JP2000051835A - Method for cleaning soil by using iron powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for leaning soil which removes pollutants effectively from the soil polluted with heavy metals and reduces the pollution load on process drainage. SOLUTION: Soil polluted with heavy metal is added with water, iron powder, and a chemical agent which promotes the movement of the heavy metals, the mixture is agitated, the heavy metals in the soil are adsorbed on the iron powder, the iron powder is separated from the soil by using a wet magnetic separator, so that the pollutants are removed efficiently, and a pollutant load on drainage can bed reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying soil contaminated with heavy metals such as lead, arsenic, cadmium, hexavalent chromium, selenium, mercury, antimony, copper and zinc.

[0002]

2. Description of the Related Art As a general method for obtaining purified soil by removing heavy metals from soil contaminated with heavy metals, there are a method by classification and a method by chemical extraction. According to the former classification method, for example, according to JP-A-6-343948, particulate matter contaminated by at least one of heavy metals, radioactive substances, and organic substances is washed with a contaminant fluidizing solution, and large particles are first mechanically removed. Separately wash. The fine particles together with the contaminants are separated from the medium particles by a fluidizing solution, and the medium particles are further subjected to attrition polishing to remove attached fine particles, and the obtained fine particles are separated from the medium particles. Methods by the latter chemical extraction, such as Herry M. Freeman, Eugene F. Harris
By Hazardous Waste Remediation (1995 Technomic Publ
ishing Company, Inc.) According to P103-112 “Soil washing method”, in Table 10.3, in the soil washing method using classification and chemical extraction, chemicals such as acid, alkali, chelating agent, etc. Are disclosed. On the other hand, the present applicant has filed a patent application for a method using iron powder as a method for detoxifying soil contaminated with organochlorine compounds (Japanese Patent Application No. 9-367177).

[0003]

However, in the method based on classification, the soil is purified by the uneven distribution of the particle size of the soil and the contaminants. Therefore, when the contaminants existing at the same particle size as the purified soil are present, the efficiency of removal is reduced. Is bad. In the method using chemical extraction, the removal rate of contaminants from soil is increased, but the amount of contaminants eluted from purified soil is likely to be increased by the extracted chemical. For this reason, an additional step such as performing an insolubilization treatment after the extraction is required, and most of the contaminants move into the extract, so that the burden of the detoxification treatment of the wastewater increases. Therefore, an object of the present invention is to provide a purification method for removing soil contaminated with heavy metals, effectively removing contaminants from the soil, and reducing the pollution load on process wastewater.

[0004]

Means for Solving the Problems The inventors of the present invention have made intensive studies to achieve the above object, and have found that if the method of the prior application is further improved, heavy metals can be purified. That is, to the contaminated soil containing heavy metals, water and iron powder as a mass transfer medium, an agent that promotes the transfer of heavy metals is added, and the contaminants are carried on the iron powder, that is, physically adsorbed and chemically bonded,
Then, if the contaminants and the iron powder are separated from the soil using a magnetic separator, it has been found that the contaminants can be removed more effectively than the classification method, and that the pollutant load on the wastewater can be reduced. Reached.

That is, the present invention firstly provides a method for purifying soil contaminated with heavy metals, in which an agent for promoting the movement of water, iron powder and heavy metals is added to the soil contaminated with heavy metals, followed by stirring. A first step of adsorbing heavy metals in the soil to the iron powder, and a second step of separating the iron powder carrying the heavy metals in the first step from the purified soil. Secondly, the iron powder is added in an amount of 1 to 10% by weight based on the soil; thirdly, a separation method of separating the heavy metal-supporting iron powder from the purified soil. Fourth, the purification method according to the first or second aspect, wherein the agent is a magnetic separator; Fourth, the agent for promoting the movement of the heavy metal is at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and acetic acid. Or sodium hydroxide, potassium hydroxide, slaked lime, carbonic acid At least one alkali selected from the group consisting of calcium or various chelating agents,
Or a purification method according to the first or second aspect, which is a combination thereof.

[0006]

DETAILED DESCRIPTION OF THE INVENTION The method of the present invention is directed to soil contaminated with heavy metals such as lead, arsenic, cadmium, hexavalent chromium, selenium, mercury, antimony, copper, zinc, and the like. After the contaminated soil is excavated from the original position, the contaminants (concrete, rebar, wood chips, etc.) are removed as a pretreatment, and are preferably sized with a sieve or a crusher. Absorb and bind the contaminants in the soil to the iron powder by the method described above.

