SK4052001A3 - 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

Technical field

The present invention relates to a process for the electrochemical treatment of an aqueous compound comprising one or more organic substances present in waste water by means of an electrochemical reactor comprising an anode portion and a cathode portion, said portions being separated by a suitable separator.

The present invention also relates to an electrochemical device with liquid electrolyte, in particular for carrying out this treatment method.

BACKGROUND OF THE INVENTION

It is known that the degradation of organic compounds in solution can be accomplished using various known techniques, such as the generation of ozone in a solution to be treated (P.Kôtz & S.Stucki - J. Ectroanal, Chem. 228, 4071987), the use of bacteria which specifically destroy certain compounds (SWHooper, CAPettigrew & GSSailer Environ. Toxicol. Chem. Vol. 9 - pp. 655-1990), the use of electrochemistry that allows the removal of several readily oxidizable organic compounds (J.O'M.Bockris - "Environmental Chemistry", Plenum Press, NY -1977) or the use of ultrasound, which was used to break down some molecules containing aromatic nuclei (GJPrice, P. Matthias & EJLenz-Trans. IchemE., Vol. 72 Part B, February 1994) ).

For example, the electrochemical combustion of benzoquinone is suggested in an article by J.Feng, LLHouk, DCJohnson, SNLowery & JJCarey (J.Ectrochem. Soc. Vol. 142, n ° 11, November 1995 - pages 3626/3631). ). Used • ···················

The double-shell reactor contains a single-compartment chamber containing one titanium anode coated with an iron / lead oxide maintained at 60 ° C by means of an adapted circulation. and one stainless steel cathode 316. This assembly allows the authors to electrochemically degrade p-benzoquinone in an acetate composition (pH 5), but the authors noted the formation of brown to black compounds resulting from the anticipated formation of humic compounds during condensation / degradation under experimental conditions.

The degradation kinetics observed under these conditions is slow, as a 40 hour treatment is proposed before the COD (chemical oxygen demand) drops to a value of substantially zero; electrode damage is even recorded at low currents (10 mA / cm ² ).

In another article, the anode oxidation of phenol in the presence of NaCl is suggested during brackish water treatment (Ch.Comninellis & A. Nerini in Journal of Applied Electrochemistry 25 (1995) pp 23/28); the proposed treatment is carried out in a 4 liter closed-loop reactor in which the solution is circulated during treatment on anodes from various compounds: Pt, SnO ₂ / Ti, IrO ₂ / Ti, RuO ₂ / Ti, PbO ₂ / Ti. The catalytic role of NaCl has been demonstrated when iridium oxide is used. At the same time, in the better case, the proposed treatment leads to a complete degradation of phenol (10 ² M) in three to four hours for a 4 liter volume (ie 5 x 10 ⁴ mol / h degradation rate): current densities used range from 50 mA / cm ² to 300 mA / cm ² at 50 ° C and pH 12.2. Thus, phenol degradation is obtained in a ratio of from 45 mAh / g to 55 mAh / g to the composition.

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

In another article, K. Kawabata & Umemura (Ultrasonic Symposium (1992)) propose to utilize the effect of sonochemical reactions by ultrasound focusing. However, their work only concerns the oxidation of iodides and relies on others

-3 · ···· 9 ·········································· works that support the hypothesis that ultrasound causes high local temperatures and high local pressures near the formed bubbles (cavitations).

In another article, GJ Price, P. Matthias & EJLenz (Trans IchemE, vol. 72, part B, February 1994-pp 27/31) suggested the use of ultrasound of very high intensity to degrade aromatic compounds in aqueous solution. The targeted compounds were PBCs (polychloro-biphenyl derivatives) as well as other organic compounds containing chlorine, but also very carcinogenic compounds such as certain polyaromatic hydrocarbons. The stability of these compounds makes them difficult to degrade, leading the authors to propose the use of ultrasound of very high intensity. This technique was designed as early as the 1950s to degrade compounds such as benzene, phenol, halobenzene, etc. at low concentrations. However, in this way only compounds were slowly degraded (at least one hour of treatment to an ultrasound intensity of 30W / cm ² ). The proposed intensities for carrying out the process are very high: 10 kW / liter. However, any degradation compounds formed have not yet been analyzed.

