JP2002219451A - Treatment method of soil contaminated by sexivalent chrome - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method to control sexivalent chromium under the level specified in the environmental quality standards for soil, where the soil is contaminated with sexivalent chromium above the standard level that is eluted from soil even if solidified with cement. SOLUTION: The soil solidified with cement but containing sexivalent chromium above the standard level is exposed to carbon dioxide gas and fired with at least one additive selected among carbides and organic matters, where the excessive sexivalent chromium is eliminated from the soil.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for treating hexavalent chromium-contaminated soil.

[0002]

2. Description of the Related Art Hexavalent chromium contamination at a site of a plating factory has become a social problem and causes a significant reduction in the price of the land.

[0003] It is difficult to suppress the elution of harmful heavy metals below environmental standards only by solidifying such a soil containing hexavalent chromium by adding an ordinary cement hardener. For this reason, hexavalent chromium-containing soil can be purified by removing contaminated soil completely and disposing it at a disposal site,
A method of inserting an electrode into a contaminated soil and flowing a large current to vitrify the soil between the electrodes, a method of mixing a chelate resin with a cement-based solidifying material, and solidifying the contaminated soil using the same are adopted.

However, of these methods, the method of removing contaminated soil is not a preferable method due to a shortage of landfills and the like.

[0005] In addition, the method of passing a large current by inserting an electrode requires a large-scale device, and it is difficult to put it into practical use in terms of cost.

The method using a cement-based solidifying material containing a chelate resin has a problem in stability of the chelate resin, and there is a possibility that the fixed heavy metal may be re-eluted in the long term.

As another method, a method of mixing a reducing agent such as ferrous sulfate has been considered. However, it is difficult to prevent elution of hexavalent chromium for a long period of time, Is toxic.

[0008]

SUMMARY OF THE INVENTION The main object of the present invention is to:
It is an object of the present invention to provide a method capable of suppressing the elution amount of hexavalent chromium from a soil environment standard to a level lower than or equal to the soil environment standard even in a cement-solidified state.

[0009]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, the soil contaminated by hexavalent chromium, even when solidified with cement, According to a method in which soil having a hexavalent chromium elution amount exceeding the environmental standard is subjected to carbonation treatment by contacting with carbon dioxide gas, and then at least one component selected from carbides and organic substances is added and calcined. For example, they have found that it is possible to stably fix the chromium component contained in soil and to suppress the elution amount of hexavalent chromium below the soil environmental standard, and thus completed the present invention.

That is, the present invention provides the following method for treating hexavalent chromium-contaminated soil. 1. After bringing cement-solidified soil having a hexavalent chromium elution amount exceeding the soil environmental standard into contact with carbon dioxide gas, adding at least one component selected from carbides and organic substances and firing the mixture. How to treat chromium contaminated soil. 2. The amount of addition of at least one component selected from carbides and organic substances is 0.1 to 5 parts by weight with respect to 100 parts by weight of cement-solidified soil, and the firing temperature is 150 to 80 parts.
Item 2. The method according to Item 1, wherein the temperature is 0 ° C.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the method of the present invention, a cement-solidified soil having a hexavalent chromium elution amount exceeding a soil environmental standard even when solidified with cement is used as a treatment target.

[0012] The method of adding cement to the soil and solidifying it is not particularly limited.
Add about 50-400kg of cement to ³ ,
What is necessary is just to mix for about 2 to 5 minutes. The cement is not particularly limited, and for example, ordinary Portland cement, blast furnace cement, fly ash cement and the like can be used.

Next, the soil solidified by the cement is brought into contact with carbon dioxide gas to carry out a carbonation treatment.

The method for bringing the cement-solidified soil into contact with carbon dioxide gas is not particularly limited. For example, even if the soil to be treated is exposed to the atmosphere, the soil is very slowly released by the carbon dioxide gas contained in the air. Although carbonation is performed, it is preferable to directly contact the soil to be treated with carbon dioxide gas in order to terminate the treatment within a practical time. For example, the soil to be treated may be placed in an appropriate container, and the container may be filled with carbon dioxide gas. In this case, in order to shorten the time required for the carbonation treatment, the pressure of the carbon dioxide gas may be increased.
In order to finish the treatment in a shorter time, the soil to be treated may be put into a container in which carbon dioxide is brought into a supercritical state.

[0015] The shape of the cement-solidified soil is not particularly limited, but may be used to shorten the time required for carbonation by contact with carbon dioxide gas, or to mix with at least one component selected from carbides and organic matter. In order to facilitate mixing, it is preferable to grind the particles to a size of about 5 cm or less, preferably about 1 cm or less.

[0016] The end time of the carbonation treatment is, for example, 1 hour by adding water to the soil to be treated so as to have a water solids ratio of 10: 1.
Stir for 0 minutes or more and can be determined by the pH of the solution. In this method, when the pH of the solution becomes 8.5 or less, the carbonation treatment is terminated.

