Method for treating water containing organofluorine compounds

Abstract

To provide a treatment method of organofluorine compound-containing water, the method capable of using cavitation to decomposition processing of a persistent organofluorine compound contained environmental water, the cavitation being used in decomposition processing of an organic compound.SOLUTION: A treatment method generates cavitation by injecting water added with bubbles of less than 1 μm of inert gas from an injection nozzle 2 to water that contains an organofluorine compound and decomposes the organofluorine compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a method for treating water containing organofluorine compounds. [Background technology]

[0002] In recent years, attention has been focused on the pollution of environmental water and other substances by perfluoroalkyl compounds and polyfluoroalkyl compounds (hereinafter referred to as "PFAS"), which are organofluorine compounds. In particular, perfluorooctanesulfonic acid (sometimes referred to as "PFOS" in this specification) and perfluorooctanoic acid (sometimes referred to as "PFOA" in this specification) are designated as Class I Specified Chemical Substances under the "Act on Examination and Regulation of Manufacture, etc., of Chemical Substances," based on their inclusion in Annexes A and B of the Stockholm Convention on Persistent Organic Pollutants (POPs Convention), and their manufacture, import, and use are prohibited in principle.

[0003] However, because these compounds possess water-repellent and oil-repellent properties, as well as excellent chemical and thermal stability, they have been widely used for many years as water repellents, coatings, and foam extinguishing agents. As a result, surveys by the Ministry of the Environment have also detected PFOS / PFOA in a wide range of sources, including river water and groundwater. In 2020, PFOS / PFOA were added to the water quality management target setting items of the drinking water quality standards and the items requiring monitoring of the water quality environmental standards, respectively, with a target value and guideline value (provisional) of 50 ng / L (combined value of PFOS and PFOA) set.

[0004] PFOS / PFOA are chemically extremely stable, water-soluble, and non-volatile substances. Therefore, when released into the environment, they readily migrate into water systems and are thought to persist in the environment for long periods due to their poor biodegradability. Furthermore, given the widespread detection of PFOS / PFOA in river water, groundwater, and other sources, there has been a need for the development of low-cost methods to decompose PFAS such as PFOS / PFOA contained in environmental water (sometimes referred to as "environmental water" in this specification), such as river water, lake water, and groundwater. However, there have been no effective methods available.

[0005] Specifically, accelerated oxidation is widely used for the decomposition of organic compounds. However, accelerated oxidation is considered unsuitable for the decomposition of PFOS / PFOA because of its extremely low reactivity with PFOS / PFOA's OH radicals. In fact, accelerated oxidation (O3+H2O2, O3+UV) was attempted using groundwater containing 2-6 ng / L of PFOS and 40-58 ng / L of PFOA, but no significant decomposition effect on PFOS / PFOA could be confirmed.

[0006] On the other hand, methods utilizing cavitation have been proposed as a decomposition treatment for organic compounds other than accelerated oxidation (see, for example, Patent Documents 1-3).

[0007] Here, cavitation refers to a physical phenomenon in which a liquid boils when the pressure drops to the liquid's saturated vapor pressure due to a pressure difference in the flow of the liquid, resulting in the rapid generation of tiny bubbles that then disappear when the velocity decreases (when the pressure recovers).

[0008] Cavitation generates a strong shock wave at the gas-liquid interface when the bubbles created by cavitation collapse. This can cause noise and vibration in pumps, for example, and lead to erosion of the impeller surface. In the decomposition of organic compounds, the shock wave causes thermal decomposition of the organic compounds.

