JPH0684998B2 - Method for treating wastewater containing radioactive organic iodine - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method suitable for treating wastewater containing a radioactive iodine-labeled organic compound discharged from, for example, a medical institution or a biotechnology-related laboratory.

[Conventional technology]

With the development of biotechnology in recent years, the amount of radioisotopes used is increasing year by year.
In the fiscal year, it was eight times that of 1975 (Statistics of the Japan Isotope Association in 1984, Source: Isotope News June 1985 issue). Along with this, wastewater treatment in hospitals and biotechnology-related laboratories that use radioisotopes has become a problem.

In such wastewater, various used radioisotopes are contained, and due to the peculiarity of biotechnology, many of these radioisotopes have a short half-life of several days, but most difficult to drain. Wastewater containing radioactive iodine-labeled organic compounds (organic compounds combined with radioactive iodines I ¹³¹ and I ¹²⁵ ). This is because these radioiodines have relatively long half-lives of 8 days and 60 days, respectively, and the effluent standards established by law are stricter by one digit or more than other radioisotopes.

Until now, wastewater treatment has mainly used the attenuation dilution method (Makino: "Characteristics of hospital wastewater and its treatment technology")
MOL July 1985 issue). This method works for a certain period (50-100
(Sun) After storing wastewater and waiting for radioactivity to decay, dilute it with a large amount of water (about 10 times), discharge it after confirming that it has reached the wastewater standard concentration specified by law.

As an example of another treatment method, an adsorption method using silver-impregnated activated carbon was reported in March _1st, 1985 in the lectures No. G ₁ and G ₂ of the Atomic Energy Society of Japan "Showa 60th Annual Meeting" . This method uses a column packed with activated carbon in the first stage and silver-impregnated activated carbon in the second stage, and removes organic substances (including organic substances labeled with radioactive iodine) in the first stage and inorganic radioactive iodine in the second stage.

[Problems to be solved by the invention]

In the above conventional attenuation dilution method, since waste water is stored for a long time,
Equipment becomes large. Therefore, it is necessary to secure a sufficient site.

In addition, in the adsorption method using silver-impregnated activated carbon, the equipment becomes small, but since the wastewater generally contains a large amount of organic substances such as proteins, the life of the activated carbon tower in the previous stage is short,
Spent activated carbon is generated as secondary waste, and its amount cannot be ignored.

In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a method capable of treating wastewater containing an organic compound combined with radioactive iodine with a small facility and a small amount of secondary waste generation.

[Means for solving problems]

The above-mentioned purpose is to desorb radioactive iodine as an inorganic substance by reduction from an organic compound combined with radioactive iodine in wastewater,
This is achieved by selectively adsorbing and removing only the radioactive iodine that has become this inorganic substance with a hydrophilic adsorbent.

[Action]

Radioiodine is eliminated as an inorganic substance only by reduction from an organic substance combined with radioactive iodine. The desorbed inorganic radioactive iodine is selectively adsorbed and removed by the hydrophilic adsorbent, while the organic substance after desorption of radioactive iodine is not adsorbed by the adsorbent and can be discharged as a non-radioactive substance.

〔Example〕

The present inventors firstly conducted an experiment to determine whether or not organic iodine (organic compound combined with iodine) can be adsorbed by an adsorbent such as an ion exchange resin or a silver impregnated adsorbent capable of adsorbing inorganic iodine. The results are shown in Table 1.

In this experiment, anion-exchange resin (strongly basic) was used as the adsorbent, metallic silver-impregnated alumina (activated alumina was impregnated with silver nitrate, and then hydrogen-reduced), and organic iodine was iodotyrosine (official name: 3-iodo-L-tyrosine), and I ⁻ and I ₃ ⁻ were used as the inorganic iodine. Iodotyrosine used as organic iodine is an iodide of a typical amino acid (tyrosine) that chemically binds to iodine among amino acids that make up proteins and hormones, and is obtained by the following reaction. It is a thing.

