Contaminated soil remediation method

Abstract

To provide a method to resolve problems of prior arts, i.e., to provide an effective cleaning technique other than the technique described in the patent literature 1, wherein this soil cleaning method is capable of recovering 1,4-dioxane by gas suction from the sites where gas suction is difficult in the conventional arts.SOLUTION: The method for cleaning a contaminated soil of the present invention is a method for cleaning a soil contaminated with 1,4-dioxane and involving a soil heating step and a gas suction step. In the soil heating step, an objective soil and its soil interstitial water are heated until the soil temperature reaches a target temperature. In the gas suction step, the soil interstitial water having been converted to steam by the heating is sucked. By recovery of the soil interstitial water, 1,4-dioxane dissolved in the soil interstitial water is recovered.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a technology for purifying contaminated soil, and more specifically, to a purification method in which soil contaminated with 1,4-dioxane is heated and the generated soil gas containing water vapor is sucked in and collected. [Background technology]

[0002] Soil contamination refers to a state in which soil has been contaminated with substances harmful to human health, which can occur when harmful substances seep from the ground surface due to careless handling during operational activities, or when harmful substances contained in flue gas fall to the ground surface and accumulate or seep, and in other cases, when contaminated soil containing harmful substances is brought in during embankment or burial. These harmful substances are designated by the Soil Contamination Countermeasures Act, and currently a total of 26 substances, including volatile organic compounds, heavy metals, and pesticides, are designated as specified harmful substances.

[0003] In addition to the specific hazardous substances designated by the Soil Contamination Countermeasures Act, other substances such as dioxins and PAHs (polycyclic aromatic hydrocarbons) are known to be harmful to human health, and 1,4-dioxane, which is suspected to be carcinogenic, has also been attracting attention in recent years. 1,4-dioxane is added to organic solvents as a reactant or stabilizer, and can also be produced as a by-product in the manufacturing process of surfactants and PET (Polyethyleneterephthalate) resin. 1,4-dioxane can also be mixed into raw materials or products as an impurity, or can be unintentionally included in wastewater from manufacturing processes, so it is not uncommon for 1,4-dioxane to be discharged even when it is not directly used. In Japan, due to an increase in the number of cases of 1,4-dioxane being detected in rivers and groundwater and its emergence at illegal dumping sites, it was added to the list of substances subject to the Environmental Quality Standards for Water Quality and Groundwater in November 2009, and in May 2012 it was designated as a hazardous substance under the Water Pollution Prevention Act. Furthermore, in March 2016, 1,4-dioxane, along with chloroethylene, was added to the environmental standards for soil pollution (soil environmental standards) under the Basic Environment Act, and its release into the environment is now subject to stricter restrictions through effluent standards, underground infiltration regulations, etc.

[0004] 1,4-dioxane is a persistent synthetic chemical that dissolves easily in water and does not volatilize easily, and its properties differ from those of volatile organic compounds (VOCs). 1,4-dioxane also has the characteristic of having low adsorption to soil particles, and its properties differ from those of heavy metals and pesticides, which tend to remain in shallow layers. Therefore, the behavior of 1,4-dioxane in the underground environment (permeability, spread of groundwater contamination, etc.) differs from that of VOCs, heavy metals, and pesticides, and therefore the purification methods used for VOCs, heavy metals, etc. may not be able to properly purify 1,4-dioxane.

[0005] Since 1,4-dioxane is easily dissolved in water, it can exist in high concentrations in aquifers (layers containing groundwater), and therefore pumping and treating groundwater is an effective purification method. Bioremediation (purification of soil contamination using microorganisms), which is used for VOCs and oils, is also considered as a purification method, but although the existence of microorganisms capable of decomposing 1,4-dioxane has already been confirmed, in-situ purification has not yet been put to practical use.

[0006] The groundwater pumping method for purifying 1,4-dioxane is effective not only as a direct purification measure, but also in terms of preventing diffusion. However, there are difficulties in obtaining the expected purification effect unless appropriate well design (number, arrangement, pumping volume, etc.) is performed, and problems have been pointed out, such as the need for advanced water treatment equipment to treat the pumped groundwater to below the standard value. Furthermore, when there is groundwater with low fluidity, such as perched water or pore water in a low-permeable layer, especially when the target is a clayey soil layer (such as a loam layer) containing an unsaturated zone, pumping treatment is difficult in the first place, and the groundwater pumping method cannot fully achieve its purpose.

