Soil washing method

Abstract

This invention provides a soil washing method that offers good cleaning performance and suppresses oily odors from the soil. [Solution] A method for washing soil, comprising the following steps. Step 1: A step of preparing a slurry by mixing oil-contaminated soil with a washing solution containing water and a surfactant. Step 2: A step following Step 1, in which part or all of the washing solution is removed from the slurry in which the soil and washing solution are mixed. Step 3: A step in which, after step 2, the slurry from which part or all of the cleaning solution has been removed is mixed with rinse water containing (A) persulfate (hereinafter referred to as component (A)) and water.

Description

Technical Field

[0001] The present invention relates to a method for cleaning soil.

Background Art

[0002] In recent years, illegal dumping of industrial waste, waste treatment in factories, soil contamination due to harmful substance leakage accidents from final disposal sites, and leakage accidents and long-term leakage of various oils at sites and former sites of oil refineries, gas stations, chemical factories, etc. have led to frequent cases of serious soil contamination in various scenarios.

[0003] Conventionally, for such methods of restoring contaminated soil, techniques such as incineration treatment after excavation, solidification / immobilization, containment treatment, bioremediation, and methods for decomposing and removing contaminants have been used. Also, techniques for cleaning contaminated soil with a soil cleaning agent composition and techniques for decomposing and removing organic compounds in soil using the Fenton reaction are known.

[0004] Patent Document 1 discloses a soil cleaning method that has excellent detergency and low environmental impact, in which a soil cleaning agent composition containing (A) a polyoxyalkylene alkyl or alkenyl ether having an alkyl or alkenyl group with 8 to 20 carbon atoms and an average addition mole number of oxyalkylene of 3 to 20, (B) a persulfate, and water is brought into contact with soil. Patent Document 2 discloses a method for purifying contaminated soil, which includes a mixing step of adding a liquid to contaminated soil contaminated with a hydrocarbon compound and mixing the contaminated soil and the liquid, a liquid removal step of removing the liquid from the contaminated soil, and an oxidative decomposition step of oxidatively decomposing the hydrocarbon compound in the contaminated soil. It is disclosed that the method for purifying the contaminated soil can efficiently purify contaminated soil contaminated with oils having low to high carbon numbers such as gasoline, heavy oil, and insulating oil on-site. Patent Document 3 discloses a method for oxidizing organic compounds, which involves contacting organic compounds present in soil, sludge, sediment, bedrock, groundwater, factory process water, or wastewater with a composition containing a water-soluble peroxygen compound and a pH adjusting agent that maintains the pH of the composition at approximately 10 or higher. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2024-31285 [Patent Document 2] Japanese Patent Publication No. 2020-195970 [Patent Document 3] Special Publication No. 2008-506511 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Soil washing requires not only excellent soil cleaning capabilities but also the suppression of oily odors from the soil. Previously, soil washing sometimes resulted in a slight residual oily odor, and in severe cases, re-washing was necessary. However, re-washing incurs significant labor and cost burdens, highlighting the need for a simpler method to reduce residual oily odors.

[0007] The present invention provides a soil cleaning method that has good cleaning properties and suppresses oily odors from the soil. [Means for solving the problem]

[0008] The present invention relates to a method for washing soil, comprising the following steps. Step 1: A step of preparing a slurry by mixing oil-contaminated soil with a washing solution containing water and a surfactant. Step 2: A step following Step 1, in which part or all of the washing solution is removed from the slurry in which the soil and washing solution are mixed. Step 3: A step in which, after step 2, the slurry from which part or all of the cleaning solution has been removed is mixed with rinse water containing (A) persulfate (hereinafter referred to as component (A)) and water. [Effects of the Invention]

[0009] The present invention provides a method for cleaning soil that has good cleaning properties and suppresses oily odors from the soil. [Modes for carrying out the invention]

[0010] [Soil washing method] The reason why the soil washing method of the present invention has good washing performance and suppresses oily odors from the soil is not entirely clear, but it is presumed to be as follows: Component (A) has the effect of decomposing volatile organic compounds that cause oily odors, but it is generally known that this requires a long reaction time. In this invention, it is presumed that by washing oil-contaminated soil with a cleaning solution containing a surfactant, modifying the oil with the surfactant, and then adding rinse water containing component (A), the residual surfactant in the soil increases the permeability of component (A), improving its reactivity with volatile organic compounds and suppressing the oily odor. Furthermore, the present invention is not limited in any way to the mechanism of action described above.

