CH715104A2 - Method of removing radioactive iodide from waste water. - Google Patents

Abstract

Radioactive iodide is removed from waste water by being precipitated by an excess of silver and copper ions. The remaining heavy metal ions are removed from the waste water in a second step by precipitation using a halide salt. The treated wastewater contains a reduced radiation level as well as a low heavy metal content and can be discharged to public wastewater.

Description

Description In radiochemistry, radioactive sodium iodide preparations are used to treat thyroid carcinomas. Since the excretions (faeces, urine, sweat) of the patients radiate radioactively over the treatment period, the wastewater of the patients in clinics must be collected separately in decay tanks. The wastewater is stored in these decay tanks until it falls below a threshold value specified by the Federal Office of Public Health (in Switzerland, in other countries) (emission of permitted radiation in MBq per week). Based on the half-life of iodine-131, the radioactive sodium iodide doses administered to the patients, and the permitted delivery limits, the required storage time for the waste water in the decay tank is determined (typically 1-2 months). The size of the decay tanks that a given clinic has available (permanently installed in the space provided) limits the number of patients who can be treated with radioactive iodine.

Thus, in order to be able to treat more patients, a hospital has to carry out great construction work (increase the decay tank volume), or methods have to be found to remove the radioactivity from the decay tanks, so that the water can be drained off more quickly. The method is also suitable for waste water other than clinical waste water.

In order to reduce the radioactivity in the corresponding hospital wastewater, the radioactive iodide-131 must be removed from the water. There are various methods for removing iodide from water, which can be found in a standard chemistry textbook, such as binding iodide using starch and precipitating iodide using silver (formation of insoluble silver iodide). Due to the low iodine concentrations in the wastewater, the starch method has proven to be unusable; the iodide can only be effectively precipitated via the extremely low solubility product of silver iodide, even at very low initial concentrations. The precipitation of iodide by means of silver ions has also already been described for the treatment of radioactive water from nuclear power plants (US Pat. No. 5,352,367). Alternatively, the removal of iodine using copper has been described (Tachikawa et al. Int. J. Appi. Radiation Isotopes, 1975, 26, 758). Copper (l) ions are of particular interest here, since the salt Cui has a similarly low solubility. However, these two methods of precipitation for the removal of radioactive iodide cannot be used for the treatment of hospital wastewater due to the following problem-specific disadvantages (restrictions):

- In addition to iodine, hospital wastewater also contains high chlorine loads (as chloride), whereby the chloride concentration is usually considerably higher than the iodine concentration. If, as in US Pat. No. 5,352,367, a 1x-4x excess of silver or copper is added to iodine, the amount of metal is not sufficient to bind all of the iodide and chloride. Even if significantly more metal is added, a large part of the metal ions is bound by the chloride, so that only a small part remains for the reaction with the iodide. As a result, the precipitation of the iodide is only incomplete.

- An excess of the metal salt must be used, with metal ions remaining in the solution after the precipitation of iodide and chloride. This heavy metal water must not be disposed of as waste water.

- When iodide and chloride are precipitated using silver or copper (l) ions, there is a very fine precipitation in the waste water (precipitated silver iodide and silver chloride particles) that cannot be filtered in a reasonable time (the small particles clog the filter pores).

The object of the present invention is to remove radioactive iodine by precipitation with silver or copper ions from water without silver or copper ions remaining in the water, and is feasible in such a way that the metal chloride / metal iodide particles are simple and are quickly detachable.

The object is achieved in several steps by adding chemicals to the radioactive water in a fixed predetermined order. Each step fulfills a specific part of the task:

1., Add non-radioactive iodide (as iodide-containing salt, e.g. Nal) to the waste water. In order to promote the precipitation of radioactive iodide, non-radioactive iodide is added to the water. Since the precipitation process (in step 2) is given by the solubility product of the metal iodide (for silver iodide K = 8.52 E-17), a residual concentration of iodide always remains in solution after the precipitation of iodide with heavy metal ions. The greater the non-radioactive portion of the original iodide concentration, the greater the portion of the non-radioactive iodide that remains in solution after the precipitation, and the more radioactive iodide is precipitated.