First, water is added to soil as a transport medium for pollutants. It is appropriate to select the ratio of soil: added water in the range of 1: 0.5 to 1:20 mainly depending on the particle size of the soil. That is, when the soil particles are gravel larger than several millimeters, it is preferable to use a kneader, a mixer, a blender, or the like as the subsequent stirring and mixing device. Therefore, reduce the amount of water and reduce the ratio of soil: added water to 1:
0.5 to about 1: 2. Also, when the soil particles are sandy smaller than several millimeters or clay smaller than a few millimeters, it is effective to use agitator as a stirring and mixing device. : About 5 to 1:20 is desirable.

When the iron powder is mixed to adsorb and bind the contaminants, the use of an agent for releasing the contaminants from the soil is used together, whereby the contaminant removal rate from the soil can be improved. Drugs that release pollutants from soil include:
Drugs commonly used in chemical extraction are available. That is, soil contamination using hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, etc. as an acid, sodium hydroxide, potassium hydroxide, slaked lime, calcium carbonate, etc. as an alkali, and various chelating agents as a metal extractant. The substance can be released into water as a transfer medium, and can be adsorbed and bound to iron powder.

The iron powder to be mixed is desirably a powdery iron powder containing 90% by weight or more of particles having a particle size of 150 μm or less in order to enhance reactivity and adsorptivity. Reactivity when the particle size is coarse
Adsorption is poor, and extremely fine particles reduce the actual yield in magnetic separation. The ratio of addition to soil is 1-2
0% by weight, preferably between 1 and 10% by weight is suitable. If the amount is smaller than this, the effect of removing contaminants is insufficient, and if it is too large, it is economically disadvantageous. The physical adsorption capacity of iron powder itself is effective for almost all pollutants. When the pollutant is susceptible to the oxidation-reduction state, such as chromium or copper, or when the heavy metal reacts with iron and chemically bonds by ferrite formation, the effect of supporting the iron powder is further enhanced.

As a second step, soil and iron powder are separated as follows. That is, as a method of separating iron powder from soil, it is effective to use a wet magnetic separator. When a drum-type wet magnetic separator is used, a magnetic force of 1200 gauss or more, preferably 1500 gauss or more is necessary for effectively separating iron powder from soil. Although there are expensive models of magnetic separators designed to increase magnetic force using rare earth magnets and electromagnets, it is not always necessary to use such expensive magnetic separators to separate iron powder from soil. Absent. As described above, according to the method of the present invention, heavy metals can be removed from soil contaminated with various heavy metals.

[0011]

EXAMPLE A slurry was prepared by adding 5 L of water to 1 kg of air-dried soil contaminated with lead, arsenic and cadmium. After sulfuric acid was added until pH 2 to release lead, arsenic, and cadmium into water, 50 g of iron powder was added, the mixture was sealed in a polyethylene bottle, and the mixture was stirred with reciprocating shaking (200 reciprocations / minute) for 30 minutes. Thereafter, sodium hydroxide was added to neutralize the solution to pH 8, and 5 L of water was further added to obtain a slurry concentration suitable for magnetic separation.
Wet drum type magnetic separator (Drum 12
Inch, magnetic force 1500 gauss) to separate the soil slurry and iron powder. Soil slurry was prepared using a Buchner funnel
The mixture was separated into soil and drainage by filtration through 5C filter paper. By the above operation, 1 kg of the contaminated soil shown in Table 1 was converted from the purified soil 830 shown in Table 2.
g could be obtained.

[0012]

[Table 1]

[0013]

[Table 2]

The concentration of heavy metals in the waste water is as shown in Table 3.

[Table 3]

As described above, iron powder as a support substance is added to soil contaminated with lead, arsenic, and cadmium to adsorb and bind lead, arsenic, and cadmium, and then iron powder is removed by magnetic separation. Purified soil satisfying the soil environmental standards was obtained with a yield of 83%. In addition, the content of lead and arsenic was lower than the reference value of the environmental standard. The water quality of the wastewater discharged by this method sufficiently satisfied the wastewater standards for lead, arsenic, and cadmium. Tables 2 and 3 show soil environmental standards regulated by the concentration in the test solution (extracted water into pure water adjusted to pH 5.8 to 6.3 with hydrochloric acid) and to public water bodies for harmful substances. The effluent standards are also shown for reference.