[0011] <Process 1> Step 1 is a step of mixing soil contaminated with oil with a cleaning solution containing water and a surfactant (hereinafter referred to as the cleaning solution of the present invention) to prepare a slurry. Soil contaminated with oil may, for example, be soil contaminated with petroleum compounds. The petroleum-based compound may be one or more selected from, for example, gasoline, kerosene, diesel fuel, heavy oil, and machine oil. The soil contaminated with the aforementioned oil may contain one or more compounds selected from benzothiophene derivatives and benzothiazole derivatives, which cause an oily odor. These compounds that cause the oily odor are compounds included in the aforementioned petroleum compounds. The soil washing method of the present invention can remove oily contaminants such as petroleum compounds from the soil, and furthermore, it can decompose the aforementioned compounds that cause oily odors, thereby suppressing oily odors.

[0012] The cleaning solution used in step 1 of the present invention contains a surfactant and water. Surfactants include one or more selected from nonionic surfactants, anionic surfactants, cationic surfactants, and amphoteric surfactants.

[0013] Examples of nonionic surfactants include alkyl monoglyceryl ethers, polyoxyalkylene monoalkyl or alkenyl ethers, alkyl (poly)glycosides (glycoside-type nonionic surfactants), sorbitan-based nonionic surfactants, aliphatic alkanolamides, fatty acid monoglycerides, sucrose fatty acid esters, and amidates of alkanolamines such as monoethanolamine, diethanolamine, and methylmonoethanolamine with fatty acids such as lauric acid and myristic acid. The alkyl or alkenyl group of the nonionic surfactant has, for example, 8 to 22 carbon atoms. The average number of added moles of the oxyalkylene group, such as the oxyethylene group, of the nonionic surfactant is, for example, 1 to 25.

[0014] From the viewpoint of cleaning properties and soil penetration, nonionic surfactants are preferably polyoxyalkylene monoalkyl or alkenyl ethers, and more preferably polyoxyalkylene alkyl or alkenyl ethers in which the alkyl or alkenyl group has 8 to 20 carbon atoms and the average number of added moles of oxyalkylene is 3 to 20.

[0015] The number of carbon atoms in the alkyl or alkenyl group of the polyoxyalkylene alkyl or alkenyl ether is 8 or more, preferably 10 or more, more preferably 12 or more, from the viewpoint of washability and soil penetration, and 20 or less, preferably 18 or less, more preferably 16 or less.

[0016] The average number of moles of oxyalkylene added to the polyoxyalkylene alkyl or alkenyl ether is 3 or more, preferably 4 or more, more preferably 5 or more, and from the perspective of detergency at low addition amounts, 20 or less, preferably 18 or less, more preferably 16 or less, still more preferably 14 or less, from the perspective of solubility in the cleaning liquid. From the perspective of solubility in the cleaning water, oxyalkylene is preferably one or more selected from oxyethylene and oxypropylene, and more preferably oxyethylene.

[0017] Examples of the anionic surfactant include one or more selected from alkyl or alkenyl sulfate esters, polyoxyalkylene alkyl or alkenyl ether sulfate esters, alkane sulfonates, alkyl or alkenyl benzene sulfonates, higher fatty acids or their salts, polyoxyethylene alkyl or alkenyl ether carboxylic acids or their salts, N-acyl amino acids or their salts, alkyl or alkenyl phosphate esters, and polyoxyethylene alkyl or alkenyl ether phosphates. The alkyl group or alkenyl group of the anionic surfactant has, for example, 8 to 22 carbon atoms. The average number of moles of the oxyalkylene group of the anionic surfactant, for example, the oxyethylene group, is, for example, 0 to 10. The salt of the anionic surfactant is, for example, an alkali metal salt such as a sodium salt or a potassium salt.

