In-situ organic fluorine compound remediation system and in-situ organic fluorine compound remediation method

Abstract

To provide an in-situ purification system capable of decontamination of soils and underground waters contaminated with organofluorine compounds such as PFOS and PFOA, and a purification method using the same, solving problems of conventional arts.SOLUTION: An organofluorine compound in-situ purification system is a system for in-situ decontamination of soils contaminated with organofluorine compounds and is assembled with a soil warming device and a decomposition treatment device. The soil warming device is a device for warming soils by applying voltage to 3 or more of electrode wells constructed in soils for feeding electric current to the soils. The decomposition treatment device is a portable type to be installed on a ground, and is a device for decomposition of organofluorine compounds by blowing ozone gas to contaminated water prepared by cooling contaminated water vapor and irradiating the contaminated water with UV light.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a technology for in-situ purification of contaminated soil and groundwater, and more specifically, to a technology for purifying soil and groundwater contaminated with organofluorine compounds (PFCs) such as perfluorooctanesulfonic acid and perfluorooctanoic acid, by utilizing an electrothermal method, which is a soil heating technology, as well as physical recovery and chemical decomposition. [Background technology]

[0002] Due to their properties of water repellency, oil repellency, and chemical stability, organic fluorine compounds have been used in a variety of applications, including water repellents, surface treatment agents, emulsifiers, fire extinguishers, insecticides, and coatings for cooking utensils. However, due to their properties, organic fluorine compounds are virtually non-degradable in nature, and their environmental persistence and bioaccumulation have been raised as concerns. In particular, perfluorooctanesulfonic acid (hereinafter referred to as "PFOS") and perfluorooctanoic acid (hereinafter referred to as "PFOA"), which have eight carbon atoms, have been shown to exhibit significant persistence, bioaccumulation, and toxicity, raising concerns about their harmfulness to humans and other living organisms.

[0003] Therefore, at the Fourth Conference of the Parties to the Stockholm Convention on Persistent Organic Pollutants (POPs Convention) in 2009, it was decided to add PFOS and its salts to Annex B (substances whose manufacture, use, import and export should be restricted). In response to this, in Japan, PFOS and its salts were designated as Class 1 Specified Chemical Substances under the Act on the Evaluation of Chemical Substances and Regulation of Their Manufacture, etc. (the so-called "Chemical Substances Control Act"), and as Class 1 Designated Chemical Substances under the Act on Reporting, etc. of Releases of Specific Chemical Substances into the Environment and Promotion of Improvements to Their Management (PRTR Act), and in principle, the manufacture, import, use, etc. of PFOS and its salts are prohibited.

[0004] Meanwhile, at the 9th Conference of the Parties to the POPs Convention (2019), it was decided that PFOA and related substances would be added to Annex A (substances whose manufacture, use, import and export should be prohibited), and in Japan they are designated as Type 2 Monitoring Chemical Substances under the Chemical Substances Control Law.

[0005] Because PFOS and PFOA hardly decompose in the environment, leaks from manufacturing plants can persist in the environment for long periods, potentially contaminating rivers and groundwater. Long-term drinking of tap water from these sources has also been reported to increase the risk of kidney cancer, thyroid disease, and ulcerative colitis. Therefore, there is a movement to focus on PFOS and PFOA in water quality management in the water supply industry. While the World Health Organization (WHO) has not yet established target or guideline values ​​for PFOS and PFOA in drinking water, Europe and the United States have established their own targets. In Japan, at the "Second Meeting of the Study Group on Sequential Revision of Water Quality Standards for FY2019" held by the Ministry of Health, Labor, and Welfare, a provisional target value for PFOS and PFOA in drinking water quality was agreed upon at a combined level of 50 ng / L, effective from April 1, 2020.

[0006] While efforts are being made to combat PFOS and PFOA in the water supply sector, there have been no notable efforts made in the areas of soil and groundwater contamination. So However, soil contamination caused by PFOS contained in firefighting foam in Okinawa Prefecture has become a problem, and it is easy to predict that in the future, attention will be focused on the remediation of PFOS and PFOA in the fields of soil and groundwater contamination.

[0007] Common methods for in-situ decontamination of soil and groundwater include physical, chemical, and biological methods. However, in cases of contamination with organofluorine compounds such as PFOS and PFOA, none of these methods are effective due to the properties of these compounds. For example, physical methods involve recovering organofluorine compounds contained in soil gas or groundwater using gas suction or pumping. However, organofluorine compounds have low volatility, making their recovery by gas suction difficult. Furthermore, their low solubility in water limits the effectiveness of pumping. In particular, when PFOS or PFOA have penetrated the pores of the soil, their mobility at room temperature is extremely low, making recovery by pumping even more difficult.

