EP4267520A1

EP4267520A1 - Mineralization of organic compounds with boron-doped-diamond electrode during radionuclides stripping process

Info

Publication number: EP4267520A1
Application number: EP20848795.9A
Authority: EP
Inventors: Peter Zeh; Ayhan SEN; Sven Wegener
Original assignee: Framatome GmbH
Current assignee: Framatome GmbH
Priority date: 2020-12-24
Filing date: 2020-12-24
Publication date: 2023-11-01
Also published as: AR124508A1; CA3199877A1; WO2022135731A1; CN116635335A; KR102814628B1; CN116635335B; KR20230110573A

Abstract

The present application relates to a recirculation method for the continuous mineralization of an organic acid in an aqueous solution used for the stripping of radionuclides from an ion exchanger, by means of an electrolyzer comprising a boron-doped diamond electrode, the apparatus for conducting said method and the method for regenerating an ion exchanger containing immobilized radionuclides involving said recirculation method.

Description

MINERALIZATION OF ORGANIC COMPOUNDS WITH BORON-DOPED-DIAMOND ELECTRODE DURING RADIONUCLIDES STRIPPING PROCESS

The present invention concerns the field of nuclear reactors, and more specifically the extraction of radioisotopes, such as carbon-14 (¹⁴C) that are produced in water reactors, such as CANDU (CANada Deuterium Uranium) reactors.

¹⁴C is generated mainly by neutron activation of stable nitrogen, oxygen ¹⁷O, and, to a lesser extent, ¹³C. In nuclear reactors, ¹⁴C is produced in the fuel, from core structural materials, and in reactor coolant, due to the presence of the stable parent isotopes that are activated by the neutrons generated from nuclear fission reactions occurring in the fuel. ¹⁴C is particularly generated in CANDU reactors because this type of reactors uses heavy water (D₂O) (both as moderator and coolant), which has a higher isotopic abundance in ¹⁷O compared to the light water.

¹⁴C that is generated is then retained in an ion exchange resin used to purify the heavy water from the circuit. The ¹⁴C immobilized on the ion exchanger is predominantly in the form of carbonate and/or bicarbonate derivatives.

The ¹⁴C-containing carbonates and bicarbonates may then be released from the ion exchanger by eluting an organic acid, resulting mainly in the release of ¹⁴CO₂ in a stripping step. The organic acids used for elution are then to be decomposed into CO₂ following the ion exchanger. This is generally done by mineralization, by carrying out a UV Fenton process in batch operation, to minimize the mobilization of radionuclides from the ion exchanger. Typically, hydrogen peroxide (in an over- stoichiometric concentration to avoid polymerization of the organic acid due to the irradiation with UV light) and a catalyst are introduced into the batch tank; the content of the tank is then pumped over a UV photolysis lamp.

As it is performed in batch, this mineralization step thus leads to a longer process time and requires the use of additional chemicals (hydrogen peroxide, metal catalysts).

The mineralization process of the organic acid is therefore time-consuming and is largely responsible for the duration of the stripping process of ion exchangers.

Therefore, the organic acid discharged from the ion exchanger is only decomposed gradually in the solution pumped around in a small cycle. The solution should not flow directly through the ion exchangers during the degradation of the organic acid, as the chemicals required for the acid oxidation would otherwise mobilize radionuclides from the ion exchanger. The ion exchanger is only reintegrated into the stripping cycle when a pH value of 5 to 7 has been reached and the oxidizing agent (hydrogen peroxide) has been eliminated.

In fact, the introduction of the treated solution into the ion exchanger creates a new equilibrium between the ion exchanger and the solution, so that the pH value falls to 3 - 5, caused by the entry of organic acid. After equilibrium establishes, the ion exchanger has to be isolated again for further decomposition of the organic acid. The full mineralization of the acid thus succeeds very slowly with this gradual degradation of the organic acid.

Further, the UV Fenton process involves several issues: When using hydrogen peroxide, the ion exchanger is also decomposed. Due to the loss of the ability to bind ions, already bound radioactive nuclides enter the stripping process cycle. Also, with the use of metal catalysts in the UV Fenton process, radioactive nuclides are inevitably washed into the stripping process cycle through exchange reactions on the ion exchanger.

