EP1115662A1 - Method for treating organic substances in an aqueous medium, in particular effluents and electrochemical device for implementing said method - Google Patents

Abstract

The invention concerns a method for electrochemical treatment of an aqueous composition comprising one or several organic substances, using an electrochemical reactor comprising an anode section and a cathode section, said sections being separated by a suitable separator. The invention is characterised in that it consists in introducing said aqueous composition into said electrochemical reactor anode section, said anode section containing a liquid support wherein suspended electrochemically active particles are present and in simultaneously or successively subjecting the medium derived from the introduction of said aqueous composition to an oxidation potential and the action of ultrasounds. The invention also concerns en elctrochemical device, in particular for implementing said method.

Description

PROCESS FOR TREATING ORGANIC SUBSTANCES IN AQUEOUS MEDIA, ESPECIALLY AFFLUENTS AND ELECTROCHEMICAL DEVICE FOR CARRYING OUT THE METHOD

The present invention relates to a method of electrochemical treatment of an aqueous composition comprising one or more organic substances, such as those present in the effluents, by means of an electrochemical reactor comprising an anode compartment and a cathode compartment, said compartments being separated by an appropriate separator. The present invention also relates to the electrochemical device with liquid electrolyte, in particular for implementing the treatment method.

It is already known that degradation of organic compounds in solution can be achieved using different techniques such as the generation of ozone in the solution to be treated (P. Kôtz & S. Stucki - J. Electroanal, Chem. 228, 407 - 1987), the use of bacteria which specifically destroy certain compounds (SW Hooper, CA. Pettigrew & GS, SAILER Environ. Toxicol. Chem. Vol. 9 - p 655-1990), the use of electrochemistry which eliminates some easily oxidizable organic compounds (J. O'M. Bockris - "Environmental Chemistry", Plenum Press, NY - 1977) or the use of ultrasound which have been used to break certain molecules containing aromatic rings ( GJ Price, P. Matthias & EJ Lenz - Trans. IchemE., Vol. 72 part B, Feb. 1994).

The electrochemical incineration of p-benzoquinone is for example proposed in an article by J. Feng, LL. Houk, DC Johnson, SN Lowery & JJ Carey (J. Electrochem. Soc. Vol. 142, n ° 11 of November 1995- ρ3626 / 3631). The double envelope reactor used comprises a cell with a single compartment in which are placed a titanium anode covered with a mixed iron / lead oxide maintained at a temperature of 60 ° C. by suitable circulation, and the cathode of stainless steel 316 This arrangement allows the authors to electrochemically degrade p-benzoquinone in acetate medium (pH 5), but they noted the formation of brownish to black compounds, originating from the supposed formation of The degradation kinetics observed under these conditions are slow since a 40-hour treatment is proposed before the COD (chemical oxygen demand) decreases to a substantially zero value; moreover, deterioration of the electrodes is noted, even at low currents (10 mA / cm ² ).

In another article the anodic oxidation of phenol in the presence of NaCI is proposed during the treatment of brackish water (Ch. Comninellis & A. Nerini in Journal of Applied Electrochemistry 25 (1995) pp23 / 28); the proposed treatment is carried out in a 4-liter reactor in a closed circuit in which a circulation of the solution is carried out during the treatment on anodes of various compositions: Pt, SnO ₂ / Ti, lrO ₂ / Ti, RuO ₂ / Ti, PbO ₂ / Ti. The catalytic role of NaCl has been shown in the case of the use of iridium oxide. However, in the best of cases, the proposed treatment leads to complete degradation of the phenol (10 ^"2 M) in three to four hours for a volume of 4 liters (ie a degradation rate of 5 10 ^{" 4} moles / h) : the current densities used are in the range 50-300 mA / cm ² at the temperature of 50 ° C and at pH 12.2. The degradation of phenol is therefore obtained at a rate of 45 to 55 mAh / g of the product.

In another article, JL Boudenne, O. Cerclier, J. Galéa, E. Van der Vlist (Applied Catalysis A: General 143 (1996) 185-202 describe the oxidation of an aqueous solution of phenol using a carbon black electrode in suspension.

In another article, Kl. Kawabata & SI. Ùmemura (Ultrasonics Symposium (1992)) proposes to use the effect of sonochemical reactions by focusing ultrasound. However, their work relates only to the oxidation of iodides and is based on other work which supports the thesis that ultrasound causes high local temperatures and high pressures, in the vicinity of the bubbles formed (cavitations).

