Mineralization of organic compounds using boron-doped diamond electrodes during radionuclide stripping

Abstract

The invention relates to a recycling method for the continuous mineralization of organic acids in aqueous solutions by means of an electrolyser comprising boron-doped diamond electrodes, an apparatus for carrying out said method and a method for regenerating ion exchangers containing immobilized radionuclides, the organic acids being used to strip the radionuclides from the ion exchangers, the method for regenerating ion exchangers involving said recycling method.

Description

Technical Field

[0001] The present invention relates to the field of nuclear reactors, and more particularly to the production of radioactive isotopes (e.g. carbon-14 ( 14 C)) extraction. Background Art

[0002] 14 C mainly through stable nitrogen, oxygen 17 O and to a lesser extent 13 In nuclear reactors, due to the presence of stable parent isotopes, 14 C is produced in the fuel and in the reactor coolant by core structural materials, where the parent isotope is activated by neutrons produced by nuclear fission reactions occurring in the fuel. 14 C is produced especially in CANDU reactors, since this type of reactor uses heavy water (D2O) (both as a moderator and as a coolant), which is more volatile than light water. 17 O has a higher isotopic abundance.

[0003] Generated 14 C is retained in the ion exchange resin used to purify heavy water from the circuit. 14 C is mainly in the form of carbonate and / or bicarbonate derivatives.

[0004] Contains 14 The carbonate and bicarbonate of C can then be released from the ion exchanger by elution with an organic acid, resulting mainly in 14 Release of CO2. After the ion exchanger, the organic acid used for elution is then decomposed into CO2. This is usually done by mineralization to minimize the migration of radionuclides from the ion exchanger, and the mineralization is carried out by UV Fenton treatment in a batch operation. Typically, hydrogen peroxide (at superstoichiometric concentrations to avoid polymerization of organic acids due to irradiation with ultraviolet light) and a catalyst are introduced into a batch tank; the contents of the tank are then pumped to a UV photolysis lamp.

[0005] This mineralization step results in longer processing times as it is carried out in batches and requires the use of additional chemicals (hydrogen peroxide, metal catalysts).

[0006] Therefore, the mineralization process of organic acids is time-consuming and largely determines the duration of the stripping process in the ion exchanger.

[0007] Therefore, the organic acids discharged from the ion exchanger are only gradually decomposed in the solution pumped in the small loop. During the organic acid degradation process, the solution should not flow directly through the ion exchanger, because the chemicals required for acid oxidation will move radionuclides from the ion exchanger. Only when the pH value reaches pH 5 to pH 7 and the oxidant (hydrogen peroxide) has been eliminated, the ion exchanger is reintegrated into the stripping cycle.

[0008] In fact, the treated solution is introduced into the ion exchanger to establish a new equilibrium between the ion exchanger and the solution, so that the pH value caused by the entry of organic acids drops to pH 3 to pH 5. After the equilibrium is established, the ion exchanger must be separated again to further decompose the organic acids. Therefore, with the gradual degradation of organic acids, the complete mineralization of the acids is very slowly successful.

[0009] In addition, UV Fenton treatment involves several problems: When hydrogen peroxide is used, the ion exchanger also decomposes. Due to the loss of the ability to bind ions, the already bound radionuclides enter the stripping treatment cycle. In addition, with the use of metal catalysts in UV Fenton treatment, radionuclides are inevitably washed into the stripping treatment cycle through the exchange reaction on the ion exchanger.

[0010] The metal catalysts used in the UV Fenton treatment process may also undergo hydrolysis and can cause the solution to become turbid in the treatment loop. Furthermore, organic acids can polymerize under the action of UV photolysis lamps: the decomposition of the polymeric products takes much longer than the decomposition of the monomeric organic acids.

[0011] Therefore, there is a need to provide an alternative mineralization of organic acids which avoids the above-mentioned disadvantages.

[0012] EP 2 949 630 discloses a system and method for recovering water from contaminated water, such as urine. Specifically, the system comprises a cation exchange device for desalting the sewage and an organic component decomposition device comprising diamond electrodes. The water is then discharged into an electrodialysis device, where acid is produced, which can be used to regenerate the cation exchange device.