[0008] Chemical methods involve using chemicals to decompose organic fluorine compounds, but because organic fluorine compounds are chemically stable and do not decompose in the environment, accelerated oxidation is required for their decomposition. For this reason, although there is a track record of decomposition using "ozone + hydrogen peroxide" in the field of water treatment (water supply), there are many challenges to applying ozone to in-situ remediation of soil and groundwater, such as the technical difficulty of supplying high concentrations of ozone underground and the fact that ozone itself is harmful.

[0009] Biological methods utilize the action of microorganisms to decompose organofluorine compounds, but organofluorine compounds are biologically stable and are not easily decomposed by microorganisms either aerobic or anaerobic, and there is no track record of their use in the field of water treatment.Similarly, the application of biological treatment to in-situ remediation of soil and groundwater is not realistic.

[0010] When physical, chemical, or biological methods alone are not effective in decontaminating the contamination, a decontamination method that combines these methods can be an effective solution. For example, Patent Document 1 proposes a decontamination technology that decomposes 1,4-dioxane by injecting persulfate into heated soil, etc. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-34086 Summary of the Invention [Problem to be solved by the invention]

[0012] The technology disclosed in Patent Document 1 utilizes an electric heating method, which allows the soil temperature to be efficiently raised to a predetermined temperature (e.g., 40 to 90°C) and stably maintained at that predetermined temperature. As a result, 1,4-dioxane can be effectively decomposed by the injected persulfate. However, the technology disclosed in Patent Document 1 is a technology for decomposing 1,4-dioxane, and is not intended to purify soil or groundwater contaminated with organofluorine compounds, which have completely different physical and chemical properties from 1,4-dioxane.

[0013] The object of the present invention is to solve the problems that have been encountered in the past, that is, to provide an in-situ remediation system that can purify soil and groundwater contaminated with organofluorine compounds such as PFOS and PFOA, and a remediation method using the same. [Means for solving the problem]

[0014] The present invention is characterized by the fact that, as the soil temperature rises, water vapor is generated, the volume of water expands, and the viscosity of water decreases, which improves the movement of the water itself, thereby increasing the recovery efficiency of organic fluorine compounds contained in pore water. Furthermore, by increasing the temperature of the soil or groundwater, chemical reactions are promoted, allowing the decomposition of organic fluorine compounds that could not be decomposed by conventional methods (or minutes This invention focuses on the fact that it can simultaneously solve problems (which take time to solve) and is based on an unprecedented idea.

[0015] The in-situ remediation system for organic fluorine compounds of the present invention is a system for remediating soil contaminated with organic fluorine compounds. or groundwaterThis system purifies contaminated water in situ and is equipped with a soil heating device and a decomposition treatment device. The soil heating device heats the soil by applying an electric current to three or more electrode wells constructed in the soil, and the decomposition treatment device decomposes the organic fluorine compounds contained in the contaminated water vapor generated by heating the soil heating device. The decomposition treatment device is a portable device installed on the ground, and decomposes the organic fluorine compounds by injecting ozone gas into the contaminated water that has been cooled from the contaminated water vapor and irradiating it with ultraviolet light.

[0016] The in-situ organic fluorine compound purification system of the present invention can also be configured to inject ozone gas into contaminated water adjusted to a predetermined temperature while irradiating it with ultraviolet light.

[0017] The in-situ purification system for organic fluorine compounds of the present invention can also be configured to decompose organic fluorine compounds by blowing ozone gas and hydrogen peroxide into contaminated water.

[0018] The in-situ remediation system for organofluorine compounds of the present invention can further include a groundwater pumping device. This groundwater pumping device pumps up contaminated groundwater (groundwater contaminated with organofluorine compounds) from a remediation well constructed in the soil. In this case, the decomposition treatment device treats the contaminated water containing cooled contaminated water vapor and contaminated groundwater.

[0019] The in-situ organic fluorine compound purification system of the present invention may further include a removal device that treats the treated water discharged from the decomposition treatment device with inorganic fluorine.

[0020] The in-situ remediation system for organic fluorine compounds of the present invention is a system for in-situ remediation of soil or groundwater contaminated with organic fluorine compounds, and is equipped with a soil heating device and an chemical injection device. The soil heating device heats the soil by applying an electric current to three or more electrode wells constructed in the soil, and the chemical injection device injects persulfate dissolved in room temperature water into the heated soil or groundwater.

