JP5956745B2 - Method for removing radioactive cesium from an aqueous solution containing radioactive cesium - Google Patents

Description

The present invention relates to an improvement in a method for adsorbing and removing radioactive cesium from an aqueous solution in which a compound of radioactive cesium is dissolved. In particular, the method of the present invention removes radioactive cesium from the incinerated ash or fly ash generated by incineration of combustible materials to which radioactive cesium has been adhered, which has been proposed by some of the inventors, and then incinerated ash or fly ash It is useful when used in the implementation of a method for safely treating sewage, but is not limited thereto, and can also be used for decontamination of radioactive cesium contaminating soil.

Among the radioactive material released from the Tokyo Electric Daini, (sometimes hereinafter referred to as ^{"137 Cs")} radioactive cesium 137, and it has spread to a wide area, intensively present in a particular location Since so-called hot spots are formed, the countermeasure is an urgent issue, and a decontamination technique for removing ¹³⁷ Cs together with ¹³⁴ Cs safely and efficiently is required.

In order to remove radioactive cesium, it is known to adsorb it to zeolite. For example, one method for treating radioactive cesium and strontium in high-level radioactive waste is to selectively adsorb to synthetic mordenite, zeolite, or a mixture thereof, and form the adsorbent into a sintered solidified body. It has been proposed (Patent Document 1). However, since the adsorption of radioactive cesium by zeolite is not performed at a very high concentration, a large amount of zeolite is required to decontaminate a small amount of radioactive cesium.

As a decontamination technique when the released radioactive cesium adheres to a combustible material such as fallen leaves, some of the inventors left the radioactive cesium in the incineration ash or fly ash generated in the municipal waste incinerator. Focusing on the fact that the incineration ash or fly ash is melted and transferred in the form of concentrated radioactive cesium in the molten fly ash, the radioactive cesium is extracted by washing the molten fly ash with water and radioactive cesium A concentration process is used, in which an adsorbent (or coagulant precipitation agent) capable of adsorbing cesium compounds is introduced into this aqueous solution, and radioactive cesium is adsorbed (or coagulated and precipitated) on the adsorbent. A decontamination method was invented and already proposed (Patent Document 2). Of course, zeolite is also useful as an adsorbent for the cesium compound used in this decontamination method, and is a suitable adsorbent in terms of availability and cost.
JP-A-6-138298 Japanese Patent Application No. 2011-232269

The practical problem of the technology for adsorbing and removing radioactive cesium using zeolite is, as is typically seen in the molten fly ash extract proposed by some of the inventors, in the aqueous solution of cesium compounds. , And other alkali metal compounds such as sodium and potassium (the presence of these alkali metals in the molten fly ash are mostly chlorides) In the treatment used, most of the adsorption sites are occupied by sodium and potassium, and the ratio of the sites used for adsorption of cesium is low, that is, the utilization rate of zeolite is low. In addition to alkali metals, alkaline earth metal compounds, if present, occupy zeolite adsorption sites. Alkaline earth metals have a higher melting point / boiling point than alkali metals, so the amount transferred to the molten fly ash is small, but if present, it is still a hindrance. Under such circumstances, despite the decontamination work for adsorbing a small amount of radioactive cesium, a large amount of secondary waste is generated as a result, and a new problem arises that the treatment is difficult. (In the following description, the terms “sodium”, “potassium”, “cesium”, “alkali metal”, or “alkaline earth metal” are used to generically refer to these compounds or ions.)

When the inventors conducted the above-described invention and tested whether there was a concern that cesium adsorbed on the zeolite would be eluted by the subsequent treatment, it was found that there was no elution. Next, I thought that there might be a difference in the strength of adsorption by zeolite depending on the type of alkali metal, and if so, the size of the atomic weight might affect it. As a result, when an adsorption-elution test was performed on a mixed aqueous solution of sodium, potassium and cesium, it was confirmed that once adsorbed sodium and potassium were eluted, but cesium was not eluted. Based on this result, the inventors clarified that the elution of sodium and potassium means that the adsorption site of the zeolite was vacant, so that cesium should be able to be adsorbed there again. There was an expectation that efficient use of this zeolite would become possible. As a result of experiments, this expectation became a reality.