The metal catalysts used during the UV Fenton process may also be subject to hydrolysis and may cause the solution to become cloudy in the process circuit. Still further, the organic acid may polymerize under the action of the UV photolysis lamps: The decomposition of the polymerization products is much longer than that of the monomeric organic acid.

There is therefore a need to provide alternative mineralization of organic acid, which may avoid the above disadvantages.

EP 2 949 630 discloses a system and a process for the recovery of water from contaminated water such as urine. Specifically, the system comprises a cation-exchange device for desalting the contaminated water and an organic-component decomposition device comprising a diamond electrode. Water is then discharged into an electrodialysis device, where an acid is produced, which may be used for the regeneration of the cationexchange device.

EP1 522 526 discloses a process of treatment of chemical plating waste liquor containing phosphorus components and organic compounds, for reducing the amount of impurities in the liquor. Specifically, a solution such as comprising disodium hydrogenphosphite, hydrophosphorous acid and sodium citrate is subjected to electrolytic treatment in an electrolytic cell having a conductive boron-doped diamond electrode as an anode for oxidizing the hydrogenphosphite and hydrophosphorous acid to orthophosphoric acid and decomposing the citrate into carbon dioxide and water. US 5,399,247 teaches a doped diamond anode electrolysis to oxidize solutes in waste water.

WO 2015/127918 discloses an electrochemical oxidative process for the degradation of nitrogen-containing compounds in waste water by means of a diamond electrode as an anode.

Still, it is desired to provide improved mineralization of the organic acid to be implemented after the stripping process, which may be easily carried out, in that does not require a batch step and/or the addition of various chemicals.

It has now been discovered a novel mineralization process that involves an electrolysis by using a boron-doped diamond electrode, which performs the electrochemical oxidation of the organic acid used in the separation of C-14 from ion exchangers.

Such electrolysis-based mineralization of the organic acid does not require the addition of chemicals. Additionally, the ion exchanger can be discharged continuously to the electrolysis step, so that the organic acid is degraded much more quickly.

According to a first object, the present invention thus concerns a recirculation method for the continuous mineralization of an organic acid in an aqueous solution used for the stripping of radionuclides from an ion exchanger, characterized in that said process comprises the following steps: a) transferring the aqueous solution comprising the organic acid following the stripping step to an electrolyzer comprising a boron-doped diamond electrode; b) contacting said organic acid with the boron-doped diamond electrode in the electrolyzer, thereby performing an electrolytic oxidation of the organic acid so as to mineralize at least a part of the organic acid in the aqueous solution, c) degassing CC>2(gas) from the aqueous solution of step (b) circulating from the electrolyzer, and d) transferring the resulting solution to the ion exchanger for continued stripping.

According to the invention, the mineralization of the organic acid now takes place through hydroxyl groups, which are produced in situ on the electrode surface (anode). The mineralization of the acid can begin immediately after the complete stripping, without the ion exchanger having to be isolated from the stripping circuit. As a result, a batch operation (gradual degradation of the acid) is no longer necessary. Further, no additive is used to mineralize the organic acid. As hydrogen peroxide is no longer required, there is no risk of degradation of the ion exchanger and the absence of a catalyst means that there is no risk of metallic radioactive nuclides being mobilized from the ion exchanger any longer. Due to the continuous operation of the mineralization, the equilibrium settings between the ion exchanger and the process solution are gradually changing, thus creating the opportunity that the previously mobilized radioactive nuclides are bound again on the ion exchanger before they are discharged into the process cycle. This leads to an improved efficiency of the process. The overall process is therefore improved in that the electrochemical decomposition of the organic acid by the boron-diamond electrode is quicker and more straightforward than the UV-Fenton step. It can thus be favorably performed continuously and in a cost- and time- efficient manner. Detailed description The embodiments described below are to be understood alone or in anyone of their combinations. The term « radionuclide » as used herein refers to a radioactive isotope (also called radioisotope). Typically, the radionuclide according to the invention is typically a radioisotope present in a nuclear wastewater purification circuit. It may be in particular ¹⁴C; said ¹⁴C may be in the form of carbonates CO3^2– and bicarbonates HCO^3–. These radionuclides containing species are immobilized in an ion exchanger during the wastewater purification process. The ion-exchanger as used herein defines a container comprising a solid phase, typically a resin that is able to remove an undesired chemical species from a liquid phase by retaining (or immobilizing) the undesired chemical species on the resin. In the present case, the undesired chemical species are ¹⁴C containing derivatives, such as carbonates and/or bicarbonate derivatives, that were contained in the nuclear wastewater liquid phase and that are retained on the ion exchanger.