[0007] Therefore, the applicant of the present application has proposed a method of decomposing 1,4-dioxane using thermally activated sulfate radicals without performing pump-and-water treatment. More specifically, as shown in Patent Document 1, the applicant has proposed a new purification technology in which the soil is heated and persulfate is injected into the heated soil and groundwater to generate thermally activated sulfate radicals, which decompose 1,4-dioxane. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2018-34086 A Summary of the Invention [Problem to be solved by the invention]

[0009] The technology disclosed in Patent Document 1 enables soil purification by an accelerated oxidation method without adding an iron catalyst, and is a method that can reduce labor and costs without requiring special equipment, making it an extremely efficient and effective technology. However, since the conditions of each site that requires soil contamination countermeasures are different, it is possible that the technology of Patent Document 1 cannot be adopted. For example, at sites where persulfate cannot be procured or where the use of persulfate is prohibited, other methods must be adopted. Therefore, it is desirable to establish several effective purification methods, including the technology of Patent Document 1, so that a method suitable for each site can be selected.

[0010] In addition to the advanced oxidation method and groundwater pumping method, a gas suction method is known in which harmful substances (such as VOCs) that cause soil contamination are gasified and then sucked up. However, when the harmful substance is 1,4-dioxane, this gas suction method is not effective. This is because 1,4-dioxane has a small Henry's constant at room temperature and is low volatile, so its mobility at room temperature is low and 1,4-dioxane cannot be sufficiently sucked up. In particular, at sites where the target soil is a clayey soil layer (such as a loam layer), its mobility is further reduced, so the effect of gas suction is extremely limited.

[0011] The object of the present invention is to solve the problems associated with the conventional technology, that is, to provide an effective purification method other than the technology of Patent Document 1, and to provide a soil purification method capable of recovering 1,4-dioxane by gas suction even at sites where gas suction has been conventionally difficult. [Means for solving the problem]

[0012] The present invention focuses on the fact that, rather than volatilizing 1,4-dioxane itself, groundwater containing 1,4-dioxane is vaporized and the water vapor is inhaled along with the 1,4-dioxane, and is an invention based on a previously unconventional idea.

[0013] The method for remediating contaminated soil of the present invention is a method for remediating soil contaminated with 1,4-dioxane, and includes a soil heating step and a gas suction step. In the soil heating step, the target soil and its pore water are heated until the soil temperature reaches a target temperature. In the gas suction step, water vapor generated by heating is sucked in. The pore water in the soil is collected, and 1,4-dioxane dissolved in the soil pore water is collected.

[0014] The contaminated soil remediation method of the present invention may further include a water injection step of injecting water into the target soil. In this case, the soil heating step heats the soil and the soil pore water together with the water injected in the water injection step (hereinafter referred to as "injected water"), and the gas suction step sucks in the vaporized soil pore water and the injected water.

[0015] The contaminated soil remediation method of the present invention can also be a method of heating by electrical heating. In this case, in the soil heating step, a current is applied to three or more electrode wells constructed in the soil, and the soil and water (soil pore water and poured water) are heated by passing an electric current through the soil.

[0016] The method for remediating contaminated soil of the present invention may also be a method in which the soil is heated to a temperature of 40 to 90°C in the soil heating step. Effect of the Invention

[0017] The method for remediating contaminated soil according to the present invention has the following effects. (1) Rather than volatilizing 1,4-dioxane itself, the groundwater and soil pore water in which 1,4-dioxane is dissolved are vaporized, making it possible to recover even low-volatility 1,4-dioxane by gas suction. (2) Because the groundwater is vaporized by heating, it is possible to suck up the groundwater (i.e., water vapor) even in sites where the target soil is a clayey soil layer, and this makes it possible to recover the 1,4-dioxane contained in the water vapor. (3) This may be carried out at an operating site without the need to stop operations. [Brief description of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view showing an example of equipment for carrying out the contaminated soil remediation method of the present invention. [Diagram 2] 1 is a flow chart showing the main steps of the contaminated soil remediation method of the present invention. [Diagram 3] Cross-section showing three electrodes placed in soil. [Figure 4] FIG. 4 is a plan view showing electrode tubes arranged in a plane to form a triangle. [Diagram 5] FIG. 1 is a graph showing the results of a treatability test using simulated contaminated soil. [Figure 6] FIG. 1 is a graph showing the results of a treatability test using actual contaminated soil. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] An embodiment of the method for remediating contaminated soil according to the present invention will be described with reference to the drawings.