[0018] Examples of the cationic surfactant include quaternary ammonium salt type cationic surfactants. As the quaternary ammonium salt type cationic surfactant, among the groups bonded to the nitrogen atom, one or two are hydrocarbon groups having 8 to 22 carbon atoms, and the rest are groups selected from the group consisting of alkyl groups having 1 to 3 carbon atoms, hydroxyalkyl groups having 1 to 3 carbon atoms, and arylalkyl groups (such as benzyl groups).

[0019] Examples of amphoteric surfactants include one or more selected from N-acylaminopropyl-N,N-dimethylamine oxide, N-alkyl or alkenyl-N,N-dimethylamine oxide, N-acylaminopropyl-N,N-dimethyl-N-carboxymethylammonium betaine, N-alkyl or alkenyl-N,N-dimethyl-N-carboxymethylammonium betaine, N-alkyl or alkenyl-N,N-dimethyl-N-sulfopropylammonium sulfobetaine, N-alkyl or alkenyl-N,N-dimethyl-N-(2-hydroxy-sulfopropyl)ammonium sulfobetaine, N-acylaminopropyl-N,N-dimethyl-N-sulfopropylammonium sulfobetaine, and N-acylaminopropyl-N,N-dimethyl-N-(2-hydroxy-sulfopropyl)ammonium sulfobetaine. In these, the alkanoyl group has 8 to 22 carbon atoms, and examples include lauroyl or myristoyl. Also, in these, the alkyl or alkenyl group has 8 to 22 carbon atoms, and examples include lauryl group or myristyl group.

[0020] In the cleaning liquid of the present invention, from the viewpoint of detergency, the content of the surfactant is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, still more preferably 0.2% by mass or more, and preferably 3% by mass or less, more preferably 1% by mass or less, still more preferably 0.5% by mass or less. In the present invention, when an anionic surfactant is included as the surfactant, the mass of the anionic surfactant shall be the value converted to the sodium salt. Also, when a cationic surfactant is included as the surfactant, the mass of the cationic surfactant shall be the value converted to the chloride salt.

[0021] From the viewpoint of reducing oil odor, the cleaning liquid of the present invention may contain the persulfate of component (A). Examples of the persulfate of component (A) include alkali metal persulfates such as sodium persulfate and potassium persulfate, and alkaline earth metal persulfates. From the viewpoints of availability and economy, alkali metal persulfates are preferred, and sodium persulfate is more preferred.

[0022] When the cleaning solution of the present invention contains component (A), the content of component (A) is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, even more preferably 0.05% by mass or more, and preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less, from the viewpoint of reducing oil odor.

[0023] The cleaning solution of the present invention may contain other optional components as appropriate, depending on the purpose, as long as they do not impair the effects of the present invention. For example, the cleaning solution of the present invention may contain components such as gelling inhibitors, thickeners, fragrances, dyes, pigments, disinfectants, preservatives, pH adjusters, pH buffers, and masking agents.

[0024] The cleaning solution of the present invention contains water. The water can be tap water, groundwater, river water, lake water, well water, etc. The water may be the remainder of the cleaning solution after removing the surfactant, component (A), and the aforementioned optional components. For example, the water content in the cleaning solution of the present invention is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and preferably 100% by mass or less, more preferably 99.9% by mass or less, and even more preferably 99.8% by mass or less.

[0025] From the viewpoint of cleaning performance and reduction of oil odor, the pH of the cleaning solution of the present invention is preferably 7.0 or higher, more preferably 8.0 or higher, even more preferably 9.0 or higher, and preferably 14.0 or lower, more preferably 12.0 or lower, and even more preferably 11.0 or lower. The aforementioned pH may be the pH at which the washing solution is at a temperature of 20°C.

[0026] In step 1, the cleaning solution of the present invention, which has been prepared in advance, may be mixed with the soil. Alternatively, the soil may be mixed with water, a surfactant, and the aforementioned optional components, while preparing the cleaning solution of the present invention.

[0027] In step 1, in the mixture of soil and the washing solution of the present invention, the ratio of liquid (washing solution of the present invention) to solid (soil) (liquid / solid mass ratio: liquid-solid ratio) is preferably 0.4 or more, more preferably 0.5 or more, even more preferably 0.6 or more, and even more preferably 0.8 or more, from the viewpoint of efficient washing, and preferably 2.0 or less, more preferably 1.5 or less, and even more preferably 1.2 or less, from the viewpoint of reducing washing wastewater.

