JP2012081380A - Method for treating fluorine-contaminated soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating fluorine-contaminated soil that does not always need to add a calcium phosphate compound for insolubilization of fluorine in the fluorine-contaminated soil, can reduce the addition amount compared to before even when the compound is added, and has the small impact to a soil ecosystem and the enough fluorine insolubilization action to make the elution amount of fluorine at most an environmental standard.SOLUTION: The method insolubilizes the fluorine contained in soil by adding acid materials to the fluorine-contaminated soil to make a pH of the soil be 5.5 or more and 6.5 or less. The acid materials is preferably at least one selected from aluminum sulfate and an aluminum chloride. The fluorine insolubilization action is further improved by adding the calcium phosphate compound more to the soil after the pH of the soil is made to be 5.5 or more and 6.5 or less by the addition of acid materials. The calcium phosphate compound is preferably a hydroxyapatite.

Description

  The present invention relates to a method for treating soil contaminated with fluorine, and more specifically, to a method for insolubilizing fluorine in soil so that the amount of fluorine eluted from the soil (fluorine elution amount) is less than or equal to the environmental standard. .

  Contamination of soil with heavy metals or toxic chemicals is a problem. If the soil contamination is neglected, the contamination gradually spreads using rainwater and groundwater as a medium, and the damage increases. One of the treatment methods for contaminated soil is insolubilization treatment of contaminants in soil, although the method that can be taken varies depending on the type of contamination and the degree of contamination. By the insolubilization treatment, diffusion of the substance using water as a medium is suppressed.

  When the pollutant is fluorine, an environmental standard of “elution amount of 0.8 mg / L or less” is established. In the method for treating soil contaminated with fluorine (fluorine-contaminated soil), it is necessary to satisfy at least this environmental standard in the treated soil.

  Patent Document 1 includes a first step of adding a mineral acid to a fluorine-contaminated soil to make it acidic (pH 2 to 4), and then adding and mixing at least one of an aluminum salt or an iron salt. The process is then insolubilized by adding alkali to adjust to weakly acidic or alkaline range (pH 3-10, preferably neutral pH 7-8) to produce aluminum or iron hydroxide. And a third process is disclosed (claims 3 and 4). In Patent Document 1, when the soil to be treated becomes the above acidic range by adding at least one of aluminum salt or iron salt, or when it is in the above acidic range from the beginning, It is described that the process can be omitted (paragraph 0007, claims 1 and 2).

  Patent Document 2 discloses a treatment method in which calcium hydrogen phosphate dihydrate is mixed with fluorine-contaminated soil.

JP 2002-326081 A JP 2007-216156 A

  The method of Patent Document 1 requires that the pH of the soil be 2 to 4 in the first step in order to make the amount of fluorine elution in the treated soil less than or equal to the environmental standard, which has an impact on the ecosystem in the soil. Very big. In the method of Patent Document 2, the amount of calcium hydrogen phosphate whose surface is activated is increased in order to make the fluorine elution amount in the treated soil below the environmental standard, specifically, 5 to 100 parts by weight of dry soil. It is necessary to add about 20 parts by weight. Phosphorus resources are feared to be exhausted worldwide, and it is known that prices are on the rise. It is desirable that the amount of phosphate compound added to the contaminated soil be as small as possible.

  In the present invention, it is not always necessary to add a calcium phosphate compound for insolubilization of fluorine in fluorine-contaminated soil, and even when a calcium phosphate compound is added, the amount of addition can be reduced as compared with the method of Patent Document 1, and the soil The purpose is to provide a method for treating fluorine-contaminated soil that has a sufficient fluorine insolubilization effect that suppresses the impact on the ecosystem and suppresses the amount of fluorine eluted below the environmental standard.

  The method for treating fluorine-contaminated soil according to the present invention is a method for insolubilizing fluorine contained in the soil by adding an acidic material to the fluorine-contaminated soil and adjusting the pH of the soil to 5.5 or more and 6.5 or less. is there.

  The treatment method of the present invention has a sufficient fluorine insolubilizing action for the fluorine-contaminated soil so that the amount of eluted fluorine is below the environmental standard. In the treatment method of the present invention, it is not always necessary to add a calcium phosphate compound for insolubilization of fluorine in fluorine-contaminated soil, and even when a calcium phosphate compound is added, the amount added is reduced as compared with the conventional method. The treatment method of the present invention can insolubilize fluorine without setting the soil pH to a strong acidic range such as pH 2 to 4, and has an impact (damage) on the ecosystem in the soil as compared with the method of Patent Document 1. small.

