Method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy

Abstract

The application discloses a method for realizing deep defluorination and mineralization of perfluorooctanoic acid (PFOA) based on reduction-oxidation synergy, which comprises the following steps: firstly, adding a reducing agent, sulfite, into sewage containing PFOA, and then performing a photoreduction reaction under ultraviolet irradiation; and then, converting PFOA into intermediate products containing -CH2- structure such as fluoropolymer carboxylic acid by means of H / F exchange reaction; after the reduction reaction reaches a bottleneck, without separating the intermediate products, directly adding an oxidizing agent, persulfate, into the reaction system, and then continuing to perform a photooxidation reaction under ultraviolet irradiation, so as to break the C-H bond of the intermediate products and realize deep defluorination and mineralization of PFOA. The method has the advantages of reduction defluorination and oxidation bond breaking by means of step-by-step connection of a UV / SF reduction system and a UV / PDS oxidation system, and effectively overcomes the problems of defluorination bottleneck of single reduction technology and incomplete defluorination of single oxidation technology; the defluorination rate of PFOA can reach more than 99% in continuous flow operation, and the method has the characteristics of simple operation, high treatment efficiency and strong environmental adaptability.

Description

Technical Field

[0001] This invention belongs to the field of new pollutant control and water treatment technology, specifically relating to a method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy. Background Technology

[0002] Perfluorooctanoic acid (PFOA) is a typical perfluoroalkyl and polyfluoroalkyl substance (PFAS). Due to the high bond energy of the CF bond in its molecule and its chemical stability, it is extremely difficult to degrade in the natural environment and has been widely detected in surface water, groundwater, and drinking water systems. PFOA has bioaccumulation and potential toxicity, and long-term exposure may have adverse effects on human health and ecosystems.

[0003] Therefore, there is an urgent need to develop a new method that can achieve high-efficiency, deep defluorination and mineralization of PFOA. Summary of the Invention

[0004] The purpose of this invention is to provide a method for deep defluorination and mineralization of perfluorooctanoic acid (PFOA) based on a reduction-oxidation synergy. This method achieves deep defluorination of PFOA in water by combining ultraviolet / sulfite reduction with ultraviolet / persulfate oxidation, thereby solving the technical problem that existing PFOA degradation methods cannot achieve deep defluorination and mineralization.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this invention provides a method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy, comprising the following steps: S1, Reduction Defluorination Stage: Sulfite is added to the PFOA-containing water body to initiate a photoreduction reaction under ultraviolet light irradiation, utilizing the generated hydrated electrons (e... aq The CF bond in the PFOA molecule is attacked to induce reductive defluorination, generating a hydrogen-containing fluorinated intermediate. S2, Oxidation and Mineralization Stage: After the reaction in step S1 reaches the defluorination bottleneck, persulfate is added directly to the reaction solution without separating the intermediate products. A photo-oxidation reaction is initiated under ultraviolet light irradiation, utilizing the generated oxidizing free radicals to degrade the hydrogen-containing fluorinated intermediates, achieving deep defluorination and mineralization of PFOA. It should be noted that the defluorination rate of the reaction system is monitored in real time; when its growth slows significantly, the defluorination bottleneck has been reached.

[0006] Furthermore, in step S1, the sulfite is selected from sodium sulfite (Na2SO3) or potassium sulfite (K2SO3), and the initial addition concentration is 10~80 mM.

[0007] Furthermore, in step S2, the persulfate is selected from sodium persulfate (Na2S2O8) or potassium persulfate (K2S2O8), and the concentration is 20~100 mM.

[0008] Furthermore, the reaction time of step S1 is 10-12 hours, during which PFOA undergoes H / F exchange to generate fluorinated telomer carboxylic acids (FTCAs) as intermediate products, which are the hydrogen-containing fluorinated intermediate products mentioned above; the reaction time of step S2 is 10-12 hours, which further oxidizes and defluorinates the FTCAs.

[0009] Furthermore, in steps S1 and S2, the water containing PFOA contains Cl... NO3 HCO3 The reaction can proceed under inorganic anionic conditions and / or with an initial pH value of 3.4 to 11.0 in the water body. Therefore, the method provided by this invention has good adaptability and can be applied to various actual water bodies; it can also be carried out under natural pH conditions without the need for additional pH adjustment.

[0010] Furthermore, the method achieves a defluoridation rate of over 95% for water containing PFOA.

[0011] Furthermore, in steps S1 and S2, the power of the ultraviolet light is 75 W and the wavelength is 254 nm.