[0013] EP1 522 526 discloses a method for treating chemical plating waste liquid containing phosphorus components and organic compounds, for reducing the amount of impurities in the waste liquid. Specifically, a solution containing, for example, disodium hydrogen phosphite, phosphorous acid and sodium citrate is electrolytically treated in an electrolytic cell having a conductive boron-doped diamond electrode as an anode for oxidizing disodium hydrogen phosphite and phosphorous acid to orthophosphoric acid and decomposing citrate to carbon dioxide and water.

[0014] US 5,399,247 teaches a doped diamond anode electrolysis to oxidize solutes in wastewater.

[0015] WO 2015 / 127918 discloses an electrochemical oxidation method for degrading nitrogen-containing compounds in wastewater using a diamond electrode as an anode.

[0016] There remains a need to provide an improved mineralization of organic acids after a stripping treatment, which can be performed easily since no batch steps and / or addition of various chemicals are required.

[0017] A new mineralization method has now been discovered that involves electrolysis using boron-doped diamond electrodes that electrochemically oxidize organic acids used to separate C-14 from ion exchangers.

[0018] This electrolysis-based mineralization of organic acids does not require the addition of chemicals. In addition, the ion exchanger can be continuously discharged to the electrolysis step, resulting in faster degradation of the organic acids. Summary of the invention

[0019] Therefore, according to a first object, the present invention relates to a process for the continuous mineralization of an aqueous solution containing organic acids used to strip radionuclides from ion exchangers,

[0020] Characterized in that the method comprises the following steps:

[0021] a) after the stripping step, transferring the aqueous solution comprising the organic acid to an electrolyser comprising a boron-doped diamond electrode;

[0022] b) contacting the organic acid with a boron-doped diamond electrode in an electrolyzer to electrolytically oxidize the organic acid so as to mineralize at least a portion of the organic acid in the aqueous solution,

[0023] c) degassing CO2 (gas) from the aqueous solution circulated from the electrolyser of step (b), and d) transferring the resulting solution to an ion exchanger for continued stripping.

[0024] According to the invention, the mineralization of the organic acid now takes place via hydroxyl radicals which are generated in situ on the electrode surface (anode).

[0025] Mineralization of the acid can start immediately after complete stripping, without the need to separate the ion exchanger from the stripping circuit. Therefore, batch operation (gradual degradation of the acid) is no longer necessary.

[0026] Furthermore, no additives are used to mineralize the organic acids. Since hydrogen peroxide is no longer required, there is no risk of ion exchanger degradation, and the absence of catalysts means there is no risk of metallic radionuclides migrating from the ion exchanger.

[0027] Due to the continuous operation of the mineralization, the equilibrium setting between the ion exchanger and the treatment solution changes gradually, thus creating an opportunity for previously mobile radionuclides to bind again on the ion exchanger before they are discharged into the treatment cycle. This leads to an improvement in the treatment efficiency.

[0028] Thus, the overall process is improved since the electrochemical decomposition of the organic acid by the boron-diamond electrode is faster and more direct than the UV-Fenton step and can therefore advantageously be performed continuously and in a cost-effective and time-efficient manner.

[0029] Detailed Description

[0030] The embodiments described below are to be understood individually or in any combination thereof.

[0031] The term "radionuclide" as used herein refers to a radioactive isotope (also referred to as a radioisotope). In general, the radionuclide according to the present invention is usually a radioisotope present in a nuclear waste water purification circuit. It can in particular be 14 C; 14 C can be carbonate CO3 2- and bicarbonate HCO 3- In wastewater purification, these substances containing radionuclides are fixed in ion exchangers.

[0032] As used herein, an ion exchanger defines a container containing a solid phase, typically a resin, which is capable of removing unwanted chemicals from a liquid phase by retaining (or fixing) the unwanted chemicals on the resin. 14 C derivatives, such as carbonate and / or bicarbonate derivatives, which are contained in the liquid phase of the nuclear wastewater and are retained on the ion exchanger.