[0009] Furthermore, regarding the decomposition of PFOS / PFOA using cavitation, it has been reported that PFOS / PFOA in pure water can be thermally decomposed by cavitation generated by ultrasonic irradiation (see Non-Patent Document 1). [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2006-253 [Patent Document 2] Japanese Patent Publication No. 2006-305546 [Patent Document 3] Japanese Patent Publication No. 2017-192914 [Non-patent literature]

[0011] [Non-Patent Document 1] Hiroshi Moriwaki, Youichi Takagi, Masanobu Tanaka, Kenshiro Tsuruho, Kenji Okitsu, and Yasuaki Maeda: Sonochemical Decomposition of Perfluorooctane Sulfonate and Perfluorooctanoic Acid, Environ. Sci. Technol., Vol.39, No.9, pp.3388-3392, 2005. [Overview of the project] [Problems that the invention aims to solve]

[0012] As mentioned above, cavitation is used to decompose organic compounds, but there was no method available to decompose persistent organofluorine compounds contained in environmental water.

[0013] The present invention aims to provide a method for treating water containing organic fluorine compounds, which allows cavitation, a method used for the decomposition of organic compounds, to be used for the decomposition of persistent organic fluorine compounds contained in environmental water. [Means for solving the problem]

[0014] To achieve the above object, the method for treating water containing an organic fluorine compound of the present invention is a method for treating water containing an organic fluorine compound, which decomposes the organic fluorine compound contained in the water by utilizing cavitation. The method is characterized in that water added with bubbles of an inert gas having a size of less than 1 μm is injected from an injection nozzle into the water containing the organic fluorine compound to cause cavitation and decompose the organic fluorine compound.

[0015] In this case, bubbles of an inert gas having a size of less than 1 μm can be added to the water containing the organic fluorine compound, and the water can be injected from an injection nozzle.

[0016] Further, the method for treating water containing an organic fluorine compound of the present invention can be used for decomposing a hardly decomposable organic fluorine compound contained in environmental water.

Advantages of the Invention

[0017] Since the method for treating water containing an organic fluorine compound of the present invention causes cavitation by injecting water added with bubbles of an inert gas having a size of less than 1 μm from an injection nozzle into the water containing the organic fluorine compound to decompose the organic fluorine compound, it is possible to treat a large amount of water containing an organic fluorine compound with a simple mechanism. And this method for treating water containing an organic fluorine compound can be used for decomposing a hardly decomposable organic fluorine compound contained in environmental water.

Brief Description of the Drawings

[0018] [Figure 1] It is an explanatory diagram showing a test facility for implementing the method for treating water containing an organic fluorine compound of the present invention. [Figure 2] It is an FFT analysis screen. [Figure 3] It is a graph showing the relationship between the outlet pressure and the acceleration decibel value. [Figure 4] It is a graph showing the relationship between the cavitation number and the acceleration decibel value. [Figure 5]This graph shows the results of the decomposition treatment of sample water A. [Figure 6] This graph shows the results of the decomposition treatment of sample water B. [Figure 7] This graph shows the results of the decomposition treatment of sample water C1. [Figure 8] This graph shows the results of the decomposition treatment of sample water C2. [Figure 9] This graph shows the results of the decomposition treatment of sample water C2 (PFOS). [Figure 10] This graph shows the results of the decomposition treatment of sample water C2 (PFOAs). [Modes for carrying out the invention]

[0019] Hereinafter, embodiments of the method for treating water containing organofluorine compounds according to the present invention will be described with reference to the drawings.

[0020] The present invention relates to a method for treating water containing organic fluorine compounds, which involves decomposing organic fluorine compounds contained in water such as environmental water using cavitation. The method is characterized by inducing cavitation and decomposing the organic fluorine compounds by spraying water to which inert gas bubbles of less than 1 μm are added from an injection nozzle into the water containing the organic fluorine compounds. In this context, in addition to nitrogen gas, noble gases such as argon can be used as the inert gas. Furthermore, examples of organofluorine compounds that can be decomposed include perfluoroalkyl compounds and polyfluoroalkyl compounds, particularly perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA).