Next, at least one component selected from carbides and organic substances is added to the carbonized soil, mixed, and then fired.

The amount of addition of at least one component selected from carbides and organic substances may be about 0.1 to 5 parts by weight based on 100 parts by weight of cement-solidified soil. It is not preferable that the addition amount of these components is too small, because the elution of hexavalent chromium cannot be sufficiently prevented even if the baking treatment is performed. On the other hand, when the addition amount of these components is too large, there is no problem about the elution prevention effect after firing, but if the firing treatment is performed within a time period considered practically preferable, unreacted carbides or organic substances will In order not to leave the remaining unreacted carbides and organic substances, it is necessary to perform calcination for a long time, which is not preferable.

As the carbide, for example, coal, charcoal, activated carbon and the like can be used. As the organic matter, for example,
Waste paper, wood chips, sewage sludge, sludge, and the like can be used. These components can be used alone or in combination of two or more. The shape of these components is not particularly limited, and various shapes such as powder and granules can be used.

The firing temperature is preferably about 150 to 800 ° C. If the firing temperature is too low, the elution of hexavalent chromium cannot be sufficiently prevented even when the firing treatment is performed. On the other hand, if the firing temperature is too high, the required amount of carbides and organic substances increases, which is not efficient because it is not preferable.

The firing atmosphere is not particularly limited, but it is preferable that the supply of oxygen is as small as possible so that a reducing atmosphere is formed by the added carbide or organic substance.

The firing time is usually from 10 to 100.
Minutes.

According to the above method, the amount of hexavalent chromium eluted can be suppressed below the soil environmental standard for soils with hexavalent chromium elution exceeding the soil environmental standard.

The reason why the elution of hexavalent chromium is suppressed by the method of the present invention is as follows. The firing atmosphere is reduced by adding and mixing a carbide or an organic substance, and the water-soluble hexavalent chromium is insoluble. It is presumed to be due to reduction to valent chromium. The effect of carbonation is not always clear, but hexavalent chromium adsorbed on cement hydrate by cement solidification
Although it is difficult to reduce to trivalent chromium as it is, by contacting it with carbon dioxide gas, the cement hydrate collapses and hexavalent chromium becomes a free state. It is considered that it is converted to valent chromium and is stably immobilized.

[0025]

According to the present invention, the amount of hexavalent chromium eluted below the soil environmental standard can be suppressed by a simple method for a soil in which the hexavalent chromium elution exceeds the soil environmental standard even in the solidified state of cement. be able to.

[0026]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples.

Examples 1 to 8 and Comparative Examples 1 to 6 Soil contaminated with hexavalent chromium (Kanto loam, hexavalent chromium elution amount 3.6 ppm, wet density 1.25 g / cm ³ , water content 114.3%) against) 1m ^3, cement 200k
g was added and mixed for 3 minutes using a 5-liter capacity Hobart mixer (manufactured by Aiko). After leaving this soil in a closed container for 3 months, the following treatment was performed.

First, this soil is crushed and sieved to a diameter of 5 mm or less, then put into a pressure chamber, carbon dioxide gas is introduced from a carbon dioxide gas cylinder, and a pressure of 0.2 MPa is applied.
The carbonation treatment was performed for an hour.

Next, a mixture of soil subjected to carbonation and activated carbon was placed in a crucible having a diameter of 15 cm and a height of 12 cm, and the mixture was covered with a porcelain plate and fired.

Table 1 shows the amount (parts by weight) of activated carbon, the calcination temperature and the calcination time for 100 parts by weight of the soil to be treated.
Shown in

The amount of hexavalent chromium eluted from the soil subjected to the above-mentioned treatment was measured in accordance with Test No. 46 of the Notification of the Environment Agency.
The analysis was performed according to JIS K0102-65.2.4. Performed according to the method described in The results are shown in Table 1 below. The criterion was evaluated as ○ when the amount of hexavalent chromium eluted was 0.05 ppm or less, which is the soil environmental standard, and × when the amount exceeded this value.

[0032]

[Table 1]

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2E191 BA02 BB01 BC01 BC05 BD11 4D004 AA16 AA41 AA50 AB03 CA04 CA15 CA30 CA34 CC11 CC15 DA03 DA06 DA10

Claims (2)

[Claims] Claims 1. A cement-solidified soil having an elution amount of hexavalent chromium exceeding a soil environmental standard is brought into contact with carbon dioxide gas, and then at least one component selected from carbides and organic substances is added, followed by firing. A method for treating hexavalent chromium-contaminated soil. 2. The amount of at least one component selected from the group consisting of carbides and organic substances is 0.1 to 5 parts by weight per 100 parts by weight of cement-solidified soil, and the firing temperature is 15 parts by weight.
The method according to claim 1, wherein the temperature is 0 to 800C.