FR3085969A1 - METHOD FOR IN SITU REGENERATION OF A BIO-ANODE OF A BIO-ELECTROCHEMICAL SYNTHESIS DEVICE - Google Patents

Abstract

Method for regenerating a working electrode of an electrochemical device operating in electrolyser mode comprising a working electrode (bio-anode) covered with a biofilm, a cathode and possibly a reference electrode, a potential difference (ddp) nominal positive being applied, in nominal operation, either between the bio-anode and the cathode, or between the bio-anode and the reference electrode, characterized in that it comprises either a decrease in the ddp of at least 10 % of ddp applied between the bio-anode to be regenerated and the cathode, in the absence of reference electrode, i.e. a reduction in the working potential imposed on the working electrode (the bio-anode) to be regenerated compared to the reference electrode of at least 0.05 V / ESH, enabling the nominal operation of the electrolyser to be restored. Regeneration can be continued by producing H2 on the surface of the bio-anode by changing its polarity by inserting a counter electrode in the anode compartment of the bio-anode.

Description

FIELD OF THE INVENTION

The present invention relates to bioelectrochemical electrolysis processes, in particular the processes using electrodes called bioelectrodes in contact with microorganisms, and more particularly relates to a process for regenerating these bioelectrodes and its use from solutions containing organic waste.

STATE OF THE ART

These bioelectrochemical electrolysis processes make it possible in particular, from solutions containing organic waste, to produce molecules of interest such as organic acids and / or alcohols.

Patent application WO2016 / 051064 describes a bio-electrochemical device, which comprises both a bio-anode and a bio-cathode, the electrolyte of the anode compartment as well as the electrolyte of the cathode compartment containing microorganisms in suspension or under form of biofilm (s). In this device, the activity of the bio-cathode is optimized for the production of particular chemical species in the electrolyte, such as acetic, lactic and / or propionic acids or alcohols. These microbial syntheses of organic molecules are carried out using electroactive bacteria present on the surface of the electrode.

One of the current problems to be solved is to improve the reliability and durability of these bioelectrochemical processes, with a view to applications at the industrial stage.

More particularly in the process mentioned above, the main objective is to increase the durability of the bio-anode. It has indeed been observed that the activity of this bioanode decreases considerably after a few weeks of operation. This phenomenon has been defined as the “aging” of the bioanode, probably due to clogging of the biofilm on this electrode. Indeed, a biofilm composed of electro-active bacteria (in particular of the genus Geobacter) is necessary for the functioning of the bio-anode. Other non-electroactive microorganisms also develop on this biofilm and thus inhibit its activity. The deposition of insoluble particles further aggravates this effect.

An object of the invention is therefore to propose a means of acting against the "aging" of this bio-anode.

The prior art gives examples of methods for cleaning measurement sensors in contact with biological fluids, subject to fouling by the formation of a bacterial film. These cleaning methods often involve emptying the electrochemical cell to wash said sensor by passing a cleaning fluid (aqueous, organic or gaseous solution).

Another solution is presented in patent application WO 2014/108689 which provides for an air circulation step, in a laminar flow, after incorporation of a sample to be tested, in the continuous flow measurement cell, in order to clean the cell and the sensor during the measurement protocol.

Such a solution, with air injection, is completely inadvisable for electrodes (in particular bio-anodes) operating in an anaerobic medium, in particular in electrochemical synthesis devices. It is also not possible to empty the anode or cathode compartment to clean the corresponding electrode, which would interrupt the operation of the electrochemical device and would require readjustment of the electrodes to restore the electro-active microorganisms on the latter.

The document JP5924241 describes a device for continuous measurement of the oxygen dissolved in water by means of a probe placed in a cylindrical envelope in which the water circulates. To avoid deposits which could affect the measurement of the probe, a strong upward current of water around the probe is created by the injection of air bubbles in the upper part of the envelope, above said probe.