[0021] The in-situ remediation method for organic fluorine compounds of the present invention is a method for remediating soil contaminated with organic fluorine compounds. or groundwaterThis method uses the in-situ organic fluorine compound remediation system of the present invention to purify the contaminated water in situ. The method includes a soil heating process and a decomposition process. In the soil heating process, the soil is heated by applying an electric current to three or more electrode wells constructed in the soil to pass through the soil. In the decomposition process, the organic fluorine compounds contained in the contaminated water vapor generated by heating in the soil heating process are decomposed. In the decomposition process, a portable decomposition treatment device installed on the ground is used to decompose the organic fluorine compounds by injecting ozone gas into the contaminated water that has been cooled from the contaminated water vapor and irradiating it with ultraviolet light.

[0022] The method for in-situ purification of organic fluorine compounds of the present invention can also be a method for decomposing organic fluorine compounds by blowing ozone gas and hydrogen peroxide into contaminated water.

[0023] The in-situ remediation method for organic fluorine compounds of the present invention can also be a method further comprising a groundwater pumping step, in which contaminated groundwater is pumped up from a remediation well constructed in the soil. In this case, the contaminated water containing the cooled contaminated water vapor and contaminated groundwater is treated in the decomposition treatment step.

[0024] The in-situ decontamination method for organic fluorine compounds of the present invention can also be a method in which treated water obtained after treatment in the decomposition treatment step (after decomposition of organic fluorine compounds) is reconstituted into soil.

[0025] The in-situ remediation method for organic fluorine compounds of the present invention is a method for purifying soil or groundwater contaminated with organic fluorine compounds in situ using the in-situ remediation system for organic fluorine compounds of the present invention, and is a method comprising a soil heating step and a chemical injection step. In the soil heating step, the soil is heated by applying an electric current to three or more electrode wells constructed in the soil to pass through the soil, and in the chemical injection step, persulfate dissolved in room temperature water is injected into the heated soil or groundwater to chemically decompose the organic fluorine compounds. [Effects of the Invention]

[0026] The in-situ organic fluorine compound remediation system and in-situ organic fluorine compound remediation method of the present invention have the following effects. (1) Heating the soil raises the groundwater temperature, which in turn generates steam, expands the volume of water, and reduces the viscosity of water, improving the movement of the water itself and increasing the solubility of organic fluorine compounds. As a result, by recovering the steam and groundwater, the organic fluorine compounds contained in them can also be efficiently recovered. (2) In the field of water treatment, PFOS and PFOA can be treated at large-scale treatment facilities such as tap water sources. However, in the field of soil and groundwater contamination, where contaminated sites are scattered, such concentrated measures are difficult. Therefore, the present invention uses a portable decomposition treatment device. In other words, by making the decomposition treatment device portable, it is possible to flexibly and efficiently perform purification treatment on scattered contaminated sites. Furthermore, by treating groundwater in a heated state, the treatment time can be shortened. (3) When injecting an aqueous solution of persulfate into soil or groundwater, there is no need to heat the agent itself (it dissolves at room temperature), which reduces losses due to self-decomposition of persulfate. Meanwhile, because reactivity increases in heated soil, the amount of agent used can be reduced compared to when the method is carried out at room temperature. Furthermore, it can also decompose organic fluorine compounds that do not decompose at room temperature. (4) The electric heating method does not require a high-temperature heat medium such as a heat conduction (heater), and there are no localized high-temperature areas underground, so polyvinyl chloride pipes (or heat-resistant rigid polyvinyl chloride pipes, depending on the heating temperature) can be used as the material for the chemical injection well. (5) Since an electric heating method is used, which uses Joule heat to heat the soil itself, it is easy to control the temperature, and therefore it is easy to raise the soil temperature to the desired temperature, and moreover, the entire soil can be heated uniformly. (6) Electric heating methods have better thermal efficiency than heating methods that use thermal conduction (heaters), and therefore can reduce the power consumption required for heating. (7) When heated treated water is condensed underground, heat loss due to pumping is reduced, and the temperature of the soil and groundwater can be maintained with less electricity. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a block diagram showing a schematic diagram of the main configuration of a physical treatment-type in-situ purification system for organic fluorine compounds. [Figure 2] FIG. 1 is a cross-sectional view showing a schematic diagram of a soil heating device that uses an electric heating method. [Figure 3] FIG. 1 is a plan view showing electrode wells arranged in a plane to form a triangle. [Figure 4] FIG. 1 is a block diagram showing a schematic diagram of the main configuration of a chemical treatment-type in-situ purification system for organic fluorine compounds. [Figure 5] 1 is a flow chart showing the flow of main steps in a method for in-situ remediation of organic fluorine compounds using a physical treatment-type in-situ remediation system for organic fluorine compounds. [Figure 6] 1 is a flow chart showing the flow of the main steps of an in-situ organic fluorine compound remediation method using a chemical treatment type in-situ organic fluorine compound remediation system. [Figure 7] This is a test result diagram showing the decomposition effect of a physical treatment type in-situ organic fluorine compound purification system. [Figure 8] Test results showing the decomposition effect of a chemical treatment-type in-situ organic fluorine compound purification system. DETAILED DESCRIPTION OF THE INVENTION

