A method for the adsorptive enrichment-desorption resolution-hydration electron reduction of perfluoro- and polyfluoroalkyl substances in water

Abstract

The application discloses a method for reducing perfluoroalkyl substances in water by adsorption enrichment-desorption analysis-hydration electron, which comprises the following steps: adsorbing PFAS in water by using fluorine-doped covalent triazine porous materials; desorbing the fluorine-doped CTF materials adsorbing PFAS in the above step by using a mixed solvent system containing saturated KI solution, transferring the high-concentration PFAS solution after desorption to a quartz reactor, adding Na2SO3, and irradiating under ultraviolet light to generate hydration electrons for reduction and defluorination. By using the combined technology, low-concentration PFAS in wastewater / environmental water can be effectively enriched. For a single pollutant solution, the solution after desorption can be recycled. For mixed pollutants, PFAS in the desorption solution can be reduced by hydration electron, and the subsequent biological treatment process can be promoted.

Description

Technical Field

[0001] This invention relates to a method for adsorption and reduction defluorination of low concentrations of perfluorinated and polyfluoroalkyl substances in water, belonging to the field of organic pollutant treatment technology. Background Technology

[0002] The use of innovative and low-cost methods to remove and degrade perfluorinated and polyfluoroalkyl substances (PFAS) in water has become a current research hotspot. Advanced oxidation processes (AOPs) have been extensively studied for PFAS degradation; however, strong CF bonds (536 kJ / mol) hinder the free radical oxidation of PFAS molecules, especially the inert hydroxyl radicals (·OH) to PFAS. Various photocatalysts have been developed to promote the oxidative destruction of PFAS, such as TiO2, In2O3, Gd2O3, and SiC / graphene, but these photocatalysts are typically used to treat bulk water, which often consumes excessive energy due to the large processing volumes. Studies have shown that adsorption is the simplest, fastest, and most effective method among existing techniques for removing or separating PFAS from environmental samples, especially for water bodies with low PFAS concentrations. Currently, various adsorbents, such as CTAB micelle-encapsulated SiO2, cross-linked chitosan beads, and anion exchange resins, have been applied to remove PFAS from water. These adsorbents are mainly based on hydrophobic interactions, electrostatic attraction, and ion exchange recognition of PFAS. However, the organic solvents used in the adsorbent regeneration process often cause secondary pollution.

[0003] The treatment of the desorption solution after PFAS adsorption is a crucial step in the complete removal of PFAS. Therefore, this invention employs a method of adsorption enrichment-desorption desorption-hydration electron reduction of PFAS in water to completely remove PFAS from the water to be treated. Covalent triazine frameworks (CTFs) are a new type of non-swellable polymer synthesized through the ionic thermal trimerization of aromatic nitriles. They possess a very large specific surface area, uniform and rigid pore structure, high chemical and thermal stability, and relatively low cost. PFASs exhibit hydrophobic and oleophobic properties. In water, fluorocarbon chains repel water molecules and are not affinity for hydrocarbons, while fluorocarbon chains tend to aggregate. Therefore, the principle of "like dissolves like" can be utilized by modifying the surface of the CTF adsorbent with organic fluorination, allowing the fluorocarbon chains on the material surface to adsorb PFAS from the water. The hydrophobic and oleophobic properties of the fluorocarbon chains can also repel hydrocarbon organic matter, thereby achieving the selective removal of PFAS. The fluorinated CTF material containing PFAS was desorbed and adsorbed using a mixed solvent system containing saturated KI solution. Then, the desorbed PFAS was reduced by hydrated electrons, which can effectively remove PFAS from the water to be treated. This method can efficiently treat PFAS while avoiding secondary pollution.

[0004] Chinese patent application number 201710122426.8 discloses a method for photocatalytic degradation of perfluorinated compounds. This patent involves preparing dual-model mesoporous SiO2 materials (BMMs) and using phosphotungstic acid as the active component. An HPW / BMMs catalyst is prepared via impregnation. The catalyst is then added under ultraviolet light to achieve the degradation and defluorination of perfluorinated compounds. However, this system is ineffective at treating low concentrations of perfluorinated compounds; a 10 mg / L PFAS solution irradiated under ultraviolet light for 4 hours only removes 40% of the perfluorinated compounds.