[0021] [Cavitation by high-speed water jets underwater] [Testing Equipment] Figure 1 shows a test facility for implementing the method for treating water containing organic fluorine compounds according to the present invention. In this test facility, rectangular (cylindrical) and cylindrical jet boxes 1 (manufactured by YBM Co., Ltd.) were used as cavitation generators. A high-pressure plunger pump 3 (7.5kW, manufactured by Super Industries) was used to spray sample water from the water tank 4 upwards into the inside of the jet box 1, which was filled with sample water, from the injection nozzle 2 (CP-type straight nozzle, manufactured by Ikeuchi Co., Ltd.) at the inlet (lower end) of the jet box 1. The overflowed sample water was returned to the water tank 4 and circulated. The circulating sample water was cooled in a cooling coil 5, and the water temperature was maintained in the range of 45-48°C. In tank 4, a foam jet (FJP-3; 50 L / min small test machine) manufactured by YBM Corporation was installed as a bubble generator (UFB generator) 6 that generates bubbles smaller than 1 μm (ultrafine bubbles (hereinafter sometimes referred to as "UFB")), and inert gas UFBs (nitrogen UFBs, argon UFBs) that can serve as nuclei for cavitation were generated. Here, the strength of cavitation generated within the jet box 1 was evaluated by measuring vibrations propagating on the surface of the jet box 1 with an acceleration pickup 71 (PV-90B manufactured by Rion Corporation) and noise with a precision sound level meter 72 (NL-52 manufactured by Rion Corporation), and then performing FFT analysis using an FFT analyzer 73 (EZA-2 manufactured by Omega Wave Corporation).

[0022] [Optimization of cavitation generation] We confirmed that the strength of cavitation generation is significantly affected by changes in outlet pressure, in addition to injection conditions such as the diameter and discharge pressure of the injection nozzle 2 of the jet box 1. In particular, in the FFT analysis of the acceleration detection signal, the decibel value in the 25kHz to 35kHz band (ultrasonic range) fluctuated significantly in response to changes in outlet pressure. An example of the FFT analysis screen is shown in Figure 2. Using the cavitation number defined by the dimensionless number shown in equation (1), the cavitation strength The optimal injection conditions that maximize the performance were determined by finding the conditions that show the maximum acceleration decibel value in the 25kHz to 35kHz range. Cavitation count σ =(P2-P ν ) / ((1 / 2)×ρU2) =(P2-P ν ) / (P1-P2) ···(1) Here, P ν : saturated vapor pressure, ρ: density, U: jet discharge velocity, P1: discharge pressure, P2: outlet pressure. Figure 3 shows the acceleration decibel values ​​in the 25kHz-35kHz range for outlet pressure at a nozzle diameter of φ1.6mm, and Figure 4 shows the acceleration decibel values ​​in the 25kHz-35kHz range for cavitation number σ at a nozzle diameter of φ1.6mm. The higher the discharge pressure, the greater the acceleration decibel value, and it was confirmed that the acceleration decibel value peaked at a cavitation number of around 0.06 to 0.07, corresponding to each discharge pressure. Therefore, in decomposition treatment tests utilizing cavitation generated by a high-speed water jet underwater, using actual groundwater and river water containing PFOS / PFOA as the treatment target (sample water), the outlet pressure was optimized according to the nozzle type, nozzle diameter, and discharge pressure, and the tests were conducted under injection conditions that maximized the cavitation strength. In the following tests, a straight nozzle was used for the injection nozzle 2. However, an ejector nozzle can also be used, for example (in this case, the water to be treated is used as the driving fluid, and an inert gas that can act as a nucleus for cavitation generation is used as the intake fluid, so that water containing fine bubbles of the inert gas is discharged from the outlet).

[0023] [Decomposition treatment test using high-speed water jet cavitation in water] [Sample water to be treated] For the decomposition treatment test using high-speed water jet cavitation in water, actual groundwater containing PFASs such as PFOS / PFOA (sample water A) and river water (sample water B, sample water C) were used as sample water. The pH, TOC, iron concentration, manganese concentration, and concentrations of PFOS / PFOA etc. of the sample water are shown in Table 1.

