Electrochemical short-cut denitrification method

Abstract

The present application relates to a kind of electrochemical short-range denitrification method, copper electrode is used as cathode, shape stable electrode is anode, a certain amount of sulfamic acid is added in nitrate wastewater in advance, then adjust the pH value of nitrate wastewater to suitable range, then nitrate wastewater is introduced into electrolytic cell and is electrolyzed to carry out denitrification treatment.Compared with the traditional denitrification method using electrochemical reduction nitrate to ammonia, the present application has the advantages of power saving, no gaseous ammonia pollution, no need to add chlorides, denitrification rate is fast, in addition, using metal copper as electrode material, not only electrode preparation is fast and convenient, preparation cost is low, and its catalytic activity has high stability, the present application shows broad application prospect in the field of wastewater denitrification.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment, and particularly relates to an electrochemical short-range denitrification method. BACKGROUND

[0002] Nitrogen pollution in water environment has become a global environmental problem. In many areas, nitrate nitrogen has become one of the main pollutants in surface water and groundwater due to the large use of nitrogen fertilizer, improper disposal of domestic sewage and nitrogen-containing industrial wastewater, etc. At present, the main treatment methods for nitrate nitrogen in wastewater include ion exchange, reverse osmosis, biological denitrification, catalytic hydrogenation and electrochemical reduction. Electrochemical reduction method has the advantages of high treatment efficiency, simple equipment, easy operation and low running cost, and gradually attracts widespread attention. The reduction reaction of nitrate on the cathode is the core of the whole electrochemical denitrification process, and the key to the denitrification efficiency of wastewater lies in the selection of cathode material.

[0003] The electrochemical reduction products of nitrate include nitrite, nitrogen and ammonia. Although non-toxic nitrogen is the most ideal product, the electrode materials developed at present do not have the performance of efficiently reducing nitrate to nitrogen, which cannot meet the needs of industrial wastewater treatment. The current electrochemical denitrification treatment scheme is to use noble metals (such as ruthenium, iridium, palladium, etc.) or non-noble metals (such as iron, brass, zinc, etc.) as cathode, first reduce nitrate in wastewater to ammonia nitrogen, and then couple with anode chlorine evolution method (i.e. breakpoint chlorine breaking) or stripping method to further remove ammonia nitrogen, so as to ultimately realize the complete removal of total nitrogen. However, this traditional electrochemical denitrification technology has the following defects: 1) high energy consumption: nitrate reduction to ammonia nitrogen needs to go through an 8-electron reaction process, which requires high current (kiloampere level), and the reaction has a high overpotential, which easily leads to the increase of electrolytic cell pressure; 2) high-salt wastewater is produced: the reduction of nitrate to ammonia nitrogen will consume a large amount of hydrogen ions in water, thereby producing strong alkaline wastewater, which needs to be neutralized by further adding acid; 3) a large amount of free ammonia is produced in the electrochemical reaction process, and ammonia is easy to volatilize from alkaline wastewater to the atmosphere, thereby causing serious gaseous ammonia pollution problem; 4) if anode chlorine evolution method is used to further remove ammonia nitrogen, a large amount of chlorine salt needs to be added to the wastewater, which will cause the rapid increase of salt concentration in the wastewater, in addition, a large amount of chlorine gas produced at the anode will also cause serious corrosion of equipment and safety hazards to personnel; 5) if stripping method is used to remove ammonia nitrogen, absorption tower and other equipment need to be added, which not only causes the increase of treatment cost, but also does not realize the complete removal of ammonia nitrogen - i.e. conversion to nitrogen, which still needs to be combined with other treatment methods for further degradation of ammonia nitrogen. The above key technical problems seriously restrict the popularization and application of electrochemical denitrification technology. SUMMARY

[0004] The present application aims to provide an electrochemical short-range denitrification method, which does not pollute with gaseous ammonia, does not need to add chloride salt, and does not produce strong alkaline wastewater.

[0005] The present application can be achieved by the following technical solution: an electrochemical short-range denitrification method, which uses copper electrode as cathode, shape-stable electrode as anode, pre-adds a certain amount of sulfamic acid to nitrate wastewater, adjusts the pH value of the nitrate wastewater to a suitable range, and then passes the nitrate wastewater into an electrolytic cell for electrolytic denitrification treatment.