[0033] The solid phase can be a cation exchanger that exchanges positively charged ions (cations), an anion exchanger that exchanges negatively charged ions (anions), or a mixed cation / anion exchanger. The resin can typically be a functionalized porous or gel polymer.

[0034] According to an embodiment, the ion exchanger is a mixed strong cation / strong anion exchanger, such as DUPONT AmberLiete from Dupont. TM LANXESS from Lanxess From Purolite PPC150 and PPA500Plus.

[0035] The radionuclide-containing material retained in the ion exchanger may be subjected to stripping (ie, release) of the radionuclide-containing material from the ion exchanger and their transformation for disposal.

[0036] 14 In the case of C, carbon stripping usually involves 14 The release of carbonate and bicarbonate of C and their conversion into gaseous CO2(g) (here 14 CO2).

[0037] This stripping is based on the following equilibrium:

[0038] (1)

[0039] (2)

[0040] (3)

[0041] Typically reactions (2) and (3) occur in an ion exchanger.

[0042] These reactions are easily transferred to gaseous carbon dioxide under acidic conditions.

[0043] Acidic conditions are provided by washing the ion exchanger with an aqueous solution comprising an organic acid.

[0044] Typically, the organic acid is a weak organic acid. The organic acid is not limited and may be selected from lactic acid, acetic acid, formic acid, citric acid, oxalic acid, uric acid, malic acid, tartaric acid, preferably formic acid.

[0045] Because organic acids and 14 C, the above reactions (2) and (3) occur in the ion exchanger, so the organic acid and dissolved 14 The aqueous solution of CO2(aq) is discharged at the outlet of the ion exchanger.

[0046] Then dissolved 14 CO2(aq) can be 14 The CO2 (gas) form is separated from the organic acid, corresponding to step (1) above, and then collected for waste disposal.

[0047] According to an embodiment, the recycling method of the present invention may further include extracting and collecting from the ion exchanger before performing the mineralization step (b). 14 Further steps of CO2 (gas).

[0048] 14 The extraction and collection of CO2(gas) can be carried out in a known manner using a stripping unit, as described below.

[0049] Mineralization step (b):

[0050] After stripping the radionuclides from the ion exchanger, the solution containing the organic acid (also called circulating solution) is then further treated in an electrolyser for the mineralization step (b).

[0051] As used herein, "electrolyzer" refers to an electrochemical device configured to use an electric current to drive a chemical reaction. Typically, oxidation (loss of electrons) occurs at the anode and reduction (gain of electrons) occurs at the cathode. An electrolyzer device typically consists of an anode and a cathode.

[0052] A typical electrode arrangement is known from EP 1 730 080 B1.

[0053] Electrolyzers are commercially available. Representative electrolyzers include the "Susi" and model.

[0054] In the present case, the organic acid is oxidized in a chemical reaction called mineralization (ie, decomposition of the organic acid) to form carbon dioxide (CO2).

[0055] According to the invention, the mineralization under step (b) is carried out into an electrolyser comprising a boron doped diamond electrode as anode.

[0056] Depending on the embodiment, the cathode may be made of the same material as the anode, or of another material, such as platinum or stainless steel.

[0057] As used herein, the term "boron-doped diamond electrode" refers to a layer of diamond crystals deposited on a metallic or non-metallic conductive substrate, wherein the diamond layer is doped with boron for conductivity.

[0058] The substrate is typically composed of niobium, titanium, tungsten, tantalum or SiO2.

[0059] Coating of the diamond layer on the electrode substrate may generally be performed by chemical vapor deposition (CVD) and / or by applying or adapting methods known in the art, such as those described in US Pat. No. 5,399,247.

[0060] Boron-doped diamond electrodes are commercially available, for example they can be purchased from Diaccon GmbH (Fürth, Germany) or Condias GmbH (Itzehoe, Germany).

[0061] The advantage of diamond electrodes is their ability to achieve high overvoltages, by which strong oxidants such as ozone, hydrogen peroxide and OH radicals are generated in situ from wastewater. Diamond electrodes can therefore be used to produce oxidation products that are difficult to obtain otherwise. In wastewater treatment, oxidation is carried out using diamond electrodes until the organic acids are completely or almost completely mineralized, i.e. degraded to CO2.