[0024] [Table 1]

[0025] Analysis of PFOS / PFOA in sample water and treated water was performed by solid-phase extraction-LC / MS according to the methods specified in Appendix 1 of the Ministry of the Environment notification and JIS K 0450-70-10. Using mixed standard solutions, PFOS compounds were quantified as C4-C10 (PFBS-PFDS, excluding C9:PFNS; C5:PFPeS was semi-quantified using C6:PFHxS standard), and PFOA compounds were quantified as C4-C14 (PFBA-PFTeDA) congeners in both linear and linear + branched isomer forms. However, this specification only shows the results for linear compounds. pH was measured according to JIS K 0102-12.1, TOC according to JIS K 0102-22.1, iron according to JIS K 0102-57.4, and manganese according to JIS K 0102-56.4. Sample water A was a groundwater sample with a combined PFOS / PFOA level of 58 ng / L, slightly exceeding 50 ng / L. However, it contained C4-C6 congeners for PFOS and C4-C9 congeners for PFOA. Sample water B was river water with high concentrations of PFOA at 3900 ng / L and C6 PFHxA at 21000 ng / L, and mainly contained PFOA compounds along with C4-C12 congeners. Sample water C (C1 and C2 were collected on the same day but measured at different times) was river water different from sample water B. The combined PFOS / PFOA levels were 640-650 ng / L, PFHxS in C6 was 1600-1700 ng / L, and PFHxA was 630-670 ng / L. The PFOS group contained congeners C4-C7, and the PFOA group contained congeners C4-C11. In addition, the iron and manganese concentrations were higher compared to the other samples.

[0026] [Processing conditions] Table 2 shows the processing conditions for the decomposition treatment test using high-speed water jet cavitation in water.

[0027] [Table 2]

[0028] The volume of water to be treated was fixed at 30L. In the cases of sample water A, B, and C1 using a rectangular jet box, batch processing was performed with a processing time of 180 minutes (3 hours), and the treated water was collected and analyzed. Here, nitrogen was used as the gas type for the UFB as the basis, but in the cases of sample water B and C1, cases using argon were also performed for comparison to investigate its effect on the treatment. In the case of sample water C2, which was treated with a cylindrical jet box, batch treatment was performed for a treatment time of 360 minutes (6 hours). To confirm the treatment effect over time, the minimum necessary amount of water was collected and analyzed as analytical samples at 1.5 hours, 3 hours, 5 hours, and 6 hours.

[0029] [Disassembly and Treatment Test Results] [Rectangular jet box] Figure 5 shows the results of the decomposition treatment test of sample water A (groundwater). Cavitation-based decomposition treatment (nozzle diameter: 1.8 mm, discharge pressure: 10 MPa) reduced the concentrations of PFOS from 5 to <1 ng / L (reduction rate of over 80%) and PFOA from 53 to 17 ng / L (reduction rate of 67.9%), resulting in a combined PFOS / PFOA concentration of 58 to 17 ng / L (reduction rate of 70.7%). The concentrations of other congeners (PFOS: C4-C6, PFOA: C4-C9) were also generally reduced. Sample water A contained 2.8 mg / L of manganese, but no discoloration or other abnormalities were observed in the treated water.

[0030] Figure 6 shows the results of the decomposition treatment test for sample water B (river water). Through decomposition treatment by cavitation (nozzle diameter: 2.0 mm, discharge pressure: 12 MPa), in the case of nitrogen UFB, the concentrations of PFOS decreased from 8 to <1 ng / L (reduction rate of 87.5% or more) and PFOA decreased from 3900 to 2200 ng / L (reduction rate of 43.6%), resulting in a combined PFOS / PFOA value of 3908 to 2200 ng / L (reduction rate of 43.7%). Other congeners besides PFOS / PFOA (PFOS group: C4~C6, PFOA group: C4~ The concentration of C12) was generally reduced by decomposition treatment by cavitation, excluding PFOA compounds C4-C6. In the argon UFB case, the concentrations of PFOS decreased from 8 to 2 ng / L (75% reduction) and PFOA decreased from 3900 to 2500 ng / L (35.9% reduction). However, comparing the results of the nitrogen UFB case and the argon UFB case, the nitrogen UFB case showed a slightly higher decomposition effect due to cavitation. Furthermore, no discoloration or other abnormalities were observed in the treated water in either case.