The solid phase can be either a cation exchanger, which exchanges positively charged ions (cations), or an anion exchanger, which exchanges negatively charged ions (anions), or a mixed cation/anion exchanger. The resin can typically be a functionalized porous or gel polymer.

According to an embodiment, the ion exchanger is a mixed strong cationic/strong anionic exchanger, such as i.e. DUPONT AmberLiete™ from Dupont, LANXESS LEW ATIT® from Lanxess, Purolite®PPC150 and Purolite®PPA500Plus from Purolite.

The radionuclides containing species retained in the ion exchanger may be subjected to stripping (ie) the release of the radionuclides containing species from the ion exchanger and their transformation for disposal.

In the case of ¹⁴C, carbon stripping typically involves the release of carbonates and bicarbonates containing ¹⁴C and their transformations into gaseous CO₂(g) (here ¹⁴COs) . This stripping is based on the following equilibria:

(1 ) CO₂(g) ±5 CO₂(aq)

(2) CO₂(aq) + H2O ±5 HCO₃‘ +H⁺

(3) HCO₃- ±5 CO₃ ²' + H⁺

Typically reactions (2) and (3) occur in the ion exchanger.

These reactions can be easily shifted in acidic conditions towards gaseous carbon dioxide.

The acidic conditions are provided by eluting the ion exchanger with an aqueous solution comprising the organic acid.

Typically, the organic acid is a weak organic acid. It is not limited and can be chosen from lactic acid, acetic acid, formic acid, citric acid, oxalic acid, uric acid, malic acid, tartaric acid, preferably formic acid.

As a result of the contacting of the organic acid with the ¹⁴C-containing species, reactions (2) and (3) above occur in the ion exchanger, so that an aqueous solution containing the organic acid and dissolved ¹⁴CO₂(aq) are discharged at the outlet of the ion exchanger. The dissolved ¹⁴CO₂(aq) may then be separated from the organic acid in the form of ¹⁴CO₂(gas), corresponding to step (1) mentioned above, and then collected for disposal. According to an embodiment, the recirculation method of the invention may also include the further step of extracting and collecting ¹⁴CO2(gas) from ion exchanger before conducting the mineralization step (b). This extraction and collection of ¹⁴CO2(gas) may be conducted in a known manner using a stripping unit, as discussed below. The mineralization step (b): Following the stripping of the radionuclides from the ion exchanger, the organic acid- containing solution (also called circulating solution) is then further processed in an electrolyzer for mineralization step (b). An « electrolyzer » as used herein refers to an electrochemical device configured to use an electrical current to drive a chemical reaction. Typically, oxidation (loss of electrons) occurs at the anode and reduction (gain of electrons) occurs at the cathode. The electrolyzer arrangement typically consists of an anode, a cathode. A typical electrode arrangement is disclosed known from EP 1730080 B1. Electrolyzers are commercially available. Representative electrolyzers include the "Susi" and "Bärbel" models available from Diaccon. In the present case, the organic acid is oxidized in a chemical reaction called mineralization (ie) the decomposition of the organic acid so as to form carbon dioxide (CO2). According to the invention, the mineralization under step (b) is conducted into an electrolyzer comprising as an anode a boron-doped diamond electrode. According to an embodiment, the cathode may be made of the same material as the anode, or made of another material such as platinum or stainless steel. The term “boron-doped diamond electrode” as used herein refers to metallic or non- metallic conductive substrate on which a layer of diamond crystals is deposited, where the diamond layer is doped with boron so as to be conductive. Said substrate typically consists of niobium, titanium, tungsten, tantalum or SiO₂. The coating of the diamond layer on the electrode substrate can be typically carried out by chemical vapor deposition (CVD), and/or by application or adaptation of methods known in the art such as US 5,399,247. Boron-doped diamond electrodes are commercially available, for example they may be purchased from Diaccon GmbH (Fürth, Germany) or Condias GmbH (Itzehoe, Germany). Diamond electrodes have the advantage that they enable a high overvoltage, by means of which strong oxidizing agents such as ozone, hydrogen peroxide and OH radicals are generated in situ from the wastewater. It is therefore possible with diamond electrodes to produce oxidation products that are otherwise difficult to obtain. In wastewater treatment, oxidation is carried out using the diamond electrodes until the organic acid is completely or almost completely mineralized, i.e. degraded into CO2. The setting of the current strengths suitable for an electrochemical treatment cell used and the operating parameters used can easily be determined experimentally by measuring the concentrations of acid. It is also possible to determine the reduction in the total acid content by the usual methods. The duration of the mineralization depends on the organic acid concentration in the circulating solution. Increasing of the organic acid in the circulating solution may generally increase the TOC depletion rate of the organic acid. According to an embodiment, the concentration of the organic acid in the electrolyzer is more than 0.1 ppm, typically more than 1 ppm, preferably comprised above 1 ppm. According to a further embodiment, the anode current density may be comprised between 0.02 A / cm² and 0.2 A / cm². This current density may be measured by an ammeter According to the invention, the mineralization step (b) leads to the formation of an aqueous mixture comprising the unreacted organic acid and dissolved CO₂. The degassing step (c) : According to the invention, the method comprises a degassing step wherein the dissolved CO₂ is extracted as gas from the circulating solution. This degassing step may typically be conducted in a surge tank containing a stripping unit.