[0062] The setting of the current intensity suitable for the electrochemical treatment cell used and the operating parameters used can be easily determined experimentally by measuring the acid concentration. The reduction in the total acid content can also be determined by conventional methods.

[0063] The duration of mineralization depends on the concentration of organic acids in the circulating solution. An increase in organic acids in the circulating solution can generally increase the TOC consumption rate of organic acids.

[0064] According to an embodiment, the concentration of the organic acid in the electrolyzer is greater than 0.1 ppm, typically greater than 1 ppm, and preferably greater than 1 ppm.

[0065] According to another embodiment, the anode current density can be 0.02A / cm 2 ~0.2A / cm 2 The current density can be measured by an ammeter.

[0066] According to the invention, the mineralization step (b) results in the formation of an aqueous mixture comprising unreacted organic acid and dissolved CO2.

[0067] Degassing step (c):

[0068] According to the invention, the method comprises a degassing step, wherein dissolved CO2 is extracted from the circulating solution as a gas.

[0069] This degassing step can typically be performed in a buffer tank comprising a stripping unit.

[0070] The resulting degassed aqueous solution comprising unreacted organic acid is then recycled in a loop to the ion exchanger for a further stripping step.

[0071] According to another embodiment, the recycling method of the present invention further comprises increasing the concentration of the organic acid in the circulating solution. As described below, the concentration step can be performed in a concentration unit.

[0072] Typically, this step may be performed after the stripping step and before the mineralization step.

[0073] It was found that the reaction kinetics of electrochemical oxidation using a boron-doped diamond electrode can be increased by increasing the acid concentration.

[0074] This increase in organic acid concentration can usually be achieved by a concentrating unit (eg, a reverse osmosis membrane and / or a bipolar electrodialyzer) located after the stripping unit outlet and before the electrolyzer inlet.

[0075] The combination of boron-doped diamond electrolysis with this concentrating unit will also prevent the decrease in the decomposition rate of organic acids.

[0076] This is because the electrochemical decomposition remains constant only up to a certain acid concentration limit, whereas the decomposition rate decreases exponentially if the organic acid concentration is below a determined minimum value.

[0077] Thus, according to an embodiment, the method of the invention may comprise a concentration step before carrying out the mineralization step (b) in order to prevent the acid concentration from falling below a defined minimum range.

[0078] According to this embodiment, the aqueous solution obtained after the stripping step passes through a concentration unit, thus resulting in an organic acid solution with increased acid concentration, which is then fed to the mineralization step (b), for example in an electrolyser.

[0079] According to an embodiment, the pH of the aqueous circulating solution is between pH 2 and pH 9, typically between pH 3 and pH 5. It is typically measured in a measuring section that is integrated downstream of the degassing step.

[0080] instrument:

[0081] According to a further object, the invention relates to a recycling plant for the continuous mineralization of an organic acid in aqueous solution, said organic acid being used for stripping radionuclides retained in an ion exchanger, comprising:

[0082] An ion exchanger comprising an ion exchange resin,

[0083] An electrolyser comprising a boron-doped diamond electrode,

[0084] Degassing unit, and

[0085] A fluid circuit, wherein the fluid circuit comprises a loop configuration for continuously circulating the aqueous solution from the outlet of the ion exchanger through the electrolyzer, then through the degassing unit, and then from the outlet of the degassing unit to the inlet of the ion exchanger.

[0086] According to an embodiment, the degassing unit is typically a buffer tank comprising a stripping unit to extract CO2(g) from the aqueous solution downstream of the electrolyser.

[0087] According to one embodiment, the device may further include another stripping unit located at the outlet of the ion exchanger to separate the ions dissolved in the aqueous solution discharged from the ion exchanger. 14 CO2(aq), and further 14 Extracted in the form of CO2 (gas).

