JP2002059150A - Cleaning agent and stabilizer for selenium-contaminated soil and method of repairing selenium-contaminated soil using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cleaning agent and a stabilizer for selenium- contaiminated soil, which is capable of efficiently cleaning selenium from the selenium-contaminated soil and a method of cleaning and repairing selenium- contaminated soil using the same and capable of remarkably reducing the environment loading to decrease the risk. SOLUTION: The soil cleaning agent comprises a phosphoric acid aqueous solution or a sulfuric acid aqueous solution. The stabilizer for stabilizing selenium in the soil is composed of a lanthanum salt, cerium (III) salt or iron (II). The method of cleaning and repairing the selenium-contaminated soil uses the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the remediation of selenium-contaminated soil that can occur as a result of actions such as improper handling of harmful chemicals, illegal dumping and exploitation of mineral resources. In more detail, by removing selenium from soil contaminated with selenium with high efficiency, the environmental load due to selenium is significantly reduced and the risk is reduced, and the elution of selenium by stabilizing trace amounts of selenium is suppressed. The present invention relates to a selenium-contaminated soil cleaner, a stabilizer, and a method for remediating selenium-contaminated soil using the same, which satisfy soil environmental standards and enable the reuse of soil.

[0002]

2. Description of the Related Art Selenium is a nutritionally essential metal element, and it is required that 0.05 to 0.2 ppm of selenium be present in foods. (Literature: Atsushi Wada, Metals and People, pp 134-144, Asakura Shoten, 1986) However, there is a risk of poisoning if 0.5 ppm or more of selenium is present in foods, and cultivated on soil contaminated with selenium, for example. Selenium is concentrated in agricultural products and the like, and selenium poisoning occurs if the selenium is continuously ingested. Therefore, selenium-containing soil is designated as a target of national soil environmental standards.

[0003] By the way, the report document of the Environment Agency "Summary of the survey results on soil contamination cases and response status in 1998".
(Soil Pesticide Division, Water Quality Conservation Bureau, Environment Agency March 22, 2000
According to Japan), the total number of soil contamination cases reported from 1975 to 1998 is 292 in total, of which 13 cases of selenium-contaminated soil have been reported. For such contaminated soil, measures such as excavation removal, containment, shatterproofing, solidification and insolubilization with cement, and landfill disposal at final disposal sites have been taken.

[0004] However, in these methods, since the entire amount of selenium remains in the soil as it is, the countermeasure effect is not sustained for a long time. It is feared that selenium in the soil elutes due to wind and rain or infiltration of groundwater, causing secondary environmental pollution.

[0005]

SUMMARY OF THE INVENTION The present invention has been made under the above-mentioned technical background, and a first object of the present invention is to provide a selenium-contaminated soil capable of efficiently extracting and removing selenium from selenium-contaminated soil. A second purpose is to provide a soil cleaning agent, which stabilizes selenium in selenium-contaminated soil efficiently.
The third object is to provide a selenium-contaminated soil stabilizing agent that can be insolubilized.The third purpose is to significantly reduce the environmental load caused by selenium by effectively extracting and removing selenium from contaminated soil by washing treatment. A fourth object of the present invention is to provide a method for cleaning selenium-contaminated soil that can be reduced.The fourth object of the present invention is to insolubilize trace selenium remaining even after the cleaning treatment and minimize the elution of the selenium, thereby enabling the reuse of soil. It is an object of the present invention to provide a method for repairing selenium-contaminated soil.

[0006]

Means for Solving the Problems In order to achieve the above object, the present inventor has conducted extensive trial and error studies on the effectiveness of washing treatment of selenium-contaminated soil with a large number of acids. Phosphoric acid alone exhibits specific action and effect, and lanthanum compound, cerium compound and iron compound stabilize (insolubilize) selenium in soil
The present inventors have found that the present invention is extremely effective, and have completed the present invention.

