Method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride and application

Abstract

The invention relates to a method for removing pollutants in water by persulfate oxidation, in particular to a method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride and application, and the method comprises the following steps: mixing a modified carbon nitride activating agent and a persulfate oxidizing agent with water containing perfluorooctanoic acid, and carrying out temperature-controlled stirring for reaction; the modified carbon nitride activating agent is obtained by calcining trithiocyanuric acid. Compared with the prior art, the defects that in the prior art, a persulfate treatment activating agent is complex in preparation, many in limitation and poor in effect are overcome, according to the scheme, the PFOA in the water is treated through the modified carbon nitride serving as the activating agent and the persulfate, and the PFOA is efficiently removed from the water in a low-limitation and easy-to-operate mode.

Description

Technical Field

[0001] The invention relates to a method for removing pollutants in water by persulfate oxidation, and in particular to a method for removing perfluorooctanoic acid in water by carbon nitride-activated persulfate and application thereof. Background Art

[0002] Perfluorooctanoic acid (PFOA) is a typical persistent organic pollutant with the characteristics of strong toxicity, high stability, bioaccumulation and long-range migration, which poses serious hazards to human health and the natural environment. In June 2022, the U.S. Environmental Protection Agency released the final health advisory levels for various pollutant indicators in drinking water, which recommended a PFOA concentration of 0.004ng / L in drinking water. Therefore, there is an urgent need for a simple and effective technical method for removing PFOA pollution from water environments. The adsorption technologies currently used (such as the use of carbon materials, etc.) often have poor removal effects; and the existing degradation technologies are generally costly and complex (such as the need for ultraviolet rays, ion beam assistance, etc.). Therefore, it is of great significance to develop a PFOA degradation and removal technology that is simple to operate, efficient and easy to implement.

[0003] Persulfate oxidation technology is considered to be a new type of green advanced oxidation technology because of its advantages such as no secondary pollution, easy operation, high efficiency and long reaction life. However, the oxidation efficiency of persulfate is low in its original state, so it is usually necessary to use other technologies (such as ultraviolet irradiation, ultrasound, etc.) or materials (such as metals, non-metals and their oxides, etc.) to activate and accelerate the oxidation reaction, thereby improving the reaction efficiency. However, these technologies are often harsh and difficult to operate, and the preparation of activated materials is complex and costly, which greatly limits the application of persulfate advanced oxidation technology in removing environmental pollutants.

[0004] For example, "Study on the Oxidative Degradation of Perfluorooctanoic Acid (PFOA) in Groundwater by Activated Persulfate" (Yin Penghua, Nanjing University, 2016) discloses a method for degrading PFOA by activated persulfate, which, under the reaction conditions of pH = 2.0, 50°C, and 100 times the amount of sodium persulfate added, can achieve a PFOA degradation rate of 89.9% after 100 hours of reaction. However, this method requires a large amount of dosage and a long reaction time, resulting in low overall treatment efficiency.

[0005] Therefore, it is of great significance to develop a PFOA degradation and removal technology that uses persulfate oxidation technology and is simple to operate, highly efficient and easy to implement. Summary of the invention

[0006] The purpose of the present invention is to solve at least one of the above problems and to provide a method and application of removing perfluorooctanoic acid in water by activating persulfate with carbon nitride, so as to solve the shortcomings of the prior art that the activator for activating persulfate is complex to prepare, has many restrictions and has poor effect. This scheme uses modified carbon nitride as an activator to treat PFOA in water together with persulfate, thereby achieving efficient, low-restriction and easy-to-operate removal of PFOA from water.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] The first aspect of the present invention discloses a method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride, comprising the following steps:

[0009] The modified carbon nitride activator and the persulfate oxidant are mixed with water containing perfluorooctanoic acid, and stirred at controlled temperature to react;

[0010] The modified carbon nitride activator is obtained by calcining thiocyanate.

[0011] The modified carbon nitride used in this scheme as an activator of persulfate can accelerate the reaction and improve the efficiency of PFOA degradation. In addition, the modified carbon nitride material used in this scheme is a modified carbon nitride containing sulfur element directly calcined by trithiocyanate. Due to the presence of sulfur element, the bandwidth and other properties of the material are better than those of the carbon nitride material without any doping elements.