The resulting degassed aqueous solution comprising the unreacted organic acid is then recirculated in a loop into the ion exchanger, in order to undergo a further stripping step.

According to a further embodiment, the recirculation method of the invention further comprises increasing the concentration of the organic acid in the circulating solution. This concentration step may be conducting in a concentration unit, as discussed below.

Typically, this step may be conducted after the stripping step and before the mineralization step.

It was found that the reaction kinetics of the electrochemical oxidation using boron-doped diamond electrodes can be increased by increasing the acid concentration.

This increase of the organic acid concentration may typically be achieved by a concentration unit such as a reverse osmosis membrane and/or a bipolar electrodialyser, located after the outlet of the stripping unit and before the inlet of the electrolyzer.

The combination of boron-doped diamond electrolysis together with such a concentration unit will also prevent a drop in the decomposition rate of the organic acid.

This is because the electrochemical decomposition is only constant up to a certain concentration limit of the acid concentration, whereas the decomposition rate drops exponentially if the organic acid concentration is lower to a determined minimal value.

According to an embodiment, the method of the invention may thus comprise a concentration step before conducting the mineralization step (b), to prevent the acid concentration from decreasing below a defined minimal range.

According to this embodiment, the aqueous solution obtained after the stripping step is passed through a concentration unit, thus leading to an organic acid solution with an increased concentration in acid which is then fed into the mineralization step (b) (e.g.) in the electrolyzer.