That is, according to the present invention, first, there is provided a selenium-contaminated soil cleaner comprising at least one aqueous solution selected from a sulfuric acid aqueous solution and a phosphoric acid aqueous solution. Second, lanthanum salt solution, cerium (III) salt solution,
And a selenium-contaminated soil stabilizer comprising at least one aqueous solution selected from aqueous solutions of iron and iron (III) salts.
Third, a selenium-contaminated soil stabilizer comprising at least one oxide selected from lanthanum oxide, lanthanum hydroxide, cerium (IV) oxide, and cerium (IV) hydroxide is provided. Fourth, there is provided a selenium-contaminated soil cleaning method characterized by treating selenium-contaminated soil with the selenium-contaminated soil cleaning agent of the first invention to extract and remove selenium. Fifthly, the soil treated by the selenium-contaminated soil cleaning method of the fourth invention is mixed with the stabilizer of the second invention, and further neutralized with alkali to insolubilize selenium in the soil. Selenium-contaminated soil remediation method is provided. Sixth,
A selenium-contaminated soil remediation method characterized by adding and mixing the stabilizer of the third invention to soil treated by the selenium-contaminated soil cleaning method of the fourth invention to insolubilize selenium in the soil.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In order to achieve the above object, the present inventor has proposed a method of using sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, perchloric acid, hydrobromic acid, hydrofluoric acid, acetic acid and hydrogen peroxide. The effectiveness of cleaning treatment of soils contaminated with selenium by many widely industrially used acids, high selenium extraction and removal effect, presence or absence of damage to soil itself after cleaning treatment, durability, ease of post treatment, treatment As a result of intensive studies using the reduction of agent cost as an index, it was found that among these, only sulfuric acid and phosphoric acid are significantly different from other acids, and are extremely effective as a cleaning agent for selenium in contaminated soil. did.

That is, the present inventor has proposed sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, perchloric acid, hydrobromic acid, hydrofluoric acid, acetic acid, hydrogen peroxide, mixed acid (3: 1 hydrochloric acid + nitric acid), mixed acid Selenium-contaminated soil was added to an aqueous solution in which the concentration of (2: 1 nitric acid + perchloric acid) was adjusted stepwise, and a study was conducted on the soil washing effect at 20 ° C. (Example 1; FIG. 1). As a result, as for the effect of extracting and removing selenium by various acids at a concentration of about 10 (w / v)%, sulfuric acid showed a high selenium removal rate of 99% or more and phosphoric acid showed a high selenium removal rate of 87% or more. On the other hand, even aqua regia (hydrochloric acid + nitric acid), which is a very strong acid, was 88% or less. In the case of aqua regia, toxic gases such as chlorine and nitrosyl chloride are generated and are not suitable as soil cleaners. In addition, the selenium removal rates of other acids were all 70% or less, and it was found that the effect was low. From the above, the present inventor has determined that sulfuric acid and phosphoric acid are the most suitable as the selenium-contaminated soil detergent. Therefore, when selenium-contaminated soil is washed with these acids, most selenium can be removed with high extraction efficiency at a low price without damaging the soil, greatly reducing the environmental burden of selenium. Risk can be reduced.

The soil detergent of the present invention is usually used in the form of an aqueous solution. The concentration of sulfuric acid or phosphoric acid in the aqueous solution depends on the concentration of selenium contaminated in the soil to be treated, but it is usually sufficient to set the concentration to 10 (w / v)%. The amount of the soil detergent of the present invention may be determined by appropriately considering the concentration of selenium in the soil and the concentration of the aqueous solution of the detergent to be used. However, it is usually sufficient to set the amount to 10 L per kg of contaminated soil.

FIGS. 2 and 3 show the results of Example 2 described later.
As shown in (1), the removal rate of selenium from contaminated soil by the sulfuric acid or phosphoric acid aqueous solution according to the present invention is very large in the early stage of the reaction, and quickly reaches equilibrium. Therefore, a short washing treatment time is sufficient. In addition, the elution amount of silica and calcium in the soil components by the detergent treatment according to the present invention is several percent or less, which indicates that the soil after the washing treatment is relatively stable.

The elution of aluminum, iron and magnesium is eluted by the acid washing treatment, but the elution of these components is slower than the removal rate of selenium. By doing so, it is possible to keep the elution of soil components low. From these facts, the cleaning treatment of the contaminated soil with the acid according to the present invention with the acid is about 1 hour.
Time is desirable.