As a result of this experiment, as shown in Table 1, both the anion exchange resin and the metal silver-impregnated alumina showed adsorbability for inorganic iodine, but no adsorbability for organic iodine. This is because in iodotyrosine, since iodine is bonded to the benzene ring, the reverse reaction of (1) and (2) is difficult to proceed, and iodotyrosine is not decomposed by simply removing I ⁻ ion. On the other hand, organic iodine that is not bonded to the benzene ring, that is, alkyl iodide such as methyl iodide and ethyl iodide, can be easily removed with an anion exchange resin, metallic silver-impregnated alumina, or the like.

From the above points, with regard to organic iodine such as iodotyrosine, it was concluded that it is necessary to desorb iodine from the organic matter as inorganic iodine, and then to adsorb and remove the desorbed inorganic iodine. .

Therefore, the inventors conducted experiments on two methods, a reduction method and an acid decomposition method, as means for performing this desorption. The reduction method is to decompose iodotyrosine into tyrosine and I ⁻ ion as represented by the following formula.

In addition, the acid decomposition method utilizes the following reactions that proceed at pH 5 or higher (Biochemistry Experiment Course 6 "Tracer Experiment Method (2)" edited by the Biochemical Society of Japan).

As a result of the experiment, the acid decomposition method did not show sufficient decomposability.
This is considered to be due to the following reasons. That is, in the acid decomposition method, I ₂ generated by the formula (4) causes a decomposition reaction as in the formula (5) to generate HIO, and the labeling reaction of the formula (2) proceeds with the HIO.

Next, an experiment of an electrolytic reduction method was performed as a reduction method for iodine desorption. Figure 2 shows the outline of the reaction in electrolytic reduction. It is electrolytically reduced at the cathode 1 of organic iodine (here, iodotyrosine is taken as an example). At this time, the electrolysis of the water of the treatment liquid 4 in the electrolytic bath 3 occurs. Oxygen O ₂ is generated at the anode 2 and hydrogen H ₂ is generated at the cathode due to the electrolysis of water. Electrolysis of water is inevitable for decomposing organic iodine, but it was found by the experiments of the inventors. Also,
It was found that the current consumed for electrolysis of water occupies most of the current applied to both electrodes, and organic iodine is difficult to decompose.

Therefore, the decomposition rate of organic iodine was measured using cathodes of various materials. The results are shown in FIG. Here, considering the case where the treatment target is the wastewater of an actual hospital, etc., iodotyrosine (formally named 3-iodo-L-tyrosine) is adjusted to 0.5 mM in 0.02 M Na ₂ HPO ₄ solution as the treatment liquid. Was dissolved, and electrolytic reduction was carried out for 3 hours under conditions of a current density of 10 mA / cm ² and a distance between electrodes of 5 cm. The concentration of inorganic iodine in the treated solution after electrolysis was measured with an iodine ion electrode after adjusting the chemical form of inorganic iodine to I ⁻ by adding ascorbic acid. FIG. 3 shows the results of electrolytic reduction organized into hydrogen overvoltage. The hydrogen overvoltage represents a deviation from a redox potential calculated thermodynamically, and is generated due to a difference in surface physical property depending on the material of the cathode. The higher this voltage, the less likely hydrogen will be generated. From Fig. 3, substances with a hydrogen overvoltage of 0.5 V or more (in Fig. 3, C, Al, S
It can be seen that (n, Zn) is a suitable cathode material for the decomposition of iodotyrosine. Also, it can be seen that the decomposition rate of iodotyrosine becomes almost 100% when the hydrogen overvoltage is 0.7 V or more.

From the above results, it was clarified that organic iodine can be effectively decomposed into inorganic iodine by electrolytic reduction using a substance having a hydrogen overvoltage of 0.5 V or more as a cathode material.

The reduction method for desorbing iodine as inorganic iodine from organic iodine can also be achieved by adding a reducing agent. In this case, a hydrogen overvoltage having a reducing power of 0.5 V or more may be used. Since the use of such a reducing agent may cause explosive reaction with water at the same time as hydrogen is generated, it is necessary to control the reaction (for example, control the rate of adding the reducing agent) to avoid this. is there. As a usable reducing agent, a reducing agent having a hydrogen overvoltage of 0.5 V or more and a relatively mild reaction with water, for example, chromium chloride (CrCl ₂ ), metallic zinc (Zn), metallic aluminum (Al ) And the like. On the other hand, the electrolytic reduction is easy to control the reaction, and the decomposition product of the reducing agent does not mix into the treatment liquid to deteriorate the water quality as in the case of using the reducing agent, which is more advantageous than the use of the reducing agent. Is.