The purpose of the present invention is to utilize the knowledge of the inventors obtained as described above, and in the decontamination work for removing radioactive cesium, cesium in an aqueous solution is replaced with other alkali metals such as sodium and potassium or calcium. By repeating the operation of removing at least part of the alkali metal or alkaline earth metal other than cesium and adsorbing cesium again to the vacant adsorption site from the zeolite adsorbed together with alkaline earth metal such as magnesium and magnesium , Making it possible to use most of the adsorption sites that zeolite has for adsorption of cesium, thereby making high use of zeolite as an adsorbent and secondary waste generated by decontamination work as much as possible. The purpose is to provide decontamination technology that keeps the amount small.

The method of the present invention that achieves this object comprises the following steps. In the following description, the term “cesium” represents radioactive cesium compounds and ions, and the term “other alkali metals” refers to alkali metal compounds and ions other than cesium, and alkaline earth metal compounds and ions. It means to include ions.
A) An aqueous solution containing other alkali metals together with radioactive cesium is brought into contact with zeolite, and cesium is adsorbed on the zeolite together with other alkali metals.
B) treating the zeolite adsorbed with cesium and other alkali metal with water to elute at least a part of the other alkali metal;
C) contacting an aqueous solution containing cesium and another alkali metal with zeolite from which at least a part of the other alkali metal is eluted, and adsorbing cesium together with the other alkali metal on the zeolite;
D) repeating the above steps (B) and (C) as many times as necessary,
And
E) The zeolite adsorbed with cesium should be cemented or housed in a radiation shielding container to make it ready for final disposal.

Since the water containing the alkali metal produced by the elution treatment in the step (B) does not contain harmful radioactive cesium, it can be discharged as it is or after necessary treatment.

According to the decontamination method of the present invention, when a decontamination technique for adsorbing and removing radioactive cesium from an aqueous solution containing radioactive cesium using zeolite is carried out, Thus, by expelling those alkali metals other than cesium that are consumed by adsorbing alkali metals other than cesium and making them available for zeolite adsorption, the zeolite can be added to more parts of the adsorption site. It is possible to adsorb and use the adsorbent more efficiently.

If adsorption of radioactive cesium by zeolite is performed more efficiently, a small amount of zeolite is required to perform decontamination of the same amount of radioactive cesium. In the implementation of the decontamination technology proposed earlier, the radioactive cesium adhering to the combustible material is concentrated in the molten fly ash of the incinerator and extracted into an aqueous solution to remove it by adsorption. However, it is possible to prevent a large amount of zeolite consumed by the adsorption of other coexisting alkali metals. In this way, the volume of secondary waste generated by decontamination can be smaller, and the volume of secondary waste can be reduced. What should be done with the treatment of a large amount of secondary waste generated as a result of decontamination? This problem is significantly reduced.

The decontamination method of the present invention has no special equipment and chemicals necessary for its implementation, and can be easily carried out by those who have ordinary knowledge and experience. Therefore, in many local governments and clean centers under its control. Can be implemented easily and at low cost.

As an object which can be processed by the decontamination method of the present invention, the water extract of molten fly ash in an incinerator containing the concentrated radioactive cesium in the invention previously proposed by some of the inventors previously mentioned, Although it is typical, not only it but the aqueous solution which contains radioactive cesium with another alkali metal can be processed whatever. Wastewater generated by the cleaning work of buildings and structures contaminated with radioactive cesium, and washing water generated by the work of treating contaminated objects and soil are targeted.

As zeolite for adsorbing cesium, mordenite-type zeolite is suitable. Although the contact for carrying out the adsorption can be carried out to a high degree by putting the zeolite in an aqueous solution containing cesium and other alkali metals and stirring it, an industrially advantageous embodiment is to place the zeolite in a fixed bed. It is to pass water there. Similarly, various modes of elution of other alkali metals are possible. As water for eluting other alkaline earth metals adsorbed from zeolite, it can be said that it is also advantageous to use, for example, water to which a surfactant is added. In the above step (B), “elution with water” is a term that includes washing with an aqueous solution to which such an additive is added.

The temperature at the time of adsorption is preferably high from the viewpoint of adsorption speed, but is preferably low from the viewpoint of adsorption equilibrium. Since it is not preferable to consume energy in this step, it is usually best to carry out at normal temperature. It was experienced that elution of other alkali metals becomes high at pH slightly higher than neutral, but at the same time, cesium is also eluted. To greatly change the pH from the neutral range requires a neutralization treatment of the discharged water generated by the treatment, which is also not a good idea. It is usually recommended to proceed with the operation without adjusting the pH.