[0005] Chinese patent application number 201710098400.4 discloses a sludge-based activated carbon with high PFAS adsorption capacity, its preparation method, and its application. This patent describes a process involving drying, activation, high-temperature carbonization, acid leaching, washing, and drying to prepare sludge-based activated carbon, achieving adsorption and removal rates of up to 99.9% and 71.0% for low-concentration PFAS. However, this adsorbent has a narrow effective pH range, poor reusability, and relatively poor adsorption effect on hydrophobic and lipophilic PFCs.

[0006] Chinese patent application number 202210813328.X discloses the preparation of a cerium-doped highly crystalline carbon nitride adsorbent and its application in the adsorption of emerging pollutants. The cerium-doped highly crystalline carbon nitride adsorbent prepared by this patent has an increased specific surface area, an increased number of adsorption active sites, and improved adsorption performance and rate, but it has the problem of secondary pollution caused by metal leaching.

[0007] Chinese patent application number 202210339442.3 discloses an electrochemical method for the synergistic degradation of perfluorinated compounds by activating persulfate and O2. This patent utilizes a one-step synthesis method to in-situ form an iron- and nickel-doped bimetallic carbon aerogel electrode. On the surface of this electrode, a two-electron oxygen reduction reaction (ORR) can occur in situ, generating H2O2. Simultaneously, the abundant metal active sites on the electrode surface catalyze the generation of hydroxyl radicals (·OH) from H2O2 and also activate peroxymonosulfonate (PMS) to generate sulfate radicals (SO4). - The system degrades perfluorinated compounds through the synergistic effect of these two free radicals at the cathode. However, the electrode plates of this system are prone to metal leaching, causing secondary pollution, and have poor reusability. Summary of the Invention

[0008] For water bodies with low concentrations of PFAS, this invention aims to provide a method for adsorption enrichment-desorption desorption-hydration electron reduction of PFAS, which to a certain extent solves the technical problems of treating water bodies with low concentrations of PFAS.

[0009] The technical solution to achieve the purpose of this invention is: a method for adsorption enrichment-desorption desorption-hydration electron reduction of perfluorinated and polyfluoroalkyl substances in water, comprising:

[0010] (1) Use fluorine-doped CTF material to adsorb low concentrations of PFAS in the water to be treated;

[0011] (2) The fluorine-doped CTF material adsorbed in step (1) was desorbed using a mixed solvent system containing saturated KI solution;

[0012] (3) Use hydrated electrons to reduce the high concentration of PFAS that has been desorbed.

[0013] Preferably, the fluorine-doped CTF material is prepared by the following steps: thoroughly mixing tetrafluoroterephthalonitrile and ZnCl2 through grinding, calcining under vacuum at 400-500℃ for 35-40 hours, and then cleaning, drying, and sieving.

[0014] Specifically, the mass ratio of tetrafluoroterephthalonitrile to ZnCl2 is 1:6.

[0015] Specifically, after drying, it is passed through a 200-300 mesh sieve.

[0016] Preferably, in step (1), the fluorinated CTF material is placed in the water to be treated, with 5 mg of fluorinated CTF material added to every 30-150 ml of water to be treated, and the adsorption time is more than 4 hours.

[0017] Preferably, the concentration of PFAS in the water to be treated is 10–250 ppm.

[0018] Preferably, in step (2), a mixed solvent system containing saturated KI solution is used to desorb the fluorinated CTF material that adsorbed PFAS in step (1). The process is as follows: the fluorinated CTF material that adsorbed PFAS in step (1) is placed in a mixed solvent system containing saturated KI solution and desorbed for more than 4 hours under shaking conditions. 5 mg of fluorinated CTF material that adsorbed PFAS is added to every 30-150 ml of mixed solvent system containing saturated KI solution.

[0019] Preferably, in the mixed solvent system containing the saturated KI solution, the mixed solvent consists of methanol and water in a volume ratio of 7:3, and the concentration of the saturated KI solution in the system is 1 vol%.

[0020] Preferably, in step (3), the high concentration of PFAS desorbed is reduced by hydrated electrons. The process is as follows: the PFAS solution obtained by adding water to the desorbed PFAS is placed in a quartz glass container, and Na2SO3 is added to it under UV lamp irradiation to reduce PFAS. The concentration of PFAS solution is 30-100 mg / L, and 100-600 mg of Na2SO3 is added to every 400 ml of PFAS solution.

