JP4838028B2 - Purification method for contaminated soil - Google Patents

Description

  The present invention relates to a method for purifying contaminated soil, and in particular, in contaminated soil containing hexavalent chromium and / or volatile organic compounds, it is possible to efficiently purify contaminants such as hexavalent chromium and / or volatile organic compounds. The present invention relates to a method for purifying contaminated soil.

Conventionally, soils in the surrounding areas such as factories and their former sites contain organic halogen compounds such as heavy metals and trichlorethylene, and soil contamination has become a major social problem.
Such soil contamination makes it difficult to reuse and develop the land, and also contaminates groundwater with heavy metals and organic halogen compounds.
Accordingly, various methods for purifying contaminated soil have been studied.

In the method for treating contaminated soil, a slurry comprising an inorganic reducing agent, an organic thickener (CMC, polyacrylamide, alginic acid, guar gum, MC, HEC) and water is used. A method of injecting and processing is disclosed in, for example, Japanese Patent Laid-Open No. 8-281246.
However, in this publication, guar gum which is an organic thickener is mixed to prevent contact between the inorganic reducing agent and air, but separation of the inorganic reducing agent cannot be suppressed in the slurry. Therefore, it is difficult to obtain a uniform composition.

On the other hand, in order to suppress material separation in the soil injection material slurry of the inorganic reducing agent, which is a metal powder having a large specific gravity, when a large amount of the organic thickener is used alone to ensure slurry viscosity However, due to the rapid thickening action of the organic thickener, the metal powder cannot be mixed uniformly, resulting in uneven slurry.
Furthermore, since guar gum is an organic substance, the amount of organic substances in groundwater increases, and BOD may increase.

Also, without using an organic thickener as described above, for example, a bentonite containing montmorillonite, which is a smectite clay mineral, is used as a thickener, and a slurry composed of a reducing agent and water is used. A method of injecting and treating contaminated soil is actually implemented.
However, when bentonite is used, in order to obtain a slurry viscosity that suppresses separation of heavy metal powder in the slurry in the slurry, the amount of bentonite added is excessively increased, and the initial slurry viscosity is increased. Cannot be mixed uniformly, resulting in an uneven slurry.
Japanese Patent Laid-Open No. 2005-200262 (Patent Document 1) contains bentonite to prevent the separability of iron powder slurry, but it interferes with the strength development when the soil is solidified with cement after soil purification treatment. Therefore, there is a problem that the range of use of the land after purification is narrowed.

Moreover, although the purification method of contaminated soil is disclosed by Unexamined-Japanese-Patent No. 2003-126838 (patent document 2) and Unexamined-Japanese-Patent No. 2004-202349 (patent document 3), the microorganisms in soil are activated. Therefore, it is a method of injecting a nutrient, and in this method, undesired microorganisms may be activated to cause harmful effects.
Furthermore, Japanese Patent Application Laid-Open No. 2005-313123 (Patent Document 4) describes that active molecules generated from hydrogen peroxide solution and iron ions directly decompose pollutants. Hydrogen water may flow into groundwater.

Other methods for treating contaminated soil include excavating contaminated soil and using a paddle (biaxial) mixer, concrete mixer, self-propelled soil treatment machine, etc. on the ground to contaminate soil, reducing agent (metal powder, etc.), sand and gravel. Alternatively, an agitation and mixing method has been proposed in which the oxidant is mixed while forcibly agitating and the mixed soil is returned to the original position (Japanese Patent Laid-Open No. 2003-47978 (Patent Document 5) and the like).
In addition, as an in-situ agitation mixing method, a method is proposed in which metal iron powder is charged into the contaminated soil, and then the contaminated soil and metal iron powder are mixed and agitated using a ground agitation mixer. (Unexamined-Japanese-Patent No. 2003-112158).
Furthermore, a method for purifying contaminated soil by pumping metal powder having a large specific gravity into the contaminated soil using compressed air and mixing and stirring the same has been proposed (Japanese Patent Laid-Open No. 2002-326080 (Patent Document 6), etc.) ).