[0031] Figure 7 shows the results of the decomposition treatment test of sample water C1 (river water). Through cavitation decomposition treatment (nozzle diameter: 2.0 mm, discharge pressure: 12 MPa), in the case of nitrogen UFB, the concentrations of PFOS decreased from 400 to 58 ng / L (reduction rate 85.5%) and PFOA decreased from 240 to 170 ng / L (reduction rate 29.2%), resulting in a combined PFOS / PFOA concentration of 640 to 228 ng / L (reduction rate 64.4%). For congeners other than PFOS / PFOA (PFOS group: C4~C7, PFOA group: C4~C11), the concentrations were generally reduced by cavitation decomposition treatment, with the exception of PFOA group C7. In the argon UFB case, the concentrations of PFOS decreased from 400 to 100 ng / L (75% reduction) and PFOA decreased from 240 to 170 ng / L (29.2% reduction). However, comparing the results of the nitrogen UFB case and the argon UFB case, similar to the case of sample water B, the nitrogen UFB case showed a slightly higher decomposition effect due to cavitation. Furthermore, sample water C1 contained 2.5 mg / L of iron and 17 mg / L of manganese, but no particular changes such as discoloration were observed in the treated water in either case.

[0032] [Cylindrical jet box] Figures 8 to 10 show the results of the decomposition treatment test of sample water C2 (river water). Cavitation decomposition treatment using a cylindrical jet box (nozzle diameter: 2.0 mm, discharge pressure: 12 MPa) reduced the concentrations of PFOS from 410 to 48 ng / L (88.3% reduction after 3 hours) and to 17 ng / L (95.9% reduction after 6 hours), and PFOA from 240 to 130 ng / L (45.8% reduction after 3 hours) and to 81 ng / L (66.3% reduction after 6 hours). The combined PFOS / PFOA concentration decreased from 650 to 178 ng / L (72.6% reduction after 3 hours) and to 98 ng / L (84.9% reduction after 6 hours). A tendency for the concentrations of PFOS / PFOA to decrease with the progression of treatment time was observed. Furthermore, when comparing the results of treating sample water C1 with a rectangular jet box and the reduction rate after 3 hours, the cylindrical jet box showed a higher reduction rate in both PFOS and PFOA concentrations. When examining the relationship between the treatment time and concentration of congeners other than PFOS / PFOA (PFOS congeners: C4-C7, PFOA congeners: C4-C11), the following was observed: For PFOS congeners, C7:PFHpS and C6:PFHxA gradually decreased similarly to C8:PFOS; C5:PFPeS gradually decreased until 3 hours later, then gradually increased; and C4:PFBS slightly increased after 5 hours, but slightly decreased after 6 hours. For PFOA congeners, C11:PFUnDA, C10:PFDA, and C9:PFNA gradually decreased similarly to C8:PFOA; C7:PFHpA and C5:PFPeA showed a gradual decrease, increase, and decrease; and C6:PFHxA and C4:PFBA showed a gradual increase and decrease. Excluding PFPeS, the concentrations ultimately showed a decreasing trend. Furthermore, although sample water C2 contained 2.1 mg / L of iron and 17 mg / L of manganese, no particular changes such as discoloration were observed in the treated water. In a study investigating the decomposition of PFOS / PFOA by ultrasonic irradiation (Non-Patent Literature 1), the decomposition products after 60 minutes were observed using ESI / MS. PFOS contains CF3 (CF2) with PFOA. n COO - (n=0~6) and CF3(CF2)6SO3 - CF3(CF2)5SO33 - The corresponding peak is CF3 (CF2) from PFOA. n CO O - (n = 0 to 5) corresponding peaks have been confirmed. These indicate that dissociation of sulfonic acid groups (-SO3H) and CF2 from PFOS, and dissociation of CF2 from PFOA have occurred. Due to thermal decomposition by cavitation, the perfluoroalkyl group (CF3(CF2)n - ) is depolymerized (decrease in n), and recombination of the perfluoroalkyl group and sulfonic acid group is considered to have occurred. Also, CF3(CF2) n COO-(n = 0 to 6) generated from PFOS, and CF3(CF2) n COO - (n = 0 to 5) concentration changes have been observed by LC / MS / MS. Based on the increasing or increasing and decreasing trends of the concentrations of each substance, it is shown that the perfluoroalkyl group (CF3(CF2) n - ) is depolymerized during the decomposition process by cavitation. In this decomposition treatment test of PFOS / PFOA, etc. by high-speed water jet cavitation in water, similar thermal decomposition as in the above ultrasonic irradiation has occurred, and it is considered that PFOS / PFOA, etc. are decomposed.