[0006] As one of the products of nitrate reduction reaction, nitrate is reduced to nitrite only by undergoing a 2-electron reduction process, and compared with reduction to ammonia, the theoretical electricity required by the former is only 1 / 4 of the latter. In addition, in a suitable pH range, nitrite can rapidly undergo diazotization reaction with sulfamic acid and ultimately produce nitrogen. The present application first adds sulfamic acid to wastewater and adjusts the pH to a suitable range, and then uses a suitable electrode material to reduce nitrate to nitrite (i.e. electrochemical short-range denitrification process). Once nitrite is generated, it will immediately react with sulfamic acid in the solution and produce nitrogen. This new type of electrochemical denitrification method not only avoids the generation of by-product ammonia nitrogen, but also greatly saves electricity and simplifies the treatment process of wastewater containing nitrate.

[0007] Preferably, during the electrolytic treatment process, the nitrogen content of the wastewater in the electrolytic cell is monitored at the same time, and the treatment process is completed when the effluent meets the indicators.

[0008] Preferably, the preparation method of the copper electrode comprises the following steps: cleaning and removing oil from the copper metal material, then removing the oxide layer, air calcining at a certain temperature, and then placing the calcined copper metal material in an electrolyte for electrochemical reduction treatment. After the treatment is completed, the electrode for electrochemical short-range denitrification is obtained.

[0009] Further preferably, the copper content in the copper metal material is not less than 90wt%, and the form of the material includes but is not limited to one of sheet, plate, net, and foam.

[0010] Further preferably, the cleaning agent used in the cleaning and oil removal process includes but is not limited to one of anhydrous ethanol, acetone, and acetonitrile, and the soaking time is 1-30 min.

[0011] Further preferably, the solvent used in the oxide layer removal process includes but is not limited to one of hydrochloric acid, sulfuric acid, and phosphoric acid, and the concentration is 0.1-1 mol / L, and the soaking time is 1-30 min.

[0012] Further preferably, the calcination temperature is 100-500℃, and the calcination time is 0.5-5 h.

[0013] Further preferably, the electrolyte used in the electrochemical reduction treatment process is a sodium sulfate solution with a concentration of 0.01-1 mol / L, the calcined copper metal material is used as the cathode in the electrolysis process, the shape-stable electrode is used as the anode (including but not limited to one of IrO2-RuO2 / Ti and PbO2 / Ti), the current density is 5-30 mA / cm 2 , the current-on time is 1-30 min, and the distance between the electrodes is 1-5 cm.

[0014] Preferably, the shape-stable electrode is one of IrO2-RuO2 / Ti and PbO2 / Ti.

[0015] Preferably, the acid solution used in the pH adjustment process is sulfuric acid with a concentration of 1-18.4 mol / L.

[0016] Preferably, the alkali solution used in the pH adjustment process is a sodium hydroxide solution with a concentration of 1-20 mol / L.

[0017] Preferably, the pH adjustment range is 2-5.

[0018] Preferably, the sulfamic acid has a purity of not less than 97%, and the ratio of the sulfamic acid addition concentration to the nitrate concentration in the wastewater is 0.5:1-1:1 (both in terms of the mass concentration of nitrogen).

[0019] Preferably, in the electrolytic denitrification process, the distance between the cathode and the anode is controlled to be 1-5 cm, and the current density is 1-15 mA / cm 2 .

[0020] The copper metal is used as the cathode material, and a large number of catalytically active sites are generated on the surface of the copper metal after the heat treatment-electrochemical reduction combined process; when electricity is supplied, the electrode can efficiently and selectively reduce the nitrate in the wastewater into nitrite, and at the same time, the chemical reaction between the sulfamic acid and the nitrite (to produce nitrogen) can be coupled to achieve efficient denitrification of the nitrate wastewater. The present application has broad application prospects in the field of wastewater denitrification.

[0021] Compared with the prior art, the present application has the following beneficial effects:

[0022] 1. The cathode of the present application selectively reduces nitrate into nitrite, which then reacts with sulfamic acid to produce nitrogen. Compared with the traditional electrochemical denitrification method of reducing nitrate into ammonia by electrochemical reduction, the novel electrochemical denitrification method proposed by the present application has the advantages of saving electricity, no gaseous ammonia pollution, no need to add chlor-alkali salt, no production of strong alkaline wastewater, fast denitrification rate, stable effluent indicators, and realization of large-scale and automated industrial wastewater treatment.

