 17: In situ active chlorine generation using Ti/lrO? circular mesh anode for the treatment of dye containing effluents

15 The efficacy of the electrolytic cell using Cell-2 or C₂ (anode: Ti/lrO₂; cathode: Ti) for active chlorine production is evaluated. The assays are performed using distilled water enriched with chloride ions added in form of chloride sodium (NaCI, Fisher Scientific, ACS reagent). Electrolysis is conducted at a pH around the neutral value (pH 6.0 -7.0) for various current densities. Samples are drawn periodically and analyzed to assess the performance of the

.0 system for active chlorine production.

The overall concentration of dissolved chlorine in water is termed the "active" chlorine and is the sum of the three possible species, Cl₂, HCIO and CIO^", CEO^". In the present study, the concentration is measured as active chlorine in mol.l^"1 and converted into mg.l^"1 , this

.5 conversion being based on the atomic weight of Cl (35.45 g mol^"1). The dependence of the active chlorine production on chloride ion concentration for chloride concentrations up to 17.1 mM (1000 mg NaCI. I^"1) with a current density of 15 mA.cm-² is shown in Fig. 12. It can be seen that, the active chlorine concentration, in the loop of the unit increases with elapse time. For both lower concentrations of chlorides imposed (1.71 mM and 3.42 mM), the

30 active chlorine concentration tends towards a plateau from 30 min of treatment time compared to higher chloride concentration imposed. The active chlorine concentration increases with increasing chloride ion concentration. For instance, at a treatment time of 30 min, the chloride concentration imposed of 17.1 mM allowed producing 3.1 times as much active chlorine as a chloride concentration imposed of 1.71 mM (100 mg NaCI. I^"1).

As shown in Figure 13, the rate of active chlorine production is proportional to current density. The specific chlorine production rate recorded is 2.8 mg.min^"1 A^'1 with 121 mg.1^"1 of chloride concentration. This discrepancy can be mainly attributed to the design of the electrolytic cell which can be greatly influenced the mass transfer inside the reactor. The present assays are carried out using a single-cell process (without ion exchanged membrane) and the electrolyte circulates in a closed loop at 3 I. min^"1. This relatively HRT (high flow rate) created turbulence effect inside the electrolytic cell so that chloride ions are easily transferred towards the electrode and easily oxidized at the anode.

Example 18: Electrochemical oxidation of methyl violet (MV2B) dye-containing solution

Once characterized in terms of active chlorine production, the experimental unit is used for the treatment of a synthetic solution containing a methyl violet 2B dye (MV2B)™ (J. T. Baker Chemical, New Jersey, USA). These experiments consist to test successively different operating parameters such as, current densities (3.8 to 30 nriA.cm-²), retention times (10 to 60 min), initial dye concentration (25 to 150 mg.l^'1), nature of anode electrode (Ti/lrO₂ and Ti/SnO₂) and concentration of electrolyte (1.7 to 17 mM.Cr) in order to determine the best conditions (reduce cost and increase effectiveness) in treating MV2B- containing solution.

The effect of current density on the electro-oxidation of MV2B dye solution is evaluated by comparing the rates of color removal (by measuring the residual MV2B concentration) at current densities of 3.8, 7.6,15, 23 and 30 mA.cm^"2 in 17.1 mM of chloride (1.O g NaCI I^"1). Figure 14 shows time course changes in the normalized concentration of MV2B. The initial MV2B concentration imposed during these assays was 50 mg.l^"1. With the exception of current density of 3.8 mA.cm^"2, 100% degradation of MV2B is reached for all current density imposed. At the higher current density of 30 mA.cm^"2, faster degradation is obtained as shown in Figure 14. After only 5 min of electrolysis at the higher current density, there is complete MV2B degradation from the solution, compared to 15 and 30 min required for current densities of 15 and 7.6 mA.cm-^"2, respectively. While imposing the lower current density, the yield of MV2B degradation reaches 80% at the end of the treatment (40 min). As discussed above, the rate of active chlorine production is proportional to current density. Generally, the higher the concentration of active chlorine in the bulk solution, the more effective the oxidation is for organic pollutant degradation.