According to an embodiment, the pH of the aqueous circulating solution is comprised between 2 and 9, typically comprised between 3 and 5. It is typically measured in the measuring section integrated downstream the degassing step. The apparatus : According to a further object, the present invention concerns a recirculation apparatus for the continuous mineralization of an organic acid in an aqueous solution used for the stripping of radionuclides retained in an ion exchanger, the apparatus comprising an ion exchanger comprising an ion exchange resin, an electrolyzer comprising a boron-doped diamond electrode, a degassing unit, and a fluid circuit, wherein the fluid circuit comprises a loop configuration for continuously circulating said aqueous solution from the outlet of said ion exchanger through the electrolyzer, then through the degassing unit, and then from the outlet of said degassing unit to the inlet of said ion exchanger. According to an embodiment, the degassing unit is typically a surge tank comprising a stripping unit to extract CO2(g) form the aqueous solution downstream the electrolyzer. According to an embodiment, the apparatus may further comprise another stripping unit at the outlet of the ion exchanger to separate the ¹⁴CO2(aq) dissolved in the aqueous solution discharged from the ion exchanger and to further extract it in the form of ¹⁴CO2(gas). As used herein, a stripping unit may be for example a membrane degasser contactor or trickling deaerator. Such a stripping unit may be in the surge tank (downstream the electrolyzer). A similar or different stripping unit may also be present at the outlet of the ion exhanger to extract ¹⁴CO2(aq). According to a further embodiment, the apparatus may further comprise a collecting unit to collect the gas, (ie) either CO2(gas) or ¹⁴CO2(gas) that is extracted from the stripping unit(s) discussed above. Such a collecting unit may be chosen for example from molecular sieves, scrubbing bottles or soda traps. According to an embodiment, the apparatus further comprises at least one concentration unit between the outlet of the ion exchanger and the inlet of the electrolyzer. According to this embodiment, the aqueous solution obtained downstream the ion exchanger is fed into at least one concentration unit before circulating through the electrolyzer. The concentration unit is typically configured to generate from the organic acid solution that is fed a concentrate solution and a permeate solution, where: the concentrate solution is more concentrated in organic acid than the fed solution and is further circulated from the concentration unit into the electrolyzer, and the permeate solution is less concentrated in organic acid than the fed solution and is routed in a bypass from the outlet of the concentration unit into the main recirculation flow downstream the outlet of the electrolyzer. One or more concentration units may be present in the recirculation circuit. They may be the same or different. They may be arranged in parallel or in series. The concentration unit may be chosen from reverse osmosis membranes and electrodialysers, such as bipolar electrodialyzers. Reverse osmosis membranes and electrodialysers are commercially available from BWT AG (Mondsee, Austria), Hydranautics (California, USA) and PCCell GmbH (Heusweiler, Germany). For example, a reverse osmosis membrane and an electrodialyser may be present, in parallel, or in series. By connecting a reverse osmosis membrane and a bipolar electrodialyser in series, the concentrate solution supplied discharged from the second concentration unit before the inlet of the electrolyzer can be further concentrated. According to a further embodiment, the electrolyzer is configured so that the gap width between the anode and the cathode is comprised between 0.5 mm and 10 mm. According to an embodiment, the temperature of the mineralization-step is comprised between 14 °C and 47 °C. A detailed description of specific embodiments of the invention is given hereafter with reference to the accompanying drawings. According to a further object, the present invention also concerns a method for the mineralization of organic acid used for stripping of radionuclides immobilized in an ion exchanger, said method comprising the above recirculation method of the invention. According to a still further object, the present invention additionally concerns the regeneration of an ion exchanger comprising immobilized radionuclides with an organic acid, said method comprising conducting a recirculation method of the invention. Description of the drawings Figure 1 is a flow diagrammatic representation illustrating an embodiment of the method of the invention, comprising a ion exchanger, a stripping unit and an electrolyzer with boron-doped diamond electrodes. Figure 2 is a flow diagrammatic representation illustrating an illustration of an embodiment of the apparatus of the invention, further comprising two concentration units, according to a first alternative where the concentration units are arranged in parallel. Figure 3 is a flow diagrammatic representation illustrating an illustration of an embodiment of the apparatus of the invention, further comprising two concentration units, according to a second alternative where the concentration units are arranged in parallel. Figure 4 represents the test setup for TOC degradation that was used in the experimental part Figure 5 illustrates the TOC degradation rates vs initial TOC concentration Figure 6 illustrates the model calculation for TOC degradation with (lower curve) and without (upper curve) increase in the concentration Detailed description of specific embodiments The apparatus of Figure 1 is configured for the continuous stripping of radionuclides immobilized on an ion exchanger 3, with an aqueous solution of organic acid. This is particularly designed for the carbon stripping of ¹⁴C. The fluid lines are illustrated in continuous lines. The fluid circuit is arranged in a loop and comprises at least one pump 2 arranged for circulating the fluid through the circuit. The pump 2 may be controlled by a control unit (not represented). In a known manner, the ion exchanger 3 comprises an ion exchange resin comprising immobilized ¹⁴C containing carbonates and bicarbonates. The ion exchange resin is eluted with a solution of organic acid that releases the ¹⁴C in the form of dissolved ¹⁴CO₂ throughout the ion exchanger 3. The circulating aqueous solution comprising the organic acid, and the dissolved ¹⁴CO2 flows from the tail of the ion exchanger. The apparatus may also comprise a stripping unit downstream the ion exchanger 3, to extract ¹⁴CO2(gas) which is discharged from the circulating solution and may then further be collected. According to the invention, the apparatus also comprises an electrolyzer 4 downstream the ion exchanger 3. The electrolyzer 4 is driven by a power supply unit (not represented in Figures 1-3) to generate oxidating species (such as hydroxyl radicals) to carry out the electrochemical oxidation of organic acid present in the circulating solution. The organic acid is mineralized into CO2 that is dissolved. in the aqueous solution circulating from the electrolyzer 4. The apparatus also comprises a degassing unit 1 to release the dissolved CO2 into CO2(gas) from the mixture circulating from the electrolyzer 4. The extracted CO2(gas) may be freely released or may be collected (not shown). A representative detailed arrangement of the electrodialyser 4 is represented in Figure 4 for illustrative purpose. According to an embodiment, the apparatus may comprise one or more concentration units. Figure 2 and Figure 3 respectively represent two alternative arrangements of this embodiment. In Figure 2, the apparatus comprises two concentration units 5 and 6 arranged in parallel. As an illustration, concentration unit 5 is a reverse osmosis membrane and concentration unit 6 is an electrodialyser. The two concentration units are in parallel: the fluid circuit is configured to divide the circulating solution into two feeding flows and is further configured to ensure circulation of the feeding flows either in the unit 5 or in the unit 6. Concentration unit 5 is configured to generate a downstream concentrated solution C(5) and a downstream permeate solution P(5), whereas concentration unit 6 is configured to generate a downstream concentrated solution C(6) and a downstream permeate solution P(6). The acid concentration in C(5) and in C(6) is respectively higher than the acid concentration in each of the corresponding feeding flow. The fluid circuit is configured for circulating the concentrated solutions C(5) and C(6) into the electrolyzer 4, and for circulating the permeate solutions P(5) and P(6) in two bypass lines (that may be collected into one single bypass, not represented) and connecting the permeate solutions P(5) and P(6) with the main circulating solution downstream the electrolyzer 4.