[0028] In step 1, the soil and the washing solution of the present invention are mixed and the contact time is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 15 minutes or more, and even more preferably 30 minutes or more, from the viewpoint of washing performance, and preferably 24 hours or less, more preferably 16 hours or less, even more preferably 10 hours or less, even more preferably 5 hours or less, and even more preferably 1 hour or less, from the viewpoint of work efficiency. In step 1, the mixture of soil and the washing solution of the present invention may be stirred. From the viewpoint of uniformity of mixing, the time for stirring the mixture is preferably 1 minute or more, more preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more. From the viewpoint of energy consumption of the stirrer, it is preferably 2 hours or less, more preferably 1 hour or less, and even more preferably 30 minutes or less.

[0029] In step 1, the cleaning solution of the present invention is mixed with soil such that the surfactant contained in the cleaning solution is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more, from the viewpoint of cleaning performance, and preferably 3.0% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.0% by mass or less, from the viewpoint of wastewater treatment performance.

[0030] In step 1, the cleaning solution of the present invention is mixed with soil such that, if the cleaning solution contains component (A), the amount of component (A) contained in the cleaning solution is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, even more preferably 0.05% by mass or more, from the viewpoint of reducing oil odor, and preferably 5.0% by mass or less, more preferably 3.0% by mass or less, even more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less, from the viewpoint of reducing costs.

[0031] In step 1, the pH of the slurry obtained by mixing the soil with the washing solution of the present invention is preferably 7.0 or higher, more preferably 8.0 or higher, even more preferably 9.0 or higher, and preferably 14.0 or lower, more preferably 13.0 or lower, even more preferably 12.0 or lower, and even more preferably 11.0 or lower, from the viewpoint of washing performance. The pH of the slurry may be the pH at 20°C.

[0032] <Process 2> Step 2 is a step performed after Step 1 to remove some or all of the washing solution from the slurry in which the soil and washing solution are mixed. In step 2, a separation operation can be performed to remove some or all of the washing solution from the slurry in which the soil and washing solution are mixed. Examples of separation operations include allowing the soil to settle by standing or centrifugation and removing the supernatant liquid which is the washing solution, and separating the soil and washing solution by filtration using a mesh or the like.

[0033] In step 2, it is preferable to centrifuge the soil to remove some or all of the washing solution from the slurry in which the soil and the washing solution of the present invention are mixed. The conditions for centrifugation are, for example, a centrifugal acceleration of 100G or more and 1200G or less, and a processing time of 1 minute or more and 10 minutes or less. Centrifugation promotes the separation of the soil from the liquid phase, making it easier to recover and reuse (backfill) the soil. If a centrifuge is not available, the mixture can be allowed to settle naturally, and only the supernatant liquid can be removed. The time for natural settling is preferably 10 minutes or more, more preferably 15 minutes or more, and even more preferably 30 minutes or more.

[0034] From the viewpoint of re-contamination, the washing liquid removed from the slurry, which is a mixture of soil and washing liquid, is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more of the washing liquid mixed with the soil, and from the viewpoint of dewatering, it is 100% by mass or less, preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less.

[0035] After completing step 2, the cycle of step 1 and step 2 may be repeated multiple times. In other words, in the soil washing method of the present invention, when soil washing is performed multiple times, one soil washing step consists of performing step 1 and step 2, and this step may be repeated two or more times. In the soil washing method of the present invention, the soil washing process, which is a cycle of steps 1 and 2, may be performed one or more times, preferably two or more times, more preferably three or more times, from the viewpoint of improving the efficiency of washing, and preferably five or fewer times, from the viewpoint of treating the recovered water.

[0036] <Process 3> Step 3 is a step in which, after step 2, the slurry from which part or all of the cleaning solution has been removed is mixed with rinse water (hereinafter referred to as the rinse water of the present invention) which contains component (A) and water.