The degradation of MV2B dye occurs due to the generation of active chlorine that is powerful oxidizing specie. On the other hand, OH° radical may also be generated on the catalytic anode (such as Tϊ/lrCb). In fact, during electrolysis the organics in the solution can be decomposed by both direct anodic electrochemical oxidation (by means of OH°) and indirect electrochemical oxidation via mediators, such as hypochlorite ion and hypochlorous acid. Both situations would lead to the formation of powerful oxidizing agents capable of degrading the MV2B dye. Likewise, hydroxyl radical production could be greatly influenced by anode material.

In order to verify these hypotheses, complementary experiments using two types of supporting electrolyte (NaCI and Na₂SO₄) and two type of anode material (Ti/lrθ₂ and

Ti/Snθ₂) are carried out. During these assays, a current density of 15 mA.cm^"2 is imposed in the presence of 1.0 g NaCU^"1 (17.1 mM NaCI) and 1.0. g Na₂SO₄.!^"1 (7.0 mM Na₂SO₄), respectively. Residual MV2B dye concentrations are monitored and are shown in Figure

15. The control assay consists only in circulating the MV2B dye solution in the experimental unit without imposing any current density. The results indicate that MV2B dye removal occurred slowly in the sulphate media using either Ti/lrO₂ or Ti/Sπθ₂, reaching a maximum discoloration of only 20% after 90 min of electrolysis. The decrease in MV2B dye concentration during the control assay is probably attributed to the deposition of a small fraction of dye on the electrolytic tank or on the pipes of the experiment unit. Considering the possible deposition of MV2B dye, the real contribution of direct anodic electrochemical oxidation can be obtained by subtracting the yields of MV2B removal (while imposing current density) from the yields recorded without current density. Thus, in the present experimental conditions, the real yield of MV2B degradation by direct oxidation is around 10%. By comparison, a remarkable difference in the MV2B dye decomposition is observed when sodium chloride us used as supporting electrolyte. Indirect electrochemical oxidation contributed to more than 80 % in the MV2B dye decomposition using either TiVIrO₂ or Ti/SnC>2 anode, TiZIrO₂ being more effective than Ti/Snθ₂- Finally, Ti/lrθ₂ anode and NaCI used as electrolyte support is retained for the next step of this study. A concentration of 3.42 mM NaCI is selected (rather than 17.1 mM NaCI) in order to minimize chlorine gas production during electrolysis.

Example 19: Application of experimental design method to study the performance of electrolytic cell Cell-2 (anode: Ti/lrOg or Ti/SnO∑; cathode; Ti) for pesticide degradation

For the optimization operation, four factors are selected and given in Table 19, each having the lowest and highest levels designated by (-1 ) and (+1), respectively, that defined the domains of variation. The full factorial 24 appeared to be most appropriate for this study. The type molecular of pesticide studied is atrazine (1-chloro-3-ethyl-amino-5- isopropylamino-2,4,6-triazine) and a concentration of 0.1 mg.L^"1 is initially imposed.

The matrix of the effects for the two anode materials is given in Table 20. The sixteen assays described in the factorial design are carried out and the results are summarized in Table 21. At first sight, the atrazine degradation rate varies in the wide range from 5.8% to 95%, indicating the pertinence of the different domains of the definition of the factors. The best results (atrazine removal of 95%) is obtained by imposing the current intensity of 2.0 A, a treatment time of 40 min, a chloride sodium concentration of 1.0 g L^"1 using TiZIrO₂ anode electrode.

The factorial design is exploited using Design-Expert™ 7.0 software (Design Expert 7, 2007, Stat-Ease Inc., Minneapolis). The experimental data is well fitted to linear model with double interaction. By using the least square method, the coefficients of the polynome of the model that described the behavior of the system, are determined. The coefficient of the first order terms indicates the effects, and those of second order express the interaction among the studied parameters. The equation of the model is shown below:

y = 43.38 + 14.38X₁ + 13.75x₂ + 7.38x₃ + 1Ox₄ + 2X₁X₂

-1.38X₁X₃ + 4.5X₁X₄ + 1.Sx₂X₃ - 2.38x₂x₄ + 3.5x₃x₄

It worth noting that the four main factors have a positive influence on the response (degradation rate of atrazine). The current intensity is the predominant factor of the process, followed by the electrolyze time, and the anode material. The process is less sensitive to the electrolyte concentration (by comparison to three factors mentioned above).