Research on original and inexpensive techniques for degrading organic compounds in solution has progressed in recent years, so that they are able to deliver the result with increasingly stringent constraints associated with current environmental concerns; these techniques also aim to save raw materials by reducing waste volumes associated with current polluting industrial activities.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for degrading organic compounds, particularly in aqueous solution, which is based on the combination of two commonly known known techniques; however, in comparison with the known techniques, at least as far as the degradation of these compounds is concerned, the electrode used is in dispersed form and according to a customized experimental protocol which makes it possible to eliminate virtually all of the passivation of said electrode. ··· • ··· · · ··· · ·

-4 • ···· ··· · · · · · · · · · in terms of concentrations encountered in the industry (generally up to 1 (/ 710 'M ^2, but which can reach saturation), and this can to suggest a possible continuous treatment of the treatment compound, in particular of the solution containing these compounds.

It is a further object of the invention to reduce COD (chemical oxygen demand) or the amount of total organic carbon in the wastewater in such proportions that the wastewater can be discharged directly into the river or into a biologically treated tank. This treatment mainly concerns phenols because phenols are very toxic to microorganisms that degrade wastewater in biologically treated tanks. Oxide derivatives increase COD.

According to the invention, the electrochemical treatment method is characterized in that the aqueous compound is introduced into the anode part of the electrochemical reactor, the anode part comprising a liquid carrier in which electrochemical active particles are present in suspension, the potential of which is sufficiently high to allow oxidation of organic substances; wherein the resulting composition, which results from the introduction of said aqueous compound, is simultaneously or sequentially exposed to oxidative potential and ultrasound.

The term "aqueous compound", in particular waste water, means a compound that contains organic substances and which is introduced into an electrochemical reactor for treatment.

The term "liquid carrier" means a carefully selected liquid electrolyte for carrying out the treatment without major difficulty. Particular mention may be made of aqueous liquids containing a carbonate buffer.

The term "resulting composition" refers to a liquid carrier into which an aqueous compound has been introduced.

This aqueous compound is generally an aqueous solution of organic matter. However, the aqueous compound may also contain organic substances in suspension.

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The term "active electrochemical particles in suspension" means that the solid particles are kept in suspension by agitation: rotating blades, air flow or other similar means.

It has been found that the combination of an electrode in the form of electrochemically active particles in suspension with the effect of ultrasound has made it possible to avoid particle passivation due to physical and possibly chemical degradation caused by the effect of ultrasound.

By "concurrently or sequentially" is meant that ultrasound can be applied at the same time as the oxidation potential, or ultrasound can also be applied after the oxidation reaction. It is also possible to apply both agents at the same time and then proceed with the application of ultrasound.

The separators used are well known and generally have the main feature of being either microporous or cathode conductors to prevent the transfer of contaminants towards the cathode section.

The current collector disposed in the anode portion is made of a durable metal, for example platinum titanium, and the cathode disposed in the cathode portion is generally composed of a steel or nickel dispersion.

The dispersion electrode consists of a suspension of small particle size electrochemically active particles, of which the electron-conductive metal oxides may be preferred.

Among the metal oxides we may mention MnO ₂ , PbO ₂ , SnO ₂ , perovskites such as LaNiO ₃ , spinels such as Fe ₃ O ₄ .

A suitable ratio of electrochemically active particles present in the composition is from 1 to 10 g / L.

By electrochemically active particles with small granulometry is meant particles whose average particle size does not exceed 1000 µm, generally 5 µm to 100 µm, meaning that the size may vary during electrochemical treatment.

Among the organic substances we mention mainly oxidizable substances, for example those that have alcohol or phenolic functions, including oxidic functions. · 99 99 9

The derivatives as well as other organic substances can be degraded by the treatment according to the invention.

The method has the advantage that it can modify the compositions resulting from the introduction of aqueous compounds, the concentrations of which may be up to 10 @ -1 M, which makes it possible to propose a possible continuous treatment of the treatment solution containing these compounds. As mentioned above, the concentration of the various solutes may nevertheless reach the solubility threshold.

The aqueous composition to be treated preferably has a pH of high, especially above 7, preferably between pH 8 and pH 14, more preferably between pH 10 and pH 11, and more particularly when the buffer is carbonate, the pH is slightly less than pH 11.

During the oxidation reaction, this pH may vary and may drop to pH 5.