[0020] FIG. 1 is a cross-sectional view showing an example of equipment for carrying out the contaminated soil remediation method of the present invention. As shown in this figure, a gas suction pipe 10 and a heating device are installed to carry out the contaminated soil remediation method of the present invention. The heating device heats the soil, and the heating method can be a conventional method using steam or a heater, or an electric heating method can be used as shown in FIG. 1. The electric heating method is a method of heating the soil by Joule heat by applying voltage to an electrode tube 20 inserted into the target soil (underground). In addition, a temperature observation tube 30 may be installed to confirm that the soil has been heated to a target temperature. When the soil and groundwater are heated to a target temperature by the heating device, the soil pore water is vaporized, and the vaporized soil pore water (hereinafter simply referred to as "pore water gas") is sucked in by the gas suction pipe 10 by the gas suction device 40.

[0021] Figure 2 is a flow diagram showing the main steps of the method for remediating contaminated soil according to the present invention. For example, once the equipment shown in Figure 1 has been constructed, the soil and groundwater are first heated (Step 10). One of the features of the present invention is that it does not volatilize 1,4-dioxane, but rather vaporizes the soil pore water and groundwater that contain 1,4-dioxane. Therefore, when there is no groundwater (i.e., the groundwater level is low) and the soil moisture content is low, the soil heating step may be combined with a water injection step. Harmful substances are dissolved in the injected water, and then vaporized and recovered.

[0022] As mentioned above, the heating method can be the conventional method using steam or a heater, or it can be an electric heating method. However, if the target soil is a clayey soil layer such as a loam layer, it may be difficult to heat it using steam or a heater, in which case it is better to use an electric heating method. The electric heating method has the following features: it can heat clayey soil layers relatively easily, the equipment required for heating can be reduced, temperature can be easily adjusted, and it is economical due to its high thermal efficiency. Heating using the electric heating method is explained in detail below.

[0023] In carrying out the electric heating method, multiple electrode tubes 20 (three in this figure) are installed in the soil to be purified, as shown in FIG. 3. The electrode tubes 20 are electrode wells formed with steel casings, and by connecting the casings of each electrode tube 20 to a power supply unit 50 as shown in FIG. 3, current flows between the electrode tubes 20, and Joule heat can be generated in the soil along the way (between the electrode tubes 20). For this reason, it is advisable to arrange each electrode tube 20 so as to surround the soil to be purified, and for example, they can be arranged to form a triangle as shown in FIG. 4. Note that each of the four triangles shown in FIG. 4 is an equilateral triangle with one side of about 2.5 m, but the electrode tubes 20 can be arranged in various shapes without being limited to this.

[0024] As shown in Figure 2, the soil heating process (Step 10) continues until the soil temperature reaches a preset target temperature. In other words, if it is determined that the soil temperature has reached the target temperature (Yes in Step 20), the process proceeds to the next step, and if it is determined that the soil temperature has not reached the target temperature (No in Step 20), the soil continues to be heated. This target temperature is the temperature required to vaporize soil pore water and poured water (hereinafter collectively referred to as "soil pore water, etc."), and as a result of testing conducted by the inventors, it has been confirmed that it can be set within the range of 40°C to 90°C.

[0025] Whether or not the soil temperature has reached the target temperature can be determined directly by measuring the soil temperature using a temperature observation tube 30 (Fig. 1), or indirectly by determining whether or not the required heating period for the soil temperature to reach the target temperature has elapsed. This required heating period is the heating period required for the soil temperature to reach the target temperature, and should be calculated in advance by trial calculation (simulation calculation) or the like.

[0026] When the temperature is increased to the target temperature, the soil pore water etc. vaporizes, and a gas suction process (Step 30) is carried out to suck in the pore water gas and vaporized water (hereinafter collectively referred to as "pore water gas etc."). Specifically, a gas suction device 40 (Fig. 1) using a pump etc. is operated, and the soil pore water gas etc. is sucked in through the gas suction tube 10. At this time, it is advisable to continue applying electricity using the heating device and keep the temperature constant.

[0027] As mentioned above, 1,4-dioxane has low volatility, so even if it is heated to a target temperature (for example, 40°C to 90°C), the amount that vaporizes directly is limited, meaning that it is not possible to directly aspirate the 1,4-dioxane vaporized from soil at the target temperature. On the other hand, 1,4-dioxane has a small octanol / water partition coefficient and is easily soluble in water, so it is dissolved in soil pore water, etc., and therefore by aspirating the pore water gas, etc., the 1,4-dioxane dissolved in it can also be recovered.

[0028] Once the interstitial water gas etc. has been sucked up, a gas-liquid separation process (Step 40) is carried out to separate the collected gas into a gas phase and a liquid phase. The liquid phase containing 1,4-dioxane is then sent to an existing treatment facility for treatment (Step 50). If no treatment facility is available, it is advisable to treat it as industrial waste or to set up an appropriate water treatment facility. As for the gas phase, it is expected that it may contain harmful substances other than 1,4-dioxane, so it is adsorbed by activated carbon and then released into the atmosphere (Step 60).

