JP5490035B2 - How to clean contaminated soil - Google Patents

Description

  The present invention relates to a method for cleaning soil contaminated with, for example, harmful substances. Specifically, the present invention exhibits high purification efficiency even when applied to soil with low purification efficiency by conventional classification removal, and cleaning treatment of cleaning liquid And a method for cleaning contaminated soil that is easy to reuse.

  As a method of purifying soil contaminated with harmful substances, a method of washing by adding a liquid such as water to the soil is used. In this method, the target substance is removed mainly in two processes.

  One is a process in which the target substance to be removed is dissolved in the added liquid, and the added liquid is separated from the soil (this process is hereinafter referred to as “dissolution removal” if necessary).

  The other is that when adding and mixing the liquid, the fine particles of the soil that adsorbed or occluded the target substance to be removed and the target substance itself to be removed are dispersed in the liquid as unit particles or fine particles. This is a process in which fine particles having a diameter or less are separated in a suspended state (hereinafter, this process is referred to as “classification removal” if necessary).

  For example, in the case of solid-liquid separation after mixing an organic solvent such as isopropyl alcohol with the purpose of purifying soil contaminated with a water-insoluble substance such as PCB, dissolution removal is the main removal process ( Patent Document 1).

  On the other hand, in order to purify soil contaminated with lead, arsenic, etc., when water is added to the soil and mixed to disperse the fine particles of the soil, the suspension containing fine particles of a certain particle size or less is separated and removed. Is the main process of classification removal, but at the same time, dissolution removal proceeds.

  Generally, heavy metals, arsenic, fluorine, boron, etc. are often adsorbed or occluded by fine particles in the soil, or they themselves exist as fine-grained compounds, so when these substances are to be removed In many cases, a cleaning method that uses classification removal as a main process is used. The size of the particles to be classified and removed is arbitrary, but is considered to be about several tens of μm or less in consideration of the amount of adsorption of heavy metals, the reusability of soil after purification, etc., and water is used as the cleaning liquid. There are many cases.

  If the pollutant (substance to be removed) is adsorbed or occluded by fine particles with a particle size of tens of μm or less in the soil, or exists as fine particles with a particle size of several tens of μm or less, the cleaning solution is added. If these fine particles can be completely dispersed as unit particles by mixing or stirring, the soil can be purified by classification removal.

  However, even if heavy metals are adsorbed or occluded by fine particles having a particle size of several tens of μm or less, these fine particles are bonded together by humic substances, oxides or hydroxides of iron, aluminum, etc. If it is present as a body and cannot be completely dispersed in the original fine particles by adding a cleaning liquid, mixing and stirring, etc., purification by classification removal is incomplete.

  In addition, even when the compound to be removed exists as coarse particles or is fixed to the surface of the coarse particles of the soil, the removal by classification removal is incomplete.

JP-A-2005-131641

  In soil, it is normal for fine particles to gather and become coarse particles. In soil size analysis in geotechnical engineering and soil science, the aggregate of fine particles is destroyed and the soil particles are used as unit particles. In addition, it is indispensable to add a dispersant and perform shaking or sonication for a long time.

  When fine particles that adsorb or occlude pollutants (substances to be removed) exist as aggregates as described above, classification and removal are incomplete. In addition, even when the compound to be removed exists as coarse particles or is fixed to the surface of the coarse particles of the soil, the removal by classification removal is incomplete.

  In such a case, in order to purify so as to satisfy a predetermined standard, it is necessary to rely on dissolution and removal.

  However, in general, the solubility of the substance to be removed is often low and is strongly adsorbed or occluded on the surface of the soil particles, so that the concentration of elution into the cleaning liquid is extremely low. For this reason, it is necessary to use a very large amount of cleaning liquid.

  For example, when calcium fluoride is attached to the surface of coarse particles of soil, classification removal is difficult. In addition, the solubility of calcium fluoride in water is about 8 mg / L as fluorine. Even when 10 cubic meters of water is used as a washing liquid for 1 ton of soil, the maximum removal amount per washing is only about 75 g. The dissolution removal efficiency is extremely low. When the calcium concentration of the washing water is high, the removal efficiency further decreases.