  However, when mixing processing by adding powder of contaminated soil and reducing agent (metal powder), the surface layer part processing in plant mixing processing or in-situ processing directly reduces the reducing agent (metal powder) to the contaminated soil (ground). While mixing and stirring can be performed, in the mixing treatment of the deep layer part, the step of charging the reducing agent (metal powder) into the contaminated ground first, and the charged reducing agent and the contaminated ground are mixed and stirred. Since the two steps of the process are performed, the construction period and the construction cost become extremely high.

In addition, when reducing agent (metal powder) is pumped into the contaminated ground using compressed air, there is a risk of heat generation due to friction of metal powder or dust explosion due to static electricity, etc., and there is a problem in construction safety. The plant and the construction machine for the construction have a special problem, and the construction cost is expensive.
JP 2002-326080 A Japanese Patent Laid-Open No. 2003-126838 JP 2004-202349 A JP-A-2005-313123 JP 2003-47978 A JP 2002-326080 A

  The object of the present invention is to solve the above-mentioned conventional problems, increase the viscosity to suppress the separation of the metal powder, and allow the metal powder in the slurry to be uniformly mixed with the soil. It is an object of the present invention to provide a method for purifying contaminated soil, which can effectively purify pollutants such as valent chromium and organic halogen compounds.

By using a slurry obtained by combining a specific clay mineral with a metal as a reducing agent, particularly iron powder, the present inventors can contaminate heavy metals such as hexavalent chromium and organic halogen compounds in contaminated soil. The present inventors have found that a substance can be effectively purified and completed the present invention.
That is, the method for purifying contaminated soil according to claim 1 of the present invention is characterized in that iron powder and sepiolite are added to and mixed with contaminated soil containing hexavalent chromium and / or volatile organic compounds.
The method for purifying contaminated soil according to claim 2 is the method for purifying contaminated soil according to claim 1, wherein the step of stirring and kneading water and sepiolite is added, and then iron powder is added to and mixed with the material obtained in the step. Preparing the slurry, injecting the obtained slurry into the contaminated soil containing hexavalent chromium and / or volatile organic compounds, and stirring and mixing the contaminated soil into which the slurry has been injected. It is characterized by

The method for purifying contaminated soil according to claim 3 is the method for purifying contaminated soil according to claim 1 or 2, wherein the iron powder and sepiolite have a mass ratio of 1: 1 to 1: 3. Features.
Furthermore, the method for purifying contaminated soil according to claim 4 is the method for purifying contaminated soil according to claim 2 or 3, wherein the water / sepiolite ratio is 8 to 16 in terms of mass ratio.

  When the contaminated soil purification method of the present invention is injected into the contaminated soil using a specific slurry purification material and mixed with stirring, a metal powder such as iron powder as a reducing agent and a heavy metal such as hexavalent chromium in the soil And pollutants such as organic halogen compounds can be contacted uniformly and sufficiently, so that the pollutants can be efficiently purified.

  Further, by adding a hydraulic substance such as cement to the slurry, in addition to the above effects, the heavy metals contained in the contaminated soil can be further fixed by the generation of a hardly soluble substance, or can be replaced with a hydrated product. By fixing by dissolution or surface adsorption, or by confining by hardening the hardened tissue, it is possible to efficiently solidify the pollutant and to secure the strength of the softened soil.

  In addition, the contaminated soil purification method of the present invention enables efficient purification of contaminants such as hexavalent chromium and organic halogen compounds in the contaminated soil, and enables effective use of the soil containing the contaminant. To do.