In another article, the use of very high power ultrasound for the destruction of aromatic compounds in aqueous solution was proposed by G.J. Price, P. Matthias & E.J. Lenz (Trans IChemE, Vol 72,

Part B, February 1994 - pp27 / 31). The target compounds were PCBs

(polychlorinated biphenyls) as well as other organic compounds chlorinated, but also highly carcinogenic compounds such as certain polyaromatic hydrocarbons. The stability of these compounds makes them difficult to destroy, which has led the authors to consider the use of very high power ultrasound. This technique was proposed as early as the 1950s to destroy compounds of the benzene, phenol, halogen-benzenes type, etc. at low concentrations. However, this process only results in slow degradation of the compounds (at least one hour of treatment for an ultrasonic power of 30 W / cm ² ). The powers proposed for implementing the process are very high: 10 kW / liter. However, the degradation compounds possibly formed have not yet been analyzed.

The search for original and inexpensive techniques for the degradation of organic compounds in solution has developed in recent years insofar as they are likely to provide a solution to the increasingly severe constraints linked to current ecological concerns; these techniques also aim to save raw materials while reducing the volumes of waste linked to currently polluting industrial activities.

The object of the present invention is to provide an original method for degrading organic compounds, in particular in aqueous solution, consisting of the combination of two techniques conventionally proposed; however, unlike the techniques already known, at least for the degradation of such compounds, the electrode used is used in the dispersed form and according to an adapted experimental protocol which makes it possible to suppress practically any passivation of the cited electrode under conditions concentration encountered in industry (in general up to 10 ^" 3/10 ^{" 2} M but possibly going as far as saturation), and thus to be able to envisage a possibly continuous treatment of a composition to be treated, in particular a solution, comprising such compounds. Another object of the present invention is to reduce the COD (chemical oxygen demand) or the amount of total organic carbon in the effluents in proportions such that these treated effluents can be discharged directly into the river or into a biological treatment basin. . This treatment concerns in particular the phenols because the phenols are very toxic vis-à-vis the microorganisms destroying the effluents in the biological treatment basins. Osidic derivatives increase COD.

According to the invention, the electrochemical treatment process is characterized in that the said aqueous composition is introduced into the anode compartment of the electrochemical reactor, the said anode compartment containing a liquid support in which are present electrochemically active particles of sufficiently high potential. high to allow the oxidation of organic substances and in that simultaneously or successively the medium resulting from the introduction of said aqueous composition is subjected to an oxidation potential and to the action of ultrasound.

By the expression "aqueous composition", in particular the effluents, is meant the composition which comprises the organic substances and which is introduced into the electrochemical reactor for the purposes of treatment.

By the expression "liquid carrier" is meant a liquid electrolyte judiciously chosen to carry out the treatment without major inconvenience. Mention may in particular be made of aqueous liquids containing a carbonate buffer. The expression “resulting medium” means the liquid support into which the aqueous composition has been introduced.

This aqueous composition is generally an aqueous solution of organic substances. However, the aqueous composition may also contain organic substances in suspension. By “electrochemically active particles in suspension”, it is meant that the solid particles are kept in suspension by a well-known stirring means: rotating blades, air flow or other equivalent means. It has in fact been found that the combination of an electrode, in the form of electrochemically active particles in suspension, with the action of ultrasound makes it possible to avoid passivation of said particles, due to the physical and possibly chemical degradation caused by the action of said ultrasound.

By the expression "simultaneously or successively" is meant that the ultrasound can be applied at the same time as the oxidation potential, but that this ultrasound can also be applied after the oxidation reaction. It is also possible to simultaneously apply the two means and then continue the application of ultrasound.

The separators used are well known and generally have the particularity of being either microporous or cationic conductors in order to avoid transfers of polluting compounds to the cathode compartment. The current collector located in the anode compartment is made of unassailable metal, for example platinum titanium and the cathode located in the cathode compartment generally consists of an expanded steel or nickel.

The dispersed electrode consists of a suspension of electrochemically active particles of small particle size, among which there may be mentioned preferably electronic conductive metal oxides.

Among the metal oxides, mention is preferably made of MnO ₂ , PbO ₂ , SnO ₂ , Fe ₃ θ ₄ , perovskites, for example LaNiO ₃ , spinels, for example Fe ₃ θ4. Preferably said electrochemically active particles are present in the medium in a proportion between 1 to 10 g / l.

By electrochemically active particles of small particle size is meant particles whose average particle size will not exceed 1000 μm, generally 5 to 100 μm, it being understood that the size may vary during the electrochemical treatment. Among the organic substances, mention may in particular be made of oxidizable substances, for example those having alcoholic or phenolic functions, including the osidic derivatives although other organic substances may also be destroyed in the treatment process according to the invention.