  In addition, as described above, in the classification removal process, dissolution and removal also proceed at the same time, so that harmful substances exceeding the standard may be dissolved in the cleaning liquid after use. In such a case, it is necessary to remove the substance before discharging the cleaning liquid. It is also inconvenient for reuse of the cleaning liquid.

  Therefore, the problem of the present invention is that it can be used without modifying existing equipment for classifying and removing, and fine particles adsorbed or occluded with pollutants form coarse aggregates, or pollutants or compounds thereof are soiled. Provided a method for cleaning contaminated soil that adheres to coarse particles of soil, exhibits high purification efficiency even when applied to soil with low purification efficiency by conventional classification removal, and is easy to clean and reuse the cleaning liquid There is.

  The other subject of this invention becomes clear by the following description.

  The above problems are solved by the following inventions.

(Claim 1)
Mixing and stirring the soil contaminated by the pollutant, the liquid, and fine particles having the ability to adsorb the pollutant;
After the mixture stirred for step performs rinsing by spraying high-pressure water to the soil containing the fine particles having adsorbed the contaminants, the particles having adsorbed the contaminants, together with soil particles under Tokoro Teitsubu size or less Classifying and removing to obtain washed soil exceeding the predetermined particle size;
A method for cleaning contaminated soil, comprising:
(Claim 2)
The method for cleaning contaminated soil according to claim 1, wherein the fine particles having the ability to adsorb the pollutant are any one of the following (1) and (2) or a combination thereof.
(1) Fine particles having the capability of adsorbing the pollutant in the liquid containing the soil (2) Fine particles generated in the liquid containing the soil and adsorbing the pollutant Fine particles having ability (Claim 3)
The contamination according to claim 1 or 2, wherein, before the mixing and stirring step, soil particles having a predetermined particle size or less are removed by classification with water on the soil contaminated with the contaminant. How to wash the soil.

  ADVANTAGE OF THE INVENTION According to this invention, even if it applies to the soil with the low purification efficiency by the conventional classification removal, the high cleaning efficiency is exhibited, and the cleaning method of the contaminated soil which can be easily purified and reused of the cleaning liquid can be provided. .

Electron micrograph of aluminum hydroxide particles produced when aluminum sulfate is added to water and the pH is adjusted to 6-7 Electron micrograph of iron hydroxide fine particles produced when ferric chloride is added to water

  Hereinafter, preferred embodiments of the present invention will be described.

  The method for cleaning contaminated soil according to the present invention comprises a step of mixing and stirring soil contaminated with a contaminant, a liquid, and fine particles having the ability to adsorb the contaminant (hereinafter referred to as the first step), and the contamination. It consists of a step (hereinafter referred to as a second step) of classifying and removing fine particles adsorbing substances together with soil particles having a predetermined particle size or less.

  In the present specification, the phrase “adsorption” includes both physical adsorption and chemical adsorption, and in the case of physical adsorption, it also includes occlusion inside the solid in addition to solid surface adsorption.

  In the present invention, the term “fine particles” means a size that can be fractionated into a removal fraction in an apparatus used for classification removal, but is several tens μm or less in normal classification removal. Point.

  Further, the liquid used in the present invention is preferably water, but is not limited thereto. Hereinafter, the case where water is used will be described.

<First step>
In the first step, the fine particles having the ability to adsorb pollutants are (1) the fine particles having the ability to adsorb the pollutants in the liquid containing soil, or (2) the liquid containing soil. Any one of the fine particles generated in the medium and having the ability to adsorb the contaminant may be used, or two or more types may be combined.

  The “fine particles having the ability of adsorbing the pollutant in the liquid containing soil” in the above (1) vary depending on the pollutant.

  For example, if the pollutant is an inorganic arsenic compound, use one or a combination of particulate iron oxide, iron hydroxide, magnesia, cerium oxide, magnesium-aluminum layered double hydroxide, etc. However, it is not limited to these.

  When the pollutant is inorganic fluorine, it is preferable to use one kind of aluminum hydroxide, iron hydroxide, magnesia or the like in combination, but it is not limited thereto.