[0012] The second aspect of this invention provides an application of the above-mentioned method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy in the field of PFOA-containing wastewater treatment.

[0013] Furthermore, the PFOA-containing wastewater includes PFOA-polluted water bodies with tap water as the substrate, PFOA-polluted water bodies with reservoir water as the substrate, and secondary effluent from wastewater treatment plants.

[0014] Furthermore, the method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy is implemented in a continuous flow reactor, which includes an ultraviolet reactor. Specifically, PFOA-containing wastewater first flows through the ultraviolet reactor containing added sulfite. After the required reduction reaction residence time is met, persulfate is added online to the ultraviolet reactor, and ultraviolet irradiation continues, thereby achieving continuous deep defluorination of PFOA-containing wastewater.

[0015] In a continuous flow reactor, the method of the present invention can stably maintain an ultra-high defluorination rate of over 99% for PFOA under long-term operation (>120 hours), proving its stability and reliability in actual continuous processing.

[0016] After the ultraviolet oxidation reactor, a low-cost adsorption unit (such as activated carbon or ion exchange resin) can be connected in series to further remove residual fluoride ions or trace amounts of incompletely mineralized organic fluoride, achieving deep and safe emissions.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Breaking the defluorination limit: This invention utilizes a UV / SF reduction system to convert highly stable PFOA into hydrogen-containing FTCAs, and then utilizes a UV / PDS oxidation system to efficiently degrade the FTCAs. Experimental results show that this combined process significantly increases the defluorination rate of PFOA from ~55% of single oxidation technology and ~80% of single reduction technology to over 95%.

[0018] (2) High stability of continuous flow: In a continuous flow reactor, the method of the present invention can maintain an ultra-high defluorination rate of more than 99% of PFOA under long-term operation (>120 hours), which proves its stability and reliability in actual continuous processing.

[0019] (3) Strong environmental adaptability: When treating complex secondary effluent, the method of the present invention can still maintain a high defluorination efficiency (4h defluorination rate >43%, and can eventually reach deep mineralization), showing good anti-interference ability and practical application potential. Attached Figure Description

[0020] Figure 1 This is a comparison chart showing the change in defluorination rate of PFOA over time in Example 1 of the present invention and Comparative Examples 1-3, which are treated with different processes. Figure 2 This is a comparison chart of the effects of different initial concentrations of sodium persulfate (PDS) on the PFOA defluorination rate in Example 2 of the present invention; Figure 3 This is a comparison chart showing the effect of different initial concentrations of sodium sulfite (SF) on the defluorination rate of PFOA during the UV / SF reduction stage in Example 3 of the present invention. Figure 4 This is a comparison chart of the PFOA defluorination rate over time under different initial pH conditions in Example 4 of the present invention; Figure 5 This is a comparison chart showing the effect of different coexisting inorganic anions on the defluorination rate of PFOA in Example 5 of the present invention; Figure 6This is a schematic diagram of the continuous flow ultraviolet photocatalytic combined reaction device used to implement the method of the present invention in Embodiment 6 of the present invention; Figure 7 This is a diagram illustrating the long-term defluorination effect of PFOA wastewater treated using a continuous flow reactor with stepwise addition of reducing and oxidizing agents in Embodiment 6 of the present invention. Detailed Implementation

[0021] The inventors discovered that current degradation technologies for PFOA mainly include oxidation and reduction technologies.

[0022] Advanced oxidation technologies (AOPs): such as UV / PDS technology, which utilize sulfate radicals (SO4) • The strong oxidizing properties of UV / PDS attack the carboxyl group at the terminal of PFOA, initiating a decarboxylation-hydroxylation-elimination-hydrolysis (DHEH) cycle. However, studies have shown that the UV / PDS system has an upper limit (usually <60%) for the defluorination efficiency of PFOA, making it difficult to achieve complete mineralization, and the reaction rate is limited by the molecular structure.

[0023] Advanced reduction techniques (ARPs): such as UV / SF technology, utilize the strong reducing properties of hydrated electrons (e... aq Direct attack on the CF bond for reductive defluorination. Although its defluorination rate is relatively fast, studies have found that as the reaction proceeds, the hydrogen-containing intermediates (such as the -CH2- structure) generated by the H / F exchange of the PFOA molecule significantly increase the bond energy of the remaining CF bond, leading to further defluorination being hindered. The defluorination rate usually stagnates at around 80%, making complete defluorination impossible.