The water extract of molten fly ash, which is an important target of the decontamination method of the present invention, may contain harmful heavy metals as well as cesium. Such a water extract is generated when the process of washing molten fly ash with water, which is a previous stage of the steps (A) to (E) of the present invention, is simply performed. If treated as it is, there is a risk that harmful heavy metals will be mixed into the discharged water. As a countermeasure, it is desirable to carry out the following steps prior to the elution of molten fly ash with water.
f) immobilizing heavy metals by applying a liquid chelating agent to the molten fly ash;
Representative liquid chelating agents include dioctylcarbamic acid salts, which are also useful in this case.

In the following examples, the radiation dose was measured using a germanium semiconductor detector (Canberra Model GC2520).

Mordenite at a rate of 10 g / L in an aqueous solution (hereinafter referred to as “radioactive raw water”) containing radioactive cesium 157Bq / L ( ¹³⁴ Cs: 72 Bq / L, ¹³⁷ Cs: 85 Bq / L) together with sodium and potassium A zeolite powder (“Nitto Zeolite” manufactured by Nitto Kako Co., Ltd.) was charged and adsorbed on a shaker. After 24 hours, the amount of radioactive cesium present in the liquid was measured, and the ratio of radioactive cesium adsorbed and removed by the zeolite was calculated. The same amount of distilled water as the above aqueous solution was added to the cesium-adsorbed zeolite, and the mixture was shaken for 24 hours. The zeolite was separated by filtration, and the state of cesium elution was examined for the obtained filtrate, but it was not detected. Next, the same amount of radioactive raw water as above was added to the zeolite that had been subjected to the dissolution test, and a third shaking was performed for 24 hours to cause adsorption again. The properties of the filtrate obtained by filtration and separation are shown in Table 1 together with the removal rate of radioactive cesium.

「△／ｇ−ゼオライト」は、ゼオライト１ｇが吸着除去した放射性セシウムの放射線量である。
「ファクター」は、処理水の放流の可否を決定する数値であって、^１３４Ｃｓ／６０＋^１３７Ｃｓ／９０により算出される。この値が１未満であれば、処理水を放流することができる。 Table 1 Solution after adsorption

“Δ / g-zeolite” is the radiation dose of radioactive cesium adsorbed and removed by 1 g of zeolite.
The “factor” is a numerical value that determines whether or not the treated water can be discharged, and is calculated by ¹³⁴ Cs / 60 + ¹³⁷ Cs / 90. If this value is less than 1, treated water can be discharged.

The alkali metals other than cesium contained in the radioactive raw water were 5.2 mg / L for sodium and 3.1 mg / L for potassium, but the filtrate after the above dissolution test contained almost the same amount of sodium and It was confirmed that it contained potassium and could be separated from radioactive cesium adsorbed on the zeolite substantially completely by elution.

Claims (5)

A decontamination method for removing radioactive cesium from an aqueous solution containing other alkali metals together with radioactive cesium, comprising the following steps:
A) with radioactive cesium, to an aqueous solution containing other alkali metals is brought into contact with the zeolite, radioactive cesium is adsorbed on the zeolite with other alkali metals,
B) treating the zeolite adsorbed with radioactive cesium and other alkali metal with water to elute at least a part of the other alkali metal;
C) To the zeolite from which at least a part of other alkali metal is eluted,
Contacting the aqueous solution used in step (A) above, and adsorbing radioactive cesium in the aqueous solution together with other alkali metals to the zeolite;
D) repeating the above steps (B) and (C) as many times as necessary,
And
E) The zeolite adsorbed with radioactive cesium should be solidified in cement or placed in a radiation shielding container to make it ready for final disposal. An aqueous solution containing other alkali metal together with radioactive cesium is generated in the process of carrying out the decontamination method of (a) to (c) below, which removes radioactive cesium from the combustible material attached with radioactive cesium. The decontamination method of claim 1:
a) Combustible material with radioactive cesium attached is incinerated in an incinerator to produce incinerated ash or fly ash containing radioactive cesium and other alkali metals .
b) Incineration ash or fly ash is melted in an ash melting facility, and radioactive cesium and other alkali metals are concentrated in the molten fly ash, and c) radioactive cesium and other by washing the molten fly ash with water the alkali metal is extracted, to obtain an aqueous solution containing radioactive cesium and other alkali metals.   The decontamination method according to claim 1, wherein the zeolite used as the adsorbent is a mordenite type zeolite. If the molten fly ash contains harmful heavy metals, prior to the step (c) of washing the molten fly ash with water,
f) immobilizing heavy metals by applying a liquid chelating agent to the molten fly ash;
The decontamination method of Claim 2 which added.   The decontamination method of Claim 4 implemented using the salt of dioctyl carbamic acid as a liquid chelating agent.