[0023] 2. The copper electrode has high catalytic activity, strong stability, low electrode preparation cost, low price, simple preparation steps and convenient preparation, and is particularly suitable for popularization and application of electrochemical short-range denitrification technology;

[0024] 3. The cathode copper material in the electrochemical denitrification method can selectively reduce nitrate to nitrite, and the electrochemical reaction and the chemical reaction (reaction of nitrite and sulfamic acid) are perfectly coupled in a suitable reaction environment (pH, current density, etc.), so as to prevent the nitrite product from being oxidized back to nitrate on the anode again;

[0025] 4. The copper electrode selectively reduces nitrate to nitrite (electrochemical short-range denitrification), and the process only involves 2 electrons, which can not only save a large amount of electric energy, but also will not produce gaseous ammonia pollution. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a scanning electron microscope image of the foamed copper before calcination in Example 1;

[0027] Figure 2 It is an X-ray powder diffraction spectrum of the foamed copper before calcination in Example 1;

[0028] Figure 3 It is a scanning electron microscope image of the foamed copper after calcination in Example 1;

[0029] Figure 4 It is an X-ray powder diffraction spectrum of the foamed copper after calcination in Example 1;

[0030] Figure 5 It is a scanning electron microscope image of the foamed copper after electro-reduction treatment in Example 1;

[0031] Figure 6 It is an X-ray powder diffraction spectrum of the foamed copper after electro-reduction treatment in Example 1;

[0032] Figure 7 It is a schematic diagram of the electrochemical short-range denitrification technology of the embodiment of the application;

[0033] Figure 8 It is a total nitrogen content change trend graph with time in Example 1;

[0034] Figure 9 It is a total nitrogen content change trend graph with time in Comparative Example 1;

[0035] Figure 10 It is a total nitrogen content change trend graph with time in Comparative Example 2;

[0036] Figure 11 It is a total nitrogen removal rate, Faraday efficiency and nitrogen selectivity change trend graph with reaction times in Example 2;

[0037] Figure 12 Figure 4 is a graph showing the trend of total nitrogen content over time in Example 3. DETAILED DESCRIPTION

[0038] The application will be described in greater detail with reference to the accompanying drawings and specific embodiments. The embodiments are implemented on the basis of the technical solutions of the application, and detailed implementation and specific operation processes are given, but the protection scope of the application is not limited to the following examples.

[0039] Example 1

[0040] In this example, the commercial copper material used is a copper foam with a copper content of more than 95%, a size of 25 mm x 32 mm (length x width), a thickness of 1.6 mm, and a pore size of 100 ppi.

[0041] Preparation of electrode:

[0042] 1) The copper foam was immersed in ethanol for 10 min, then taken out and the surface was rinsed with water, and then the copper foam was immersed in a 0.1 mol / L sulfuric acid solution for 5 min, then taken out and the surface was rinsed with water, and then dried, and the scanning electron microscope image of the surface morphology and the X-ray powder diffraction spectrum thereof are shown in Figure 1 , Figure 2 .

[0043] 2) The cleaned copper foam was placed in a muffle furnace for calcination at a temperature of 300°C for 3 h, and the scanning electron microscope image of the surface morphology of the copper foam after calcination and the X-ray powder diffraction spectrum thereof are shown in Figure 3 , Figure 4 . It can be found that after calcination, Cu2O and CuO are generated on the surface of the copper foam, and the surface roughness increases.

[0044] 3) The calcined copper foam was used as the cathode, and IrO2-RuO2 / Ti was used as the anode (size 25 mm x 20 mm, thickness 1.5 mm), and the oxides on the surface of the copper foam were reduced by electro-reduction, with a current density of 15 mA / cm 2 , a current-on time of 10 min, an electrode distance of 2 cm, and an electrolyte of a 0.1 mol / L sodium sulfate solution. After the current was turned off, the copper foam was dried, and the scanning electron microscope image of the surface morphology and the X-ray powder diffraction spectrum thereof are shown in Figure 5 , Figure 6 . It can be found that the surface roughness of the copper foam is still high, and the oxides on the surface have been reduced.