The present invention will be described with reference to the following preferred examples, but is not limited thereto.
The method for purifying contaminated soil of the present invention is a method of adding and mixing metal powder, particularly iron powder and sepiolite, to contaminated soil containing hexavalent chromium and / or volatile organic compounds.
Particularly preferably, the method for purifying contaminated soil of the present invention comprises a step of stirring and kneading water and sepiolite, a step of preparing a slurry by adding and mixing iron powder to the material obtained in the above step, and A step of injecting the resulting slurry into contaminated soil containing hexavalent chromium and / or volatile organic compounds, and a step of stirring and mixing the contaminated soil into which the slurry has been injected.
Thus, by injecting and mixing the slurry combining iron powder and sepiolite into the contaminated soil, the contact between the iron powder and air can be prevented, and the iron powder can be prevented from being oxidized by air. It is possible to maintain the purification effect of pollutants in the soil for a long time.

As the metal powder used in the method for purifying contaminated soil of the present invention, any metal powder can be used as long as it is a reducible metal powder. Examples thereof include iron powder and magnesium powder, and particularly, iron is inexpensive. It can be easily obtained on the market and can be effectively used in combination with sepiolite.
The particle size of the metal particles should be as small as possible because the reaction area of the particles can be increased and it is difficult to separate them in the slurry.

Sepiolite group minerals such as sepiolite are used in the method for purifying contaminated soil of the present invention.
Sepiolite is mixed with water and swells when water and iron powder are used in combination.
Sepiolite can also be used effectively in that it does not adversely affect strength development when the ground is improved with a cement-based solidified material.
The mixing ratio is not particularly limited, and may be any ratio that increases the slurry viscosity after the time that the initial slurry viscosity is suppressed and the iron powder can be uniformly mixed has elapsed. It is desirable that the sepiolite ratio is 5 to 20, preferably 8 to 16 in terms of mass ratio, since the above action is particularly excellent.

  The method for preparing the slurry is not particularly limited as long as water, sepiolite, and iron powder can be uniformly mixed, but particularly preferably, water and sepiolite are stirred and kneaded and then obtained in the above step. It is preferable to prepare by adding and mixing iron powder to the kneaded material because iron powder can be uniformly kneaded in a short time.

The blending ratio of sepiolite and iron contained in the slurry is not particularly limited as long as the iron powder can be uniformly dispersed in the slurry. Preferably, the blending ratio is 1: 1 to 1: 3, preferably by mass ratio. The ratio is 1: 1.6 to 1: 2.4. By setting the mixing ratio as described above, in particular, the viscosity of the slurry immediately after kneading can be suppressed, and the added metal powder can be uniformly kneaded, and after kneading. The advantage that the material separation of the metal powder can be suppressed is obtained.
Further, sepiolite covers the periphery of the iron powder, and even if it touches air or the like, it can efficiently reduce contaminants in the soil without oxidizing.

  In addition, the slurry used in the method for purifying contaminated soil of the present invention includes, as necessary, salicylic acid, benzoic acid, sodium benzoate, sorbic acid, sodium sorbate, dehydroacetic acid, sodium dehydroacetate, p-hydroxybenzoate ester, methyl Preservatives such as naphthoquinone, organic nitrogen compounds, halogen-containing nitrogen-sulfur compounds, and nitrogen-containing cyclic compounds can be added to enhance storage stability.

Furthermore, the slurry used in the method of the present invention is mixed with at least one additive selected from the group consisting of cement, blast furnace slag, limestone powder, fly ash, silica powder, calcium carbonate, and gypsum as necessary. be able to.
By mixing these additives, the unit water amount of the slurry can be reduced and bleeding can be suppressed.

In the method for purifying contaminated soil of the present invention, the slurry containing the sepiolite and iron powder is mixed with the contaminated soil to reduce heavy metals contained in the contaminated soil or decompose organic substances such as organic halogen compounds. You can do it.
Furthermore, in addition to sepiolite and iron powder, the slurry containing a solidifying material such as cement is mixed with the contaminated soil, so that in addition to the above action, it is fixed by the generation of a hardly soluble substance, or the hydrated product It is possible to solidify and insolubilize pollutants efficiently and to secure the strength of softened soil by immobilizing by substitution solid solution and surface adsorption by, or by confining by hardening the hardened tissue. it can.