2. Precipitation of iodide and chloride by adding copper (l) ions (in excess). Since copper (l) ions are difficult to dissolve in water without the addition of appropriate complexing agents, it is advantageous to introduce the copper as copper (II) salt and convert it into copper (l) using a suitable reducing agent (e.g. ascorbic acid, reducing sugars). Alternatively, metallic copper shavings can be converted into copper (l) ions using a suitable oxidizing agent (e.g. Cu (II) by disproportionation, air).

3. Precipitation of iodide and chloride by adding silver ions (in excess). Since the initial concentrations of iodide and chloride in the wastewater are not constant and cannot be determined with sufficient accuracy, an excess of silver or copper ions (e.g. as silver nitrate, copper nitrate) must be added. An increase in the metal ion concentration during the precipitation process further promotes a reduction in the iodide concentration remaining in the solution after the precipitation (via solubility product).

4. Precipitation of the excess heavy metal ions by adding another salt. Water-soluble chloride, iodide and bromide salts are suitable for this, since these form poorly water-soluble salts with the silver and copper ions. For this purpose, an excess of the soluble salt must be used again, and it is therefore important to ensure that the salt is not hazardous to health, is inexpensive and is not regulated / limited in the local wastewater regulation. The use of sodium chloride (NaCl) is particularly suitable for this, but KCl, Nal, Kl can also be used. This process increases the salinity of the wastewater, which favors the easier flocculation of the precipitated substances and thus also significantly simplifies the separability of the precipitated particles. It is also possible to use a salt, which can be removed by a further flocculation step. An example of this is iron (III) chloride and subsequent flocculation of the iron ions as hydroxides by increasing the pH. In this variant, the flocculation of the iron hydroxides helps to separate the previously precipitated salts by aggregation with copper iodide, copper chloride, silver iodide and silver chloride particles.

5. Optionally, a polymeric flocculant can be added after the excess heavy metal has been precipitated. Anionic polymers such as e.g. Polyacrylic acids, cellulose derivatives, polysaccharides, polyamines, polyvinyl alcohol, and soaps.

6. The precipitated substances, which largely consist of silver iodide, copper iodide (radioactive and non-radioactive), silver chloride, copper iodide and optionally iron hydroxides, are separated by suitable solid / liquid processes (eg by filtration, sedimentation in a tank and / or via a hydrocyclone ) removed from the waste water. As a result, the radioactive content of the wastewater is significantly reduced and the water contains no further ions or solids that limit the direct release of the water to the public wastewater network. This means that the wastewater can be disposed of more quickly (with a significantly shorter decay time, ideally without further decay in tanks). From an experimental point of view, the use of methods in which the viscosity of the liquid media is low has proven itself. In practice this is especially important during the separation process, preferred viscosity values are <0.01 Pa s. Methods with viscosity values below 0.002 Pa s are particularly preferred.

7. The solid (which is radioactive by containing 1-131) is stored for decay. After the radioactive iodide has completely subsided, the silver is purified from the solid (e.g. by dissolving the Agl and AgCI in acid and chemically reducing the silver with glucose). The silver can now be used again (e.g. as water-soluble nitrate) in step 2.

In a preferred embodiment, the hospital waste water is treated in several steps, and the radioactivity of the waste water is gradually reduced. In the first step, at least 90% of the available halide ions (Cl-, I-) are generated by generating Cu (l) ions immediately before or in the wastewater and with the optional addition of non-radioactive halide ions (depending on the specific composition of the hospital wastewater). , Br-) precipitated in the water. In a further step, silver ions are added to the water, which leads to a majority (> 80%) of the halide ions remaining after the 1st treatment step being precipitated.

In other preferred embodiments, the temperature before or after the addition of the metal salts and / or additional halide ions is reduced or increased by heating or cooling the waste water by at least 10 ° C. Suitable methods for this are well known to the person skilled in the art and include the use of immersion heaters, instantaneous water heaters, heat exchangers or the addition of ice.