In the case of potentiostatic control, the oxidation potential applied to the composition is from 500 mV / ECS to 2000 mV / ECS. Preferably, the oxidation potential is about 800 mV / ECS. During the application of the oxidation potential, an oxidation current can be observed which shows an oxidation peak whose intensity decreases exponentially.

In the case of the galvanostatic method, the anode current density is suitable from 5 mA / cm ² to 200 mA / cm ^{2 of the} surface of the anode collector.

The frequency of ultrasound applied to the composition is preferably from 16 kHz to 500 kHz, especially about 22 kHz. According to a preferred variant after the sonoelectrochemical cycle described above, after a suitable treatment time an oxidation potential of greater than 1100 mV / ECS is applied to initiate oxygen release and substantially increase the ultrasound effect on the surface of the electrochemically active particles and prevent passivation and / or mechanically destroy it. a very thin layer of passivation formed optionally on powdered oxide.

The object of the invention is to combine two techniques, ultrasound and electrochemistry, and to define a specific degradation cycle; this protocol allows the compounds to be degraded without significant passivation of the used electrode in suspension, in the range of concentrations tested (up to 10 ^-2 M).

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The process of the invention can be carried out continuously by blowing off, filtering, removing small particles that could remain in the suspension and recycling the suspension.

The invention also relates to an electrochemical device with a liquid electrolyte, in particular for carrying out the treatment method according to the invention, characterized in that it comprises a cathode part, an anode part, these parts being separated by a separator, the cathode part may comprise a reference electrode and anode part may comprise a current collector. an ultrasonic generator, a mixing means and a means for introducing an aqueous compound containing one or more organic substances, the liquid electrolyte in the anode portion comprising electrochemical active particles in suspension.

The attached figure 6 shows such a device. The figure is a longitudinal schematic cross-sectional view of a device which enables the method according to the invention to be implemented.

A Grignard 1 reactor is used in which a separating membrane 4 is fixed which defines the anode part 2 and the cathode part 3. The platinum titanium current collector 5 is around this membrane in the anode part as close to its surface as possible to reduce the ohmic loss in the system. and the inert counterelectrode 6 formed by the nickel dispersion is also closest to the membrane surface in the cathode part for the same reasons. The two electrodes 5 and 6 are connected to a generator 7. The electrochemical reactor comprises a solution of sodium carbonate buffer in both portions, which is used as a support solution for attempts to degrade organic substances in the aqueous compound.

The electrochemical reactor has an opening 8 that allows the introduction of the aqueous compound. The ultrasonic transducer 9 is positioned such that the tip of the transducer 10, whose diameter is 19 mm, is immersed in the liquid carrier of the anode portion.

The ultrasonic transducer is connected to an ultrasonic generator (not shown).

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The liquid carrier contains small particle size PbO ₂ particles (from 5 µm to 10 µm) held in suspension by means of a mixer (not shown) at a ratio of 1 g / 25 ml.

The following examples illustrate the invention without limiting it in any way.

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DETAILED DESCRIPTION OF THE INVENTION

To explain the effectiveness of the treatment method of the invention, there are several examples to compare (Examples 3, 4 and 5) with this method as described in Examples 1 and 2.

Example 1 shows the effect associated with electrochemistry and ultrasound by maintaining the electrode potential in a lead slurry where oxidation takes place at +800 mV / ECS;

Example 2 shows the effect of the cleaning time proposed for the electrode portion, by periodically reversing the potential of the operating electrode (lead oxide slurry) with a higher value: 1000 mV / ECS to 1200 mV / ECS over time to cause oxygen release and thus mechanical cleaning electrodes in suspension as a result of the application of ultrasound.

Example 3 shows the electrochemical degradation of phenol on a platinum titanium oxide-free electrode alone in suspension;

Example 4 shows the electrochemical degradation alone (without ultrasound) of a selected model compound: phenol, on a powdered metal oxide electrode in suspension maintained at a potential of +800 mV / ECS;

In Example 5, the experiment is carried out under the same conditions as in Example 2 except for the absence of the metal oxide in the slurry in order to demonstrate the advantage of the process according to the invention.

In all of the following examples, a Grignard reactor was used, into which a Grignard reactor was used.

A membrane separating the two parts of the reactor was mounted; then a platinum titanium current collector was placed in the vicinity of the membrane in the anode portion as close as possible to its surface to reduce the ohmic loss in the chamber, then the counter electrode in the cathode portion, also as close as possible to the membrane surface for the same reasons; has introduced the necessary amount of sodium carbonate buffer solution as a liquid carrier for attempts to degrade organic compounds in solution.