In the present invention, as a method for adding and mixing the slurry into the contaminated soil, the soil is mechanically used by using an in-situ processing method for injecting and dispersing the slurry into the soil at the site, or a civil engineering machine such as a kneader or a blender. An excavation processing method of stirring and mixing can be employed.
By performing the soil treatment as described above, it is possible to efficiently purify contaminants such as hexavalent chromium, organic halogen compounds, and volatile organic compounds contained in the soil.
As the pollutant, not only the hexavalent chromium described above but also various substances that can be oxidized, such as heavy metals and organic chlorine compounds, which are problematic in terms of environment, are symmetrical.

The following examples, comparative examples and test examples of the present invention will be described.
Materials used The following raw materials were used in the following Examples and Comparative Examples.
Product name; E-200 Specific gravity 7.11 Dowa Iron Powder Industry Co., Ltd. thickener;
Guam gum: F-50 (Mitsubishi Corporation)
Bentonite: Super Clay (Toyoshun Yoko Co., Ltd.)
Sepiolite: Miracle P-800V (Omi Sangyo Co., Ltd.)
Cement-based solidifying material; Tough rock type 3 (Sumitomo Osaka Cement Co., Ltd.)
Kneading water; tap water

Examples 1-2 and Comparative Examples 1-4
(slurry)
Using the above-mentioned raw materials, in a blending ratio shown in Table 1, a predetermined amount of thickener and water are sufficiently kneaded using a mixer, then a predetermined amount of iron powder is added and further sufficiently kneaded, and soil A purification material slurry was prepared.
The addition amount of the thickener added in Examples and Comparative Examples was obtained because the amount was not such that separation of materials (bleeding and sedimentation of iron powder) did not occur after 6 hours from the preparation of the slurry. The slurry is a slurry that can uniformly knead metal powder and does not cause separation of the metal powder.

(soil)
Simulated contaminated soil was prepared by adding hexavalent chromium (potassium dichromate) as a heavy metal and TCE (trichloroethylene) as a volatile organic compound (VOC) to two types of real soil shown in Table 2 below.
Chromium (Cr ⁶⁺ ) and VOC for the two types of soils obtained from the Environmental Agency Notification No. 46 test, and for organic carbon (TOC), assuming the elution from the original location, the Environmental Agency Notification The dissolution test was conducted according to the 13th test. The results are shown in Table 2.

(Purification method)
As shown in Table 1 above, the slurry shown in Table 1 is added to 1 dm ³ each of the two types of sandy soil or marine clay in Table 2, and a mixer (ACM-20: manufactured by Aikosha Seisakusho Co., Ltd.) is sufficient. After kneading, it was scraped off and further sufficiently stirred and kneaded to prepare treated soil.

(Test Example 1)
Each treated soil was cured in a constant temperature room at 20 ° C. for 7 days and eluted by the Environmental Agency Notification No. 46 test, and the hexavalent chromium concentration and the VOC concentration were measured.
The results are shown in Table 3.
However, the hexavalent chromium concentration in the eluate was measured according to JIS K 0102 “Industrial Wastewater Test Method”. The evaluation criteria are indicated by ◯ when the environmental standard is 0.05 mg / l or less and x when the reference value is exceeded.