Next, a means for adsorbing and removing the desorbed inorganic iodine will be described. There are two types of adsorbents: hydrophobic adsorbents typified by activated carbon and porous plastics, and hydrophilic adsorbents such as ion exchange resins and activated alumina. Organic substances are well adsorbed by the hydrophobic adsorbent, but hardly adsorbed by the hydrophilic adsorbent. On the other hand, contrary to the organic substance, the inorganic substance is hardly adsorbed by the hydrophobic adsorbent but is well adsorbed by the hydrophilic adsorbent. In the case of the present invention as well, it was confirmed by experiments by the inventors that this general law holds. In the present invention, by utilizing this, the inorganic iodine desorbed and produced from a large amount of organic substances by the reduction is adsorbed and removed by the hydrophilic adsorbent. As this adsorbent, anion exchange resin,
Metallic silver-impregnated alumina, metallic silver-impregnated silica gel, metallic copper-impregnated alumina and the like can be used.

As described above, the method of the present invention desorbs inorganic iodine from organic iodine by reduction, and adsorbs and removes the desorbed inorganic iodine.

Hereinafter, some specific examples will be described.

Embodiment 1 FIG. 4 shows an embodiment suitable for carrying out the present invention. In FIG. 4, 1 is a cathode, 2 is an anode, 3 is an electrolytic cell, 4 is a processing solution containing organic iodine, 5 is a processing solution tank, 6 is an adsorption tower, 7 is an iodine adsorbent, 8 is an intermediate electrode, 20 , 21 are valves and 22 are pumps. The electrolytic cell 4 is a rectangular parallelepiped with an open upper part, and a cathode 1 made of a substance having a hydrogen overvoltage of 0.5 V or more is fixed to the side surface thereof, and an anode 2 is fixed to the side surface opposite to the cathode. Intermediate electrodes 8 at equal intervals between
Has a fixed structure. Anode 1 by power supply
Then, is applied to the cathode 2. No voltage is applied to the intermediate electrode 8. The adsorption tower 6 is filled with a hydrophilic iodine adsorbent 7 that hardly adsorbs organic substances.

The treatment liquid 4 is put into the electrolysis tank 3 from the treatment liquid tank 5 through the valve 20 and electrolyzed therein for a predetermined time. FIG. 5 shows the relationship between the decomposition rate of organic iodine and the electrolysis time. here,
As the treatment liquid, 0.02M Na ₂ HPO ₄ solution with 0.5 mM of iodotyrosine dissolved was used, and the current density was 10 mA / c.
Electrolytic reduction was performed at m ² and a distance between electrodes of 5 cm. When carbon is used as the cathode 1, the decomposition is completed in 3 hours. After the electrolytic reduction is completed, the valve 21 is opened and the pump 22 is used to send a liquid containing the desorbed inorganic iodine and the organic substance after the iodine desorption to the adsorption tower 6, where iodine that has become an inorganic substance by the electrolytic reduction is removed. The iodine adsorbent 7 is removed. Thus, the liquid containing the organic substances free of iodine is discarded as general wastewater.

According to this embodiment, the amount of secondary waste can be reduced and the equipment can be reduced. This effect will be described with a specific example. Treatment liquid 4 contained 300 mg / l of an organic compound combined with radioactive iodine I-131 (half-life of 8 days), and required radioiodine removal efficiency was 99% or more, and treatment capacity was 1 m ³ / d
Calculate for the case. Approximately 50 with the conventional attenuated dilution method
It requires daily storage and a minimum storage tank of 50 m ³ is required. Moreover, in the conventional activated carbon adsorption method, since the organic matter adsorption capacity of activated carbon is about 10 mg / g, 3 kg activated carbon / day,
The equipment scale will be 1 m ^{3 when} used for 1 year, and the amount of secondary waste generated will be 1 m ³ / year. On the other hand, in this embodiment,
The electrolyzer is 1 (m ³ / day) / 24 for 24 hours operation at 3hr / batch
(Hr) from a × 3 (hr / batch) = 0.125 m ³ / batch, it is sufficient that the volume of 125L. Further, since the adsorption tower only needs to adsorb inorganic iodine, about 1/100 of that of the activated carbon tower is sufficient. Also, the amount of electricity required for electrolysis is as low as 1.6kW. As a result, the apparatus volume of this embodiment is 0.2 m ^3, which is 1/5 to 1/200 of the conventional method, and the amount of secondary waste generated is about 1/100 of that of the conventional activated carbon adsorption method.

