Method for removing nitrate in water by using modified zero-valent iron aerogel electrocatalyst

Abstract

The invention provides a method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst, which comprises the following steps: preparing a mixed solution of ferrous sulfate and sodium sulfide, preparing a sodium borohydride solution, mixing and magnetically stirring the mixed solution and the sodium borohydride solution to obtain black colloid, and finally obtaining vulcanized zero-valent iron aerogel, dispersing the obtained sulfurized zero-valent iron aerogel and a Nafion solution in absolute ethyl alcohol to prepare a catalyst mixed solution; potassium nitrate and potassium hydroxide are dissolved in a pure water solution to serve as electrolyte, a sulfurized zero-valent iron aerogel electrode is used as a cathode, a platinum sheet is used as an anode, a mercuric oxide electrode is used as a reference electrode, and an electrocatalytic reduction reaction is carried out by using a three-electrode system. The technology not only can improve the efficiency of water nitrate pollution treatment, but also conforms to the concept of green and low-carbon development, and has a wide environment application prospect.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of water treatment, specifically a method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst. Background Art

[0002] The rapid development of modern industry has greatly promoted social and economic progress, but at the same time has led to the emission of a large number of pollutants, especially the pollution problem of nitrate-containing wastewater is becoming increasingly serious. Nitrate (NO3⁻), as the main pollutant in industrial, agricultural and urban wastewater, is widely sourced from industries such as fertilizer application, mining, metal smelting, electroplating wastewater, and food processing. For a long time, due to the high water solubility and chemical stability of nitrate, its enrichment in the water environment has become one of the environmental problems of global concern.

[0003] Currently, the treatment methods for nitrate-containing wastewater mainly include physical methods (ion exchange, reverse osmosis), chemical methods (chemical reduction, catalytic reduction), biological methods (heterotrophic and autotrophic denitrification), etc. However, these methods still have many limitations in practical applications. For example, although ion exchange and reverse osmosis can effectively remove nitrate, they have high energy consumption, the resin is easily saturated and needs to be replaced frequently; the biological denitrification method is relatively sensitive to environmental conditions, is easily affected by factors such as temperature and pH value, and has a long treatment time; traditional chemical catalytic reduction methods (such as Cu-Zn reduction method) usually require the use of external reducing agents, such as hydrogen or formic acid, which not only increases the operating cost but also may bring secondary pollution problems. Therefore, the development of an efficient, low-cost and environmentally friendly nitrate removal technology has become a research hotspot in the field of water pollution treatment.

[0004] In recent years, electrocatalytic nitrate reduction technology has received extensive attention due to its advantages such as green sustainability, simple operation, and strong selectivity. This technology utilizes the catalytic reaction on the electrode surface and, by regulating the electron transfer process, nitrate is gradually reduced to nitrogen gas (N2) under the action of an external electric field. Compared with traditional methods, electrocatalytic nitrate reduction not only avoids the addition of chemical agents but also can achieve efficient nitrogen generation at normal temperature and pressure. However, existing electrocatalytic materials still have problems such as low catalytic activity, insufficient selectivity, and excessive by-products (such as ammonia, nitrite), which limit the large-scale application of this technology.

[0005] Zero-valent iron (Fe 0 ) shows great potential in electrocatalytic nitrate reduction due to its excellent electron transfer ability and low cost. However, the catalytic efficiency and stability of traditional zero-valent iron materials are relatively low, and surface oxidation passivation is likely to occur, resulting in a decrease in catalytic activity. Summary of the Invention

[0006] To address the deficiencies of current technologies, the present invention combines existing technologies and starts from practical applications to provide a method for removing nitrate in water using a modified zero-valent iron aerogel electrocatalyst. By introducing an appropriate amount of sulfur doping into the zero-valent iron aerogel, an effective sulfur site network is constructed, thereby significantly improving the selectivity and catalytic efficiency of the electrocatalytic reaction and achieving the efficient conversion of nitrate to nitrogen. This technology can not only improve the efficiency of treating nitrate pollution in water bodies but also conforms to the concept of green and low-carbon development, having broad environmental application prospects.