It is to be noted that, the interaction effects are found to be weak compared to the main factors. Indeed, an increase of 36 % of atrazine degradation is recorded while the current intensity increase from 0.5 to 2.0 A in the best experimental conditions (anode = TiZIrO₂; t =

40 min; CN_SCI = 1.0 g. L^"1), whereas an increase of 25 % and 20% are recorded while increasing both the treatment time and electrolyte concentration at their high level (+1), respectively. Considering now the effects of the interaction, it can be seen that, the most significant interaction include the electrode material (X₄), with a predominance interaction

X₁X₄ (current intensity and electrode material) and X₃X₄ (electrolyte concentration and anode material. The effect of current density is slightly significant while TiVIrO₂ anode is used. An increase of 42 % of atrazine degradation was recorded while the current intensity ranged from 0.5 A to 2.0 A for Ti/lrO₂ compared to 24 % recorded for Ti/SnO₂. This can be explained by the fact that active chlorine production is proportional to the current intensity and Ti/lrO₂ anode (contrary to Ti/SnO₂) favors indirect effect of electrolysis.

TABLES 1 TO 21:

TABLE 1. Physical and chemical properties of PAHs identified in the creosote

PAHs Parameters

AbbreMolecular Numb Molecular Aqueous Octanol/ Vapour structure er of weight solubility pressure (20- viation water aromat (g/mol) (25°C, 25°C, mm partition ic rings mg/L) coeff. (log Hg)

C

Naphthalene NAP CioH₈ 2 128 31.7 3.37 8.7 x 10²

2-Methyl- MEN C-πH-_to 2 142 24.6 3.87 6.8 x 10-² naphtalene

Acenaphtyleπe ACN 3 152 3.93 4.07 2.9 x 10-²

Acenaphtene ACA C12H10 3 154 1.93 3.98 4.5 x 10 '

Fluorene FLU C13H10 3 166 1.83 4.18 3.2 x 10-⁴

Phenanthrene PHE C14H10 3 178 1.20 4.45 6.8 x 10⁴

Anthracene ANT C14H10 3 178 0.076 4.45 1.7 x 10⁵

Fluoranthene FLE CiβHio 4 202 0.23 4.90 5.O x 10 ^c'

Pyrene PYR CiβHio 4 202 0.077 4.88 6.8 x 10⁷

Benzo(a) BAA C₁₈H₁₂ 4 228 0.0094 5.61 2.2 x 10 » anlhracene

Chrysene CHR C₁₈H₁₂ 4 228 0.0018 5.63 6.3 x 10 '

Benzo(b,j,k) BJK C2θH-|2 5 252 0.0015 6.04 5.O x IO ⁷ fluoranthene

Benzo(a)pyrene BAP C_2C)H₁₂ 5 252 0.0016 6.06 5.6 x 10 ^•>

Dibenzo(a,h) DAN C₂₂H₁₄ 5 278 0.0005 6.84 1.0 x 10 «¹ anthracene

Indeno(l,2,3- INP C₂₂H₁₂ 6 276 0.062 6.58 10 » - 10 '' c,d)pyrene

Benzo(ghi) BPR C₂₂H₁₂ 6 276 0.0003 6.50 1.0 \ 10-'" perylene TABLE 2. PAHs solubilization (mg/L) from creosote

PAHs Creosote i (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-rιng 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-πng 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 1 1 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-πng 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-rιng 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-rιng 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

Σ PAHs 123 97 9 74 7 53 3 538 471 404 274 TABLE 3. PAHs concentration (mg/L) before and after treatment using different current densities^*:

PAHs Initial Control Current densities (mA/cm²) solution

0.00 3.08 4.62 6.15 9.23 12.30

NAP 70.9 70.1 22.5 21.7 21.7 13.6 1 1.5

MEN 64.1 53.7 13.1 13.0 11.6 8.83 7.30

ACN 3.91 3.02 1.98 1.52 1.08 0.68 0.66

ACA 62.8 57.1 12.3 12.2 11.1 9.40 9.59

FLU 45.3 38.8 10.0 9.87 10.1 9.51 9.12

PHE 90.4 84.4 22.7 22.4 21.9 17.7 18.2

ANT 16.3 15.3 4.11 4.09 4.21 2.95 3.63

FLE 53.2 40.5 10.8 10.7 10.6 8.42 8.99

PYR 41.3 31.5 8.74 8.55 8.07 5.91 5.53

BAA 8.00 6.02 2.37 2.29 2.28 1.26 1.76

CHR 9.48 8.31 2.48 2.26 2.48 1.78 1.94

BJK 6.29 5.86 1.50 1.43 1.37 1.25 1.24

BAP 2.51 2.23 0.63 0.62 0.55 0.47 0.49

DAN 1.21 1.09 0.31 0.28 0.25 0.20 0.21

INP 0.18 0.17 0.08 0.08 0.06 0.05 0.05

BPR 0.53 0.43 0.19 0.17 0.16 0.08 0.08

Σ PAHs (mg/L) 476 418 1 14 1 11 108 82.1 80.4

Removal (%) - 13.4 72.0 73.5 75.7 82.1 81.6

(^*) Operating conditions: treatment time = 180 min, without initial pH adjustment (pHj 6.0), [Na₂SO₄] = 0 mg/L, T = 21⁰C.

TABLE 4. PAHs concentrations (mg/L) before and after treatment using different retention times^*

PAHs 1^st set of experiments set of experiments

Initial Treatment time Initial Treatment :time (mm) solution (min) solution

0 10 20 30 60 90 120 150 180

NAP 461 406 425 387 623 156 149 136 127 124 11 1

MEN 422 351 220 186 623 157 127 762 688 782 796

ACN 373 332 247 211 601 1 12 098 093 090 095 092

ACA 782 780 553 449 658 187 147 100 107 114 115

FLU 449 445 435 349 451 160 143 104 103 917 8.13

PHE 153 143 104 857 114 348 31 7 256 245 214 203

ANT 157 127 105 807 13.7 4.16 364 300 301 198 247

FLE 655 616 438 347 704 170 153 146 133 872 152

PYR 39.5 321 260 205 527 119 104 990 851 951 974

BAA 852 797 480 338 902 269 233 194 192 117 126

CHR 932 832 578 410 132 282 236 205 202 230 239

BJK 364 336 172 1 38 600 1 52 133 1 19 1 11 088 094

BAP 124 1 16 086 086 257 059 046 042 035 043 047

DAN 077 077 067 063 097 010 011 011 011 013 012

INP 025 023 019 016 016 009 009 005 005 005 004

BPR 042 031 014 011 064 011 015 014 015 015 014

Σ PAHs 513 474 364 299 525 143 126 102 967 885 927 (mg/L)

Removal 968 327 442 736 761 806 81 3 826 823

(%)

(^*) Operating conditions current density = 923 mA/cm , without initial pH adjustment (pH, = 60), [Na₂SO₄] = 0 mg/L, T = 21⁰C TABLE 5. 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 724 117 141 130 109 167

MEN 690 102 11 3 970 788 137

ACN 325 075 078 076 059 083

ACA 753 112 11 9 11 9 919 1432

FLU 501 108 117 105 948 120

PHE 113 258 224 246 173 227

ANT 133 237 236 225 206 291

FLE 551 106 134 109 834 125

PYR 354 715 949 765 650 881

BAA 794 154 126 129 141 224

CHR 970 221 235 209 234 292

BJK 494 144 124 141 058 155

BAP 1 56 024 026 026 019 027

DAN 100 021 025 024 023 034

INP 030 004 006 005 004 007

BPR 030 006 008 005 006 006

Σ PAHs (mg/L) 513 963 103 967 771 112

Removal (%) - 805 784 802 836 757

Operating conditions current density = 923 mA/cm², treatment time = 90 mm, [Na₂SO₄] = 0 mg/L, T = 21⁰C

TABLE 6. PAHs concentrations (mg/L) before and after treatment using different concentrations of supporting electrolyte (Na₂SO₄)^*

PAHs Initial Na ₂SO₄ 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 1 1.8 1 1.1 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 1 1.8 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 2.1 1 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

Σ 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 (pHj = 6.0), T = 21⁰C.

TABLE 7. 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 1 1.0 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 1 1.0 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 1 1.7 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 I.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

Σ 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 (pHj = 6.0), [Na₂SO₄] = 500 mg/L, T = 21⁰C. TABLE 8. PAHs concentrations (mg/L) before and after treatment in optimal conditions^*:

PAHs Solution Removal

Initial Final (%)

NAP 65.3 ±0.3 14.410.2 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

∑ 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 (pHj = 6.0), [Na₂SO₄] = 500 mg/L, T = 21⁰C.