[0012] Specifically: The addition of sulfur can replace the N atoms in C3N4 and promote the electron transfer on the adjacent C, because S-C3N4 (S-doped carbon nitride) has an electronegativity (2.58V) comparable to that of C (2.55V). In this case, the spin density and positive charge density of the C atoms increase, destroying the chemical inertness of the C network and promoting the transfer of electrons from C3N4 to persulfate for activation. Moreover, the addition of sulfur reduces the energy difference between the HOMO and LUMO (Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital) of carbon nitride to the maximum. The lower HOMO level in the narrow band gap of C3N4 is conducive to improving the compatibility between its HOMO and the LUMO of persulfate, thereby improving the electron transfer efficiency of persulfate activation. The narrower band gap will also accelerate the activation of persulfate to generate reactive oxygen species (ROS), accelerating the reaction.

[0013] The S doping energy level is above the valence band maximum of heteroatom-doped C3N4. The extra valence electrons generated by sulfur are transferred to the adjacent C atoms, resulting in the formation of N vacancies below the conduction band minimum. These N vacancy states play a crucial role in narrowing the band gap.

[0014] The modified carbon nitride activator in this scheme is directly synthesized by calcination, which is a simple method. The pore size is fine, uniform and spherical. According to the pore size characterization, the maximum pore size of the material is 0.24 cm 3 / g, the most concentrated pore size is 0.21cm 3 / g. The modified carbon nitride material has a larger specific surface area and a finer and more uniform pore size, which can increase the number of activation sites, improve the electron transfer efficiency and thus accelerate the PFOA degradation reaction. In addition, the fine pore size and large specific surface area are conducive to increasing the adsorption amount of the persulfate surface, thereby combining with more persulfate surface functional groups to improve the reaction efficiency. In addition, persulfate can also act as an electron acceptor to capture the electrons generated on the carbon nitride surface and promote the electron (e - ) and holes (h + ) separation and improve activation performance.

[0015] Persulfate advanced oxidation technology is an oxidation technology that produces sulfate radicals as the main free radicals. It has the advantages of long free radical lifetime, high oxidation potential, low requirements for the pH of the reaction system and good selectivity. It is a powerful removal technology for removing environmental pollutants. Carbon nitride materials are a type of non-metallic semiconductor materials composed of carbon and nitrogen elements. They have a stable structure and are insoluble in acids, alkalis and organic solvents. The material has the advantages of simple synthesis methods and easy surface modification and modification. In addition, the modification method of element (S) doping of carbon nitride materials can effectively increase its specific surface area, improve charge density and electron transfer ability, thereby improving its ability to remove target pollutants as a persulfate activator.

[0016] Preferably, the modified carbon nitride activator is obtained by calcining thiocyanate at 550° C. for 4 hours and then naturally cooling to room temperature.

[0017] Preferably, the amount of the modified carbon nitride activator added to the water containing perfluorooctanoic acid is 0.01-1 g / L; the amount of the persulfate oxidant added to the water containing perfluorooctanoic acid is 10-200 mmol / L.

[0018] More preferably, the amount of the persulfate oxidant added to the water containing perfluorooctanoic acid is in the range of 50-200 mmol / L.

[0019] Most preferably, the amount of the modified carbon nitride activator added to the water containing perfluorooctanoic acid is 0.1 g / L; the amount of the persulfate oxidant added to the water containing perfluorooctanoic acid is 100 mmol / L.

[0020] Preferably, the temperature of the temperature-controlled stirring is 25-60° C., and the time is 0.5-6 h.

[0021] Preferably, the temperature of the temperature-controlled stirring is 60° C., the time is 6 hours, and the rotation speed is 350 r / min.