Hospital wastewater is stored for weeks today in order to ensure the decay of radioactivity, this naturally requires large tanks with residence times of weeks to months. In a preferred embodiment, the hospital wastewater is therefore treated continuously or semi-continuously. This is characterized by the fact that the residence time of the water in the cleaning process is less than 24 hours and the untreated, radioactive hospital wastewater is stored for less than 3 days. This can drastically reduce the space required for the storage of radioactive waste water for a typical hospital. The metering of the metal ions and / or optional halide ions to the hospital wastewater is preferably carried out via a T-piece, followed (in the direction of flow) by a static mixer, e.g. commercially available from Sulzer-Chemtech, followed by a solid / liquid separation process. Corresponding separation processes are well known to the person skilled in the art and include how they are used in traditional wastewater treatment. Filtration is the preferred method. Optionally, the filter load can be reduced using a hydrocyclone or an upstream sedimentation basin. One-way filtration elements of compact construction are preferably used, e.g. commercially available from PALL or 3M.

In a preferred embodiment, an ion-selective electrode or an overall conductance electrode is used for process monitoring in the process described above. Appropriate electrodes are commercially available from Mettler-Toledo.

Since the water has to be stored in the tanks for less time by the method, larger volumes of waste water can be processed with the tank volume remaining the same, and thus more patients with thyroid carcinomas can be treated with radioactive iodine therapy.

Examples

Example 1: Reference example according to US Pat. No. 5,352,367 with excess AgNO _3. One liter of water with a radiation exposure of 190 kBc / l was removed from a hospital decay basin. 1 g of silver nitrate was added to this solution, after which a milky precipitate could be observed. This precipitation could only be separated with great difficulty (and after a waiting time of more than 1 hour) using a pleated filter. However, the entire precipitate could not be separated off, so the water remained cloudy and had a radiation exposure of over 50 kBc / l. After the filtered water was left standing in a glass overnight, it was further observed that a metallic mirror film formed on the glass wall, indicating that the silver ion concentration in the water was high (and the water cannot be disposed of in the waste water).

Example 2: without addition of Nal, with precipitation of silver ions One liter of water with a radiation exposure of 190 kBc / l was removed from a hospital decay basin. 1 g / l of silver nitrate was first added to this solution, after which a milky precipitate could be observed. After a reaction time of 10 minutes, 400 mg of NaCl were added to the water, after which the precipitate was compacted (precipitation of Ag as AgCl). The precipitate was quickly removed from the water using a pleated filter. The clear, remaining water had a radiation exposure of 6.7 kBc / l. No silver film was observed even after the water was left standing in a glass for a long time.

Example 3: with addition of Nal with precipitation of silver ions One liter of water with a radiation exposure of 190 kBc / l was removed from a hospital decay basin. 0.02 g of sodium iodide was first added to this solution (without any visible effect). Then 1 g / l silver nitrate was added, after which a milky precipitate could be observed (precipitation of iodide and chloride as Agl and AgCl). After a reaction time of 10 minutes, 400 mg of NaCl were added to the water, after which the precipitate was compacted (precipitation of Ag as AgCl). The precipitate was quickly removed from the water using a pleated filter. The clear, remaining water had a radiation load of 3.4 kBc / l and could therefore be disposed of as waste water without further decay. The solid was stored for decay.

Example 4: Precipitation of sodium iodide with copper ions A 100 ml of a solution with 1 mg / ml sodium iodide in drinking water was treated with 1.06 mg / ml copper (II) sulfate. The resulting slightly cloudy solution immediately became milky white by adding ascorbic acid (reduction of Cu (II) to Cu (I)). The white solid (1.22 mg / ml) could be filtered off from the solution and could be identified as copper (l) iodide by X-ray diffraction. The stoichiometry of the reaction Cu ⁺ + I ^- -> Cui can be used to calculate that at least 96% of the originally dissolved iodide was precipitated.