[0088] As used herein, a stripping unit can be, for example, a membrane degasser contactor or a trickle degasser. Such a stripping unit can be present in the buffer tank (downstream of the electrolyzer). A similar or different stripping unit can also be present at the outlet of the ion exchanger to extract 14 CO2(aq).

[0089] According to a further embodiment, the apparatus may further comprise a collecting unit to collect the gas extracted from the stripping unit discussed above, (i.e.) CO2 (gas) or 14 CO2 (gas). Such a collection unit can be selected, for example, from a molecular sieve, a scrubber bottle or a soda trap.

[0090] According to one embodiment, the device further comprises at least one concentrating unit between the outlet of the ion exchanger and the inlet of the electrolyser.

[0091] According to this embodiment, the aqueous solution obtained downstream of the ion exchanger is fed to at least one concentration unit before circulating through the electrolyser.

[0092] The concentration unit is generally configured to produce a concentrate solution and a permeate solution from a feed organic acid solution, wherein:

[0093] The concentrate solution is more concentrated in organic acid than the feed solution and is further circulated from the concentration unit to the electrolyser, and

[0094] The permeate solution is less concentrated in organic acids than the feed solution and is conveyed in a bypass from the outlet of the concentrating unit to the main recycle stream downstream of the outlet of the electrolyser.

[0095] One or more concentrating units may be present in the recirculation loop.

[0096] The concentrating units may be identical or different. The concentrating units may be arranged in parallel or in series.

[0097] The concentrating unit may be selected from a reverse osmosis membrane and an electrodialyser, such as a bipolar electrodialyser.

[0098] Reverse osmosis membranes and electrodialyzers were commercially available from BWT AG (Mondsee, Austria), Hydranautics (California, USA), and PCCell GmbH (Heusweiler, Germany).

[0099] For example, the reverse osmosis membrane and the electrodialyser may be present in parallel or in series.

[0100] By connecting a reverse osmosis membrane and a bipolar electrodialyzer in series, the provided concentrate solution can be further concentrated, and the concentrate solution is discharged from the second concentration unit before the inlet of the electrolyzer.

[0101] According to a further embodiment, the electrolyser is configured such that the gap width between the anode and the cathode is 0.5 mm to 10 mm.

[0102] According to one embodiment, the temperature of the mineralization step is between 14°C and 47°C.

[0103] The specific implementation modes of the present invention are described in detail below with reference to the accompanying drawings.

[0104] According to a further object, the present invention also relates to a method for the mineralization of organic acids used for stripping radionuclides fixed in ion exchangers, said method comprising the above-described recycling method of the present invention.

[0105] According to a yet further object, the present invention furthermore relates to the regeneration of an ion exchanger comprising immobilized radionuclides with an organic acid, said method comprising carrying out the recycling method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Figure 1 is a flow chart illustrating an embodiment of the process of the present invention, comprising an ion exchanger, a stripping unit and an electrolyzer with boron doped diamond electrodes.

[0107] Figure 2 is a flow chart illustrating an exemplary embodiment of the apparatus of the present invention, further comprising two concentrating units according to a first alternative, wherein the concentrating units are arranged in parallel.

[0108] Figure 3 is a flow chart illustrating an exemplary embodiment of the apparatus of the present invention, further comprising two concentrating units according to a second alternative, wherein the concentrating units are arranged in parallel.

[0109] Figure 4 Represents the test setup for TOC degradation used in the experimental part.

[0110] Figure 5 The relationship between TOC degradation rate and initial TOC concentration is explained.

[0111] Figure 6 Model calculations for TOC degradation with (lower curve) and without (upper curve) concentration increase are illustrated. DETAILED DESCRIPTION

[0112] Figure 1 The apparatus is configured to continuously strip the radionuclides fixed on the ion exchanger 3 with an aqueous solution of an organic acid. This is particularly for 14 Designed for carbon stripping of C.

[0113] Fluid lines are indicated in solid 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 shown).