Document JPH0416746 describes a system for cleaning probes intended to measure certain physicochemical parameters continuously in wastewater, in particular the concentration of dissolved Oz. An air bubbling system generates currents to detach deposits formed on the surface of the optical detection window, which alter the measurement, for example of dissolved oxygen. The generation of these bubbles takes place inside the measuring cell either by the introduction of electrodes allowing the electrolysis of water, said electrodes being arranged respectively on either side of the measurement window. to be cleaned, either by the introduction of a heating resistance placed below the measuring window causing the boiling. Such solutions are also not adaptable to bio-anodes (heating is to be excluded so as not to destroy the active biofilm and the generation of oxygen by electrolysis is to be avoided for operation in an anaerobic medium).

The solutions of the prior art are by no means satisfactory for solving the aging of bio-anodes which causes a significant reduction in current density, in a bio-electrochemical electrolysis process.

GOALS OF THE INVENTION

A first object of the invention is to overcome the drawbacks of the prior art by proposing, in a bioelectrochemical electrolysis process, a means making it possible to rid the surface of a bioanode of impurities or microorganisms which interfere or inhibit its operation.

Another object of the invention is to propose a process for regenerating the surface of the bioanode without completely destroying it, that is to say by maintaining at least in part, the electroactive biofilm present on this surface.

Another object of the invention is to propose a method for regenerating the electrochemical activity of an "aging" bio-electrode, without introducing complex means into the electrochemical device, without emptying the anode compartment or without changing the anode

DESCRIPTION OF THE INVENTION

To this end, the present invention relates to a method for regenerating a working electrode of an electrochemical device operating in electrolyser mode comprising:

at least one working electrode, called a bio-anode, having a surface immersed in an electrolyte containing electro-active microorganisms in an anode compartment, said bio-anode being covered with a biofilm,

- a counter electrode, called a cathode, immersed in an electrolyte of a cathode compartment,

- and possibly a reference electrode, a positive nominal potential difference (ddp) being applied, in nominal operation, either between the working electrode, corresponding to the bioanode, and the counter electrode, corresponding to the cathode, or between l working electrode and the reference electrode (corresponding to an imposed potential) characterized in that the regeneration process of the bio-anode covered with a biofilm comprises one of the following actions I or II:

-Either faction I): a step of decreasing the ddp by at least 10% of the value of the potential difference applied between the working electrode, corresponding to the bio-anode to be regenerated, and the counter electrode, called primary counter-electrode, during the operation of the electrochemical device in electrolyser mode not involving a reference electrode,

- Or action II): when the device comprises a reference electrode, a first step of reducing the potential imposed on the working electrode (the bioanode) to be regenerated by at least 0.05 V relative to the reference electrode, said imposed potential being expressed in Volts relative to the standard hydrogen electrode (ESH), making it possible to restore the nominal operation of the electrolyser.

The inventors have in fact found that the decrease in the ddp of at least 10% of the value of the potential difference applied between the bio-anode to be regenerated and the cathode (here against primary electrode) (action I) or when the device includes a reference electrode, the reduction of at least 0.05 V / ESH in the working potential imposed on the bio-anode to be regenerated (action II) could be sufficient to regenerate the bio-anode and restore the nominal functioning of the electrolyser.

Within the meaning of the invention, a “bio-electrode” (“bio-anode” or “bio-cathode”) is an electrode covered at least in part with a bacterial biofilm comprising electro-active organisms, that is to say ie covered at least over part of its surface immersed in the electrolyte by a bacterial biofilm. According to one embodiment, the entire submerged surface of the bio-electrode is covered with biofilm. Alternatively, according to another embodiment, only part of the surface of the bio-electrode is covered with biofilm. In this latter embodiment, the surface covered with biofilm is sufficient to generate the desired activity, in particular in the case of oxidation of organic waste or of bio-electrochemical synthesis.