[0028] An example of an embodiment of the in-situ organic fluorine compound remediation system and in-situ organic fluorine compound remediation method of the present invention will be described with reference to the drawings. Note that the in-situ organic fluorine compound remediation method of the present invention is a method for purifying soil, etc., using the in-situ organic fluorine compound remediation system of the present invention. Therefore, the in-situ organic fluorine compound remediation system of the present invention will be described first, and then the in-situ organic fluorine compound remediation method of the present invention will be described.

[0029] 1. In-situ purification system for organic fluorine compounds The in-situ organic fluorine compound remediation system of the present invention purifies soil and groundwater contaminated with organic fluorine compounds such as PFOS and PFOA in situ, and is characterized by using an electrothermal heating method to heat the soil and then decompose the organic fluorine compounds. The in-situ organic fluorine compound remediation system of the present invention can be broadly divided into two types based on the method used to decompose the organic fluorine compounds: those that decompose organic fluorine compounds on the ground (hereinafter, for convenience, referred to as "physical treatment type in-situ organic fluorine compound remediation systems") and those that decompose organic fluorine compounds in the soil (hereinafter, for convenience, referred to as "chemical treatment type in-situ organic fluorine compound remediation systems"). Each embodiment will be described below in order.

[0030] (Physical treatment type in-situ purification system for organic fluorine compounds) 1 is a block diagram showing a schematic diagram of the main components of a physical treatment-type in-situ organic fluorine compound remediation system 100a. As shown in this figure, the physical treatment-type in-situ organic fluorine compound remediation system 100a includes a soil heating device (electrode well 111 and power supply device 112) and a decomposition treatment device 120, and can also include a gas suction device 130, a groundwater pumping device 140, and the like. Each of the main components that make up the physical treatment-type in-situ organic fluorine compound remediation system 100a will be described in detail below.

[0031] A soil heating device heats the soil in a target area to a predetermined temperature and maintains that temperature. While other known soil heating methods, such as those using thermal conduction (heaters), are used in the present invention, an electric heating method, a type of electrical resistance heating, is preferred. This electric heating method allows for uniform heating by generating heat from the soil itself, and temperature control is easy. Clay layers, which allow for easy electrical current flow (low electrical resistance), tend to heat up more easily. Furthermore, its superior thermal efficiency reduces the amount of power consumed to heat the soil compared to thermal conduction (heaters).

[0032] Figure 2 is a cross-sectional view of a soil heating device 110 that uses electrical heating. As shown in this figure, the soil heating device 110 includes electrode wells 111 constructed within the soil of interest and a power supply 112 installed above ground. The electrode wells 111 are constructed with steel casings. The power supply 112 applies a three-phase AC voltage to the casings of each electrode well 111, causing current to flow between the electrode wells 111. This generates Joule heat in the soil between the wells, thereby heating the soil. Therefore, it is recommended that the electrode wells 111 be installed in three or more locations surrounding the soil of interest. For example, they can be arranged to form multiple triangles, as shown in Figure 3. While the four triangles shown in Figure 3 are each equilateral triangles with sides of approximately 3.5 m, the electrode wells 111 can be arranged in various shapes.

[0033] Even if soil temperature is raised to, for example, 40-90°C using electrical heating or other methods, PFOS, PFOA, and other contaminants do not volatilize, let alone decompose. Therefore, conventional common knowledge does not lead to the technical concept of heating soil to purify PFOS, PFOA, and other contaminants. However, the inventors of the present invention came up with the idea of ​​recovering PFOS, PFOA, and other contaminants together with water vapor and groundwater, and came up with the technical concept of heating soil to recover groundwater. PFOS, PFOA, and other contaminants contained in soil pore water are virtually immobile at normal temperatures. However, as soil temperature increases, water vapor is generated, the volume of water expands, and the viscosity of water decreases, improving the mobility of the groundwater (pore water). By recovering this groundwater, PFOS, PFOA, and other contaminants contained in the soil can be efficiently recovered. Furthermore, the solubility of PFOS, PFOA, and other contaminants also increases with increasing temperature, allowing for efficient recovery of these contaminants. Furthermore, even when PFOS and PFOA are present in their original liquid form, their fluidity increases, enabling efficient recovery.