[0037] The rinse water of the present invention contains component (A). (A) The component is the same as described in step 1. From the viewpoint of reducing oil odor, the content of component (A) in the rinse water of the present invention is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, even more preferably 0.1% by mass or more, and preferably 5.0% by mass or less, more preferably 3.0% by mass or less, and even more preferably 1.0% by mass or less.

[0038] The rinse water of the present invention may contain other optional components as appropriate, depending on the purpose, as long as they do not impair the effects of the present invention. For example, the rinse water of the present invention may contain components such as gelling inhibitors, thickeners, fragrances, dyes, pigments, disinfectants, preservatives, pH adjusters, pH buffers, and masking agents.

[0039] The rinse water of the present invention contains water. The water can be tap water, groundwater, river water, lake water, well water, etc. The water may be the remainder of the rinse water after removing component (A) and the aforementioned optional components. For example, the water content in the rinse water of the present invention is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and preferably 100% by mass or less, more preferably 99.9% by mass or less, and even more preferably 99.8% by mass or less.

[0040] From the viewpoint of cleaning performance and wastewater treatment, the pH of the rinse water of the present invention is preferably 6.5 or higher, more preferably 7.0 or higher, even more preferably 7.5 or higher, and preferably 12.0 or lower, more preferably 11.0 or lower, even more preferably 10.5 or lower, even more preferably 9.5 or lower, and even more preferably 8.5 or lower. The aforementioned pH may be the pH at which the washing solution is at a temperature of 20°C.

[0041] In step 3, the rinse water of the present invention, which has been prepared in advance, may be mixed with the slurry from which part or all of the washing solution has been removed. Alternatively, the slurry may be mixed with water, component (A), and the aforementioned optional components, and the rinse water of the present invention may be prepared while mixing with the soil. The slurry may contain the cleaning solution of the present invention that remains unremoved in step 2.

[0042] In step 3, in the mixture of the slurry from which part or all of the cleaning solution has been removed and the rinsing water of the present invention, the ratio of liquid (total of the cleaning solution and rinsing water of the present invention) to solid (soil) (liquid / solid mass ratio: liquid-solid ratio) is preferably 0.4 or more, more preferably 0.5 or more, even more preferably 0.6 or more, and still more preferably 0.8 or more, from the viewpoint of efficient rinsing and suppression of oil odor, and preferably 2.0 or less, more preferably 1.5 or less, and still more preferably 1.2 or less, from the viewpoint of reducing rinsing wastewater.

[0043] In step 3, the slurry and the rinsing water of the present invention are mixed and the contact time is preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more, from the viewpoint of efficient rinsing and suppression of oil odor, and preferably 24 hours or less, more preferably 16 hours or less, even more preferably 10 hours or less, even further 5 hours or less, and even further 1 hour or less, from the viewpoint of work efficiency. In step 3, the mixture obtained by mixing the slurry with the rinsing water of the present invention may be stirred. The time for stirring the mixture is preferably 1 minute or more, more preferably 5 minutes or more, even more preferably 10 minutes or more, and even more preferably 15 minutes or more, from the viewpoint of efficient rinsing and suppression of oil odor, and preferably 2 hours or less, more preferably 1 hour or less, and even more preferably 30 minutes or less, from the viewpoint of energy consumption of the stirrer.

[0044] In step 3, the rinse water of the present invention is mixed with the slurry such that, from the viewpoint of reducing oil odor, component (A) contained in the rinse water is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, even more preferably 0.1% by mass or more, and from the viewpoint of reducing costs, preferably 5.0% by mass or less, more preferably 3.0% by mass or less, even more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less.

[0045] In step 3, the pH of the slurry obtained by mixing the slurry with the rinse water of the present invention is preferably 7.0 or higher, more preferably 7.5 or higher, even more preferably 8.0 or higher, and preferably 12 or lower, more preferably 11.0 or lower, even more preferably 10.5 or lower, even more preferably 9.5 or lower, and even more preferably 8.5 or lower, from the viewpoint of cleaning performance and wastewater treatment performance. The pH of the slurry may be the pH at 20°C.