[0114] In a known manner, the ion exchanger 3 comprises an ion exchange resin comprising a fixed 14 The ion exchange resin is eluted with an organic acid solution, which is dissolved in the entire ion exchanger 3. 14 Released in the form of CO2 14 C. Contains organic acids and dissolved 14 The circulating aqueous solution of CO2 flows from the tail of the ion exchanger. The device may also include a stripping unit downstream of the ion exchanger 3 to extract the CO2 discharged from the circulating solution. 14 The CO2 (gas) can then be further collected.

[0115] According to the invention, the device further comprises an electrolyser 4 downstream of the ion exchanger 3. The electrolyser 4 is powered by a power supply unit ( Figures 1 to 3 The electrolyzer 4 is driven by a oxidizing agent (not shown) to produce oxidizing species (e.g., hydroxyl radicals) to electrochemically oxidize organic acids present in the circulating solution. The organic acids are mineralized into CO2, which is dissolved in the aqueous solution circulating from the electrolyzer 4.

[0116] The device also comprises a degassing unit 1 to release dissolved CO 2 from the mixture circulating in the electrolyser 4 as CO 2 (gas).

[0117] The extracted CO2 (gas) may be released freely or may be collected (not shown).

[0118] For the purpose of illustration, a representative detailed arrangement of the electrodialyser 4 is shown in FIG. Figure 4 Displayed in.

[0119] According to one embodiment, the device may comprise one or more concentrating units. Figure 2 and Figure 3 Two alternative arrangements of this embodiment are shown respectively.

[0120] exist Figure 2 In the embodiment, the device comprises two concentrating units 5 and 6 arranged in parallel. As an example, the concentrating unit 5 is a reverse osmosis membrane and the concentrating unit 6 is an electrodialyzer. The two concentrating units are connected in parallel: the fluid circuit is configured to divide the circulating solution into two feed streams and is further configured to ensure that the feed stream circulates in the unit 5 or in the unit 6.

[0121] Concentration unit 5 is configured to produce a downstream concentrated solution C( 5 ) and a downstream permeate solution P( 5 ), while concentration unit 6 is configured to produce a downstream concentrated solution C( 6 ) and a downstream permeate solution P( 6 ).

[0122] The acid concentrations in C(5) and C(6) are higher than the acid concentrations in each corresponding feed stream.

[0123] The fluid circuit is configured to circulate the concentrated solutions C(5) and C(6) to the electrolyzer 4 and to circulate the permeate solutions P(5) and P(6) in two bypass lines (which may be collected in a single bypass, not shown) and to connect the permeate solutions P(5) and P(6) to the main circulating solution downstream of the electrolyzer 4.

[0124] The fluid circuit is configured for circulating the main circulating solution from the electrolyser 4 to the degassing unit 1 for further stripping.

[0125] exist Figure 3 In the embodiment, the device comprises two concentrating units 5 and 6 arranged in series. As an example, the concentrating unit 5 is a reverse osmosis membrane and the concentrating unit 6 is an electrodialyzer. The two concentrating units are arranged in a downstream configuration: as shown, the concentrating unit 6 is downstream of the concentrating unit 5, but the opposite arrangement can also be considered (not shown).

[0126] The fluid circuit is configured to feed the circulating solution to the first concentration unit 5 (here a reverse osmosis membrane).

[0127] The concentration unit 5 is configured to produce a downstream concentrate solution C ( 5 ) and a downstream permeate solution P ( 5 ).

[0128] The acid concentration in C(5) is higher than the acid concentration of the circulating solution in the feed concentration unit 5.

[0129] The fluid circuit is configured for circulating the concentrated solution C (5) to the second concentrating unit 6, here an electrodialyser.

[0130] Concentration unit 6 is configured to produce a downstream concentrate solution C ( 6 ) and a downstream permeate solution P ( 6 ).

[0131] The acid concentration in C(6) is higher than the acid concentration in the concentrated solution C(5) fed to the second concentration unit 6, respectively.

[0132] The fluid circuit is configured for circulating the concentrated solution C(6) flowing from the concentration unit 6 to the electrolyzer 4 for electrochemical oxidation.

[0133] The fluid circuit is also configured for circulating permeate solutions P(5) and P(6) in two separate bypass lines (which may be collected in a single bypass, not shown) and for connecting permeate solutions P(5) and P(6) to the main circulating solution downstream of electrolyzer 4.