Carbon, graphite, aluminum, tin, zinc can be used as the cathode, but aluminum, tin, zinc, etc., which tend to be cations due to metal oxidation at the anode and intermediate electrode (the surface facing the cathode becomes the anode) It cannot be used. As the adsorbent 7, anion exchange resin, metallic silver-impregnated alumina, metallic silver-impregnated silica gel,
Metallic copper-impregnated alumina or the like can be used.

Embodiment 2 FIG. 1 shows another embodiment of the present invention. In FIG.
1 to 7 are the same as in FIG. Reference numeral 9 is a conductive filler made of a substance having a hydrogen overvoltage of 0.5 V or more, 10 is a perforated plate, and 23 is a pump. The electrolytic cell 3 is a rectangular parallelepiped having an open upper portion, the bottom portion of which is partitioned by a perforated plate 10, and a flow path 11 which communicates from below the perforated plate 10 to the upper overflow portion is provided. An anode 2 and a cathode 1 are fixed in an electrolytic cell 3 so as to face each other, and a space between both electrodes is filled with a filler 9. In this embodiment, the filler 9 is conductive such as graphite particles and has an adsorbing property to organic substances.

The treatment liquid 4 is supplied from the treatment liquid tank 5 to the electrolytic bath 3 by the pump 23.
Is continuously supplied to. The organic substance in the treatment liquid 4 is the filler 9
Perforated plate with treatment liquid while repeating adsorption and desorption to and from
An overflow and an outflow from the electrolytic cell 3 through the flow path 11 and the flow path 11 are made so as to pass through the adsorption tower 6.

In the above configuration, the organic substance adsorbed on the filler 9 is
Electrolytic reduction is performed by the potential difference generated on the surface of the filler 9 caused by the difference in electric conductivity between the filler 9 and the treatment liquid 4. This is shown in FIG. In FIG. 6 (a), only the lowermost layer of the filler 9 is shown to simplify the illustration. The flow of ions is as shown. The potential gradient at this time is shown in FIG. 6 (b). The electric potential of the treatment liquid changes linearly with respect to the distance. On the other hand, when the electric conductivity in each particle 9 of the filler is higher than that of the treatment liquid, the electric potential in the particle 9 becomes constant and the particle surface A potential difference occurs in the
As shown by a and c, an anodic reaction (oxidation reaction) and a cathodic reaction (reduction reaction) occur in each filler particle. In this way, organic iodine is electrolytically reduced to produce inorganic iodine. This inorganic iodine is sent to the adsorption tower 6 together with the treatment liquid, and is adsorbed by the iodine adsorbent 9.

According to this embodiment, unlike the first embodiment, continuous processing is possible, and the apparatus is simple. Furthermore, since the apparent concentration of organic substances in the electrolytic cell 3 increases due to the adsorption of organic substances on the filler 9, the device can be downsized.

Embodiment 3 FIG. 7 shows still another embodiment of the present invention. 1-23 is the first
It is the same as the figure. The structure of the electrolytic cell 3 is also the same as in FIG. However, in this embodiment, the same filler 9 and iodine adsorbent 7 as in Embodiment 2 are mixed and filled between the cathode 1 and the anode 2. The filler 9 needs to be a substance such as graphite particles that is electrically conductive and has a higher electrical conductivity than the treatment liquid 4. The iodine adsorbent 7 is effectively activated alumina or silica gel impregnated with metallic silver or metallic copper. In the iodine adsorbent 7, it is necessary that the carrier in which metallic copper or metallic silver is attached is an insulator. Because, if the carrier is a conductor and the condition to reduce organic iodine,
This is because an electric potential is generated on the surface as described with reference to FIG. 6, and the attached metallic silver or metallic copper is oxidized by this electric potential and eluted as ions.