The method has the advantage of being able to treat media resulting from the introduction of aqueous compositions, the concentrations of which can range up to approximately 10 ⁻² M, which makes it possible to envisage an optionally continuous treatment of a solution to be treated comprising As indicated above, the concentration of the various solutes can nevertheless go up to the solubility threshold.

The treated aqueous medium preferably has a high pH at the start of the reaction, in particular greater than 7, advantageously between pH 8 and pH 14, even more advantageously between 10 and 11 and more precisely when the buffer is a carbonate buffer, a pH slightly lower. at around pH 11.

During the oxidation reaction, this pH may vary and may drop to a pH close to 5. In the case of a potentiostatic control, the oxidation potential applied to the medium is preferably between 500 and 2000 mV / DHW. Advantageously, the oxidation potential is close to 800 mV / DHW. During the application of the oxidation potential, an oxidation current is observed which reveals an oxidation peak whose intensity decreases in a manner resembling an exponential.

In the case of an operation in galvanostatic mode, the anodic current density is preferably between 5 mA / cm ² and 200 mA / cm ² of the apparent surface of the anodic collector.

The frequency of ultrasound applied to the medium is preferably between 16 kHz and 500 kHz, especially around 22 kHz. According to a preferred variant following the sono-electrochemical cycle described above, we applies after an appropriate treatment time a higher oxidation potential close to 1100 mV / DHW in order to cause the release of oxygen and to significantly amplify the effect of ultrasound on the surface of the grains of electrochemically active particles to avoid passivation and / or mechanically destroying the very fine passivation layer possibly formed on the oxide powder.

The originality of the invention therefore lies in the association of two techniques, that of ultrasound and that of electrochemistry and by defining a specific degradation cycle; this protocol makes it possible to destroy the compounds without significant passivation of the suspension electrode used, in the range of the concentrations tested (up to approximately 10 ^"2 M).

The process according to the invention can be carried out continuously by decantation, filtration, elimination of small particles which could remain in suspension and recycling of a suspension. The invention also relates to an electrochemical device with liquid electrolyte, in particular for implementing the treatment method according to the invention, characterized in that it comprises a cathode compartment, an anode compartment, said compartments being separated by a separator, said cathode compartment which may include a reference electrode and said anode compartment comprising a current collector, an ultrasonic generator, a stirring means and a means for introducing an aqueous composition comprising one or more organic substances, the liquid electrolyte of the anode compartment comprising electrochemically active particles in suspension.

Figure 6 attached describes such a device. This figure is a schematic longitudinal sectional view of a device for implementing the method according to the invention.

A Grignard reactor 1 is used in which is fixed a separating membrane 4 defining an anode compartment 2 and a cathode compartment 3. A current collector 5 in platinum titanium is present around this membrane in the anode compartment, as close as possible to its surface in order to reduce the ohmic drop in the cell and an inert counter electrode 6 made up of a Expanded nickel is present in the cathode compartment also as close to the surface of the membrane for the same reasons. These two electrodes 5 and 6 are connected to a generator 7. The electrochemical reactor contains a buffer solution of sodium carbonate in the two compartments, which serves as a support solution for the experiments of destruction of the organic compounds of the aqueous composition.

The electrochemical reactor comprises an opening 8 allowing the aqueous composition to be introduced. An ultrasonic probe 9 is positioned so that the probe tip 10, the diameter of which is approximately 19 mm, plunges into the support liquid of the anode compartment. The ultrasound probe is connected to an ultrasound generator (not shown).

The liquid support comprises PbO ₂ particles of small particle size (close to 5 to 10 μm) maintained in suspension by an agitator (not shown) at a rate of 1 g / 250 ml. The following examples illustrate the invention without limiting it in any way.

In order to demonstrate the effectiveness of the treatment method according to the invention, several examples are provided for comparison (examples 3, 4 and 5) with the present method, which is described in examples 1 and 2. • The example 1 shows the associated action of electrochemistry and ultrasound, while maintaining the potential of the lead oxide suspension electrode where oxidation takes place, at +800 mV / DHW,

• Example 2 shows the effect of the proposed cleaning period on the behavior of the electrode, by periodic incursion of the potential of the working electrode (lead oxide suspension) to a higher value: 1000 to 1200 mV / DHW for a defined time aimed at producing oxygen release and therefore mechanical cleaning of the electrode in suspension due to the application of ultrasound. Example 3 shows the electrochemical degradation of phenol on platinum titanium electrode alone, without oxide in suspension,