[0134] The fluid circuit is configured for circulating the main circulating solution from the electrolyser 4 to the degassing unit 1 for further stripping.

[0135] Example

[0136] Degradation of Total Organic Carbon (TOC) Using Boron-doped Diamond Electrode

[0137] Experiments on the mineralization of organic compounds were performed by analyzing the total parameter TOC (Total Organic Carbon) measured with boron-doped diamond electrodes in a radiochemical laboratory. The experimental setup was handled in a fume hood.

[0138] The circuit consists of the following components, expansion tank -> pump -> electrolyzer -> expansion tank, see Figure 4 .

[0139] The components used in the experiment are:

[0140] Expansion tank (20L)

[0141] Pump (EHEIM; 20L / min)

[0142] Electrolyser (Diaccon; model "Susi" - laboratory scale and – Industrial scale)

[0143] Cold trap for condensate recovery

[0144] ·Container cooling

[0145] Gas port 1 and gas port 2 (Swagelok; 12mm stainless steel valve)

[0146] Mass flow controller (MFC) (Alicat; Vmax = 10NLMP)

[0147] Flow meter (NATEC Sensor; Vmax = 100NLPM)

[0148] Nitrogen (flushing gas for container flushing)

[0149] Sampling valve (liquid sample)

[0150] Hydrogen sensor (gas detector, Testo)

[0151] Power supply unit (TDK-Lambda; model GEN16-150A)

[0152] Catalyst (Infiltec; model: IAC-114 / IAC-124; Pd / Pt mixture 1:1)

[0153] Temperature sensor (PT-100)

[0154] Chemicals, tools and equipment:

[0155] Formic acid (Merck, 98%-100%)

[0156] Commercial detergent powder (ethylenediaminetetraacetic acid (EDTA) as complexing agent)

[0157] Deionized water (conductivity ~1.5μS / cm)

[0158] TOC quicktesters LCK 385 and LCK 386 (Hach-Lange)

[0159] UV-visible spectrometer (Hach-Lange; DR5000)

[0160] pH indicator strips (Merck; pH 0-14)

[0161] Conductivity meter (WTW; LF 315)

[0162] In order to minimize the volume loss due to evaporation, the condensed water return is built into the gas flow and maintained at 10°C by a cooling unit. The circulating solution is regulated with a cooling circuit, which is operated with a cooling unit or with tap water. With external cooling, the circulating solution is maintained at about 30°C during operation. A constant nitrogen flow is continuously introduced into the expansion tank by a mass flow controller, resulting in the gas volume in the expansion tank being continuously conducted through the gas nozzle 1, the catalyst (cat.) and the gas nozzle 2 via a cold trap. The catalytic converter is placed vertically in the exhaust gas flow, and the exhaust gas flow flows in reverse from below the catalytic converter. The temperature in the catalytic converter is measured by a PT-100 element directly on the metal cylinder of the catalytic converter. A flow meter (FM) is installed after the gas nozzle 2 and the outlet volume flow is monitored. Gas samples are collected before and after the catalyst and the generation of hydrogen and oxygen is checked. Liquid samples are collected from the loop at irregular time intervals to determine the pH value, conductivity and TOC. In order to monitor the hydrogen concentration in the fume hood, a hydrogen sensor is set in the middle of the fume hood. When hydrogen is detected, the measuring device provides an optical and acoustic signal. In order to be able to compare the TOC degradation rate of electrolyzers of different sizes, the current density applied in the electrolyzer was kept constant. Two different electrolyzers from Diaccon were used for the TOC degradation tests. The characteristics of the electrolyzers used are listed in Table 1.

[0163] Table 1: Comparison of the electrolysers used

[0164]

[0165]

[0166] A total of 9 TOC degradation experiments on boron-doped diamond electrodes were conducted and are listed in Table 2.