According to this embodiment, not only is the adsorption tower 6 unnecessary,
Since the adsorption and removal of I ⁻ proceeds promptly, it is possible to prevent the recombination of iodine and organic matter due to I ₂ → HIO generated by the oxidation at the anode, and it becomes possible to downsize the device. Further, I ₂ is a substance that is easily vaporized and may come out in a gas phase together with oxygen from the anode. However, in the present embodiment, since I ⁻ and I ₂ are adsorbed and removed at the same time as they are generated, I ₂ is vaporized. It can be prevented.

In Examples 2 and 3, it is more effective if the filler 9 is activated carbon. Activated carbon has a large organic matter adsorption capacity, so
Even if the electrolyzer fails, there is no risk of organic matter staying there and being discharged downstream. If the power failure is corrected, the processing can be restarted immediately. In addition, since the organic matter adsorbed on the activated carbon is decomposed when electricity is applied, the adsorption capacity of the activated carbon can be maintained for a long period of time, and not only does the amount of secondary waste generated increase, but also chemical purification of the treatment liquid is possible. Can be done at the same time.

Example 4 The reduction in the present invention can also be carried out by using other means having a reducing power higher than the hydrogen generation potential. For example,
Using a photocatalyst is also one of the effective means. A photocatalyst that can be used within the state of the art is selected from the viewpoint of redox potential. The results are shown in FIG. The vertical axis represents the oxidation-reduction potential (E _v ) based on the hydrogen standard electrode, and the horizontal axis represents the pH of the treatment liquid. As you can see from this figure, SiC, GaP
Photocatalysts such as can be used. If a photocatalyst is used, a new effect such that the reaction can be controlled by the irradiation light amount and the treatment liquid can be purified by the sterilizing action of ultraviolet rays used for irradiation can be expected.

FIG. 9 shows a preferred embodiment when a photocatalyst is used. In the figure, 4 to 7 are the same as those in FIG. 11 is a photocatalyst, 12 is a photocatalyst tank, 13 is a reaction tank, 14 is a stirrer, 15 is a light source, 20, 21 are valves, and 22 is a pump. Reaction tank 13
Is provided with a quartz glass window so that the ultraviolet light from the light source 15 can be transmitted, and the reaction tank 13 is provided with a stirrer 14.

Process liquid 4 from process liquid tank 5 via valve 20 to reaction tank
Put in 13. Then, a photocatalyst 11 such as 10 to 20 μm of SiC or GaP is put into the reaction tank 13 from the photocatalyst tank 12 via the valve 24. Irradiate ultraviolet rays from the light source 15 and stirrer 14
Is rotated and reduction by the photocatalyst 11 is performed. After this,
The stirrer 14 is stopped, waiting for the photocatalyst 11 to settle at the bottom of the reaction tank 13, and the supernatant liquid is sent to the adsorption tower 6 via the valve 21 and the pump 22. Here, the inorganic iodine produced by the reduction is removed by the iodine adsorbent 7. In the case of subsequent treatment, the photocatalyst 11 is recovered by sedimentation, and therefore the photocatalyst 11 is hardly required to be supplied.

As described above, in the present embodiment using the photocatalyst, since the irradiation of ultraviolet rays is performed, it is possible to stop the decomposition of the organic matter in the treatment liquid, and it is also possible to prevent the occurrence of clogging of the adsorption tower in the subsequent stage. However, the photocatalyst is expensive and has an economical problem.

Example 5 An example using a reducing agent will be described below. When a reducing agent is used, the same device as that shown in FIG. 9 can be used, and the reducing agent may be supplied instead of the photocatalyst, and the light source 15 is unnecessary. The reducing agent that can be used is one that has a reduction power of 0.5 V higher than the hydrogen generation potential and a relatively mild reactivity with water. Specifically, chromium chloride (CrCl ₂ ), metallic zinc powder (Z
n), metallic aluminum powder, etc. can be used. When using a reducing agent, it is necessary that the reducing agent before use does not absorb moisture in the atmosphere (because water decomposes the reducing agent).