[0022] Preferably, the method comprises the following steps:

[0023] 0.1 g / L of modified carbon nitride activator and 100 mmol / L of persulfate oxidant were mixed with water containing perfluorooctanoic acid, and stirred at 60°C and 350 r / min for 6 hours to react;

[0024] The modified carbon nitride activator is obtained by calcining trithiocyanate at 550° C. for 4 hours and then naturally cooling to room temperature;

[0025] The removal rate of perfluorooctanoic acid in the water containing perfluorooctanoic acid (calculated by the percentage of the PFOA concentration at the end of the reaction to the initial PFOA concentration, and subtracted from 100%) is 98.5%.

[0026] Preferably, the concentration of perfluorooctanoic acid in the water containing perfluorooctanoic acid is determined by the following method:

[0027] Methanol was added to the mixed solution after the reaction to terminate the reaction, and the mixture was mixed and refrigerated for standing; the supernatant after standing was taken and filtered; the filtrate was diluted and the concentration of perfluorooctanoic acid was determined by UPLC-MS / MS.

[0028] Preferably, the volume ratio of the mixed solution to methanol is 5:0.4; the temperature of the refrigerated storage is 4°C; the filtration is performed through a 0.22 μm polyethersulfone filter membrane; and the dilution multiple is 50 times.

[0029] The second aspect of the present invention discloses the application of any of the above methods for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride in the field of water treatment.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] This scheme uses modified carbon nitride as a persulfate activator to activate persulfate under certain temperature conditions to produce active species to remove PFOA in water. This method has the advantages of simple operating conditions and good removal effect.

[0032] (1) The carbon nitride preparation method used in the present invention is simple and has a stable structure. The carbon nitride precursor material used, thiocyanate, is easily available and low in price.

[0033] (2) The persulfate advanced oxidation system used in the present invention has simple conditions and is easy to implement. The reaction process is green and efficient and does not produce secondary pollution.

[0034] (3) The carbon nitride activated persulfate reaction system constructed by the present invention has a high removal rate of PFOA in aqueous solution, which can reach 98.5% in 6 hours.

[0035] (4) The degradation reaction system constructed by the present invention is expected to be widely used in other water environment pollutant treatment processes, with low operating costs and simple operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is the dynamic removal rate diagram of PFOA in the reaction system under different temperature conditions;

[0037] Figure 2 This is the dynamic removal rate diagram of PFOA in the reaction system under different concentrations of sodium persulfate;

[0038] Figure 3 is the dynamic removal rate of PFOA in the reaction system under different concentrations of carbon nitride. DETAILED DESCRIPTION

[0039] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] In the following description, unless otherwise specified, the reagents used may be conventional commercial products, and the methods used are well known in the art.

[0041] The modified carbon nitride material is used as an activator for persulfate to construct a degradation and removal technology for PFOA in water, which has the advantages of convenient operation, high removal rate, and economic efficiency. The specific concept is as follows: (1) preparation of modified carbon nitride materials; (2) optimization of reaction temperature; (3) optimization of persulfate concentration (sodium persulfate is used as an example later); (4) optimization of the amount of modified carbon nitride material added; (5) determination of PFOA removal rate under optimal conditions.

[0042] Example

[0043] Take 100mL of reaction system (modified carbon nitride activator + sodium persulfate oxidant + PFOA aqueous solution) as an example:

[0044] (1) The reaction system was added into a beaker according to the target test concentration, wherein the initial concentration of PFOA was controlled at 10 mg / L.

[0045] (2) Add a magnetic stirrer to the beaker and place the beaker on a temperature-controlled magnetic stirrer. Insert a thermometer into the reaction system and control the height to prevent the magnetic stirrer from lifting the thermometer. This completes the experimental preparation.

[0046] (3) The reaction system is stirred at a stirring speed of 350 r / min under controlled temperature to allow the reaction to occur.

[0047] (4) After stirring at the target test temperature for a certain period of time, take 5 mL of the sample solution and add 0.4 mL of methanol to the sample solution to terminate the reaction. After vortex mixing, immediately transfer to a refrigerator at 4°C and refrigerate for at least half an hour.

[0048] (5) After the sample solution has been fully refrigerated and allowed to stand, the supernatant is taken and filtered through a 0.22 μm polyethersulfone filter membrane to obtain a filtered sample solution.