  If the contaminant is a substance that easily elutes as a cation, such as lead, use one or a combination of iron oxide, iron hydroxide, aluminum hydroxide, particulate zeolite, particulate cation exchange resin, etc. However, it is not limited to these.

  When the pollutant is a nonionic organic compound, it is preferable to use one kind of particulate activated carbon, particulate zeolite, or a combination thereof, but it is not limited thereto.

  When adding the above-mentioned fine particles to water, these substances may be mixed with water in advance and then added to the soil, or the soil and water may be mixed in advance and these fine particles are added thereto. It's okay.

  The amount of fine particles added depends on the content of the substance to be removed contained in the contaminated soil to be purified and the adsorption or occlusion capacity of the fine particles, so it cannot be determined in advance and is determined by conducting a preliminary test before construction. Although it should be, it is preferable to add 1-100g with respect to 1L of water, More preferably, it is 1-20g, More preferably, it is 2-6g.

  Further, the adsorption or occlusion ability of the particulate matter mentioned as the above example may depend on the conditions of the solution in contact therewith. For example, when adding iron hydroxide fine particles to remove lead, the lead ion adsorption ability of iron hydroxide fine particles depends greatly on the pH of the water in which it is suspended, and the higher the pH, the greater the adsorption ability. . Similarly, the lead ion adsorption capacity by soil minerals increases as the pH increases.

  In such a case, the adsorbing ability of the added fine particles is increased sufficiently, but when it is possible to find a pH that does not increase the adsorption capacity of the soil mineral, the pH of the washing solution can be adjusted by adding acid or alkali. It is effective to control. Although not all cases can be enumerated, it is not possible to add substances to control the pH conditions of cleaning water so that the addition or absorption capacity of the added fine particles is relatively improved while adding fine particles. It is valid.

  In selecting a substance to be added, in addition to the ability of the added fine particles to adsorb or occlude the target substance to be removed, the laws and regulations to be observed when disposing the classified and removed fractions must be considered.

  The above-mentioned (2) “fine particles generated in a liquid containing soil and having the ability to adsorb the pollutant” is (A) one kind of substance that generates fine particles in a liquid containing soil or Fine particles generated in a liquid using a plurality of types may be used, or (B) a liquid using one or more types of substances that react with contaminants to generate fine particles (compounds) having low solubility. Fine particles produced therein may be used.

  In addition to the above substances, these substances may be mixed with water in advance and added to the soil, or the soil and water may be mixed in advance and these substances may be added thereto. .

  The “substance that generates fine particles in a liquid containing soil” in the above (A) is a substance that generates fine particles having the ability to adsorb or occlude the removal target substance in water. Lead, cadmium and their compounds eluting as ions, Arsenic, hexavalent chromium, fluorine and their compounds easily eluting as oxoacid ions, salts of polyvalent metals (typically iron and aluminum), solutions thereof Alternatively, it is appropriate to use a solution of these metal hydroxide clusters, monosilicic acid and polysilicate, or a solution thereof, but is not limited thereto.

  Particularly when fluorine is to be removed, aluminum sulfate is preferably used.

  In the case where a compound that easily elutes as ions is to be removed, a fired magnesium-aluminum double hydroxide is also effective.

  In addition, when it is difficult to produce fine particles when added to the soil, it is effective to add acid, alkali, or the like so that the fine particles are easily produced.

  Even in the case of performing classification and removal by adding a substance that generates fine particles having the ability to adsorb or occlude the substance to be removed, the addition amount of the substance is the content of the substance to be removed contained in the soil to be purified and Since it depends on the adsorption or occlusion ability of fine particles, it cannot be determined in advance and should be determined by conducting a preliminary test before construction, but it is preferable to add 1 to 100 g, more preferably, to 1 L of water. 1 to 20 g, more preferably 2 to 6 g.

  By adding a substance capable of generating fine particles capable of being adsorbed or occluded in water, the work of producing the fine particles in advance can be omitted.