[0034] As mentioned above, the electric heating method allows for easy temperature control, making it possible to raise the soil temperature to a desired temperature and maintain it at that desired temperature. The temperature at which the soil is raised or maintained (hereinafter referred to as the "planned temperature") should be determined in advance based on the type of soil, the normal soil temperature, and other factors. After conducting various tests, the inventors of the present invention found that PFOS, PFOA, and other compounds can be efficiently recovered by setting the planned temperature in the range of 40 to 90°C.

[0035] To fully recover PFOS, PFOA, and the like, the soil is heated using a soil heating device 110 at a planned temperature for a planned period (e.g., several weeks to several months). However, it is quite conceivable that the actual temperature of the heated soil or groundwater will differ from the predicted (or analyzed) temperature. Therefore, it is possible to apply a voltage to the electrode wells 111 while observing the actual temperature. In this case, as shown in FIG. 3, it is preferable to construct temperature observation wells 160 between each electrode well 111 and use these temperature observation wells 160 to observe the temperature of the soil or groundwater. Note that these temperature observation wells 160 can also be configured with temperature measurement sections installed at different depths (e.g., every 1 meter), which is advantageous in that it allows the depth distribution of temperature to be ascertained.

[0036] The water vapor containing PFOS, PFOA, and other contaminants (hereinafter referred to as "contaminated water vapor" for convenience) generated by the soil heating device 110 is collected through a well. For example, as shown in Figure 1, the contaminated water vapor can be collected through a countermeasure well CW constructed in the contaminated target area (hereinafter simply referred to as the "target area"), or the contaminated water vapor can be collected using an electrode well 111 instead of the countermeasure well CW. Of course, the contaminated water vapor can also be collected using both the countermeasure well CW and the electrode well 111. Note that while the soil is heated by the soil heating device 110, the electrical heating method used in the present invention does not require a high-temperature heat medium like thermal conduction (heater). Therefore, relatively inexpensive and easily procurable polyvinyl chloride pipe (or heat-resistant rigid polyvinyl chloride pipe, depending on the heating temperature) can be used as the countermeasure well CW.

[0037] When collecting contaminated water vapor, it is possible to collect the water vapor that naturally rises through a ventilation pipe, or to collect the water vapor by sucking it up using a gas suction device 130. This gas suction device 130 can utilize various conventional vacuum pumps, etc., and is preferably placed on the ground as shown in Figure 1, and sucks up the contaminated water vapor through the ventilation pipe.

[0038] The physical treatment-type in-situ organic fluorine compound remediation system 100a can also be configured with a groundwater pumping device 140 in addition to the gas suction device 130. This groundwater pumping device 140 pumps (pumps) the groundwater containing PFOS, PFOA, and other contaminants (hereinafter referred to as "contaminated groundwater" for convenience) heated by the soil heating device 110 through a control well CW constructed within the soil. The groundwater pumping device 140 can be a ground-mounted self-priming pump, or, if the pump head is insufficient, a submersible pump installed within the control well CW, as shown in Figure 1. However, because a three-phase AC voltage is applied to the soil to heat it, if a submersible pump is used, it is recommended to avoid motor-driven pumps, which are prone to malfunction due to three-phase AC voltage, and instead use a compressed air-powered or bladder-type pump.

[0039] The contaminated water vapor collected above ground (or the contaminated water vapor and contaminated groundwater if a groundwater pumping device 140 is installed) is sent to a decomposition treatment device 120 installed above ground, where PFOS, PFOA, and other substances contained in the contaminated water vapor are decomposed by this decomposition treatment device 120. It is advisable to use a portable decomposition treatment device 120. As mentioned above, in the field of water treatment, centralized treatment can be carried out using large-scale treatment facilities, but in the field of soil and groundwater contamination, where contaminated sites are scattered, such centralized measures are difficult. If the decomposition treatment device 120 is portable, it can be easily transported and installed (i.e., repurposed), allowing for flexible and efficient purification treatment of scattered contaminated sites.

[0040] To decompose PFOS, PFOA, and other contaminants in the decomposition treatment device 120, the recovered contaminated water vapor (or contaminated water vapor and contaminated groundwater) is liquefied by natural or forced cooling, and the resulting liquid (hereinafter referred to as "contaminated water" for convenience) is then chemically treated. Specifically, ozone gas is injected into the contaminated water stored in a tank or the like, and ultraviolet (UV) light is irradiated. While PFOS, PFOA, and other contaminants can be decomposed by exposing room-temperature contaminated water to ozone gas and UV light, the inventors of the present invention have found that a medium-high temperature (40-50°C) contaminated water is more effective. Therefore, it is recommended to set a predetermined medium-high temperature in advance, heat cooled contaminated water to that medium-high temperature, or cool recovered contaminated water vapor or contaminated groundwater to that medium-high temperature, and then inject ozone gas and irradiate UV light.