[0046] <Step 4> The soil washing method of the present invention may further include the following step 4. Step 4: After step 3, the slurry mixed with rinse water is allowed to stand, and the supernatant liquid of the slurry is collected. In step 4, the slurry is allowed to stand and settle naturally, and only the supernatant liquid can be removed. The time for natural settling is preferably 10 minutes or more, more preferably 15 minutes or more, and even more preferably 30 minutes or more.

[0047] After step 3, the supernatant liquid removed from the slurry mixed with rinse water is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more of the rinse water mixed with the soil, from the viewpoint of preventing re-contamination, and from the viewpoint of dewatering, 100% by mass or less, preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less.

[0048] After completing step 4, the cycle of steps 3 and 4 may be repeated multiple times. In other words, in the soil washing method of the present invention, when rinsing the soil multiple times, steps 3 and 4 are performed as one soil rinsing step, and this step may be repeated two or more times. In the soil washing method of the present invention, the soil rinsing step, which is a cycle of steps 3 and 4, may be performed one or more times, preferably two or more times, more preferably three or more times, from the viewpoint of suppressing oil odor, and preferably five or fewer times, from the viewpoint of treating recovered water.

[0049] In the soil washing method of the present invention, excavated soil may be washed at the location where the soil exists (site) or at a location different from the original location (outside the site). For example, excavated soil can be washed at a washing facility set up at a location different from the site. In this case, for example, soil collected by excavation from the site can be transported to a washing facility set up at a location different from the site and washed. It is preferable to set up the washing facility near the site. For example, soil can be collected at a predetermined location within a certain area (site), and a washing facility can be set up at a location different from the collection site within the same area. In this way, excavated soil can be washed by setting up a washing facility as a so-called on-site plant. Furthermore, when cleaning in situ, for example, a soil cleaning agent composition can be added to the excavated area and then simply mixed and cleaned using heavy machinery (such as a backhoe or power shovel). In this case, groundwater can also be used as part of the cleaning solution.

[0050] Soil treated by the washing method of the present invention can be reused by backfilling it at the place where it was collected or at another location. [Examples]

[0051] The components used in the examples are shown below. <(A) component> • Sodium persulfate: Sodium peroxodisulfate, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. <Surfactants> • Polyoxyethylene alkyl ether: A polyoxyethylene alkyl ether in which the alkyl group has 12 carbon atoms and the average number of added moles of oxyethylene groups is 8. Manufactured by Kao Corporation.

[0052] [Evaluation method for soil washability and oil odor suppression] (1) Preparation of simulated soil Commercially available river sand (Akagi Horticulture's Watarase River sand from Gunma Prefecture, washed, treated to remove impurities, sieved, and sorted) was dried at 105°C for more than 24 hours and then cooled to room temperature. 0.5% by mass (5000 mg / kg) of light oil was added to this river sand, and it was mixed uniformly with a hand mixer. The mixture was then cured at room temperature for more than 7 days to prepare a simulated soil. The simulated soil contains benzothiophene derivatives and benzothiazole derivatives, which are the cause of the oily odor derived from diesel fuel.