[0007] The technical solution of the present invention is as follows:

[0008] A method for removing nitrate in water using a modified zero-valent iron aerogel electrocatalyst, comprising the following steps:

[0009] Step 1: Dissolve ferrous sulfate and sodium sulfide simultaneously in a pure aqueous solution treated with nitrogen aeration, and set aside after magnetic stirring evenly.

[0010] Step 2: Dissolve sodium borohydride in a pure aqueous solution treated with nitrogen aeration, and set aside after magnetic stirring evenly.

[0011] Step 3: Rapidly mix the solution obtained in Step 1 with the solution obtained in Step 2, and perform aging treatment after magnetic stirring to obtain a black colloid.

[0012] Step 4: Filter and collect the black colloid product obtained in Step 3, wash it multiple times with deionized water and tert-butanol respectively, and freeze-dry the washed colloid to obtain sulfurized zero-valent iron aerogel.

[0013] Step 5: Disperse the sulfurized zero-valent iron aerogel obtained in Step 4 and 5wt.% Nafion solution in absolute ethanol, and perform ultrasonic treatment to ensure uniform mixing to prepare a catalyst mixture; subsequently, apply the prepared catalyst mixture on both sides of the carbon paper to form a working electrode, and then place the coated carbon paper in an anaerobic glove box for drying treatment at room temperature.

[0014] Step 6: Dissolve potassium nitrate and potassium hydroxide in a pure aqueous solution as the electrolyte.

[0015] Step 7: Inject the electrolyte prepared in Step 6 into the anode chamber and cathode chamber of the H-type electrolytic cell; use the sulfurized zero-valent iron aerogel electrode prepared in Step 5 as the cathode, use a platinum sheet as the anode, and a mercury oxide electrode as the reference electrode, and perform an electrocatalytic reduction reaction using a three-electrode system.

[0016] Step 8: Continuously introduce argon into the cathode chamber, apply a constant voltage, start the electrocatalytic reduction reaction of nitrate, and after a period of reaction, the selective conversion of nitrate pollutants to harmless nitrogen can be achieved.

[0017] Further, in Step 1, the molar concentration ratio of ferrous sulfate to sodium sulfide after being dissolved in pure aqueous solution is 10:1;

[0018] In Step 2, the concentration of sodium borohydride after being dissolved in pure aqueous solution and the molar concentration of ferrous sulfate after being dissolved in pure aqueous solution in Step 1 have a ratio of 5:1;

[0019] In Step 3, the solution obtained in Step 1 and the solution obtained in Step 2 are mixed according to a volume ratio of 9:1.

[0020] Further, in Step 1, the concentration of ferrous sulfate is 10 mmol / L and the concentration of sodium sulfide is 1 mmol / L;

[0021] In Step 2, the concentration of sodium borohydride is 50 mmol / L.

[0022] Further, in Step 3, the aging time of the black colloid is not less than 2 hours; in Step 4, the alternate freeze-drying time after cleaning is not less than 20 hours.

[0023] Further, in Step 5, the zero-valent iron sulfide aerogel, Nafion solution and absolute ethanol are in the following ratio: 5 mg of zero-valent iron sulfide aerogel corresponds to 20 μL of 5 wt.% Nafion solution and 980 μL of absolute ethanol.

[0024] Further, in Step 6, the molar concentration ratio of potassium nitrate to potassium hydroxide after being dissolved in pure aqueous solution is 1:(25 - 30).

[0025] Further, in Step 6, the concentration of potassium nitrate after being dissolved in pure aqueous solution is 360 mg / L, and the concentration of potassium hydroxide after being dissolved in pure aqueous solution is 0.1 mol / L.

[0026] Advantages of the present invention:

[0027] (1) The preparation process of the zero-valent iron sulfide aerogel catalyst used in the present invention is simple, with high stability and high selectivity for the reduction of nitrate pollutants.