The reason why the detergent comprising an aqueous sulfuric acid solution or an aqueous phosphoric acid solution according to the present invention has such an excellent selenium extraction / removal function is not clear at present, but these acids have an ability to adsorb selenium in soil. It is presumed that by acting on the free aluminum oxide or free iron oxide to dissolve them, the adsorbed selenium is also eluted.

In order to treat selenium-contaminated soil using the soil cleaning agent according to the present invention, the selenium-contaminated soil is taken out of the ground by means such as excavation, carried into a predetermined cleaning device, and then washed with the cleaning agent. An appropriate amount of a certain sulfuric acid aqueous solution or phosphoric acid aqueous solution is added, mixed and stirred, selenium is extracted, and then a washing solution containing the extracted selenium may be collected. The collected washing liquid can be recycled until the selenium concentration and the concentration of the dissolved soil component reach predetermined values.

According to the present invention, most of selenium can be removed with high extraction efficiency at low cost without damaging the properties of selenium-contaminated soil by using the above-mentioned soil detergent. In addition, at least one aqueous solution selected from lanthanum salt aqueous solution, cerium (III) salt aqueous solution, and iron (III) salt aqueous solution, or lanthanum oxide, lanthanum hydroxide, cerium (IV) oxide, and By mixing a selenium-contaminated soil stabilizer composed of at least one oxide selected from cerium (IV) hydroxide, the environmental load due to selenium can be further reduced.

That is, the use of such a stabilizer allows the selenium-contaminated soil to be rehabilitated and the land and soil to be reused. .01 mg / L) or less. ”Soil Environmental Standards (Operational Guidelines for Surveys and Countermeasures for Soil and Groundwater Contamination, edited by the Water Quality Bureau, Environment Agency, page 133, Ministry of Finance, March 8, 1999) Printing bureau).

For example, as shown in Table 3 of the results of Example 3 below, when the contaminated soil of Reference Example 1 was washed with an aqueous solution of sulfuric acid or phosphoric acid for 2 hours in accordance with Reference Example 2 described later, the selenium removal rate was as follows. , 99.8% and 87.1% respectively
After the washing treatment, a very small amount of selenium remains in the soil. Then, when a dissolution test was performed on the soil after the washing treatment with these acids, 0.140 and 0.1
32 mg / L selenium elutes.

However, the soil after the acid washing treatment is treated with an aqueous lanthanum salt solution, an aqueous cerium (III) salt solution, or an iron (III) salt solution.
The soil treated with a salt solution and then neutralized with an alkali to insolubilize selenium in the soil or the soil after the acid washing treatment is treated with lanthanum oxide, lanthanum hydroxide, cerium (IV) oxide or cerium hydroxide (IV). ), The soil in which selenium in the soil was insolubilized, even if a subsequent elution test was performed, the amount of selenium eluted as shown in Example 3 described below.
It becomes extremely small and satisfies the environmental standard.

In order to stabilize (insolubilize) selenium in the soil using a lanthanum salt, a cerium (III) salt or an iron (III) salt among the stabilizers according to the present invention, the contaminated soil is treated as described above. After performing the washing treatment, an appropriate amount of the aqueous stabilizer solution is added, mixed and stirred, neutralized with an alkali, left for a while, and then dehydrated of excess water.

The lanthanum salt, cerium (III) salt and iron (III) salt include, for example, chlorides, nitrates and sulfates. These stabilizers are usually used in the form of an aqueous solution.

Lanthanum salt in aqueous solution, cerium (III)
The concentration of the salt or iron (III) salt depends on the residual selenium concentration in the soil to be treated, but it is usually sufficient to set the concentration to 1 equivalent. The amount of these stabilizers to be used may be determined in consideration of the residual selenium concentration in the soil as appropriate, but it is usually sufficient to set the amount to 1 L per 1 kg of the soil.