^• Example 4 shows the electrochemical degradation alone (without ultrasound) of the chosen model compound: phenol, on an electrode in suspension of powder of a metal oxide brought to the potential of +800 mV / ECS,

• In Example 5, in order to highlight the advantage of the technique according to the invention, the experiment is carried out under the same conditions as in Example 2, with the exception of the absence of metal oxide suspended in this case,

In all of the examples below, a Grignard reactor was used in which the separating membrane of the two reactor compartments was fixed; the platinum titanium current collector was then placed around this membrane in the anode compartment, as close as possible to its surface in order to reduce the ohmic drop in the cell, then the counter electrode in the cathode compartment, also as close as possible of the surface of the membrane for the same reasons, and the necessary quantity of a sodium carbonate buffer solution was then introduced into the two compartments, which will serve as a support solution for the experiments of destruction of the organic compounds in solution .

Example 1

According to this example, the cell used is supplemented by the installation of an agitator and addition of the quantity of metal oxide required in the anode compartment. The working electrode is brought to a potential of + 800 mV / ECS and an injection of phenol is then carried out, leading to a concentration of the order of 10 ⁻³ M in the agitated solution of the anode compartment (approximately 250 ml), in the presence of ultrasound at a frequency of 20 kHz and the corresponding oxidation current is recorded for injections of phenol at increasing concentrations. The results are indicated in the table below and on the curve of FIG. 1 in which the horizontal axis represents the time elapsed since the start of the experiment and the vertical axis the measured oxidation currents. TABLE 1

phenol no. C eq. m Ah mAh yield te. ech. (mfi) (M) (theoretical) (measured) (%) (mn)

1 1.01 0.11 10 ° 8.09 0.93 11.5 20

2 5.07 0.54 10 ° 40.45 5.84 14.4 25

3 5.07 0.54 10- ³ 40.45 6.32 15.6 30

4 8.11 0.86 10 ° 64.72 7.98 12.3 40

5 10.13 1.08 10 ^"3 80.89 9.02 11.2 45

6 10.13 1.08 10 ° 80.89 8 9.9 50

Example 2

The experimental conditions defined in Example 1 were used. The working electrode is therefore brought to the potential of 800 mV / DHW and after each injection / oxidation of the compound and return of the oxidation current to a value close to the value of the base current, a so-called 'in situ' cleaning phase is carried out, bringing the potential of the electrode to a value greater than 1000 mV / DHW in order to cause the release of oxygen and facilitate the mechanical degradation of the products passivators formed on the surface of the suspended working electrode due to the application of ultrasound which cause very significant mechanical effects on the oxygen bubbles which are released.

The results are collated in the table below. The shape of the curves representing the oxidation current of the compound to be destroyed is reproduced in appended FIG. 2 in which there is very little change in its shape during the successive injections of the compound to be destroyed; indeed, no significant deformation of the shape of the curves of the oxidation current, nor appearance of a second oxidation peak was noted, at least until the equivalent concentrations reached, that is to say approximately 10 ^"2 M cumulated. TABLE 2

phenol no. C eq. m A h m Ah yield t „, scale. (mg) (M) (theoretical) (measured) (%) (min)

1 7 0.75 10 ° 55.88 8.57 15.33 30

2 10 1, 06 10 ° 79.83 10.46 13, 1 30

3 12 1.28 10 ° 95.8 13, 15 13.73 30

4 15 1.6 10 ° 1 19.74 15.05 12.57 20

5 18.75 2.0 10 ° 149.68 18.47 12.34 20

6 18.75 2.0 10 ° 149.68 18.47 12.43 20

7 5.02 0.53 10 ° 40.07 6.78 16.9 20

8 10.24 1.09 10 ° 81.75 12.81 15.67 20

Example 3

In this example, the cell is used as it is, without suspension of metal oxide and without ultrasound. The working electrode is brought to the potential of 800 mV / DHW and the measured oxidation currents are recorded as a function of time during the successive injections; the measured currents are reported in attached figure 3 in which the horizontal axis represents the time elapsed since the start of the experiment and the vertical axis the measured oxidation currents. It can be noted that the measured oxidation current is extremely low from the second injection, a sign of a significant passivation of the titanium-platinum working electrode which then becomes unusable very quickly.