〔The invention's effect〕

According to the present invention, from wastewater containing a large amount of organic compounds combined with radioactive iodine, for example, medical wastewater, only radioactive iodine can be selectively adsorbed and removed, and the amount of secondary waste generated is small,
In addition, the equipment can be downsized.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the principle of electrolytic reduction, FIG. 3 is a diagram showing hydrogen overvoltage of a cathode material and decomposition rate of organic iodine, and FIG. The figure shows another embodiment of the present invention, FIG. 5 shows the electrolytic reduction time in the embodiment of FIG. 4, and FIGS. 6 (a) and 6 (b) show the embodiment of FIG. FIG. 7 is a diagram showing another embodiment of the present invention, FIG. 8 is a graph showing a photocatalyst usable for reduction in the present invention, and FIG. 9 is a diagram showing an embodiment using a photocatalyst. Is. 1 ... Cathode, 2 ... Anode 3 ... Electrolysis tank, 4 ... Treatment liquid 5 ... Treatment liquid tank, 6 ... Adsorption tower 7 ... Iodine adsorbent, 8 ... Intermediate electrode 9 ... Filler, 11 …… Photocatalyst 13 …… Reaction tank, 14 …… UV light source

Front Page Continuation (72) Inventor Tatsuo Izumida 1168 Moriyama-cho, Hitachi City, Ibaraki Prefecture, Hitachi Energy Research Institute Co., Ltd. (72) Inventor Masao Matsuda 1168 Moriyama-cho, Hitachi City, Ibaraki Energy Research Institute, Hitachi, Ltd. In-house (72) Eiichi Miura 1168 Moriyama-cho, Hitachi, Hitachi, Ibaraki Energy Research Institute, Hitachi, Ltd. (72) Inventor Toru Komori 1168, Moriyama-cho, Hitachi, Ibaraki Energy Research Institute, Hitachi (72) ) Inventor Toshio Sawa 4026 Kujimachi, Hitachi City, Ibaraki Prefecture, Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Yoshio Yamazaki, 4-13-1, Higashimachi, Hitachi City, Ibaraki Hitachi Chemical Co., Ltd., Ibaraki Laboratory (72) ) Inventor Toshinori Ohashi 4-13-1, Higashi-cho, Hitachi-shi, Ibaraki Hitachi Chemical Co., Ltd. Ibaraki Laboratory

Claims (7)

[Claims] 1. A method for treating wastewater containing an organic substance combined with radioactive iodine, wherein radioactive iodine is desorbed as an inorganic substance from the organic substance by reduction, and the desorbed radioactive iodine is adsorbed and removed by a hydrophilic adsorbent. A method for treating wastewater containing radioactive organic iodine, which is characterized by the above. 2. The method for treating radioactive organic iodine-containing wastewater according to claim 1, wherein the reduction is performed by electrolytic reduction using a substance having a hydrogen overvoltage of 0.5 V or more as a cathode. 3. The method for treating radioactive organic iodine-containing wastewater according to claim 1, wherein the reduction is carried out by electrolytic reduction in an electrolytic cell filled with particles of a substance having a hydrogen overvoltage of 0.5 V or more. 4. The method according to claim 1, wherein the reduction is electrolytic reduction, and the electrolytic reduction and the adsorptive removal are performed in an electrolytic cell filled with particles of a substance having a hydrogen overvoltage of 0.5 V or more and a hydrophilic adsorbent. The method for treating wastewater containing radioactive organic iodine according to the item. 5. The method for treating wastewater containing radioactive organic iodine according to claim 1, wherein the reduction is carried out by adding a reducing agent having a reducing power of hydrogen overvoltage of 0.5 V or more. 6. The method for treating wastewater containing radioactive organic iodine according to claim 1, wherein the reduction is carried out in the presence of a photocatalyst. 7. The hydrophilic adsorbent is a metal silver-impregnated alumina,
A method for treating wastewater containing radioactive organic iodine according to any one of claims 1 to 6, which is silica gel impregnated with metal silver or alumina impregnated with metal copper.