[0033] [Analysis of decomposition treatment test results] As a result of conducting a decomposition treatment test of PFOS / PFOA, etc. by high-speed water jet cavitation in water using environmental water (actual groundwater, river water) containing PFAS such as PFOS / PFOA, the following was found. · It was confirmed that the concentrations of PFOS / PFOA decreased due to cavitation generated by high-speed water jets in water. · It was confirmed that the reduction rate of PFOS / PFOA concentration was higher in the cylindrical jet box than in the rectangular one. · It was confirmed that nitrogen UFB was more effective than argon UFB as a cavitation nucleus. · It was confirmed that reduction of concentrations was also possible for homologues other than PFOS / PFOA. · It was confirmed that no change such as coloration was observed in the treated water even in the case where environmental water contained iron or manganese.

[0034] Furthermore, in a separate decomposition treatment test using ozone UFB and high-speed water jet cavitation in water, a sufficient reduction in PFOS / PFOA concentration could not be confirmed compared to the case using nitrogen UFB. In addition, when using ozone UFB, significant discoloration of the treated water was observed in cases where the environmental water contained iron or manganese. Therefore, it can be said that the decomposition treatment method using ozone UFB (ultra-fast underwater water jet cavitation) is not suitable for treating environmental water, which often contains iron and manganese.

[0035] The present invention has described the method for treating water containing organofluorine compounds based on an embodiment (decomposition treatment test by high-speed water jet cavitation in water (circulating batch treatment test)). However, the present invention is not limited to the configuration described in the above embodiment, and its configuration can be appropriately modified without departing from the spirit of the invention, for example, by performing the treatment in a continuous manner. [Industrial applicability]

[0036] The present invention provides a method for treating water containing organofluorine compounds, which utilizes cavitation, a method used for the decomposition of organic compounds, to decompose persistent organofluorine compounds contained in water. In particular, because it can treat large quantities of water containing organofluorine compounds with a simple mechanism, it is suitable for low-cost decomposition of PFAS such as PFOS / PFOA contained in environmental water such as river water, lake water, and groundwater. [Explanation of Symbols]

[0037] 1. Jet box 2. Spray nozzle 3. High-pressure plunger pump 4 Aquariums 5 Cooling tube 6. Bubble Generator (UFB Generator) 71 Accelerometer 72 Precision Sound Level Meter 73 FFT Analyzer

Claims

1. A method for treating water containing organic fluorine compounds, comprising decomposing organic fluorine compounds contained in water using cavitation, characterized in that a bubble generator that generates inert gas bubbles of less than 1 μm is used, and inert gas is supplied to the bubble generator, thereby causing cavitation by injecting water to which inert gas bubbles of less than 1 μm have been added from a spray nozzle into water containing organic fluorine compounds, and decomposing organic fluorine compounds including at least one or more of PFHxS, PFHpS, PFOS, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, and PFDoDA.

2. The method for treating water containing an organofluorine compound according to claim 1, characterized in that an inert gas bubble less than 1 μm in size is added to water containing an organofluorine compound, and the water is sprayed from a spray nozzle.

3. The method for treating water containing an organofluorine compound according to claim 1 or 2, characterized in that the water containing the organofluorine compound is environmental water.

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