It is a flowchart which shows an example of the processing method of this invention. It is a flowchart which shows another example of the processing method of this invention. It is a figure which shows the relationship between the pH of the soil after implementation of a 1st process, and the amount of fluorine elution evaluated in Example 1. FIG. It is a figure which shows the relationship between the pH of the soil after implementation of a 1st process, and the amount of fluorine elution evaluated in Example 1 and 2. FIG. The relationship between the pH of the soil after execution of the first step and the amount of fluorine elution evaluated in Example 2 and the relationship between the pH of the soil after execution of the second step and the amount of fluorine elution evaluated in Example 3 are shown. FIG. The relationship between the amount of hydroxyapatite added and the amount of fluorine eluted from the soil after the second step evaluated in Example 4, and the amount of hydroxyapatite added and the amount of fluorine eluted from the soil after hydroxyapatite evaluated in Comparative Example It is a figure which shows the relationship. The relationship between the amount of hydroxyapatite added and the amount of fluorine elution from the soil when the acid addition elution test was further performed on the soil after the second step, evaluated in Example 4, and after the addition of hydroxyapatite evaluated in the comparative example It is a figure which shows the relationship between the amount of hydroxyapatite addition at the time of performing an acid addition elution test to the soil of no, and the fluorine elution amount of soil. Relationship between the amount of hydroxyapatite added and the amount of fluorine elution from the soil when the alkali addition dissolution test was further performed on the soil after the second step, evaluated in Example 4, and after the addition of hydroxyapatite evaluated in the comparative example It is a figure which shows the relationship between the amount of hydroxyapatite addition at the time of performing an alkali addition elution test to the soil of no, and the fluorine elution amount of soil.

  FIG. 1 shows an example of the processing method of the present invention. In the treatment method shown in FIG. 1, an acidic material is added to fluorine-contaminated soil to adjust the pH of the soil to 5.5 or more and 6.5 or less (first step). When the pH of the fluorine-contaminated soil is adjusted to 5.5 or more and 6.5 or less by the addition of acidic materials, fluoride ions are adsorbed on the clay fraction in the soil and fluorine is insolubilized. The efficiency of this insolubilization is high, and depending on the amount of fluoride ions contained in the soil before treatment, the amount of fluorine elution in the treated soil is below the environmental standard without necessarily adding a calcium phosphate compound. Become. When the pH of the soil after adjusting the pH by addition of an acidic material is less than 5.5 and exceeds 6.5, such efficient adsorption of fluoride ions is not performed, and insolubilization of fluorine becomes insufficient. Typical clay fractions in the soil are layered silicate compounds; oxides or hydroxides of iron, aluminum, manganese, silicon, etc .; allophane; imogolite; humic substances. The particle size of the clay fraction is generally 5 μm or less, preferably 2 μm or less.

  The pH of the soil is expressed by the pH of the suspension obtained by drying the soil and then adding water 10 times the weight of the soil (dry weight) to the soil and shaking for 6 hours. Is done.

  The environmental standards in this specification are based on the designated standards for the amount of soil elution of heavy metals and the like (second-type specified hazardous substances) in the Soil Contamination Countermeasures Law enacted in 2003. With respect to fluorine, the standard is that the amount of elution from the soil is 0.8 mg / L. Fluorine-contaminated soil is soil whose fluorine elution amount exceeds the standard.

  The acidic material is not limited as long as the pH of the fluorine-contaminated soil can be adjusted to 5.5 or more and 6.5 or less. An acidic material is a substance that exhibits acidity, for example, by contact with mineral acids such as hydrochloric acid and sulfuric acid, and water, and more specifically by dissolution in water.

  The acidic material is preferably at least one selected from aluminum sulfate and aluminum chloride. Aluminum sulfate and aluminum chloride are substances that show acidity when dissolved in water, and have a function as a pH adjuster that adjusts the pH of the soil to 5.5 or more and 6.5 or less. In addition to this, both can be expected to function as an insolubilizing agent for insolubilizing fluoride ions. Specifically, in the said pH range, aluminum exists in soil in the state which precipitated as aluminum hydroxide. Aluminum hydroxide has a large number of surface hydroxyl groups, and positively charged surface sites are dominant in the pH range. Fluoride ions are insolubilized by adsorbing fluoride ions to the surface hydroxyl groups. That is, by using at least one selected from aluminum sulfate and aluminum chloride as the acidic material, the fluorine insolubilizing action in the treatment method of the present invention is improved.