TABLE 9. 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-C₅₀) (mg/L) 170 27 84.1

COD (mg/L) 2, 102 809 61.5

TOC (mg/L) 237 174 26.6

Toxicity

Daphnia magπa test (TU) 4,762 453 90.5

Vibrio fischeri test (Microtox) (TU) 1 ,000 200 80.0

Operating conditions: current density = 9.23 mA/cm , treatment time = 90 min, without initial pH adjustment (pH| = 6.0), [Na₂SO₄] = 500 mg/L, T = 21⁰C.

TABLE 10. Characterization of the creosote oily effluent

Parameters Means values and standard deviation pH 60±01

Conductivity (μS/cm) 322 ±8

POR (mV) 213 + 8

Temperature 20 ± 1

PAHs (mg/L) Aromatic rings

Naphthalene (NAP) 2 218±38 2-Methyl-naphtalene (MEN) 2 188 + 34 Acenaphtylene (ACN) 3 210±05 Acenaphtene (ACA) 3 436 ±175 Fluorene (FLU) 3 184± 20 Phenanthrene (PHE) 3 283 ± 58 Anthracene (ANT) 3 114 + 39 Fluoranthene (FLE) 4 427 + 198 Pyrene (PYR) 4 308 + 109 Benzo(a)anthracene (BAA) 4 91 +38 Chrysene (CHR) 4 87±32 Benzo(b,j,k)f!uoranthene (BJK) 5 58±19 Benzo(a)pyrene (BAP) 5 35±15 Dιbenzo(a,h)anthracene (DAN) 5 08±03 lndeno(1,23-c,d)pyrene (INP) 6 03±02 Benzo(ghι)perylene (BPR) 6 06 + 03 ΣPAHs 247 ± 55

TABLE 11. Treatment of creosote oily effluent using different electrolytic cells:

Parameters Electrolytic cells

C 1 C₂ C₃

Anodic current density 9 12 9 12 9 12 (mA/cm²)

Current intensity (A) 2.4 3.2 1.2 1.6 1.2 1.6

Anode electrode Ti/lrC-2 Ti/lrO₂ Ti/lrO₂ TiZIrO₂ Ti/SnC-₂ Ti/SnO₂

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

Σ PAHs (before 146 140 146 146 155 155 treatment)

Σ 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

TABLE 12. PAHs concentration (mg/L) before and after treatment using experimental C₃ (TiZSnO₂) operated at different current densities^*

PAHs Control Current density (mA/cm²)

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 1 1 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

Σ 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 mm, recycling rate = 3.6 L/min

TABLE 13. PAHs concentration (mg/L) before and after treatment using experimental C₃ (TVSnO₂) 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 1 1 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 1 1 0 10 0 09 0 12

Σ 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/cm², treatment time = 90 mm

TABLE 14. 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 O₂

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

Σ 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.

TABLE 15. PAHs concentration before and after treatment using experimental C3 (Tι/Snθ2) and the optimal conditions^*

PAHs Effluent Degradation

10/.

Untreated Treated

NAP 235 ± 08 486 ± 020 793 ± 04

MEN 205 ± 05 326 + 012 840 ± 08

CAN 246 ±018 039 ± 003 843± 05

ACA 595±51 910 ±039 847 ± 09

FLU 191 ±11 390+ 051 796 ± 34

PHE 245 ± 36 378 ± 042 845 ± 06

ANT 143+ 17 289 ± 074 798 ± 45

FLE 550 + 31 894 ± 070 837 ± 14

PYR 383 ± 22 678 ± 052 822 ± 23

BAA 114± 17 212±014 81 1 ± 33

CHR 108± 11 211 ±031 805 ± 19

BJK 699 ± 095 128 ±010 814± 38

BAP 413 ± 123 083 ± 011 792 ± 32

DAN 096 ± 028 018 ±003 805 ± 27

INP 042 ± 021 007 ± 002 830 ± 29

BPR 071 ±022 016 ±005 777 ± 07

Σ PAHs (mg/L) 293 + 24 505 ± 43 Removal (%) 81 6±22

* Operating conditions current density = 15 mA/cm , treatment time = 90 mm, recycling rate = 36 L/min, without oxygen injection

TABLE 16. Combining batch and continuous mode operations for the treatment of creosote oily effluent using experimental C3 (Ti/Snθ₂):

Exp. Operating Current Recycling Inlet Outlet Retention TreatΣ PAH Σ PAH runs mode density flow rate flow rate flow rate time ment inlet outlet

(mA/cm²) (Umin) (mL/min) (mUmin) (min) time cone. cone.