[0029] In order to confirm the effect of the method for remediating contaminated soil of the present invention, the inventors carried out a treatability test in a room. The test procedure is as follows. (Laboratory test 1: Test using simulated contaminated soil) Weigh out 30g of drilled soil into a 37ml glass bottle. Inject 6 μl of 10-fold diluted 1,4-dioxane (equivalent to 0.6 mg of dioxane) using a microsyringe. Seal tightly and leave at 20℃ for 4 days. Measure the 1,4-dioxane concentration (elution) and water content as the initial concentration - Drill a hole of about 3 mm in the lid of the remaining sample bottle and cure it in different temperature environments (60℃ and 40℃). Measure 1,4-dioxane concentration (elution) every week

[0030] (Laboratory test 2: Test using actual contaminated soil) Weigh out 30g of the boring soil (contaminated) into a 37ml glass bottle. Measure the 1,4-dioxane concentration (elution) and water content as the initial concentration Drill a hole of about 3 mm in the lid of the remaining sample bottle and cure it in different temperature environments (13℃, 40℃, and 50℃). Measure the 1,4-dioxane concentration (elution) and moisture content after curing

[0031] Figure 5 shows the results of a treatability test (Laboratory Test 1) using simulated contaminated soil. As shown in this figure, in the case of the simulated contaminated soil, when it was cured at 60°C, the concentration fell below the lower limit of quantification (set at 0.005 mg / L) in two weeks, and even when it was cured at 40°C, it fell below the lower limit of quantification in four weeks.

[0032] FIG. 6 is a graph showing the results of a treatability test (Laboratory Test 2) using actual contaminated soil. As shown in this figure, in the case of actual contaminated soil, in the case of 13°C (local soil temperature), no significant decrease in the 1,4-dioxane concentration was confirmed. In other words, purification by gas absorption measures cannot be expected at room temperature. On the other hand, in the case of 50°C, the concentration fell below the lower limit of quantification (set at 0.005 mg / L) in 10 days, and even in the case of curing at 40°C, it fell below the standard value (0.05 mg / L) in 13 days. This confirmed that the contaminated soil purification method of the present invention is an extremely effective purification method. [Industrial Applicability]

[0033] The method for remediating contaminated soil of the present invention can be used at operational sites (or former operational sites) where 1,4-dioxane is used, discharged, or by-produced. Considering that the present invention is extremely beneficial for improving the environment in Japan, it can be said to be an invention that can be expected to not only be used industrially, but also to make a great contribution to society. [Explanation of symbols]

[0034] 10 Gas suction tube 20 electrode tube 30 Temperature observation tube 40 Gas suction device 50 Power supply

Claims

1. A method for purifying soil contaminated with 1,4-dioxane, comprising: A soil heating step of heating the soil and soil pore water until the soil temperature reaches a preset target temperature; A gas suction step of suctioning the soil pore water vaporized by heating, The target temperature is a temperature required for the soil pore water to evaporate, and is set within a range of 40 to 80°C; In the soil heating step, a current is applied to three or more electrode wells constructed in the soil, and an electric current is passed through the soil to heat the soil and the soil pore water; The vaporized soil pore water is recovered to recover 1,4-dioxane dissolved in the vaporized soil pore water. A method for remediating contaminated soil.

2. A method for purifying soil contaminated with 1,4-dioxane, comprising: A water injection step of injecting water into the target soil; A soil heating step of heating the target soil, the soil pore water, and the water injected in the water injection step until the soil temperature reaches a preset target temperature; A gas suction process for suctioning the soil pore water vaporized by heating and the injected water, The target temperature is a temperature required for the soil pore water and the injected water to be vaporized, and is set within a range of 40 to 80°C; In the soil heating step, a current is applied to three or more electrode wells constructed in the soil, and a current is passed through the soil to heat the soil, the soil pore water, and the injected water; The vaporized soil pore water and the injected water are collected to collect 1,4-dioxane dissolved in the vaporized soil pore water and the injected water. A method for remediating contaminated soil.

3. In the gas suction step, the gas is continuously applied to the electrode well to keep the soil temperature constant and to suction the gas.

3. The method for remediating contaminated soil according to claim 1 or 2.

4. The method further comprises a gas-liquid separation step of separating the vaporized soil pore water into a gas phase and a liquid phase, The liquid phase containing 1,4-dioxane separated in the gas-liquid separation step is subjected to a treatment.

3. The method for remediating contaminated soil according to claim 1 or 2.

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