[0041] The decomposition treatment device 120 can also perform chemical treatment using ozone gas and hydrogen peroxide instead of chemical treatment using ozone gas and ultraviolet light. In this case, the collected contaminated water vapor or contaminated groundwater is liquefied by natural or forced cooling, and ozone gas and hydrogen peroxide are then blown into the contaminated water.

[0042] As shown in Figure 1, the treated water in which PFOS, PFOA, etc. have been decomposed by the decomposition treatment device 120 can be discharged outside the target area (for example, a public water body), or it can be condensed and circulated within the target area through the control well CW or electrode well 111. In this case, depending on the conditions of the contaminated site, it is preferable to treat the treated water with an inorganic fluorine treatment device (hereinafter referred to as a "fluorine removal device") and then discharge the treated water outside the target area, or circulate the treated water within the target area. In this case, the fluorine removal device is naturally installed downstream of the decomposition treatment device 120.

[0043] (Chemical treatment type in-situ purification system for organic fluorine compounds) 4 is a block diagram showing the main components of a chemical treatment-type in-situ organic fluorine compound remediation system 100b. As shown in this figure, the chemical treatment-type in-situ organic fluorine compound remediation system 100b includes a soil heating device 110 (electrode well 111 and power supply device 112) and an agent injection device 150.

[0044] The chemical treatment type in-situ organic fluorine compound remediation system 100b heats the soil using the soil heating device 110 described above, and decomposes PFOS, PFOA, etc. by injecting a chemical into the soil or groundwater that has been heated to a predetermined temperature (for example, 40 to 90°C). For example, when persulfate is injected as a chemical into the heated soil or groundwater, thermally activated persulfate is produced through various reactions, such as sulfate radicals in a thermally activated state, and this thermally activated persulfate promotes the decomposition of PFOS, PFOA, etc.

[0045] As shown in Figure 4, chemical injection device 150 injects chemicals into soil or groundwater and is composed of a chemical adjustment tank and a pressure-transfer means (such as a pump) for pumping the chemical from the adjustment tank. The chemical is a solution of persulfate in water at room temperature (15 to 25°C) (persulfate aqueous solution). Examples of persulfate used here include sodium persulfate, potassium persulfate, and ammonium persulfate. The reason for using room-temperature water to dissolve persulfate is that persulfate does not undergo reactions such as thermal activation at room temperature, so dissolving it in room-temperature water prevents the generation of thermally activated persulfate or autolysis before it reaches the soil or groundwater. In other words, dissolving it in room-temperature water allows it to be delivered to the soil or groundwater in its persulfate form.

[0046] The chemicals pumped by the chemical injection device 150 can be injected into the soil or groundwater through a well. For example, as shown in Figure 4, chemicals can also be injected through a countermeasure well (CW) constructed within the target area. While the soil heating device 110 heats the soil, the electrical heating method used in the present invention does not require a high-temperature heat medium like thermal conduction (heater). Therefore, relatively inexpensive and easily procured polyvinyl chloride pipes (or heat-resistant hard polyvinyl chloride pipes, depending on the heating temperature) can be used as the countermeasure well (CW). Therefore, the conventional "multipoint injection method using injection pipes made of polyvinyl chloride pipes (or heat-resistant hard polyvinyl chloride pipes, depending on the heating temperature)" can also be adopted. In this case, there is no need to use steel materials for the countermeasure well (CW), eliminating concerns about metal corrosion due to persulfates.

[0047] Even after chemical injection, the soil temperature is maintained continuously by the soil heating device 110. As explained in the physical treatment type in-situ organic fluorine compound remediation system 100a, the temperature to which the soil is heated and the planned temperature to be maintained can be determined in advance based on the type of soil, the normal soil temperature, and other factors. The inventors of the present invention conducted various tests and found that setting the planned temperature between 40 and 90°C effectively generates thermally activated persulfate, which can efficiently decompose PFOS, PFOA, and other compounds. In this case, too, it is advisable to construct temperature observation wells 160 between each electrode well 111, as shown in Figure 3, and use these temperature observation wells 160 to monitor the temperature of the soil and groundwater.

[0048] By continuously applying the agent to the soil, the soil and groundwater are maintained at the planned temperature (e.g., 40-90°C), and the agent that reaches the soil or groundwater also raises its temperature. As a result, thermally activated persulfates are produced through various reactions, such as sulfate radicals in a thermally activated state, and these thermally activated persulfates promote the decomposition of PFOS, PFOA, etc.