(min) (mg/L) (mg/L)

A Batch 15 3 6 00 00 90 90 150 302

B Continuous 15 00 50 50 90 60 150 25 4

C Continuous 15 00 50 55 90 300 150 19 1

D Continuous 15 0 0 50 55 90 360 150 21 2

E Continuous 15 00 50 54 90 1080 150 23 5

F Continuous 15 0 0 50 52 90 1440 150 34 4

G Batch 15 3 6 00 00 90 90 176 27 6

H Continuous 15 00 75 75 60 60 176 38 1

I Continuous 15 00 75 77 60 360 rβ 40 8

J Continuous 15 0 0 75 76 60 720 176 60 2

K Continuous 15 00 75 ^"6 60 1080 176 80 0

L Batch 15 3 6 0 0 0 0 90 90 176 26 6

M Continuous 15 00 100 100 45 60 176 65 7

N Continuous 15 00 100 100 45 360 176 84 5

O Continuous 15 00 100 100 45 720 176 90 2

TABLE 17. PAH concentration (mg kg ¹) before and after electro-oxidation of FCO-AIW while imposing the best experimental conditions (current density = 9 mA cm ², treatment time = 90 mm, Na₂SO₄ = 0 25 g L^'1, TS = 20 g L ¹)

FCOAIW Degradation (%)

Untreated Treated

2- ring PAH

NAP 33±01 13±01 610±12

MEN 48±01 15±02 685±31

3- ring PAH

ACN 40±03 13±01 662+04

ACA 37±01 14±01 622±16

FLU - - -

PHE 389+17 235±8 396+07

ANT 266±13 135±06 492±22

4- ring PAH

FLE 1137±23 624±12 451±10

PYR 1807±13 837±14 537±05

BAA 1697±27 984±5 42 O±O 3

CHR 5315±20 2340±53 56 O±O 8

5- ring PAH

BJK 6857±13 3995+120 417±13

BAP 1442+9 978±59 322±35

DAN 1418+18 788±20 444+11

6 - ring PAH

INP 618116 354±11 427+10

BPR 1657±13 938+90 434±50

∑ PAH (mg kg ¹) 22380±171 12091±302

Total degradation (%) - - 499±09 Table 18. Electrochemical treatment of foam concentrate suspensions from soils (FCO- Soil)

Parameters FCO-soil

Untreated Treated

Electrochemical treatment procedure

Current density (mA cm ²) - 9 2

Power consumption (kWh t ¹) - 1 582

Electrolyte (Na₂SO₄) (g L ¹) - 0 25

Form total solids (%) - 2 0

Conductivity (μS cm ²) - 1 696+28

Final temperature - 27±1

Final pH - 2 3+0 1

Residual PAH concentration in foam suspension (mg kg ¹)

2- ring PAH

NAP 514±8 197±16

MEN 382±8 116±19

3- ring PAH

ACN 1236±26 771 ±27

ACA 48 6±1 7 22 8±1 3

FLU 183±5 89 6±10 4

PHE 1670±11 845±41

ANT 390±6 170+11

4- ring PAH

FLE 2980±13 1 960±39

PYR 4480±17 3 703±29

BAA 1615±17 803±37

CHR 2020+ 13 1 730+17

5- ring PAH

BJK 2210+18 1 024±45

BAP 1240+17 583±24

DAN 760±47 408+18

6 - ring PAH

INP 1877±185 1 454±30

BPR 1280±18 825±13

∑ PAH (mg kg ¹) 22 886±395 14 675±90

Total degradation (%) - 44 1±1 8

Treatment cost (USD $ f¹)

Energy - 95

Na₂SO₄ - 4

Total cost - 99 TABLE 19. Data for optimization operation:

Variable Factors levels

(-1) (+1 )

Xi Current intensity 0.5 A 2.0 A

X₂ Electrolysis time 10 min 40 min

X₃ NaCI Cone. 0.2 g.L^"1 1.0 g.L^"1

X₄ Type of anode Ti/lrC-2 Ti/SnO₂

TABLE 20. Matrix for calculation of effects for different anode materials (Ti/lrO₂ and Ti/SnO₂):