[0049] (6) The filtered sample was diluted 50 times and then detected by high performance liquid chromatography-triple quadrupole tandem mass spectrometry (UPLC-MS / MS) to detect the change in PFOA concentration during the reaction and calculate the PFOA removal rate.

[0050] in,

[0051] The modified carbon nitride activator is obtained by calcining trithiocyanate at 550°C in a muffle furnace for 4 hours and then cooling to room temperature in a natural state.

[0052] Target test concentrations include: sodium persulfate concentration gradient of 10mmol / L, 50mmol / L, 100mmol / L and 200mmol / L, and carbon nitride dosage concentration gradient of 1g / L, 0.1g / L and 0.01g / L; target test temperatures include: 25℃, 40℃ and 60℃; certain time refers to sampling 5mL of liquid at 0.5h, 1h, 2h, 3h, 4h, 5h and 6h, respectively, to determine the dynamic removal rate of PFOA within 6h of reaction.

[0053] The concentration of PFOA was detected by UPLC-MS / MS using the external standard method for calibration.

[0054] The PFOA removal rate was obtained by comparing the PFOA concentration at the end of the reaction with the initial PFOA concentration. The initial PFOA concentration was measured by taking the initial sample solution after stirring for 5 minutes in step (3) to make the reaction system uniform (avoiding feeding errors, reducing experimental errors, and ensuring data accuracy).

[0055] 1. Investigation of the temperature of the reaction system:

[0056] The reaction temperature was adjusted to 25°C, 40°C and 60°C, and each group was repeated three times, and the initial concentration sample solution was taken as C0. Other reaction parameters were: sodium persulfate 200mmol / L, modified carbon nitride 0.1g / L.

[0057] Start timing when the corresponding temperature condition is reached. At 0.5h, take 5mL samples from each of the three parallel groups of the three temperature groups, add 0.4mL methanol, vortex and quickly cool in a 4℃ refrigerator. Continue to take samples at 1h, 2h, 3h, 4h, 5h and 6h of the reaction, and repeat the above operation. The sampled liquid taken at different times is recorded as Ct (t=0.5, 1, 2, 3, 4, 5, 6). Each group of samples is taken out after cooling and precipitation in a 4℃ refrigerator for at least half an hour, and filtered through a polyethersulfone (PES) filter membrane. Dilute the filtered supernatant 50 times and add it to the injection vial and wait for machine detection.

[0058] The results of the machine test are referenced Figure 1 As shown in the results, it can be seen that the PFOA removal effect at 60°C is significantly better than that at 25°C and 40°C. The 6h removal rates of PFOA at 25°C and 40°C are 34.61% and 66.35% respectively, while the 6h removal rate of the PFOA removal system at 60°C can reach 96.9%. Therefore, 60°C was selected as the temperature condition for the subsequent reaction optimization.

[0059] 2. Investigation of sodium persulfate concentration in the reaction system:

[0060] The sodium persulfate concentrations were set to 10 mmol / L, 50 mmol / L, 100 mmol / L and 200 mmol / L respectively in a reaction system containing 10 mg / L PFOA solution and 0.1 g / L modified carbon nitride at a reaction temperature of 60°C for 6 hours.

[0061] The sampling, filtration and dilution process are the same as 1.

[0062] The test results are as follows: Figure 2 As shown in the figure, as the sodium persulfate concentration increases from 10mmol / L to 100mmol / L, the 6h removal rate of PFOA continues to rise, and the PFOA removal rates of 10mmol / L, 50mmol / L and 100mmol / L are 59.8%, 92.6% and 98.5%, respectively. The 6h removal rate of PFOA in the relatively high concentration 200mmol / L sodium persulfate system is 96.9%, indicating that the 100mmol / L sodium persulfate concentration has better overall performance. Therefore, the 100mmol / L sodium persulfate concentration is selected as the optimal sodium persulfate concentration condition.

[0063] 3. Research on the dosage of modified carbon nitride in the reaction system:

[0064] The dosage of modified carbon nitride was set to 0.01 g / L, 0.1 g / L and 1 g / L respectively, and the reaction was carried out in a 100 mL system of 60°C, 10 mg / LPFOA and 100 mmol / L sodium persulfate for 6 h.