[0024] Based on this, the present invention aims to provide a new method that can overcome the bottleneck of single oxidation or reduction technology and achieve high-efficiency, deep defluorination and mineralization of PFOA.

[0025] The present invention will now be described in detail with reference to specific embodiments.

[0026] Example 1: Defluorination effect of UV / SF-UV / PDS stepwise treatment of PFOA. First, an aqueous solution of PFOA with an initial concentration of 20 mg / L was prepared.

[0027] S1 (Reduction Stage): Add Na2SO3 to the above PFOA solution to an initial concentration of 10–80 mM (specifically 10 mM, 20 mM, 40 mM, 60 mM, and 80 mM). Place the solution in a photochemical reactor, turn on a 254 nm wavelength, 75 W UV lamp, and carry out the photoreduction reaction under stirring. The reaction vessel is sealed with a silicone stopper, and the reaction temperature is controlled at approximately 25°C using condensate.

[0028] S2 (Oxidation Stage): The defluorination rate of the reaction system was monitored in real time. When its growth slowed significantly (in this example, the reaction was carried out for 12 hours), Na2S2O8 was directly added to the same reaction vessel until the initial concentration was 20~100 mM (specifically 20 mM, 40 mM, 60 mM, 80 mM, and 100 mM), and the photo-oxidation reaction continued under ultraviolet light irradiation. During the reaction, samples were taken at intervals, and 5 mL of each sample was stored in a 4°C refrigerator for subsequent determination of fluoride ion concentration and intermediate products.

[0029] Comparative Example 1: Using the same apparatus and initial PFOA concentration as in Example 1, 10-100 mM Na2S2O8 was added, and the reaction was carried out continuously for 24 hours under ultraviolet light irradiation to investigate its defluorination effect (experimental results are shown in...). Figure 1 ).

[0030] Comparative Example 2: Using the same apparatus and initial PFOA concentration as in Example 1, 10-100 mM Na2SO3 was added, and the reaction was carried out continuously for 24 hours under ultraviolet light irradiation to investigate its defluorination effect (experimental results are shown in...). Figure 1 ).

[0031] Comparative Example 3: The same apparatus and initial PFOA concentration as in Example 1 were used.

[0032] S1 (oxidation stage): First, add 20~100 mM Na2S2O8 and carry out ultraviolet light oxidation reaction for 12 hours; S2 (reduction stage): Then add 10~80 mM Na2SO3 to the system and carry out the ultraviolet light reduction reaction for 12 hours.

[0033] Experimental results and analysis: such as Figure 1 As shown: Comparative Example 1: After 12 hours of reaction, the defluorination rate was approximately 81.18%, and after extending to 24 hours, the defluorination rate only increased to 83.51%, indicating significant incomplete defluorination. Comparative Example 2: After 12 hours of reaction, the defluorination rate was approximately 83.17%, and after extending to 24 hours, there was almost no improvement (84.06%), indicating that the intermediate products generated during reduction (containing -CH2- structures) hindered further defluorination. Comparative Example 3: The final defluorination rate of this reverse combined process was only 54.60%. Example 1: The final defluorination rate of the reduction-oxidation process of this invention reached as high as 95.89%.

[0034] The above results indicate that the processing sequence is crucial. The reduction-oxidation process employed in this invention first converts stable PFOA into easily oxidizable hydrogen-containing intermediates through a reduction stage, and then deeply mineralizes these intermediates through an oxidation stage. This overcomes the defluorination bottleneck of single-process methods and achieves highly efficient defluorination of PFOA. In contrast, the oxidation-reduction process is less effective because the oxidation stage cannot effectively break the CF bond, and the residual oxidant in the system consumes the subsequently added reducing agent, leading to system failure.

[0035] Example 2: Effect of persulfate concentration on the final defluorination rate of PFOA Using the same apparatus and initial PFOA concentration as in Example 1, different initial concentrations of Na2S2O8 were added: 20 mM, 40 mM, 60 mM, 80 mM, and 100 mM. After the reaction was completed, the final defluorination rate of each group was measured.

[0036] Experimental method: The sulfite concentration was fixed at 10 mM in the first stage. In the second stage, sodium persulfate was added at concentrations of 20 mM, 60 mM, 80 mM, and 100 mM, respectively.

[0037] Experimental Results and Conclusions: The experimental results are as follows Figure 2 As shown, when the sodium persulfate concentration increased from 20 mM to 40 mM, the final defluorination rate of PFOA significantly improved. Further increases in concentration to 60 mM, 80 mM, and even 100 mM resulted in slight increases in the defluorination rate, but no significant jumps. Considering both treatment efficiency and reagent cost, a concentration of 40 mM achieved the best cost-effectiveness.