Electroactive microorganisms are typically anaerobic microorganisms. Microorganisms differ depending on the electrode on which they grow as a biofilm, and the characteristics of the electrolyte in which they are immersed. For example, when wastewater or bio-waste is injected into the anode electrolyte, there is a large population affiliated with the genus Geobacter. On the other hand, in a saline environment, other genera such as Geoalkalibacter or Desulforomonas can become dominant. Thus, when the microorganisms are located on the anode, we bet anodic electroactive microorganisms, while when the microorganisms are located on the cathode, we speak of cathodic or electrotrophic electroactive microorganisms.

According to one embodiment of the invention, when a first regeneration step, comprising one of the actions I or II, is insufficient for restoring the nominal operation of the electrolyser, these actions are followed by a second step including:

- the introduction into the anode compartment of a secondary counter electrode, or the connection of a secondary electrode already in place in the anode compartment, or the connection of a conductive surface already present in the anode compartment in contact with the electrolyte then playing the role of secondary electrode,

- the disconnection of the primary counter electrode (the initial cathode),

- then a decrease in the potential difference in the case of action I, or in the potential imposed in the case of action Ii, between the bio-anode to be regenerated and the secondary counter electrode until the appearance of d a reduction current on the bio-anode to be regenerated, the latter then operating temporarily in cathode mode, this operation temporarily leading to the appearance of hydrogen bubbles on the bio-anode to be regenerated, said bubbles allowing the biofilm to detach of the bio-anode to be regenerated.

According to a particular embodiment of the method according to the invention, the electrochemical device can comprise several bio-anodes in the anode compartment. In this case, in the second regeneration step of one of the bioanodes, called the first bioanode, one of the bioanodes, called the second bioanode, serves as a secondary counter electrode.

Preferably, in the first regeneration step:

or said action I comprises a decrease in the ddp of between 10% and 70% of the value of the potential difference applied to the working electrode (bio-anode) and the cathode during the operation of the electrochemical device in electrolyser mode.

either said action II, in the first regeneration step, said action II comprises a reduction in the potential imposed on the working electrode (the bioanode) to be regenerated up to a potential imposed on the working electrode of between 0, 05 V and -0.15 V with respect to the reference electrode, said imposed potential being indicated with respect to the standard hydrogen electrode (ESH).

Preferably, when the second regeneration step is implemented:

- in the case of an electrochemical device which does not involve a reference electrode, the second step comprises the application of a negative potential difference (ddp) between the working electrode (bio-anode) and said second against electrode, until a reduction current of less than -1A / m2 is established on the bio-anode to be regenerated, which then operates temporarily as a cathode;

- in the case of an electrochemical device involving a reference electrode, the second step comprises the application to the bio-anode to be regenerated of a potential less than or equal to -0.65 V / ESH leading to a current of reduction of less than -1A / m ² on said bio-anode to be regenerated. As a variant, the second step may include the application to the bio-anode to be regenerated of a potential of between -0.65 V / ESH and -1.7 V / ESH, intensifying the production of the hydrogen bubbles.

Such a process makes it possible to trigger the production of dihydrogen at the surface of the working electrode (bio-anode) to be treated and it has been found that this production of dihydrogen generates the formation of fine bubbles of this gas which make it possible to "Unhooking" impurities from the biofilm, as well as microorganisms from the outer layer of the biofilm covering said bioelectrode.

The hydrogen bubbles generated do not negatively impact the anaerobic medium in the anode compartment necessary for the oxidation of soluble organic materials from waste such as bio-waste hydrolysates.

This method can be applied for a short time, for example a minute, or a longer time if necessary.

A fraction of the electroactive microorganisms nevertheless remain "attached" to said bioelectrode. The electro-active biofilm is thus partly preserved, and can again develop on the unclogged areas on the surface of the electrode, allowing the resumption of the nominal operation of the electrochemical device.

According to an advantageous embodiment, the cathode of the electrochemical device is a bio-cathode. The invention advantageously applies to bio-electrochemical synthesis devices comprising at least one bio-anode and one bio-cathode as described in patent application WO2016 / 051064.