[0021] Specifically, the UV lamp has a power of 10W or more.

[0022] Compared with the prior art, the present invention has the following advantages and technical effects:

[0023] 1. The F-CTF material of the present invention utilizes electrostatic interaction, hydrophobic interaction, and the strong adsorption effect of PFAS on the surface of the material on the fluorocarbon chain, and the removal rate of PFAS in wastewater is about 4 times that of existing CTF materials.

[0024] 2. According to the embodiments of the present invention, the desorbed PFAS can be treated to avoid secondary pollution and to complete the harmless treatment of PFAS.

[0025] 3. Utilizing KI in a mixed solvent system containing a saturated KI solution as a co-activator of hydrated electrons, the desorbed PFAS can be efficiently reduced, thereby achieving the goal of saving resources. Attached Figure Description

[0026] Figure 1 This is a SEM image of the F-CTF of this invention.

[0027] Figure 2 This is the XRD pattern of the CTF of this invention.

[0028] Figure 3 This is a schematic diagram comparing the adsorption effects of F-CTF material and other CTF materials on PFAS in water to be treated in one application example of the present invention. The inset shows a schematic diagram comparing the adsorption effects of F-CTF material and other CTF materials on PFAS in water to be treated within one hour.

[0029] Figure 4 This is a schematic diagram illustrating the adsorption and removal effect of F-CTF on PFAS after six consecutive regenerations, as an application example of the present invention.

[0030] Figure 5 In one application example of the present invention, the change curve of PFAS concentration is shown when sodium sulfite is added and UV irradiation is applied.

[0031] Figure 6 This is a flowchart of the method for adsorption enrichment-desorption desorption-hydration electron reduction of perfluorinated and polyfluoroalkyl substances in water according to the present invention. Detailed implementation method:

[0032] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0033] The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. It should be noted that the terms "first," "second," etc., used herein are for convenience of description only and should not be construed as indicating or implying relative importance, nor as indicating a sequential relationship between them. In the description of the present invention, unless otherwise stated, "a plurality of" means two or more.

[0034] The following inventions and concepts form the basis for the inventor's creation of this invention:

[0035] CTFs possess a very large specific surface area, a uniform and rigid pore structure, high chemical and thermal stability, and relatively low cost. PFASs exhibit hydrophobic and oleophobic properties; in water, fluorocarbon chains repel water molecules and are not affinity for hydrocarbons, while also tending to aggregate. Therefore, the principle of "like dissolves like" can be utilized by modifying the surface of CTF adsorbents with organic fluorination, allowing the fluorocarbon chains on the material surface to adsorb PFASs from water. Furthermore, the hydrophobic and oleophobic properties of the fluorocarbon chains can repel hydrocarbon organic compounds, thereby achieving the selective removal of PFASs.

[0036] Adsorbent regeneration typically requires the use of organic solvents such as methanol and ethanol. The PFAS eluted off remain in the regeneration solution, and improper handling can cause secondary pollution. Considering this problem, this invention provides a method for adsorption and reduction of PFAS. The desorbed PFAS is placed in a UV quartz glass reactor, and sodium sulfite is added. The PFAS are efficiently reduced by the hydrated electrons generated by sodium sulfite under UV irradiation, thereby achieving complete removal of PFAS from the water to be treated.

[0037] By using a mixed solvent system containing a saturated KI solution to desorb PFAS, KI can act as a co-activator of Na2SO3 in the field of hydrated electrons. Compared with using Na2SO3 or KI alone, it has higher reduction efficiency and can reduce PFAS more efficiently and quickly.

[0038] The following examples are provided to better understand the present invention, but are not intended to limit the invention. Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0039] The method for adsorbing and reducing low concentrations of PFAS in water, as described above, will be explained in detail below with specific examples.

[0040] Example 1

[0041] The preparation method of the F-CTF adsorbent of the present invention includes the following specific steps:

[0042] (1) 1g of tetrafluoroterephthalonitrile and 6.82g of ZnCl2 were thoroughly mixed by grinding and the mixture was transferred to a quartz ampoule.