  The amount of the acidic material added to the fluorine-contaminated soil should be selected so that the change in the pH of the soil due to the addition of the acidic material stops, that is, the pH after the soil is stabilized is 5.5 or more and 6.5 or less. That's fine.

  In the first step, two or more acidic materials may be added to the fluorine-contaminated soil. In this case, it is preferable that at least one acidic material is at least one selected from aluminum sulfate and aluminum chloride.

  The specific implementation method of a 1st process is not limited. A known method of adding a fluorine insolubilizer to soil can be applied. In order to uniformly add the acidic material into the soil, it is preferable to add a solution-like acidic material, for example, an aqueous solution, to the soil. When the acidic material is insoluble or hardly soluble, the acidic material may be added to the soil as a powder or a slurry. The first step may be performed at a site where fluorine-contaminated soil exists, and the soil may be cured as it is. Curing conditions such as curing time and temperature can be adjusted as appropriate. Alternatively, fluorine-contaminated soil may be collected locally, and after the first step is separately performed on the collected soil, the soil in which the fluorine is insolubilized may be backfilled locally. The treatment method of the present invention can also be applied to a method for containing fluorine-contaminated soil. When the method is used, the first step may be performed as a pretreatment before containment. Curing, backfilling and containment can be carried out by applying known methods.

  FIG. 2 shows another example of the processing method of the present invention. In the treatment method shown in FIG. 2, after the first step described above is performed, a calcium phosphate compound is added to the soil (second step). In this method, for example, the amount of fluorine contained in the fluorine-contaminated soil to be treated is very large, and insolubilization of fluorine is insufficient only by the first step, or the amount of further fluorine elution is higher than the environmental standard. What is necessary is just to implement when reduction is required. In the processing method of the present invention, the second step is a step that is performed as necessary.

  The calcium phosphate compound is added after the first step, that is, after the pH of the soil is adjusted to 5.5 or more and 6.5 or less by adding an acidic material. At this time, it is preferable to maintain the pH of the soil after adding the calcium phosphate compound at 5.5 or more and 6.5 or less.

  When a calcium phosphate compound is added to the soil in the second step, fluorine (fluoride ions) that have not yet been insolubilized in the soil are combined with the compound and insolubilized. That is, by performing the second step, the fluorine insolubilization effect in the treatment method of the present invention is further improved.

  In addition to this, when the second step is performed, the effect of fluorine insolubilization is more reliably maintained even when the pH of the soil is changed later regardless of the acidic side or the alkaline side. The treatment method of the present invention including the second step is particularly effective when the pH of the soil after insolubilization is expected to change, particularly when the pH of the soil is expected to fluctuate to the alkali side. It is. As a case where the pH of the soil is expected to fluctuate to the alkali side, for example, when a concrete structure is present in the soil, the ground strength is reduced by the addition of a solution-like oxidizing material or a solution-like calcium phosphate compound. In order to prevent or to prevent a decrease in ground strength due to backfilling of the treated soil, it is conceivable that an alkaline solidifying agent such as cement is used in combination.

The calcium phosphate compound is not particularly limited. For example, hydroxyapatite (Ca ₅ (PO ₄ ) ₃ OH), or calcium dihydrogen phosphate (Ca (H ₂ PO ₄ ) ₂ ), calcium hydrogen phosphate (CaHPO ₄ ), tricalcium phosphate (Ca ₃ ( Hydroxyapatite is preferred because it is an anhydride or hydrate of PO ₄ ) ₂ ) and has a high ability to bind fluoride ions. Hydroxyapatite is changed to fluoroapatite (Ca ₅ (PO ₄ ) ₃ F) by binding with fluoride ions. This change is particularly likely to proceed in the region where the pH of the soil is 5.5 or higher.

  The amount of calcium phosphate compound added to the soil after the first step is not particularly limited. What is necessary is just to set suitably according to the addition amount of the acidic material in a 1st process, the quantity of the fluorine contained in the fluorine-contaminated soil, and the fluorine elution amount requested | required of the soil after a process. Since the fluorine insolubilizing action in the treatment method of the present invention is strong, the amount added is, for example, less than 5 parts by weight, 3 parts by weight or less, and further 2 parts by weight or less with respect to 100 parts by weight of dry soil.