The process described above aims to regenerate a bio-anode whose performance is degraded, but it is also possible to apply this process as a preventive measure, even before the performance of the electrochemical device begins to deteriorate.

The present invention also relates to the use of the regeneration process, as described above, of a working electrode of an electrochemical device operating in electrolyser mode for the regeneration of bio-anode immersed in an electrolyte supplied with carbon substrates organic.

More particularly, it can be applied to the regeneration of a working electrode of an electrochemical device operating in electrolyser mode to the regeneration of bio-anode coupled to a bio-cathode of a microbial electrosynthesis reactor, putting in uses organic carbon substrates.

Said organic carbon substrates can be organic waste chosen from: bio-waste hydrolysates, hydrolysed sludge from treatment plants, various organic liquid fractions from treatment plants, urban waste water after primary decantation, organic industrial effluents , effluents from the food industry, digestates from sewage treatment plants, or a mixture of several of these.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will appear in the description below of nonlimiting exemplary embodiments, with reference to the attached diagrams, in which:

Figure 1 shows schematically an electrochemical laboratory device comprising a bio-anode and a bio-cathode;

Figure 2 is a top perspective view of a pilot electrochemical device;

FIG. 3 is a diagram showing the variation of the current density, as a function of time, of the device of FIG. 1 with change of anodic potential (black arrow);

FIG. 4 is a diagram showing the variation of the current density, as a function of time, of the device of FIG. 2 with change of anodic potential (black arrow);

EXAMPLES

Free 1

The electrochemical device, presented in FIG. 1, is an electrolyser 1 with two compartments comprising a bio-anode 3 and a bio-cathode 5. The two anode 13 and cathode 15 compartments consist of 1.5 L glass containers of total volume separated by a cation exchange membrane 14 (MEC, Fumasep® FKE, Germany). The two compartments are closed by a cover and gas-tight.

The basic electrolyte 12A, 12C used is the synthetic medium for the BMP test (Biochemical Methane Potential ISO 11734).

Substrate

Food bio-waste was used as the substrate for the anode compartment. It is composed of potatoes (8.1%), tomatoes (3.4%), ground beef (8.1%), milk powder (0.7%), dry cookies (4, 1%) and water (75.6%). After mixing all the fractions, the bio-waste is left to ferment for 5 days at 35 degrees C °. Thus, the composition of biowaste in volatile fatty acids (VFAs) is: lactic acid (55%), butyric acid (24%), propionic acid (10%), acetic acid (7%) , formic acid (3%) and valeric acid (1%). The bio-waste was centrifuged at 7000 g for 5 min to collect its liquid fraction (the supernatant). The average COD (chemical oxygen demand) of this fraction is around 100 g / L. The biowaste supernatant was injected into the anode compartment to have a COD of 2.5 g / L each time the substrate was fed.

The basic material of the bio-anode is a piece of 4 cm * 4 cm of carbon fabric (Paxitech®, France) it is connected to the electrical circuit by a wire 23 of platinum. The material of the bio-cathode is a stainless steel plate (Outokumpu®, 254 SMO) of 4 cm * 4 cm connected to the electrical circuit by a steel rod 25.

An ECS saturated calomel reference electrode is also present in the anode compartment 13. The bio-anode was used as working electrode and the bio-cathode as a counter-electrode.

Anode 3 is polarized at + 0.158 V relative to the ECS reference electrode by means of a potentiostat (BioLogic®, France, VMP3 not shown, controlled by EC-Lab software).

In order to quantify the activity of a bio-anode, the most used method is to measure the maximum current density that it is capable of producing in the presence of an organic substrate. The current density (A / m ² ) at the bio-anode was thus monitored as a function of time in two electrochemical devices (see the curve in FIG. 3 showing the current density in solid lines for reactor No. 1 and in dotted lines for reactor No. 2).