[0043] (2) After vacuuming, the tube is sealed and calcined in a muffle furnace at 400°C for 40 hours, then cooled to room temperature.

[0044] (3) The calcined product was stirred and washed with 1 mol / L hydrochloric acid for 48 h, then washed with deionized water until the solution was neutral, and dried at 60 °C to obtain material F-CTF.

[0045] Comparative Example 1

[0046] The preparation method of FUM-CTF adsorbent using trans-butenedionitrile as a precursor is as follows:

[0047] (1) 0.39g of trans-butenedionitrile and 6.82g of ZnCl2 were thoroughly mixed by grinding and the mixture was transferred to a quartz ampoule.

[0048] (2) After vacuuming, the tube is sealed and calcined in a muffle furnace at 400°C for 40 hours, then cooled to room temperature.

[0049] (3) The calcined product was stirred and washed with 1 mol / L hydrochloric acid for 48 h, then washed with deionized water until the solution was neutral, and dried at 60 °C to obtain the material FUM-CTF.

[0050] Comparative Example 2

[0051] The specific steps for preparing DCB-CTF adsorbent using terephthalonitrile as a precursor are as follows:

[0052] (1) 0.64 g of terephthalonitrile and 6.82 g of ZnCl2 were thoroughly mixed by grinding and the mixture was transferred to a quartz ampoule.

[0053] (2) After vacuuming, the tube is sealed and calcined in a muffle furnace at 400°C for 40 hours, then cooled to room temperature.

[0054] (3) The calcined product was stirred and washed with 1 mol / L hydrochloric acid for 48 h, then washed with deionized water until the solution was neutral, and dried at 60 °C to obtain the material DCB-CTF.

[0055] The structural characteristics of the adsorbents obtained in Example 1 and Comparative Examples 1-2 were characterized, including:

[0056] (1) Morphological observation

[0057] The morphology of F-CTF was characterized using scanning electron microscopy (SEM), such as... Figure 1As shown, F-CTF is an aggregate of layered CTF nanoscale objects, composed of irregular rectangular aggregates.

[0058] (2) Crystal form

[0059] XRD patterns of CTF materials prepared from different precursors are shown below. Figure 2 As shown in the figure, CTF has a distinct characteristic peak at 2θ = 7.2°, reflecting the (100) crystal plane of the triazine functional group hexagonal structure; and a distinct broad peak at 26.2°, reflecting the (001) crystal plane of the aromatic lamellar structure.

[0060] Application Example 1

[0061] This application example mainly examines the adsorption equilibrium time and adsorption rate of PFAS in water by the F-CTF adsorbent prepared in Example 1, the FUM-CTF adsorbent prepared in Comparative Example 1, and the DCB-CTF adsorbent prepared in Comparative Example 2. The specific steps are as follows:

[0062] First, weigh 5 mg of the corresponding adsorbent into a 50 ml polypropylene centrifuge tube, add 30 ml of 250 mg / L PFAS into the centrifuge tube, and shake at 160 r / min at 25 °C. Take 1 ml samples of each adsorbent at the beginning of the experiment (0, 0.5, 1, 2, 3, 5, 7, 10, 15, 30, 45, 60, 90, 120, 180, 240, and 360 min). After filtering the samples through a 0.22 μm filter to remove the adsorbent, determine the concentration of PFAS in the samples by high-performance liquid chromatography (HPLC). The adsorption amount at time t is q. t Calculate using the following formula:

[0063] q t =V×(C0-C t ) / m

[0064] Where V is the solution volume (L), C0 is the initial concentration of the PFAS solution (mg / L), and C t t represents the solution concentration (mg / L) at time t, and m represents the mass of adsorbent added (g).

[0065] Depend on Figure 3It can be seen that, compared with the FUM-CTF adsorbent prepared in Comparative Example 1 and the DCB-CTF adsorbent prepared in Comparative Example 2, the F-CTF adsorbent synthesized in Example 1 reached adsorption equilibrium for PFAS at a concentration of 250 ppm in solution within 120 min, and the adsorption capacity was significantly higher than that of the control group, reaching 580 mg / g. This is mainly attributed to the organofluorine modification on the surface of the CTF adsorbent. PFAS has hydrophobic and oleophobic properties. In water, the fluorocarbon chains repel water molecules and are not affinity for hydrocarbons, while the fluorocarbon chains tend to aggregate. By modifying the surface of the CTF adsorbent with organofluorine, the fluorocarbon chains on the material surface adsorb PFAS in water, achieving the purpose of selective adsorption of PFAS.