Example 4

Under the same conditions as Example 3, and after setting up an agitator and adding the quantity of metal oxide required in the anode compartment, the working electrode is still brought to the potential of +800 mV / DHW ; one then proceeds to an injection of phenol leading to a concentration of the order of 10 ^{~ 3} M in the agitated solution of the anode compartment, and the corresponding oxidation current is recorded; several injections of phenol at increasing concentrations are carried out. The results are collated in the table below. As can be seen in the appended figure 4, if one proceeds to a series of injections having waited after each for the oxidation current to return to a value close to the base current, it is noted that one still gradually leads to a blockage of the activity of the suspension electrode used. It is also interesting to note the evolution of the shape of the curve representing the oxidation current; as the injections progress, it presents a second oxidation peak which is more spread out and with retention times much longer than that of the oxidation peak which takes place at the time of the injection of the product; this change in the shape of the curve of the oxidation current is representative of a passivation of the electrode in suspension, but it can be seen that the blocking of the electrode is much slower than without this metal oxide in suspension. TABLE 3

Example 5

In this example, the same experimental conditions were used as in Example 2, except that no metallic oxide in suspension was introduced. The working electrode is therefore brought to the potential of 800 mV / DHW and after each injection / oxidation of the compound and return of the oxidation current to a value close to the value of the base current, a so-called cleaning phase is carried out. 'in situ' by bringing the potential of the electrode at a value greater than 1000 mV / DHW in order to cause the release of oxygen and facilitate the mechanical degradation of the passivating products formed on the surface of the working electrode in suspension due to the application of ultrasound which causes very important mechanical effects on the oxygen bubbles which are released.

The results are collated in the table below.

FIG. 5 represents the shape of the oxidation current of the compound injected during the first two injections; from the second injection, the oxidation current becomes extremely low, showing the ineffectiveness of the protocol under these conditions and thus the catalyzing role of degradation played by the metal oxide in suspension used.

TABLE 4

In Figures 2 and 5 on the abscissa axis, the symbol "SS" represents a break in continuity in the scale.

Claims

1. A method of electrochemical treatment of an aqueous composition comprising one or more organic substances, by means of an electrochemical reactor comprising an anode compartment and a cathode compartment, said compartments being separated by an appropriate separator, characterized in that one introduces said aqueous composition in the anode compartment of the electrochemical reactor, said anode compartment containing a liquid support in which electrochemically active particles are present in suspension and in that simultaneously or successively the medium resulting from the introduction of said aqueous composition is subjected to oxidation potential and the action of ultrasound. 2. Treatment process according to claim 1, characterized in that the organic substances, in particular oxidizable, have alcoholic functions. 3. Treatment process according to claim 2, characterized in that the organic substances are chosen from alcoholic derivatives, phenolic derivatives, osidic derivatives. 4. Treatment process according to one of claims 1 to 3, characterized in that the organic substance or substances are present in the medium up to approximately 10 ^{* 2} M. 5. Treatment process according to claim 1, characterized in that the electrochemically active particles are chosen from metal oxides. 6. Treatment process according to claim 5, characterized in that the metallic electronic conductive oxides are chosen from MnO ₂ , PbO ₂ , SnO ₂ , Fe ₃ O ₄ , perovskites, spinels. 7. Treatment method according to claim 1, characterized in that said electrochemically active particles are present in the medium in a proportion between 1 to 10 g. 8. Treatment process according to one of the preceding claims, characterized in that the pH of the medium is greater than approximately 7 at the start of the reaction. 9. Treatment process according to claim 8, characterized in that the pH of the medium is between 8 and 14 at the start of the reaction. 10. Treatment method according to one of the preceding claims, characterized in that the oxidation potential is between 500 mv / DHW and 2000 mv / DHW in the case of a potentiostatic control and the anode current density is understood between 5 mA / cm ² and 200 mA / cm ² of the apparent surface of the anode collector. 11. Treatment method according to one of the preceding claims, characterized in that the frequency of the ultrasound is between 16 kHz and 500 kHz. 12. Treatment method according to one of the preceding claims, characterized in that, following the application of the oxidation potential, the medium is subjected to a second oxidation potential such that it allows oxygen to be released , while continuing to apply ultrasound. 13. Treatment method according to one of claims 1 to 12, useful in particular for the treatment of polluting effluents. 14. electrochemical device with liquid electrolyte, in particular for implementing the treatment method according to one of claims 1 to 13, characterized in that it comprises a cathode compartment (3), an anode compartment (2), said compartments being separated by a separator (4), said anode compartment comprising a current collector (5), an ultrasonic generator (10), a stirring means and a means for introducing (8) an aqueous composition comprising a or several organic substances, the liquid electrolyte of the anode compartment comprising electrochemically active particles in suspension.