After approximately 30 days of operation at this potential difference, the diagram in FIG. 3 shows that the current density at the bio-anode drops sharply, despite regular substrate injections (thick white arrows).

The potential imposed on the bio-anode of reactor no. 2 was reduced to 90 days to a value of -0.2 V / DHW (corresponding to -0.04 V / ESH).

The diagram in FIG. 3 then shows a very marked increase in the current density, confirming the restoration of the activity at the bio-anode, and therefore the regeneration of the latter.

Example 2

The bioelectrochemical reactor 20 shown diagrammatically in FIG. 2, was designed to mimic industrial conditions. This electrolyser comprises three compartments separated by two ion exchange membranes: an anode compartment 43 which encloses two bio-anodes 32 and 33 (electrically connected to the outside of the reactor). This compartment is separated by a cation exchange membrane, from an inter-membrane compartment 30 which is itself separated by an anion exchange membrane from the cathode compartment 45 which encloses the bio-cathode 35. The volumes of these three compartments are 5.25 L, 2 L and 5.25 L respectively.

The bio-cathode 35 is a granular electrode comprising carbon grains, housed in metal baskets themselves held in a metal frame. The frame is connected to a current collector. The bio-anodes 32, 33 each consist of a metal frame formed by two parallel parts which enclose between them two stainless steel grids housing between them a carbon fabric. This carbon fabric can be in the form of a single element or in the form of a fabric strip element.

These bio-electrodes are connected to a potentiostat (BioLogic®, France, VMP3 not shown, controlled by the EC-Lab software), a potential difference of 1.1 V being imposed between the bio-anodes and the bio-cathode.

Reference electrodes (Ag / AgCI) 36, 37 are present respectively in the anode 43 and cathode 45 compartments. In an industrial scale reactor, these reference electrodes may be absent.

The cathode electrolyte is the BMP medium modified with 30 g / L of NaHCCh. The basic anodic electrolyte is composed of 12.5 g / L of NaaHPCUJHaO, 3 g / L of KH ₂ PO ₄ , 0.5 g / L of NaCl, 1 g / L of NH ₄ CI and 30 g / L NaHCO ₃ . The electrolyte of the inter-membrane compartment 30 is composed of 35 g / L of KCI and 32.6 g / L of KH2PO4.

The same substrate as in Example 1 was introduced into the anode compartment.

The pH of the electrolyte in the anode compartment is kept at 7 by automatic injection of a K2CO3 solution. Anodic 40 and / or cathodic 41 level regulation systems can be provided.

Despite a daily supply of the anode compartment with substrate which generates a slight increase in activity at the bio-anode (see in Figure 4 the current density in A / m2), the overall activity decreases over time and becomes very low from the 15th day. At this stage, a regeneration treatment 10 was then carried out by applying a potential of -1 V to the bio-anode (dotted arrow) which caused the production of dihydrogen in the form of bubbles.

The curve in FIG. 4 then shows a significant increase in the current density, proof of an in situ restoration of the activity at the bio-anode, and therefore of the regeneration of the latter.

Claims (12)