Example 1

According to this example, the chamber used is supplemented by installing a stirrer and adding the necessary amount of metal oxide in the anode portion. The operating electrode is applied to a potential of +800 mV / ECS and then phenol is injected up to a concentration of 10 ³ M mixed anode portion solution (250 mL) in the presence of 20 kHz ultrasound, and the corresponding oxidation current for increasing phenol injection is recorded.

The results are shown in the following table and the curve in Figure 1, in which the horizontal axis is the time from the start of the experiment and the vertical axis the oxidation currents are measured.

Table 1

sample phenol C eq. mAh mAh yield tox number (Mg) M (Theory) (Found) (%) (Min) 1 1.01 0.11 x W ⁵ 8.09 0.93 11.5 20 2 5.07 0.54X10 · ³ 40.45 5.84 14.4 25 3 5.07 0.54 x1ο · ³ 40.45 6.32 15.6 30 4 8.11 0.86X10 · ³ 64.72 7.98 12.3 40 5 10.13 1,08x10 ' ³ 80.89 9.02 11.2 45 6 10.13 1,08x10 ' ³ 80.89 8.00 9.9 50

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Example 2

The test conditions defined in Example 1 were used. The operating electrode is applied to a potential of +800 mV / ECS and after each injection / oxidation of the compound and the oxidation current return to a value close to the baseline current, the in-situ purification phase is proceeded. The electrode is added to a value greater than 1000 mV / ECS to induce oxygen release and facilitate mechanical degradation of the passivation means formed on the surface of the process electrode in suspension due to the application of ultrasound, which causes considerable mechanical effects on the oxygen bubbles being released.

The results are summarized in the following table:

Table 2

sample phenol C eq. mAh mAh yield tox number (Mg) M (Theory) (Found) (%) (Min) 1 7 0.75X10 · ³ 55,88 8.57 15.33 30 2 10 1,06x10 ' ³ 79.83 10.46 13.1 30 3 12 1.28 x1ο · ³ 95.80 13.15 13.73 30 4 15 1,60x10 ' ³ 119.74 15,05 12.57 20 5 18.75 2.00X10 · ³ 149.68 18.47 12.34 20 6 18.75 2.00X10 · ³ 149.68 18.47 12.43 20 7 5.02 0,53x10 ' ³ 40.07 6.78 16.90 20 8 10.24 1,09x10 ' ³ 81.75 12.81 15.67 20

Example 3

In this example, the chamber is used without treatment, without a metal oxide slurry and without ultrasound. The operating electrode is applied to a potential of +800 mV / ECS and the measured oxidation currents are recorded as a function of time over a period of time. ···································································

-11following injection; the observed currents are recorded in the enclosed figure 3, in which the horizontal axis is the time elapsed since the start of the experiment and on the vertical axis the oxidation currents are measured.

It can be noted that the measured oxidation current has been extremely weak since the second injection, which is a sign of the large passivation of the platinum titanium working electrode, which thus becomes very useless very quickly.

Example 4

Under the same conditions as in Example 3 and after installing the stirrer and adding the necessary amount of metal oxide to the anode portion, the process electrode is brought back to a potential of +800 mV / ECS; phenol up to a concentration of 10 * ³ is injected into the stirred solution of the anode portion, and the corresponding oxidation current is recorded; several phenol injections of increasing concentration are performed.

The results are summarized in the following table.

As can be seen in the accompanying Figure 4, if several injections are made while waiting for each of them before the oxidation current returns to a value close to that of the base current, it can be seen that blockage of the electrode activity in suspension is progressively achieved. . It is also interesting to note the change in the shape of the curve which represents the oxidation current; according to injection, the second oxidation peak curve is more spaced and at retention times much higher than the oxidation peak that occurred at the time of injection of the composition; this change in the shape of the oxidation current curve represents passivation of the electrode in suspension, but it can be seen that blocking the electrode is much slower than without the metal oxide in the suspension.