[0045] Electrochemical denitrification process:

[0046] 1) A certain amount of sulfamic acid was added to three simulated nitrate-containing wastewater with different concentrations (506.3 mg / L, 993.3 mg / L and 5043.5 mg / L, all with a volume of 40 mL, and all with an electrolyte of 0.1 mol / L Na2SO4 solution), and the ratio of the added concentration to the nitrate concentration was 1:1 (calculated on the basis of nitrogen), and the pH value was adjusted to 3.

[0047] 2) The prepared foam copper electrode and IrO2-RuO2 / Ti (with a size of 25 mm x 20 mm and a thickness of 1.5 mm) were respectively connected to the negative and positive poles of the power supply.

[0048] 3) Power was turned on for electrolysis, and the current density was 3 mA / cm 2 , 5 mA / cm 2 , and 10 mA / cm 2 (corresponding to the three kinds of nitrate-containing wastewater in order), and the electrode distance was 1.5 cm.

[0049] When the power was turned on, the nitrate was reduced to nitrite at the cathode, and at the same time, the nitrite immediately underwent diazotization with sulfamic acid and finally generated nitrogen gas. The technical principle of the electrochemical denitrification process is shown in Figure 7 .

[0050] Total nitrogen concentration = nitrate concentration + nitrite concentration + ammonia nitrogen concentration

[0051]

[0052]

[0053]

[0054] After the reaction, the relevant index results are shown in Table 1 and Table 2, and the total nitrogen concentration change with time is shown in Figure 8 .

[0055] Table 1

[0056]

[0057]

[0058] Table 2

[0059]

[0060] The above results show that the electrochemical short-range denitrification method proposed in the application exhibits excellent denitrification effect for each concentration of nitrate wastewater, especially in the treatment of high-concentration nitrate wastewater, the Faraday efficiency is close to 100%, which means that the effective utilization rate of electrons is close to 100%, and the electrons consumed by the side reactions (including hydrogen evolution reaction and nitrate reduction to ammonia reaction) are very few.

[0061] Comparative Example 1

[0062] In order to verify the influence of the heat treatment-electrochemical reduction composite process on the activity of the electrode, in this comparative example, only the copper metal material is subjected to oil removal and oxide removal treatment.

[0063] In this comparative example, the commercial metal copper material used is a copper foam with a copper content of more than 95%, a size of 25mm x 32mm (length x width), a thickness of 1.6mm, and a pore size of 100ppi.

[0064] Electrode preparation: immerse the copper foam in ethanol for 10min, then take it out and rinse the surface with water, then immerse the copper foam in a 0.1mol / L sulfuric acid solution for 5min, then take it out and rinse the surface with water, and then perform drying treatment.

[0065] Electrochemical denitrification process:

[0066] 1) A certain amount of sulfamic acid is added to the simulated nitrate-containing wastewater (40mL, electrolyte is 0.1mol / L Na2SO4 solution) in advance (the ratio of the addition concentration to the nitrate concentration is 1:1, calculated on the basis of nitrogen), and the pH value is adjusted to 3.

[0067] 2) The prepared copper foam electrode and IrO2-RuO2 / Ti (size of 25mm x 20mm, thickness of 1.5mm) are respectively connected to the negative and positive electrodes of the power supply.

[0068] 3) Power on for electrolysis, current density is 3mA / cm 2 , electrode distance is 1.5cm.

[0069] After the reaction is completed, the relevant index results are shown in Tables 3 and 4, and the total nitrogen concentration change with time is shown in Table 5. Figure 9

[0070] Table 3

[0071]

[0072] Table 4

[0073] Total nitrogen removal rate (%) Faradaic efficiency (%) Nitrogen selectivity (%) 35.5 28.6 70.5

[0074] ​In the present comparative example, the foam copper without the heat treatment-electrochemical reduction composite process has poor catalytic activity, which shows that the two electrode treatment steps are essential to improve the catalytic activity of the electrode.

[0075] Comparative Example 2

[0076] To verify the effect of sulfamic acid, in the present comparative example, no sulfamic acid is added to the nitrate-containing wastewater in advance, and the preparation process of the electrode is consistent with that of Example 1.