[0049] 2. In-situ purification method for organic fluorine compounds Next, the in-situ organic fluorine compound remediation method of the present invention will be described with reference to the drawings. The in-situ organic fluorine compound remediation method of the present invention is a method for remediating soil, etc., using the in-situ organic fluorine compound remediation system described above. Therefore, we will avoid overlapping explanations with those described for the in-situ organic fluorine compound remediation system and will only describe the details unique to the in-situ organic fluorine compound remediation method of the present invention. In other words, details not described here are the same as those described in "1. In-situ Organic Fluorine Compound Remediation System." The in-situ organic fluorine compound remediation method of the present invention can be broadly divided into methods using a physical treatment-type in-situ organic fluorine compound remediation system 100a and methods using a chemical treatment-type in-situ organic fluorine compound remediation system 100b. Furthermore, the physical treatment and chemical treatment types can also be used together. Below, we will sequentially describe embodiments of both a physical treatment-type in-situ organic fluorine compound remediation system and a chemical treatment-type in-situ organic fluorine compound remediation system.

[0050] (Method using a physical treatment type in-situ purification system for organic fluorine compounds) Figure 5 is a flow diagram showing the main steps of the in-situ organic fluorine compound remediation method using a physical treatment-type in-situ organic fluorine compound remediation system 100a. As shown in this figure, first, a control well CW and an electrode well 111 are constructed at appropriate locations within the target area (Step 11 in Figure 5). This control well CW can be excavated using various conventional construction methods, and various materials, including polyvinyl chloride pipe (heat-resistant rigid polyvinyl chloride pipe), can be used.

[0051] Like the countermeasure well CW, the electrode well 111 can be excavated using a variety of conventional construction methods. The electrode wells 111 should preferably be placed in three or more locations surrounding the soil of the target area (for example, forming multiple triangles as shown in Figure 3). After the countermeasure well CW and the electrode well 111 are constructed, a portable decomposition treatment device 120 is installed on the ground, and a temperature observation well 160 is constructed as needed.

[0052] After the countermeasure well CW and the electrode well 111 are constructed and the decomposition treatment device 120 is installed, a three-phase AC voltage is applied to the casing of each electrode well 111 by the power supply device 112, causing the soil to heat up (Step 12 in FIG. 5). Installation If so, it is advisable to use a temperature observation well 160 to observe the temperature of the soil and groundwater while raising the temperature of the soil.

[0053] When the soil is heated, the contaminated water vapor generated as a result of the heating is collected by the gas suction device 130, and if a groundwater pumping device 140 is provided, the heated contaminated groundwater is collected by the groundwater pumping device 140 (Step 13 in FIG. 5). The collected contaminated water vapor and contaminated groundwater are then cooled to become contaminated water (Step 14 in FIG. 5), and are then chemically treated by the decomposition treatment device 120 to decompose PFOS, PFOA, and the like (Step 15 in FIG. 5). This decomposition treatment device 120 can perform chemical treatment using ozone gas and ultraviolet light, or chemical treatment using ozone gas and hydrogen peroxide. When chemical treatment using ozone gas and ultraviolet light is performed, it is recommended that ozone gas be blown into the contaminated water while ultraviolet light is irradiated onto the water at a predetermined medium-high temperature.

[0054] The treated water obtained after treatment by the decomposition treatment device 120 in the decomposition treatment step (after decomposition of the organic fluorine compounds) is condensed and circulated within the target area via the control well CW or electrode well 111 (Step 16 in Figure 5). Alternatively, the treated water may be discharged outside the target area (for example, to a public water body). In this case, depending on the conditions of the contaminated site, it may be preferable to treat the treated water in a fluoride removal device and then circulate the treated water within the target area, or to discharge the treated water outside the target area after making it comply with fluoride effluent standards. Condensing heated treated water underground is advantageous because it reduces heat loss and allows the temperature of the soil and groundwater to be maintained with less power.

[0055] The soil is heated by the soil heating device 110 at the planned temperature for a planned period (for example, several weeks to several months), DecompositionOnce the PFOS, PFOA, etc. in the contaminated water have been decomposed by the treatment device 120, the application of the three-phase AC voltage by the power supply device 112 is stopped (Step 17 in FIG. 5). Note that before stopping the application of the voltage by the power supply device 112, it is advisable to confirm by inspection or the like that the organic fluorine compounds in the target range have been sufficiently purified.

[0056] (Method using a chemical treatment type in-situ purification system for organic fluorine compounds) Figure 6 is a flow diagram showing the main steps of an in-situ organic fluorine compound remediation method using a chemical treatment-type in-situ organic fluorine compound remediation system 100b. As shown in this figure, first, a control well CW and an electrode well 111 are constructed at appropriate locations within the target area (Step 21 in Figure 6). This control well CW can be excavated using various conventional construction methods, and various materials, including polyvinyl chloride pipe (heat-resistant rigid polyvinyl chloride pipe), can be used.