[0066] Application Example 2

[0067] This application example mainly examines the reusability of the F-CTF adsorbent prepared in Example 1. The specific steps are as follows:

[0068] (1) First, weigh 5 mg of F-CTF adsorbent into a 30 ml polypropylene centrifuge tube (the F-CTF adsorbent that underwent adsorption-desorption in Application Example 1), add 20 ml of 250 mg / L PFAS into the centrifuge tube, and shake at 160 r / min at 25 °C. After adsorption for 4 h, take 1 ml of F-CTF adsorbent as a sample. Filter the sample through a 0.22 μm pore size filter to remove the adsorbent, and then determine the PFAS concentration in the sample by high performance liquid chromatography. The adsorption amount q at time t is determined. t Calculate using the following formula:

[0069] q t =V×(C0-C t ) / m

[0070] Where V is the solution volume (L), C0 is the initial concentration of the PFAS solution (mg / L), and C t t represents the solution concentration (mg / L) at time t, and m represents the mass of adsorbent added (g).

[0071] (2) The reacted F-CTF material was placed in a 20 ml mixed solvent system of methanol and water containing 1 vol% saturated KI solution and shaken at 160 r / min for 4 h in a constant temperature shaker. Then the centrifuge tube was placed in a high-speed centrifuge and centrifuged at 14500 rpm for 10 min. The desorbed F-CTF material was taken out, washed, dried and re-adsorbed with PFAS at a concentration of 250 ppm. The above desorption was repeated 6 times.

[0072] The adsorption effect of the adsorbent after six cycles is as follows: Figure 4 As shown, by Figure 4It can be seen that after three adsorption cycles, the removal rate of PFAS in water by F-CTF decreased slightly, by about 15% compared with the first adsorption. The reason for the decrease in adsorption capacity may be that a small amount of PFAS failed to desorb during the desorption and regeneration process, and these PFAS occupied a certain number of adsorption sites. F-CTF regenerated using a mixed solvent system of methanol and water containing 1 vol% saturated KI solution exhibited strong stability in six adsorption-desorption-regeneration cycles, demonstrating the high usability and stability of the F-CTF material.

[0073] Application Example 3

[0074] Combination Figure 6 This invention provides a method for adsorption-enrichment-desorption-hydration electron reduction of perfluorinated and polyfluoroalkyl substances in water, comprising:

[0075] (1) The F-CTF material prepared in Example 1 was used to adsorb PFAS in the water to be treated;

[0076] (2) The fluorinated CTF material that adsorbed PFAS in step (1) was desorbed using a mixed solvent system of methanol and water containing 1 vol% saturated KI solution.

[0077] (3) The PFAS desorbed by hydrated electrons is reduced, and the specific process is as follows:

[0078] The desorption solution of the F-CTF material was placed in a UV quartz glass reactor. Nitrogen gas was introduced into the reactor for 30 min, and 2 mmol of sodium sulfite was added. After the solution was stirred evenly, a 10W UV lamp was turned on. 1 ml samples were taken at 0, 1, 3, 5, 7, 10, 15, 20, and 30 min at the beginning of the experiment. The samples were filtered through a 0.22 μm pore size filter to remove the adsorbent, and the concentration of PFAS in the samples was determined by high-performance liquid chromatography (HPLC). The experimental results are as follows: Figure 5 As shown.

[0079] Comparative Application Example 1

[0080] (1) The F-CTF material prepared in Example 1 was used to adsorb PFAS in the water to be treated;

[0081] (2) The fluorine-doped CTF material that adsorbed PFAS in step (1) was desorbed using a mixed solvent system of methanol and water containing 1 vol% saturated NaCl solution.

[0082] (3) The PFAS desorbed by hydration electron reduction was performed as follows: The desorption solution of the F-CTF material was placed in a UV quartz glass reactor. Nitrogen gas was introduced into the reactor for 30 min, and 5 mmol / L sodium sulfite was added. After the solution was stirred evenly, a 10W UV lamp was turned on. 1 ml samples were taken at 0, 1, 3, 5, 7, 10, 15, 20, and 30 min at the beginning of the experiment. The samples were filtered through a 0.22 μm pore size filter to remove the adsorbent, and the concentration of PFAS in the samples was determined by high performance liquid chromatography. The experimental results are as follows: Figure 5 As shown.