[0077] Electrochemical denitrification process:

[0078] 1) The pH value of the simulated nitrate-containing wastewater (40 mL, electrolyte is 0.1 mol / L Na2SO4 solution) is adjusted to 3.

[0079] 2) The prepared foam copper electrode and IrO2-RuO2 / Ti (size is 25 mm x 20 mm, thickness is 1.5 mm) are respectively connected to the negative and positive electrodes of the power supply.

[0080] 3) Power on for electrolysis, and the current density is 3 mA / cm 2 , and the electrode distance is 1.5 cm.

[0081] After the reaction, the relevant index results are shown in Tables 5 and 6, and the total nitrogen concentration change with time is shown in Table 7. Figure 10

[0082] Table 5

[0083]

[0084] Table 6

[0085] Total nitrogen removal rate (%) Faradaic efficiency (%) Nitrogen selectivity (%) 0.4 0.3 4.8

[0086] In the present comparative example, no sulfamic acid is added, which causes the accumulation of nitrite generated in the cathode, and the nitrite is further oxidized to nitrate in the anode, which ultimately leads to the complete loss of the effect of the entire electrochemical denitrification reaction, and the total nitrogen removal rate is only 0.4%.

[0087] Example 2

[0088] To verify the high stability of the activity of the copper electrode prepared in the present application, in the present example, the cycle test of the electrode activity is carried out, and the electrode preparation process is consistent with that of Example 1.

[0089] Electrochemical denitrification process:

[0090] 1) The pH value of the simulated nitrate-containing wastewater (40 mL, electrolyte is 0.1 mol / L Na2SO4 solution) is adjusted to 3. ​

[0091] 2) The prepared copper foam electrode and IrO2-RuO2 / Ti (size 25mm x 20mm, thickness 1.5mm) are respectively connected with the negative and positive poles of the power supply.

[0092] 3) Power on for electrolysis, the current density is 3mA / cm 2 , and the electrode distance is 1.5cm. After the single electrolysis experiment is finished, the electrode is washed with water, and then is directly used for the next electrolysis experiment. The total test number is 30.

[0093] After the reaction is finished, the relevant index results are shown in Table 7 and Table 8, and the results of the total nitrogen removal rate, the Faraday efficiency and the nitrogen selectivity with the reaction number are shown in Table 9. Figure 11

[0094] Table 7

[0095]

[0096] Table 8

[0097]

[0098]

[0099] The copper electrode prepared in the application still shows excellent denitrification effect after multiple experiments, which indicates that the activity has high stability.

[0100] Example 3

[0101] In this embodiment, the actual wastewater is used as the treatment object, and the water treatment capacity is improved. The wastewater is from the part pickling workshop of China Commercial Aircraft Shanghai Aircraft Manufacturing Co., Ltd. The workshop produces high-concentration nitrate wastewater all year round.

[0102] In this embodiment, the commercial copper material used is foam copper (five pieces), the copper content of which is greater than 95%, the size is 100mm x 100mm (length x width), the thickness is 2.0mm, and the pore size is 80ppi.

[0103] Preparation of electrode:

[0104] 1) The foam copper is soaked in ethanol for 15min, then taken out and washed with water, and then the foam copper is soaked in a sulfuric acid solution with a concentration of 0.1mol / L for 10min, then taken out and washed with water, and then dried.

[0105] 2) The cleaned foam copper is placed in a muffle furnace for calcination treatment, the calcination temperature is 350℃, and the calcination time is 4h.

[0106] ​3) The calcined foam copper is used as a cathode, and IrO2-RuO2 / Ti is used as an anode (100 mm x 100 mm in size and 1.5 mm in thickness), and the oxides on the surface of the foam copper are reduced by electric reduction, the current density is 15 mA / cm 2 , the power-on time is 20 min, the electrode distance is 2 cm, and the electrolyte is a sodium sulfate solution with a concentration of 0.1 mol / L. After power-on, the foam copper is dried.

[0107] Electrochemical denitrification process:

[0108] 1) A certain amount of sulfamic acid is added to the nitrate wastewater (4.7 L, the wastewater quality is shown in Table 9), the ratio of the sulfamic acid concentration to the nitrate nitrogen concentration is 1:1 (calculated by nitrogen), and the pH value is adjusted to 3.