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Table 3

sample phenol C eq. mAh mAh yield tox number (Mg) M (Theory) (Found) (%) (Min) 1 7 0,96x1 θ ' ³ 71.85 6.00 8.35 10 2 18 1.91 x W ³ 143.7 14.3 9.97 20 3 36 3.83Χ10 · ³ 287.4 27.6 9.6 30 4 54 5.75 x1ο · ³ 431.1 40 9.42 45 5 72 7,66x10 ' ³ 575 50.7 8.83 60 6 90 9.57x10 ' ³ 718.5 11.85 1.65 90 7 9 0.96X10 · ³ 71.85 0 0

Example 5

In this example, the same experimental conditions were used as in Example 2 except that no metal oxide was introduced in the slurry. The operating electrode is applied to a potential of +800 mV / ECS and after each injection / oxidation of the device and the oxidation current back to a value close to the base current, the in-situ cleaning phase is advanced and the electrode potential is applied to a value greater than 1000 mV / ECS to induce oxygen release and facilitate mechanical degradation of the passivation means formed on the surface of the process electrode in suspension due to the application of ultrasound, which causes considerable mechanical effects on the oxygen bubbles that are released.

The results are summarized in the following table.

Figure 5 shows the shape of the oxidation stream of the compound injected during the two first injections; from the second injection, the oxidation stream becomes very weak and demonstrates the ineffectiveness of the protocol under these conditions and thus the role of the degradation catalyst that the used metal oxide has in the slurry.

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Table 4

4 sample number phenol (Mg) C eq. M mAh (Theory) mAh (Found) yield (%) tox (Min) 1 2 5.02 10.04 0,53x10 ' ³ 1,00x10' ³ 40.07 80,15 1.66 1.0 4.14 1.25 50 15 • In the pictures scale. 2 and 5 on the axis x indicates "SS interruption continuity

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Claims (14)

A method of electrochemical treatment of an aqueous compound comprising one or more organic substances by means of an electrochemical reactor comprising an anode portion and a cathode portion, said portions being separated by a suitable separator, characterized by tm. wherein the aqueous compound is introduced into the anode portion of the electrochemical reactor, said anode portion comprising a liquid carrier in which electrochemical active particles are present in suspension, and the resulting composition, starting from introducing said aqueous compound, simultaneously or sequentially exposed to oxidation potential and action ultrasound. Treatment method according to claim 1, characterized by tm. that organic substances, especially oxidizable ones, have alcoholic functions. Treatment method according to claim 1, characterized by tm. The organic compounds are selected from alcohol derivatives, phenol derivatives, and osido derivatives. Treatment method according to claims 1 to 3, characterized by tm. wherein the organic substance (s) is (are) present in the composition up to 10 ^-2 M. Treatment method according to claim 1, characterized by tm. The electrochemical active particles are selected from metal oxides. Treatment method according to claim 5, characterized by tm. The electron conductive metal oxides are selected from MnO ₂ , PbO ₂ SnO ₂ , Fe ₃ O _{4 1} perovskites, spinels. ························ -15 • ········································ Treatment method according to claim 1, characterized by tm. wherein the electrochemical active particles are present in the composition in a ratio of from 1 g to 10 g. Treatment method according to any one of the preceding claims, characterized by tm. wherein the pH of the composition is greater than 7 at the start of the reaction. Treatment method according to claim 8, characterized by tm. wherein the pH of the composition is from 8 to 14 at the start of the reaction. Treatment method according to any one of the preceding claims, characterized by tm. that the pH oxidation potential is from 500 mV / ECS to 2000 mV / ECS in the case of potentiostatic control and the anode current density is from 5 mA / cm ² to 200 mA / cm ^{2 of the} apparent surface of the anode collector. Treatment method according to any one of the preceding claims, characterized by t. The ultrasonic frequency is from 16 kHz to 500 kHz. Treatment method according to any one of the preceding claims, characterized by tm. that after application of the oxidation potential, the composition is exposed to the effect of a second oxidation potential so as to allow the release of oxygen all through the application of ultrasound. Treatment method according to one of Claims 1 to 12, characterized by tm. It is suitable for polluting waste water. Electrochemical device with a liquid electrolyte, in particular for carrying out the treatment method according to one of claims 1 to 13, characterized in that it comprises: comprising an cathode part (3), an anode part (2), these parts being separated by a separator (4), the anode part comprising a current collector (5), an ultrasonic generator (10), a stirring means and a means for introducing (8) the aqueous compound; which ·· ·· ·· * · · · · ··· · ··· ι · • · It contains one or more organic substances, the liquid electrolyte in the anode part contains electrochemical active particles in suspension.