[0053] (2) Evaluation of washing and rinsing tests Washing solution and rinse water were prepared to have the composition shown in Table 1. However, the washing solution and rinse water consisted of water as the remainder of the components shown in Table 1. 500g of the simulated soil prepared in (1) above was placed in a 500ml glass container (20cm x 15cm x 7cm), and the prepared washing solution was mixed with the simulated soil to achieve the liquid-to-solid ratio (the ratio of liquid (washing solution) to solid (simulated soil) (liquid / solid mass ratio)) shown in Table 1. The mixture was then stirred manually with a spoon for 1 minute to prepare a slurry in which the soil and washing solution were mixed (Step 1). Table 1 also shows the amount (mass %) of each component contained in the washing solution mixed with the simulated soil. After stirring, the mixture was allowed to stand for 30 minutes, and the supernatant was removed to obtain a slurry from which the washing solution from one wash had been removed (Step 2). The glass container holding the slurry from which the supernatant had been removed was then mixed with the prepared washing solution again, in an amount calculated from the amount of supernatant liquid removed, so that the liquid-to-solid ratio with respect to the simulated soil was as shown in Table 1 (the same liquid-to-solid ratio as the first wash). After stirring, the mixture was allowed to stand and the supernatant was removed again, and Steps 1 and 2 were repeated twice to obtain a slurry from which the washing solutions from two washes had been removed. The slurry from which the supernatant liquid was removed was mixed with the prepared rinse water in a glass container with the simulated soil at the liquid-to-solid ratio (the ratio of liquid (rinse water) to solid (simulated soil) (liquid / solid mass ratio)) shown in Table 1 for 1 minute and stirred (Step 3). (Note that in Comparative Example 1, water was used as the rinse water.) The mixture was then left to stand for 30 minutes, and the supernatant liquid was removed to obtain the rinsed sample. Table 1 shows the amount (mass %) of each component contained in the rinse water mixed with the simulated soil. Table 1 also shows the pH at 20°C of the slurry obtained by mixing the washing solution and simulated soil, and the pH at 20°C of the slurry obtained by mixing the rinse water and simulated soil. Furthermore, to improve cleaning performance, the pH of the slurry, which was a mixture of soil and cleaning solution (or rinse water), was made alkaline. Separately, the amount of monoethanolamine required to obtain the pH value shown in Table 1 was determined for the slurry, which was a mixture of soil and cleaning solution (or rinse water) prepared in advance. A pH meter was used to measure the pH. Then, the amount of monoethanolamine determined in step 1 or step 3 was added.

[0054] (3) Evaluation of cleanability After the first and second washes, and after rinsing, a portion of the soil (sediment) was sampled, and TPH (Total Petroleum Hydrocarbon) was measured. The TPH measurement was performed using Document-3 of Section 2.2.1 of the Oil Pollution Control Guidelines (March 2006) (Oil Pollution Control Guidelines, [online], Ministry of the Environment, Internet).<URL:https: / / www.env.go.jp / water / dojo / oil / full.pdf> The TPH test was conducted according to the GC / FID method described in [the relevant document]. A lower TPH value indicates better soil washability. The results are shown in Table 1.

[0055] (4) Evaluation of oil odor suppression After washing, 10g of the soil (sediment) from the first, second, and final rinses was measured into 50ml screw-top tubes, sealed, and left airtight for 10 minutes. After 10 minutes, the lids were opened and the oily odor was assessed. The sensory evaluation of the oily odor was based on the judgment of one experimenter. The oily odor of the soil was evaluated sensorily based on the following evaluation criteria. Soil washing methods with lower sensorily evaluated oily odors result in less light oil remaining in the soil after washing, and are therefore superior in cleaning oil-contaminated soil. The results are shown in Table 1. (Evaluation Criteria) 0... Odorless. 1. This is an odor that can just barely be detected (detection threshold concentration). 2. The smell is faint enough that it can be identified (recognition threshold concentration). 3. It is an easily detectable odor. 4. It has a strong smell. 5. It has a very strong smell.

[0056] Table 1

Claims

1. A method for washing soil, comprising the following steps. Step 1: A step of preparing a slurry by mixing oil-contaminated soil with a washing solution containing water and a surfactant. Step 2: A step following Step 1, in which part or all of the washing solution is removed from the slurry in which the soil and washing solution are mixed. Step 3: A step in which, after step 2, the slurry from which part or all of the cleaning solution has been removed is mixed with rinse water containing (A) persulfate (hereinafter referred to as component (A)) and water.

2. The soil washing method according to claim 1, further comprising the following step 4. Step 4: After step 3, the slurry mixed with rinse water is allowed to stand, and the supernatant liquid of the slurry mixed with rinse water is collected.

3. The soil washing method according to claim 1 or 2, wherein in step 3, the liquid-to-solid ratio (liquid / solid mass ratio) of the slurry after mixing with rinse water is 0.5 or more and 2.0 or less.

4. The soil washing method according to claim 1 or 2, wherein in step 1, the pH of the slurry obtained by mixing soil and washing solution is 8.0 or higher and 13.0 or lower.

5. The soil washing method according to claim 1 or 2, wherein the soil contaminated with the aforementioned oil contains one or more selected from benzothiophene derivatives and benzothiazole derivatives.

6. (A) The method for washing soil according to claim 1 or 2, wherein the component is sodium persulfate.

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