1. Method for regenerating a working electrode of an electrochemical device (1, 20) operating in electrolyser mode comprising: - at least one working electrode, called bio-anode (3; 32, 33), having a surface immersed in an electrolyte (12A) containing electro-active microorganisms in an anode compartment (13; 43), said bio-anode being covered with a biofilm, - a counter electrode, called cathode (5; 35), immersed in an electrolyte (12C) of a cathode compartment (15; 45), - and possibly a reference electrode (DHW; 36), a positive nominal potential difference (ddp) being applied, in nominal operation, either between the working electrode, corresponding to the bioanode, and the counter electrode, corresponding to the cathode, or between the working electrode and the reference electrode (corresponding to an imposed potential) characterized in that the process of regeneration of the bio-anode covered with a biofilm comprises one of the following actions I or II : - Or action I): a step of reducing the ddp by at least 10% of the value of the potential difference applied between the working electrode, corresponding to the bio-anode to be regenerated, and the counter electrode , called the primary counter-electrode, during the operation of the electrochemical device in electrolyser mode which does not involve a reference electrode, - Either faction II): when the device comprises a reference electrode, a step of reducing the potential imposed on the working electrode (the bio-anode) to be regenerated by at least 0.05 V relative to the electrode reference, said imposed potential being indicated with respect to the standard hydrogen electrode (ESH), making it possible to restore the nominal operation of the electrolyser. 2. Method according to claim 1, characterized in that when a first regeneration step, comprising one of the actions I or II, is insufficient for restoring the nominal operation of the electrolyser, these actions are followed by a second step including: - the introduction into the anode compartment (13; 43) of a secondary counter electrode, or the connection of a secondary electrode already in place in the anode compartment, or the connection of a conductive surface already present in the compartment anodic in contact with the electrolyte then playing the role of secondary electrode, - disconnection of the primary counter electrode (the initial cathode), - then a decrease in the potential difference in the case of action I, or in the potential imposed in the case of action II, between the bio-anode to be regenerated and the secondary counter electrode until the appearance of d 'a reduction current on the bio-anode to be regenerated, the latter then operating temporarily in cathode mode, this operation temporarily leading to the appearance of hydrogen bubbles on the bio-anode to be regenerated, said bubbles allowing a "dropout" of the bio-anode biofilm to be regenerated. 3. Method according to claim 2, characterized in that the electrochemical device comprises several bio-anodes (32, 33) in the anode compartment (43), and in the second stage of regeneration of one of the bio-anodes, called first bio-anode, one of the bio-anodes, called the second bioanode, serves as a secondary counter-electrode. 4. Method according to any one of the preceding claims, characterized in that said action I comprises a reduction in the ddp of between 10% and 70% of the value of the potential difference applied between the working electrode (bioanode) and the cathode during the operation of the electrochemical device in electrolyser mode. 5. Method according to any one of claims 1 to 3 characterized in that in the first regeneration step, said action II comprises a reduction in the potential imposed on the working electrode (the bio-anode) to be regenerated up to a potential imposed on the working electrode between 0.05 V and -0.15 V relative to the reference electrode, said working potential being expressed in Volt relative to the standard hydrogen electrode (ESH) . 6. Method according to any one of claims 2 to 4 characterized in that, in the case of an electrochemical device not involving a reference electrode, the second step comprises the application of a potential difference (ddp ) negative between the working electrode (bio-anode) and said second counter electrode, until a current of less than -1A / m ^{2 is established} on the bio-anode to be regenerated. 7. Method according to any one of claims 2 or 3 characterized in that, in the case of an electrochemical device involving a reference electrode (ECS; 36), the second step comprises application to the bio-anode to regenerate with a potential less than or equal to -0.65 V / ESH leading to a current less than -1A / m ² on said bio-anode to be regenerated. 8. Method according to claim 7 characterized in that the second step comprises the application to the bio-anode to be regenerated of a potential between -0.65 V / ESH and - 1.7 V / ESH, intensifying the production of hydrogen bubbles. 9. Method according to any one of the preceding claims, characterized in that the cathode is a bio-cathode. 10. Use of the regeneration method according to any one of the preceding claims, of a working electrode of an electrochemical device operating in electrolyser mode in the regeneration of bio-anode immersed in an electrolyte supplied with organic carbon substrates. 11. Use of the regeneration method, according to any one of claims 1 to 9, of a working electrode of an electrochemical device operating in electrolyser mode in the regeneration of bio-anode coupled to a bio-cathode of microbial electrosynthesis reactor, using organic carbon substrates. 12. Use according to one of claims 10 or 11, in which the organic carbon substrates are organic waste chosen from: bio-waste hydrolysates, hydrolysed sludge from treatment plants, various organic liquid fractions from treatment plants purification, urban wastewater after primary decantation, organic industrial effluents, effluents from the food industry, digestates from treatment plants, or a mixture of several of these.