[0065] The sampling, filtration and dilution process are the same as 1.

[0066] The test results are as follows: Figure 3 As shown in the figure, overall, the PFOA removal rates in the groups with different modified carbon nitride contents were not much different. The 6h PFOA removal rates corresponding to the modified carbon nitride dosages of 0.01g / L, 0.1g / L and 1g / L were 97.5%, 98.5% and 95.2%, respectively. In contrast, the 0.1g / L dosage had the best effect, so 0.1g / L modified carbon nitride was selected as the optimal dosage.

[0067] 4. Under the optimal reaction conditions, a 100 mL PFOA aqueous solution reaction system consisting of 100 mmol / L sodium persulfate and 0.1 g / L modified carbon nitride (added concentration is 10 mg / L) was stirred continuously for 6 hours under the action of a temperature-controlled magnetic stirrer, and the temperature was controlled at 60°C. After reacting for 6 hours, 5 mL of the sample was taken, 0.4 mL of methanol was added, and the system was vortexed to make it uniform and then placed in a 4°C refrigerator for refrigeration. After precipitation for at least half an hour, the supernatant was taken and passed through a 0.22 μm polyethersulfone filter membrane. After being diluted 50 times, it was analyzed and detected by high performance liquid chromatography-triple quadrupole tandem mass spectrometry (UPLC / MS / MS), and the removal rate of PFOA was 98.5%.

[0068] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for removing perfluorooctanoic acid from water by activating persulfate with carbon nitride, characterized in that: The steps include: The modified carbon nitride activator and the persulfate oxidant are mixed with water containing perfluorooctanoic acid, and stirred at controlled temperature to react; The modified carbon nitride activator is obtained by calcining thiocyanate.

2. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 1, characterized in that: The modified carbon nitride activator is obtained by calcining trithiocyanate at 550° C. for 4 hours and then naturally cooling to room temperature.

3. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 1, characterized in that: The amount of the modified carbon nitride activator added to the water containing perfluorooctanoic acid is 0.01-1 g / L; the amount of the persulfate oxidant added to the water containing perfluorooctanoic acid is 10-200 mmol / L.

4. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 3, characterized in that: The amount of the modified carbon nitride activator added to the water containing perfluorooctanoic acid is 0.1 g / L; the amount of the persulfate oxidant added to the water containing perfluorooctanoic acid is 100 mmol / L.

5. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 1, characterized in that: The temperature of the temperature-controlled stirring is 25-60° C., and the time is 0.5-6 hours.

6. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 5, characterized in that: The temperature of the temperature-controlled stirring is 60° C., the time is 6 hours, and the speed is 350 r / min.

7. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 1, characterized in that: The method comprises the following steps: 0.1 g / L of modified carbon nitride activator and 100 mmol / L of persulfate oxidant were mixed with water containing perfluorooctanoic acid, and stirred at 60°C and 350 r / min for 6 hours to react; The modified carbon nitride activator is obtained by calcining trithiocyanate at 550° C. for 4 hours and then naturally cooling to room temperature; The removal rate of perfluorooctanoic acid in the water containing perfluorooctanoic acid is 98.5%.

8. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 1, characterized in that: The concentration of perfluorooctanoic acid in the water containing perfluorooctanoic acid is determined by the following method: Methanol was added to the mixed solution after the reaction to terminate the reaction, and the mixture was mixed and refrigerated for standing; the supernatant after standing was taken and filtered; the filtrate was diluted and the concentration of perfluorooctanoic acid was determined by UPLC-MS / MS.

9. The method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride according to claim 8, characterized in that: The volume ratio of the mixed solution to methanol is 5:0.4; the temperature of the refrigerated storage is 4°C; the filtration is performed through a 0.22 μm polyethersulfone filter membrane; and the dilution multiple is 50 times.

10. Use of the method for removing perfluorooctanoic acid in water by activating persulfate with carbon nitride as claimed in any one of claims 1 to 9 in the field of water treatment.

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