JP4540554B2 - Treatment method of fluorine-contaminated soil - Google Patents

Description

  The present invention relates to a method for treating fluorine-contaminated soil that physicochemically purifies soil contaminated with a fluorine compound. The method for treating fluorine-contaminated soil according to the present invention can be suitably used for purification of soil contaminated with hydrofluoric acid or the like, for example.

Fluorine is frequently used, for example, as hydrofluoric acid in a semiconductor factory cleaning process. Such a fluorine compound sometimes leaks into the ground due to damage to the tank or an accident at the time of transfer to the tank, and may cause fluorine contamination of the soil. It has been pointed out that if a large amount of fluorine is taken into the body, it not only shows acute toxicity due to inactivation of proteolytic enzymes and glycolytic enzymes, but also causes disorders such as osteofluorosis and patchy teeth. Yes. For this reason, the fluorine elution concentration in the soil is defined as an environmental standard value of 0.8 mg / L or less, but at present, there is no other way but to rely on soil cleaning with excavation. However, in recent years, fluoride ions are immobilized in the soil as fluoroapatite with low solubility by coexisting calcium ions and phosphate ions together with fluoride ions, which has been applied in the conventional water treatment field as a purification method for fluorine-contaminated soil. A technique for preventing elution has been developed (for example, Patent Document 1 and Patent Document 2).
JP 2002-331272 A JP 2004-305833 A

In the above method, calcium chloride, which is inexpensive and has high solubility, is frequently used as a calcium ion source. As a phosphorus source, it is preferable to use a sodium salt from the viewpoint of solubility. The sodium salt of phosphoric acid includes trisodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate, but trisodium phosphate shows strong alkalinity when dissolved in water, so use with caution. In addition, since the soil is alkaline even after the reaction with calcium chloride and the fluorine of fluoroapatite is substituted with the hydroxyl group and the fluorine is eluted, disodium hydrogen phosphate and sodium dihydrogen phosphate are preferably used. . However, when weakly alkaline substances such as disodium hydrogen phosphate and sodium dihydrogen phosphate are used as the phosphorus source, the reaction formula is as follows, and hydrochloric acid is generated. As a result, the pH of the soil decreases.
10CaCl ₂ + 6Na ₂ HPO ₄ + 2NaF → Ca ₁₀ (PO ₄ ) ₆ F ₂ + 14NaCl + 6HCl
(Uses disodium hydrogen phosphate)
10CaCl ₂ + 6NaH ₂ PO ₄ + 2NaF → Ca ₁₀ (PO ₄ ) ₆ F ₂ + 8NaCl + 12HCl
(Uses sodium dihydrogen phosphate)

  When the pH is lowered and the soil is acidified as described above, undesirable effects such as inhibition of plant growth, re-elution of fluorine from fluoroapatite, and elution of heavy metals in the soil are caused. In addition, when calcium hydroxide is used as the calcium source, the soil pH after treatment should be close to neutral unless it is adjusted very tightly due to phosphate, coexisting substances in the soil, pH buffering capacity of the soil, etc. It becomes very difficult to adjust. The structure in the soil is not uniform, and it is difficult to inject the drug according to the set value. For this reason, in particular, in the in-situ purification method in which a chemical is added and mixed in soil without excavating the soil, it has been difficult to control the pH in the conventional methods.

  Accordingly, an object of the present invention is to provide a method for treating fluorine-contaminated soil, which makes it possible to prevent the elution of fluorine while keeping the pH of the soil at an appropriate value in view of the above situation.

  In order to solve the above-mentioned problems, the present inventor added a calcium compound and a phosphate compound to soil contaminated with fluorine so that phosphate ions and calcium ions exist in a dissociated state in the soil. In particular, it has been reported that it is possible to reduce the concentration of fluorine elution from soil by forming water-insoluble calcium fluoroapatite in water. Highly calcium chloride is preferably used, but together with this, a calcium compound that is hardly soluble in water is added, and the hydrochloric acid produced by the reaction is reacted with this hardly soluble calcium compound to neutralize the pH. It has been found that it is possible to maintain the vicinity, and the present invention has been completed.

That is, the processing method of the fluorine contaminated soil according to the present invention, a calcium compound and a phosphate compound, a processing method of the fluorine contaminated soil was added to soil containing fluorine to prevent the elution of fluoride ions, calcium After mixing and adding calcium chloride and calcium carbonate as the compound , the method comprises adding disodium hydrogen phosphate or sodium dihydrogen phosphate as the phosphoric acid compound .