Moreover, VOC measured the trichlorethylene density | concentration by following GC-MS. However, what added 3 g of NaCl immediately before the measurement was used. As the evaluation criteria, the environmental standard 0.03 mg / l or less of trichlorethylene was evaluated as ◯, and the standard value exceeded was evaluated as x.
・ GCMS measurement conditions (headspace method)
Vials: 20 mL, temperature chamber temperature: 80 ° C
Heating time: 20 minutes, loop temperature: 130 ° C, transfer line temperature: 180 ° C
Column: HP-VOC
Injection mode: Pulsed split injection method, split ratio: 7: 1, inlet temperature: 200 ° C
Pulse pressure: 20 psi, carrier gas flow rate: 2.0 mL / min (constant flow mode), carrier gas: He
Oven temperature: 40 ° C (7 minutes) → 7 ° C / minute → 180 ° C (0 minute) → 18 ° C / minute → 250 ° C (0 minute)
Quantitative method: quantified by 4 inspection curve lines of VOC mixed standard solution 0,0.001,0.01,0.1ppm 0.1ppm-5ppm is quantified by 2 inspection curve lines of VOC mixed standard solution 0.1ppm, 5ppm

(Test Example 2)
Each treated soil after curing for 7 days prepared in the same manner as in Test Example 1 was eluted in accordance with Environmental Agency Notification No. 13 and each of the treated soils was prepared using a TOC measuring device (TOC-650: manufactured by Toray Engineering). The TOC concentration of the eluate was measured. The evaluation criteria were evaluated as 100% for the elution amount of each soil itself before treatment, ○ for 100% or less, Δ for 100 to 130%, and × for 130% or more.
The results are shown in Table 3.

(Test Example 3)
200 kg / m ^{3 of} solidified material (Tough Rock 3 type: manufactured by Sumitomo Osaka Cement) was added to each treated soil after 7 days curing prepared in the same manner as in Test Example 1, and a mixer (same as Test Example 1) ) Was used for 1 minute, scraped off, and further kneaded for 1 minute.
Next, using the obtained solidified material-containing treated soil, a specimen having φ = 50 mm × height of 100 mm was prepared, and after curing at 20 ° C., a compressive strength test was performed on a specimen having a material age of 7 according to JIS A 1216. The compressive strength was measured in accordance with “Soil Uniaxial Compression Test Method”. The evaluation criteria, the intensity upon addition of a solidifying material 200kg water 350kg soil 1 m ³ as 100%, ○ more than 100%, the 100% to 75% △, and rated 75% or less ×. However, the compressive strength of sandy soil itself is 1100 kN / m ² , and the compressive strength of marine clay itself is 1600 kN / m ² .
The results are shown in Table 3.

From these results, it was found that the amount of elution of chromium, VOC and TOC was small and the compressive strength on the seventh day was good in the examples.
In Comparative Example 1, the elution amount of VOC is high and the compressive strength is not sufficient.
In Comparative Example 2, the elution amounts of chromium, VOC and TOC are high, and the compression strength on the seventh day is not good.
Further, in Comparative Example 3, the elution amount of VOC is high and the compression strength on the seventh day is not sufficient, and in Comparative Example 4, the elution amount of VOC and TOC is high and the compression strength on the seventh day is not sufficient. Recognize.

The method for purifying contaminated soil according to the present invention can be applied to ground such as factories, factory ruins, and the like, which has conventionally been difficult to effectively utilize due to contaminants, and can effectively use the land.

Claims (4)

  A method for purifying contaminated soil, comprising adding and mixing iron powder and sepiolite to soil containing hexavalent chromium and / or volatile organic compounds.   The method for purifying contaminated soil according to claim 1, wherein a step of stirring and kneading water and sepiolite, a step of adding and mixing iron powder to the material obtained in the step, and preparing a slurry, and the resulting slurry A method for purifying contaminated soil, comprising: a step of injecting into a contaminated soil containing hexavalent chromium and / or a volatile organic compound; and a step of stirring and mixing the contaminated soil into which the slurry has been injected.   The method for purifying contaminated soil according to claim 1 or 2, wherein the iron powder and sepiolite are mixed at a mass ratio of 1: 1 to 1: 3. 4. The method for purifying contaminated soil according to claim 2, wherein the water / sepiolite ratio is 8 to 16 in terms of mass ratio.