[0109] 2) The prepared foam copper electrode (five pieces) and IrO2-RuO2 / Ti (four pieces, 100 mm x 100 mm in size and 1.5 mm in thickness) are connected to the negative and positive poles of the power supply, respectively, and the cathode and anode are arranged in an interlaced and equal height manner, and the electrode is immersed in water to a depth of 70 mm.

[0110] 3) Power-on electrolysis is performed, the current density is 5 mA / cm 2 , and the electrode distance is 1.5 cm.

[0111] Table 9

[0112]

[0113]

[0114] After the reaction, the relevant index results are shown in Tables 10 and 11, and the total nitrogen concentration change with time is shown in Figure 12 .

[0115] Table 10

[0116]

[0117] Table 11

[0118] Total nitrogen removal rate (%) Faradaic efficiency (%) Nitrogen selectivity (%) 95.8 74.2 96.9

[0119] In this embodiment, for actual nitrate wastewater, the electrochemical short-cut denitrification denitrification proposed by the present application still has excellent denitrification efficiency, and shows broad application prospects.

[0120] The foregoing description of the embodiments has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the application to the precise form disclosed. Modifications and variations are possible in light of the above teachings or can be acquired from practice of the application. As well, the description is presented in the context of the preferred embodiments as a number of alternatives. It is not intended to limit the application to the precise form described.

Claims

1. An electrochemical short-cut denitrification method for removing nitrogen, characterized by, The copper electrode is used as a cathode, a shape-stable electrode is used as an anode, a certain amount of sulfamic acid is added into the nitrate wastewater in advance, the pH value of the nitrate wastewater is adjusted to a proper range, and then the nitrate wastewater is introduced into an electrolytic cell for electrolytic denitrification treatment; The preparation method of the copper electrode comprises the following steps: cleaning and removing oil from a copper metal material, removing an oxide layer, air calcining at a certain temperature, and then placing the calcined copper metal material in an electrolyte for electrochemical reduction treatment; The calcining temperature is 100-500 DEG C, and the calcining time is 0.5-5 h. The pH value is adjusted to 3. In the process of electrolytic denitrification, the distance between cathode and anode is controlled at 1-5 cm, and the current density is 1-15 mA / cm 2 ; The copper metal is used as a cathode material, a large number of catalytic active sites are generated on the surface of the copper metal after a heat treatment-electrochemical reduction composite process, the electrode can efficiently reduce nitrate to nitrite in wastewater when power is on, and nitrogen is generated by coupling the chemical reaction of sulfamic acid and nitrite, so that efficient denitrification of nitrate wastewater is realized. The purity of the sulfamic acid is not less than 97%, and the ratio of the dosing concentration of the sulfamic acid to the concentration of nitrate in the wastewater is 0.5:1-1:

1.

2. The electrochemical short-cut denitrification method according to claim 1, characterized in that, The copper content in the copper metal material is not less than 90wt%, and the shape of the material includes one of a sheet, a plate, a net, and a foam; the cleaning agent used in the cleaning and oil removal process includes one of anhydrous ethanol, acetone, and acetonitrile, and the soaking time is 1-30 min.

3. The electrochemical short-cut denitrification method according to claim 1, characterized in that, The solvent used in the oxide layer removal process includes one of hydrochloric acid, sulfuric acid, and phosphoric acid, the concentration is 0.1-1 mol / L, and the soaking time is 1-30 min.

4. The electrochemical short-cut denitrification method according to claim 1, wherein, The electrolyte used in the electrochemical reduction treatment process is sodium sulfate solution with a concentration of 0.01-1 mol / L, the calcined copper metal material is used as cathode, the shape-stable electrode is used as anode, the current density is 5-30 mA / cm 2 , the current time is 1-30 min, and the distance between electrodes is 1-5 cm.

5. The electrochemical short-cut denitrification method of claim 1, wherein, The shape-stable electrode is one of IrO2-RuO2 / Ti and PbO2 / Ti.

6. The electrochemical short-cut denitrification method of claim 1, wherein, The acid used in the pH adjustment process is sulfuric acid, the concentration is 1-18.4 mol / L, the base used in the pH adjustment process is sodium hydroxide solution, the concentration is 1-20 mol / L, and the pH value is adjusted to 3.

Images