[0057] Like the countermeasure well CW, the electrode well 111 can be excavated using various conventional construction methods. The electrode wells 111 should be placed in three or more locations surrounding the soil of the target area (for example, forming multiple triangles as shown in Figure 3). After the countermeasure well CW and the electrode well 111 are constructed, a chemical injection device 150 is installed on the ground, and a temperature observation well 160 is constructed as needed.

[0058] Once the countermeasure wells CW and electrode wells 111 are constructed and the chemical injection device 150 is installed, a three-phase AC voltage is applied to the casing of each electrode well 111 by the power supply device 112, raising the temperature of the soil (Step 22 in Figure 6). If a temperature observation well 160 is installed, it is advisable to use the temperature observation well 160 to observe the temperature of the soil and groundwater (Step 24 in Figure 6) while raising the temperature of the soil.

[0059] Once the soil has been heated by the soil heating device 110, the chemical is injected into the soil and groundwater by the chemical injection device 150 (Step 23 in FIG. 6). Note that even after the chemical injection, the soil temperature is maintained continuously by the soil heating device 110.

[0060] The soil is heated by the soil heating device 110 at the planned temperature for a planned period (for example, several weeks to several months), and the chemical is injected by the chemical injection device 150. Once the PFOS, PFOA, etc. contained in the soil and groundwater have been decomposed, the application of the three-phase AC voltage by the power supply device 112 is stopped (Step 24 in FIG. 6). Note that before stopping the application by the power supply device 112, it is advisable to confirm by inspection or the like that the organofluorine compounds within the target area have been sufficiently purified.

[0061] (Test results) Figure 7 shows the test results confirming the decomposition effect of the physical treatment-type in-situ organic fluorine compound remediation system 100a. As shown in this figure, it was confirmed that PFOS and PFOA were decomposed in both cases of chemical treatment using ozone gas and hydrogen peroxide and chemical treatment using ozone gas and ultraviolet light for contaminated water containing PFOS and PFOA. It was also found that the effect was further improved by extending the treatment time and increasing the amount of sodium persulfate added.

[0062] Figure 8 shows the test results confirming the decomposition effect of the chemical treatment type in-situ organic fluorine compound remediation system 100b. As shown in this figure, it was confirmed that PFOS and PFOA were decomposed when sodium persulfate was injected into contaminated water at medium to high temperatures (40°C, 60°C). Although not shown in Figure 8, it was also found that the effect was further improved by increasing the soil temperature to 60°C and injecting sodium persulfate multiple times (with injection periods spaced about two weeks apart). [Industrial Applicability]

[0063] The in-situ organic fluorine compound remediation system and in-situ organic fluorine compound remediation method of the present invention can be used at operational sites (or former operational sites) where organic fluorine compounds such as PFOS and PFOA are manufactured, used, discharged, or by-produced, as well as illegal dumping sites, etc. 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 make a great contribution to society. [Explanation of symbols]

[0064] 100a Physical treatment type in-situ purification system for organic fluorine compounds of the present invention 100b Chemical treatment type in-situ purification system for organic fluorine compounds of the present invention 110 Soil heating device (for in-situ purification of organic fluorine compounds) 111 Electrode well (for soil heating devices) 112 Power supply (for soil heating devices) 120 Decomposition treatment equipment (for physical treatment type in-situ purification systems for organic fluorine compounds) 130 Gas suction device (for physical treatment type in-situ purification system for organic fluorine compounds) 140 Groundwater pumping equipment (for physical treatment type in-situ purification systems for organic fluorine compounds) 150 Chemical injection device (for in-situ chemical treatment of organic fluorine compounds) 160 Temperature monitoring well (for in-situ purification of organic fluorine compounds) CW Control Well

Claims

1. A system for in-situ remediation of soil or groundwater contaminated with organofluorine compounds, comprising: a soil heating device that heats the soil by applying current to three or more electrode wells constructed in the soil and passing it through the soil; a chemical injection device that injects an aqueous solution of persulfate into soil or groundwater, The chemical injection device injects persulfate dissolved in room temperature water into heated soil or groundwater. An in-situ purification system for organic fluorine compounds.

2. 1. A method for in-situ remediation of soil or groundwater contaminated with organofluorine compounds, comprising: a soil heating step of applying an electric current to three or more electrode wells constructed in the soil to heat the soil and groundwater; and an agent injection step of injecting an aqueous solution of persulfate into soil or groundwater, In the chemical injection step, persulfate dissolved in room temperature water is injected into heated soil or groundwater.

1. A method for in-situ purification of organic fluorine compounds.

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