[0038] Example 3: Effect of sulfite dosage concentration on defluorination efficiency during the reduction stage Using the same apparatus and initial PFOA concentration as in Example 1, the initial Na₂SO₃ concentrations were varied to 10 mM, 20 mM, 40 mM, 60 mM, and 80 mM. The UV / SF reduction reaction was performed alone for 24 hours to investigate its upper limit of defluorination. The experimental results are as follows: Figure 3 As shown.

[0039] Experimental results show that as the Na2SO3 concentration increases from 10 mM to 80 mM, the upper limit of the defluorination rate of PFOA in the UV / SF single reduction system increases from about 60% to about 80%. However, the defluorination curves at all concentrations tend to flatten out in the later stages of the reaction, indicating the existence of an insurmountable defluorination bottleneck.

[0040] Experimental Conclusion: Based on the results of Example 1, it is evident that even with a high concentration of 80 mM sodium sulfite, a single reduction process cannot achieve deep defluorination of PFOA (defluorination rate >95%). Therefore, this invention preferably uses 10 mM sodium sulfite as the dosage concentration for the reduction stage. At this concentration, although the defluorination rate of the reduction stage itself is limited (approximately 60-70%), it is sufficient to efficiently convert PFOA into oxidizable hydrogen-containing intermediates. Subsequently, through an oxidation stage (as shown in Example 1), the total defluorination rate can be easily increased to over 95%. This approach ensures highly efficient final defluorination while significantly reducing the cost of the reducing agent.

[0041] Example 4: Effect of initial pH value on PFOA defluorination efficiency Experimental Method: Following the steps in Example 1, the initial pH of the PFOA reaction solution was adjusted to 8.0, 11.0, and 12.0 with dilute NaOH before the reaction began, followed by UV / PDS oxidation. The unadjusted pH (natural pH ≈ 3.4) group served as the control group. After the reaction, the final defluorination rate of each group was measured.

[0042] Experimental results: The experimental results are as follows Figure 4 As shown, the defluorination rate of PFOA in this study was essentially the same as that in the control group over a wide initial pH range of 3.4 to 11.0.

[0043] Conclusion: The results of this embodiment demonstrate that the method of the present invention has good tolerance to initial pH values ​​and maintains extremely high defluorination efficiency over a wide pH range. This means that in practical applications, no additional pH adjustment of the wastewater is required; it can be operated directly under natural pH conditions, greatly simplifying the operation process and reducing treatment costs.

[0044] Example 5: Effect of coexisting inorganic anions on the defluorination effect of PFOA Experimental Method: Based on Example 1, before the reaction began, 10 mM of chloride, nitrate, carbonate, and bicarbonate ions were added to the PFOA reaction solution to simulate the anion background commonly found in actual water bodies. The reaction was then carried out according to the steps of Example 3, with a control group not containing any added anions. Changes in the defluorination rate were monitored during the reaction.

[0045] Experimental Results and Analysis: The experimental results are as follows Figure 5 As shown in the figure, compared with the control group, the addition of chloride ions resulted in a stronger anti-interference ability. Analysis suggests this is due to SO42-. • •OH will react with Cl A reaction occurs. Add HCO3. Afterward, the defluorination rate slows down slightly in the initial stage of the reaction, but after 24 hours, the final defluorination rate can still be maintained above 85%.

[0046] Conclusion: The results of this embodiment demonstrate that the method of the present invention has strong resistance to interference from common inorganic anions in water. Even at a concentration of 10 mM, these anions only have a slight impact on reaction kinetics and do not hinder the ultimate achievement of deep defluorination. This further proves the good application potential of the method of the present invention in treating real wastewater with complex compositions.

[0047] Example 6: Long-term operational performance evaluation of the method of the present invention in a continuous flow reactor Experimental Apparatus and Methods: A continuous flow photocatalytic reaction apparatus was constructed (see schematic diagram below). Figure 6 A 25 L solution of PFOA with an initial concentration of 20 mg / L was prepared using tap water as the treatment target. A peristaltic pump was used to control the influent flow rate at 20 mL / min. The reaction apparatus consisted of a single ultraviolet reactor. Reduction section: The concentration of Na2SO3 in the water entering this section is maintained at 10~80 mM by online dosing.

[0048] Oxidation section: After the defluorination rate in the reduction section tends to stabilize, Na2S2O8 is added online to maintain its concentration at 20~100mM.