In order to stabilize selenium in the soil by using lanthanum oxide or cerium (IV) oxide among the stabilizers according to the present invention, the above-mentioned washing treatment was performed on the contaminated soil. Thereafter, an appropriate amount of the stabilizer may be added as a powder or suspended in a small amount of water, mixed and stirred, left for a while, and then dehydrated with excess water. Also, instead of lanthanum oxide or cerium (IV) oxide,
Lanthanum hydroxide or cerium (IV) hydroxide may be used.

The amount of these stabilizers to be used may be determined in consideration of the concentration of residual selenium in the soil and the concentration of the aqueous solution of the stabilizer to be used.
It is sufficient to set the amount to 0 to 20 g.

The method for remediating selenium-contaminated soil according to the present invention is summarized as shown in FIG. That is, the soil contaminated with selenium is excavated and carried into a predetermined cleaning device. Add an appropriate amount of sulfuric acid or phosphoric acid aqueous solution, and stir for about 1 hour. Next, solid-liquid separation is performed by a gravity sedimentation method, centrifugation, or the like, and the washing liquid is collected and recycled. However, if the concentration of selenium or dissolved components in the cleaning solution is higher than a predetermined value,
Combine with the waste liquid from the next step. The washed soil is
In order to remove the remaining acid, it is appropriately washed with water. The washing solution and the used washing solution are treated by an existing water treatment method. Such acid washing removes most of the selenium in the contaminated soil and reduces environmental risks. Since a small amount of selenium remains in the washed soil, selenium is insolubilized by one of the following stabilization methods. First, lanthanum chloride, cerium (III) chloride,
Alternatively, neutralization is performed after adding an aqueous solution of iron (III) chloride. The other is a method of adding and mixing a powder or suspension of lanthanum oxide or cerium (IV) oxide. The soil subjected to the stabilization treatment in this way satisfies the soil environmental standard set by the Environment Agency, and the soil can be reused.

[0025]

The present invention will be described in more detail with reference to the following examples. The preparation of model selenium-contaminated soil is described in Reference Example 1.
And the soil washing experiment was based on Reference Example 2.

Reference Example 1 (Preparation of Model Selenium-Contaminated Soil) The Andosol collected in Tsukuba City, Ibaraki Prefecture was air-dried for one week, and then the soil having a particle size of 2 mm or less was sieved with a standard sieve. 1.5 kg of the solution was placed in a 5 L vinyl chloride container, and 4.0 L of a 25 mM sodium selenite aqueous solution was added thereto. The pH was adjusted to about 4, and the mixture was treated for 3 months while optionally stirring to contaminate. The aqueous phase was separated by filtration, and the obtained filtration residue was washed with water several times and air-dried to prepare model selenium-contaminated soil. The properties and chemical composition of the contaminated soil are as shown in Tables 1 and 2, respectively. The selenium content of the contaminated soil was 47.2 mmol / kg = 3,730 m
g / kg (as Se). The soil is one of the representative soils in Japan and is rich in organic matter and shows weak acidity. Further, since it is rich in free aluminum oxide and free iron oxide, it has a relatively high selenium adsorption capacity.

[0027]

[Table 1]

[0028]

[Table 2]

Reference Example 2 (Soil Washing Experiment) 1 g of the selenium-contaminated soil prepared in Reference Example 1 and 25 mL of a detergent aqueous solution were put into a 35 mL polycarbonate centrifugal sedimentation tube, and stirred with a shaker in a thermostat kept at 20 ° C. The specimen was stiffened in the lateral direction (amplitude: 10 cm) for a predetermined time. Then
20 at 9,000 rpm using a high-speed centrifuge with a refrigerator
After centrifugation for minutes, the separated supernatant was filtered through a membrane filter having a pore size of 0.45 μm. The selenium concentration of the obtained filtrate was measured by hydride generation atomic absorption spectrometry, and the concentrations of aluminum, iron, magnesium, calcium, and silica were measured by ICP emission spectrometry.