Assays Xi X₂ X₃ X₄ Assays Xi X₂ X₃ X₄

1 - - - - 9 - - - +

2 - + - - 10 - + - +

3 - - + - 1 1 - - + +

4 - + + - 12 - + + +

5 + - - - 13 + - - +

6 + + - - 14 + + - +

7 + - + - 15 + - + +

8 + + + - 16 + + + +

TABLE 21. Factorial experiment design and experimental results

Assays Ui U₂ U₃ U₄ Response (%)

(Atrazine degradation efficay)

1 05A 10 min 0 2 g L^"1 IrO₂ 166

2 0.5 A 40 min 0 2 g L-¹ IrO₂ 308

3 05A 10 min 1 0g_.ι_-¹ IrO₂ 334

4 0.5 A 40 min 1 •0 g.L^"1 IrO₂ 597

5 2.0A 10 mm 0 2 g L-¹ IrO₂ 490

6 2.0A 40 mm 0 2 g.L-¹ IrO₂ 757

7 2 OA 10 mm 1 O g L^"1 IrO₂ 872

8 20A 40 mm 1 Og L^"1 IrO₂ 951

9 05A 10 mm 0 2 g L^"1 SnO₂ 5.8

10 05A 40 mm 0 2 g L-¹ SnO₂ 286

11 05A 10 mm 1 0 g.L^"1 SnO₂ 14.3

12 05A 40 mm 1 0 g.L^"1 SnO₂ 449

13 2 OA 10 mm 0 2 g L¹ SnO₂ 244

14 2 OA 40 mm 0 2 g L-¹ SnO₂ 599

15 2OA 10 mm 1 0 g.L¹ SnO₂ 25.0

16 20A 40 mm 1 0 g.L-¹ SnO₂ 64.5

Although the present invention has been explained hereinabove by way of preferred embodiments thereof, it should be pointed out that any modifications to these preferred embodiments within the scope of the appended claims is not deemed to alter or change the nature and scope of the present invention

Claims

1. A method for decontaminating wastes containing toxic organic molecules and for degrading said toxic organic molecules, the method comprising the steps of:

a) obtaining an aqueous mixture by mixing said wastes with an effective amount of a surfactant to form micelles of surfactants, said micelles entrapping said toxic organic molecules;

b) extracting from said aqueous mixture obtained in step a) an aqueous solution comprising said micelles entrapping said toxic organic molecules;

c) filling an electrolytic cell with said aqueous solution obtained in step b), said electrolytic cell comprising at least one pair of electrodes; and

d) applying to said at least one pair of electrodes a given current for a period of time suitable to electro-oxidize said toxic organic molecules.

2. The method of claim 1 , wherein said wastes are hazardous or polluted municipal or industrial wastewaters, soils, sands, sediments or sludge.

3. The method of claim 1 or 2, wherein said wastes are stemmed from aluminum industry.

4. The method of any one of claims 1 to 3, wherein said toxic organic molecules are creosotes, polycyclic aromatic hydrocarbons, hydrocarbons from oils, hydrocarbons from greases, hydrocarbons from petroleum, chlorinated molecules, pesticides, endocrine disruptors, polychlorinated biphenyl molecules, polychlorinated, dyes or mixtures thereof.

5. The method of any one of claims 1 to 4, wherein said toxic organic molecules comprise polycyclic aromatic hydrocarbons and hydrocarbons from oils and/or hydrocarbons from grease.

6. The method of any one of claims 1 to 5, wherein said surfactant is an amphoteric or non-ionic surfactant.

7. The method of claim 6, wherein the surfactant is cocamidopropyl hydroxysultaine, polyoxoethylene(20)sorbitan monooleate, polyoxyethylene(10)isooctylphenyl ether, polyoxyethylene-(12)isooctylphenyl ether, or polyethylenglycol dodecyl ether (Brij 35).

8. The method of any one of claims 1 to 7, wherein the aqueous solution comprises polycyclic aromatic hydrocarbons at a concentration up to about 30 g by kg of said aqueous solution.

9. The method of any one of claims 1 to 8, wherein the electrodes are cylindrical electrodes.

10. The method of any one of claims 1 to 9, wherein each pair of electrodes comprises a dimensionally stable anode with high oxygen overvoltage and a cathode.

11. The method of claim 10, wherein said dimensionally stable anode is made of titanium coated with iridium oxide, ruthenium oxide or tin oxide.