  Next, the “substance that reacts with the pollutant to produce fine particles (compound) having low solubility” in the above (B) is a substance that has the ability to react with the removal target substance to produce fine particulate compounds having low solubility. When the object to be removed is a substance that easily forms a hardly soluble sulfide, it is useful to use a sulfide salt. Also effective are organic compounds that produce sparingly soluble chelate compounds, such as chelating agents having two dithiocarboxy groups. When the substance to be removed is hexavalent chromium and its compound, ferrous salt is also effective.

  In this case, the concentration of the substance in the washing water does not increase by reacting directly with the target substance to be removed to produce a particulate substance with low solubility. For this reason, the aqueous solution for washing is not saturated with the substance, and the elution from the substance fixed to the coarse particles of the soil continues. In addition, desorption and elution of substances adsorbed or occluded by the soil particles continues. As a result, it becomes possible to purify the soil without removing coarse particles.

  Since the product is fine particles, it is removed from the soil during the classification process. Since the concentration of the substance to be removed in water is kept low, the aqueous solution can be discharged or reused immediately after the solid-liquid separation. Furthermore, since the harmful substances are insolubilized in the classified and removed fraction, it is convenient for disposal.

  Even when water containing a substance that has the ability to react with the substance to be removed and generate a particulate compound with low solubility is added to the soil for classification and removal, the amount of the substance added to the soil to be purified Since it depends on the content of the substance to be removed and the adsorption or occlusion capacity of fine particles, it cannot be determined in advance and should be determined by conducting a preliminary test before construction, but 1 to 100 g is added to 1 L of water. More preferably, it is 1-20 g, More preferably, it is 2-6 g.

  The first step is a step of mixing and stirring the soil contaminated with the pollutant, the liquid, and the fine particles having the capability of adsorbing the pollutant, and has the capability of adsorbing the pollutant by this mixing and stirring. The fine particles adsorb pollutants in the contaminated soil.

  In other words, the first step is a step of causing the contaminants in the soil to be adsorbed on the fine particles and generating fine particles adsorbing the contaminants.

  The liquid and fine particles may be added to the soil after mixing the liquid and fine particles in advance, or the liquid may be mixed with the soil in advance, and the fine particles may be added thereto. Since the mixture can be handled as a cleaning solution, it is excellent in mixing with soil and easy to handle.

  After adding the liquid and fine particles to the soil, it is preferable to provide a contact time of 30 minutes or longer, preferably 30 minutes to 5 hours, more preferably 1 hour to 3 hours. During this contact time, the added liquid and fine particles may be lightly stirred so as to spread over the entire soil and then allowed to stand, or mechanical stirring may be added using a stirrer or the like.

<Second step>
Next, the second step of the present invention will be described.

  In the second step, the fine particles adsorbing the contaminant are classified and removed together with soil particles having a predetermined particle size or less.

  Here, the term “less than the predetermined particle size” means less than 180 μm, preferably less than 150 μm, more preferably less than 125 μm. In this embodiment, the particle size is less than 125 μm or less than 75 μm.

  The classification method is not particularly limited, and classification is performed using a wet shake screen, a high mesh separator, a cyclone, or the like. The smallest fraction contains particulates adsorbed to contaminants in addition to the particulates originally contained in the soil that are smaller than the smallest sieve pore size, i.e. below the specified particle size. To separate the pollutants from the soil.

  After classification, dehydration may be performed as necessary.

  Unlike the method of reducing the amount of pollutant elution by so-called “insolubilization”, the cleaning method of the present invention requires that the fine particles adsorbing the pollutant do not remain in the soil. Therefore, in order to increase the accuracy of classification and removal of fine particles, it is also a preferred aspect of the present invention to perform rinsing before classification in the second step.

  “Rinse” is to add water to the contaminated soil, liquid, and fine particle mixture after the first step to diffuse or disperse the fine particles in water.

  Water is preferably added while being sprayed onto the soil as water (high pressure water) to which a pressure of about 0.5 to 10 MPa is applied with a high pressure pump or the like.