[0167] Table 2: TOC degradation experiment

[0168]

[0169] Five experiments were conducted using the Susi electrolyser, using Four experiments were performed in the electrolyzer. Washing powder and formic acid were used as test substances. The concentration of the test substances was adjusted in such a way that the largest possible starting TOC concentration interval was achieved. The current density set in all experiments was about 0.09 A / cm 2 , which means that the "Susi" electrolyzer is running at 14.5A, The electrolyzer was operated at a current of 135 A. All experiments were mineralized to a TOC concentration of approximately 5 ppm. The TOC degradation rate determined from the starting TOC concentration is shown in Figure 5 shown.

[0170] In the TOC concentration range of 0.01 g TOC / L (10 ppm TOC) to 0.23 g TOC / L (230 ppm TOC), the TOC degradation rate changes linearly from the starting TOC concentration. It can be seen that a higher TOC degradation rate is achieved with a high TOC concentration.

[0171] In order to keep the mineralization time of the organic acids used to separate C-14 from the ion exchanger using boron-doped diamond electrodes as short as possible, a high concentration of organic acids must be set in the electrolyzer. This can be achieved by membrane separation (e.g. reverse osmosis) and / or electrodialysis treatment. Both components must be connected upstream of the electrolyzer (boron-doped diamond electrodes) so that the concentrated organic acids are mineralized and the TOC decomposition rate is significantly increased. Reverse osmosis or electrodialysis treatment can increase the concentration of organic compounds in the concentrate by 10 times. This leads to a significantly accelerated TOC reduction. Different TOC degradation rates with and without previous concentrations were simulated and Figure 6 Here it can be clearly seen that the organic matter in the solution can be degraded faster if the electrolyser is equipped with a concentrating unit (e.g. a reverse osmosis membrane) connected upstream.

Claims

1. A method for the recycling of organic acids in a continuous mineralization aqueous solution for stripping radionuclides from an ion exchanger, It is characterized in that The method comprises the following steps: a) after the stripping step, transferring the aqueous solution comprising the organic acid to an electrolyser comprising a boron-doped diamond electrode; b) contacting the organic acid with a boron-doped diamond electrode in an electrolyzer to electrolytically oxidize the organic acid to mineralize at least a portion of the organic acid in the aqueous solution, and c) degassing CO2 (gas) from the aqueous solution of step (b) of the electrolyser cycle, and d) The resulting solution is transferred to an ion exchanger for further stripping.

2. The recycling method according to claim 1, wherein: The radionuclide is 14 C.

3. The recycling method according to claim 1, further comprising: 14 Extraction and collection of CO2.

4. The recycling method according to claim 1, wherein: The organic acid is a weak organic acid.

5. The recycling method according to claim 4, wherein: The weak organic acid is formic acid.

6. The recycling method according to claim 1, wherein: The organic acid concentration in the electrolyzer is greater than 0.1 ppm.

7. The recycling method of claim 1, further comprising the step of increasing the concentration of the organic acid in the aqueous solution upstream of the electrolyzer.

8. A recycling device for the continuous mineralization of organic acids in aqueous solution for stripping radionuclides retained in an ion exchanger, the device comprising: an ion exchanger comprising an ion exchange resin, An electrolyzer comprising a boron-doped diamond electrode, Degassing unit, and A fluid circuit, wherein the fluid circuit comprises a loop structure for passing the aqueous solution from the outlet of the ion exchanger through the electrolyzer and through the degassing unit, and then continuously circulating from the outlet of the degassing unit to the inlet of the ion exchanger.

9. The device according to claim 8, wherein: The anode current density in the electrolyzer is 0.02 A / cm 2 ~0.2A / cm 2 .

10. The device according to claim 8, wherein: The electrolyzer is configured so that the gap width between the anode and the cathode is 0.5 mm to 10 mm.

11. The device according to claim 8, wherein: The degassing unit is a buffer tank comprising a stripping unit.

12. The apparatus of claim 8, further comprising at least one concentrating unit between an outlet of the ion exchanger and an inlet of the electrolyzer.

13. The device according to claim 12, comprising a reverse osmosis membrane and an electrodialyser arranged in series as a concentration unit, wherein: The electrodialyser is downstream of the reverse osmosis membrane.

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