The fluid circuit is configured for circulating the main circulating solution from the electrolyzer 4 to the degassing unit 1 , for conducting further stripping.

In Figure 3, the apparatus comprises two concentration units 5 and 6 arranged in series. As an illustration, concentration unit 5 is a reverse osmosis membrane and concentration unit 6 is an electrodialyser. The two concentration units are respectively arranged in a downstream configuration: As represented, concentration unit 6 is downstream concentration unit 5, although the reverse arrangement may also be considered (not represented).

The fluid circuit is configured to feed the circulating solution into the first concentration unit 5 (here a reverse osmosis membrane)

Concentration unit 5 is configured to generate a downstream concentrated solution C(5) and a downstream permeate solution P(5)

The acid concentration in C(5) is higher than the acid concentration in the circulating solution feeding concentration unit 5.

The fluid circuit is configured for circulating the concentrated solution C(5) into the second concentration unit 6 (here an electrodialyser)

Concentration unit 6 is configured to generate a downstream concentrated solution C(6) and a downstream permeate solution P(6).

The acid concentration in C(6) is respectively higher than the acid concentration in the concentrated solution C(5) feeding the second concentration unit 6.

The fluid circuit is configured for circulating the concentrated solution C(6) flowing from concentration unit 6 into the electrolyzer 4, to undergo electrochemical oxidation.

It is also configured for circulating the permeate solutions P(5) and P(6) in two respective bypass lines (that may be collected into one single bypass, not represented), and for connecting the permeate solutions P(5) and P(6) with the main circulating solution downstream the electrolyzer 4.