  Since the fine particles of the present invention chemically adsorb pollutants, even when high-pressure water is jetted, the adsorbed contaminants and fine particles are hardly destroyed by the physical action of the pressure water, and the adsorbed state is maintained. Can be maintained. On the other hand, since it is stirred by the physical action of pressure water, the fine particles are efficiently diffused or dispersed in water.

  By rinsing, the fine particles adsorbing the soil particles and the contaminants are separated, and the fine particles are diffused or dispersed in water, so that the fine particles are easily separated in the subsequent classification.

<Principle>
The following mechanism can be considered that the fine particles of the present invention have a high cleaning effect.

  Fine particles that have the ability to adsorb or occlude in washing water remove contaminants eluted from the soil by adsorption or occlusion, so the concentration of the substance in water does not increase.

  For this reason, the aqueous solution is not saturated with the substance, but rather the concentration in the aqueous solution is lowered, so that the elution from the substance fixed to the coarse particles of the soil continues. In addition, desorption and elution of substances adsorbed or occluded by the soil particles continues. Dissolution is also promoted for poorly soluble compounds.

  This makes it possible to wash without removing coarse particles, and also to those that are made up of coarse particles that are aggregated from hardly soluble compounds that adhere to coarse soil particles and fine clay particles that have adsorbed contaminants. Correspondingly, the soil can be purified.

  Since the added material is fine particles, it is removed from the soil during the classification process.

  In addition, since the contaminants are adsorbed or occluded by the fine particles, the concentration of the contaminants dissolved in the aqueous solution for washing is low. If the solids are separated after washing to remove the fine particles, the liquid component is released immediately or It can be reused.

  In the classification and removal described above, fine particles and substances that generate fine particles in an aqueous solution for cleaning may be added from the beginning. However, at first, the usual classification and removal treatment or the present invention can be performed without the addition of these substances. A classification removal process including rinsing and classification shown in the second step may be performed, and a classification removal process in which these substances are added may be performed a second time or later.

  In contaminated soil, contaminants are adsorbed or occluded on the surface of the soil particles, and contaminants are often eluted in the pore water of the soil. It can be removed by doing.

  Therefore, if the normal classification removal treatment or the classification removal treatment consisting of rinsing and classification shown in the second step of the present invention is performed only with water before the first step, fine particles are produced in the aqueous solution for cleaning. The amount of substance used can be reduced.

  Examples of the present invention will be described below, but the present invention is not limited to such examples.

Reference example 1
Aluminum sulfate was added to water and the pH was adjusted to 6-7 to produce aluminum hydroxide particles (FIG. 1). The particles are 50 nm or less in size (scale is 50 nm).

  If the pH of the soil was 6 or more, hydroxide fine particles were generated by adding the aluminum salt without adjusting the pH due to the buffering action of the soil itself.

Reference example 2
Ferric chloride was added to water to produce fine iron hydroxide particles (FIG. 2). The particle size of the produced particles is 5 nm or less (scale is 50 nm).

  Since iron ions tend to be hydroxides, hydroxides are generated just by adding them to water.

<Case where washing was performed by mixing and mixing>
Example 1
200 g of highly contaminated soil (fluorine elution amount 9.6 mg / L) is collected, and the cleaning solution (aluminum sulfate aqueous solution (aluminum concentration: 8% as aluminum oxide)) is 5% by weight with respect to the soil sample. After soaking for 60 minutes, it was washed and classified with 2000 g of water, and the fraction less than 75 μm that passed through the 75 μm sieve was removed.

  Then, when the fluorine elution amount of the soil from which the fraction less than 75 μm was removed (washed soil) was measured, it was 0.5 mg / L, and the elution amount reference value was 0.8 mg / L or less.

  In addition, the measurement of the fluorine elution amount in the following Examples and Comparative Examples was carried out according to “A method for determining a measurement method for soil elution amount (March 6, 2003, Ministry of the Environment Notification No. 18)”.

Example 2
The soil with a medium concentration of fluorine contamination (fluorine elution amount 1.8 mg / L) was washed in the same manner as in Example 1, and the fluorine elution amount was measured, and it was less than 0.1 mg / L.