[0049] Both reaction sections use 75 W UV lamps with a wavelength of 254 nm. After startup, the system runs continuously for an extended period, with periodic monitoring of the fluoride ion concentration in the effluent from the reduction section and the final effluent to calculate the defluorination rate.

[0050] Execution results: Long-term running results are as follows Figure 7 As shown.

[0051] Standalone reduction section operation phase (0~120 hours): During the first 120 hours, only the reduction section (UV / SF) is turned on, and no oxidant is added. At this time, the PFOA defluorination rate of the system effluent is stable at about 79%, and cannot be further improved, which is consistent with the defluorination bottleneck phenomenon of the single reduction process in Example 1.

[0052] Combined process operation phase (after 120 hours): When the reaction has proceeded for 120 hours, the dosing system of the oxidation section is started. Thereafter, the defluoridation rate of the final effluent from the system rapidly increases and remains stable at over 99% for about 150 hours until the end of operation (240 hours), demonstrating extremely high stability and thorough defluoridation capability.

[0053] Conclusion: The results of this embodiment demonstrate that the proposed combined reduction-oxidation process (UV / SF-UV / PDS) remains stable and reliable in continuous flow operation, achieving deep defluorination of PFOA (>99%). This process fully leverages the synergistic advantages of the efficient start-up defluorination of the UV / SF system and the deep mineralization of hydrogen-containing intermediates by the UV / PDS system, successfully solving the problem of incomplete defluorination during PFOA treatment and providing a highly promising technical solution for the practical engineering treatment of perfluorinated pollutants.

[0054] Those skilled in the art will understand that the above embodiments are specific examples of implementing the present invention, and in practical applications, various changes in form and detail can be made without departing from the spirit and scope of the present invention. Any person skilled in the art can make their own modifications and alterations without departing from the spirit and scope of the present invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy, characterized in that, Includes the following steps: S1, Reduction Defluorination Stage: Sulfite is added to water containing PFOA, and a photoreduction reaction is initiated under ultraviolet light irradiation. The generated hydrated electrons attack the CF bond in the PFOA molecule to carry out reduction defluorination, generating hydrogen-containing fluorinated intermediates. S2, Oxidation and Mineralization Stage: After the reaction in step S1 reaches the defluorination bottleneck, there is no need to separate the intermediate products. Persulfate is directly added to the reaction solution, and the photo-oxidation reaction is initiated under ultraviolet light irradiation. The generated oxidizing free radicals are used to degrade the hydrogen-containing fluorinated intermediate products, thereby achieving deep defluorination and mineralization of PFOA.

2. The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to claim 1, characterized in that, In step S1, the sulfite is selected from sodium sulfite or potassium sulfite, and the concentration is 10~80 mM.

3. The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to claim 1, characterized in that, In step S2, the persulfate is selected from sodium persulfate or potassium persulfate, and the concentration is 20~100 mM.

4. The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to claim 1, characterized in that, The reaction time for step S1 is 10-12 hours, and the hydrogen-containing fluorinated intermediate is a fluorinated telomer carboxylic acid intermediate; the reaction time for step S2 is 10-12 hours.

5. The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to claim 1, characterized in that, In steps S1 and S2, the water containing PFOA contains Cl... NO3 HCO3 The reaction can proceed under inorganic anionic conditions and / or in water with an initial pH range of 3.4 to 11.

0.

6. The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to claim 1, characterized in that, The method achieves a defluoridation rate of over 95% for water containing PFOA.

7. The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to claim 1, characterized in that, In steps S1 and S2, the power of the ultraviolet light is 75 W and the wavelength is 254 nm.

8. The application of a method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy according to any one of claims 1 to 7 in the field of PFOA-containing wastewater treatment.

9. The application according to claim 8, characterized in that, The PFOA-containing wastewater includes PFOA-polluted water bodies with tap water as the substrate, PFOA-polluted water bodies with reservoir water as the substrate, and secondary effluent from wastewater treatment plants.

10. The application according to claim 8, characterized in that, The method for deep defluorination and mineralization of perfluorooctanoic acid based on reduction-oxidation synergy is implemented in a continuous flow reactor, which includes an ultraviolet reactor. Specifically, wastewater containing PFOA first flows through the ultraviolet reactor containing sulfite. After the reduction reaction residence time is satisfied, persulfate is added online to the ultraviolet reactor, and ultraviolet irradiation continues, thereby achieving continuous deep defluorination of wastewater containing PFOA.

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