[0028] (2) When using the novel electrocatalytic reduction method constructed by the present invention to treat nitrate pollutants in water, it can effectively avoid the limitations of traditional electrocatalytic reduction methods such as high energy consumption, high material consumption and high carbon emissions, without the need to add a large amount of chemical reagents, and the reaction is carried out at normal temperature and pressure, with good environmental adaptability.

[0029] (3) The zero-valent iron sulfide aerogel catalyst prepared by the present invention can achieve higher nitrate conversion rate and nitrogen-selective yield, and has a relatively long service life. Description of the Drawings

[0030] Figure 1It is the high-magnification transmission electron microscope image of the zero-valent iron sulfide aerogel in Example 1;

[0031] Figure 2 It is the performance comparison chart of nitrate removal rate and nitrogen selectivity of Example 1 and Comparative Examples 1-2 under different electrode conditions;

[0032] Figure 3 It is the product distribution chart of electrocatalytic reduction of nitrate at different times by the zero-valent iron sulfide aerogel in Example 1;

[0033] Figure 4 It is the performance chart of continuous flow cyclic use of the zero-valent iron sulfide aerogel electrode in Example 1;

[0034] Figure 5 It is the performance comparison chart of nitrogen selectivity of Example 1 and Comparative Examples 3-6 under different initial potassium nitrate concentration conditions. Detailed implementation manners

[0035] In combination with the attached drawings and specific embodiments, the present invention will be further described. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by this application.

[0036] Example 1:

[0037] This example provides a method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst, and the steps are as follows:

[0038] (1) Dissolve ferrous sulfate and sodium sulfide in the pure aqueous solution after nitrogen aeration treatment, and stir magnetically until evenly mixed for later use; among them, the concentration of ferrous sulfate is 10 mmol / L; the concentration of sodium sulfide is 1 mmol / L;

[0039] (2) Dissolve sodium borohydride in the pure aqueous solution after nitrogen aeration treatment, and stir magnetically until evenly mixed for later use; among them, the concentration of sodium borohydride is 50 mmol / L;

[0040] (3) Rapidly mix the solution obtained in step 1 and the solution obtained in step 2, stir magnetically to obtain a black colloid, and age for 2 hours;

[0041] (4) Filter and collect the black colloid product obtained in step 3, wash it three times with deionized water and tert-butanol respectively, and freeze-dry the washed colloid for 20 hours to obtain zero-valent iron sulfide aerogel;

[0042] (5) Weigh 5 mg of the zero-valent iron sulfide aerogel obtained in step 4 and disperse it in 980 μL of absolute ethanol together with 20 μL of 5 wt.% Nafion solution. Sonicate for 30 minutes to ensure uniform mixing, and prepare a catalyst mixture. Subsequently, coat 80 μL of the catalyst ink on both sides of a 1×1 cm 2 carbon paper, and control the working electrode area to 2 cm 2 ; then place the coated carbon paper in an anaerobic glove box and dry it at room temperature for 6 hours;

[0043] (6) Dissolve potassium nitrate and potassium hydroxide in pure aqueous solution as the electrolyte, with the concentrations of potassium nitrate and potassium hydroxide being 360 mg / L and 0.1 mol / L respectively;

[0044] (7) Inject the electrolyte prepared in step 6 into the anode chamber and cathode chamber of an H-type electrolytic cell; use the zero-valent iron sulfide aerogel electrode prepared in step 5 as the cathode, use a platinum sheet (1 × 1 cm 2 ) as the anode, and a mercury oxide electrode as the reference electrode, and use a three-electrode system to conduct an electrocatalytic reduction reaction;

[0045] (8) Continuously introduce argon into the cathode chamber, apply a constant voltage, and initiate the electrocatalytic reduction reaction of nitrate. After reacting for a period of time, the harmless treatment of nitrate pollutants can be directed and selectively converted into nitrogen; among them, the applied constant voltage is −0.6 V vs. RHE; the reaction time is 8 hours.