[0083] The UV / sulfite system has been shown to produce e with extremely strong reducing properties. aq - e aq - Potassium iodide can efficiently reduce PFAS, and it can act as a co-activator of sulfite, increasing the yield of hydrated electrons, thereby reducing PFAS more efficiently. Figure 5 As shown, adding 5 mmol / L sodium sulfite to a desorption solution containing 50 ppm resulted in a PFAS removal rate of only 50% within 30 min. The fluorinated CTF material adsorbed PFAS was desorbed using a mixed solvent system of methanol and water containing 1 vol% saturated KI solution. When the PFAS concentration in the desorption solution was 50 ppm, the calculated KI concentration was approximately 3 mmol / L. Adding 2 mmol / L sodium sulfite to a desorption solution containing 3 mmol / L potassium iodide resulted in a PFAS removal rate of 95% within 30 min, significantly better than the removal effect of sodium sulfite alone. This is mainly attributed to potassium iodide acting as a co-activator of sodium sulfite, which increases the yield of hydrated electrons, thereby increasing the reduction efficiency of PFAS.

[0084] This invention provides a method for adsorption enrichment-desorption desorption-hydration electron reduction of PFAS in water with low concentrations of PFAS. It innovatively combines adsorption and advanced reduction methods, which can efficiently enrich low concentrations of PFAS with a removal efficiency of up to 90% compared with other treatment methods. Furthermore, the subsequent use of hydration electron reduction for defluorination can efficiently treat mixed pollutants and solve the problem of secondary pollution of desorption solution.

Claims

1. A method for the adsorptive enrichment-desorptive elution-hydration electron reduction of perfluoro- and polyfluoroalkyl substances in water, characterized in that, include: (1) Use fluorine-doped CTF material to adsorb low concentrations of PFAS in the water to be treated; (2) The fluorinated CTF material adsorbed in step (1) was desorbed using a mixed solvent system containing saturated KI solution; (3) Utilize hydrated electrons to reduce the high concentration of PFAS that has been desorbed; The process of desorbing the fluorinated CTF material adsorbed with PFAS in step (1) using a mixed solvent system containing saturated KI solution is as follows: the fluorinated CTF material adsorbed with PFAS in step (1) is placed in a mixed solvent system containing saturated KI solution and desorbed for more than 4 hours under shaking conditions. 5 mg of the fluorinated CTF material adsorbed with PFAS is added to every 30~150 ml of mixed solvent system containing saturated KI solution. The fluorine-doped CTF material is prepared by the following steps: tetrafluoroterephthalonitrile and ZnCl2 are thoroughly mixed by grinding, calcined under vacuum at 400~500℃ for 35~40h, and then washed, dried and sieved. The mass ratio of tetrafluoroterephthalonitrile to ZnCl2 is 1:6; In step (3), the high concentration of PFAS desorbed is reduced by hydrated electrons. The process is as follows: the PFAS solution obtained by adding water to the desorbed PFAS is placed in a quartz glass container, and Na2SO3 is added to it under UV lamp irradiation to reduce PFAS. The concentration of PFAS solution is 30~100mg / L, and 100~600mg of Na2SO3 is added to every 400ml of PFAS solution.

2. The method as described in claim 1, characterized in that, After drying, pass through a 200-300 mesh sieve.

3. The method as described in claim 1, characterized in that, In step (1), the fluorinated CTF material is placed in the water to be treated. 5 mg of fluorinated CTF material is added to every 30-150 ml of water to be treated, and the adsorption time is more than 4 hours.

4. The method as described in claim 1, characterized in that, The concentration of PFAS in the water to be treated is 10~250ppm.

5. The method as described in claim 1, characterized in that, In the mixed solvent system containing saturated KI solution, the mixed solvent consists of methanol and water in a volume ratio of 7:3, and the concentration of saturated KI solution in the system is 1 vol.

6. The method as described in claim 1, characterized in that, The UV lamp has a power of 10W or more.

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