In addition, calcium chloride and calcium carbonate can be supplied to the soil as a mixed slurry.

  According to the method for treating fluorine-contaminated soil according to the present invention, when adding a calcium compound and a phosphate compound, by adding a combination of a soluble calcium compound and a hardly soluble calcium compound as a calcium compound, By reacting the generated hydrochloric acid with a sparingly soluble calcium compound, the pH can be maintained near neutrality, and the contaminated soil can be appropriately purified while avoiding the problems associated with soil acidification. .

  Such a method according to the present invention can be constructed in-situ by using a heavy machinery for civil engineering, and therefore is superior in cost compared to existing methods.

Hereinafter, the present invention will be described in detail together with preferred embodiments.
The soil to be purified in the present invention is soil contaminated with fluorine compounds, sediment, etc., and these are collectively referred to as fluorine-contaminated soil in the present application. Hereinafter, the purification of soil contaminated with hydrogen fluoride will be described in detail as an example.

In the present invention, for example, after specifying a soil site contaminated with fluorine, the purification target range is determined. Next, calcium chloride as a calcium salt soluble in water and calcium carbonate as a hardly soluble calcium salt are simultaneously added to the soil to be treated. Also, solid, added by the slurry are contemplated, although it is possible to add also to the soil by any method, not I especially accompanied drilling soil, the application of in situ using underground kneader When performing, the method of mixing each calcium salt and inject | pouring as a slurry is employ | adopted suitably.

At this time, as the solubility of the calcium salt using calcium chloride, to use calcium carbonate as a low-solubility calcium salt. The amount of each calcium salt added may be determined depending on the fluorine concentration in the soil. Generally, calcium chloride is 1 to 350 kg per 1 ton of soil (more preferably 5 to 200 kg-Ca salt / t-soil). Calcium carbonate is added in the range of 1 to 200 kg (more desirably 5 to 100 kg-Ca salt / t-soil) per 1 ton of soil. After mixing of the calcium salt, it performs the addition of phosphate, where that use of disodium hydrogen phosphate or sodium dihydrogen phosphate as the phosphate in the vicinity of neutral. Since these phosphates solid, aqueous solution, but the slurry or the like can be supplied to the soil, phosphate relatively high solubility, and it is relatively difficult to slurry, solution It is easy to add as. The amount of phosphate added is determined by the fluorine concentration in the soil in the same manner as the amount of calcium salt added, but is generally added in the range of 0.5 to 100 kg (more desirably 2 to 50 kg) as phosphate per 1 t of soil.

  The mixing method of the above compound is not particularly limited, but when excavated, a mixer having two or more shafts or a hammer crusher type mixer can be preferably used. Further, in the present invention, purification is possible only by mixing the above compounds. Therefore, mixing in situ without drilling is particularly effective in terms of cost and shortening the purification period. A general heavy machine for soil improvement can be used as the mixer.

When the calcium compound and the phosphate compound are added to the soil in this way, it reacts with soluble fluorine ions to form hardly soluble calcium fluoroapatite Ca ₁₀ (PO ₄ ) ₆ F ₂ in the soil. Therefore, the addition amount of the calcium compound is determined by the fluorine content or the elution amount in the soil, and is usually added as calcium ions so as to be between 10 times and 1000 times the fluorine elution molar concentration. The addition amount of the phosphoric acid compound is similarly determined by the fluorine content or the elution amount, and is usually added as phosphate ions so as to be between 5 times and 500 times the fluorine elution molar concentration. Calcium ions and phosphate ions added to the soil react with fluorine ions in the soil to form fluoroapatite, and the excess added forms calcium hydroxyapatite and calcium phosphate, which becomes insoluble. When the molar ratio of phosphoric acid is high, phosphoric acid exists in a free state and easily dissolves in groundwater, causing eutrophication, etc., so add 3 or more calcium to phosphate ion 2 Is particularly desirable. In addition, when the calcium compound or phosphate compound is present in the soil in advance, these addition amounts can be determined by taking these concentrations into consideration.