[0046] The high-resolution transmission electron microscopy image of the zero-valent iron sulfide aerogel prepared in Example 1 is as Figure 1 shown. The image in the upper left frame in the figure is an enlarged view of the crystal region. The image shows that the aerogel has a unique core-shell structure, with its crystalline Fe core being wrapped by a thin amorphous S-rich shell. The results of fast Fourier transform and inverse fast Fourier transform show that it conforms to the lattice parameters. And it can be Figure 2 seen that using the zero-valent iron sulfide aerogel electrode, nitrate can be 100% removed within 8 hours, and the selectivity for nitrogen can reach 85.6%. This result indicates that the electrocatalytic reduction system with zero-valent iron sulfide aerogel as the cathode can simultaneously achieve the removal of nitrate and its harmless conversion into nitrogen under environmental conditions. In addition, after the zero-valent iron sulfide aerogel electrode is continuously recycled 20 times, its catalytic activity is not significantly inhibited, indicating its high stability ( Figure 4 ).

[0047] Comparative Example 1:

[0048] Comparative Example 1 used the method of Example 1 to achieve the removal of nitrate in water, and the differences were as follows: in step (1), only ferrous sulfate with a concentration of 10 mmol / L was used, and sodium sulfide was not used, and the obtained product in step (4) was zero-valent iron aerogel.

[0049] Comparative Example 2:

[0050] Comparative Example 2 used the method of Comparative Example 1 to achieve the removal of nitrate in water, and the differences were as follows: the solution obtained in step 1 and the solution obtained in step 2 were slowly mixed and magnetically stirred to obtain black particles, and the obtained product in step (4) was zero-valent iron nanoparticles.

[0051] From Figure 2 it can be obtained the comparative graph of nitrate removal rate and nitrogen selectivity performance of Example 1, Comparative Example 1, and Comparative Example 2 under different electrode conditions. It can be seen that when using zero-valent iron aerogel and zero-valent iron nanoparticle electrodes as the cathode, although 100% removal of nitrate in water can be achieved within 8 hours, the selective yields for nitrogen are only 7.3% and 15.2% respectively, indicating that the zero-valent iron aerogel catalyst has no selectivity for the catalytic reduction of nitrate to nitrogen.

[0052] From Figure 2 and Figure 3 it can be known that when using sulfided zero-valent iron aerogel as the cathode, while 100% removal of nitrate in water can be achieved within 8 hours, the by-products of ammonia nitrogen and nitrite nitrogen are significantly inhibited, and its selectivity for nitrogen can reach 85.6%. Compared with the catalytic performance of zero-valent iron aerogel, it can be found that the introduction of sulfur elements can promote the harmless directional conversion of electrocatalytic reduction of nitrate by sulfided zero-valent iron aerogel to nitrogen.

[0053] Comparative Example 3:

[0054] The difference between Comparative Example 3 and Example 1 was that the initial potassium nitrate concentration in the electrolyte in step (6) was adjusted to 180 mg / L.

[0055] Comparative Example 4:

[0056] The difference between Comparative Example 4 and Example 1 was that the initial potassium nitrate concentration in the electrolyte in step (6) was adjusted to 720 mg / L.

[0057] Comparative Example 5

[0058] The difference between this comparative example and Example 1 was that the initial potassium nitrate concentration in the electrolyte in step (6) was adjusted to 1080 mg / L.

[0059] Comparative Example 6

[0060] The difference between this comparative example and Example 1 is that the initial potassium nitrate concentration in the electrolyte in step (6) is adjusted to 1440 mg / L.

[0061] The selectivities of nitrate conversion to nitrogen in the electrocatalytic reduction systems in Example 1 and Comparative Examples 3 to 6 under different initial potassium nitrate concentrations are as Figure 5 shown. The results show that zero-valent iron sulfide aerogel can achieve the harmless and directional conversion of nitrate to nitrogen in a wide range of nitrate concentrations, but the effect is the best when the potassium nitrate concentration is 360 mg / L.