  The specific implementation method of a 2nd process is not limited. A known method of adding a fluorine insolubilizer to soil can be applied. In order to uniformly add the calcium phosphate compound to the soil, it is preferable to add a solution-like calcium phosphate compound such as an aqueous solution to the soil. If the calcium phosphate compound is insoluble or sparingly soluble, a powder or slurry of the compound may be added to the soil. The first and second steps may be performed at a site where fluorine-contaminated soil exists and the soil may be cured as it is. Curing conditions after the second step, such as curing time and temperature, can be adjusted as appropriate. Alternatively, fluorine-contaminated soil may be collected locally, and after the first and second steps are separately performed on the collected soil, the soil in which fluorine is insolubilized may be backfilled locally. When taking the method of containing fluorine-contaminated soil, you may implement a 1st and 2nd process as pre-processing before containment. Curing, backfilling and containment can be carried out by applying known methods.

  The treatment method of the present invention may include any step other than the first and second steps as long as the effect of the present invention is obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples.

  First, the soil evaluation method is described.

[Soil pH]
After sufficiently drying the soil to be evaluated (for example, spreading the soil in a vat and air-drying for one week), a fraction having a particle size of 2 mm or less is collected by evaluation on a polypropylene sieve (opening of 2 mm) and evaluated. A sample was used. Next, 10 times the weight of water relative to the dry weight of the sample was added to the sample. Next, the mixture of the sample and water was shaken at room temperature for 6 hours using a shaker (manufactured by EYELA Tokyo Rika Kikai Co., Ltd., NTS-1300) to obtain a suspension. The pH of the resulting suspension was measured with a pH meter (HORIBA, F-52), and the value was taken as the pH of the soil.

[Soil fluorine elution amount]
The amount of fluorine elution from the soil was evaluated by the method shown in Environmental Agency Notification No. 46 (August 23, 1991) (normal elution test). The acid addition elution test and the alkali addition elution test on the fluorine elution amount, assuming the pH change of the soil after the fluorine insolubilization treatment, were performed according to the method proposed by the Soil Environment Center. For these evaluations, samples prepared in the same manner as the evaluation samples for measuring the pH of the soil were used.

Example 1
After the fluorine-contaminated soil collected from the site of the establishment using the fluorine compound was air-dried by the above method, a fraction having a particle size of 2 mm or less was collected and used as an evaluation sample. The fluorine elution amount of the sample was 3.64 mg / L, and the pH was 8.63. The sample contained a layered silicate compound, hydroxide, iron oxide, and humic substance as a clay fraction.

  Next, an aqueous hydrochloric acid solution having a concentration of 0.1 M as an acidic material was added to the sample prepared as described above while changing the addition amount, and the whole was shaken and stirred for 10 minutes to carry out the first step. At this time, water was added separately from the hydrochloric acid aqueous solution so that the mixing ratio (weight ratio) of the sample and water was 1:10. The water in the mixing ratio includes water in the aqueous hydrochloric acid solution.

  The following Table 1 and FIG. 3 show the pH (of soil) and fluorine elution amount of the sample after the first step.

  As shown in Table 1 and FIG. 3, when the pH of the soil is changed to the acidic side by addition of an aqueous hydrochloric acid solution which is an acidic material, and the pH is set to 5.5 or more and 6.5 or less, the fluorine elution amount of the soil Greatly reduced to 0.8 mg / L or less, which is the environmental standard. On the other hand, when the pH of the soil was less than 5.5 and more than 6.5, the fluorine elution amount was high and the environmental standard was not satisfied.

(Example 2)
To the sample prepared in Example 1, an aqueous solution of aluminum sulfate (concentration is 8% by weight in terms of Al ₂ O ₃ ) was added as an acidic material while changing the addition amount, and the whole was shaken and stirred for 10 minutes. One step was performed. At this time, water was added separately from the aluminum sulfate aqueous solution so that the mixing ratio (weight ratio) of the sample and water was 1:10. Water in the mixing ratio includes moisture in the aqueous aluminum sulfate solution.

  The following Table 2 and FIG. 4 show the pH (of soil) and the fluorine elution amount of the sample after the first step. In addition, in FIG. 4, the result of Example 1 is shown collectively.