Examples carried out using the method of the present invention are shown below. Note that this example does not limit the scope of the present invention.
Examples 1 and 2 and Comparative Examples 1 to 3
Fluorine simulated contaminated soil was treated. As simulated contaminated soil, fluorine-contained soil that is prepared by adding NaF to 100 mg / kg as fluorine and mixing it with artificial soil made by mixing commercially available river sand and clay, and storing it for about 1 week after mixing. Used as. For fixation of fluorine, except for Comparative Example 1 (pure water addition), slurry prepared using the following predetermined amounts of CaCl ₂ and CaCO ₃ and 200H of NaH ₂ PO ₄ were added to 200 g of the simulated contaminated soil. And a well-mixed aqueous solution.

The test conditions are as follows.
1: 40 mL of pure water added to 200 g of fluorine-contaminated soil (Comparative Example 1)
2: NaH ₂ PO ₄ solution (2.25 g as NaH ₂ PO ₄ ) was added immediately after adding CaCl ₂ (5.8 g as CaCl ₂ ) in a slurry state to 200 g of fluorine-contaminated soil and allowed to stand overnight (Comparative Example) 2)
3: After mixing CaCO ₃ (1.0 g as CaCO ₃ ) in slurry form with 200 g of fluorine-contaminated soil, add NaH ₂ PO ₄ solution (2.25 g as NaH ₂ PO ₄ ) and let stand overnight ( Comparative Example 3)
4: After mixing (2.25 g as _{NaH 2 PO 4) NaH 2 PO} 4 solution to fluorine contaminated soil 200 g, as CaCl ₂ (5.8 g as CaCl ₂₎ and CaCO ₃ (CaCO ₃ was slurried 1.0g ) Was added and allowed to stand overnight ( Reference Example 1)
5: After adding a slurry of CaCl ₂ (5.8 g as CaCl ₂ ) and CaCO ₃ (1.0 g as CaCO ₃ ) mixed with 200 g of fluorine-contaminated soil, NaH ₂ PO ₄ solution (NaH ₂ PO ₄ 2.25 g) as a mixture and let stand overnight (Example 2)

<Dissolution test>
According to the method shown in Environment Agency Notification No. 46 (Heisei 3), the sample solution was added to pure water at a weight ratio of 10% and shaken for 6 hours to elute. The supernatant was prepared by filtering through a membrane filter having a diameter of 45 μm. The fluorine concentration was measured by a lanthanum-alizarin complexone spectrophotometric method described in JIS K0102 (1998). The test results are shown in Table 1.

As shown in Table 1, in Comparative Example 2, in which soluble calcium compound and phosphoric acid compound were added, Comparative Example 1, Reference Example 1 and Example 2 in which calcium compound and phosphorous compound were not added, fluorine elution It was possible to treat the amount up to 0.8mgF / L, which is the environmental standard value. However, in Comparative Example 2, a decrease in pH was confirmed, and it was assumed that there was a risk of increasing the environmental burden such as a decrease in the pH buffering capacity of the soil and promotion of elution of heavy metals in the soil. Moreover, the amount of phosphorus elution is increasing, which may cause eutrophication in groundwater. On the other hand, in Comparative Example 3 using only the hardly soluble calcium compound, it was confirmed that the elution concentration of fluorine was high, the treatment to the environmental standard value was difficult, and unreacted phosphate remained. It was. Regarding Reference Example 1 and Example 2 to which the soluble calcium compound and the hardly soluble calcium compound were added, it was found that the fluorine elution concentration was also low and the pH was appropriately adjusted around neutrality. In Reference Example 1, the elution concentration of phosphorus was slightly high, and it was confirmed that it was more desirable to add the phosphoric acid compound after mixing the calcium compound in advance as in Example 2.

  Thus, it was possible to suppress the elution of fluorine by appropriately adding and mixing the calcium compound and the phosphate compound to the fluorine-contaminated soil.

Claims (3)

A calcium compound and a phosphate compound, a processing method of the fluorine contaminated soil to prevent the elution of fluoride ions by forming calcium fluoroapatite added to soil containing fluorine, calcium chloride as a calcium compound and a carbonic acid A method for treating fluorine-contaminated soil, characterized by adding disodium hydrogen phosphate or sodium dihydrogen phosphate as a phosphate compound after mixing and adding calcium . The method for treating fluorine-contaminated soil according to claim 1, wherein the calcium compound and the phosphate compound are added in situ to the soil. The method for treating fluorine-contaminated soil according to claim 1 or 2, wherein the calcium chloride and the calcium carbonate are mixed and added in a slurry state with water .