JP7653217B2 - Contaminated soil remediation method - Google Patents

Description

This invention relates to technology for purifying contaminated soil, and more specifically, to a purification method in which soil contaminated with 1,4-dioxane is heated and the resulting soil gas, including water vapor, is sucked in and collected.

Soil pollution refers to a state in which soil has been contaminated with substances that are harmful to human health. It can occur when harmful substances seep up 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 up. It can also occur 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.

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 of being 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 where 1,4-dioxane has been detected in rivers and groundwater, and its occurrence at illegal dumping sites, it was added to the list of substances subject to the water quality environmental standards and groundwater environmental standards in November 2009, and designated as a hazardous substance under the Water Pollution Prevention Act in May 2012. Furthermore, in March 2016, 1,4-dioxane was added to the environmental standards for soil pollution (soil environmental standards) under the Basic Environment Act, along with chloroethylene, and its release into the environment is now subject to stricter restrictions through wastewater standards and underground infiltration regulations.

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 purification methods used for VOCs, heavy metals, etc. may not be able to properly purify 1,4-dioxane.

1,4-dioxane is easily soluble in water, so 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 contaminated soil using microorganisms), which is used to treat 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.

The groundwater pumping method for purifying 1,4-dioxane is effective not only as a direct purification measure, but also in preventing diffusion. However, there is the difficulty that the expected purification effect cannot be achieved unless appropriate well design (number, arrangement, pumping volume, etc.) is carried out, and there is also the problem that advanced water treatment equipment is required 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) that includes an unsaturated zone, pumping treatment is difficult in the first place, and the purpose cannot be fully achieved with the groundwater pumping method.

The applicant of this application has therefore proposed a method of decomposing 1,4-dioxane using thermally activated sulfate radicals, without the need for pumping 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 then decompose 1,4-dioxane.

JP 2018-34086 A

The technology disclosed in Patent Document 1 enables soil purification by an accelerated oxidation method without the addition of 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 at each site requiring soil contamination countermeasures are different, it is possible that the technology in 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 in Patent Document 1, so that a method suitable for each site can be selected.

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 effectiveness of gas suction is extremely limited.

The objective of the present invention is to solve the problems inherent in the conventional technology, that is, to provide an effective purification method in addition to the technology of Patent Document 1, and to provide a soil purification method that can recover 1,4-dioxane by gas suction even at sites where gas suction has traditionally been difficult.

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, making it an invention based on a previously unconventional idea.

The contaminated soil remediation method of the present invention is a method for remediating soil contaminated with 1,4-dioxane, and is a method that includes a soil heating process and a gas suction process. In the soil heating process, the target soil and its soil pore water are heated until the soil temperature reaches a target temperature. In the gas suction process, water vapor generated by heating is suctioned. By collecting the soil pore water, 1,4-dioxane dissolved in the soil pore water is collected.

The contaminated soil remediation method of the present invention can also be a method further comprising a water injection process in which water is injected into the target soil. In this case, the soil heating process heats the soil and the soil pore water as well as the water injected in the water injection process (hereinafter referred to as "injected water"), and the gas suction process sucks in the vaporized soil pore water and the injected water.

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.

The contaminated soil remediation method of the present invention can also be a method in which the soil is heated until the soil temperature reaches 40 to 90°C in the soil heating step.

The method for remediating contaminated soil according to the present invention has the following effects.
(1) Since 1,4-dioxane itself is not volatilized but groundwater or soil pore water in which 1,4-dioxane is dissolved is vaporized, even low-volatility 1,4-dioxane can be recovered 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 thereby recover the 1,4-dioxane contained in the water vapor.
(3) It may be carried out at an operating site without the need to stop operations.

1 is a cross-sectional view showing an example of equipment for carrying out the contaminated soil remediation method of the present invention. 1 is a flow chart showing the main steps of the contaminated soil remediation method of the present invention. A cross-sectional view showing three electrodes placed in soil. FIG. 4 is a plan view showing electrode tubes arranged in a plane to form a triangle. FIG. 1 is a graph showing the results of a treatability test using simulated contaminated soil. FIG. 1 is a graph showing the results of a treatability test using actual contaminated soil.

An example of an embodiment of the contaminated soil remediation method of the present invention will be described with reference to the drawings.

Figure 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 Figure 1. The electric heating method is a method of heating the soil by Joule heat by applying it 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 the target temperature. When the soil and groundwater are heated to the 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.

Figure 2 is a flow diagram showing the main steps of the contaminated soil remediation method of 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, if 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.

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 apply 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.

When performing the electric heating method, multiple electrode tubes 20 (three in this figure) are installed in the soil to be purified, as shown in Figure 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 Figure 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 that it surrounds the soil to be purified, and for example, they can be arranged to form a triangle as shown in Figure 4. Note that each of the four triangles shown in Figure 4 is an equilateral triangle with one side of approximately 2.5 m, but the electrode tubes 20 can be arranged in various shapes without being limited to this.

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), proceed to the next step, and if it is determined that the soil temperature has not reached the target temperature (No in Step 20), heating continues. 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 the inventors have confirmed through testing that it can be set within the range of 40°C to 90°C.

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 (Figure 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.

When the temperature is increased to the target temperature, the soil pore water etc. vaporizes, and a gas suction process (Step 30) is performed 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 (Figure 1) using a pump or the like 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.

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 cannot be expected to directly aspirate vaporized 1,4-dioxane 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. Therefore, by aspirating pore water gas, etc., the 1,4-dioxane dissolved in it can also be recovered.

Once the interstitial water gas etc. has been sucked in, 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).

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 30 g of soil from the borings into a 37 ml glass bottle. ・Inject 6 μl of 10-fold diluted 1,4-dioxane (equivalent to 0.6 mg of dioxane) using a microsyringe. ・Separate the bottle tightly and leave it at 20°C for 4 days. ・Measure the 1,4-dioxane concentration (elution) and water content as the initial concentration. ・Punch approximately 3 mm holes in the lids of the remaining sample bottles and store them in different temperature environments (60°C and 40°C). ・Measure the 1,4-dioxane concentration (elution) every week.

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

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, it 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.

Figure 6 shows 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, at 13°C (local soil temperature), no significant decrease in 1,4-dioxane concentration was confirmed. In other words, purification by gas absorption measures cannot be expected at room temperature. On the other hand, at 50°C, the concentration fell below the lower limit of quantification (set at 0.005 mg/L) in 10 days, and even when cured 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 technique.

The contaminated soil remediation method of the present invention can be used at operational sites (or former operational sites) where 1,4-dioxane is used, discharged, or by-produced. Given 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 significant contribution to society.

10 Gas suction tube 20 Electrode tube 30 Temperature observation tube 40 Gas suction device 50 Power supply device

Claims (4)

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. 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 recovered, thereby recovering the vaporized soil pore water and 1,4-dioxane dissolved in the injected water.
A method for remediating contaminated soil. In the gas suction step, the soil temperature is kept constant by continuously applying the gas to the electrode well, and the gas is suctioned.
3. The method for remediating contaminated soil according to claim 1 or 2. The method further includes 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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