The fluid circuit is configured for circulating the main circulating solution from the electrolyzer 4 to the degassing unit 1 , for conducting further stripping. Examples Total organic carbon (TOC) degradation with boron-doped diamond electrodes The experiments on the mineralization of organic compounds, measured by the analytical sum parameter TOC (Total Organic Carbon), with boron-doped diamond electrodes were carried out at in a radiochemical laboratory. The experimental setup was handled in a fume cupboard. The circuit consisted of the following components expansion tank -> pump -> electrolyzer -> expansion tank, see Figure 4 The components used in the experiments were: • Expansion tank (20 L) • Pump (EHEIM; 20 l / min) • Electrolyzer (Diaccon Company; models "Susi" - laboratory scale and "Bärbel" - technical scale) • Cold trap for condensate return • Container cooling • Gas mouse 1 and gas mouse 2 (Swagelok; 12 mm stainless steel valves) • Mass flow controller (MFC) (Alicat; Vmax = 10 NLMP) • Flow meter (NATEC Sensors GmbH; Vmax = 100 NLPM) • Nitrogen (flushing gas for container flushing) • Sampling valve (liquid samples) • Hydrogen sensor (gas detector testo) • Power supply unit (TDK-Lambda; model GEN16-150A) • Catalyst (Infiltec GmbH; type: IAC-114 / IAC-124; Pd / Pt mixture 1: 1) • Temperature sensor (PT-100) • Chemicals, tools and devices: Formic acid (Merck, 98-100%) Commercial washing powder (ethylenediaminetetraacetic acid (EDTA) as complexing agent) Deionized water (conductivity ~ 1.5 µS / cm) TOC quicktesters LCK 385 and LCK 386 (Hach-Lange) UV-VIS spectrometer (Hach-Lange; DR5000) pH indicator strips (Merck; pH 0-14) Conductivity meter (WTW; LF 315) In order to minimize the volume loss due to evaporation, a condensate return was built into the gas stream, which was kept at 10°C with a cooling unit. The circulating solution was tempered with a cooling loop, which was operated either with a cooling unit or with tap water. With the external cooling, the circulating solution was kept at about 30 ° C. during operation. A constant stream of nitrogen was continuously introduced into the expansion tank via a mass flow controller, with the result that the gas volume in the expansion tank was continuously conducted via the cold trap through the gas mouse 1 , the catalyst (cat.) and the gas mouse 2. The catalytic converter was positioned vertically in the exhaust gas flow and flowed against it from below. The temperature in the catalytic converter was tapped with a PT-100 element directly on the metal cartridge of the catalytic converter. After the gas mouse 2 a flow meter (FM) was installed and the outlet volume flow was monitored. Gas samples were taken before and after the catalyst and examined for hydrogen and oxygen evolution. For the determination of pH, conductivity and TOC, liquid samples were taken from the circuit at irregular time intervals. To monitor the hydrogen concentration in the fume cupboard, a hydrogen sensor was positioned in the middle of the fume cupboard. When hydrogen was detected, the measuring device provided optical and acoustic signals. In order to be able to compare the TOC degradation rates with the differently dimensioned electrolyzers, the applied current density in the electrolyzer was kept constant. Two different electrolyzers from Diaccon were used for the TOC degradation tests. The properties of the electrolyzers used are listed in Table 1 .