[0062] From the above examples and comparative examples, it can be seen that the method for selectively converting nitrate in water to nitrogen by electrocatalytic reduction provided by the present invention can not only achieve the effect of purifying wastewater, but also simultaneously realize the harmless and directional conversion of nitrate.

Claims

1. A method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst, characterized in that, It includes the following steps: Step 1: Dissolve ferrous sulfate and sodium sulfide simultaneously in the pure aqueous solution treated by nitrogen aeration. After magnetic stirring evenly, reserve it for later use; Step 2: Dissolve sodium borohydride in the pure aqueous solution treated by nitrogen aeration. After magnetic stirring evenly, reserve it for later use; Step 3: Rapidly mix the solution obtained in Step 1 with the solution obtained in Step 2. After magnetic stirring to obtain a black colloid, conduct an aging treatment; Step 4: Filter and collect the black colloid product obtained in Step 3, and wash it multiple times with deionized water and tert-butanol respectively. After freeze-drying the washed colloid, obtain zero-valent iron sulfide aerogel; Step 5: Disperse the zero-valent iron sulfide aerogel obtained in Step 4 and 5wt.% Nafion solution in absolute ethanol, and conduct ultrasonic treatment to ensure uniform mixing to prepare a catalyst mixture; Subsequently, coat the prepared catalyst mixture on both sides of the carbon paper to form a working electrode, and then place the coated carbon paper in an anaerobic glove box for drying treatment at room temperature; Step 6: Dissolve potassium nitrate and potassium hydroxide in the pure aqueous solution as the electrolyte; Step 7: Inject the electrolyte prepared in Step 6 into the anode chamber and cathode chamber of the H-type electrolytic cell; Use the zero-valent iron sulfide aerogel electrode prepared in Step 5 as the cathode, use a platinum sheet as the anode, and a mercury oxide electrode as the reference electrode, and conduct an electrocatalytic reduction reaction using a three-electrode system; Step 8: Continuously introduce argon into the cathode chamber, apply a constant voltage, start the electrocatalytic reduction reaction of nitrate, and after reacting for a period of time, the harmless treatment of selectively converting nitrate pollutants into nitrogen can be realized.

2. The method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst according to claim 1, characterized in that, In Step 1, the molar concentration ratio of ferrous sulfate and sodium sulfide after being dissolved in the pure aqueous solution is 10:1; In Step 2, the concentration of sodium borohydride after being dissolved in the pure aqueous solution and the molar concentration of ferrous sulfate after being dissolved in the pure aqueous solution in Step 1 have a ratio of 5:1; In Step 3, the solution obtained in Step 1 and the solution obtained in Step 2 are mixed according to a volume ratio of 9:

1.

3. The method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst according to claim 2, characterized in that, In Step 1, the concentration of ferrous sulfate is 10 mmol / L, and the concentration of sodium sulfide is 1 mmol / L; In Step 2, the concentration of sodium borohydride is 50 mmol / L.

4. The method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst according to claim 1, characterized in that, In Step 3, the aging time of the black colloid is not less than 2 hours; In Step 4, the alternate freeze-drying time after washing is not less than 20 hours.

5. The method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst according to claim 1, wherein In Step 5, the zero-valent iron sulfide aerogel, Nafion solution and absolute ethanol are in the following ratio: 5 mg of zero-valent iron sulfide aerogel corresponds to 20 μL of 5wt.% Nafion solution and 980 μL of absolute ethanol.

6. The method for removing nitrate in water by using a modified zero-valent iron aerogel electrocatalyst according to claim 1, characterized in that, In Step 6, the molar concentration ratio of potassium nitrate and potassium hydroxide after being dissolved in the pure aqueous solution is 1:(25 - 30).

7. The method for removing nitrate in water by using the modified zero-valent iron aerogel electrocatalyst according to claim 6, wherein, In Step 6, the concentration of potassium nitrate after being dissolved in the pure aqueous solution is 360 mg / L, and the concentration of potassium hydroxide after being dissolved in the pure aqueous solution is 0.1 mol / L.

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