Claims (20)

claims 1. A method for removing radioactive iodine from water, consisting of the following steps a-c: a. Add a soluble silver salt in excess to the expected chloride and iodide loading of the water. b. Add a water-soluble, halide-containing salt in excess to the silver salt used before. c. Separate the precipitated radioactive solid. 2. A method for removing radioactive iodine from water, consisting of the following steps a-d: a. Add a water-soluble salt containing iodine. b. Add a soluble silver salt in excess to the expected chloride and iodide loading of the water. c. Add a water-soluble, halide-containing salt in excess to the silver salt used before. d. Separate the precipitated radioactive solid. 3. A method for removing radioactive iodine from water, consisting of the following steps a-e: a. Add a soluble silver salt in excess to the expected chloride and iodide loading of the water. b. Add a water-soluble, halide-containing salt in excess to the silver salt used before. c. Add a flocculant. d. Adjustment of the pH value. e. Separate the precipitated radioactive solid. 4. A method for removing radioactive iodine from water, consisting of the following steps a-f: a. Add a water-soluble salt containing iodine. b. Add a soluble silver salt in excess to the expected chloride and iodide loading of the water. c. Add a water-soluble, halide-containing salt in excess to the silver salt used before. d. Add a flocculant. e. Adjustment of the pH value. f. Separate the precipitated radioactive solid. 5. The method according to any one of claims 1-4, characterized in that the radioactive solid is thermally and / or chemically worked up after decay, and the silver contained therein is recovered. 6. The method according to any one of the preceding claims, characterized in that the radioactivity of the water after the treatment has less than 10% of the initial activity. 7. The method according to any one of the preceding claims characterized in that the radioactivity of the water after treatment has less than 10 kBc / l. 8. The method according to any one of the preceding claims characterized in that the silver concentration in the water after the treatment is less than 5 mg / l. 9. The method according to any one of claims 1-7 characterized in that the silver concentration in the water after the treatment is less than 50 pg / l. 10. The method according to any one of claims 1-7 characterized in that the silver concentration in the water after the treatment is less than 1 pg / l. 11. A method for removing radioactive iodine from water, consisting of the following steps a-c: a. Add a soluble copper (II) salt in excess to the expected chloride and iodide loading of the water. b. Add a reducing agent to convert the copper (II) ions into copper (I). c. Separate the precipitated radioactive solid. 12. A method for removing radioactive iodine from water, consisting of the following steps a-d: a. Add a soluble copper (II) salt in excess to the expected chloride and iodide loading of the water. b. Add a reducing agent to convert the copper (II) ions into copper (I). c. Add a water-soluble, halide salt in excess to the copper salt used before. d. Separate the precipitated radioactive solid. 13. A method for removing radioactive iodine from water, consisting of the following steps a-g: a. Add a water-soluble halide salt. b. Add a soluble copper (II) salt in excess to the expected chloride and iodide loading of the water. c. Add a reducing agent to convert the copper (II) ions into copper (I). d. Add a water-soluble, halide salt in excess to the copper salt used before. e. Add a flocculant. f. Adjustment of the pH value. G. Separate the precipitated radioactive solid. 14. A method for removing radioactive iodine from water, consisting of the following steps a-h: a. Add a water-soluble halide salt. b. Add a soluble copper (II) salt in excess to the expected chloride and iodide loading of the water. c. Add a reducing agent to convert the copper (II) ions into copper (I). d. Add a water-soluble silver salt. e. Add a water-soluble, halide salt in excess to the copper and silver salts used before. f. Add a flocculant. G. Adjustment of the pH value. H. Separate the precipitated radioactive solid. 15. The method according to any one of claims 11-14, characterized in that it is silver concentration and the copper concentration in the water after the treatment is less than 1 mg / l. 16. The method according to any one of the preceding claims, characterized in that the viscosity when separating the radioactive solid from the remaining liquid is less than 0.01 Pa s. 17. The method according to claim 1-15, characterized in that the viscosity when separating the radioactive solid from the remaining liquid is less than 0.002 Pa s. 18. Use of one of the preceding methods for increasing the capacity of a radionucleotide decay system in a hospital or clinic. 19. Use of one of the preceding methods for increasing the capacity of a radionucleotide decay system in a hospital or clinic, characterized in that the originally radioactive hospital wastewater can be disposed of in the public sewage network within 10 days. 20. Use of one of the methods of claims 1-18 for increasing the capacity of a radionucleotide decay system in a hospital or clinic, characterized in that the originally radioactive hospital waste water can be disposed of in the public waste water network within one day.