Table 1 : Comparison of the used electrolyzers A total of 9 experiments on TOC degradation with boron-doped diamond electrodes were carried out, which are listed in Table 2 Table 2: TOC degradation experiments Five experiments were carried out with the Susi electrolyzer and four with the Bärbel electrolyzer. Washing powder and formic acid were used as test substances. The concentration of the test substance was adjusted in such a way that the largest possible TOC starting concentration interval is achieved. The set current density was around 0.09 A/cm² in all experiments, which means that the "Susi" electrolyzer was operated with a current flow of 14.5 A and the "Bärbel" electrolyzer with a current flow of 135 A. All experiments were mineralized to a TOC concentration of about 5 ppm. The TOC degradation rates determined depending on the TOC starting concentration are shown in Figure 5. In the TOC concentration interval from 0.01 g TOC / L (10 ppm TOC) to 0.23 gTOC / L (230 ppm TOC) there is a linear course of the TOC breakdown rate from the TOC start concentration. It can be seen that a higher TOC degradation rate is achieved with a high TOC concentration. In order to keep the duration of the mineralization of the organic acid used in the separation of C-14 from ion exchangers by means of boron-doped diamond electrodes as short as possible, a high concentration of the organic acid must be set in the electrolyzer. This can be achieved with a membrane separation (e.g. reverse osmosis) and/or electrodialysis process. Both components must be connected upstream of the electrolyzer (boron-doped diamond electrodes) so that the concentrated organic acid is mineralized with a significantly improved TOC breakdown rate. Reverse osmosis or electrodialysis processes can increase the concentration of organic compounds in the concentrate by a factor of 10. This results in a significantly faster TOC reduction. The different TOC degradation rates with and without previous concentration were modeled and are shown in Figure 6. Here it becomes clear that the organic matter of a solution can be degraded much faster if the electrolyzer is equipped with a concentrating unit such as e.g. reverse osmosis membrane is connected upstream.

Claims

1. A recirculation method for the continuous mineralization of an organic acid in an aqueous solution used for the stripping of radionuclides from an ion exchanger, characterized in that said process comprises the following steps: a) transferring the aqueous solution comprising the organic acid following the stripping step to an electrolyzer comprising a boron-doped diamond electrode; b) contacting said organic acid with the boron-doped diamond electrode in the electrolyzer, thereby performing an electrolytic oxidation of the organic acid so as to mineralize at least a part of the organic acid in the aqueous solution, and c) degassing CC>2(gas) from the aqueous solution of step (b) circulating from the electrolyzer, and d) transferring the resulting solution to the ion exchanger for continued stripping.

2. The recirculation method according to claim 1 wherein the radionuclide is ¹⁴C.

3. The recirculation method according to claim 1 or 2 further comprising extraction and collection of gaseous ¹⁴CC>2.

4. The recirculation method according to anyone of the preceding claims wherein the organic acid is a weak organic acid.

5. The recirculation method according to claim 4 wherein the weak organic acid is formic acid.

6. The recirculation method according to anyone of the preceding claims wherein the concentration of the organic acid in the electrolyzer is more than 0.1 ppm.

7. The recirculation method according to anyone of the preceding claims further comprising the step of increasing the concentration of the organic acid in the aqueous solution upstream electrolyzer.

8. A method for the mineralization of organic acid used for stripping of radionuclides immobilized in an ion exchanger, said method comprising the recirculation method according to anyone of the preceding claims.

9. A method for regenerating an ion exchanger comprising immobilized radionuclides with an organic acid, said method comprising conducting a recirculation method according to anyone of claims 1 to 7.

10. A recirculation apparatus for the continuous mineralization of an organic acid in an aqueous solution used for the stripping of radionuclides retained in an ion exchanger, the apparatus comprising an ion exchanger comprising an ion exchange resin, an electrolyzer comprising a boron-doped diamond electrode, a degassing unit, and a fluid circuit, wherein the fluid circuit comprises a loop configuration for continuously circulating said aqueous solution from the outlet of said ion exchanger through the electrolyzer, and through the degassing unit, and then from the outlet of said degassing unit to the inlet of said ion exchanger.

11. The apparatus according to claim 10 wherein the anode current density in the electrolyzer is comprised between 0.02 A / cm² and 0.2 A / cm².

12. The apparatus according to claim 10 or 11 wherein the electrolyzer is configured so that the gap width between the anode and the cathode is comprised between 0.5 mm and 10 mm.

13. The apparatus according to anyone of claims 10 to 12, wherein the degassing unit is a surge tank comprising a stripping unit.

14. The apparatus according to anyone of claims 10 to 13 further comprising at least one concentration unit between the outlet of the ion exchanger and the inlet of the electrolyzer.

15. The apparatus according to claim 14 which comprises as concentration units a reverse osmosis membrane and an electrolyzer, arranged in serie, where the electrodialyser is downstream the reverse osmosis membrane.