Example 1 The purpose of the cleaning treatment of selenium-contaminated soil is to remove as much selenium from the soil as possible to reduce the environmental risk due to selenium. In this example, the cleaning effect on selenium-contaminated soil was compared and studied for many acids widely used industrially. Sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, perchloric acid, hydrobromic acid, hydrofluoric acid, acetic acid, hydrogen peroxide, mixed acid (3: 1 hydrochloric acid + nitric acid), mixed acid ( Reference Example 1 for 2: 1 nitric acid + perchloric acid) aqueous solution
Of selenium-contaminated soil was added, and a washing treatment was performed for 6 hours according to Reference Example 2. The result is shown in FIG. The selenium removal rate on the vertical axis of the graph is a ratio of the amount of selenium eluted in the aqueous phase to the selenium content in the untreated soil. In FIG. 1, the effect of extracting and removing selenium by various acids at an acid concentration of about 10 (w / v)% is shown. Sulfuric acid> phosphoric acid> hydrochloric acid = mixed acid (hydrochloric acid + nitric acid)> nitric acid = mixed acid (nitric acid + peroxide) Chloric acid)> hydrobromic acid = hydrofluoric acid> perchloric acid> hydrogen peroxide> acetic acid. That is, sulfuric acid and phosphoric acid show a high selenium removal rate of 87% or more. Therefore,
Of these acids, sulfuric acid and phosphoric acid are the most suitable detergents for selenium-contaminated soil because of their high selenium extraction / removal effect, low harmfulness and price. Thus, it can be seen that by extracting and removing most of selenium, the environmental load due to selenium can be significantly reduced and the risk can be reduced.

Example 2 9.4 (w / v)% phosphoric acid or 10.7 (w / v)% sulfuric acid aqueous solution 50
To 0 mL, 20 g of the selenium-contaminated soil of Reference Example 1 was added, and the mixture was stirred continuously using a magnetic stirrer. About 10 mL of the suspension sample was taken at predetermined time intervals using a plastic syringe and immediately filtered through a membrane filter having a pore size of 0.45 μm. The selenium concentration of the obtained filtrate was measured by hydride generation atomic absorption spectrometry, and the concentrations of aluminum, iron, magnesium, calcium, and silica were measured by I.
It was measured by CP emission analysis. The obtained results are shown in FIGS. From these results, it can be seen that in any of the acid cleaning treatments, the extraction and removal of selenium almost reached equilibrium 60 minutes after the start of the treatment, and the time required for the cleaning treatment of the contaminated soil was extremely short. In addition, the elution amount of silica and calcium among soil components is several percent or less, which indicates that the washing treatment according to the present invention is relatively stable. On the other hand, the elution rates of aluminum, iron, and magnesium reached 35 to 40%, 30 to 40%, and 25%, respectively, 10 hours after the start of the treatment. However, since the elution of these components is slower than the removal rate of selenium, it is possible to suppress the elution of soil components by ending the washing treatment with an acid in a short time.

Example 3 1 g of the selenium-contaminated soil shown in Reference Example 1 was added to 25 mL of 9.4 (w / v)% phosphoric acid or 10.7 (w / v)% sulfuric acid, and washed for 2 hours according to Example 1. did. A small amount of water was added to the treated soil and washed with water. 1 mL of an aqueous solution of lanthanum chloride, cerium (III) chloride, iron (III) chloride, or calcium chloride having an equivalent concentration of 1 equivalent and 20 mL of water were added to the obtained washed soil, and the mixture was thoroughly stirred. Neutralization was performed using a 0.1 N aqueous sodium hydroxide solution so that the pH value became about 7. After standing overnight, the mixture was centrifuged, and the obtained residue was subjected to a dissolution test. On the other hand, instead of the above-mentioned stabilizer aqueous solution, lanthanum oxide, cerium (IV) oxide,
The suspensions of iron hydroxide and calcium hydroxide were added and mixed well. After standing overnight, the mixture was centrifuged, and the obtained residue was subjected to a dissolution test. Table 3 shows the results. The removal rates of selenium by sulfuric acid and phosphoric acid are 99.8% and 87.1%, respectively, and a very small amount of selenium remains in the soil even after the washing treatment. When an elution test is performed on the soil after the washing treatment with these acids, 0.140 mg / L and 0.132 mg / L of selenium are eluted, respectively. But,
Lanthanum chloride, cerium chloride (II
I) or an aqueous solution of iron (III) chloride was added and mixed, and in order to further hydrolyze the metal salt, the solution was neutralized with an aqueous solution of sodium hydroxide to a pH value of about 7, and left overnight. As a result of performing a dissolution test on soil, the dissolution amount of selenium was 1.3 ppb or less and 9.6 ppp, respectively.
b and 8.9 ppb or less, indicating that the stabilized soil satisfies the soil environmental standard. On the other hand, those treated with calcium chloride, which is widely used as a coagulating sedimentation agent, did not satisfy the soil environmental standards in any case, and did not show any stabilizing effect. Also, from Table 3, it was confirmed that selenium elution can be significantly suppressed by adding and mixing an appropriate amount of lanthanum oxide or cerium (IV) oxide to the soil that has been washed with sulfuric acid or phosphoric acid. However, the addition of iron (III) hydroxide and calcium hydroxide was not effective.