Example 3
The soil with a low concentration of fluorine contamination (fluorine elution amount 0.8 mg / L) was washed in the same manner as in Example 1, and the fluorine elution amount was measured and found to be 0.2 mg / L.

  The results of Examples 1 to 3 are shown in Table 1.

<Case where washing was performed by mixing and stirring, rinsing, and classification>
Example 4
Contaminated soil sample (fluorine elution amount 2.6mg / L), which was sieved in advance with 2mm sieve and made only particles of 2mm or less, and washing solution (aluminum sulfate aqueous solution (aluminum concentration: 8% as aluminum oxide)) on soil sample After adding for 5% by weight and soaking for 60 minutes, rinsing with high-pressure water was carried out with a wet shake sieve while flowing water, and the fraction less than 125 μm that passed through the 125 μm sieve was removed.

  Then, when the fluorine elution amount of the soil from which the fraction less than 125 μm was removed (washed soil) was measured, it was 0.1 mg / L.

Comparative Example 1
In Example 4, the amount of fluorine eluted from the washed soil was measured in the same manner except that the aluminum sulfate solution was replaced with water, and it was 2.8 mg / L.

Example 5
For another contaminated soil sample with a fluorine elution amount of 1.0 mg / L, the fluorine elution amount of the washed soil obtained by washing and classification in the same manner as in Example 4 was 0.1 mg / L.

Comparative Example 2
In Example 5, the amount of fluorine eluted from the washed soil was measured in the same manner except that the aluminum sulfate solution was replaced with water, and it was 1.3 mg / L.

Example 6
For another contaminated soil sample with a fluorine elution amount of 0.6 mg / L, the fluorine elution amount of the washed soil obtained by washing and classification in the same manner as in Example 4 was 0.1 mg / L.

Comparative Example 3
In Example 6, the amount of fluorine eluted from the washed soil was measured in the same manner except that the aluminum sulfate solution was replaced with water, and it was 1.2 mg / L.

  Table 2 shows the results of Examples 4 to 6 and Comparative Examples 1 to 3.

<Rinsing with water only, classification, adding a cleaning solution, mixing, stirring, rinsing, and classification>
Example 7
A contaminated soil sample (fluorine elution amount 3.6 mg / L) that has been sieved in advance with a 2 mm sieve and made only particles of 2 mm or less is rinsed with high-pressure water without adding fine particles. Classification was performed, and a fraction less than 125 μm that passed through a 125 μm sieve was removed.

  After adding a washing liquid (aluminum sulfate aqueous solution (aluminum concentration: 8% as aluminum oxide)) to 1% by weight with respect to the dry weight of the soil sample, to the soil (A) from which the fraction less than 125 μm was removed, Rinsing was performed with high-pressure water, and classification was performed with a wet shaking sieve while flowing water, and a fraction less than 125 μm that passed through a 125 μm sieve was removed.

  Then, when the fluorine elution amount of the soil from which the fraction less than 125 μm was removed (washed soil) was measured, it was 0.22 mg / L, and a high washing effect was obtained even if the amount of washing solution added was reduced.

  The results of Example 7 are shown in Table 3.

Claims (3)

Mixing and stirring the soil contaminated by the pollutant, the liquid, and fine particles having the ability to adsorb the pollutant;
After the mixture stirred for step performs rinsing by spraying high-pressure water to the soil containing the fine particles having adsorbed the contaminants, the particles having adsorbed the contaminants, together with soil particles under Tokoro Teitsubu size or less Classifying and removing to obtain washed soil exceeding the predetermined particle size;
A method for cleaning contaminated soil, comprising: The method for cleaning contaminated soil according to claim 1, wherein the fine particles having the ability to adsorb the pollutant are any one of the following (1) and (2) or a combination thereof.
(1) Fine particles having the capability of adsorbing the pollutant in the liquid containing the soil (2) Fine particles generated in the liquid containing the soil and adsorbing the pollutant Fine particles with ability   The contamination according to claim 1 or 2, wherein, before the mixing and stirring step, soil particles having a predetermined particle size or less are removed by classification with water on the soil contaminated with the contaminant. How to wash the soil.