[0016]

[Comparative Example 1] 1 kg of contaminated soil same as that used in the example
With air opening 600 μm, 300 μm, 150 μm
Classified in order with a sieve of m. 10 soils finer than 150 μm
It was repulped with L of water and classified using a 38 μm sieve. Three
Slurry finer than 8 μm is treated with a Buchner funnel
o Separated into soil and drainage by filtration through 5C filter paper. Table 4 shows the soil weight and the contents of lead, arsenic, and cadmium for each grain group obtained from this. Table 4 for lead
Since the content increases in the order of C, B, D, E, and A in the particle size categories, recovery of the purified soil as shown in Table 5 is expected. As for arsenic, the particle size classification in Table 4 is A → B → C → D → E
Therefore, the recovery of the purified soil as shown in Table 6 is expected. Furthermore, for cadmium,
Since the content increases in the order of granular soil classification A → E → B → C → D in Table 4, recovery of purified soil as shown in Table 7 is expected.

[0017]

[Table 4]

[0018]

[Table 5]

[0019]

[Table 6]

[0020]

[Table 7]

The purified soil shown in Table 2 obtained in the examples and Table 5
Comparing 6 and 7, when the purified soil was collected with the actual yield of 83% as in the example by classification, the heavy metal content in the purified soil was higher for both lead, arsenic, and cadmium than in the example. . Further, the content of lead 430 was the same as in the example.
When the purified soil is limited to mg / kg, arsenic 38 mg / kg, and cadmium 32 mg / kg, the actual yield of the purified soil is lower than 83% in all cases. Further, the heavy metals in the wastewater generated by the classification operation are as shown in Table 8, and in each case, the pollution load on the wastewater is larger than in the case of the embodiment.

[0022]

[Table 8]

[0023]

[Comparative Example 2] 1 kg of the same air-dried soil as used in the examples
Was added with 5 L of water to form a slurry. Add sulfuric acid until pH 2 to release lead, arsenic, and cadmium into water, then seal in a polyethylene bottle and shake back and forth (20
(0 reciprocations / min) for 30 minutes. The slurry was filtered through a No. 5C filter paper using a Buchner funnel while being acidic to separate it into soil and drainage. 5 L of water was added to the filtered soil to form a slurry, which was neutralized to pH 8 by adding sodium hydroxide, and filtered through a No. 5C filter paper using a Buchner funnel to separate the soil into drainage. The results of analysis of the purified soil obtained from this are shown in Table 9. The heavy metal concentration in the acidic filtrate obtained as waste water and the filtrate neutralized to pH 8 is shown in Table 10.
It was as follows.

[0024]

[Table 9]

[0025]

[Table 10] Compared to the example, the heavy metal load on the wastewater is clearly higher. Further, in the chemical extraction, the elution value of the purified soil was not suppressed, and the superiority of the example was confirmed.

[0026]

As described above, according to the method of the present invention, a heavy metal is adsorbed on iron powder as a supporting substance from soil contaminated with the heavy metal, and this is physically separated and separated by a magnetic separator. Since the contaminated soil can be purified by a simple process, it is extremely effective for soil restoration.

Claims (4)

[Claims] Claims 1. A method for purifying soil contaminated with heavy metals, comprising adding an agent that promotes movement of water, iron powder, and heavy metals to the soil contaminated with heavy metals, stirring the mixture, and removing heavy metals in the soil. A soil purification method, comprising: a first step of supporting iron powder on iron powder; and a second step of separating the iron powder supporting heavy metal in the first step from soil. 2. The iron powder is 1 to 10% by weight based on the soil.
The purification method according to claim 1, which is added. 3. The purification method according to claim 1, wherein the separator that separates the soil from the iron powder carrying the heavy metal is a magnetic separator. 4. The method according to claim 1, wherein the agent for promoting the movement of the heavy metal is hydrochloric acid.
At least one acid selected from the group consisting of sulfuric acid, nitric acid, phosphoric acid, and acetic acid, or at least one alkali selected from the group consisting of sodium hydroxide, potassium hydroxide, slaked lime, calcium carbonate, or various chelating agents, or 3. The purification method according to claim 1, which is a combination thereof.