[0033]

[Table 3]

[0034]

Industrial Applicability The selenium-contaminated soil detergent of the present invention has a high selenium extraction / removal effect, does not damage the soil itself even after washing, has excellent durability, is easy to perform post-treatment, and is inexpensive. , And so on. Therefore, when selenium-contaminated soil is washed with these acids, most selenium can be removed with high extraction efficiency at a low price without damaging the soil, greatly reducing the environmental burden of selenium. Risk can be reduced. The selenium-contaminated soil stabilizing agent of the present invention has a function of strongly insolubilizing a trace amount of selenium in soil and suppressing the elution of selenium to an extent that the soil environmental standards are satisfied in an elution test, and It is cheap. Therefore, the method for remediating contaminated soil using the soil cleaning agent and the stabilizer of the present invention can easily extract and remove selenium from selenium-contaminated soil, greatly reducing the environmental load due to selenium, and reduce It stabilizes the remaining trace amount of selenium to surely repair the contaminated soil and makes it possible to reuse the soil.

[Brief description of the drawings]

FIG. 1 Sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, perchloric acid, hydrobromic acid, hydrofluoric acid, acetic acid, hydrogen peroxide, mixed acid (3: 1)
Explanatory drawing of the selenium removal effect from selenium contaminated soil by hydrochloric acid + nitric acid) and mixed acid (2: 1 nitric acid + perchloric acid).

FIG. 2 is a diagram showing a time change of a selenium removal rate and a soil component elution rate in a washing treatment of selenium-contaminated soil with a sulfuric acid aqueous solution.

FIG. 3 is a diagram showing a time change of a selenium removal rate and a soil component elution rate in a washing treatment of selenium-contaminated soil with a phosphoric acid aqueous solution.

FIG. 4 is a schematic explanatory view of a method for repairing selenium-contaminated soil using a soil detergent and a stabilizer according to the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) // C09K 101: 00

Claims (6)

[Claims] 1. A selenium-contaminated soil cleaner comprising at least one aqueous solution selected from an aqueous solution of sulfuric acid and an aqueous solution of phosphoric acid. 2. A selenium-contaminated soil stabilizer comprising at least one aqueous solution selected from an aqueous lanthanum salt solution, an aqueous cerium (III) salt solution, and an aqueous iron (III) salt solution. 3. A selenium-contaminated soil stabilizer comprising at least one oxide selected from lanthanum oxide, lanthanum hydroxide, cerium (IV) oxide, and cerium (IV) hydroxide. 4. A method for cleaning selenium-contaminated soil, comprising treating selenium-contaminated soil with the selenium-contaminated soil cleaning agent according to claim 1 to extract and remove selenium. 5. The method according to claim 4, wherein the soil treated by the method for cleaning selenium-contaminated soil is mixed with the stabilizer according to claim 2, and further neutralized with an alkali to insolubilize selenium in the soil. Selenium-contaminated soil remediation method. 6. The selenium contamination characterized by adding and mixing the stabilizer according to claim 3 to the soil treated by the selenium-contaminated soil cleaning method according to claim 4, and insolubilizing the selenium in the soil. Soil repair method.