12. The method of claim 10 or 11 , wherein said cathode is made of titanium or stainless steel.

13. The method of any one of claims 1 to 12, wherein said current has a voltage lower than 40 Volts.

14. The method of any one of claims 1 to 13, wherein said current has a density ranging from about 3.0 to 30 mA/cm².

15. The method of any one of claims 1 to 14, wherein in step d), said suitable period of time is ranging from about 10 to 200 minutes.

16. The method of any one of claims 1 to 15, further comprising adding a given amount of air or oxygen to the toxic aqueous solution before or during steps c) or d).

17. The method of any one of claims 1 to 16, further comprising adding a given amount of at least one supporting electrolyte to the toxic aqueous solution before or during steps c) or d).

18. The method of claim 17, wherein the supporting electrolyte comprises Na₂SO₄, NaCI, KCI, MgCI₂, CaCI₂, HCI, H₂SO₄, MgSO₄, (NH₄)₂SO₄, NH₄CI or a mixture thereof, and is present in the toxic aqueous solution with a concentration ranging from about 0.5 to about 4 0 g/L.

19. The method of any one of claims 1 to 18, further comprising the steps of: before steps c) or d), measuring an initial pH of the toxic aqueous solution, and in the case said measured initial pH is lower than 4, adjusting said initial pH to a new pH ranging from 4 to 7 by adding to the solution a given amount of a base, or in the case said measured initial pH is greater than 7, adjusting said initial pH to the new pH ranging from 4 to 7 by adding to the solution a given amount of an acid.

20. The method of any one of claims 1 to 19, further comprising the steps of:

e) analysing the aqueous solution after step d) for detecting remaining toxic organic molecules; and f) in case remaining toxic organic molecules are detected in step e), applying again step c) and d) of said method until clearing said toxic organic molecules from said aqueous solution.

21. The method of any one of claims 1 to 20, further comprising providing turbulences to the solution during step d) in order to enhance the electro-oxidation of said toxic organic molecules.

22. The method of any one of claims 1 to 22, wherein in said step b), said aqueous solution is extracted from the wastes by decantation or flotation.

23. The method of any one of claims 1 to 22, wherein said method is carried out in a batch, semi-continuous or continuous mode.

24. An electrolytic cell for electro-oxidizing organic molecules contained in an electrolyte, said cell comprising:

an electrolytic vessel comprising an inlet for filling said vessel with said electrolyte and an outlet for draining said electrolyte;

a tubular weir having a closed end, an open end and a tubular wall provided with a plurality of perforations, the tubular weir being installed into the vessel with its open end in connection with the inlet for receiving said electrolyte; and

a pair of cylindrical electrodes installed into the vessel for passing an electric current through the electrolyte, said pair of electrodes comprising a first perforated cylindrical electrode surrounding said tubular weir and a second cylindrical electrode surrounding said first perforated cylindrical electrode.

25. The electrolytic cell of claim 24, further comprising a wall installed into the vessel above the inlet for supporting said weir and electrodes, said wall and vessel forming /b

as such an inlet zone, said wall being provided with at least one orifice allowing the electrolyte to flow through the wall from the inlet zone toward the weir.

26. The electrolytic cell of claim 25, further comprising an agitator installed into the inlet zone for providing turbulences to the electrolyte.

27. The electrolyte cell of any one of claims 24 to 26, wherein the vessel and/or the tubular weir are made of a plastic material.

28. The electrolyte of claim 27, wherein the plastic material is polyvinylchlohde (PVC).

29. The electrolyte cell of any one of claims 24 to 28, wherein the first perforated electrode is an anode and the second electrode is a cathode.

30. The electrolyte cell of claims 29, wherein the anode is a dimensionally stable anode.

31. The electrolytic cell of claims 29 or 30, wherein the anode is made of titanium coated with iridium oxide, ruthenium oxide or tin oxide.

32. The electrolytic cell of any one of claims 29 to 31 , wherein the cathode is made of titanium or stainless steel.

33. The electrolytic cell of any one of claims 24 to 32, wherein the second electrode is perforated.

34. An electrolytic system for electro-oxidizing organic molecules comprising at least one electrolytic cell as defined in any one of claims 24 to 33.

35. The electrolytic system of claim 34, wherein said organic molecules are creosotes, polycyclic aromatic hydrocarbons, hydrocarbons from oils, hydrocarbons from greases, hydrocarbons from petroleum, chlorinated molecules, pesticides, endocrine disruptors, polychlorinated biphenyl molecules, polychlorinated dibenzodioxins, dyes or mixtures thereof.