  As shown in Table 2 and FIG. 4, when the pH of the soil is changed to the acidic side by addition of aluminum sulfate, which is an acidic material, and the pH is set to 5.5 or more and 6.5 or less, the fluorine elution amount of the soil Was greatly reduced. The effect of reducing the fluorine elution amount was higher than that in the case of using a hydrochloric acid aqueous solution as an acidic material.

(Example 3)
In the same manner as in Example 2, after adding aluminum sulfate to adjust the pH of the sample to 5.4 to 6.7 (first step), 2.5 g of hydroxyapatite powder as a calcium phosphate compound was added to 100 g of the sample. Then, the second step was carried out by shaking and stirring for 10 minutes so that the whole was uniform.

  Table 3 and FIG. 5 below show the (soil) pH of the sample after the first step, that is, the hydroxyapatite addition, and the fluorine elution amount of the sample (soil) after the second step. In addition, in FIG. 5, the result of Example 2 is shown collectively.

  As shown in Table 3 and FIG. 5, the amount of fluorine elution from the soil became very small by performing the second step.

Example 4 and Comparative Example
As in Example 2, after adding aluminum sulfate to adjust the pH of the sample to 6.1 (first step), hydroxyapatite powder was added as the calcium phosphate compound while changing the addition amount so that the whole became uniform. The second step was carried out with shaking for 10 minutes.

  Table 4 and FIG. 6 below show the amount of hydroxyapatite added, the pH (of soil) and the amount of fluorine eluted of the sample after the second step. Table 4 and FIG. 7 show the (soil) pH and fluorine elution amount of the sample when the acid addition elution test was further performed on the sample after the second step. Table 4 and FIG. 8 show the (soil) pH and fluorine elution amount of the sample when the alkali addition elution test was further performed on the sample after the second step.

  Separately, hydroxyapatite powder as a calcium phosphate compound was added to the sample prepared in Example 1 without changing the amount of addition, and shaken for 10 minutes so that the whole was uniform. Stir.

  Table 5 and FIG. 6 below show the amount of hydroxyapatite added, the pH (of soil) and the amount of fluorine elution of the sample after the addition of hydroxyapatite. Table 5 and FIG. 7 show the (soil) pH and fluorine elution amount of the sample when the acid addition elution test was further performed on the sample to which hydroxyapatite was added. Table 5 and FIG. 8 show the (soil) pH and fluorine elution amount of the sample when the alkali addition elution test was further performed on the sample to which hydroxyapatite was added.

  As shown in Tables 4 and 5 and FIGS. 6 to 8, compared with the comparative example in which only the addition of hydroxyapatite, which is a calcium phosphate compound, was performed without performing the first step, the hydroxyapatite was added after the first step. In Example 4, even when the amount of hydroxyapatite added was small, the fluorine elution amount was very small, and a high fluorine insolubilizing action was obtained. Moreover, in Example 4, compared with the comparative example, the fluorine elution amount was largely suppressed in the acid addition elution test and the alkali addition elution test.

  About Example 4, in the acid addition elution test, even when the amount of hydroxyapatite added was zero, that is, when only the first step was performed, the amount of fluorine elution in the treated soil was very small. On the other hand, in the alkali addition elution test, when the amount of hydroxyapatite added was zero, the amount of fluorine elution in the treated soil increased, but by adding 2.4 g or more of hydroxyapatite to 100 g of dry soil, The amount of fluorine elution in the later soil became very small.

  The treatment method of the present invention realizes efficient fluorine insolubilization treatment for fluorine-contaminated soil. Thereby, the expansion of soil contamination by fluorine can be suppressed, which contributes to environmental protection.

Claims (4)

  A method for treating fluorine-contaminated soil, wherein insoluble fluorine is contained in the soil by adding an acidic material to the fluorine-contaminated soil to adjust the pH of the soil to 5.5 or more and 6.5 or less.   The method for treating fluorine-contaminated soil according to claim 1, wherein the acidic material is at least one selected from aluminum sulfate and aluminum chloride. After the pH of the soil is set to 5.5 or more and 6.5 or less by the addition of the acidic material,
The method for treating fluorine-contaminated soil according to claim 1, wherein a calcium phosphate compound is further added to the soil. The method for treating fluorine-contaminated soil according to claim 3, wherein the calcium phosphate compound is hydroxyapatite.

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